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The molecular structures of dimethylbis(trichlorosilyl)germane [Me 2 Ge(SiCl 3 ) 2 ] and trimethyl(trichlorosilyl)germane (Me 3 GeSiCl 3 ) have been determined in the gas phase. Me 3 GeSiCl 3 was found ab initio to possess C 3 v symmetry, with a low-lying torsional motion of the SiCl 3 group relative to the GeMe 3 group. The gas electron diffraction data were modelled with C 3 symmetry, although little deviation from C 3 v symmetry was observed. Me 2 Ge(SiCl 3 ) 2 ( C 2 v symmetry) was also found to have a very low-lying vibrational frequency relating to the rotation of the SiCl 3 groups. This led to the gas electron diffraction data being modelled in C 2 symmetry, with the observed combined deviation of the SiCl 3 groups being 14° from the eclipsed structure calculated ab initio. The Ge–C bond lengths were unaffected by the addition of an extra SiCl 3 group to the central germanium atom, but the Ge–Si bond lengths were observed to increase by over 1 pm.

Combined gas electron diffraction/mass spectrometry (GED/MS) was used to determine the molecular structure of tris(hexafluoroacetylacetonato)scandium(III), Sc(hfa) 3 (hfa=1,1,1,5,5,5-hexafluoropentane-2,4-dionato). The experimental data recorded at 310(5) K indicate that the molecules have D 3 symmetry. The bond distances ( r h 1 ) in the chelate ring are Sc–O 2.084(5), C–O 1.268(8) and C–C 1.411(10) A. Theoretical computations at the HF and DFT levels with basis sets up to Ahlrich valence triple-ξ (VTZ) afford structures similar to those found experimentally, with a distorted ScO 6 antiprism.

The molecular structure and conformation of 1, 1, 1,4,4,4-hexachloro- 1,4-disilabutane in the gas-phase have been determined by electron diffraction and computational methods. The lowest-energy conformation has the trichlorosilyl groups anti to one another. The gauche conformation also has a shallow potential minimum, but lies about 19 kJ mol(-1) above the anti form. Calculations on related butane derivatives, in which terminal methyl groups have been replaced by CCl3, SiH3 and SiCl3 groups, reveal that the conformational preferences are primarily caused by steric interactions between the terminal groups, and that it is the presence of chlorine atoms that destabilises gauche conformations. The electronegativity of the chlorine atoms has only small effects, mainly limited to the Si-Cl bond lengths. (c) 2004 Elsevier B.V. All rights reserved.

The gas-phase molecular structures of 6,8-difluoro-1,3λ4δ2,2,4-benzodithiadiazine and 5,6,7-trifluoro-1,3λ4δ2,2,4-benzodithiadiazine have been determined using quantum chemical calculations and electron diffraction via the SARACEN method of structural analysis. Of particular interest was the planarity or nonplanarity of the heterocyclic fragments of the molecules. It was shown that the difluoro compound is planar with respect to the heterocyclic fragment [folding angle: 0.0(5)° (figures in parenthesis indicate e.s.d.s in the final digits)] and that the trifluoro compound probably deviates from planarity to a small extent [folding angle: 4.0(−30;+3)°; the large uncertainty in the value of this angle is due to the low-frequency, large-amplitude vibrational motion associated with the folding of the heterocyclic fragment]. Pertinent structural parameters from the electron diffraction data (rh1 structure) of the difluoro compound are av[r(S=N)] = 155.7(7) pm, r(S−N) = 175.8(5) pm, r(C−S) = 176.6(8) pm and r(C−N) = 141.6(2) pm. For the trifluoro compound the equivalent structural parameters are av[r(S=N)] = 155.5(7) pm, r(S−N) = 170.7(6) pm, r(C−S) = 180.6(8) pm and r(C−N) = 140.6(2) pm. The gas-phase electron diffraction structures have been compared with results from ab initio and DFT calculations and also with experimental solid-state structures. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The determination of structure in the gas phase is an important technique for obtaining accurate structural information for molecules, free from intermolecular interactions that could affect solid-state structures. For large asymmetric molecules, obtaining the full gas-phase structure using experimental data alone is virtually impossible in most cases. The utilisation of theoretical methods in the determination of the experimental molecular structure has allowed many more molecules to be studied, revealing important structural information. This information can be used by theoreticians to develop their methods further, and so both experimentalists and theoreticians benefit. This article describes the development of these methods and uses some examples to demonstrate their importance. These include the determination of the gaseous molecular structures of ferrocene and isopropylferrocene, and of the bis[bis(trimethylsilyl)methyl]phoshido radical, for which Density Functional Theory is realistically the only theoretical method available to aid the determination of the gas-phase structure. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

