N. Ragunathan

Measurement of Vibrational Circular Dichroism Using a Polarizing Michelson Interferometer

Fourier transform infrared (FT-IR) vibrational circular dichroism (VCD) has been observed for the first time with the use of a polarizing Michelson interferometer (PMI). VCD spectra have been recorded for (–)-α-pinene between 900 and 1350 cm−1 at 4 cm−1 resolution with excellent signal-to-noise ratio. The FT-VCD spectra are virtually the same as those recorded for (–)-α-pinene under the same conditions with the use of the more conventional double modulation FT-IR approach.

Chapter 3 Instrumental methods of infrared and Raman vibrational optical activity

Abstract We present the basic concepts and methods for the measurement of infrared and Raman vibrational optical activity (VOA). These two forms of VOA are referred to as infrared vibrational circular dichroism (VCD) and Raman optical activity (ROA), respectively The principal aim of the article is to provide detailed descriptions of the instrumentation and measurement methods associated with VCD and ROA in general, and Fourier transform VCD and multichannel CCD ROA, in particular. Although VCD and ROA are closely related spectroscopic techniques, the instrumentation and measurement techniques differ markedly. These two forms of VOA will be compared and the reasons behinds their differences, now and in the future, will be explored.

Vibrational circular dichroism in ephedra molecules. Experimental measurement and ab initio calculation

Vibrational circular dichroism (VCD) spectra in the OH- and NH-stretching regions have been measured for six pharmaceutical molecules in the ephedra class, (1S,2R)-norephedrine, (1), (1S,2S)-norpseudoephedrine (2), (1S,2R)-ephedrine (3), (1S,2S)-pseudoephedrine (4), (1S,2R)-N-methylephedrine (5) and (1S,2S)-N-methylpseudoephedrine (6), all in dilute C2Cl4 solution. Ab initio calculations of geometries and vibrational frequencies for a number of conformers of 1 to 6 have been carried out. The recently developed locally distributed origin gauge model for VCD was used to calculate VCD spectra of the conformers for each drug. This ground-state ab initio model, which does not require sum-over-states, magnetic field perturbation or localized molecular orbitals, has been found to give good agreement with experiment in this frequency region. Composite spectra obtained from weighted averages of the calculated conformer spectra agree within a factor of two with observed IR and VCD intensities for five of the ephedra drugs. For (1S,2S)-pseudoephedrine, the discrepancy between experiment and calculation has been interpreted in terms of a coupled-oscillator effect absent in the other molecules.

Vibrational Circular Dichroism of Ephedrines and Related Molecules

Vibrational circular dichroism (VCD), the difference in absorbance of left and right circularly polarized infrared radiation during vibrational excitation, provides information on the solution conformation of biological molecules.1,2 The fast time scale of vibrational excitation allows observation of discrete vibrational bands for conformers that are averaged on the NMR time-scale, and the added dimensions of sign and conformational sensitivity introduced with circular dichroism permit additional resolution of features that overlap in the parent infrared absorption spectra. We have recently investigated a series of molecules with two chiral centers and a variety of possible intramolecularly hydrogen-bonded conformations. These molecules include six ephedra drugs,3 where R and R’ are H or CH3, and the chirality at C1 is S and that at C2 is R for ephedrines and S for pseudoephedrines. The other molecules studied, related structurally to ephedrines, are the antiarrhythmic drug propranolol and a series of substituted 1,2-diphenylethanes Ephedrines [(R,R)-hydrobenzoin, (1R,2R)-diphenylethylenediamine and (1S,2S)-2-amino-l,2-diphenylethanol]. The absorbance and VCD spectra in the OH-and NH-stretching regions have been measured and interpreted by comparison with ab initio geometry, vibrational frequency and intensity calculations for a group of low energy conformers of each compound.

Vibrational circular dichroism of 1-amino-2-propanol and 2-amino-1-propanol: experiment and calculation

The vibrational circular dichroism (VCD) spectra of (S)-1-amino-2-propanol and (S)-2-amino- 1-propanol in non-aqueous solution have been investigated. The vibrational frequencies, infrared intensities and VCD intensities have been calculated for all the hydrogen-bonded conformers with ab initio wave functions at a 6-31G(0.3) basis set level. The VCD intensities have been calculated with the vibronic coupling theory a priori formalism. Comparison of the experimental and calculated spectra allows correlation of the major VCD features with a few predominant conformers.

Spectral and kinetic investigations of chirally deuterated three-membered ring molecules using Fourier transform vibrational-circular-dichroism spectroscopy

Vibrational circular dichroism (VCD) and kinetic measurements have been carried out for several three-membered ring molecules that are optically active by virtue of isotopic substitution, (S,S)-cyclopropane-1,2-2H2, (2S,3S)-cyclopropane-1- 13C,2H-2,3-2H2, and (S,S)-oxirane-2,3-2H2.

Circular Dichroism Measurement Using Fourier Transform Interferometry

The first measurements of circular dichroism (CD) using a polarizing Michelson interferometer (PMI) are presented. PMI-CD was observed for infrared electronic transitions in the rare-earth metal complex, praseodymium-L-tartrate between 2400-2000 cm -1. Following further improvements in instrumental technique, vibrational CD was measured for (-)-α-pinene in the region 1350 to 900 cm-1. Comparisons to previous infrared CD measurements of these molecules shows excellent agreement.