Search

The crystal structure and electronic spectra of the Th symmetry hexanitritoytterbate(III) anion have been studied in Cs2NaY0.96Yb0.04(NO2)6, which crystallizes in the cubic space group Fm3¯. The emission from Yb3+ can be excited via the NO2- antenna. The latter electronic transition is situated at more than twice the energy of the former, but at room temperature, one photon absorbed at 470 nm in the triplet state produces no more than one photon emitted. Some degree of quantum cutting is observed at 298 K under 420 nm excitation into the singlet state and at 25 K using excitation into either state. The quantum efficiency is ~10% at 25 K. The energy level scheme of Yb3+ has been deduced from excitation and emission spectra and calculated by crystal field theory. New improved energy level calculations are also reported for the Cs2NaLn(NO2)6 (Ln = Pr, Eu, Tb) series using the f-Spectra package. The neat crystal Cs2NaYb(NO2)6 has also been studied, but results were unsatisfactory due to sample decomposition, and this chemical instability makes it unsuitable for applications. [ABSTRACT FROM AUTHOR]

Spectroscopically relevant properties in photosynthetic reaction centers change during charge separation. In this paper, we focus on incorporation of the complete set of environmental fluctuations in the modeling of the nonlinear spectra of molecular aggregates. The model is applied in simulations of two-dimensional electronic spectra of a photosynthetic reaction center at low temperature (5 K), where spectral lines are narrow, such that more features can be resolved. We show that vertical cross sections of the simulated two-dimensional spectra (with all populations in the lowest excited state) reveal transient hole-burned spectra excited resonantly within the B band in agreement with experiment, thus providing new insight into environmental fluctuation parameters of Rhodobacter sphaeroides at low temperatures. Correlated fluctuations of molecular parameters are found to be necessary to describe charge separated configurations of molecular excited states. [ABSTRACT FROM AUTHOR]

Electronic spectra often exhibit vibronic structure when vibrational and electronic transitions occur in concert. Theory reveals (1) that orbital symmetry considerations determine specific roles played by the nuclear degrees of freedom and (2) that the vibrational excitation is often highly regiospecific, that is, attributable to an identifiable subset of atoms within the molecule. Spectra obtained from a chromium(III) complex involving a macrocyclic ligand and two axially disposed butadiynide groups nicely illustrate many of the concepts involved. [ABSTRACT FROM AUTHOR]

A fully quantum mechanical (QM) treatment to calculate electronic absorption (UV-vis) and circular dichroism (CD) spectra of typical biomolecules with thousands of atoms is presented. With our highly efficient sTDA-xTB method, spectra averaged along structures from molecular dynamics (MD) simulations can be computed in a reasonable time frame on standard desktop computers. This way, nonequilibrium structure and conformational, as well as purely quantum mechanical effects like charge-transfer or exciton-coupling, are included. Different from other contemporary approaches, the entire system is treated quantum mechanically and neither fragmentation nor system-specific adjustment is necessary. Among the systems considered are a large DNA fragment, oligopeptides, and even entire proteins in an implicit solvent. We propose the method in tandem with experimental spectroscopy or X-ray studies for the elucidation of complex (bio)molecular structures including metallo-proteins like myoglobin. [ABSTRACT FROM AUTHOR]

Photofragmentation electronic spectra of isolated single-isomeric N-protonated quinoline (quinolinium) and isoquinoline (isoquinolinium) ions have been measured at a temperature of ~40 K using a mass-selective, 10 cm-1 spectral resolution, photodissociation spectrometer. Additionally, ab initio adiabatic transition energies calculated using the RI-ADC(2) method have been employed to assist in the assignment of the spectra. Three electronic transitions having ππ* character were clearly evidenced for both protonated ions within the UV and deep-UV spectral ranges. The corresponding spectra at room temperature were previously reported by Hansen et al., together with TD-DFT calculations and a careful analysis of the possible fragmentation mechanisms. This information will be complemented in the present study by appending better resolved spectra, characteristic of cold ions, in which well-defined vibrational progressions associated with the S1 ← S0 and S3 ← S0 transitions exhibit clear 0-0 bands at hν0-0 = 27868 and 42230 cm-1, for protonated quinoline, and at hν0-0 = 28043 and 41988 cm-1, for protonated isoquinoline. Active vibrations in the spectra were assigned with the help of calculated normal modes, looking very similar to those of the structurally related protonated naphthalene. Finally, we have observed that the bandwidths associated with the deep-UV S3 ← S0 transition denote a lifetime for the S3 excited state in the subpicosecond time scale, in contrast with that of S1. [ABSTRACT FROM AUTHOR]

The lowest two electronically excited singlet states of indole and its derivatives are labeled as La or Lb, based on the orientation of the transition dipole moment (TDM) and the magnitude of the permanent electric dipole moment. Rotationally resolved electronic Stark spectroscopy in combination with high level ab initio calculations offers the possibility to determine these characteristics and thus the electronic nature of the excited states. In the present contribution this approach was pursued for the systems 4- and 6-fluoroindole and the results compared to the previously investigated system 5-fluoroindole. Changing the position of the fluorine atom from 5 to 4 or 6 is accompanied by an increasing amount of La character in the S1 state. This dramatically influences the orientation of the TDM and erases its ability to be a reasonable identifier of the nature of the excited states for both molecules. However, for 4-fluoroindole, where the influence of the La is weak, the nature of the S1 state can still be assigned to be mainly Lb based on the excited state dipole moment. For 6-fluoroindole, this is not the case anymore, and the La/Lb nomenclature completely breaks down due to heavily mixed excited states. [ABSTRACT FROM AUTHOR]

A model for simulating the transient electronic absorption spectra of donor-acceptor dyads undergoing ultrafast intramolecular charge transfer in solution has been developed. It is based on the stochastic multichannel point-transition approach and includes the reorganization of high-frequency intramolecular modes (treated quantum mechanically) and of low frequency intramolecular and solvent modes (described classically). The relaxation of the slow modes is assumed to be exponential with time constants taken from experiments. The excited-state dynamics is obtained by simulating the population distribution of each quantum state after optical excitation and upon electronic and vibrational transitions. This model was used to simulate the transient electronic absorption spectra measured previously with a pyrylium phenolate in acetonitrile. A very good agreement between the simulated and measured spectra was obtained assuming a three-level model including the ground state, the optically excited state, and a dark state with large charge-transfer character and a substantially different geometry relative to that of the optically excited state. The merit of this approach to disentangle the contributions of both population changes and relaxation processes to the ultrafast spectral dynamics will be discussed. [ABSTRACT FROM AUTHOR]

