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The gas-phase molecular structure of iso-propyl(tert-butyl)(trichlorosilyl)phosphine has been determined using a combination of gas electron diffraction and computational methods. The structure presents a conformational challenge that required use of the SARACEN method to combine theoretical observations into the least-squares refinement process, a great advance on the techniques used to solve the structure of the parent trisilyl phosphine. Five conformers were found on the potential-energy surface for iso-propyl(tert-butyl)(trichlorosilyl)phosphine using the UCONGA program, and following a series of individual structure refinements a combined model with the two most abundant confirmers was evaluated. Key structural parameters (ra) include rP–Si [225.5(6) pm], rSi–Clmean [204.0(1) pm] and rP–Cmean [204.0(1) pm], ∠P–C–H 101.5(5)°, ∠C–C–C (iPr) 110.5(5)°, ∠C–C–C (tBu) 109.2(5)° and ∠C–P–C 105.4(5)°.

The structures of the molecules C(SiXMe2)4 (X=H, F, Cl, Br) have been determined by gas electron diffraction (GED). Ab initio calculations revealed nine potential minima for each species, with significant ranges of energies. For the H, F, Cl, and Br derivatives nine, seven, two, and two conformers were modelled, respectively, as they were quantum-chemically predicted to be present in measurable quantities. Variable-temperature 1H and 29Si solution-phase NMR studies and, where applicable, 13C NMR, 1H/29Si NMR shift-correlation, and 1H NMR saturation-transfer experiments are reported for C(SiXMe2)4 (X=H, Cl, Br, and also I). At low temperature in solution two conformers (one C1-symmetric and one C2-symmetric) are observed for each of C(SiXMe2)4 (X=Cl, Br, I), in agreement with the isolated molecule ab initio calculations carried out as part of this work for X=Cl, Br. C(SiHMe2)4 is present as a single C1-symmetric conformer in solution at the temperatures at which the NMR experiments were performed.

The molecular structures of 1-Br-4-F-C 6H 4 and 1-Cl-4-F-C 6H 4 have been studied in the gas phase using gas electron diffraction (GED) and ab initio methods. The structure of 1-Cl,4-F-C 6H 4 in the crystalline phase has also been studied, but whilst the gaseous structures were found to possess C 2v symmetry, the solid-state structure was found to be quite distorted, with three molecules in the asymmetric unit. These fragments only possess C s symmetry in the plane of the molecules, as opposed to the C 2v symmetry observed in the gas phase. The bonding motifs within the solid-state structure are very unusual and unexpected, with quite different C-F bond lengths for the three moieties, and are a result of weak hydrogen-halogen interactions within the structure. © 2010 Springer Science+Business Media, LLC.

The gas-phase structure of 1-adamantylphosphine has been determined by electron diffraction, supplemented with data from ab initio and DFT calculations. The adamantyl fragment was modeled with local C 3v symmetry and the phosphino group was found to be in a position almost bisecting a mirror plane of the adamantyl group, giving the molecule overall approximate C s symmetry. There is a small displacement of the C–P bond from the local threefold axis of the adamantyl group. Geometry optimizations were also performed for bis-(1-adamantyl)phosphine (C 1 point-group symmetry) and tris-(1-adamantyl)phosphine (C 3 symmetry), demonstrating extremely crowded environments around the phosphorus atoms leading to adamantyl groups that were much less symmetric. The adamantyl groups were also found to twist by a significant amount to minimize the strain.

The structure and conformations of 2-methylacetophenone (1) have been investigated by ab initio calculations carried out at the MP2(full)/6-311++G** level and by gas electron diffraction (GED). According to both methods, 1 exists predominantly as a form with the C=O bond synclinal with respect to the C(ar)-C(O) bond (1B), with a torsional angle [C(6)-C(1)-C=O] of 32.7(24) degrees as determined by GED and 26.0 degrees from MP2(full)/6-311++G**. Calculations also predict the presence of a second conformer, the anticlinal structure (1C), with phi = 140.0 degrees, with an abundance of less than 6%, an amount hardly detectable by GED. Different DFT computational protocols both support a nonplanar form of the predominant conformer (B2PLYP) and are in contradiction (B3LYP, M052x, B98, B97-D) with this experimental finding. The GED results, supported by the calculations that involve long-range correlation, are in a good agreement with (13)C NMR spectroscopic investigations, UV spectra, and dipole moment studies. However, previous claims that assumed steric inhibition of resonance caused by a significantly nonplanar conformation with phi close to 90 degrees have been disproved. Steric crowding is evident from the geometrical parameters, particularly from the C(1)-C(2) bond length and from the C(1)-C(2)-C(H(3)) and C(2)-C(1)-C(O) bond angles. It is concluded that any explanation of reactivity by steric inhibition of resonance and by other steric factors must be supported by experimental and/or theoretical investigation of the actual molecular shape.

