Ab initio tools for the accurate prediction of the visible spectra of anthraquinones

The UV/vis absorption spectra of 101 anthraquinones solvated in two protic solvents (methanol and ethanol) has been theoretically predicted using the time-dependent density functional theory (TD-DFT) for the excited state calculations and the polarizable continuum model (PCM) for evaluating bulk solvent effects. Two functionals (B3LYP and PBE0) have been used and they provide similar mean absolute deviations (approximately 0.09 eV) but mean signed errors presenting opposite signs. The errors can be minimized by using simple or multiple linear regression, the latter combining the results of both functionals to reach an optimal estimation of the lambda(max) (mean absolute error 0.06 eV). Specific fittings for the two media have been performed and it turned out that our approach is even more efficient for anthraquinones solvated in ethanol.     

The search for bishomoaromatic semibullvalenes and barbaralanes: computational evidence of their identification by UV/Vis and IR spectroscopy and prediction of the existence of a blue bishomoaromatic semibullvalene

Time-dependent B3LYP/6-31G calculations have been performed at the optimized C(2) or C(2v) geometries of several substituted semibullvalenes (1(deloc)) and barbaralanes (2(deloc)), to compare the computed vertical electronic excitation energies with the temperature-dependent, long-wavelength absorptions that have been observed in the UV/vis spectra of some of these compounds by Quast and co-workers. The excellent agreement between the calculated vertical excitation energies and the observed values of lambda(max) provides strong support for the identification of the bishomoaromatic species 1(deloc) and 2(deloc) as the source of these absorptions. Furthermore, the CN stretching frequencies, computed for the C(2) geometry of 1,5-dimethyl-2,6-dicyano-4,8-diphenylsemibullvalene (1f(deloc)), fit the low-frequency absorptions seen in the IR spectrum of 1f, thus furnishing independent evidence that bishomoaromatic geometries of semibullvalenes have, in fact, been observed spectroscopically. B3LYP/6-31G calculations predict that 2,6-dicyano-4,8-diphenylsemibullvalene 1c has a C(2) equilibrium geometry (1c(deloc)) and that the long-wavelength UV/vis absorption (lambda(max) = 585 nm) and CN stretching frequencies (2192 and 2194 cm(-1)) computed for 1c(deloc) should serve to identify this bishomoaromatic semibullvalene when it is synthesized.     

Assessing alkyl-, silyl-, and halo-substituent effects on the electron affinities of silyl radicals

Neutral anion energy differences for a large class of alpha-substituted silyl radicals have been computed to determine the effect of alkyl, silyl, and halo substituents on their electron affinities. In particular, we report theoretical predictions of the adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) for a series of methyl-, silyl-, and halo-substituted silyl radical compounds. This work utilizes the carefully calibrated DZP++ basis set, in conjunction with the pure BLYP and OLYP functionals, as well as with the hybrid B3LYP, BHLYP, PBE1PBE, MPW1K, and O3LYP functionals. Bromine has the largest effect in stabilizing the anions, and the BLYP/DZP++ AEA for SiBr(3) is 3.29 eV. The other predicted electron affinities are for SiH(3) (1.37 eV), SiH(2)CH(3) (1.09 eV), SiH(2)F (1.54 eV), SiH(2)Cl (1.94 eV), SiH(2)Br (2.05 eV), SiH(2)(SiH(3)) (1.77 eV), SiH(CH(3))(2) (0.92 eV), SiHF(2) (1.86 eV), SiHCl(2) (2.53 eV), SiHBr(2) (2.67 eV), Si(CH(3))(3) (0.86 eV), SiF(3) (2.66 eV), SiCl(3) (3.21 eV), Si(SiH(3))(3) (2.25 eV), and SiFClBr (3.13 eV). For the five silyl radicals where experimental data are available, the BLYP functional gives the most accurate determination of AEAs; the average absolute error is 0.04(1) eV, whereas the corresponding errors for the O3LYP, MPW1K, PBE1PBE, B3LYP, OLYP, and BHLYP functionals are 0.05(8), 0.06(0), 0.06(3), 0.08(5), 0.11(5), and 0.15(3) eV, respectively.     

