Parallel experiments with the cis/trans isomer8 (eq 5) provide a series of control experiments. In structure 6, both intermolecular 7 ON ... (5) For earlier studies involving hydrogen bonding and stacking in models, see: Kyogoku, Y.; Lord, RG; Rich, A. Proc. Natl. Acad. Sci. USA 1967, 57, 250-257. Hammes, GC; Park, AC J. Am. Chem. SOC. 1968, 90, 4151-4157. Chan, S. I.; Schweitzer, MP; Tso, P. 0. P.; Helmkamp, GK J. Am. Chem. SOC. 1964, 86, 4182. ...