Chirality-sensitive broadband microwave spectroscopy was performed on mixtures of carvone enantiomers and conformers to distinguish enantiomers, measure enantiomeric excesses, and determine the absolute configurations of the enantiomers. This method uses microwave three-wave mixing and is inherently well-suited to the analysis of mixtures-a unique advantage over other techniques. In contrast to conventional microwave spectroscopy, the phase of the received signal is also exploited. This phase depends upon the signs of the molecules' dipole-moment components and is used to identify the excess enantiomer. The measured signal amplitude determines the size of the excess. The broadband capabilities of the spectrometer were used to simultaneously excite and measure two conformers of carvone, demonstrating the analysis of a sample with multiple chiral species. Employing quantum chemical calculations and the measured phases, the absolute configurations of the enantiomers are determined.

broadband microwave spectroscopy is a proven tool to precisely determine molecular properties of gas-phase molecules. Recent developments make it applicable to investigate chiral molecules. Enantiomers can be differentiated, and the enantiomeric excess and, indirectly, the absolute configuration can be determined in a molecule-selective manner. The resonant character and high resolution of rotational spectroscopy provide a unique mixture compatibility. Future directions, such as extending the technique to chemical analysis, are discussed.

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We apply chirality sensitive microwave three-wave mixing to 4-carvomenthenol, a molecule previously uncharacterized with rotational spectroscopy. We measure its rotational spectrum in the 2-8.5 GHz range and observe three molecular conformers. We describe our method in detail, from the initial step of spectral acquisition and assignment to the final step of determining absolute configuration and enantiomeric excess. Combining fitted rotational constants with dipole moment components derived from quantum chemical calculations, we identify candidate three-wave mixing cycles which were further tested using a double resonance method. Initial optimization of the three-wave mixing signal is done by varying the duration of the second excitation pulse. With known transition dipole matrix elements, absolute configuration can be directly determined from a single measurement.

The two-step laser excitation scheme of stimulated emission pumping (SEP) induces shifts of a single water molecule between two remote hydrogen bonding sites on trans-formanilide. This reaction can be initiated by selective excitation of either isomer (CO-bound or NH-bound) with different SEP excitation wavelengths. Energy (E) thresholds for isomerization in both directions have been measured [796 wave numbers NH) CO)

Resonant two-photon ionization (R2PI), UV hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopies have been used to record single-conformation infrared and ultraviolet spectra of three model synthetic foldamers with heterogeneous backbones, alpha/beta-peptides Ac-beta(3)-hAla-L-Phe-NHMe (betaalphaL), Ac-beta(3)-hAla-D-Phe-NHMe (betaalphaD), and Ac-L-Phe-beta(3)-hAla-NHMe (alphabetaL), isolated and cooled in a supersonic expansion. BetaalphaL and betaalphaD are diastereomers, differing only in the configuration of the alpha-amino acid residue; betaalphaL and alphabetaL contain the same residues, but differ in residue order. In all three alpha/beta-peptides the beta(3)-residue has S absolute configuration. UVHB spectroscopy is used to determine that there are six conformers of each molecule and to locate and characterize their S(0)-S(1) transitions in the origin region. RIDIR spectra in the amide NH stretch region reflect the number and strength of intramolecular H-bonds present. Comparison of the RIDIR spectra with scaled, harmonic vibrational frequencies and infrared intensities leads to definite assignments for the conformational families involved. C8/C7(eq) double-ring structures are responsible for three conformers of betaalphaL and four of betaalphaD, including those with the most intense transitions in the R2PI spectra. This preference for C8/C7(eq) double rings appears to be dictated by the C7(eq) ring of the alpha-peptide subunit. Three of the conformers of betaalphaL and betaalphaD form diastereomeric pairs (A/A', C/C', and G/G') that have nearly identical S(0)-S(1) origin positions in the UV and belong to the same conformational family, indicating no significant change associated with the change in chirality of the alpha-peptide subunit. However, betaalphaL favors formation of a C6/C5 conformer over C11, while the reverse preference holds in betaalphaD. Calculations indicate that the selective stabilization of the lowest-energy C11(g(+)) structure in betaalphaD occurs because this structure minimizes steric effects between the beta(2) methylene group and C=O(1). In the alpha/beta-peptide alphabetaL, two conformers dominate the spectrum, one assigned to a C5/C8 bifurcated double-ring, and the other to a C5/C6 double-ring structure. This preference for C5 rings in the alpha/beta-peptide occurs because the C5 ring is further stabilized by an amide NH...pi interaction involving an NH group on the adjacent amide, as it is in the alpha-peptides. Comparison of the NH stretch spectra of C8/C7(eq) structures in betaalphaL with their C7(eq)/C8 counterparts in alphabetaL shows that the central amide NH stretch is shifted to lower frequency by some 50-70 cm(-1) due to cooperative effects associated with the central amide accepting and donating a H-bond to neighboring amide groups. This swaps the ordering of the C8 and C7 NH stretch fundamentals in the two molecules.

Laser-induced fluorescence (LIF), ultraviolet hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopies were carried out on isolated benzo-15-crown-5 ether (B15C) and 4'-amino-benzo-15-crown-5 ether (ABC) cooled in a supersonic expansion. Three conformational isomers of B15C and four of ABC were observed and spectroscopically characterized. Full optimizations and harmonic frequency calculations were undertaken for the full set of almost 1700 conformational minima identified in a molecular mechanics force field search. When compared with TDDFT predictions, the S(0)-S(1) origin positions serve as a useful diagnostic of the conformation of the crown ether near the phenyl ring responsible for the UV absorption and to the position of the NH(2) substituent. In-plane orientations for the beta carbons produce red-shifted S(0)-S(1) origins, while out-of-plane "buckling" produces substantial blue shifts of 600 cm(-1) or more. Comparison between the alkyl CH stretch spectra of B15C and ABC divide the spectra into common subgroups shared by the two molecules. The high-frequency CH stretch transitions (above 2930 cm(-1)) reflect the number of CH...O interactions, which in turn track in a general way the degree of buckling of the crown. On this basis, assignments of each of the observed conformational isomers to a class of structure can be made. All the observed structures have some degree of buckling to them, indicating that in the absence of a strong-binding partner, the crown folds in on itself to gain additional stabilization from weak dispersive and CH...O interactions.

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