An ab initio study of the absorption spectra of indirubin, isoindigo, and related derivatives

The UV/visible spectra of a series of indirubin, isoindigo, and other indigo/thioindigo related dyes have been evaluated in various solvent environments by using the time-dependent density functional theory in conjunction with the polarizable continuum model. Even for molecules of the same family, significant differences in the excitation processes have been noted. Two hybrid functionals have been selected: B3LYP and PBE0. For a set of the 50 selected molecular cases, both functionals provide accurate lambda(max), with mean absolute deviations limited to 0.1 eV. Actually, isoindigo is the main challenging series, with systematically underestimated excitation energies, due to the different nature of the excitation process. In most cases, we found that PBE0 is more efficient in reproducing the experimental values than B3LYP for sulfur-containing dyes not featuring internal hydrogen bonds, the reverse assertion being also true. In addition, the spectra of a series of unknown dyes have been predicted.     

Thioindigo dyes: highly accurate visible spectra with TD-DFT

The structure and visible spectra of a large panel of thioindigo dyes and derivatives have been evaluated using a TD-PBE0/6-311+G(2d,p)//PBE0/6-311G(d,p) approach explicitly taking bulk solvent effects into account by means of the polarizable continuum model. The influence of the solvent characteristics, the trans-cis isomerization, and the chemical substitution on the benzene rings have been investigated. In addition, hemi-thioindigo dyes, thiazine-indigo, chromophore-like molecules, and selenoindigo have been considered. Though the relative oscillator strengths of the two allowed visible transitions in the nonplanar cis isomers are not always correctly reproduced by theory, the agreement between theoretical and experimental results is far above expectations. For the 170 cases studied, we obtained a mean unsigned error on the predicted lambda(max) limited to 6.9 nm or 0.03 eV, with only 6 (4) cases for which the difference exceeds 20 nm (0.10 eV). These errors are 1 order of magnitude smaller than what has previously been reported for indigoids. A linear correlation between the central double bond length and the lambda(max) has been established, while the bond length and vibrational frequency of the carbonyl groups do not correlate with the thioindigo color. The higher excitation energies of the cis conformers, compared to the trans structures, result from a less stabilized LUMO in the former case. Indeed, for cis thioindigo, the two electron-rich (in the excited state) carbonyl units lie close to each other.     

Modulation of the lowest metal-to-ligand charge-transfer state in [Ru(bpy)2(N-N)]2+ systems by changing the N-N from hydrazone to azine: photophysical consequences

Two Ru(II) complexes, [Ru(bpy)2L](ClO4)2 (1) and [Ru(bpy)2L'](BF4)2 (2), where bpy is 2,2'-bipyridine, L is diacetyl dihydrazone, and L' 1:2 is the condensate of L and acetone, are synthesized. From X-ray crystal structures, both are found to contain distorted octahedral RuN(6)(2+) cores. NMR spectra show that the cations in 1 and 2 possess a C2 axis in solution. They display the expected metal-to-ligand charge transfer (1MLCT) band in the 400-500 nm region. Complex 1 is nonemissive at room temperature in solution as well as at 80 K. In contrast, complex 2 gives rise to an appreciable emission upon excitation at 440 nm. The room-temperature emission is centered at 730 nm (lambda(em)(max)) with a quantum yield (Phi(em)) of 0.002 and a lifetime (tau(em)) of 42 ns in an air-equilibrated methanol-ethanol solution. At 80 K, Phi(em) = 0.007 and tau(em) = 178 ns, with a lambda(em)(max) of 690 nm, which is close to the 0-0 transition, indicating an 3MLCT excited-state energy of 1.80 eV. The radiative rate constant (5 x 10(4) s(-1)) at room temperature and 80 K is almost temperature independent. From spectroelectrochemistry, it is found that bpy is easiest to reduce in 2 and that L is easiest in 1. The implications of this are that in 2 the lowest (3)MLCT state is localized on a bpy ligand and in 1 it is localized on L. Transient absorption results also support these assignments. As a consequence, even though 2 shows a fairly strong and long-lived emission from a Ru(II) --> bpy CT state, the Ru(II) --> L CT state in 1 shows no detectable emission even at 80 K.     

