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 to 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 (C(2v)) 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.

Analysis of the Coriolis perturbed rovibrational spectrum of the C O asymmetric stretch and C C symmetric stretch of trimethylene oxide

“…For comparison, the ring puckering mode of silacyclobutane (158 cm -1 ) is described by a deeper double-well potential with barrier to planarity (440 cm -1 ) above the lowest vibrational levels and in this case, the frequency is predicted to within 7.5% (146 cm -1 ) using harmonic calculations at the B3LYP/6-311++(2d,2p) level with the anharmonic terms providing a slightly worse estimate (142 cm -1 ) but still within 10%. 12 For a more harmonic potential, such as that of 3-oxetanone (with only a small quartic term in the potential), the level of agreement with experiment for the ring puckering mode (140 cm -1 ) 39 is similar with the harmonic calculations matching within 6.4% (149 cm -1 harmonic) and the anharmonic corrections predicting the value to within 13% (158 cm -1 anharmonic) at the B3LYP/6-311++(2d, 3p) level). 35 The quantum chemical results for TMO in this work are far less accurate and demonstrate that generic corrections are insufficient for modeling the low frequency modes of molecules with large amplitude motions and that in such cases, a more detailed theoretical treatment is needed.…”

“…In both cases, the barriers to planarity (274.2(2) cm -1 (TMS) 7 ; 440(3) cm -1 (SCB) 8 ) are considerably higher and give rise to inversion tunnelling features in the microwave and far infrared spectra. 7,[9][10][11][12][13][14] TMO has been the subject of numerous spectroscopic studies in the microwave, 3,4,15,16 millimeterwave, 17 far infrared 5,6,[18][19][20] and infrared regions. [21][22][23][24][25][26][27] These investigations have largely focused on the ground and ring puckering vibrational states.…”

Analysis of the rotation-vibration-inversion infrared spectrum of the ν 10 and ν 14 bands of silacyclobutane

“…We recently reported the first such study of SCB and described the assignment of 8 255 far infrared transitions corresponding to the weak ν30 ring puckering (~158 cm -1 ) and medium intensity ν29 SiH2 rocking (~410 cm -1 ) vibrations using spectra recorded at room temperature at the Canadian Light Source. 10 Both vibrational bands have B1 symmetry which leads to c-type rotational structure and in the high resolution spectrum, these rotation-vibration transitions were observed to be doubled due to ring inversion tunnelling. By symmetry, transitions that cross between inversion states of opposite parity are also allowed and are expected to follow a-type selection rules.…”

“…The far infrared spectra of small heterocycles, for example, provide information about the lowest energy states of these systems and have been the subject of numerous reviews. 6,7,8 In recent years, we have reported high resolution spectroscopic investigations of SCB in both the microwave 9 and far infrared 10 regions with the latter focusing on vibrations below 500 cm -1 . Although such four-membered rings have been under spectroscopic scrutiny for half a century, modern techniques have enabled the study of molecules such as SCB with greater detail.…”

“…Following individual band analysis, the assigned frequencies were added to those reported for the ν30 ring puckering and ν29 SiH2 rocking below 500 cm -1 . 10 The resulting data set consists of 20 626 lines and was used to derive accurate spectroscopic constants for each of the ground and four excited vibrational states as well as their ring inversion tunnelling splittings.…”