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The title compound is a small molecule with many structural variations, it can illustrate a variety of internal hydrogen bonds, among other noncovalent interactions. Here we examine structures displaying hydrogen bonding between carbonyl oxygen and hydroxyl H, between carbonyl oxygen and amino H, hydroxyl H and amino N, hydroxyl O and amino H. We also consider H-bonding in its tautomer 2-oxopropanamide. By extrapolation algorithms applied to Hartree-Fock and correlation energies as estimated in HF, MP2, and CCSD calculations using the cc-pVNZ correlation-consistent basis sets (N = 2, 3, and 4) we obtain reliable relative energies of the isomeric forms. Assuming that such energy differences may be attributed to the presence of the various types of hydrogen bonding, we attempt to infer relative strengths of types of H-bonding. The Atoms in Molecules theory of Bader and the Local Vibrational Modes analysis of Cremer and Kraka are applied to this task. Hydrogen bonds are ranked by relative strength as measured by local stretching force constants, with the stronger =O…HO- &gt, NH…O= &gt, OH…N well separated from a cluster &gt, NH…O= ≈ &gt, NH…OH ≈ CH…O= of comparable and intermediate strength. Weaker but still significant interactions are of type CH…N which is stronger than CH…OH.

The possibility of carbon tetravalence in geometries other than tetrahedral and of carbon hypervalence has been taken seriously since the 1970s. Computational modeling and subsequent experimental validation have established the existence of molecules with carbon atoms with planar tetravalence and as many as six objects in carbon’s coordination sphere. In this work, we develop insight into the nature of bonding to carbon in these unusual environs as provided by Bader’s Atoms in Molecules (AIM) analysis of the electron density, along with the electron localization function (ELF) and the non-covalent index (NCI). We review several well-established systems (spiropentadiene dication, hexamethyl benzene dication, dimethanospiro[2.2]octaplane dication, and 1,8-dimethoxy-9-dimethoxyanthracene cation) and propose new D2d–symmetric variants of a hexacoordinated species.

Purely organic phosphors have important applications in imaging, sensing, informatics and illumination. Methoxy-substituted difluoroboron dibenzoylmethane (BF2dbm) complexes exhibit intense fluorescence with an almost unity quantum yield. Here we show that by simply introducing an sp3 oxygen-bridged methoxylphenyl group as a pendant to BF2dbm, the boron complex exhibits a triplet quantum yield of 0.16, a more than 100-fold increase compared to that of BF2dbm.

Room-temperature phosphorescence (RTP)-based sensors have distinctive advantages over the fluorescence counterparts, such as larger Stokes shifts and longer lifetimes. Unfortunately, almost all RTP sensors are operated on quenching-based mechanisms given the sensitive nature of the emissive triplet state. Here we report a type of thioether RTP molecules that shows RTP "turn-on" when volatile acid vapors such as HCl are in contact. To elucidate the underlying mechanism, model thioethers containing different donor/acceptor combinations are investigated via fluorescence spectroscopy and theoretical calculations aided by molecular coordinates obtained from single-crystal X-ray diffraction. It is revealed that a charge-transfer character in the phosphorescence state is crucial. The "turn-on" design concept may significantly broaden the sensing application scope for organic RTP molecules.

The preparation and properties of the complex (RMo,R)-MoTp(NO)(DMAP)(η2-α-pinene) are described (∼10 g scale; DMAP = 4-(dimethylamino)pyridine; Tp = hydridotris(pyrazolyl)borate). This complex undergoes exchange of the pinene with a wide range of other π ligands including acetone, ethyl acetate, N,N-dimethylformamide, acetonitrile, and naphthalene. Treatment of the α-pinene complex with iodine results in the complex (S)-MoTp(NO)(DMAP)(I), which is recovered in enantioenriched form (er = 99:1; yield >90%; scale 4.6 g). Reduction of this molybdenum(I) precursor results in enantioenriched molybdenum(0) complexes, including (R)-MoTp(NO)(DMAP)(η2-trifluorotoluene). Sequential treatment of this arene complex with acid, a masked enolate, and iodine regenerates MoTp(NO)(DMAP)(I) along with an alkylated 1-(trifluoromethyl)cyclohexa-1,3-diene with an er value as high as 99:1. This process demonstrates the efficient transfer of asymmetry from α-pinene to the diene product. Accompanying studies with (1R)-myrtenal rev...