Structure and stability of salicylic acid-water complexes and the effect of molecular hydration on the spectral properties of salicylic acid

Density functional theory with B3LYP parametrization and 6-311++G(d,p) basis set has been used to investigate the structure and stability of salicylic acid-water complexes. The vertical excitation energies for these complexes have been computed using time-dependent density functional theory (with B3LYP parametrization and a 6-311++G(d,p) basis set). It is shown that the hydrogen bond between the carboxylic hydrogen and the oxygen of water is the strongest among all possible hydrogen bonds in the system. The hydrogen bond strength in salicylic acid-water complexes seems to be nearly additive. The change in absorption maximum (lambda(max)) corresponding to the vertical excitation energy for the first three excited singlet and triplet states of the complex with 1-3 water molecules is nominal (approximately 1-3 nm). But with the addition of the fourth water molecule, the lambda(max) for S(1) and T(1) decreases by approximately 17 nm and it increases for S(2) and S(3) by about the same amount. The decrease in lambda(max) for transition to the T(2) state on the addition of the fourth water molecule is only approximately 9 nm. There seems to be an intersystem crossing between the S(1) and T(3) states that could account for the observed fluorescence quenching of salicylic acid in water.     

An iridium(III) complex that exhibits dual mechanism nonlinear absorption

The photophysical properties of the complex (L)Ir(ppy)(2)(+), where ppy = 2-phenylpyridine and L = 4,4'-(2,2'-bipyridine-5,5'-diylbis(ethyne-2,1-diyl))bis(N,N-dihexylaniline), have been investigated under one- and two-photon excitation conditions. In THF solution, the complex exhibits broad ground-state absorption with lambda(max) approximately 500 nm and weak photoluminescence with lambda(max) approximately 730 nm. Excitation of (L)Ir(ppy)(2)(+) at 355 nm produces a long-lived excited state (tau approximately 1 mus) that features a strong excited-state absorption in the near-infrared (lambda(max) approximately 875 nm, Deltaepsilon approximately 6.1 x 10(4) M(-1) cm(-1)). Photoluminescence and transient absorption studies of (L)Ir(ppy)(2)(+) carried out using 5 ns, 1064 nm pulsed excitation demonstrate that the same long-lived and strongly absorbing excited state can be efficiently produced by two-photon absorption. Solutions of the complex in THF display nonlinear absorption of 5 ns, 1064 nm pulses in a process that is believed to involve a combination of two-photon absorption and reverse saturable absorption.     

Determination of ruthenium and osmium in each other's presence in chloride solutions by direct and third-order derivative spectrophotometry

Ruthenium and osmium (up to 20 mug Ru(Os) ml(-1)) can be determined in chloride solutions directly after absorption of RuO(4) and OsO(4) in hydrochloric acid. In 9 M HCl, RuO(4) and OsO(4) are quantitatively converted into RuCl(6)(2-) (lambda(max) = 480.0 nm, epsilon = 4.8 x 10(3) l mol(-1) cm(-1)) and OsCl(6)(2-) (lambda(max) = 334.8 nm, epsilon = 8.4 x 10(3) l mol(-1) cm(-1)) respectively. Osmium does not interfere with the determination of ruthenium in the form of the RuCl(6)(2-) complex by direct spectrophotometry. The absorbance of the obtained solution at lambda(max) = 480.0 nm corresponds only to the concentration of ruthenium. A derivative spectrophotometric method using numerical calculation of absorption spectra of the RuCl(6)(2-) and OsCl(6)(2-) complexes has been developed for the determination of osmium in a mixture with ruthenium. The interfering effect of ruthenium on the determination of osmium can be eliminated by measuring the value of a third-order derivative spectrum of the OsCl(6)(2-) complex at 350.0 nm ("zero-crossing point" of ruthenium). Simple and rapid determination of ruthenium and osmium in a calibration standard solution of the noble metals (Ru, Rh, Pd, Os, Ir, Pt and Au) for plasma spectroscopy using the proposed methods has been achieved.     

Ligand effects on luminescence of new type blue light-emitting mono(2-phenylpyridinato)iridium(III) complexes

