Spectroscopic and theoretical studies of some 2?(methoxy)?2?[(4?substituted)?phenylsulfanyl]?(4′?substituted) acetophenones

Article abstract of DOI:10.1016/j.saa.2018.11.010

The conformational analysis of some 2?(methoxy)?2?[(4?substituted)?phenylsulfanyl]?(4′?substituted) acetophenones was performed through infrared (IR) spectroscopic analysis of the carbonyl stretching band (ν_CO), supported by B3LYP/6-31+G(d,p) calculations and X-ray diffraction. Five (1–5) of the seven studied compounds (1–7) presented Fermi resonance (FR) on the ν_CO fundamental transition band. Deuteration of these compounds (1a–5a) precluded the occurrence of FR, revealing a ν_CO doublet for all compounds in all solvents used. The computational results indicated the existence of three conformers (c₁, c₂ and c₃) for the whole series whose relative abundances varied with solvent permittivity. The higher ν_CO frequency c₁ conformer was assigned to the higher frequency component of the carbonyl doublet, while both c₂ and c₃ were assigned to the lower frequency one. Anharmonic vibrational frequencies and Potential Energy Distribution (PED) calculations of compound 3 indicated that the combination band (cb) between the methyne δ_CH and one skeletal mode couples with the ν_CO mode giving rise to the FR on the c₂ conformer in vacuum and on the c₁ one in non-polar solvents. The experimental data indicated a progressive increase in c₁ conformer stability with the increase of the solvent dielectric constant, which is in good agreement with the polarizable continuum model (PCM) calculations. The higher ν_CO frequency and the stronger solvation of the c₁ conformer is a consequence of the repulsive field effect (RFE) originated by the alignment and closeness of the C^δ+[dbnd]O^δ? and C^δ+–O^δ? dipoles. Finally, the balance between orbital and electrostatic interactions dictates the conformational preferences. X-ray single crystal analysis for compound 6 revealed the c₁ geometry in the solid state and its stabilization by C–H…O hydrogen bonds.

Full text of DOI:10.1016/j.saa.2018.11.010

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic and theoretical studies of some
2 (methoxy) 2 [(4 substituted) phenylsulfanyl]
(4′ substituted) acetophenones
a
b
Henrique J. Traesel a, Paulo R. Olivato a, Daniel N.S. Rodrigues a, , Jéssica Valença , Alessandro Rodrigues ,
⁎
Julio Zukerman-Schpector c, Maurizio Dal Colle d
a
Conformational Analysis and Electronic Interactions Laboratory, Institute of Chemistry, University of São Paulo, CP 26077, 05513-970 São Paulo, SP, Brazil
b
Department of Chemistry, Federal University of São Paulo, UNIFESP, 09972-270 Diadema, SP, Brazil,
Department of Chemistry, Federal University of São Carlos, CP 676, 13565-905 São Carlos, SP, Brazil
Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, 44121 Ferrara, Italy
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2018
Received in revised form 16 October 2018
Accepted 8 November 2018
Available online 12 November 2018
The conformational analysis of some 2 (methoxy) 2 [(4 substituted) phenylsulfanyl] (4′ substituted)
acetophenones was performed through infrared (IR) spectroscopic analysis of the carbonyl stretching band
(νCO), supported by B3LYP/6-31+G(d,p) calculations and X-ray diffraction. Five (1–5) of the seven studied com-
pounds (1–7) presented Fermi resonance (FR) on the νCO fundamental transition band. Deuteration of these
compounds (1a–5a) precluded the occurrence of FR, revealing a νCO doublet for all compounds in all solvents
used. The computational results indicated the existence of three conformers (c1, c2 and c3) for the whole series
whose relative abundances varied with solvent permittivity. The higher νCO frequency c1 conformer was assigned
to the higher frequency component of the carbonyl doublet, while both c2 and c3 were assigned to the lower fre-
quency one. Anharmonic vibrational frequencies and Potential Energy Distribution (PED) calculations of com-
pound 3 indicated that the combination band (cb) between the methyne δCH and one skeletal mode couples
with the νCO mode giving rise to the FR on the c2 conformer in vacuum and on the c1 one in non-polar solvents.
The experimental data indicated a progressive increase in c1 conformer stability with the increase of the solvent
dielectric constant, which is in good agreement with the polarizable continuum model (PCM) calculations. The
higher νCO frequency and the stronger solvation of the c1 conformer is a consequence of the repulsive field effect
(RFE) originated by the alignment and closeness of the Cδ+_Oδ− and Cδ+\\Oδ− dipoles. Finally, the balance be-
tween orbital and electrostatic interactions dictates the conformational preferences. X-ray single crystal analysis
for compound 6 revealed the c1 geometry in the solid state and its stabilization by C\\H…O hydrogen bonds.
© 2018 Published by Elsevier B.V.
Keywords:
Conformational analysis
Infrared spectroscopy
Fermi resonance
Theoretical calculations
Solvent effect
X-ray diffraction
2 (Methoxy) 2 [(4 substituted)
phenylsulfanyl] (4′ substituted) acetophenones
1. Introduction
enzyme inhibitors containing selenium-sulfur atoms [5–7], and in mo-
lecular docking studies [8] to understand the mechanism of its inhibition.
α Thio keto compounds constitute an important organic framework
widely applied as intermediates in many synthetic transformations [1],
and frequently used as building blocks in the synthesis of drugs and bio-
active compounds [2]. In this context, α keto thioacetals and their ana-
logues are often used in various synthetic applications, such as in the
preparation of complex intermediates [3] and bioactive natural products
[4]. With the objective of exploring these themes we have been
performing studies in searching for selective cyclooxygenase 2 (COX-2)
The X-ray crystal and molecular structure of compound (6) was
determined, described, and compared with those already published,
such as 2 (4 chlorophenylsulfanyl) 2 methoxy 1 phenylethanone [9],
2 methoxy 2 (4 methylphenylsulfanyl) 1 phenylethanone [10], and
2 methoxy 2 (4 methoxyphenylsulfanyl) 1 phenylethanone [11].
In the last thirty years, the conformational analysis of several differ-
ent carbonyl compounds performed by our research group, based on a
number of spectroscopic (IR, UV, NMR, UPS and ETS) and X-ray studies
supported by theoretical calculations, disclosed some relevant findings.
The studies of some (α alkylthio)- and (α phenylthio)- acetones,
acetophenones, N,N diethylamides, esters, thioesters [12,13],
N methoxy N methyl amides [14], and N methyl δ valerolactams [15],
⁎
Corresponding author at: Conformational Analysis and Electronic Interactions
Laboratory, Institute of Chemistry, University of São Paulo, P. O. Box 26077, 05513-970
São Paulo, SP, Brazil.
indicated that the simultaneous occurrence of the nS → π*CO, σC\\S
→
E-mail address: dannoppper@gmail.com (D.N.S. Rodrigues).
π*CO, and πCO → σ*C\\S orbital interactions is the main controlling factor
https://doi.org/10.1016/j.saa.2018.11.010
1386-1425/© 2018 Published by Elsevier B.V.

H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
83
that determines the preference of the gauche (axial) conformer(s) over
the cis (equatorial) one(s).
supported by theoretical calculations and X-ray diffraction data. The oc-
currence of unexpected Fermi resonance is fully experimentally re-
solved and also discussed in terms of the normal coordinate analysis.
In particular, these compounds became spectroscopically interesting
as the α substituents are expected to compete for the syn-clinal or anti-
clinal (gauche) and syn-periplanar (cis) geometries with respect to the
carbonyl group, due to differences in the orbital and electrostatic inter-
actions that stabilizes the conformers. Finally, this work is an extension
of recent conformational studies on the 2 (phenylselanyl) 2
(methoxy) acetophenones,
The
IR
carbonyl
stretching
analysis
of
the
α (p substituted phenoxy) p′ substituted acetophenones, X\\C6H4\\C
(O)\\CH2\\O\\C6H4\\Y (X and Y = NO2, H, and OMe) [16], indicated
the existence of cis-gauche rotational isomerism. The sum of the field
(F), inductive (−Iσ), and mesomeric (M) effects in the cis rotamers of
the derivatives with (X = H and OMe) for (Y = NO2, H, and OMe)
causes a similar carbonyl bond order, which accounts for the almost
constant positive carbonyl frequency shifts observed. Additionally, for
any Y substituent, the gauche conformers in the p-nitroacetophenones
(X = NO2) are more stable than the cis ones, while in the
methoxyacetophenones (X = OMe) and acetophenones (X = H), the
cis conformers are slightly more stable. This behavior was ascribed to
the π*CO(LUMO)/nO(OC6H4\\Y) orbital interaction, which stabilizes the
gauche conformers of the (X = NO2) derivative to a larger extent than
the (X = OMe and H) ones.
2 (phenylseleno) 2 (ethylsulfanyl) acetophenones and their mono- and
di-oxygenated derivatives [5–7,22].
2. Experimental Section
2.1. Materials
The experimental photoelectron spectra of the Me2X (X = S and
O) compounds indicated that both the nS lone pair (8.71 eV) and the
All solvents for IR measurements were spectrograde and were used
without further purification. The 2 (methoxy) [(4 substituted)
phenylsulfanyl] (4′ substituted) acetophenones 1–7 are new com-
pounds and were prepared according to the literature [23] as follows:
a THF solution of 2 (methoxy) 4′ substituted acetophenone was slowly
added to a stirred solution of LDA in THF at 195 K. After 30 min, a solu-
tion of 1,2 bis(4 substituted phenyl) disulfide with HMPA [24,25] in THF
was added dropwise, with immediate discoloration of the reactant.
Water was added, and crude product extracted with ethyl ether or di-
chloromethane after the reaction mixture reached room temperature
(ca. 3 h). The organic layer was washed with a saturated NH4Cl solution
till neutral pH and dried over anhydrous magnesium sulphate. The ob-
tained crude product was purified by flash chromatography on silica
gel, and after solvent evaporation a solid was obtained.
The corresponding α methyne deuterated compounds 1a–5a were
obtained from an adaptation of the literature procedure [26] as follows:
5 mg of anhydrous K2CO3 was added to a stirring solution of
2 (methoxy) 2 4 substituted phenylsulfanyl) acetophenone in a 50%
methanol-d4 (CD3OD) and CDCl3 solution at room temperature. The
progress of the isotopic substitution was followed through 1H NMR.
When the progress of hydrogen/deuterium exchange reached an equi-
librium of ca. 95% (ca. 30 min) the mixture was centrifuged. The organic
layer was then carefully collected and filtered through a thin pad of
Celite® 545 (SiO2), giving the 2 deutero 2 (methoxy) 2 (4
substituted phenylsulfanyl) acetophenones 1a–5a. The reagents
2 methoxy acetophenone (Sigma-Aldrich), 2 methoxy (4′ methoxy) ,
and 2 methoxy (4′ chloro) acetophenones (Novel Chemical Solutions)
were commercially available. Suitable crystals of 6 for X-ray analysis
were obtained by vapor diffusion from chloroform/n hexane at 283 K.
The 1H and 13C NMR data and elemental analysis for compounds 1–7
are presented in Table 1.
σC\\S orbital (11.28 eV) [17] ionization energies are lower than those
of the nO lone pair (10.04 eV) and the σC\\O orbital (11.91 eV) [17]. In ad-
dition, the σ*C\\S (3.25 eV) [18] electron-affinity energy is higher than
that of σ*C\\O (6.0 eV [19] or 4.2 eV [20,21]). Therefore, as shown in
our previous papers [12–16], then X→π*CO, σC\\X→π*CO, πCO→σ*C\\X inter-
actions are stronger in the gauche (axial) conformer(s) of the
α thio carbonyl compounds (X = S) than for the analogous conformer
in the α oxa acetophenones (X = O).
Stimulated by the above discussion, this study reports the conforma-
tional analysis and the electronic interactions of some mixed
acetophenones bearing both the phenylthio- and the methoxy-
substituents in the α position. This study analyses the 2 (methoxy)
[(4 substituted) phenylsulfanyl] (4′ substituted)acetophenones 1–7
(Scheme 1). The occurrence of conformational isomerism is investi-
gated through the νCO infrared band decomposition method in solvents
of increasing dielectric constant. The obtained spectroscopic results are
2.2. IR Spectroscopy
The IR spectra for the fundamental carbonyl region
(1800–1600 cm−1) were recorded with a FTIR Michelson Bomem
MB100 spectrometer with 1.0 cm−1 resolution at a concentration of
1.0 × 10−2 mol dm−3 in n hexane, carbon tetrachloride, chloroform, di-
chloromethane, and acetonitrile solutions using a 0.519 mm sodium
chloride cell. The spectra for the carbonyl first overtone region
(3600–3100 cm−1) were recorded in a carbon tetrachloride solution
(1.0 × 10−2 mol dm−3) using a 1.00 cm quartz cell. The overlapping car-
bonyl bands (fundamental and first overtone) were deconvoluted using
the Grams/32 curve fitting program version 4.04 [27]. The relative in-
tensities and the conformer populations for the title compounds 1–7
and 1a–5a in the referred solvents were estimated from the percentage
of the integrated absorbance B = ∫band ln (I0/I)νdν (in cm−1) for each re-
solved carbonyl multiplet (doublet or triplet) component assuming
Scheme 1. Atoms labelling of 2 (methoxy) 2 [(4 substituded) phenylsulfanyl]
[(4′ substituted) acetophenones] (1–7), the corresponding methyne deuterated
compounds (1a–5a), and definition of relevant dihedral angles.

