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Vertical Excitation Energies (vertical + excitation_energy)
Selected AbstractsVertical excitation energies for ribose and deoxyribose nucleosidesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2007Remmick So Abstract Vertical excitation energies for DNA and RNA nucleosides are determined with electron structure calculations using the time-dependent density functional theory (TDDFT) method at the B3LYP/6-311++G(d,p) level for nucleoside structures optimized at the same level of theory. The excitation energies and state assignments are verified using B3LYP/aug-cc-pVDZ level calculations. The nature of the first four excited states of the nucleosides are studied and compared with those of isolated bases. The lowest n,* and ,,* transitions in the nucleoside remain localized on the aromatic rings of the base moiety. New low-energy n,* and ,,* transitions are introduced in the nucleosides as a result of bonding to the ribose and deoxyribose molecules. The effect on the low-lying excited state transitions of the binding to phosphate groups at the 5,- and 3,,5,-hydroxyl sites of the uracil ribose nucleoside are also studied. Some implications of these calculations on the de-excitation dynamics of nucleic acids are discussed. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007. [source] Computational analyses of singlet,singlet and singlet,triplet transitions in mononuclear gold-capped carbon-rich conjugated complexesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2005Zexing Cao Abstract Density functional theory and CASSCF calculations have been used to determine equilibrium geometries and vibrational frequencies of metal-capped one-dimensional ,-conjugated complexes (H3P)Au(CC)n(Ph) (n = 1,6), (H3P)Au(CCC6H4)(CCPh), and H3PAu(CCC6H4)CCAuPH3 in their ground states and selected low-lying ,,* excited states. Vertical excitation energies for spin-allowed singlet,singlet and spin-forbidden singlet,triplet transitions determined by the time-dependent density functional theory show good agreement with available experimental observations. Calculations indicate that the lowest energy 3(,,*) excited state is unlikely populated by the direct electronic excitation, while the low-lying singlet and triplet states, slightly higher in energy than the lowest triplet state, are easily accessible by the excitation light used in experiments. A series of radiationless transitions among related excited states yield the lowest 3(,,*) state, which has enough long lifetimes to exhibit its photochemical reactivities. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1214,1221, 2005 [source] Intruder state avoidance multireference Møller,Plesset perturbation theoryJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2002Henryk A. Witek Abstract A new perturbation approach is proposed that enhances the low-order, perturbative convergence by modifying the zeroth-order Hamiltonian in a manner that enlarges any small-energy denominators that may otherwise appear in the perturbative expansion. This intruder state avoidance (ISA) method can be used in conjunction with any perturbative approach, but is most applicable to cases where small energy denominators arise from orthogonal-space states,so-called intruder states,that should, under normal circumstances, make a negligible contribution to the target state of interests. This ISA method is used with multireference Møller,Plesset (MRMP) perturbation theory on potential energy curves that are otherwise plagued by singularities when treated with (conventional) MRMP; calculation are performed on the 13, state of O2; and the 21,, 31,, 23,, and 33, states of AgH. This approach is also applied to other calculations where MRMP is influenced by intruder states; calculations are performed on the 3,u state of N2, the 3, state of CO, and the 21A, state of formamide. A number of calculations are also performed to illustrate that this approach has little or no effect on MRMP when intruder states are not present in perturbative calculations; vertical excitation energies are computed for the low-lying states of N2, C2, CO, formamide, and benzene; the adiabatic 1A1,3B1 energy separation in CH2, and the spectroscopic parameters of O2 are also calculated. Vertical excitation energies are also performed on the Q and B bands states of free-base, chlorin, and zinc,chlorin porphyrin, where somewhat larger couplings exists, and,as anticipated,a larger deviation is found between MRMP and ISA-MRMP. © 2002 Wiley Periodicals, Inc. J Comput Chem 10: 957,965, 2002 [source] Ultraviolet Absorption Spectra of Substituted Phenols: A Computational Study,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2006Lei Zhang ABSTRACT Vertical excitation energies for electronic transitions from the ground state to the first two excited states of phenol, mono- and disubstituted methoxyphenols and methyl-substituted phenols have been characterized with the Time-Dependent Density Functional Theory (TD-DFT), the Complete Active Space Self-Consistent Field method (CASSCF) and the Coupled