We have developed methodology for the determination of solution structures of small molecules from residual dipolar coupling constants measured in dilute liquid crystals. The power of the new technique is demonstrated by the determination of the structure of methyl β-d-xylopyranoside (I) in solution. An oriented sample of I was prepared using a mixture of C12E5 and hexanol in D2O. Thirty residual dipolar coupling constants, ranging from −6.44 to 4.99 Hz, were measured using intensity-based J-modulated NMR techniques. These include 15 DHH, 4 1DCH, and 11 nDCH coupling constants. The accuracy of the dipolar coupling constants is estimated to be

Gas-phase electron diffraction is a powerful technique for structural analysis of molecules in the gas phase, where they are free from packing forces that can occur in crystals. The compound B(BF2)3CO has been studied by gas-phase electron diffraction to compare its structure to that seen in the solid phase by low-temperature X-ray crystallography. Results show the gas-phase structure to be similar to that seen in the crystal. A model with C3 symmetry refined to give a C–O bond length of 115.8 pm and a C–B bond distance of 150.2 pm, which compare to values of 111.7 and 152.2 pm for the solid phase. The family of borane carbonyl compounds B(BX2)3CO (X = F, Cl, Br or I) have all been studied by ab initio calculations to show the effects of halogen substitution and to gauge the effects of electron correlation and basis set on each structure. Compounds X = F, Cl and Br give calculated structures with C3 symmetry in which the boron–halogen bonds lie coplanar with the C–O bond. In the case of X = I, the BI2 groups are twisted by approximately 35° from coplanar at the DFT level as a result of the large steric interactions between iodine atoms.

The molecular structures of pyrrolizin-3-one 1 and 1,2-dihydropyrrolizin-3-one 2 have been investigated in the gas phase by ab initio calculations (both compounds) and electron diffraction using the SARACEN method of structural analysis (parent compound only), and in the solid phase at 150 K by low-temperature X-ray diffraction. Important structural parameters (rh1 structure) for a free molecule of 1 are: r(C–O) 1.215(4) and r[N–C(CO)] 1.437(4) A. For 2 these (re structure) are: r(C–O) 1.2095 and r[N–C(CO)] 1.4068 A. The corresponding values in the crystal for 1 are r(C–O) 1.207(2) and r[N–C(CO)] 1.408(2) A, and for 2r(C–O) 1.216(2) and r[N–C(CO)] 1.392(2) A.

The molecular structures of the stable phosphinyl and arsinyl radicals, •PnR2 [Pn = P (2); As (4); R = CH(SiMe3)2], have been determined by gas-phase electron diffraction (GED) in conjunction with ab initio molecular orbital calculations. The X-ray crystal structures of the corresponding dipnictines, the “dimers”, R2PnPnR2 [Pn = P (1), As (3)], and the chloro derivatives R2PnCl [Pn = P (5), As (6)] have also been determined. Collectively, these structural investigations demonstrate that large distortions of the ligands attached to Pn occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers. Principally, it is the shape and flexibility of the CH(SiMe3)2 ligands that permit the formation of the P−P and As−As bonds in 1 and 3, respectively. However, theoretical studies indicate that in the process of pnictinyl radical dimerization to form 1 and 3, both molecules accumulate substantial amounts of potential energy and are thus primed to spring apart upon release from the solid state ...

The series of compounds ButX2SiSiX2But (X = Cl, Br and I) have been studied by X-ray crystallography, vibrational spectroscopy and ab initio calculations. The solids were all found to be anti and the vibrational spectra are consistent with this observation. The molecular structure of 1,2-di-tert-butyltetrachlorodisilane, ButCl2SiSiCl2But, has also been determined in the gas phase by electron diffraction (GED) and ab initio molecular-orbital calculations. The conformation is distorted slightly from the fully staggered arrangement, with ϕ(CSiSiC) 167.7(11)°. Important structural parameters (ra) include: Si–Si 238.0(7) pm, Si–C 187.2(7) pm, C–C 154.3(2) pm, C–H 114.2(4) pm, Si–Si–C 119.8(6)°, Cl–Si–Cl 105.5(8)° and C–Si–Si–C 167.7(11)°. These experimental observations are supported by theoretical predictions obtained at the MP2/6-31G* level.

