Microwave Spectra and Barrier to Internal Rotation in Cyclopropylmethylsilane

Abstract

Rotational spectra for 3 silicon isotopologues (28Si, 29Si, 30Si) of cyclopropylmethylsilane (c-C3H5SiH2CH3) have been observed in natural abundance using Fourier-transform microwave spectroscopy, and the dipole moment of the most abundant (28Si) isotopologue has been determined using the Stark effect. The observed rotational constants (A = 8800.5997(9) MHz; B = 2238.6011(3) MHz; C = 2001.0579(3) MHz) and dipole moment components (mu(a) = 0.195(2) D, mu(b) = 0.674(11) D, mu(c) = 0.362(19) D, mu(total) = 0.790(13) D) for the 28Si species are consistent with ab initio predictions (MP2/6-311+G(d)) for a gauche conformation about the Si-cyclopropyl bond. All of the observed transitions were split into doublets due to internal rotation of the methyl group, allowing a determination of the V3 barrier to internal rotation of 6.671(9) kJ mol(-1) for the most abundant isotopologue. This barrier will be compared to those for other Si-CH(3) containing compounds and will be related to a partial structure determination from the available microwave and ab initio data.

Rotational spectrum of five isotopologues of cyclopropylmethylgermane: Ab initio calculations, barrier to internal rotation and nuclear quadrupole coupling constants for the 73Ge nucleus

“…The gauche conformer of this molecule was the only conformer observed in the supersonic expansion of the microwave study and a barrier to rotation of the methyl group of 6.672(9) kJ mol À1 was obtained from an analysis of the internal rotation doublets [2]. The vibrational work agreed that the gauche conformer was the more stable although evidence for a cis form was also seen in the IR and Raman studies [1].…”

“…It can be seen that agreement is reasonable, with l c agreeing exactly while the l a and l b components are slightly overestimated at MP2 level (having percent differences of 13% and 6%, respectively); the largest difference is for the component of smallest magnitude. Comparison of these values with those obtained for the silicon analog, cyclopropylmethylsilane (CMS) [2] reveals similar charge distribution in the two molecules, although the total dipole moment of CMS is about 16% higher than CMG (CMG: l a = 0.1782 (10) …”

“…V 3 in CMG is about 71% of the Table 2 Spectroscopic constants for the gauche conformer of cyclopropylmethylgermane. Rotational constants, centrifugal distortion constants, and internal rotation parameters are tabulated; nuclear quadrupole coupling constants for the 73 Ge species are listed in Table 4 value obtained for the silicon analog, CMS (6.671(9) kJ mol À1 ), by similar analysis of the internal rotation splittings [2]; in the case of CMS, use of the IR torsional frequency [4] We can also compare this microwave barrier in CMG with a reasonable (although crude) ab initio estimate of this parameter of 4.2 kJ mol À1 (Table 2), computed by taking an energy difference between the staggered and the eclipsed conformation of the methyl group.…”

“…Recent studies reported IR and Raman measurements [1] and a separate analysis of the microwave spectrum and determination of the barrier to rotation [2] for cyclopropylmethylsilane (CMS). The gauche conformer of this molecule was the only conformer observed in the supersonic expansion of the microwave study and a barrier to rotation of the methyl group of 6.672(9) kJ mol À1 was obtained from an analysis of the internal rotation doublets [2].…”

“…In this paper we confirm the existence of the most stable (gauche) conformer of CMG and focus on a detailed analysis of the Fourier-transform microwave spectroscopic results, outlining the assignment of rotational spectra for five Ge isotopologues, a determination of the barrier to rotation for the methyl group, measurement of dipole moment components and finally an interpretation of nuclear quadrupole hyperfine structure of the 73 Ge nucleus. A comparison of these parameters, both with other Ge containing molecules and in the related Si analog (CMS) [1,2] will also be made.…”