Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.     

Photodissociation and electronic spectroscopy of [Re(H)(CO)(3)(H-dab)] (H-dab=1,4-diaza-1,3-butadiene): quantum wavepacket dynamics based on ab initio potentials

The photodissociation dynamics of [Re(H)(CO)(3)(H-dab)] (H-dab=1,4-diaza-1,3-butadiene) were studied by means of wavepacket propagations on CASSCF/MR-CCI potentials calculated for the electronic ground state and low-lying excited states as a function of two coordinates, q(a) and q(b), that correspond to the Re-H bond homolysis and to the axial CO loss, respectively. The theoretical absorption spectrum is characterized by two bands, one intense peak centered at lambda=500 nm (21,000 cm(-1)) and one broad band centered at 310 nm (32,500 cm(-1)). The visible band was assigned to the low-lying metal-to-ligand charge-transfer (MLCT) states with a main contribution of the a(1)A'-->c(1)A' transition corresponding to the 3d(xz)-->pi*(dab) excitation. The second band calculated in the UV energy domain was assigned to the d(1)A' (sigma(Mn-H)-->pi*(dab)) state corresponding to a sigma-bond-to-ligand charge-transfer (SBLCT) state. The photodissociation dynamics of the low-lying (1)MLCT and (3)SBLCT states following irradiation in the visible energy domain was simulated by wavepacket propagation on the two-dimensional diabatic potentials V(q(a), q(b)) coupled by the spin-orbit. In contrast to what was found for the manganese analogue, the (1)MLCT state is nonreactive and a rather slow (beyond the ps time scale), nontotal and indirect homolysis of the Re-H bond occurs through (1)MLCT-->(3)SBLCT intersystem crossing.     

Excited states of thiophene: ring opening as deactivation mechanism

(Time-dependent) Kohn-Sham density functional theory and a combined density functional/multi-reference configuration interaction method (DFT/MRCI) were employed to explore the ground and low-lying electronically excited states of thiophene. Spin-orbit coupling was taken into account using an efficient, nonempirical mean-field Hamiltonian. Phosphorescence lifetimes were calculated by means of spock.ci, a selecting direct multi-reference spin-orbit configuration interaction program. Throughout this paper, we use the following nomenclature: S1, S2,..., T1, T2,..., denominate electronic structures in their energetic order at the ground state minimum geometry, whereas S1, S2,..., T1, T2,..., refers to the actual order of electronic states at a given nuclear geometry. Multiple minima were found on the first excited singlet (S1) potential energy hypersurface with electronic structures S1 (piHOMO-1-->pi+piHOMO-->pi), S2 (piHOMO-->pi), and S3 (piHOMO-->sigma*) corresponding to the 2 1A1 (S1), 1 1B2 (S2), and 1 1B1 (S3) states in the vertical absorption spectrum, respectively. The S1 and S2 minimum geometries show out-of-plane deformations of the ring. The S3 electronic structure yields the global minimum on the S1 surface with an adiabatic excitation energy of merely 3.81 eV. It exhibits an asymmetric planar nuclear arrangement with one significantly elongated C-S bond. A constrained minimum energy path calculation connecting the S1 and S3 minima suggests that even low-lying vibrational levels of the S1 potential well can access the global minimum of the S1 surface. Nonradiative decay of the electronically excited singlet population to the electronic ground state via a close-by conical intersection will be fast. According to our work, this ring opening mechanism is most likely responsible for the lack of fluorescence in thiophene and the ultrafast decay of the S1 vibrational levels, as observed in time-resolved pump-probe femtosecond multiphoton ionization experiments. An alternative relaxation pathway leads from the S1 minimum via vibronic coupling to the S2 potential well followed by fast inter-system crossing to the T2 state. For an estimate of individual rate constants a quantum dynamical treatment will be required. The global minimum of the T1 surface has a chair-like nuclear conformation and corresponds to the T1 (1 3B2, piHOMO-->pi) electronic structure. Phosphorescence is weak here with a calculated radiative lifetime of 0.59 s. For the second potential well on the T1 surface with T3 (1 3B1, piHOMO-->sigma*) electronic structure, nonradiative processes are predicted to dominate the triplet decay.     

Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(pi,pi*) excited state

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(pi,pi*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (nu45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu43) and ring-twisting (nu44) vibrations. Despite some interaction with nu43 and nu44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(pi,pi*) state coumaran is quasiplanar with a barrier to planarity of 34 cm(-1) and with energy minima at puckering angles of +/-14 degrees. The corresponding ground state (S0) values are 154 cm(-1) and +/-25 degrees . As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the pi-->pi* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm(-1) for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(pi,pi*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.     

On the ultraviolet photodissociation of H(2)Te

The photodissociation of H(2)Te through excitation in the first absorption band is investigated by means of multireference spin-orbit configuration interaction (CI) calculations. Bending potentials for low-lying electronic states of H(2)Te are obtained in C(2v) symmetry for Te-H distances fixed at the ground state equilibrium value of 3.14a(0), as well as for the minimum energy path constrained to R(1)=R(2). Asymmetric cuts of potential energy surfaces for excited states (at R(1)=3.14a(0) and theta;=90.3 degrees ) are obtained for the first time. It is shown that vibrational structure in the 380-400 nm region of the long wavelength absorption tail is due to transitions to 3A('), which has a shallow minimum at large HTe-H separations. Transitions to this state are polarized in the molecular plane, and this state converges to the excited TeH((2)Pi(1/2))+H((2)S) limit. These theoretical data are in accord with the selectivity toward TeH((2)Pi(1/2)) relative to TeH((2)Pi(3/2)) that has been found experimentally for 355 nm H(2)Te photodissociation. The calculated 3A(')<--XA(') transition dipole moment increases rapidly with HTe-H distance; this explains the observation of 3A(') vibrational structure for low vibrational levels, despite unfavorable Franck-Condon factors. According to the calculated vertical energies and transition moment data, the maximum in the first absorption band at approximately 245 nm is caused by excitation to 4A("), which has predominantly 2(1)A(") ((1)B(1) in C(2v) symmetry) character.     

A theoretical insight into the photophysics of psoralen

Psoralen photophysics has been studied on quantum chemistry grounds using the multiconfigurational second-order perturbation method CASPT2. Absorption and emission spectra of the system have been rationalized by computing the energies and properties of the low-lying singlet and triplet excited states. The S1 pipi* state has been determined to be responsible of the lowest absorption and fluorescence bands and to initially carry the population in the photophysical processes related to the phototherapeutic properties of psoralen derivatives. The low-lying T1 pipi* state is, on the other hand, protagonist of the phosphorescence, and its prevalent role in the reactivity of psoralen is suggested to be related to the elongation of the pyrone ring C3-C4 bond, where the spin density is distributed on both carbon atoms. Analysis of energy gaps and spin-orbit coupling elements indicates that the efficient photophysical process leading to the population of the lowest triplet state does not take place at the Franck-Condon region but along the S1 relaxation path.     

Photoactivity and UV absorption spectroscopy of RCo(CO)4 (R = H, CH3) organometallic complexes

