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Vibrational circular dichroism (VCD) of a series of proteins in H2O solution with differing secondary structure are reported for the first time in the near-infrared (NIR) region as well as the NH-stretching region. The Fourier transform (FT) near-infrared (NIR) measurements were carried out between 6000 to 4000 cm−1. FT-VCD measurements were simultaneously carried out for the mid-infrared (mid-IR) region from 2000 to 800 cm−1 for direct comparison to VCD in the NIR region. The NIR VCD spectra of proteins show distinct spectral features for different protein structural motifs, indicating a valuable new method to study protein conformations. The principal VCD transitions in the NIR region are two combination bands, the amide A-II and B-II bands, of the amide A and B fundamentals with the amide II fundamental, and the second overtone of the amide II, referred to as the amide 3 X II band. VCD in the amide A and B band region consisting primarily of NH stretching motions were successfully obtained in H2O for the first time for an insulin fibril sample. Similar to the enhanced VCD signal observed in amide I and II regions, the amide A and B VCD of insulin fibril shows strong intensity enhancements, providing an additional valuable probe of protein fibril growth and development in solution. The relative sensitivities of the mid-IR, N–H stretching, and NIR regions are discussed.

Near-infrared (near-IR) and mid-infrared (mid-IR) vibrational circular dichroism (VCD) spectra in the region 800–10,000 cm−1 were recorded for nine enantiomeric pairs (or enantiomer and racemate pairs) of molecules with three ChiralIR Fourier transform (FT) VCD spectrometers equipped with various light sources and detectors and modified for dual polarization modulation (DPM) operation. Correlations across these sets of spectra permit conclusions regarding expected VCD intensities in the near-infrared compared to the conventional mid-IR regions. These VCD spectra also provide spectral standards for future studies in the near-infrared region. Compared with recent near-IR measurements carried out elsewhere using dispersive instrumentation, better signal-to-noise ratios were obtained for all but the frequency region above 8000 cm−1 where VCD spectra of comparable quality were obtained. These results are the first comprehensive set of VCD spectra for a set of simple chiral molecules with continuous coverage from the mid-infrared to nearly the visible region of the spectrum. This study demonstrates that with suitably equipped FT-VCD spectrometers, VCD and IR spectra can be measured with high quality across a wide spectral range.

Two-dimensional correlation spectroscopy (2D-COS) has been applied to Fourier transform (FT) vibrational circular dichroism (VCD) in multiple spectral regions for the first time. FT-IR and FT-VCD measurements of l -alanine in D 2 O have been carried out in the mid-IR (800–2000 cm −1 ) region, the hydrogen-stretching region (2000–4000 cm −1 ), and the near-IR region (4000–6000 cm −1 ) as a function of pD. A variety of different 2D-COS plots are possible from the IR and VCD spectral data in these three spectral regions, and examples are presented that show particularly interesting features of these spectra. Homo-spectral-region and hetero-spectral-region VCD–VCD and IR–IR 2D-COS plots are presented for l -alanine. Relative sensitivities to spectral changes in the methine and methyl vibrational regions are elucidated by analysis of both IR and VCD 2D-COS spectra. Near-IR band assignments are facilitated by hetero-spectral-region 2D-COS spectra that correlate the combination band region to that of the underling fundamental modes. These studies demonstrate that the method of 2D-COS IR-VCD is a useful tool that provides enhanced chemical information over traditional 1D spectra.

The first use of Fourier transform vibrational circular dichroism (FT-VCD) to follow changes in the percent enantiomeric excess (% EE) of chiral molecules in time using a flow cell sampling apparatus is reported. FT-VCD, as opposed to dispersive scanning VCD, eliminates the need to scan the VCD spectrum in time to monitor the % EE at more than one spectral location. The first use of partial least-squares chemometric analysis to determine % EE values from kinetic sets of VCD spectral data is also reported. These two advances have been used to monitor simultaneously changes in the fractional composition and the % EE of a mixture of two different chiral molecules. This simulates the progress of the chemical reaction from a chiral reactant to a chiral product where the % EE of both molecules can change with time. For the molecules studied, alpha-pinene, camphor, and borneol, the accuracy of following % EE changes for one species alone is approximately 1%, while for simultaneously following % EE changes in two species is approximately 2% for 10-20-min sampling periods at 4 cm(-)(1) spectral resolution. This accuracy can be increased for the same collection times or maintained for shorter periods of collection by lowering the spectral resolution. These findings demonstrate the potential for VCD to be used for real-time monitoring of the composition and % EE of chemical reactions involving the synthesis chiral molecules.