The molecular structure of 1,1,2-tri-tert-butyldisilane, But2HSiSiH2But, has been determined in the gas phase by electron diffraction (GED) and ab initio molecular-orbital calculations. Vibrational spectra are consistent with a vapour consisting of one conformer, identified by the structural study as a syn arrangement in which each of the butyl groups eclipses an Si–H bond. Important structural parameters (ra) for the conformer are: Si–Si 236.3(8), Si–C (mean) 191.1(3), C–C 154.5(1), C–H 112.4(1) pm, Si(1)–Si(2)–C(21) 116.0(8), Si(2)–Si(1)–C(11) 111.2(10), Si(2)–Si(1)–C(12) 108.7(9), C(11)–Si(1)–C(12) 121.1(11) and C(21)–Si(2)–Si(1)–H(13) –6.2(11)°, where C(11), C(12) and C(21) are the central carbon atoms of the three tert-butyl groups. These experimental observations are supported by theoretical predictions obtained at the D95*/MP2 level, which also identify two higher-energy conformers.

The gas-phase molecular structure of iodotrimethylsilane (ITMS) has been determined from electron diffraction data. Infrared and Raman spectra have been completely assigned. The experimental work is supported by ab initio HF and MP2 calculations for the gas-phase structure determination and DFT(B3LYP) calculations, combined with Pulay's SQM method, for the vibrational spectra data. © the Owner Societies 2006. Fil: Montejo, Manuel. Universidad de Jaén; España Fil: Hinchley, Sarah L.. University of Edinburgh; Reino Unido Fil: Ben Altabef, Aida. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Tucumán; Argentina Fil: Robertson, Heather E.. University of Edinburgh; Reino Unido Fil: Ureña, Francisco Partal. Universidad de Jaén; España Fil: Rankin, David W. H.. University of Edinburgh; Reino Unido Fil: López González, Juan J.. Universidad de Jaén; España

A series of nine tripodal tetradentate ligands based on tris(pyridyl-2-methyl)amine TPA with hydrogen bond donors R in one, two and three of the pyridine 6-positions (R = NH2 amino, L(Am-1,2,3); NHCH2(t)Bu neopentylamino, L(Np-1,2,3); NHCO(t)Bu pivaloylamido, L(Piv-1,2,3)) and TPA are used to investigate the effect of different hydrogen bonding microenvironments on electrochemical properties of their LCuCl complexes. The hydrogen bond donors are rigidly preorganised and suitably oriented for intramolecular N-H...Cl-Cu hydrogen bonds. Cyclic voltammetry studies show that the reduction potential of the Cu(II)/Cu(I) couple as a function of the ligand follows the order TPAL(Am-n)or approximately L(Np-n)L(Piv-n), and that the magnitude of the effect increases with the number of hydrogen bonding groups. These trends could be explained in terms of the steric and electronic effects exerted by these groups stabilising the Cu(I) oxidation state. In fact, the X-ray structure of the air-stable [(L(Piv-3))Cu(I)Cl] complex is reported and shows elongated Cu-N and Cu-Cl bonds, presumably due to the combination of steric and electron withdrawing effects exerted by the three pivaloylamido groups. We reasoned that the strength of hydrogen bonding in the Cu(I) and Cu(II) oxidation states could differ and therefore contribute also to the aforementioned redox changes; this hypothesis is tested using IR and NMR spectroscopy. IR studies of the [(L(Piv-1,2,3))Cu(I)Cl] and [(L(Piv-1,2,3))Cu(II)Cl]+ complexes in acetonitrile show that the intramolecular N-H...Cl-Cu hydrogen bonding weakens in the order L(Piv-1)L(Piv-2)L(Piv-3), and that it is stronger in the Cu(I) complexes. The 1H NMR spectra of the [(L(Piv1,2,3))Cu(I)Cl] complexes are in complete agreement with the IR data, and reveal that the stability of the Cu(I) complexes to oxidation in air increases in the order L(Piv-1)L(Piv-2)L(Piv-3). The hydrogen bonds in the Cu(I) complexes are stronger because of the higher electron density on the Cl ligand, when compared to the Cu(II) complexes. This is consistent with ab initio MP2 calculations performed on the complexes [(L(Piv-3))Cu(I)Cl] and [(L(Piv-3))Cu(II)Cl]+. Thus, the electron density of a metal-bound ligand acting as hydrogen bond acceptor is revealed as the major factor in determining the strength of the hydrogen bonds formed. From the IR data the energies of the N-H...Cl-Cu hydrogen bonds is estimated, as is the contribution of changes in hydrogen bond strength with the oxidation state of the copper centre and number of interactions to stabilising the Cu(I) state. Some of the implications of this result in dioxygen activation chemistry are discussed.

