Angle and bond-length dependent C6 coefficients for H2 interacting with H,Li, Be and rare gas atoms

Summary Accurate new C₆ dispersion energy coefficients, and their dependence on the diatom orientation and bond length, are calculated for molecular hydrogen interacting with an atom of H, Li, Be, He, Ne, Ar, Kr or Xe. They are generated from accurateab initio pseudo dipole oscillator strength distributions (DOSD) for H₂, H, He and Be, and reliable semiempirical ones for Li, Ne, Ar, Kr and Xe. Compact power series expansions for the diatom bond-length dependence of these coefficients, suitable for incorporation into representations of full potential energy surfaces for these systems, are determined and assessed.     

Hartree-Fock calculation of cohesive energies and equilibrium lattice constants for solid Li and Be

A method is described for calculating cohesive energies of solids in the single-determinant approximation including the full Hartree-Fock exchange. The method involves (1) the construction of a rapidly convergent series in vectors of the direct and reciprocal lattice for the Fock matrix, (2) a decoupling procedure for the k -dependence of the Fock matrix, which works even in the case of strong interatomic overlap. An application to Li and Be is given. Agreement with experiment to 10% is achieved for the cohesive energies and to 5% for the equilibrium lattice constants.     

Structural and electronic properties of complexes between Li+, Na+, Be2+, and Mg2+ ions and HF,H2O,and NH3 molecules

Theoretical and Experimental Chemistry -     

Ti-doped LiAlH4 for hydrogen storage: synthesis, catalyst loading and cycling performance

The direct synthesis of LiAlH(4) from commercially available LiH and Al powders in the presence of TiCl(3) and Me(2)O has been achieved for the first time. The effects of TiCl(3) loadings (Ti/Al = 0, 0.01, 0.05, 0.2, 0.5, 1.0 and 2.0%) and various other additives (TiCl(3)/Al(2)O(3), metallic Ti, Nb(2)O(5), and NbCl(5)) on the formation and stability of LiAlH(4) have been systematically investigated. The yield of LiAlH(4) initially increases, and then decreases, with increasing TiCl(3) loadings. LiH + Al → LiAlH(4) yields above 95% were obtained when the molar ratios of Ti/Al were 0.05 and 0.2%. In the presence of a very tiny amount of TiCl(3) (Ti/Al = 0.01%), LiAlH(4) is still generated, but the yield is lower. In the complete absence of TiCl(3), LiAlH(4) does not form. Addition of metallic Ti, Nb(2)O(5), and NbCl(5) to commercial LiH and Al does not result in the formation of LiAlH(4). Preliminary tests show that TiCl(3)-doped LiAlH(4) can be cycled, making it a suitable candidate for hydrogen storage.     

Aromatic N-Heterocyclic superalkali M@C4H4N2 complexes (M=Li,Na, K); A promising potential hydrogen storage system

In search of a potential hydrogen storage material, we have used three different aromatic N-heterocyclic systems; Pyrazine, Pyrimidine, and Pyridazine. The alkali (Li, Na, and K) metallated heterocyclic systems (M@C₄H₄N₂) are stabilized as cation found to be superalkali with a vertical electron affinity ranges from 4.42 to 3.97 ​eV. First principle calculations reveal that M@heterocylclic systems can adsorb up to 10–18H₂ molecules with gravimetric wt% of 17. It has been observed that all the trapping process belongs to physisorption as H₂ are almost in a molecular form with a slight increase in bond length. Binding energy values favor gradual H₂ adsorbtion. Superalkali nature is gradually increasing upon the increasing number of H₂ molecules but attains a constant value after certain numbers. Among three isomers Pyrazine, Pyrimidine, and Pyridazine, Li@Pyrazine complex is found to be the most suitable for H₂ storage.     

Use of electrostatic method for calculating the bond energy of H 2 + , H2, and Li2 molecules and the force constant of H2 and Li2 molecules

Journal of Structural Chemistry -     

A normal equilibrium isotope effect for oxidative addition of H2 to (eta6-anthracene)Mo(PMe3)3

Oxidative addition of H2 and D2 to the anthracene complex (eta6-AnH)Mo(PMe3)3 giving (eta4-AnH)Mo(PMe3)3X2 (X = H, D) is characterized by a normal equilibrium isotope effect (KH/KD > 1) at temperatures close to ambient; calculations on (eta4-AnH)Mo(PH3)3H2 indicate that this is a consequence of relatively low energy Mo-H vibrational modes.     

Infrared spectra of Li2SeO4.H2O,Li2SO4.H2O and Li2(S,Se)O4.H2O

On of the hydrogen bonds formed by water molecules in lithium selenate monohydrate is evidently stronger than in the corresponding sulfate, whereas the other one is weaker. The temperature dependence of the stretching and bending modes of water is similar in both compounds, their frequencies decreasing on lowering the temperature. The study of mixed sulfate—selenate compounds made it possible to clearly show that the effective symmetry of the tetrahedral ions is higher than their local crystallographic one.     

