Table 4. Optimum and mean D–A distances.

	Optimum D–A distance [Å]	Mean D–A distance [Å]	σ [±Å]
15a	9.98	12.27	1.38
15b	18.96	18.77	0.24
16a	11.16	11.89	1.83
16b	19.87	20.57	1.59
17a	6.25	8.23	3.53
17b	6.13	6.32	0.13