Effects of alkyl-group substitution on the proton-transfer barriers in ammonium and hydroxylammonium nitrate salts

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.