Volatile and Thermally Stable Polymeric Tin Trifluoroacetates.

Tin trifluoroacetates are effective vapor phase single-source precursors for F-doped SnO2, but their structures have been poorly understood for decades. Here we undertook a comprehensive structural analysis of these compounds in both the solid and gas phases through a combined single-crystal X-ray crystallography, gas phase electron diffraction, and density functional theory investigation. Tin(II) bis(trifluoroacetate) (1) thermally decomposes into a 1:1 mixture of 1 and ditin(II) µ-oxybis(µ-trifluoroacetate) (2) during sublimation, which then polymerize into hexatin(II)-di-µ3-oxyoctakis(µ-trifluoroacetate) (3) upon solidification. Reversible depolymerization occurred readily upon heating, making 3 a useful vapor phase precursor itself. Tin(IV) tetrakis(trifluoroacetate) (5) was also found to be polymeric in the solid state, but it evaporated as a monomer over 130 °C lower than 3. This counterintuitive improvement in volatility by polymerization was possibly due to the large entropy change during sublimation, which offers a strategic new design feature for vapor phase deposition precursors.

Icosahedral Carbaboranes with Peripheral Hydrogen-Chalcogenide Groups: Structures from Gas Electron Diffraction and Chemical Shielding in Solution.

Carbaboranes 1,2-(EH)2 -closo-1,2-C2 B10 H10 (E=S, Se) were prepared, in the case of E=Se for the first time. Their semi-experimental equilibrium molecular structures were established by the concerted use of quantum-chemical calculations and gas electron diffraction. A method was developed and implemented to quantify the contribution of experimental data to each refined structural parameter. The accuracy of the experimental structures and those calculated at the MP2 level of theory were gauged by comparison of experimental 11 B NMR chemical shifts with quantum-chemically computed values; the inclusion of electron correlation (GIAO-MP2) provided superior results. For the purpose of geometrical prediction, the remaining group 16 elements were considered, and the icosahedral structures for E=O and Te were also computed; for E=O the same theoretical approach was used as for E=S, and for E=Te a description similar to that for E=Se was employed.

Direct Experimental Observation of in situ Dehydrogenation of an Amine-Borane System Using Gas Electron Diffraction.

In situ dehydrogenation of azetidine-BH3, which is a candidate for hydrogen storage, was observed with the parent and dehydrogenated analogue subjected to rigorous structural and thermochemical investigations. The structural analyses utilized gas electron diffraction supported by high-level quantum calculations, while the pathway for the unimolecular hydrogen release reaction in the absence and presence of BH3 as a bifunctional catalyst was predicted at the CBS-QB3 level. The catalyzed dehydrogenation pathway has a barrier lower than the predicted B-N bond dissociation energy, hence favoring the dehydrogenation process over the dissociation of the complex. The predicted enthalpy of dehydrogenation at the CCSD(T)/CBS level indicates that mild reaction conditions would be required for hydrogen release and that the compound is closer to thermoneutral than linear amine boranes. The entropy and free energy change for the dehydrogenation process show that the reaction is exergonic, energetically feasible, and will proceed spontaneously toward hydrogen release, all of which are important factors for hydrogen storage.

Structure of 4-(Dimethylamino)benzonitrile Using Gas Electron Diffraction: A New Lease of Life for the Only Gas Electron Diffractometer in the U.K. .

The continued demand for gas-phase molecular structures has led to the recommissioning of a gas electron diffractometer, formerly housed at the University of Reading. The gas electron diffractometer, now the only one of its kind in the U.K., is currently housed at the University of York, where it is now used routinely to determine directly structures of isolated molecules in the gas phase. The instrument has been fitted with an air-heated nozzle assembly to increase the range of molecules accessible to study in the gas phase; the efficacy of this assembly is demonstrated in this article via the determination of the gas-phase structure of 4-(dimethylamino)benzonitrile (DMABN) at high temperature. A series of complementary theoretical calculations using the B2PLYP DFT functional of Grimme et al. with correlation-consistent basis sets of double, triple, and quadruple-ζ quality are also presented. The agreement between the experimental and theoretical structural parameters attests to the accuracy of the applied theoretical calculations and of our gas-phase structural solution.

