Molecular Structures and Microwave Spectra of the Gas-Phase Homodimers of 3-Fluoro-1,2-epoxypropane and 3,3-Difluoro-1,2-epoxypropane.

Molecular structures for the heterochiral and homochiral gas-phase homodimers of 3-fluoro-1,2-epoxypropane and 3,3-difluoro-1,2-epoxypropane are investigated using both ab initio and density functional quantum chemistry calculations. Although microwave spectra for the heterochiral dimers are not observed as the lowest-energy isomers lack an electric dipole moment and others are presumably too high in energy, rotational spectra are observed for the homochiral dimers of each molecule that are consistent with the lowest-energy isomers of each. The presence of hydrogen atoms in the fluoromethyl groups makes it possible for these groups to participate in the intermolecular interactions that stabilize these dimers, resulting in a distinctly different bonding motif than is observed in the homodimers of 3,3,3-trifluoro-1,2-epoxypropane where the lack of a hydrogen atom prevents this possibility. The rotational spectra and energy ordering of the dimers are sufficiently well predicted with modest calculational methods to enable straightforward assignment of the observed spectra and to identify the molecular carrier of an assigned spectrum.

Microwave Spectrum and Novel Molecular Structure of (Z)-1-Chloro-3,3,3-trifluoropropene-Acetylene.

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.

Examining the gas-phase homodimers of 3,3,3-trifluoro-1,2-epoxypropane using quantum chemistry and microwave spectroscopy.

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.

Straining to Put the Pieces Together: The Molecular Structure of (E)-1-Chloro-1,2-difluoroethylene-Acetylene from Microwave Spectroscopy.

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 Importance of How the Pieces Fit Together: The Microwave Spectrum and Molecular Structure of 2-Chloro-1,1-Difluoroethylene-Acetylene.

Fourier transform microwave spectroscopy is used to obtain the rotational spectrum of the gas-phase heterodimer formed between 2-chloro-1,1-difluoroethylene and acetylene between 5.1 and 20.0 GHz. Rotational constants derived from the analysis of the spectra for the normal isotopologue, the singly substituted 37Cl isotopologue, observed in natural abundance, and two isotopologues singly substituted with 13C, obtained using an isotopically enriched HC13CH sample, are used to determine the structure of the complex. Although the formation of a hydrogen bond to fluorine, considered in isolation, would be electrostatically favored, the angle strain induced in forming the secondary interaction between the acetylene triple bond and the hydrogen atom on 2-chloro-1,1-difluoroethylene coupled with the relaxed steric requirements of hydrogen bonding to chlorine lead to the heterodimer adopting what we have previously termed the side-binding configuration as the lowest energy structure. In this arrangement, the acetylene forms a hydrogen bond with the chlorine atom and a secondary interaction with the hydrogen atom, which is geminal to the chlorine. Comparisons with acetylene complexes of (Z)-1-chloro-2-fluoroethylene and vinyl chloride show the effects of increasing fluorine substitution on this bonding motif.

Microwave Spectrum and Molecular Structure of 3-Fluoro-1,2-epoxypropane and the Unexpected Structure of Its Complex with the Argon Atom.

In common with the homologous 3,3-difluoro- and 3,3,3-trifluoro-species, 3-fluoro-1,2-epoxypropane is a small chiral molecule with a simple rotational spectrum, making it potentially useful for chiral analysis via conversion of enantiomers into spectroscopically distinct diastereomers through formation of noncovalently bound complexes. The rotational spectrum of 3-fluoro-1,2-epoxypropane (FO) and of its heterodimer with the argon atom are obtained, along with several isotopologues of each, using Fourier transform microwave spectroscopy from 5.6 to 18.1 GHz, and their structures determined. Surprisingly, the structure of 3-fluoro-1,2-epoxypropane-argon does not show a strong similarity to those previously determined for 3,3-difluoro-1,2-epoxypropane-argon and 3,3,3-trifluoro-1,2-epoxypropane-argon but instead is more analogous to that of propylene oxide-argon. Equilibrium structural parameters and mapped electrostatic potential surfaces obtained via quantum chemistry calculations are used in rationalizing this result.

