1. 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

2. 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.

3. 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.

4. 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.

5. 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.

6. Experimental methods
Chirped pulse Fourier transform microwave spectrometer: GHz
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

7. 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

8. 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

9. 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/ G(2d,2p) results:
A = 2085 MHz
B = 1283 MHz
C = 921 MHz
μa = 0.07 D
μc = 1.82 D

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

11. Spectroscopic Constants (MHz)
Ar-(Z)-35ClHCCH35Cl
Ar-(Z)-37ClHCCH35Cl A
(94)
(16) B
(85)
(13) C
(27)
(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)
(98)
χcc (35Cl)
(66)
(19)
χaa (37Cl)
N/A
35.605(52)
χbb (37Cl)
7.4131(96)
χcc (37Cl)
(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/ G(2d,2p) results:
A = 2085 MHz
B = 1283 MHz
C = 921 MHz
μa = 0.07 D
μc = 1.82 D

12. 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

13. 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 °

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

15. 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