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The molecular structure of 1,2-di-tert-butyldisilane has been accurately determined by gas-phase electron diffraction (GED) and ab initio calculations. These techniques show that the large majority of molecules at room temperature have the anti conformation with overall symmetry C2, and vibrational spectra confirm this conclusion. Infrared spectra of the gas and liquid phases, and Raman spectra of the liquid and solid phases, have been recorded for (CH3)3CSiH2SiH2C(CH3)3 and (CH3)3CSiD2SiD2C(CH3)3. The most striking feature of this structure (ra) is a relatively large deviation of the SiSiC angle from the parent tetrahedral angle 109.5° (113.7(3)°, GED; 114.4°, SCF 6-31 G ∗ as calculated for the anti form). That the SiSi bond length does not show any substantial deviation from its usual value (234.8(3) pm, GED; 236.8 pm, SCF 6-31 G ∗ computed for the anti form) is also substantiated by the value of the SiSi valence force constant (169 N m−1) given by normal coordinate analysis. The t-butyl groups are tilted so that the SiC bonds (GED ( SCF 6-31 G ∗ ): 190.1(1) (191.9) pm) do not coincide with the local C3 axes of the C(CH3)3 groups in which the CC bond length is 154.1(1) (GED); 154.0 ( SCF 6-31 G ∗ ) pm. The conformations along all the single bonds are more or less staggered.

The structure of gaseous 1-(dichloroboryl)pentaborane(9), 1-(Cl2B)B5H8, has been determined by electron diffraction. The results confirm that the molecule consists of a pentaborane(9) cage substituted at the apical boron atom, B(1), by a dichloroboryl group; the BCl2 moiety is essentially free to rotate about the exo B–B bond. Salient experimental structural parameters (ra) are r(B–B)(base–base) 182.1(14), r(B–B)(base-apex) 173.9(26), r(B–B)(apex–exo) 170.1(18), r(B–Cl) 172.0(15), r(B–Ht) 126.2(22), and r(B–Hb) 134.6(16) pm (Ht= terminal hydrogen, Hb= bridging hydrogen); Cl–B–Cl 121.7(28)°, B–Ht‘rise’(above basal-boron plane) 15.1(26)°, and B–Hb‘dip’(below basal-boron plane) 62.0(39)°. These conclusions are supported by ab initio(MP2/6–31G* or MP2/DZP level) optimisations of the molecular geometry, and by comparison of the calculated 11B NMR chemical shifts {individual gauge for localised orbitals (double zeta)//gas-phase electron diffraction level [IGLO(DZ)//GED level]} with the experimental NMR data.

The structure of (E)-3,4-dibromotetrahydrothiophene 1, 1-dioxide, C4H6SO2Br2, has been determined at 150 K by X-ray crystallography and in the gas phase by electron diffraction constrained using bond length and angle differences calculated ab initio. The results show that in both phases the ring adopts the half twist conformation with C2 symmetry. However, in the solid phase the bromine atoms occupy equatorial positions, whereas in the gas phase the optimum fit is for a mixture of axial and equatorial conformers. These are present in effectively equal amounts [proportion of axial conformer 47.2(23)%], as also predicted by ab initio calculations at the MP2/6-31 G* level. Salient mean structural parameters (r/pm, angles/degrees) for the gas phase (ra) and the solid phase, respectively, are: r(SO)142.5(1), 143.8(7); r(S–C) 179.0(3), 179.7(18); r(C–C) 152.9(4), 153.6(10); r(C–Br) 194.8(3), 194.7(10); 050117.9(10), 117.7(8); CSC 92.8(7), 96.4(6); CCCC 55.9(10), 57.7(15); BrCCBr (axial)–167.9(18) and (equatorial)–70.8(11), –64.3(20). These values are in good agreement with those obtained in an ab initio(MP2/6-31G* level) study of the molecular geometry.

