Molecular Structure of 3,4-Dimethylenehexa-1,5-diene ([4]Dendralene), C(8)H(10), in the Gas Phase As Determined by Electron Diffraction and ab Initio Calculations.

The molecular structure of 3,4-dimethylenehexa-1,5-diene ([4]dendralene), C(8)H(10), has been determined in the gas phase. A single conformer with C(2) symmetry, having two almost planar, anti butadiene groups orientated with a dihedral angle C(2)C(3)C(4)C(5) of 71.7(19) degrees, is detected by electron diffraction employing flexible restraints derived from ab initio computations. Other experimental structural parameters (r(alpha)/pm, angle(alpha)/ degrees ) are: C(1)=C(2) 133.4(1), C(3)=C(7) (not in main chain) 134.0(1), C(2)-C(3) 147.4(2), C(3)-C(4) 149.6(3), C(1)C(2)C(3) 124.4(3), C(2)C(3)C(4) 119.2(5), C(4)C(3)C(7) 117.6(7), and C(7)C(3)C(2)C(1) -174.8(28). Ab initio computations at the MP2/6-311G level predict that the vapor consists of ca. 90% of the conformer found experimentally, the other 10% comprising four other conformers.

Molecular Structure of Chlorodifluoronitrosomethane, CClF(2)NO, As Determined in the Gas Phase by Electron Diffraction and ab Initio Calculations.

The gas-phase structure of chlorodifluoronitrosomethane, CClF(2)NO, has been determined by electron diffraction and calculated ab initio. Theoretically, CClF(2)NO is predicted to consist of two conformers having C(s)() (phi = 0 degrees ) and C(1) (phi = 105 degrees ) symmetry, which are near degenerate in energy, DeltaE = 1.1 kJ mol(-1) [TZ2P/MP2 + ZPE(DZP/MP2)], and separated by a barrier of around 1 kJ mol(-1). Equivalent C(1) conformers are predicted to be connected by a barrier of around 5-10 kJ mol(-1). The low predicted barriers to interconversion of the two conformers suggest that the rotation of the nitroso group can be regarded as being barely restricted over most values of the torsional angle at room temperature. This conclusion is supported by the gas-phase electron diffraction data, for which a dynamic model employing 11 conformations was needed to obtain an accurate fit to the experimental data. The final refined values of structural parameters for the two conformers (C(s)()/C(1)) are (r(alpha)/pm,<(alpha)/deg) as follows: C(1)-N(2) 156.7(5)/155.9(5), N(2)-O(3) 117.5(3)/117.9(3), C(1)-Cl(4) 173.9(2)/174.2(2), C(1)-F 132.0(2)/132.1(2) and 131.0(2), C(1)-N(2)-O(3) 110.8(12)/110.7(12), N(2)-C(1)-Cl(4) 117.5(5)/108.9(5), N(2)-C(1)-F 103.7(2)/104.2(2) and 111.6(2), Cl(4)-C(1)-N(2)-F 123.6(14)/119.2(14) and 123.7(14).

Synthesis of Volatile Cyclic Silylamines and the Molecular Structures of Two 1-Aza-2,5-disilacyclopentane Derivatives.

An optimized synthetic procedure for alpha,omega-bis(bromosilyl)alkanes, BrH(2)Si(CH(2))(n)()SiH(2)Br (with n = 2 and 3), is proposed. 1,2-Bis(bromosilyl)ethane reacts with ammonia to give 1,4-bis(1-aza-2,5-disilacyclopentane-1-yl)-1,4-disilabutane, traces of 1,6-diaza-2,5,7,10,11,14-hexasilabicyclo[4.4.4]tetradecane and nonvolatile products. The primary reaction products undergo slow redistribution reactions whereby (1-aza-2,5-disilacyclopentane-1-yl)-1,4-disilabutane is formed as the major product. Reactions of alpha,omega-bis(bromosilyl)alkanes, BrH(2)Si(CH(2))(n)()SiH(2)Br (with n = 2 and 3), with isopropylamine afford the heterocycles 1-isopropyl-1-aza-2,5-disilacyclopentane and 1-isopropyl-1-aza-2,6-disilacyclohexane, whereas the analogous reaction with bis(bromosilyl)methane gives 1,5-diisopropyl-1,5-diaza-2,4,6,8-tetrasilacyclooctane rather than a four-membered ring compound. All compounds have been characterized by elemental analysis, mass spectrometry, and IR and NMR spectroscopy [(1)H, (13)C, (15)N and (29)Si including the measurement of (1)J((29)Si(15)N) coupling constants]. The molecular structure of 1-isopropyl-1-aza-2,5-disilacyclopentane, determined by analysis of gas-phase electron-diffraction data augmented by restraints derived from ab initio calculations, is compared with the molecular structure of the isoelectronic 1-(dimethylamino)-1-aza-2,5-disilacyclopentane. The latter also was determined by gas-phase electron-diffraction (supported by ab initio calculations) and by low-temperature crystallography. The presence of a beta-donor Si.N interaction in the latter compound, leading to a narrow Si-N-N angle, is apparent from a significant distortion of the molecular structure as compared with the isoelectronic reference compound.