The photoactivity of RCo(CO)4 (R = H, CH3) complexes has been investigated and compared by means of state correlation diagrams connecting the low-lying singlet (1)E (d(Co) --> sigma*(Co-R) and d(Co) --> pi*(CO)) and (1)A1 (d(Co) --> pi*(CO)) electronic states accessible through UV irradiation, and the low-lying triplet states ((3)E and (3)A1), to the corresponding states of the primary products R + Co(CO)4 and CO(ax) + RCo(CO)3. The electronic absorption spectra have been calculated by time-dependent wave packet propagations on two-dimensional potential energy surfaces describing both channels of dissociation, namely the homolysis of the R-Co and the CO(ax)-Co bonds. It is shown that the absorption spectrum of HCo(CO)4 is characterized by two peaks; the most intense peaks for each set are located respectively at 42,659 and 45,001 cm(-1). The CH(3)Co(CO)4 absorption spectrum also gives two sets of signals with maximum intensities found at 42,581 and 51,515 cm(-1). These bands for both molecules are assigned to the two metal-to-ligand-charge-transfer (MLCT; d(Co) --> pi*(CO)) states. Three photoactive states have been determined in both molecules, namely the singlet metal-to-sigma-bond-charge-transfer (MSBCT) states (a(1)E and b(1)E), simultaneously dissociative for both the homolysis of CO and the R-Co bond, and the (3)A1 (sigma(Co-R) --> sigma*(Co-R)), dissociative along the R-Co bond.     

Theory of laser enhancement and suppression of cold reactions: the fermion-boson 6Li+7Li2<-->(variant Planck's over 2pi omega0) 6Li7Li+7Li radiative collision

We present a nonperturbative time-dependent quantum mechanical theory of the laser catalysis and control of a bifurcating A+BC<-->(variant Planck's over 2pi omega(0))ABC*(v)<-->(variant Planck's over 2pi omega(0) )AB+C reaction, with ABC*(v) denoting an intermediate, electronically excited, complex of ABC in the vth vibrational state. We apply this theory to the low collision energy fermion-boson light-induced exchange reaction, (6)Li((2)S)+(7)Li(2)((3)Sigma(u)(+))<-->(variant Planck's over 2pi omega(0))((6)Li(7)Li(7)Li)*<-->(variant Planck's over 2pi omega(0))(6)Li(7)Li((3)Sigma(+))+(7)Li((2)S). We show that at very low collision energies and energetically narrow (approximately 0.01 cm(-1)) initial reactant wave packets, it is possible to tune the yield of the exchange reaction from 0 to near-unity (yield >or=99%) values. Controllability is somewhat reduced at collisions involving energetically wider (approximately 1 cm(-1)) initial reactant wave packets. At these energetic bandwidths, the radiative reactive control, although still impressive, is limited to the 0%-76% reactive-probabilities range.     

Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm: intramolecular orbital alignment of the phenylthiyl radical

The photoinduced hydrogen (or deuterium) detachment reaction of thiophenol (C(6)H(5)SH) or thiophenol-d(1) (C(6)H(5)SD) pumped at 243 nm has been investigated using the H (D) ion velocity map imaging technique. Photodissociation products, corresponding to the two distinct and anisotropic rings observed in the H (or D) ion images, are identified as the two lowest electronic states of phenylthiyl radical (C(6)H(5)S). Ab initio calculations show that the singly occupied molecular orbital of the phenylthiyl radical is localized on the sulfur atom and it is oriented either perpendicular or parallel to the molecular plane for the ground (B(1)) and the first excited state (B(2)) species, respectively. The experimental energy separation between these two states is 2600+/-200 cm(-1) in excellent agreement with the authors' theoretical prediction of 2674 cm(-1) at the CASPT2 level. The experimental anisotropy parameter (beta) of -1.0+/-0.05 at the large translational energy of D from the C(6)H(5)SD dissociation indicates that the transition dipole moment associated with this optical transition at 243 nm is perpendicular to the dissociating S-D bond, which in turn suggests an ultrafast D+C(6)H(5)S(B(1)) dissociation channel on a repulsive potential energy surface. The reduced anisotropy parameter of -0.76+/-0.04 observed at the smaller translational energy of D suggests that the D+C(6)H(5)S(B(2)) channel may proceed on adiabatic reaction paths resulting from the coupling of the initially excited state to other low-lying electronic states encountered along the reaction coordinate. Detailed high level ab initio calculations adopting multireference wave functions reveal that the C(6)H(5)S(B(1)) channel may be directly accessed via a (1)(n(pi),sigma(*)) photoexcitation at 243 nm while the key feature of the photodissociation dynamics of the C(6)H(5)S(B(2)) channel is the involvement of the (3)(n(pi),pi(*))-->(3)(n(sigma),sigma(*)) profile as well as the spin-orbit induced avoided crossing between the ground and the (3)(n(pi),sigma(*)) state. The S-D bond dissociation energy of thiophenol-d(1) is accurately estimated to be D(0)=79.6+/-0.3 kcalmol. The S-H bond dissociation energy is also estimated to give D(0)=76.8+/-0.3 kcalmol, which is smaller than previously reported ones by at least 2 kcalmol. The C-H bond of the benzene moiety is found to give rise to the H fragment. Ring opening reactions induced by the pi-pi(*)n(pi)-pi(*) transitions followed by internal conversion may be responsible for the isotropic broad translational energy distribution of fragments.     

