Rebecca A. Peebles

A generalizable method of intensity ratio analysis has facilitated identification and deconvolution of rotational spectra for nine mixed trifluoroethylene (TFE)/CO clusters, (TFE) (CO ) , within a single spectroscopic data set. Molecular complexes with = 1-2 and = 1-6 have been investigated using chirped-pulse (CP) Fourier-transform microwave (FTMW) spectroscopy. Collection of two spectra with different CO concentrations allowed determination of intensity ratios of rotational transitions, which were then correlated with cluster size and formula. The wide composition range of observed TFE/CO clusters provides an abundance of data with which to benchmark computational methods for determining compositions and energy ordering of large mixed clusters. Initial computational structural results for experimentally observed species are also presented.

[Display omitted] •Intensity variation was used to identify rotational spectra within a dense dataset.•Variation of CO2 concentration allowed differentiation of clusters up to pentamers.•Cluster structures mimic pure CO2 complexes, with an added fluoroethylene tag. Chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy has been used to measure the spectra of ten previously unobserved fluoroethylene (FE)/CO2 clusters, (FE)x(CO2)y, with x from 1 to 4 and y from 0 to 4. Multiple spectra were recorded with varying concentrations of CO2 in the sample, and at least 400,000 free induction decays averaged per data set. This allowed implementation of data-centered approaches, using intensity variation, to identify subsets of transitions belonging to the same cluster species or those of similar composition, simplifying the assignment process for the complex mixture of clusters present. All spectra were fitted to Watson A-reduction Hamiltonians, including quartic distortion constants, with very few species requiring higher order distortion constants for satisfactory fits to be obtained. Computational data at MP2/6-311++G(2d,2p) and ωB97X-D/6-31+G(d,p) levels suggested FE:CO2 ratios and likely structural arrangements of each cluster based on comparisons with experimental rotational constants.

•The chirped-pulsed Fourier-transform microwave spectrum of 2-fluorophenylacetylene was measured.•The rotational constants for the parent and 13C isotopomers were obtained.•Dipole moments were determined from stark effect measurements.•The structure of 2-fluorophenylacetylene is determined by density functional theory calculation (re), Kraitchman substitution (rs) and semi-experimental method (reSE).•The structural parameter comparisons were made using the above methods. The rotational spectra of the parent and eight 13C isotopomers of 2-fluorophenylacetylene were measured using chirped-pulsed Fourier-transform microwave spectroscopy in the frequency range 6–18 GHz. The rotational constants for the parent isotopomer were determined to be A0 = 2949.41825(17) MHz, B0 = 1495.78081(13) MHz, and C0 = 992.23008(9) MHz. The total dipole moment was determined to be 1.720(2) Debye from Stark measurements. The molecular geometry parameters were obtained experimentally using Kraitchman substitution and semi-experimental equilibrium structure. These structural parameters and dipole moments were compared with the data calculated at the B3LYP/6-311+G(2d,p) level and analyzed using the natural bond orbital method. [Display omitted]

•Rotational spectra of 2-bromobutane were measured using a CP-FTMW spectroscopy.•Hyperfine structures for 79Br and 81Br species of G+, A, and G− conformers were assigned.•Relative abundances of conformers were estimated by relative intensity measurements.•Spectroscopic constants and structural parameters were compared to ab initio calculations.•Dipole moments of the G+ conformer were determined by the stark effect measurements. The rotational spectra of 2-bromobutane were obtained by using a chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer in the frequency range 8–18 GHz. Hyperfine structures of both bromine isotopologues (I = 3/2) for the three conformers (G+, A, and G−) have been investigated to determine the rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants of bromine (Br). The conformational compositions for G+, A, and G− in the supersonic jet, estimated by relative intensity measurements, are 55%, 28%, 17%, respectively, for the 79Br species, and 52%, 33%, 15%, respectively, for the 81Br species. However, the small zero-point energy differences of isotopomers can be negligible and can be considered as an average value instead of individual values, which is 54%, 30%, 16% for G+, A, and G− species. The nuclear quadrupole coupling tensors (χxx, χyy, and χzz) of Br and the quadrupolar angles were obtained, and the χzz value of 2-bromobutane is compared with those of other alkyl bromides. The spectroscopic constants, rs coordinates of Br and the dipole moments of the most stable G+ conformer are predicted well from the ab initio calculations at the MP2/6–311++G(2d,2p) level. [Display omitted]