Time-resolved transient absorption spectroscopy and computational analysis of D-pi-A complexes comprising Fe-II donors and Ti-IV acceptors with the general formula (Cp2Ti)-Cp-R(C(2)Fc)(2) (where RCp = Cp*, Cp, and Cp-MeOOC) and (Cp2Ti)-Cp-TMS(C(2)Fc)(C2R) (where R = Ph or CF3) are reported. The transient absorption spectra are consistent with an Fe-III/Ti-III metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer ((MMCT)-M-1) excited state for the Fe-II/Ti-IV complexes is not purely MMCT in nature but is contaminated with the higher-energy (1)Fc (d-d) state. For the diferrocenyl complexes, (Cp2Ti)-Cp-R(C(2)Fc)(2), the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the Cp-MeOOC complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased (1)Fc character. The effect of Cu-I coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp2Ti(C(2)Fc)(2).CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of Ti-IV-Fe-3(II)-Cu-I.

The production of complex molecules in ammonia&ndash, carbon dioxide ices is presumed to pass through species of formula H3N:CO2 with further addition of ammonia and carbon dioxide. One possible landmark, carbamic acid, H2NCOOH, has been implicated among the products of warming and irradiation of such ices. Experimental study of the IR spectra of residues has suggested the presence of related species, including weakly bound 1:1 and 2:1 complexes of ammonia with carbon dioxide, zwitterionic carbamic acid, ammonium carbamate, and the dimer of carbamic acid. We computed the energetics and vibrational spectra of these species as well as the complex between ammonia and carbamic acid for gas and condensed phases. By means of a new spectrum-matching scoring between computed and observed vibrational spectra, we infer species that are most probably present. The leading candidates are ammonium carbamate, the carbamic acid&ndash, ammonia complex, and the carbamic acid dimer.

The simplest ideas of antiaromaticity refer to regular monocyclic systems and the eigenfunctions of the H\"{u}ckel Hamiltonian for 4n $\pi $ electrons in such systems. The antiaromaticity is expressed in the energy penalty for such idealized systems relative to the H\"{u}ckel energy for 2n noninteracting $\pi $ pairs. Observed systems seldom achieve the regular planar geometry assumed in this picture, owing to their ability to ease the antiaromaticity penalty by departures from the regular geometry and also by export of the 4n $\pi $ electrons' charge to substituents. In this report we estimate numerical values for the stabilization derived from such departures from the structure and the charge distribution of the idealized antiaromatic cyclopropenyl anion for a specific case, 3-dehydro-3-methyl carboxylate cyclopropenyl anion 1(-) using the thermochemical scheme CBSQB3 supplemented by CCSD(T) calculations. According to the isodesmic reaction, the anion 1(-) is destabilized by about 10--15 kcal/mol relative to the saturated 3-dehydro-3-methylcarboxylate cyclopropyl anion 2(-). We propose that the anion relieves a portion of the antiaromatic destabilization by (a) pyramidalization of one carbon of the ring, and (b) export of negative charge into the ester substituent. Both of these responses are expressed in the equilibrium structure of the anion. In the course of the study we estimate the acidity of several related anions and the enthalpy of formation of their neutral conjugate acids, and describe the interconversion of 1 to the dehydrotriafulvalene anion 3(-) by reaction with CO$_{2}$.

The binding strength and collective effects of multiple H-bonds in the glycolic acid-water dimer were studied in comparison to the aromatic analog, 9-hydroxy-9-fluorene carboxylic acid (9HFCA). Quantitative analysis by the generalized Kohn-Sham energy decomposition analysis shows that the energy difference in each specific physical interaction, from a glycolic acid-water dimer to a 9HFCA-water dimer, is small and amounts to less than 5% of the binding energy of the 9HFCA-water dimer. Extensive comparison of further, similar H-bonded complexes with widely varying binding strengths reinforces their excellent analogy in that the fluorene group acts as a non-interfering spectator for intermolecular H-bonding interactions. With reference to the spectroscopic measurement on the 9HFCA-water dimer (8.51 ± 0.09 kcal mol-1), the binding energy of the glycolic acid-water dimer is estimated to be 8.51 ± 0.31 kcal mol-1, a much better accuracy than previous reports. Furthermore, correlating the infrared spectra of 9HFCA H-bonded complexes provides a circumstantial probing of the existence and consequences of cooperative and anti-cooperative behaviors in the glycolic acid-water dimer. Our studies point to the interesting H-bonding phenomena in the glycolic acid-water dimer, which may inspire challenging experiments in future.

Hydrogen (H) bonds are of fundamental importance in a wide range of molecular sciences. While the study of two-center H-bonding A⋯H is well advanced, much remains to be learned in a quantitative and definitive manner for complexes with multiple H-bonds. Exemplary cases are in the category of alpha hydroxy carboxylic acids, i.e., the complexes of glycolic acid with water and formic acid. In glycolic acid, an intramolecular H-bond forms between the carboxyl group and the alpha OH group. The alpha OH stretching frequency may be affected upon complexing with water or formic acid. Direct study of glycolic acid complexes is unfortunately very difficult. However, an aromatic analogue, 9-hydroxy-9-fluorene carboxylic acid (9HFCA), permits detailed and accurate gas phase spectroscopic studies. Since computational analysis establishes that H-bonding is very similar from glycolic acid complexes to 9HFCA complexes, direct studies on 9HFCA complexes by laser spectroscopy also deduce new and subtle information for glycolic acid complexes in terms of molecular structures, binding strength, and collective effects of multiple H-bonds associated with anti-cooperativity and cooperativity.

