Stephen Cradock

The ground state structures of disilane, methyl silane and the silyl halides, and an SiH bond length correlation with stretching frequency

Abstract Infrared spectra of the molecules Si 2 HD 5 , SiHD 2 CH 3 , SiH 3 CHD 2 , SiHD 2 X (X = F, Cl, Br, I) have been recorded at 0.05 cm −1 resolution. Analyses of “perpendicular” fundamentals of these very slightly asymmetric top molecules permit the accurate determination of their ground state ( A 0 — B 0 ) constants, from which the A 0 rotational constants may be obtained. When combined with all known Raman and microwave constants, this enables the ground state structures to be determined with precision, allowance being made for the shortening of SiH(CH) bonds on deuteration. For disilane this work represents the first spectroscopic determination of a complete structure, while for the silyl halides it demonstrates overestimates in the SiH bond length of nearly 1% by previous workers. In the case of methyl silane, comparison with a recent ab initio structure calculation reveals a considerable overestimate of the CSi bond length in the latter. A correlation between isolated SiH stretching frequency and ground state bond length over 15 molecules reveals a somewhat reduced sensitivity compared with the corresponding CH correlation. Nevertheless, a predictive capability of better than ±0.003 A seems possible from the isolated SiH stretching frequency, at least for molecules with fully saturated bonding.

The gas-phase molecular structure of silyl(methyl)-acetylene (1-silabut-2-yne), determined by electron diffraction

Abstract The molecular structure of CH3C  CSiH3 in the gas phase has been investigated using electron diffraction. Mean amplitudes of vibration and perpendicular amplitude correction factors calculated from spectroscopic data enabled refinements of both ra and rαo structures to be carried out, allowing a comparison with the B rotation constant, corrected to Bz. Ths r0α refinement leads to a linear skeleton, with parameters: r(Si-C), 180.2(4) pm; r(CC), 121.9(4) pm; r(C-C), 145-0(5) pm; r(Si-H), 150.6(10) pm; r(C-H), 105.3(7) pm; ∠-HSiC, 110.7(10)°; ∠HCC, 110.9(12)°. The ra structure shows the usual apparent bends in the skeleton due to shrinkage.

Microwave and infrared spectra of deuterium-substituted germyl halides and the structures of the germyl halides

Abstract We have obtained microwave spectra of fully and partially deuterated germyl halides GeD3X and GeHD2X (X = F, Cl, Br, I) and analysed the infrared spectra in the GeH stretching region of the asymmetric species. The resulting Ao and Bo values have been combined with existing data on GeH3X to give improved structures for the germyl halides. The values of the HGeX angles obtained show a correlation with GeH stretching frequencies, as in methyl compounds, but there appears to be no such correlation between stretching frequencies and GeH bond lengths.

Electron diffraction investigation of the molecular structures of ethyl isocyanate and ethyl isothiocyanate

Abstract Electron diffraction (ED) measurements and published microwave spectra are used to define the molecular structures of ethyl isocyanate and ethyl isothiocyanate in the gas phase. A cis -conformation is compatible with the data in both cases, while ED data for EtNCO are also consistent with a deviation of 45° from the cis conformation. An interpretation (in general terms) of the very complex microwave spectra in terms of an anharmonic two-dimensional motion combining the bend at nitrogen and the CN torsion is proposed. An average cis structure ( r * av basis) consistent with ED and microwave data with removal of the effects of the torsional motion is proposed in each case, with skeletal parameters (NCO, NCS) r CN 144.8 pm, 143.8; r CC 152.4, 152.0 pm; r NC 121.8 pm, 118.7; r CO 117.4 pm, r CS 158.0 pm; ∠CCN 114.7°, 111.0°; ∠CNC 132.2°, 147,4°; ∠NCO 192.2°, ∠NCS 184.5°.

The molecular structure of chlorobenzene, determined by the combined analysis of electron diffraction, rotation constant and liquid crystal nuclear magnetic resonance data

Abstract The molecular structure of chlorobenzene has been determined from new electron diffraction data and existing microwave data in the gas phase and direct dipolar couplings between spin- 1 2 nuclei in the NMR spectrum of a sample dissolved in a nematic liquid crystal solvent. The data, when corrected to a common basis (r0α), are compatible with a single structure in which the ring geometry is distorted by the chlorine substituent so that CC bonds adjacent to Cl are shorter than the others, and the CCC bond angle at the substituted carbon atom is larger than 120°. The refined structural parameters are rC(1)C(2) 139.08(18), rC(2)C(3) 139.42(16), rC(3)C(4) 140.00(12), rCCl 173.90(23), rC(2)H 107.79(21), rC(3)H 108.72(21), rC(4)H 107.95(21) pm; ∠C(6)C(1)C(2) 121.65(23), ∠C(1)C(2)C(3) 119.05(15), ∠C(2)C(3)C(4) 120.24(12), ∠C(3)C(4)C(5) 119.79(11), ∠C(1)C(2)H 119.67(25), ∠C(2)C(3)H 120.41(19) degrees.