We provide the first experimental evidence for resonance Raman optical activity (RROA). In non-aqueous solution (+)-naproxen ((S)-6-methoxy-α-naphthalenacetic acid) and its deuterated methyl ester exhibit monosignate negative RROA, whereas the RROA spectrum of (−)-naproxen sodium is monosignate positive (aqueous solution above pH 10), for 514 nm excitation. In all cases, the RROA bands exhibit the same relative intensities as those in the parent resonance Raman (RR) spectrum, and the ratio of RROA to RR is comparable in magnitude, but opposite in sign, to the electronic circular dichroism (CD) anisotropy ratio of the lowest energy electronic state near 330 nm. These findings are in accord with the theory of RROA in the limit of strong resonance with a single electronic state.

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.

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.

Infrared vibrational circular dichroism (VCD) and vibrational Raman optical activity (ROA) have been measured and compared quantitatively over the frequency range from 835 to 1345 cm−1 for trans-pinane, cis-pinane, α-pinene, and β-pinene. For these molecules in this region of spectral overlap between VCD and ROA, the average ratio of VCD or ROA to its parent vibrational intensity favors ROA by a factor of two to three. Several vibrational modes in each molecule yield both large VCD and large ROA, while several other modes show little propensity toward significant VCD or ROA intensity. Beyond this general property of a few strongly chiral and strongly achiral vibrational modes, little additional correlation between VCD and ROA intensity is found. This quantitative compilation of VCD, infrared, ROA, and Raman intensities provides an experimental basis for computational intensity studies of VCD, ROA, and their theoretical comparison.

The backscattering in-phase dual circular polarization (DCPI) Raman and Raman optical activity (ROA) for a series of terpene molecules is reported. The selected molecules were studied in three groups of closely related structures and correlations between ROA features and structural elements among molecules within these groups are discussed. One aim of this study was to uncover additional details regarding the sensitivity of ROA to stereostructural features in molecules and to elucidate further the degree of local character of this new stereochemical probe. Another was to compare the backscattering DCPI ROA intensities reported here with those obtained using depolarized right-angle incident circularly polarized (ICP) ROA for some of the same molecules more than a decade ago.

We report the first measurements of Raman optical activity (ROA) in ephedrine molecules. In-phase dual circular polarization (DCP I ) ROA spectra have been recorded for (1S,2R)-ephedrine, (1S,2R)-norephedrine, (1S,2S)-pseudoephedrine and (1S,2S)-norpseudoephedrine as hydrochloride salts in H 2 O solution. The spectra are interpreted in relation to the small changes in molecular structure among these four molecular species.

The first use of near-infrared (NIR) Fourier transform vibrational circular dichroism (FT-VCD) to follow changes in the enantiomeric excess (EE) of chiral sample molecules in time using a flow-cell sampling apparatus is reported. Simultaneous changes in the fractional composition and the EE of a mixture of two different chiral molecules were monitored as a function of time. This simulates the progress of the chemical reaction from a chiral reactant to a chiral product where the mole fractions and EE values of both species may change with time. For the molecules studied, α-pinene, camphor, and borneol, the accuracy of following EE changes for one species alone is approximately 2%, while for simultaneously following EE changes in two species it is approximately 3% for 30 min sampling periods at 16 cm−1 spectral resolution. These findings demonstrate the potential for VCD to be used in the NIR region for real-time monitoring of the composition and %EE of chemical reactions involving the synthesis of chiral molecules.