The molecular structure and conformational properties of ethyl trifluoroacetate, CF3CO2CH2CH3, were determined in the gas phase by electron diffraction, and vibrational spectroscopy (IR and Raman). The experimental investigations were supplemented by ab initio (MP2) and DFT quantum chemical calculations at different levels of theory. Experimental and theoretical methods result in two structures with Cs (anti–anti) and C1 (anti–gauche) symmetries, the former being slightly more stable than the latter. The electron-diffraction data are best fitted with a mixture of 56% anti–gauche and 44% anti–anti conformers. The conformational preference was also studied using the total energy scheme, and the natural bond orbital scheme. Also, the infrared spectra of CF3CO2CH2CH3 are reported for the gas, liquid and solid states, as is the Raman spectrum of the liquid. The comparison of experimental averaged IR spectra of Cs and C1 conformers provides evidence for the predicted conformations in the IR spectra. Harmonic vibrational wavenumbers and scaled force fields have been calculated for both conformers. Copyright © 2009 John Wiley & Sons, Ltd.

A conformational study of diethyldichlorosilane and the elucidation of the gas-phase molecular structures of its four conformers have been performed using the combined approach of gas-phase electron diffraction and computational techniques. Moreover, the Raman spectrum of the liquid and the IR spectra of the gas and liquid phases have been recorded and thoroughly analyzed on the basis of the scaled quantum-mechanical force field methodology. The results of the vibrational assignment have given spectroscopic evidence of the presence of the different conformers in the samples. Copyright c � 2009 John Wiley & Sons, Ltd.

The molecular structures of allyl-, allenyl-, propargyl-, vinyl-, ethynyl-, phenyl-, benzyl-, and chloromethyl-phosphine have been determined from gas-phase electron diffraction data employing the SARACEN method. The experimental geometric parameters are compared with those obtained using ab initio calculations performed at the MP2 level using both Pople-type basis sets and the correlation-consistent basis sets of Dunning. The structure and conformational behavior of each molecule have been analyzed and, where possible, comparisons made to the analogous amine. For systems with multiple conformers, differences in the CCP bond angle of approximately 5 degrees between conformers are common. Trends in the key parameters are identified and compared with those found in similar systems.

The molecular structure of methyl trifluoroacetate (CF3C(O)OCH3) has been determined in the gas phase from electron-diffraction data supplemented by ab initio (MP2) and DFT calculations using different basis sets. Experimental data revealed an anti conformation with a dihedral angle θ (CCOC) = 180°. Quantum mechanical calculations indicate the possible existence of two conformers, differing by a rotation about the C(O)O bond. The global minimum represents a Cs-symmetric structure in which the CF3 group has the anti orientation with respect to the CH3 group, but there is another potential minimum, much higher in energy, representing a Cs-symmetric structure with a cis conformation. The preference for the anti conformation was studied using the total energy scheme and the natural bond orbital (NBO) partition scheme. Additionally, the total potential energy has been deconvoluted using a six-fold decomposition in terms of a Fourier-type expansion, showing that the electrostatic and steric contributions are dominant in stabilizing the anti conformer. Infrared spectra of CF3C(O)OCH3 were obtained for the gaseous and liquid phases, while the Raman spectrum was recorded for the liquid phase. Harmonic vibrational frequencies and a scaled force field have been calculated, leading to a final root mean-square deviation of 9 cm−1 when comparing experimental and calculated frequencies. Copyright © 2009 John Wiley & Sons, Ltd.

The development of modern computational methods, linked to improved methods for analysis of experimental gas-phase structural data, has allowed the stereochemistry of many boranes and heteroboranes to be determined with great accuracy over the past two decades. Many of these compounds have been prepared in the Institute of Inorganic Chemistry of the Academy of Sciences of the Czech Republic, and gas-phase electron diffraction (GED) data have been obtained mainly at the University of Edinburgh. Structural tools based on the concerted use of GED and computations of the geometries and 11B chemical shifts (MOCED, SARACEN) have also been employed. The results of this extended programme of research are reviewed here. Many different closo-, nido-, arachno-, and hypho- geometrical shapes, as well as some that do not obey Wade's rules, are included. The outlook for the future of this type of structural chemistry research is assessed.

