During optimization, the remaining bond lengths involving heavy atoms were fixed at values found in the crystal structures, which can be measured reliably. C-H and O-H bond lengths in the two sets of DFT calculations are summarized in Table . Geometry optimization of all hydrogen atoms resulted in the lengthening of C-H and O-H bonds by ~0.1 Å. These changes led to larger calculated 1 J C1,C2 and 1 J C5,C6 values by 3-6 Hz even though C-C bond lengths were fixed at lengths found in their crystal structures in both the Set 1 and 2 calculations (Table ).