The molecular structure of trimethylsilylisocyanate in the gas phase redetermined by electron diffraction

Abstract We have redetermined the molecular structure of trimethylsilylisocyanate in the gas phase by electron diffraction. An r a structure is defined with bond distances (pm) SiN 174.0(4). SiC 186.4(2), NC 120.2(16), CO 117.6(10) and CH 109.9(5), bond angles (degrees) HCSi 109.0(9), CSiC 108.8(25), SiNC 156.9(30) and NCO 165.8(36), with the N C bond eclipsing one SiC bond and the Me 3 Si group tilted slightly. The methyl groups are twisted 26.9(35) a from the position giving the Me 3 Si group C 3v symmetry in an (assumed) concerted fashion. The apparent deviations from linearity of the SiN CO skeleton are shown to be compatible with a pseudolinear structure similar to that of SiH 3 NCO.

The photoelectron spectra of GeH4 and GeF4

Abstract The photoelectron spectra of GeH 4 and GeF 4 up to 30 eV are reported. Each shows the expected two bonding levels; in addition GeH 4 gives two bands attributed to Ge 3d level, and GeF 4 gives three bands due to F 2p “lone pair” levels.

The molecular structure of pyridazine, determined from electron-diffraction and microwave data in the gas phase and direct dipolar couplings in the 1H-NMR spectrum in a liquid-crystal solvent

Abstract The structure of pyridazine has been studied using new electron-diffraction data, existing microwave data and new dipolar direct couplings from nmr spectra of a sample dissolved in an oriented nematic liquid-crystal solvent. This combination allows the structure to be defined fully, without any assumptions, although each data set on its own unable to provide more than a partial structure, and even taking the data in pairs leaves strong correlations between geometrical parameters that lead to ambiguities in the structure. The refined r 0 α structure is compatible with earlier incomplete results: r NN = 133.70(21) pm, r N(2)C(3) = 133.79(15) pm, r C(3)C(4) = 140.00(9) pm, r C(4)C(5) = 138.46(19) pm, r C(3)H(7) = 107.87(20) pm, r C(4)H(8) = 107.07(20) pm, ∠NNC = 119.38(5)°, ∠ NCC = 123.76(&)°, ∠ CCC = 116.86(5)°, ∠ NCH = 114.91(17)° and ∠ C(3)c(4)H(8) = 120.67(13)°. The ring structure is very similar to that found in the crystal structure of pyridazine-3,6-dicarboxylic acid monohydrate.

Infrared and Raman spectra and normal coordinate calculations for methylisothiocyanate

Abstract The infrared spectra (3200-50 cm −1 ) of gaseous and solid CH 3 NCS and CD 3 NCS and the Raman spectra (3200-10 cm −1 ) of the liquids and solids have been recorded. The spectra have been interpreted on the basis of a “pseudo-symmetric top” with C 3v symmetry. An assignment of the fundamental vibrations in both molecules, based on their infrared band contours, depolarization values and group frequencies, is given and discussed. Particularly interesting is the low-frequency region where band maxima were observed at 152 and 80 cm −1 for CH 3 NCS and 139 and 71 cm −1 for CD 3 NCS in the infrared spectra of the gases. A normal coordinate analysis has also been carried out based on C 3v symmetry. Considerable mixing was found between the C α N stretch and NCS symmetric stretch in both isotopic species. The other normal modes in CH 3 NCS are reasonably pure but, for the CD 3 NCS molecule, considerable mixing was found between the CD 3 stretches and NCS antisymmetric stretch. The proposed vibrational assignment and the results of the normal coordinate calculations are discussed and compared with the results obtained for similar molecules.

Electron diffraction investigation of the molecular structures of isopropyl isocyanate and isopropyl isothiocyanate

Abstract The molecular structures of isopropyl isocyanate and isopropyl isothiocyanate have been determined by electron diffraction in the gas phase. For both molecules the data are consistent with the presence of a single conformer at room temperature, with the NCX group eclipsing one of the CC bonds of the isopropyl group (a skew conformation). The structures found are consistent with the limited information available from microwave studies. The skeletal parameters found are (NCO, NCS; distances in pm, angles in degrees): r (CN) 146.0(8), 145.9(13); r (CC) 153.4(5), 15.8(7); r (NC) 121.4(6), 120.1(6); r (CO) 118.4(4); r (CS) 159.8(5); ∠NCC 110.0(5), 110.4(10); ∠CCC 114.7(9), 115.7(13); ∠CNC 132.6(10), 135.9(17); ∠NCX 159(3), 166(3); CN torsion 125(3), 115(6) (where a torsion angle of 120° corresponds to the skew form). The results are compared with those for some related molecules.