Structural and thermochemical investigation of 1,3-bis(λ4-boraneyl)-1λ4,3λ4-imidazolidine adduct for chemical hydrogen storage.

The structure, thermochemical properties and reaction pathways of a cyclic amine diborane complex (1,3-bis(λ4-boraneyl)-1λ4,3λ4-imidazolidine) were investigated using quantum chemical calculations. Structural and thermochemical analysis revealed that the simultaneous and spontaneous elimination of both hydrogen molecules from this complex is predicted to occur under thermoneutral conditions. This observation is further supported by the investigation of the BH3-catalysed dehydrogenation pathway. The calculated thermochemical parameters indicate that the energy requirements for hydrogen release from this complex are minimal, suggesting efficient hydrogen release capability under suitable conditions. Additionally, the activation barriers, ∼75 and ∼20 kJ mol-1 for the first and second dihydrogen release from the catalysed dehydrogenation reactions of this compound exhibit moderate kinetics, confirmed by kinetic studies. These findings and the ability of the system to easily release two molecules of dihydrogen emphasize the potential of 1,3-bis(λ4-boraneyl)-1λ4,3λ4-imidazolidine as a highly effective hydrogen storage material.

A Diverse View of Science to Catalyse Change.

Valuing diversity leads to scientific excellence, the progress of science and most importantly, it is simply the right thing to do. We can value diversity not only in words, but also in actions.

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.

Gas-Phase Structures of Ketene and Acetic Acid from Acetic Anhydride Using Very-High-Temperature Gas Electron Diffraction.

The gas-phase molecular structure of ketene has been determined using samples generated by the pyrolysis of acetic anhydride (giving acetic acid and ketene), using one permutation of the very-high-temperature (VHT) inlet nozzle system designed and constructed for the gas electron diffraction (GED) apparatus based at the University of Canterbury. The gas-phase structures of acetic anhydride, acetic acid, and ketene are presented and compared to previous electron diffraction and microwave spectroscopy data to show improvements in data extraction and manipulation with current methods. Acetic anhydride was modeled with two conformers, rather than a complex dynamic model as in the previous study, to allow for inclusion of multiple pyrolysis products. The redetermined gas-phase structure of acetic anhydride (obtained using the structure analysis restrained by ab initio calculations for electron diffraction method) was compared to that from the original study, providing an improvement on the description of the low vibrational torsions compared to the dynamic model. Parameters for ketene and acetic acid (both generated by the pyrolysis of acetic anhydride) were also refined with higher accuracy than previously reported in GED studies, with structural parameter comparisons being made to prior experimental and theoretical studies.

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.

Structural, thermochemical and kinetic insights on the pyrolysis of diketene to produce ketene.

Diketene (4-methylideneoxetan-2-one) is a precursor to the formation of either two molecules of ketene, or allene and CO2 using pyrolysis techniques. It is not known experimentally which of these pathways is followed, or indeed if both are, during the dissociation process. We use computational methods to show that the formation of ketene has a lower barrier than formation of allene and CO2 under standard conditions (by 12 kJ/mol). According to CCSD(T)/CBS, CBS-QB3 and M06-2X/cc-pVTZ calculations the formation of allene and CO2 is favoured thermodynamically under standard conditions of temperature and pressure; however, kinetically the formation of ketene is favoured from transition state theory calculations at standard and elevated temperatures.

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

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

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.

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.

Supersilyl radicals from the dissociation of superdisilane observed by gas electron diffraction.

The vapour produced upon mild heating of hexa-tert-butyldisilane (superdisilane) has been studied by gas electron diffraction and ab initio molecular orbital calculations; the disilane is not observed in the vapour, and the observed radical structure is not the lowest energy structure predicted ab initio.

Embracing chemical and structural diversity with UCONGA: A universal conformer generation and analysis program.

Molecular properties depend on molecular structure, so the first step in any computational chemistry investigation is to generate all thermally accessible conformers. Typically it is necessary to make a trade-off between the number of conformers to be explored and the accuracy of the method used to calculate their energies. Ab initio potential energy surface scans can, in principle, be applied to any molecule, but their conformational cost scales poorly with both molecular size and dimensionality of the search space. Specialized conformer generation techniques rely on parameterized force fields and may also use knowledge-based rules for generating conformers, and are typically only available for drug-like organic molecules. Neither approach is well-suited to generating or identifying chemically sensible conformers for larger non-organic molecules. The Universal CONformer Generation and Analysis (UCONGA) program package fills this niche. It requires no parameters other than built-in atomic van der Waals radii to generate comprehensive ensembles of sterically-allowed conformers, for molecules of arbitrary composition and connectivity. Analysis scripts are provided to identify representative structures from clusters of similar conformers, which may be further refined by subsequent geometry optimization. This approach is particularly useful for molecules not described by parameterized force fields, as it minimizes the number of computationally intensive ab initio calculations required to characterize the conformer ensemble. We anticipate that UCONGA will be particularly useful for computational and structural chemists studying flexible non-drug-like molecules.

