I.A. Godunov

A quantum-chemical study of the structure and conformational dynamics of the acrolein molecule in the ground electronic state

The geometric parameters (including vibrationally averaged parameters), energy differences (ΔE) between the s-cis and s-trans conformers, and barrier to internal rotation (Vt) were calculated for the acrolein molecule CH2=CHCHO by various quantum-chemical methods (MP2, DFT, CASSCF, QCISD, CCSD(T), and MR-AQCC). The MP2 and B3LYP methods were used to calculate internal rotation potential functions and vibrational frequencies; the calculations were performed in various anharmonic approximations. To refine the ΔE and Vt values, two-dimensional (using a basis set of atomic orbitals) VFPA extrapolation procedure was applied, which allowed the results to be estimated in the FCI/CBS approximation taking into account nonadiabaticity, core correlation effects, and changes in the difference between zero point energies.

Theoretical study of the structure of propanal in the first excited singlet and triplet electronic states: three-dimensional model for torsional and inversion vibrations

Abstract Potential energy surfaces of the conformationally nonrigid propanal molecule (CH 3 CH 2 CHO) in the lowest excited triplet (T 1 ) and singlet (S 1 ) electronic states were calculated by the multiconfigurational self-consistent field method CASSCF. Electronic excitations of the propanal conformers from the ground state (S 0 ) to the excited states (T 1 and S 1 ) were demonstrated to cause essential changes in the molecular structure, viz. the rotation of the ethyl tops and the out-of-plane distortion of carbonyl fragments. Nuclear vibrational motions in the propanal molecule in the excited electronic states were analyzed both in the harmonic approximation and by the use of the one-, two-, and three-dimensional variational calculations to describe correlated large-amplitude vibrations.

Determination of the potential functions of molecular inversion from experimental data: Current state and problems

Various approximations of the method of determining two-well potential functions of molecular inversion from experimental data (geometrical parameters and inversion level energies) are considered. The potential of the method is illustrated by reference to carbonyl molecules in the lowest excited states. Some of the current problems are discussed.

The Barriers to Internal Rotation of Benzaldehyde and Benzoyl Fluoride: “Reconciliation” Between Theory and Experiment

This work is devoted to investigation of the known problem of the discrepancy between the experimental and theoretical values for the barrier to internal rotation of the molecules comprised of a benzene ring and a π-conjugated substituent, such as benzaldehyde and benzoyl fluoride. The possible reasons of such discrepancy are considered. As a result the conclusion was drawn that the origin of the problem is incorrectness of the assignments of the torsional levels higher than first, both for benzaldehyde and benzoyl fluoride. In addition the significant kinematic interaction between torsional vibration and out-of-plane CHO deformation was found for benzaldehyde.

Structure and dynamics of conformationally non-rigid molecules in excited electronic states: Ab initio calculations of the R2CO (R=H, F, Cl)

Abstract Potential energy surfaces of electonically excited H 2 CO (S 1 , T 1 ), F 2 CO (S 1 ), and Cl 2 CO (S 1 ) molecules are investigated by means of multiconfigurational ab initio approaches. The theoretical estimations of inversion barrier heights calculated by different ab initio method and the relative energies of inversion vibrational levels are compared to the experimental data.

Structure and dynamics of acrolein in 1,3(π,π*) excited electronic states: A quantum-chemical study

The geometrical structure, conformer energy differences, and conformational and vibrational dynamics of acrolein in (1,3)(pi,pi(*)) electronic states were investigated using a number of single- and multi-reference quantum-chemical methods. Peculiarities of acrolein in the (1)(pi,pi(*)) state were described with both conformers being significantly non-planar. A Valence Focal-Point Analysis of the conformer energy difference in the (3)(pi,pi(*)) state was performed. The coupling of the internal rotation about C-C and C=C bonds with large amplitude molecular motions, such as non-planar distortions of carbonyl, methylene, and methyne fragments was also investigated. The corresponding two-dimensional PES sections were constructed.