In this article, density functional theory in conjunction with Monte Carlo statistical mechanical simulation was used to investigate the electronic structure, reduction potential, solvation, and solvent effects on the electronic spectra of nitrosyl ammine complexes using [Ru(NH3)5(NO)]2+/3+ as model compounds. In addition, ligand exchange reactions with solvent water molecules were also investigated. It is shown that the complexes are involved in strong hydrogen bonds in aqueous solution, with mean average energies of −13.5 ± 0.4 and −22.4 ± 0.4 kcal mol-1 for Ru(II) and Ru(III), respectively. Interestingly, for all the complexes studied, the NO ligand is not involved in hydrogen bonding interactions in aqueous solution. These strong hydrogen bonds are responsible for the high stability of these complexes in aqueous solution, showing formation constants Kf greater than 1021. The complex [Ru(NH3)5(NO)]3+ can easily be reduced by biological reducing agents in both the singlet and triplet states; however, the reduction is easier in the triplet state, which has a positive reduction potential of 1.70 V. The formation of [Ru(NH3)5(NO)]3+ in its most stable singlet state may take place through at least two singlet-triplet surface crossings leading to nonadiabatic effects. The existence of the minimum-energy crossing points makes the release of NO from the triplet state more favorable, with an activation energy almost seven times lower (∼6 kcal mol-1). [ABSTRACT FROM AUTHOR]

We report the electronic spectra of mass selected [(bpy)(tpy)Ru-OH2]2+·(H2O)n clusters (bpy = 2,2′-bipyridine, tpy =2,2′:6′2″-terpyridine, n = 0-4) in the spectral region of their metal-to-ligand charge transfer bands. The spectra of the mono- and dihydrate clusters exhibit partially resolved individual electronic transitions. The water network forming at the aqua ligand leads to a rapid solvatochromic shift of the peak of the band envelope: addition of only four solvent water molecules can recover 78% of the solvatochromic shift in bulk solution. The sequential shift of the band shows a clear change in behavior with the closing of the first hydration shell. We compare our experimental data to density function theory (DFT) calculations for the ground and excited states. [ABSTRACT FROM AUTHOR]

The electronic and infrared spectra of the 5-methyl-2-furanylmethyl (MFM) radical have been characterized under jet-cooled conditions in the gas phase. This resonance-stabilized radical is formed by H atom loss from one of the methyl groups of 2,5-dimethylfuran (DMF), a promising second-generation biofuel. As a resonance-stabilized radical, it plays an important role in the flame chemistry of DMF. The D0-D1 transition was studied using two-color resonant two-photon ionization (2C-R2PI) spectroscopy. The electronic origin is in the middle of the visible spectrum (21934 cm-1 = 455.9 nm) and is accompanied by Franck-Condon activity involving the hindered methyl rotor. The frequencies and intensities are fit to a one-dimensional methyl rotor potential, using the calculated form of the ground state potential. The methyl rotor reports sensitively on the local electronic environment and how it changes with electronic excitation, shifting from a preferred ground state orientation with one CH in-plane and anti to the furan oxygen, to an orientation in the excited state in which one CH group is axial to the plane of the furan ring. Ground and excited state alkyl CH stretch infrared spectra are recorded using resonant ion-dip infrared (RIDIR) spectroscopy, offering a complementary view of the methyl group and its response to electronic excitation. Dramatic changes in the CH stretch transitions with electronic state reflect the changing preference for the methyl group orientation. [ABSTRACT FROM AUTHOR]

Electronic spectra of C6H are measured in the 18 950-21 100 cm-1 domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C6H are presented, all probing the vibrational manifold of the B²Π electronically excited state. The assignments are guided by electronic spectra available from matrix isolation work, isotopic substitution experiments (yielding also spectra for 13C6H and C6D), predictions from ab initio calculations, and rotational fitting and vibrational contour simulations using the available ground state parameters as obtained from microwave experiments. Besides the 00 0 origin band, three nondegenerate stretching vibrations along the linear backbone of the C6H molecule are assigned: the ν6 mode associated with the C-C bond vibration and the ν4 and ν3 modes associated with CC triple bonds. For the two lowest ν11 and ν10 bending modes, a Renner-Teller analysis is performed identifying the μ²Σ(ν11) and both μ²Σ(ν10) and κ²Σ(ν10) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as μ²Σ(ν9) and μ²Σ(ν8) levels. In the excitation region below the first nondegenerate vibration (ν6), some ²Π-²Π transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a ²Π ²Π transition found above the ν6 level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B²Π state of C6H is derived and presented. [ABSTRACT FROM AUTHOR]

The Fenna-Matthews-Olson (FMO) trimer (composed of identical subunits) from the green sulfur bacterium Chlorobaculum tepidum is an important protein model system to study exciton dynamics and excitation energy transfer (EET) in photosynthetic complexes. In addition, FMO is a popular model for excitonic calculations, with many theoretical parameter sets reported describing different linear and nonlinear optical spectra. Due to fast exciton relaxation within each subunit, intermonomer EET results predominantly from the lowest energy exciton states (contributed to by BChl a 3 and 4). Using experimentally determined shapes for the spectral densities, simulated optical spectra are obtained for the entire FMO trimer. Simultaneous fits of low-temperature absorption, fluorescence, and hole-burned spectra place constraints on the determined pigment site energies, providing a new Hamiltonian that should be further tested to improve modeling of 2D electronic spectroscopy data and our understanding of coherent and dissipation effects in this important protein complex. [ABSTRACT FROM AUTHOR]

Tetrapyrrole-based pigments play a crucial role in photosynthesis as principal light absorbers in light-harvesting chemical systems. As such, accurate theoretical descriptions of the electronic absorption spectra of these pigments will aid in the proper description and understanding of the overall photophysics of photosynthesis. In this work, time-dependent density functional theory (TD-DFT) at the CAM-B3LYP/6-31G* level of theory is employed to produce the theoretical absorption spectra of several tetrapyrrole-based pigments. However, the application of TD-DFT to large systems with several hundreds of atoms can become computationally prohibitive. Therefore, in this study, TD-DFT calculations with reduced orbital spaces (ROSs) that exclude portions of occupied and virtual orbitals are pursued as a viable, computationally cost-effective alternative to conventional TD-DFT calculations. The effects of reducing orbital space size on theoretical spectra are qualitatively and quantitatively described, and both conventional and ROS results are benchmarked against experimental absorption spectra of various tetrapyrrole-based pigments. The orbital reduction approach is also applied to a large natural pigment assembly that comprises the principal light-absorbing component of the reaction center in purple bacteria. Overall, we find that TD-DFT calculations with proper and judicious orbital space reductions can adequately reproduce conventional, full orbital space, TD-DFT results of all pigments studied in this work. [ABSTRACT FROM AUTHOR]

