The first ground-state rotational spectrum of 3-methylstyrene (3MS) was measured by Fourier transform microwave spectroscopy under supersonic jet-cooled conditions. Transitions were assigned for two conformers: cis-3MS and trans-3MS. In the cis conformer, the vinyl group is oriented toward the methyl group, while in the trans conformer, it is positioned away from the methyl. The energy difference between the two conformers was calculated to be only 2.1 cm−1, with the cis conformer having lower energy. Significant tunneling splitting, caused by the low-barrier internal rotation of the methyl group, was observed and analyzed using the XIAM and BELGI-Cs codes. The BELGI results show that the V3 barrier is 30.6688(87) cm−1 for the cis conformer and 11.0388(88) cm−1 for the trans conformer. The experimental rotational and torsional parameters are compared to their density functional theory counterparts. The planarity of the molecular geometry of cis- and trans-3MS is discussed, contributing to the long-standing topic of discussion about the planarity of styrene derivatives.

1.
N. C.
Craig
,
M. J.
Tubergen
,
R.
Gurusinghe
,
A. R.
Conrad
, and
P.
Groner
, “
Microwave spectra for the three 13C1 isotopologues of propene and new rotational constants for propene and its 13C1 isotopologues
,”
J. Mol. Spectrosc.
328
,
1
6
(
2016
).
2.
J.
Demaison
,
N. C.
Craig
,
R.
Gurusinghe
,
M. J.
Tubergen
,
H. D.
Rudolph
,
L. H.
Coudert
,
P. G.
Szalay
, and
A. G.
Császár
, “
Fourier transform microwave spectrum of propene-3-d1 (CH2=CHCH2D), quadrupole coupling constants of deuterium, and a semiexperimental equilibrium structure of propene
,”
J. Phys. Chem. A
121
,
3155
3166
(
2017
).
3.
R. M.
Gurusinghe
,
A.
Fox-Loe
, and
M. J.
Tubergen
, “
Structures of guaiacol and the guaiacol-argon van der Waals complex from rotational spectroscopy of guaiacol isotopologues
,”
J. Mol. Struct.
1246
,
131233
(
2021
).
4.
D.
Jelisavac
,
D. C.
Cortés Gómez
,
H. V. L.
Nguyen
,
L. W.
Sutikdja
,
W.
Stahl
, and
I.
Kleiner
, “
The microwave spectrum of the trans conformer of ethyl acetate
,”
J. Mol. Spectrosc.
257
,
111
115
(
2009
).
5.
M. D.
Mills
,
R. E.
Sonstrom
,
Z. P.
Vang
,
J. L.
Neill
,
H. N.
Scolati
,
C. T.
West
,
B. H.
Pate
, and
J. R.
Clark
, “
Enantioselective synthesis of enantioisotopomers with quantitative chiral analysis by chiral tag rotational spectroscopy
,”
Angew. Chem., Int. Ed.
61
,
e202207275
(
2022
).
6.
R. M.
Gurusinghe
,
N.
Dias
,
A. M.
Mebel
, and
A. G.
Suits
, “
Radical–radical reaction dynamics probed using millimeterwave spectroscopy: Propargyl + NH2/ND2
,”
J. Phys. Chem. Lett.
13
,
91
97
(
2022
).
7.
N.
Dias
,
R. M.
Gurusinghe
,
B. M.
Broderick
,
T. J.
Millar
, and
A. G.
Suits
, “
Direct D-atom incorporation in radicals: An overlooked pathway for deuterium fractionation
,”
Astrophys. J.
944
,
77
(
2023
).
8.
R. M.
Gurusinghe
,
N.
Dias
,
R.
Krueger
, and
A. G.
Suits
, “
Uniform supersonic flow sampling for detection by chirped-pulse rotational spectroscopy
,”
J. Chem. Phys.
156
,
014202
(
2022
).
9.
C. C.
Lin
and
J. D.
Swalen
, “
Internal rotation and microwave spectroscopy
,”
Rev. Mod. Phys.
31
,
841
892
(
1959
).
10.
H. V.
Nguyen
,
W.
Caminati
, and
J.
Grabow
, “
The LAM of the rings: Large amplitude motions in aromatic molecules studied by microwave spectroscopy
,”
Molecules
27
,
3948
(
2022
).
11.
R. M.
Gurusinghe
and
M. J.
Tubergen
, “
Probing the electronic environment of methylindoles using internal rotation and 14N nuclear quadrupole coupling
,”
J. Phys. Chem. A
120
,
3491
3496
(
2016
).
12.
D.
Marasinghe
,
R. M.
Gurusinghe
, and
M. J.
Tubergen
, “
Identification of two stable side-chain orientations of valine methyl ester by microwave spectroscopy
,”
J. Phys. Chem. A
128
,
3266
3272
(
2024
).
