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Rotational Spectrum and Structure of the 1,1Difluoroethylene···Carbon Dioxide Complex Ashley M. Anderton, Rebecca A. Peebles, and Sean A. Peebles* Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, Illinois 61920, United States S Supporting Information *
ABSTRACT: Rotational spectra for five isotopologues of the 1:1 weak complex between 1,1-difluoroethylene (H2CCF2) and carbon dioxide (CO2) have been measured using 480 MHz bandwidth chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy between 5.5 and 18.5 GHz. The observed structure of the complex is planar, with the CO2 aligned roughly parallel to the CC bond, and experimental structural parameters derived from rotational constants are consistent with the most stable geometry predicted by basis set superposition error and zero point energy corrected ab initio geometry optimizations at the MP2/6-311++G(2d,2p) level. Comparisons with the recently characterized vinyl fluoride···carbon dioxide complex reveal slightly longer intermolecular distances in the present complex, but very similar binding energies. with hydrogen and fluorine atoms of the fluorinated ethylene (as was observed in VF···CO2) because the CF2 end of DFE precludes the possibility of an analogous side-bonded form of DFE···CO2. Despite the nonpolar nature of CO2, its relatively large quadrupole moment can allow for sizable electrostatic interactions between it and other molecules. Successive fluorination of ethylene has significant effects on the charge distribution of the resulting fluoroethylene, as is emphasized by a change in sign of the out of plane molecular electric quadrupole moment in going from difluoroethylene (DFE) to trifluoroethylene (TFE).3 The consequences for the preferred geometry of resulting complexes are nicely illustrated by comparison of the structures of the DFE and TFE complexes with difluoromethane (CH2F2); in DFE···CH2F24 the difluoromethane is aligned such that its H atoms lie out of the symmetry plane, but in TFE···CH2F2 the F atoms of difluoromethane lie out of plane to maximize the favorable quadrupole−quadrupole interactions. Literature data on weakly bound complexes with 1,1difluoroethylene are relatively scarce. Complexes have been reported with the rare gases Ar5,6 and Ne6 and with hydrogen bond donors of the HX type (where X = F,7 Cl,8 and CCH9). Some dimers exhibiting F···H contacts, DFE···dimethyl ether10 and DFE···difluoromethane, were studied by Ogata,4 whereas the DFE···VF complex has been recently studied in our own lab.11
1. INTRODUCTION The carbon dioxide complex with ethylene was studied in 1995 by IR spectroscopy1 and it was determined that CO2 was located above the C2H4 plane and aligned parallel to the CC bond, as would be expected on the basis of a dominant quadrupole−quadrupole interaction between the two monomers. When one of the hydrogen atoms is replaced by a fluorine atom to yield the polar molecule vinyl fluoride (VF), zero-point energy (ZPE), and basis set superposition error (BSSE) corrected ab initio optimizations at the MP2/6-311+ +G(2d,2p) level revealed three almost isoenergetic structures,2 all lying within 7 cm−1 of each other, with the most stable predicted geometry placing CO2 above the plane of the molecule. However, our microwave spectroscopic results revealed two planar isomeric forms of the complex with CO2 were present in the supersonic expansion,2 with CO2 either interacting with the CHF end (“side-bonded”) or aligned along the HCCF edge of the VF (“top-bonded”), with the former predicted to be marginally more stable, by 6 cm−1 at ZPE/BSSE corrected level. The logical next step is to add a second fluorine atom to the ethylene subunit (giving two possible disubstituted alkenes: 1,1difluorethylene and 1,2-difluoroethylene, the latter of which can exist in either a polar cis or nonpolar trans form). The present paper reports on an ab initio and rotational spectroscopic determination of the first of these possibilities complexed with CO2, namely the 1,1-difluoroethylene···carbon dioxide (DFE··· CO2) dimer. It will be of interest to determine whether ab initio calculations once again predict a structure with the CO2 above the plane of the fluoroethylene to be most stable. In DFE··· CO2, only one possible orientation of CO2 with respect to the DFE will allow simultaneous interaction of the CO2 moiety © 2016 American Chemical Society
Received: November 19, 2015 Revised: December 16, 2015 Published: January 8, 2016 247
DOI: 10.1021/acs.jpca.5b11345 J. Phys. Chem. A 2016, 120, 247−253