Properties of the tetraphenylcyclobutadienecyclopentadienylnickel(II) cation 1 and its tetra-o-fluoro derivative 1a have been measured and calculated. The B3LYP/TZP optimized geometry of the free cation 1 agrees with a single-crystal X-ray diffraction structure except that in the crystal one of the phenyl substituents is strongly twisted to permit a close-packing interaction of two of its hydrogens with a nearby BF4- anion. The low-energy parts of the solution electronic absorption and magnetic circular dichroism (MCD) spectra of 1 and 1a have been interpreted by comparison with TD-DFT (B3LYP/TZP) results. Reduction or pulse radiolysis lead to a neutral 19-electron radical, whose visible absorption and MCD spectra have been recorded and interpreted as well. The reduction is facilitated by ~0.1 V upon going from 1 to 1a. Unsuccessful attempts to prepare several other aryl substituted derivatives of 1 by the classical synthetic route are described in the Supporting Information. [ABSTRACT FROM AUTHOR]

An effective time dependent approach based on a method that is similar to the Gaussian wave packet propagation (GWP) technique of Heller is developed for the computation of vibrationally resolved electronic spectra at finite temperatures in the harmonic, Franck-Condon/Hertzberg-Teller approximations. Since the vibrational thermal density matrix of the ground electronic surface and the time evolution operator on that surface commute, it is possible to write the spectrum generating correlation function as a trace of the time evolved doorway state. In the stated approximations, the doorway state is a superposition of the harmonic oscillator zero and one quantum eigenfunctions and thus can be propagated by the GWP. The algorithm has an O(N3) dependence on the number of vibrational modes. An application to pyrene absorption spectrum at two temperatures is presented as a proof of the concept. [ABSTRACT FROM AUTHOR]

We report electronic spectra of a series of ruthenium polypyridine complexes of the form [(trpy)(bipy)RuII-L]2+ (bipy = 2,2'-bipyridine and trpy = 2,2':6',2"-terpyridine), where L represents a small molecular ligand that occupies the last coordination site. Species with L = H2O, CO2, CH3CN, and N2 were investigated in vacuo using photodissociation spectroscopy. All species exhibit bright metal-to-ligand charger transfer (MLCT) bands in the visible and near UV, but with different spectral envelopes and peak energies, encoding the influence of the ligand L on the electronic structure of the complex. Several individual electronic bands can be resolved for L = H2O and CO2, while the spectra for L = N2 and CH3CN are more congested, even at low ion temperatures. The experimental results are discussed in the framework of time-dependent density functional theory. [ABSTRACT FROM AUTHOR]

Silver clusters develop within DNA strands and become optical chromophores with diverse electronic spectra and wide-ranging emission intensities. These studies consider a specific cluster that absorbs at 400 nm, has low emission, and exclusively develops with single-stranded oligonucleotides. It is also a chameleon-like chromophore that can be transformed into different highly emissive fluorophores. We describe four characteristics of this species and conclude that it is highly oxidized yet also metallic. One, the cluster size was determined via electrospray ionization mass spectrometry. A common silver mass is measured with different oligonucleotides and thereby supports a Ag10 cluster. Two, the cluster charge was determined by mass spectrometry and Ag L3-edge X-ray absorption near-edge structure spectroscopy. Respectively, the conjugate mass and the integrated white-line intensity support a partially oxidized cluster with a +6 and +6.5 charge, respectively. Three, the cluster chirality was gauged by circular dichroism spectroscopy. This chirality changes with the length and sequence of its DNA hosts, and these studies identified a dispersed binding site with ~20 nucleobases. Four, the structure of this complex was investigated via Ag K-edge extended X-ray absorption fine structure spectroscopy. A multishell fitting analysis identified three unique scattering environments with corresponding bond lengths, coordination numbers, and Debye-Waller factors for each. Collectively, these findings support the following conclusion: a Ag10+6 cluster develops within a 20-nucleobase DNA binding site, and this complex segregates into a compact, metal-like silver core that weakly links to an encapsulating silver-DNA shell. We consider different models that account for silver-silver coordination within the core. [ABSTRACT FROM AUTHOR]

Superatom state-resolved dynamics of the Au25(SC8H9)18- monolayer-protected cluster (MPC) were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The electronic ground state of the Au25(SC8H9)18- MPC is described by an eight-electron P-like superatom orbital. Hot electron relaxation (200 ± 15 fs) within the superatom D manifold of lowest-unoccupied molecular orbitals was resolved from hot hole relaxation (290 ± 20 fs) in the superatom P states by using 2DES in a partially collinear pump-probe geometry. Electronic relaxation dynamics mediated by specific superatom states were distinguished by examining the time-dependent cross-peak amplitudes for specific excitation and detection photon energy combinations. Quantification of the time-dependent amplitudes and energy positions of cross peaks in the 2.21/1.85 eV (excitation/detection) region confirmed that an apparent energetic blue shift observed for transient bleach signals results from rapid hot electron relaxation in the superatom D states. The combination of structurally precise MPCs and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices. [ABSTRACT FROM AUTHOR]

The ground and many excited states of the Mg+He van der Waals molecular system have been explored using a one-electron pseudopotential approach. In this approach, effective potentials are used to consider the Mg2+ core and the electron-He effects. Furthermore, a core-core interaction is included. This has reduced the number of active electrons of the Mg+He, to be considered in the calculation, to a single valence electron. This has permitted to use extended Gaussian basis sets for Mg and He. Therefore, the potentianl energy and dipole moments calculations are carried out at the Hartree-Fock level of theory, and the spin-orbit effect is included using a semiclassical approach. The core-core interaction for the Mg2+He ground state is included using accurate CCSD(T) calculations. The spectroscopic constants of the Mg+He electronic states are extracted and compared with the existing theoretical works, where very good agreement is observed. Moreover, the transition dipole function has been determined for a large and dense grid of internuclear distances including the spin-orbit effect. [ABSTRACT FROM AUTHOR]

