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The lowest energy optical electronic absorption band of the three-chromophore system tris(4-bromophenyl)amine radical cation is analyzed. The lowest energy electronic transition corresponds to a p-bromophenyl orbital to nitrogen p orbital transition that places the positive charge on three equivalent p-bromophenyl chromophores. The excited electronic state is an example of excited-state mixed valence (ESMV), and the spectrum is interpreted using two ESMV models. The simplest model invokes the concept of an “effective coupling” between the three identical chromophores with an excited-state energy splitting equal to three times the coupling. A more accurate model, the “neighboring orbital model”, utilizes the coupling between the bridge’s and charge-bearing unit’s orbitals closest in energy. The three-chromophore system provides a striking illustration of the failure of an effective coupling term to account for ESMV splitting. The calculated relative energies of the diabatic and adiabatic states are different, but the calculated absorption spectra of the two models show nearly identical vibrational fine structure. Resonance Raman data and the time-dependent theory of electronic and resonance Raman spectroscopies are used to calculate the spectra. [ABSTRACT FROM AUTHOR]

We present the results of time-dependent density functional theory calculations of the optical absorption spectra of synthetic nucleobases and of their hydrogen-bonded and stacked base pairs. We focus on size-expanded analogues of the natural nucleobases obtained through the insertion of a benzene ring bonded to the planar heterocycles (x-bases), according to the protocol designed and realized by the group of Eric Kool (e.g., see:  Gao, J.; Liu, H.; Kool, E.T. Angew. Chem., Int. Ed.2005, 44, 3118, and references therein). We find that the modifications of the frontier electron orbitals with respect to natural bases, which are induced by the presence of the aromatic ring, also affect the optical response. In particular, the absorption onset is pinned by the benzene component of the HOMO of each x-base (xA, xG, xT, xC). In addition, the main trait of the H-bonding interbase coupling is a conspicuous red shift of spectral peaks in the low-energy range. Finally, the hypochromicity, a well-known fingerprint of stacking, is more pronounced in stacked xG−C and xA−T pairs than that in stacked G−C and A−T pairs, an index of enhanced stacking. [ABSTRACT FROM AUTHOR]

Two-photon absorption spectra for a range of bis(acceptor)-substituted bis(dibutoxythienyl)ethene and bis(N-hexylpyrrolyl)ethene chromophores have been recorded using Z-scan and white-light-continuum pump−probe techniques. All the chromophores studied show strong near-infrared two-photon absorption with cross sections in the range of 2400−5900 GM (1 GM 1 × 10-50cm4s/photon) at photon wavelengths between 1.0 and 1.3 m; cross sections as high as 10000 GM can be accessed close to the one-photon absorption edge. Quantum-chemical calculations reproduce the experimentally observed variations of the two-photon properties with the chemical structure. [ABSTRACT FROM AUTHOR]

TmAl3(BO3)4 crystallizes in the trigonal system R32 (No. 155) with a = b = 9.2741(13) à, c = 7.218(3) Å, oα = β = 90°, γ = 120°, V = 537.7(2) ų, Dc = 4.494 g cm-3, and Z = 3. The absorption spectrum of this crystal was recorded at room temperature. The Judd-Ofelt (J-O) theory was applied to the absorption intensities of TmAl3(BO3)4 to obtain the three J-O parameters: Ω2 = 2.40 × 10-20 cm², Ω4 = 0.48 × 10-20 cm², and Ω6 = 1.09 × 10-20 cm². The radiative probabilities, radiative lifetimes, and branching ratios of TmAl3(BO3)4 were calculated. The absorption and emission cross sections, together with the potential laser gain near 1.9 μm, were investigated. The potential laser gain curves indicate that the tunability range is about 200 nm. [ABSTRACT FROM AUTHOR]

