Ion-molecule reactions involving methyl isocyanide and methyl cyanide

Ion-molecule reactions involving methyl isocyanide and methyl cyanide have been performed in a new rf-only hexapole collision cell inserted in a large-scale tandem mass spectrometer. Beside protonation processes, N-methyl cyanogen ions (CH(3)N(+)CCN) and 1-methyleneiminium-1- ethylenium ions (CH(2)CN(+)CH(2)) have been produced in high yield and fully characterized by high-energy collisional activation. The unimolecular chemistry of the molecular ions of caffeine (1,3,7-trimethyl xanthine) has been revisited on the basis of these new results.     

Heats of formation of diphosphene, phosphinophosphinidene, diphosphine, and their methyl derivatives, and mechanism of the borane-assisted hydrogen release

The heats of formation of diphosphene (cis- and trans-P2H2), phopshinophosphinidene (singlet and triplet H2PP) and diphosphine (P2H4), as well as those of the P2H and P2H3 radicals resulting from PH bond cleavages, have been calculated by using high-level ab initio electronic structure theory. Energies were calculated using coupled-cluster theory with a perturbative treatment for triple excitations (CCSD(T)) and employing augmented correlation consistent basis sets with additional tight d-functions on P (aug-cc-pV(n+d)Z) up to quadruple- or quintuple-zeta, to perform a complete basis set extrapolation for the energy. Geometries and vibrational frequencies were determined with the CCSD(T) method. Core-valence and scalar relativistic corrections were included, as well as scaled zero-point energies. We find the following heats of formation (kcal/mol) at 298 [0] K: DeltaH(degree)(f)(P2H) = 53.4 [54.4]; DeltaH(degree)(f)(cis-P2H2) = 32.0 [33.9]; DeltaH(degree)(f)(trans-P2H2) = 28.7 [30.6]; DeltaH(degree)(f)(H2PP) = 53.7 [55.6]; DeltaH(degree)(f)(3H2PP) = 56.5 [58.3]; DeltaH(degree)(f)(P2H3) = 32.3 [34.8]; DeltaH(degree)(f)(P2H4) = 5.7 [9.1] (expt, 5.0 +/- 1.0 at 298 K); and DeltaH(degree)(f)(CH3PH2) = -5.0 [-1.4]. We estimate these values to have an accuracy of +/-1.0 kcal/mol. In contrast to earlier results, we found a singlet ground state for phosphinophosphinidene (H2PP) with a singlet-triplet energy gap of 2.8 kcal/mol. We calculated the heats of formation of the methylated derivatives CH3PPH, CH3HPPH2, CH3PPCH3, CH3HPP, (CH3)2PP, (CH3)2PPH2, and CH3HPPHCH3 by using isodesmic reactions at the MP2/CBS level. The calculated results for the hydrogenation reactions RPPR + H2 --> RHPPHR and R2PP + H2 --> R2PPH2 show that substitution of an organic substituent for H improves the energetics, suggesting that secondary diphosphines and diphosphenes are potential candidates for use in a chemical hydrogen storage system. A comparison with the nitrogen analogues is given. The mechanism for H2-generation from diphosphine without and with BH3 as a catalyst was examined. Including tunneling corrections, the rate constant for the catalyzed reaction is 4.5 x 1015 times faster than the uncatalyzed result starting from separated catalyst and PH2PH2.     

Computational study of the release of H2 from ammonia borane dimer (BH3NH3)2 and its ion pair isomers