Extending our earlier studies on cyclophanes, we here report the structure, chemical shifts, spin-spin coupling constants, absorption and emission properties of [m.n]paracyclophanes, m, n = 2-4, obtained using a combination of experimental and computational techniques. Accurate values of proton chemical shifts as well as of JHH for the bridges are determined. The experimental chemical shifts, coupling constants, absorption and emission wavelengths are satisfactorily reproduced using density functional theory calculations, using both the B3LYP and ?B97X-D functionals. The geometries predicted using a functional that includes dispersion corrections (?B97X-D) are in a better agreement with available experimental values than those obtained using the B3LYP method. Up to 8 UV-vis absorption/emission bands have been observed (or anticipated in the region below 200 nm) and assigned on the basis of quantum-chemical calculations. Optimized excited-state geometries showed that the distances between the aromatic bridgehead carbon atoms of all the [m.n]paracyclophanes in the excited state decrease compared to the ground-state geometries by ca. 0.2-0.9 Å, the largest being for [4.4]paracyclophane, though the rather large differences in the calculated emission wavelength compared to experiment cast some doubts on the accuracy of the excited-state geometries. [ABSTRACT FROM AUTHOR]

Using an ab initio approach based on nonempirical pseudopotentials for the Mg2+ and Li+ cores, Gaussian basis sets, effective core polarization potentials and full configuration interaction calculations, the adiabatic potential energy curves, the spectroscopic constants, the permanent and transition electric dipole moments of the several lowest electronic states of the alkali-alkaline earth ion MgLi+ have been performed. These states dissociate into Mg+(3s and 3p) + Li (2s, 2p, 3s, 3p, 3d, 4s, 4p, and 4d) and Mg (3s², 3s3p, 3s4s, 3s3d, 3s4p, 3s5s, and 3s4d) + Li+. The spectroscopic constants (Re, De, Te, ωe, ωexe, and Be) of the ground state and nearly 53 excited states of 1,3Σ+, 1,3Π, and 1,3Δ symmetries are derived. Most of them are computed for the first time. Moreover, several avoided crossings between the electronic states of 1,3Σ+, 1,3Π symmetries are localized and analyzed. Their existence is related to the interaction between the potential energy curves and to the charge transfer process between the two ionic systems Mg+Li and MgLi+. Furthermore, accurate adiabatic permanent and transition dipole moments for several transitions have been calculated for a large and dense grid of internuclear distances for the first 15 ¹Σ+ electronic states. A linear behavior is observed in the permanent dipole moments for several electronic states. Additionally, the transition electric dipole moments between neighbor states have shown many peaks situated around the avoided crossing positions. [ABSTRACT FROM AUTHOR]

Transient electronic and vibrational absorption spectroscopy unravel the mechanisms and dynamics of bimolecular reactions of CN radicals with acetone in deuterated chloroform solutions. The CN radicals are produced by ultrafast ultraviolet photolysis of dissolved ICN. Two reactive forms of CN radicals are distinguished by their electronic absorption bands: "free" (uncomplexed) CN radicals, and "solvated" CN radicals that are complexed with solvent molecules. The lifetimes of the free CN radicals are limited to a few picoseconds following their photolytic production because of geminate recombination to ICN and INC, complexation with CDCl3 molecules, and reaction with acetone. The acetone reaction occurs with a rate coefficient of (8.0 ± 0.5) × 1010 M-1 s-1 and transient vibrational spectra in the C-N and C-O stretching regions reveal that both the nascent HCN and 2-oxopropyl (CH3C(O)CH2) radical products are vibrationally excited. The rate coefficient for the reaction of solvated CN with acetone is 40 times slower than for free CN, with a rate coefficient of (2.0 ± 0.9) × 109 M-1 s-1 obtained from the rise in the HCN product v1(C-N stretch) IR absorption band. Evidence is also presented for CN complexes with acetone that are more strongly bound than the CN-CDCl3 complexes because of CN interactions with the carbonyl group. The rates of reactions of these more strongly associated radicals are slower still. [ABSTRACT FROM AUTHOR]

Molecular conformational changesin electronic excited states play a key role in numerous light-activatedprocesses. In the case of porphyrin oligomers intramolecular twistinginfluences energy and charge transport dynamics. Here we address thetwisting reaction in both ground and excited states in a model porphyrindimer, employing two-dimensional electronic spectroscopy (2D ES).By spreading the information over excitation and detection frequencies,cross-peaks reveal the twisting reaction in both the ground and excitedstates unambiguously and distinctly from other dynamics. A quasi-barrierlessplanarization reaction is observed in the excited state on a tensof picoseconds time scale. This is accompanied by a spectral narrowing,indicative of a reduction in conformational disorder. The reversereaction is suppressed in the excited state due to a steep activationenergy barrier. However, in the ground state the barrier is withinthe thermal energy distribution, and therefore contributions fromreverse and forward reactions could be observed on the subnanosecondtime scale. Crucially 2D ES enables simultaneous assessment of groundand excited state reactions through analysis of different spectralregions on the 2D spectral maps. [ABSTRACT FROM AUTHOR]

Center line slope (CLS) analysisin 2D infrared spectroscopy has been extensively used to extract frequency–frequencycorrelation functions of vibrational transitions. We apply this conceptto 2D electronic spectroscopy, where CLS is a measure of electronicgap fluctuations. The two domains, infrared and electronic, possessdifferences: In the infrared, the frequency fluctuations are classical,often slow and Gaussian. In contrast, electronic spectra are subjectto fast spectral diffusion and affected by underdamped vibrationalwavepackets in addition to Stokes shift. All these effects resultin non-Gaussian peak profiles. Here, we extend CLS-analysis beyondGaussian line shapes and test the developed methodology on a solvatedmolecule, zinc phthalocyanine. We find that CLS facilitates the interpretationof 2D electronic spectra by reducing their complexity to one dimension.In this way, CLS provides a highly sensitive measure of model parametersdescribing electronic–vibrational and electronic–solventinteraction. [ABSTRACT FROM AUTHOR]

Surface enhanced Raman spectroelectrochemistry (SERS) spectroelectrochemistry provides a very sensitive technique to investigate the vibrational characteristics of coordination compounds and their particular behavior under the influence of plasmonic surfaces, concomitant with the exploitation of their redox properties and electronic spectra. The results, however, depend upon the mechanisms involved in the intensification of Raman spectra associated with the electromagnetic, resonance Raman and charge-transfer excitation at the Fermi levels. By probing the model complex [(Ru3O)(CH3COO)6(4,4′-bipy)3]n (n = 1, 0, −1) adsorbed onto rough gold electrode surfaces, contrasting SERS profiles were obtained at several successive redox potentials and oxidation states, which enables a critical discussion on the role of the complex interaction with the gold surface, and the influence of the specific electronic bands in the triruthenium acetate cluster. Density functional theory (DFT) and time-dependent DFT calculations were carried out for the complex bound to an Au20 cluster to show the participation of active lowest unoccupied molecular orbital levels centered on the gold atoms. The corresponding charge-transfer band was predicted around 1200 nm, which supports a charge-transfer interpretation for the SERS response observed at λexc = 1064 nm. The selective enhancement of the vibrational modes was discussed based on the Raman theoretical calculations. [ABSTRACT FROM AUTHOR]

