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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 CF3and SO2CF3groups 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 CF3group 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 CF3groups on the same side, i.e.denoted ap, ap.These conformers have very different SNS angles, 126.9(9)° and 117.1(17)° respectively. In the crystal all molecules have the ap, spconformation, with parameters similar to those found for this conformer in the gas phase.

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 initioand 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.

Dimethylamine–gallane is relatively slow to decompose in a closed system and vaporises at low temperature primarily as Me2(H)N·GaH3molecules which can be trapped in a solid Ar matrix and characterised by their IR spectrum. Under the conditions needed to secure a useful gas electron diffraction (GED) pattern, however, the vapour was found to consist of dimeric dimethylamidogallane molecules, [Me2NGaH2]2, formed from the secondary amine adduct by elimination of H2, and the most reliable structure for which has been determined. Salient structural parameters (rhlstructure) were found to be: r(Ga–N) 202.6(2), r(Ga–H) 155.6(8), r(N–C) 148.0(3), r(C–H) 111.2(6) pm; Ga–N–Ga 90.7(1), C–N–C 109.3(5), N–C–H 109.9(10) and H–Ga–H 119.4(42)°.

The molecular structures of SeSCH32and TeSCH32were investigated using gas-phase electron diffraction GED and ab initioand DFT geometry optimisations. While parameters involving H atoms were refined using flexible restraints according to the SARACEN method, parameters that depended only on heavy atoms could be refined without restraints. The GED-determined geometric parameters rh1 are: rSe–S 219.11, rS–C 183.21, rC–H 109.64 pm; ∠S–Se–S 102.93, ∠Se–S–C 100.62, ∠S–C–H mean 107.45, S–Se–S–C 87.920, Se–S–C–H 178.819° for SeSCH32, and rTe–S 238.12, rS–C 184.13, rC–H 110.06 pm; ∠S–Te–S 98.96, ∠Te–S–C 99.74, ∠S–C–H mean 109.29, S–Te–S–C 73.048, Te–S–C–H 180.119° for TeSCH32. Ab initioand DFT calculations were performed at the HF, MP2 and B3LYP levels, employing either full-electron basis sets 3-21Gd or 6-31Gd or an effective core potential with a valence basis set LanL2DZd. The best fit to the GED structures was achieved at the MP2 level. Differences between GED and MP2 results for rS–C and ∠S–Te–S were explained by the thermal population of excited vibrational states under the experimental conditions. All theoretical models agreed that each compound exists as two stable conformers, one in which the methyl groups are on the same side gg−conformer and one in which they are on different sides ggconformer of the S–Y–S plane Y Se, Te. The conformational composition under the experimental conditions could not be resolved from the GED data. Despite GED R-factors and ab initioand DFT energies favouring the ggconformer, it is likely that both conformers are present, for SeSCH32as well as for TeSCH32.

The structure of B8F12has been shown by gas electron diffraction and computational methods up to MP26-31G to have the same highly asymmetric form observed in crystalline phases. The structure can be regarded as derived from a central B2group, bridged by two BF2groups to give a central B4core that is folded, not planar, and with a very short bond 164.3 pm calculated, 164.219 pm experimental along the fold line. There are also four terminal BF2groups. 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 BF interactions, particularly those between core boron atoms and fluorines of the terminal BF2groups. 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 BCl2groups 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 B4I6is 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 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 initiomethods 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 226.48 pm; X-ray 223.41 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 eded, which is described in detail. The molecular structure of gaseous P2But4is compared to that of the isoelectronic 1,1,2,2-tetra-tert-butyldisilane. The molecules adopt a conformation with C2symmetry. The P–P–C angles returned from the gas electron diffraction refinement are 118.86 and 98.96°, a difference of 20°, whilst the C–P–C angle is 110.38°. The corresponding parameters in the crystal are 120.91, 99.51 and 109.51°. There are also large deformations within the tert-butyl groups, making the DYNAMITE analysis for this molecule extremely important.

