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.

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ABSTRACT:The equilibrium geometry, barrier to internal rotation, and harmonic and anharmonic vibrational frequencies of the CH 3 NO molecule in the ground electronic state were calculated by different modern quantum chemical techniques. Special attention was given to the problem of torsional motion. The dependence of the results of calculations on the atomic orbital and molecular orbital basis sets and correlated methods was examined. The results of calculations were compared with experimental data. Some methods provided excellent agreement with the experiment.

Systematic calculations of the structures of the H 2 CO, F 2 CO, Cl 2 CO, HClCO, HFCO, and FClCO molecules in the S 0 and T 1 states were performed using the B3LYP and MP2 methods with different AO basis sets and also at the CCSD(T)/cc pV(T+d)Z level of theory. The saturation of the correlation consistent sequence of basis sets cc pV(N+d)Z (N = D, T, Q, and 5) and aug cc pV(N+d)Z (N = D, T, and Q) was studied. Recommendations for choosing the calculation method are given. The relativistic corrections were estimated. The influence of the number and type of halogen atoms on the geometric parameters of the molecules in the S 0 and T 1 states and the heights of inversion barriers in the T 1 state was investigated.Key words: quantum chemical calculations, systematic basis sets, structures of molecules in excited electronic states, inversion motion, formaldehyde, halogen substituted formalde hydes.Experimental studies of vibronic spectra of carbonyl compounds demonstrated that the carbonyl fragment in particular compounds becomes nonplanar upon excita tion from the ground (S 0 ) to the lowest triplet (T 1 ) and singlet (S 1 ) electronic states due to which a large ampli tude internal motion, viz., the inversion of the nonplanar carbonyl fragment, can occur. 1 The molecules of formaldehyde (H 2 CO) and its halo gen substituted derivatives are the simplest representa tives of carbonyl compounds and serve as convenient mod els for studying the structural features of this class of mol ecules. Many of these compounds have found use in in dustry as chemicals for organic synthesis (for example, phosgene Cl 2 CO is used as the starting material in the production of dyes, polycarbonates, urea and its deriva tives, pesticides, drugs, solvents, etc.). Some of them are generated upon photolysis and oxidation of fluoro and chloro derivatives of hydrocarbons, including freons, in the troposphere and stratosphere; it is supposed that these compounds catalyze ozone decomposition. 2-5 Hence, the physical and chemical properties of these compounds are of considerable interest.Earlier, the molecules of halogen substituted formal dehydes have been studied primarily in the ground elec tronic state by vibrational spectroscopy and gas electron diffraction. 6-10 The structure of the H 2 CO molecule in the S 0 state has been investigated in depth by micro wave and rovibrational spectroscopy. 11 Analysis of the rovibronic spectra provided information 11 on the struc tures of these molecules in the S 1 and (only for formalde hyde) T 1 states. We performed theoretical study of the structures and vibrational dynamics of formaldehyde and its halogen substituted derivatives in the S 0 and T 1 states. Our investigation included two steps. In the first step, we estimated the reliability of different methods for solving the electronic Schroedinger equation. In the second step, the chosen methods were used for studying the potential energy surface (PES) in the region, where vibrational wavefunctions are localized, and solved the nuclear Schroedinger equ...

Equilibrium geometries, harmonic vibrational frequencies, and barriers to internal rotation of the CF 3 NO molecule in the ground singlet (S 0 ) and lowest excited triplet (T 1 ) and singlet (S 1 ) electronic states are calculated using various ab initio techniques and compared with the experimental data. The sensitivity of the results to the choice of atomic orbital basis set, in particular, to the presence of f-symmetry functions, and to the dynamical correlation treatment is analyzed. Anharmonic vibrational frequencies and dipole moment of the CF 3 NO molecule in the ground state are computed. The results of all approaches employed are in good agreement with each other and with available experimental data. Conclusions concerning the accuracy of different methods are drawn.

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The molecular structure of 2.2-difluoroethanal ~,DFE) in the ground (S 0) and lowest excited triplet (Ti) electronic states was investigated by ab initio quantum-chemical methods.In the S O state, the DFE molecule exists as the only stable cis conformer. The Tle--S 0 electronic excitation is accompanied by the rotation of the top and the deviation of the carbonyl fragment from planarity. For the DFE molecule in the T I state, six minima corresponding to three pairs of enantiomers were found on the potential energy surface. Based on this potential energy surface, the problems on torsion and inversion nuclear motions were solved in the one-and two-dimensional approximations, and the interaction between these motions was revealed.Key words: ab initio quantum-chemical calculations, carbonyl compounds, geometric parameters, vibration frequencies, potentials of internal rotation and inversion, ground and excited electronic states.The structures of many conformationally flexible molecules of carbonyl compounds, which have symmetrical tops (C3v), in the ground state (So) and in the lowest excited singlet (Sz) and triplet (T0 electronic states have been studied. It was found that the St<--S 0 and Tt+-S 0 electronic excitations of these molecules are accompanied by rotations of tops and pyramidalization of carbonyl Fragments (though this is not necessarily the case). I-5 Systems with lower-symmetry tops remain poorly studied, and the data on these compounds are more ambiguous. 6 This gave impetus to our studies of the structures of RCHO molecules (R = CH2C1 or CHF 2) in the S 0, S l, and T 1 states by molecular spectroscopy and quantum chemistry.In this work, we report the results of ab initio studies of the conformational behavior of the 2,2-difluoroethanal molecule (DFE; CHF2CHO) in the S o and Ti states. Most attention was given to large-amplitude nuclear motions, viz., to torsion (in the S o and T L states) and inversion (in the T t state) motions, for which one-and two-dimensional vibrational problems were solved.Taking into account our experience in investigations of molecules of carbonyl compounds in the ground and lowest excited electronic states, 6-10 we carried out calculations for the DFE molecule with the use of the restricted (RHF) and unrestricted (UHF) Hartree--Fock methods, the second-order M611er--Plesset perturbation theory (MP2), and the multiconfigurational method of the self-consistent field (the CASSCF technique) with the active space including molecular orbitals, which are close to frontier orbitals and are localized predominantly on the carbonyl fragment, via,., OCO, rr.co, no, rtCO*+ and ~co* (CASSCF(6e-SMO)). The principal calculations were carried out with the 6-31G** basis set. The correctness of the results was estimated by the configuration interaction method taking into account single and double excitations (CISD) and with the use of larger AO basis sets. The structure of the DFE molecule in the S O state was also studied by the density functional theory (DFT). The calculations were carried o...