Bonding and isomerism in SF(n-1)Cl (n = 1-6): a quantum chemical study

Using high-level MRCI and CCSD(T) quantum chemical calculations, we report structures, energetics, and other properties of the sulfur fluoromonochloride family (SF(n-1)Cl, n = 1-6). Our group previously studied the sulfur fluoride family (SF(n), n = 1-6) and found that several of the excited states of SF and SF(2) as well as the ground states of SF(3)-SF(6) exhibited a new type of bonding, called recoupled pair bonding. Comparing the SF(n-1)Cl and SF(n) species allows us to study isomerism, apicophilicities, and substituent effects due to the Cl substitution. The primary findings of this work are twofold. First, replacing F with Cl weakens the adjacent S-F bonds by destabilizing the molecule with respect to the pure SF(n) analog. Second, an isomer with a singly occupied S-Cl antibonding orbital is more stable than the analogous isomer with a singly occupied S-F antibonding orbital, thus explaining apicophilicities. This work has also allowed us to further refine and expand our understanding of the nature of the recoupled pair bond model. Finally, we discovered the presence of bond-stretch isomers in the first excited ((3)A') state of SFCl.     

Bonding in SCln (n = 1-6): a quantum chemical study

Following a previous study of bonding and isomerism in the SF(n) and singly chloro-substituted SF(n-1)Cl (n = 1-6) series, we describe bonding in the ground and low-lying excited states of the completely substituted series, SCl(n) (n = 1-6). All structures were characterized at least at the RCCSD(T)/aug-cc-pV(Q+d)Z level of theory. Both differences and similarities were observed between SCl(n) and our previous results on SF(n-1)Cl and SF(n). Several minimum structures that exist in SF(n) and SF(n-1)Cl are absent in SCl(n). For example, the optimized structure of SCl(2)((3)A(2)) is a transition state in C(s) symmetry, whereas the analogous states are minima in SF(n) and SF(n-1)Cl. Second, we found a continuation of a trend discovered in the SF(n-1)Cl series, where Cl substitution has a destabilizing effect that weakens bonds with respect to SF(n). This effect is much stronger in the SCl(n) series than it is in the SF(n-1)Cl series, which is why SCl(2) is the most stable observed species in the family and why SCl(4), SCl(5), and SCl(6) are unstable (SCl(n-2) + Cl(2) additions are endothermic for n = 4-6).     

Ab initio calculations on SCl2 and low-lying cationic states of SCl2+: Franck-Condon simulation of the UV photoelectron spectrum of SCl2

Geometry optimization calculations were carried out on the (approximate)X (1)A(1) state of SCl(2) and the (approximate)X(2)B(1), (approximate)A(2)B(2), (approximate)B(2)A(1), (approximate)C(2)A(1), (approximate)D(2)A(2), and (approximate)E (2)B(2) states of SCl(2) (+) at the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] level with basis sets of up to the augmented correlation-consistent polarized quintuple-zeta [aug-cc-pV(5+d)Z] quality. Effects of core electron correlation, basis set extension to the complete basis set limit, and relativistic contributions on computed minimum-energy geometrical parameters and/or relative electronic energies were also investigated. RCCSD(T) potential energy functions (PEFs) were calculated for the (approximate)X (1)A(1) state of SCl(2) and the low-lying states of SCl(2)(+) listed above employing the aug-cc-pV(5+d)Z basis set. Anharmonic vibrational wave functions of these neutral and cationic states of SCl(2), and Franck-Condon (FC) factors of the lowest four one-electron allowed neutral photoionizations were computed employing the RCCSD(T)aug-cc-pV(5+d)Z PEFs. Calculated FC factors with allowance for the Duschinsky rotation and anharmonicity were used to simulate the first four photoelectron (PE) bands of SCl(2). The agreement between simulated and observed He I PE spectra reported by Colton et al. [J. Electron Spectrosc. Relat. Phenom. 3, 345 (1974)] and Solouki et al. [Chem. Phys. Lett. 26, 20 (1974)] is excellent. However, our FC spectral simulations indicate that the first observed vibrational component in the first PE band of SCl(2) is a "hot" band arising from the SCl(2)(+)(approximate)X(2)B(1)(0,0,0)<--SCl(2)(approximate)X (1)A(1)(1,0,0) ionization. Consequently, the experimental adiabatic ionization energy of SCl(2) is revised to 9.55+/-0.01 eV, in excellent agreement with results obtained from state-of-the-art ab initio calculations in this work.     

