Gas electron diffraction then and now: from trisilyl phosphine to iso-propyl(-butyl)(trichlorosilyl)phosphine.

The gas-phase molecular structure of iso-propyl(-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(-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.

A computational analysis of the apparent nido vs. hypho conflict: are we dealing with six- or eight-vertex open-face diheteroboranes?

A series of computational studies have been undertaken to investigate the electronic structures and bonding schemes for six hetero-substituted borane cages, all of which have been presented in the literature as potential hypho structures. The six species are hypho-7,8-[C2B6H13](-) (1a), hypho-7,8-[CSB6H11](-) (1b), hypho-7,8-[S2B6H9](-) (1c), hypho-7,8-[NSB6H11] (1d), exo-7-Me-hypho-7,8-[NCB6H12] (1e), and endo-7-Me-hypho-7,8-[NCB6H12] (1f) and the so-called mno rule has been applied to each of them. As no structural data are known for the carbathia-, azathia-, and dithiahexaboranes, we have also applied the ab initio/GIAO/NMR structural tool for 1b-1d, with 1c having been prepared for this purpose.

Equilibrium gas-phase structures of sodium fluoride, bromide, and iodide monomers and dimers.

The alkali halides sodium fluoride, sodium bromide, and sodium iodide exist in the gas phase as both monomer and dimer species. A reanalysis of gas electron diffraction (GED) data collected earlier has been undertaken for each of these molecules using the EXPRESS method to yield experimental equilibrium structures. EXPRESS allows amplitudes of vibration to be estimated and correction terms to be applied to each pair of atoms in the refinement model.

Dimethylalkoxygallanes: monomeric versus dimeric gas-phase structures.

The molecular structures of the vapors produced on heating dimethylalkoxygallanes of the type [Me(2)Ga(OR)](2) have been determined by gas electron diffraction and ab initio molecular orbital calculations. In the solid state [Me(2)Ga(OCH(2)CH(2)NMe(2))](2) (1) and [Me(2)Ga(OCH(2)CH(2)OMe)](2) (2) adopt dimeric structures, although only the monomeric forms [Me(2)Ga(OCH(2)CH(2)NMe(2))] (1a) and [Me(2)Ga(OCH(2)CH(2)OMe)] (2a) were observed in the gas phase. For comparison the structure of the vapor produced on heating [Me(2)Ga(O(t)Bu)](2) (3) was also studied by gas electron diffraction.

Using molecular-dynamics simulations to understand and improve the treatment of anharmonic vibrations. II. Developing and assessing new Debye-Waller factors.

Two new anharmonic forms for the Debye-Waller factor, aimed at modelling curvilinear and asymmetric motion, have been introduced. These forms permit the refinement of structures with these types of anharmonic motion using a small number of additional parameters. Molecular-dynamics-derived numerical probability density functions (PDFs) have been used to assess the merit of these new functions in real space.

Using molecular-dynamics simulations to understand and improve the treatment of anharmonic vibrations. I. Study of positional parameters.

Molecular-dynamics-derived numerical probability density functions (PDFs) have been used to illustrate the effect of different models for thermal motion on the parameters refined in a crystal structure determination. Specifically, anharmonic curved or asymmetric PDFs have been modelled using the traditional harmonic approximation and the anharmonic Gram-Charlier series treatment. The results show that in cases of extreme anharmonicity the mean and covariance matrix of the harmonic treatment can deviate significantly from physically meaningful values.

Why is the antipodal effect in closo-1-SB9H9 so large? A possible explanation based on the geometry from the concerted use of gas electron diffraction and computational methods.

The molecular structure of 1-thia-closo-decaborane(9), 1-SB(9)H(9), has been determined by the concerted use of gas electron diffraction and quantum-chemical calculations. Assuming C(4v) symmetry, the cage structure was distorted from a symmetrically bicapped square antiprism (D(4d) symmetry) mainly through substantial expansion of the tetragonal belt of boron atoms adjacent to sulfur. The S-B and (B-B)(mean) distances are well determined with r(h1) = 193.

Multiple bonding versus cage formation in organophosphorus compounds: the gas-phase structures of tricyclo-P3(CBu(t))2Cl and P≡C-Bu(t) determined by electron diffraction and computational methods.