A full multiple theoretical model (MXAN) is applied to fit picosecond difference X-ray absorption spectra at the ruthenium L3edge upon photoexcitation of aqueous RuII(bpy)32. We show that fitting difference spectra allows an increase in sensitivity, such that slight structural changes can be retrieved, which are not detected in fitting full spectra. The Ru−N bond distances of the excited complex in the 3MLCT state are in good agreement with recently published values. The implementation of the present approach to L-edge spectra and its high sensitivity opens opportunities for its extension to a large class of experiments where difference X-ray absorption spectra are recorded. [ABSTRACT FROM AUTHOR]

The high resolution, single site emission and absorption spectra of magnesium isobacteriochlorin (MgiBC) in n-octane matrixes at 7 K are reported. Its emission and Q and Soret band absorption regions have been investigated. The vibrational frequencies of the ground and the lowest energy ππ* singlet excited states were determined from luminescence and excitation spectra, respectively. The emission from MgiBC at 7 K seems to be entirely fluorescence. The luminescence and Qy region spectra of the complex are similar, having intense, narrow origin bands followed by relatively weak, but orderly vibrational structure. The Qx region does not have a clear origin and exhibits complex vibrational structure that increases in intensity going to higher energy. In the Soret region the individual ππ* origins are clearly identifiable and some vibrational structure was also observed. [ABSTRACT FROM AUTHOR]

The six multichromophoric species 1−6, containing the potentially luminescent Ru(II) polypyridine subunits and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene fluorophores (dipyrromethene−Mi2 dyes, herein after called bodipy), have been prepared and their absorption spectra, luminescence properties (both at room temperature in fluid solution and at 77 K in rigid matrix), and redox properties have been investigated (for the structural formulas of all the compounds, see Figure 1). For comparison purposes, also the same properties of the bodipy-based free ligands have been examined. Three of the multichromophoric species (1−3) are based on the Ru(bpy)3-type metal subunit, whereas 4−6 are based on the Ru(terpy)2-type metal subunit. Transient absorption spectroscopy at room temperature of all the compounds has also been performed. The absorption spectra of all the metal complexes show features that can be assigned to the Ru(II) polypyridine subunits and to the bodipy centers. In particular, the lowest energy spin-allowed π−π* transition of the bodipy groups dominates the visible region, peaking at about 530 nm. All the new complexes exhibit a rich redox behavior, with reversible processes attributed to specific sites, indicating a small perturbation of each redox center and therefore highlighting the supramolecular nature of the multichromophoric assemblies. Despite the good luminescence properties of the separated components, 1−6 do not exhibit any luminescence at room temperature; however, transient absorption spectroscopy evidences that for all of them a long-lived (microsecond time scale) excited state is formed, which is identified as the bodipy-based triplet state. Pump−probe transient absorption spectroscopy suggests that such a triplet state is formed from the promptly prepared bodipy-based 1π−π* state in most cases by the intervention of a charge-separated level. At 77 K, all the complexes except complex 1 exhibit the bodipy-based fluorescence, although with a slightly shortened lifetime compared to the corresponding free ligand(s), and 4−6 also exhibit a phosphorescence assigned to the bodipy subunits. Phosphorescence of bodipy species had never been reported in the literature to the best of our knowledge:  in the present cases we propose that it is an effective decay process thanks to the presence of the ruthenium heavy atom and of the closely lying 3MLCT state of the Ru(terpy)2-type subunits. [ABSTRACT FROM AUTHOR]

In this work, we describe the spectroscopic properties of a series of platinum complexes containing one acetylide ligand per platinum, having the chemical formula trans-Pt(PBu3)2((C≡CC6H4)n—H)Cl, n = 1−3 (designated as half-PEn-Pt) and compare their spectroscopic behavior with the well-characterized series trans-Pt(PBu3)2((C≡CC6H4)n—H)2, n = 1−3 (designated as PEn-Pt). This comparison aims to determine if the triplet state of PEn-Pt is confined to one ligand or delocalized across the central platinum atom. We measured ground-state absorption spectra, fluorescence spectra, phosphorescence spectra, and triplet-state absorption spectra. The ground-state absorption spectra and fluorescence spectra both showed a blue shift when comparing half-PEn-Pt with PEn-Pt, showing the S1 state is delocalized across the platinum. In contrast, the phosphorescence spectra of the two types of compounds had the same 0−0 band energy, showing the T1 state was confined to one ligand in PEn-Pt. The triplet state absorption spectra blue shifted when comparing half-PEn-Pt with PEn-Pt, showing the Tn state was delocalized across the central platinum. This comparison supports recently published work that suggested this confinement effect (Rogers, J. E et al. J. Chem. Phys. 2005, 122, 214701). [ABSTRACT FROM AUTHOR]