The structures of the D3h-symmetric molecules dodecafluoro-1,6-distibatriptycene and dodecafluoro-1,6-dibismatriptycene [Z2(C6F4)3 (Z = Sb, Bi)] have been determined in the gas phase by electron diffraction, using the SARACEN method, with restraints obtained from quantum chemical calculations. Several methods of ab initio and density functional theory geometry calculations have been performed and recommendations made as to their relative suitabilities for determining the structures of such species. Calculations using the MP2 method with a small-core pseudopotential (aug-cc-pVQZ-PP) on the Sb and Bi atoms and the 6-311G* basis set on the light atoms were found to give the closest correlation with the experimental results for both molecules. Differences in structure were found depending on whether a large-core or small-core pseudopotential was used on the heavy atoms.

Molecular dynamics simulations were performed under conditions of constant volume and temperature and of constant pressure and temperature to elucidate the structure activity relationships of a series of non-ionic surfactant molecules derived from vegetable fat and employed as friction modifiers in commercial engine oils. The simulations show the extent to which intermolecular hydrogen bonding is important in determining the stability of the monolayer formed by the surfactant molecules and show that mono-alkanoyl glyceride molecules are able to pack more efficiently, forming significantly more intermolecular hydrogen bonds and occupying approximately half the volume needed by di-alkanoyl glyceride molecules. Density profiles are presented which show significant mixing of the hydrophobic tail groups and a non-polar solvent. The distribution of torsion angles in the tail groups shows that the conformation is consistent with a liquid at finite temperature rather than a crystal structure. The measured friction coefficients of equimolar solutions of the glycerides show that the efficacy as friction modifiers varies in the order mono-, di- and the tri-oleyl glyceride, which is consistent with the efficacy of film formation predicted by the molecular dynamics calculations.

The structure of B8F12 has been shown by gas electron diffraction and computational methods (up to MP2/6-31+G*) to have the same highly asymmetric form observed in crystalline phases. The structure can be regarded as derived from a central B2 group, bridged by two BF2 groups to give a central B4 core that is folded, not planar, and with a very short bond [164.3 pm calculated, 164.2(19) pm experimental] along the fold line. There are also four terminal BF2 groups. One of the other four bonds in the core is consistently 20-30 pm longer than the others. This asymmetry has been attributed to many intra-molecular B...F interactions, particularly those between core boron atoms and fluorines of the terminal BF2 groups. Calculations for the chloro analogue lead to a structure similar to that for B8F12, but with the long core bond extended so that one of the bridging BCl2 groups may now be regarded as terminal. With bromine as the halogen the structure changes again, with one bromine atom taking up a bridging position. With iodine, this process continues further, and there are three bridging iodine atoms. However, in this case this is not the lowest energy structure, and instead a loosely associated dimer of B4I6 is preferred. In all these cases, and particularly with the heavier halogens, there are huge differences between the results obtained with different computational methods.

The molecular structures of a family of eight aminodifluorophosphines, (PF2)NRR' (R, R' = H, CH3, SiH3, GeH3, PF2), have been redetermined using gas-phase electron diffraction data and high-level ab initio molecular-orbital calculations. The SARACEN method has allowed the application of flexible restraints, giving greater accuracy and precision of structure, while the SHRINK program has allowed curvilinear corrections for vibrational effects to be applied to intramolecular distances. The more accurate structures of these eight compounds show consistent patterns of effects attributable to the various substituents, while conformations are dominated by the requirement that adjacent phosphorus and nitrogen lone pairs of electrons should be orthogonal.