The neutral and cationic forms of tryptamine–water dimer present a variety of noncovalent interactions. To characterize these interactions, a series of complementary methods, including the quantum theory of atoms in molecules, noncovalent interaction plots, natural bond orbital analysis, and energy decomposition analysis, were used. For the first time, the existence of the three intermolecular H-bonds in the conformer-locked tryptamine–water dimer A–H2O are identified, highlighting a single water's role as one proton donor and two proton acceptors as it binds to tryptamine. Furthermore, upon threshold ionization of the A–H2O dimer, the network of the three intermolecular H-bonds is indeed preserved while the individual H-bonds’ binding strengths are subject to change; this is attributed to the existence of the optically accessible minimum energy isomer A+–H2O in the cation. In addition, it is found that the global minimum energy isomer H+–H2O contains a single intermolecular H-bond, but is more stable, by ca. 3 kcal mol−1, than the local minimum energy isomer A+–H2O; this is due to the stronger intramolecular interaction of H+–H2O as opposed to A+–H2O.

Gas phase conversion of acetylene to benzene, assisted by a single metal cation such as Fe(+), Ru(+) and Rh(+), offers an attractive prospect for application of computational modelling techniques to catalytic processes. Gas phase processes are not complicated by environmental effects and the participation of a single metal atom is a significant simplification. Still the process is complex, owing to the possibility of several low-energy spin states and the abundance of alternative structures. By density functional theory modelling using recently developed models with range and dispersion corrections, we locate and characterise a number of extreme points on the FeC6H6(+) surface, some of which have not been described previously. These include eta-1, eta-2 and eta-3 complexes of Fe(+) with the C4H4 ring. We identify new FeC6H6(+) structures as well, which may be landmarks for the Fe(+)-catalysed production of benzene from acetylene. The Fe(+) benzene complex is the most stable species on the FeC6H6 cation surface. With the abundant energy of complexation available in the isolated gas phase species, detachment of the Fe(+) and production of benzene can be efficient. We address the issue raised by other investigators whether multi-configurational self-consistent field methods are essential to the proper description of these systems. We find that the relative energy of intrinsically multi-determinant doublets is strongly affected, but judge that the density functional theory (DFT) description provides more accurate estimates of energetics and a more plausible reaction path.

Charge transfer (CT) transitions are relevant in the fields of solar energy conversion and nonlinear optical materials. Herein, a series of complexes with an alkynyl linkage between an aryl amine donor and a TiIV (titanocene) acceptor is reported. Each complex displays a strong (15000 < e < 24,000 M–1cm–1), low-energy (520 < λ < 560 nm) absorption ascribed to an amine to TiIV ligand-to-metal CT. This characterization is supported by UV–vis spectroscopy, cyclic voltammetry, and TD-DFT calculations. These complexes are not photostable; therefore, an alternate architecture, wherein the amine donor is appended to the titanocene cyclopentadienyl ligand, has been designed. The molar absorptivity of the amine to TiIV CT in this latter architecture is lower (2100 M–1 cm–1), indicating weaker donor–acceptor coupling. This architecture is indeed much more photostable.

Purely organic materials with room-temperature phosphorescence (RTP) are currently under intense investigation because of their potential applications in sensing, imaging, and displaying. Inspired by certain organometallic systems, where ligand-localized phosphorescence ((3) π-π*) is mediated by ligand-to-metal or metal-to-ligand charge transfer (CT) states, we now show that donor-to-acceptor CT states from the same organic molecule can also mediate π-localized RTP. In the model system of N-substituted naphthalimides (NNIs), the relatively large energy gap between the NNI-localized (1) π-π* and (3) π-π* states of the aromatic ring can be bridged by intramolecular CT states when the NNI is chemically modified with an electron donor. These NNI-based RTP materials can be easily conjugated to both synthetic and natural macromolecules, which can be used for RTP microscopy.