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H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
Table 1
1H, 13C NMR and elemental analysis data for 4 and 4′ substituted 2 (methoxy) 2 (phenylsulfanyl) acetophenones 4′ Y PhC(O)CH[OMe][SPh 4 X] (1–7).
Comp.
Y
X
1H NMRa
13C NMRa
Mp (°C)
Molecular
formula
Elemental
analysis/%
7.95–7.94 (2H, m), 7.57 (1H, m), 7.46–7.44 (2H, m),
OCH3 7.26–7.24 (2H, m), 6.81–6.80 (2H, m), 5.76 (1H, s), 3.78
(3H, s), 3.68 (3H, s)
7.97–7.95 (2H, m), 7.57 (1H, m), 7.46–7.44 (2H, m),
CH3 7.25–7.24 (2H, m), 7.10–7.08 (2H, m), 5.81 (1H, s), 3.67
(3H, s), 2.33 (3H, s)
190.26, 160.49, 136.75, 134.53, 133.28,
128.84, 128.49, 120.83, 114.64, 89.79,
56.16, 55.28
190.55, 139.07, 134.60, 134.44, 133.32,
129.86, 128.91, 128.49, 127.25, 90.01,
56.13, 21.25
Calc. 66.64 5.59
Found 66.52 5.53
1
2
H
H
120.5–121.2
86.3–86.8
C
C
16H16O3S
16H16O2S
Calc. 70.56 5.92
Found 69.96 6.02
7.97–7.95 (2H, m), 7.58 (1H, m), 7.46–7.43 (2H, m),
7.39–7.37 (2H, m), 7.32–7.28 (3H, m), 5.86 (1H, s), 3.68
(3H, s)
7.95–7.93 (2H, m), 7.59 (1H, m), 7.47–7.44 (2H, m),
7.29–7.24 (4H, m), 5.86 (1H, s), 3.67 (3H, s)
7.94–7.92 (2H, m), 7.58 (1H, m), 7.47–7.44 (2H, m),
7.41–7.39 (2H, m), 7.22–7.20 (2H, m), 5.87 (1H, s), 3.67
(3H, s)
190.65, 134.34, 134.19, 133.40, 131.23,
129.04, 128.93, 128.72, 128.50, 90.07,
56.10
190.20, 135.60, 135.25, 134.23, 133.55,
129.22, 128.84, 128.59, 89.37, 56.13
190.16, 135.73, 134.18, 133.53, 132.13,
129.92, 128.81, 128.57, 123.41, 89.28,
56.10
Calc. 69.74 5.46
Found 69.74 5.72
3
4
5
H
H
H
H
Cl
Br
54.8–55.3
85.5–85.8
84.0–84.5
C
C
C
15H14O2S
Calc. 61.53 4.48
Found 61.47 4.55
15H13ClO2S
15H13BrO2S
Calc. 53.42 3.89
Found 53.19 3.85
189.63, 163.77, 133.98, 131.77, 131.43,
129.02, 128.55, 127.00, 113.73, 90.55,
56.01, 55.50
189.57, 139.83, 134.22, 132.55, 130.95,
130.43, 129.10, 128.86, 128.81, 90.39,
56.21
7.98–7.97 (2H, m), 7.41–7.39 (2H, m), 7.31–7.28 (3H, m),
6.93–6.91 (2H, m), 5.81 (1H, s), 3.87 (3H, s), 3.64 (3H, s)
Calc. 66.64 5.59
Found 66.60 5.57
6
OCH3
Cl
H
H
70.3–70.8
64.4–64.9
C16H16O3S
7.91–7.87 (2H, m), 7.42–7.40 (2H, m), 7.37–7.36 (2H, m),
7.32–7.28 (3H, m), 5.77 (1H, s), 3.67 (3H, s)
Calc. 61.53 4.48
Found 61.07 4.49
7
C15H13ClO2S
a
1H and 13C chemical shifts in ppm relative to TMS, in CDCl3.
R
1
cl
ꢀ
based on multiple scans [29] and reduced using standard methods
[30]. The structure was solved by direct methods [31] and refined on
F2 (anisotropic displacement parameters, C-bound H atoms in the riding
model approximation) using the SHELXL2014/6 program [32] through
the WinGX Interface [33]. Crystal data and refinement details are col-
lated in Table 2. The molecular structure diagram was drawn with
ORTEP-3 for Windows [33]. Crystallographic data for the structural
analysis have been deposited with the Cambridge Crystallographic
Data Centre as CCDC 1859035. Copies of this information may be ob-
tained free of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (fax: 44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk).
equal integrated molar absorption coefficient A ¼
lnðI0=IÞνdν
band
(km mol−1) for all the conformers [28].
2.3. NMR Spectroscopy
1H NMR and 13C NMR spectra were recorded on a Bruker DRX 500
spectrometer operating at 500.130 and 125.758 MHz, respectively, for
0.1 mol dm−3 solutions in CDCl3. 1H and 13C chemical shifts are reported
in ppm relative to the internal standard TMS.
2.4. X-ray Measurements
The X-ray diffraction measurements for the colorless crystal of 6
were performed at 293 K on a Bruker SMART APEX II CCD diffractometer
fitted with MoKα radiation. The data set was corrected for absorption
2.5. Theoretical Calculations
The conformational search was performed using Monte Carlo's
method in HF/STO-3G theory level with Spartan '06 software [34]. The
obtained conformer geometries were used as initial inputs in all calcula-
tions carried out at 298 K using the methods and basis sets implemented
in the Gaussian package of programs (G09-D01) [35] with a hybrid
Hartree-Fock density functional B3LYP method [36–38] and the 6-31
+G(d,p) basis set. Full geometrical optimizations and analytical har-
monic vibrational frequency calculations were performed on the most
stable conformers. Frequency analysis was carried out to verify the na-
ture of the minimum state of the stationary points and to calculate the
zero-point vibrational energy (ZPVE) corrections. To estimate the solva-
tion effects on the relative stability of the most relevant conformers, cal-
culations were conducted on the optimized structures using the
polarizable continuum model (PCM) [39,40]. Due to the lack of symme-
try of the three conformers found (C1 point group), the thermodynamic
probability factor was the same (ω = 2) for all of them. The NBO 6.0
program [41] was used as implemented in the Gaussian 09 package,
and the reported NBO delocalization energies (E2) were those given
by second-order perturbation theory. The partial atomic charges were
calculated using the charges from electrostatic potentials using a grid-
based method (ChELPG) [42]. In order to compare the performance of
B3LYP functional with others that include dispersion energy correc-
tions, M06-2X [43] and B3LYP-D3 [44] calculations along with the 6-
31+G(d,p) basis set were performed for compound 3, both in gas and
in condensed phase (PCM) (Tables S1 and S2).
Table 2
Crystal data and structure refinement for 6.
Chemical formula
C16H16O3S
Formula mass (g mol−1
T (K)
)
288.35
293
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
Monoclinic
P21/c
10.9628(4)
6.0281(2)
22.0394(7)
90
α (°)
β (°)
γ (°)
94.358(1)
90
1452.26(9)
4
1.319
608
Volume (Å3)
Z
Dcalc (g/cm3)
F(000)
μ (MoKα)/mm−1
0.227
θmax (°)
26.4
Measured data
Unique data
18,360
2992
Observed data (I ≥ 2.0σ(Ι))
R, obs. data; all data
a, b in weighting scheme
Rw, obs. data; all data
2627
0.0370; 0.0423
0.0545, 0.3736
0.1036; 0.1087

H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
85
The anharmonic vibrational frequency calculations were obtained
3. Results and Discussion
with the inclusion of the anharmonic potential as denoted in the Gauss-
ian 09 revision A02 [45] program. The analysis of anharmonic frequen-
cies along with a search for coupling vibrational modes calculations
were performed for compound (3), representative for the series, in
order to check if Fermi resonance operates in the system. The obtained
fundamental modes of vibrations were analyzed in the potential energy
distribution (PED) terms using the VEDA [46] program.
3.1. Infrared Analysis
Table 3 lists the stretching frequencies and the integrated absor-
bance percentages of the analytically resolved components of the car-
bonyl band for the 2 (methoxy) [(4 substituted) phenylsulfanyl]
(4′ substituted) acetophenones 1–7 in solvents of increasing relative
.25
.2
(a)
(b)
.2
.15
.1
.15
.1
.05
0
.05
0
1760
1740
1720
1700
1680
1660
1640
1740 1730 1720 1710 1700 1690 1680 1670 1660 1650 1640
Wavenumbers (cm-1)
Wavenumbers (cm-1)
.6
(c)
(d)
.035
.5
.4
.3
.2
.1
0
.03
.025
.02
.015
.01
.005
0
3420
3400
3380
3360
3340
3320
3300
1740
1720
1700
1680
1660
1640
Wavenumbers (cm-1)
Wavenumbers (cm-1)
.25
.2
.3
.25
.2
(e)
(f)
.15
.1
.15
.1
.05
0
.05
0
1740 1730 1720 1710 1700 1690 1680 1670 1660 1650 1640
1730
1720
1710
1700
1690
1680
1670
1660
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Fig. 1. IR spectra of 2 (phenylsulfanyl) 2 (methoxy) acetophenone 3, showing the analytically resolved carbonyl stretching band, in: n hexane (a), carbon tetrachloride [fundamental (b)
and first overtone (c)], chloroform (d), dichloromethane (e) and acetonitrile (f).

