Structures and Properties of trans-1,3,3,3-Tetrafluoro- propene (HFO-1234ze) and 2,3,3,3-Tetrafluoropropene (HFO-1234yf) Refrigerants

Article abstract of DOI:10.1002/open.202000172

The refrigerant trans-1,3,3,3-tetrafluoropropene (HFO-1234ze) is used as a replacement for former cooling agents that have been phased-out due to their global warming potential or ozone depleting potential. Although it is used on a large scale, only a few vibrational data and no structural data of HFO-1234ze are known. We report structure determinations based on low-temperature single-crystal X-ray diffraction data as well as gas-phase diffraction data of HFO-1234ze and HFO-1234yf (2,3,3,3-tetrafluoropropene). Furthermore, vibrational spectra of HFO-1234ze in all phases are described. The results are discussed together with quantum-chemical calculations on the PBE0/cc-pVTZ level of theory. Combustion experiments of HFO-1234ze show carbonyl difluoride, carbon dioxide and hydrogen fluoride to be the main combustion products.

Full text of DOI:10.1002/open.202000172

Full Papers
doi.org/10.1002/open.202000172
ChemistryOpen
1
2
3
4
5
6
7
8
9
Structures and Properties of trans-1,3,3,3-Tetrafluoro-
propene (HFO-1234ze) and 2,3,3,3-Tetrafluoropropene
(HFO-1234yf) Refrigerants
Jan Schwabedissen,[a] Timo Glodde,[a] Yury V. Vishnevskiy,[a] Hans-Georg Stammler,[a]
Lukas Flierl,[b] Andreas J. Kornath,*[b] and Norbert W. Mitzel*[a]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
The refrigerant trans-1,3,3,3-tetrafluoropropene (HFO-1234ze) is
used as a replacement for former cooling agents that have
been phased-out due to their global warming potential or
ozone depleting potential. Although it is used on a large scale,
only a few vibrational data and no structural data of HFO-
1234ze are known. We report structure determinations based
on low-temperature single-crystal X-ray diffraction data as well
as gas-phase diffraction data of HFO-1234ze and HFO-1234yf
(2,3,3,3-tetrafluoropropene). Furthermore, vibrational spectra of
HFO-1234ze in all phases are described. The results are
discussed together with quantum-chemical calculations on the
PBE0/cc-pVTZ level of theory. Combustion experiments of HFO-
1234ze show carbonyl difluoride, carbon dioxide and hydrogen
fluoride to be the main combustion products.
1. Introduction
Hydrofluoroolefines (HFO) are discussed as environmentally
more friendly replacements[1] of common refrigerants like
hydrofluorocarbons (HFC) as they have zero ODP (ozone
depletion potential) and low GWP (global warming potential).[2]
Among the tetrafluoropropenes shown in Scheme 1 the isomers
HFO-1234yf and HFO-1234ze(E) are used as refrigerants.
Many previous investigations aim to reveal information
about thermodynamic properties[3] or structural behavior in the
liquid phase[4] or even as building blocks in polymers.[5] Hitherto
only HFO-1234yf (2,3,3,3-tetrafluoropropene) was structurally
characterized in the gas phase by microwave spectroscopy[6]
and in the solid state by single crystal X-ray diffraction,[7] but no
elucidation of the structural behavior of the isomer HFO-1234ze
(E) (trans-1,3,3,3-tetrafluoropropene) has been reported yet.
Herein we report about the molecular structures of trans-
1,3,3,3-tetrafluoropropene (trans-1333TFP, 1) and 2,3,3,3-
tetrafluoropropene (2333TFP, 2) in the vapor phase by gas-
Scheme 1. Isomers of tetrafluoropropene (commercial abbreviations).
phase electron diffraction (GED) and in the solid state by high
angle X-ray diffraction along with the electron density distribu-
tions of both substances. Furthermore, the vibrational spectra
and combustion properties for 1 are described.
2. Results and Discussion
[a] Dr. J. Schwabedissen, T. Glodde, Dr. Y. V. Vishnevskiy, Dr. H.-G. Stammler,
Prof. Dr. N. W. Mitzel
Lehrstuhl für Anorganische Chemie und Strukturchemie
Fakultät für Chemie Universität Bielefeld
Universitätsstraße 25
33615 Bielefeld (Germany)
E-mail: mitzel@uni-bielefeld.de
2.1. Calculations
For the determination of possible conformers of 1,3,3,3-tetra-
fluoropropene (1) and 2,3,3,3-tetrafluoropropene (2), the poten-
tial energy curves at the PBE0[8]/cc-pVTZ[9] level of theory were
cal-culated for the rotation of the trifluoromethyl group about
the C–C bond (Figure 1). As for 1 trans and cis isomers exist,
both were studied regarding the barrier for the rotation of the
CF3 group.
[b] L. Flierl, Prof. Dr. A. J. Kornath
Department of Chemistry
Ludwig-Maximilian University
Butenandtstraße 5–13 (Haus D)
81377 Munich (Germany)
E-mail: Andreas.kornath@cup.uni-muenchen.de
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.202000172
In Figure 1 all three graphs show a minimum at a dihedral
°
°
© 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any med-
ium, provided the original work is properly cited, the use is non-commercial
and no modifications or adaptations are made.
angle φ(FCCX) of 60 and 180 due to local C3v symmetry of the
CF3 group. In all three cases, this is a conformation of Cs
symmetry with the trifluoromethyl group being staggered
regarding the substituent at the adjacent carbon atom, fluorine
ChemistryOpen 2020, 9, 921–928
921
© 2020 The Authors. Published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/open.202000172
ChemistryOpen
significantly longer when a fluorine atom is attached to C2,
1
2
3
4
5
which confirms the higher steric demand of fluorine compared
to hydrogen. The largest angle at the carbon scaffold is found
for the staggered conformer of the cis-isomer of 1, which might
be due to the steric interaction of the trifluoromethyl group
with the fluorine atom at carbon C1.
