A vibrational circular dichroism study of optically active polymers

The absorption and vibrational circular dichroism (VCD) spectra of optically active poly(menthyl methacrylate), poly(menthyl acrylate), poly(menthyl vinyl ether), poly(isomenthyl vinyl ether), poly(neomenthyl vinyl ether), poly(triphenylmethyl methacrylate) and the corresponding monomeric species were reported from 830--1750 cm-1. The VCD spectra of poly(menthyl methacrylate) and poly(menthyl acrylate) were similar to one another, and to the corresponding monomeric species. The VCD spectrum of poly(menthyl vinyl ether) was different than the two former polymers, yet remarkably similar to that obtained for poly(neomenthyl vinyl ether). The VCD spectrum of poly(triphenyhmethyl methacrylate) was the most distinctive of each of the polymers, and was characterized by conservative bisignate peaks.; As a first step to probing the conformation of the optically active polymers an attempt was made to reproduce theoretically the absorption and VCD spectra of the parent alcohol and monomeric species, and to correlate these spectra with a specific conformation or conformational equilibrium. Absorption and VCD spectra were calculated at the DFT/Becke3LYP/6-31G** and VCT/HF/6-31G levels, respectively for (1S,2R,5S)-(+)-menthol, (1S,2R,5R)-(+)-isomenthol, and (1S,2S,5R)-(+)-neomenthol. There were at least ten low energy conformers calculated for each alcohol, two or three of which constituted the majority of the conformational equilibrium, and were rotamers of the hydroxyl group.; The absorption and VCD spectra of menthyl vinyl ether were calculated at the DFT/BPW91/6- 31G** and the EXC/BPW91/6-31G** levels, respectively, while the absorption and VCD spectra of the menthyl acrylate were calculated at the DFT/BPW91/6-31G* and the EXC/BPW91/6-31G* levels. The simulated spectra of both monomers agreed admirably with experiment. The low energy conformers were skeletally similar to those obtained for menthol, and differed only in the conformation of the vinyl pendants which could be described as one of two near planar orientations.; The conformation of poly(menthyl vinyl ether) was investigated using oligomers of two, three and five repeat units. Their absorption and VCD spectra were calculated with a fragment approach, in which the molecular properties needed to calculate the absorption and VCD spectra were transferred from smaller fragments of the oligomer to the larger oligomer. The agreement between experiment and theory was reasonable, showing the promise of this method in calculating the solution conformation of optically active polymers.