Exploring the sterically disfavored binding of acetylene to a geminal olefinic hydrogen-fluorine atom pair: The microwave spectrum and molecular structure of cis-1,2-difluoroethylene-acetylene.

The microwave rotational spectrum of the gas-phase bimolecular heterodimer formed between cis-1,2-difluoroethylene and acetylene is obtained using Fourier transform microwave spectroscopy from 5.9 to 21.2 GHz. Rotational constants derived from the analysis of the spectra for the normal isotopologue and singly substituted 13C isotopologues, obtained in natural abundance, allow the determination of the structure of the complex, which, in the absence of a fluorine-hydrogen atom pair located cis to each other, adopts a sterically disfavored geometry ("side-binding") in which the acetylene interacts with a geminal fluorine-hydrogen atom pair. Structural details are found to be similar to those of previously studied heterodimers with side-binding of acetylene to fluorine while reflecting the degree of halosubstitution. A detailed comparison with the (Z)-1-chloro-2-fluoroethylene-acetylene complex reveals information regarding the relaxed steric requirements for hydrogen bonding to chlorine as opposed to hydrogen bonding to fluorine.

Exploring the Forces Contributing to Noncovalent Bonding by Microwave Spectroscopy and Structural Characterization of Gas-Phase Heterodimers of Protic Acids with Haloethylenes.

A detailed comparison of structural parameters obtained via microwave rotational spectroscopy in a systematic study of protic acid-haloethylene heterodimers is used to investigate the forces contributing to intermolecular interactions. Conclusions reached using structural data and chemical intuition are supplemented with information obtained from quantum-chemistry calculations to refine the understanding of the various contributions to complex formation. The observed structures, representative of the global minimum on the potential energy surface, are found to reflect a balance between optimal electrostatics and steric requirements, or in other words, how well the two interacting molecules fit together. Structural trends are rationalized in terms of familiar chemical concepts of the electrophilicity or nucleophilicity of interaction sites as modulated by electron-withdrawing and electron-donating groups along with the geometric requirements for optimal interactions between the two molecules.

Finding the Better Fit: The Microwave Spectrum and Sterically Preferred Structure of trans-1,2-Difluoroethylene-Hydrogen Chloride.

The rotational spectrum of the gas-phase bimolecular heterodimer formed between trans-1,2-difluoroethylene and hydrogen chloride is obtained using Fourier transform microwave spectroscopy from 5.6 to 20.6 GHz. Analysis of the spectrum provides rotational constants and nuclear quadrupole coupling constants that are used to determine the structure of the complex. The HCl molecule forms a hydrogen bond with one of the two electrostatically equivalent fluorine atoms of the difluoroethylene, and this hydrogen bond bends from linearity to allow a secondary interaction between the chlorine atom and the hydrogen atom located cis to the fluorine atom in the hydrogen bond. Because the two hydrogen atoms are likewise electrostatically equivalent, the structure indicates that this is the sterically preferred arrangement in HCl binding to a fluoroethylene rather than the one with the secondary interaction to the geminal hydrogen atom. Detailed comparisons among the geometries of the complexes formed between HCl and HF, on the one hand, and vinyl fluoride, 1,1-difluoroethylene, trans-1,2-difluoroethylene, 1,1,2-trifluoroethylene, and ( E)-1-chloro-2-fluoroethylene, on the other, reveal structural trends accompanying increasing fluorination and substitution of chlorine for fluorine.

Microwave Spectrum and Molecular Structure of the Chiral Tagging Candidate, 3,3,3-Trifluoro-1,2-epoxypropane and Its Complex with the Argon Atom.

The rotational spectrum of the chiral tagging candidate molecule, 3,3,3-trifluoro-1,2-epoxypropane (TFO), and of its heterodimer with the argon atom, is obtained using Fourier transform microwave spectroscopy from 5.6 to 18.1 GHz. With a strong, simple rotational spectrum, TFO shows promise for applications in chiral analysis through the conversion of enantiomers into spectroscopically distinct diastereomeric species through noncovalent attachment. The structure of the argon complex of TFO, determined from analysis of the microwave spectrum, is extremely similar to that previously found for ethylene oxide-argon but quite different from that suggested for propylene oxide-argon.

