The 2-18 GHz microwave spectrum of jet-expanded allyl fluoride in argon is obtained using both broadband, chirped-pulse and cavity-enhanced, Balle-Flygare Fourier transform microwave spectrometers. Rotational transitions for both the achiral, cis, and chiral, gauche, rotamers of allyl fluoride are observed and assigned in addition to the gas-phase heterodimers of each with the argon atom. The spectra of the argon complexes are analyzed to provide the position of the argon atom in the heterodimer, which is found to be driven by a preference for interaction with the electron rich fluorine atom while simultaneously interacting with other heavy atoms as geometrically possible. Tunneling between the two equivalent (and enantiomerically related) forms of the gauche rotamer is observed via splitting of c-type rotational transitions, which was previously not resolved in the ground torsional state. In addition to this large amplitude motion, atypically large values of the centrifugal distortion constants for both rotamers of this fluoropropene suggest the existence of fairly soft vibrational modes that are sufficiently altered by the presence of argon in the heterodimer so that the usual assumption of unchanged monomer geometry upon complex formation is not completely valid.

The gas-phase heterodimers formed by argon and acetylene with 3,3,3-trifluoropropene are investigated using ab initio calculations and Fourier transform microwave spectroscopy. Spectroscopic constants of the most abundant isotopologues of these two complexes, as well as their minor isotopologues containing a single 13C substitution, reveal their structures. Ar is located above the planar HCCCF cavity of the trifluoropropene so that it can interact with a maximum number of heavy atoms. Acetylene binds to the H atom of trifluoropropene geminal with the CF3 group, and lies in the symmetry plane of trifluoropropene. One of the acetylenic H atoms forms a bifurcated hydrogen bond with both out-of-plane fluorine atoms, and the acetylenic bond interacts with the geminal H atom. The proximity of this H atom to the CF3 group suggests that this motif is driven by electrostatic factors.

The cis and trans isomers of the chiral tagging candidate molecule, 2-fluoro-3-(trifluoromethyl)oxirane, as well as the lowest energy gas-phase heterodimer of each with the argon atom, are characterized via quantum chemistry calculations and microwave rotational spectroscopy from 5 to 18 GHz and their ground state, vibrationally averaged structures, are determined. Apart from the cis/trans nature of the ring substitution and small differences in the dihedral angle specifying the rotation of the trifluoromethyl group, the two oxirane molecules and their respective argon complexes each have remarkable structural similarity. In contrast, the binding mode of argon to the oxirane, while similar for the two complexes here, is distinct from those modes observed in previous argon-fluorooxirane species. The ability to tune the preferred mode of binding with differing levels of fluorine substitution may prove advantageous in applications of chiral tagging to a wide variety of analytes.

The microwave spectra of three isotopologues of the gas-phase heterodimer formed between cis-1,2-difluoroethylene and hydrogen chloride are obtained in the 5–21 GHz region using Fourier transform microwave spectroscopy. The molecular structure, determined from the analysis of the spectra and supported by quantum chemistry calculations, has the hydrogen atom of the hydrogen chloride molecule interacting with both fluorine atoms of the fluoroethylene and no interaction between the chlorine atom and the olefin. Although the equilibrium structure has two inequivalent H⋯F interactions, zero-point motion averages over the two equivalent choices for these interactions, rendering the pairs of like atoms (C, H, and F) of the fluoroethylene equivalent, retaining the C2v symmetry of the olefin. This results in only one unique singly substituted 13C isotopologue and in the observed effects on transition intensities due to nuclear spin statistics. The heterodimer structure allows for a strong, linear hydrogen bond between the HCl donor and the fluoroethylene acceptor that is more important here than in the analogous acetylene containing complex, where the interaction between the π electrons of acetylene and an electrophilic hydrogen atom on the olefin compensates for the loss of linearity required for binding to a geminal F/H pair.