Cluster with Single and Double Excitations Equation-of-Motion approach (CCSD-EOM) to simulate and interpret experimental ultraviolet absorption spectra. While CASSCF excitation energies for the first two transitions either are grossly overestimated or exhibit a weak correlation with experimental data, both TD-DFT and CCSD-EOM perform very well, reproducing the spectral shifts of both the primary band and secondary band observed upon substitution. The conformational dependence of the calculated excitation energies is generally smaller than the shifts caused by substitution. [source] Electronic spectrum of 2-pyridone+: Ab initio and time-dependent density functional calculationsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2010D. Hammoutène Abstract In a comparative study, the doublet and quartet electronic states of the 2-pyridone+ cation are calculated using the PBE0/6-311+G(d,p) technique and the CASSCF and MRCI(+Q) methods in connection with the cc-pVDZ and cc-pVTZ Dunning's basis sets. Our data show that TD-DFT describes quite well the vertical excitation energies of these electronic states, whereas, multiconfiguration methods should be used for the investigation of the fragmentation and the dynamics of this molecular species. This is related to the change of the nature of the wavefunction of these electronic states along the corresponding reactive coordinates not accounted for by TD-DFT methods. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Theoretical studies on structures and electronic spectra of linear carbon chains C2nH+ (n = 1,5)INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2009Jinglai Zhang Abstract The density functional theory (DFT) and the complete active space self-consistent-field (CASSCF) method have been used for full geometry optimization of carbon chains C2nH+ (n = 1,5) in their ground states and selected excited states, respectively. Calculations show that C2nH+ (n = 1,5) have stable linear structures with the ground state of X3, for C2H+ or X3,, for other species. The excited-state properties of C2nH+ have been investigated by the multiconfigurational second-order perturbation theory (CASPT2), and predicted vertical excitation energies show good agreement with the available experimental values. On the basis of our calculations, the unsolved observed bands in previous experiments have been interpreted. CASSCF/CASPT2 calculations also have been used to explore the vertical emission energy of selected low-lying states in C2nH+ (n = 1,5). Present results indicate that the predicted vertical excitation and emission energies of C2nH+ have similar size dependences, and they gradually decrease as the chain size increases. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Temperature effects on the UV,Vis electronic spectrum of trans-stilbeneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001S. P. Kwasniewski Abstract The ultraviolet (UV),Visible absorption spectrum of trans-stilbene (tS) is computed at different temperatures by coupling molecular dynamics (MD) simulations with the classical MM3 force field to ZINDO/S-CIS calculations of vertical excitation energies and transition dipole moments. The selection of a large number of structures along the MD trajectories enables a consistent treatment of temperature effects in the vacuum, whereas the ZINDO/S-CIS calculations permit a reliable treatment of electron correlation and relaxation, taking account of multistate interactions in the final state. Thermal motions are found to alter very differently the width and shape of bands. Structural alterations such as the stretching and the torsion of the vinyl single and double bonds very strongly influence the appearance of the first valence state, pertaining to the highest occupied and lowest unoccupied molecular orbital (HOMO,LUMO) transition. At temperatures less than 400 K, these are found to yield a merely Gaussian and very pronounced thermal broadening of the related band (A), up to nearly 30 nm, together with a minor blue shift of its maximum ,max. In contrast, a red shift by several nanometers occurs due to thermal motions for the remaining three valence bands. As can be expected, the broadening intensifies at higher temperatures, and for the A-band, becomes markedly asymmetric when T exceeds 400 K. The combination of MD(MM3) and ZINDO/S-CIS computations enables also consistent calculations of hot bands, which are forbidden by symmetry at 0 K. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] QM/MM calculation of solvent effects on absorption spectra of guanineJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2010Maja Parac Abstract Electronic spectra of guanine in the gas phase and in water were studied by quantum mechanical/molecular mechanical (QM/MM) methods. Geometries for the excited-state calculations were extracted from ground-state molecular dynamics (MD) simulations using the self-consistent-charge density functional tight binding (SCC-DFTB) method for the QM region and the TIP3P force field for the water environment. Theoretical absorption spectra were generated from excitation energies and oscillator strengths calculated for 50 to 500 MD snapshots of guanine in the gas phase (QM) and in solution (QM/MM). The excited-state calculations used time-dependent density functional theory (TDDFT) and the DFT-based multireference configuration interaction (DFT/MRCI) method of Grimme and Waletzke, in combination with two basis sets. Our investigation covered keto-N7H and keto-N9H guanine, with particular focus on solvent effects in the low-energy spectrum of the keto-N9H tautomer. When compared with the vertical excitation energies of gas-phase guanine at the optimized DFT (B3LYP/TZVP) geometry, the maxima in the computed solution spectra are shifted by several tenths of an eV. Three effects contribute: the use of SCC-DFTB-based rather than B3LYP-based geometries in the MD snapshots (red shift of ca. 0.1 eV), explicit inclusion of nuclear motion through the MD snapshots (red shift of ca. 0.1 eV), and intrinsic solvent effects (differences in the absorption maxima in the computed gas-phase and solution spectra, typically ca. 0.1,0.3 eV). A detailed analysis of the results indicates that the intrinsic solvent effects arise both from solvent-induced structural changes and from electrostatic solute,solvent interactions, the latter being dominant. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010 [source] A combined ab initio and Franck-Condon factor simulation study on the photodetachment spectrum of ScO2,JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2009Edmond P. F. Lee Abstract Restricted-spin coupled-cluster single-double plus perturbative triple excitation {RCCSD(T)} potential energy functions (PEFs) of the 2B2 state of ScO2 and the 1A1 state of ScO2, were computed, employing the augmented correlation-consistent polarized-weighted core-valence quadruple-zeta (aug-cc-pwCVQZ) basis set for Sc and augmented correlation-consistent polarized valence quadruple-zeta (aug-cc-pVQZ) basis set for O, and with the outer core Sc 3s23p6 electrons being explicitly correlated. Franck-Condon factors, which include allowance for Duschinsky rotation and anharmonicity, were calculated using the computed RCCSD(T) PEFs, and were used to simulate the first photodetachment band of ScO2,. The simulated spectrum matches well with the corresponding experimental 355 nm photodetachment spectrum of Wu and Wang, J Phys Chem A 1998, 102, 9129, confirming the assignment of the photodetachment spectrum and the reliability of the RCCSD(T) PEFs used. Further calculations on low-lying electronic states of ScO2 gave adiabatic relative electronic energies (Te's) of, and vertical excitation energies (Tv's) to, the 2A1, 2B1, and 2A2 states of ScO2 (from the 2B2 state of ScO2), as well as electron affinities (EAs) and vertical detachment energies (VDEs) to these neutral states from the 1A1 state of ScO2,. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Intruder state avoidance multireference Møller,Plesset perturbation theoryJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2002Henryk A. Witek Abstract A new perturbation approach is proposed that enhances the low-order, perturbative convergence by modifying the zeroth-order Hamiltonian in a manner that enlarges any small-energy denominators that may otherwise appear in the perturbative expansion. This intruder state avoidance (ISA) method can be used in conjunction with any perturbative approach, but is most applicable to cases where small energy denominators arise from orthogonal-space states,so-called intruder states,that should, under normal circumstances, make a negligible contribution to the target state of interests. This ISA method is used with multireference Møller,Plesset (MRMP) perturbation theory on potential energy curves that are otherwise plagued by singularities when treated with (conventional) MRMP; calculation are performed on the 13, state of O2; and the 21,, 31,, 23,, and 33, states of AgH. This approach is also applied to other calculations where MRMP is influenced by intruder states; calculations are performed on the 3,u state of N2, the 3, state of CO, and the 21A, state of formamide. A number of calculations are also performed to illustrate that this approach has little or no effect on MRMP when intruder states are not present in perturbative calculations; vertical excitation energies are computed for the low-lying states of N2, C2, CO, formamide, and benzene; the adiabatic 1A1,3B1 energy separation in CH2, and the spectroscopic parameters of O2 are also calculated. Vertical excitation energies are also performed on the Q and B bands states of free-base, chlorin, and zinc,chlorin porphyrin, where somewhat larger couplings exists, and,as anticipated,a larger deviation is found between MRMP and ISA-MRMP. © 2002 Wiley Periodicals, Inc. J Comput Chem 10: 957,965, 2002 [source] Electronic Structure of the Ground and Excited States of ,-CarbolineCHEMPHYSCHEM, Issue 6 2008Vicente Pérez Mondéjar Abstract Coupled-cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of ,-carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects. [source] | ||||||||||