Theoretical studies on structures and spectroscopic properties of a series of novel mixed-ligand Ir(III) complexes [Ir(Mebib)(ppy)X

The series of novel mixed-ligand iridium(III) complexes Ir(Mebib)(ppy)X (Mebib = bis(N-methylbenzimidazolyl)benzene and ppy = phenylpyridine; X = Cl, 1; X = -C[triple band]CH, 2; X = CN, 3) have been investigated theoretically to explore their electronic structures and spectroscopic properties. The ground and excited state geometries have been fully optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ levels, respectively. The optimized geometry structural parameters agree well with the corresponding experimental results. The HOMO of 1 and 3 are mainly localized on the Ir atom, Mebib, and ppy ligand, but that of 2 has significant X ligand composition. Absorptions and phosphorescences in CH2 Cl2 media have been calculated using the TD-DFT level of theory with the PCM model based on the optimized ground and excited state geometries, respectively. The lowest lying absorptions of 1 and 3 at 444 and 416 nm are attributed to a {[d(yz)(Ir) + pi(Mebib) + pi(ppy)] --> [pi*(Mebib)]} transition with metal-to-ligand, ligand-to-ligand, and intra-ligand charge transfer (MLCT/LLCT/ILCT) character, whereas that of 2 at 458 nm is related to a {[d(yz)(Ir) + pi(Mebib) + pi(ppy) + pi(C[triple band]CH)] --> [pi*(Mebib)]} transition with MLCT/LLCT/ILCT and X ligand-to-ligand charge transfer (XLCT) transition character. The phosphorescence of 1 and 3 at 565 and 543 nm originates from the 3{[dy(yz)(Ir) + pi(Mebib) + pi(ppy)] [pi*(Mebib)]} excited state, while that of 2 at 576 nm originates from the 3{[d(yz)(Ir) + pi(Mebib) + pi(ppy) + pi(C[triple band]CH)] [pi*(Mebib)]} excited state. The calculation results show that the absorption and emission transition character can be changed by altering the pi electron-withdrawing ability of the X ligand and the phosphorescent color can be tuned by adjusting the X ligand.     

Electrochemical and spectroscopic characterization of the novel charge-transfer ground state in diimine complexes of ytterbocene

