Thermochemical Studies of N-Methylpyrazole and N-Methylimidazole

Abstract

The 351.1 nm photoelectron spectra of the N-methyl-5-pyrazolide anion and the N-methyl-5-imidazolide anion are reported. The photoelectron spectra of both isomers display extended vibrational progressions in the X2A' ground states of the corresponding radicals that are well reproduced by Franck-Condon simulations, based on the results of B3LYP/6-311++G(d,p) calculations. The electron affinities of the N-methyl-5-pyrazolyl radical and the N-methyl-5-imidazolyl radical are 2.054 +/- 0.006 eV and 1.987 +/- 0.008 eV, respectively. Broad vibronic features of the A(2)A' ' states are also observed in the spectra. The gas-phase acidities of N-methylpyrazole and N-methylimidazole are determined from measurements of proton-transfer rate constants using a flowing afterglow-selected ion flow tube instrument. The acidity of N-methylpyrazole is measured to be Delta(acid)G(298) = 376.9 +/- 0.7 kcal mol(-1) and Delta(acid)H(298) = 384.0 +/- 0.7 kcal mol(-1), whereas the acidity of N-methylimidazole is determined to be Delta(acid)G(298) = 380.2 +/- 1.0 kcal mol(-1) and Delta(acid)H(298)= 388.1 +/- 1.0 kcal mol(-1). The gas-phase acidities are combined with the electron affinities in a negative ion thermochemical cycle to determine the C5-H bond dissociation energies, D(0)(C5-H, N-methylpyrazole) = 116.4 +/- 0.7 kcal mol(-1) and D(0)(C5-H, N-methylimidazole) = 119.0 +/- 1.0 kcal mol(-1). The bond strengths reported here are consistent with previously reported bond strengths of pyrazole and imidazole; however, the error bars are significantly reduced.