1. Helen O. Leung, Mark D. Marshall & Joseph P. Messenger Department of Chemistry Amherst College Supported by the National Science Foundation

2. 2.59 (1) Å 2.319 (6) Å 19.8 (3)° 2.02 (4) Å 2.752 (4) Å 41.6 (51)° Leung, H. O.; Marshall, M. D., J. Chem. Phys. 126, 114310 (2007). Leung, H. O.; Marshall, M. D., J. Phys. Chem. A, 118, 9783 (2014). “Top” “Side”

3. 1.892 (14) Å 2.123 (1) Å 2.441 (4) Å 18.7 (15)° 18.3 (1)° 36.5 (2)° Cole, G. C.; Legon, A. C. Chem. Phys. Lett. 400, 2004. Kisiel, Z.; Fowler, P. W.; Legon, A. C. J. Chem. Phys. 93, 1990. Cole, G. C.; Legon, A. C. Chem. Phys. Lett. 369, 2003.

4. 2.59 (1) Å 2.319 (6) Å 19.8 (3)° Leung, H. O.; Marshall, M. D., J. Phys. Chem. A, 118, 9783 (2014). Leung, H.O.; Marshall, M. D.; Feng, F., J. Phys. Chem. A 117, 13419 (2013). 2.939 (4) Å 3.01 (1) Å 58.5 (5)°

5. Energy (cm -1 )035.0552.07 Gaussian 09 MP2/6-311++G(2d,2p)

6.  Chirped pulse Fourier transform microwave spectrometer: 5.6-18.1 GHz  Balle-Flygare Fourier transform microwave spectrometer: 6.0-19.2 GHz  Mixture was 1% vinyl chloride and 1% HCl in argon Photo courtesy of Aaron Bozzi, Amherst College Photo courtesy of Jessica Mueller, Amherst College

7. C 2 H 3 35 Cl-H 35 ClC 2 H 3 37 Cl-H 35 ClC 2 H 3 35 Cl-H 37 ClC 2 H 3 37 Cl-H 37 Cl A / MHz5703.7 (24)5599.27 (70)5715.19 (74)5597.8 (14) B / MHz1590.1 (12)1572.00 (15)1535.28 (13)1519.79 (17) C / MHz1267.8 (11)1253.240 (98)1235.16 (10)1219.63 (13) Highest J6444 Highest K a 2111 Number of transitions 2610128 Number of a- type transitions 21796 Number of b- type transitions 5332 RMS (MHz)3.1541.0541.1271.302

8. 6 06 -5 05 Most abundant CH 2 CH 37 Cl-H 35 ClCH 2 CH 35 Cl-H 37 Cl CH 2 CH 37 Cl-H 37 Cl

9. Most abundant CH 2 CH 37 Cl-H 35 Cl CH 2 CH 35 Cl-H 37 Cl CH 2 CH 37 Cl-H 37 Cl Hyperfine patterns match

11. CH 2 CH 35 Cl-H 37 Cl Observed Predicted

12. C 2 H 3 35 Cl-H 35 ClC 2 H 3 37 Cl-H 35 ClC 2 H 3 35 Cl-H 37 ClC 2 H 3 37 Cl-H 37 Cl A5713.9802 (43)5554.12 (57)5672.4 (25)5599.21(34) B1588.8062 (26)1572.0212 (30)1535.234 (14)1520.9331(26) C1268.4775 (21)1254.2436(42)1236.081 (15)1219.32091(95)  J /10  3 2.6420(96)7.137(43)6.92(20)1.86(10)**  J K /10  3 37.28(68)  aa (HCl) –41.367(17)–41.953(27)–32.6361(72)–33.1028(97)  bb (HCl) 14.369(14)14.966(18)11.3647(84)11.7992(93)  cc (HCl) 26.998(14)26.987(20)21.2715(84)21.3036(91) |  ab \ (HCl) 29.59(70)30.6(11)23.39(40)21.19(46)  aa (v.c.) 24.3635(55)19.6229(94)24.4065(34)19.6271(46)  bb (v.c.)  53.653(14) –42.864(18)–53.5171(91)–43.186(11)  cc (v.c.) 29.289(11)23.241(14)29.1105(75)23.5586(81) |  ab \ (v.c.) 25.73(65)19.08(84)*27.05(37)18.40(39) rms / kHz3.419.317.458.27 * |  bc \ = 36.0(25)**  J /10  3 =0.395(76)

13. Assume no efg perturbation upon complexation,  g = angle between HCl and the g inertial axis. aa 30.56 o bb 116.96 o cc 105.01 o

14. Assume no efg perturbation upon complexation, and principal axis along C–Cl bond.  g = angle between C–Cl bond and the g inertial axis. aa 73.84 o bb 18.74 o cc 80.76 o

15. 2.5989(39) Å 3.0942(26) Å 29.573(15)° 96.2289(69)°  P cc is over 4 amu Å 2 – complex is not planar  Use quadrupole coupling constants to determine angles  Fit Cl–Cl separation to I a, I b and I c for three isotopologues  RMS = 1.59 amu Å 2

16.  Nuclear quadrupole coupling hyperfine splitting analyzed for several transitions in three isotopologues in ground tunneling state of vinyl chloride–HCl  Deviations observed in some transitions  Often one half of an asymmetry doublet appears “normal,” while the other can’t be fit  Angular information from quadrupole coupling constants is helpful in structure determination

17. C 2 H 3 35 Cl-H 35 ClC 2 H 3 37 Cl-H 35 ClC 2 H 3 35 Cl-H 37 ClC 2 H 3 37 Cl-H 37 Cl 3 03 – 2 02 4 04 – 3 03 5 05 – 4 04 6 06 – 5 05 4 14 – 3 13 5 15 – 4 14 5 14 – 4 13 5 15 – 4 14 3 13 – 2 02