The accurate structures of silsesquioxanes Si8O12H8 and Si8O12Me8 have been determined by gas-phase electron diffraction methods in order to obtain experimental data on single molecules unconstrain...

The gas-phase structures of Al(But)3 and Ga(But)3 have been investigated by electron diffraction and are shown to consist of monomeric units with very slightly pyramidal geometries. Salient structural parameters (rh1) include r(Al-C) = 2.008(2) Å and r(Ga-C) = 2.032(2) Å. For both compounds the ligand orientations and geometries are controlled by interligand interactions. The structures of M(But)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(But)3 with those of Al(But)3 and Ga(But)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.

Molecular structures of dimethylbis(trimethylsilylketyl)silane (Me 2 Si[C(SiMe 3 ) C O] 2 ), dimethylbis(trimethylgermylketyl)silane (Me 2 Si[C(GeMe 3 ) C O] 2 ), and dimethylbis(trimethylstannylketyl)germane (Me 2 Ge[C(SnMe 3 ) C O] 2 ) have been studied in the gas phase by electron diffraction accompanied by high level ab initio and DFT calculations. Extensive theoretical conformational analyses of the molecules in the vapour predicted a possibility of existence of two types of conformers with small energy differences. The first type had gauche – gauche arrangements of the ketenyl groups in the central C( C O)XC( C O) fragments directed away from each other. The second type had nearly syn – gauche arrangements of the ketenyl groups. In addition, the energy differences were found to depend on the level of computations used. The experimental analysis, in turn, was unable to distinguish between different conformers due to the large number of similar overlapping distances. The experimental data were fitted by an averaged single-conformer model, which nevertheless allowed reliable determination of bonds and bonded angles in the molecules. Main experimental ( r h1 ) structural parameters for Me 2 Si[C(SiMe 3 ) C O] 2 , Me 2 Si[C(GeMe 3 ) C O] 2 , and Me 2 Ge[C(SnMe 3 ) C O] 2 , i.e. Me 2 X[C(YMe 3 ) C O] 2 (X,Y = Si, Ge, Sn), are (X–C) mean 187.7(1) pm, 194.6(2) pm, 216.1(3) pm; (Y–C) mean , 187.7(1) pm, 188.8(8) pm, 194.6(4) pm; (C C) mean , 135.3(5) pm, 131.6(5) pm, 131.5(13) pm; (C O) mean , 117.0(7) pm, 117.4(7) pm, 119.0(11) pm; (C–H) mean , 110.6(7) pm, 110.0(4) pm, 109.1(13) pm; (X(Y)–C C) mean , 114.4(2)°, 115.6(1)°, 115.6(2)°; (C–X(Y)–C Me ) mean , 108.3(3)°, 108.4(3)°, 108.9(13)°; C(2)–C(1)–Y(4)–C(10), −19(6)°, 5(4)°, −9(10)°; C(7)–C(6)–Y(9)–C(38),−22(7)°, −32(3)°, −9(10)°; C(2)–C(1)–X(5)–C(6), 128(4)°, 142(1)°, 108(9)°; C(7)–C(6)–X(5)–C(1), 92(6)°, 115(2)°, 108(9)°, respectively.

A new method of incorporating ab initio theoretical data dynamically into the gas-phase electron diffraction (GED) refinement process has been developed to aid the structure determination of large, sterically crowded molecules. This process involves calculating a set of differences between parameters that define the positions of peripheral atoms (usually hydrogen), as determined using molecular mechanics (MM), and those which use ab initio methods. The peripheral-atom positions are then updated continually during the GED refinement process, using MM, and the returned positions are modified using this set of differences to account for the differences between ab initio and MM methods, before being scaled back to the average parameters used to define them, as refined from experimental data. This allows the molecule to adopt a completely asymmetric structure if required, without being constrained by the MM parametrization, whereas the calculations can be performed on a practical time scale. The molecular structures of tri-tert-butylphosphine oxide and tri-tert-butylphosphine imide have been re-examined using this new technique, which we call SEMTEX (Structure Enhancement Methodology using Theory and EXperiment).

Conformational studies of triethylchlorosilane (TECS) and triethylbromosilane (TEBS) and the elucidation of their gas phase molecular structures have been accomplished by the combined use of theoretical (ab initio and density functional theory) calculations and experimental data from gas phase electron diffraction experiments. Additionally, analysis of the experimental features observed in the IR and Raman spectra recorded for both compounds has allowed us to propose a complete description of the vibrational spectra of both compounds, including an explanation of certain bands, which can only be correctly assigned when more than one conformer is considered to be present.

