Low temperature matrix-isolation and solid state vibrational spectra of tetrazole

Abstract

Infrared spectra of tetrazole (CN4H2) isolated in an argon matrix (T = 10 K) and in the solid state (at room temperature) were investigated. In the crystalline phase, tetrazole exists in its 1H-tautomeric form and new assignments of the vibrational spectra (both infrared and Raman) of this phase are presented. The infrared spectrum of the matrix-isolated monomeric form of tetrazole is now reported and assigned for the first time, showing essentially the expected signature of the 2H-tetrazole tautomer. From relative intensities of the infrared bands ascribable to the two tautomers, the amount of the 1H-tautomer in the argon matrix was estimated to be ca. 10% of the most stable tautomer. Assuming that gas-phase relative populations of the two tautomers could be efficiently trapped in the argon matrix during deposition, the energy difference between 1H- and 2H-tetrazole (ΔE1H–2H) was then obtained. The experimental value, ΔE1H–2H = 6.95 ± 1.50 kJ mol−1, now determined for the first time, compares fairly well with the theoretical predictions for the molecule in vacuum (e.g., the zero point vibrational energy corrected energy difference obtained at the B3LYP/6–31G* level of theory is 9.96 kJ mol−1).