The novel charge-transfer ground state found in alpha,alpha'-diimine adducts of ytterbocene (C(5)Me(5))(2)Yb(L) [L = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen)] in which an electron is spontaneously transferred from the f(14) metal center into the lowest unoccupied (pi*) molecular orbital (LUMO) of the diimine ligand to give an f(13)-L(*)(-) ground-state electronic configuration has been characterized by cyclic voltammetry, UV-vis-near-IR electronic absorption, and resonance Raman spectroscopies. The voltammetric data demonstrate that the diimine ligand LUMO is stabilized and the metal f orbital is destabilized by approximately 1.0 V each upon complexation for both bpy and phen adducts. The separation between the ligand-based oxidation wave (L(0/-)) and the metal-based reduction wave (Yb(3+/2+)) in the ytterbocene adducts is 0.79 V for both bpy and phen complexes. The UV-vis-near-IR absorption spectroscopic data for both the neutral adducts and the one-electron-oxidized complexes are consistent with those reported recently, but previously unreported bands in the near-IR have been recorded and assigned to ligand (pi*)-to-metal (f orbital) charge-transfer (LMCT) transitions. These optical electronic excited states are the converse of the ground-state charge-transfer process (e.g., f(13)-L(*-) <--> f(14)-L(0)). These new bands occur at approximately 5000 cm(-1) in both adducts, consistent with predictions from electrochemical data, and the spacings of the resolved vibronic bands in these transitions are consistent with the removal of an electron from the ligand pi* orbital. The unusually large intensity observed in the f --> f intraconfiguration transitions for the neutral phenanthroline adduct is discussed in terms of an intensity-borrowing mechanism involving the low-energy LMCT states. Raman vibrational data clearly reveal resonance enhancement for excitation into the low-lying pi* --> pi* ligand-localized excited states, and comparison of the vibrational energies with those reported for alkali-metal-reduced diimine ligands confirms that the ligands in the adducts are reduced radical anions. Differences in the resonance enhancement pattern for the modes in the bipyridine adduct with excitation into different pi* --> pi* levels illustrate the different nodal structures that exist in the various low-lying pi* orbitals.     

Accurate ab initio potential for the Na+...I* complex

High-level ab initio calculations employing the multireference configuration interaction and coupled clusters methods with a correlation-consistent sequence of basis sets have been used to obtain accurate potential energy curves for the complex of the sodium cation with the iodine atom. Potential curves for the first two electronic Lambda-S states have very different characters: the potential for the 2pi state has a well depth of approximately 10 kcal/mol, while the 2sigma state is essentially unbound. This difference is rationalized in terms of the anisotropic interaction of the quadrupole moment of the iodine atom with the sodium cation, which is stabilizing in the case of the 2pi state and destabilizing in the case of the 2sigma state. The effects of spin-orbit coupling have been accounted for with both ab initio and semiempirical approaches, which have been found to give practically the same results. Inclusion of spin-orbit interactions does not affect the X(omega = 32) ground state, which retains its 2pi character, but it results in two omega = 12 spin-orbit states, with mixed 2sigma and 2pi characters and binding energies roughly half of that of the ground spin-orbit state. Complete basis set (CBS) extrapolations of potential curves, binding energies, and equilibrium geometries were also performed, and used to calculate a number of rovibronic parameters for the Na+...I* complex and to parameterize model potentials. The final CBS-extrapolated and zero-point vibrational energy-corrected binding energy is 10.2 kcal/mol. Applications of the present results for simulations of NaI photodissociation femtosecond spectroscopy are discussed.     

Symmetry switching of the fluorescent excited state in alpha,omega-diphenylpolyynes

By using MO calculations based on DFT, absorption, and fluorescence spectroscopy, we have comprehensively studied the low-lying excited singlet states of alpha,omega-diphenylpolyynes (DPY) having 1-6 triple bonds. The a(g) vibrational modes of the C(triple bond)C stretching and of the phenyl ring motion were observed in the fluorescence spectra of diphenylacetylene and 1,4-diphenylbutadiyne. On the other hand, in the fluorescence spectra of the long DPY with the triple-bond number (N) more than two, the phenyl ring motion with a(g) symmetry disappeared and the b(1g) modes of the phenyl ring twisting (approximately 400 cm(-1)) and of the C-H bending (approximately 900 cm(-1)) were detected. The observed fluorescent states of DPY with N < or = 2 and N > or = 3 are assigned to the 1(1)B(1u) (pi(x)pi(x*)) and 1(1)A(u) (pi(x)pi(y*) and/or pi(y)pi(x*)) states, respectively, based on the vibronic structures, the relatively short lifetimes, and the solvatochromic shifts of the fluorescence spectra. Not only the allowed transition of 1(1)B(1u) <-- S(0) but also the forbidden transition of 1(1)A(u) <-- S(0) was detected in the fluorescence excitation spectra of the long DPY with N > or = 3. The low-lying excited state with A(u) symmetry is characteristic in polyyne, which does not exist in polyene. The oscillator strength (f) of the first absorption band in DPY decreases with an increase in N, which is the opposite behavior of the all-trans-alpha,omega-diphenylpolyenes. The N-dependence of the f value is understood by the configuration interaction between the 1(1)B(1u) and 2(1)B(1u) (pi(y)pi(y*)) states, which is consistent with the reduction of the nonlinear optical response of polyyne.     