6
7
8
9
2.2. Crystal Structures
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
For the determination of the molecular structures of trans-1 and
2 in the solid state, a small amount of each substance was
condensed into a capillary, which was flame sealed. Suitable
crystals[10] were grown from the liquid by the in situ crystalliza-
tion technique by manually generating a crystal seed at a
temperature just below the melting point and subsequent
cooling. For details of the procedure as well as crystallographic
details, see the Supporting Information (Table S2). For both
crystals high-angle scattering data were recorded to determine
their charge density topologies.
Figure 1. Scan of the potential energy for the rotation of the trifluoromethyl
group around the CÀ C single bond for the three different isomers of
tetrafluoropropene. X=H for both 1,3,3,3-tetrafluoropropenes and X=F for 2.
Molecular structures in the solid state are depicted in
Figure 3 and selected structural parameters of 1 and 2 are listed
in Table 1. In the solid state, 1 resides on a crystallographic
mirror plane, accordingly with
a
dihedral angle
°
φ(H2À C2À C3À F2) of 180 . However, 2 shows only approximately
molecular Cs symmetry with the corresponding dihedral angle
°
φ(F1À C2À C3À F4) measuring 178.7(1) . In line with the computa-
tionally determined structures, the distance C1À C2 is affected
only slightly by the position of the fluorine atom (1.318(1) Å
resp. 1.313(1) Å). Anyhow, the bond C2À C3 to the CF3 group is
almost 0.02 Å longer in 2 than the corresponding bond in 1
(1.496(1) resp. 1.479(1) Å). In addition to the fact that the
fluorine atom is sterically more demanding than the hydrogen
atom, its high electronegativity induces a higher positive charge
at the carbon atom. The charge can be obtained from the
experimentally determined electron density distribution. Hence,
in 1 the charge of C2 amounts to +0.07 e and for C3 +1.93 e
whereas in 2 the charges are even larger at +0.60 e and
+2.05 e for C2 and C3, respectively. Both properties lead to a
longer C2À C3 bond in 2 compared to that in 1. Concerning the
trifluoromethyl group, the CÀ F bond eclipsing the CÀ C double
bond, is in both structures shorter by 0.01 Å than that to the
two out-of-plane fluorine atoms. The same structural feature is
observed in the solid state structure of hexafluoropropene.[11]
Figure 2. Optimized minimum structures of the conformers of three isomers
of tetrafluoropropene.
°
for 2 and hydrogen for 1. The barriers of rotation found at 120
are 10 kJmolÀ 1 for 2 and 8 kJmolÀ 1 for trans-1. At this point,
about 2 kJmolÀ 1 more energy is required for the rotation of the
trifluoromethyl group in 2, because the fluorine atom is
sterically more demanding than the hydrogen atom. However,
the energy profile of cis-1 differs from the two mentioned
above in a manner that a relatively low barrier for the rotation
of about 1 kJmolÀ 1 can be found. The reason for this low barrier
is steric repulsion occurring in the minimum structure between
the CF3 group and the fluorine atom at C1, which can be
considered as a manifestation of 1,3-interactions. The local
maximum is found, as for all three isomers of tetrafluoropro-
pene, for an eclipsed conformation of the CF3 group regarding
the substituent at C2. In Figure 2 all three minimum structures
are depicted.
°
The ∡(H1À C1À X) angle in the case of X=F for 1 (114.0(4) ) is
°
°
about 9 smaller than in 2 (123.3(5) ) where X=H. Furthermore,
The double bond between C1 and C2 as well as the bond to
the fluorine atom from a planar coordinated carbon atom are
not much affected by the variation of the position of the
fluorine atom. In contrast, the carbonÀ carbon single bond is
Figure 3. Molecular structures in the solid state of trans-1 (left) and 2 (right)
1
(50% probability displacement ellipsoids). Symmetry code: (À = +x, 1= À y,
2
2
1= À z).
2
ChemistryOpen 2020, 9, 921–928
www.chemistryopen.org
922
© 2020 The Authors. Published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/open.202000172
ChemistryOpen
Table 1. Structural parameters in the solid state (XRD) and in the gas
phase (GED, re) of 1,3,3,3-tetrafluoropropene and 2,3,3,3-tetrafluoropro-
pene. Distances r are given in Å and angles ∡ in degrees. Errors in XRD are
given as three times standard deviations (3σ) and total errors (from the
Monte-Carlo method) are reported for the gas-phase structures.
1
2
3
4
5
Parameter[a]
1 (XRD)
rα
1 (GED)
re
2 (XRD)
rα
2 (GEDcomb)
re
6
7
8
9
r(C1À C2)
1.3181(3)
1.4789(3)
1.054(17)
1.3407(5)
1.084(14)
1.3350(5)
1.3468(5)
1.3468(5)
121.53(2)
114.0(4)
121.8(4)
116.7(4)
106.98(3)
111.81(2)
111.81(2)
105.63(5)
1.321(4)
1.481(6)
1.101(9)
1.326(4)
1.101(9)
1.336(4)
1.337(4)
1.337(4)
120.8(8)
116.0(15)
121.6[b]
117.6(8)
113.0(5)
111.7(5)
111.7(5)
106.3(5)
1.3134(4)
1.4963(4)
1.071(4)
1.323(8)
1.498(4)
1.083(13)
1.085(13)
1.333(3)
1.331(3)
1.334(3)
1.334(3)
125.8(2)
120.4[b]
122.8(4)
111.4(4)
111.3(4)
111.0(4)
111.0(4)
107.3(7)
r(C2À C3)
r(C1À H1)
r(C1À X)
1.075(3)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
r(C2À Y)
1.3373(5)
1.3302(6)
1.3412(6)
1.3422(6)
126.69(3)
123.3(5)
122.59(4)
110.73(4)
111.66(3)
111.63(4)
111.40(4)
107.19(5)
r(C3À F2/4)c
r(C3À F3/2)c
r(C3À F3'/3)c
∡(C1-C2-C3)
∡(H1-C1-X)
∡(C1-C2-Y)
∡(C3-C2-Y)
∡(F2-C3-C2)d
∡(F3-C3-C2)d
∡(F4-C3-C2)d
∡(F3-C3-F4)d
Figure 4. Aggregation in the solid-state structure of trans-1,3,3,3-tetrafluoro-
propene.
tion, Figure S11). The distance between the planes is half of the
crystallographic b axis (3.28 Å).