Newly prepared hydrido iridium(III) complexes [Ir(ppy)(PPh3)2(H)L](0,+) (ppy = bidentate 2-phenylpyridinato anionic ligand; L = MeCN (1b), CO (1c), CN(-) (1d); H being trans to the nitrogen of ppy ligand) emit blue light at the emission lambda(max) (452-457, 483-487 nm) significantly shorter than those (468, 495 nm) of the chloro complex Ir(ppy)(PPh3)2(H)(Cl) (1a). Replacing ppy of 1a-d with F2ppy (2,4-difluoro-2-phenylpyridinato anion) and F2Meppy (2,4-difluoro-2-phenyl-m-methylpyridinato anion) brings further blue-shifts down to the emission lambda(max) at 439-441 and 465-467 nm with CIE color coordinates being x = 0.16 and y = 0.18-0.20 to display a deep-blue photoemission. No significant blue shift is observed by replacing PPh3 of 1a with PPh2Me to produce Ir(ppy)(PPh2Me)2(H)(Cl) (1aPPh 2Me), which displays emission lambda max at 467 and 494 nm. The chloro complexes, [Ir(ppy)(PPh3)2(Cl)(L)](0,+) (L = MeCN (2b), CO (2c), CN(-) (2d)) having a chlorine ligand trans to the nitrogen of ppy also emit deep-blue light at emission lambda(max) 452-457 and 482-487 nm.     

Density functional theory study of trans-dioxo complexes of iron, ruthenium, and osmium with saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), and detection of [Fe(qpy)(O)2]n+ (n=1, 2) by high-resolution ESI mass spectrometry

Density functional theory (DFT) calculations on trans-dioxo metal complexes containing saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), were performed with different types of density functionals (DFs): 1) pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta-GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta-GGAs, a singlet d2 ground state for trans-[Fe(O)2(NH3)2(NMeH2)2]2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at lambda approximately 509 and 515 nm in the respective ground-state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand-to-metal charge-transfer (LMCT) transition with PW91, but a pi(Fe-O)-->pi*(Fe-O) transition with B3LYP, in which pi and pi* are the bonding and antibonding combinations between the dpi(Fe) and ppi(O(2-)) orbitals. The FeVI/V reduction potential of trans-[Fe(O)2(NH3)2NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The [Fe(qpy)(O)2](n+) (qpy=2,2':6',2':6',2':6',2'-quinquepyridine; n=1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high-resolution ESI-MS spectroscopy.     

Scratching the surface of buckminsterfullerene: the barriers for Stone-Wales transformation through symmetric and asymmetric transition states

General-gradient approximation (PBE) and hybrid Hartree-Fock density functional theories (B3LYP) in conjunction with basis sets of up to polarized triple-zeta quality have been applied to study the Stone-Wales transformation of buckminsterfullerene (BF) to yield a C(60) isomer of C(2)(v) symmetry with two adjacent pentagons (#1809). In agreement with earlier investigations, two different transition states and reaction pathways could be identified for the rearrangement from BF to C(60)-C(2)(v) on the C(60) potential energy surface (PES). One has C(2) molecular point group symmetry with the two migrating carbon atoms remaining close to the fullerene surface. The other one has a high-energy carbene-like (sp(3)) structure where a single carbon atom is significantly moved away from the C(60) surface. The carbene intermediate and the second transition state along the stepwise reaction path characterized previously at lower levels of theory do not exist as stationary points with the density functionals utilized here. The classical barriers of both mechanisms are essentially identical, 6.9 eV using PBE and 7.3 eV with B3LYP.     

Molecular structure, infrared spectrum, and photochemistry of squaric acid dimethyl ester in solid argon

Squaric acid dimethyl ester (C(6)O(4)H(6); 3,4-dimethoxycyclobut-3-ene-1,2-dione; DCD) was studied by matrix isolation infrared spectroscopy and by density functional theory (B3LYP) and ab initio (MP2) calculations with the 6-31++G(d,p) and 6-311++G(d,p) basis sets. Three conformers of the compound were theoretically predicted. The two most stable conformers were identified in low-temperature argon matrixes and the energy gap between them was determined. The trans-trans conformer (C(2)(v)) was found to be more stable than the cis-trans form (C(s)) by 4.2 kJ mol(-1), in consonance with the theoretical predictions (MP2 calcd = 3.9 kJ mol(-1)). In situ broadband UV irradiation (lambda > 337 nm) of the matrix-isolated compound was found to induce the ring-opening reaction leading to production of the bisketene, 2,3-dimethoxybuta-1,3-diene-1,4-dione as well as the trans-trans --> cis-trans conformational isomerization. The latter phototransformation allowed separation of the infrared spectra of the two conformers initially trapped into a low-temperature matrix. Upon higher energy irradiation (lambda > 235 nm), the main observed photoproducts were CO and deltic acid dimethyl ester (C(5)O(3)H(6); 2,3-dimethoxycycloprop-2-en-1-one), the latter being obtained in two different conformations (trans-trans and cis-trans). According to the experimental data, deltic acid dimethyl ester is produced by decarbonylation of the initially formed bisketene and not by direct CO extrusion from DCD.     