The molecular structures of tert -butyldimethylsilanol and its hemihydrate in the gas phase have been determined by electron diffraction. The two diffraction patterns are indistinguishable, indicating that the hydrate is essentially completely dissociated in the vapour phase under the conditions of the experiment. The most significant geometrical parameters for Bu t Me 2 SiOH ( r a ) are: r (SiC) (mean) 188.4(1) pm, r (SiO) 166.2(3) pm, r (CC) 154.6(2) pm, ∠SiCC 107.8(2)°, ∠C Bu SiO 106.8(17)°, ∠C Bu SiC Me 11.23(10)° and ∠C Me SiO 109.2(10)°. The molecule adopts an almost perfectly staggered conformation about the SiC(butyl) bond.

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. In contrast to 1 and 2, compound 3 is dimeric in the gas phase, as well as in the solid state. The gas-phase structures of 1a and 2a exhibit five-membered rings formed by a dative bond between Ga and the donor atom (N or O) from the donor-functionalized alkoxide. In 3 there is no possibility of a monomeric structure being stabilized by the formation of such a dative bond since only a monofunctional alkoxide is present in the molecule.

The structure of gaseous 1-(difluoroboryl)pentaborane(9), 1-(F 2 B)B 5 H 8 , has been determined by electron diffraction. The results confirm that the molecule consists of a pentaborane(9) cage substituted at the apical boron atom, B(1), by a difluoroboryl group; the BF 2 moiety is free to rotate about the exo B-B bond. Salient structural parameters (r a ) are r(B-B)(base-base)=181.2(6), r(B-B)(base-apex)=170.6(4), r(B-B)(apex-exo)=167.6(7), r(B-F)= 132.2(3), r(B-H t )=1 19.5(13), and r(B-Hb)=138.9(11) pm; FBF=1 15.4(6) o , B-H t «rise» (above basal plane) =1.9(33) o , and B-Hb «dip» (below basal plane)=67.7(29) o

The structure of gaseous 2,4-ethanotetraborane(10), B 4 H 8 (CH 2 ) 2 , more commonly referred to as dimethylene-tetraborane, has been determined by electron diffraction and ab initio computations. The results confirm that the molecule consists of a tetraborane(10) cage substituted at the «wing» boron atoms, B(2) and B(4), by a bridging «ethene», C 2 H 4 , moiety. The molecular symmetry is C 2ν , the conformation about the C-C bond being eclipsed

The electron-diffraction patterns of gaseous arachno -B 5 H 11 and arachno -B 6 H 12 have been reanalysed. Inclusion of ab initio (MP2/6-31G*) computed bond-length differences in the refinements afforded new optimum geometries with improved R factors ( R G = 0.053 and 0.057, respectively) compared with the structures resported previously. In contrast to the latter, the new geometries cna be employed to calculate 11 B NMR chemical shifts (DZ//GED level) which are in good agreement with the experimental NMR data.

The molecular structure and conformation of dichloromethylvinylsilane in the gas phase have been determined by electtron diffraction. A force-field, which reproduced experimental vibrational frequencies, was used to calculate mean-square amplitudes of vibration and shrinkage corrections, so that the r α structure could be determined. At room temperature, 67(19)% of the molecules were in a gauche conformation in which the CCSiC dihedral angle was 104(5)°, while the remaining molecules adopted a conformation in which this angle was 0°. The principal refined parameters were r (CC) 131.9(7) pm, r (SiCl) 204.8(2) pm, ∠CSiCl 108.4(4)° and ∠CSiC 115.6(13)°. The SiC(methyl) and SiC(vinyl) distances were refined as a single parameter, with the difference between them being fixed at 2.3 pm. The SiC(methyl) distance was then 186.0(3) pm. The SiCC angle refined to 124.3(13)° in the gauche conformer, with the angle in the other conformer fixed at 1.2° larger than this.