The gas-phase structures of tricyclo-P(3)(CBu(t))(2)Cl and P≡C-Bu(t) have been determined by electron diffraction and associated quantum chemical calculations. Efforts to obtain detailed solid-state data for tricyclo-P(3)(CBu(t))(2)Cl have been thwarted by inability to prepare suitable crystalline material. Additional calculations for another tricyclic isomer of P(3)(CBu(t))(2)Cl and for two phosphorus-containing cyclopentadiene derivatives with pseudo-planar five-membered rings show that the experimentally observed isomer is more stable by at least 52 kJ mol(-1).

The gas-phase equilibrium structures of Si8O12(OSiMe3)8 and Si8O12(CHCH2)8.

The equilibrium molecular structure of Si(8)O(12)(OSiMe(3))(8) has been determined in the gas phase by electron diffraction (GED). With OSi-containing substituents on the cage silicon atoms, this molecule contains a moiety, which would, if reproduced in a periodic manner, yield a zeolite-type structure. Extensive ab initio calculations were used to identify two conformers of this molecule, with D(4) and D(2) point-group symmetries; the D(4)-symmetric conformer was approximately 1.

Low symmetry in molecules with heavy peripheral atoms. The gas-phase structure of perfluoro(methylcyclohexane), C6F11CF3.

When refining structures using gas electron diffraction (GED) data, assumptions are often made in order to reduce the number of required geometrical parameters. Where these relate to light, peripheral atoms there is little effect on the refined heavy-atom structure, which is well defined by the GED data. However, this is not the case when heavier atoms are involved.

The gas-phase structure and some reactions of the bulky primary silane (Me(3)Si)(3)CSiH(3) and the solid-state structure of the bulky dialkyl disilane [(Me(3)Si)(3)CSiH(2)](2).

The molecular structure of the bulky primary silane, (Me(3)Si)(3)CSiH(3), in the gas phase has been determined by electron diffraction. Photolysis of (Me(3)Si)(3)CSiH(3) affords a convenient route to the bulky dialkyl disilane, [(Me(3)Si)(3)CSiH(2)](2), which is the first 1,2-dialkyldisilane to be structurally characterised by single-crystal X-ray diffraction. The disilane has an unusually large Si-Si-C angle of 120.

Does 2-methylacetophenone comply with steric inhibition of resonance? A direct experimental proof of its nonplanar conformation from a joint ab initio/electron diffraction analysis.

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.

Simulating thermal motion in crystalline phase-I ammonia.

Path-integral molecular dynamics have been used to simulate the phase-I crystalline form of ammonia, using an empirical force field. This method allows quantum-mechanical effects on the average geometry and vibrational quantities to be evaluated. When these are used to adjust the output of a high-temperature density functional theory simulation, the results are consistent with those given by the most recent structural refinement based on powder neutron diffraction data.

Determination of the experimental equilibrium structure of solid nitromethane using path-integral molecular dynamics simulations.

Path-integral molecular dynamics (PIMD) simulations with an empirical interaction potential have been used to determine the experimental equilibrium structure of solid nitromethane at 4.2 and 15 K. By comparing the time-averaged molecular structure determined in a PIMD simulation to the calculated minimum-energy (zero-temperature) molecular structure, we have derived structural corrections that describe the effects of thermal motion.

A conformational and vibrational study of CF(3)COSCH(2)CH(3).

The molecular structure and conformational properties of S-ethyl trifluorothioacetate, CF(3)COSCH(2)CH(3), were determined in the gas phase by electron diffraction and vibrational spectroscopy (IR and Raman). The experimental investigations were supplemented by ab initio (Moller Plesset of second order) and density functional theory quantum chemical calculations at different levels of theory. Both experimental and theoretical methods reveal two structures with C(s) (anti, anti) and C(1) (anti, gauche) symmetries, although there are disagreements about which is more stable.

The gas-phase structure of the decasilsesquioxane Si(10)O(15)H(10).

