Symmetry-adapted Cluster-configuration Interaction Theory Research Articles

Electronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study.

Electronic structures of azaindole were studied using symmetry-adapted cluster configuration interaction theory utilizing Dunning's cc-pVTZ basis set augmented with appropriate Rydberg spd functions on carbon and nitrogen atoms. The results obtained in the present study show good agreement with the available experimental values. Importantly, and contrary to previous theoretical studies, the excitation energy calculated for the important n-π(∗) state agrees well with the experimental value. A recent study by Pratt and co-workers concluded that significant mixing of π-π(∗) and n-π(∗) states leads to major change in the magnitude and direction of the dipole moment of the upper state vibrational level in the 0,0 + 280 cm(-1) band in the S1←S0 transition when compared to that of the zero-point level of the S1 state. The present study, however, shows that all the four lowest lying excited states, (1)Lb π-π(∗), (1)La π-π(∗), n-π(∗), and π-σ(∗), cross each other in one way or another, and hence, significant state mixing between them is likely. The upper state vibrational level in the 0,0 + 280 cm(-1) band in the S1←S0 transition benefits from this four-state mixing and this can explain the change in magnitude and direction of the dipole moment of the S1 excited vibrational level. This multistate mixing, and especially the involvement of π-σ(∗) state in mixing, could also provide a route for hydrogen atom detachment reactions. The electronic spectra of benzimidazole, a closely related system, were also investigated in the present study.

The low-lying singlet electronic excited states of TiO2: A symmetry adapted cluster-configuration interaction (SAC-CI) study

The ground electronic state (X1A1) and nine low-lying singlet electronic excited states (21A1, 31A1, 11A2, 21A2, 11B1, 21B1, 31B1, 11B2, and 21B2) of TiO2 were studied using SAC-CI theory. The geometries of the nine electronic excited states were optimized at the SAC-CI/def2-SVP and SAC-CI/def2-SVPD levels. The adiabatic excitation energies of the nine electronic excited states were obtained from single-point calculations at the SAC-CI/def2-TZVPD//SAC-CI/def2-SVPD and SAC-CI/def2-TZVPPD//SAC-CI/def2-SVPD levels. Stable Cs structures for the 11B1 (A″) and 31B1 (A″) states were determined in this study. For comparison, the X1A1, 11A2, 11B1, and 11B2 states were also studied using the unrestricted DFT theory (UBPW91) with the def2-SVP, def2-SVPD, def2-TZVPD, def2-TZVPPD, and aug-cc-pVTZ basis sets.

Conformational Dependence of the Circular Dichroism Spectrum of α-Hydroxyphenylacetic Acid: A ChiraSac Study

The conformational dependence of the circular dichroism (CD) spectrum of a chiral molecule, α-hydroxyphenylacetic acid (HPAA) containing phenyl, COOH, OH and H groups around a chiral carbon atom, has been studied theoretically by using the SAC-CI (symmetry adapted cluster-configuration interaction) theory. The results showed that the CD spectrum of HPAA depends largely on the rotations (conformations) of the phenyl and COOH groups around the single bonds. The first band is due to the excitation of electrons belonging to the phenyl region and therefore sensitive to the phenyl rotation. The second band is due to the excitation of electrons belonging to the COOH region and therefore sensitive to the COOH rotation. From the comparison of the SAC-CI CD spectra calculated at various conformations of phenyl, COOH, and OH groups with the experimental spectrum, we could predict the stable geometry of this molecule, which agreed well with the most stable conformation deduced from the energy criterion. We also calculated the Boltzmann averaged spectrum and obtained better agreement with the experiment. Further, we performed preliminary investigations of the temperature dependence of the CD spectrum of HPAA. In general, the CD spectra of chiral molecules are very sensitive to low-energy processes like the rotations around the single bonds. Therefore, one should be able to study the natures of the weak interactions by comparing the SAC-CI spectra calculated at different geometries and conditions with the experimental spectrum using a new methodology we have termed ChiraSac.

Electronic excitation spectra of radical anions of cyanoethylenes and cyanobenzenes: Symmetry adapted cluster–configuration interaction study

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.

Vibrational effects on the electron momentum distributions of valence orbitals of formamide

The ionization energy spectra and electron momentum distributions of formamide were investigated using the high-resolution electron momentum spectrometer in combination with high level calculations. The observed ionization energy spectra and electron momentum distributions were interpreted using symmetry adapted cluster-configuration interaction theory, outer valence Green function, and DFT-B3LYP methods. The ordering of 10a(') and 2a(") orbitals of formamide was assigned unambiguously by comparing the experimental electron momentum distributions with the corresponding theoretical results, i.e., 10a(') has a lower binding energy. In addition, it was found that the low-frequency wagging vibration of the amino group at room temperature has noticeable effects on the electron momentum distributions. The equilibrium-nuclear-positions-approximation, which was widely used in electron momentum spectroscopy, is not accurate for formamide molecule. The calculations based on the thermal average can evidently improve the agreement with the experimental momentum distributions.