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H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
permittivity [47], from n hexane (ε = 1.9) to acetonitrile (ε = 38), ex-
cept for compounds 1, 4, and 5, which are insoluble in n hexane. It
should be pointed out that a νCO triplet was found in solvents of low po-
larity for compounds 2 and 3 (in n hexane) and 1–5 (in carbon tetra-
chloride) in the fundamental region, but not in the first overtone
region, for which only a doublet is present. Moreover, for 1–3 as the sol-
vent polarity increases, from CHCl3 to CH3CN, only a doublet is detected
unless for 1 in CH2Cl2 where a triplet occurs. As for 4 and 5, a triplet is
present in the polar solvents CHCl3, CH2Cl2 and CH3CN. For compounds
6 and 7 bearing 4′-substituents in the acetophenone moiety, only dou-
blets are observed in all solvents, in the fundamental region, as well as
in the first overtone, in CCl4. The solvent effect on the carbonyl band
components for compounds 1–7 is illustrated in Figs. 1 and S1–S4.
The occurrence of three carbonyl band components in the funda-
mental region (carbon tetrachloride) and only a doublet in the first
overtone region for 1–5 is indicative of the occurrence of the Fermi res-
onance in the fundamental transitions [48,49] (see below). On the other
hand, the doublet found in all solvents in the fundamental region for 6
and 7 was reproduced in the first overtone region (in carbon tetrachlo-
ride). Moreover, the occurrence of two carbonyl band components in
the first overtone at frequencies twice those of fundamental transition,
minus ca. 18 cm−1 (twice the mechanical anharmonicity ωeχe) [28,48],
and intensity ratios comparable to those of the fundamentals is indica-
tive of the presence of at least two conformers. This behavior rules out
the existence of the FR in the fundamental transition of the νCO mode
[48,49].
table shows the presence of only carbonyl doublets in the fundamental
region for 1a–5a in all solvents, and the occurrence of a doublet in the
first overtone region in CCl4, with each component frequency twice
that of fundamental transition minus ca. 17 cm−1 (twice the mechanical
anharmonicity ωeχe) and intensity ratios close to those of the funda-
mentals. This behavior is similar to that of compounds 6 and 7, being in-
dicative of the presence of at least two conformers in solution. In
addition, the intensity of the higher frequency component for 1a–5a in-
creases with respect to the lower frequency as the solvent polarity in-
creases, as shown in Figs. 2 and S5–S8. Likewise, the analogous solvent
effect is observed for derivatives 6 and 7 (Fig. 3).
The comparison between carbonyl stretching IR bands of com-
pounds (1–5) (Table 3) with the corresponding deuterated derivatives
(1a–5a) (Table 4) indicates the absence of the highest triplet frequency
component for compounds 2, 3 in n-C6H14, 1–5 in CCl4, 1 in CH2Cl2, and
4 and 5 in CHCl3, CH2Cl2 and CH3CN.
The disappearance of this component in the deuterated derivatives
suggests the occurrence in compounds 1–5 of the anharmonic Fermi
resonance between the carbonyl stretching fundamental of one con-
former (the intense middle frequency triplet component) and a combi-
nation band involving the C\\H deformation and skeletal modes (the
less intense highest frequency component) (see Section 3.3). The fre-
quencies of the unperturbed transitions can be estimated from the ex-
perimental shifted values through the following Eq. (1) [28,52]:
ꢀ
νn þ νm νn−νm ρ−1
ꢀ
ρ þ 1
ꢁ
ν0n;m
¼
ð1Þ
It is well known that the isotopic substitution of one or more atoms
in a molecule can significantly change the vibrational levels, breaking
the accidental degeneracy and destroying the FR phenomena
[26,50,51]. In fact, the classical example of the FR is the carbonyl
stretching region in the IR spectra of the cyclopentanone, which
shows a doublet in solution, but α, α, α′, α′ cyclopentanone d4 destroys
the FR, displaying only a single carbonyl stretching band.
Therefore, in this work we undertook the deuteration of the α
methyne hydrogen atom of compounds 1–5 obtaining the
2 (deutero) 2 (methoxy) 2 (4 substituted phenylsulfanyl)
2
2
where ν°n and ν°m are the unperturbed corrected frequencies, νn and
νm are the observed frequencies, and ρis the intensity ratio An/Am
expressed in absorbance at the referred band maxima. All of the rele-
vant data is collected in Table 5 for 1–5, along with the higher doublet
frequency component of the deuterated derivatives (1a–5a) in solution.
The large frequency shift (Δνm,n = νm–νn, ca. 10 cm−1) observed in
non-polar solvents is a consequence of the closeness of the unperturbed
transitions (Δν°m,n ca. 3.5 cm−1). On the contrary, in polar solvents the
Fermi resonance slightly affects the excited states, thus leading to a
acetophenones, whose carbonyl region is presented in Table 4. This
Table 3
Experimental frequencies (ν, cm−1) and intensities of the carbonyl stretching bands in the IR spectra of 4 and 4′ substituted 2 (methoxy) 2 (phenylsulfanyl) acetophenones 4′ Y PhC(O)CH
[OMe][SPh 4 X] (1–7) in solvents of increasing relative permittivity.
Comp.
Y
X
n-C6H14 (ε = 1.9)
Pb
CCl4 (ε = 2.2)
CHCl3 (ε = 4.8)
CH2Cl2 (ε = 9.1)
CH3CN (ε = 38)
ν
ν
P
2νc
P
ν
P
ν
P
ν
P
a
d
d
d
d
d
1
2
3
4
5
6
H
OMe
Me
H
–
–
–
–
25.3
27.5
47.2
24.4
42.2
33.4
–
1705
1695
1680
1705
1696
1683
1704
1695
1683
1705
1696
1682
1704
1696
1684
1697
1686
3341
1699
1686
14.5
45.1
40.4
22.0
32.6
45.4
19.9
38.4
41.7
25.1
40.5
34.6
18.4
43.9
37.7
24.1
77.6
69.2
37.7
62.3
–
–
–
–
1703
1691
1675
–
1692
1676
–
1696
1684
1703
1692
1679
1702
1692
1680
1684
7.1
–
–
–
–
3377
3346
–
3378
3349
–
3378
3351
–
3378
3349
–
59.5
40.5
1691
1682
68.0
32.0
78.9
14.0
–
85.2
14.8
–
56.8
43.2
18.9
62.9
18.2
21.1
61.1
17.8
55.3
1693
1684
90.0
10.0
d
d
d
d
d
H
H
H
H
1708
1698
1688
1708
1695
1686
–
–
–
–
–
53.1
46.9
1694
1685
41.8
58.2
1695
1689
64.8
35.2
d
d
d
d
d
–
–
–
–
–
50.9
49.1
1695
1683
1703
1691
1676
1701
1689
1670
3367
48.4
51.6
14.7
68.9
16.4
16.7
71.6
11.7
30.8
1697
1686
1704
1693
1676
1703
1693
1682
1685
64.5
35.5
13.8
78.9
7.3
23.4
61.9
14.7
61.8
a
d
Cl
–
–
–
–
–
–
–
–
59.8
40.2
a
d
Br
–
–
–
–
H
3377
3349
1692
59.4
40.6
29.0
–
OMe
1683
7
75.9
H
1679
1703
1790
71.0
25.9
74.1
1674
3378
3353
44.7
43.0
57.0
1674
1694
1683
38.2
57.2
42.8
1675
1695
1684
22.4
65.6
34.4
Cl
1696
1685
81.0
19.0
ε Relative permittivity.
a
Compound not soluble in this solvent.
Intensity of each component of the carbonyl doublet or triplet expressed in percentage of integrated absorbance.
First overtone.
The highest triplet frequency component is absent.
b
c
d

H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
87
.3
.25
.2
.3
(a)
(b)
.25
.2
.15
.1
.15
.1
.05
0
.05
0
1740
1720
1700
1680
1660
1640
1730 1720 1710 1700 1690 1680 1670 1660 1650 1640
Wavenumbers (cm-1)
Wavenumbers (cm-1)
.6
.5
.4
.3
.2
.1
0
.05
.04
.03
.02
.01
0
(c)
(d)
3420
3400
3380
3360
3340
3320
3300
1730
1720
1710
1700
1690
1680
1670
1660
1650
1640
Wavenumbers (cm-1)
Wavenumbers (cm-1)
.5
.4
.3
.2
.1
0
.6
.5
.4
.3
.2
.1
0
(e)
(f)
1730 1720 1710 1700 1690 1680 1670 1660 1650
1720
1710
1700
1690
1680
1670
1660
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Fig. 2. IR spectra of 2 (deutero) 2 (phenylsulfanyl) 2 (methoxy) acetophenone 3a, showing the analytically resolved carbonyl stretching band, in: n hexane (a), carbon tetrachloride
[fundamental (b) and first overtone (c)], chloroform (d), dichloromethane (e) and acetonitrile (f).
negligible splitting, that is Δνm,n of ca. 12 cm−1 in comparison with the
unperturbed one Δν°m,n ca. 11 cm−1. As expected, the Fermi coupling
coefficient W for compound 4, obtained with Eq. (2) [50], is larger in
the non-polar solvent CCl4 (W = 4.8 cm−1) and smaller in the polar sol-
vent CH3CN (W = 3.8 cm−1).
These behaviors are clearly summarized in the bar diagrams in
Schemes 2 and S1. Additionally, both Table 5 and Scheme 2 show that
the higher doublet frequency component of the deuterated compounds
displays νCO frequency close or slightly lower (due to some mass effect)
to that of the lower computed unperturbed ν°m frequency. This proxim-
ity is further evidence that the lowest doublet frequency corresponds to
the fundamental ν°m(CO) transition and the higher one (ν°n) is due to
the unperturbed combination band.
Δ2ðn;mÞ ¼ Δ20ðn;mÞ þ 4W2
ð2Þ