We have developed methodology for the determination of solution structures of small molecules from residual dipolar coupling constants measured in dilute liquid crystals. The power of the new technique is demonstrated by the determination of the structure of methyl beta-d-xylopyranoside (I) in solution. An oriented sample of I was prepared using a mixture of C(12)E(5) and hexanol in D(2)O. Thirty residual dipolar coupling constants, ranging from -6.44 to 4.99 Hz, were measured using intensity-based J-modulated NMR techniques. These include 15 D(HH), 4 (1)D(CH), and 11 (n)D(CH) coupling constants. The accuracy of the dipolar coupling constants is estimated to be+/- 0.02 Hz. New constant-time HMBC NMR experiments were developed for the measurement of (n)D(CH) coupling constants, the use of which was crucial for the successful structure determination of I, as they allowed us to increase the number of fitted parameters. The structure of I was refined using a model in which the directly bonded interatom distances were fixed at their ab initio values, while 16 geometrical and 5 order parameters were optimized. These included 2 CCC and 6 CCH angles, and 2 CCCC and 6 CCCH dihedral angles. Vibrationally averaged dipolar coupling constants were used during the refinement. The refined solution structure of I is very similar to that obtained by ab initio calculations, with 11 bond and dihedral angles differing by 0.8 degrees or less and the remaining 5 parameters differing by up to 3.3 degrees . Comparison with the neutron diffraction structure showed larger differences attributable to crystal packing effects. Reducing the degree of order by using dilute liquid crystalline media in combination with precise measurement of small residual dipolar coupling constants, as shown here, is a way of overcoming the limitation of strongly orienting liquid crystals associated with the complexity of (1)H NMR spectra for molecules with more than 12 protons.

The molecular structure of tri-tert-butylphosphine imide has been re-determined using the recently developed DYNAMITE method, which allows all assumptions about local symmetry to be removed without increasing the number of refining structural parameters excessively. The imide hydrogen causes the NPBu(t)(3) group to deviate hugely from local C(3) symmetry, with N-P-C angles returned as 99.2(9), 110.9(7), and 111.5(11) degrees, while the C-P-C angles also deviate from symmetry, being 109.8(8), 110.5(9), and 113.9(9) degrees, so that the NPC(3) fragment is close to C(s) rather than C(3) symmetry. The application of the DYNAMITE method to HNPBu(t)(3) also allows the methyl groups to be asymmetric, which has been shown to be important by ab initio methods. The re-determination of this structure using these more sophisticated methods has also resulted in a much shorter P-N bond than was previously determined, and is consistent with the molecule being regarded as HN=PBu(t)(3), rather than HN(-)-P(+)Bu(t)(3).

The molecular structure of tetra-tert-butyldiphosphine has been determined in the gas phase by electron diffraction using the new DYNAMITE method and in the crystalline phase by X-ray diffraction. Ab initio methods were employed to gain a greater understanding of the structural preferences of this molecule in the gas phase, and to determine the intrinsic P-P bond energy, using recently described methods. Although the P-P bond is relatively long [GED 226.4(8) pm; X-ray 223.4(1) pm] and the dissociation energy is computed to be correspondingly small (150.6 kJ mol(-1)), the intrinsic energy of this bond (258.2 kJ mol(-1)) is normal for a diphosphine. The gaseous data were refined using the new Edinburgh structure refinement program ed@ed, which is described in detail. The molecular structure of gaseous P(2)Bu(t)(4) is compared to that of the isoelectronic 1,1,2,2-tetra-tert-butyldisilane. The molecules adopt a conformation with C(2) symmetry. The P-P-C angles returned from the gas electron diffraction refinement are 118.8(6) and 98.9(6) degrees, a difference of 20 degrees, whilst the C-P-C angle is 110.3(8) degrees. The corresponding parameters in the crystal are 120.9(1), 99.5(1) and 109.5(1) degrees. There are also large deformations within the tert-butyl groups, making the DYNAMITE analysis for this molecule extremely important.

A new method to aid the determination of structures of sterically crowded molecules in the gas phase by dynamically linking the gas-phase electron diffraction (GED) refinement process with computational methods has been developed. The procedure involves refining the heavy-atom skeleton of the molecule using the GED data while continually updating the light-atom positions during the refinement using computational methods, in this case molecular mechanics. This removes errors associated with the assumption of local symmetry for the light-atom groups, which can affect the final values of the heavy-atom parameters. The refinement of the molecular structure of tri-tert-butyl phosphine oxide has been used to illustrate this new technique, which we call the DYNAMITE (DYNAMic Interaction of Theory and Experiment) method. Re-examination of the structure using this method has resulted in a shorter P-O distance than was found in a less sophisticated anaylsis, and is consistent with the molecule being regarded as O=PBut3, rather than O(-)-P+But3.