Extensive ab initio study of the electronic states of SCl including spin-orbit coupling

The effect of different basis sets for calculation of the spectroscopic constants of the ground state of sulfur monochloride (SCl) was analyzed using scalar relativistic multireference configuration interaction with single and double excitations plus Davidson correction. Then the generally contracted all-electronic correlation-consistent polarized valence quintuple zeta basis sets were selected to compute the electronic states of SCl including 12 valence and 9 Rydberg lambda-S states. The spin-orbit coupling effect was calculated via the state interaction approach with the full Breit-Pauli Hamiltonian. This effect splits these lambda-S states into 42 omega states. Potential-energy curves of all these states are plotted with the help of the avoided crossing rule between the electronic states of the same symmetry. The structural properties of these states are analyzed. Spectroscopic constants of bound excited states that have never been observed in experiment are obtained. The transition dipole moments and the Franck-Condon factors of several transitions from low-lying bound excited states to the ground state were also calculated.     

CEPA calculations on open-shell molecules. V. The vibration frequencies of SF and SCl

Ab initio calculations for the ² ground states of SF and SCl have been performed on Hartree-Fock level and with inclusion of valence shell correlation effects by means of the CI and CEPA approaches. The calculated properties are: Equilibrium distances, vibration frequencies, and dipole moment curves in the vicinity of the respective equilibrium geometries. Our best estimates for the 0 1 infrared absorption frequencies _o for SF and SCl are 786 cm^–1 and 520 cm^–1, respectively, both with an uncertainty of about 10 cm^–1. This confirms a recent experimental value obtained by Willner for SF (791 cm^–1), but indicates that for SCl both experimental values reported previously in the literature (617 cm^–1 and 574 cm^–1) are wrong. The S—F and S—Cl bonds in SF and SCl are very similar to the ones in SF₂ and SCl₂, being essentially single p-bonds in either case. In the analogous oxygen-halogen molecules the situation is different, the O—F and O—Cl bonds in the diatomic radicals OF and OCl have partial double bond character and are much stronger than those in OF₂ and OCl₂ or in HOF and HOCl.     

Reactivity and electronic structure of the ground and excited states of oxazole, 2-phenyloxazole, and 2-phenylthiazole

The relationship of the electronic structure, of oxazole, 2-phenyloxazole, and 2-phenylthiazole to the properties of the electronic excited states and transitions is examined. Spectral properties of these compounds in the free state (no effect from external perturbations) and in complexes with the proton and aprotic acids are measured and calculated by quantum-chemical PPP/S (-approximation), and INDO/S (sp-basis) methods. Features of the electronic excitation of the atoms and the vibronic interaction of bonds in the singlet and triplet states are examined for a change of the various structural forms of azoles, which determine their spectral fluorescence properties and reactivity. Possible direction control of reactions and optimized syntheses of new compounds with given properties are discussed based on a study of the properties of the ground and excited states.Russian People's Friendship University, Moscow 117302 N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow 117913 Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 948–974, July, 1999.     

