Vladimir S. Mastryukov

STRUCTURE AND CONFORMATION OF CYCLOPENTENE, CYCLOHEPTENE AND TRANS-CYCLOOCTENE

Abstract The molecular geometries of different conformations of cycloalkenes, C n H 2 n -2 , with n = 5, 7 and 8 were optimized by restricted Hartree-Fock calculations using the 6–31G∗ basis set followed by second-order Moller-Plesset perturbation theory (MP2) treatment of electron correlation. For cyclopentene, C 5 H 8 , the potential function for the ring-puckering motion was constructed, followed by solving for the vibrational eigenvalues in terms of distributed Gaussian bases. Good agreement was obtained with the observed frequencies in the far-infrared spectrum of the molecule. For cycloheptene, C 7 H 12 , geometries were optimized for the chair, boat, twist-boat, and three possible transition states. The chair-chair interconversion mechanism was investigated and compared with available experimental evidence and with the results of a previous molecular mechanics calculation. The computed potential barrier compares well with NMR evidence, but the conformation of the relevant transition state is found to be different from the one assumed in the experimental study. The structure of the smallest isolable trans -cycloalkene, trans -cyclooctene, C 8 H 14 , was optimized, yielding a structure in reasonable agreement with a previous gas phase electron diffraction study. The agreement includes the pyramidality of the olefinic carbon atoms which was also compared with available X-ray data on related compounds. The bond angles and torsion angles were in better agreement with the experiment than were those obtained in earlier molecular mechanics studies, although it is remarkable how well that method works for these highly strained cyclic systems.

The molecular structure of copper- and nickel-phthalocyanine as determined by gas-phase electron diffraction and ab initio/DFT computations

Abstract The structure of two metal phthalocyanines (MPcs) where M=Ni and Cu, have been determined by gas-phase electron diffraction for the first time. Structural analysis is assisted by ab initio/DFT calculations, carried out in the present work for NiPc and taken from the literature for CuPc. The focus of these studies is the position of the metal ion relative to the 16-membered C8N8 ring. Both molecules were found to be planar. Slight non-planarity found in the previous study is now removed by a new procedure of structural analysis; this is in accordance with the results of the X-ray studies and ab initio/DFT calculations for both compounds. From comparisons presented here we can conclude that there is a close structural similarity of MPcs in the gas phase and in the crystal.

ELECTRON DIFFRACTION STUDIES OF METAL PHTHALOCYANINES, MPC, WHERE M=SN, MG, AND ZN (REINVESTIGATION)

The structure of two metal phthalocyanines, MPc where M=Sn and Mg, have been determined by gas-phase electron diffraction for the first time; furthermore, the structure of ZnPc established earlier by the same technique has been reanalyzed. The focus of these studies is the position of the metal ion relative to the 16-membered C8N8 ring. Tin(II)phthalocyanine we find to be nonplanar with the Sn(II) ion 1.0(1) A above the molecular plane in accordance with its structure in crystal established earlier; the two other molecules were found to be planar. Slight nonplanarity of ZnPc found in the previous study is now removed by a new procedure of structural analysis; this is in accordance with the results of ab initio/DFT calculations for this molecule performed by Pulay. From comparisons presented here we can conclude that there is a close structural similarity of metal phthalocyanines in the gas phase and in crystal.

Structural changes as a function of torsional motion studied by ab initio calculations part 4. The dependence of the dynamic behavior of the structural parameters for H2O2 and H2S2 on basis sets and comparison with experimental results

Abstract The dynamical behavior of the structural parameters of H 2 O 2 and H 2 S 2 has been calculated using 6-31G∗ and 6-311 + + G∗∗ basis sets and correlation is included at the MP2 level. Comparison is made with experimental results which for the first time have been obtained by analysing the rotational constants of the excited torsional states. The agreement between theoretical and experimental results is good, particularly considering the difficulties in obtaining such results. Another approach to analyse the experimental data has been proposed.

