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When refining structures using gas electron diffraction (GED) data, assumptions are often made in order to reduce the number of required geometrical parameters. Where these relate to light, peripheral atoms there is little effect on the refined heavy-atom structure, which is well defined by the GED data. However, this is not the case when heavier atoms are involved. We have determined the gas-phase structure of perfluoro(methylcyclohexane), C6F11CF3, using three different refinement methods and have shown that our new method, which makes use of both MP2 and molecular mechanics (MM) calculations to restrain the peripheral-atom geometry, gives a realistic structure without the need for damaging constraints. Only the conformer with the CF3group in an equatorial position was considered, as ab initio calculations showed this to be 25 kJ mol−1lower in energy than the axial conformer. Refinements combining both high-level and low-level calculations to give constraints were superior both to those based only on molecular mechanics and to those in which assumptions about the geometry were imposed. [ABSTRACT FROM AUTHOR]

The molecular structures of methylphosphine (CH3PH2) and methylphosphine–borane (CH3PH2·BH3) have been determined from gas-phase electron diffraction data and rotational constants, employing the SARACEN method. The experimental geometric parameters generally showed a good agreement with those obtained using ab initio calculations and previous microwave spectroscopy studies. In order to assess the accuracy of the calculated structures a range of ab initio methods were used, including the CCSD(T) method, with correlation-consistent basis sets. The structural environment around the phosphorus atom was found to change significantly upon complexation with borane, with the P–C bond length shortening and the bond angles widening. [ABSTRACT FROM AUTHOR]

The gas-phase structures of 1,2,3-trifluorobenzene, 1,3,5-trifluorobenzene, 2,6-difluoropyridine, and 2,6-dichloropyridine have been determined using electron diffraction and computational methods. The accuracy afforded by modern experimental methods has allowed subtle trends to be identified in the geometrical parameters of the rings. Comparisons have also been made between experimental and theoretical structures. Although the effects of multiple fluorine substituents on a benzene ring are shown to be more or less additive, distortions caused by replacement of a ring carbon atom by nitrogen and by halogen substituents are not. [ABSTRACT FROM AUTHOR]

The vapour produced upon mild heating of hexa-tert-butyldisilane (superdisilane) has been studied by gas electron diffraction and ab initio molecular orbital calculations; the disilane is not observed in the vapour, and the observed radical structure is not the lowest energy structure predicted ab initio. [ABSTRACT FROM AUTHOR]

The molecular structure of trimethylgermane has been determined by gas electron diffraction experiments. Infrared spectra for the gaseous, liquid, and solid phases were also recorded. Parallel and perpendicular polarized Raman spectra for the liquid were measured to obtain depolarization values. The experimental studies were supported by a series of computational calculations using HF, B3LYP, and MP2 methods and a variety of basis sets. The force fields obtained from density functional theory using both B3LYP/6-31G* and B3LYP/6-311+G** were scaled with both Pulay’s SQM methodology and Yoshida’s WLS procedure to simulate the vibrational spectra and assist in the assignment of fundamental bands. The Raman intensities were obtained from polarizability derivatives. The vibrational spectra of trimethylgermane were completely assigned on the basis of the experimental data and the theoretical prediction of vibrational frequencies and intensities. [ABSTRACT FROM AUTHOR]

The geometry of iodotrimethylgermane has been determined by experimental and computational methods. Fourier transform infrared spectra have been recorded over a range of temperatures along with the Raman spectrum to obtain comprehensive vibrational data for the fundamental modes. The stretching, rocking, and deformation bands of the methyl groups have been resolved into their components with the aid of low-temperature infrared spectroscopy using Fourier self-deconvolution and curve-fitting methods. The optimized geometries and vibrational harmonic frequencies were calculated by density functional theory methods employing Pople-type basis sets, as well as those with descriptions for an effective core potential describing both germanium and iodine atoms. A scaled quantum mechanical analysis was carried out to yield the best set of harmonic force constants and obtain a transferable set of scale factors that can be applied to the (CH3)3GeX (X H, Cl, Br, I) series. [ABSTRACT FROM AUTHOR]

1-Trimethylsilyltetrazole, 1, has been synthesised from chlorotrimethylsilane and tetrazole in the presence of triethylamine as an auxiliary base. The structure of this compound has been determined in the crystalline phase by X-ray diffraction of a crystal grown in situ. The gas-phase structures of 1 and 1-trimethylsilyl-1,2,4-triazole, 2, have been determined by gas-phase electron diffraction (GED). An extensive investigation of these and related compounds by ab initio calculations is also reported. The angles between the rings and the substituents on N were of particular interest. It was found that the calculated difference of 15.5° between the CNSi and NNSi angles in 1 was mostly, but not entirely, an inherent property of the tetrazole ring and not due to a short Si⋯N interaction. [ABSTRACT FROM AUTHOR]