Gas-phase electron diffraction (GED) data for 1,7-dichloro-1,7-dicarba-closo-dodecaborane(12). 1,7-Cl2-1,7-C2B10H10, were fitted by a structure possessing overall C2v, symmetry. Not all of the independent parameters could be refined, and some differences between C–B and B–B bond lengths were fixed at values calculated ab initio.Assumption of local C5v symmetry for the CB5 pentagonal pyramids led to the same GED fit (RG= 0.065) as with a model in which the B5 rings adjacent to the carbon atoms were not quite regular, as found by ab initio optimisations [HF/6-31G* and MP2(fc)/6-31G*]. Nearest-neighbour C–B and B–B separations do not deviate significantly from normal values, and distortion of the cage from regular icosahedral symmetry is small. The C–Cl bond length, 175.9(9) pm, [MP2/6-31G* 174.9 pm], is shorter than in C(sp3)–Cl systems and longer than in C(sp2)–Cl systems. The geometrical parameters calculated at the MP2(fc)/6-31G* level, as well as 11B NMR chemical shifts, calculated by the individual gauge for localised orbitals method, are in satisfactory agreement with the experimental observations.

The structure of phenylsulfanylacetylene, PhSCCH, has been determined at 150 K by X-ray crystallography and in the gas phase by electron diffraction. The results indicate that although the dihedral angle between the planes of the benzene ring and the sulfanylacetylene group is ca. 10° in the solid phase, in the gas phase there is effectively free rotation about the C(ring)–S bond. Salient structural parameters are (a) for the solid phase, r[S(1)–C(2)] 168.3(2), r[S(1)–C(5)] 177.8(2), r[C(2)–C(3)] 119.2(3) and r(C–C)ring(mean) 138.8(1) pm; C(2)S(1)C(5) 102.7(1)° and C(6)C(5)S(1)C(2) 9.6(2)°; and (b) gas phase (ra), r[S(1 )–C(2)] 169.6(5), r[S(1)–C(5)] 177.3(4), r[C(2)–C(3)] 119.5(6) and r(C–C)ring(mean) 139.9(1) pm; C(2)S(1)C(5) 103.8(6)°. These values agree well with those obtained in an ab initio(HF/6-31G* and MP2/6-31G* level) study of the molecular geometry.

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. The comparison is favourable particularly for the curvilinear PDF based on a parabolic coordinate system change of a trivariate Gaussian distribution. The initial results also suggest that high-order even terms from the Gram-Charlier series may be important for modelling methyl-group libration. The molecular-dynamics data sets provide useful insights into the nature of anharmonic thermal motion. Addressing the problem in real space allows intuitive PDFs to be developed but numerical methods may be necessary for these methods to be implemented in refinement programs as an analytical Debye-Waller factor cannot always be obtained.

