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Effects of Alkyl-Group Substitution on the Proton-Transfer Barriers in Ammonium and Hydroxylammonium Nitrate Salts

From National Research Council Canada

DOIResolve DOI: https://doi.org/10.1021/jp040189r
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Abstract
The effects of alkyl (CH3 and C2H5) substitutions for hydrogen on the proton-transfer barriers in ammonium nitrate (AN) and hydroxylammonium nitrate (HAN) are studied by using ab initio electronic structure calculations. The optimized hydrogen-bonded neutral-pair structures and the ion-pair transition states for proton transfer are determined at the B3LYP/6-311++G(d,p) level. Zero-point energies, basis set superposition corrections, and single-point MP2 calculations on the optimized structures are applied to obtain binding energies for these hydrogen-bonded molecules. The alkyl substituents strengthen the hydrogen bonding in both the neutral- and ion-pair complexes, but the ion-pair forms are stabilized to a greater extent, which results in a decrease in the barrier to proton transfer and exchange. The energy barrier to proton transfer in AN is 8.1 kcal/mol, whereas in methylammonium (MeA), ethylammonium (EtA), and dimethylammonium (diMeA) nitrate this barrier decreases to 4.1, 3.7, and 1.4 kcal/mol, respectively. The alkyl substitution reduces the proton-transfer barrier, and the dialkyl substitution reduces it even further. A similar trend holds for HAN and methylhydroxylammonium nitrate (MeHAN); the barrier to proton transfer from the most stable neutral-pair HAN to the lowest-energy ion-pair configuration is 13.6 kcal/mol, whereas this barrier decreases to 9.5 kcal/mol in the corresponding MeHAN complex. The effect of the alkyl substitutions on the basicity and strength of hydrogen bonds in the complex is discussed.
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  • The Journal of Physical Chemistry A 108, no. 41 (14 October 2004): 8801–8809.
LanguageEnglish
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NPARC number12327066
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Record identifier29339493-2c5f-48ea-9f0d-86457b95274c
Record created2009-09-10
Record modified2020-04-17
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