Preparation, properties and infrared spectral studies of N-(p-ethylphenyl)thiobenzohydroxamic acid

Article abstract of DOI:10.1016/j.saa.2007.02.021

The preparation of N-(p-ethylphenyl)thiobenzohydroxamic acid and its spectral properties are described in this paper. The preferred conformation of the acid is investigated by both infrared techniques and theoretical calculations at the DFT level. It is f

Full text of DOI:10.1016/j.saa.2007.02.021

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Spectrochimica Acta Part A 68 (2007) 1362–1369
Preparation, properties and infrared spectral studies of
N-(p-ethylphenyl)thiobenzohydroxamic acid
Rita Kakkar∗, Amita Dua, Sheza Zaidi
Department of Chemistry, University of Delhi, Delhi 110 007, India
Received 27 November 2006; received in revised form 27 February 2007; accepted 27 February 2007
Abstract
The preparation of N-(p-ethylphenyl)thiobenzohydroxamic acid and its spectral properties are described in this paper. The preferred conformation
of the acid is investigated by both infrared techniques and theoretical calculations at the DFT level. It is found that the acid exists in the cis thione
(Z) form, rather than the trans form (E) in the gas phase. Both infrared spectroscopy and theoretical calculations show that this structure is stabilized
by intramolecular hydrogen bonding.
© 2007 Published by Elsevier B.V.
Keywords: Thiobenzohydroxamic acid; IR; DFT; B3LYP; Conformation
1. Introduction
thioacylation of hydroxylamines with dithiocarboxylic acids,
dithiocarboxylic acid esters, or thioacyl chlorides, and the reac-
tionofhydroxamicacidchloridesornitrileoxideswiththiols, the
thioacylation of hydroxylamines with alkyl thiocarboximidates,
the reaction of nitro compounds with thiols, and the S-alkylation
of thiohydroxamic acids.
A few methods have been described in the literature for
the preparation of hydroxamic acids [15–23]. N-monoacyl-N-
phenylhydroxylamines have been prepared by reacting phenyl
hydroxylamines with acylchloride. A weak base is added to
remove the hydrochloric acid formed during the reaction. In
such reactions, N-acylation is preferred over O-acylation. A lit-
tle of N-phenyl-N-diacylhydroxylamine is also formed during
the reaction; this is separated from the monoacyl derivative by
treatment with aqueous ammonia.
Thiohydroxamic acids (N-hydroxythioamides) are an impor-
tant class of compounds. They play various roles in analytical
and biological chemistry, as they can coordinate with metal
ions [1–6], including Fe3+, Ni2+, and Cu2+ to form colored
metal complexes by using their sulfur and oxygen atoms. Thus,
thiohydroxamic acids are utilized in the detection and quan-
titative determination of metals [3]. Also, the tris-chelates of
thiohydroxamic acids and Fe3+ are believed to participate in bac-
terial iron-transport systems [1]. Thiohydroxamic acids and their
metal complexes exhibit antimicrobial activity [7]. They have
been used as polymer regulators [8]. The properties of cyclic
thiohydroxamic acids and their derivatives have been studied
[9,10]. The properties of the thiohydroxamate system have been
reviewed recently [11]. In spite of their biological importance,
very little theoretical work [12–14] is done on this system.
In this work, we have prepared a thiohydroxamic acid and
carried out a determination of its spectral and other properties,
and its structure has been interpreted by these data. Furthermore,
theoretical studies on this compound have also been undertaken
to supplement our findings.
Several methods for the preparation of thiohydroxamic acids
are reported in the literature [24–26]. They have been pre-
pared by treatment of the corresponding hydroxamic acid with
phosphorous pentasulfide in the requisite amount of dioxane at
45–65 ◦C for 9–12 h. The carbonyl oxygen is replaced by sulfur
in thiohydroxamic acids.
Several conventional methods have been developed for the
preparation of thiohydroxamic acids [3]. These include the
∗
Corresponding author. Tel.: +91 11 27666313.
E-mail address: rita kakkar@vsnl.com (R. Kakkar).