Unusual Cage Rearrangements in 10-Vertex nido-5,6-Dicarbaborane Derivatives: An Interplay between Theory and Experiment.

The reaction between selected X-nido-5,6-C2B8H11 compounds (where X = Cl, Br, I) and "Proton Sponge" [PS; 1,8-bis(dimethylamino)naphthalene], followed by acidification, results in extensive rearrangement of all cage vertices. Specifically, deprotonation of 7-X-5,6-C2B8H11 compounds with one equivalent of PS in hexane or CH2Cl2 at ambient temperature led to a 7 → 10 halogen rearrangement, forming a series of PSH+[10-X-5,6-C2B8H10]- salts. Reprotonation using concentrated H2SO4 in CH2Cl2 generates a series of neutral carbaboranes 10-X-5,6-C2B8H11, with the overall 7 → 10 conversion being 75%, 95%, and 100% for X = Cl, Br, and I, respectively. Under similar conditions, 4-Cl-5,6-C2B8H11 gave ∼66% conversion to 3-Cl-5,6-C2B8H11. Since these rearrangements could not be rationalized using the B-vertex swing mechanism, new cage rearrangement mechanisms, which are substantiated using DFT calculations, have been proposed. Experimental 11B NMR chemical shifts are well reproduced by the computations; as expected δ(11B) for B(10) atoms in derivatives with X = Br and I are heavily affected by spin-orbit coupling.

Competition between Halogen, Hydrogen and Dihydrogen Bonding in Brominated Carboranes.

Halogen bonds are a subset of noncovalent interactions with rapidly expanding applications in materials and medicinal chemistry. While halogen bonding is well known in organic compounds, it is new in the field of boron cluster chemistry. We have synthesized and crystallized carboranes containing Br atoms in two different positions, namely, bound to C- and B-vertices. The Br atoms bound to the C-vertices have been found to form halogen bonds in the crystal structures. In contrast, Br atoms bound to B-vertices formed hydrogen bonds. Quantum chemical calculations have revealed that halogen bonding in carboranes can be much stronger than in organic architectures. These findings open new possibilities for applications of carboranes, both in materials and medicinal chemistry.

Influence of Antipodally Coupled Iodine and Carbon Atoms on the Cage Structure of 9,12-I2-closo-1,2-C2B10H10: An Electron Diffraction and Computational Study.

Because of the comparable electron scattering abilities of carbon and boron, the electron diffraction structure of the C2v-symmetric molecule closo-1,2-C2B10H12 (1), one of the building blocks of boron cluster chemistry, is not as accurate as it could be. On that basis, we have prepared the known diiodo derivative of 1, 9,12-I2-closo-1,2-C2B10H10 (2), which has the same point-group symmetry as 1 but in which the presence of iodine atoms, with their much stronger ability to scatter electrons, ensures much better structural characterization of the C2B10 icosahedral core. Furthermore, the influence on the C2B10 geometry in 2 of the antipodally positioned iodine substituents with respect to both carbon atoms has been examined using the concerted application of gas electron diffraction and quantum chemical calculations at the MP2 and density functional theory (DFT) levels. The experimental and computed molecular geometries are in good overall agreement. Molecular dynamics simulations used to obtain vibrational parameters, which are needed for analyzing the electron diffraction data, have been performed for the first time for this class of compound. According to DFT calculations at the ZORA-SO/BP86 level, the (11)B chemical shifts of the boron atoms to which the iodine substituents are bonded are dominated by spin-orbit coupling. Magnetically induced currents within 2 have been calculated and compared to those for [B12H12](2-), the latter adopting a regular icosahedral structure with Ih point-group symmetry. Similar total current strengths are found but with a certain anisotropy, suggesting that spherical aromaticity is present; electron delocalization in the plane of the hetero atoms in 2 is slightly hindered compared to that for [B12H12](2-), presumably because of the departure from ideal icosahedral symmetry.