Trifluoromethyl trifluoromethanesulfonate, CF3SO2OCF3, has been synthesised and its gas-phase structure determined from electron-diffraction data. This structural study was supported by HF, MP2 and DFT (B3LYP and B1B95) calculations, which revealed a strong dependence of the theoretical structure on the polarisation of the basis set. Infrared spectra for the gas and solid and a Raman spectrum for the liquid were obtained, and the observed bands were assigned. The experimental vibrational data, along with theoretical (B3LYP) force constants, were used to define a scaled quantum mechanical force field, which enabled reproduction of the measured frequencies with a final root-mean-square deviation of 6 cm–1. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

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.

The electrophilic addition reactions of methylsulfenyl chloride to the double bonds of functionalized ethenes have been studied theoretically. Density functional theory (DFT) calculations have been applied for starting species and ethene-based sulfonium intermediates bearing substitutes at α-carbon atom to study geometrical parameters and electronic states of plausible intermediate forms. The quantum chemical optimizations of intermediates indicate that the episulfonium ion is the most likely methyl- or carboxyl-substituted ethane-based intermediate. However, with phenyl substituents the intermediate is more like a carbonium than an episulfonium ion. The role of sulfur appears to be that of directing the stereochemistry of the addition reaction of chloride, forming the trans product upon nucleophilic attack on the C—C bond of the episulfonium ion. The regioselectivity features of the opening of the episulfonium ion by the chloride anion depend on the LUMO and LUMO+1 of the episulfonium ion and the approaching HOMO of chlorine. The results of the theoretical investigations are in agreement with experiment. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:695–703, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20378

The gas-phase structure of 6,9-CSB8H12 has been determined by electron diffraction and ab initio calculations, and that of 6,9-CNB8H13 has also been calculated. The accuracy of each structure has been confirmed by 11B NMR calculations. The position of the sulfur atom is very close to that of the boron atom occupying the equivalent position in the parent molecule [B10H14](2-), reflecting the similarity of sizes of sulfur and boron atoms. The nitrogen and carbon atoms, on the other hand, lie much closer to the centers of the cages. The B8-X9-B10 angles increase from 98.7 degrees for X = S to 122.8 degrees for X = N. There are also large changes in relative lengths of bonds, with some bonds lengthening by up to 14.6 pm on introduction of a sulfur atom.

Ti(NMe2)4 is a monomeric species in the gas phase and in solution, and this remains true on crystallisation. The crystal structure is triclinic, with space group P 1 ¯ , and unit cell dimensions a = 8.277(6), b = 8.407(6), c = 11.167(7) A, α = 105.36(4)°, β = 97.03(3)° and γ = 116.01(2)° at 105 K. Electron-diffraction measurements show that in the gas-phase Ti(NMe2)4 takes the form of a slightly flattened tetrahedron with overall S4 symmetry. In the solid state the symmetry changes to C1; the reduction of point-symmetry results from the twisting of one of the amino groups away from its gas-phase configuration. DFT calculations indicate that the energy of the C1 structure lies only 0.2 kJ mol−1 above that of the S4 conformer. The change in structure on crystallisation can be ascribed to the avoidance of prohibitively short intermolecular methyl–methyl interactions which would have been formed had the S4 structure been retained.

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)

Nine organotin fluorocarboxylates R n SnO 2 CR f (n = 3, R = Bu, R f = CF 3 , C 2 F 5 , C 3 F 7 , C 7 F 15 ; R = Et, R f = CF 3 , C 2 F 5 ; R = Me, R f = C 2 F 5 ; n = 2, R = Me, R f = CF 3 ) have been synthesized; key examples have been used to deposit fluorine-doped SnO 2 thin films by atmospheric pressure chemical vapour deposition. Et 3 SnO 2 CC 2 F 5 , in particular, gives high-quality films with fast deposition rates despite adopting a polymeric, carboxylate-bridged structure in the solid state, as determined by X-ray crystallography. Gas-phase electron diffraction on the model compound Me 3 SnO 2 CC 2 F 5 shows that accessible conformations do not allow contact between tin and fluorine, and that direct transfer is therefore unlikely to be part of the mechanism for fluorine incorporation in SnO 2 films. The structure of Me 2 Sn(O 2 CCF 3 ) 2 (H 2 O) has also been determined and adopts a trans-Me 2 SnO 3 coordination sphere about tin in which each carboxylate group is monodentate.

