Low temperature matrix-isolation and solid state vibrational spectra of 5-chlorotetrazole

The vibrational spectra of 5-chlorotetrazole (CN₄HCl) isolated in an argon matrix (T = 8.5 K) and in the solid state (at room temperature) were studied. The infrared spectrum of monomers of 5-chlorotetrazole isolated in an argon matrix agrees well with the spectrum predicted theoretically (DFT(B3LYP)/6-31G*) for the 2H-tautomer of the compound. The bands assigned to the 1H-tautomer appear in the experimental spectrum as very low intensity features. Based on the relative intensities of the bands in the spectra of the 1H- and 2H-tautomers, the relative amount of the first tautomer in this matrix can be estimated as 1%. Three matrixes were deposited with different nozzle temperatures and the enthalpy difference between the tautomers H = 8.0 kJ mol^-1 was estimated using the Van't Hoff relation. The internal energy difference between the two tautomers was predicted theoretically (DFT B3LYP/6-31G*) as 12.6 kJ mol^-1. This is in reasonable agreement with experimental observations. In the crystalline phase, this compound exists in its 1H-tautomeric form. Accordingly, the IR spectrum of polycrystalline 5-chlorotetrazole is well reproduced by the spectrum predicted theoretically for the 1H- tautomer.