TACTICITY-DEPENDENT C-13 NMR CHEMICAL-SHIFTS FOR POLY(VINYL ALCOHOL) MODELS STUDIED BY AB-INITIO GAUGE-INCLUDED ATOMIC ORBITAL CALCULATIONS

Large differences in the experimental C-13 NMR chemical shifts for three resonance peaks assigned to the methine carbons in solid poly(vinyl alcohol) (PVA) are attributed not to the intramolecular hydrogen-bond effect but to the conformation effect due to tacticity on the basis of ab initio GIAO calculations for model compounds: 1,3-propanediol, 1,3,5-pentanetriol, pentane, and 4-methylheptane. The experimental chemical-shift differences of the methine carbons are well reproduced by the calculated shifts for not only the triad models of 1,3,5-pentanetriol but also those of 4-methylheptane. The strong intramolecular hydrogen bonds in the m tacticity maintain the all-anti conformation of the main chain of PVA. Both the inter- and intramolecular hydrogen-bond effects on C-13 chemical shifts are found to be small for carbons connected to hydroxyl groups forming a hydrogen-bond system, where the hydroxyl oxygens can serve as either donor or acceptor in the hydrogen bonding.