New gas phase infrared spectroscopy is reported for an aromatic alpha hydroxy carboxylic acid homo dimer of 9-hydroxy-9-fluorene carboxylic acid (9HFCA)2, and the hetero dimer of 9HFCA with glycolic acid. In terms of the 9-hydroxy stretching frequency, the 16 cm−1 blue-shift in the homo dimer and the 17 cm−1 blue-shift in the hetero dimer, relative to that in 9HFCA monomer, are attributed to collective effects with anti-cooperativity stronger than cooperativity. Furthermore, for the hetero dimer, the two alpha hydroxy groups’ stretching frequencies are clearly resolved, and differ by 30 cm−1. This difference represents a modest, quantitative enhancement of the intramolecular H-bond by the fluorene moiety in 9HFCA monomer, as opposed to that in glycolic acid. Accurate vibrational frequencies of the alpha OH, 3568 cm−1 in the bare glycolic acid, and 3584 cm−1 in the glycolic acid homo dimer are determined for the first time by comparison to 9HFCA monomer, homo and hetero dimers. The quantitative studies by infrared spectroscopy reveal subtle interactions among intra- and intermolecular H-bonds in the alpha hydroxyl acid dimers, which are also uniquely extended to probe each monomer's subtle intramolecular interactions.

Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti(IV) has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and Cp2Ti(C4Fc)2. Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, (TMS)Cp, and comprise the complexes (TMS)Cp2Ti(C2Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2Ph)2, which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti(IV/III) reduction wave and an Fe(II/III) oxidation that is irreversible for all complexes except (TMS)Cp2Ti(C2Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M(-1)cm(-1)ε8000 M(-1)cm(-1)) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti(IV/III) and Fe(III/II) reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, λ, have been determined using band shape analysis (2600 cm(-1)λ5300 cm(-1)) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti(IV) and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp2Ti(C2Ph)2 rapidly decomposes upon photolysis.

We report modeling studies of high spin and low spin forms of the test set of problematic cases of pseudo-octahedral Fe(II) complexes defined by Ye and Neese. These include the most thoroughly studied spin crossover compound Fe(SCN)2(phen)2 and a more recently discovered Fe(II)-2,13-dimethyl-6,9-dioxa-3,12,18-triaza-bicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentene. Reliable estimates of the often small high-spin to low-spin energy gap require balanced treatment of electronic structure in very different bonding environments. Density functionals differ in their bias toward low spin or high spin configurations. For the test set defined by Ye and Neese, we describe the estimates of their energy gaps between high-spin and low-spin forms for 75 functionals; geometries are obtained by optimization with B3LYP with zero-order relativistic approximation (ZORA) in a triple-zeta quality basis. Fairly reliable results for high-spin to low-spin energy gaps are obtained by functionals OLYP, HCTH, OPBE, and B3LYP∗, and with the ωB97XD functional expressed in the cc-pVTZ basis, about half the size of the def2-QZVPP used by Ye and Neese. The best functionals are still biased systematically toward the high spin by about 20 kJ/mol. We apply these methods in a new study of a pseudo C3v four coordinate crossover species reported by Scepaniak et al. Their four coordinate spin crossover complex of Fe with aryl-substituted scorpionate and N = P(Ph)3 ligands is modeled by a truncated structure lacking aromatic substituents. These substituents appear to stabilize the lower spin. Reproducing the 80 K crossover temperature requires an empirical correction of about 12 kJ/mol in favor of the singlet for this truncated model system.

Difluoroboron beta-diketonates (BF(2)bdks) show both fluorescence (F) and room-temperature phosphorescence (RTP) when confined to a rigid matrix, such as poly(lactic acid). These materials have been utilized as optical oxygen sensors (e.g., in tumors, wounds, and cells). Spectral features include charge transfer (CT) from the major aromatic donor to the dioxaborine acceptor. A series of naphthyl-phenyl dyes (BF(2)nbm) (1-6) were prepared to test heavy-atom placement effects. The BF(2)nbm dye (1) was substituted with Br on naphthyl (2), phenyl (3), or both rings (4) to tailor the fluorescence/phosphorescence ratio and RTP lifetimeimportant features for designing O-2 sensing dyes by means of the heavy atom effect. Computational studies identify the naphthyl ring as the major donor. Thus, Br substitution on the naphthyl ring produced greater effects on the optical properties, such as increased RTP intensity and decreased RTP lifetime compared to phenyl substitution. However, for electron-donating piperidyl-phenyl dyes (5), the phenyl aromatic is the major donor. As a result, Br substitution on the naphthyl ring (6) did not alter the optical properties significantly. Experimental data and computational modeling show the importance of Br position. The S-1 and T-1 states are described by two singly occupied MOs (SOMOs). When both of these SOMOs have substantial amplitude on the heavy atom, passage from S-1 to T-1 and emission from T-1 to S-0 are both favored. This shortens the excited-state lifetimes and enhances phosphorescence.

The thermochemical properties of superbase species derived from cyclopropeneimine are estimated by computations on systems of isodesmic reactions. Proton affinities, gas phase basicities, and pKa values are well represented by computations incorporating G4 and CBS-QB3 schemes augmented for large systems by DFT calculations with functionals M06-2X and ωB97DX in the cc-pVTZ Dunning basis. Our calculations show that relative base strengths in gas are enhanced by alkyl substitution, either by methyl groups or larger species. For acetonitrile solution, alkyl substitution seems to weaken the base. © 2013 Wiley Periodicals, Inc.