The structure of fluoroethylene dimer ((C2H3F)(2), (FE)(2)) has been determined using chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy. Although the most stable structure was expected to be nonpolar, a planar, polar configuration was observed with sufficient intensity to measure spectra for four unique C-13 substituted isotopologues in natural abundance. This allowed a least-squares fit of the structural parameters of the dimer to observed moments of inertia. The resulting structure has a T-shaped arrangement of C=C bonds, similar to the recently studied FE center dot center dot center dot 1,1-difluoroethylene complex. Both dimers contain a cyclic arrangement of CH center dot center dot center dot F contacts between the H-C-F end of one monomer and the H-C=C-F side of the second monomer. omega B97X-D/6-31+G(d,p), MP2/6-311++G(2d,2p) and MP2/aug-cc-pVDZ calculations give the expected nonpolar end-to-end structure, ranging from 9 cm(-1) (DFT) to 63 cm(-1) and 57 cm(-1) (MP2/6-311++G(2d,2p) and aug-cc-pVDZ, respectively) more stable than the experimentally observed orientation. The omega B97X-D/6-31+G(d,p) calculations also indicate a nonplanar, polar configuration that is over 60 cm(-1) more favorable than the nonpolar structure; however, MP2 calculations predict significantly higher energy, and spectroscopic results provide no evidence of that structure's existence. The rotational constants of the observed T-shaped (FE)(2) structure are A = 6582.8323(12) MHz, B = 1256.6203(4) MHz, C = 1060.3474(3) MHz, and the dipole moment components, determined via Stark effect measurements, are mu(a) = 0.683 (3) D, mu(b) = 0.301 (5) D, mu(tot) = 0.746 (5) D. (C) 2020 Elsevier B.V. All rights reserved.

The microwave spectrum of the dimer Ar–1,3-difluorobenzene from 2 to 18 GHz is reported. The spectrum has been observed using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer that has recently been expanded to include the 2–6 GHz region of the electromagnetic spectrum. Details of this upgraded spectrometer are reported. Eighty-seven transitions were observed for the parent dimer spectrum, which was adequately fit to a semirigid rotational Hamiltonian consisting of A, B, and C as well as four quartic centrifugal distortion constants. Observations of 13C species in natural abundance were aided by utilizing smaller chirp ranges of 7–9 and 9–11 GHz for 1.9 million and 3.73 million averages, respectively. Assignment of 13C isotopologues allowed for determination of the Kraitchman coordinates of the carbon atoms as well as inertial fits of the complex. The quantum-chemical structure predicts an Ar to monomer center of mass distance of 3.48 Å, compared with 3.564(1) Å determined from experimental structural analysis. This new study indicates that in fluorinated benzene–Ar dimers, when the fluorines are separated by more carbon atoms, the Ar–ring center distance decreases.

Understanding structural changes during microsolvation of a solute by increasing numbers of CO2 molecules provides a path towards improved models of supercritical CO2 solvent properties. Microwave spectroscopic observations of fluoroethylene (FE) microsolvation by CO2 give two FE-(CO2)(2) isomers, each with fragments similar to previously studied planar FE-CO2 dimers. In each trimer, a second CO2 is located above the FE plane, in a position not observed for FE-CO2. Current work on FE/CO2 mixtures focuses on deconvoluting interleaved spectra in the microwave scan, aiming to identify FE-(CO2)(3) and other clusters, with patterns from at least five additional cluster spectra already identified. (C) 2018 The Authors. Published by Elsevier B.V.

