AB-INITIO AND DENSITY-FUNCTIONAL THEORY STUDIES OF PROTON-TRANSFER REACTIONS IN MULTIPLE HYDROGEN-BOND SYSTEMS

We have carried out both nb initio molecular orbital theory and density functional theory studies of mechanisms of proton transfer reactions in multiple hydrogen bond systems using formamidine and its mono-, di-, and trihydrated complexes as model systems. The highest level of nb initio theory, namely CCSD(T)/6-31G(d,p) at the optimized MP2 geometries, predicts the tautomerization in formamidine to have a high classical barrier of 48.8 kcal/mol. Adding one, two, or three waters to form cyclic hydrogen bond clusters stabilizes the transition state assisting proton transfer via a concerted mechanism and reduces this barrier to 21.9, 20.0, or 23.7 kcal/mol, respectively. Compared to our best ab initio CCSD(T)/6-3 IG(d,p)//MP2 results, we found that, among the local DFT JMW and VWN and nonlocal DFT B-LYP, B-P86, B3-P86, BH&H-LYP, and B3-LYP methods, only the hybrid BH&H-LYP method is capable of predicting the structure and energetic information of both the minimum energy and transition structures at a comparable accuracy with the MP2 level. We also found that using numerical atomic orbital or DFT-based Gaussian-type-orbital (GTO) basis sets yields slightly more accurate DFT results than using an HF-based GTO basis set at the 6-31G(d,p) level.