Quantum–chemical study of the structure of the acetyl fluoride molecule in the ground and lowest excited singlet and triplet electronic states
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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 ...
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 ...
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...
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