Benchmarking a new segmented K-band chirped-pulse microwave spectrometer and its application to the conformationally rich amino alcohol isoleucinol

Abstract

Isoleucinol, a potential precursor to the essential α-amino acid isoleucine, has been studied using microwave spectroscopy from 2–26 GHz. The measurements between 18–26 GHz were performed with a newly developed segmented chirped-pulse Fourier transform microwave spectrometer, which has reduced the cost of the instrument by half compared to a single pulse excitation and direct detection chirped-pulse microwave spectrometer in the same frequency range. The performance of the instrument has been demonstrated and found to be comparable to the previous design. For isoleucinol, the flexibility of the sec-butyl side chain (R = –CH(CH₃)CH₂CH₃) can result in more than 200 different conformers from its five dihedral angles, and experimentally, seven conformers have been assigned. A fit including the hyperfine splitting due to nitrogen nuclear quadrupole coupling for the rotational transitions is reported for all conformers, along with the experimental structures of the three lowest energy conformers. The observed conformers have intramolecular N⋯H–O hydrogen bond interactions, similar to the second energetically favorable conformer of the analogous amino acid, isoleucine. A complete linelist has been provided to facilitate a search for isoleucinol in the interstellar medium.