一些密度泛函方法预测电子亲和势

选取了杂化泛函B3LYP, B3PW91, O3LYP, PBE0, 以及与之相对应的GGA泛函BLYP, BPW91, OLYP和PBE, 还选取了能更好地兼顾强相互作用和弱相互作用的X3LYP泛函和在预测NMR的化学位移有较好表现的OPBE泛函, 以及两种meta-GGA泛函VSXC和TPSS, 共12种泛函, 详细地考察了这些泛函在预测EA方面的准确性.     

Photolabile ruthenium nitrosyls with planar dicarboxamide tetradentate N(4) ligands: effects of in-plane and axial ligand strength on NO release

Four ruthenium nitrosyls, namely [(bpb)Ru(NO)(Cl)] (1), [(Me(2)bpb)Ru(NO)(Cl)] (2), [(Me(2)bpb)Ru(NO)(py)](BF(4)) (3), and [(Me(2)bqb)Ru(NO)(Cl)] (4) (H(2)bpb = 1,2-bis(pyridine-2-carboxamido)benzene, H(2)Me(2)bpb = 1,2-bis(pyridine-2-carboxamido)-4,5-dimethylbenzene, H(2)Me(2)bqb = 1,2-bis(quinaldine-2-carboxamido)-4,5-dimethylbenzene; H is the dissociable amide proton), have been synthesized and characterized by spectroscopy and X-ray diffraction analysis. All four complexes exhibit nu(NO) in the range 1830-1870 cm(-)(1) indicating the [Ru-NO](6) configuration. Clean (1)H NMR spectra in CD(3)CN (or (CD(3))(2)SO) confirm the S = 0 ground state for all four complexes. Although the complexes are thermally stable, they release NO upon illumination. Rapid NO dissociation occurs when solutions of 1-3 in acetonitrile (MeCN) or DMF are exposed to low-intensity (7 mW) UV light (lambda(max) = 302 nm). Electron paramagnetic resonance (EPR) spectra of the photolyzed solutions display anisotropic signals at g approximately 2.00 that confirm the formation of solvated low-spin Ru(III) species upon NO release. The ligand trans to bound NO namely, anionic Cl(-) and neutral pyridine, has significant effect on the electronic and NO releasing properties of these complexes. Change in the in-plane ligand strength also has effects on the rate of NO release. The absorption maximum (lambda(max)) of 4 is significantly red shifted (455 nm in DMF) compared to the lambda(max) values of 1-3 (380-395 nm in DMF) due to the extension of conjugation on the in-plane ligand frame. As a consequence, 4 is also sensitive to visible light and release NO (albeit at a slower rate) upon illumination to low-intensity visible light (lambda > 465 nm). Collectively, the photosensitivity of the present series of ruthenium nitrosyls demonstrates that the extent of NO release and their wavelength dependence can be modulated by changes of either the in-plane or the axial ligand (trans to bound NO) field strength.     

Theoretical Study and Luminescence Properties of the Cyclic Cu(3)(dppm)(3)OH(2+) Cluster. The First Luminescent Cluster Host at Room Temperature

The [Cu(3)(dppm)(3)OH](BF(4))(2) cyclic cluster host is found to be luminescent at 298 K (lambda(max) = 540 nm; tau(e) = 89 +/- 9 &mgr;s; Phi(e) = 0.14 +/- 0.01) in degassed ethanol solutions and at 77 K (lambda(max) = 480 nm; tau(e) = 170 +/- 40 &mgr;s; Phi = 0.73 +/- 0.07) also in ethanol. The nature of the lowest energy excited states has been addressed theoretically using density functional theory and experimentally using UV-visible, luminescence, and polarized luminescence spectroscopy and is found to be (1,3)A(2) arising from the.(18e)(4)(7a(2))(1)(13a(1))(1) electronic configuration. The excited state geometry optimization for the model Cu(3)(PH(3))(6)OH(2+) compound in its T(1) state ((3)A(2)) has been performed using density functional theory and compared to its ground state structure. The Cu.Cu bond length is expected to decrease greatly in the excited state (calculated DeltaQ approximately 0.47 ?), in agreement with the d(10) electronic configuration. The perturbation of the photophysical properties by the addition of two guest carboxylate anions has been investigated. From the Stern-Volmer plots, the quenching constants, k(q), are 1.65 x 10(8) and 5.10 x 10(8) M(-)(1) s(-)(1) for acetate and 4-aminobenzoate, respectively, which are also proportional to the relative binding strengths of the substrates with Cu(3)(dppm)(3)OH(2+) (i.e., acetate < 4-aminobenzoate).     

