Equilibrium bond lengths, harmonic frequencies, electron affinities, and dissociation energies (as appropriate) were determined for I, I-,I2, I2 -, and I3 - by CCSD(T) and four DFT methods (BLYP, BPW91, B3LYP, and B3PW91) using a basis set consisting of a relativistic effective core potential and a triple-zeta plus 2df polarization functions (ECP-TZ(2df)) for the valence electrons. Comparison of the DFT results with the CCSD(T) results and available experimental information indicates that the B3PW91 approach does particularly well describing the bonding in these species. B3PW91/ECP-TZ(2df) calculations of I5 - in linear (D ∞ h ) and bent (C 2 v ) geometries indicate that the linear structure is a low-energy transition state lying only about 0.1−0.2 eV above the C 2 v global minimum-energy structure. Harmonic frequencies and infrared and Raman intensities are calculated for both structures and compared to available experimental information.

The reactions of B+ + nH2 to produce BH2 +(H2) n - 1 have been studied by high-level ab initio techniques. The reaction mechanism and associated activation energy is found to depend dramatically on the number of H2 molecules present. For n = 1, the reaction proceeds stepwise:  first breaking the H2 bond and forming one BH bond followed by forming the second BH bond. This process has an activation energy of about 57 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism though a planar cyclic transition state where two H2 bonds are broken while simultaneously two BH bonds and one new H2 bond are formed. The activation energy for this process decreases dramatically from the n =1 value to only about 11 kcal/mol. For n = 3, the reaction proceeds through a true insertion mechanism; however, the actual insertion occurs late in the reaction after over 75% of the exothermicity has been realized. The addition of the third H2 molecule decreases the activation energy to only about 3.4 kcal/mol. For n = 4, the reaction mechanism is essentially identical to that of the n = 3 case. However, the fourth H2 causes the activation energy to increase by about 2 kcal/mol relative to the n = 3 case because the additional H2 molecule causes one of the other three H2 molecules to be slightly further away from the boron ion in the transition state geometry. The computational results are compared with the experimental results of Kemper, Bushnell, Weis, and Bowers (J. Am Chem. Soc. 1998 , 120, xxxx) and are in full agreement with the experimental conclusion that the n = 3 electrostatic cluster ion is the most reactive. On the basis of a comparison of experimentally determined magnitude and isotopic dependence of the activation energies with the computed adiabatic reaction barriers, it is suggested that the observed reaction rate may be dominated by a nonclassical tunneling contribution.

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.

The reactions of Al+ + nH2 to produce AlH2 +(H2) n - 1 have been studied by high-level ab initio electronic structure techniques motivated by the σ bond activation by cooperative interaction observed experimentally and theoretically for the isovalent B+ + nH2 reaction systems. For n = 1, the reaction proceeds stepwise:  first breaking the H2 bond and forming one AlH bond followed by the formation of the second AlH bond. This process has an activation energy of 85.0 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism through a planar, cyclic transition state where two H2 bonds are broken simultaneously while two AlH bonds and one new H2 bond are formed. The activation energy for this process decreases from the n = 1 value to about 55.0 kcal/mol. These two cases are qualitatively very similar to what was observed for B+ + nH2 with the major quantitative differences being that corresponding activation energies were 30−40 kcal/mol lower and reaction energetics were 60−80 kcal/mol more exothermic in the boron systems. For n = 3, no additional activation energy lowering was observed with Al+, which contrasts significantly with the behavior observed with B+. This difference is rationalized in terms of the special ability of boron to form strong three center−two electron bonds.

Ab initio investigations at the MP2, CCSD(T), and MRCISD levels of theory with augmented triple-basis sets have identified and characterized various stationary points on the Be/(H 2 ) n , n ) 1-3, hypersurfaces. The van der Waals complexes, Be(H 2 ) n , are very weakly bound (D e ) 0.08-0.32 kcal/mol with respect to H 2 loss) with H 2 /H 2 interactions playing an important role in determining equilibrium structures which can be understood in terms of the various relevant long-range potentials. The covalent molecule, BeH 2 , is found to have a linear, centrosymmetric structure and to be strongly bound with respect to Be + H 2 , in agreement with previous calculations. BeH 2 interacts weakly with additional H 2 molecules (D e < 0.75 kcal/mol) which are positioned parallel to the near-linear BeH 2 moiety in the equilibrium structures of the BeH 2 (H 2 ) n-1 complexes. Of particular interest is the dramatic change in the nature of the transition state for BeH 2 production depending on the number of H 2 molecules present. For n ) 1, the reaction proceeds stepwise: first breaking the H 2 bond and forming one BeH bond followed by forming the second BeH bond. This process has an activation energy of about 56 kcal/mol. For n ) 2, the reaction proceeds via a pericyclic mechanism through a planar cyclic transition state where two H 2 bonds are broken while simultaneously two BeH bonds and one new H 2 bond are formed. The activation energy for this process decreases from the n ) 1 value to about 38 kcal/mol. For n ) 3, the reaction proceeds through a true insertion mechanism with the addition of the third H 2 molecule, decreasing the activation energy to about 33 kcal/mol. The results are discussed in comparison to the isoelectronic B + /nH 2 systems where significant σ bond activation through a cooperative interaction mechanism has been identified.

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