Study of the structural and conformational properties of fluoro-substituted thioacetanilide derivatives

Article abstract of DOI:10.1016/j.molstruc.2020.128768

The assessment of the influence of a halogen atom in the structural, vibrational and conformational properties of o-, m- and p-fluorothioacetanilide derivatives was achieved through different methodologies. For all compounds infrared (FTIR) and nuclear ma

Full text of DOI:10.1016/j.molstruc.2020.128768

Journal Pre-proof
Study of the structural and conformational properties of fluoro-substituted
thioacetanilide derivatives
José Osvaldo Guy Lezama, Ana Graciela Iriarte, Rodrigo E. Domínguez, Norma Lis
Robles
PII:
S0022-2860(20)31093-0
DOI:
https://doi.org/10.1016/j.molstruc.2020.128768
MOLSTR 128768
Reference:
To appear in: Journal of Molecular Structure
Received Date: 23 December 2019
Revised Date: 11 June 2020
Accepted Date: 24 June 2020
Please cite this article as: José.Osvaldo. Guy Lezama, A.G. Iriarte, R.E. Domínguez, N.L. Robles, Study
of the structural and conformational properties of fluoro-substituted thioacetanilide derivatives, Journal of
Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.128768.
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Credit Author Statement
José Osvaldo Guy Lezama: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Data curation, Writing-Original draft, Writing - Review & Editing,
Visualization
Ana Graciela Iriarte: Resources, Writing - Review & Editing, Visualization, Funding
acquisition
Rodrigo Ezequiel Domínguez: Methodology, Writing - Review & Editing
Norma Lis Robles: Conceptualization, Formal analysis, Resources, Writing-Original draft,
Writing - Review & Editing, Visualization, Supervision, Project administration, Funding
acquisition

Study of the structural and conformational properties of
fluoro-substituted thioacetanilide derivatives
José Osvaldo Guy Lezama,a Ana Graciela Iriarte,b Rodrigo E. Domínguez b and Norma Lis
Robles *a
a INQUINOA UNT CONICET. Facultad de Ciencias Exactas y Tecnología. Universidad
Nacional de Tucumán. Av. Independencia 1800. S. M. de Tucumán, Tucumán. República
Argentina.
b INFIQC – Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad
Nacional de Córdoba. Ciudad Universitaria, 5000 Córdoba, República Argentina.
nrobles@herrera.unt.edu.ar
Abstract
The assessment of the influence of a halogen atom in the structural, vibrational and
conformational properties of o-, m- and p-fluorothioacetanilide derivatives was achieved
through different methodologies. For all compounds infrared (FTIR) and nuclear magnetic
resonance (NMR) data were obtained; for m-fluorothioacetanilide, FT Raman and GC/MS
analysis were additionally performed. The substances now prepared from corresponding
aniline derivatives reveal a conformational equilibrium of anti and syn conformers, anti or
syn with respect to the N-H and C=S bonds, respectively. The evidence of the contribution
of each form varies upon experimental conditions of the spectra, being the presence of the
least stable alternative evident in room temperature NMR spectra of samples dissolved in
CDCl3. The assignment of the signals was supported by quantum chemical calculations and
data reported in literature for related compounds. Natural Bond Orbital (NBO) analysis was
employed for studying the stability of the conformers upon charge delocalization in order to
rationalize dominant electronic effects on each case.
Keywords: thioacetanilide derivatives; substituent effect; conformational equilibrium;
spectral characterization; DFT calculations
1

1. Introduction
Being one of the smallest molecules with a thiopeptide group, and among the smallest ones
capable of associating to another thiopeptide group to form hydrogen bonds, thioacetanilide
has been attracting the attention of diverse research groups since long time ago and still
remains being of interest nowadays [1]. In contrast to structural analogues such as
thioamides and thiobenzamide, thioacetanilide is not able to form oxides, which implies
that no acute hepatotoxicity action is expected to be observed [2]. It was recently
discovered to possess an interesting potential as inhibitor of some specific proteins
involved in the melanin biosynthesis [3] and to act as second-line drug in the treatment of
multidrug resistant pathologic bacteria [4].
Considering structures and conformations, several articles were devoted to the
conformational analysis of these thiocarbonyl compounds [5–8]. The two most stable
conformers found in thioacetamides result from a hindered rotation around the central C-N
bond of the -C(S)-NH- group and are called anti and syn forms (see Scheme 1). In the case
of the aromatic derivatives, the planar or non-planar disposition of the aromatic ring with
respect to the thioamide group was also considered, together with the presence of N-H···S
intermolecular bridges.
An early analysis of the infrared spectra of thioacetanilide in CHCl3 gathered the
vibrational signals belonging to the N-H stretching modes in two different groups: one
group, including signals with well-defined structure and higher wavenumber values (3399
and 3368 cm-1), which were attributed to “isolated” molecules, i.e. low concentration
solutions of thioacetanilide; the other group including signals at lower wavenumbers and
poorer definition (3250 and 3200 cm-1) were attributed to the corresponding vibrations in
“associated” molecules [9].
2