The crystal structure of 2 contains dimers formed by H···F
contacts of 2.60(1) Å length. These dimers are linked to each
other by H···F contacts of 2.59(1) Å length (Figure 5). The central
H1-F4-H1'-F4' planes of the different dimers are tilted by
[a] X=F for 1 and X=H for 2, Y=H for 1 and Y=F for 2, [b] Parameter
fixed at the computational value. [c] Numbering for 2. [d] Analogous
parameters for 2 are given.
°
24.6(3) . In the analogous ethene, 1,1-difluoroethene, columns
are formed with the positively charged hydrogen atoms
pointing at the negatively charged fluorine atoms, but no short
H···F contacts are reported.
°
in hexafluoropropene the angle FÀ C1À F measures 111.9(3) .
This is in good agreement with the explanation that fluorine
atoms generally prefer orbitals of their binding partners with
small s and thus higher p character, when forming covalent
polar bonds.[12] In comparison, the double bond in the
perfluorinated propene is slightly shorter at 1.307(5) Å due to
the higher degree of fluorination, whereas the length of the
carbonÀ carbon single bond in hexafluoropropene (1.486(5) Å)
falls between the corresponding ones in 1 and 2. A systematic
study on solid-state structures of fluorinated ethenes has been
performed to investigate the structural effects on the double
bond, but revealed no systematic behavior of increase or
decrease of the bond length of the double bond upon
fluorination of the C=C backbone.[13]
Aggregation of single molecules in the solid state occurs
mainly through F···H contacts like it was previously examined
for partially fluorinated olefins 1,1,4,4-tetrafluorobutadiene,[14]
1,1,2-trifluorobuta-1,3-diene[15] as well as for a series of fluori-
nated ethenes.[13] In the crystal structure of 1 the shortest H···F
contacts can be found within the crystallographic mirror plane
with 2.44(1) Å and 2.63(1) Å length (Figure 4). Both contacts are
less than the sum of the corresponding van der Waals radii
(2.66 Å).[16] Because the ethylene units are facing each other, a
planar double strand polymer is build up. These strands are
linked with H···F contacts of 2.72(1) Å length within the mirror
plane from the H2 atom to the symmetric equivalent fluorine
atoms. These planes are linked with H···F contacts of 2.83(1) Å.
In the related ethene (E)-1,2-difluoroethene molecules are
connected by H···F contacts of 2.59 Å length.
The extended solid-state structure shows a herringbone like
arrangement of the molecules along the c-axis (Supporting
Information, Figure S12). A similar pattern was also observed for
1,1,4,4-tetrafluorobutadiene.[14]
2.3. Charge Density and Topological Analysis
On the basis of high-resolution X-ray diffraction at low tempera-
ture, the charge density 1(r) was determined[17] for both crystal
structures mentioned above. Along with the experimental
deduced densities the theoretical ones, from MP2(full)/aug-cc-
pVTZ calculation and evaluated with the AIMALL program,[18]
Similar to the extended solid-state structure of (E)-1,2-
difluoroethene, the extended structure of 1 in the crystalline
phase shows parallel layers of molecules (Supporting Informa-
Figure 5. Aggregation of 2,3,3,3-tetrafluoropropene in the crystal phase.
ChemistryOpen 2020, 9, 921–928
www.chemistryopen.org
923
© 2020 The Authors. Published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/open.202000172
ChemistryOpen
Table 2. Values taken from the experimental electron density for both molecules in the crystal phase in comparison with computed values at the MP2(full)/
1
2
2
aug-cc-pVTZ level (experiment/theory). Electron densities at bond critical points 1(rbcp) are given in e ÅÀ 3 the Laplacian À r 1(rbcp) in e ÅÀ 5 and the ellipticity ɛ
is dimensionless, d1 is defined as the distance of the first atom to the bond critical point along the bond path.
3
4
2
Bonda
Bond path length
d1
1
1(rbcp
1
)
À r 1(rbcp
)
ɛ
1
1
2
2
2
1
2
2
5
6
7
8
C1À C2
C2À C3
C1À H1
C1À X
1.3181(3)/
1.3211
1.4789(3)/
1.4766
1.054(17)/
1.058
1.3407(5)/
1.3311
1.084(14)/
1.057
1.3350(5)/
1.3398
1.3468(5)/
1.3134(4)/
1.3180
1.4963(4)/
1.4946
1.071(4)/
1.057
1.075(3)/
1.055
1.3373(5)/
1.3351
1.3302(6)/
1.3328
1.3422(6)/
0.700/
0.728
0.638/
0.687
0.722/
0.716
0.469/
0.432
0.713/
0.705
0.433/
0.441
0.433/
0.443
0.638/
0.555
0.697/
0.719
0.760/
0.702
0.736/
0.702
0.507/
0.435
0.424/
0.439
0.433/
0.441
0.430/
0.441
2.41(1)/
2.48
1.95(1)/
1.95
1.88(1)/
2.07
1.93(1)/
1.83
1.79(1)/
1.98
2.03(1)/
1.91
1.99(1)/
2.47(1)/
2.47
1.95(1)/
1.96
1.76(1)/
2.00
1.71(1)/
1.99
2.03(1)/
1.82
1.98(1)/
1.94
1.93(1)/
À 27.15(2)/
À 31.88
À 28.25(4)/
À 31.23
0.33/
0.43
0.04/
0.05
0.11/
0.04
0.03/
0.10
0.14/
0.01
0.05/
0.12
0.13/
0.12
0.23/
0.39
0.04/
0.09
0.05/
0.01
0.04/
0.01
0.15/
0.20
0.25/
0.12
0.13/
0.12
0.07/
0.12
À 20.70(2)/
À 21.78
À 21.23(3)/
À 21.99
À 22.19(8)/
À 31.53
À 19.68(2)/
À 29.13
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
À 19.08(4)/
5.62
À 17.14(2)/
À 28.89
C2À Y
À 18.54(5)/
À 23.52(8)/
À 21.78
4.69
C3À F2/
C3À F4b
C3À F3/
C3À F2b
C3À F3b
À 21.27(7)/
5.46
À 7.44(10)/
À 5.01
À 18.90(6)/
À 12.07(10)/
À 5.78
1.3421
1.3363
1.3421(6)/
1.3363
1.91
1.93
1.89(1)/
1.93
À 6.18
À 9.28(10)/
À 5.78
[a] X=F for 1 and X=H for 2, Y=H for 1 and Y=F for 2. [b] Numbering for 2,3,3,3-tetrafluoropropene 2.