Calculation of Dyson orbitals using a symmetry-adapted-cluster configuration-interaction method for electron momentum spectroscopy:N2andH2O

The symmetry-adapted-cluster (SAC) configuration-interaction (CI) theory was introduced to interpret the non-coplanar symmetric ($e$,$2e$) results. Dyson orbitals derived from the bench-marked SAC CI general-R method were utilized for computing the electron momentum distributions. The corresponding excitation energies and spectroscopic factors can be used to reproduce the ionization spectra. The implementation was demonstrated by examples of ${\mathrm{N}}_{2}$ and ${\mathrm{H}}_{2}$O. The electron momentum distributions calculated using SAC CI method were compared with recent experimental results, as well as the Hartree-Fock and density-functional-theory calculations. The SAC CI method gave the best performance on the description of the experimental momentum distributions. It was found that the electron momentum distributions of Dyson orbitals related to the satellite lines can be notably different from those of their parent orbitals due to the electron correlation in the initial target states. Present work demonstrated that the SAC CI theory is a very useful and accurate tool for interpreting high-resolution electron momentum spectroscopy results.

Dyson orbitals of N2O: Electron momentum spectroscopy and symmetry adapted cluster-configuration interaction calculations

Electron momentum spectroscopy and symmetry adapted cluster-configuration interaction (SAC-CI) theory were combined to study electron correlation effects in nitrous oxide molecule (N(2)O). The SAC-CI General-R method accurately reproduced the experimental ionization spectrum. This bench-marked method was also introduced for calculating the momentum distributions of N(2)O Dyson orbitals. Several calculated momentum distributions with different theoretical methods were compared with the high resolution experimental results. In the outer-valence region, Hartree-Fock (HF), density functional theory (DFT), and SAC-CI theory can well describe the experimental momentum distributions. SAC-CI presented a best performance among them. In the inner-valence region, HF and DFT cannot work well due to the severe breaking of the molecular orbital picture, while SAC-CI still produced an excellent description of experimental momentum profiles because it can accurately take into account electron correlations. Moreover, the thermally averaged calculation showed that the geometrical changes induced by the vibration at room temperature have no noticeable effects on momentum distribution of valence orbitals of N(2)O.

Electronic transitions in cis- and trans-dichloroethylenes and tetrachloroethylene

Electronic structures of trans- and cis-dichloroethylenes and tetrachloroethylene were studied using symmetry-adapted cluster configuration interaction theory. Basis sets up to the aug-cc-pVTZ of Dunning, Jr., augmented with appropriate Rydberg functions were used for the calculations. The results derived in the present study show good agreement with the available experimental values. In all cases, the main bright excitation was the pi-->pi( *) transition. The other vertical excitations, pi-->sigma( *), n-->sigma( *), and n-->pi( *), which have not been studied before, were also investigated. First Rydberg series involving transitions from the pi orbitals to one 3s, three 3p, and five 3d orbitals were identified clearly. Several new assignments and reassignments of features in the experimental spectra were suggested. Contrary to earlier prediction, two n-sigma( *) states, along with a pi-sigma( *) state in the dichloroethylenes, were calculated to be located above the main pi-pi( *) state. Accordingly, crossing between both the n-sigma( *) states with the bright pi-pi( *) state is highly likely, unlike conclusions made in the earlier studies. This indicates that the photodissociation mechanism proposed by the earlier calculations warrants revision. Several low-lying triplet excited states were also studied. Electronic spectra of trans-1-chloro-2-fluoroethylene and cis-1-chloro-2-fluoroethylene were also calculated. The pi-->pi( *) transitions of these haloethylenes are compared and interpreted in terms of the inductive and resonance effects.

Electronic excitations of fluoroethylenes

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 pi-->pi(*) 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 pi-pi(*) and pi-3ppi states in trifluoroethylene. No pure pi-sigma(*) 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 pi-pi(*) and sigma-sigma(*) states is the reason behind the blueshift of around 1.25 eV in the pi-pi(*) 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.

C 4 Cl : Bent or linear?

The ground state structure for the CCCCCl radical was computed by using symmetry-adapted cluster configuration-interaction (SAC-CI) theory along with density functional theory to overcome the differences raised in the recently published paper [Y. Sumiyoshi et al., Chem. Phys. Lett. 414, 82 (2005)] between the theory and the experiment. SAC-CI results clearly support the earlier experimental conclusion that the radical has the bent ground state structure corresponding to 2Pi symmetry. Contrarily, probably due to spin contamination, mixing of a bent doublet ground state with the quartet components of a linear structure, coupled-cluster singles and doubles (CCSD) calculations were unable to provide reliable results. Results obtained using density functional theory also show that the radical has a bent structure. Some low-lying doublet excited states were also studied using the SAC-CI theory. The energy difference between the ground Pi state and the nearby Sigma state is around 0.2 eV. The excitation energy for the transition with the largest oscillator strength agrees with the strongest absorption peak.