Scheme 1. Anti and syn conformers of thiocarbonyl compounds (anti or syn with
respect to the N-H and C=S bonds, respectively; R = CH3, phenyl, benzyl).
The reported analysis of the infrared spectra obtained for solutions of
thioformanilide, thioacetanilide and several derivatives in solvents with different
polarity, showed the presence of a conformational equilibrium of the anti and syn
conformers, presumably favoured by some specific solute-solvent interactions [10].
For the series of compounds analysed it was stated that the thioamide group does not
lay in the ring plane unless constrained to do so by hydrogen bonding between the
thioamide proton and a suitable ortho substituent. Later, a configurational
equilibrium of anti and syn conformers in the infrared spectra of diverse
ortho-substituted derivatives of thioacetanilide dissolved in different solvents was
found [5]. According to that study, as the concentration of the substance diminishes,
the relative proportion of syn form increases because of the prevalence of B type
hydrogen-bond intermolecular interaction over A type (see Scheme 2). The ratio
between these forms modifies with changes in the polarity of the solvent, as it was
observed for the series of substances solved in CCl4, CDCl3 and DMSO.
Scheme 2. Intermolecular arrangement types.
3

Although the results indicated a clear influence of the solvent in the conformational
equilibrium, the effect of different functional groups bonded to the aromatic ring should
also be considered. Since no link between electron-donor and electron-withdrawing
properties of the substituents was established in the display of the conformational
equilibrium, the presence of either a single or both conformers was rationalized by
considering its position in the aromatic ring [5]. As in most cases anti was the only
conformational isomer found in ortho X substituted derivatives of thioacetanilide (X = Cl,
OCH3, NO2, COCH3, CH2OCH3), it was stated that the presence of a suitable substituent,
able to form an intramolecular N-H···X interaction, forced the adoption of this
configuration (see Scheme 3). Electrostatic and steric effects were proposed as the factors
determining the adoption of a given configuration.
Scheme 3. ortho X substituted derivatives of thioacetanilide (X = Cl, OCH3, NO2,
COCH3, CH2OCH3).
A very detailed assessment regarding the origin of the higher rotational barriers
characteristic of para substutitued thioacetanilide derivatives, in comparison with the
corresponding oxygenated compounds, determined that mesomeric effects might be
involved [6]. The relative energy of rotation around the C-N bond in both series of
compounds was linked to the fluctuations of the atomic charges in the amide group, which
in turn, was rationalized in terms of resonance interaction between the para substituents
4

and the amide group. Since the most stable conformational isomer for each compound
considered possesses anti configuration with a planar disposition of the thioamide (and
amide) function and the aromatic ring, the prevalence of this geometry was expected.
However, recent X ray crystallographic data for solid thioacetanilide at 120 K demonstrated
its being formed by two independent infinite chains in the α-axis direction formed only by
anti forms with an almost planar -C(S)-NH group rotated 40-50° with respect to the plane
of the phenyl ring [7]. In this study, the room temperature infrared spectra of a solid sample
showed a 400 cm-1 red shift in the N-H stretching vibration, being this change a direct
evidence of the intermolecular N-H···S interaction. Similar results were found recently for
3-methylacetanilide and N-benzylthioacetamide [11].
Even though electrostatic, steric and mesomeric effects provided good explanations to
account for the vibrational and conformational properties of thioacetanilide derivatives, a
more complete analysis should be addressed to fully understand the conformational
properties of substituted sulfur derivatives of acetanilide. The present article rationalizes the
influence of fluoro substituents bonded to the aromatic ring in the conformational
equilibrium through the analysis of experimental spectra (FTIR and NMR) obtained for o-,
m- and p-fluorothioacetanilide and FT Raman and GC/MS for m-fluorothioacetanilide,
abbreviated in the following text as o-, m- or p-FTA. The compounds were prepared by
subsequent thionation of the corresponding fluoroacetanilide precursors with Curphey's
thionation reagent and microwave irradiation for efficient heating of reaction mixtures
[5,8,12–14]. The analysis of the potential energy surface (PES) and possible conversion
paths between rotameric forms, together with theoretical simulations of the vibrational and
NMR spectra and relevant orbital interactions, was achieved by quantum chemical
calculations derived from Density Functional Theory.
2. Materials and methods
2.1. Synthesis
All reactions were performed in a microwave reactor Biotage model Initiator UE using
continuously stirred 10 ml vials. Temperature and reaction time varied according to each
5

