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Electronic excitation spectra of the radical anions of cyanoethylenes (trans-dicyanoethylene and tetracyanoethylene) and cyanobenzenes (1,2-dicyanobenzene: o-DCNB, 1,3-dicyanobenzene: m-DCNB, and 1,4-dicyanobenzene: p-DCNB) were studied by the symmetry adapted cluster-configuration interaction (SAC-CI) method. Theoretical calculations predicted positive electron affinities for all the molecules in good agreement with the experimental observations. Electronic excitation spectra of open-shell radicals is a topic that has not been studied as much as such spectra of closed-shell molecules, but this can be easily addressed using SAC-CI theory. The present paper systematically describes the calculation procedures for radical anions by investigating several basis sets, including anion diffuse and Rydberg functions. The calculated excitation energies were in good agreement with the experimental UV/NIR (near infrared region) spectra, which had been observed by one of the present authors in 2-methyltetrahydrofuran matrix frozen to transparent glassy solids at 77 K. For p-DCNB, the SAC-CI theoretical spectrum agreed particularly well with the experimental spectrum. An extremely weak π*(SOMO) - π* excitation at 1.41 eV predicted in the present work, but had been overlooked in the previous experimental spectrum published in 1988, was confirmed to be real by a careful re-examination of the old spectrum. [ABSTRACT FROM AUTHOR]

In this paper we present the theory and implementation of the symmetry-adapted cluster (SAC) and symmetry-adapted cluster-configuration interaction (SAC-CI) method, including the solvent effect, using the polarizable continuum model (PCM). The PCM and SAC/SAC-CI were consistently combined in terms of the energy functional formalism. The excitation energies were calculated by means of the state-specific approach, the advantage of which over the linear-response approach has been shown. The single-point energy calculation and its analytical energy derivatives are presented and implemented, where the free-energy and its derivatives are evaluated because of the presence of solute-solvent interactions. We have applied this method to s-trans-acrolein and metylenecyclopropene of their electronic excitation in solution. The molecular geometries in the ground and excited states were optimized in vacuum and in solution, and both the vertical and adiabatic excitations were studied. The PCM-SAC/SAC-CI reproduced the known trend of the solvent effect on the vertical excitation energies but the shift values were underestimated. The excited state geometry in planar and nonplanar conformations was investigated. The importance of using state-specific methods was shown for the solvent effect on the optimized geometry in the excited state. The mechanism of the solvent effect is discussed in terms of the Mulliken charges and electronic dipole moment. [ABSTRACT FROM AUTHOR]

Several lowest-lying singlet electronic states of vinyl fluoride, trans-, cis-, and 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroethylene were investigated by using symmetry-adapted cluster configuration interaction theory. Basis sets up to Dunning’s aug-cc-pVTZ augmented with appropriate Rydberg functions were utilized for the calculations. Calculated excitation energies show a good agreement with the available experimental values. Even in the troublesome π→π* transitions, the excitation energies obtained in the present study agree well with the experimental values except in one or two fluoroethylenes. Strong mixing between different states was noticed in a few fluoroethylenes; especially the mixing is very strong between π-π* and π-3pπ states in trifluoroethylene. No pure π-σ* excited state was found in almost all the fluoroethylenes. Several assignments and reassignments of features in the experimental spectra were suggested. The present study does not support the existing argument that the interaction between the π-π* and σ-σ* states is the reason behind the blueshift of around 1.25 eV in the π-π* excitation energy of tetrafluoroethylene. Possible reasons, including structural changes, for this shift are discussed in detail. Several low-lying triplet excited states were also studied. [ABSTRACT FROM AUTHOR]