Vibrational spectra in the ν(SiH) and ν(SiD) regions of chloro and bromodisilanes and ab initio geometry studies of C2H5Cl, Si2H5Cl and 1,1-Si2H4Cl2

Abstract Infrared measurements in the gas phase are reported for the ν(SiH) and ν(SiD) regions of Si 2 H 5 X, Si 2 D 5 X, 1,1-Si 2 H 4 X 2 and 1,1-Si 2 D 4 X 2 species where X = Cl, Br. Incomplete Raman data have also been obtained. All three possible isolated SiH stretching frequencies are observed in the spectra of the Si 2 D 4 X 2 samples, but only two from the Si 2 D 5 X ones. The missing ν is (SiH) values are obtained by use of the frequency sum rule, and by harmonic local mode force field treatments of all the available ν(SiH) and ν(SiD) data, using a procedure previously tested on disilane. Ab initio calculations of the geometries of C 2 H 5 Cl, Si 2 H 5 Cl and 1,1-Si 2 H 4 Cl 2 using the 6-31G* basis set are reported. Trends in r e (CH) or r e (SiH) values reflect trends in ν is (CH) or ν is (SiH) ones. The alpha, trans and gauche effects of halogen are similar in CH and SiH compounds, although smaller in the latter. In both cases, ab initio calculations predict larger effects than are observed in the spectra, especially for the α effect of halogen. A kinetic isotope effect in the halogenation of disilane may occur. Reassignment of earlier spectra of disilyl iodide species is proposed.

Ab initio and molecular mechanics studies of thianthrene and similar molecules

Abstract The structures of six molecules (dibenzodioxine, phenoxathiin, thianthrene, 1-azathianthrene, 1,4-diazathianthrene and 1,4,6,9-tetraazathianthrene) previously studied by X-ray and electron diffraction have been calculated using MM3 and ab initio (3-21G∗ basis set) methods. These computational methods reproduce correctly the experimental bond lengths, bond angles and conformations. Calculations helped to classify all the molecules into two families, where the thianthrene is a common member of both. The calculated barriers to planarity are found to correlate with both the calculated and the observed puckering angles of the molecules.

Structural changes as a function of torsional motion studied by ab initio calculations. Part 2. X2A-AX2 (A = N, P and X = H, F, Cl) and some implications for electron diffraction analysis

Abstract Structural changes as a function of the torsional angle about the central A-A bond have been studied for molecules of the type X 2 A-AX 2 (A = N, P; X = H, F, Cl) by ab initio calculations. The results from the calculations are compared with the corresponding experimental values. The implications of the results from the ab initio calculations are discussed in connection with electron diffraction analysis.

Structural changes as a function of torsional motion studied by ab initio calculations: Part 1. X3A-AX3 (A = C, Si, Ge and X = H, F, Cl)

Abstract Structural changes as a function of the torsional angle about the central A-A bond have been studied for X3A-AX3-type molecules (A = C, Si, Ge and X = H, F, Cl) by ab initio calculations. The results from the calculations are compared with the corresponding experimental values. Simple empirical formulas are given for simulation of the structural changes.

Structure and conformation of some saturated four-membered rings, CH2CH2CH2X⎵

Abstract Complete geometry optimizations of the ground state of three four-membered molecules with structures known from experiment (oxetane. silacyclobutane and thietane) have been carried out by ab initio SCF calculations at the 6-31G level. Additionally, calculations were made for two similar compounds CH 2 CH 2 CH 2 X ⎵ H (X = B, Al) not yet studied experimentally. Although comparison with available experimental data confirms that this level of theory somewhat underestimates both the dihedral angles in these molecules and the barriers to ring planarity, this study fills the gaps previously existing in structural investigations of four-membered ring molecules at a consistent level of theory, making possible the development of a uniform picture of seven monosubstituted heterocyclic compounds together with the parent molecule cyclobutane. Seven structural relationships common to all four-membered rings are systematized and discussed giving emphasis to the changes of C-C bond lengths and angular deformations around tetrahedral carbon and silicon atoms.

Structural changes as a function of torsional motion studied by ab initio calculations: Part 3. XA-AX (A = O, S and X = H, F, Cl) and implications on comparisons of structural parameters from ab initio calculations and electron diffraction, and on inclusion of rotational constants in an electron diffraction analysis

Abstract Structural changes as a function of the torsion angle about the central A-A bond have been studied for XA-AX type molecules with A = O, S and X = H, F, Cl by ab initio calculations using standard 6-31G∗ basis sets. The results from the calculations are compared with the corresponding experimental values. The implications of the results for a comparison of the equilibrium structure with the average structure from electron diffraction are discussed. The implications on the inclusion of rotational constants from microwave spectroscopy and intensity data from electron diffraction in a joint analysis are also considered. These results are the first quantitative estimation of how the average structure changes with the potential and temperature and of the consequences for practical structural work.