Photolysis of (3-methyl-2H-azirin-2-yl)-phenylmethanone: direct detection of a triplet vinylnitrene intermediate

The photoreactivity of (3-methyl-2H-azirin-2-yl)-phenylmethanone, 1, is wavelength-dependent (Singh et al. J. Am. Chem. Soc. 1972, 94, 1199-1206). Irradiation at short wavelengths yields 2P, whereas longer wavelengths produce 3P. Laser flash photolysis of 1 in acetonitrile using a 355 nm laser forms its triplet ketone (T(1K), broad absorption with λ(max) ~ 390-410 nm, τ ~ 90 ns), which cleaves and yields triplet vinylnitrene 3 (broad absorption with λ(max) ~ 380-400 nm, τ = 2 μs). Calculations (B3LYP/6-31+G(d)) reveal that T(1K) of 1 is located 67 kcal/mol above its ground state (S(0)) and has a long C-N bond (1.58 ?), and the calculated transition state to form 3 is only 1 kcal/mol higher in energy than T(1K) of 1. The calculations show that 3 has significant 1,3-carbon iminyl biradical character, which explains why 3 reacts efficiently with oxygen and decays by intersystem crossing to the singlet surface. Photolysis of 1 in argon matrixes at 14 K produced ketene imine 7, which presumably is formed from 3 intersystem crossing to 7. In comparison, photolysis of 1 in methanol with a 266 nm laser produces mainly ylide 2 (λ(max) ~ 380 nm, τ ~ 6 μs, acetonitrile), which decays to form 2P. Ylide 2 is formed via singlet reactivity of 1, and calculations show that the first singlet excited state of the azirine chromophore (S(1A)) is located 113 kcal/mol above its S(0) and that the singlet excited state of the ketone (S(1K)) is 85 kcal/mol. Furthermore, the transition state for cleaving the C-C bond in 1 to form 2 is located 49 kcal/mol above the S(0) of 1. Thus, we theorize that internal conversion of S(1A) to a vibrationally hot S(0) of 1 forms 2, whereas intersystem crossing from S(1K) to T(1K) results in 3.     

On the performance of quantum chemical methods to predict solvatochromic effects: the case of acrolein in aqueous solution

The performance of the Hartree-Fock method and the three density functionals B3LYP, PBE0, and CAM-B3LYP is compared to results based on the coupled cluster singles and doubles model in predictions of the solvatochromic effects on the vertical n-->pi* and pi-->pi* electronic excitation energies of acrolein. All electronic structure methods employed the same solvent model, which is based on the combined quantum mechanics/molecular mechanics approach together with a dynamical averaging scheme. In addition to the predicted solvatochromic effects, we have also performed spectroscopic UV measurements of acrolein in vapor phase and aqueous solution. The gas-to-aqueous solution shift of the n-->pi* excitation energy is well reproduced by using all density functional methods considered. However, the B3LYP and PBE0 functionals completely fail to describe the pi-->pi* electronic transition in solution, whereas the recent CAM-B3LYP functional performs well also in this case. The pi-->pi* excitation energy of acrolein in water solution is found to be very dependent on intermolecular induction and nonelectrostatic interactions. The computed excitation energies of acrolein in vacuum and solution compare well to experimental data.     

Enhancement of metal-metal coupling at a considerable distance by using 4-pyridinealdazine as a bridging ligand in polynuclear complexes of rhenium and ruthenium