We refine analytical representationsof the potential energy surfaces for the 1 2A′,2 2A′, and 1 2A″ electronic statesof BeOH in simultaneous least-squares fittings of experimental and ab initiodata. These optimizations are carried out withthe MORBID and RENNER programs, using input data from Mascaritolo,Merritt, Heaven, and Jensen (Experimental and Theoretical Characterization of the 22A′-12A′ Transition of BeOH/D. J. Phys. Chem. A.2013, 117, 13654−13663). Withthe refined potential energy surfaces, we make predictions of energylevels for BeOH/D that have not been experimentally determined sofar. We hope that these predictions will permit further assignmentsto be made in the electronic spectra of BeOH/D. [ABSTRACT FROM AUTHOR]

We have performed broad-band two-dimensional(2D) electronic spectroscopyof light-harvesting complex II (LHCII) at ambient temperature. Wefound that electronic dephasing occurs within ∼60 fs and inhomogeneousbroadening is approximately 120 cm–1. A three-dimensionalglobal fit analysis allows us to identify several time scales in thedynamics of the 2D spectra ranging from 100 fs to ∼10 ps andto uncover the energy-transfer pathways in LHCII. In particular, theenergy transfer between the chlorophyll b and chlorophyll a poolsoccurs within ∼1.1 ps. Retrieved 2D decay-associated spectraalso uncover the spectral positions of corresponding diagonal peaksin the 2D spectra. Residuals in the decay traces exhibit periodicmodulations with different oscillation periods. However, only oneof them can be associated with the excitonic cross-peaks in the 2Dspectrum, while the remaining ones are presumably of vibrational origin.For the interpretation of the spectroscopic data, we have applieda refined exciton model for LHCII. It reproduces the linear absorption,circular dichroism, and 2D spectra at different waiting times. Severalcomponents of the energy transport are revealed from theoretical simulationsthat agree well with the experimental observations. [ABSTRACT FROM AUTHOR]

Reduction of (4,0)-Ru2(chp)4Cl (1) (chp = 6-chloro-2-oxypyridinate) with Zn or FeCl2 yields a series of axial ligand adducts of the Ru2(II,II) species Ru2(chp)4(L), with L = tetrahydrofuran (2), dimethyl sulfoxide (DMSO; 3), PPh3 (4), pyridine (5), or MeCN (6). Zn reduction in noncoordinating solvents such as toluene or CH2Cl2 leads to the dimeric species [Ru2(chp)4]2 (7) or [Ru2(chp)4]2(ZnCl2) (8), whereas addition of strongly σ-donating ligands such as CO causes cleavage of the Ru-Ru bond. Density functional theory (DFT) models of these complexes, the axially free species, and the axial adducts of several other potential ligands (H2O, NH3, CH2Cl2, S-bound DMSO, N2, and CO) indicate that these compounds can be divided into three distinct categories, based on their Ru-Ru bond length and electronic structure. Compounds 2, 3, 5, 6, 7, and 8, the hypothetical axially free species, and adducts of H2O and NH3 fit in Category 1 with a (δ*)²(π*)² ground state, as indicated by their electronic spectra, magnetic properties, and Ru-Ru bond distances. However, compound 4 and the CH2Cl2 adduct (Category 2) show a pseudo-Jahn-Teller distortion and spectroscopic signs of δ*/π* orbital mixing suggestive of a new electronic ground state intermediate between the (δ*)²(π*)² and (δ*)¹(π*)³ configurations. Category 3 consists of the hypothetical adducts of N2, S-bound DMSO, and CO, all of which are predicted to have a (δ*)¹(π*)³ configuration. Electronic spectra were recorded and assigned using time-dependent DFT, allowing assignment of a band in the 10 000-13 000 cm-1 range as the δ → π* transition. The axial ligand's π-acid character heavily influences the δ*−π* gap, and thereby the ground-state electronic configuration, but not the axial ligand binding strength, which is dictated more by the σ-donor character of the ligands. Thus, this work greatly expands the number of axial ligand adducts known for Ru2(II,II) complexes supported by N,O-donor ligands and provides a predictive theoretical framework for their stability and electronic structures. [ABSTRACT FROM AUTHOR]

The electronic spectroscopy of isolated toluene in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 4.0-10.8 eV energy range, with absolute cross-section measurements derived. We present the first set of ab initio calculations (vertical energies and oscillator strengths), which we use in the assignment of valence and Rydberg transitions of the toluene molecule. The spectrum reveals several new features not previously reported in the literature, with particular relevance to 7.989 and 8.958 eV, which are here tentatively assigned to the p*(17a') ? s(15a') and 1p*(10a?) ? 1p(14a') transitions, respectively. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of toluene in the upper stratosphere (20-50 km). [ABSTRACT FROM AUTHOR]

The first borate-vacant deca-vanadoborateof formula (NH3CH2CH2CH2NH3)5[(NH3CH2CH2CH2NH3)V10B24O66H8]·13.23H2O with an occluded1,3-propanediammonium cation, using moltenmethylboronic acid flux reaction is reported. This cluster lacks fourborate units with respect to all known deca-vanadoborates. It crystallizesin the orthorhombic P21212space group which is non-centrosymmetric, leading to the first non-centrosymmetricdeca-vanadoborate reported in the literature. Even though the deca-vanadoboratesformed by 28 borate units have an inner space large enough to hosta molecule, no examples have been reported before. The electronicproperties were studied using electronic spectroscopy, corroboratinga fully reduced species, due to the lack of intervalence charge transfertransitions in the 1000 to 1500 nm region. The magnetic behavior providedevidence that the studied cluster presents strong antiferromagnetismamong the ten VIVspin-carriers, with an S= 0 ground state. Using a model considering three different exchangepathways, three different Jvalues were obtained,all of them antiferromagnetic in nature. [ABSTRACT FROM AUTHOR]