were topologically characterized based on Bader's quantum
theory of atoms in molecules (QTAIM).[19] Table 2 contains the
characteristic values for the electron density topology at the
bond critical points [bcp, at the location rbcp where r1(rbcp)=0].
The experimental densities for both species are well reproduced
by the ab initio method, whereas larger discrepancies in the
2
Laplacian of the electron density À r 1(rbcp) especially for the
CÀ F bonds are observed. Particularly, the Laplacians in the
bonds to the vinylic fluorine atoms feature charge depletions in
the theoretical electron density (5.62 and 4.69 e ÅÀ 5 for 1 and 2,
respectively) whereas the experimentally determined Laplacians
have large negative values of À 19.08(4) and À 23.52(8) e ÅÀ 5,
respectively; this indicates a strongly covalent character of the
bonds. The frequently reported failure of theory to describe the
electron density distribution in CÀ F bonds properly has also
been observed in preceding studies on fluorinated olefins and
aromatics.[13,14,20] Regarding the experimental findings for the
bonds to the vinylic fluorine atoms, the charge density is higher
Figure 6. Isodensity surfaces (1(r)=2.0 e ÅÀ 3) of the electron densities of
trans-1 (upper) and 2 (lower).
in magnitude for the fluorine atom in 2 position 1(rbcp)=2.03
versus 1.93(1) e ÅÀ 3) as well as the Laplacian À r 1(rbcp
)
2
(À 23.52(8) versus À 19.08(4) e ÅÀ 5). Along with this, the bond
length is slightly shorter in the 2 isomer (1.3373(5) versus
1.3407(5) Å).
2.4. Gas-Phase Structures
These findings might indicate a conjugative effect with the
double bond also manifest in the depicted charge density
(Figure 6) where electron densities higher than 2.0 e ÅÀ 3 are
also found along the corresponding bonds. Substitution of a
fluorine atom by a CF3 group takes no significant effect on the
electron densities in the double bonds. While for 1,1-difluor-
oethene 1(rbcp)=2.55(5) / 2.56(5) e ÅÀ 3, the charge density at
the critical point of the double bond in 2 is 2.47(1) e ÅÀ 3, which
is the same trend for (E)-difluoroethene (2.47(3) e ÅÀ 3) and 1
(2.41(1) e ÅÀ 3).
Structures of free molecules of trans-1 and 2 were determined
by gas-phase electron diffraction (GED). For details of the
experiment as well as the structural analysis, see Supporting
Information. The radial distribution curves are depicted in
Figure 7 and Figure 8 for trans-1 and 2, respectively. Structural
parameters are listed in Table 1. Both refined model structures
show good agreement with the experimental data with overall
Rf =3.3% (1) and 2.3% (2). For the determination of total overall
errors for the structural parameters, the earlier described
Monte-Carlo method[21] was applied. Microwave spectroscopy
data were available for 2,[6] and consequently its structure was
refined based on the combination of electron-diffraction data
ChemistryOpen 2020, 9, 921–928
www.chemistryopen.org
924
© 2020 The Authors. Published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/open.202000172
ChemistryOpen
Table 3. Reported experimental (MW)[6] rotational constants, calculated
corrections Be–B0, Be calculated from the commonly refined (GEDcomb
structure and difference between refined and reported value in Be for
1
2
)
3
different isotopologues of 2.
4
5
6
7
8
9
B0 from MW
Be(calcd)
À B0
Be from
GEDcomb
Δ(Be) [MW À
GEDcomb]
CF3CFCH2
A0
3714.71903(50) 18.886
2465.51465(39) 17.878
2001.06445(38) 11.884
3733.571
2483.452
2012.860
0.034
B0
C0
À 0.056
0.118
13CF3CFCH2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
A0
B0
3671.50110(17) 18.549
2431.99032(97) 17.724
1967.24810(93) 11.754
2689.667
2450.990
1978.921
0.383
À 0.276
C0
0.080
CF313CFCH2
A0
B0
3714.13390(15) 18.773
2454.73533(91) 17.620
1993.82750(97) 11.697
3723.895
2472.440
2005.424
0.012
À 0.084
C0
0.100
CF3CF13CH2
A0
B0
C0
3714.98250(12) 18.603
2462.24116(65) 17.631
1998.93819(69) 11.772
3733.559
2479.954
2010.558
0.027
À 0.082
0.102
Figure 7. Experimental (O) and model (—) radial distribution curves of 1. The
difference curve is below. Vertical bars indicate interatomic distances;
selected ones are labelled.
CF3 group vanish in the gas phase within the error ranges,
because it cannot be resolved satisfactorily.
