The Conformational Landscape, Internal Rotation, and Structure of 1,3,5-Trisilapentane using Broadband Rotational Spectroscopy and Quantum Chemical Calculations

The rotational spectrum of 1,3,5-trisilapentane was observed on a chirped-pulse Fourier transform microwave spectrometer and is reported. During assignment, multiple conformations of the molecule were identified in the molecular beam. Prior quantum-chemical calculations performed on the molecule show that the identified spectra correspond to the lowest three calculated energetic structures. These structures are of C ₂ (Conf.1), C _2ѵ (Conf.2), and C ₁ (Conf.3) symmetry, with relative energy ordering of Conf.1 < Conf.3 < Conf.2, which is in stark contrast to n-pentane and all known silicon-substituted n-pentane derivatives. This is found to most likely arise from the elongation of the Si-C bond and the size of the silicon atoms providing for the C ₂ and C ₁ structures relieving steric hindrance in comparison to that of the C _2ѵ . In the C _2ѵ and C ₁ conformers, splitting in the spectra due to internal rotation of the -SiH3 end groups of 1,3,5-trisilapentane was observed and determined. The C _2ѵ equivalent V ₃ values are 368.46(33) cm ^-1 , and the C ₁ V ₃ values are 347.78(21) and 360.18(88) cm ^-1 , respectively. These barriers are compared to similar species in order to help verify their veracity and are determined to be accurate based on similar molecular silyl rotors.