Ab initio calculation of the electronic structures of the 3Phi ground and 5Phi excited states of CoH

The electronic structures and the spectroscopic constants of the electronic ground 3Phi and low-lying 5Phi electronic excited states of the CoH molecule were studied by multireference single and double excitation configuration interaction (MR-SDCI)+Davidson's correction (Q) calculations and size-consistent multireference coupled pair approximation (MRCPA) calculations. Calculations were performed under Cinfinityv symmetry using Slater-type basis functions. The electronic ground state was confirmed to be the 3Phi state. It was found that at least four reference configurations were needed to describe the ground 3Phi state correctly at the MR-SDCI+Q level, while the 5Phi state can be described well by one reference configuration, namely, the Hartree-Fock configuration. Larger dynamical electron correlation for the low-spin 3Phi state than that for the high-spin 5Phi state is discussed. Spectroscopic constants, i.e., equilibrium bond lengths (re), harmonic frequency (omegae), and excitation energy, obtained by the MR-SDCI+Q method showed good correspondence with experimental values. MRCPA calculations gave a slightly shorter value for re than experimental values, but improved omegae and the excitation energy bringing them very close to experimental values.     

Multireference configuration interaction studies on the ground and excited states of N2O2: the potential energy curves of N2O2 along N-N distance

In this paper, the ground and excited states of N2O2 were studied at the multireference configuration interaction (MRCI) level of theory with Dunning's [J. Chem. Phys. 90, 1007 (1985); 96, 6796 (1992)] correlation consistent basis sets augo-cc-pVDZ and aug-cc-pVTZ. The geometry optimizations were performed for the ground state of N2O2. The vertical excitation energies and transition moments were calculated for the low-lying singlet states of N2O2 including the lowest three 1A1 states, two 1B1 states, one 1B2 state, and two 1A2 states at the MRCI level of theory with Dunning's correlation consistent basis sets aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ. Furthermore, for the first time, the potential energy curves were calculated at the complete active space self-consistent-field and MRCI levels of theory for as many as 12 N2O2 singlet electronic states along the N-N distance. The dissociation asymptotes of these 12 N2O2 singlet electronic states were discussed.     

Spin-orbit coupling in O2(upsilon)+O2 collisions: I. Electronic structure calculations on dimer states involving the X 3Sigmag-, a 1Deltag, and b 1Sigmag+ states of O2

The importance of vibrational-to-electronic (V-E) energy transfer mediated by spin-orbit coupling in the collisional removal of O2(X 3Sigmag-,upsilon>or=26) by O2 has been reported in a recent communication [F. Dayou, J. Campos-Martinez, M. I. Hernandez, and R. Hernandez-Lamoneda, J. Chem. Phys. 120, 10355 (2004)]. The present work provides details on the electronic properties of the dimer (O2)2 relevant to the self-relaxation of O2(X 3Sigmag-,upsilon>0) where V-E energy transfer involving the O2(a 1Deltag) and O2(b 1Sigmag+) states is incorporated. Two-dimensional electronic structure calculations based on highly correlated ab initio methods have been carried out for the potential-energy and spin-orbit coupling surfaces associated with the ground singlet and two low-lying excited triplet states of the dimer dissociating into O2(X 3Sigmag-)+O2(X 3Sigmag-), O2(a 1Deltag)+O2(X 3Sigmag-), and O2(b 1Sigmag+)+O2(X 3Sigmag-). The resulting interaction potentials for the two excited triplet states display very similar features along the intermolecular separation, whereas differences arise with the ground singlet state for which the spin-exchange interaction produces a shorter equilibrium distance and higher binding energy. The vibrational dependence is qualitatively similar for the three studied interaction potentials. The spin-orbit coupling between the ground and second excited states is already nonzero in the O2+O2 dissociation limit and keeps its asymptotic value up to relatively short intermolecular separations, where the coupling increases for intramolecular distances close to the equilibrium of the isolated diatom. On the other hand, state mixing between the two excited triplet states leads to a noticeable collision-induced spin-orbit coupling between the ground and first excited states. The results are discussed in terms of specific features of the dimer electronic structure (including a simple four-electron model) and compared with existing theoretical and experimental data. This work gives theoretical insight into the origin of electronic energy-transfer mechanisms in O2+O2 collisions.     

