Various versions of the Karplus equation (eqn 15.27) have been used to correlate data on vicinal proton coupling constants in systems of the type RjR2CHCHR3R4. The original version, (M. Karplus, f. Am. Chem. Soc. 85, 2870 (1963», is 3JHH=A cos'1>HH+ B. When R3= R4 = H, 3JHH= 7.3 Hz; when R3 = CH3 and R4 = H, 3JHH= 8.0 Hz; when R3= R4 =CH3, 3JHH = 11.2 Hz. Assume that only staggered conformations are important and determine which version of the Karplus equation fits the data better.