The molecular structures of 1,2-closo-P(2)B(10)H(10) (1) and 1,2-closo-As(2)B(10)H(10) (2) have been determined by gas electron diffraction and the results obtained compared with those from computation at the MP2/6-31G** level of theory. The level of agreement is good for 2 (root-mean-square [rms] misfit for As and B atoms 0.0297 Å) and very good for 1 (rms misfit for P and B atoms 0.0082 Å). In comparing the structures of 1 and 2 with that of 1,2-closo-C(2)B(10)H(12) (I) it is evident that expansion of the polyhedron from I to 1 to 2 is restricted only to the heteroatom vertices and the B(6) face to which these are bound. Following deboronation (at B3) and subsequent metallation, compounds 1 and 2 have been converted into the new metalladiheteroboranes 3-(η-C(9)H(7))-3,1,2-closo-CoAs(2)B(9)H(9) (4), 3-(η-C(10)H(14))-3,1,2-closo-RuAs(2)B(9)H(9) (5), 3-(η-C(5)H(5))-3,1,2-closo-CoP(2)B(9)H(9) (6), 3-(η-C(9)H(7))-3,1,2-closo-CoP(2)B(9)H(9) (7) and 3-(η-C(10)H(14))-3,1,2-closo-RuP(2)B(9)H(9) (8), the last three constituting the first examples of metalladiphosphaboranes. Together with the known compound 3-(η-C(5)H(5))-3,1,2-closo-CoAs(2)B(9)H(9) (3), compounds 4-8 have been analysed by NMR spectroscopy and (except for 8) single-crystal X-ray diffraction. The (11)B NMR spectra of analogous pairs of metalladiphosphaborane and metalladiarsaborane (6 and 3, 7 and 4, 8 and 5) reveal a consistently narrower (9-10 ppm) chemical shift range for the metalladiarsaboranes, the combined result of a deshielding of the lowest frequency resonance (B6) and an increased shielding of the highest frequency resonance (B8) via an antipodal effect. In crystallographic studies, compounds 3 and 5B (one of two crystallographically-independent molecules) suffer As/B disorder, but in both cases it was possible to refine distinct, ordered, components of the disorder, the first time this has been reported for metalladiarsaboranes. Moreover, whilst the Cp compounds 6 and 3 are disordered, their indenyl analogues 7 and 4 are either ordered or significantly less disordered, a consequence of both the reduced symmetry of an indenyl ligand compared to a Cp ligand and the preference of the former for a distinct conformation relative to the cage heteroatoms. Unexpectedly, whilst this conformation in the cobaltadiphosphaborane 7 is cis-staggered (similar to that previously established for the analogous cobaltadicarborane), in the cobaltadiarsaborane 4 the conformation is close to cis-eclipsed.

The gas-phase structures of tricyclo-P3(CBut)2Cl and PCBut have been determined by electron diffraction and associated quantum chemical calculations. Efforts to obtain detailed solid-state data for tricyclo-P3(CBut)2Cl have been thwarted by inability to prepare suitable crystalline material. Additional calculations for another tricyclic isomer of P3(CBut)2Cl 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. Calculations for the equivalent structures with P atoms replaced by CH fragments have demonstrated that a ring structure is more favourable by over 200 kJ mol-1 compared to each of two cage structures.

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 1.2 kJ mol(-1) lower in energy. With 132 atoms in each conformer, this is one of the largest studies to be undertaken using gas electron diffraction. Semiempirical molecular-dynamics (SE-MD) calculations were used to give amplitudes of vibration, vibrational distance corrections (differences between interatomic distances in the equilibrium structure and the vibrationally averaged distances that are given directly by the diffraction data), and anharmonic constants. The structure of Si(8)O(12)(CHCH(2))(8) has also been determined by GED. Calculations showed that the vinyl groups are fairly unhindered and rotate between three minimum-energy positions. Ultimately, all possible combinations of the vinyl groups in these low-energy positions were accounted for in the GED model.

The molecular structure of [Rh(η5-C5H5)(C2H4)2] has been determined in the gas phase by electron diffraction at 473 K. The molecule has C2v local symmetry for the Rh(C2H4)2 fragment and C5v for the Rh(C5H5) fragment, with the local rotation axes coincident. The distance from the rhodium atom to the center of the cyclopentadienyl ring is 190.7(3) pm, and that from the rhodium atom to the centres of the C–C bonds of the ethene ligands is 198.0(3) pm. The C–C bond distances in the cyclopentadienyl ligands refined to 143.2(2) pm, as expected for such five-co-ordinated rings, but the C–C bonds of the ethene ligands. [145.7(7) pm] are longer than those previously reported for rhodium-ethene compounds. The angle between the lines linking the midpoints of the CC bonds of the ethene ligands to the rhodium atom is not uniquely determined by the electron diffraction data, and models with values of 91.2(13) and 111.8(12)° both fit well. The former value is preferred, as it gives a lower R factor, and is consistent with corresponding angles reported for bis(ethene) complexes in th e crystalline phase. The two ethene ligands are tilted about their CC axes away from one another by 11.7(18)°.

