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

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 (DeltaE(1H-2H)) was then obtained. The experimental value, AE(1H-2H) = 6.95 +/- 1.50 kJ mol(-1), 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)).