The S1 ← S0 fluorescence excitation spectrum and structure of propanal in the S1 excited electronic state.

We have obtained and analyzed the S1 ← S0 fluorescence excitation spectra of jet-cooled propanal-h1 (CH3CH2CHO) and -d1 (CH3CH2CDO). Using the results of theoretical studies of the structure of propanal molecule in the S1 lowest excited singlet electronic state, we have assigned the bands of both spectra to the vibronic transitions of the cis conformer (in the S0 ground electronic state) to the 1 and 3 conformers (in the S1 state) differed by the angle of the C2H5 ethyl group rotation around the central C-C bond. The origins of the 1 ← cis and 3 ← cis electronic transitions have been observed at 29 997 and 30 075 cm(-1) for propanal-h1 and at 30 040 and 30 115 cm(-1) for propanal-d1, respectively. The high activity of torsional (C2H5 ethyl groups) and inversional (CCHO/CCDO carbonyl fragments) vibrations and the intensity distribution of the bands in torsional sequences (passing through maximum) are in agreement with the theoretical prediction that the S1 ← S0 electronic excitation of the cis conformer causes (after geometrical relaxation) the pyramidalization of carbonyl fragments and the rotation of ethyl groups around the central C-C bond. A number of energy levels have been found for torsional and inversional vibrations, and also fundamentals of ν10 (CCO bend) and ν13 (CCC bend) for the both 1 and 3 conformers of propanal-h1 and -d1 have been found. Then the "experimental" potential functions of inversion for the pair of the 1 and 3 conformers have been determined. The heights of potential barriers to inversion and the angle values corresponding to the minima of potential functions of inversion are 900 cm(-1) and 35° for propanal-h1 and 820 cm(-1) and 34° for propanal-d1, respectively.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, trimethylgermyl, trimethylstannyl, and trimethylplumbyl derivatives of 3,3-dimethylcyclopropene X. Some regularities.

The changes in the vibrational frequencies of 1-tert-butyl and 1,2-di-tert-butyl derivatives of 3,3-dimethylcyclopropene brought about by substitution of the central carbon atom (X) of the tert-butyl moieties by Si, Ge, Sn, or Pb atoms are examined. The most important decrease in the vibrational frequencies implicating the X(CH(3))(3) moieties is noted for substitution of X=C by Si. The substitutions of Si by Ge or Ge by Sn or Sn by Pb are not accompanied by the pronounced frequency shifts observed for the C-->Si transition. An explanation is given for trends in these vibrational frequencies for the transitions X=C-->Si-->Ge-->Sn-->Pb. It is concluded that there are lower limiting values of the vibrational frequencies of a molecular moiety which are approached when the mass of its isovalent atom is increased. This leads to the formation of cluster regions in the vibrational spectra for the frequencies of the SnC(3) and PbC(3) moieties.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, trimethylgermyl, trimethylstannyl, and trimethylplumbyl derivatives of 3,3-dimethylcyclopropene IX. 3,3-Dimethyl-1-(trimethylplumbyl)cyclopropene.

The geometrical parameters and quantum mechanical force fields (QMFF's) of 3,3-dimethyl-1-(trimethylplumbyl)cyclopropene (I), 3,3-dimethyl-1-(t-butyl)cyclopropene (II), 3,3-dimethyl-1-(trimethylsilyl)cyclopropene (III), 3,3-dimethyl-1-(trimethylgermyl)cyclopropene (IV), and 3,3-dimethyl-1-(trimethylstannyl)cyclopropene (V) were calculated at the pseudopotential (HF/SDDAll) level. Analysis of the optimised geometrical parameters was performed. The set of scale factors for correction of the pseudopotential QMFF of III was determined using its earlier well-characterised vibrational spectrum. Transferral of the set of scale factors obtained for III to the QMFF's of I, II, IV and V was followed by calculation of the fundamental vibrational frequencies. Analysis of the results for these molecules revealed some peculiarities in the vibrational frequencies obtained at the pseudopotential level.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, trimethylgermyl, and trimethylstannyl derivatives of 3,3-dimethylcyclopropene VII. 3,3-Dimethyl-1-(trimethylstannyl) cyclopropene.

