Barriers to internal rotation in methylimidazole isomers determined by rotational spectroscopy

The rotational spectra of N-, 2-, 4-, and 5-methylimidazole are reported and analyzed. Liquid N-methylimidazole was vaporized from a reservoir, and each of 2-, 4-, and 5-methylimidazole was laser-vaporized from a solid target prior to mixing with argon buffer gas and undergoing supersonic expansion from a pulsed nozzle. The spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy in the 7.0-18.5 GHz frequency range. Rotational constants, A₀, B₀, and C₀, centrifugal distortion constants, D_J, D_JK, D_K, d₁, and d₂, and nuclear quadrupole coupling constants of nitrogen atoms, χ_aa(N1), χ_bb(N1) - χ_cc(N1), χ_aa(N3), and χ_bb(N3) - χ_cc(N3), are determined from experimentally measured transition frequencies. Data recorded for isotopologues containing ¹³C or ¹⁵N are used to determine the r_s coordinates of all heavy atoms in N-, 2-, and 4-methylimidazole. The results allow fitting of parameters in the Hamiltonian that describes internal rotation of the CH₃ group about its local C₃ axis. The V₃ terms in the periodic potential energy functions that describe the internal rotation in N-, 2-, 4-, and 5-methylimidazole are determined to be 185.104(11), 122.7529(38), 317.20(14), and 386.001(19) cm^-1, respectively, by the internal axis method. The experiments are supported by density functional theory calculations. Observed variations in barrier height are explained with reference to the symmetry of overlap between a π-like orbital on the CH₃ group and π-orbitals on the aromatic ring.