88
H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
.35
.25
.2
(a)
(b)
.3
.25
.2
.15
.1
.15
.1
.05
0
.05
0
1715 1710 1705 1700 1695 1690 1685 1680 1675 1670 1665 1660
Wavenumbers (cm-1)
1710
1700
1690
1680
1670
1660
1650
Wavenumbers (cm-1)
.05
.04
.03
.02
.01
0
.35
.3
(c)
(d)
.25
.2
.15
.1
.05
0
3400
3380
3360
3340
3320
3300
3280
1700
1690
1680
1670
1660
1650
1640
Wavenumbers (cm-1)
Wavenumbers (cm-1)
.35
.3
.35
.3
(e)
(f)
.25
.2
.25
.2
.15
.1
.15
.1
.05
0
.05
0
1710 1705 1700 1695 1690 1685 1680 1675 1670 1665 1660 1655
1710
1700
1690
1680
1670
1660
1650
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Fig. 3. IR spectra of 2 (phenylsulfanyl) 2 (methoxy) 4′ methoxy acetophenone 6, showing the analytically resolved carbonyl stretching band, in: n hexane (a), carbon tetrachloride
[fundamental (b) and first overtone (c)], chloroform (d), dichloromethane (e) and acetonitrile (f).
3.2. Geometries and Properties
≈ 130°) geometry between the C\\O and C_O groups, and an anti-
clinal orientation between the C\\S and C_O groups, with α′ ≈ 101°
for c1 and c2 and α′ ≈ 111° for c3, for the whole series. The computed
structures of the c1, c2, and c3 conformers of 3, taken as representative
of the series 1–7, are reported in Fig. 4.
The c2 conformers are the most stables in the series (1–7), and their
relative abundances increase from compound 1 (74.8%) to compound 7
(86.7%) (Table 6). On the contrary, the relative population of the second
The relevant DFT data, calculated at the B3LYP/6-31+G(d,p) level, of
the optimized geometries for compounds 1–7 are reported in Table 6,
along with the vibrational frequencies of the minimum energy confor-
mations and the X-ray dihedral angles for 6.
The calculations indicate the existence of three distinct conformers,
which assume a synperiplanar (c1, α ≈ 25°) and anti-clinal (c2 ≈ c3, α

H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
89
Table 4
Experimental frequencies (ν, cm−1) and intensities of the carbonyl stretching bands in the IR spectra of 2 (deutero) 2 (methoxy) 2 (4 substituted phenylsulfanyl) acetophenones 4′ Y PhC
(O)CD[OMe][SPh 4 X] (1a–5a), in solvents of increasing relative permittivity.
Comp.
Y
X
n-C6H14 (ε =
CCl4 (ε = 2.2)
CHCl3 (ε = 4.8)
CH2Cl2 (ε = 9.1)
CH3CN (ε = 38)
1.9)
ν
Pa
ν
P
2νb
P
ν
P
ν
P
ν
P
c
1a
2a
3a
4a
5a
H
H
H
H
H
OMe
Me
H
–
–
–
1698
1683
1697
1683
1698
1684
1698
1685
1697
1684
34.2
65.8
36.3
63.7
38.9
61.1
36.4
63.6
36.0
64.0
3376
3347
3377
3348
3379
3351
3378
3351
3378
3352
44.9
55.1
49.2
50.8
41.9
58.1
38.8
61.2
43.7
56.3
1689
1674
1690
1682
1691
1679
1690
1674
1690
1673
83.4
16.6
70.3
29.7
67.7
32.3
77.1
25.1
71.6
28.4
1689
1673
1691
1680
1692
1679
1692
1678
1692
1677
85.9
14.1
71.2
28.8
73.6
26.4
74.9
25.1
72.4
27.6
1692
1679
1693
1687
1694
1685
1692
1680
1693
1675
89.2
10.8
78.3
21.7
80.1
19.9
87.2
12.8
78.3
21.7
–
1700
1687
1702
1687
34.6
65.4
28.3
71.7
–
–
–
–
c
Cl
–
–
c
Br
–
–
ε Relative permittivity.
a
Intensity of each component of the carbonyl doublet expressed in percentage of integrated absorbance.
First overtone.
Compound not soluble in this solvent.
b
c
stable c3 conformer and the less stable c1 one decreases on going from
In order to check if the inclusion of dispersion energy corrections
would significantly change the properties of the conformers, B3LYP-
D3 and M06-2X calculations along with the 6-31+G(d,p) basis set
were performed for 3, taken as representative for the whole series,
both in gas and in condensed phase (PCM). The molecular geometries,
carbonyl frequencies and relative energy trends in the gas phase, pre-
sented in Table S1, are quite similar to those obtained at B3LYP/6-31
+G(d,p) level (Table 6). Furthermore, computations in condensed
phase (PCM), carried out at the above mentioned levels of theory
(Table S2), lead to the same trend found at B3LYP/6-31+G(d,p) level
(Table 7) which closely reproduces the experimental data (Table 4)
conducting to the same general conclusions.
compound 1 (c3 20.7%, c1 4.5%) to 7 (c3 11.1%, c1 2.1%). Moreover, the
c1 conformers have the highest carbonyl frequency in the series, while
the c2 and c3 ones have similar low values. Therefore, the less intense
carbonyl doublet component at high frequency in the IR spectra for 6
and 7 (Table 3), as well as for 1a–5a (Table 4) in the non-polar solvents
n-C6H14 and CCl4, could be reasonably ascribed to the c1 conformer.
Consequently, the intense component at low frequency corresponds to
both the c2 and c3 conformers. These assignments are fully supported
by the PCM calculations (Table 7), which indicate an increase in the c1
conformer relative abundance and the concomitant decrease in the c2
and c3 relative abundances, as the solvent polarity increases, which is
in agreement with the experimental behavior in solution.
This experimental and theoretical trend can be rationalized taking
into account the suitable geometry of the c1 conformer which favor, in
solvent of increasing polarity, a local stabilizing solvation of the quasi
parallel Cδ+_Oδ− and Cδ+\\Oδ− dipoles.
Calculations at the same level have been performed for the
α methyne deuterated analogue 3a, representative of the 1a–5a series.
As expected, the c1, c2, and c3 conformers of 3a exhibit the same geom-
etries and dipole moments of the corresponding non-deuterated com-
pound 3, with negligible difference in the relative energies.
Furthermore, their νCO(D) frequencies (Table 6) were lowered by ca.
2 cm−1 with respect to the corresponding frequencies in compound 3
due to small mass effects of the substituted deuterium atom.
3.3. Anharmonic Frequencies and Potential Energy Distribution Analyses
Detailed anharmonic frequencies studies were carried out at the
B3LYP/6-31+G(d,p) level for the representative compound 3 in order
to identify the vibrational modes that give rise to the combination
band, which interacts with the νCO mode causing the Fermi resonance.
The results indicate that in the c2 conformer the carbonyl symmetric
stretching excited level can be mixed with the excited vibrational
state of a combination band involving the α hydrogen bending
(δC3\\H4) and a skeletal mode and that this interaction is responsible
for the calculated FR frequency splitting at 1709.0 cm−1 and
1696.1 cm−1, respectively (Table 8). It is noteworthy that twice the
Table 5
Experimental νCO stretching frequencies (νn/m, cm−1) and the unperturbed frequencies (ν0n/m, cm−1) (1–5) along with the highest doublet frequency component [νCO(C-D), cm−1] of the
corresponding deuterated compounds (1a–5a).
Comp.
1
1a
2
2a
3
3a
4
4a
5
5a
Solvent
n-C6H14
νn/m
ν0
νCO(C-D)
νn/m
ν0
νCO(C-D)
νn/m
ν0
νCO(C-D)
νn/m
ν0
νCO(C-D)
νn/m
ν0
νCO(C-D)
n/m
n/m
n/m
n/m
n/m
a
a
a
–
–
–
1708.2
1697.9
1705.1
1695.7
1703.8
1702.3
1701.3
1699.4
–
1708.4
1695.2
1704.3
1694.5
1703.6
1700.0
1700.9
1697.8
–
–
–
–
–
–
–
1700.4
1702.2
CCl4
1704.8b
1694.7b
1702.4c
1697.2c
–
1705.3
1695.5
1702.7
1690.5
1702.5
1692.1
1703.6
1693.0
1701.6
1699.3
1700.6
1692.6
1700.1
1694.5
1702.0
1694.6
1704.4
1696.1
1701.4
1689.4
1701.7
1691.8
1702.8
1692.9
1702.4
1698.2
1699.5
1691.3
1699.4
1694.2
1700.2
1695.6
1697.5d
–
1697.2
–
1697.7
–
1697.6
1690.1
1691.6
1692.4
1697.3
1690.0
1692.0
1692.6
a
a
a
CHCl3
CH2Cl2
CH3CN
–
–
–
a
a
1703.0
1691.2
1702.0
1692.2
–
–
–
–
–
–
–
–
–
–
–
1688.8
–
a
a
a
–
–
–
Note: the values for frequencies are presented as fractionated numbers reflecting in more accurate unperturbed levels.
a
For details see Tables 3 and 4.
Refers to the frequencies of the experimental highest and medium triplet νCO stretching perturbed frequencies.
Refers to the unperturbed frequencies corrected for the shift caused by Fermi Resonance.
Refers to single νCO frequency of the corresponding methyne deuterated compounds.
b
c
d