Analytical energy surfaces for the collinear H+H2 and Li+H2 exchange reactions

A simple four-parameter function is shown to possess adequate flexibility to fit the H + H₂ →H₂ + H and Li + H₂ → LiH + H exchange reaction energy surfaces to good accuracy along the reaction paths.     

Water exchange rates and mechanisms in tetrahedral [Be(H2O)4]2+ and [Li(H2O)4]+ complexes using DFT methods and cluster‐continuum models

The water exchange reactions in aquated Li⁺ and Be²⁺ ions were investigated with density functional theory calculations performed using the [Li(H₂O)₄]⁺·14H₂O and [Be(H₂O)₄]²⁺·8H₂O systems and a cluster‐continuum approach. A range of commonly used functionals predict water exchange rates several orders of magnitude lower than the experimental ones. This effect is attributed to the overstabilization of coordination number four by these functionals with respect to the five‐coordinated transition states responsible for the associative ( A ) or associative interchange ( I_a ) water exchange mechanisms. However, the M06 and M062X functionals provide results in good agreement with the experimental data: M062X/TZVP calculations yield a concerted I_a mechanism for the water exchange in [Be(H₂O)₄]²⁺·8H₂O that gives an average residence time of water molecules in the first coordination sphere of 260 μs. For [Li(H₂O)₄]⁺·14H₂O the water exchange reaction is predicted to follow an A mechanism with a residence time of inner‐sphere water molecules of 25 ps.     

Ab initio calculations for ground states of CH2Li− and CH2Be

The ground state of CH₂Li^? and CH₂Be molecules has been investigated by an SCF calculation using a contracted Gaussian basis set. Only for the second system a bound state with respect to the ground states of the molecular fragments has been found.     

Re-examination of a conformational equilibrium isotope effect for hydrogen in 1,1,3,3-tetramethylcyclohexane - the importance of intrinsic isotope effects

Neglect of intrinsic isotope effects on ¹³C chemical shifts is a major source of error in the conformational equilibrium isotope effect for deuteriation of one methyl group in 1,1,3,3-tetramethylcyclohexane; there is no significant solvent effect.     

溶剂对电荷转移复合物平衡常数的影响

研究了溶剂与电荷转移复合物平衡常数K_(CT)之间的关系,并建立了简略模型,选择N-乙基咔唑作为给体,分别与2,4,7-三硝基芴酮和2,4,5,7-四硝基芴酮作为受体进行复合,得到了两对电荷转移复合物。用VPO法测量了在不同溶剂中复合物的K’_(CT)。发现,K’_(CT)随溶剂偶极矩μs的减小而增大,InK'_(CT)和μ_s~2间存在着良好的线性关系。其关系可表达为InK'_(CT)=In K_(CT)—Bμ_s~2-C或InK'_(CT)=A—Bμ_s~2。     

Electronic charge density and mean kinetic energy of lithium orbitals in LiH,LiH+, Li2, Li2+, LiH2+, and Li2H+

The electron density near the lithium nucleus in the species LiH, LiH⁺, Li₂, Li₂⁺, LiH₂⁺, and Li₂H⁺ was analyzed by transforming the SCF molecular orbitals into a sum of atomic contribnutions, for both core and valence orbitals. These “hybrid-atomic” orbitals were used to compare: electron densities, orbital polarizations, and orbital mean kinetic energies with the corresponding lithium atom quantities. Core-orbital electron densities at the lithium nucleus were observed to increase by up to 0.5% relative to the lithium atom 1s orbital. Lithium cores also exhibited polarization but, surprisingly, in the direction away from the internuclear region. Similar dramatic changes were seen in the electron densities of the valence orbitals of lithium: The electron density at the nucleus for these orbitals increased two-fold for homonuclear species and twenty-fold for heteronuclear triatomic species relative to the electron density at the nucleus in lithium atom. The polarization of the valence orbital electronic charge, in the vicinity of the lithium nucleus, was also away from the internuclear region. The mean “hybrid-atomic” orbital kinetic energies associated with the lithium atom in the molecules also showed changes relative to the free lithium atom. Such changes, accompanying bond formation, were relatively small for the lithium core orbitals (within 0.2% of the value for lithium atom). The orbital kinetic energies for the lithium valence electrons, however, increased considerably relative to the lithium atom: By a factor of about 2 in homonuclear diatomics, by a factor of 7 in heteronuclear diatomics, and by a factor of 11 in the triatomic species. In summary, the total electronic density (core plus valence) at the lithium nucleus remained remarkably constant for all of the species studied, regardless of the effective charge on lithium. Thus, the drastic changes noted in the individual lithium orbitals occurred in a cooperative fashion so as to preserve a constant total electron density in the vicinity of the lithium nucleus. In all cases, bond formation was accompanied by an increase in the orbital kinetic energy of the lithium valence orbital. We suggest that these two observations represent important and significant features of chemical bonding which have not previously been emphasized.