The ground state, singlet, and triplet excited states, and ionized states of ferrocene Fe(C[SUB5]H[SUB5])[SUB2] were studied by the symmetry adapted cluster (SAC)/SAC-configuration-interaction method. The calculated ionization energies and intensities fairly well reproduced the observed photoelectron spectrum in the wide region of 6-14 eV. In particular, the first two peaks ([SUP2]E′[SUB2] and [SUP2]A′[SUB1]) were assigned to the ionizations from the occupied 3 d orbitals of Fe, mixed already with the two-electron shake-up processes. This is the first ab initio quantitative assignment that is consistent with the experimental data. For the singlet states, three d-d transitions were calculated at 2.12, 2.26, and 4.02 eV, which correspond to the experimental peaks observed at 2.69, 2.97, and 3.82 eV. We propose possible assignments for other absorption bands in the range of 2.12-6.57 eV. In another three triplet d-d transition states we calculated, we found that the energy order of these states (1 [SUP3]E″[SUB1], 1 [SUP3]E″[SUB2], 2 [SUP3]E″[SUB1]) differs from that of singlet states (1 [SUP1]E″[SUB2], 1 [SUP1]E″[SUB1], 2 [SUP1]E″[SUB1]). [ABSTRACT FROM AUTHOR]

Electronic ground and excited states of anion radicals of cyanobenzene derivatives: 1,3,5-tricyanobenzene, 1,2,4,5-tetracyanobenzene, and tetracyanoquinodimethane (TCNQ) and nitrobenzene derivatives: nitrobenzene,p-nitroaniline,m-nitroaniline, ando-nitroaniline were theoretically investigated by the symmetry adapted cluster-configuration interaction (SAC-CI) method, which is able to produce accurate theoretical electronic excitation spectra even for radical doublet states. For all the target molecules, the present calculations reproduced the positive electron affinities, which were mostly in good agreement with the experimental values, and their features, especially for TCNQ, were characterised by singly occupied molecular orbitals as well as the number of the electron-withdrawing terminal groups. The excitation energies and their oscillator strengths by the SAC-CI method were also in good agreement with the corresponding experimental UV/VIS/NIR spectra observed by one of the authors and other experimental evidences. Except for TCNQ, the present theoretical calculations were successful to first predict the existences of the forbidden (or very low intense) pure valence excited states in near-infrared region. The physical natures of the observed intense spectral bands were clarified and some new assignments to their electronic states were provided. By extending the present work, photo-related molecular designs of new functional electron acceptors may be challenged. [ABSTRACT FROM PUBLISHER]

Theconformational dependence of the circular dichroism (CD) spectrumof a chiral molecule, α-hydroxyphenylacetic acid (HPAA) containingphenyl, COOH, OH and H groups around a chiral carbon atom, has beenstudied theoretically by using the SAC-CI (symmetry adapted cluster–configurationinteraction) theory. The results showed that the CD spectrum of HPAAdepends largely on the rotations (conformations) of the phenyl andCOOH groups around the single bonds. The first band is due to theexcitation of electrons belonging to the phenyl region and thereforesensitive to the phenyl rotation. The second band is due to the excitationof electrons belonging to the COOH region and therefore sensitiveto the COOH rotation. From the comparison of the SAC-CI CD spectracalculated at various conformations of phenyl, COOH, and OH groupswith the experimental spectrum, we could predict the stable geometryof this molecule, which agreed well with the most stable conformationdeduced from the energy criterion. We also calculated the Boltzmannaveraged spectrum and obtained better agreement with the experiment.Further, we performed preliminary investigations of the temperaturedependence of the CD spectrum of HPAA. In general, the CD spectraof chiral molecules are very sensitive to low-energy processes likethe rotations around the single bonds. Therefore, one should be ableto study the natures of the weak interactions by comparing the SAC-CIspectra calculated at different geometries and conditions with theexperimental spectrum using a new methodology we have termed ChiraSac. [ABSTRACT FROM AUTHOR]

Abstract: We developed the direct SAC-CI (symmetry-adapted cluster-configuration interaction) method for accurate calculations of electronic excited states of large molecules. The direct SAC-CI method uses an efficient MO (molecular orbital) direct scheme that is compatible with the perturbation-selection technique with using sparse linear algebra techniques. The direct SAC-CI can calculate large molecules with moderate computational cost. Using the direct SAC-CI method, excited states of free-base tetraazaporphyrin, phthalocyanine, naphthalocyanine, and anthracocyanine were studied. An interesting feature of the observed spectra is that the splitting of the Q-bands decreases with increasing molecular size. The SAC-CI calculation well reproduced the observed trend of excitation energies and showed the importance of electron correlation effects in the excited states. [Copyright &y& Elsevier]