Aromatic difluoroboron β-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. To gain a better understanding of their emissive properties in both solution and polymer matrices, BF2bdks with varying aromatic groups were synthesized and their photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Absorption spectra showed systematic variations that are well correlated with structural features, including the size of the aryl substituent and the presence of a para electron-donating methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)//B3LYP/6-31G(d) formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems show intense π-π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO - k, k = 0, 1, 2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium. Based on spectroscopic and lifetime studies, BF2bdks exhibit comparable fluorescence properties in both solutions and polymers when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF2bdks with larger arene ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both solution and in PLA. There are differences in relative intensities of peaks arising from π-π* and ICT excitations depending upon dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen atmosphere in PLA matrices.

We characterize the geometry and electronic structure of isomers of the five atom Manganese cluster for its highest spin state, multiplicity 26. DFT calculations with various functionals and basis sets. The lowest energy form is either a D3h symmetric trigonal bipyramid or disphenoid C2v structure which can be considered a distorted trigonal bipyramid. The regular pentagon (D5h) is higher in energy but occupies a shallow relative minimum on the potential surface. The lowest energy square pyramid (C4v) is a saddle point; it spontaneously rearranges to the bisphenoid structure or the trigonal bipyramid very similar in energy. Analysis of the wave functions shows that the clusters can be considered distinct and weakly bound atoms with stabilization derived from interatomic charge-transfer interactions. Dispersion attraction seem to play a lesser role. The clusters have moderate static polarizabilities α but large first hyperpolarizabilities β, comparable with the reference system para-nitro aniline. Dynamic polarizabilities for an exciting frequency of 0.1 atomic units (wavelength 450 nm) are higher still.

The ionization potential (IP) of the aromatic alpha hydroxy carboxylic acid, 9-hydroxy-9-fluorene carboxylic acid (9HFCA), is shifted by complexation with hydrogen bonding ligands such as water and formic acid. Generalized Kohn-Sham energy decomposition analysis decomposes the intermolecular binding energies into a frozen energy term, polarization, correlation, and/or dispersion energy terms, as well as terms of geometric relaxation and zero point energy. We observe that in each dimer the attractive polarization always increases upon ionization, enhancing binding in the cation and shifting the IP toward the red. For 9HFCA—H2O, a substantial decrease of the repulsive frozen energy in cation further shifts the IP toward red. For 9HFCA—HCOOH, the increase of the frozen energy actually occurs in the cation and shifts the IP toward blue. Consistent with the experimental measurements, our analysis provides new, non-intuitive perspectives on multiple hydrogen bonds interactions in carboxylic acids and water complexes.

Enhanced spin-orbit coupling through external heavy-atom effect (EHE) has been routinely used to induce room-temperature phosphorescence (RTP) for purely organic molecular materials. Therefore, understanding the nature of EHE, i.e., the specific orbital interactions between the external heavy atom and the luminophore, is of essential importance in molecular design. For organic systems, halogens (e.g., Cl, Br, and I) are the most commonly seen heavy atoms serving to realize the EHE-related RTP. In this report, we conduct an investigation on how heavy-atom perturbers and aromatic luminophores interact on the basis of data obtained from crystallography. We synthesized two classes of molecular systems including N-haloalkyl-substituted carbazoles and quinolinium halides, where the luminescent molecules are considered as "base" or "acid" relative to the heavy-atom perturbers, respectively. We propose that electron donation from a π molecular orbital (MO) of the carbazole to the σ* MO of the C-X bond (π/σ*) and n electron donation to a π* MO of the quinolinium moiety (n/π*) are responsible for the EHE (RTP) in the solid state, respectively.