One-dimensional coordination polymer [Co(Piv)2(4-ptz)(C2H5OH)2]n (compound 1, Piv- = pivalate, 4-ptz = 2,4,6-tris(4-pyridyl)-1,3,5-triazine) was synthesized by interaction of CoII pivalate with 4-ptz. Desolvation of 1 led to formation of [Co(Piv)2(4-ptz)]n (compound 2), which adsorbed N2 and H2 at 78 K as a typical microporous sorbent. In contrast, absorption of methanol and ethanol by 2 at 295 K led to structural transformation probably connected with coordination of these alcohols to CoII. Formation of 2 from 1 was accompanied by change of color of sample from orange to brown and more than 2-fold decrease of molar magnetic susceptibility (χM) in the temperature range from 2 to 300 K. Resolvation of 2 by ethanol or water resulted in restoration of spectral characteristics and χM values almost to the level of that of 1. χMT versus T curves for 1 and samples, obtained by resolvation of 2 by H2O or C2H5OH, were fitted using a model for CoII complex with zero-field splitting of this ion. [ABSTRACT FROM AUTHOR]

The Light Harvesting 2 (LH2) complexis a vital part of the photosystemof purple bacteria. It is responsible for the absorption of lightand transport of the resulting excitations to the reaction centerin a highly efficient manner. A general description of the chromophoresand the interaction with their local environment is crucial to understandthis highly efficient energy transport. Here we include this interactionin an atomistic way using mixed quantum-classical (molecular dynamics)simulations of spectra. In particular, we present the first atomisticsimulation of nonlinear optical spectra for LH2 and use it to studythe energy transport within the complex. We show that the frequencydistributions of the pigments strongly depend on their positions withrespect to the protein scaffold and dynamics of their local environment.Furthermore, we show that although the pigments are closely packedthe transition frequencies of neighboring pigments are essentiallyuncorrelated. We present the simulated linear absorption spectra forthe LH2 complex and provide a detailed explanation of the states responsiblefor the observed two-band structure. Finally, we discuss the energytransfer within the complex by analyzing population transfer calculationsand 2D spectra for different waiting times. We conclude that the energytransfer from the B800 ring to the B850 ring is mediated by intermediatestates that are delocalized over both rings, allowing for a stepwisedownhill energy transport. [ABSTRACT FROM AUTHOR]

The structural, electronic, and spectroscopicproperties of methanediolin aqueous solutions have been studied by a combined approach basedon Car–Parrinello molecular dynamics simulations and ab initiocalculations. The hydrogen bond interactions between the solute andwater have been characterized, showing the important role of the solventin the stabilization of the methanediol conformers in solution. Firstinsights on the experimental vibrational spectra have been obtainedby the analysis of the simulation results, with particular regardto the most prominent band at 1050 cm–1that hasbeen attributed to both the symmetric and antisymmetric CO stretchingmodes. The assignment has been completed adopting both electric andmechanical anharmonic calculations considering the interactions withthe solvent using a polarizable continuum model. [ABSTRACT FROM AUTHOR]

The vibronic coupling model of Köppel,Domcke, and Cederbaum is a powerful means to understand, predict,and analyze electronic spectra of molecules, especially those thatexhibit phenomena that involve breakdown of the Born–Oppenheimerapproximation. In this work, we describe a new parallel algorithmfor carrying out such calculations. The algorithm is conceptuallyfounded upon a “stencil” representation of the requiredcomputational steps, which motivates an efficient strategy for coarse-grainedparallelization. The equations involved in the direct-CI type diagonalizationof the model Hamiltonian are presented, the parallelization strategyis discussed in detail, and the method is illustrated by calculationsinvolving direct-product basis sets with as many as 17 vibrationalmodes and 130 billion basis functions. [ABSTRACT FROM AUTHOR]

The title compound, η5-2,7-dimethoxyfluorenido-η5-pentamethylcyclopentadienylcobalt(II)(1), was synthesized from Cp*Co(acac) and sodium 2,7-dimethoxyfluorenide. 1was oxidized with AgBF4to yield the cobaltoceniumcongener 1BF4. The structuralcomparison of both compounds reveals structural changes in the ligandscaffold. In the electronic spectra of 1, an MLCT transitionwas found that could be assigned by TD-DFT calculations. In this transitiona significant quantity of spin-density is transferred to the fluorenidoligand, allowing for possible spin-filter or spin-injection applications. [ABSTRACT FROM AUTHOR]

Electronic theory calculations areapplied to the study of theUO molecule and the UO誉. Relativistic effective corepotentials are used along with the accompanying valence spin–orbitoperators. Polarized double-ς and triple-ς basis setsare used. Molecular orbitals are obtained from state-averaged multiconfigurationself-consistent field calculations and then used in multireferencespin–orbit configuration interaction calculations with a numberof millions of terms. The ground state of UO has open shells of 5f37s1, angular momentum Ω = 4, and a spin–orbit-inducedavoided crossing near the equilibrium internuclear distance. ManyUO excited states are studied with rotational constants, intensities,and experimental comparisons. The ground state of UOof 5f3nature with Ω = 9/2. Many UO笗⊵states are also studied. The open-shell nature of both UO and UO闦 to many low-lying excited states. [ABSTRACT FROM AUTHOR]

The electronic absorption spectrumof 9,9-dimethyl-2,7-dinitrofluoreneradical anion in HMPA displays both a NIR intervalence charge transferand a visible excited state mixed valence transition. These transitionscontain a similar vibronic progression resulting from molecular orbitalsthat are common to both transitions. Vibrational frequency and intensitydata are acquired from the resonance Raman spectrum and used to calculatea best fit for the absorption spectrum. The normal coordinate distortionsare analyzed in terms of the electronic changes for both transitionsto explain their similarity. The Raman scattering intensity decreasesat lower excitation wavelength as a result of Raman de-enhancementcaused by interference between neighboring excited states. [ABSTRACT FROM AUTHOR]

