Chirped Pulse Fourier Transform Microwave Spectroscopy of Nitrogen-Containing Organic Molecules

Abstract

Chirped Pulse Fourier Transform Microwave (CP-FTMW) spectroscopy is a powerful analytical technique. Its versatile applications include molecular structure determination, generating molecular rotational signatures for astrochemistry and probing weak inter- and intra-molecular interactions as well as large amplitude motions. For this thesis, experiments were performed with the CP-FTMW Spectrometer at Newcastle University operating in the 2.0-18.5 GHz region, featuring a pulsed valve for supersonic expansion and a laser ablation source for gas-phase generation of non-volatile species. The microwave spectra of i) phosphine-carboxamide and a van der Waals complex with argon, ii) two monohydrate isomers of imidazole, iii) methylimidazole isomers and iv) monohydrate complexes of methylimidazole isomers are reported and analyzed. Molecular rotations probed in the microwave spectra are directly correlated to the geometry of these species. Detection of isotopologues, in natural isotopic abundance or by using isotopically enriched samples, allows for full or partial structure determination and rationalization of large amplitude motions. Phosphine-carboxamide is a phosphorus-bearing analogue of urea. Coordination of an argon atom highlights that the electronic density is concentrated at the amide site. The monohydrate complexes of imidazole exhibit bifunctional hydrogen bonds with imidazole acting either as a proton donor or acceptor. The spectra of methylimidazole isomers exhibit internal rotation of the CH3 group in each isomer. The relative energy required to surmount the barrier to internal rotation in each isomer is relatable to the position where the CH3 substitutes on the imidazole ring. The barrier depends on the chemical environment and orbital overlap between the CH3 group and the adjacent C-H bonds. In the monohydrate complexes of methylimidazole only one type of hydrogen bonding was detected and internal rotation of CH3 group was observed. Results are supported by ab initio and density functional theory (DFT) calculations.