1386-1425/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.saa.2007.02.021

R. Kakkar et al. / Spectrochimica Acta Part A 68 (2007) 1362–1369
1363
ona1710Perkin-ElmerFTinfraredspectrophotometer. Elemen-
tal analyses were performed on a Haraeus CHN-Rapid analyzer,
and satisfactory results were obtained for all the compounds
(within 0.04%).
Recognizing the multiple roles of thiohydroxamic acids and
thiohydroximates, we have prepared a thiohydroxamic acid, N-
(p-ethylphenyl)thiobenzohydroxamic acid (N-(4-ethylphenyl)-
N-hydroxythiobenzamide), and describe its structure and
properties in this paper. Thiohydroxamic acids can exist as
thione or thiol tautomers.
2.2. Preparation
The preparation of thiohydroxamic acids involves two stages:
(i) Preparation of the hydroxamic acid.
(ii) Thionation of the hydroxamic acid.
The details of the preparation of the thiohydroxamic acid are
described below.
In addition, E and Z diastereomers of both are possible [13].
However, since the prepared compound is a secondary thiohy-
droxamic acid, and lacks a hydrogen at the nitrogen atom, only
thethionetautomersarepossible. Todistinguishbetweenthetwo
possible conformers, we have employed infrared measurements
and performed theoretical calculations.
2.2.1. Preparation of the hydroxamic acid
2.2.1.1. Preparation of p-ethylphenyl hydroxylamine. Ammo-
nium chloride (1.20 mol), p-nitroethylbenzene (0.15 mol) and
water (85 mol) were taken in a 2-l beaker fitted with a ther-
mometer and a mechanical stirrer. The mixture was stirred
vigorously. Zinc dust (1.1 mol) was added in small amounts.
The reaction being exothermic, the addition of zinc dust was
so adjusted that the temperature did not exceed 55–60 ◦C. The
stirring was further continued for about 20 min for complete
reduction, till the temperature began to fall. The reaction mix-
ture was filtered under suction and the filtrate was saturated
with common salt in a conical flask. It was cooled in an ice
bath for about 45 min to ensure maximum crystallization of
(p-ethylphenyl)hydroxylamine.
2. Experiment
2.1. Materials and apparatus
The glassware for handling the solutions were cleaned with
warm chromic acid, thoroughly washed with distilled water, sub-
sequently rinsed with acetone and dried at about 340 K for more
than 12 h.
Water was distilled successively over acidified potassium
dichromate and alkaline permanganate in an all ‘Corning’ glass
apparatus and then passed through two Cole–Parmer mixed bed
ion exchange columns. It was used within 3 h of its preparation.
The following chemicals were purchased: p-nitroethyl-
benzene (High Pure Chemicals, Delhi), ammonium chloride
(BDH), zinc dust (BDH), sodium hydroxide (Merck), potas-
sium dichromate (S.D. Fine Chem.), potassium permanganate
(Speckpure), DMF (Ranbaxy), silver nitrate (Glaxo), ammo-
nium acetate (BDH), acetone (Ranbaxy) and phosphorous
pentasulphide (S.D. Fine Chem.). All the reagents were A.R.
grade.
2.2.1.2. Preparation of N-(p-ethylphenyl)benzohydroxamic
acid. 0.1 mol of the prepared hydroxylamine in 25 ml of
benzene, water 0.56 mol and sodium bicarbonate 0.02 mol were
taken in a 250 ml Erlenmayer’s flask. Benzoyl chloride, 0.2 mol,
was gradually added to the solution, with constant shaking,
till effervescence ceased. The water layer was kept alkaline to
litmus by the gradual addition of sodium bicarbonate. Addition
of benzoyl chloride changed the colour of the reaction mixture
from yellow to pink near completion of the reaction, but on
shaking, it changed again to yellow. At this stage, before adding
more benzoyl chloride, a drop of the reaction mixture was tested
with Tollen’s reagent. A negative test indicated completion of
the reaction. Approximately 6 h are required for completion of
the reaction. The benzene layer was separated and evaporated
off on a water bath. The residue was purified using aqueous
ammonia. The crude mass was subjected to crystallization from
a benzene-petroleum ether mixture.