The absorption spectrum of the propargyl radical in the region from 180 to 400 nm is investigated in detail by means of theory. Vertical excitation energies and potential energy surfaces are determined by highly accurate multireference configuration interaction (MRCI) calculations. The vibrational dynamics of several electronic states is studied, and line intensities and positions are calculated with respect to the electronic and vibrational ground state. Four electronic states absorb in the region of interest:  1 2B2, 2 2B1, 2 2B2, and 3 2B1. However, electronic excitations into the 2B2 states are dipole-forbidden from the X̃ 2B1 ground state and corresponding vibronically allowed transitions are shown to be weak. The spectrum is dominated by the strong 2 2B1 ← 1 2B1 band which is computed in overall good agreement with available experiments. A strong absorption at 242 nm, which has been assigned to propargyl, is not confirmed by the calculations, and only very weak absorptions are found at wavelengths shorter than 280 nm. The present results strongly suggest that the 242 nm feature must be due to a different species. [ABSTRACT FROM AUTHOR]

Electronic absorption and resonance Raman (RR) spectra are reported for hydroxide and aqua complexes of iron(II)-protoporphyrin IX (Fe(II)PP) respectively formed in alkaline and neutral aqueous solutions. These compounds with weak axial ligand(s) represent a biomimetic approach of the unusual coordination of the atypical heme ci of membrane cytochrome b6f complexes. Absorption spectra and spectrophotometric titrations show that Fe(II)PP in alkaline aqueous cetyltrimethylammonium bromide (CTABr) binds one hydroxide ion, forming a five-coordinated high-spin (HS) complex. In alkaline aqueous ethanol, we confirm the formation of a dihydroxy complex of Fe(II)PP. In the RR spectra of Fe(II)PP dissolved in neutral aqueous CTABr, a mixture of a four-coordinated intermediate spin form with an HS monoaqua complex (Fe(II)PP(H2O)) was observed. The spectroscopic information obtained for Fe(II)PP(OH-), Fe- (II)PP(H2O), and Fe(II)PP(OH-)2 was compared with that previously reported for the 2-methylimidazole and 2-methylimidazolate complexes of Fe(II)PP, representative of the most common axial ligation in HS heme proteins. This investigation reveals a very remarkable analogy in the spectral properties of, in one hand, the Fe(II)PP(H2O) and mono-2-methylimidazole complexes and, in the other hand, the Fe(II)PP(OH-) and mono-2-methylimidazolate complexes. The comparisons of the absorption and AR spectra of Fe(II)PP(OH-) and Fe(II)PP(OH-)2 clearly establish that both a redshift of the π-π electronic transitions and an upshift of the v8 RR frequency are spectral parameters indicative of porphyrin doming in HS ferrous complexes. Based upon isotopic substitutions (16OH-,16OD-, and 18OH-), stretching modes of the Fe-OH bond(s) of a ferrous porphyrin were assigned for the first time, i.e., at 435 cm-1 for Fe(II)PP(OH) (v(Fe- (II)-OH-)) and at 421 cm-1 for Fe(II)PP(OH-)2 (vs(Fe(II)-(OH-)2). The spectroscopic and redox properties of Fe(II)PP(H2O), Fe(II)PP(OH-), and heme ci were discussed and favor a water coordination for the heme ci iron. [ABSTRACT FROM AUTHOR]