The Microwave Spectrum and Molecular Structure of (Z)-1-Chloro-2-Fluoroethylene-Acetylene: Demonstrating the Importance of the Balance Between Steric and Electrostatic Interactions in Heterodimer Formation.

The structure of the gas-phase heterodimer formed between (Z)-1-chloro-2-fluoroethylene and acetylene is determined via Fourier transform microwave spectroscopy from 5.5 to 20.8 GHz. In the first instance where in the presence of both a fluorine atom and a chlorine atom on the haloethylene the protic acid binds to the chlorine atom, the acetylene adopts a configuration similar to that in the analogous complex with vinyl chloride. Positioned in a manner to interact favorably with both the chlorine atom and the hydrogen atom geminal to it, the acetylene molecule is able to maximize the overall electrostatic stabilization even though other regions of the haloethylene offer individual sites of greater positive or negative electrostatic potential. Detailed comparison with the vinyl chloride-acetylene complex suggests that the presence of the fluorine atom weakens the hydrogen bond but strengthens the interaction between the geminal hydrogen atom and the triple bond.

The gas-phase structure of the asymmetric, trans-dinitrogen tetroxide (N2O4), formed by dimerization of nitrogen dioxide (NO2), from rotational spectroscopy and ab initio quantum chemistry.

We report the first experimental gas-phase observation of an asymmetric, trans-N2O4 formed by the dimerization of NO2. In additional to the dominant 14N216O4 species, rotational transitions have been observed for all species with single 15N and 18O substitutions as well as several multiply substituted isotopologues. These transitions were used to determine a complete substitution structure as well as an r0 structure from the fitted zero-point averaged rotational constants. The determined structure is found to be that of an ON-O-NO2 linkage with the shared oxygen atom closer to the NO2 than the NO (1.42 vs 1.61 Å). The structure is found to be nearly planar with a trans O-N-O-N linkage. From the spectra of the 14N15NO4 species, we were able to determine the nuclear quadrupole coupling constants for each specific nitrogen atom. The equilibrium structure determined by ab initio quantum chemistry calculations is in excellent agreement with the experimentally determined structure. No spectral evidence of the predicted asymmetric, cis-N2O4 was found in the spectra.

Effect of Chlorine Substitution in Modulating the Relative Importance of Two Intermolecular Interactions: The Microwave Spectrum and Molecular Structure of (E)-1-Chloro-2-fluoroethylene-HCl.

Fourier transform microwave rotational spectroscopy is used to determine the structure of the gas-phase bimolecular complex formed between (E)-1-chloro-2-fluoroethylene and hydrogen chloride. Extensively split by nuclear quadrupole hyperfine structure and isotopic dilution, the spectrum is first identified via weak features observed using a broadband chirped pulse spectrometer in the 5.6-18.1 GHz range and studied in detail with greater sensitivity and resolution over 6.0-20.0 GHz with a Balle-Flygare, narrowband instrument. The complex has a geometry similar to that of vinyl fluoride-HCl, with HCl binding across the C═C double bond, forming a hydrogen bond to the fluorine atom of the haloethylene and bending to allow a secondary interaction to develop with the hydrogen atom in the cis position. Further consideration of structural details among the complexes of hydrogen fluoride and hydrogen chloride with (E)-1-chloro-2-fluoroethylene and vinyl fluoride suggests that the addition of a trans Cl atom in vinyl fluoride enhances the significance of the secondary interaction while deemphasizing that of the hydrogen bond.

The Microwave Spectrum and Molecular Structure of (E)-1-Chloro-2-Fluoroethylene-HF: Revealing the Balance among Electrostatics, Sterics, and Resonance in Intermolecular Interactions.

