Geometrical parameters of tetraatomic carbonyl molecules X 2 CO and XYCO (X, Y ϭ H, F, Cl) in the ground (S 0 ) and lowest excited singlet (S 1 ) and triplet (T 1 ) electronic states as well as values of barriers to inversion in S 1 and T 1 states and S 1 4 S 0 and T 1 4 S 0 adiabatic transition energies were systematically investigated by means of various quantumchemical techniques.

Electronic structure of the low-lying excited states of the CH 3 NO molecule is considered. The detailed analysis of the first excited triplet and singlet states of this molecule is performed by different ab initio methods to estimate equilibrium geometry, barriers to internal rotation, harmonic frequencies, and adiabatic transition energies. Anharmonic vibrational approximations are also considered. The multidimensional VibSCF scheme and the 1-D variational method for the section of the potential energy surface along the torsion coordinate are used. Theoretical results are found to be in good agreement with experimental data.

Structure, conformer energy difference, and dynamics of acrolein molecule in 1,3 (n,*) electronic states were investigated by means of diverse single-and multireference quantum-chemical methods. Valence focal-point analysis of conformer energy difference in 3 (n,*) state was performed. Some reassignments of fundamentals of s-trans conformer were proposed. Internal rotation in 1,3 (n,*) states was shown to be coupled with nonplanar vibration of carbonyl fragment. Two-dimensional PES sections were constructed, and the corresponding two-dimensional vibrational problem was solved.

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