1. Microwave Spectrum and Molecular Structure of the Argon-(E )-1-Chloro-1,2-Difluoroethylene Complex Mark D. Marshall, Helen O. Leung, Hannah Tandon, Joseph Messenger, and Eli Mlaver Department of Chemistry Amherst College Supported by the National Science Foundation

2. Fluoroethylene-Argon Complexes As noted by Kisiel, Fowler, and Legon, J. Chem. Phys. 95, 2283 (1991) Argon binds in either XCCF or FCF “cavity” Maximizing number of contacts with preferably heavy atoms 3.46 Å 3.55 Å FF Ar 3.47 Å FF F Ar 3.53 Å3.45 Å Fine balance between dispersion and hard sphere repulsion Nearly constant Ar–F distance Argon provides structureless probe of electron density away from ethylene plane

3. Extension to Fluorochloroethylenes Our results show that trends continue Ar-vinyl chloride Ar-(Z)-1-chloro-2-fluoroethylene Ar-2-chloro-1,1-difluoroethylene All non-planar Ar-vinyl chloride exhibits tunneling motion

4. Argon-(E )-1-chloro-1,2-difluoroethylene Relaxed potential scan  Gaussian 09  MP2/6-311++G(2d,2p)  Scan , optimize R,  θ R Φ x y z

5. Optimized Ab Initio Structure 3.476 Å 3.979 Å 3.665 Å 71.2˚ Top view Side view A = 1971 MHz B = 1229 MHz C = 894 MHz  a = 0.40 D  b = 0.81 D  c = 1.48 D

6. Chirped Pulse Experiment Studied in the 6.0 – 18.0 GHz region with a CP-FTMW spectrometer Ar- 35 ClFCCHF Ar- 37 ClFCCHF 1% ClFCCHF in Ar at 2 atm through 0.8 mm pulsed nozzle Spectra obtained as 1.5 GHz portions, 20 W power, 4  s chirp Ten 10-  s FIDs per gas pulse 624,000 to 732,000 FIDs averaged 200 kHz FWHM

7. Narrow Band Experiment Lines re-measured and additional ones found in the 5.0 – 22.0 GHz region with a Balle-Flygare cavity FTMW spectrometer Ar- 35 ClF 13 CCHF 1% ClFCCHF in Ar at 2 atm through 0.8 mm pulsed nozzle 0.5 MHz BW Nozzle is mounted parallel to the resonator axis Each spectral line is Doppler doubled 7 – 10 kHz FWHM

8. Typical Ar– 35 ClFCCHF Spectrum c - type 4 22 – 3 12 b - type 6 16 – 5 05 672,000 FIDs

9. Spectroscopic Constants Watson A-reduced I r representation AABS SPFIT 4 Sextic CD constants required For Ar-CHFCF 35 Cl: |  ab |= 1.72(27) MHz |  ac | = 7.3(34) MHz A = 1971 MHz B = 1229 MHz C = 894 MHz Ab initio predictions

10. Ar (ExtremeubstitutionCoordinates Ar (Extreme) Substitution Coordinates |a| / Å0.091(17) |b| / Å1.1916(12) |c| / Å3.35069(45) Structures above and below plane are equivalent Four possible argon locations All are consistent with rotation of quadrupole tensor In principal axis system of monomer:

11. Cl substitution strongly suggests one possibility 13 C isotopologues provide additional confirmation Use four possibilities to predict coordinates of Cl in principal axis system of dimer Substitution coordinates of 37 Cl strongly suggests #3 Also consistent with ab initio prediction

12. 13 C substitution confirms this choice Use four possibilities to predict coordinates of C1 in principal axis system of dimer Substitution coordinates of 13 C1 also indicates #3

13. Experimental Structure Fix haloethylene at monomer geometry Vary argon position Fit to 3 moments of inertia for 35 Cl and 37 Cl complexes using Kisiel’s STRFIT ( 13 C data too recent)  rms = 0.091 amu Å 2  Ar locates in FCCl cavity.  Difference between Ar–F and Ar–Cl distances exceeds difference in vdW radii.

14. Comparison with Fluoroethylene Complexes 3.58 Å 3.56 Å Kisiel, Fowler, and Legon, J. Chem. Phys. 95, 2283 (1991)

15. Comparison with Chloroethylene Complexes Distance from argon to center of C=C bond decreases with increasing halogen substitution C1–Ar distance may be more consistent than Ar–F distance Argon prefers to locate closer to fluorine atoms, that side of plane tilts toward Ar 3.52 Å3.59 Å3.58 Å

16. Summary The rotational spectra of three isotopologues of Ar–ClFCCHF have been observed and analyzed. Ar–ClFCCHF has a non-planar structure, with binding most likely in the ClCF cavity to maximize contacts with heavy atoms as suggested by Kisiel, Fowler and Legon. Ar-F distance is shorter than the consistent 3.5 Å seen in previous species, further increasing the Ar-Cl distance over the difference in van der Waals radii. Ar-C1 distance, however, is a consistent 3.5 – 3.6 Å

17. Modifications to Amherst CP-FTMW Instrument Bandwidth limited to 5 GHz 25 W power amplifier Tektronix DPO72004C scope  100 Gs s –1, 20 GHz analog BW  Direct detection of FID  250 MB fast memory  Only transfer summary frame Rb 10 MHz standard 10 MHz crystal oscillator