The structure of the gas-phase bimolecular complex formed between (E)-1-chloro-2-fluoroethylene and hydrogen fluoride is determined via Fourier transform microwave spectroscopy from 6.9-21.6 GHz. Although the complex adopts a geometry very similar to that of previously studied dihalosubstituted ethylene-HF species, trends observed in the values of structural parameters such as bond lengths, bond angles, and deviations of the primary hydrogen-bonding interaction from linearity provide information regarding the balance among electrostatic, steric, and resonance effects in the structures of these complexes. Consideration of the ab initio interaction potential between (E)-1-chloro-2-fluoroethylene and hydrogen fluoride suggests that it is the strength of the intermolecular bond formed between the hydrogen atom of HF and the fluorine atom of the substituted ethylene that plays the significant role in determining the geometry. In addition to determining the complete nuclear quadrupole coupling constant tensor for the (E)-1-chloro-2-fluoroethylene-HF complex, the corresponding tensor for (E)-1-chloro-2-fluoroethylene itself was measured with greater precision than previously availabe, including the first reported determination of the single, nonzero off-diagonal element, χab.

Infrared rovibrational spectroscopy of OH-C2H2 in (4)He nanodroplets: Parity splitting due to partially quenched electronic angular momentum.

The T-shaped OH-C2H2 complex is formed in helium droplets via the sequential pick-up and solvation of the monomer fragments. Rovibrational spectra of the a-type OH stretch and b-type antisymmetric CH stretch vibrations contain resolved parity splitting that reveals the extent to which electronic angular momentum of the OH moiety is quenched upon complex formation. The energy difference between the spin-orbit coupled (2)B1 (A″) and (2)B2 (A') electronic states is determined spectroscopically to be 216 cm(-1) in helium droplets, which is 13 cm(-1) larger than in the gas phase [Marshall et al., J. Chem. Phys. 121, 5845 (2004)]. The effect of the helium is rationalized as a difference in the solvation free energies of the two electronic states. This interpretation is motivated by the separation between the Q(3/2) and R(3/2) transitions in the infrared spectrum of the helium-solvated (2)Π3/2 OH radical. Despite the expectation of a reduced rotational constant, the observed Q(3/2) to R(3/2) splitting is larger than in the gas phase by ≈0.3 cm(-1). This observation can be accounted for quantitatively by assuming the energetic separation between (2)Π3/2 and (2)Π1/2 manifolds is increased by ≈40 cm(-1) upon helium solvation.

Effect of acid identity on the geometry of intermolecular complexes: the microwave spectrum and molecular structure of vinyl chloride-HF.

The structure of the gas-phase bimolecular complex formed between vinyl chloride and hydrogen fluoride is determined using Fourier transform microwave spectroscopy from 6.3 to 21.4 GHz. Although all previous examples of complexes formed between protic acids and haloethylenes are observed to have similar modes of binding regardless of the specific identity of the acid, HF, HCl, or HCCH, the planar vinyl chloride-HF complex has HF located at the "top" of the vinyl chloride with the secondary interaction occurring with the cis hydrogen atom as opposed to the "side" binding configuration found for vinyl chloride-HCCH. Nevertheless, the details of the structure, such as hydrogen bond length (2.32 Å) and amount of deviation from linearity (19.8°), do reflect the strength of the interaction and show clear correlations with the gas-phase acidity. Comparison with analogous complexes allows the determination of the relative importance of electrostatic interactions and steric requirements in leading to the observed structures.

A longitudinal assessment of periodontal disease in 52 Miniature Schnauzers.

BACKGROUND: Periodontal disease (PD) is the most widespread oral disease in dogs and has been associated with serious systemic diseases. The disease is more prevalent in small breeds compared to large breeds and incidence increases with advancing age. In prevalence studies 84% of Beagles over the age of 3 and 100% of Poodles over the age of 4 were diagnosed with PD. Current knowledge of the rate of progression of PD is limited. The objective of this study was to determine the rate of PD progression in Miniature Schnauzers, an at risk small breed of dog. Dogs (n = 52, age 1.3-6.9 years) who had received a regular oral care regime prior to this study were assessed for levels of gingivitis and periodontitis around the whole gingival margin in every tooth under general anaesthetic. Assessments were conducted approximately every six weeks for up to 60 weeks following the cessation of the oral care regime. RESULTS: All of the 2155 teeth assessed entered the study with some level of gingivitis. 23 teeth entered the study with periodontitis, observed across 12 dogs aged between 1.3 and 6.9 years. 35 dogs had at least 12 teeth progress to periodontitis within 60 weeks. Of the teeth that progressed to periodontitis, 54% were incisors. The lingual aspect of the incisors was significantly more likely to be affected (p < 0.001). The severity of gingivitis in periodontitis-affected teeth was variable with 24% of the aspects affected having very mild gingivitis, 36% mild gingivitis and 40% moderate gingivitis. Periodontitis progression rate was significantly faster in older dogs. Only one dog (age 3.5) did not have any teeth progress to periodontitis after 60 weeks. CONCLUSIONS: This is the first study to have assessed the progression rate of periodontitis in Miniature Schnauzers and highlights that with no oral care regime, the early stages of periodontitis develop rapidly in this breed. An oral care regime and twice yearly veterinary dental health checks should be provided from an early age for this breed and other breeds with similar periodontitis incidence rates.

