ijms-15-19562-t002_Table 2 Table 2  Comprehensive summary of the applied experimental and theoretical methods for the conformational/tautomeric equilibria of neurotransmitters in aqueous solution a.   a In cases, when more than one quantum mechanical methods were used, the level producing the best result is indicated. The “+” sign indicates that ZPE/Gth was calculated; b Reference [57]; c Reference [59]; d Reference [52].