Ab initio investigations of Li−+nH2→LiH2−(H2)n−1,n=1–3

Abstract

Correlated ab initio investigations on the intermolecular and intramolecular potential energy surfaces in the ground electronic state of the O 2 − ( X Π g 2 ) − HF ( X Σ + 1 ) complex J. Chem. Phys. 138, 014304 (2013); 10.1063/1.4772653Ab initio analytical potential energy surface and quasiclassical trajectory study of the O + ( 4 S)+ H 2 (X 1 Σ g + )→ OH + (X 3 Σ − )+ H ( 2 S) reaction and isotopic variants Ab initio investigations at the coupled-cluster single double ͑triple͒ ͓CCSD͑T͔͒ and MRCISD level with augmented triple and quadruple zeta basis sets have identified various stationary points on the Li Ϫ /͑H 2 ͒ n ,nϭ1 -3, hypersurfaces. The electrostatic complexes, Li Ϫ ͑H 2 ͒ n , are very weakly bound (D e Ͻ0.25 kcal/mol with respect to H 2 loss͒ and H 2 /H 2 interactions play a contributing role in determining the equilibrium structures within the electrostatic constraint of a linear or near-linear Li Ϫ -H-H orientation. The covalent molecular ion, LiH 2 Ϫ , is found to have a linear centrosymmetric structure and to be bound with respect to Li Ϫ ϩH 2 in agreement with previous calculations. The interaction of LiH 2Ϫ with additional H 2 is purely electrostatic but with a D e larger than those of the Li Ϫ ͑H 2 ͒ n complexes. LiH 2 Ϫ ͑H 2 ͒ is found to have a linear equilibrium structure and LiH 2 Ϫ ͑H 2 ͒ 2 is found to have two almost isoenergetic structures: linear with an H 2 on either end of the LiH 2Ϫ , and C 2v with both H 2 on the same end of the LiH 2 Ϫ . Of particular interest is the dramatic change in the nature of the transition state for LiH 2 Ϫ production depending on the number of H 2 molecules present. For nϭ1, the reaction proceeds through a conical intersection between the lowest energy 1 B 2 and 1 A 1 electronic surfaces in C 2v symmetry. For nϭ2, the reaction occurs on a single surface in a pericyclic mechanism through a transition state consisting of a planar five-member ring where simultaneously two H 2 bonds are broken while two LiH bonds and one new H 2 bond are formed. For nϭ3, the reaction proceeds by direct insertion of Li Ϫ into one of the H 2 molecules with the two additional H 2 molecules providing substantial stabilization of the transition state by taking on part of the negative charge in a weakly covalent interaction. The results are discussed in comparison to the isoelectronic B ϩ /͑H 2 ͒ n systems where significant sigma bond activation through a cooperative interaction mechanism has been identified recently.

Nonrelativistic molecular quantum mechanics without approximations: Electron affinities of LiH and LiD

“…The theoretical interest in the LiH Ϫ has increased since the electron affinity ͑EA͒ of LiH and its deuterated counterpart LiD were measured with the use of the photoelectron spectroscopy by Bowen and co-workers. 4 The adiabatic electron affinities of 7 LiH and 7 LiD determined in that experiment were 0.342Ϯ0.012 eV for the former and 0.337Ϯ0.012 eV for the latter system. The appearance of these data posed a challenge for theory to reproduce those values in rigorous calculations based on the first principles.…”

“…In essence, one should be able to describe this situation well with such methods as the coupled cluster ͑CO͒ approach ͓with single ͑S͒, double ͑D͒, and triple ͑T͒ excitations͔ or with MRCI method provided that a sufficiently complete basis set is used. However, despite the use of the most advanced techniques and extended basis sets the calculated values ͑CCSDT of 0.327 eV 7,8 and MRCI of 0.319 eV 7 ͒ have come short of the experimental result.…”

“…[5][6][7][8][9] The only smaller diatomic anion, according to the theoretical calculations 10,11 is HeH Ϫ , but its parent neutral system HeH is known to be unbound. Since LiH Ϫ contains only five electrons it can be calculated with the highest levels of theory available for molecular systems.…”

Color Tuning in Short Wavelength-Sensitive Human and Mouse Visual Pigments: Ab initio Quantum Mechanics/Molecular Mechanics Studies

“…SORCI can be considered as an individually selecting approximation to the iterative DDCI3 method,40 which includes active-space configurations only with two holes and one particle or two particles and one hole, in addition to DDCI2 excitations. The most straightforward MRCI method applied here includes all single and double excitations (MRCISD) 44,45. The effects of excitations higher than doubles were included in all the MRCI excitation energies with the MR-Davidson +Q corrections 43.…”

First principles determination of the bound levels of Li−(H2)

“…3 In contrast, the isomeric geometric configuration of the weakly bound electrostatic complex, Li Ϫ (H 2 ), has attracted much less attention. Recently, this complex was studied theoretically 4 in the context of the insertion reactions, Li Ϫ ϩnH 2 →HLiH Ϫ (H 2 ) nϪ1 , with special emphasis placed on the mechanistic changes induced by the number of H 2 molecules present. Exploration of the nϭ1 potential energy surface ͑PES͒ in that study 4 identified some regions where the anionic and neutral PESs cross all of which were far from the electrostatic complex geometry.…”