1. Department of Chemistry
The microwave spectra and molecular structures of 2-(trifluoromethyl)-oxirane and 2-vinyloxirane, two candidates for chiral analysis via noncovalent chiral tagging
Mark D. Marshall, Helen O. Leung, Desmond Acha, Kevin Wang, Olivia Febles, Alexandra Gomez
Department of Chemistry
Amherst College
Supported by the National Science Foundation

2. Noncovalent chiral tagging

3. Requirements for chiral tag
Easily available in enantiopure form
Small, chiral molecule
Easy to introduce into free-jet expansion
Functionalized for noncovalent interactions
Simple rotational spectrum
Minimal isotopic dilution (13C, 18O helpful)
No hyperfine
No internal rotation
Spectroscopically characterized
Structure determined
2-(trifluoromethyl)-oxirane (TFO)
2-vinyloxirane (VO)

4. Experimental methods
Chirped pulse Fourier transform microwave spectrometer
6.1 – 18.1 GHz (TFO)
GHz (VO)
1% TFO in argon or liquid VO entrained in argon is expanded through two pulsed valves with 0.8 mm nozzle
Spectra obtained as 4 GHz portions, 20 W power, 4 s chirp
Ten 10-s FIDs per gas pulse
618,000 – 900,000 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

5. TFO spectrum - overview
μa = 1.49 D
μb = 1.82 D
μc = 1.17 D
a-type transitions
(red)
c-type transitions
(purple & blue)
b-type transitions
(green & teal)

6. TFO spectrum – 500 MHz portion

7. TFO – spectroscopic constants
CH2CH(CF3)O
13CH2CH(CF3)O
CH213CH(CF3)O
CH2CH(13CF3)O
CH2CH(CF3)18O
A / MHz
(43)
(79)
(99)
(86)
(13)
B / MHz
(21)
(50)
(71)
(52)
(89)
C / MHz
(21)
(50)
(70)
(56)
(82)
J / 10-3 MHz
0.2531(22)
0.235(10)
0.263(14)
0.2370(92)
0.283(30)
JK / 10-3 MHz
1.5833(19)
1.549(19)
1.493(20)
1.554(16)
1.586(49)
K / 10-3 MHz
−0.9442(88)
−0.926(42)
−0.908(45)
−0.916(43)
−1.50(18)
J / 10-3 MHz
(11)
0.0221(26)
0.0239(20)
0.0249(16)
0.0225(16)
K / 10-3 MHz
−0.8504(57)
−0.79(10)
−0.57(12)
−0.559(86)
[−0.8504]
J range
0 – 16
0 – 10
Ka range
0 – 8
0 – 4
0 – 3
rms / kHz
8.43
8.44
10.93
9.89
6.38

8. TFO - structure 1.4309(4) Å 1.46(1) Å 59.7(8)o 1.44(1) Å 1.498(4) Å
Kisiel’s STRFIT used to fit A, B, C of 5 isotopologues to 6 structural parameters
2 bond lengths and one angle in ring
–CF3 carbon located relative to O and plane of ring
All other parameters fixed at ab initio values
rms = u Å2 and well-behaved correlations
EVAL used to calculate chemically relevant parameters

9. VO spectrum - overview
μa = 0.81 D
μb = 1.78 D
μc = 0.41 D

10. VO spectrum – 1200 MHz portion

11. VO – spectroscopic constants
C4H6O
13CH2CHOC2H3
CH213CHOC2H3
CH2CHO13CHCH2
CH2CHOCH13CH2
C2H318OC2H3
A / MHz
(62)
(24)
(68)
(18)
(24)
(54)
B / MHz
(16)
(49)
(16)
(41)
(53)
(90)
C / MHz
(23)
(65)
(18)
(48)
(52)
(72)
J / 10-3 MHz
0.4697(72)
[0.470]
JK / 10-3 MHz
10.098(81)
[10.10]
K / 10-3 MHz
10.08(34)
[10.08]
J / 10-3 MHz
0.0096(13)
[0.0096]
K / 10-3 MHz
-11.14(78)
[ ]
RMS / kHz
5.155
6.145
4.691
4.068
5.692
7.127
Highest J 13 7 6 4 10 9
Highest Ka 3 2 1

12. VO - structure 1.436(3) Å 1.439(7) Å 1.474(6) Å 61.7(4)o 122.9(3)o
1.477(4) Å
1.338(2) Å
Kisiel’s STRFIT used to fit A, B, C of 6 isotopologues to 9 structural parameters
2 bond lengths and one angle in ring
C=C bond length, C–C=C angle, and C–C=C–C dihedral [–150.3(8)°]
–C=CH2 carbon located relative to O and plane of ring
All other parameters fixed at ab initio values
rms = u Å2 and well-behaved correlations
EVAL used to calculate chemically relevant parameters

13. Ar-TFO – spectroscopic constants
Ar−CH2CH(CF3)O
Ar−13CH2CH(CF3)O
Ar−CH213CH(CF3)O
Ar−CH2CH(13CF3)O
A / MHz
(36)
(21)
(27)
(25)
B / MHz
(11)
(11)
(13)
(13)
C / MHz
(11)
(11)
(13)
(13)
J / 10-3 MHz
(30)
(24)
(29)
(28)
JK / 10-3 MHz
−2.0995(20)
−2.1322(24)
−2.1026(33)
−2.1150(31)
K / 10-3 MHz
31.812(15)
31.773(22)
31.885(30)
31.867(29)
J / 10-3 MHz
(12)
(10)
(15)
(13)
K / 10-3 MHz
0.896(28)
0.864(52)
0.860(58)
0.860(59)
J range
1 – 16
1 – 12
Ka range
0 – 6
0 – 3
rms/kHz
8.23
1.14
1.41
1.39

14. Ar-TFO - Structure
3.788(1) Å
3.7491(7) Å
3.454(2) Å
Observed structure is the anti conformer in contrast to the syn conformer seen in methyloxirane!
Blanco, Maris, Millemaggi, Caminati, J.Mol. Struct., 612, 309 (2002).
Kisiel’s STRFIT used to fit A, B, C of 4 isotopologues to 3 structural parameters locating the Ar atom
TFO fixed at monomer values
rms = u Å2 and well-behaved correlations
EVAL used to calculate chemically relevant parameters

15. Summary
The microwave spectra and molecular structures for two potential chiral tags are determined.
Both 2-(trifluoromethyl)-oxirane and 2-vinyloxirane have uncomplicated, easily assigned spectra.
Both are easily available in racemic and enantiopure forms.
2-(trifluoromethyl)-oxirane is easier to incorporate into free-jet expansion.
Alternate functionalizations may prove useful in forming non-covalant interactions with different partners.
Argon–2-(trifluoromethyl)-oxirane is observed as the anti conformer in contrast to the syn conformer found for propylene oxide (2-methyloxirane).