Experimental and theoretical investigation of the geometry and vibrational frequencies of 1,2,3-triazole, 1,2,4-triazole, and tetrazole anions

Optimized geometries, vibrational frequencies, scale factors, and IR absorption intensities were calculated for the 1,2,3-triazole anion (1) at the B3LYP, LDA, BLYP, BVWN, HF, and MP2 levels of theory using the 6-31G**(BS1), 6-31++G**(BS2), cc-pVDZ (BS3) and aug-cc-pVDZ (BS4) basis sets and for 1,2,4-triazole (2) and tetrazole (3) anions at the same lavels of theory using the BS1 and BS2 basis sets only. The calculations were also carried out at the AM1, PM3 and MNDO semiempirical levels. For comparison with the calculated results, the IR and Raman spectra of 2 and 3 were measured, while for 1 the experimental assignment reported in the literature was used. Measurement of the spectra of 2 and 3 resulted in a complete assignment of the fundamental vibrations of both anions except for few bands corresponding only to the C-H stretching and out-of-plane modes. The force fields were scaled with respect to the experimental frequencies using one-scale-factor (1SF) scaling. The agreement between the calculated and experimental frequencies was excellent, especially with the B3LYP method, scaled and unscaled. The agreement between the experimental and calculated IR absorption intensities was generally satisfactory. The calculated scale factors for the studied molecules were larger than commonly calculated for other neutral molecules. The basis set effect on the calculated results was generally small. Differences and similarities between the calculated frequencies and scale factors by the different methods were rationalized in terms of the optimized geometries.