The carbonÀ carbon double bond in 3,3,3-trifluoropropene[22]
(rg =1.318(8) Å) is shorter than that in propene (rg =1.342(2) Å),
likely due to induction of a negative charge at the carbon atom
bearing the CF3 group. Changing the CF3 to a fluorine
substituent does not have the same shortening effect on the
carbonÀ carbon double bond in fluoroethene (rg(C=C)=
1.333(1).[23] For the here examined species the length of the
C=C bond is between those of propene and trifluoropropene, at
rg(C=C)=1.328(4) (1) and 1.330(8) Å (2). Thus, the fluorine
substituents in 1 and 2 are decreasing the effect of the
trifluoromethyl group regardless the position of substitution.
This tendency for a shorter C=C bond upon exchange of CF3 by
fluorine was also observed for the pair trans-1,2-difluoroethene
(1.329(4) Å)[23] and trans-1,1,1,4,4,4-but-2-ene (1.296(20) Å).[24]
However, for 1,1-bis(trifluoromethyl)ethene (1.373(13) Å)[25] and
1,1-difluoroethene (1.311(35),[26] 1.316(2),[23] 1.340(6) Å[27]) the
trend is reversed. Furthermore, the gas-phase structure of
hexafluoropropene[28] provides no hint for the systematic
behavior of fluorine or CF3 substitution on the C=C double
bond. Hence, no conclusions on the systematic effects of
fluorine substituents and CF3 substituents can be drawn.
Figure 8. Experimental (O) and model (—) radial distribution curves of 2. The
difference curve is below. Vertical bars indicate interatomic distances;
selected ones are labelled.
and the reported rotational constants. Table 3 lists the experi-
mental rotational constants (B0) for different isotopologues
along with the constants modelled from the structure deter-
mined by GED and the corrections (Be–B0). Corrections were
taken from anharmonic frequency calculations. The small differ-
ences between the rotational constants derived from the
2.5. Vibrational Spectra
The gas-phase infrared spectrum and the Raman spectra of all
three states of trans-1 are displayed in Figure 9. Additionally,
Raman spectra in noble gas matrices were performed in order
to obtain fundamental vibrations not affected by intermolecular
interactions (see Supporting Information). Selected vibrational
data together with quantum-chemically calculated frequencies
at the PBE0/cc-pVTZ level of theory are listed in Table 4. The
assignments were made by consideration of the Cartesian
combined microwave/electron diffraction experiment (GEDcomb
)
and the ones measured by microwave spectroscopy prove the
satisfying accordance of both methods.
In both cases, the structural parameters of free molecules
coincide well with the respective parameters in the solid state.
However, all bonds are slightly shorter in the solid state than in
the gas phase and the difference between the CÀ F bonds in the
ChemistryOpen 2020, 9, 921–928
www.chemistryopen.org
925
© 2020 The Authors. Published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/open.202000172
ChemistryOpen
observed in their typical regions. The CÀ C valence mode appear
1
2
3
4
5
6
7
8
9
at 867 cmÀ 1 which is quite a low frequency for carbon single
bonds, but it is still in the expected region between 850 and
1150 cmÀ 1.[29,30] This is surprising considering the short CÀ C
bond (crystal 1.4789(3), gas 1.481(6) Å, see above) and thus we
attribute the low frequency to coupling effects. A similar
observation was made for 2 with ν(CÀ C) at 790 cmÀ 1,[7] whereas
the bond lengths are more on the shorter range of CÀ C values
(crystal 1.4963(4), gas 1.498(4) Å, see above). The assignments
of the deformation vibrations are listed in the Supporting
Information (Table S9).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
2.6. Combustion
For the 2 several combustion experiments as well as flamma-
bility, burning velocity and heat of combustion were
determined.[3,7,31–34] Far less is known about trans-1 which is
claimed to be not readily flammable, in contrast to the highly
flammable 2.[35,36] Therefore we performed combustion experi-
ments at different ratios of trans-1 with oxygen and synthetic
air. The combustion products were analyzed by infrared
spectroscopy. Carbon dioxide, hydrogen fluoride and carbonyl
fluoride were found as the main combustion products. As
already reported for 2, hydrogen fluoride was detected
indirectly as SiF4; this was formed by the reaction of HF with the
glass wall of the reaction vessel. Carbon tetrafluoride was
observed as a minor species.[7]
Figure 9. Infrared spectrum (gas (5 hPa), top) and Raman spectra (gas
(500 hPa), liquid, solid) of 1.
With
a sufficient amount of oxygen the combustion
Table 4. Selected experimental and calculated vibrational frequencies
[cmÀ 1] of 1.
proceeds accordingly to Equation (1).
IR[a]
(gas)
Raman
(solid)
Raman
(Ar matrix)
Calc.[b]
Assignment
2 CF3CHCHF þ 5 O2 ! 4 CO2 þ 2 COF2 þ 4 HF
(1)
3110 (vw)
3111(30)
3092(93)
1675(73)
1323(17)
1160(18)
3112(20)
3096(100)
1681(43)
1321(15)
1168(26
1132(15)
1082(22)
867(37)
3110 (1/74)
3103 (5/40)
1664 (170/15)
1303 (121/4)
1152 (77/3)
1119 (283/2)
1068 (314/2)
853 (21/5)
ν(CÀ Ht)
ν(CÀ Hm)
ν(C=C)
νs(CF3)
ν(CÀ Ft)
νas(CF3)
νas(CF3)
ν(C–C)
The combustion behavior is similar to that of 2. Differences
occur at sub-stoichiometric ratios of oxygen. The amount of
COF2 decreases, a part of trans-1 remains unchanged, but
considerable amounts of a black soot are formed. In general,
the combustion of hydrofluorocarbons does not lead to the
formation of considerable amounts of carbonyl fluoride. The
hydrogen and fluorine atoms preferentially form the thermody-
namically favored hydrogen fluoride during combustion. Con-
sequently, the formation of COF2 is only possible when the
number of fluorine atoms is larger than that of hydrogen atoms
in a molecule. But such molecules are usually non-flammable,
thus formation of COF2 has been only observed during pyrolysis
on air.[37] In this respect tetrafluoropropene 1 is an exception.
