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 thr ee 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 o f ab initio GIAO calculations for model compounds: 1,3-propanediol, 1, 3,5-pentanetriol, pentane, and 4-methylheptane. The experimental chemi cal-shift differences of the methine carbons are well reproduced by th e calculated shifts for not only the triad models of 1,3,5-pentanetrio l but also those of 4-methylheptane. The strong intramolecular hydroge n bonds in the m tacticity maintain the all-anti conformation of the m ain chain of PVA. Both the inter- and intramolecular hydrogen-bond eff ects on C-13 chemical shifts are found to be small for carbons connect ed to hydroxyl groups forming a hydrogen-bond system, where the hydrox yl oxygens can serve as either donor or acceptor in the hydrogen bondi ng.