We report the first vibrational circular dichroism (VCD) spectra with continuous coverage from 800 cm−1 in the mid-infrared (MIR) region to 10 000 cm−1 in the near-infrared (NIR) region. This coverage is illustrated with MIR and NIR absorbance and VCD spectra of 2,2-dimethyl-dioxolane-4-methanol (DDM), α-pinene, and camphor that serve as calibration samples over this entire region. Commercially available, dual-source Fourier transform (FT) MIR and NIR VCD spectrometers were equipped with appropriate light sources, optics, and detectors, and were modified for dual-polarization-modulation (DPM) operation. The combination of liquid-nitrogen- and thermoelectric-cooled HgCdTe (MCT) detectors, as well as InGaAs and Germanium (Ge) detectors operating at room temperature, permitted collection of the desired absorbance and VCD spectra across the range of vibrational fundamental, combination band, and overtone frequencies. The spectra of DDM and α-pinene were measured as neat liquids and recorded for both enantiomers in the various spectral regions. Spectra for camphor were all measured in CCl4 solution at a concentration of 0.6 M, except for the carbonyl-stretching region, where a more dilute concentration was used. The typical anisotropy ratios ( g) of the three molecules were estimated with respect to their strongest VCD bands in each spectral region. It was found that for all three molecules in the spectral regions above 2000 cm−1, anisotropy ratios are approximately the same order (10−5) of magnitude. However, in the MIR region, the typical anisotropy ratios are significantly different for the three molecules. This study demonstrates that with modern FT-VCD spectrometers modified for DPM operation, VCD spectra can be measured continuously across a wide spectral range from the MIR to nearly the visible region with an unsurpassed combination of signal-to-noise ratio and spectral resolution.

Advances in the measurement, calculation, and application of vibrational circular dichroism (VCD) for the determination of absolute configuration are described. The purpose of the review is to provide an up-to-date perspective on the capability of VCD to solve problems of absolute stereochemistry for chiral molecules primarily in the solution state. The scope of the article covers the experimental methods needed for the accurate measurement of VCD spectra and the theoretical steps required to systematically deduce absolute configuration. Determination of absolute configuration of a molecule by VCD requires knowledge of its conformation or conformational distribution, and hence VCD analysis necessarily provides solution-state conformation information, in many cases available by no other method, as an additional benefit. Comparisons of the advantages and limitations of VCD relative to other available chiroptical methods of analysis are also presented.

A comparison of step-scan and rapid-scan Fourier transform vibrational circular dichroism (FT-VCD) spectroscopy in the hydrogen-stretching region has been carried out on Nicolet Magna 850 and Bruker IFS-55 FTIR spectrometers, each equipped with an accessory VCD bench. Single enantiomer VCD spectra of (+)- and (−)-camphor with flat baselines are obtained on each instrument, both in step-scan mode and rapid-scan mode. On both instruments, the RMS noise level is less than 1×10−6 (absorbance units) in less than two-hour collections, and the noise level obtained with the step-scan method is slightly lower than that for rapid-scan. The calibrated noise level of VCD spectra obtained on the Nicolet instrument is about twice that obtained on the Bruker instrument for a comparable collection time. Our preliminary results show that with an InSb detector on the Nicolet instrument, the noise level of VCD spectra is about a half of that obtained with an MCT detector.

Raman optical activity (ROA)1, the difference in Raman scattering intensity for right and left circularly polarized incident and/or Raman scattered radiation, has emerged as a technique for investigating the solution structure of chiral molecules, and has been applied to biomolecules in aqueous solution. 1, 2 Resonance Raman spectroscopy is well established as a technique for probing the structure and dynamics of appropriate chromophores in biomolecules. However, resonance Raman optical activity (RROA) has not been reported. Recently, the theory of Raman optical activity has been extended to the case of strong resonance with a single electronic state.3 The theory predicts that, in this limit, the RROA bands will all have the same sign (opposite in sign to the pure electronic circular dichroism of the resonant electronic transition) and will be proportional in intensity to the parent resonance Raman bands. We provide here evidence for both resonance and non-resonance ROA in the spectra of the analgesics naproxen and ibuprofen in basic and nonaqueous solution.

Infrared, vibrational circular dichroism, dual circular polarization Raman optical activity and Raman spectra have been used in conjunction for investigation of the vibrational modes of trans- and cis-pinane, and α- and β-pinene with a view of evaluating the usefulness of these techniques. It has been concluded that vibrational circular dichroism and Raman optical activity provide highly complementary and non-redundant information, of comparable spectral quality.

