Ab initio study of the structure of 2,2-difluoroethanal in the ground and lowest excited triplet electronic states

Abstract

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...

Features of the fluorine‐substituted acetaldehydes dynamics in the low‐lying electronic states: Quantum mechanical study of the CHF2CHO molecule lowest excited singlet state

“…The vibrational frequencies estimated in harmonic approximation are given in Table IV for CHF 2 CHO (S 1 ) in comparison with those for S 0 and T 1 states 6. The experimental data 4 are in agreement with the calculated data.…”

“…The results of ab initio studies of the molecule CHF 2 CHO in the S 0 state indicate the existence of only one stable cis conformer (the H atom of the top is located in the cis position with respect to the O atom) 6. The geometric parameters of this conformer are given in Table I.…”

“…But, the unusual form of the torsional potential is conserved for different types of the AO set and the method's variations, for example, for the density functional calculations of the B3LYP/6‐31+G* type (for details see Ref. 6). It is worth noting that experimental data on infrared (IR) and vibronic spectra agree with the calculated stability of the one conformation of CHF 2 CHO (S 0 ) only 3, 4, 6.…”

“…6). It is worth noting that experimental data on infrared (IR) and vibronic spectra agree with the calculated stability of the one conformation of CHF 2 CHO (S 0 ) only 3, 4, 6.…”

“…Earlier we studied the structure of CF 3 CHO 1, 2 and CHF 2 CHO 3, 4 molecules in the excited S 1 and T 1 electronic states by methods of vibronic spectroscopy. We also have used quantum mechanical calculations to investigate the structure of CH 2 FCHO (S 0 , S 1 , T 1 ) 5, CHF 2 CHO (S 0 , T 1 ) 6, and CF 3 CHO (S 0 ) 7. There is no experimental data for the molecule CH 2 FCHO.…”