Studies on tautomerism: Benchmark quantum chemical calculations on formamide and formamidine

Tautomerism is a ubiquitous phenomenon in both chemistry and molecular biology and numerous computational studies of variable accuracy are available on it. In the present work, we report on some benchmark quantum chemical calculations on the formamide ↔ formamidic acid (1) and formamidine ↔ formamidine (2) systems. Some results on the acetaldehyde ↔ vinyl alcohol and acetaldimine ↔ vinyl amine pairs are also presented. A central question of the study is the role of water as a catalyst, investigated in form of the monohydrates of 1 and 2. Optimized structures have been obtained for the tautomers and the transition states, including their monohydrates. The calculations represent the most comprehensive and highest level study up to now, with the goal of converging the relevant energy differences to an accuracy of 0.5 kcal/mol. To this aim, basis sets cover a range from simple 6-31G(d,p) up to 6-311++G(3df,3pd) and from cc-pVTZ up to aug-cc-pV5Z. The electron correlation treatment has been varied from RHF and DFT up to CCSD(T), plus one case with CCSDT (true triples). The convergence of the results with respect to the computational level is, unfortunately, different. The tautomerization energies of the free molecules converge at CCSD/aug-cc-pVTZ or /cc-pVQZ, the triples being not necessarily required. However, in the general case (hydrates and transition states) one needs the triples, and the basis set should be of quadruple-zeta quality, to achieve really strict convergence.