Novel polynuclear complexes of rhenium and ruthenium containing PCA (PCA = 4-pyridinecarboxaldehyde azine or 4-pyridinealdazine or 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a bridging ligand have been synthesized as PF(6-) salts and characterized by spectroscopic, electrochemical, and photophysical techniques. The precursor mononuclear complex, of formula [Re(Me(2)bpy)(CO)(3)(PCA)](+) (Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine), does not emit at room temperature in CH(3)CN, and the transient spectrum found by flash photolysis at lambda(exc) = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me(2)bpy)(CO)(3)Re(II)(PCA(-))](+), with lambda(max) = 460 nm and tau < 10 ns. The spectral properties of the related complexes [[Re(Me(2)bpy)(CO)(3)}(2)(PCA)](2+), [Re(CO)(3)(PCA)(2)Cl], and [Re(CO)(3)Cl](3)(PCA)(4) confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(II)(NH(3))(5)](3+), presents an intense absorption in the visible spectrum that can be assigned to a MLCT d(pi)(Ru) --> pi(PCA); in CH(3)CN, the value of lambda (max) = 560 nm is intermediate between those determined for [Ru(NH(3))(5)(PCA)](2+) (lambda(max) = 536 nm) and [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](4+) (lambda(max) = 574 nm), indicating a significant decrease in the energy of the pi-orbital of PCA. The mixed-valent species, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(III)(NH(3))(5)](4+), was obtained in CH(3)CN solution, by bromine oxidation or by controlled-potential electrolysis at 0.8 V in a OTTLE cell of the [Re(I),Ru(II)] precursor; the band at lambda(max) = 560 nm disappears completely, and a new band appears at lambda(max) = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) Re(I) --> Ru(III). By using the Marcus-Hush formalism, both the electronic coupling (H(AB)) and the reorganization energy (lambda) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (approximately 15.0 Angstroms), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](5+) (K(c) approximately 10(2), in CH(3)CN), puts into evidence an unusual enhancement of the metal-metal coupling in the bridged PCA complexes. This effect can be accounted for by the large extent of "metal-ligand interface", as shown by DFT calculations on free PCA. Moreover, lambda is lower than the driving force -DeltaG degrees for the recombination charge reaction [Re(II),Ru(II)] --> [Re(I),Ru(III)] that follows light excitation of the mixed-valent species. It is then predicted that this reverse reaction falls in the Marcus inverted region, making the heterodinuclear [Re(I),Ru(III)] complex a promising model for controlling the efficiency of charge-separation processes.     

Characterization of reactive intermediates generated during photolysis of 4-acetoxy-4-aryl-2,5-cyclohexadienones: oxenium ions and aryloxy radicals

Aryloxenium ions 1 are reactive intermediates that are isoelectronic with the better known arylcarbenium and arylnitrenium ions. They are proposed to be involved in synthetically and industrially useful oxidation reactions of phenols. However, mechanistic studies of these intermediates are limited. Until recently, the lifetimes of these intermediates in solution and their reactivity patterns were unknown. Previously, the quinol esters 2 have been used to generate 1, which were indirectly detected by azide ion trapping to generate azide adducts 4 at the expense of quinols 3, during hydrolysis reactions in the dark. Laser flash photolysis (LFP) of 2b in the presence of O(2) in aqueous solution leads to two reactive intermediates with lambda(max) 360 and 460 nm, respectively, while in pure CH(3)CN only one species with lambda(max) 350 nm is produced. The intermediate with lambda(max) 460 nm was previously identified as 1b based on direct observation of its decomposition kinetics in the presence of N(3)(-), comparison to azide ion trapping results from the hydrolysis reactions, and photolysis reaction products (3b). The agreement between the calculated (B3LYP/6-31G(d)) and observed time-resolved resonance Raman (TR(3)) spectra of 1b further confirms its identity. The second intermediate with lambda(max) 360 nm (350 nm in CH(3)CN) has been characterized as the radical 5b, based on its photolytic generation in the less polar CH(3)CN and on isolated photolysis reaction products (6b and 7b). Only the radical intermediate 5b is generated by photolysis in CH(3)CN, so its UV-vis spectrum, reaction products, and decay kinetics can be investigated in this solvent without interference from 1b. In addition, the radical 5a was generated by LFP of 2a and was identified by comparison to a published UV-vis spectrum of authentic 5a obtained under similar conditions. The similarity of the UV-vis spectra of 5a and 5b, their reaction products, and the kinetics of their decay confirm the assigned structures. The lifetime of 1b in aqueous solution at room temperature is 170 ns. This intermediate decays with first-order kinetics. The radical intermediate 5b decomposes in a biphasic manner, with lifetimes of 12 and 75 mus. The decay processes of 5a and 5b were successfully modeled with a kinetic scheme that included reversible formation of a dimer. The scheme is similar to the kinetic models applied to describe the decay of other aryloxy radicals.