MILLIMETER-WAVE AND SUBMILLIMETER-WAVE SPECTROSCOPY OF DIBRIDGED SI2H2 ISOTOPOMERS - EXPERIMENTAL AND THEORETICAL STRUCTURE

Various isotopomers of the free dibridged disilyne molecule, Si(H-2)Si , have been observed by millimeter- and submillimeter-wave spectroscop y in a silane-argon plasma produced by an abnormal electric discharge. In order to make measurement of the weak absorption lines possible a novel computer processing treatment has been developed. From the molec ular constants measured for Si-28(H-2)Si-28, Si-29(H-2)Si-28, Si-30(H- 2)Si-28, and Si-28(D-2)Si-28 an accurate substitution structure has be en deduced with r(s)(Si-Si)=2.2154 Angstrom, r(s)(Si-H)=1.6680 Angstro m, and angle(HSiSiH)=104.22 degrees The spectrum analysis and the stru cture determination have been aided by correlated level ab initio calc ulations resulting in accurate estimates of the equilibrium geometry a nd rotational constants, the cubic force field, the quartic and sextic centrifugal distortion constants, and the inversion barrier height of dibridged disilyne. The barrier to inversion of the ''butterfly-type' ' Si(H-2)Si molecule must be relatively high as no splitting due to in version could be experimentally observed.