Synthesis and Characterization of Cl2HSi-O-NMe2

O-(Dichlorosilyl)-N,N-dimethylhydroxylamine, Cl2HSiONMe2, was synthesised by the reaction of O-lithio-N,N-dimethylhydroxylamine with an excess of trichlorosilane. The compound was characterised by multinuclear NMR spectroscopy (1H, 13C, 15N, 29Si), gas-phase IR spectroscopy and mass spectrometry. The structure of Cl2HSiONMe2 was determined by gas-phase electron diffraction. It exists as two conformers in the vapour, named anti and gauche after the position of the SiH hydrogen atom relative to the SiON skeleton. The anti:gauche ratio in the vapour was determined to be 40:60. The anti conformer shows only a weak attractive interaction between the geminal donor and acceptor centres N and Si [angle Si-O-N 111.1(20)­°], whereas these interactions are stronger in the gauche conformer [angle Si-O-N 98.8(12)°]. Further structure-determining forces from weak hydrogen bridges of the Si-Cl ···H-C type and van der Waals repulsive forces are also discussed. The experimental results are in reasonable agreement with ab initio calculations at the MP2/6-311++G** level of theory.

Conformational stability, infrared and Raman spectra, and vibrational assignment of ethyl bromogermane

The infrared spectra (3200–30 cm–1) of ethyl bromogermane, CH3CH2GeH2Br, in the gaseous phase and of amorphous and annealed solid samples as well as the Raman spectrum (3200–100 cm–1) of the liquid have been recorded. Variable-temperature (–105 to –150 °C) studies of the infrared spectra of a sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference between the gauche and trans conformers has been determined to be 158 ± 16 cm–1 (1.89 ± 0.20 kJ/mol), with the gauche conformer being the more stable form. This result is consistent with the prediction from ab initio calculations at both the Hartree–Fock level and with full electron correlation by the perturbation method to second order. It is estimated that at ambient temperature 81 ± 2% of the sample is in the gauche form. A relatively complete vibrational assignment is proposed for both the gauche and trans conformers based on infrared band contours, relative intensities, depolarization values, and group frequencies which is supported by normal coordinate calculations utilizing the force constants from the ab initio MP2/6-31G(d) calculations. The optimized geometries and conformational stabilities have also been obtained from ab initio calculations utilizing several different basis sets with full electron correlation by the perturbation method up to MP2/6-311+G(2d,2p). Additionally, the potential functions governing the conformation interchange have been estimated from ab initio MP2/6-31G(d) and density functional theory calculations by the B3LYP method, with barriers to conformational interchange of 560, 924, and 852 cm–1 from MP2/6-31G(d) and 418, 771, and 606 cm–1 from B3LYP/6-31G(d) calculations for the trans to gauche, gauche to gauche, and gauche to trans conformers, respectively. The results are discussed and compared to some corresponding quantities for several related molecules.Key words: conformational stability, Raman spectra, infrared spectra; ab initio calculations, ethyl bromogermane.

The Low-Temperature Polymorph of Tetramethyldiphosphinebis( monoborane): Insight into the Stabilization of Different Rotational Isomers in the Solid State

The low-temperature (LT) polymorph of tetramethyldiphosphine-bis(monoborane), Me2(H3B)PP( BH3)Me2, was obtained by crystallization from diethyl ether at 4 °C. It crystallizes in the monoclinic space group P21/c, a = 6.464(1), b = 7.605(1), c = 11.867(2) Å , β = 119.99(1)° (at 153 K) with 2 molecules per unit cell. This implies that the individual molecules have crystallographic inversion symmetry and a strict anti arrangement with respect to the central B-P-P-B skeleton. At 87.6 °C (DTA) the LT polymorph transforms to the high-temperature (HT) modification which contains the anti and gauche conformers in a 1:2 ratio (P21/c, Z = 6; H. L. Carrell, J. Donohue, Acta Crystallogr. B24, 699 (1968)). This strongly suggests that the gauche conformer is higher in energy and stabilized by the crystal packing of the HT modification. The P-P-B angle in the anti LT form (113.91(6)°) ascertains the value of the anti conformer in the HT form (114.4(6)°) which was found to be significantly different from the gauche conformer which centered around 110 °C.