Thermochemical Studies of N-Methylpyrazole and N-Methylimidazole
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- 作者：Stephanie M. Villano ; Adam J. Gianola ; Nicole Eyet ; Takatoshi Ichino ; Shuji Kato ; Veronica M. Bierbaum ; W. Carl Lineberger
- 刊名：Journal of Physical Chemistry A
- 出版年：2007
- 出版时间：September 6, 2007
- 年：2007
- 卷：111
- 期：35
- 页码：8579 - 8587
- 全文大小：219K
- 年卷期：v.111,no.35(September 6, 2007)
- ISSN：1520-5215
文摘
The 351.1 nm photoelectron spectra of the N-methyl-5-pyrazolide anion and the N-methyl-5-imidazolide anionare reported. The photoelectron spectra of both isomers display extended vibrational progressions in the 
2A'ground states of the corresponding radicals that are well reproduced by Franck-Condon simulations, basedon the results of B3LYP/6-311++G(d,p) calculations. The electron affinities of the N-methyl-5-pyrazolylradical 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 Ã2A' ' states are also observed in the spectra. The gas-phase acidities ofN-methylpyrazole and N-methylimidazole are determined from measurements of proton-transfer rate constantsusing a flowing afterglow-selected ion flow tube instrument. The acidity of N-methylpyrazole is measured tobe 
acidG298 = 376.9 ± 0.7 kcal mol-1 and acidH298 = 384.0 ± 0.7 kcal mol-1, whereas the acidity ofN-methylimidazole is determined to be acidG298 = 380.2 ± 1.0 kcal mol-1 and acidH298 = 388.1 ± 1.0 kcalmol-1. The gas-phase acidities are combined with the electron affinities in a negative ion thermochemicalcycle to determine the C5-H bond dissociation energies, D0(C5-H, N-methylpyrazole) = 116.4 ± 0.7 kcalmol-1 and D0(C5-H, N-methylimidazole) = 119.0 ± 1.0 kcal mol-1. The bond strengths reported here areconsistent with previously reported bond strengths of pyrazole and imidazole; however, the error bars aresignificantly reduced.
2A'ground states of the corresponding radicals that are well reproduced by Franck-Condon simulations, basedon the results of B3LYP/6-311++G(d,p) calculations. The electron affinities of the N-methyl-5-pyrazolylradical 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 Ã2A' ' states are also observed in the spectra. The gas-phase acidities ofN-methylpyrazole and N-methylimidazole are determined from measurements of proton-transfer rate constantsusing a flowing afterglow-selected ion flow tube instrument. The acidity of N-methylpyrazole is measured tobe acidG298 = 376.9 ± 0.7 kcal mol-1 and acidH298 = 384.0 ± 0.7 kcal mol-1, whereas the acidity ofN-methylimidazole is determined to be acidG298 = 380.2 ± 1.0 kcal mol-1 and acidH298 = 388.1 ± 1.0 kcalmol-1. The gas-phase acidities are combined with the electron affinities in a negative ion thermochemicalcycle to determine the C5-H bond dissociation energies, D0(C5-H, N-methylpyrazole) = 116.4 ± 0.7 kcalmol-1 and D0(C5-H, N-methylimidazole) = 119.0 ± 1.0 kcal mol-1. The bond strengths reported here areconsistent with previously reported bond strengths of pyrazole and imidazole; however, the error bars aresignificantly reduced.