experience. All reactants and solvents were purchased to Sigma-Aldrich Argentine and
used without further purification.
Fluoroacetanilides: Fluorinated acetanilides were synthesized from the corresponding
fluoro-substituted anilines following the methodology published by Brahmachari et al [15]
Fluorothiocetanilides: Starting fluoroacetanilide derivative (12 mmol), P4S10 (2.28 mmol)
and hexamethyldisiloxane, HMDO (20 mmol) were placed in a 10 ml sealed microwave
glass tube and solved with dichloromethane (3 ml). The reaction mixture was heated at
150-200°C for 30-60 minutes with constant stirring. The reaction progress was monitored
by TLC and final products were purified following procedures published elsewhere [14].
2.2. Spectroscopic measurements
FTIR spectra of the samples at room temperature were recorded in the region 3500–400
cm-1 (4 cm-1 spectral resolution) using a Perkin Elmer GX1 Fourier Transform infrared
instrument provided with a deuterated-triglycine sulfate (DTGS) detector constantly purged
with dry air. KBr disc for solid (p-FTA) and KRS-5 windows were employed for liquid
(o-FTA and m-FTA) samples. For the Raman measurements a FTIR Perkin Elmer GX1
spectrometer, equipped with Raman accessory, an Nd-YAG⁄ laser (excitation line of 1064
nm, 1900 mW of laser power) and InGaAs detector at room temperature was used. The FT
Raman spectra of m-FTA were recorded in the 3500 - 100 cm-1 region with a resolution of
1 cm-1 and 200 scans.
The GC/MS analysis was carried out using a Perkin Elmer CLARUS 560 GC/MS with an
ELITE 5MS (30 m length, 0.25 µm thickness, 250 µm ID) capillary column. Split-less
injection was used for this study. The initial temperature of the oven was 120°C. After
maintaining that temperature for 4 min, the oven temperature was increased at a rate of
8°C/min to reach a final oven temperature of 280°C. The final temperature was maintained
for 2 min and the total run time was 26 min. Helium was used as carrier gas. Quantification
of the peaks was based on peak area.
The 1H, 13C and 19F nuclear magnetic resonance (NMR) spectra of all samples were
measured in chloroform-d solution (CDCl3) with a Bruker Avance 500 spectrometer at
frequencies of 500.13; 125.76 and 470.59 MHz, respectively. The chemical shifts were
6

reported in ppm relative to tetramethylsilane (TMS) for 1H, 13C spectra and referenced to
CFCl3 for 19F spectra.
2.3. Computational details
Based on the Density Functional Theory (DFT), the Becke-style three parameter hybrid
functional for the exchange part [16] with the Lee-Yang-Parr correlation function (B3LYP)
[17] and the 6-311++G(df) and 6 311++G(3df,3pd) bases sets was applied to determine the
energies of the stable minima in the potential energy surfaces (PES). Subsequent full
geometry optimizations and calculation of vibrational spectra were conducted for stable
structures. For the study of the orbital interactions the natural bond orbital analysis (NBO)
[18] was used as implemented in the GAUSSIAN03 program package [19]. The 1H, 13C and
19F chemical shifts were predicted by the Gauge-Including Atomic Orbital (GIAO) [20]
method, with and without solvent (chloroform) was represented by the IEFPCM (Integral
Equation Formalism variant Polarizable Continuum Model ) [21].
3. Results and Discussion
3.1. Conformational analysis
Scheme 1 shows conformational isomers derived from a hindered rotation around the amide
C-N bond. In the present case, the analysis of the PES was performed with
B3LYP/6-311++G(d,p) approximation for each fluoro-substituted conformer by geometry
optimizations at fixed H-N-C=S angles in steps of 30 degrees (see Fig. S1 in supplementary
material). Full geometry optimization of minima found indicate the prevalence of the anti
conformers (see Table 1). The same analysis performed by using a larger basis set
(6-311++G(3df,3pd)) produced equivalent results in terms of the greater stability of the anti
form, but the corresponding syn rotamers were predicted much less stable. Two transition
states (TS1 and TS2) were obtained through intrinsic reaction coordinate (IRC) calculations
with this last method for p-FTA being the conformational change between anti and syn
forms more likely to occur via the lower energy TS1; full geometry optimizations of
minima found predicted a rotational barrier of ca. 19 kcal.mol-1 (see Fig. 1) in good
7