The molecular structure of 1,1,2,2-tetra-tert-butyldisilane has been determined by gas-phase electron diffraction supported by ab initio calculations, in the solution phase by Raman spectroscopy, and in the solid phase by Raman spectroscopy and X-ray crystallography. The gas-phase structure (C2 symmetry) was found to be almost anticlinal, a most unusual and unexpected result. In the favoured conformation, contact between tert-butyl groups at each end of the molecule is avoided by a large deviation of the angles around the silicon atoms from the parent tetrahedral angle of 109.5 degrees. In fact, the Si-Si-C angles returned from the gas electron diffraction refinement are 117.0(5) and 110.7(6) degrees, indicating the large degree of flexibility about the silicon centres. The ab initio methods and gas electron diffraction results indicate that there is only one conformer of But2HSiSiHBut2 in the gaseous mixture. Variable temperature Raman studies indicate the possibility of a further higher energy conformer existing in the liquid phase. However, this seems quite improbable from other observations made for the Raman spectra at all temperatures. The X-ray structure is close to that observed in the gas phase, with phiHSiSiH = 94.2(18) degrees. There is a large amount of disorder about one of the silicon postions and one of the tert-butyl groups within the crystal structure, which makes detailed direct comparison with the gaseous structure difficult.

The simple silylhydrazines F(3)SiN(Me)NMe(2) (1), F(2)Si(N(Me)NMe(2))(2) (2), and F(3)SiN(SiMe(3))NMe(2) (3) have been prepared by reaction of SiF(4) with LiN(Me)NMe(2) and LiN(SiMe(3))NMe(2), while F(3)SiN(SnMe(3))NMe(2) (4) was prepared from SiF(4) and (Me(3)Sn)(2)NNMe(2) (5). The compounds were characterized by gas-phase IR and multinuclear NMR spectroscopy ((1)H, (13)C, (14/15)N, (19)F, (29)Si, (119)Sn), as well as by mass spectrometry. The crystal structures of compounds 1-5 were determined by X-ray crystallography. The structures of free molecules 1 and 3 were determined by gas-phase electron diffraction. The structures of 1, 2, and 4 were also determined by ab initio calculations at the MP2/6-311+G** level of theory. These structural studies constitute the first experimental proof for the presence of strong Si.N beta-donor-acceptor bonds between the SiF(3) and geminal NMe(2) groups in silylhydrazines. The strength of these non-classical Si.N interactions is strongly dependent on the nature of the substituent at the alpha-nitrogen atom of the SiNN unit, and has the order 3>4>1. The valence angles at these extremely deformed alpha-nitrogen atoms, and the Si.N distances are (crystal/gas): 1 104.2(1)/106.5(4) degrees, 2.438(1)/2.510(6) A; 3 83.6(1)/84.9(4) degrees, 2.102(1)/2.135(9) A; 4 89.6(1) degrees, 2.204(2) A.

The molecular structures of trans-1,2-dichloro-1,2-disilylethene and 1-bromo-1-silylethene have been determined by gas-phase electron diffraction (GED) and ab initio molecular orbital calculations (MP2/6-311G). Both compounds were found to have highly asymmetric coordination around the carbon atoms with [ab initio (r(e))/GED (r(a))] C=C-Cl [117.0/117.0(2) degrees] and C=C-Si [126.2/128.1(1) degrees] in the C(2)(h) structure of trans-1,2-dichloro-1,2-disilylethene and C=C-Br [119.2/120.7(4) degrees] and C=C-Si [125.0/125.0(4) degrees] in the C(s) structure of 1-bromo-1-silylethene. Other important structural parameters for trans-1,2-dichloro-1,2-disilylethene are C=C [135.2/134.5(3) pm], C-Si [189.4/187.9(2) pm], and C-Cl [175.1/174.9(1) pm], and C=C [134.2/133.4(2) pm], C-Si [187.8/187.2(3) pm], and C-Br [191.3/191.0(3) pm] for 1-bromo-1-silylethene. Further ab initio calculations were carried out on CH(2)CRX and trans-(CRX)(2) (R = SiH(3), CH(3), or H; X = H, F, Cl, or Br) to gauge the effects of electron-withdrawing and electron-donating groups on the structures. They reveal some even more distorted structures. The asymmetric appearance of these molecules can largely be accounted for by valence shell electron pair repulsion theory.