Influence of geometry relaxation on the energies of the S1 and S2 states of violaxanthin, zeaxanthin, and lutein

Precise knowledge of the excitation energies of the lowest excited states S(1) and S(2) of the carotenoids violaxanthin, lutein, and zeaxanthin is a prerequisite for a fundamental understanding of their role in light harvesting and photoprotection during photosynthesis. By means of density functional theory (DFT) and time-dependent DFT (TDDFT), the electronic and structural properties of the ground and first and second excited states are studied in detail. According to our calculations, all-s-cis-zeaxanthin and s-cis-lutein conformers possess lower total ground-state energies than the corresponding s-trans conformers. Thus, only s-cis isomers are probably physiologically relevant. Furthermore, the influence of geometric relaxation on the energies of the ground state and S(1) and S(2) states has been studied in detail. It is demonstrated that the energies of these states change significantly if the carotenoid adopts the equilibrium geometry of the S(1) state. Considering these energetic effects in the interpretation of S(1) excitation energies obtained from fluorescence and transient absorption spectroscopy shifts the S(1) excitation energies about 0.2 eV to higher energy above the excitation energy of the chlorophyll a.     

Ground and valence-excited states of SF: a multireference configuration interaction with single and double excitations+Q study

Ab initio calculations on the ground and valence-excited states of the sulfur monofluoride radical have been performed using entirely uncontracted all-electron augmented correlation consistent polarized valence quintuple zeta basis sets and the internally contracted multireference configuration interaction with single and double excitations method and Davidson correction (+Q). Potential-energy curves of all valence electronic states and the spectroscopic constants of several bound states are fitted. It is the first time that the entire 27-omega states generated from the 12 valence lambda-S states which come from the S(3P(g)) and F(2P(u)) atomic states of SF radical have been studied theoretically. The effects of spin-orbit coupling and the avoided crossing rule between omega states of the same symmetry are analyzed. The calculated results reproduce well the available experimental values and predict the properties of several bound excited states that have never been observed in experiment. The transition properties of the dipole-allowed transitions from bound excited states to the ground state are predicted for the first time, including the transition dipole moments, the Franck-Condon factors, and the radiative lifetimes.     

Electronically excited states of water clusters of 7-azaindole: structures, relative energies, and electronic nature of the excited states

The geometries of 1H-7-azaindole and the 1H-7-azaindole(H(2)O)(1-2) complexes and the respective 7H tautomers in their ground and two lowest electronically excited pi-pi(*) singlet states have been optimized by using the second-order approximated coupled cluster model within the resolution-of-the-identity approximation. Based on these optimized structures, adiabatic excitation spectra were computed by using the combined density functional theory/multireference configuration interaction method. Special attention was paid to comparison of the orientation of transition dipole moments and excited state permanent dipole moments, which can be determined accurately with rotationally resolved electronic Stark spectroscopy. The electronic nature of the lowest excited state is shown to change from L(b) to L(a) upon water complexation.     

Geometries,vibrational frequencies,and excitation energies of a series of fluorine‐substituted carbenes,FCX (X = H,F, Cl,Br, and I): A high‐level multireference configuration interaction study

High‐level calculations using internally contracted multireference configuration interaction including Davidson correction (icMRCI+Q) method have been carried out for the ground singlet states, the first excited states, and the lowest triplet states of a series of fluorine‐substituted carbenes FCX (X = H, F, Cl, Br, and I). Equilibrium geometries and vibrational frequencies of the three electronic states, adiabatic transition energy of the first excited singlet state, as well as the ground singlet—lowest triplet energy gap (S‐T gap) of each of FCX carbenes have been obtained. Effects of the basis set of icMRCI+Q calculation on the geometries and energies have been investigated. In addition, various corrections, including the scalar relativistic effect, spin‐orbit coupling, and core‐valence correlation, have been studied in calculating the transition energies and the S‐T gaps, especially for heavy‐atom carbenes. This results have been compared with previous calculations using a variety of methods. Our icMRCI+Q results are in very good agreement with the high‐resolution laser‐based spectroscopic results where available. Some structure and spectroscopic constants of the fluorine‐substituted carbenes which are void in the literature have been provided with consistent high‐level calculations. © 2013 Wiley Periodicals, Inc.     

