Teresa B. Freedman

Vibronic coupling theory of infrared vibrational transitions

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.

Scattered circular polarization Raman optical activity

Abstract 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.

Dual circular polarization Raman optical activity

Abstract 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.

Chirality and Diastereoselection of Δ/Λ-Configured Tetrahedral Zinc Complexes through Enantiopure Schiff Base Complexes: Combined Vibrational Circular Dichroism, Density Functional Theory, 1H NMR, and X-ray Structural Studies

The metal-centered Δ/Λ-chirality of four-coordinated, nonplanar Zn(A(^)B)(2) complexes is correlated to the chirality of the bidentate enantiopure (R)-A(^)B or (S)-A(^)B Schiff base building blocks [A(^)B = (R)- or (S)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-κ(2)N,O with X = OCH(3), Cl, Br]. In the solid-state the (R) ligand chirality induces a Λ-M configuration and the (S) ligand chirality quantitatively gives the Δ-M configuration upon crystallization as deduced from X-ray single crystal studies. The diastereoselections of the pseudotetrahedral zinc-Schiff base complexes in CDCl(3) solution were investigated by (1)H NMR and by vibrational circular dichroism (VCD) spectroscopy. The appearance of two signals for the Schiff-base -CH═N- imine proton in (1)H NMR indicates an equilibrium of both Δ- and Λ-diastereomers with a diastereomeric ratio of roughly 20:80% for all three ligands. VCD proved to be very sensitive to the metal-centered Δ/Λ-chirality because of a characteristic band representing coupled vibrations of the two ligands C═N stretch modes. The absolute configuration was assigned on the basis of agreement in sign with theoretical VCD spectra from Density Functional Theory calculations.

Experimental observation of resonance Raman optical activity

Abstract 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.

Structure Elucidation and Absolute Stereochemistry of Isomeric Monoterpene Chromane Esters

Six novel monoterpene chromane esters were isolated from the aerial parts of Peperomia obtusifolia (Piperaceae) using chiral chromatography. This is the first time that chiral chromane esters of this kind, ones with a tethered chiral terpene, have been isolated in nature. Due to their structural features, it is not currently possible to assess directly their absolute stereochemistry using any of the standard classical approaches, such as X-ray crystallography, NMR, optical rotation, or electronic circular dichroism (ECD). Herein we report the absolute configuration of these molecules, involving four chiral centers, using vibrational circular dichroism (VCD) and density functional theory (DFT) (B3LYP/6-31G*) calculations. This work further reinforces the capability of VCD to determine unambiguously the absolute configuration of structurally complex molecules in solution, without crystallization or derivatization, and demonstrates the sensitivity of VCD to specify the absolute configuration for just one among a number of chiral centers. We also demonstrate the sufficiency of using the so-called inexpensive basis set 6-31G* compared to the triple-ζ basis set TZVP for absolute configuration analysis of larger molecules using VCD. Overall, this work extends our knowledge of secondary metabolites in plants and provides a straightforward way to determine the absolute configuration of complex natural products involving a chiral parent moiety combined with a chiral terpene adduct.

Extension of Fourier Transform Vibrational Circular Dichroism into the Near-Infrared Region: Continuous Spectral Coverage from 800 to 10 000 cm -1

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.

Chlorofluoroiodomethane as a potential candidate for parity violation measurements

CHFClI is among the more favorable molecules for parity violation (PV) measurements in molecules. Despite the fact that calculated PV effects are two orders of magnitude smaller than in some organometallic compounds, CHFClI displays interesting features which could make possible a new experimental PV test on this molecule. Indeed, ultrahigh resolution spectroscopy using an ultrastable CO(2) laser is favored by several intrinsic properties of this molecule. For example, the high vapor pressure of CHFClI allows investigation by supersonic beam spectroscopy. Indeed, the spectroscopic constants have been accurately determined by microwave and millimetre wave spectroscopy. This is important for the subsequent selection of an appropriate absorption band of CHFClI that could be brought to coïncide with the absorption of CO(2). Partially resolved (+)- and (-)-CHFClI enantiomers with respectively 63.3 and 20.5% ees have been recently prepared and analyzed by molecular recognition using chiral hosts called cryptophanes. Finally, the S-(+)/R-(-) absolute configuration was ascertained by vibrational circular dichroïsm (VCD) in the gas phase.

Comparison of Step-Scan and Rapid-Scan Approaches to the Measurement of Mid-Infrared Fourier Transform Vibrational Circular Dichroism

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.

Vibrational optical activity calculations using infrared and Raman atomic polar tensors

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.