Russian Chemical Bulletin, International Edition, Vol. 54, No. 1, pp. 62—70, January, 2005 62 Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 62—70, January, 2005. 1066-5285/05/5401-0062 © 2005 Springer Science+Business Media, Inc. Structure of acetyl chloride molecule isotopomers CH 3 COCl and CD 3 COCl in the ground and lowest excited singlet and triplet electronic states: a quantum-mechanical study A. V. Kudich, V. A. Bataev,  and I. A. Godunov Department of Chemistry, M. V. Lomonosov Moscow State University, 1 Leninskie Gory, 119992 Moscow, Russian Federation. Fax: +7 (095) 939 3689. E-mail: lant@phys.chem.msu.ru The structures of isotopomers of conformationally flexible acetyl chloride molecule, CH 3 COCl and CD 3 COCl, in the ground (S 0 ) and lowest excited singlet (S 1 ) and triplet (T 1 ) electronic states were calculated by the RHF, MP2, and CASSCF methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these elec- tronic states were estimated. According to calculations, electronic excitation causes consider- able conformational changes involving rotation of the CH 3 (CD 3 ) top and a substantial devia- tion of the CCOCl fragment from planarity. The results of calculations agree with experimental data. Two-dimensional torsional-inversion sections of the potential energy surface were calcu- lated and analyzed. Vibrational problems for large-amplitude vibrations (torsional vibration in the S 0 state and both torsional and inversion vibrations in the T 1 and S 1 states) were solved in one- and two-dimensional approximations. Key words: ab initio quantum-chemical calculations, carbonyl compounds, acetyl chloride, isotopomers, vibrational frequencies, potential energy surface, ground electronic state, excited electronic states. Studies on the structures of conformationally flexible molecules in the ground and excited electronic states are of considerable importance from both practical and theo- retical standpoints. The structures of stable conformers, the energy differences between them, and the heights of potential barriers to conformational transitions are re- sponsible for a number of the most important properties of molecules and compounds. 1—3 These are the electrical and optical properties, thermodynamic characteristics, reactivity and biological activity, mechanisms of chemi- cal (including photochemical) reactions, etc. Prediction of the structure and properties of such molecules first of all requires obtaining and accumulation of relevant ex- perimental and theoretical data. Besides, of considerable interest is to elucidate (i) the physical nature of the poten- tials of internal rotation and inversion and (ii) conforma- tional changes accompanying the electronic excitation of molecules (see, e.g., Refs 1—5). It was experimentally established that transitions of certain molecules of carbonyl compounds with symmetri- cal CR 3 (R = H, D, F, Cl, CH 3 ) tops from the ground state S 0 to the lowest excited singlet (S 1 ) and triplet (T 1 ) electronic states are accompanied by significant changes in the equilibrium geometric structures, namely, by rota- tion of the tops by about 60° and by pyramidalization of carbonyl fragments. 5—11 Experimental studies of the structure of conforma- tionally flexible molecules (especially if they are in ex- cited electronic states) face severe difficulties, which are first of all associated with possible ambiguity of band as- signment in the vibronic spectra and instability of inverse problems of the determination of potentials of internal rotation and inversion. Therefore, quantum-mechanical calculations can serve as both an additional source of structural information and a method of analysis of the nature of conformational effects. 5 Theoretical studies 12—15 of aldehyde molecules CX 3 CHO (X = H, F, Cl) in the S 0 , S 1 , and T 1 states showed that the S 1 ←S 0 and T 1 ←S 0 electronic excitations of these molecules cause changes in the orientation of the CX 3 top and a pyramidal distortion of the CCHO fragment, which is planar in the S 0 state. The results ob- tained in those studies are in agreement with experimen- tal data. The structures of carboxylic acid haloanhydrides were studied much poorer. However, experimental studies of, e.g., HFCO, HClCO, F 2 CO, Cl 2 CO, CH 3 COF, and CH 3 COCl also revealed a pyramidal distortion of the car- bonyl fragment upon electronic excitation of the mol- ecules to the S 1 state. 16—18 For CH 3 COF and CH 3 COCl we showed that excitation also changes the orientation of the methyl top relative to the molecular skeleton.