Deuterium Nuclear Quadrupole Coupling and cis−trans Isomerization in Poly(phenylacetylene-d₁)

Abstract

Chain conformations due to thermal isomerization in poly(phenylacetylene-d 1) (PPA-d 1) were investigated by 2H solid state NMR. The observed 2H NMR line shapes at −100 °C for PPA-d 1 and an annealed sample, An-PPA-d 1, consisted of both large and small Pake doublet peaks. The former doublet peak was attributed to the static segments in the polymer chain. Deuterium nuclear quadrupole coupling constants of the large doublet peaks for PPA-d 1 and An-PPA-d 1 were 164 and 168 kHz, respectively. A quadrupole coupling constant and the formation energy of various chain conformations were calculated using semiempirical molecular orbital methods (AM1 and PM3 Hamiltonians). Agreement between theory and experiment confirmed that an exothermic peak at 177 °C in the DSC thermogram for PPA-d 1 is due to the cis − trans isomerization from the cis-transoidal form to 80°-deflected trans-transoidal one.

Cis-trans isomerization and13C-NMR chemical shift of polyphenylacetylene

“…The 80°-deflected transpoly(phenylacetylene) (deflected trans-PPA) was prepared in the following way. 8,9,17 The powder sample of cis-PPA was sealed under nitrogen atmosphere in a glass tube and annealed in an oil bath at 180°C for 1 h, because the exotherm of the cis-trans isomerization began at 150°C and was brought to an end at 180°C. After cooling, the sample was dissolved in chloroform and then poured into n-hexane to remove the pyrolysis byproducts of triphenylbenzene and low molecules, etc.…”

“…In the MO calculations, we take account of not only planar zigzag configurations such as cis-transoidal, trans-cisoidal, and transtransoidal forms, but also the same deflected configurations as the TGЈTGЈ-type for the trans chain in poly(vinylidene fluoride) reported by Hasegawa et al 19 The cis-cisoidal forms of the chain was not used in the calculation, because the steric hindrance of phenyl groups in PPA chain did not allow the considerable helical structure of the chain. 6,8,17 Figure 1 shows the backbone geometry of the same deflected trans chain conformation projected on a perpendicular plane and a perpendicular axis to its axis. The deflected angle of corresponds to the dihedral angle between two double bonds in the main chain.…”

“…of C3, C4, C5, and C6. In a previous report, 8 we determined that the trans form of the main chain after thermal isomerization was 80°-deflected trans-transoidal, based on theoretical analysis of the observed nuclear quadrupole coupling constant of the deuteron in PPA-d 1 .…”

“…In a previous article, 8 we clarified that the cis-transoidal poly(phenylacetylene-d 1 ) was thermally isomerized to 80°-deflected trans-transoidal conformation from deuterium quadrupole cou-pling analysis of 2 H-NMR spectra in the solid and semiempirical molecular orbital calculations. The finding first confirmed that the DSC exotherm for each polymer between 170 and 180°C is attributed to the thermal cis-trans isomerization.…”

“…The finding first confirmed that the DSC exotherm for each polymer between 170 and 180°C is attributed to the thermal cis-trans isomerization. 8,9 Terao et al determined the 13 C chemical shift tensors of cis-and trans-poly(acetylene)s (PAs) by measurements of two-dimensional 13 C-NMR powder patterns in a rotating solid. 10 The isotropic 13 C chemical shift of the main chain was 127 and 137 ppm for cis-and trans-PAs, respectively.…”