Electronic spectra and structures of d2 molybdenum-oxo complexes. Effects of structural distortions on orbital energies,two-electron terms,and the mixing of singlet and triplet states

Molybdenum-oxo ions of the type [Mo(IV)OL(4)Cl](+) (L = CNBu(t), PMe(3), (1)/(2)Me(2)PCH(2)CH(2)PMe(2)) have been studied by X-ray crystallography, vibrational spectroscopy, and polarized single-crystal electronic absorption spectroscopy (300 and ca. 20 K) in order to investigate the effects of the ancillary ligand geometry on the properties of the MotriplebondO bond. The idealized point symmetries of the [Mo(IV)OL(4)Cl](+) ions were established by X-ray crystallographic studies of the salts [MoO(CNBu(t)())(4)Cl][BPh(4)] (C(4)(v)), [MoO(dmpe)(2)Cl]Cl.5H(2)O (C(2)(v)), and [MoO(PMe(3))(4)Cl][PF(6)] (C(2)(v)()); the lower symmetries of the phosphine derivatives are the result of the steric properties of the phosphine ligands. The Motbd1;O stretching frequencies of these ions (948-959 cm(-)(1)) are essentially insensitive to the nature and geometry of the equatorial ligands. In contrast, the electronic absorption bands arising from the nominal d(xy)() --> d(xz), d(yz) (n --> pi(MoO)) ligand-field transition exhibit a large dependence on the geometry of the equatorial ligands. Specifically, the electronic spectrum of [MoO(CNBu(t)())(4)Cl](+) exhibits a single (1)[n --> pi(xz)(,)(yz)] band, whereas the spectra of both [MoO(dmpe)(2)Cl](+) and [MoO(PMe(3))(4)Cl](+) reveal separate (1)[n --> pi(xz)] and (1)[n --> pi(yz)] bands. A general theoretical model of the n --> pi state energies of structurally distorted d(2) M(triplebondE)L(4)X chromophores is developed in order to interpret the electronic spectra of the phosphine derivatives. Analysis of the n --> pi transition energies using this model indicates that the d(xz) and d(yz) pi(MotriplebondO) orbitals are nondegenerate for the C(2)(v)-symmetry ions and the n --> pi(xz) and n --> pi(yz) excited states are characterized by different two-electron terms. These effects lead to a significant redistribution of intensity between certain spin-allowed and spin-forbidden absorption bands. The applicability of this model to the excited states produced by delta --> pi and pi --> delta symmetry electronic transitions of other chromophores is discussed.     

Time-dependent quantum wave-packet description of the 1pi sigma* photochemistry of phenol