Avoided crossings, conical intersections, and low-lying excited states with a single reference method: The restricted active space spin-flip configuration interaction approach

The restricted active space spin-flip CI (RASCI-SF) performance is tested in the electronic structure computation of the ground and the lowest electronically excited states in the presence of near-degeneracies. The feasibility of the method is demonstrated by analyzing the avoided crossing between the ionic and neutral singlet states of LiF along the molecular dissociation. The two potential energy surfaces (PESs) are explored by means of the energies of computed adiabatic and approximated diabatic states, dipole moments, and natural orbital electronic occupancies of both states. The RASCI-SF methodology is also used to study the ground and first excited singlet surface crossing involved in the double bond isomerization of ethylene, as a model case. The two-dimensional PESs of the ground (S(0)) and excited (S(1)) states are calculated for the complete configuration space of torsion and pyramidalization molecular distortions. The parameters that define the state energetics in the vicinity of the S(0)/S(1) conical intersection region are compared to complete active space self-consistent field (CASSCF) results. These examples show that it is possible to describe strongly correlated electronic states using a single reference methodology without the need to expand the wavefunction to high levels of collective excitations. Finally, RASCI is also examined in the electronic structure characterization of the ground and 2(1)A(g) (-), 1(1)B(u) (+), 1(1)B(u) (-), and 1(3)B(u) (-) states of all-trans polyenes with two to seven double bonds and beyond. Transition energies are compared to configuration interaction singles, time-dependent density functional theory (TDDFT), CASSCF, and its second-order perturbation correction calculations, and to experimental data. The capability of RASCI-SF to describe the nature and properties of each electronic state is discussed in detail. This example is also used to expose the properties of different truncations of the RASCI wavefunction and to show the possibility to use an excitation operator with any number of α-to-β electronic promotions.     

Electronic structure of carbon trioxide and vibronic interactions involving Jahn-Teller states

The electronic structure of CO3 is characterized by equation-of-motion and coupled-cluster methods. C(2v) and D(3h) isomers are considered. Vertical excitation energies, transition dipoles, and the molecular orbital character of the excited states are presented for singlet and triplet manifolds. Ground-state equilibrium structures and frequencies are strongly affected by vibronic interactions with low-lying excited states. At D(3h) geometries, the vibronic interactions are enhanced by the Jahn-Teller character of the excited states. The curvature of the potential energy surface and the existence of the D(3h) minimum are very sensitive to the correlation treatment and the basis set. The correlation effects are stronger at D(3h), in agreement with a smaller HOMO-LUMO gap.     

CASSCF and CASPT2 studies on the structures, transition energies, and dipole moments of ground and excited states for azulene

The electronic structure of azulene molecule has been studied. We have obtained the optimized structures of ground and singlet excited states by using the complete active space self-consistent-field (CASSCF) method, and calculated vertical and 0-0 transition energies between the ground and excited states with second-order M?ller-Plesset perturbation theory (CASPT2). The CASPT2 calculations indicate that the bond-equalized C(2v) structure is more stable than the bond-alternating C(s) structure in the ground state. For a physical understanding of electronic structure change from C(2v) to C(s), we have performed the CASSCF calculations of Duschinsky matrix describing mixing of the b(2) vibrational mode between the ground (1A(1)) and the first excited (1B(2)) states based on the Kekule-crossing model. The CASPT2 0-0 transition energies are in fairly good agreement with experimental results within 0.1-0.3 eV. The CASSCF oscillator strengths between the ground and excited states are calculated and compared with experimental data. Furthermore, we have calculated the CASPT2 dipole moments of ground and excited states, which show good agreement with experimental values.     

