Complex hyperfine-fine structure overlapping in the microwave spectrum of 3,4-lutidine

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 <i>meta</i> methyl group and 426 cm^-1 for the <i>para</i> methyl group. Unlike in other xylene derivatives with adjacent <i>meta</i>-<i>para</i> methyl groups, where the <i>para</i> barrier is typically higher, the reversal observation in 3,4-lutidine is attributed to the strong <i>meta</i>-directing effect of the nitrogen atom in the aromatic ring, which enhances electron density at the <i>meta</i> position. A comparative analysis with structurally related molecules highlights the interplay between steric and electrostatic effects in determining methyl torsional barriers.