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. We have determined the gas-phase structure of perfluoro(methylcyclohexane), C(6)F(11)CF(3), using three different refinement methods and have shown that our new method, which makes use of both MP2 and molecular mechanics (MM) calculations to restrain the peripheral-atom geometry, gives a realistic structure without the need for damaging constraints. Only the conformer with the CF(3) group in an equatorial position was considered, as ab initio calculations showed this to be 25 kJ mol(-1) lower in energy than the axial conformer. Refinements combining both high-level and low-level calculations to give constraints were superior both to those based only on molecular mechanics and to those in which assumptions about the geometry were imposed.

The molecular structure of 1,2,3-trichlorobenzene (A) has been determined by using data from gas electron diffraction experiments and direct dipolar couplings from 1H NMR experiments in a joint structural analysis. The data are consistent with an assumed molecular model of C2v symmetry and all 11 structural parameters could be refined simultaneously, with standard deviations between 0.3 and 0.7 pm for bond lengths and less than 0.44° for valence angles. The crystal and molecular structures of A have also separately been determined by X-ray crystallography. The mean structural parameters in the solid state are almost equal to those obtained from the joint analysis.

The molecular structure of tris(trimethylsilyl)phosphine in the gas phase has been determined by electron diffraction. Results of three refinements are reported. In the first refinement, overall C 3 symmetry and local C 3 symmetry for the SiMe 3 groups were assumed, but these groups were allowed to tilt away from one another so that their C 3 local axes no longer coincided with the PSi bonds. The major geometrical parameters were r a (PSi) 225.9 (1), r (SiC) 188.2(1) pm, ∠ SiPSi 105.1(2) and ∠ CSiC 107.9(1)°. The tilt angle was 5.9(3)°, and the SiMe 3 groups were twisted 17.2(2)° away from the staggered position, at which the overall molecular symmetry was C 3v ; the PSiC angles were therefore 106.8, 109.3 and 116.7°. In the other two refinements the restriction of C 3 symmetry within the SiMe 3 groups was removed, and the differences between SiC distances and between CSiC angles were first fixed at values calculated by molecular mechanics (MM2), and then these new restrictions were partially relaxed. Neither of these refinements fitted the experimental data quite as well as the first one.

As a precursor for trisilylmethane, tris(phenylsilyl)methane is prepared by a Merker-Scott reaction of chlorophenylsilane, bromoform, and magnesium turnings in boiling tetrahydrofuran. Chlorophenylsilane is formed in a new synthesis starting from phenylsilane and hydrogen choride/AlCl3 in diethyl ether. The gas phase structure of trisilylmethane (H3Si)3CH, obtained from (PhSiH2)3CH via (BrSiH2)3CH, has been determined by electron diffraction. Data refinement confirmed a model of C3 molecular symmetry, with local C3v symmetry for the silyl groups. — As a precursor for 1,1,1-trisilylethane, 1,1,1-tris(phenylsilyl)ethane has been prepared similarly from chlorophenylsilane, 1,1,1-trichloroethane and magnesium and converted via 1,1,1-tris(bromosilyl)ethane into CH3C(SiH3)3.

Simultaneous analysis of gas-phase electron diffraction data and dipolar coupling constants reported for solutions of 1,3,5-trichlorobenzene in several liquid crystal solvents has yielded an r 0 α structure in which all parameters have great precision. Bond lengths are 139.39(10), 172.78(12) and 108.07(21) pm for r (CC), r (CCl) and r (CH) respectively, and the CCC angles at chlorine-substituted carbon atoms are 122.36(18)°.

