¹⁴N Hyperfine Structure and Low Methyl Torsional Barrier in 3-Picoline: Insights from Microwave Spectroscopy and Quantum Chemistry

The microwave spectrum of 3-picoline (3-methylpyridine) was investigated using a pulsed molecular jet Fourier transform microwave spectrometer and the newly developed Passage And Resonance In Synergy (PARIS) spectrometer across a frequency range of 2-26.5 GHz. The presence of a methyl internal rotor introduces torsional splitting, while the nitrogen nucleus of the pyridine ring gives rise to a hyperfine structure. A total 637 hyperfine-resolved rotational transitions were assigned and fitted using the programs <i>XIAM</i> and <i>BELGI-C</i>_<i>s</i><i>-hyperfine</i>, with the latter achieving measurement accuracy. The assignment was further validated by separately fitting the torsional species, which also reached measurement accuracy. The barrier to internal rotation was determined to be 60.198904(29) cm^-1, consistent with the values observed for the 2- and 4-isomers of picoline, where steric and electrostatic effects influence the barrier height. Quantum chemical calculations guided the spectral assignment, and benchmarking computations indicated that the rotational constants calculated at the MP2/6-31G(d,p) level could be useful as assignment guidance.