1,2,4-Triazole: Vibrational spectra, normal coordinate calculations, and hydrogen bonding

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Abstract
The influence of hydrogen bonding on the vibrational spectra of 1,2,4‐triazole has been studied using infrared and Raman spectroscopy and normal coordinate calculations. The infrared vapor phase fundamentals have been assigned. The polarized infrared and Raman spectra of triazole single crystals (Pbca,Z=8) have been investigated in the 4000 to 10 cm−1 region. An assignment of the 18 intramolecular fundamentals is given with help of −d1 and −d3 isotopic derivatives and normal coordinate calculations. Correlation field splitting is discussed. Thirty‐two of the 45 lattice modes have been assigned to their symmetry species and in terms of rotational, translational, and hydrogen bond vibrations. They are compared to those calculated on the ground of the rigid body and flexible molecule models assuming appropriate intermolecular atom–atom potentials. The variation of the NH stretching and bending force constants upon hydrogen bond formation is correlated with the hydrogen bond force constant (0.35 mdyn/Å), N–N distance and low (1.25) νNH/νND ratio, indicating a positive isotope effect. The structure and breadth of the NH stretching band are discussed; the half‐width of the corresponding Raman band decreases from 400 to 10 cm−1 when the temperature is lowered from 300 to 10 K.