The solutions were prepared fresh, by weight, using a Mett-
lar AE-163 electronic balance precise to 10−5 g. All melting
points were determined by the Electrothermal Melting Point
apparatusandareuncorrected. IRspectra(inKBr)wererecorded

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R. Kakkar et al. / Spectrochimica Acta Part A 68 (2007) 1362–1369
Table 1
2.2.2. Thionation of the hydroxamic acid
Elemental analysis and physical characteristics of the prepared compound, N-
(p-ethylphenyl)thiobenzohydroxamic acid
2.2.2.1. Preparation of N-(p-ethylphenyl)thiobenzohydroxamic
acid. 0.02 mol of N-(p-ethylphenyl)benzohydroxamic acid was
made to react with 0.02 mol phosphorous pentasulphide in 40 ml
dioxane (50%). The temperature during the reaction was main-
tained at 45 ◦C for about 10 h. The crude product, obtained from
the filtrate, was crystallized from an ethanol–chloroform mix-
ture.
Property
Shape and color
Molecular mass
Melting point
Yellow flakes
341.352
125 ◦C
Elemental analysis
Required
Found
H
C
N
S
6.264%
74.646%
5.803%
6.236%
74.616%
5.763%
13.285%
13.225%
4.2.1. Infrared spectral characteristics
The infrared spectrum, recorded for the prepared thiohydrox-
amic acid, is available given in the Electronic Supplementary
Information. The molecule has 33 atoms and, therefore, 93
modes of vibration are expected. However, although the most
characteristic bands associated with the hydroxamic acid func-
tional grouping are due to the O–H and C O stretching
vibrations and these can be assigned rather unambiguously; the
same is not true for the thio analogues. Similarly, the C–N and
N–O stretching vibrations and the O–H deformation vibration
can be assigned with less certainty because of overlap by several
other modes of vibrations and because of the non-availability of
systematic data on the assignment of these bands in the infrared
spectra of thiohydroxamic acids.
Of the 93 modes of vibration, we have here analyzed only
the ones that give a clue to the thiohydroxamic acid grouping.
Hydroxamic acids are characterized in the solid state by three
bands between 3300 and 2800 cm−1, a band near 1640 cm−1
(C O), a band near 1550 cm−1 (CNH), a variable intensity band
at 1440–1360 cm−1, and a strong band [32] near 900 cm−1. By
analogy, thiohydroxamic acids must also show similar spectra,
but with the band at 1640 cm−1 replaced by the C S band around
1450 cm−1. However, the C S group does not give rise to as
characteristic a band as the C O group due to the complication
of interaction [33–35]. Due to the greater mass of sulphur, the
C S vibration is expected to occur at considerably lower fre-
quencies than the C O vibration, in the N–O or C–N region.
This means that the C S and attached single bonds can interact,
with the result that more than one band involves C S stretch-
ing. Bands have been assigned as involving C S stretching at
1420–700 cm−1. In the prepared compound, which lacks a NH,
the band due to CNH bending is also expected to be replaced
by a band at lower frequency, corresponding to the CNC group-
ing. A tentative assignment of the bands is given in Table 2,
in which the most intense bands are analyzed. It is found that
there are deviations from the expected values. There is large
conjugation in the molecule under study, and this shifts the
bands.
The product was a yellow colored flaky substance. It was
stable at room temperature. The yield obtained was 5.8 g.
3. Computational details
The DFT calculations were performed with the B3LYP three-
parameter density functional, which includes Becke’s gradient
exchange correction [27], the Lee, Yang, Parr correlation func-
tional [28], and the Vosko, Wilk, Nusair correlation functional
[29].
The geometries were fully optimized with respect to the
energy using the basis set 6-31G(d) and single-point calcula-
tions were performed with the 6-311 + G(d,p) basis set using the
Gaussian 03W suite [30] of programs. In the latter calculations,
the SCF = tight option was used. Zero-point energies, calculated
at the B3LYP/6-31G(d) level and scaled by a factor of 0.9806
[31], were added to the energies calculated with the higher basis
set.
Vibrational analysis was performed to confirm the nature of
the stationary points. For the 6-31G(d) basis set, the harmonic
vibrational frequencies were scaled down by a factor of 0.9614
[31] to account for anharmonicity and other factors.