The ground and excited states of five nucleic acid bases (adenine, guanine, cytosine, uracil and thymine) were calculated by employing the SAC⁄/SAC-CI (symmetry adapted cluster/SAC configuration-interaction) method. The absorption spectra with the SAC-CI method were compared with the experimental spectra. The spectra obtained with the SAC-CI method were in good agreement with the experimental spectra in gas phase. Comparisons with the calculations with other methods were made. [ABSTRACT FROM AUTHOR]

Electronic excitation spectrum of thiophene was investigated by the symmetry-adapted cluster (SAC)/SAC configuration interaction method. Seventy singlet and four lowest triplet electronic states of thiophene were computed to give a detailed satisfactory theoretical interpretation of the vacuum ultraviolet (VUV) spectrum and the electron energy loss spectrum of thiophene. The present calculations gave the 2 [sup 1]A[sub 1] valence state at 5.41 eV and the 1 [sup 1]B[sub 2] valence state at 5.72 eV with oscillator strengths 0.0911 and 0.1131, respectively, and the 5 [sup 1]A[sub 1] valence state at 7.32 eV and the 4 [sup 1]B[sub 2] valence state at 7.40 eV with oscillator strengths 0.3614 and 0.1204, respectively. These valence-excited states were assigned to the two strong absorption bands of the VUV spectrum centered around 5.5 and 7.05 eV, respectively. A number of Rydberg transitions were obtained and assigned to the 6.0, 6.6, and 7.5-8.7 eV, etc. energy regions. The similarities and differences in the electronic excitations between thiophene and other five-membered ring compounds were discussed. The accuracy and assignment of the present results are compared with those of the recent theoretical studies by CASPT2 and multireference double configuration interaction methods. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electronic excitation spectra of furan and pyrrole are reinvestigated by the symmetry-adapted cluster configuration-interaction method. The 47 and 46 lowest singlet and triplet electronic states are computed for furan and pyrrole, respectively. Two series (1a[sub 2] and 2b[sub 1]) of low-lying Rydberg states and the valence π-π* excited states strongly influence each other in both furan and pyrrole. The present calculations give detailed and satisfactory theoretical assignments of the vacuum ultraviolet spectra and the electron energy-loss spectra of the two molecules. The similarities and differences in the electronic excitations between furan and pyrrole are discussed in detail. The accuracy and assignments of recent theoretical studies, i.e., complete active space second-order perturbation, multireference Møller-Plesset perturbation, second-order algebraic-diagrammatic construction, multireference double configuration interaction, and CC3, are compared. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electronic excitation and ionization spectra of cyclopentadiene (CP) were reinvestigated by the symmetry-adapted cluster (SAC) and SAC-configuration interaction (SAC-CI) method with an extended basis set and a wide active orbital space. To give a satisfactory interpretation of the general profile of the observed excitation spectrum, 40 low-lying excited singlet and triplet states (with excitation energies of up to 9.5 eV) were computed. The calculated excitation energies were greatly improved compared to those reported previously. All of the peaks in the experimental spectrum were reassigned theoretically with small deviations. The natures of the low-lying valence and Rydberg-excited states were discussed in detail, and the results were also compared with those of some other recent theoretical studies. The ionization energies calculated by the SAC-CI general-R method agree well with the experimental peaks in the photoelectron spectrum. A number of two-electron shake-up states were calculated below 23 eV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The two lowest singlet and triplet Σ+ potential energy curves of LiH were calculated using the free complement (FC) local Schrödinger equation (LSE) method. The overall potential curves and the properties calculated therefrom, like equilibrium bond length, dissociation energy, adiabatic and vertical excitation energies, zero point energy, vibrational spacings, etc., demonstrated the high accuracy of the FC LSE method for both the ground and excited states in comparison to the reference calculations and experiments. [ABSTRACT FROM AUTHOR]