The instability towards oxidation is based on the presence of
the carbonÀ carbon double bond. This instability towards
oxidants is also the reason for the short lifetime in the
atmosphere and consequently its relatively low global warming
potential of GWP100 =7.[1]
1698(vs)
1336(vs)
1160(vs)
1106(vs)
883(w)
1079[c]
863(65)
[a] Abbrevations for IR intensities: vs=very strong, s=strong, m=medium,
w=weak. [b] Calculated on the PBE0/cc-pVTZ level of theory; Scaling
factor 0.95; IR intensities in kmmolÀ 1; Raman intensities in Å4/u. [c]
recalculated from the Fermi resonance doublet at 1098(15) and 1056(12)
cmÀ 1
.
displacement coordinates. For the trans-1 with Cs symmetry 21
fundamentals (14 A’+7 A’’) are expected.
The CÀ H vibrations occur at 3112 [C(1)H] and 3096 cmÀ 1
[C(2)H] in argon matrices and do not considerably differ from
the frequencies observed in the solid state. The hydrogen
atoms are involved in weak CÀ H···F hydrogen bonds in the solid
phase and therefore a red-shift would be expected, but it
should be kept in mind that guest-host interactions in matrices
cause also a small red-shift.
The classification of the flammability according to the
European transportation standard ADR, which is worldwide
used for transportation of dangerous goods, is based on the
explosion region of a gas/air mixture. To the best of our
knowledge the explosion region has been only determined by
the ASHRAE method which has a different classification for
The C=C stretching vibration at 1681 cmÀ 1 as well as the
CÀ F stretching vibrations between 1132 and 1321 cmÀ 1 are
ChemistryOpen 2020, 9, 921–928
www.chemistryopen.org
926
© 2020 The Authors. Published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/open.202000172
ChemistryOpen
flammability.[34–36] Therefore we determined the explosion (or
flammability) region of trans-1 with air according to the
European standard protocol EN 1839.[38] The lower flammability
limit (LFL) was found at 5.6% and the upper flammability limit
(UFL) at 14.2%. These values are comparable with the ASHRAE
method which yields values of LFL=6.25% and UFL=12.4%. In
case of 2 the ASHRAE method yielded LFL=6.82% and UFL=
12.0% whereas with the European standard setup values of
LFL=6.2% and UFL=14.4% have been determined. The differ-
ent values of both methods are probably caused by the
differences in the experimental setups. The flammability region
of trans-1 determined in our laboratories leads to a classification
of a highly flammable gas. This finding together with the fact
that highly toxic gases are formed in the case of fire should
lead to a reconsideration of the safety risk assessments of trans-
1.
in Table S3 of the Supporting Information; instrumental details are
reported elsewhere.[44] The electron diffraction patterns, three for
each, long and short nozzle-to-plate distances, were measured on
Fuji BAS-IP MP 2025 imaging plates, which were scanned by using
a calibrated Fuji BAS 1800II scanner. The intensity curves (Figures S1
and S2, Supporting Information) were obtained by applying the
method described earlier.[45] Electron wavelengths were refined[46]
using carbon tetrachloride diffraction patterns, recorded in the
same series of experiments as the substances under investigation.
Analysis of the measured electron diffraction intensities has been
done using least-squares method. Geometrical models of the
molecules have been defined in form of Z-matrices (see Supporting
Information). Initial values of parameters and fixed differences
between values of parameters in each group were taken from MP2/
cc-pVTZ calculations. Amplitudes of interatomic vibrations and
vibrational corrections have been calculated for both molecules
with the VibModule program[47] on the basis of harmonic and cubic
force fields from PBE0/cc-pVTZ computations. For 2,3,3,3-tetra-
fluoropropene (2) a combined structure refinement was performed,
using GED intensities and rotational constants, the relative weights
for the latter were adjusted so that both GED R-factors and
discrepancies in rotational constants were acceptable.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
3. Conclusions
The Raman spectroscopic measurements were executed on a
Bruker® MultiRAMII FT-Raman spectrometer equipped with a Nd:
YAG laser (λ=1064 nm). The interpretation of the spectra was
carried out with the aid of the software Advanced Chemistry
Development, Inc.® (ACD/Labs 2015). Infrared spectra were re-
corded on a Bruker® Vertex-80 V-FT-IR spectrometer. The spectra
were evaluated using the same software as for the Raman spectra.
The matrix isolation apparatus and the general procedure for the
preparation of matrix layers are described elsewhere.[48] The matrix
layers were prepared by condensation of 6 mmol premixed trans-1
and noble gas mixture at a continuous flow rate of 6 mmol hÀ 1
(130 cm3 hÀ 1) onto the cold tip at 10 K.
The refrigerants trans-1,3,3,3-tetrafluoropropene (HFO-1234ze,
1) and 2,3,3,3-tetrafluororopene (HFO-1234yf, 2) were investiga-
ted by high angle X-ray diffraction which allowed performing a
topological analysis of their charge density distributions. Gas-
phase electron diffraction yielded sets of structure parameters
of the molecules free of intermolecular interactions. Vibrational
spectra of trans-1,3,3,3-tetrafluoropropene (1) in all aggregation
states and in matrices have been studied and the results were
related to quantum-chemical values calculated at the PBE0/cc-
pVTZ level of theory. Combustion experiments of trans-1,3,3,3-
tetrafluoropropene (1) revealed carbon dioxide, carbonyl
fluoride and hydrogen fluoride to be the main combustion
products. The determination of the flammability region leads to
the classification of trans-1,3,3,3-tetrafluoropropene (1) as a
highly flammable gas.