Transient UV absorption spectra and kinetics of the CH2I radical in the gas phase have been investigated at 313 K. Following laser photolysis of 1−3 mbar CH2I2 at 308 nm, transient spectra in the wavelength range 330−390 nm were measured at delay times between 60 ns and a few microseconds. The change of the absorption spectra at early times was attributed to vibrational cooling of highly excited CH2I radicals by collisional energy transfer to CH2I2 molecules. From transient absorption decays measured at specific wavelengths, time-dependent concentrations of vibrationally “hot” and “cold” CH2I and CH2I2 were extracted by kinetic modeling. In addition, the transient absorption spectrum of CH2I* radicals between 330 and 400 nm was reconstructed from the simulated concentration−time profiles. The evolution of the absorption spectra of CH2I* radicals and CH2I2* due to collisional energy transfer was simulated in the framework of a modified Sulzer−Wieland model. Additional master equation simulations for the collisional deactivation of CH2I* by CH2I2 yield 〈ΔE〉 values in reasonable agreement with earlier direct studies on the collisional relaxation of other systems. In addition, the simulations show that the shape of the vibrational population distribution of the hot CH2I* radicals has no influence on the measured UV absorption signals. The implications of our results with respect to spectral assignments in recent ultrafast spectrokinetic studies of the photolysis of CH2I2 in dense fluids are discussed. [ABSTRACT FROM AUTHOR]

Excited-state absorption spectroscopy and ionization threshold measurements for coumarin 314 (C314) adsorbed to the surface of NaCl aerosols characterize the chemical environment of the particle surface as a function of relative humidity (RH). An atmospheric pressure flow of aerosol passes through an ionization cell where two-photon laser ionization of the adsorbed molecules produces a net charge on the particle. Monitoring this charge as a function of the laser wavelength produces either the absorption spectrum of the S1 ← S0 transition or the ionization threshold. The wavelength of maximum absorption for the S1 excited-state shifts from 448 nm for RH < 5% to 441 nm for RH = 60%, indicating that adsorbed water decreases the polarity of the surface. Similarly, the ionization threshold increases from 5.10 to 5.27 eV over a similar range of RH. The decrease in polarity is attributable to changes in the local electric field experienced by C314, which is on the order of 1 × 107 V/cm, and is correlated with changes in the surface topography. Using a continuum model, we estimate the contributions to the measured thresholds of the polarization response of the surface ions and the electric field and calculate an effective dielectric constant for the adsorbed water film. For a multilayer water coverage (RH = 65%), the effective dielectric constant is approximately 2.4. These results demonstrate that the changes in surface topography with adsorbed water are as important as direct water−solute interactions in determining the solvent character of the surface. [ABSTRACT FROM AUTHOR]

An isosbestic point in the temperature dependence of an absorption spectrum is commonly viewed as a sign of interconverting chemical or structural species. Here I explore an alternative microscopic origin of temperature-independent absorbance in inhomogeneously broadened spectra: While the equilibrium probability of observing a particular frequency generally changes with the magnitude of thermal fluctuations, the weight of at least one frequency is insensitive to small changes in temperature. The range of temperature accessible to aqueous solutions at ambient pressure is sufficiently small that such insensitivity would appear in experiments as an isosbestic point. This mechanism depends only on the continuity of equilibrium distributions, not on the presence of multiple components. Even the simplest model of a single solute species in a fluctuating environment exhibits isosbestic points, van't Hoff behavior, and, with appropriate dependence of transition frequency on configuration, multiple absorption peaks. [ABSTRACT FROM AUTHOR]

