Table Relative energies of different conformations of monomer (X) and donor-acceptor (X-Y) species as well as transition states (X-H-Y) with (∆E 0 ) and without (∆E e in parentheses) inclusion of harmonic zero point energy for all non-imaginary modes at B3LYP/TZ level. The acceptor atom for hydrogen bonds is given in parentheses. Also included in brackets is a previous computational barrier at MPW1K/6-31+G(d,p) level.