Gas-phase structures of sterically crowded disilanes studied by electron diffraction and quantum chemical methods: 1,1,2,2-tetrakis(trimethylsilyl)disilane and 1,1,2,2-tetrakis(trimethylsilyl)dimethyldisilane.

The gas-phase structures of the disilanes 1,1,2,2-tetrakis(trimethylsilyl)disilane [(Me3Si)2HSiSiH(SiMe3)2] (1) and 1,1,2,2-tetrakis(trimethylsilyl)dimethyldisilane [(Me3Si)2MeSiSiMe(SiMe3)2] (2) have been determined by density functional theoretical calculations and by gas electron diffraction (GED) employing the SARACEN method. For each of 1 and 2 DFT calculations revealed four C2-symmetric conformers occupying minima on the respective potential-energy surfaces; three conformers were estimated to be present in sufficient quantities to be taken into account when fitting the GED data. For (Me3Si)2RSiSiR(SiMe3)2 [R = H (1), CH3 (2)] the lowest energy conformers were found by GED to have RSiSiR dihedral angles of 87.7(17)° for 1 and -47.0(6)° for 2. For each of 1 and 2 the presence of bulky and flexible trimethylsilyl groups dictates many aspects of the geometric structures in the gas phase, with the molecules often adopting structures that reduce steric strain.

The re-determination of the molecular structure of antimony(III) oxide using very-high-temperature gas electron diffraction (VHT-GED).

The molecular structure of a fundamental binary compound [antimony(III) oxide] has been re-determined to a far higher accuracy and precision than previously reported. The structure is compared to those determined by various ab initio methods, X-ray diffraction and to a previous gas-phase structure determined by electron diffraction prior to modern-day developments for data extraction and manipulation. The experiments utilised a new very-high-temperature (VHT) inlet nozzle system that has been designed and constructed for the gas electron diffraction (GED) apparatus now based at the University of Canterbury (formerly at the University of Edinburgh). The VHT-GED system is capable of heating samples to temperatures of up to 1100 K. The VHT system was tested by studying the gas-phase structure of antimony(III) oxide. Data were collected at 750 K and the contents of the gas flow analysed by mass spectrometry. In the gas phase at this temperature, antimony(III) oxide (Sb(2)O(3)) exists as a dimer (Sb(4)O(6)) with tetrahedral symmetry. The Sb-O bond length (r(a3,1)) was determined to be 195.66(4) pm and the O-Sb-O bond angle was 98.15(3)°. Unusually, the Sb-O bond length observed from X-ray diffraction investigation of Sb(4)O(6) was longer than the gas phase bond length [197.7(1) pm as compared to 195.66(4) pm].

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). 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 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 120.05(9)°.

The gas-phase structure of octaphenyloctasilsesquioxane Si8O12Ph8 and the crystal structures of Si8O12(p-tolyl)8 and Si8O12(p-ClCH2C6H4)8.

The equilibrium molecular structure of octaphenyloctasilsesquioxane Si(8)O(12)Ph(8) in the gas phase has been determined by electron diffraction. It was found to have D(4) point-group symmetry, with Si-O bond lengths of 1.634(15)-1.645(19) A, and a narrow range [147.5(45)-149.8(24) degrees] of Si-O-Si angles. The structures of Si(8)O(12)(p-tolyl)(8) and Si(8)O(12)(p-ClCH(2)C(6)H(4))(8) have been determined by X-ray diffraction and are found to have Si(8)O(12) cages significantly distorted from the symmetry found for Si(8)O(12)Ph(8) in the gas phase. Thus, Si-O-Si angles range between 144.2(2)-151.64(16) degrees for Si(8)O(12)(p-tolyl)(8), and between 138.8(2)-164.2(2) degrees for Si(8)O(12)(p-ClCH(2)C(6)H(4))(8). These three structures show how much a Si(8)O(12) cage may be distorted away from an ideal structure, free from intermolecular forces, by packing forces in a crystalline lattice.