4. Results and discussion
4.1. Properties of N-(p-ethylphenyl)thiobenzohydroxamic
acid
The following studies were carried out:
(1) Experimental elemental analysis and IR spectral studies.
(2) Theoretical evaluation of the experimental IR spectra.
4.2. Experimental
The expected bands are given on the basis of Jensen and
Nielsen’s [36] interpretation and other considerations.
The assignment of the various bands is briefly discussed
below.
The elemental analysis and physical characteristics of the
prepared N-(p-ethylphenyl)thiobenzohydroxamic acid are given
in Table 1, confirming the composition of the prepared acid.

R. Kakkar et al. / Spectrochimica Acta Part A 68 (2007) 1362–1369
1365
Table 2
broad, whichisusefulfordistinguishingitfromabsorptionpeaks
due, for example, to the phenyl group, which are generally sharp
in shape. Furthermore, this band is very sensitive to complex for-
mation and S-alkylation. Generally it is found between 1500 and
Infrared absorption frequencies for the prepared compound (N-(p-
ethylphenyl)thiobenzohydroxamic acid)
Band
Wavenumbers (cm−1
Found
)
1600 cm−1
.
Expected
In view of the major shift to higher frequencies observed by
S-alkylation, a decisive feature of this band must be the C–N
vibration, the shift thus being explained by the increased double
bond character of the C–N bond following alkylation of the
sulphur atom. However, a full explanation of this band must
obviously be founded on the concept of mixed vibrations, the
extent and type of which seem to be highly dependent on the
environment of the C–N bond. In this case, it is found at lower
frequency, 1348 cm−1, implying that the C–N bond does not
have marked double bond character.
O–H stretch
C–N–C stretch
C–N stretch
N–C–S stretch
N–O stretch
C–S stretch
3270(m)
1599(s)
1348(s)
1138(m)
961(m)
837(w)
683(m)
3700–3100(s)
<1550(s)
1400–1600(s)
1000–1200(m)
900(m)
600–800(w)
700(m)
N–C–S bend
4.2.1.1. O–H stretching vibrations. In the N-arylthiohy-
droxamicacidexaminedhere, thebandduetotheO–Hstretching
vibrations has been assigned at 3270 cm−1. It is well known that
the absorption bands due to O–H stretching vibrations, when
free, appear around 3600 cm−1 [37]; hydrogen bonding shifts
these bands to lower frequencies. It is thus deduced that this
thiohydroxamic acid is involved in strong hydrogen bonding.
This implies its existence in the Z form.
Most of the changes in the O–H stretching vibrations are
mainly due to the ability of the acidic hydrogen of the hydroxyl
group to form hydrogen bonds with electron rich atoms. In
thiohydroxamic acids, the acidic O–H group is placed in very
close proximity of the polar thiocarbonyl group. This situation
is highly favorable for the formation of strong intramolecular
hydrogen bonding.
Thus the formation of a strong hydrogen bond causes a large
shift (of the order of 500 cm−1) in the absorption band to lower
frequencies, and may be ascribed to resonance stabilization. A
consequence of this resonance stabilization should be to lower
the force constant of the carbon–sulfur bond and to increase the
contribution of the single bond form, thereby causing a fall in
the frequency of the C S stretching vibration. That this is true
will be seen in a later section.
The assignment of bands made here is supported by the work
ofHadzi andPrevorsek [32] andothers. Thehydrogenbondingis
of the intramolecular type and is confirmed by “solution studies”
made by other workers.
4.2.1.4. N–C–S stretching vibrations. A band of medium
strength can be found in the 1000–1200 cm−1 range in the spec-
tra of most thioamides and thioureas, with the exception of
symmetrically aromatic disubstituted thioureas. Collard-Charon
and Renson [38] considered this band to be due to the C S
group; however the changes induced on selenation seem to be
much too small to justify this assignment [36]. Further, Jensen
and Nielsen’s deuteration studies showed it to be due chiefly to
NH vibrations in secondary thioamides and most thioureas. Only
in a few compounds, e.g. thioacetamide and thiourea, does the
CS character seem to be recognizable, as this band of such com-
pounds is shifted to lower frequencies on S-alkylation, reflecting
the diminished double bond character of the CS bond following
S-alkylation. However, even in these cases, the shifts are very
small.
