Ae. Tonelli et Db. Chesnut, MODEL-CALCULATIONS OF THE C-13 NMR SHIELDINGS IN THE CRYSTALLINE CYCLIC PENTAMER OF POLY(P-PHENYLENE SULFIDE), Macromolecules, 29(7), 1996, pp. 2537-2542
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.