Article
The Journal of Physical Chemistry A
whereas spectra for the other two 13C substitutions on the DFE subunit (13CH2CF2···CO2 and CH213CF2···CO2) were measured in natural abundance. Broadband microwave spectra were assigned and fit to a Watson A-reduced Hamiltonian in the Ir representation21 using the AABS package of Kisiel,22 which provides a graphical overlay for the SPFIT/SPCAT programs of Pickett.23 Stark effect experiments were carried out on the resonantcavity instrument allowing determination of dipole moment components for DFE···CO2. Voltages of up to ±4.5 kV (corresponding to an electric field of ca. 300 V cm−1) were applied to a pair of parallel steel mesh plates placed within the vacuum chamber and straddling the molecular expansion. Calibration of the applied electric field was carried out by measurement of the J = 1 ← 0 transition of OCS and assuming a dipole moment of μ = 0.71519(3) D.24
Aside from the goals of revealing subtle differences in binding geometry and strengths that occur upon systematic variation of degree of fluorination, characterization of small dimer clusters containing CO2 may help shed light on mechanisms responsible for enhanced solubility of fluorocarbon compounds in supercritical CO2.12,13 Such information can aid in improvement of designs for new materials for efficient capture of CO214 or by improving the effectiveness of supercritical CO2 as an industrial solvent.15
2. AB INITIO OPTIMIZATIONS Dimer geometries were optimized using Gaussian 0916 starting with CO2 placed at a number of different locations around the DFE. Full geometry optimizations were carried out at the MP2/6-311++G(2d,2p) level, using OPT = CALCALL and OPT = TIGHT keywords and identified six stationary points; rotational constants, dipole moment components and relative energies are listed in Supporting Information, Table S1. Zeropoint energy (ZPE) corrections to the energies were determined from harmonic vibrational frequency calculations. To examine the effects of basis set superposition error (BSSE) on the structures and relative energies, the four most stable geometries were reoptimized, employing the COUNTERPOISE keyword, which includes BSSE corrections at each step of the optimization process according to a Boys and Bernardi17 counterpoise correction scheme. The remaining two structures (V and VI) that were identified in the initial computational survey were considerably less stable (lying approximately 250 cm−1 higher in energy than the most stable configuration) and were therefore not considered further.
4. RESULTS 4.1. Ab Initio Calculations. Table 1 lists results of BSSE corrected geometry optimizations for the four most stable Table 1. MP2/6-311++G(2d,2p) Predicted Spectroscopic Parameters, Dipole Moment Components, and Relative Energiesa for the Four Lowest Energy Structures of DFE··· CO2b A/MHz B/MHz C/MHz μa/D μb/D μc/D ΔEMP2+ZPE/cm−1 ΔEBSSE+ZPE/cm−1
3. EXPERIMENTAL SECTION A sample mixture consisting of 1% CO2 (99.8%, SigmaAldrich) and 1% CH2CF2 (98%, Synquest Laboratories) seeded in a He/Ne carrier gas (17.5% He:82.5% Ne, BOC Gases) was delivered at a pressure of 2.5 atm to a General Valve Series 9 pulsed nozzle with a 0.8 mm orifice oriented perpendicularly to the microwave horns, and pulsing at 2 Hz. The rotational spectrum was initially scanned on a 480 MHz bandwidth chirped-pulse Fourier-transform microwave (FTMW) spectrometer18 over the 6−18.5 GHz region in overlapping 480 MHz segments (although very few dimer transitions were seen above 16 GHz because intensity drops off somewhat due to limitations of various instrumental components). A total of 5000 free induction decays (FID’s) were averaged at each frequency step, with 4 FID’s collected per gas pulse. A LabVIEW program was used to determine absolute transition frequencies and assemble the scan segments into a full broadband spectrum. Measurements of additional weaker parent isotopologue transitions, and of 13C and 18O substituted species (see below) were carried out on a resonant-cavity FTMW spectrometer of the Balle−Flygare design.19,20 Stronger transitions for the parent isotopic species were also remeasured on the Balle− Flygare instrument for consistency and to confirm the measurement precision. This spectrometer utilized a General Valve Series 9 nozzle identical to that in the chirped-pulse FTMW spectrometer, although running at a higher repetition rate of 10 Hz, and with only one FID collected per gas pulse. Isotopically enriched samples were used for measurement of the 13CO2 (13CO2: 99% 13C,
