The relative stabilities of benzotriazole tautomers determined by a rotational band contour analysis of the N-H stretching vibration

The FTIR spectrum of the N-H stretching vibration of 1H- and 2H-benzotriazo le (BT) has been measured in the gas phase in the region between 3440 cm(-1 ) and 3540 cm(-1) at six different temperatures. From the deconvolution of the partially resolved rotational band contours the contribution of the two tautomers of BT at each temperature is derived. From this temperature depe ndency the 1H-tautomer could be determined to be stabilized by 417 cm(-1) ( 5 kJ/mol) with respect to the 2H-tautomer. This experimental value is compa red to results of ab initio calculations on the Hartree-Fock, second order Moller-Plesset and coupled cluster level of theory as well as to different density functional methods. It will be pointed out that the correct order o f energies can only be obtained if electron correlation and zero point ener gy correction is taken into account in the ab initio calculations. (C) 1999 Elsevier Science B.V. All rights reserved.