We use a variant of the focal point analysis to refine estimates of the relative energies of the four low-energy torsional conformers of glycolaldehyde. The most stable form is the cis-cis structure which enjoys a degree of H-bonding from hydroxyl H to carbonyl O; here dihedral angles τ1 (OCCO) and τ2 (CCOH) both are zero. We optimized structures in both CCSD(T)/aug-cc-pVDZ and aug-cc-pVTZ; the structures agree within 0.01 A for bond lengths and 1.0 degrees for valence angles, but the larger basis brings the rotational constants closer to experimental values. According to our extrapolation of CCSD(T) energies evaluated in basis sets ranging to aug-cc-pVQZ the trans-trans form (180°, 180°) has a relative energy of 12.6 kJ/mol. The trans-gauche conformer (160°, ±75°) is situated at 13.9 kJ/mol and the cis-trans form (0°, 180°) at 18.9 kJ/mol. Values are corrected for zero point vibrational energy by MP2/aug-cc-pVTZ frequencies. Modeling the vibrational spectra is best accomplished by MP2/aug-cc-pVTZ with anharmonic corrections. We compute the Watsonian parameters that define the theoretical vibrational-rotational spectra for the four stable conformers, to assist the search for these species in the interstellar medium. Six transition states are located by G4 and CBS-QB3 methods as well as extrapolation using energies for structures optimized in CCSD(T)/aug-cc-pVDZ structures. We use two isodesmic reactions with two well-established thermochemical computational schemes G4 and CBS-QB3 to estimate energy enthalpy and Gibbs energy of formation as well as the entropy of the gas phase system. Our extrapolated electronic energies of species appearing in the isodesmic reactions produce independent values of thermodynamic quantities consistent with G4 and CBS-QB3. © 2013 Wiley Periodicals, Inc.

We use the energies obtained by a focal point analysis including extrapolation from results with basis sets cc-pVnZ and aug-cc-pVnZ with n up to 4 and correlation corrections through CCSD(T), to estimate thermodynamic functions for the syn and anti isomers of cyclopropane carboxaldehyde (CPCA). These agree with values obtained by well-established thermochemical schemes CBS-QB3 and G4. The structures obtained in these studies also conform to the best experimental determination of the rotational constants in the gas phase. The kinetics of gas phase interconversion of the syn- and anti-isomers of CPCA have been studied by a chirped-pulse dynamic rotational spectroscopy. Computational modeling of the internal rotational potential allows the estimate of the interconversion rates by statistical (RRKM) methods. RRKM rates using a range of barrier heights including a CBS-Q estimate are more than 10× the rates deduced from the dynamical rotational spectra. This suggests that nonstatistical effects may be limiting the rate. Detailed study of the interconversion potential by a variant of the focal point analysis suggests that previous estimates of the barrier may be too low, and thus, the inferred rice-ramsperger-kassel-marcus (RRKM) rate could be too high. These results cast some doubt on the presence of nonstatistical effects and suggest that molecular dynamics studies should be conducted to characterize the energy flow in detail. © 2013 Wiley Periodicals, Inc.

A suite of Ru(II) complexes in which one ligand is pH responsive and the other two are varied in an effort to achieve improved photophysics has been synthesized and their potential as pH reporters assessed. The more general purpose of the study was to examine the role of the accessory ligands in heteroleptic reporter complexes and the degree to which such ligands can affect the performance of luminescent reporters. For this suite of complexes, judicious choice of the accessory ligand can alter both the pK(a)* and the dynamic range of response. It was found that the emission color and brightness were influenced by pH, but the lifetimes were only weakly affected. Surprisingly, some accessory ligands which should have improved luminescent properties essentially turned off the pH response. Several possible reasons for this observation are explored. It is suggested, and density functional theory (DFT) calculations support, that the relative π* levels of the pH sensitive and the accessory ligands are critical.

At least one bisaminoacyl-tRNA is synthesized in nature (by Thermus thermophilus phenylalanyl-tRNA synthetase), and many disubstituted tRNAs have been prepared in vitro. Such misacylated tRNAs are able to participate in protein synthesis, even though they lack the free 2'-OH group of the 3'-terminal adenosine moiety. Their ready participation in protein synthesis implies significant chemical reactivity. The basis for this reactivity has been documented previously. Surprisingly, the aminoacyl moieties of these tRNAs also exhibit exceptional chemical stability. In the present report, bisaminoacylated nucleotides are investigated computationally and experimentally to define the basis for the stability of such species. Molecular modeling of bisalanyl-AMP in the absence of solvent and in the presence of a limited number of water molecules revealed two common features among the low-energy structures. The first was the presence of H-bonding interactions between the two aminoacyl moieties. The second was the presence of a H-bonding interaction between the 2'-O-alanyl moiety and the N-3 atom of the adenine nucleobase, typically mediated through a water molecule. The prediction of an interaction between an aminoacyl moiety and the adenine nucleobase was confirmed experimentally by comparing the behavior of bisalanyl-AMP and bisalanyl-UMP in the presence of model nucleophiles. This study suggests a possible role for the adenosine moiety at the 3'-end of aminoanyl-tRNAs in controlling the stability and reactivity of the aminoacyl moiety and has important implications for the reactivity and stability of normal aminoacyl-tRNAs.

