The sensitivity of calculated J CC values to C-H bond lengths was investigated further by conducting two sets of calculations. In the first, starting from the crystal structure of 1 1,2 , the C1-H1 and C2-H2 bonds were both lengthened incrementally by 0.02 Å and 1 J C1,C2 values calculated in the resulting structures. In these calculations, only the C-H bond lengths involving the coupled carbons were changed; all remaining structural parameters were fixed to those found in the crystal structure. The results of these calculations are shown in Figure . Calculated 1 J C1,C2 values increase linearly as both r C1,H1 and r C2,H2 increase. Increasing C-H bond lengths from 1.0 Å to 1.12 Å results in an increase in 1 J C1,C2 values from ~44 Hz to ~50 Hz. Importantly, the experimental 1 J C1,C2 measured in crystalline 1 1,2 (48 Hz; Table ) is in better agreement with that calculated in Set 2 than with that calculated in Set 1, as expected since C-H bond lengths