The structure of dimethyl-bismethoxyethynyl germanium has been determined in the gas phase by electron diffraction utilising flexible restraints from quantum chemical calculations. Theoretical methods B3LYP6-311G and MP26-311G predict a low barrier to rotation of the methoxy groups in the molecule in addition to low-frequency vibrations of the long ethynyl chains. In the equilibrium structure the Ge–CC angles of the two methoxyethynyl fragments in the molecule are computed to deviate by up to 4° from the linear arrangement. As a consequence of low-frequency large-amplitude vibrational motion the experimental structure of these fragments without applying vibrational corrections deviates considerably from linearity, while the structure corrected for vibrational effects using the harmonic approximation and taking into account a non-linear transformation between internal and Cartesian coordinates rh1 shows closer agreement with theory. The main experimental structural parameters of dimethyl-bismethoxyethynyl germanium rh1 are: rGe–Cmean, 192.51 pm; ΔGeC rGe–Cmethyl − rGe–Cethynyl, 4.55 pm, rCCmean, 122.82 pm; rC–Omean, 138.93 pm; ΔCO rCmethyl–O − rCethynyl–O, 14.52 pm, rC–Hmean, 109.14 pm; ∠X–C–HmeanX Ge,O, 1091°; ∠Cethynyl–Ge–Cethynyl, 108.14°; ∠Cmethyl–Ge–Cmethyl, 113.45°; ∠Ge–CC, 1631°; ∠CC–O, 1762°; ∠C–O–C, 115.26°; methoxy group torsion, τ, 369° from the position in which the C–O bond eclipses the further Ge–Cethynylbond.

Treatment of PCl[N(SiMe₃)₂](NPri₂) (1) with potassium–graphite in thf afforded the colourless, crystalline diphosphine {P[N(SiMe₃)₂](NPri₂)}₂ (2) in good yield. Sublimation of 2 in vacuo yielded the yellow phosphinyl radical P[N(SiMe₃)₂](NPri₂) (3), which upon cooling reverted to 2; the latter in C₆D₆ at 298 K was a mixture of rac and meso diastereoisomers. The yellow, crystalline phosphenium salt {P[N(SiMe₃)₂](NPri₂)}[AlCl₄] (4) was obtained from 1 and ½ Al₂Cl₆ in CH₂Cl₂. By single-crystal X-ray diffraction (XRD) the structures of the known compound 1 and of 2 and 4 were determined. The structure of the radical 3, formed by the thermal homolytic dissociation of the diphosphine 2, was determined in the gas phase by electron diffraction (GED), utilising data from UMP2/6-31+G* ab initio calculations. The model of the molecule in the GED structure analysis was described by a set of internal coordinates and an initial set of Cartesian coordinates from ab initio calculations, facilitating the structure analysis. The experimental data were found to be consistent with the presence of a single conformer of the radical in the gas phase. The computed standard homolytic dissociation enthalpy of the P–P bond in the corresponding diphosphine 2, corrected for BSSE, 54 kJ mol−1, is substantially reduced compared to the dissociation enthalpy of tetramethyldiphosphine by the reorganisation energies of the fragments that form upon dissociation. The intrinsic energy content of the P–P bond in the diphosphine 2 was estimated to be 286 kJ mol−1, in agreement with the results of previous work on a series of crowded diphosphines.

The molecular structures of 1-bromo–pentaborane(9) and 2-bromo–pentaborane(9) in the gas phase have been determined by electron diffraction and ab initio and DFT computational methods. Computational methods have also been applied to the fluoro and chloro analogues, to 1,2-dibromo-pentaborane(9), and to the parent unsubstituted borane. The electronic effects of halogen substitution on the borane cage are remarkably small, particularly for chlorine and bromine substituents, and steric effects are also minimal, even in the compound with two bromine atoms. The largest effects are (a) lengthening of B_(base)–B_(apex) bonds adjacent to the halogen in the 2-isomers, with an associated shortening of the opposite base–apex bond, (b) shortening of the B_(base)–B_(apex) bond in the 1-fluoro compound, and (c) increase of the B_(base)–B_(apex)–F angle in 1-F–B₅H₈, but a decrease in this angle in the 2-bromo compounds.

