Millimeter-Wave and High-Resolution Infrared Spectroscopy of the Ground and Seven Lowest Fundamental States of 1 H -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 H -1,2,4-triazole is reported. While the lowest-energy vibrationally excited state (ν 18 ) is well-treated using a single-state distorted-rotor Hamiltonian, the second (ν 17 ) and third (ν 16 ) vibrationally excited states are involved in strong c -type Coriolis coupling and require an appropriate two-state Hamiltonian. The oblate nature of 1 H -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 r representation in order to achieve low σ fit values. The coupling between ν 17 (A″) and ν 16 (A″) resulted in many transitions with slightly perturbed frequencies, many highly displaced resonant intrastate transitions, and 165 nominal interstate transitions. Modeling the spectra of ν 17 and ν 16 required three c -axis Coriolis-coupling terms ( F ab , F ab J , and F ab K ) to treat the interaction. Many of the nominal interstate transitions form clearly discernible Q-branch bands, comprising degenerate sets of a - and b -type transitions. The rotational spectra of four higher-energy vibrationally excited states (ν 15 , ν 14 , ν 13 , and ν 12 ), 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: ν 18 = 542.601 824 3 (28) cm -1 , ν 17 = 665.183 128 5 (43) cm -1 , ν 16 = 682.256 910 5 (43) cm -1 , and ν 15 = 847.557 400 (11) cm -1 .