Structures of tetrasilylmethane derivatives (XMe2Si)2C(SiMe3)2 (X = H, Cl, Br) in the gas phase, and their dynamic structures in solution.

The structures of the molecules (XMe2Si)2C(SiMe3)2, where X = H, Cl, Br, have been determined by gas electron diffraction (GED) using the SARACEN method of restraints, with all analogues existing in the gas phase as mixtures of C1- and C2-symmetric conformers. Variable temperature (1)H and (29)Si solution-phase NMR studies, as well as (13)C NMR and (1)H/(29)Si NMR shift correlation and (1)H NMR saturation transfer experiments for the chlorine and bromine analogues, are reported. At low temperatures in solution there appear to be two C1 conformers and two C2 conformers, agreeing with the isolated-molecule calculations used to guide the electron diffraction refinements. For (HMe2Si)2C(SiMe3)2 the calculations indicated six conformers close in energy, and these were modeled in the GED refinement.

Molecular structure of 1,2-bis(trifluoromethyl)-1,1,2,2-tetramethyldisilane in the gas, liquid, and solid phases: unusual conformational changes between phases.

The molecular structure of 1,2-bis(trifluoromethyl)-1,1,2,2-tetramethyldisilane has been determined in three different phases (solid, liquid, and gas) using various spectroscopic and diffraction techniques. Both the solid-state and gas-phase investigations revealed only one conformer to be present in the sample analyzed, whereas the liquid phase revealed the presence of three conformers. The data have been reproduced using computational methods and a rationale is presented for the observation of three conformers in the liquid state.

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. These quantities are calculated from the ab initio potential-energy surfaces corresponding to the vibrational modes of the monomer and dimer. Because they include many of the effects associated with large-amplitude modes of vibration and anharmonicity, we have been able to determine highly accurate experimental structures. These results are found to be in good agreement with those from high-level core-valence ab initio calculations and are substantially more precise than those obtained in previous structural studies.

Structures of, and related consequences of deprotonation on, two C(s)-symmetric arachno nine-vertex heteroboranes, 4,6-X2B7H9 (X = CH2; S) studied by gas electron diffraction/quantum chemical calculations and GIAO/NMR.

Gas-phase structure determinations have been performed for arachno-4,6-(CH2)2B7H9 and arachno-4,6-S2B7H9 by combining quantum-chemical calculations and gas electron diffraction (GED) data. In addition, the monoanion derivatives of each of the aforementioned species have been studied using ab initio calculations. In all cases, comparison with experimental (11)B NMR chemical shifts have been achieved by calculating the appropriate NMR chemical shifts using GIAO-MP2 methods and the IGLO-II basis set for various geometries, both experimental and calculated. The NMR parameters calculated for the geometry obtained from the SARACEN GED refinement appeared to be quite reasonable, and in general, the fit between theoretical and experimental δ((11)B) NMR was found to be consistently good for all four species investigated.

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. 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 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 1.2 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.

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 26.0 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.

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. 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.

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. The crystal structures have also been determined for methylamine-, dimethylamine-, and trimethylamine-borane, Me(n)H(3-n)N.BH(3) (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 Me(n)H(3-n)N.BH(3) (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 Me(n)H(3-n)N 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.

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. Compound 1 has been used to synthesize novel heterobimetallic derivatives W(NBut)4[Rh(COD)]2 (3) and W(NBut)4[Pd(eta3-C3H5)]2 (4) via the dilithiated intermediate Li2[W(NBut)4] (2). In both cases, the [W(NBut)4] moiety bridges the two organometallic fragments. Reaction of 1 with Me2Zn has produced [Me(tBuN)W(mu-NBut)2ZnMe(NH2But)] (5). The structures of 3, 4, and 5 have been determined. Thermal decomposition of 4 under an autogenerated pressure at 700 degrees C has formed the hitherto uncharacterized bimetallic alloy WPd2.

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. 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.

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. 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.

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. 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.