For other compounds, a shift, equally small, to higher fre-
quencies is observed on S-alkylation, which shows that this band
also has some C–N character. These observations indicate that a
major contribution to this band might be the symmetrical stretch-
ing vibration of the N–C–S grouping. S-alkylation will affect
the C–N and C–S bond in an opposite way and the net result
may therefore be small. In the present case, the band appears at
1138 cm−1
.
4.2.1.5. N–O stretching vibrations. In N-arylhydroxamic acids
[39], this band appears around 915 cm−1. In aromatic hydrox-
amic acids such as benzohydroxamic acid (BHA) it appears
near 900 cm−1. In several oximes, this band appears at around
950 cm−1. It therefore appears reasonable to look for this band
in the 1000–900 cm−1 region. A weak band at 961 cm−1 may
be attributed to the N–O stretching mode. The band is rather
inconspicuous in the spectrum examined here. This assignment
is supported by Pilipenko et al. [40], who assigned this band
in the spectra of N-phenylbenzohydroxamic acid (PBHA) and
N-phenyl-2-furohydroxamic acid at 915 and 912 cm−1, respec-
tively. Earlier Hadzi and Prevorsek [32] assigned this band at
890 cm−1 in PBHA.
4.2.1.2. C–N–C stretching vibrations. A strong band appears at
1599 cm−1. This has been assigned to the CNC stretching mode.
As stated previously, the CNH stretching vibration at 1550 cm−1
in primary thioamides is replaced by a band at lower frequency
in secondary and tertiary amides. In this case, its occurrence
at a higher frequency, namely 1599 cm−1, signifies that this is
mixed with other modes, probably the NOH bending mode.
Other workers also found a band in this region, which disap-
pears on complexation, implying that it involves the hydroxyl
hydrogen.
4.2.1.3. C–N stretching vibrations. A strong band in the
1400–1600 cm−1 range appears in the spectra of all thioamides,
thioureas, thiohydroxamides, and thiosemicarbazides. This is
a very characteristic band, usually very strong and somewhat
It may be noted that this portion of the infrared spectrum
contains several aromatic and other bands and hence caution
must be exercised in assigning the N–O band. We shall take the
help of theoretical calculations to aid our assignment of bands.

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R. Kakkar et al. / Spectrochimica Acta Part A 68 (2007) 1362–1369
4.2.1.6. C S stretching vibrations. The weak absorption bands
due to thiocarbonyl stretching vibrations are reported to occur
at much lower frequencies, where it is much more susceptible
to coupling effects. Identification of the bands in this crowded
part of the spectrum becomes much more difficult and somewhat
uncertain. The absorption bands in the region 1180–1150 cm−1
have been assigned to the C S stretching vibration in thio-
hydroxamic acids. Mecke and Mecke [41] have reported an
absorption band near 1120 cm−1 in thioamides. Contribution
of C S stretching vibrations have also been reported [33] in the
absorption bands observed at 1099, 955 and 670 cm−1 for sec-
ondary amides. Moreover, the position of the C S stretching
band is much influenced by molecular structure and is gen-
erally shifted to lower frequencies. For carbonyl systems, it
is reported that hydrogen bonding lowers the C O frequency
by 10–45 cm−1. Conjugation of the carbonyl group with C C
lowers the absorption band by about 30 cm−1 for the first con-
jugation. When an aromatic ring is directly attached to the
carbon atom of the carbonyl group, the frequency shift of
the carbonyl group is generally less than that occurring with
a full double bond in conjugation. The substituents of the
aromatic ring and the ring strain also lower the carbonyl fre-
quency. A similar behaviour is expected for thiohydroxamic
acids.
Fig. 1. Z and E forms of N-(p-ethylphenyl)thiobenzohydroxamic acid.