FTIR absorption spectra of water-containing ethene:Ar matrices, with compositions of ethene up to 1:10 ethene:Ar, have been recorded. Systematically increasing the concentration of ethene reveals features in the spectra consistent with the known 1:1 ethene:water complex, which subsequently disappear on further increase in ethene concentration. At high concentrations of ethene, new features are observed at 3669 and 3585 cm-1, which are red-shifted with respect to matrix-isolated &ngr;3 and &ngr;1 O−H stretching modes of water and the 1:1 ethene:water complex. These shifts are consistent with a &pgr;···H interaction of a 2:1 ethene:water complex of the form (C2H4···H−O−H···C2H4). The analogous (C2D4)2H2O complex shows little shifting from positions associated with (C2H4)2H2O, while the (C2H4)2D2O isotopomer shows large shifts to 2722.3 and 2617.2 cm-1, having identical &ngr;3(H2O)/&ngr;3(D2O) and &ngr;1(H2O)/&ngr;1(D2O) values when compared with monomeric water isotopomers. Features at 3626.1 and 2666.2 cm-1 are also observed and are attributed to (C2H4)2HDO. DFT calculations at the B3LYP/6-311+G(d,p) level for each isotopomer are presented, and the predicted vibrational frequencies are directly compared with experimental values. The interaction energy for the formation of the 2:1 ethene:water complex from the 1:1 ethene:water complex is also presented. [ABSTRACT FROM AUTHOR]

A series of 4′-tolylterpyridyl platinum(II) complexes with different arylacetylide ligands, namely, phenylacetylide, 4-bromophenylacetylide, 4-nitrophenylacetylide, 4-methoxyphenylacetylide, 4-dimethylaminophenylacetylide, 1-naphthylaoetylide, and 3-quinolinylacotylide, were synthesized. Their photophysical properties, such as electronic absorption spectra, emission characteristics at room temperature and 77 K, and transient difference absorption spectra, have been investigated. All of these complexes exhibit a metal-to-ligand charge-transfer (1MLCT) transition at ca. 420–430 nm in their electronic absorption spectra. For ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, an additional solvatochromic charge-transfer band appears at ca. 460–540 nm. This band is sensitive to the para substituents on the phenylacetylide ligand and is tentatively assigned to a metal- or/and acetylide-to-terpyridyl charge-transfer transition (i.e., a 1MLCT or/and 1LLCT transition). All of the complexes exhibit room- temperature phosphorescence. The emission can be attributed to a 3MLCT state except for ttpy-C6H4NO2-4, for which the emission likely originates from an intraligand 3π,π∗ state involving the nitrophenylacetylide ligand. For ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, there probably is more than one low-energy state in close energy proximity, resulting in multiple exponential decays. In addition, the triplet transient absorption difference spectra of ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4NO2-4, and ttpy-Quin exhibit moderately intense, broad absorption bands in the visible region and extending into the near-IR region, which likely originate from the same excited state that emits or from a state that is in equilibrium with the emitting state. It appears that the electron-rich arylacetylide ligands, especially 4-methoxyphenylacetylide and 4-dimethylaminophenylacetylide, cause a decrease of the emission efficiency and disappearance of the transient absorption. In contrast, the complexes that exhibit positive absorption bands in the visible spectral region of the triplet transient difference absorption spectra show substantial optical limiting for nanosecond laser pulses at 532 nm. [ABSTRACT FROM AUTHOR]

Femtosecond time-resolved absorption spectroscopy has been used to elucidate the excited- state dynamics associated with formation of the ²E excited state in a CrIII transition metal complex. Cr- (acac)3 (where acac is the deprotonated monoanion of acetylacetone) exhibits monophasic decay kinetics with τ = 1.1 ± 0.1 Ps following excitation into the lowest-energy ligand-field absorption band; the time constant is found to be independent of both excitation and probe wavelength across the entire 4A2 → 4T2absorption envelope. The lack of a significant shift in the excited-state absorption spectrum combined with the observed spectral narrowing is consistent with an assignment of this process as vibrational cooling (kvib) in the ²E state. The data on Cr(acac)3 indicate that intersystem crossing associated with the 4T2→ ²E conversion occurs at a rate kISC > 1013 S-1 and furthermore competes effectively with vibrational relaxation in the initially formed 4T2 state. Excitation into the higher energy 4LMCT state (&lemda;ex = 336 nm) gives rise to biphasic kinetics with τ1 = 50 ± 20 fs and τ2 2 = 1.2 ± 0.2 Ps. The slower component is again assigned to vibrational cooling in the ²E state, whereas the subpicosecond process is attributed to conversion from the charge-transfer to the ligand-field manifold. In addition to detailing a process central to the photophysics of CrIII, these results reinforce the notion that the conventional picture of excited-state dynamics in which kvib > kIC > kISC does not generally apply when describing excited-state formation in transition metal complexes. [ABSTRACT FROM AUTHOR]

Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O ↔ HO2−H2O was determined to be K2 = (5.2 ± 3.2) × 10-19 cm3 molecule-1 at 297 K. Substituting K2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2−H2O was estimated to be (1.5 ± 0.1) × 10-11 cm3 molecule-1 s-1 at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2−H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2−H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2−H2O]/[HO2] was estimated from the value of K2 to be 0.19 ± 0.11. [ABSTRACT FROM AUTHOR]

A combined synthetic/spectroscopic/computational approach has been employed to prepare and characterize a series of Fe(II)-thiolate complexes that model the square-pyramidal [Fe(II)(N(His))4(SCys)] structure of the reduced active site of superoxide reductases (SORs), a class of enzymes that detoxify superoxide in air-sensitive organisms. The high-spin (S = 2) Fe(II) complexes [(Me4cyclam)Fe(SC6H4-p-OMe)]OTf (2) and [FeL]PF6 (3) (where Me4cyclam = 1,4,8,11-tetramethylcyclam and L is the pentadentate monoanion of 1-thioethyl-4,8,11-trimethylcyclam) were synthesized and subjected to structural, magnetic, and electrochemical characterization. X-ray crystallographic studies confirm that 2 and 3 possess an N4S donor set similar to that found for the SOR active site and reveal molecular geometries intermediate between square pyramidal and trigonal bipyramidal for both complexes. Electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies were utilized, in conjunction with density functional theory (DFT) and semiemperical INDO/S-CI calculations, to probe the ground and excited states of complexes 2 and 3, as well as the previously reported Fe(II) SOR model [(L8py2)Fe(SC6H4-p-Me)]BF4 (1) (where L8py2 is a tetradentate pyridyl-appended diazacyclooctane macrocycle). These studies allow for a detailed interpretation of the S→Fe(II) charge transfer transitions observed in the absorption and MCD spectra of complexes 1-3 and provide significant insights into the nature of Fe(II)-S bonding in complexes with axial thiolate ligation. Of the three models investigated, complex 3 exhibits an absorption spectrum that is particularly similar to the one reported for the reduced SOR enzyme (SORred), suggesting that this model accurately mimics key elements of the electronic structure of the enzyme active site; namely, highly covalent Fe-S π- and σ-interactions. These spectral similarities are shown to arise from the fact that 3 contains an alkyl thiolate tethered to the equatorial cyclam ring, resulting in a thiolate orientation that is very similar to the one adopted by the Cys residue in the SORred active site. Possible implications of our results with respect to the electronic structure and reactivity of SORred are discussed. [ABSTRACT FROM AUTHOR]

The UV absorption spectrum of the permanganate anion is a prototype transition-metal complex spectrum. Despite this being a simple d0 Td system, for which a beautiful spectrum with detailed vibrational structure has been available since 1967, the assignment of the second and third bands is still very controversial. The issue can be resolved only by an elucidation of the intricate vibronic structure of the spectrum. We investigate the vibronic coupling by means of linear-response time-dependent density functional calculations. By means of a diabatizing scheme that employs the transition densities obtained in the TDDFT calculations in many geometries around Re, we construct a Taylor series expansion in the normal coordinates of a diabatic potential energy matrix, coupling 24 excited states. The simulated vibronic structure is in good agreement with the experimental absorption spectrum after the adjustment of some of the calculated vertical excitation energies. The peculiar blurred vibronic structure of the second band, which is a very distinctive feature of the experimental spectrum, is fully reproduced in the calculations. It is caused by the double-well shape of the adiabatic energy surface along the Jahn−Teller active e mode of the allowed 1E state arising from the second 1T2 state, which exhibits a Jahn−Teller splitting into 1B2 and 1E states. We trace the double-well shape to an avoided crossing between two diabatic states with different orbital-excitation character. The crossing can be explained at the molecular orbital level from the Jahn−Teller splitting of the set of 7t2{3dxy, 3dxz, 3dyz} orbitals (the LUMO + 1), to which the excitations characterizing the diabatic states take place. In contrast to its character in the two well regions, at Re the 21T2 state is not predominantly an excitation to the LUMO + 1, but has more HOMO − 1 → LUMO (2e = {3dx2-y2, 3dz2}) character. The changing character of the 21T2 − 1E state along the e mode implies that the assignment of the experimental bands to single orbital transitions is too simplistic intrinsically. This spectrum, and notably the blurring of the vibronic structure in the second band, can be understood only from the extensive configurational mixing and vibronic coupling between the excited states. This solves the long-standing assignment problem of these bands. [ABSTRACT FROM AUTHOR]