13.
G. D.
Darling
and
J. M. J.
Frechet
, “
Chemical modification of polystyrene resins. Approaches to the binding of reactive functionalities to polystyrene resins through a two-carbon spacer
,”
J. Org. Chem.
51
,
2270
2276
(
1986
).
14.
A.
Akelah
and
D. C.
Sherrington
, “
Application of functionalized polymers in organic synthesis
,”
Chem. Rev.
81
,
557
587
(
1981
).
15.
G. S.
Georgiev
,
I. G.
Dakova
, and
S. J.
Simpson
, “
Solvent effect on 3-methylstyrene radical copolymerization with methacrylic acid and methyl methacrylate
,”
J. Macromol. Sci., Part A
32
,
497
514
(
1995
).
16.
M.
Abbasian
,
M.
Jaymand
, and
S. E.
Shoja Bonab
, “
Synthesis and characterization of a terpolymer derived from styrene, methyl styrene, and polyaniline and its organoclay nanocomposite
,”
J. Appl. Polym. Sci.
125
,
E131
E140
(
2012
).
17.
J. M.
Hollas
and
P. F.
Taday
, “
Methyl and vinyl torsional potentials in cis- and trans-3-methylstyrene from supersonic jet fluorescence spectra
,”
J. Chem. Soc., Faraday Trans.
87
,
3585
3593
(
1991
).
18.
V. H.
Grassian
,
E. R.
Bernstein
,
H. V.
Secor
, and
J. I.
Seeman
, “
Conformational study of jet-cooled styrene derivatives: Demonstration of the planarity of nonsterically hindered styrenes
,”
J. Phys. Chem.
93
,
3470
3474
(
1989
).
19.
J. M.
Hollas
and
M. Z.
Bin Hussein
, “
The C(1)-C(α) torsional potential function of cis- and trans-3-fluorostyrene by supersonic jet spectroscopy
,”
Chem. Phys. Lett.
154
,
228
233
(
1989
).
20.
I.
Meurisse
,
P.
Ribeiro-Claro
,
J.
Teixeira-Dias
, and
C.
Pouchan
, “
Conformational equilibria for 3-methylstyrene: Raman and FTIR spectra and ab initio calculations
,”
J. Raman Spectrosc.
26
,
1033
1037
(
1995
).
21.
V. V.
Ilyushin
,
Z.
Kisiel
,
L.
Pszczólkowski
,
H.
Mäder
, and
J. T.
Hougen
, “
A new torsion–rotation fitting program for molecules with a sixfold barrier: Application to the microwave spectrum of toluene
,”
J. Mol. Spectrosc.
259
,
26
38
(
2010
).
22.
V. V.
Ilyushin
,
E. A.
Alekseev
,
Z.
Kisiel
, and
L.
Pszczółkowski
, “
High-J rotational spectrum of toluene in |m| ⩽ 3 torsional states
,”
J. Mol. Spectrosc.
339
,
31
39
(
2017
).
23.
T.
Bruhn
and
H.
Mäder
, “
The microwave spectrum of m-tolunitrile: Methyl internal rotation and 14N nuclear quadrupole coupling
,”
J. Mol. Spectrosc.
200
,
151
161
(
2000
).
24.
A.
Hellweg
,
C.
Hättig
,
I.
Merke
, and
W.
Stahl
, “
Microwave and theoretical investigation of the internal rotation in m-cresol
,”
J. Chem. Phys.
124
,
204305
(
2006
).
25.
A.
Roucou
,
I.
Kleiner
,
M.
Goubet
,
S.
Bteich
,
G.
Mouret
,
R.
Bocquet
,
F.
Hindle
,
W. L.
Meerts
, and
A.
Cuisset
, “
Towards the detection of explosive taggants: Microwave and millimetre-wave gas-phase spectroscopies of 3-nitrotoluene
,”
ChemPhysChem
19
,
1056
1067
(
2018
).
26.
A. J.
Shirar
,
D. S.
Wilcox
,
K. M.
Hotopp
,
G. L.
Storck
,
I.
Kleiner
, and
B. C.
Dian
, “
Impact of molecular conformation on barriers to internal methyl rotation: The rotational spectrum of m-methylbenzaldehyde
,”
J. Phys. Chem. A
114
,
12187
12194
(
2010
).
27.
D. A.
Obenchain
,
P.
Pinacho
,
S.
Zinn
, and
M.
Schnell
, “
The low-barrier methyl internal rotation in the rotational spectrum of 3-methylphenylacetylene
,”
J. Mol. Struct.
1213
,
128109
(
2020
).