The rotational spectra of four isotopologues of an isolated complex formed between an argon atom and imidazole, Ar...imidazole, have been recorded in the 6-19 GHz region by Fourier transform microwave spectroscopy. Rotational transition frequencies have been fitted to Watson's S-reduced Hamiltonian to yield rotational, centrifugal distortion and nuclear quadrupole coupling constants for the complex. Rotational constants determined for the parent and three N-15-containing isotopologues allow the three-dimensional structure of the complex to be described. The two angles, theta and phi , which define the orientation of the Ar atom relative to the imidazole ring have been determined for the first time in addition to the distance between Ar and the center of mass of the imidazole sub-unit, R. Fitting of structural parameters to the experimentally-determined moments of inertia yields a structure where Ar is positioned above the ring plane at a distance of 3.519 & Aring; from the center of mass of the imidazole sub-unit. In the experimentally determined, average geometry, the intermolecular axis (drawn through Ar and the center of mass of the imidazole sub-unit) is oriented at 6 degrees from the normal to the ring plane. The experimental results allow for four alternative possibilities for phi with 62.0(39)degrees being that which is most consistent with expectations for this parameter based on previous work. The experimentally-determined nuclear quadrupole coupling constants imply that the electric field gradient at each of the nitrogen nuclei of imidazole does not significantly change on formation of the complex with Ar.

Microwave spectra of both the E and Z isomers of 1,2,3,3,3-pentafluoropropene along with all three of the singly substituted 13C isotopologues for each are obtained using broadband chirped-pulse Fourier transform microwave spectroscopy from 2.0-18.1 GHz. Associated quantum chemistry calculations show that the barrier to internal rotation of the CF3 group is significantly higher for the Z isomer, which is stabilized by an intramolecular hydrogen bond, although the barriers in both isomers are sufficiently high to prevent the observation of any effects due to internal rotation. The normal isotopologues of the argon heterodimers for both isomers are also observed in the broadband spectrum and a Balle-Flygare cavity Fourier transform microwave spectrometer is used to obtain the 5.0-20.6 GHz spectra of the corresponding 13C isotopologues. In each case, the argon atom locates so as to maximize its interactions with areas of significant electron density. However, mapped electrostatic potential surfaces indicate that the areas of greatest nucleophilicity are different for the two isomers, suggesting that they may interact differently in forming heterodimers with protic acids.

The microwave spectra of (Z)-1-chloro-3,3,3-trifluoropropene and its gas-phase heterodimer with the argon atom in the 5.6 to 18.1 GHz frequency range are first observed and assigned using broadband, chirped pulse, Fourier transform microwave (FTMW) spectroscopy. Subsequent analysis of higher-resolution spectra obtained between 5 and 21 GHz with a narrowband, Balle-Flygare FTMW instrument provides spectroscopic constants, including nuclear quadrupole coupling constants, for five isotopologues of the propene molecule and two isotopologues of the complex with argon. Structural parameters obtained from these spectra show the existence of an intra-molecular hydrogen bond between one of the fluorine atoms of the trifluoromethyl group and the hydrogen atom on the adjacent carbon atom. No evidence is seen for internal rotation of the trifluoromethyl group. The location of the argon atom in the heterodimer is consistent with the expectation that it will be positioned so to interact with the greatest number of heavy atoms, and in particular, the polarizable chlorine atom.

The gas-phase heterodimer formed between (Z)-1-chloro-3,3,3-trifluoropropene and acetylene is investigated using quantum chemistry calculations and observed via chirped-pulse Fourier transform microwave (FTMW) spectroscopy. Subsequent analysis of higher resolution spectra, including those using a sample enriched in H13C13CH, obtained with a Balle-Flygare FTMW spectrometer reveals a novel structure, as predicted by theory, for the complex, in which the acetylene functions as the gas-phase (Lewis) base and the halopropene as the acid. In the equilibrium structure, the acetylene molecule is located perpendicular to the symmetry plane of (Z)-1-chloro-3,3,3-trifluoropropene with the triple bond interacting with the two olefinic hydrogens. Mapped electrostatic potential surfaces suggest that this structure results from a reduction in the nucleophilicity of the halogen atoms as compared to previously studied acetylene halo-olefin complexes and a concomitant increase in the electrophilicity of the hydrogen atoms.