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

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.

From the whole plant of Senecio tsoongianus, two pairs of epimers, tsoongianolides A (1) and B (2), and tsoongianolides C (3) and D (4), and two new sesquiterpene lactones, tsoongianolides E (5) and F (6), all of which possess the eremophilane skeleton, were isolated. Two known eremophilane dimers (7) and (8) were also obtained from this species. Their absolute structures were elucidated by 1D- and 2D-NMR techniques and by X-ray diffraction studies, along with chemical evidence.

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

The molecular structures of two carbaboranes, closo-2,3-C(2)B(9)H(11) and nido-2,9-C(2)B(9)H(13), were determined experimentally for the first time using gas-phase electron diffraction (GED). For closo-2,3-C(2)B(9)H(11), a model with C(2)(v)() symmetry was refined to give C-B bond distances ranging 158.3-167.0 pm and B-B distances ranging 177.4-200.0 pm. The structure of nido-2,9-C(2)B(9)H(13) was refined using a model with C(s)() symmetry to give C-B bond lengths ranging 160.3-171.9 pm and B-B lengths ranging 173.0-196.1 pm. Ab initio computations (up to MP2/6-311+G) were also carried out on these and the related nido-7,8-C(2)B(9)H(13), which was not sufficiently stable to allow determination of its molecular structure by GED.

The molecular structures of the diphosphines P(2)[CH(SiH(3))(2)](4), P(2)[C(SiH(3))(3)](4), P(2)[SiH(CH(3))(2)](4), and P(2)[Si(CH(3))(3)](4) and the corresponding radicals P[CH(SiH(3))(2)](2), P[C(SiH(3))(3)](2), P[SiH(CH(3))(2)](2), and P[Si(CH(3))(3)](2) were predicted by theoretical quantum chemical calculations at the HF/3-21G*, B3LYP/3-21G*, and MP2/6-31+G* levels. The conformational analyses of all structures found the gauche conformers of the diphosphines with C(2) symmetry to be the most stable. The most stable conformers of the phosphido radicals were also found to possess C(2) symmetry. The structural changes upon dissociation allow the release of some of the energy stored in the substituents and therefore contribute to the decrease of the P-P bond dissociation energy. The P-P bond dissociation enthalpies at 298 K in the compounds studied were calculated to vary from -11.4 kJ mol(-1) (P(2)[C(SiH(3))(3)](4)) to 179.0 kJ mol(-1) (P(2)[SiH(CH(3))(2)](4)) at the B3LYP/3-21G* level. The MP2/6-31+G* calculations predict them to be in the range of 52.8-207.9 kJ mol(-1). All the values are corrected for basis set superposition error. The P-P bond energy defined by applying a mechanical analogy of the flexible substituents connected by a spring shows less variation, between 191.3 and 222.6 kJ mol(-1) at the B3LYP/3-21G level and between 225.6 and 290.4 kJ mol(-1) at the MP2/6-31+G* level. Its average value can be used to estimate bond dissociation energies from the energetics of structural relaxation.

The structure of the gallane adduct Me3P·GaH3 in the vapour and crystalline states has been investigated. The gas-phase electron-diffraction (GED) pattern has been analysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rh1 structure) were found to be: r(Ga–H) 159.0(11), r(Ga–P) 244.3(6), r(P–C) 184.0(2), r(C–H) 108.3(7) pm; H–Ga–P 98.4(12) and Ga–P–C 117.7(3)°. The structure of a single crystal at 150 K shows that the adduct retains the same monomeric unit in the crystalline phase, with dimensions generally close to those of the gaseous molecule and an eclipsed conformation of the C3PGaH3 skeleton. The results are discussed and analysed in the light of quantum chemical calculations and of the properties of related adducts of Group 13 metal hydrides.

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.

A molecular mechanics force field has been developed for the conformational analysis of amido- and α-aminoferrocenes. Parameterization for ring-substituent rotational barriers in amidoferrocenes and other cross-conjugated derivatives have been calculated using DFT on both the free and complexed cyclopentadienyl ligand. Modeled structures of (diisopropylamido)- and (dimethylamido)ferrocene and N,N-dimethyl-α-ferrocenylethylamine are in agreement with those determined through single-crystal X-ray diffraction. The diastereoselective lithiation of N,N-dimethylferrocenylethylamine and sparteine-mediated enantioselective lithiation of (diisopropylamido)ferrocene using MeLi have been modeled through an assumed reversible adduct formation at the amine nitrogen or amide oxygen, followed by an irreversible ring lithiation. Results indicate that selectivity results from ring lithiation via the adduct conformer with the shortest C−Hring- - -H3C−Li interaction.