The quantum mechanical force fields (QMFF's) of 3,3-dimethyl-1-(tert-butyl)cyclopropene (I), 3,3-dimethyl-1-(trimethylsilyl)cyclopropene (II), 3,3-dimethyl-1-(trimethylgermyl)cyclopropene (III), and 3,3-dimethyl-1-(trimethylstannyl)cyclopropene (IV) were calculated at the HF/3-21G*//HF/3-21G* level. The set of scale factors for the correction of HF/3-21G*//HF/3-21G* QMFF of II was determined using its well-characterised vibrational spectrum. Transferral of the set of scale factors obtained for II to the QMFF's of I, III and IV and calculation of the fundamental frequencies resulted in good agreement between the calculated and previously assigned experimental frequencies of III. This again demonstrates the feasibility of transferral of a set of scale factors obtained for the correction of the QMFF of a molecule to others containing heteroatoms from the same column of the Mendeleyev Periodic Table. Thus the calculations performed permitted the accurate assignment of the fundamental vibrational frequencies in the experimental IR spectrum of IV. The vibrational frequencies of 3,3-dimethyl-1-(tert-butyl)cyclopropene (I) were also calculated from the HF/6-31G*//HF/6-31G* QMFF, scaled by the set of scale factors used previously for the HF/6-31G*//HF/6-31G* QMFF's of II and III. Regularities in the trends of some vibrational frequencies with increasing atomic number of the heteroatom are observed.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, trimethylgermyl, and trimethylstannyl derivatives of 3,3-dimethylcyclopropene VIII. 3,3-Dimethyl-1,2-bis-(trimethylstannyl)cyclopropene.

The quantum mechanical force fields of 3,3-dimethyl-1,2-bis-(tert-butyl)cyclopropene (I), 3,3-dimethyl-1,2-bis-(trimethylsilyl)cyclopropene (II), 3,3-dimethyl-1,2-bis-(trimethylgermyl)cyclopropene (III), and 3,3-dimethyl-1,2-bis-(trimethylstannyl)cyclopropene (IV) were calculated at the HF/3-21G*//HF/3-21G* level. The scale factors which were optimized previously for the HF/3-21G*//HF/3-21G* quantum mechanical force field of 3,3-dimethyl-1-(trimethylsilyl)cyclopropene were used for correction of the force fields of these molecules. Good agreement between the frequencies calculated from these scaled force fields and the well-analyzed and assigned experimental frequencies of II and III suggests the transferability of these scale factors and the possibility of the spectroscopically accurate prediction of the vibrational spectrum of IV. Some regularities in the changes of the vibrational frequencies were found for this molecular series.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, and trimethylgermyl derivatives of 3,3-dimethylcyclopropene. VI: Application of observed trends to stannyl derivatives.

The effects of substitution of X = C by Si or Ge in X(CH(3))(3) moieties attached to the formal double bond of 3,3-dimethylcyclopropene are examined. Regularities in observed trends of vibrational frequencies implicating the moieties containing the X atom, as the X atomic mass is increased, are extrapolated to X = Sn. The results of this extrapolation made it possible to assign the known experimental vibrational frequencies of 3,3-dimethyl-1-(trimethylstannyl)cyclopropene and 3,3-dimethyl-1,2-bis(trimethylstannyl)cyclopropene.

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

The structure of the conformationally flexible acetyl fluoride molecule (CH3CFO and CD3CFO) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was calculated by different quantum-chemical methods (RHF, UHF, MP2, CASSCF). The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. The calculations demonstrated that the electronic excitation causes considerable conformational changes involving the rotation of the CH3(CD3) top and a substantial deviation of the CCFO carbonyl fragment from planarity. For large-amplitude vibrations, namely, for the torsional vibration in the S0 state and the torsional and inversion (nonplanar carbonyl fragment) vibrations in the T1 and S1 states, the quantum-mechanical problems were solved in one-dimensional (1D) and two-dimensional (2D) approximations. The results of calculations are in good agreement with experimental data.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, and trimethylgermyl derivatives of 3,3-dimethyl cyclopropene V. 3,3-dimethyl-1-(trimethylgermyl)cyclopropene.