Excited electronic states of the cyclic isomers of O2 and SO2

The low-lying electronic states of O3 and SO2 in their bent and cyclic isomers up to about 10 eV are calculated using the multireference configuration interaction (MRCI) method with a standard Gaussian correlation consistent polarized triple-zeta (cc-pVTZ) basis set. The vertical excitation energies, electron configurations, and oscillator strengths of these states are reported. The molecular orbital structures and excited states of the cyclic isomers are discussed in relation to the bent ones. Coherent anti-Stokes Raman spectroscopy (CARS) schemes for detecting the synthesis of the cyclic isomers are suggested.     

Study of 7-azaindole in its first four singlet states

The molecular structure and properties of 7-azaindole in its first four singlet states were studied with a view to improving current understanding of the photophysical behavior of its C(2h) dimer. This dimer, which exhibits a double proton transfer via its two hydrogen bonds upon electronic excitation, has for 35 years been used as a model for the photophysical behavior of DNA base pairs. Electronic excitation of 7-azaindole simultaneously increases its acidity and basicity; these changes facilitate a concerted mechanism for the double proton transfer in the dimer. In this work, we found the acidity and basicity changes to occur only in its first pi,pi(*) excited singlet state.     

Insights into the unusual barrierless reaction between two closed shell molecules, (CH3)2S + F2, and its H2S + F2 analogue: role of recoupled pair bonding

Early flowtube studies showed that (CH(3))(2)S (DMS) reacted very rapidly with F(2); hydrogen sulfide (H(2)S), however, did not. Recent crossed molecular beam studies found no barrier to the reaction between DMS and F(2) to form CH(2)S(F)CH(3) + HF. At higher collision energies, a second product channel yielding (CH(3))(2)S-F + F was identified. Both reaction channels proceed through an intermediate with an unusual (CH(3))(2)S-F-F bond structure. Curiously, these experimental studies have found no evidence of direct F(2) addition to DMS, resulting in (CH(3))(2)SF(2), despite the fact that the isomer in which both fluorines occupy axial positions is the lowest energy product. We have characterized both reactions, H(2)S + F(2) and DMS + F(2), with high-level ab initio and generalized valence bond calculations. We found that recoupled pair bonding accounts for the structure and stability of the intermediates present in both reactions. Further, all sulfur products possess recoupled pair bonds with CH(2)S(F)CH(3) having an unusual recoupled pair bond dyad involving π bonding. In addition to explaining why DMS reacts readily with F(2) while H(2)S does not, we have studied the pathways for direct F(2) addition to both sulfide species and found that (for (CH(3))(2)S + F(2)) the CH(2)S(F)CH(3) + HF channel dominates the potential energy surface, effectively blocking access to F(2) addition. In the H(2)S + F(2) system, the energy of the transition state for formation of H(2)SF(2) lies very close to the H(2)SF + F asymptote, making the potential pathway a roaming atom mechanism.     

Theoretical study of the photoinduced transfer among the ground state and two metastable states in [Fe(CN)5NO]2-

Ab initio calculations were performed to investigate photoinduced transfers among the ground state (GS) and two metastable states (MS1 and MS2) of [Fe(CN)5NO]2-. We obtained the global potential energy surface of the electronic ground state by a scheme of multireference singly and doubly excited configuration interaction followed by a Davidson-type quadruple correction (MRSDCI+Q). The ground state surface has three local minima corresponding to GS, MS1, and MS2. The character of bond between Fe and the nitrosyl group are discussed. We carried out calculations of the lower five electronic excited states by MRSDCI+Q. The main configurations of these lower five excited states were represented by the dFe-->pi*NO transition accompanied by considerable back-donation. The potential energy surfaces of the six states, including the ground state, were obtained by state averaged complete active space self-consistent field calculations. The surfaces have several conical intersections and avoided crossings in the reaction pathway. The photoinduced transfers among GS, MS1, and MS2 are caused by the nonadiabatic effect near these crossings.