Structure of acetyl chloride molecule isotopomers CH3COCl and CD3COCl in the ground and lowest excited singlet and triplet electronic states: a quantum mechanical study

Abstract

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. 5-11Experimental 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 ...

Physics behind the Barrier to Internal Rotation of an Acetyl Chloride Molecule: A Combined Approach from Density Functional Theory, Car–Parrinello Molecular Dynamics, and Time-Resolved Wavelet Transform Theory

“…The “ V 6 ” value has been estimated to be −0.019 kcal/mol. The negative sign of “ V 6 ” for the AC molecule is in accordance with that reported by Kudich et al, where the values of “ V 6 ” had been predicted to vary between −10.5 and −11.4 cm –1 , depending upon the level of theories computed over different basis sets. In this connection, it is also worth to mention that for the acetaldehyde molecule, the sign of the corresponding “ V 6 ” term is also negative for fully relaxed optimizations, signifying strong coupling of methyl torsion with the out-of-plane γ (C aldehyde –H) wagging motion of the molecule.…”

Insights into Mechanistic Photodissociation of Acetyl Chloride by ab Initio Calculations and Molecular Dynamics Simulations

“…Sumathi and Chandra optimized the geometries of CH 3 COCl and CH 3 CO at the MP2 level of theory with the 6-31G(d) and 6-31lG(d,p) basis sets 22a. In addition, energy barrier for CH 3 CO dissociation is predicted to be 19.1 kcal/mol at the MP2/cc-pVTZ level by Deshmukh et al The structures of CH 3 COCl in the S 1 and T 1 electronic states were calculated by the MP2 and CASSCF methods . It was found that electronic excitation causes considerable conformational changes involving rotation of the CH 3 top and a substantial deviation of the CCOCl fragment from planarity.…”