Unimolecular dissociation dynamics of disilane
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M. 
Published: July 2002

The unimolecular dissociation dynamics of disilane are investigated using classical trajectory methods with a global potential-energy surface fitted to the available experimental data and the results of various ab initio calculations. The potential surface is written as the sum of 52 many-body terms containing 86 adjustable parameters which are fitted to experimental and/or calculated data for stationary point geometries, fundamental vibrational frequencies, reaction endo- and exothermicities, and potential-energy barrier heights for reactions of disilane and molecules derived from disilane. In general, the equilibrium bond lengths and angles for Si2 H6 , Si2 H5 , H3 Si–SiH, H2 Si=SiH2, H2 Si=SiH, H2 Si=Si, HSi=Si, Si2 , H2 , and SiH2 given by the global potential agree with ab initio results to within 0.03 A and 2°, respectively, or better. The predicted heats of reaction for 13 reactions involving disilane or its derivatives are in good accord with the experimental and ab initio results.

The average absolute deviation is 3.55 kcal/mol. The average absolute difference between the normal-mode frequencies given by the global potential for Si2 H6 , Si2 H5, and H3 Si–SiH and those obtained from scaled MP4 calculations are 58.7, 52.1, and 62.8 cm−1 , respectively. If two low-frequency Si–Si–H deformation modes for each of these molecules are omitted from consideration, the average absolute differences are all in the range 34–36 cm−1 . The calculated barrier height for the hydrogen-atom transfer process leading to SiH4 +SiH2 products is 56.7 kcal/mol. For three- and four-center H2 elimination reactions, the barrier heights given by the global surface are 60.1 and 91.1 kcal/mol, respectively. These values are all within 1.2 kcal/mol of the results obtained by Ho et al. from MP4 calculations. The Si2 H6 dissociation dynamics at seven internal energies ranging from 5.31 to 9.31 eV have been investigated. At low internal energy, dissociation leading to SiH4 +SiH2 dominates the dynamics.

At internal energies in the range 5.31≤E≤6.31 eV, the various Si2 H6 decomposition channels are, in order of importance, hydrogen-atom transfer leading to SiH4 +SiH2 , Si–Si bond rupture giving two SiH3 radicals, three-center H2 elimination, Si–H bond rupture to give Si2 H5 +H, and four-center H2 elimination to give H2 Si=SiH2 . At higher internal energies, entropy effects cause an inversion of this ordering such that Si–Si bond rupture becomes the major decomposition channel followed by three-center H2 elimination, SiH4 +SiH2 formation, Si–H bond rupture, and four-center H2 elimination. The present results suggest that the formation of disilene in disilane pyrrolysis occurs predominantly via the formation of H3 Si–SiH from three-center H2 elimination followed by a low-barrier hydrogen transfer process. For all decomposition channels, most of the available energy is partitioned in vibrational modes of the products. To a large extent, product energy partitioning is found to be governed by statistical considerations. Exceptions to this generalization are found in three-center H2 elimination and for any product which involves the formation of a new bond. We find that while three-center H2 elimination is a concerted reaction, it probably does not occur along a symmetric pathway. Hydrogen transfer to form SiH4 +SiH2 is found to be a concerted process, but four-center H2 elimination involves the rupture of one Si–H bond followed by hydrogen transfer and a subsequent H2 abstraction reaction to give the H2 +H2 Si=SiH2 products.