N,N-Dimethylaminoxygermane, H3GeONMe2, was prepared by the reaction of H3GeBr with LiONMe2 in dimethyl ether at -96 degreesC. The identity of H3GeONMe2 was proven by gas-phase IR and solution NMR spectroscopy (H-1, C-13, N-15, O-17). It is unstable volatile liquid compound. It decomposes by cleavage of a Ge-O and a Ge-H bond giving HONMe2 and an insoluble germanium hydride polymer (GeH2)(n). This decomposition reaction has been modeled at the MP2/6-311G(d,p) level of theory by the homodesmotic reaction H3GeONMe2 + Ge2H6 --> Ge3H8 + HONMe2, which is predicted slightly exothermic by 14 kJ mol(-1). The molecular structure of H3GeONMe2 was determined by gas-phase electron diffraction supported by an ab initio geometry [MP2/6-311G(d,p)] and a force field [MP2/6-31G(d)]. The structure of the compound in the crystal lattice was determined by low-temperature crystallography using a single crystal of H3GeONMe2 grown in situ [C2H9NOGe, orthorhombic, Pnma, Z = 4, a = 8.1280(12) Angstrom, b = 9.7037(15) Angstrom, c = 7.0722(12) Angstrom]. Important bond lengths and angles (gas phase/solid state, Angstrom /deg) are Ge-O 1.785(2)/1.815(1), O-N 1.462(7)/1.460(2), N-C 1.460(4)/1.453(2), Ge-O-N 105.2(5)/104.6(1), O-N-C 105.8(5)/105.8(1), C-N-C 110.8(9)/111.2(2), Ge...N 2.587(6)/2.601(1). In the solid state the compound forms infinite chains by intermolecular Ge...O contacts of 2.808 Angstrom. The question of the attraction between Ge and N atoms is discussed with respect to reference Ge/O and N/O compounds, which have wider angles at oxygen than H3GeONMe2. For comparison the structures of the compounds H3CONMe2, H3SiONMe2, and H3SnONMe2 were also calculated to reflect the influence of the group 14 atom on the structure and to discuss the occurrence of weak E...N interactions in the compounds H3EONMe2.

The structure of the phosphinyl radical, ṖR2 [R = CH(SiMe3)2], has been determined by gas-phase electron diffraction (GED) together with ab initio molecular orbital calculations, and that of the corresponding diphosphine, (PR2)2, has been established by X-ray crystallography; the diphosphine behaves as an energy storage reservoir.

The molecular structure of 2,4-di-tert-butyl-η4-1,3-diphosphacyclobutadiene tin has been determined in the gas phase by electron diffraction using both the DYNAMITE and SARACEN methods. The suitability of many different theoretical methods for the calculation of structures of half-sandwich main-group metal complexes has been investigated, and, by comparison of the results with the experimental structures, suggestions have been made as to the most suitable methods for this class of compound.

Combined gas electron diffraction/mass spectrometry (GED/MS) was used to determine the molecular structure of tris(dipivaloylmethanato)lutetium(III), Lu(dpm)(3)(dpm = 2,2,6,6-tetramethyl-heptane-3,5-dionato). Up to about 520-570 K the vapour consisted only of molecules Lu(dpm)(3). The experimental data recorded at 408(5) K indicate that the molecules have D(3) symmetry. The bond distances (r(h1)) in the chelate ring are Lu-O 2.197(6) Angstrom, C-O 1.270(4) Angstrom and C-C 1.390(6) Angstrom . Theoretical computations at the HF and DFT levels with basis sets up to 6-311G* afford structures similar to those found experimentally, with a distorted LuO(6) antiprism.

