Identity-reaction proton transfers from nitrogen acids yielding localized vs. delocalized conjugate bases. An ab initio study

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Identity-reaction proton transfers from a series of nitrogen acids to the corresponding conjugate bases have been studied by ab initio methods at the MP2/6-31+G*//MP2/6-31+G* level. The acids are NH4+, H2NNH3+, CH3NH3 +, CH2NH2, OCNH 2+, OCHNH3+, H2NCHNH2+ , HNCHNH 3+, NH3, CH3NH2, CH2NH, OCNH, OCHNH2,and HNCHNH2. Gas-phase acidities were calculated at the G2(MP2) level where experimental values are not available in order to have benchmark values for all acidities. Barriers to proton transfer relative to the separated reactants, ΔHTS, show a straight-line relation to acidity for all of the neutral acids and for all but four of the cationic acids. Three show ΔHTS values well above the line: HNCHNH3+, OCNH2+, and OCHNH3 +, in increasing order of positive deviation. One shows a small negative deviation: H2NNH3+ . The first three acids have localized pi systems but can yield delocalized transition structures and conjugate bases. The barriers result from a lag in delocalization relative to proton transfer in the transition structures. All of the other acids give transition structures that can only be localized, or if they can be delocalized they prefer to adopt conformations in which the unshared pair on nitrogen delocalizes rather than the electrons of the N---H bond. The negative deviation for H2NNH3 + is attributed to polarizability of the NH2 group.Key words: ab initio, nitrogen acids, proton transfer, acidity.