Molecular Structures of Methyldifluoroarsine, CH(3)AsF(2), and Dimethylfluoroarsine, (CH(3))(2)AsF, in the Gas Phase As Determined by Electron Diffraction and ab InitioCalculations.

The structures of gaseous CH(3)AsF(2) and (CH(3))(2)AsF have been determined by electron diffraction incorporating vibrational amplitudes derived from ab initio force fields scaled by experimental frequencies and, for the difluoride, restrained by microwave constants. The following parameters (r(alpha) degrees structure, distances in pm, angles in degrees) have been determined for CH(3)AsF(2): r(As-C) = 194.6(4), r(As-F) = 173.1(1), angleCAsF = 95.2(1), angleFAsF = 97.0(1). For (CH(3))(2)AsF structural refinement gives r(As-C) = 195.1(1), r(As-F) = 175.4(1), angleCAsF = 95.3(5), and angleCAsC = 96.9(8). For the series (CH(3))(3)As, (CH(3))(2)AsF, CH(3)AsF(2), and AsF(3), both As-C and As-F bond lengths are shortened with increasing numbers of F atoms, but the angles CAsF and FAsF are almost invariant.

Molecular structures of M(Bu(t))3 (M = Al, Ga, In) using gas-phase electron diffraction and ab initio calculations: experimental and computational evidence for charge-transfer processes leading to photodissociation.

The gas-phase structures of AI(Bu')3 and Ga(Bu')3 have been investigated by electron diffraction and are shown to consist of monomeric units with very slightly pyramidal geometries. Salient structural parameters (r(hl)) include r(A1-C) = 2.008(2) A and r(Ga-C) = 2.032(2) A. For both compounds the ligand orientations and geometries are controlled by interligand interactions. The structures of M(Bu(t))3 (M = Al, Ga, In) have been calculated ab initio and those for the aluminium and gallium derivatives are in good agreement with the electron-diffraction structures. Comparison of the ab initio calculated structure of In(Bu')3 with those of Al(Bu(t))3 and Ga(Bu(t))3 suggests that the significantly different photochemistry exhibited by the former does not result from structural factors. In fact the compounds undergo a charge-transfer process in the UV region, with the wavelength required calculated to be slightly longer for the indium compound than for the other two.

The molecular structure of N-fluorobis(trifluoromethanesulfonyl)imide, NF(SO(2)CF(3))(2), as studied in the gas phase by electron diffraction restrained by ab initio calculations.

The structure of N-fluorobis(trifluoromethylsulfonyl)imide, prepared by a relatively safe and easy method, has been determined by gas-phase electron diffraction (GED), employing the SARACEN method, with flexible restraints based on the MP2/6-311G* structure, and by X-ray crystallography at 150 K. The strongly electron-withdrawing CF(3) and SO(2)CF(3) groups make the C-S and N-S distances long, averaging 187.7(3) and 171.7(3) pm, respectively, in the gas phase. The gas consists of two conformers, one (75%) with a CF(3) group on each side of the SNS plane, one anti-periplanar and one syn-periplanar to the further N-S bond (ap, sp), and the other with both CF(3) groups on the same side, i.e. denoted ap, ap. These conformers have very different SNS angles, 126.9(9) degrees and 117.1(17) degrees respectively. In the crystal all molecules have the ap, sp conformation, with parameters similar to those found for this conformer in the gas phase.