The structure of the weakly bound dimer of 1,1-difluoroethylene with vinyl fluoride was studied using Fourier-transform microwave spectroscopy in the 6-19 GHz range. In the observed dimer structure each monomer acts as both a weak bond donor and acceptor, giving a cyclic arrangement of C-H center dot center dot center dot F contacts. The difluoroethylene C=C bond is roughly perpendicular to the vinyl fluoride C=C bond, forming the cross of a T-shaped carbon atom framework, resembling the lowest energy structure predicted by MP2/6-311++G(2d,2p) calculations. Observed C-H center dot center dot center dot F distances are similar to those of vinyl fluoride and 1,1-difluoroethylene complexes with difluoromethane and HCCH. The dipole moment of the dimer was measured using the Stark effect, giving values of mu(a) = 0.9003(19) mu(b), = 0.030(8) D and mu(total) = 0.9008 (22) D. A second ab initio structure, with the C=C bonds in a slipped parallel arrangement, was predicted to be about 38 cm(-1) higher in energy than the T-shaped conformation. This higher energy arrangement has not been observed experimentally.. (C) 2017 Elsevier Inc. All rights reserved.

Using a chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer, a 6–18 GHz spectrum of 4-fluorophenylacetylene (4FPA) was measured and only a-type R-branch transitions were observed up to J = 9. Rotational constants and quartic centrifugal distortion constants for the normal isotopic species were determined based on Watson-S reduction: A = 5652.812(22) MHz, B = 966.92885(11) MHz, C = 825.67680(11) MHz, DJ = 0.01377(60) kHz, and DJK = 0.2468(61) kHz, with other three distortion constants fixed as DK = 0.6629 kHz, d1 = 2.386 Hz, and d2 = 0.989 Hz from ab initio results. For six kinds of carbon-13 isotopic species, 10–15 transitions were detected by a resonant cavity FTMW spectrometer in natural abundance, and rotational constants of each species were also determined by fitting transition frequencies. Gaseous molecular structures of 4FPA were derived via the least-squares fitting (r0) and substitution (rs) methods, and ab initio optimization (re). They were compared to the structures of benzene derivatives having fluorine and the acetylenic group as substituents. In addition, dipole moment component of 4FPA was also determined to be μa = μtotal = 0.8935(9) D from Stark effect measurements. •Microwave spectrum for the parent and six 13C isotopic species has been measured.•Rotational constants and centrifugal distortion constants of 4FPA have been determined.•The r0 and rs structures have been compared with the ab initio structure.•Dipole moments were derived from Stark effect coefficients.

The 6–18 GHz rotational spectrum of 3-fluorophenylacetylene (3FPA) was measured by chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy. Rotational constants and quartic centrifugal distortion constants based on a Watson-A reduction were determined with 89 transitions; A = 3383.73821 (33) MHz, B = 1180.97617 (16) MHz, C = 875.26172 (12) MHz, ΔJ = 0.0382 (13) kHz, ΔK = 1.316 (21) kHz, δJ = 13.93 (56) Hz, and δK = 180.3 (60) Hz. An additional 12-13 transitions for each of eight 13C isotopic species and Stark effects to determine dipole moment components were observed by Balle-Flygare FTMW spectroscopy. Gas phase molecular structures of 3FPA were derived via the least-square fitting (r0) and substitution (rs) methods using the moments of inertia of the isotopic species. The ring geometry is discussed and compared with previous studies of structures of monosubstituted benzene and crystalline solid structures of 3FPA. •Microwave spectrum for the parent and eight 13C isotopic species were measured.•Rotational constants and centrifugal distortion constants of 3FPA were determined.•The r0 and rs structure were defined and compared to crystal structure.•Dipole moments were derived from Stark effect measurements.