Combustion experiments were performed using the same appara-
tus and in the same manner as already reported for 2.[7] For the
determination of the explosion region of trans-1 the protocol
accordingly to the DIN EN 1839 standard was used (tube
method).[38] The experimental set-up shown in the Supporting
Information (Figure S23) was calibrated with methane and synthetic
air prior to use. For the experiments with trans-1 the synthetic air
was moistened with distilled water as recommended for halogen-
ated gases by the protocol. The explosion region of trans-1 was
determined between 5.4% and 15.6%.
Experimental Section
Materials
Samples (ca. 10 g) of trans-1,3,3,3-tetrafluoropropene (HFO-1234ze
(E), Tyczka Industriegase, 1) and 2,3,3,3-tetrafluoropropene (HFO-
1234yf, Honeywell, 2), respectively, were condensed into carbon-
steel cylinders using a stainless-steel vacuum line. Residues of
Acknowledgements
We gratefully acknowledge the financial support of the Deutsche
Forschungsgemeinschaft (DFG), the Ludwig-Maximilians-Univer-
sität (LMU). We gratefully acknowledge computation time and QC
programs provided by the RRZK (Universität zu Köln) and the PC2
(Universität Paderborn). This work was funded by Deutsche
Forschungsgemeinschaft DFG (German Research Foundation)
through a core facility GED@BI grant MI477/35-1, project no.
324757882) and by a grant for YuVV (VI713/1-2, project no.
243500032). Open access funding enabled and organized by
Projekt DEAL.
°
nitrogen were removed at À 196 C in dynamic vacuum. The
samples were checked for purity by GC-MS (Varian CP3800, mass
spectrometer Saturn).
Crystals were measured on an Agilent SuperNova, Single Source at
offset, Eos diffractometer using Mo-Kα radiation (λ=0.71073 Å) at
93.0(4) resp. 95.0(2) K. Using Olex2.[39], the structures were solved
with the ShelXT[40] structure solution program using Dual Space and
refined using XD-2006.[41] All atoms were refined anisotropically.
The data were corrected for absorption using SADABS.[42] Plots
about distribution of outliers, normal probability and fractal
dimension in the SI were produced by WINGX.[43]
Electron diffraction patterns for 1 and 2 were recorded on the
heavily improved Balzers Eldigraph KD-G2 gas-phase electron
diffractometer at Bielefeld University. Experimental details are listed
ChemistryOpen 2020, 9, 921–928
www.chemistryopen.org
927
© 2020 The Authors. Published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/open.202000172
ChemistryOpen
Bach, S. Scheins, P. Luger, Org. Biomol. Chem. 2003, 1, 409; c) H.-G.
Conflict of Interest
1
2
3
4
5
Stammler, Yu. V. Vishnevskiy, C. Sicking, N. W. Mitzel, CrystEngComm
2013, 15, 3536; d) J. Schwabedissen, P. C. Trapp, H.-G. Stammler, B.
Neumann, J.-H. Lamm, Yu. V. Vishnevskiy, L. A. Körte, N. W. Mitzel, Chem.
Eur. J. 2019, 25, 7339.
The authors declare no conflict of interest.
[21] Yu. V. Vishnevskiy, J. Schwabedissen, A. N. Rykov, V. V. Kuznetsoc, N. N.
Makhova, J. Phys. Chem. A, 2015, 119, 10871.
[22] I. Tokue, T. Fukuyama, K. Kuchitsu, J. Mol. Struct. 1973, 17, 207.
[23] J. L. Carlos Jr., R. R. Karl Jr., S. H. Bauer, J. Chem. Soc. Faraday Trans. 2
1974, 70, 177.
Keywords: refrigerants
vibrational spectra · matrix isolation · combustion analysis
·
high-angle X-ray diffraction
·
6
7
[24] H. Bürger, G. Pawelke, H. Oberhammer, J. Mol. Struct. 1982, 84, 49.
[25] R. L. Hilderbrandt, A. L. Andreassen, S. H. Bauer, J. Phys. Chem. 1970, 74,
1586.
[26] I. L. Karle, J. Karle, J. Chem. Phys. 1950, 18, 963.
[27] F. C. Mijlhoff, G. H. Renes, K. Kohata, K. Oyanagi, K. Kuchitsu, J. Mol.
Struct. 1977, 39, 241.
[28] A. H. Lowrey, C. George, P. D'Antonio, J. Karle, J. Mol. Struct. 1979, 53,
189.
[29] T. Soltner, N. R. Goetz, A. Kornath, Eur. J. Inorg. Chem. 2011, 3076.
[30] J. Weidlein, K. Dehnicke, U. Müller, Schwingungsspektroskopie: eine
Einführung, Georg Thieme Verlag, Stuttgart, 1988.
[31] K. Takizawa, K. Tokuhashi, S. Kondo, J. Hazard. Mater. 2009, 172, 1329.
[32] B. H. Minor, D. Herrmann, R. Gravell, Process Saf. Prog. 2010, 29, 150.
[33] M. E. Koban, D. D. Herrmann, Process Saf. Prog. 2011, 30, 27.
[34] S. Kondo, K. Takahashi, K. Takizawa, K. Tokuhashi, Fire Saf. J. 2011, 46,
289.
8
9
[1] C. Hogue, Chem. Eng. News 2011, 89, 31.
[2] Ø. Hodnebrog, M. Etminan, J. S. Fuglestvedt, G. Marston, G. Myhre, C. J.
Nielsen, K. P. Shine, T. J. Wallington, Rev. Geophys. 2013, 51, 300.
[3] a) R. Akasaka Int. J. Thermophys. 2011, 32, 1125; b) C. Di Nicola, G.