3,3-dimethyl-1-(trimethylgermyl)cyclopropene (I) was synthesised using a standard procedure. The IR and Raman spectra of I in the liquid phase were measured. The molecular geometry of I was optimised completely at the HF/6-31G* level. The HF/6-31G*//HF/6-31G* force field was calculated and scaled using the set of scale factors transferred from those determined previously for scaling the theoretical force fields of 3,3-dimethylbutene-1 and 1-methyl-, 1,2-dimethyl-, and 3,3-dimethylcyclopropene. The assignments of the observed vibrational bands were performed using the theoretical frequencies calculated from the scaled HF/6-31G*//HF/6-31G* force field and the ab initio values of the IR intensities, Raman cross-sections and depolarisation ratios. The theoretical spectra are given. The completely optimised structural parameters of I and its vibrational frequencies are compared with corresponding data of related molecules.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, and trimethylgermyl derivatives of 3,3-dimethylcyclopropene III. 3,3-Dimethyl-1-(trimethylsilyl)cyclopropene.

The experimental Raman and IR vibrational spectra of 3,3-dimethyl-1-(trimethylsilyl)cyclopropene in the liquid phase were recorded. Total geometry optimisation was carried out at the HF/6-31G* level and the HF/6-31G*//HF/6-31G* force field was computed. This force field was corrected by scale factors determined previously (using Pulay's method) for correction of the HF/6-31G*//HF/6-31G* force fields of 3,3-dimethylbutene-1, 1-methyl-, 1,2-dimethyl-, and 3,3-dimethylcyclopropene. The theoretical vibrational frequencies calculated from the scaled quantum mechanical force field and the theoretical intensities obtained from the quantum mechanical calculation were used to construct predicted spectra and to perform the vibrational analysis of the experimental spectra.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, and trimethylgermyl derivatives of 3,3-dimethylcyclopropene II. 3,3-Dimethyl-1,2-bis(trimethylsilyl)cyclopropene.

The IR and Raman spectra of 3,3-dimethyl-1,2-bis(trimethylsilyl)cyclopropene (I) (synthesised using standard procedures) were measured in the liquid phase. Total geometry optimisation was performed at the HF/6-31G* level. The HF/6-31G*//HF/6-31G* quantum mechanical force field (QMFF) was calculated and used to determine the theoretical fundamental vibrational frequencies, their predicted IR intensities, Raman activities, and Raman depolarisation ratios. Using Pulay's scaling method and the theoretical molecular geometry, the QMFF of I was scaled by a set of scaling factors used previously for 3,3-dimethyl-1,2-bis(tert-butyl)cyclopropene (17 scale factors for a 105-dimensional problem). The scaled QMFF obtained was used to solve the vibrational problem. The quantum mechanical values of the Raman activities were converted to differential Raman cross sections. The figures for the experimental and theoretical Raman and IR spectra are presented. Assignments of the experimental vibrational spectra of I are given. They take into account the calculated potential energy distribution and the correlation between the estimations of the experimental IR and Raman intensities and Raman depolarisation ratios and the corresponding theoretical values (including Raman cross sections) calculated using the unscaled QMFF.

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, and trimethylgermyl derivatives of 3,3-dimethylcyclopropene IV. 3,3-dimethyl-1,2-bis(trimethylgermyl)cyclopropene.

The infrared (IR) and Raman spectra of 3,3-dimethyl-1,2-bis(trimethylgermyl)cyclopropene (I) were measured in the liquid phase. Total geometry optimisation was performed at the HF/6-31G* level. The HF/6-31G*//HF6-31G* quantum mechanical force field (QMFF) was calculated and used to determine the theoretical fundamental vibrational frequencies, their predicted IR intensities, Raman activities, and Raman depolarisation ratios. Using Pulay's scaling method and the theoretical molecular geometry, the QMFF of I was scaled by a set of scaling factors comprised of elements transferred from the sets used to correct the QMFF's of 3,3-dimethylbutene-1, and 1-methyl-, 1,2-dimethyl-, and 3,3-dimethylcyclopropene (17 scale factors for a 105-dimensional problem). This set of scale factors was used previously to correct the QMFF of 3,3-dimethyl-1,2-bis(tert-butyl)cyclopropene and 3,3-dimethyl-1,2-bis(trimethylsilyl)cyclopropene. The scaled QMFF obtained was used to solve the vibrational problem. Differential Raman cross-sections were calculated using the quantum mechanical values of the Raman activities. The appropriate theoretical spectrograms for the Raman and IR spectra of I were constructed. Assignments of the experimental vibrational spectra of I are given. They take into account the calculated potential energy distributions and the correlation between the estimations of the experimental IR and Raman intensities and Raman depolarisation ratios and the corresponding theoretical values calculated using the unscaled QMFF.