A combinationof ultrafast transient electronic absorption spectroscopy(TEAS) and transient vibrational absorption spectroscopy (TVAS) isused to investigate whether photoinduced N–H bond fission,mediated by a dissociative 1πσ* state, is activein aqueous adenine (Ade) at 266 and 220 nm. In order to isolate UV/visibleand IR spectral signatures of the adeninyl radical (Ade[-H]), formedas a result of N–H bond fission, TEAS and TVAS are performedon Ade in D2O under basic conditions (pD = 12.5), whichforms Ade[-H]−anions via deprotonation at the N7or N9 sites of Ade’s 7H and 9H tautomers. At 220 nm we observeone-photon detachment of an electron from Ade[-H]−, which generates solvated electrons (eaq–) together with Ade[-H] radicals,with clear signatures in both TEAS and TVAS. Additional wavelengthdependent TEAS measurements between 240–260 nm identify a thresholdof 4.9 ± 0.1 eV (∼250 nm) for this photodetachment processin D2O. Analogous TEAS experiments on aqueous Ade at pD= 7.4 generate a similar photoproduct signal together with eaq–afterexcitation at 266 and 220 nm. These eaq–are born from ionization ofAde, together with Ade+cations, which are indistinguishablefrom Ade[-H] radicals in TEAS. Ade+and Ade[-H] are foundto have different signatures in TVAS and we verify that the pD = 7.4photoproduct signal observed in TEAS following 220 nm excitation issolely due to Ade+cations. Based on these observations,we conclude that: (i) N–H bond fission in aqueous Ade is inactiveat wavelengths ≥220 nm; and (ii) if such a channel exists inaqueous solution, its threshold is strongly blue-shifted relativeto the onset of the same process in gas phase 9H-Ade (≤233nm). In addition, we extract excited state lifetimes and vibrationalcooling dynamics for 9H-Ade and Ade[-H]−. In both cases, excited state lifetimes of <500fs are identified, while vibrational cooling occurs within a timeframe of 4–5 ps. In contrast, 7H-Ade is confirmed to have alonger excited state lifetime of ∼5–10 ps through bothTEAS and TVAS. [ABSTRACT FROM AUTHOR]

A detailed mechanistic investigationof the early stages of thePaternò–Büchi reaction following 267 nm excitationof benzaldehyde in cyclohexene has been completed using ultrafast,broadband transient UV–visible and IR absorption spectroscopies.Absorption due to electronically excited triplet state benzaldehydedecays on a 80 ps time scale via reaction with cyclohexene. The growthand subsequent decay of the biradical intermediate produced followingC–O bond formation is followed by transient vibrational spectroscopy.The biradical decays by ring closure to an oxetane or by dissociating,reforming the ground state reactants. Detailed kinetic analysis allowedderivation of quantum yields and rate constants for these competingbiradical decay processes, ϕoxetane= 0.53, ϕdiss= 0.47, koxetane= 0.27 ±0.09 ns–1and kdiss=0.24 ± 0.09 ns–1. This study provides a strikingillustration of the ways in which contemporary ultrafast transientabsorption spectroscopy methods can be used to dissect the mechanismand kinetics of a classic photoreaction. [ABSTRACT FROM AUTHOR]

Byusing a hierarchical equations-of-motion approach, we reproducethe two-dimensional electronic spectra of phycoerythrin 545 from RhodomonasCS24 at ambient temperature (294 K). The simulatedspectra are in agreement with the experimental results reported inWong et al. (Nat. Chem. 2012, 4, 396). The evolutions of cross peaks for rephasing spectraand diagonal peaks for nonrephasing spectra have also been plotted.The peaks oscillate with the population times, with frequencies, phases,and amplitudes of the oscillating curves also being qualitativelyconsistent with the experimental results. [ABSTRACT FROM AUTHOR]

9,9′-Bifluorenylidene hasbeen proposed as an alternativeand flexible electron acceptor in organic photovoltaic cells. Herewe characterize its excited state properties and photokinetics, combiningultrafast fluorescence and transient IR measurements with quantumchemical calculations. The fluorescence decay is ultrafast (sub-100fs) and remarkably independent of viscosity. This suggests that largescale structure change is not the primary relaxation mode. The ultrafastdecay populates a dark state characterized by distinct vibrationaland electronic spectra. This state decays with a 6 ps time constantto a hot ground state that ultimately populates the initial statewith a 20 ps time constant; these times are also insensitive to solventviscosity. No metastable intermediate structures are resolved in thephotocycle after population of the dark state. The implications ofthese results for the operation of 9,9′-bifluorenylidene asan electron acceptor and as a potential molecular switch are discussed. [ABSTRACT FROM AUTHOR]

Wereport a theoretical study of the structures, energetics, andelectronic spectra of the PtII/RuIImixed-metalcomplex RuPt2(tppz)2Cl2(PF6)4(tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) in acetonitrile.The hybrid B3LYP density functional theory and its TDDFT methods wereused with a complete basis set (CBS) extrapolation scheme and a conductorpolarizable continuum model (C-PCM) for solvation effects. Resultsshowed that the trinuclear complex has four types of stable conformersand/or enantiomers. They are separated by high barriers owing to therepulsive H/H geometrical constraints in tppz. A strong entropy effectwas found for the dissociation of RuPt2(tppz)2Cl2(PF6)nin acetonitrile. The UV–visibleand emission spectra of the complex were also simulated. They arein good agreement with experiments. In this work we have largely focusedon exploring the origin of anomalous electronic circular dichroism(ECD) spectra of the RuPt2(tppz)2Cl2(PF6)4complex in acetonitrile. As a result,a new mechanism has been proposed together with a clear illustrationby using a physical model. [ABSTRACT FROM AUTHOR]

Time-resolvedtwo-dimensional (2D) electronic spectra (ES) trackingthe evolution of the excited state manifolds of the retinal chromophorehave been simulated along the photoisomerization pathway in bovinerhodopsin, using a state-of-the-art hybrid QM/MM approach based onmulticonfigurational methods. Simulations of broadband 2D spectraprovide a useful picture of the overall detectable 2D signals fromthe near-infrared (NIR) to the near-ultraviolet (UV). Evolution ofthe stimulated emission (SE) and excited state absorption (ESA) 2Dsignals indicates that the S1→ SN(with N≥ 2) ESAs feature a substantialblue-shift only after bond inversion and partial rotation along the cis→ transisomerization angle,while the SE rapidly red-shifts during the photoinduced skeletal relaxationof the polyene chain. Different combinations of pulse frequenciesare proposed in order to follow the evolution of specific ESA signals.These include a two-color2DVis/NIR setup especiallysuited for tracking the evolution of the S1→ S2transitions that can be used to discriminate between differentphotochemical mechanisms of retinal photoisomerization as a functionof the environment. The reported results are consistent with the availabletime-resolved pump–probe experimental data, and may be usedfor the design of more elaborate transient 2D electronic spectroscopytechniques. [ABSTRACT FROM AUTHOR]

Theknown (tetraphenyl-η4-cyclobutadiene)-η5-cyclopentadienylcobalt (1) and a series of itsnew substituted derivatives have been prepared. The electronic statesof a few representatives have been characterized by absorption andmagnetic circular dichroism. Time-dependent density functional theoryhas been used to arrive at spectral assignments for several prominentlow-energy bands. The absorption spectra of the radical ions of 1have also been recorded. [ABSTRACT FROM AUTHOR]