The elusive binary hydride of gallium, [GaH 3 ] n , has at last been synthesized in rigorously conditioned all-glass apparatus by the reaction between monochlorogallane, [H 2 GaCl] 2 , and lithium tetrahydridogallate, LiGaH 4 , near −30°C. The compound, which decomposes to the elements at ambient temperatures, has been characterized by chemical analysis, by its vibrational and 1 H NMR spectra, and by chemical trapping with trimethylamine

The molecular structure of dimethyl(methylthio)phosphine, (CH 3 ) 2 PSCH 3 , has been determined from a combined gas-phase electron diffraction study at 20°C and ab initio calculations utilizing both the 3–21G * and 6–31G * basis sets. Two distinct conformers, trans and cis or near-cis (the cis conformer has the thiomethyl group eclipsing the non-bonded electron pair on the phosphorous atom and the trans conformer has this methyl group staggered with respect to the two methyl groups attached to the phosphorous atom) were identified with the isomeric composition of 78±5% of the cis form. The differences between the geometrical parameters were fixed at the values obtained from the ab initio calculations. The determined heavy atom structural parameters for the cis conformer are r (PS)=2.111±0.002, r (PC)=1.862±0.002, r (SC)=1.769±0.006, ∠ PSC=102.3±2.6, ∠ CPC=96.7±0.8 and ∠ CPS=96.8±1.5 where the distances are in A and the angles in degrees. The parameters which differ for the trans conformer are r (PS) =2.103±0.002, ǒ PSC=108.5±2.5 and ∠ CPS=99.9±1.5. The rms amplitudes of vibration with their associated uncertainties have been determined for some of the distances. In addition, the geometry has been calculated by ab initio Hartree-Fock gradient calculations with geometry optimization at the 3-21G * and 6-31G * levels, and the computed structural parameters are in reasonable agreement with those obtained from the electron diffraction data. Features of the structure are compared with those of similar molecules.

International audience; The molecularstructures of vinylarsine (CH2double bond; length as m-dashCHAsH2), vinyldichloroarsine (CH2double bond; length as m-dashCHAsCl2) and arsine (AsH3) have been determined from gas-phaseelectrondiffraction data and, in the case of vinylarsine, rotation constants, employing the SARACEN method. The structure of vinylarsine represents the first complete gas-phasestructure of a primary arsine. The experimental geometric parameters generally show good agreement with those obtained using ab initiocalculations. Key structural parameters (rh1) for vinylarsine are rAs-H = 150.5(4) pm, rAs-C = 195.1(1) pm and ∠C-C-As = 119.4(2)°. The bonding and conformational trends in both vinylarsine and vinyldichloroarsine are compared to those found in the analogous amines and phosphines.

Isolation of closed 6,6-bridged aziridinofullerenes 2 by reaction of [60] fullerene with singlet oxycarbonylnitrenes is accompanied by small amounts of previously undetected closed 5,6-adducts 4 , thought to arise by trapping of triplet nitrenes. Isomerization of the 6,6-bridged adducts to the corresponding oxazolo[4′,5′:1,2] [60] fullerene derivatives 3 occurs upon heating with the exception of the tert-butoxycarbonyl adduct which eliminates isobutene and CO 2 to yield the parent aziridinofullerene C 60 NH 6 .

Ligulaverin A 1, an eremophilane derivative with a novel carbon skeleton, was isolated from the medicinal plant Ligularia veitchiana, along with four analogues (ligulaverin B 2, ligulaverin C 3, ligulaverin D 4 and ligulaverin E 5). Their structures were elucidated by NMR techniques and X-ray diffraction. The biosynthetic route to this nineteen-carbon skeleton is discussed. © 1997 Elsevier Science Ltd.

Aldol reaction of (+)-isomenthone lithium enolate with aryl and alkyl aldehydes proceeds with complete diastereocontrol at the α-carbon (C6), but the stereochemical outcome at the secondary alcohol stereocentre (established by X-ray crystal structure determinations) can be unexpectedly reversed by changing reaction quenching temperature. This provides a route to a range of new homochiral ketoalcohols.

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. These corrections were subsequently used to determine the equilibrium structure of nitromethane from the experimental time-averaged structure. We find that the corrections to the intramolecular and intermolecular bond distances, as well as to the torsion angles, are quite significant, particularly for those atoms participating in the anharmonic motion of the methyl group. Our results demonstrate that simple harmonic models of thermal motion may not be sufficiently accurate, even at low temperatures, while molecular simulations employing more realistic potential-energy surfaces can provide important insight into the role and magnitude of anharmonic atomic motions.

