THE REMARKABLE MONOBRIDGED STRUCTURE OF SI2H2

Inspired by the observation of a monobridged structure of Si2H2 by Cordonnier et al. via microwave spectroscopy (see the following paper), we have reinvestigated the Si2H2 singlet state potential energy surface using large basis sets and extensively correlated wave functions. Coupled-cluster single, double, and (perturbative) triple excitation methods [CCSD(T)] in conjunction with a triple-zeta 2df (TZ2df) basis set on silicon and a triple zeta with two sets of polarization (TZ2P) basis set on hydrogen predict that the monobridged Si(H)SiH structure is indeed a minimum; in fact, Si(H)SiH is the second most stable Si2H2 isomer, as suggested by a recent theoretical study [B. T. Colegrove and H. F. Schaefer, J. Phys. Chem. 94, 5593 (1990)]. The predicted Si(H)SiH geometrical structure-which exhibits the shortest SiSi bond distance of any molecule characterized to date-and hence the rotational constants, as well as the quartic centrifugal distortion constants are in good agreement with the experimental data. We have located transition states between these pairs of minima-disilavinylidene H2SiSi and monobridged Si(H)SiH; monobridged and dibridged Si(H2)Si; trans-HSiSiH and monobridged. We predict Si(H)SiH to lie 8.7 kcal mol-1 above Si(H2)Si, with the transition state between them 3.7 kcal mol-1 higher. H2SiSi is predicted to lie 11.6 kcal mol-1 above Si(H2)Si and the transition state barrier between H2SiSi and Si(H)SiH is 2.4 kcal mol-1 above H2SiSi. Predictions of absolute 0 K heats of formation for the various structures are presented.