a Weak intermolecular interactions of imidazole, urea and thiourea studied by microwave spectroscopy

Abstract

Microwave spectroscopy is a powerful spectroscopic tool in determining the structure of gas phase molecules. The microwave spectra of imidazole, urea or thiourea containing complexes such as (C3N2H4)2, (H2N)2CO...HNCO, (H2N)2CO...C3H4N2, and H2O...CS(NH2)2 have been recorded. The spectrum of various isotopologues have been measured in order to determine structural information. The geometry of these weakly bound complexes is investigated, alongside the nature of the weak intermolecular interactions present. A chirped-pulse Fourier transform microwave spectrometer (CP-FTMW) has been used in the work detailed in this thesis to measure the spectra of such molecules. The instrument allows measurement of spectra over the full range of 6.5-18.5 GHz. The molecules presented in this thesis were formed as a result of laser vaporisation of solid organic precursors in the presence of argon introduced from a pulsed nozzle. The subsequent supersonic expansion generates rotationally cool molecules (~2 K) and stabilises the weakly bound complexes. The (C3N2H4)2 complex was found to adopt a twisted configuration with a single stabilising intermolecular hydrogen bond. The (H2N)2CO...HNCO complex was observed to adopt an almost planar structure, forming two intermolecular hydrogen bonds. The N-H group of HNCO acts as both a hydrogen bond donor and acceptor. The (H2N)2CO...C3H4N2 complex was found to adopt a conformation where two intermolecular hydrogen bonding interactions around the N-H group of imidazole are present. Finally the H2O...CS(NH2)2 complex was found to contain two intermolecular hydrogen bonds, where water acts as a donor and acceptor forming S...H and O...H interactions. The results of ab initio calculations at the DFT (density functional theory), MP2 (second order Møller-Plesset perturbation theory) and CC (coupled cluster) level of theory are presented and compared to experimental results.