MODEL-CALCULATIONS OF THE C-13 NMR SHIELDINGS IN THE CRYSTALLINE CYCLIC PENTAMER OF POLY(P-PHENYLENE SULFIDE)

The cyclic pentamer of poly(p-phenylene sulfide) [c-(PS)(5)] cystalliz es in an interesting manner. Though only a single c-(PS)(5) molecule c onstitutes the asymmetric unit of its crystalline unit cell, the confo rmation about and the geometry of the C-S bonds vary widely around the cycle. The high-resolution, solid-state C-13 NMR spectrum of c-(PS)(5 ) reflects this heterogeneity of structural environments, where multip le resonances (at least six) are observed for both protonated (C-P) an d nonprotonated (C-Q) carbons spread over 18 and 8 ppm chemical shift ranges, respectively. Crystalline c-(PS)(5) provides us with a unique model system with which we may test our ability to calculate NMR nucle ar shieldings with ab initio quantum mechanical methods. For this purp ose, a gauge including atomic orbital (GIAO) approach was implemented with the use of locally dense basis sets. Diphenyl sulfide was adopted as a fragment model for c-(PS)(5), with the S, C-Q, and two C-P atoms on a single adjacent phenyl ring treated with the locally dense 6-311 G(d,p) basis set, while the remaining atoms were treated at the 3-21G level. This approach required 10 calculations to treat each of the str ucturally unique S-C-Q-(C-P)(2) fragments in c-(PS)(5). Calculated nuc lear shieldings and a C-13 NMR spectrum generated from them resemble t he observed C-13 NMR spectrum of crystalline c-(PS)(5). The calculated nuclear shieldings are strongly correlated with the conformation (dih edral angle) about the C-Q-S bonds, exhibiting a cos(2) theta(PQ) depe ndence on the dihedral angle B-PQ. Application of the same conformatio nal dependence of nuclear shieldings found for c-(PS)(5) to-crystallin e and liquid diphenyl sulfide permitted an understanding of their obse rved C-13 NMR spectra as well as their rigid, crystalline and flexible , liquid state conformations.