Table 3. Energy difference (kcal mol-1) between completely optimized and planar structures of 4a.
 
basis HF B3LYP  MP2
6-31G* -0.09 -0.23  -1.45
6-31+G*  -0.19 -0.39 -2.58 
6-31G**  -0.06 -0.20 -1.42 
6-31+G**  -0.17 -0.34 -2.56