Gas phase homodimers of 3,3,3-trifluoro-1,2-epoxypropane (TFO), a molecule which has shown promise as an effective chiral tag for determining the absolute stereochemistry and the enantiomeric composition of chiral analytes, are explored using a variety of quantum chemistry models and rotational spectroscopy. The potential surface governing the interaction of the two molecules is rapidly explored using the artificial bee colony algorithm for homodimer candidates that are subsequently optimized by quantum chemistry methods. Although all model chemistries employed agree that the lowest energy form of the heterochiral homodimer of TFO (RS or SR) is lower in energy than that of the homochiral dimer (RR or SS), the energy spacings among the lower energy isomers of each and indeed the absolute energy ordering of the isomers of each are very model dependent. The experimental results suggest that the B3LYP-D3BJ/def2-TZVP model chemistry is the most reliable and provides excellent estimates of spectroscopic constants. In accord with theoretical predictions the non-polar lowest energy form of the heterochiral homodimer is not observed, while two isomers of the homochiral dimer are observed and spectroscopically characterized. Observation and assignment of the spectra for all three unique singly-substituted 13C isotopologues, in addition to that of the most abundant isotopologue for the lowest energy isomer of the homochiral homodimer of TFO, provide structural information that compares very favorably with theoretical predictions, most notably that the presence of three fluorine atoms on the trifluoromethyl group removes their direct participation in the intermolecular interactions, which instead comprise two equivalent pairs of CH⋯O hydrogen bonds between the two epoxide rings augmented by favorable dispersion interactions between the rings themselves.

The microwave spectra of two rotamers, connected by rotation of the -CHF2 group, of 2,3,3-trifluoropropene and all three of the singly substituted C-13 isotopologues of each are obtained in the 5.6 - 18.1 GHz region using broadband chirped pulse Fourier transform microwave spectroscopy. The lower energy rotamer, stabilized by a weak intermolecular hydrogen bond, is chiral, existing as a pair of enantiomers, while the higher energy isomer, lacking this interaction but having a plane of symmetry, is achiral. Heterodimers of both rotamers with the argon carrier gas are also observed in the broadband spectrum and further analyzed using spectra obtained from 5.0 to 20.6 GHz with a Balle-Flygare cavity Fourier transform microwave spectrometer. Although all singly substituted C-13 isotopologues are observed, in addition to the normal species, for the argon complex with the lower energy rotamer, only a single isotopologue of the argon complex, the parent, is observed with the higher energy mtamer. Despite differing slightly in detail, the argon atom occupies a similar position in the heterodimer with each rotamer, one that is also similar to the location of the argon atom in argon-2,3,3,3-tetrafluoropropene.

The capabilities of rotational spectroscopy-based methods as tools to deliver accurate and precise chirality-sensitive information are still breaking ground, but their applicability in the challenging field of analytical chemistry is already clear. In this mini review, we explore the current abilities and challenges of two emergent techniques for chiral analysis based on rotational spectroscopy. For that, we will showcase the two methods (microwave 3-wave mixing and chiral tag rotational spectroscopy) while testing their performance to solve the absolute configuration and the enantiomeric excess of a blind sample containing a mixture of enantiomers of styrene oxide.

The microwave, rotational spectrum between 5.6 and 19.7 GHz of the gas-phase heterodimer formed between acetylene and (E)-1-chloro-1,2-difluoroethylene is obtained using both broadband, chirped-pulse and narrow band, Balle-Flygare Fourier transform microwave spectrometers. The structure of the complex is determined from the rotational constants obtained via the analysis of the spectra for the normal isotopologue of the complex and three isotopically substituted species: the singly substituted 37Cl isotopologue, obtained in natural abundance, and two isotopologues singly substituted with 13C, obtained using an isotopically enriched HC13CH sample. The acetylene forms a hydrogen bond with the fluorine atom on singly halogenated carbon and a secondary interaction with the hydrogen atom on that same carbon. The angle strain induced in forming the secondary interaction is offset by the favorable electrostatics of the hydrogen bond to fluorine. Comparisons with acetylene complexes of 1,1,2-trifluoroethylene and cis-1,2-difluoroethylene show the effects of halogen substitution at the remote carbon on this bonding motif.

The microwave spectra of four isotopologues of (E)-1-chloro-1,2-difluoroethylene, and additionally, the spectra of two isotopologues of the complex formed between argon and (E)-1-chloro-1,2-difluoroethylene are obtained in the 5.6-18.1 GHz region using broadband chirped pulse Fourier transform microwave spectroscopy. The argon atom is determined to occupy a position out of the symmetry plane of the haloethylene where it can interact most strongly with the fluorine atom and the chlorine atom located geminal to each other as well as the pi electron density at the end of the double bond on carbon atom C-1. As in the previously studied argon-2-chloro-1,1difluoroethylene complex, the argon atom interacts most strongly with a fluorine atom and the chlorine atom regardless of whether they are located geminal or cis to each other.