Electronic spectra and photodissociation of vinyl chloride: A symmetry-adapted cluster configuration interaction study

The vertical absorption spectrum and photodissociation mechanism of vinyl chloride (VC) were studied by using symmetry-adapted cluster configuration interaction theory. The important vertical pi --> pi* excitation was intensively examined with various basis sets up to aug-cc-pVTZ augmented with appropriate Rydberg functions. The excitation energy for pi --> pi* transition obtained in the present study, 6.96 eV, agrees well with the experimental value, 6.7-6.9 eV. Calculated excitation energies along with the oscillator strengths clarify that the main excitation in VC is the pi --> pi* excitation. Contrary to the earlier theoretical reports, the results obtained here support that the C-Cl bond dissociation takes place through the n(Cl-)sigma(C-Cl)* state.

Potential energy curves of the 5s Rydberg excited states and ionized states of Kr 2 studied by the SAC-CI theory

Potential energy curves of the 5s Rydberg excited states and ionized states of Kr 2 are studied by symmetry adapted cluster-configuration interaction (SAC-CI) theory. New assignments for the observed peaks of the near infrared absorptions from the lowest excited state are proposed. Calculated transition energies from the ground state to the lowest excited states are comparable with those from the laser absorption experiment. The experimental vertical and adiabatic ionization potential are well reproduced from the potential curves of the ionized states.

Electronic origin of molybdenum-95 NMR chemical shifts in molybdenum complexes. Relationship between excitation energy and chemical shift

From the previous ab initio theoretical study on the electronic origin of the {sup 95}Mo NMR chemical shifts of the complexes MoO{sub 4{minus}n}S{sub n}{sup 2{minus}} (n = 0-4) and MoSe{sub 4}{sup 2{minus}}, it became clear that the d-d{sup *} excitations are the dominant factors in the perturbation theoretic point of view. This suggests an inverse proportionality between the Mo chemical shifts and the excitation energies. This relationship is examined by calculating the magnetically allowed excitation energies by the SAC-CI (symmetry-adapted cluster-configuration interaction) theory. The results show a nice linear relationship between the Mo chemical shifts and the energies of the excitations from the 4d{sigma} MO to the 4d{sigma} MO to the 4d{pi}{sup *} MO. This result quantitatively supports the previously reported mechanism of the Mo chemical shifts. 18 refs., 2 figs., 2 tabs.

Potential energy curves of the ground, excited, and ionized states of Ar2 studied by the symmetry adapted cluster-configuration interaction theory

Symmetry adapted cluster-configuration interaction theory is applied to the calculation of potential energy curves for the ground, excited, and ionized states of Ar2. The excited states studied here dissociate into Ar(3p6)+Ar(3p54s1) and Ar(3p6)+Ar(3p54p1). Spin–orbit coupling is included by a semiempirical method. The present results, especially for the 4s Rydberg states, compare very well with the experimental results of absorption and emission spectra. Some new assignments of the observed spectra are given, particularly for excitations from bound excited states of Ar2.

Calculation of hyperfine splitting constants with Slater-type cusp basis by the symmetry adapted cluster-configuration interaction theory

A method of calculating hyperfine splitting constants (hfsc) of open-shell radicals is reported. Since the hfsc reflects a very local property of the wave function at the position of the nuclei, we use Slater-type orbitals (STOs) which have cusps at the centers. Spin correlation (spin polarization) and electron correlation are taken into account with the use of the SAC-CI (symmetry adapted cluster-configuration interaction) theory. Configuration selection is not done since energy and spin density are very different properties. The dimensions of the matrices involved in the SAC-CI method are small enough to permit such a procedure. The integrals necessary for the SCF and SAC-CI methods are calculated by the STO-6G method with the use of the available Gaussian program, and the hfsc’s and the cusp values are calculated from the original STOs. Several STOs are examined for H+2 and H2O+. The resultant standard method is applied to the organic π and σ radicals CH3, CH3CH2, CH3NH, CH3O, CH3OCH+3, H2CO+, CH2CH, and HCO. The present results of hfsc’s show generally better agreement with experiment than the previous calculations based on the Gaussian-type orbitals (GTOs). In particular, the present results are free from the theoretical dilemma seen for the GTO calculations that a variationally better wave function does not necessarily give a better hfsc. This is important in developing a reliable theory for calculating hfsc’s.