The structure of Ru(C5Me5)(C5F5) has been determined by gas-phase electron diffraction and density functional theory. Comparison structures of the known compounds Ru(C5H5)2 and Ru(C5F5)(C5H5), as well as the unknown compound Ru(C5F5)2, have also been determined by density functional theory.

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 equilibrium molecular geometry of thiophene has been determined from a combination of gas-phase electron diffraction, vibrational and microwave data and ab initio and DFT calculations. The quadratic and cubic force constants of thiophene calculated theoretically and empirically improved by harmonic scale factors were incorporated in the analysis in which equilibrium distances and harmonic scale factors were refined simultaneously. The diffraction intensities were calculated by the use of first-order perturbation theory. The commonly used r a distances and amplitudes of vibration were also estimated and found to agree reasonably well with those from an earlier investigation. Anharmonic phase shift parameters for all atom pairs and the various distance correction terms are presented.

Gallaborane (GaBH6, 1), synthesized by the metathesis of LiBH4 with [H2GaCl]n at ca. 250 K, has been characterized by chemical analysis and by its IR and 1H and 11B NMR spectra. The IR spectrum of the vapor at low pressure implies the presence of only one species, viz. H2Ga(μ-H)2BH2, with a diborane-like structure conforming to C2v symmetry. The structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements afforced by the results of ab initio molecular orbital calculations. Hence the principal distances (rα in A) and angles ( α in deg) are as follows: r(Ga•••B), 2.197(3); r(Ga−Ht), 1.555(6); r(Ga−Hb), 1.800(6); r(B−Ht), 1.189(7); r(B−Hb), 1.286(7); Hb−Ga−Hb, 71.6(4); and Hb−B−Hb, 110.0(5) (t = terminal, b = bridging). Aggregation of the molecules occurs in the condensed phases. X-ray crystallographic studies of a single crystal at 110 K reveal a polymeric network with helical chains made up of alternating pseudotetrahedral GaH4 and BH4 units linked through single hydrogen bridges; the average Ga•••B distance is now 2.473(7) A. The compound decomposes in the condensed phases at temperatures exceeding ca. 240 K with the formation of elemental Ga and H2 and B2H6. The reactions with NH3, Me3N, and Me3P are also described.

The molecular structures of 1,1‘-di-tert-butylferrocene (1) and isopropylferrocene (2) have been examined by solid-state X-ray crystallography, gas-phase electron diffraction, and DFT and molecular...

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.

Interactions between silyl groups in various substituted silanes and molecules that include groups that are potential electron pair donors have been investigated by ab initio molecular orbital calculations. These calculations indicate that all the systems investigated should form weak adducts, depending upon the substituents on the silicon atom and the nature of the donor, and on the quality of the calculations. The interactions demonstrate distinct directional preferences. In general, the most stable adducts were those in which the donor was oriented opposite to the electron-withdrawing substituent at the silicon. Preparation of single crystals of these adducts proved to be impossible, but new clathrates of tri- ortho -thymotide (TOT) with phenylsilane and 1,3-disilylbenzene have been prepared and their crystal structures determined. In both clathrates the silylbenzene molecules are located in three different types of channels in the crystal structure of TOT. Close intermolecular contacts between silicon atoms of silanes and oxygen atoms of carbonyl groups of TOT were observed in two of the three channels. The geometry of the interactions was found to be consistent with computed structures of weak methylsilane–formaldehyde adducts.