Several π-allyl complexes of {TpW(NO)(PMe3)} are investigated as possible sources of η2-diene complexes. In order to prepare the diene complex as a single diastereomer, the allyl complex must undergo deprotonation stereoselectively. Allyl complexes of this tungsten system are highly distorted, with the difference between the W−C bond lengths for the two allyl termini being as much as 0.69 A. DFT calculations and several crystal structures are presented that collectively suggest that one terminus, C1, distal to the PMe3 group, tends toward an sp2 carbocation. Consistent with this interpretation, deprotonation preferentially occurs at a carbon adjacent to this allyl terminus for six-membered rings. However, in the presence of base the five-membered cyclic analogue [TpW(NO)(PMe3)(C5H7)]+ fails to form either isomer of the corresponding η2-diene complex.

The thermochemistry of acrylic acid has presented challenges owing to its high reactivity, tendency to dimerize in the gas phase, and the existence of two very nearly equal energy conformational isomers. Well-tested thermochemical schemes including G2, G3, G4, and CBS-QB3 agree in the prediction that the s-cis syn structure is the most stable of the torsional isomers, with the s-cis anti form lying 3 kJ mol−1 or less higher in energy. Microwave spectra suggest a value of 0.63 kJ mol−1. The energy barrier between these forms is in the neighbourhood of 25 kJ mol−1 according to a MP2/cc-pVDZ calculation. We present estimates of the relative energies of all four torsional isomers and the rotational barrier based on a variant of the Focal Point Analysis developed by Csaszar and co-workers. These calculations, extending to the CCSD(T)/cc-pV5Z level, predict that the s-cis anti torsional isomer is the most stable form, in contrast to prior estimates. The s-cis syn form lies about 2.9 kJ mol−1 higher, while the s...

Small multiply charged species may be long-lived without the benefit of stabilization by a medium, primarily owing to the Coulomb barrier that impedes both formation and dissociation of the systems. We extend the category of small organic dicationic structures that may possess thermodynamic stability to include derivatives of cyanoacetylene. The linear dication HCCCNH(2+) and its isomer HCCNCH(+2) are characterized in detail; according to CCSD(T)/6-31G(d,p)//MCSCF(7,13)/6-31G(d,p) estimates, HCCCNH(2+) is thermodynamically stable by 2.8 eV and the barrier to colinear dissociation is ca 4.1 eV. The isomer HCCNCH(2+) is thermodynamically stable by about 2.1 eV and the barrier to colinear dissociation is ca 4.5 eV. Preparation of these systems by charge stripping of the predecessor H 2 C 3 N monocation may be feasible.

The addition of singly and doubly activated dienophiles to TpW(NO)(PMe3)(5,6-η2-anisole) and TpW(NO)(PMe3)(5,6-η2-1,3-dimethoxybenzene) to generate dihapto-coordinated [2.2.2]bicyclooctadienes and -trienes is explored. The highly functionalized bicyclic scaffolds are isolated from the metal fragment intact. Although moderate yields hinder their synthetic application, a unique stereoelectronic effect is uncovered. Coordination diastereomers of anisoles with the methoxy substituent distal to the PMe3 ligand have faster reaction rates relative to the proximal diastereomer. DFT calculations reveal the metal fragment possesses a dipole that directs positive charge away from the PMe3 ligand, and a polarized cycloaddition transition state is invoked to explain this observation.

Ammonia and ethane undergo interesting internal motions which are not easily described within the theory of simple harmonic motion. We discuss a series of calculations on these systems based on a Hamiltonian operator including kinetic energy terms for protons, and treating proton-electron interactions as two-particle terms. A formalism derived from Frost's “Lewis orbital” model of electronic structure is outlined, and results for planar and pyramidal NH3 and staggered and eclipsed C2H6 are reported. The possibility of interconversion of energetically equivalent “geometric configurations” suggests a calculation of the frequency of interconversion by a kind of configuration interaction related to resonance theory. Preliminary results of such calculations on NH3 are summarized.

A multiconfigurational mapping formalism is developed, by which the states of a set of reaction products are described in terms of the configurations of the original reactant system. Each reactant configuration is mapped into a wave function of the products, and an optimum mix of the mapped functions is obtained by configuration interaction. The principle governing the mapping is that the topology of bonding and antibonding in the system cannot be altered, although the shapes of the bonding and antibonding regions can be distorted by bends, twists, and stretches. Results of the calculation allow predictions of allowed or forbidden nature of reactions similar to those of Woodward and Hoffmann [1], but the new analysis is fully automated and independent of point group symmetry. Identification of the particular excited state(s) leading to certain products in thermally forbidden reactions is considerably simplified by this analysis. Calculations on relatively well-understood systems, the pyrolysis of cyclobutane and the closure of both a symmetric and chiral butadiene, illustrate this approach to the study of concerted reactions.