90
H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
Scheme 2. Bar diagram showing the experimental highest and medium νCO stretching frequency components (νn/m, cm−1) and the calculated unperturbed frequencies (ν0n/m, cm−1) of
compound 4, along with the higher doublet frequency component [νCO(C-D), cm−1] of the corresponding methyne deuterated compound (4a). The relative intensities and the frequencies
of the carbonyl band components were drawn to match the experimental values.
mechanical anharmonicity (2ωeχe) calculated (ca. 19.6 cm−1) is in line
with the experimental findings (ca. 18 cm−1).
The computational results indicate that the FR occurs on the c2 con-
former, which is in disagreement with the experimental data that points
to the higher frequency c1 one. This discrepancy could be related to a
solvent effect (see below).
To better understand the nature of the vibrational modes responsi-
ble for the interaction and how they couple each other, PED analysis
was performed [46] for all conformers of compound 3 (Table 9).
For the three conformers, the vibrational mode 15 assigned as νCO
(Tables 8 and 9) is mainly composed by the νC1_O2 coordinate
(≈88%) in the range of 1761–1736 cm−1
.
Table 6
Relative energy (E, kcal mol−1), dipole moment (μ, D), carbonyl harmonic frequencies (ν, cm−1), selected dihedral angles (°) optimized for the minimum energy conformations of PhC(O)
CH[OMe][SPh 4 X] (1, 2, 4 and 5), 4′ Y PhC(O)CH[OMe][SPh] (6, 3 and 7) and PhC(O)C[D][OMe][SPh] (3a) at the B3LYP/6-31+G(d,p) level and the X-ray geometrical data of 6.
Comp
X
Conf.a
Eb
Pc (%)
μ
νCO
Dihedral anglesd
α
β
φ
δ
α′
100.1
−103.5
−110.8
100.1
−103.3
−110.8
101.0
−101.2
−110.6
101.8
β′
γ′
−72.9
φ′
71.9
75.0
162.7
72.3
75.1
163.8
72.1
75.9
164.4
71.8
76.1
164.2
1
2
4
5
OMe
Me
Cl
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
1.67
0
0.76
1.93
0
1.09
1.72
0
1.14
1.66
0
4.5
74.8
20.7
3.2
83.6
13.2
4.6
83.3
12.1
5.0
82.9
12.1
1.06
3.85
1.75
1.82
2.43
2.47
4.16
3.12
2.58
4.12
3.08
2.53
1758.4
1737.7
1734.5
1758.8
1734.8
1732.8
1758.5
1734.3
1736.3
1758.2
1733.8
1736.6
−25.9
131.4
126.2
−26.0
131.8
126.1
−25.2
134.3
126.4
−24.5
133.9
126.1
−165.0
−162.8
−74.6
−164.4
−162.7
−73.6
−164.4
−161.9
−73.0
−164.9
−161.9
−73.3
71.4
−0.2
−3.8
3.8
1.6
−3.7
3.1
−0.4
−4.7
3.1
−5.8
−3.7
3.5
−51.1
117.3
150.8
−51.5
116.8
153.1
−49.4
111.4
153.0
−48.4
110.7
152.6
−119.7
−83.6
71.2
−120.0
−81.5
73.5
−125.2
−81.4
74.4
−125.9
−81.9
−67.6
−101.7
−70.9
−60.0
−105.1
−56.2
−54.1
−109.0
−54.6
−53.3
−110.2
Br
−101.7
−111.0
1.28
Comp
Y
Conf.a
Eb
Pc (%)
μ
νCO
Dihedral anglesd
α
β
φ
δ
α′
β′
γ′
φ′
6
OMe
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
–
2.10
0
1.10
2.00
0
1.21
2.03
0
1.22
2.19
0
1.21
–
–
–
–
2.4
84.4
13.2
2.9
85.9
11.2
2.8
86.2
11.0
2.1
86.7
11.1
–
2.90
1.33
1.91
2.38
2.25
2.15
2.38
2.25
2.15
1.67
3.23
2.91
–
1751.0
1724.8
1726.2
1759.6
1735.5
1736.2
1758.2
1733.3
1733.9
1760.0
1736.3
1736.5
–
−24.4
133.8
126.9
−24.3
133.3
125.7
−24.3
133.3
125.7
−24.8
133.2
126.2
−164.4
−161.7
−73.6
−164.4
−161.9
−73.3
−164.4
−161.9
−73.3
−164.8
−162.0
−74.3
74.4
−125.4
−82.0
73.7
−123.7
−83.3
73.7
−123.7
−83.3
73.3
−124.2
−83.0
0.3
−3.4
3.3
0.2
−4.2
4.0
0.2
−4.2
4.0
2.2
−3.5
4.0
102.1
−48.7
111.2
152.4
−49.4
113.1
151.2
−49.4
113.1
151.2
−49.5
112.9
151.8
−58.5
71.6
−101.7
−110.1
102.1
−102.1
−111.2
102.1
−102.1
−111.2
101.5
−102.4
−110.9
106.39(15)
−104.5(3)
105.9(4)
−105.1(5)
−53.2
−108.0
−56.0
−54.6
−108.7
−56.0
−54.6
−108.7
−61.3
−55.9
−106.7
75.9
163.8
71.8
75.9
164.1
71.8
75.9
164.1
71.6
76.0
163.4
3
H
3a
7
H(D)e
Cl
6e
1f
2f
4f
X-ray
−18.3(2)
19.8(4)
−18.2(5)
19.3(7)
−171.88(14)
161.9(3)
−164.3(3)
163.9(5)
71.19(12)
−74.1(3)
63.2(4)
−63.7(4)
−27.4(2)
−0.6(4)
4.9(6)
−2.9(8)
−49.65(11)
46.6(3)
−57.9(3)
57.3(5)
−62.41(15)
−101.2(3)
−83.1(4)
80.5(5)
68.20(17)
−76.7(3)
74.4(4)
–
–
–
–
–
–
–
–
–
–
–
–
−75.3(6)
a
Conformer assignment.
Relative energy.
Molar fraction in percentage.
See Scheme 1.
Refers to the deuterated analogue 3a.
From refs. [9–11], respectively.
b
c
d
e
f

H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
91
c1
c2
c3
Fig. 4. Molecular structures of the conformers of 3 obtained at the B3LYP/6-31+G(d,p) level. Adopted colors: H = white, C = grey, O = red, S = yellow.
Table 7
Relative energies (E, kcal mol−1), carbonyl harmonic frequencies (νC_O, cm−1) and dipole moments (μ, D) optimized for different conformers of 1–7 at the B3LYP/6-31+G(d,p) level of
theory including the solvent effect PCM method.
Comp.
X
Conf.a n-C6H14
Eb Pc
CCl4
E
CHCl3
E
CH2Cl2
E
CH3CN
νC_O
μd
P
νC_O
μ
P
νC_O
μ
P
νC_O
μ
E
0
P
νC_O
μ
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
1.14 10.3 1747.5 1.39 1.02 12.4 1745.1 1.50 0.80 26.6 1734.6 2.04 0.06 42.4 1728.2 2.31
70.5 1729.1 4.41 69.3 1727.1 4.53 58.4 1718.7 5.00
0.77 19.2 1723.7 1.84 0.79 18.2 1721.5 1.85 0.47 15.1 1713.2 1.89 0.87 10.7 1708.8 1.92 1.37
61.6 1721.2 2.63
1
OMe
Me
Cl
0
0
0
0
46.9 1713.7 5.20 0.38 32.4 1703.4 4.61
6.0 1704.5 1.96
64.6 1720.7 2.95
50.2 1712.5 3.44 0.44 30.7 1707.7 3.60
7.6 1707.6 3.18 1.55 4.7 1703.1 3.31
50.1 1729.2 5.50 71.6 1722.5 5.81
60.9 1719.4 3.60 0.07 44.3 1715.4 3.75 0.61 25.4 1711.3 3.90
8.0 1715.4 3.20 1.30 5.6 1710.6 3.35 1.87 3.0 1706.0 3.50
1748.9 4.59 1.14 11.4 1746.0 4.72 0.42 30.2 1736.2 5.07 49.9 1730.1 5.38 75.8 1723.2 5.71
61.7 1719.5 3.56 0.07 44.4 1715.5 3.70 0.62 25.0 1711.4 3.84
8.0 1715.6 3.17 1.28 5.7 1710.5 3.23 1.85 3.2 1706.4 3.37
1.43
0
7.2
1747.6 2.11 1.26
9.3
1745.0 2.18 0.52 26.3 1734.4 2.49 0.10 42.2 1727.8 2.70
63.4 1717.1 3.28
0
2
80.0 1726.6 2.83
0
77.7 1724.9 2.93
0
0
1.08 12.8 1722.5 2.75 1.06 13.0 1720.4 2.81 1.07 10.3 1712.1 3.05 1.12
1.26
0
9.5
1748.8 4.57 1.09 12.2 1746.1 4.70 0.40 31.1 1735.4 5.20
77.2 1726.5 3.38
0
0
4
79.6 1728.5 3.32
0
0
1.18 10.9 1726.0 2.88 1.18 10.6 1723.6 2.95 1.20
1.29
0
9.0
0
0
5
Br
80.2 1728.1 3.27
0
78.1 1726.3 3.33
0
1.19 10.8 1726.0 2.85 1.18 10.6 1723.7 2.93 1.21
Comp.
Y
Conf.a n-C6H14
CCl4
E
CHCl3
E
CH2Cl2
CH3CN
Eb
Pc
νC_O
μd
P
νC_O
μ
P
νC_O
μ
E
P
νC_O
μ
E
P
νC_O
μ
6
3
7
OMe
H
c1
c2
c3
c1
c2
c3
c1
c2
c3
1.69
0
4.7
1739.0 3.21 1.52
82.5 1716.7 1.47
6.2
1736.1 3.29 0.80 18.2 1724.3 3.48 0.29 34.6 1717.5 3.69
70.7 1707.9 1.72
0
58.7 1710.3 3.91
0
80.9 1714.7 1.50
0
0
56.6 1703.3 1.85 0.29 35.7 1698.8 1.96
8.8 1698.4 2.57 1.39 5.6 1694.1 2.78
1746.9 2.66 0.74 20.2 1736.2 3.01 0.26 36.2 1729.8 3.30 60.0 1722.7 3.57
79.8 1727.4 2.83 70.2 1719.7 3.15 56.1 1715.2 3.32 0.31 35.3 1710.8 3.49
9.6 1714.7 2.81 1.18 7.7 1710.2 2.95 1.51 4.7 1706.0 3.08
1747.7 2.00 0.69 21.6 1737.1 2.31 0.15 40.8 1730.9 2.51 65.8 1724.0 2.74
83.1 1728.3 3.80 69.0 1721.5 4.12 52.1 1717.9 4.31 0.46 30.1 1713.7 4.50
9.4 1717.2 3.51 1.18 7.1 1712.7 3.61 1.64 4.1 1708.7 3.78
1.10 12.8 1714.6 2.16 1.09 12.9 1711.9 2.22 1.09 11.1 1703.4 2.50 1.10
1.48
0
1.13 12.0 1725.1 2.43 1.12 12.1 1722.9 2.48 1.18
1.68
0
6.6
1749.5 2.58 1.35
8.1
0
81.4 1729.2 2.75
0
0
0
Cl
4.9
1750.2 1.93 1.51
6.5
0
84.7 1730.0 3.71
0
0
0
1.24 10.4 1727.0 3.20 1.23 10.4 1725.2 3.32 1.18
a
Conformer attribution.
Relative energy.
Molar fraction in percentage.
Dipole moment.
b
c
d
Table 8
Harmonic, anharmonic unperturbed and perturbed frequencies (cm−1) of the νCO(15), δCH(32) and the skeletal (72) vibrational modes and mechanical νCO anharmonicity (cm−1) of 3
obtained at the B3LYP/6-31+G(d,p) level in vacuum.
Conformera
c1
c2
c3
Vibrational mode
Harmonic
Anharmonicb
Harmonic
Anharmonicb
Fermic
Harmonic
Anharmonicb
Mode 15 νC_O
Fundamental
Overtone
1760.7
3521.4
0
1728.1
3435.9
20.3
1735.6
3471.2
0
1708.2
3397.3
19.1
1709.0
–
–
1736.2
3472.4
0
1705.9
3392.4
19.4
2ωeχe
Mode 32 δC\\H
Mode 72 skeletal
72 + 32d
1273.1
430.3
1703.5
1235.8
422.8
1658.6
1296.8
441.3
1737.8
1264.6
437.3
1701.9
–
–
1286.8
425.0
1708.7
1262.2
413.2
1675.4
1696.1
a
See Scheme 1 and Table 6.
Unperturbed modes.
Fermi perturbed modes.
Resulting combination mode.
b
c
d