28.
K. R.
Nair
,
S.
Herbers
,
H. V. L.
Nguyen
, and
J.
Grabow
, “
The structure and low-barrier methyl torsion of 3-fluorotoluene
,”
Spectrochim. Acta, Part A
242
,
118709
(
2020
).
29.
A. R.
Conrad
,
N. H.
Teumelsan
,
P. E.
Wang
, and
M. J.
Tubergen
, “
A spectroscopic and computational investigation of the conformational structural changes induced by hydrogen bonding networks in the glycidol–water complex
,”
J. Phys. Chem. A
114
,
336
342
(
2010
).
30.
M. J.
Frisch
,
G. W.
Trucks
,
H. B.
Schlegel
et al, Gaussian 16 Rev. C.01.
31.
A. D.
Becke
, “
Density-functional exchange-energy approximation with correct asymptotic behavior
,”
Phys. Rev. A
38
,
3098
3100
(
1988
).
32.
C.
Lee
,
W.
Yang
, and
R. G.
Parr
, “
Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density
,”
Phys. Rev. B
37
,
785
789
(
1988
).
33.
S.
Grimme
, “
Semiempirical GGA-type density functional constructed with a long-range dispersion correction
,”
J. Comput. Chem.
27
,
1787
1799
(
2006
).
34.
C. M.
Western
, “
PGOPHER: A program for simulating rotational, vibrational and electronic spectra
,”
J. Quant. Spectrosc. Radiat. Transfer
186
,
221
242
(
2017
).
35.
H.
Hartwig
and
H.
Dreizler
, “
The microwave spectrum of trans-2,3-dimethyloxirane in torsional excited states
,”
Z. Naturforsch. A
51
,
923
932
(
1996
).
36.
J. T.
Hougen
,
I.
Kleiner
, and
M.
Godefroid
, “
Selection rules and intensity calculations for a Cs asymmetric top molecule containing a methyl group internal rotor
,”
J. Mol. Spectrosc.
163
,
559
586
(
1994
).
37.
K.
Eibl
,
W.
Stahl
,
I.
Kleiner
, and
H. V. L.
Nguyen
, “
Conformational effect on the almost free internal rotation in 4-hexyn-3-ol studied by microwave spectroscopy and quantum chemistry
,”
J. Chem. Phys.
149
,
144306
(
2018
).
38.
S.
Khemissi
,
M.
Schwell
,
I.
Kleiner
, and
H. V. L.
Nguyen
, “
Approaching the free rotor limit: Extremely low methyl torsional barrier observed in the microwave spectrum of 2,4-dimethylfluorobenzene
,”
Phys. Chem. Chem. Phys.
26
,
402
411
(
2024
).
39.
I.
Kleiner
, “
Asymmetric-top molecules containing one methyl-like internal rotor: Methods and codes for fitting and predicting spectra
,”
J. Mol. Spectrosc.
260
,
1
18
(
2010
).
40.
W.
Caminati
,
B.
Vogelsanger
, and
A.
Bauder
, “
Rotational spectrum of styrene observed by microwave Fourier transform spectroscopy
,”
J. Mol. Spectrosc.
128
,
384
398
(
1988
).
41.
J. C.
Cochran
,
K.
Hagen
,
G.
Paulen
,
Q.
Shen
,
S.
Tom
,
M.
Traetteberg
, and
C.
Wells
, “
On the planarity of styrene and its derivatives: The molecular structures of styrene and (Z)-β-bromostyrene as determined by ab initio calculations and gas-phase electron diffraction
,”
J. Mol. Struct.
413–414
,
313
326
(
1997
).
42.
J. M.
Granadino-Roldán
,
M.
Fernández-Gómez
,
A.
Navarro
,
T.
Peña Ruiz
, and
U. A.
Jayasooriya
, “
An approach to the structure and vibrational analysis of cis- and trans-3-chlorostyrene through IR/Raman and INS spectroscopies and theoretical ab initio/DFT calculations
,”
Phys. Chem. Chem. Phys.
6
,
1133
1143
(
2004
).
43.
R. M.
Villamanan
,
J. C.
Lopez
, and
J. L.
Alonso
, “
On the planarity of 2-fluorostyrene
,”
J. Am. Chem. Soc.
111
,
6487
6491
(
1989
).
44.
T.
Schaefer
and
R.
Sebastian
, “
Molecular orbital computations of the internal rotational potentials in 2-, 3-, and 4-fluorostyrene. Comparison with experiment
,”
Chem. Phys. Lett.
163
,
212
216
(
1989
).
45.
R. K.
Bohn
,
J. A.
Montgomery
,
H. H.
Michels
, and
J. A.
Fournier
, “
Second moments and rotational spectroscopy
,”
J. Mol. Spectrosc.
325
,
42
49
(
2016
).