Microwave spectrum and molecular structure of vinyl chloride-acetylene, a side binding complex.

The structure of the gas-phase bimolecular complex formed between vinyl chloride and acetylene is determined using a combination of broad-band, chirped-pulse, and narrow-band, Balle-Flygare Fourier transform microwave spectroscopy from 5.8 to 20.7 GHz. Although all previous examples of complexes formed between protic acids and haloethylenes are observed to have similar modes of binding regardless of the specific identity of the acid, HF, HCl, or HCCH, the vinyl chloride-HCCH complex has HCCH located at one end of the vinyl chloride with the secondary interaction occurring with the geminal hydrogen atom as opposed to the "top" binding configuration found for vinyl chloride-HF. Nevertheless, the details of the structure, such as hydrogen bond length (3.01 Å) and amount of deviation from linearity (58.5°), do reflect the strength of the interaction and show clear correlations with the gas-phase acidity. Comparison with analogous complexes allows the determination of the relative importance of electrostatic interactions and steric requirements in leading to the observed structures.

The molecular structure of and interconversion tunneling in the argon-cis-1,2-difluoroethylene complex.

Guided by ab initio predictions, the structure of the gas-phase complex formed between cis-1,2-difluoroethylene and an argon atom in a pulsed molecular jet is determined using microwave spectroscopy in the 5.7-21.5 GHz region of the spectrum. This is a non-planar, symmetric species, with the argon atom located in the FCCF cavity of the difluoroethylene. The transitions in the microwave spectrum are observed to be split by an interconversion tunneling motion between the two equivalent configurations for the complex with the argon atom located either above or below the difluoroethylene molecular plane. Both one- and two-dimensional discrete variable representation calculations of the tunneling splitting using the ab initio interaction potential for the complex suggest that the barrier to interconversion is overestimated by theory.

Rotational spectroscopy and molecular structure of the 1-chloro-1-fluoroethylene-acetylene complex.

Guided by ab initio calculations, Fourier transform microwave spectra in the 6-21 GHz region are obtained for seven isotopomers of the complex formed between 1-chloro-1-fluoroethylene and acetylene. These include the four possible combinations of (35)Cl- and (37)Cl-containing CH(2)CClF with the most abundant acetylene isotopic modification, HCCH, and its H(13)C(13)CH analogue, as well as three singly substituted deuterated isotopomers. Analysis of the spectra determines the rotational constants and additionally, the complete chlorine quadrupole hyperfine coupling tensors in both the inertial and principal electric field gradient axis systems, and where appropriate, the diagonal components of the deuterium quadrupole coupling tensors. The inertial information contained in the rotational constants provides the structure for CH(2)CClF-HCCH: a primary, hydrogen bonding interaction existing between the HCCH donor and the F atom acceptor on the 1-chloro-1-fluoroethylene moiety, while a secondary interaction occurs between the acetylenic bond on the HCCH molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene, which causes the hydrogen bond to deviate from linearity. This is similar to the structure obtained for 1,1-difluoroethylene-HCCH [H. O. Leung and M. D. Marshall, J. Chem. Phys. 126, 154301 (2006)], and indeed, to within experimental uncertainty, the intermolecular interactions in CH(2)CClF-HCCH and its 1,1-difluoroethylene counterpart are practically indistinguishable, even though ab initio calculations at the MP2∕6-311G++(2d, 2p) level suggest that the former complex is more strongly bound.