Vibrational circular dichroism (VCD) spectra in the region above 2000 cm−1 provides valuable information on absolute configuration or conformation of chiral molecules.1 Conventionally, VCD spectra in this region have been measured with dispersive instruments, since the high Fourier frequencies resulting from the rapid motion of a moving mirror in a conventional Fourier transform spectrometer may deteriorate the VCD signal. Step-scan FT-VCD, however, has no such difficulty (there is no Fourier frequency) and has advantages over dispersive VCD in terms of noise level, resolution, spectral coverage and repeatability. 2, 3 We have recently investigated a number of molecules of pharmaceutical interest using step-scan FT-VCD above 2000 cm−1. These molecules include S-(-)-propranolol, (S)-4-isobutyl-α- methylphenylacetic acid (ibuprofen) and (S)-6-methoxy-a-methyl-2-naphthalene- acetic acid (naproxen). The FTIR and VCD spectra have been measured and interpreted by comparison with ab initio calculations of geometry, vibrational frequency and intensity for a group of low-energy conformers of each compound. For the VCD spectra, intensities have been calculated with the locally distributed origin gauge (LDO) model and the vibronic coupling VCD theory.4

Raman optical activity (ROA), the circular intensity difference between right and left circularly polarized incident and/or Raman scattered radiation, provides a unique conformational probe of biological molecules in aqueous solution.1,2 In the classical measurement of ROA, the polarization of the incident laser radiation is modulated between the two circular polarization states, keeping the scattered linear polarization fixed, resulting in ICP-ROA. More recent methods developed in our laboratory include modulation only of the Raman scattered radiation, denoted SCP-ROA, or dual circular polarization modulation of both the incident laser and the Raman scattered radiation, either in- or out-of-phase, denoted DCPI- and DCPII-ROA, respectively. The depolarized backscattering DCPI-ROA measurement has been found to be particularly advantageous for biological samples. One emphasis of our recent research is to explore the sensitivity of ROA to locallized vs. longer range vibrational modes and to local vs. long-range stereochemistry. In addition, comparison of results from the various forms of measurement of ROA provides information on the relative importance of magnetic dipole vs. electric quadrupole mechanisms for ROA intensity. Two classes of molecules have been surveyed. Studies of simple peptides provide the opportunity to investigate the degree of localization of ROA features on specific residues and the effects of vibrational coupling among residues.3

Publisher Summary Raman optical activity (ROA) is the difference in the intensity of Raman scattered light with respect to right and left circularly polarized incident or scattered radiation, or both. The form of ROA defined with circular polarization modulation of the incident laser was first described theoretically in 1971 and was first measured in 1973. As with other forms of natural optical activity, a molecular sample must be chiral to possess a nonzero ROA spectrum. The measurement of an ROA spectrum requires particular care to avoid the presence of optical artifacts since ROA intensities are small, being typically three to four orders of magnitude below those of the parent Raman spectrum. The value of ROA as a spectroscopic technique lies in its particular sensitivity to chiral molecular environments. After nearly two decades of steady development, ROA is now emerging as an important new spectroscopic tool in the study of the stereo conformations of optically active molecules, including biological macromolecules and metal complexes. Advances in three principal areas have revolutionized the measurement of ROA spectra. These are (1) the use of alternative scattering geometries including backward scattering, (2) the use of charge-coupled device (CCD) detectors as nearly ideal, low-background, large area, multichannel detectors, and (3) the introduction of new polarization modulation techniques involving the measurement of circular polarization intensity differences associated with the scattered beam.

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.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Vibrational circular dichroism (VCD) spectra have been recorded in the NH and C equals O stretching regions for a series of tripodal peptides and peptides containing an unsaturated residue, in order to obtain information about solution conformations.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Recent advances have led to dramatic improvements in the measurement of Raman optical activity (ROA). These include the use of new scattering geometries and improvements in polarization modulation methodology and multichannel-detector technology. Several new ROA experimental configurations are selected to illustrate the capabilities of a new ROA instrument constructed recently at Syracuse University. Both general and simplified theoretical expressions are used to identify fundamental aspects of various ROA experiments and to determine preferred configurations for the measurement of ROA. The authors find for their instrument that the optimum configuration is in-phase dual circular polarization (DCPI) ROA in backscattering geometry, as illustrated using (+)-fenchone. DCPI- ROA backscattering spectra are presented for aqueous solutions of L-alanine, glycyl-L-alanine and D-mannose. The large couplet arising from the bands between 800 and 900 cm-1 in the ROA spectrum of alanine is interpreted in terms of coupled methyl rocking and the methyl carbon-carbon stretching internal coordinates. The feasibility of applying ROA to the study of biological molecules is demonstrated by the high quality of the ROA spectra obtained for these molecules.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Natural vibrational optical activity consists of two principal forms. The IR form is known as vibrational circular dichroism (VCD) and is simply the extension of electronic circular dichroism into the IR vibrational region of the spectrum. The Raman form, known as Raman optical activity (ROA), is a new form of optical activity that has no counterpart in the classical forms of optical activity. In this paper, the similarities and differences of the IR and Raman forms of vibrational optical activity will be examined. Although both VCD and ROA were discovered and confirmed in the period from 1973 to 1975, each field has evolved independently with key advances in theoretical description, instrumentation and application coming at different times over the past 20 years. The current relative strengths and weaknesses of VCD and ROA will be discussed, and specific examples of VOA spectra of (-)-alpha-pinene and the amino acid L-alanine, for which overlapping VCD and ROA data are available, will be presented.