A correlation function formalism is applied to compute the two-photon absorption spectrum of benzene. Using harmonic Hamiltonians for the ground and excited electronic states, we find that the theory agrees qualitatively with the experimentally observed sparsity of the thermal two-photon absorption spectrum as compared with the single-photon absorption spectrum. An expression for the average vibrational energy in the excited state is derived. We find that cooling of the nascent vibrational energy in the electronically excited state is not as extensive in the two-photon absorption process as compared to the single-photon case. [ABSTRACT FROM AUTHOR]

The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)]·DMF (1), and cate-cholate based (H2bispictn)[Mn2III(Cl4Cat)4(DMF)2] (2) (bispicen: N,N'-bis(2-pyridyl-methyl)-l,2-ethanediarnine, bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl4Cat = tetrachlorocatecholate dianion, and Cl4SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffrac-tion study. The electronic structures have been elucidated by UV-visNIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramole-cular electron transfer, a phenomenon known as valence tautomerism (VT), has been followed by electronic absorption spectroscopy. For 1, the isomeric form [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] is favored at Iow temperature, while at an elevated temperature, the [FeII(bispicen)(Cl4SQ)2] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the MnIII(Cat)2- moiety is admixed with its mixed valence semiquinone-catecholate isomer MnII(SQ)(Cat)-, and the electronic absorption spectrum is dominated by the mixed charged species. The origin of the intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form, MnII(Cl4Cat)(Cl4SQ)-. The observation of VT in complex 1 is the first example where a mixed valence semiquinonecatecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes. [ABSTRACT FROM AUTHOR]

The electronic structure of a monolayer-protected gold cluster, [Au13(SCH3)8]3+, has been investigated by performing density functional calculations. The cluster has a characteristic structure with Oh molecular symmetry, and eight (111) facets of a centered cuboctahedral Au13 core cluster are fully passivated by eight methanethiolates. The bond distance between two neighboring gold atoms (3.673 Å) is much larger than that of the bare Au13 cluster (2.929 Å), whereas the Au-S bond distance is 2.403 Å. This atomic rearrangement means that the methanethiolates stabilize the enlarged bare Au13 cluster by bonding to the (111) hollow sites of the bare cluster. The absorption spectrum of the [Au13(SCH3)8]3+ cluster is simulated within time-dependent density functional theory. The spectrum shows clear absorption peaks and each peak is assigned to specific excitation processes. [ABSTRACT FROM AUTHOR]

Optical absorption spectroscopy was used to characterize the acid-induced conformational transition of horse heart ferrocytochrome c in the presence of urea. By using linear extrapolation to zero denaturant concentration, an apparent pK value for denaturation was found to be 0.86 ± 0.07 at 25 °C. Visible absorption spectra in the presence of high urea concentration indicate that the dominant population is a high-spin, five-coordinate form under acidic conditions. Ferricytochrome c, used as a model reference system, shows a linear dependence of pK values versus urea concentration in the range from 0 to 4.1 M. Our data also indicate that even at a pH below 2 the iron-sulfur bond in ferrocytochrome c is present. [ABSTRACT FROM AUTHOR]