92
H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
Table 9
Potential energy distribution of conformers c1, c2 and c3, of compound 3 obtained at B3LYP/6-31+G(d,p) level.
e,f
Conf.
bEPmNa
Normal modeb
νc
Mixing coordinatesd
%
15 νCO
32 δCH
1760.7
1273.1
ν(O1_C2)
90
25
18
17
13
11
11
87
46
10
30
15
11
88
38
14
23
15
τ(H4\\C3\\O5\\C6)
ν(C2\\C22)
(C22\\C31\\C29\\C27)-τ(C25\\C27\\C29\\C31)
(C3\\C2\\C22)-δ(C3\\O5\\C6) + δ(C22\\C2\\O1) + δ(C22\\C31\\C29)
δ(O5\\C3\\S10)
(S10\\C12\\C16\\C11) + τ(C12\\C14\\C16\\C18)
ν(O1_C2)
(C2\\C22)-ν(C2\\C3)
(H32\\C31\\C29) + δ(H24\\C23\\C25) + δ(H26\\C25\\C27)-δ(H30\\C29\\C31)
(S10\\C11) + ν(C18\\C20) + ν(C11\\C20) + ν(C2\\C22)
δ(C14\\C16\\C18)
(C14\\C16\\C18\\C20)-ω(S10\\C12\\C20\\C11)-τ(C12\\C14\\C16\\C18)
ν(O1_C2)
ν(C2\\C22)
τ(H7\\C6\\O5\\C3)-τ(H4\\C3\\O5\\C6)
c1
59.15
72 skeletal
430.3
τ
δ
ω
15 νCO
32 δCH
1735.6
1296.8
ν
δ
ν
c2
60.83
59.03
72 skeletal
441.3
τ
15 νCO
32δCH
1736.2
1286.8
c3
72 skeletal
425.0
ν(S10\\C11)
δ(C14\\C16\\C18)
a
bEPmN parameter should be between 55.0 and 85.0 [46].
Analyzed Normal mode.
Harmonic frequency.
Vibrational modes coordinates involved on the resulting normal modes frequencies. Plus and minus signs design the phase.
Percentage of the coordinate on the normal mode of vibration.
Coordinates with b10% of contribution on the normal modes are not listed.
b
c
d
e
f
For the c1 conformer, the mode 32 at ν = 1273.1 cm−1 is largely
CCl4, which is almost twice than that for the c1 one, suggests that a
weaker FR should be expected, although not detected experimentally.
As for the c3 conformer, although there is a small Δν value in CH3CN
(−5.1 cm−1), the FR is precluded in any solvent as a consequence of the
lack of mechanical coupling between the δC\\H and the skeletal modes,
as evidenced by the PED analysis.
characterized by a torsion involving the H4\\C3\\O5\\C6 group
(25%) and a stretching of the C2\\C22 bond (18%). On the contrary,
the skeletal mode 72 at ν = 430.3 cm−1 consists of the mixing of four
relevant coordinates, which is an out of plane phenacyl ring vibration
(17%), several complex bending movements that include the methoxy
carbon atom, the carbonyl and the phenacyl ring groups (13%), and fi-
nally two displacements involving the S-phenyl ring (11% for each
mode). The characteristic coordinates suggest that the δC3\\H4 mode
and the skeletal mode could be mechanically coupled along the
C6\\O5\\C3(H4)\\C2\\C22 moiety.
It should be noted that the significant contribution of the H4 hydro-
gen atom to the δC3\\H4 mode implies that its frequency should be
lowered with deuterium substitution, thus inhibiting the Fermi reso-
nance, as verified for compounds 1a–5a. Moreover, the phenacyl ring
contribution to the skeletal mode suggests that the corresponding fre-
quency can differ in the mono- or in the para-substituted derivatives
and, consequently, the combination band frequency could be unsuitable
for the occurrence of the Fermi resonance. That is the case for the para-
substituted compounds 6 and 7, for which the FR was not detected.
Table 9 indicates that the δC\\H bending does not contribute to the 32
mode for the c2 conformer. Nonetheless, this mode mainly involves the
asymmetrical stretching νC2\\C22-νC2\\C3, which may couple mechani-
cally with the νC2\\C22 coordinate of the skeletal mode. On the contrary,
the δC\\H and the skeletal modes in the c3 conformer are composed of co-
ordinates that preclude any mechanical coupling.
It is well known that an increase in the dielectric constant of the
media lowers the νCO frequency [49,53] and increases the combination
band frequency [53,54], mostly due to the δCH mode [26]. Table 10 re-
ports the anharmonic unperturbed frequencies, calculated in vacuum
and in two solvents of distinct polarities, of the three modes responsible
for the FR, together with the frequency difference (Δν) between the
combination band (32 + 72) and the νCO one. The large Δν for the c1
conformer in vacuum and in the polar solvent CH3CN (69.5 and
− 33.7 cm−1, respectively), as well as for c2 in CH3CN (−53.4 cm−1), ac-
counts for the experimental absence of FR. On the contrary, the small Δν
values for c1 in the non-polar solvent CCl4 (−5.8 cm−1) and for c2
(6.3 cm−1 in vacuum) allow its occurrence. These conclusions are in
line with the experimental findings for the c1 conformer in CCl4 (Δν°
= −3.1 cm−1, Table 5) and with the calculations in vacuum for the c2
one. Nevertheless, the Δν value of −11.5 cm−1 for the c2 conformer in
3.4. Short Contacts and Natural Orbital Analyses
Tables 11, 12, and 13 report the ChELPG charges, the interatomic dis-
tances of selected atoms, and NBO energies of selected orbital interac-
tions, respectively, for the three conformers of compounds 1–7
calculated at the B3LYP/6-31+G(d,p) level.
The strongest orbital interactions for the three conformers are the
following: a) the conjugative πC22_C31 → π*C2_O1 at mean energy
values for all the conformers of ca. 22.2 kcal mol−1 for 1–5 and 7 and
ca. 2 kcal mol−1 more for 6 due to the mesomeric effect of the 4′-substit-
uent (Y = OMe); b) the LpO1 → σ*C2\\C3 and LpO1 → σ*C2\\C22 through
Table 10
Solvent effect on anharmonic frequencies (cm−1) of unperturbed vibrational modes and
difference between the carbonyl stretching band and the combination band (cm−1) of 3
obtained by PCM calculations at B3LYP/6-31+G(d,p) level.
Conf.a
Normal mode
Vacuum
CCl4
CH3CN
15 (νC2_O1
32 (δC3\\H4
72 (skeletal)
32 + 72b
Δνc
)
)
1728.1
1235.8
422.8
1658.6
69.5
1708.2
1264.6
437.3
1701.9
6.3
1705.9
1262.2
413.2
1675.4
30.5
1714.7
1296.7
423.8
1720.5
−5.8
1696.5
1268.8
439.2
1697.1
1299.4
431.4
1730.8
−33.7
1676.4
1291.8
438
1729.8
−53.4
1668.1
1257.9
415.3
c1
15 (νC2_O1
32 (δC3\\H4
)
)
c2
72 (skeletal)
32 + 72
Δν
1708
−11.5
1700.5
1256.4
413.4
1669.8
30.7
15 (νC2_O1
32 (δC3\\H4
)
)
c3
72 (skeletal)
32 + 72
Δν
1673.2
−5.1
a
Conformer assignment.
Combination band (sum).
Difference between the νCO and the combination band frequencies [15 − (72 + 32)].
b
c