The photoinduced hydrogen elimination reaction in phenol via the conical intersections of the dissociative 1pi sigma* state with the 1pi pi* state and the electronic ground state has been investigated by time-dependent quantum wave-packet calculations. A model including three intersecting electronic potential-energy surfaces (S0, 1pi sigma*, and 1pi pi*) and two nuclear degrees of freedom (OH stretching and OH torsion) has been constructed on the basis of accurate ab initio multireference electronic-structure data. The electronic population transfer processes at the conical intersections, the branching ratio between the two dissociation channels, and their dependence on the initial vibrational levels have been investigated by photoexciting phenol from different vibrational levels of its ground electronic state. The nonadiabatic transitions between the excited states and the ground state occur on a time scale of a few tens of femtoseconds if the 1pi pi*-1pi sigma* conical intersection is directly accessible, which requires the excitation of at least one quantum of the OH stretching mode in the 1pi pi* state. It is shown that the node structure, which is imposed on the nuclear wave packet by the initial preparation as well as by the transition through the first conical intersection (1pi pi*-1pi sigma*), has a profound effect on the nonadiabatic dynamics at the second conical intersection (1pi sigma*-S0). These findings suggest that laser control of the photodissociation of phenol via IR mode-specific excitation of vibrational levels in the electronic ground state should be possible.     

An ab initio study on the ground and low-lying doublet electronic states of SbO2

Geometry optimization and harmonic vibrational frequency calculations have been carried out on the low-lying doublet electronic states of antimony dioxide (SbO(2)) employing a variety of ab initio methods, including the complete active space self-consistent field/multireference configuration interaction and the RCCSD(T) methods. Both large and small core relativistic effective core potentials were used for Sb in these calculations, together with valence basis sets of up to aug-cc-pV5Z quality. Contributions from outer core correlation and off-diagonal spin-orbit interaction to relative electronic energies have been calculated. The ground electronic state of SbO(2) is determined to be the X (2)A(1) state, as is the case for dioxides of other lighter group 15 p-block (or group VA) elements. However, the A (2)B(2) and B (2)A(2) states are estimated to be only 4.1 and 10.7 kcalmole above the X (2)A(1) state, respectively, at the complete basis set limit. Reliable vertical excitation energies from the X (2)A(1) state to low-lying excited states of SbO(2) have been computed with a view to assist future spectral assignments of the absorption and/or laser-induced fluorescence spectra of SbO(2), when they become available.     

Computational investigation of the vibrational and electronic states of S2N2

The structures and vibrational frequencies of the ground and excited states of S(2)N(2) have been calculated using density functional (DF) methods. Time-dependent DF theory (TDDFT) has been used to calculate the excitation energies of the lowest 20 singlet-singlet transitions using a variety of methods. All computational methods predict a small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap. There is some disagreement in the ordering of the b(2g) and b(3g) pi orbitals. This is reflected in the ordering of the B(2u) and B(3u) states from the TDDFT calculations. The excitation energies and oscillator strengths strongly suggest it is the transitions to these states that are responsible for the experimental electronic spectrum. The calculated geometries and vibrational frequencies for these two states show that both have C(2v) equilibrium structures. Modelling of the vibrational progressions and band shapes suggest that the ordering of the states is B(2u)相似文献   

Ultrafast infrared spectroscopy of riboflavin: dynamics, electronic structure, and vibrational mode analysis

Femtosecond time-resolved infrared spectroscopy was used to study the vibrational response of riboflavin in DMSO to photoexcitation at 387 nm. Vibrational cooling in the excited electronic state is observed and characterized by a time constant of 4.0 +/- 0.1 ps. Its characteristic pattern of negative and positive IR difference signals allows the identification and determination of excited-state vibrational frequencies of riboflavin in the spectral region between 1100 and 1740 cm (-1). Density functional theory (B3LYP), Hartree-Fock (HF) and configuration interaction singles (CIS) methods were employed to calculate the vibrational spectra of the electronic ground state and the first singlet excited pipi* state as well as respective electronic energies, structural parameters, electronic dipole moments and intrinsic force constants. The harmonic frequencies of the S 1 excited state calculated by the CIS method are in satisfactory agreement with the observed band positions. There is a clear correspondence between computed ground- and excited-state vibrations. Major changes upon photoexcitation include the loss of the double bond between the C4a and N5 atoms, reflected in a downshift of related vibrations in the spectral region from 1450 to 1720 cm (-1). Furthermore, the vibrational analysis reveals intra- and intermolecular hydrogen bonding of the riboflavin chromophore.     