agreement with values reported for p-substituted anilides and thioanilides by using
B3LYP/6-31G(d,p) (18 and 20 kcal.mol-1, respectively) [6].
Fig. 1. Energy diagram for the rotation around amide C-N bond in p-FTA calculated at the
B3LYP/6-311++G(3df,3pd) level of theory.
The presence of the fluorine atom bonded in different positions to the aromatic ring in o-
and m-FTA give raise to additional conformational possibilities of diverse stability derived
from rotation around the C1-N bond (see Fig. 2). The analysis of the PES performed with
the B3LYP/6-311++G(d,p) approximation for these compounds by geometry optimizations
at fixed C-N-C-C angles in steps of 30 degrees indicated a rotational barrier of ca. 2
kcal.mol-1; steric hindrance avoids additional anti (or syn) structures for o-FTA rotamer
than those shown. Two transition states (denoted as TS1 and TS2 in Fig. S2 of the
supplementary file) were obtained with B3LYP/6-311++G(3df,3pd) approximation for each
set of m-FTA possibilities being the conformational change between them more likely to
occur once again via the lower energy TS1 (rotational barrier of ca. 18 kcal.mol-1); only
one syn-anti interconversion path (through TS1) was feasible for the ortho isomer (see Fig.
S3 in supplementary material). Subsequent full optimizations and frequency calculations at
8

the minima determined the energy differences listed in Table 1 for the stable rotamers
shown in Figure 2. From the figures it is evident that increasing the size of the bases set
lead to larger energy separations between the anti and syn forms with the consequent
decrease of estimated population in the vibrational equilibrium of the less stable
conformers, similar to the results observed for p-substituted anilides and thioanilides [6].
Fig. 2. Calculated stable conformational isomers of fluorothioacetanilide.
According to the calculations, the global minimum in the potential energy surface for each
compound possesses the expected anti configuration of the thioamide group. The planar
disposition of the amide group with respect to the aromatic ring in anti o-FTA was
expected, since it was already stated previously that an intramolecular interaction, between
the amide proton and a suitable ortho substituent, would favour this configuration [10].
No such a substitution favours the planar disposition of the amide group in anti p-FTA;
nevertheless, it is still the most stable conformer predicted by the theoretical calculations
performed and presumably the most abundant in the vibrational spectra. For m-FTA, the
low relative energy differences between anti A and B, and between syn A and B together
9

with the low barrier heights for rotation around C1-N bond (see above), prevent the
distinction between corresponding conformational isomers. Geometrical parameters
obtained with B3LYP/6-311++G(3df,3pd) approximation for fully optimized stable forms
are available as supplementary material (Tables S1-S3).
Table 1. Calculated relative free energies (
isomers of o-, m- and p-FTA.a
Δ
G°) in kcal.mol-1 for stable conformational
ortho
meta
para
6-311++G(d,p)
6-311++G(3df,3pd)
6-311++G(d,p)
0.00 (46%)
0.36 (24%)
0.63 (16%)
0.70 (14%)
6-311++G(3df,3pd)
6-311++G(d,p)
6-311++G(3df,3pd)
anti (A)
anti (B)
syn (A)
syn (B)
0.00 (95%)
0.00 (99%)
0.00 (91%)
1.63 (6%)
2.38 (2%)
2.54 (1%)
0.00 (60%)
0.00 (91%)
-
-
-
-
1.73 (5%)
-
2.61 (1%)
-
0.24 (40%)
-
1.38 (9%)
-
a Calculated by B3LYP method and indicated bases sets. In parenthesis, conformational contribution of each form in the
vibrational spectra.
3.2. Vibrational analysis
The absence of bands attributable to the carbonyl group in N-arylacetamides region (ca.
1700 cm-1) [22] in saturated vibrational spectra of FTAs indicates that the corresponding
acetanilide precursor reacted completely. The tentative assignment of selected fundamental
vibrational modes is provided as supplementary material (see Tables S4-S6). This section
focuses only on the discussion of the experimental features (see Fig. 3) and predicted
frequencies attributed to the main stretching and bending fundamental modes (see Table 2).
For the assignment of the vibrations belonging to the monofluorinated ring, data reported
for the series o-, m- and p-fluoroaniline [23,24] and o-, m- and p-fluorosulfinylaniline were
also considered [25–27].
10