Di Nicola, M. Pacetti, F. Polonara, G. Santori, J. Chem. Eng. Data 2010, 55,
3302; c) G. Di Nicola, M. Moglie, Int. J. Refrig. 2011, 34, 1098; d) L. Fedele,
S. Bobbo, F. Groppo, J. S. Brown, C. Zilio, J. Chem. Eng. Data 2011, 56,
2608; e) Y. Kano, Y. Kayukawa, K. Fujii, H. Sato, Int. J. Thermophys. 2010,
31, 2051; f) S. Lago, P. A. G. Albo, S. Brignolo, J. Chem. Eng. Data 2011,
56, 161; g) R. A. Perkins, M. L. Huber, J. Chem. Eng. Data 2011, 56, 4868;
h) M. Richter, M. O. McLinden, E. W. Lemmon, J. Chem. Eng. Data 2011,
56, 3254; i) K. Tanaka, Y. Higashi, R. Akasaka, J. Chem. Eng. Data 2010,
55, 901.
[4] I. Skarmoutsos, P. A. Hunt, J. Phys. Chem. B 2010, 114, 17120.
[5] T. Soulestin, V. Ladmiral, T. Lannuzel, F. D. D. Santos, B. Améduri, Polym.
Chem. 2017, 8, 735.
[6] M. D. Marshall, H. O. Leung, B. Q. Scheetz, J. E. Thaler, J. S. Muenter, J.
Mol. Spectrosc. 2011, 266, 37.
[7] M. Feller, K. Lux, C. Hohenstein, A. Kornath, Z. Naturforsch. 2014, 69b,
379.
[8] a) J. P. Perdew, K. Burke, Y. Wang, Phys. Rev. B 1996, 54, 16533; b) J. P.
Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1997, 78, 1396.
[9] T. H. Dunning, J. Chem. Phys. 1989, 90, 1007.
[10] More details are listed in Table S2 of the Supporting Information. CCDC
1841624 (for 1) and 1841625 (for 2) contain the supplementary
crystallographic data for this paper. These data can be obtained via
www.ccdc.cam.ac.uk/ data_request/cif free of charge from The Cam-
bridge Crystallographic Data Centre.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
[35] S. Kondo, K. Takizawa, K. Tokuhashi, J. Fluorine Chem. 2012, 144, 130.
[36] S. Kondo, K. Tokuhashi, K. Takizawa, J. Fluorine Chem. 2013, 149, 18.
[37] S. H. Modiano, K. L. McNesby, P. E. Marsh, W. Bolt, C. Herud, Appl. Opt.,
1996, 35, 4004.
[38] a) M. Krautwurst, ADR, 2nd Ed. 2019, Verkehrsverlag J. Fischer GmbH und
Co. KG, Düsseldorf; b) Deutsches Institut für Normung e.V. „Bestimmung
der Explosionsgrenzen von Gasen und Dämpfen“ EN 1839:2012, 2014,
Beuth Verlag GmbH, Berlin.
[39] O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard, H. Pusch-
mann, J. Appl. Crystallogr. 2009, 42, 339.
[40] G. M. Sheldrick, Acta Crystallogr. Sect. C 2015, 71, 3.
[41] A. Volkov, P. Macchi, L. J. Farrugia, C. Gatti, P. R. Mallinson, Richter, T.
Koritsánszky, 2006, XD2006.
[42] Sadabs, 2012/1, Bruker AXS Inc., Madison, Wisconsin, USA.
[43] L. J. Farrugia, J. Appl. Crystallogr. 2012, 45, 849.
[44] a) R. J. F. Berger, M. Hoffmann, S. A. Hayes, N. W. Mitzel, Z. Naturforsch.
2009, 64b, 1259; b) C. G. Reuter, Yu. V. Vishnevskiy, S. Blomeyer, N. W.
Mitzel, Z. Naturforsch. 2016, 71b, 1.
[45] Yu. V. Vishnevskiy, J. Mol. Struct. 2007, 833, 30.
[46] Yu. V. Vishnevskiy, J. Mol. Struct. 2007, 871, 24.
[47] Yu. V. Vishnevskiy, Yu. A. Zhabanov, J. Phys. Conf. Ser. 2015, 633, 012076.
[48] A. Kornath, J. Raman Spectrosc. 1997, 28, 9.
[11] P. Luger, A. Bach, J. Buschmann, D. Lentz, Z. Kristallogr. 2000, 215, 518.
[12] H. A. Bent, Chem. Rev. 1961, 61, 275.
[13] D. Lentz, A. Bach, J. Buschmann, P. Luger, M. Messerschmidt, Chem. Eur.
J. 2004, 10, 5059.
[14] A. Bach, D. Lentz, P. Luger, M. Messerschmidt, C. Olesch, M. Patzschke,
Angew. Chem. Int. Ed. 2002, 41, 296.
[15] D. Lentz, J. Chem. Crystallogr. 2003, 33, 977.
[16] S. Alvarez, Dalton Trans. 2013, 42, 8617.
[17] a) T. S. Koritsánszky, P. Coppens Chem. Rev. 2001, 101, 1583; b) P.
Coppens, X-Ray Charge Densities and Chemical Bonding, International
Union of Crystallography, Chester, England, Oxford, New York, 1997.
[18] T. A. Keith, “AIMALL” (version 16.05.18), TK Gristmill Software Overland
Parks KS, USA, can be found under aim.tkgristmill.com, 2016.
[19] a) R. F. W. Bader, Atoms in Molecules. A Quantum Theory, Clarendon
Press, Oxford, 1994; b) R. F. W. Bader Chem. Rev. 1991, 91, 893.
[20] a) P. Luger, J. Buschmann, T. S. Koritsánszky, D. Lentz, N. Nickelt, S.
Willemsen, Z. Kristallogr. 2000, 215, 487; b) D. Lentz, M. Patzschke, A.
Manuscript received: June 8, 2020
Revised manuscript received: July 20, 2020
ChemistryOpen 2020, 9, 921–928
www.chemistryopen.org
928
© 2020 The Authors. Published by Wiley-VCH GmbH