Treatment of PClNSiMe32NPri21 with potassium–graphite in thf afforded the colourless, crystalline diphosphine PNSiMe32NPri222 in good yield. Sublimation of 2in vacuoyielded the yellow phosphinyl radical PNSiMe32NPri23, which upon cooling reverted to 2; the latter in C6D6at 298 K was a mixture of racand mesodiastereoisomers. The yellow, crystalline phosphenium salt PNSiMe32NPri2AlCl44 was obtained from 1and ½Al2Cl6in CH2Cl2. By single-crystal X-ray diffraction XRD the structures of the known compound 1and of 2and 4were determined. The structure of the radical 3, formed by the thermal homolytic dissociation of the diphosphine 2, was determined in the gas phase by electron diffraction GED, utilising data from UMP26-31G ab initiocalculations. The model of the molecule in the GED structure analysis was described by a set of internal coordinates and an initial set of Cartesian coordinates from ab initiocalculations, facilitating the structure analysis. The experimental data were found to be consistent with the presence of a single conformer of the radical in the gas phase. The computed standard homolytic dissociation enthalpy of the P–P bond in the corresponding diphosphine 2, corrected for BSSE, 54 kJ mol−1, is substantially reduced compared to the dissociation enthalpy of tetramethyldiphosphine by the reorganisation energies of the fragments that form upon dissociation. The intrinsic energy content of the P–P bond in the diphosphine 2was estimated to be 286 kJ mol−1, in agreement with the results of previous work on a series of crowded diphosphines.

Combined gas electron diffractionmass spectrometry GEDMS was used to determine the molecular structure of trisdipivaloylmethanatolutetiumiii, Ludpm3dpm 2,2,6,6-tetramethyl-heptane-3,5-dionato. Up to about 520–570 K the vapour consisted only of molecules Ludpm3. The experimental data recorded at 4085 K indicate that the molecules have D3symmetry. The bond distances rh1 in the chelate ring are Lu–O 2.1976 , C–O 1.2704 and C–C 1.3906 . 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 LuO6antiprism.

The structure of the gaseous hafnium tetrakistetrahydroborate molecule, HfBH44, has been investigated by detailed quantum mechanical calculations and by analysis of its gas electron-diffraction GED pattern. The ground-state geometry possesses Tsymmetry with all of the triply-bridged BH4groups twisted equally about the HfB–H axes. Salient structural parameters radistances, rαangles deduced from the GED pattern by the SARACEN method were: rHfB 231.42, rHf–Hb 221.57, rB–Hb 127.65, rB–Ht 1211 pm, HfB–Hb69.43, Hb–B–Hb108.44, Hb–B–Ht110.63, BHfB–Hb1661°. A notable feature is the large magnitude of the HfB and Hf–Hbanharmonicity parameters, attributed to the fluxional hydrogen atom exchange process. The properties are compared with those of related tetrahydroborates.

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 OPBut3, rather than O−–PBut3.

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&z.dbd;PBut₃, rather than O−–P+But₃.

The structure of the gallane adduct Me3P·GaH3in 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 rh1structure were found to be: rGa–H 159.011, rGa–P 244.36, rP–C 184.02, rC–H 108.37 pm; H–Ga–P 98.412 and Ga–P–C 117.73°. 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 C3PGaH3skeleton. 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.

Structures of molecules in the gas phase, determined experimentally, provide definitive information about their identity, reactivity and other properties, free from intermolecular interactions. Available methods have not been applicable to large and asymmetric molecules. Now the SARACEN Structure Analysis Restrained by Ab initioCalculations for Electron diffractioN method, using data from computational methods to complement experimental data, has opened the door to full structure determination for all sufficiently volatile molecules.

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 (r_h1 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.

Structures of molecules in the gas phase, determined experimentally, provide definitive information about their identity, reactivity and other properties, free from intermolecular interactions. Available methods have not been applicable to large and asymmetric molecules. Now the SARACEN (Structure Analysis Restrained by Ab initio Calculations for Electron diffractioN) method, using data from computational methods to complement experimental data, has opened the door to full structure determination for all sufficiently volatile molecules.