Complex hyperfine-fine structure overlapping in the microwave spectrum of 3,4-lutidine†
Abstract
The microwave spectrum of 3,4-lutidine was measured using two pulsed molecular jet Fourier transform microwave spectrometers in the 2.0–20.0 GHz frequency range to investigate the effects of two inequivalent methyl internal rotations and 14N nuclear quadrupole coupling. Rotational transitions were observed as complex multiplets due to the combined influence of methyl torsion fine splittings and 14N hyperfine splittings. A total of 680 transitions were assigned and fitted using the XIAM program, yielding a standard deviation close to measurement accuracy. Quantum chemical calculations at various levels of theory, including B3LYP-D3BJ/6-311++G(d,p) and MP2/6-311++G(d,p), provided reliable estimates of rotational constants to guide the spectral assignment. The barriers to methyl internal rotation were determined to be 510 cm−1 for the meta methyl group and 426 cm−1 for the para methyl group. Unlike in other xylene derivatives with adjacent meta–para methyl groups, where the para barrier is typically higher, the reversal observation in 3,4-lutidine is attributed to the strong meta-directing effect of the nitrogen atom in the aromatic ring, which enhances electron density at the meta position. A comparative analysis with structurally related molecules highlights the interplay between steric and electrostatic effects in determining methyl torsional barriers.