H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
93
Table 11
ChelpG charges (e) at selected atoms obtained at the B3LYP/6-31+G(d,p) level for 4′ Y PhC(O)CH[OMe][SPh 4 X] (1–7).
Comp.
Y
X
Conf
O(1)[CO]
C(2)[CO]
C(3)
O(5)
S(10)
H(4)[CH]
H(7)[OCH3]
H(32)[o-PhC(O)]
1
H
H
H
H
H
OMe
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
−0.43
−0.48
−0.52
−0.42
−0.47
−0.52
−0.41
−0.47
−0.52
−0.41
−0.47
−0.52
−0.42
−0.47
−0.51
−0.44
−0.49
−0.55
−0.42
−0.48
−0.54
0.20
0.34
0.52
0.22
0.32
0.53
0.22
0.32
0.49
0.25
0.33
0.53
0.27
0.31
0.50
0.27
0.36
0.56
0.26
0.38
0.57
0.64
0.51
0.22
0.59
0.50
0.20
0.56
0.51
0.28
0.50
0.48
0.23
0.47
0.53
0.27
0.55
0.46
0.26
0.50
0.42
0.20
−0.52
−0.51
−0.38
−0.51
−0.52
−0.37
−0.50
−0.52
−0.38
−0.49
−0.50
−0.37
−0.50
−0.52
−0.37
−0.50
−0.51
−0.38
−0.49
−0.49
−0.37
−0.39
−0.37
−0.34
−0.39
−0.37
−0.34
−0.36
−0.36
−0.35
−0.35
−0.34
−0.34
−0.35
−0.35
−0.34
−0.37
−0.35
−0.36
−0.36
−0.34
−0.35
−0.020
−0.026
0.017
−0.005
−0.017
0.019
−0.040
−0.037
0.040
0.039
−0.046
0.038
−0.040
−0.044
0.040
−0.042
−0.041
0.040
−0.038
−0.047
0.033
−0.038
−0.045
0.041
0.129
0.160
0.054
0.123
0.163
0.056
0.111
0.169
0.052
0.180
0.182
0.048
0.101
0.169
0.044
0.125
0.180
0.061
0.133
0.185
0.074
2
3
4
5
6
7
Me
H
0.009
−0.019
−0.008
0.036
−0.010
0.006
Cl
Br
H
0.050
−0.023
−0.007
0.006
−0.007
−0.003
0.022
OMe
Cl
H
−0.039
−0.037
0.039
0.006
0.014
bond coupling interactions [55], almost constant for each conformer in
the series 1–7, at mean energy values of ca. 22.1 kcal mol−1 and
19.4 kcal mol−1, respectively. The c1 and c2 conformers are largely stabi-
lized by the LpO5 → σ*C3\\S10 interaction, which occurs for the suitable β
dihedral angle (ca. 163°), which allows the O(5) lone pair to lie almost
parallel to the antibonding σ*C3\\S10 orbital. This interaction is replaced
in the c3 conformers by the equivalent LpO5 → σ*C2\\C3 one, which has
a negligible effect on the c1 and c2 conformers. The sum of the two inter-
actions stabilizes the c1 and c2 conformers (ca. 17.2 kcal mol−1) more
superjacent orbital interactions [56], whose sum contributes to the con-
formers stabilization by only ca. 2.3 kcal mol−1 for c2 and ca.
1.6 kcal mol−1 for c1 and c3.
Additionally, the quasi-antiperiplanar geometry of the C(3)\\S(10)
and O(5)\\C(6) bonds in the c3 conformers allows the simultaneous
weak interactions σC3\\S10 → σ*O5\\C6 and σO5\\C6 → σ*C3\\S10 for a total
energy of ca. 4.7 kcal mol−1
.
The LpS10 → π*C11_C12 conjugation, whose energy is modulated by
the γ′ dihedral angle values, mainly stabilizes the c1 and c2 conformers
(γ′ ca. 60°) by a mean value of ca. 6.8 kcal mol−1 for compounds 3–7
and ca. 3.3 kcal mol−1 for derivatives 1 and 2, which have an electron-
donor substituent at the para position of the phenylthio ring. The un-
suitable γ′ dihedral angle of ca. 110° for the c3 conformers in the
whole series reduces this interaction to just ca. 1.8 kcal mol−1 in 3–7
and 1.4 kcal mol−1 in 1–2.
than the c3 ones (ca. 10.6 kcal mol−1). The analogous LpS10 → σ*C3 O5
_
and the weak LpS10 → σ*C2\\C3 interactions, which occur when the β′ di-
hedral angles have the proper values to force the S(10) lone pair to be
parallel to the σ*C3\\O5 orbital, stabilize the three conformers to the
same extent (8.5 kcal mol−1). An effect of the unsuitable β and β′ angles
is the weakening of the LPS10 → π*O1_C2 and LPO5 → π*O1_C2
Table 12
Selected interatomic distances (Å) for the minimum energy conformations of 4′ Y PhC(O)CH[OMe][SPh 4 X] (1–7) at the B3LYP/6-31+G(d,p) level and the intramolecular X-ray contacts
for 6.
Comp
Y
X
Conf.a
O1⋯O5
Δlc
O1⋯H4
Δl
O5⋯H32
Δl
S10⋯H7
Δl
S10⋯H32
Δl
1
H
H
OMe
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
c3
c1
c2
2.65
3.38
3.43
2.65
3.39
3.42
2.64
3.40
3.42
2.64
3.40
3.43
2.64
3.40
3.43
2.64
3.40
−0.39
–
–
−0.39
–
–
−0.40
–
–
−0.40
–
–
−0.40
–
–
−0.40
–
–
−0.40
–
3.23
2.42
2.41
3.23
2.41
2.41
3.22
2.41
2.41
3.23
2.41
2.41
3.22
2.41
2.41
3.22
2.41
2.41
3.22
2.42
2.42
3.05
2.72
–
4.00
2.37
2.39
4.01
2.37
2.40
4.02
2.36
2.39
4.00
2.35
2.40
3.99
2.35
2.40
4.03
2.35
2.39
4.03
2.35
2.39
4.02
2.72
–
2.93
2.97
4.22
2.93
2.96
4.23
2.93
2.96
4.23
2.93
2.95
4.23
2.92
2.95
4.23
2.92
2.94
4.23
2.93
2.96
4.23
2.85
3.00
−0.07
−0.03
–
−0.07
−0.04
–
−0.07
−0.06
–
−0.07
−0.05
–
−0.08
−0.05
–
−0.08
−0.06
–
−0.07
−0.06
–
3.16
2.99
2.95
3.12
3.00
2.94
3.10
3.02
2.94
3.14
3.04
2.94
3.25
3.05
2.94
3.10
3.06
2.96
3.08
3.03
2.95
3.60
3.00
–
d
−0.30
−0.31
–
−0.31
−0.31
–
−0.31
−0.31
–
−0.31
−0.31
–
−0.31
−0.31
–
−0.31
−0.31
–
−0.30
−0.30
–
−0.35
−0.33
–
−0.35
−0.32
–
−0.36
−0.33
–
−0.37
−0.32
–
−0.37
−0.32
–
−0.37
−0.33
–
−0.37
−0.33
–
−0.01
−0.05
–
–
−0.06
–
–
−0.06
–
–
−0.06
–
–
−0.06
–
–
−0.04
–
–
−0.05
–
2
3
4
5
6
7
Me
H
H
H
Cl
Br
H
H
OMe
Cl
c3
c1
c2
c3
3.43
2.64
3.39
3.42
H
–
6
OMe
H
X-ray
2.6157(17)
3.04
−0.42
−0.13
ΣvdWrb
a
Conformer designation.
Sum of the van der Waals radii.
Difference between nonbonded atoms distance and the sum of the van der Waals radii.
Interatomic distance larger than the sum of the van der Waals radii.
b
c
d

94
H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
Table 13
Comparison of significant NBO energies (kcal mol−1) of the interacting orbitals for the c1, c2 and c3 conformers of 4′ Y PhC(O)CH[OMe][SPh 4 X] 1–7 at the B3LYP/6-31+G(d,p) level.
Orbitals
X = OMe (1)
c1 c2
20.4
X = Me (2)
X = Cl (4)
X = Br (5)
c3
23.0
c1
c2
23.0
21.0
19.2
15.1
1.7
–
c3
23.1
c1
c2
23.4
20.9
19.0
15.4
1.7
–
c3
23.3
c1
c2
23.4
c3
23.2
π
C22_C31 → π*C2_O1
22.8
21.0
19.2
15.0
1.7
–
20.4
23.5
20.0
15.9
1.8
–
20.8
23.7
19.8
16.0
1.8
–
20.6
23.7
19.8
15.9
1.8
–
LPO1 → σ*C2\\C3
LPO1 → σ*C2\\C22
LPO5 → σ*C3\\S10
LPO5 → σ*C2\\C3
23.4
20.0
15.8
1.8
21.3
19.3
1.2
9.3
2.2
2.5
8.9
–
1.4
1.1
0.7
2.3
1.8
1.0
4.4
–
21.4
19.3
1.3
9.2
2.2
2.5
8.9
–
1.4
1.1
0.6
2.3
1.8
1.0
4.3
–
21.5
19.3
1.3
9.3
2.2
2.6
8.7
–
1.9
1.0
0.5
2.3
1.8
1.0
4.3
–
20.9
19.0
15.4
1.7
–
21.6
19.3
1.3
9.3
2.2
2.6
8.7
–
2.2
1.0
0.6
2.3
1.8
1.0
4.3
–
a
σ
σ
C3\\S10 → σ*O5\\C6
O5\\C6 → σ*C3\\S10
–
–
6.5
2.5
1.7
–
–
–
–
–
–
–
–
LPS10 → σ*C3\\O5
LPS10 → σ*C2\\C3
LPS10 → π*C11_C12
LPO5 → π*O1_C2
LPS10 → π*O1_C2
5.8
2.2
3.3
–
6.5
2.5
2.6
–
1.7
–
2.4
–
5.8
0.5
3.5
–
5.7
2.3
5.5
–
2.2
2.0
2.1
0.9
5.0
–
6.5
2.5
6.5
–
1.5
–
2.4
–
5.7
0.5
3.5
–
4.9
2.5
7.5
–
2.4
1.8
2.1
0.8
5.2
–
6.6
2.4
7.0
–
1.5
–
2.4
–
5.8
–
4.8
2.5
7.8
–
2.4
1.9
2.1
0.8
5.2
–
1.6
–
2.4
–
6.0
0.5
3.5
–
0.8
106.9
2.2
2.0
2.0
0.9
5.0
–
3.3
0.6
0.6
107.6
πC2_O1 → σ*C3\\O5
πC2_O1 → σ*C3\\S10
σC3\\O5 → π*O1_C2
σC3\\S10 → π*O1_C2
LPO1 → σ*C23\\H24
LPO5 → σ*C3\\H4
LPO5 → σ*C31\\H32
LPS10 → σ*C6\\H7
ΣE
3.2
0.9
–
3.3
0.6
0.6
110.2
3.3
0.8
–
3.2
0.7
0.7
112.2
3.4
0.8
–
3.5
–
0.8
111.8
3.2
0.8
0.7
112.6
3.3
0.8
–
0.8
107.9
0.8
112.0
104.5
104.5
105.2
105.5
Orbitals
Y = OMe (6)
Y = H (3)
c1
20.6
23.7
19.9
15.7
1.8
–
Y = Cl (7)
c1
20.3
23.7
20.3
15.9
1.8
–
c1
c2
25.0
20.9
18.7
15.0
1.7
–
c3
c2
23.1
22.9
19.1
15.1
1.7
–
c3
c2
22.9
20.9
19.3
15.0
1.7
–
c3
π
C22_C31 → π*C2_O1
22.8
23.9
19.7
15.7
1.8
24.9
21.6
19.1
1.3
9.1
2.2
2.5
8.8
–
1.7
1.0
0.6
2.3
1.8
1.0
4.3
–
23.1
21.4
19.3
1.3
9.2
2.2
2.5
8.9
–
1.8
1.0
0.6
2.3
1.8
1.0
4.3
–
22.8
21.4
19.5
1.3
9.3
2.2
2.5
8.9
–
1.4
1.1
0.6
2.3
1.8
1.0
4.4
–
LPO1 → σ*C2\\C3
LPO1 → σ*C2\\C22
LPO5 → σ*C3\\S10
LPO5 → σ*C2\\C3
a
σ
σ
C3\\S10 → σ*O5\\C6
O5\\C6 → σ*C3\\S10
–
–
6.7
2.5
5.8
–
1.4
–
2.4
–
5.5
0.5
3.5
–
0.8
113.0
–
–
–
–
–
LPS10 → σ*C3\\O5
LPS10 → σ*C2\\C3
LPS10 → π*C11_C12
LPO5 → π*O1_C2
LPS10 → π*O1_C2
4.9
2.6
7.7
–
2.3
1.9
2.1
0.8
5.1
–
6.6
2.5
6.5
–
1.5
–
2.4
–
5.6
0.5
3.5
–
5.2
2.5
7.2
–
2.3
1.9
2.1
0.9
5.1
–
6.6
2.5
5.0
–
1.5
–
2.4
–
5.7
0.5
3.6
–
5.1
2.5
6.7
–
2.4
1.9
2.1
0.9
5.2
–
πC2_O1 → σ*C3\\O5
πC2_O1 → σ*C3\\S10
σC3\\O5 → π*O1_C2
σC3\\S10 → π*O1_C2
LPO1 → σ*C23\\H24
LPO5 → σ*C3\\H4
LPO5 → σ*C30\\H31
LPS10 → σ*C6\\H7
ΣE
3.2
0.7
0.7
113.3
3.4
0.9
–
3.2
0.7
0.6
113.6
3.4
0.9
–
3.2
0.7
0.7
111.2
3.3
0.9
–
0.8
111.6
0.8
110.6
106.5
105.0
104.7
a
Interaction energy lower than 0.5 kcal mol−1
.
The orbital interactions πC2_O1 → σ*C\\Y and σC\\Y → π*O1_C2 (Y = O
or S), which are stronger for (Y = S) for the favorable overlap of the in-
volved orbitals, are maximized as the α or α′ torsional angles approach
90°. The energies of the σC3\\S10 → π*O1_C2 and the πC2_O1 → σ*C3\\S10
orbital interactions decrease smoothly from the c1 conformer (ca. 5.7
and 2.4 kcal mol−1, respectively) to the c3 one (ca. 4.3 and
1.8 kcal mol−1, respectively), in line with the increase in the α′ dihedral
angle from ca. 100° to 110°. The corresponding σC3\\O5 → π*O1_C2 and
πC2_O1 → σ*C3\\O5 interactions involving the oxygen atom show the op-
posite trend, i.e. an energy increase on going from the c1 conformers to
the c3 ones. The former has a minor effect for all conformers
(≤1.0 kcal mol−1) as their α dihedral angles diverge from the optimal
value of 90°, while the energy of the latter decreases from ca.
2.3 kcal mol−1 for c3 to 1.9 kcal mol−1 for c2 and to 0 kcal mol−1 for c1.
The anomeric effect LPO5 → σ*C3\\H4 stabilizes the three conformers
by ca. 3.4 kcal mol−1 for the whole series. The Oδ−5⋯Hδ+ and
32
Sδ−10⋯Hδδ+7 hydrogen bonds were assessed (Table 12) by the LPO5
→
σ*C31\\H32 and LPS10 → σ*C6\\H7 interactions, at a mean stabilization en-
ergy of ca. 0.8 kcal mol−1 for c2 and c3 and 0.7 kcal mol−1 for c1 and
c2, respectively. The c2 and c3 conformers are electrostatically stabilized
by the contact Oδ_1⋯Hδ+4, ca. −0.30 Å shorter than the sum of the van
Fig. 5. ORTEP view of the asymmetric unit of 6 showing atom labelling and displacement
ellipsoids at the 40% probability level.