High-level electronic structure calculations have been used to map out the relevant portions of the potential energy surfaces for the release of H2 from dimers of ammonia borane, BH3NH3 (AB). Using the correlation-consistent aug-cc-pVTZ basis set at the second-order perturbation MP2 level, geometries of stationary points were optimized. Relative energies were computed at these points using coupled-cluster CCSD(T) theory with the correlation-consistent basis sets at least up to the aug-cc-pVTZ level and in some cases extrapolated to the complete basis set limit. The results show that there are a number of possible dimers involving different types of hydrogen-bonded interactions. The most stable gaseous phase (AB)2 dimer results from a head-to-tail cyclic conformation and is stabilized by 14.0 kcal/mol with respect to two AB monomers. (AB)2 can generate one or two H2 molecules via several direct pathways with energy barriers ranging from 44 to 50 kcal/mol. The diammoniate of diborane ion pair isomer, [BH4-][NH3BH2NH3+] (DADB), is 10.6 kcal/mol less stable than (AB)2 and can be formed from two AB monomers by overcoming an energy barrier of approximately 26 kcal/mol. DADB can also be generated from successive additions of two NH3 molecules to B2H6 and from condensation of AB with separated BH3 and NH3 molecules. The pathway for H2 elimination from DADB is characterized by a smaller energy barrier of 20.1 kcal/mol. The alternative ion pair [NH4+][BH3NH2BH3-] is calculated to be 16.4 kcal/mol above (AB)2 and undergoes H2 release with an energy barrier of 17.7 kcal/mol. H2 elimination from both ion pair isomers yields the chain BH3NH2BH2NH3 as product. Our results suggest that the neutral dimer will play a minor role in the release of H2 from ammonia borane, with a dominant role from the ion pairs as observed experimentally in ionic liquids and the solid state.     

Comment on “Theoretical Investigations on Geometrical and Electronic Structures of Silver Clusters”

We would like to make a few comments on the results reported in a recent paper in this journal by Tsuneda, J. Comput. Chem, 2019, 40, 206. The structures of some pure silver clusters were not correctly assigned. © 2019 Wiley Periodicals, Inc.     

Pressure effects on EXAFS Debye-Waller factor and melting curve of solid krypton

The pressure effects on atomic mean-square displacement, extended X-ray absorption fine structure (EXAFS) Debye-Waller factor and melting temperature of solid krypton have been investigated in within the statistical moment method scheme in quantum statistical mechanics. By assuming the interaction between atoms can be described by Buckingham potential, we performed the numerical calculations for krypton up to pressure 120?GPa. Our calculations show that the atomic mean-square displacement and EXAFS Debye-Waller factor of krypton crystal depend strongly on pressure. They make the robust reduction of the EXAFS peak height. Our results are in good and reasonable agreements with available experimental data. This approach gives us a relatively simple method for qualitatively calculating high-pressure thermo-physical properties of materials. Moreover, it can be used to verify future high-pressure experimental and theoretical works.     

Mobile ion transport pathways in (LiBr) x [(Li2O)0.6(P2O5)0.4](1−x) glasses

(LiBr) _x [(Li₂O)_0.6(P₂O₅)_0.4]_{(1 − x)} glasses with 0 ≤ x ≤ 0.2 are prepared by melt quenching. Glass transition temperature (T _g), ionic conductivity (σ), and its activation energy (E _a) are determined experimentally and correlated to molecular dynamics (MD) simulations with an optimized potential, fitted to match bond lengths, coordination numbers, and ionic conductivity. Based on equilibrated MD configurations, ion transport pathways are modelled in detail by the bond valence approach to clarify the influence of the halide dopant concentration on the glass structure and its consequence for Li ion mobility. Results of experimental and computational studies are compared with our previous report on the (LiCl) _x [(Li₂O)_0.6(P₂O₅)_0.4]_{(1 − x)} system. Both T _g and σ values are higher for LiBr-doped glasses than for LiCl-doped glasses, but the effect of halide doping is unusually small.     

Far‐Infrared Spectra of Yttrium‐Doped Gold Clusters AunY (n=1–9)