Table 2. Experimental and calculated wavenumbers of anti FTA conformational isomers
and tentative assignment of selected fundamental vibrational modes.
Theoretical values
B3LYP/ 6-311++G(3df,3dp)
a
Mode Approximate description
Experimental b
νcal
(cm-1) c
IR (liq)
3164 m
1541 s
1506 vs
1289 m
1147 s
727 s
IR Intensity d
p-FTA
ν1
N–H Stretch.
N–H Def. i.p
3504
1548
1511
1301
1126
732
7
34
74
4
ν11
ν12
ν19
ν23
ν34
C–C Stretch. + C–H Def.
C–F Stretch.
C=S Stretch.
9
C=S Stretch. + ring def.
6
o-FTA
IR (liq)
3188 m
1523 s
1501 vs
1255 s
1165 s
740 vs
IR Intensity d
νcal (cm-1) c
3505
ν1
N–H Stretch.
N–H Def. i.p.
14
39
17
22
<1
5
ν11
ν12
ν20
ν22
ν34
1548
C–C Stretch. + C–H Def.
C–F Stretch.
1490
1237
C=S Stretch.
1158
C=S Stretch. + ring def.
730
m-FTA (anti A)
IR (liq)
Raman (liq)
IR Intensity d Raman Activity e
νcal (cm-1) c
3504
-
N–H Stretch.
N–H Def. i.p.
3202 w
1547 m
1489 s
1261 m
1181 s
722 m
ν1
6
53
15
6
38
19
6
1548 w
1487 vw
1258 w
1181 m
723 m
ν11
ν12
ν19
ν21
ν34
1544
C–C Stretch. + C–H Def.
C–F Stretch.
1485
1284
29
8
C=S Stretch.
1178
10
C=S Stretch. + ring def.
721
4
6
a Stretch.: stretching; Def.: deformation; i.p.: in phase; b s: strong; vs: very strong; m: medium; w: weak; vw: very weak; sh.:
shoulder. c Values scaled by 0.98, according to scaling factors in Ref. [28]. d Relative infrared intensities activities normalized to
100%. e Raman activities normalized to 100%.
11

Fig. 3. Experimental infrared spectra of o-FTA and m-FTA (liquid), and p-FTA (solid) in
the region 4000-400 cm-1.
In contrast to previous studies of acetanilide and thioacetanilide, the infrared spectra of the
series of compounds addressed in this article were performed for liquid or solid samples of
the substances at room temperature. As a shared experimental hindrance for registering
Raman spectra, the strong fluorescence phenomena shown by the dark coloured substances
avoided the obtention of spectra by using 532 and 780 nm lasers; by using a 1064 nm laser
only the corresponding spectra for the meta compound could be acquired. On account of
the analysis of the conformational equilibrium, the N-H stretching region in the infrared
spectra was reported to be one of the most useful for solutions of acetanilide and
thioacetanilide derivatives [5,9,10]. Vibrational spectra of room temperature pure samples
of thioacetanilide derivatives now studied do not evidence the presence of a conformational
equilibrium. The signals centred at 3164, 3202 and 3188 cm-1 were attributed to the N-H
stretching for the anti conformational isomer of p-, m- and o-FTA, respectively (see Table
2).
12

The analysis of the 1600-1400 cm-1 region of the infrared spectra of different disubstituted
benzene rings show features with characteristic intensity and shape which vary upon
substitution [29]. For ortho and meta conformational isomers a very intense and narrow
signal belonging to an in-plane C-H bending mode is expected at ca. 1500 cm-1; the same
appears at ca. 1515 cm-1 in para conformers. In the present case, this fundamental mode
was calculated at 1490, 1485 and 1511 cm-1, respectively (see Table 2).
The so-called amide II band in carbonyl compounds (a N-H in-plane deformation) was
observed at 1553 cm-1 in the infrared spectrum of acetanilide [30] and at 1547, 1553 and
1557 cm-1 in the IR spectrum of o-, m- and p-fluoroacetanilide, respectively. Their
counterparts in thiocarbonyl compounds were attributed to signals at lower frequencies and
intensities, centred at 1523, 1547 and 1541 cm-1, respectively for the corresponding anti
forms. This assignment is in good agreement with values recently reported for
fluoro-isomeric species [31]. A closer analysis of the frequency values for the amide II
vibrations requires considering the geometry of the thioamide functional group. Anti
structures depicted in Fig. 2 show a planar disposition of this group with respect to the
aromatic ring. Amide II vibrations belonging to para and meta conformational isomers
were assigned to signals closely similar both in frequency and intensity. In contrast, this
fundamental mode was observed at lower wavenumbers for the ortho conformer. This red
shift might be attributed to the presence of the intramolecular interaction between the amide
proton and the ortho-fluoro substituent already stated before [5], which diminishes the
strength of the N-H bond and lengthens it. The natural bond orbital analysis (NBO) allows
evaluating donor-acceptor interactions and the corresponding delocalization energies
involved which mainly contribute to the stabilization of a given structure, estimated by the
second order perturbation theory [18]. Figure 4 shows representations of the NBO for the
main donor-acceptor interactions of anti conformers together with the corresponding
calculated values for delocalization energies. The delocalization energy value and estimated
degree of orbital overlapping predicted for the lp F → σ* N-H interaction in anti o-FTA,
absent in the other conformational isomers, clearly evidence that the corresponding N-H
bond should be the weakest in the series. This anomeric interaction increases the electronic
population of the
σ* N-H orbital (0.025 e) and produces a subtle elongation of the N-H
bond. More noticeable differences were predicted for the C-F bond length and CCN angle.
13