46.
M. J.
Carrillo
,
D.
Marasinghe
,
E. B.
Feeley
,
K. M.
Sobie
,
R. J.
Zarzycki
,
K.
Carter-Fenk
,
C. J.
Fenk
, and
M. J.
Tubergen
, “
Theoretical and microwave spectroscopic characterization of cyclobutenone: Planar or puckered?
,”
J. Phys. Chem. A
127
,
9082
9087
(
2023
).
47.
X.
Liu
,
R. K.
Bohn
,
S. A.
Sorenson
, and
N. S.
True
, “
Rotational spectra of benzyl cyanide assignment of the planar conformer and evidence of a low barrier to internal rotation
,”
J. Mol. Struct.
243
,
325
339
(
1991
).
48.
R.
Giudici
,
K.
Utzat
,
E.
Trosell
, and
R. K.
Bohn
, “
The rotational spectrum and heavy-atom-planar structure of propargyl benzene (3-phenyl-1-propyne)
,”
J. Mol. Struct.
786
,
65
67
(
2006
).
49.
W. M.
Ralowski
,
P. J.
Mjöberg
, and
S. O.
Ljunggren
, “
Microwave spectrum and planarity of p-fluorostyrene
,”
J. Mol. Struct.
30
,
1
11
(
1976
).
50.
W. M.
Ralowski
,
P. J.
Mjöberg
, and
S. O.
Ljunggren
, “
Microwave spectrum of p-chlorostyrene
,”
J. Mol. Struct.
31
,
169
176
(
1976
).
51.
W. E.
Sinclair
,
H.
Yu
,
D.
Phillips
,
R. D.
Gordon
,
J. M.
Hollas
,
S.
Klee
, and
G.
Mellau
, “
C(1)-C(α) torsion potential function and vibrational assignments of trans-β-methylstyrene from S1-S0 supersonic jet fluorescence spectra
,”
J. Phys. Chem.
99
,
4386
4396
(
1995
).
52.
R. M.
Gurusinghe
,
Methyl Internal Rotation Probed by Rotational Spectroscopy
(
Kent State University
,
2016
).
53.
V. P.
Barber
and
J. J.
Newby
, “
Jet-cooled fluorescence spectroscopy of a natural product: Anethole
,”
J. Phys. Chem. A
117
,
12831
12841
(
2013
).
54.
C.
Calabrese
,
Q.
Gou
,
A.
Maris
,
S.
Melandri
, and
W.
Caminati
, “
Conformational equilibrium and internal dynamics of E-anethole: A rotational study
,”
J. Phys. Chem. B
120
,
6587
6591
(
2016
).
55.
R.
Hoffmann
, “
Interaction of orbitals through space and through bonds
,”
Acc. Chem. Res.
4
,
1
9
(
1971
).
56.
R. M.
Gurusinghe
and
M. J.
Tubergen
, “
Microwave spectra of 2-phenylethyl methyl ether and 2-phenylethyl methyl ether-argon: Conformation-dependent tunneling and complexation
,”
J. Mol. Spectrosc.
346
,
13
18
(
2018
).
57.
L.
Tulimat
,
H.
Mouhib
,
H. V. L.
Nguyen
, and
W.
Stahl
, “
Laboratory rotational spectroscopy of methyl n-propyl sulfide: Conformational analysis and methyl internal rotation
,”
J. Mol. Spectrosc.
373
,
111356
(
2020
).
58.
J. P.
Lowe
, “
The barrier to internal rotation in ethane: A qualitative, intuitively useful explanation emerges from a comparison of different theoretical approaches
,”
Science
179
,
527
532
(
1973
).
59.
T.
Kundu
,
B.
Pradhan
, and
B. P.
Singh
, “
Origin of methyl torsional potential barrier—An overview
,”
J. Chem. Sci.
114
,
623
638
(
2002
).
60.
P. R.
Schreiner
, “
Teaching the right reasons: Lessons from the mistaken origin of the rotational barrier in ethane
,”
Angew. Chem., Int. Ed.
41
,
3579
3582
(
2002
).
61.
S.
Jacobsen
,
U.
Andresen
, and
H.
Mäder
, “
Microwave spectra of o-fluorotoluene and its 13C isotopic species: Methyl internal rotation and molecular structure
,”
Struct. Chem.
14
,
217
225
(
2003
).
62.
J.
Rottstegge
,
H.
Hartwig
, and
H.
Dreizler
, “
The rotational spectrum, structure and barrier V6 to internal rotation of p-fluorotoluene
,”
J. Mol. Struct.
478
,
37
47
(
1999
).
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