The spin-forbidden production of singlet molecular oxygen promoted by the complexation of triplet oxygen with acetylene is described with the aid of spin-extended orbital correspondence analysis in maximum symmetry (OCAMS). It is shown that a strong collision complex is necessary to the change in spin state owing to angular momentum requirements. Two possible reaction pathways for the spin-forbidden process are discerned; in both cases a rotation of O2 relative to acetylene must accompany the approach to the O2-acetylene complex, and its subsequent dissociation. The sense of the rotation may in one case be parallel to the C2v (maximum) symmetry axis, and may in the other case be perpendicular to the plane of the complex four-membered ring.

McIver and Stanton's rules concerning the symmetry of transition states have a counterpart in rules concerning the permutation symmetry of single steps in degenerate rearrangements, derivable with the aid of Longuet-Higgins group theory. Examples in organometallic fluxional systems and carbonium ions illustrate the rules; the analysis leads to prediction of hitherto unexplored pathways for rearrangements in ethyl and homotetrahedryl cations.

It is intuitively appealing to refer to atomic properties within a molecular environment. In particular, the total molecular charge can be apportioned among the constituent atoms in a number of plausible ways. We evade the recurring difficulty of assigning portions of the charge in an overlap region among adjacent atoms by demanding that the total charge distribution defined by atomic orbital occupation numbers match the molecular charge distribution in a least-squares sense. The formal basis for the definition of optimum atomic charges is presented, with the results of illustrative computations on a series of diatomic molecules and polyatomics.

A generalized derivation of the EPCE-F2σmethod is obtained by employing Mulliken-type approximation and the pair correlation energy expression of the many-electron theory of Sinanoglu. The correlation energy is expressed in terms of α and β density matrix elements and a small number of atomic correlation parameters. The method is employed to obtain the ionization or the bond dissociation energies of some molecular systems. The molecular SCF results are considerably improved by addition of the correlation energy contribution, in the test cases, which include only the lowest molecular states of a particular symmetry.

Musher's theory of hypervalent molecules (2) accounts for the shape of numerous molecules containing second- and third-row atoms of groups V, VI, and VII. In an attempt to examine the hypervalent bonding mechanism in a system free of competing d orbital effects, we consider three hypervalent systems of first-row atoms, H4O, FH3, and HF3. Gaussian computations employing very small lobe-representations of Slater-type aos indicate that H4O assumes a typical hypervalent geometry, with two colinear weak OH bonds and two covalent OH bonds in a normal plane. This prediction survives an improvement in basis. Our crudest computation indicates a D3h geometry for FH3, but an improved basis leads to a T-shaped molecule consistent with the theory of hypervalent molecules. F3H does not possess a stable hypervalent geometry; it is computed to be unstable relative to rare gas structure F+3 + H-. The reliability of our small basis in a system with three fluorines is questionable.

The group theory of flexible molecules developed by Longuet–Higgins is used to establish compatibility conditions between nuclear spin states and migrational states for fermions interchanged by intramolecular rearrangements in fluxional complexes of benzyl, allyl, and cyclooctatetraenyl species.

Computational modeling of optical activity, circular dichroism (CD) and optical rotatory dispersion, is rapidly becoming a useful supplement to experimental studies of absolute configuration. Here, we investigate the predictions of two alternative formulations of the rotational strength based on time-dependent density functional theory (TD-DFT), for a series of high symmetry chiral systems. We employ the TD-DFT method as realized in Gaussian 03 suite with the hybrid functional B3LYP and as incorporated in the Amsterdam density functional (ADF) suite with PBE and SAOP functionals. The high-symmetry systems described here are somewhat larger than those used to evaluate the influence of basis sets and density functional choice, and for such large systems the very extensive basis sets recommended by most investigators may not be suitable for routine use. We observe that useful results for these systems can be obtained in modest bases, and in particular that diffuse functions may not be required for informative use of the ADF implementation. The statistical average of orbital potentials (SAOP) model developed by Baerends is essential to the success of the ADF implementation. In some cases chirality is defined by features of the molecular structure remote from the chromophore. This is a severe test of the TD-DFT theory, since high-lying excitations define the most prominent features of the CD spectra, and complicates the use of computations to guide the assignment of absolute configuration. Experimental investigation of the high symmetry systems described here is desirable.

The complex TpW(NO)(PMe3)(η2-benzene) cleanly inserts into the C−F bond of fluorobenzene to form the seven-coordinate complex TpW(NO)(PMe3)(F)(Ph) (confirmed by X-ray diffraction), while no C−H insertion is detected. Treatment of this product with triethyl- or phenyldimethylsilane results in liberation of benzene and the corresponding silyl fluoride. DFT calculations suggest a low-energy reaction pathway for C−F activation involving a fluorine-bound σ-complex that does not require the presence of an η2-arene intermediate. As the fluorine content of the benzene increases, or the benzene is replaced by fluoronaphthalene, C−F addition is no longer observed, and either C−H addition or η2-coordination dominates. Two species thought to be η2-silanes are also reported.