Cyclohexane (1), oxygen-, sulfur-, and/or nitrogen-containing six-membered heterocycles 2-5, cyclohexanone (6), and cyclohexanone derivatives 7-16 were studied theoretically [B3LYP/6-31G(cl,p) and PP/IGLO-III//B3LYP/6-31G(d,p) methods] to determine the structural (in particular C-H bond distances) and spectroscopic (specifically, one bond (1)J(C-H) NMR coupling constants) consequences of stereoelectronic hyperconjugative effects. The results confirm the importance of n(x) --> sigma*(C-Happ) (where X = O, N), sigma(C-Hax) --> pi(C=O)(*), sigma(S-C) --> sigma*(C-Happ), sigma(C)-(S)-->sigma*(C-Happ), beta-n(O) --> sigma*(C-H), and sigma(C-H) --> sigma*(C-Happ) hyperconjugation, as advanced in previous theoretical models. Calculated r(C-H) bond lengths and (1)J(C-H) coupling constants for C-H bonds participating in more than one hyperconjugative interaction show additivity of the effects.