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. The use of a Gram-Charlier anharmonic PDF gives means and covariance matrices closer to the true (numerically determined) anharmonic values. The physical significance of the maxima of the anharmonic distributions (the most probable or mode positions) is also discussed. As the data sets used for the modelling process are theoretical in origin, these most probable positions can be compared to equilibrium positions that represent the system at the bottom of its potential-energy surface. The two types of position differ significantly in some cases but the most probable position is still worthy of report in crystal structure determinations.

The molecular structure and conformational properties of S -ethyl trifluorothioacetate, CF3 COSCH2 CH3, were determined in the gas phase by electron diffraction and vibrational spectroscopy (IR and Raman). The experimental investigations were supplemented by ab initio (Møller 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 Cs (anti, anti) and C1 (anti, gauche) symmetries, although there are disagreements about which is more stable. The electron diffraction intensities are best interpreted with a mixture of 51(3)% anti, anti and 49(3)% anti, gauche conformers. This conformational preference was studied using the total energy scheme and the natural bond orbital scheme. In addition, the infrared spectra of CF3 COSCH2 CH3 are reported for the gas, liquid and solid phases as well as the Raman spectrum of the liquid. Using calculated frequencies as a guide, evidence for both Cs and C1 structures is obtained in the IR spectra. Harmonic vibrational frequencies and scaled force fields have been calculated for both conformers. Fil: Defonsi Lestard, Maria Eliana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química Física; Argentina Fil: Tuttolomondo, María Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química Física; Argentina Fil: Wann, Derek A.. University of Edinburgh; Reino Unido Fil: Robertson, Heather E.. University of Edinburgh; Reino Unido Fil: Rankin, David W. H.. University of Edinburgh; Reino Unido Fil: Ben Altabef, Aída. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química Física; Argentina

Gas-phase electron-diffraction scattering data and dipolar couplings from NMR experiments in four different liquid crystal solvents have been combined to give a high-accuracy molecular structure of 1,4-difluorobenzene. The anisotropic components of the CF and FF indirect couplings have been deduced directly from the experimental data. The resultant structure has standard deviations of around 0.2 pm for interatomic distances and less than 0.2° for inter-bond angles.

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. For the test case Si(8)O(12)Me(8), where the methyl groups rotate and large deformations of the Si(8)O(12) cage are observed, the MD simulations produced results markedly superior to those obtained using force-field methods. The experimental equilibrium structure of Si(8)O(12)H(8) has also been determined, demonstrating the use of empirical potentials rather than DFT methods when such potentials exist. We highlight the one major deficiency associated with classical MD--the absence of quantum effects--which causes some light-atom bonded-pair amplitudes of vibration to be significantly underestimated. However, using C(3)N(3)Cl(3) and C(3)N(3)H(3) as examples, we show that path-integral MD simulations can overcome these problems. The distance corrections and amplitudes of vibration obtained for C(3)N(3)Cl(3) are almost identical to those obtained from force-field methods, as we would expect for such a rigid molecule. In the case of C(3)N(3)H(3), for which an accurate experimental structure exists, the use of path-integral methods more than doubles the C-H amplitude of vibration.

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. The experimental studies were supported by a series of computational calculations using HF, B3LYP, and MP2 methods and a variety of basis sets. The force fields obtained from density functional theory using both B3LYP/6-31G; and B3LYP/ 6-311+G; ; were scaled with both Pulay's SQM methodology and Yoshida's WLS procedure to simulate the vibrational spectra and assist in the assignment of fundamental bands. The Raman intensities were obtained from polarizability derivatives. The vibrational spectra of trimethylgermane were completely assigned on the basis of the experimental data and the theoretical prediction of vibrational frequencies and intensities. Fil: Roldan, Maria Lorena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina Fil: Brandan, Silvia Antonia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina Fil: Masters, Sarah L.. University of Edinburgh; Reino Unido Fil: Wann, Derek A.. University of Edinburgh; Reino Unido Fil: Robertson, Heather E.. University of Edinburgh; Reino Unido Fil: Rankin, David W. H.. University of Edinburgh; Reino Unido Fil: Ben Altabef, Aída. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina

The structures of the molecules methylamine-borane, MeH2N·BH3, and dimethylamine-borane, Me2HN·BH3, have been investigated by gas-phase electron diffraction (GED) and quantum chemical calculations. The crystal structures have also been determined for methylamine-, dimethylamine-, and trimethylamine-borane, MenH3-nN·BH3 (n = 1-3); these are noteworthy for what they reveal about the intermolecular interactions and, particularly, the N-H···H-B dihydrogen bonding in the cases where n = 1 or 2. Hence, structures are now known for all the members of the ammonia- and amine-borane series MenH3-nN·BH3 (n = 0-3) in both the gas and solid phases. The structural variations and energetics of formation of the gaseous adducts are discussed in relation to the basicity of the MenH3-nN fragment. The relative importance of secondary interactions in the solid adducts with n = 0-3 has been assessed by the semi-classical density sums (SCDS-PIXEL) approach.

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. The results show that two conformers, with C(s) and C(2v) symmetry, exist for the S and Se compounds. The C(s) and C(2v) conformers have very different (2)J(PP) coupling constants (-12.6 and 395.2 Hz for S(PF(2))(2) at B3LYP/aug-cc-pVQZ) and thermal interconversion of these conformers explains the experimental behavior.

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 SiMe3 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. Calculated structures of a series of analogous compounds with different substituents on silicon show only minor variations in the extent of the distortion, although with hydrogen instead of a silyl group the displacement is significantly smaller.

The equilibrium molecular structure of the decasilsesquioxane, Si10O15H10, 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 D5h symmetry, and do not show the distortions that are apparent in the crystalline phase. The ten-membered silicon-oxygen rings are found to be particularly flexible in the gas phase, a phenomenon that was also seen in crystal structures. The Si-O bond lengths in the ten-membered rings are 161.6(2) pm long and in the eight-membered rings they are 162.2(3) pm, with Si-O-Si angles of 155.0(5) and 153.9(7) degrees, respectively.

The molecular structure and conformation of SiHBu t Me 2 in the gas phase have been determined by electron diffraction. Assuming local C 3 symmetry within tert -butyl and methyl groups the principal refined parameters are r [SiC(Bu)] 188.6(2), r [SiC(Me)] 188.2(2), r (CC) 154.6(1) pm, ∠CCC 108.5(3), ∠C(Bu)SiC(Me) 111.1(4) and ∠C(Me)SiC(Me) 105.5(10)°. A small steric effect is apparent, with the threefold axis of the tert -butyl group tilted 3.6(10)° away from the Si—methyl groups, so that the three SiCC angles are 114.1 (9), 109.0(6) and 108.3(8)°. The conformation adopted about the SiC (butyl) bond is almost fully staggered with the HSiCC dihedral angle 174.3 (48)°.

As a precursor for trisilylmethane, tris(phenylsilyl)methane is prepared by a Merker-Scott reaction of chlorophenylsilane, bromoform, and magnesium turnings in boiling tetrahydrofuran. Chlorophenylsilane is formed in a new synthesis starting from phenylsilane and hydrogen choride/AlCl3 in diethyl ether. The gas phase structure of trisilylmethane (H3Si)3CH, obtained from (PhSiH2)3CH via (BrSiH2)3CH, has been determined by electron diffraction. Data refinement confirmed a model of C3 molecular symmetry, with local C3v symmetry for the silyl groups. — As a precursor for 1,1,1-trisilylethane, 1,1,1-tris(phenylsilyl)ethane has been prepared similarly from chlorophenylsilane, 1,1,1-trichloroethane and magnesium and converted via 1,1,1-tris(bromosilyl)ethane into CH3C(SiH3)3.