Carbon-13 NMR chemical shifts of dimeric model compounds of poly(propyleneoxide): A proof of existence of the (C-H)center dot center dot center dot O attraction

Conformational energies of poly(propylene oxide) (PPO) have been determined from C-13 NMR chemical shifts of its six dimeric model compounds. The mode l compounds were prepared and fractionated by supercritical fluid chromatog raphy into three components: CH3OCH2CH(CH3)OCH2CH(CH3)OCH3 (head-to-tail); CH3OCH2CH(CH3)OCH(CH3)CH2OCH3 (head-to-head); CH3OCH(CH3) CH2OCH2CH(CH3)OCH 3 (tail-to-tail). Carbon-13 NMR measurements using H-1 broad-band decouplin g and DEPT techniques were carried out for the benzene solutions at 25 degr eesC, and all observed NMR peaks were assigned to the methine, methylene, p endant methyl, and terminal methoxy carbons. By the simulation based on the rotational isomeric state (RIS) scheme and gamma- and delta -substituent e ffects for the C-13 NMR chemical shifts, the gamma- and delta -effect param eters and the conformational energies were optimized; the root-mean-square error between the calculated and observed chemical shifts was minimized to 0.12 ppm. Values of the gamma -anti effect (-2.6 ppm) of oxygen, the gamma -gauche effects of carbon (-4.9 ppm) and oxygen (-7.9 ppm), and the delta - effect (2.1 ppm) of oxygen for the g(+)g(-) conformations were obtained to fall within the allowable ranges, as shown in the parentheses. The conforma tional energies evaluated here are comparable to those determined for isota ctic PPO in our previous studies. These results confirm our interpretation of the gauche oxygen effect: The gauche stability of the C-C bond in the ma in chain of PPO is due to the intramolecular (C-H). . .O hydrogen bonding.