It is well known that the amide group has a large contribution from a resonance structure
which weakens the C=O bond and strengthen the C-N bond. As a substituent of the
aromatic ring, it provides negative charge density to carbon atoms both ortho and para to
this group. On the other hand, the fluorine atom can exert opposing electronic effects: the
electron attracting inductive effect and a mesomeric electron donor effect which provides
electron density in positions ortho and para to the halogen atom [32]. Therefore, it is
expected that the conjunction of the thioamide group and the fluorine atom as substituents
of the aromatic ring bestow properties to the compound which vary with the position of the
halogen in relation to the main functional group (thioamide group, in the present case).
Considering the intramolecular F···H interaction mentioned before, together with the
calculated geometrical parameters on Fig. 4, it is expected that the C-F bond in o-FTA were
the largest of the series. Accordingly, and supported by the mesomeric effects of the
fluorine atom towards the aromatic ring, the C-F stretching modes were attributed to the
signals observed at 1255, 1261 and 1289 cm-1, for o-, m- and p-FTA, respectively. This
assignment is in good agreement with the infrared spectra data reported for o-, m- and
p-fluoroaniline series [23,24].
14

Fig. 4. Relevant NBO orbital interactions [B3LYP/6-311++G(3df,3pd)] and
corresponding energy delocalization values (kcal.mol-1), relevant bond distances (Å)
and angles (°) for anti FTA conformational isomers (isovalue for orbital plotting:
0.05).
Finally, in account to vibrations belonging to the thiocarbonyl functional group, early
studies placed the unperturbed thiocarbonyl stretching vibration at 1140 ± 80 cm-1 being
rather insensitive to polar effects but prone to undergo accidental resonance with NH2
wagging vibration, (in thioamides derivatives) or with asymmetric N-C-N stretching (in
thiourea derivatives) [33,34]. More recently, this signal was reported in the 700-810 cm-1
region [35–37]. The comparison between our experimental spectra of fluoroacetanilide and
corresponding thiofluoro isomers show new signals in both regions, centred at 1165, 1181
and 1147 cm-1, and 740, 722 and 727 cm-1 for the anti forms of o-, m- and p-FTA
conformational isomers, respectively, in agreement with results recently published by
Saeed et al [31]. As this mode is mixed with other fundamental vibrational modes, no
15

further reasoning was undertaken regarding frequency variations for the different
conformers.
3.3. GC/MS Analysis
As the compounds under study are fluoro-substituted thioacetanilide derivatives, no major
differences regarding the characteristic fragment patterns were expected for each one, being
the relative abundance of the signals the main variations [38]. For m-FTA, the only intense
signal observed at time retention 12.29 min (98.5% relative area and 169 molecular weight)
in the total-ion chromatogram accounts for the presence and high purity of this compound.
The feature at m/z 59 is the base peak and was assigned to the fragment [CH3C
Fig. 5).
≡
S]+ (See
Fig. 5. Mass spectrum of m-FTA.
The peak at m/z 169 was attributed to the molecular ion [M+·], therefore the peak at m/z
168 was assigned to its deprotonated form. The loss of the CH3CN fragment from [M+·] and
[M+·-1] accounts for the signals at m/z 128 and 127, respectively. The peak at m/z 136 were
attributed to the loss of ·SH. The assignment was based on reported mass spectra of
thioacetanilide [39]. Figure 6 depicts a scheme of the fragmentation pattern with the most
relevant fragments of interest. The assignment of [FC6H6N]+, [FC5H5]+ and [FC6H4]+
fragments are supported by data reported in the literature [13,40].
16

Fig. 6. Major fragmentation pattern of m/z peaks for m-FTA.
3.4. NMR Analysis
NMR spectra were analysed considering literature and theoretical values obtained by
B3LYP/6-311++G(3df, 3pd) approximation using the GIAO method (see section 2.3). In
general, theoretical predictions obtained considering the solvent overestimated the
experimental figures observed (see Tables S7-S10 in supplementary material). Room
temperature experimental spectra of samples in CDCl3 evidence a conformational
equilibrium of anti and syn conformational isomers.
The presence of the anti and syn forms was easily confirmed by room temperature 13C
NMR spectra, which show different thiocarbonyl and methyl signals for different
conformers (see Table 3). Thiocarbonyl chemical shifts were assigned taking into account
theoretical values and data reported for thioacetanilide in the literature [41].
17

