Microwave spectrum and molecular structure of the argon-cis-1,2-dichloroethylene complex Mark D. Marshall, Helen O. Leung, Craig J. Nelson & Leonard H. Yoon Department of Chemistry Amherst College Supported by the National Science Foundation

Intermolecular interactions: Fluoroethylenes-Protic acids 2.441(4) Å 3.159 Å 122.6(4)o 36.4(2)o Cole & Legon, Chem. Phys. Lett. 369, 31 (2003). 2.123(1) Å 3.162 Å 123.7(1)o 18.3(1)o Kisiel, Fowler & Legon, J. Chem. Phys. 93, 3054 (1990). 1.89(1) Å 2.734 Å 121.4o 19(2)o Cole & Legon, Chem. Phys. Lett. 400, 419 (2004). The vinyl fluorideHX complexes are planar and adopt the same structural motif. There are two interactions: a H-bond donated by HX, and a secondary interaction between the nucleophilic portion of HX and an H atom cis to F. As the acid strength decreases, the H bond becomes longer and less linear. The CF---H angle for each complex indicates that the electron density of F that interacts with the acid is ~120o from the CF bond.

Are there differences for chloroethylenes? Cl Cl Cl Cl is more polarizable but less electronegative than F. In several studies of haloethyleneHX when both Cl and F are present, HX prefers to bind to F instead of Cl. To investigate the manner in which Cl participates in intermolecular interactions, we turn to vinyl chlorideHX complexes.

Very different indeed! 2.319(6) Å 2.59(1) Å 19.8(3)o 102.4(2)o vinyl chlorideHF 58.5(5)o 88.7(2)o 3.01(1) Å 2.939(4) Å vinyl chlorideHCCH 100.0(8)o 39(2)o 2.6810(2) Å vinyl chlorideHCCH Unlike vinyl fluorideHX, the manner of binding for vinyl chlorideHX depends on the identity of the acid partner. The "top" binding configuration of the HF complex is less strained than a "side" binding structure. The "side" binding configuration of the HCCH complex allows it to interact with the most electropositive H atom in vinyl chloride. The nonplanar HCl complex is likely a result of dispersion interactions that arise between Cl in HCl and the electron density in vinyl chloride.

What is effect of second chlorine? Complexes of cis-1,2-dichloroethylene will reveal effect of second chlorine Characterization of complex with argon is the first step. Monomer studied (12.4 – 246 GHz) by Leal, Alonso, and Lesarri, J. Mol. Spectrosc., 165, 368 – 376 (1994) We extend down to 5.6 GHz and found a pleasant surprise.

Experimental methods Chirped pulse Fourier transform microwave spectrometer: 5.6- 18.1 GHz Argon @ 10 psig flows through liquid cis-1,2-dichloroethylene and expands through 0.8 mm nozzle Spectra obtained as 3 GHz portions, 20 W power, 4 s chirp Ten 10-s FIDs per gas pulse 630,000 to 990,000 FIDs averaged 200 kHz FWHM Spectra analyzed using Kisiel’s AABS package in conjunction with Pickett’s SPFIT/SPCAT Photo courtesy of Jessica Mueller, Amherst College

110 – 101 transition is very sensitive to χab Off-diagonal quadrupole coupling constants enter in 2nd order Typically have small effect on spectra Χab not determined previously 110 – 101 transition at 9.4 GHz is qualitatively different when it is omitted The complete tensor can provide information regarding orientation of the dichloroethylene molecule in a complex Not necessary for Ar–cis-1,2-dichloroethylene, but often crucial in structure determination of complexes with protic acids

Monomer spectroscopic constants Not previously studied a corresponds to 35Cl b corresponds to 37Cl c corresponds to Cl bonded to 12C d corresponds to Cl bonded to 13C e fixed at the value appropriate to H35ClCC35ClH Requires some creative Pickett parameter coding

Argon complex : Theory Non-planar structure 3.89 Å 3.69 Å 72.62° 90° (fixed) Non-planar structure Analogous to Ar–cis-1,2-difluoroethylene MP2/6-311++G(2d,2p) results: A = 2085 MHz B = 1283 MHz C = 921 MHz μa = 0.07 D μc = 1.82 D

Representative spectra Transition: 312  202 5.6 – 18.1 GHz 30 MHz inset shows 312 –202 Experiment on top Prediction on bottom

Spectroscopic Constants (MHz) Ar-(Z)-35ClHCCH35Cl Ar-(Z)-37ClHCCH35Cl A 2098.85572(94) 2052.3944(16) B 1288.40068(85) 1281.0045(13) C 921.0485(27) 909.2897(47) ΔJ / 10-3 -3.644(88) 0.26(13) ΔJK / 10-3 -17.73(27) -28.83(42) ΔK / 10-3 20.26(19) 27.91(30) δJ / 10-3 -1.096(43) -3.050(78) δK / 10-3 -11.43(19) -4.28(42) χaa (35Cl) 45.210(20) 45.014(46) χbb (35Cl) 9.8187(30) 10.2740(98) χcc (35Cl) -34.7074(66) -35.553(19) χaa (37Cl) N/A 35.605(52) χbb (37Cl) 7.4131(96) χcc (37Cl) -26.695(25) No. of rotational transitions 20 11 No. of hyperfine components 133 36 J range 1-6 Ka range 0-4 rms (kHz) 8.705 6.498 ΔJ cannot be well-determined for either isotopologue due to correlation primarily with ΔJK . MP2/6-311++G(2d,2p) results: A = 2085 MHz B = 1283 MHz C = 921 MHz μa = 0.07 D μc = 1.82 D

Structure Fit Kisiel’s STRFIT Program Fix argon to plane bisecting C=C bond Fit distance from C=C center and out of plane angle to three moments of inertia each for two isotopologues RMS = 0.31 amu Å2 3.89 Å 3.96(2) Å 3.61 Å 3.47(1) Å 72.62° 84.05(54) ° 90° (fixed) Theory Experiment

Compare with Ar-cis-1,2-difluoroethylene 170 cm-1 in dichloroethylene 140 cm-1 in dichloroethylene 3.96 Å 3.47 Å 84.05 °

Compare with Ar-vinyl chloride 170 cm-1 in dichloroethylene 140 cm-1 in dichloroethylene 3.96 Å 3.47 Å 84.05 °

Summary Microwave spectrum of cis-1,2-dichloroethylene is extended to the CHCl13CHCl isotopologue The single non-zero off-diagonal element of the quadrupole coupling tensor is determined for CH35ClCH35Cl and CH37ClCH37Cl in addition to improving precision for the diagonal elements. Microwave spectra for two isotopologues of Ar-cis-1,2-dichloroethylene are obtained and analyzed. The structure of Ar-cis-1,2-dichloroethylene is non-planar and has similarities with Ar-cis-1,2-difluoroethylene and Ar-vinyl chloride Tunneling between two equivalent configurations with the argon on either side of the cis-1,2-dichloroethylene plane is not observed