The S 1 ← S 0 vibronic spectra and structure of the (CH3)2 CHCHO and (CH3)2 CHCDO 2-methylpropanal molecules

A multipass cell with an optical path up to 120 m long was used to measure the vibronic absorption spectra of 2-methylpropanal-h1 (MPA-h1, (CH3)2CHCHO)) and 2-methylpropanal-d1 (MPA-d1, (CH3)2CHCDO)) over the frequency range 28200–31600 cm−1. The most intense spectral lines were assigned to transitions from vibrational levels of the cis and gauche MPA-h1 and MPA-d1 conformers in the ground electronic state (S0) to vibrational levels of conformers 1 and 3 in the lowest singlet excited electronic state (S1). According to our estimates, the origins (000) of the 1S1) ← cis(S0) and 3(S1) ← cis(S0) and also 1(S1) ← gauche(S0) and 3(S1) ← gauche(S0) electronic transitions were situated at 29147 and 29177, 29391 and 29417 cm−1, respectively, for MPA-h1 and at 29226 and 29240, 29480 and 29500 cm−1 for MPA-d1. The structure of conformers 1 and 3 in the S1 state was shown to differ from the structure of the cis and gauche conformers in the S0 state by the angle of rotation of the (CH3)2CH-isopropyl top and “pyramidal distortion” of the CCHO/CCDO carbonyl fragment. A series of fundamental frequencies of MPA conformers in different electronic states were found. The potential functions of inversion were determined for the conformer 1-conformer 3 pairs of MPA-h1 and MPA-d1 from the experimental energy levels of inversion vibrations. The potential barriers to inversion and equilibrium displacements of the CH/CD bond out of the CCO plane were found to be 735/675 cm−1 and ±34°/±32° for MPA-h1 and MPA-d1, respectively.

One- and Two-Dimensional Torsion-Inversion Models for the Fluoral Molecule (CF3CHO) in Excited Electronic States

The structure of the conformationally nonrigid fluoral molecule (CF3CHO) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was studied by ab initio quantum-chemical methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecule in these electronic states were determined. The calculations demonstrated that the electronic excitation causes substantial changes in the molecular structure involving the rotation of the CF3 top and the deviation of the CCHO carbonyl fragment from planarity. The quantum-mechanical problems for large-amplitude vibrations, namely, for the torsional vibration in the S0 state and the torsional and inversion vibrations (nonplanar carbonyl fragment) in the T1 and S1 states, were solved in the one- and two-dimensional approximations. A comparison of the results of calculations revealed the correlation between the torsional and inversion motions.

Theoretical study of the structure of the CClF2NO and CCl2FNO molecules in the ground and lowest excited singlet and triplet electronic states

The potential energy surfaces of the nitroso compounds CClF2NO and CCl2FNO in the ground and lowest excited singlet and triplet electronic states were studied by various ab initio methods (including multiconfigurational methods). The equilibrium geometric parameters, vibrational frequencies, internal rotation potential functions, and rotational contours of bands in the S1 ← S0 vibronic spectrum of the CClF2NO molecule were calculated. For the molecules under consideration, the quantum-mechanical problem on torsional motion was solved. The results of calculations are, on the whole, in good agreement with experiment.

Potential functions of inversion of R2CO (R=H, F, Cl) molecules in the lowest excited electronic states

The inversion potentials of R2CO (R=H, F, Cl) molecules in the lowest excited electronic states were determined from experimental data using various model potential functions and approximations for the kinetic energy operator of inversion motion. The estimates of the heights of the barriers to inversion and the equilibrium values of the inversion coordinate for the H2CO molecule in the S1 and T1 states are fairly stable. The results for the F2CO and Cl2CO molecules are strongly dependent on the approximation used; for these molecules, the most reliable parameters of the potential functions were chosen. The problem of qualitative characteristics of the shape of inversion potentials is discussed using the results ofab initio quantum-chemical calculations of the molecules under study.