(N,N-Dimethylaminoxy)trifluorosilane, F3SiONMe2 (1), was prepared by the reaction of LiONMe2 with SiF4 in Me2O at -96 degrees C as a colorless, air-sensitive liquid, which was identified by gas-phase IR spectroscopy and NMR spectroscopy of the nuclei H-1, C-13, N-15, O-17,F-19, and Si-29. The gas-phase geometry of 1, as determined by electron diffraction analysis refined in C-s symmetry, is influenced by weak attractive interactions between Si and N: Si ... N 2.273(17) Angstrom, Si-O-N 94.3(9)degrees, [Si-O 1.619(8) Angstrom, N-O 1.479(7) Angstrom, O-Si-Fin-plane 104.1(10)degrees, O-Si-Fout-of-plane 111.8(10)degrees]. X-ray diffraction analysis of 1 reveals that intramolecular Si N interactions are much stronger in the solid state than in the gas phase: Si ... N 1.963(1) Angstrom, Si-O-N 77.1(1)degrees [Si-O 1.639(1) Angstrom, N-O 1.508(1) Angstrom, O-Si-Fin-plane 102.5(1)degrees, O-Si-Fout-of-plane 118.0(1)degrees and 120. 1(1)degrees]. Using measured NMR chemical shifts in C6D6 solution, the geometry of 1 in solution was determined with the NMR/ab initio/DFT-IGLO method to fall between that of the gas-phase geometry and the geometry in the solid state. MP2 and DFT calculations reveal that electrostatic interactions between 1 and the surrounding medium increase with the dielectric constant epsilon since mutual charge polarization enhances the molecular dipole moment from 4 to more than 6 D, which implies a compression of the Si-O-N angle and the Si N distance. Since electrostatic attraction between N and Si supports these changes, the increase in molecular energy upon reduction of the Si ... N distance is small and compensated by the gain of stabilizing intermolecular interactions. The analysis of the calculated electron density distribution shows that the main aspects of bonding in 1 are not changed in the solid state and that the Si ... N attraction is not of covalent nature, but rather due to strong electrostatic and dipole interactions.

Molecules of the type A(OX) 4 , A(NX 2 ) 4 and A(CY 2 X) 4 are not regularly tetrahedral about the central atom A. Two of the angles about this central atom are smaller than tetrahedral and four are larger, or four are smaller and two larger. By considering that the ligand atom (O, N or C) that is bonded to the central atom A has three intramolecular ligand radii and minimising the number of ligand–ligand contact distances, as described in the theory of ligand close packing, we are able to account for the S 4 and D 2d geometries observed for C(OCH 3 ) 4 , C(OPh) 4 and CEE 4 and related molecules. The ligand close packing model also rationalises the C 2 geometries of SO 2 (OR) 2 molecules and the differences in O–C–R angles and C–R bond lengths in R 3 COX molecules. The lengths of these interligand radii can be determined either by calculating the molecular geometry or by deriving them from experimental geometries. The radii depend on the charge of the ligand, the sizes of the Y and Z groups and the angle A–X–Z. The relative sizes of these radii determine the preference for D 2d or S 4 geometries and the degree of distortion of the bond angles from the ideal tetrahedral angle.

Monochlorogallane, synthesized by the metathesis of gallium(III) chloride with an excess of trimethylsilane at ca. 250 K, has been characterized by chemical analysis, by its IR, Raman, and 1H NMR spectra, and by the products of its reaction with trimethylamine. The vibrational spectra of the vapor species isolated in solid Ar, N2, or CH4 matrixes at ca. 12 K imply the presence of only one species, viz. the dimer with an equilibrium structure conforming to D2h symmetry. The structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements augmented by the results of ab initio molecular orbital calculations. An equilibrium structure with D2h symmetry has been assumed in the analysis of the electron diffraction pattern. However, as the molecule has a very low frequency Ga(mu-Cl)2Ga ring-puckering mode, a dynamic model was used to describe it with the aid of a set of pseudoconformers spaced at even intervals (deltadelta = 5 degrees, deltamax, = 20 delta) around the ring-puckering angle delta and Boltzmann-weighted according to a quartic potential V(delta) = V4delta4 + V2delta2. The differences in bond distances and angles between the different pseudoconformers were constrained to the values derived from the ab initio calculations employing second-order Moller-Plesset (MP2) methods (with all the electrons included in the correlation calculations) and a 6-311G(d) basis set. The results for the weighted average of the principal distances (ralpha) and angles (alpha) (with estimated 1sigma uncertainties) from the combined GED/ab initio study are r(Ga-Cl) 2.350(2) A, r(Ga-H) 1.523(20) A,Cl-Ga-Cl 89.7(5)degrees,H-Ga-H 135.1 degrees, V4 3.0 x 10(-6) kcal mol(-1) deg(-4), and V2 6.0 x 10(-4) kcal mol(-1) deg(-2).