The geometric, spectroscopic, and electronic properties of neutral yttrium‐doped gold clusters Au_nY (n=1–9) are studied by far‐infrared multiple photon dissociation (FIR‐MPD) spectroscopy and quantum chemical calculations. Comparison of the observed and calculated vibrational spectra allows the structures of the isomers present in the molecular beam to be determined. Most of the isomers for which the IR spectra agree best with experiment are calculated to be the energetically most stable ones. Attachment of xenon to the Au_nY cluster can cause changes in the IR spectra, which involve band shifts and band splittings. In some cases symmetry changes, as a result of the attachment of xenon atoms, were also observed. All the Au_nY clusters considered prefer a low spin state. In contrast to pure gold clusters, which exhibit exclusively planar lowest‐energy structures for small sizes, several of the studied species are three‐dimensional. This is particularly the case for Au₄Y and Au₉Y, while for some other sizes (n=5, 8) the 3D structures have an energy similar to that of their 2D counterparts. Several of the lowest‐energy structures are quasi‐2D, that is, slightly distorted from planar shapes. For all the studied species the Y atom prefers high coordination, which is different from other metal dopants in gold clusters.     

Multi-user cooperation schemes with iterative decoding

We propose multi-user cooperation schemes in which all cooperating users simultaneously forward their partners’ signals that are successfully decoded to a multi-antenna destination to achieve high bandwidth efficiency. Various suitable signal processing and decoding techniques at the relay and at the destination are considered. Simulation results show that the proposed schemes significantly outperform direct transmission and reference schemes under the same transmit power and bandwidth efficiency.     

Structure property correlation in lithium borophosphate glasses

To investigate the influence of cation mobility variation due to the mixed glass former effect, 0.45Li₂O-(0.55 − x) P₂O₅−x B₂O₃ glasses (0 ≤; x ≤ 0.55) are studied keeping the molar ratio of Li₂O/(P₂O₅ + B₂O₃) constant. Addition of B₂O₃ into lithium phosphate glasses increases the glass transition temperature (T _g) and number density, decreases the molar volume, and generally renders the glasses more fragile. The glass system has been characterised experimentally by XRD, XPS and impedance studies and studied computationally by constant volume molecular dynamics (MD) simulations and bond valence (BV) method to identify the structural variation with increasing the B₂O₃ content, its consequence for Li⁺ ion mobility, as well as the distribution of bridging and non-bridging oxygen atoms. These studies indicate the increase of P-O-B bonds (up to Y = [B₂O₃]/([B₂O₃] + [P₂O₅]) ≈ 0.5 and B-O-B bonds, as well as the decrease of P-O-P bonds and non-bridging oxygens (NBOs) with rising B₂O₃ content. The system with Y ≈ 0.5 exhibits maximum ionic conductivity, 1.0 × 10⁻⁷ S cm⁻¹, with activation energy 0.63 V. Findings are rationalised by a model of structure evolution with varying B₂O₃ content Y and an empirical model quantifying the effect of the various structural building blocks on the ionic conductivity in this mixed glass former system.     

Disparate effects of Cu and V on structures of exohedral transition metal-doped silicon clusters: a combined far-infrared spectroscopic and computational study

The growth mechanisms of small cationic silicon clusters containing up to 11 Si atoms, exohedrally doped by V and Cu atoms, are described. We find that as dopants, V and Cu follow two different paths: while V prefers substitution of a silicon atom in a highly coordinated position of the cationic bare silicon clusters, Cu favors adsorption to the neutral or cationic bare clusters in a lower coordination site. The different behavior of the two transition metals becomes evident in the structures of Si(n)M(+) (n = 4-11 for M = V, and n = 6-11 for M = Cu), which are investigated by density functional theory and, for several sizes, confirmed by comparison with their experimental vibrational spectra. The spectra are measured on the corresponding Si(n)M(+)·Ar complexes, which can be formed for the exohedrally doped silicon clusters. The comparison between experimental and calculated spectra indicates that the BP86 functional is suitable to predict far-infrared spectra of these clusters. In most cases, the calculated infrared spectrum of the lowest-lying isomer fits well with the experiment, even when various isomers and different electronic states are close in energy. However, in a few cases, namely Si(9)Cu(+), Si(11)Cu(+), and Si(10)V(+), the experimentally verified isomers are not the lowest in energy according to the density functional theory calculations, but their structures still follow the described growth mechanism. The different growth patterns of the two series of doped Si clusters reflect the role of the transition metal's 3d orbitals in the binding of the dopant atoms.