Millimeter-Wave and High-Resolution Infrared Spectroscopy of the Ground and Seven Lowest Fundamental States of 1<i>H</i>-1,2,4-Triazole

A combined analysis of millimeter-wave (70-700 GHz) and rotationally resolved infrared (400-1200 cm^-1) spectra of the ground state and seven fundamental vibrational modes of 1<i>H</i>-1,2,4-triazole is reported. While the lowest-energy vibrationally excited state (ν₁₈) is well-treated using a single-state distorted-rotor Hamiltonian, the second (ν₁₇) and third (ν₁₆) vibrationally excited states are involved in strong <i>c</i>-type Coriolis coupling and require an appropriate two-state Hamiltonian. The oblate nature of 1<i>H</i>-1,2,4-triazole is sufficiently close to the oblate symmetric-top limit that the analysis requires the use of A-reduced, sextic centrifugally distorted-rotor Hamiltonian models in the I^<i>r</i> representation in order to achieve low σ_fit values. The coupling between ν₁₇ (A″) and ν₁₆ (A″) resulted in many transitions with slightly perturbed frequencies, many highly displaced resonant intrastate transitions, and 165 nominal interstate transitions. Modeling the spectra of ν₁₇ and ν₁₆ required three <i>c</i>-axis Coriolis-coupling terms (<i>F</i>_<i>ab</i>, <i>F</i>_<i>ab</i>^<i>J</i>, and <i>F</i>_<i>ab</i>^<i>K</i>) to treat the interaction. Many of the nominal interstate transitions form clearly discernible Q-branch bands, comprising degenerate sets of <i>a</i>- and <i>b</i>-type transitions. The rotational spectra of four higher-energy vibrationally excited states (ν₁₅, ν₁₄, ν₁₃, and ν₁₂), which form a complex polyad involving Coriolis and anharmonic coupling interactions, were analyzed by single-state models, thus producing only effective spectroscopic constants. Inclusion of rotationally resolved infrared transitions enabled the accurate and precise determination of vibrational band origins for the four lowest-energy fundamental states: ν₁₈ = 542.601 824 3 (28) cm^-1, ν₁₇ = 665.183 128 5 (43) cm^-1, ν₁₆ = 682.256 910 5 (43) cm^-1, and ν₁₅ = 847.557 400 (11) cm^-1.