The CH- and OH-stretching vibrational circular dichroism (VCD) spectra of methyl lactate and related molecules in CCl4 solution have been investigated to identify solution conformations and establish a correlation between OH- and methine-stretching VCD intensity and molecular structure. Deuterium substitution was used when possible to remove overlapping absorption features in the CH-stretching region. Anisotropy ratios between +2.1 × 10-4 and +2.8 × 10-4 were measured for the methine-stretching VCD of the molecules with both α-oxy and α-CO substituents:  (S)-methyl-d 3 lactate, (S)-methyl-d 3 2-(methoxy-d 3)-propionate, di(methyl-d 3) d-tartrate, (S)-methyl-d 3 mandelate, (S)-methyl-d 3 O-(acetyl-d 3)-mandelate, and (S)-benzoin. The methine-stretching VCD intensity serves as a marker for both absolute configuration and solution conformation in these molecules, as previously demonstrated for amino acids and peptides. In (S)-methyl 2-chloropropionate, (R)-methyl 3-hydroxy-2-methylpropionate, and (S)-methyl 3-hydroxybutyrate, the net CH-stretching VCD intensity is small relative to that in the other molecules studied. Ab initio calculations of geometries, vibrational frequencies, and unpolarized infrared absorption (IR) and VCD intensities were carried out to identify the most abundant solution conformers from the VCD spectra and to correlate OH- and methine-stretching VCD intensity with molecular conformations. Factors leading to large methine-stretching VCD anisotropy ratios are assessed.

We report mid-infrared Fourier transform vibrational circular dichroism (FT-VCD) spectra obtained by using three different measurement schemes: the traditional sequential rapid-scan method, a new simultaneous rapid-scan method, and the step-scan method. We also provide a description of the optical setup used to obtain the VCD spectra. Using this optical layout, we are able to measure FT-VCD spectra for single enantiomers of the chiral sample with low levels of interfering artifacts and for collection times as fast as one minute. For approximately 20 min of spectral collection, the three FT-VCD measurements approaches tested here yield nearly the same signal quality. The approach with the highest quality, by a small margin, was the simultaneous rapid-scan method. Next was the sequential rapid-scan approach, followed closely by the step-scan method. VCD spectra and noise curves obtained with these different measurement techniques are presented and compared, and their relative advantages and disadvantages are discussed.

Rapid scanning Fourier transform infra-red spectroscopy was used to follow the changes in the population of trans conformers and in the infra-red dichroic functions of the bands characterizing the trans (T) and gauche (G) conformers and the chains in the amorphous regions during stretching of a solution-cast amorphous poly(ethy1ene terephthalate) (PET) film A in the glassy state (67 "C) and in the rubbery state (88 "C). Stretching at 67 "C showed a yield stress peak followed by strain softening in the stress-strain curve. The population of trans conformers in the amorphous and crystalline regions, T, + T,, decreased in the pre-yielding region and then increased continuously after yielding. The T conformers formed during post-yield stretching showed high orientation, while the G conformers remained in low degree of orientation.Stretching at 88 "C showed no pronounced yield stress peak in the stress-strain curve, and the population of T conformers did not change until the beginning strain-induced crystallization. A solvent-cast PET film annealed at 67 "C for 24 h, the film N, showed a sharp and high yield stress peak in the stress-strain curve, and strain-induced crystallization occurred at lower draw ratio as compared to the film A. The film A showed a non-equilibrium high population of T, conformers, part of which reverted to G conformers during sub-T, annealing. 0 1991, Hiithig & Wepf Verlag, Basel CCC 0025-1 16X/91/$03.00

We present the results of a new Fourier transform infrared vibrational circular dichroism (FT-IR-VCD) spectrometer optimized for performance in the near-infrared region above 2000 cm−1. The instrument is capable of operating in either the conventional rapidscan mode or the step-scan mode and is designed around the Bruker IFS 55 FT-IR spectrometer. We find that in the region of hydrogen stretching, the instrument performs as well as or better than (depending on the spectral coverage and resolution desired) a corresponding dispersive grating VCD spectrometer, with noise levels at approximately 10−6 Δ A units for a two-hour collection at 8-cm−1 resolution. The performance is approximately a factor-of-2 better in signal-to-noise ratio in optimized step-scan operation in comparison to the corresponding results in rapid-scan operation. The reasons are traced to advantages in electronic filtering and undersampling efficiency in step-scan mode. It is expected that the advantages of step-scan will increase with increasing optical transition frequency, such as the region of overtone and combination bands between 4000 and 8000 cm−1.