Carbon-13 NMR Chemical Shifts of Dimeric Model Compounds of Poly(propylene Oxide): A Proof of Existence of the (C−H)···O Attraction

Abstract

Conformational energies of poly(propylene oxide) (PPO) have been determined from 13C NMR chemical shifts of its six dimeric model compounds. The model compounds were prepared and fractionated by supercritical fluid chromatography into three components: CH3OCH2CH(CH3)OCH2CH(CH3)OCH3 (head-to-tail); CH3OCH2CH(CH3)OCH(CH3)CH2OCH3 (head-to-head); CH3OCH(CH3)CH2OCH2CH(CH3)OCH3 (tail-to-tail). Carbon-13 NMR measurements using 1H broad-band decoupling and DEPT techniques were carried out for the benzene solutions at 25 °C, and all observed NMR peaks were assigned to the methine, methylene, pendant methyl, and terminal methoxy carbons. By the simulation based on the rotational isomeric state (RIS) scheme and γ- and δ-substituent effects for the 13C NMR chemical shifts, the γ- and δ-effect parameters 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 γ-anti effect (−2.6 ppm) of oxygen, the γ-gauche ...