Table 3. Selected experimental and calculated 13C NMR chemical shifts for FTA
conformational isomers.
C=S chemical shift
Experimentala Theoreticalb
anti (A) syn (A) anti (A) syn (A) anti (A) syn (A) anti (A) syn (A)
CH3 chemical shift
Experimentala Theoreticalb
Compound
p-TFA
m-TFA
o-TFA
201.00
200.93
201.05
205.14
204.81
206.16
205.87
207.11
207.52
212.86
213.03
212.25
35.85
35.93
36.41
29.96
30.25
29.71
38.59
38.90
38.56
30.00
30.78
31.14
a Reported at 125.76 MHz in CDCl3; b Calculated chemical shifts (GIAO method) using
B3LYP/6-311++G(3df, 3pd) approximation.
In addition to the expected signals for the anti and syn conformers of the FTA series, 19F
NMR spectra obtained for m-FTA show a third signal, indicating the presence of an
additional conformer, anti B (see Fig. 2). The 19F NMR chemical shifts were calculated
from the isotropic shielding (σ) as δ = σref - σ + δref, using C6F6 as reference (calculated σref
= 340.2 ppm at the same approach, tabulated δref = -164.9 ppm) [42]. Hence, experimental
values are in general agreement with the calculated ones (see Table 4).
Table 4. Experimental and calculated 19F NMR chemical shifts for FTA conformational
isomers.
F shift (δ) (ppm)
Experimentala,b
Theoreticalc
Compound
anti (A)
syn (A)
anti (B)
anti (A) syn (A) anti (B)
p-TFA
m-TFA
o-TFA
-113.82 (63.4) -112.42 (36.6)
-
-117.00 -116.50
-
-111.62 (58.4) -111.26 (33.9) -110.05 (7.7) -107.99 -110.62 -115.15
-131.60 (83.2) -126.67 (16.8) -139.67 -128.81
-
-
a Reported at 470.59 MHz in CFCl3; b In parenthesis, percentage contribution of each conformer;
c Calculated chemical shifts (GIAO method) using B3LYP/6-311++G(3df, 3pd) approximation.
18

Signals belonging to the methyl protons in the 1H NMR spectra represent suitable targets to
determine the anti-syn ratio [43], as it was proposed for series of p-substituted thioanilides
previously reported [8], where a ca. 65 and 35% contribution of forms anti and syn,
respectively were observed. In our case, we also used this signal to determine the amount of
each conformer (see Table 5). The presence of the additional anti isomer observed for m-
FTA in 19F NMR spectra was confirmed again by 1H NMR.
Table 5. Experimental and calculated chemical shifts of methyl protons in the 1H NMR for
FTA conformational isomers.
CH3 shift (δ) (ppm)
Experimentala,b
Theoreticalc
anti (B) anti (A) syn (A) anti (B)
Compound
anti (A)
syn (A)
p-TFA
m-TFA
o-TFA
2.73 (65.1) 2.47 (34.9)
2.13 (60.8) 2.70 (31.7) 2.53 (7.5)
2.22 (82.1) 2.77 (17.9)
-
2.93
2.94
3.01
2.86
3.04
3.04
-
2.96
-
-
a Reported at 500.13 MHz in CDCl3; b In parenthesis, percentage contribution of each conformer;
c. Calculated chemical shifts (GIAO method) using B3LYP/6-311++G(3df, 3pd) approximation.
Experimental contributions of each conformational isomer remarkably differ from those
listed on Table 1, as it was previously reported for solutions of thioacetanilide derivatives
using solvents of distinct polarities [5,8]. This might be a consequence of the sample
conditions in each case: quantum chemical calculations of gaseous equilibrium
overestimate the amount of anti conformer with respect to the value derived from the
signals in the experimental NMR spectra of liquid solutions of the samples.
In general, chemical shifts for thioanilides are larger than the corresponding values in
anilides, revealing greater magnetic anisotropy of a thiocarbonyl group compared to the
oxygenated counterpart [43]. As an example, in p-fluoroacetanilide [44], the amide proton
was assigned to a singlet at 7.40 ppm. The corresponding larger values in the FTA series
19

are listed on Table 6. Experimental chemical shifts for amide protons of anti forms of p-
and o-FTA are similar, confirming that mesomeric electron donating effects of the
thioamide and fluorine substituents of the ring rule in these planar structures. As expected,
the corresponding signal was observed at higher fields in form anti. For the syn forms, not
only larger experimental chemical shifts were observed, but also a different trend was
followed, which may be rationalized by considering the orbital interactions depicted on Fig.
7.
Table 6. 1H NMR experimental and calculated chemical shifts of amide proton in the for
FTA conformational isomers.
N-H chemical shift (
δ
) (ppm)
Theoretical b
Experimental a
Compound
anti (A)
8.78
syn (A)
9.49
anti (A)
8.58
syn (A)
9.39
p-TFA
m-TFA
o-TFA
8.36
10.02
9.03
8.62
9.54
8.71
9.34
9.56
a
b
Reported at 500.13 MHz in CDCl3;
B3LYP/6-311++G(3df, 3pd) approximation.
Calculated chemical shifts (GIAO method) using
20

