Sigma Bond Activation by Cooperative Interaction with ns² Atoms:  B⁺ + nH₂

Abstract

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.

The Al+–H2 cation complex: Rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations

“…Sharp et al found that the electrostatically bound Al + -H 2 complex lies 13.1 kcal/ mol lower in energy than the inserted HAlH + structure. 12,13 Although the insertion barrier is also predicted to be lowered for the larger Al + -͑H 2 ͒ n clusters, it remains appreciable ͑53.2 kcal/ mol for n =3͒. 10 The corresponding reaction for the isovalent B + ion, where the inserted HBH + form is 55.9 kcal/ mol more stable than the B + -H 2 complex, was found to be expedited in B + -͑H 2 ͒ n clusters, with a significant lowering in the insertion barrier for n =3 compared to n =1 ͑3 kcal/ mol vs 56 kcal/ mol͒.…”

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

“…2 From a third viewpoint, LiH 2 Ϫ is the simplest anion having a geometry which allows the transition state region of a neutral system to be accessed by photodetachment. 5 To effectively weaken and ultimately break an H 2 bond, the node of an occupied molecular orbital needs to bisect the H 2 molecule. Kemper et al 4 used mass spectrometric techniques to study the interaction of B ϩ ͑isoelectronic with Li Ϫ ͒ with H 2 and reported that the activation energy for forming BH 2 ϩ could be reduced from near 60 kcal/mol when only one H 2 molecule was present to about 2 kcal/mol through the interaction of two additional H 2 molecules.…”

“…The first theoretical investigation of LiH 2 Ϫ was performed only at the self-consistent field ͑SCF͒ level, 8 where the ion was calculated to be linear but not stable with respect to the lowest energy dissociation products, Li Ϫ ϩH 2 . 4,5 In addition, Senekowitsch and Romus pointed out that the energy of the transition state region may be very close to that of the neutral LiH 2 system at the same geometry, a situation which has no analog in the B ϩ ϩH 2 system. Although it was shown that LiH 2 Ϫ could be formed from LiHϩH Ϫ without a barrier, 1 the barrier for formation from Li Ϫ ϩH 2 has not been considered in detail previously.…”

Properties of the B+-H2 and B+-D2 complexes: A theoretical and spectroscopic study

“…Previous studies suggest that the [BH 2 ] + system possesses two stable structures, the electrostatically bound B + -H 2 complex (Figure 1(a)), and a covalently bound HBH + form (Figure 1(b)). [17][18][19] The electrostatic complex has a C 2v equilibrium structure, as favoured by the charge-quadrupole interaction between the B + cation and the H 2 molecule. Interconversion between the electrostatic complex (Figure 1(a)) and the more stable covalent form (Figure 1(b)) entails surmounting a 252 kJ mol −1 barrier associated with a change in electron configuration.…”

“…Interconversion between the electrostatic complex (Figure 1(a)) and the more stable covalent form (Figure 1(b)) entails surmounting a 252 kJ mol −1 barrier associated with a change in electron configuration. 18 Whereas, for B + -H 2 the valence electron configuration is 2a 2 1 3a 2 1 (corresponding to ground state B + interacting with ground state H 2 ), the covalently bound HBH + molecule has an 2a 2 1 1b 2 2 electron configuration and correlates with an electronically excited B + ion (2s 1 2p 1 z configuration). There have been several previous experimental studies of the B + +H 2 system, including ion-molecule reactions, 20 reactive scattering, [21][22][23] and chemiluminescence.…”