A new pathway for the rapid decay of electronically excited adenine

Combined density functional and multireference configuration interaction methods have been used to calculate the electronic spectrum of 9H-adenine, the most stable tautomer of 6-aminopurine. In addition, constrained minimum energy paths on excited potential energy hypersurfaces have been determined along several relaxation coordinates. The minimum of the first (1)[n-->pi*] state has been located at an energy of 4.54 eV for a nuclear arrangement in which the amino group is pyramidal whereas the ring system remains planar. Close by, another minimum on the S(1) potential energy hypersurface has been detected in which the C(2) center is deflected out of the molecular plane and the electronic character of S(1) corresponds to a nearly equal mixture of (1)[pi-->pi*] and (1)[n-->pi*] configurations. The adiabatic excitation energy of this minimum amounts to 4.47 eV. Vertical and adiabatic excitation energies of the lowest n-->pi* and pi-->pi* transitions as well as transition moments and their directions are in very good agreement with experimental data and lend confidence to the present quantum chemical treatment. On the S(1) potential energy hypersurface, an energetically favorable path from the singlet n-->pi* minimum toward a conical intersection with the electronic ground state has been identified. Close to the conical intersection, the six-membered ring of adenine is strongly puckered and the electronic structure of the S(1) state corresponds to a pi-->pi* excitation. The energetic accessibility of this relaxation path at about 0.1 eV above the singlet n-->pi* minimum is presumably responsible for the ultrafast decay of 9H-adenine after photoexcitation and explains why sharp vibronic peaks can only be observed in a rather narrow wavelength range above the origin. The detected mechanism should be equally applicable to adenosine and 9-methyladenine because it involves primarily geometry changes in the six-membered ring whereas the nuclear arrangement of the five-membered ring (including the N(9) center) is largely preserved.     

Electroswitchable photoluminescence activity: synthesis, spectroscopy, electrochemistry, photophysics, and X-ray crystal and electronic structures of [Re(bpy)(CO)3(C[triple bond]C[bond]C6H4[bond]C[triple bond]C)Fe(C5Me5)(dppe)][PF6](n) (n = 0, 1)

A novel heterobimetallic alkynyl-bridged complex, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)Me(5))(dppe)], 1, and its oxidized species, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)Me(5))(dppe)][PF(6)], 2, have been synthesized and their X-ray crystal structures determined. A related vinylidene complex, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond](H)C[double bond]C)Fe(C(5)Me(5))(dppe)][PF(6)], 3, has also been synthesized and characterized. The cyclic voltammogram of 1 shows a quasireversible reduction couple at -1.49 V (vs SCE), a fully reversible oxidation at -0.19 V, and a quasireversible oxidation at +0.88 V. In accord with the electrochemical results, density-functional theory calculations on the hydrogen-substituted model complex Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)H(5))(dHpe) (Cp = C(5)H(5), dHpe = H(2)P[bond](CH(2))(2)[bond]PH(2)) (1-H) show that the LUMO is mainly bipyridine ligand pi* in character while the HOMO is largely iron(II) d orbital in character. The electronic absorption spectrum of 1 shows low-energy absorption at 390 nm with a 420 nm shoulder in CH(2)Cl(2), while that of 2 exhibits less intense low-energy bands at 432 and 474 nm and additional low-energy bands in the NIR at ca. 830, 1389, and 1773 nm. Unlike the related luminescent rhenium(I)-alkynyl complex [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C[bond]H)], 4, complex 1 is found to be nonemissive, and such a phenomenon is attributed to an intramolecular quenching of the emissive d pi(Re) --> pi*(bpy) (3)MLCT state by the low-lying MLCT and LF excited states of the iron moiety. Interestingly, switching on of the luminescence property derived from the d pi(Re) --> pi*(bpy) (3)MLCT state can be demonstrated in the oxidized species 2 and the related vinylidene analogue 3 due to the absence of the quenching pathway.