A new formulation of the bond polarizability and vibronic coupling theories of Raman optical activity (ROA) is presented in which terms dependent upon the nuclear momentum are introduced for the first time. The new terms appear in the magnetic‐dipole electric‐dipole optical activity tensor and are directly analogous to the non‐Born–Oppenheimer, momentum‐dependent terms required in the formulation of the theory of vibrational circular dichroism. Through the use of simplifying approximations in the ROA vibronic coupling theory, a term‐for‐term correspondence with the bond polarizability theory is established. It is shown that the new terms are smaller than the original terms by the ratio of the vibrational transition frequency to the frequency of the incident radiation. The new terms also contribute oppositely to Stokes and anti‐Stokes ROA which provides a basis for their experimental isolation.

Expressions for vibrational circular dichroism (VCD) and Raman optical activity are formulated using infrared and analogous Raman atomic polar tensors. Calculations using finite electric field perturbation with CNDO wave functions are presented for the VCD of CH‐stretching vibrations in L‐alanine and (+)‐(3R)‐methylcyclohexanone. These compare favorably with experiments and previous models.

Attribution de l'activite optique Raman (AOR) a 11 mouvements du squelette basee sur les spectres Raman et l'analyse en coordonnees normales. Discussion des resultats

The theoretical description of dual circular polarization (DCP) Raman optical activity is presented. It is shown that for forward scattering, right-angle scattering and backward scattering, it is possible to measure a form of ROA in which both the incident and scattered circular polarizations are simultaneously switched between right and left circular states. DCP ROA is shown to be equal to either the sum or difference of the corresponding incident and scattered circular polarization ROA intensities for nearly the same parent Raman intensity.

The theory of vibronic coupling is developed for infrared vibrational transitions. It is shown that the lowest order nonadiabatic Born–Oppenheimer correction terms contain an important adiabatic component which may be used to describe infrared transition intensity for imaginary Hermitian operators, such as the momentum and angular momentum operators. This previously unrecognized source of adiabatic infrared intensity forms a complement to the traditional Herzberg–Teller vibronic coupling expressions, which are active for the position operator, and resolves the paradox of vanishing electronic intensity for momentum operators in the Born–Oppenheimer approximation. Expressions for infrared absorption and vibrational circular dichroism are derived that utilize only ground electronic state wave functions; LCAO wave functions are used in these expressions to provide a more detailed description of these new momentum intensity contributions.

Raman spectra have been obtained for bleomycin A2 and selected fragments. Virtually all of the features in the bleomycin Raman spectrum can be attributed to the preresonance enhanced vibrational transitions of the 2,4′-bithiazole moiety as determined by comparison to the Raman spectra of bithiazole fragments. The Raman spectrum of bleomycin is dominated by a very intense band at 1542 cm−1 that can be approximately assigned as a bithiazole ring stretching mode. This feature may serve as a probe of bithiazole environment in bleomysin -DNA interactions.

The resonance Raman spectra of the n -butylamine protonated Schiff bases of the all- trans , 9- cis , 11- cis and 13- cis isomers of retinal in ethanol are reported here. They were obtained using a rapid-flow sampling system with excitation at 514·5 nm. The resonance Raman spectra of these isomers are distinctly different from each other, showing that the spectra are very sensitive to conformational differences. A comparison of the resonance Raman spectrum of the 11- cis protonated Schiff base with that of rhodopsin reveals that the protein does not grossly distort the solution conformation of the chromophore. Similarly, the resonance Raman spectrum of isorhodopsin closely resembles that of the 9- cis protonated Schiff base.