The nature of dimethylalane, [Me2AlH]n, in the vapor phase and in hydrocarbon solutions has been reinvestigated experimentally by gas electron diffraction (GED), IR and mass spectrometric, and vapor pressure depression measurements, and also by quantum chemical calculations. The predominant vapor species at low pressures and temperatures near 470 K is the dimer (n = 2), but at temperatures near 330 K the vapor consists of a mixture of the dimer and a smaller proportion (ca. 30%) of the trimer (n = 3). The results of the GED measurements have been analyzed, together with those of ab initio calculations, using the SARACEN method to determine the most reliable structures of the doubly hydrogen-bridged dimer, Me2Al(μ-H)2AlMe2, and singly hydrogen-bridged trimer, Salient structural parameters in the ra structures were thus found to be (a) for the dimer, r(Al−H) 177.6(7), r(Al−C) 195.2(2), r(Al···Al) 262.5(6), r(C−H) 111.4(2) pm; ∠C−Al−C 117.7(4), ∠Al−H−Al 95.3(6), ∠Al−C−H 108.9(7)°; and (b) for the trimer, r(A...

The molecular structures of CH(SiMe2H)3 and CH(SiMe2Br)3 have been determined by gas-phase electron diffraction, ab initio calculations, and, for CH(SiMe2Br)3, X-ray diffraction. In each case, 11 distinct conformations, with energies lying within a range of 8.5 kJ mol−1 for CH(SiMe2H)3 and 26 kJ mol−1 for CH(SiMe2Br)3, were calculated and thus many conformers for both compounds are likely to exist in the gas phase. The structures are compared with those of related alkylsilanes to assess the changes in molecular geometry resulting from crowding at the carbon centre.

The molecular structure of 1,12-dicarba-closo-dodecaborane(12)-1,12-dithiol, 1,12-(SH)2-1,12-C2B10H10, prepared by means of an improved synthesis, has been determined by gas-phase electron diffraction restrained by ab initio calculations. The carbaborane core, shown by calculations at the MP2(fc)/6-31G* level to be very close to D5d symmetry, gave good agreement between theoretical and experimental 11B NMR chemical shifts. A model of the entire molecule in overall C2 symmetry led to an experimental geometry (RG = 0.077), in good agreement with the theoretical findings. The substituents do not distort the cage significantly. The well determined parameters, the C–B, B(2)–B(3), and B(2)–B(7) distances, 170.6(4), 177.5(3) and 176.5(9) pm respectively (ra), are consistent with the analogous parameters established experimentally for other 1,12-disubstituted 1,12-dicarbadodecaboranes. Whereas the C–B and B–B distances are found to be relatively constant in the MP2(fc)/6-31G* geometries of a series of carbaboranes 1,12-X2-1,12-C2B10H10 (in addition to SH, X = H, Li, BeH, BH2, CH3, SiH3, NH2, OH, F and Cl) the C(1)⋯C(12) distances and B(2)–C(1)–C(12) angles are appreciably sensitive to the nature of X, in a similar manner to the para-disubstituted benzene derivatives, 2 × B(2)–C(1)–C(12) being viewed as an analogue of the ipso angle in the latter.

The low-temperature reaction between 2 equiv of (perfluorovinyl)lithium, derived from CF3CH2F and butyllithium, and mercury(II) chloride results in high yields of Hg(CFCF2)2. Complete characterization of the air- and moisture-stable liquid product is afforded by multinuclear (13C, 19F, 199Hg) NMR studies. Crystals of the compound [triclinic, P1; a = 4.956(4), b = 5.733(4), c = 6.394(4) A; α = 104.57(5), β = 109.32(6), γ = 107.16(6)°; Z = 1] were obtained by slow cooling; an X-ray structural determination at 110 K represents the first such report for a (perfluorovinyl)metal complex. The mercury is coordinated linearly [r(Hg−C) = 1.998(5) A], and π-stacking of the perfluorovinyl groups is observed. There is considerable variation in the C−F bond distances [1.286(6), 1.312(6), 1.362(6) A] within each perfluorovinyl group. Structural data for the vapor-phase species were obtained by analysis of the electron-diffraction pattern. There appears to be free rotation of the perfluorovinyl groups around the Hg−C bo...

The molecular structure of 3,3-diethylpentane (tetraethylmethane) in the gas phase has been determined by electron diffraction and ab initio calculations at the MP2/6-31G* level. Five local minima on the potential energy surface were located, but only two have significant populations at room temperature. The experimental distribution of conformers was found to be 66(2)% with D2d symmetry and 34(2)% with S4 symmetry, corresponding to an energy difference ΔH° in favor of the D2d form of 3.3(2) kJ mol-1. The molecule shows significant distortion from regular tetrahedral coordination at the central carbon atom, with two CCC angles in the D2d form reduced to 106.7(8)° and two angles in the S4 form increased to 110.9(4)°. These distortions are attributed to asymmetry of the electron density distribution around the CH2 groups.