Precise equilibrium structures of 1H- and 2H-1,2,3-triazoles (C2H3N3) by millimeter-wave spectroscopy

The 1H- and 2H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130-375 GHz frequency range, we measured the spectroscopic constants for thirteen 1H-1,2,3-triazole and sixteen 2H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium (r(e)(SE)) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration-rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a best theoretical estimate (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1H tautomer are determined to <0.0008 angstrom and bond angles to <0.2 degrees. For the 2H tautomer, bond angles are also determined to <0.2 degrees, but bond distances are less precise (2 sigma <= 0.0015). Agreement between BTE and r(e)(SE) values is discussed.

Automated summary

Using millimeter-wave spectroscopy, the researchers obtained accurate equilibrium structures for 1H-1,2,3-triazole and 2H-1,2,3-triazole tautomers. These structures were determined based on spectroscopic constants and calculations for vibration-rotation interaction and electron-mass distribution. The results were compared with a theoretical estimate and showed good agreement.