The first measurements of scattered circular polarization (SCP) Raman optical activity (ROA) are reported in which the degree of circular polarization of the scattered light is determined. The SCP ROA spectra are relatively simple to measure and confirm theoretical predictions of their relation to incident circular polarization (ICP) ROA, the only previously measured form of ROA.

The localized molecular orbital (LMO) model of infrared vibrational circular dichroism (VCD) is extended to vibrational Raman optical activity (ROA). Using the equations arising from the finite perturbation approach to ordinary infrared and Raman intensities we show that VCD and ROA intensities can be determined in the same manner provided that localized molecular orbitals are used to describe the motion and response of electronic charge density in the molecule. In this approach a small perturbing electric field is used to obtain the first and second derivatives of the potential energy gradient of the molecule with respect to the electric field and these derivatives are in turn related to infrared and Raman intensities, respectively. Since only a few SCF calculations are required by this method to obtain both VCD and ROA intensities for all the vibrational modes of a molecule, the way is now open for the efficient and complete calculation of vibrational optical activity intensities using molecular orbital...

The Raman optical activity (ROA) spectrum of (+)-(3R)-methylcyclohexanone from 150 to 1400 cm −1 measured using a novel ROA spectrometer is reported. The spectrometer features a Soleil-Babinet compensator to produce high-quality circularly polarized laser radiation. The principal ROA couplets are assigned and interpreted in terms of pairs of coupled vibrational motions.

The first molecular orbital calculations of infrared transition intensities using adiabatic non-Born-Oppenheimer vibronic coupling expressions for the dipole momentum operator are reported. The calculated dipole momentum intensities for the CH stretching modes in three deuterated alanine isotopomers agree favorably with those calculated from conventional dipole length expressions.

Polarization modulation Fourier transform infrared spectroscopy can be measured by either double modulation FTIR or interferometric modulation FTIR techniques. The principles of both techniques are presented with examples of representative measurements. The relative advantages and disadvantages of each method are compared.

The vibrational circular dichroism (VCD) spectra of the N-t-BOC and N-CBZ-derivatives of alanine, proline, phenylalanine and valine in 0.2M CHC13 solutions all exhibit a characteristic, strong bisignate couplet in the carbonyl stretching region. The VCD couplet is interpreted in terms of the coupled oscillation of the urethane and acid carbonyl groups in an intramolecularly hydrogen-bonded structure.

The Fourier transform vibrational circular dichroism (VCD) in the amide I region of poly-L-lysine in D20 solution has been investigated. Signals corresponding to the random coil, a-helix and antiparallel (3-sheet have been characterized. The spectrum of the a-helix shows the presence of three distinct features in agreement with previous results on deuterated polypeptides. We were able to detect the antiparallel β-sheet conformation in solution; the signal is unexpectedly large and monosignate in contrast to that predicted from exciton theory.

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.

Expressions for vibrational circular dichroism (VCD) and Raman optical activity are formulated using infrared and analogous Raman atomic polar tensors. Calculations using finite electric field perturbation with CNDO wave functions are presented for the VCD of CH‐stretching vibrations in L‐alanine and (+)‐(3R)‐methylcyclohexanone. These compare favorably with experiments and previous models.

Oxygenation of hemocyanin gives rise to resonance Raman peaks at 742 and 282 cm−1. The 742 cm−1 peak which is in resonance with the 575 nm charge transfer band shifts to 704 cm−1 when 18O2 is substituted for 16O2. Our results establish that the bound oxygen is in the form of peroxide (O22−). The 282 cm−1 peak which is in resonance with the 340 nm optical transition is insensitive to isotopic substitution, suggesting that the 282 cm−1 peak corresponds to a vibration involving the magnetically-coupled Cu(II)··Cu(II) centers.

The Raman spectrum of µ-peroxo-binuclear cobalt complexes exhibits an intense peak at ca. 800 cm–1, assigned to O22– stretching, which is in resonance with the ca. 360 nm O22–→ CoIII charge-transfer band.