The equilibrium molecular structure of the decasilsesquioxane, Si(10)O(15)H(10), in the gas phase has been determined by gas electron diffraction. Molecular dynamics calculations were used to give amplitudes of vibration and differences between interatomic distances in the equilibrium structure and the vibrationally averaged distances that are given directly by the diffraction data. The molecules have D(5h) symmetry, and do not show the distortions that are apparent in the crystalline phase.

Primary phosphines studied by gas-phase electron diffraction and quantum chemical calculations. Are they different from amines?

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.

Experimental equilibrium structures: application of molecular dynamics simulations to vibrational corrections for gas electron diffraction.

A general method is described that allows experimental equilibrium structures to be determined from gas electron diffraction (GED) data. Distance corrections, starting values for amplitudes of vibration and anharmonic "Morse" constants (all required for a GED refinement) have been extracted from molecular dynamics (MD) simulations. For this purpose MD methods have significant advantages over traditional force-field methods, as they can more easily be performed for large molecules, and, as they do not rely on extrapolation from equilibrium geometries, they are highly suitable for molecules with large-amplitude and anharmonic modes of vibration.

Unusual chalcogen-boron ring compounds: the gas-phase structures of 1,4-B4S2(NMe2)4 and related molecules.

The structure of 1,4-B4S2(NMe2)4 has been determined by gas-phase electron diffraction and quantum chemical calculations and is compared with the known solid-state structure. While these structures are similar, with a twisted ring geometry [the dihedral angle S-B-B-S from electron diffraction is 75.4(16) degrees], they are strikingly different to the solid-state structure of 1,4-B4O2(OH)4, which is planar.

Stereochemistry of free boranes and heteroboranes from electron scattering and model chemistries.

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 "B chemical shifts (MOCED, SARACEN) have also been employed.

Why are trimethylsilyl groups asymmetrically coordinated? Gas-phase molecular structures of 1-trimethylsilyl-1,2,3-benzotriazole and 2-trimethylsilyl-1,3-thiazole.

The structures of 1-trimethylsilyl-1,2,3-benzotriazole and 2-trimethylsilyl-1,3-thiazole have been determined by gas electron diffraction and computational methods. While 1-trimethylsilyl-1,2,3-benzotriazole shows a significant asymmetry in the way the SiMe(3) groups bonds to the ring system, the same is not true for 2-trimethylsilyl-1,3-thiazole. However, it has been shown that when the positions of formal single and double bonds in the rings systems are considered, the silyl groups in both compounds are displaced towards the neighbouring ring nitrogen atom.

Combined experimental studies and theoretical calculations to yield the complete molecular structure and vibrational spectra of (CH3)3GeH.

The molecular structure of trimethylgermane has been determined by gas electron diffraction experiments. Infrared spectra for the gaseous, liquid, and solid phases were also recorded. Parallel and perpendicular polarized Raman spectra for the liquid were measured to obtain depolarization values.

What makes the huge 31P-31P coupling constants in S(PF2)2 and Se(PF2)2 vary so much with temperature?

The enormous temperature dependence of the (2)J(PP) coupling constants in S(PF(2))(2) and Se(PF(2))(2) has been explained by a theoretical investigation of their conformations and NMR coupling constants. In contrast, the coupling in O(PF(2))(2) is almost invariant. Gas electron diffraction data for S(PF(2))(2) have been reinterpreted.

Structures and aggregation of the methylamine-borane molecules, Me(n)H(3-n)N.BH(3) (n = 1-3), studied by X-ray diffraction, gas-phase electron diffraction, and quantum chemical calculations.

The structures of the molecules methylamine-borane, MeH(2)N.BH(3), and dimethylamine-borane, Me(2)HN.BH(3), have been investigated by gas-phase electron diffraction (GED) and quantum chemical calculations.

Structural tungsten-imido chemistry: the gas-phase structure of W(NBut)2(NHBut)2 and the solid-state structures of novel heterobimetallic W/N/M (M = Rh, Pd, Zn) species.

The gas-phase (electron diffraction) and solid-state (X-ray) structures of W(NBut)2(NHBut)2 (1) have been determined. In the gas phase, 1 adopts both C1 and C2 conformations in a 69:31 ratio. The solid-state structure is disordered over two equal sites, both showing approximate C2 conformation as in the gas phase; the imido and amido centers are, however, clearly distinguished.