σ Bond Activation by Cooperative Interaction with ns² Atoms:  Be + nH₂, n = 1−3

Abstract

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.

A sequential transformation approach to the internally contracted multireference coupled cluster method

“…1. 38 This model has been used extensively in the literature 6,28,[39][40][41][42][43][44] to test new multireference methods since it is a prototype for symmetry forbid- den reactions. 45 In the BeH 2 model, the reaction coordinate is parametrized by a one-dimensional variable x that falls in the range [0,4] bohrs.…”

Sigma Bond Activation by Cooperative Interaction: B+ + CH4 + nH2

“…Subsequent theoretical studies on the isoelectronic Li -+ nH 2 and Be + nH 2 systems 4,5 and the isovalent Al + + nH 2 system 3 sought to identify how much of the activation energy lowering in the cooperative interaction mechanism could be attributed to the node structure of the orbitals, how much to the effect of the positive charge and relative bond strengths, and how much to the special ability of boron to form three-center two-electron bonds (3c-2e). The results revealed that (1) the n ) 1 activation energy was dominantly controlled by M-H bond energy, (2) the positive charge and HM-H bond energy play important roles in the activation energy lowering occurring for n ) 2, and (3) 3c-2e bonding played a small role in the n ) 3 case.…”

“…The activation of sigma bonds, particularly CH sigma bonds, is an active area of experimental and theoretical research and has been referred to as one of the "Holy Grails" of modern chemical research. 1 Recent theoretical studies [2][3][4][5][6] have identified a new mode of sigma bond activation that depends critically on the number of sigma bonds simultaneously interacting with an atom in an s 2 electron configuration. Motivated by unexpected experimental results, 7 theoretical studies 2 of this cooperative interaction mechanism first focused on the gas phase reactions: B + ( 1 S g ) + nH 2 f BH 2 + + (n -1)H 2 , n ) 1-4.…”