Fig. 7. σ(N-H) → σ* (C-C) orbital interactions and corresponding delocalization energies for
the syn series of FTA conformational isomers.
The largest chemical shift observed in 1H NMR spectrum of m-FTA, attributed to the amide
proton of form syn A evidences a greater electronic density around this nucleus, in
agreement with the largest calculated delocalization energy value. The amide proton for
anti conformers were observed to be more shielded that syn forms in lactams [9]. In the
present case, calculated chemical shifts figures obtained by GIAO support these statements.
For the assignment of the aromatic protons in the FTA series, the effect of several
simultaneous factors had to be considered. On one hand, the geometry of the thioamide
group itself, which lays coplanar to the aromatic ring in the anti-series but tilts about 40
degrees in the syn forms, making intramolecular interactions less effective. On the other
hand, not only the expected proton-proton couplings should be considered, but also proton-
nitrogen and proton-fluorine couplings may complicate the resulting signals in the spectra.
For ortho-substituted anilides and thioanilides, striking deshielding effects of the H12
signal (see Fig. 2) were found because of the presence of a N-H···X intramolecular
interaction [43]. In agreement, experimental values listed on Table 7 evince larger chemical
shifts for o-FTA than those observed for the p- and m- alternatives conformers. Figure 8
21

shows the corresponding experimental features observed for H12 in anti FTA
conformational isomers; coupling constants listed in Table 8 were assigned considering
data reported for related compounds [45].
Fig. 8. Fragment of the 1H NMR spectrum showing the H12 signals for forms anti of o-, m-
and p-FTA conformational isomers and corresponding coupling constants.
22

Table 7. Experimental and calculated 1H NMR chemical shifts of H12 in the FTA series.
H12 chemical shift ( ) (ppm)
Experimentala Theoreticalb
anti (A) syn (A) anti (A) syn (A)
δ
Compound
p-TFA
m-TFA
o-TFA
7.60
7.45
8.49
7.44
7.72
-
10.20
10.56
10.82
7.17
7.20
7.68
For atom numbering see Fig. 8. a Reported at 500.13 MHz in CDCl3. The signal corresponding to
o-FTA (anti form) could not be identified in the spectrum; b Calculated chemical shifts (GIAO
method) using B3LYP/6-311++G(3df, 3pd) approximation.
Table 8. Coupling constants (Hz) in the 1H NMR spectrum for the aromatic proton H12.a
JH12-H13
JH12-H14
JH12-H15
JH12-F
Compound anti (A) syn (A) anti (A) syn (A) anti (A) syn (A) anti (A) syn (A)
p-TFA
m-TFA
o-TFA
7.3
1.5
8.0
7.5
1.9
-
-
-
-
-
-
2.0
1.5
0.2
2.2
1.9
-
4.9
11.0
6.1
4.8
10.5
-
1.7
a For atom numbering see Fig. 8.
4. Conclusions
A series of fluoro-substituted thioacetanilide derivatives were obtained in high yield and
characterized by infrared spectroscopy whereby a single conformational isomer (anti) was
observed for room temperature spectra of the pure samples. On the other hand, theoretical
calculations derived from Density Functional Theory with different bases sets predicted a
conformational equilibrium of at least two conformers in each case. Room temperature
NMR spectra of the samples solved in CDCl3 provided a better approach to assess the
conformational equilibrium. 13C NMR easily confirmed the anti/syn equilibrium through
the presence of two discernible sets of thiocarbonyl and methyl signals for each form. The
integration of well separated signals in the 1H and 19F NMR spectra allowed determining
the contribution of each form in equilibrium. The ratio found by analysing these last spectra
contrast with the calculated for each conformational isomer. However, the differing values
23

obtained were not completely unexpected because this ratio is sensitive to the nature of the
solvent and the experimental conditions. In addition, 1H and 19F NMR helped to uncover
the presence of a third form in equilibrium (anti B) for m-FTA. Through the analysis of this
series of compounds it may be stated that the structural, vibrational and conformational
properties depend on the physical state of the sample, but most importantly, on the position
of the fluorine substituent in the aromatic ring, which determine different conformational
possibilities and ratios.
Acknowledgements
Authors acknowledge CONICET (Consejo Nacional de Investigaciones Científicas y
Técnicas), ANPCyT (Agencia Nacional de Promoción Científica y Tecnológica- PICT
1167-2015) and UNT (Universidad Nacional de Tucumán) for the finantial support.
Authors also kindly acknowledge staff from LISA (Laboratorio de Investigación y
Servicios Analíticos - UNT CONICET), and Dr. Gerardo Burton, from UMYMFOR
(Unidad de Microanálisis y Métodos Físicos Aplicados a la Química Orgánica – CONICET
FCEN UBA), as well as INIQUI (Instituto de Investigaciones para la Industria Química –
CONICET UNSA for GC/MS, NMR and Raman measurements, respectively.
Supplementary Material
Graphics (transition states and PES) and tables (geometrical parameters, vibrational and
NMR data) for o-, m- and p-FTA are given in supplementary file as supporting material.
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