Article pubs.acs.org/JPCA
Characterization of Two Isomers of the Vinyl Fluoride···Carbon Dioxide Dimer by Rotational Spectroscopy Cori L. Christenholz, Rachel E. Dorris, 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 of two different structural forms of the 1:1 weak complex between vinyl fluoride (C2H3F) and carbon dioxide were measured using 480 MHz bandwidth chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy in the 5−17 GHz region. Both structures have the CO2 molecule situated in the plane of the vinyl fluoride, such that the CO2 is interacting either with a CHF side or with a HCCF edge of the vinyl fluoride subunit. Both observed structures are close to those predicted by ab initio geometry optimizations (corrected for basis set superposition error) at the MP2/ 6-311++G(2d,2p) level. Dipole moment measurements and structural fits, including determinations of principal axis coordinates for all three carbon atoms, confirm the geometries of the assigned species.
1. INTRODUCTION Experimental and theoretical investigations of weak interactions of carbon dioxide (CO2) with simple molecules can improve understanding of a number of important processes such as explaining the influences of a supercritical carbon dioxide solvent on reaction processes1,2 or aiding in design of new materials for efficient CO2 capture.3 For example, there has been disagreement over the particular mechanism responsible for the high solubility of perfluorinated compounds in supercritical CO2,4 and so studies of interactions of CO2 with small molecules may shed light on the identity of the forces behind these mechanisms. Despite the nonpolar nature of CO2, its relatively large quadrupole moment (arising from significant CO bond dipoles) can allow for sizable electrostatic interactions between it and polar compounds; the electrondeficient carbon atom of CO2 can function as a Lewis acid, while the oxygen atoms can serve as a Lewis base. Our previous studies of fluorinated methanes such as CH2F2 and CHF3 with CO25,6 naturally led us to examine complexes of CO2 with alkenes. The CO2 complex of the simplest alkene, ethylene, was studied in 1995 by IR spectroscopy,7 and despite indications of significant internal motion of the two subunits, it was deduced that the structure is stacked (with CO2 lying above the ethylene plane and parallel to the CC bond), in agreement with the expected arrangement of molecular quadrupole moments of the two monomers. Similar evidence of a stacked geometry and internal motion was observed in the IR spectrum for the ethylene···N2O complex8 and in the microwave spectrum of ethylene···OCS.9 In addition to these stacked interactions, ethylene’s π system has been observed to act as a hydrogen-bond acceptor in dimers with more acidic partners such as HCCH,10 HF,11 and HCl,12 with these © 2014 American Chemical Society
complexes determined to place the HX molecule perpendicular to the ethylene plane and bound via an XH···π interaction. Replacement of one or more hydrogen atoms in ethylene with fluorine will break the symmetry in ethylene, preventing tunneling between equivalent configurations (such as that observed in ethylene···CO2).7 Of course, it also makes the resulting fluoroethylene polar, hence increasing the number of possible configurations in which the fluorinated ethylene may interact, enabling the molecule to itself act as a weak proton donor via a CH bond, or as a weak acceptor through either its π system or F atom. Previous microwave work on the simplest of these fluoroethylenes, vinyl fluoride (VF, hereafter), has centered on complexes of VF with HX (X = F,13 Cl,14 Br,15 CCH16), with simple halogenated methanes (VF···CH2F217 and VF··· CH2ClF18) or with other fluoroethylenes (VF···1,1-difluoroethylene).19 In the HX series of complexes, HX is “top-bonded” (forming a planar dimer), sitting in the HCCF pocket and forming an XH···F bond, with a secondary interaction of X with the hydrogen atom of vinyl fluoride. Interestingly, the related vinyl chloride···HCCH20 is found to be “side-bonded” (with HCCH interacting with the CHCl end of vinyl chloride). One study of an interhalogen (VF···ClF)21 exists, and ClF is found to prefer a top-bonding position. VF···CH2F2 and VF···CH2ClF have both been determined to interact in a side-bonded configuration.17,18 This paper reports on an ab initio and rotational spectroscopic determination of two structural forms of the vinyl fluoride···carbon dioxide (VF···CO2) complex. The Received: August 4, 2014 Revised: August 21, 2014 Published: September 4, 2014 8765
dx.doi.org/10.1021/jp507869y | J. Phys. Chem. A 2014, 118, 8765−8772
The Journal of Physical Chemistry A
Article
Figure 1. Three most stable MP2/6-311++G(2d,2p) BSSE optimized structures for the CH2CHF···CO2 complex. Associated relative energies, rotational constants, and dipole moment components are listed in Table 1. Structures II (side-bonded) and III (top-bonded) are both planar, while the CO2 in Structure I lies above the VF such that the carbon atom of CO2 is located approximately 2.9 Å above the plane of VF. Relative energies given in the figure are BSSE and ZPE corrected. See text for details.
corrections to the energies were determined from harmonic frequency calculations.
present study is one of relatively few examples in the literature where more than one isomer of a weak complex has been characterized simultaneously by microwave spectroscopy in a supersonic expansion. These additional isomers are usually either identified directly from unassigned lines in the rotational spectrum after assignment of the main isomer, such as was seen for HCCH···OCS (a parallel22 and a T-shaped23 form were found), or identified at a later date, often by utilizing data from IR work (for example, OCS···CO2,24 and polar forms of the N2O25 and OCS dimers26). The present work therefore acts as a useful benchmark with which to explore relative energies and stabilities that are provided by ab initio calculations. This raises an interesting question of how close in energy must different isomers be to reasonably expect to see evidence of a second isomer in the supersonic expansion.
3. EXPERIMENTAL SECTION A sample of 1% CO2 (99.8%, Sigma-Aldrich) and 1% CH2 CHF (98%, Synquest Laboratories) 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, running at 2 Hz. The rotational spectrum was recorded on a 480 MHz bandwidth chirped-pulse Fouriertransform microwave (FTMW) spectrometer29 over the 7−19 GHz region in overlapping 480 MHz segments (although very few dimer transitions were seen above 16 GHz as a result of a drop-off in intensity due to limitations of various instrumental components). A total of 5000 free induction decays (FIDs) were averaged at each frequency step, with four FIDs collected per gas pulse, and a LabVIEW program was used to determine absolute transition frequencies and assemble the scan sections into a full spectrum. Measurements of additional weaker parent isotopologue transitions within (and beyond) the initial scan region, and of three additional 13C substituted species in natural abundance for the top-bonded form (III), were carried out on a resonantcavity FTMW spectrometer of the Balle-Flygare design.30,31 The transitions of the normal isotopologues were also remeasured on the Balle−Flygare instrument for consistency. This 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 per gas pulse. An enriched 13CO2 sample (13CO2: 99% 13C,