The molecular structures of bis(tert-butyl)sulfurdiimide [S(NBut)2] and tris(tert-butyl)sulfurtriimide [S(NBut)3] have been investigated in the gas phase by electron diffraction and ab initio calculations, and in the solid phase by low-temperature X-ray diffraction. The structures of each were found to be similar in both phases, and the calculated structures agree well with those in the gas phase. Ab initio calculations at levels up to MP2(fc)/cc-pVTZ for S(NBut)2 predict that the E/Z conformer (Cs symmetry) is the preferred arrangement by as much as 36.5 kJ mol−1 over the E/E conformer. Important structural parameters [ab initio (re)/GED (ra)/X-ray] for the E/Z conformer of S(NBut)2 are S(1)–N(2) [152.9/153.8(3)/152.8(3) pm], S(1)–N(3) [155.5/156.5(4)/154.4(3) pm], N(2)–C(4) [147.3/146.2(4)/147.7(5) pm], N(3)–C(5) [147.9/147.0(4)/148.9(4) pm], N(2)–S(1)–N(3) [116.9/117.8(6)/117.4(2)°], S(1)–N(2)–C(4) [125.9/125.9(6)/128.1(2)°] and S(1)–N(3)–C(5) [117.1/116.7(7)/118.2(2)°]. One conformer of S(NBut)3 with C3h symmetry was located at the MP2(fc)/6-31G* level. The gas and solid-phase studies both returned C3 structures, with the butyl groups moved a little out of the SN3 plane. Important structural parameters [ab initio (re)/GED (ra)/X-ray] for S(NBut)3 are SN [152.8/153.5(3)/151.0(2), 151.0(2), 151.1(2) pm], N–C [148.7/147.2(4)/148.3(3), 148.5(3), 148.3(3) pm], C–C [152.8/150.8(2)/152.4(4), 152.6(4), 153.0(3) pm], SN–C [123.2/122.9(4)/126.2(2), 125.5(2), 126.0(2)°], C–C–C (mean) [110.4/108.3/110.0°] and NSN–C (mean) [180.0/173.0(5)/179.4°]. Theoretical predictions at the MP2(fc)/6-31G* level were used to restrain some of the refining parameters for both structures using the SARACEN method. The lowest energy structure of bis(tert-butyl)sulfurdiimide was found to be the E/Z conformer, and the structure of tris(tert-butyl)sulfurtriimide is such that each fragment with two NBut ligands has the E/Z conformation.

The molecular structure of bis(trichlorosilyl)tert-butylphosphine has been determined in the gas phase by electron diffraction (GED) and high level ab initio molecular-orbital calculations. The C1 global minimum on the potential energy surface could be reached by twisting all three groups in the same sense by 15–20° from the fully staggered Cs conformation, found to be a saddle point and 26.7 kJ mol−1 higher in energy at HF/6-31G*. Important structural parameters (ra) are: (distances) P–C 190.6(6), P–Si (mean) 221.0(5), C–C (mean) 156.5(6) and Si–Cl (mean) 203.2(1) pm; (bond angles) P–C–C (mean) 109.7(6), C–P–Si 105.0(7) and 104.5(7), Si–P–Si 99.9(6), and P–Si–Cl (mean) 111.2°; (dihedral angles) Si(7)–P–C–C 87.0(17), Si(8)–P–C–C 168.3(17), Cl(17)–Si–P–C 68.3(17) and Cl(18)–Si–P–C 80.0(10)°. Theoretical predictions at the MP2(fc)/6-31G* level were used to restrain some of the refining parameters using the SARACEN method. The angles at silicon and carbon (tert-butyl) were found to vary from 105.9(7) to 117.4(5)°, indicating that the three groups are greatly distorted from regular tetrahedral geometry. The axial and equatorial components of the tilts of these groups have been investigated and the tert-butyl group was found to tilt in the direction of the phosphorus lone pair with virtually no equatorial tilt. One SiCl3 group is tilted towards the tert-butyl group and one tilted away, both with axial and equatorial components. The structures of the related molecules PBut(SiH3)2 and PBut(SiH3)(SiCl3) have also been investigated by ab initio calculations and the distortions analysed in a similar manner to those of PBut(SiCl3)2.

The gas-phase molecular structures of 1,3lambda4delta2,2,4-benzodithiadiazine and 5,6,7,8-tetrafluoro-1,3lambda4delta2,2,4-benzodithiadiazine have been investigated by ab initio calculations and electron diffraction using the SARACEN method of structural analysis. Important structural parameters (r(h1) structure) for the parent compound were found to be: 1.546(3), r(S-N) 1.697(5), r(C-S) 1.784(5), and r(C-N) 1.393(6) A. For the tetrafluoro derivative, these are (r(h1) structure): 1.552(3), r(S-N) 1.723(8), r(C-S) 1.812(9), and r(C-N) 1.396(7) A. Furthermore, the GED experiment (Gas Electron Diffraction) quite convincingly demonstrates the nonplanarity of the former and the planarity of the latter in agreement with DFT calculations; but the results contradict calculations at the MP2 level. The effect of the fluorine atoms on the conformations of the molecules is discussed.