H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
95
Fig. 6. ORTEP view of 6 showing the molecules held through C\\H…O interactions, and displacement ellipsoids at the 40% probability level.
der Waals radii (ΣvdWr), while the c3 one is weakly stabilized through
the contact Sδ−10⋯Hδ+32, ca. −0.05 Å shorter than the ΣvdWr.
Fig. 6 shows that compound 6 is stabilized in the crystal through the
following two C\\H…O interactions: C6\\H7…O1i (H…O = 2.60 Å,
C\\H…O = 123°) and C33\\H34…O5ii (H…O = 2.56 Å, C\\H…O =
129°). Symmetry operations: i = x, 1 + y, z and ii = x, 1.5 − y, 0.5 + z.
The sum of the NBO orbital interactions (ΣE) for compounds 3–7
shows that the c1 and c2 conformers are stabilized almost to the same
extent (ca. 112.1 kcal mol−1), while the c3 conformer is less stabilized
by ca. 6.8 kcal mol−1. At first sight, this difference could be ascribed to
the LPO5 → σ*C3\\S10 and LPO5 → σ*C2\\C3 orbital interactions, which
favor the c1 and c2 conformers over the c3 one by just ca.
6.6 kcal mol−1. Nevertheless, the average ΣE for the c1 and c2 con-
formers in the derivatives 1 and 2 is higher by only ca. 3.7 kcal mol−1
than the ΣE for the c3 ones. This minor stabilization is partially related
to the decrease from ca. 6.8 kcal mol−1 (compounds 3–7) to ca.
3.3 kcal mol−1 (1 and 2) of the LPS10 → π*C11_C12 conjugation energy
for c1 and c2, caused by the electron–donor substituent at the para posi-
tion of the phenylthio ring. Analogously, this conjugation energy de-
4. Conclusions
The
conformational
preferences
of
the
2 (methoxy) 2 [(4 substituted) phenylsulfanyl] (4′ substituted)
acetophenones [Y-C6H4\\C(O)\\CH(O\\CH3)\\S\\C6H4\\X] have
been determined by νCO IR analysis and theoretical calculations at the
B3LYP/6-31+G(d,p) level for compounds 1–7. The νCO fundamental
transition region of the spectra for compounds 1–5 (Y = H) suggests
the occurrence of the Fermi resonance, confirmed by the lack of corre-
spondence between the fundamental and first overtone bands profiles
in non-polar solvents. The deuteration of the methyne hydrogen (1a–
5a) inhibits the FR and indicated that the resonance takes place be-
tween the middle frequency νCO component of one conformer and the
highest frequency one assigned to a combination band. The IR spectra
of compounds 1a–5a, 6, and 7 revealed a νCO doublet, whose higher fre-
quency component intensifies as the dielectric constant of the media in-
creases. The theoretical data indicated the presence of three stable
conformers (c1, c2 and c3), which is in good agreement with the exper-
imental spectra. PCM calculations showed that the higher νCO frequency
c1 conformer is stabilized as the solvent dielectric constant increases,
while the relative populations of both the most stable c2 and the c3
one decreases. Since the c2 and c3 conformers νCO frequencies are al-
most coincident, they are both ascribed to the lower frequency compo-
nent of the experimental IR νCO band.
creases from 1.8 kcal mol−1 to 1.4 kcal mol−1 for the c3 conformers.
δ−
The short contact Oδ−(1)
O
CO…
(5)OMe, which uniquely occurs in
the c1 conformer (Table 12), gives rise to a strong repulsive destabilizing
field effect (RFE) [49] between the Cδ+_Oδ− and Cδ+\\Oδ− dipoles that
induces a decrease in the carbonyl oxygen charge (Table 11). The con-
comitant increase in the LPO1 → σ*C2\\C3 delocalization energy by ca.
2.3 kcal mol−1 and the decrease in the πC22_C31 → π*C2_O1 conjugative
one by about the same amount (2.5 kcal mol−1) for the c1 conformers
with respect to the c2 and c3 ones, lead to an increase in the carbonyl
bond order and thus to higher νCO frequency.
As discussed, the strong repulsive effect between the Cδ+_Oδ− and
Cδ+\\Oδ− dipoles significantly destabilizes the c1 conformer. Therefore,
the high stability of the c2 conformer compared to the c1 and c3 ones in
the gas phase (Table 6) is likely related to a balance between the simul-
taneous occurrence of orbital delocalization energies and coulombic re-
pulsive interactions.
Anharmonic and PED calculations for compound 3 indicate that the
FR should occur on the c2 conformer in vacuum and on the c1 one in
non-polar solvents, and that the resonance couples the νCO mode with
a combination band involving the δC3\\H4 methyne hydrogen bending
(mode 32) and a skeletal vibration which includes the phenacyl ring
(mode 72). These results are in line with the experimental data, as the
deuteration of the methyne hydrogen (1a–5a) or the substitution at 4′
position of the phenacyl ring (6 and 7) precludes the FR.
3.5. X-ray Diffraction
Relevant crystallographic information and final refinement parame-
ters for 6 are given in Table 2. The ORTEP view of the asymmetric unit of
6 is shown in Fig. 5 with atom labelling.
Table 4 shows that the geometry of compound 6 in the solid phase is
quite equivalent to those previously studied for compounds 1 [7], 2 [6]
and 3 [5]. In fact, to obtain the largest energy gain due to dipole moment
coupling in the crystal, the molecules assume the least stable conforma-
tion (c1) in the gas phase, as evidenced by the almost coincident values
of the torsional α-ϕ′ angles, whose Cδ+_Oδ− and Cδ+\\Oδ−dipoles are
quasi-parallel. It should be pointed out that the intramolecular contact
S10⋯H7 (hydrogen bond), shorter than the ΣvdWr (Δl = −0.13 Å) as
in the gas phase, also contributes to stabilization of the (c1) conformer
in the solid phase of 6 (Table 12).
The three conformers assume an anti-clinal geometry between the
C\\S and C_O groups and a synperiplanar (c1) and anti-clinal (c2 and
c3) orientation of the C\\O and C_O groups, for the whole series.
The sum of the delocalization energies (NBO) of the selected orbitals
stabilizes the c1 and c2 conformers almost to the same extent. The differ-
ence of ca. 6.8 kcal mol−1 with respect to the c3 conformers could be as-
cribed to the favorable LPO5 → σ*C3\\S10 and LPO5 → σ*C2\\C3 orbital
interactions for c1 and c2. It is noteworthy that the strong repulsive
field effect between the Cδ+_Oδ− and Cδ+\\Oδ− uniquely destabilize
the c1 conformer and increases its νCO frequency. Therefore, the order

96
H.J. Traesel et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 82–97
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by a balance between orbital and electrostatic interactions.
X-ray single crystal analysis of compound 6 reveals that in the solid
state it assumes the least stable c1 conformation found in vacuum. These
molecules are stabilized through a series of intermolecular hydrogen
bonds.
Supplementary data to this article can be found online at https://doi.
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Acknowledgments
[21] Refers to the electron-affinity of the diethyl ether.
The Brazilian authors thank the Fundação de Amparo à Pesquisa do
Estado de São Paulo (FAPESP–2016/21676-0) for financial support of
this research, the Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq) for scholarships to D.N.S.R. and J.V. as well as fel-
lowships to P.R.O. (301180/2013-0) and J.Z-S (303207/2017–5). This
study was financed in part by the Coordenação de Aperfeiçoamento
de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. The
authors also thank Prof. Lucas C. Ducati and Patrick R. Batista for
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