Internal rotation in methyl silane by avoided-crossing molecular-beam spectroscopy

The avoided-crossing molecular-beam electric-resonance technique was applied to methyl silane in the ground torsional state. A new type of anticrossing is introduced which breaks the torsional symmetry and obeys the selection rules ∆ J = 0, K = +1 / a3 -1. For these "barrier" anticrossings, the values of the crossing fields E_c yield directly the internal rotation splittings; the E_c are independent of the difference ( A- B) in the rotational constants. Such anticrossings were observed for J from 1 to 6. Studies were also conducted of several "rotational" anticrossings ( J, K) = (1, ±1) / a3 (2, 0) for which E_c does depend on ( A- B). The normal rotational transition ( J, K) = (1, 0) ← (0, 0) was observed in the ground torsional state using the molecular beam spectrometer. The present data on CH ₃²⁸SiH ₃ were combined with Hirota's microwave spectra and analyzed with the torsion-rotation Hamiltonian including all quartic centrifugal distortion terms. In addition to evaluating B and several distortion constants, determinations were made of the moment of inertia of the methyl top I_α = 3.165(5) amu-Å ², the effective rotational constant A^eff = 56 189.449(32) MHz, and the effective height of the threefold barrier to internal rotation V₃^eff = 592.3359(73) cm ^-1. The correlations leading to these two effective constants are discussed and the true values of A and V₃ are determined within certain approximations. For the isotopic species CH ₃³⁰SiH ₃, barrier and rotational anticrossings were observed. The isotopic changes in A and V₃ were determined, as well as an upper limit to the corresponding change in I_α.