Synchrotron-based far infrared study of the rotation-vibration-inversion spectrum of silacyclobutane below 500 cm-1: The ν29 and ν30 bands

Abstract

Fourier transform spectra of the four-membered heterocycle silacyclobutane (c-C3H8Si) were recorded in the far infrared region from 100-500 cm-1 with a maximum resolution of 0.000959 cm-1 using synchrotron radiation from the Canadian Light Source. The two fundamental bands observed in this region correspond to motions best described as the out-of-plane modes related to ring puckering (ν30) at ~158 cm-1 and SiH2 rocking (ν29) at ~410 cm-1. Both bands exhibit complex, dense spectral patterns that arise from ring inversion tunneling of the puckered SCB ring through a planar (C2v) intermediate configuration. Analysis of these patterns revealed rotation-vibration transitions between states of the same inversion symmetry as well as rotation-vibration-inversion transitions that connect states of different inversion symmetry. Infrared ground state combination differences from 1871 pairs of P and R branch transitions were used to accurately determine the spectroscopic parameters for the tunneling-doubled ground state based on a broad range of quantum levels. With the ground state energy levels well-determined, 8255 infrared transitions were assigned and analyzed to derive the band centers, rotational and centrifugal distortion constants for the inversion split ν29 and ν30 vibrational states. Comparison with the band centers predicted via DFT (B3LYP) and MP2 calculations [6-311++G(2d,2p)] suggests that anharmonic corrections found via perturbation theory typically agree within 2% when compared with the observed spectrum of SCB.