Resonance-enhancedmultiphoton ionization spectroscopy of supersonicallycooled gas-phase 9-hydroxy-9-fluorene carboxylic acid (9HFCA) is reportedfor its first electronic excited state, S1. The UV–UVhole-burning experiment identifies a single conformer in the molecularbeam, stabilized by an intramolecular hydrogen bond. For this Cssymmetric conformer, twolow frequencies in the S1spectrum are assigned: an in-planerocking mode of the carboxylic acid side chain lies at 58 cm–1, and an in-plane fluorene bending mode appears at 183 cm–1. The corresponding mode frequencies in the cation, 58 and 196 cm–1, are measured by zero electron kinetic energy (ZEKE)spectroscopy upon pumping the S1vibronic states. The adiabaticionization potential is measured to be 64 923 ± 5 cm–1. In addition, a feature established by ZEKE spectroscopyupon pumping the hot band is found at 67 cm–1. Thisis assigned as a hot band of the HO–C9–COOH rockingmode in the neutral ground state. [ABSTRACT FROM AUTHOR]

Recently 1-(phenylethynyl)naphthalene(1-PEN) was suggested tobe the primary dimerization product of phenylpropargyl radicals andtherefore an important polycyclic hydrocarbon in combustion processes.Here we describe a spectroscopic investigation of a genuine 1-PENsample by several complementary techniques, infrared spectroscopy,multiphoton ionization (MPI), and threshold photoelectron spectroscopy.The infrared spectrum recorded in a gas cell confirms that 1-PEN isindeed the previously observed dimerization product of phenylpropargyl.The origin of the transition into the electronically excited S1state lies at 30823 cm–1, as found by MPI.Considerable vibrational activity is observed, and a number of low-wavenumberbands are assigned to a progression in the torsional motion. Valuesof 6 cm–1(S0) and 17 cm–1(S1) were derived for the fundamental of the torsion.In the investigated energy range the excited state lifetimes are inthe nanosecond range. Spectra of the 1-PEN/Ar cluster exhibit a redshift of the electronic origin of 22 cm–1, in goodagreement with other aromatic molecules. A threshold photoelectronspectrum recorded using synchrotron radiation yields an ionizationenergy of 7.58 eV for 1-PEN. An excited electronic state of the cationis found at 7.76 eV, and dissociative photoionization does not setin below 15 eV. [ABSTRACT FROM AUTHOR]

Wereport a combined gas phase and solution phase study of 9-fluorenone.The structure and dynamics of isolated fluorenone in the S3-state were studied by resonant enhanced multiphoton ionization withpicosecond pulses in a free jet of molecules excited between 285 and312 nm. Ionization was performed with a second ps-pulse at 351 nm.The electronic spectrum is structured, and the origin of the C 1B2← X 1A1transitionwas observed at 32 122 cm–1. Several vibrational fundamentals appear in the spectrum.In the gas phase we observe a biexponential decay, which suggestsan internal conversion to the coupled S1/S2-statewithin 10–40 ps. A further decay that is assigned to intersystemcrossing was found to be longer than 500 ps. In addition to the gasphase measurements, we studied the photophysics of 9-fluorenone incyclohexane by femtosecond-time-resolved transient absorption spectroscopyand observed very similar dynamics upon excitation to the S3state: It deactivates within 8–11 ps by internal conversion,followed by intersystem crossing within 120–150 ps, forminga long-lived triplet state. Experiments in acetonitrile, however,showed marked differences. Intersystem crossing is ineffective inpolar solvents because the lowest excited singlest state is of ππ*character and does not interact with the 3ππ*. [ABSTRACT FROM AUTHOR]

In this paper we report the spectral properties of the stable radical adducts 1*-3*, which ate formed by an electron donor moiety, the carbazole ring and an electron acceptor moiety, the polychlorotriphenylmethyl radical. The molecular structure of radical adduct 1* in the crystalline state shows a torsion angle of approximately 90° between the pheny| and the carbazole rings due to steric interactions. They exhibit a charge transfer band in the visible range of the electronic spectrum. All of them are chemically oxidized with copper(II) perchlorate to the respective cation species, which show a strong charge transfer band into the near-infrared region of the spectrum. Radical adducts 1*-3* and the corresponding stable oxidized species 1+-3+ are real organic mixed-valence compounds due to the open-shell nature of their electronic structure. Charge transfer bands of the cation species are stronger and are bathochromically shifted with respect to those of the neutral species due to the greater acceptor ability of the positively charged central carbon atom of the triphenylmethyl moiety. The cationic species 1+-3+ are diamagnetic, as shown by the absence of a signal in the EPR spectrum in acetonitrile solution at room temperature, but they show an intense and unique band in frozen solutions (183 K). [ABSTRACT FROM AUTHOR]

Ruthenium clusters of the type [Ru3(μ3-O)(OAc)6(CO)(L)(nic)], where L = 4-dimethylaminopyridine (dmap) and nic = isonicotinic acid, form hydrogen-bonded mixed-valence dimers upon a single electron reduction. Electrochemical responses show two overlapping reduction waves, indicating the presence of a thermodynamically stable mixed-valence dimer with considerable electronic coupling across the hydrogen bond. Electronic spectra of the singly reduced hydrogen-bonded mixed-valence dimer reveal two intervalence charge transfer bands in the near-infrared region consistent with a Robin-Day class II system. These bands are assigned as metal-to-metal and metal-to-bridge charge transfer, and their behavior is best described by a semiclassical three state model. Infrared spectroscopy suggests localized behavior indicating electron transfer between the two clusters is slower than 1010 s–1. [ABSTRACT FROM AUTHOR]

Computationalchemistry approach was applied to explore the natureof electron attachment to cytosine-rich DNA single strands. An oligomerdinucleoside phosphate deoxycytidylyl-3′,5′-deoxycytidine(dCpdC) was selected as a model system for investigations by densityfunctional theory. Electron distribution patterns for the radicalanions of dCpdC in aqueous solution were explored. The excess electronmay reside on the nucleobase at the 5′ position (dC•–pdC) or at the 3′ position (dCpdC•–). From comparison with electron attachment to the cytosine relatedDNA fragments, the electron affinity for the formation of the cytosine-centeredradical anion in DNA is estimated to be around 2.2 eV. Electron attachmentto cytosine sites in DNA single strands might cause perturbationsof local structural characteristics. Visible absorption spectroscopymay be applied to validate computational results and determine experimentallythe existence of the base-centered radical anion. The time-dependentDFT study shows the absorption around 550–600 nm for the cytosine-centeredradical anions of DNA oligomers. This indicates that if such speciesare detected experimentally they would be characterized by a distinctivecolor. [ABSTRACT FROM AUTHOR]