Jensen and Nielsen [36] reported that this band shifts towards
lower frequencies on S-alkylation or formation of metal com-
plexes, reactions which diminish the double-bond character of
the C–S bond. It is found in the range usually cited for the single-
bond C–S vibration (600–800 cm−1) or in a few cases somewhat
higher. The thiohydroxamic acid under study shows an absorp-
tionbandnear837 cm−1 duetothethiocarbonylgroupstretching
vibration.
torsion angles O–N–C4–C3 and S–C–C13–C18 are −35.7◦ and
−47.0◦, respectively, for the E structure.
The Z form is the one suitable for chelation. There is also a
possibility of intramolecular hydrogen bonding in this form. Our
calculations indicate the existence of a hydrogen bond between
the proton of OH and the sulfur of C S, based on the following
evidence:
˚
(i) the distance (2.122 A) between the proton of OH and the
4.2.1.7. N–C–S bending. A band near 700 cm−1 seems typical
for the spectra of most thioamides and thiourea. It has been
assigned to C–N stretching or to a mixture of C S and N–C–N
sulphur atom is much smaller than the sum of the van der
Waals’ radii (ca. 3.05 A) of the hydrogen and sulfur atoms;
˚
(ii) the positive Mulliken charge density (+0.220) on the proton
of the OH group, as well as the high negative charge density
(−0.489) on the sulfur atom;
stretching [42]. This band was located at 683 cm−1
.
4.3. Theoretical calculations
˚
(iii) the C S bond (1.691 A) is longer than the normal C S bond
˚
(1.553 A);
4.3.1. Relative energies
(iv) the O–H· · ·S bond angle of 129.3◦ is higher than the cut-off
We now report the results of theoretical calculations on the
prepared thiohydroxamic acid to interpret the infrared results
reported here, and it was found that the Z stereoisomer is more
stable than E by 5.4 kcal/mol, in agreement with our experimen-
tal studies and calculations for simpler thiohydroxamic acids
[43]. Semi-empirical PM3 calculations, however, predict that
the E form is more stable by 3.9 kcal/mol.
The optimized structures of the Z and E forms are depicted in
Fig. 1. The N–C and C–C bonds connecting the thiohydroxam-
ate moiety with the ethylphenyl and phenyl rings, respectively,
are essentially single bonds, and free rotation about these is
possible. The molecule adopts a conformation in which reso-
nance with the phenyl ring is maintained while, at the same
time, minimizing steric repulsion. It can be seen from the fig-
ure that both structures are nonplanar. In Z, the O–N–C11–C19
dihedral angle is 47.3◦, while that of S–C–C6–C11 is 48.3◦. The
angle (90◦ of arc) for a hydrogen bond [44].
The Z form is thus stabilized due to hydrogen bonding.
4.3.2. Charge density analysis
Table 3 gives a portion of the NBO analysis [45] of the Z
structure. The extent of delocalizaton can be gauged from the
fact that Lewis structures account for only 97% of the resonance
structures. The depletion of electron density in the C–S bond and
formation of a C–N ␲ bond is obvious from the analysis. Most
of the C–C ␲ orbitals are found to have a large population.
Even the high-energy C–N ␲ orbital has a large population.
According to the Lewis representation, the C–N bond should be
single, but delocalization of charge from sulfur gives it double
bond character.
*
*

R. Kakkar et al. / Spectrochimica Acta Part A 68 (2007) 1362–1369
1367
Table 3
depletion of charge density from this bond, the C S bond order
decreases, while the C–N bond acquires double bond character.
Similarly, the O–H bond is weakened due to hydrogen bonding,
whose existence is confirmed by the S–H bond order of 0.126.
The two phenyl rings are connected to carbon and nitrogen, and
the respective bond orders are 1.025 and 0.977, which indicate
that these are essentially single bonds and free rotation is pos-
sible along these bonds. The S–N bond order (0.195) indicates
a hyperconjugative interaction that puts electron density in the
C–N bond, increasing its bond order.
An analysis of the energies of the molecular orbitals of the Z
form reveals interesting facts. The highest occupied molecular
orbital (HOMO) is depicted in Fig. 2, whence it is obvious that
this is concentrated mainly on the sulfur atom, which is thus
electron rich. The NBO analysis had also indicated that the sul-
fur p orbital lone pair is the highest occupied MO, but there are
several C–C ␲ orbitals nearby. The thiohydroxamate grouping
orbitals are also close in energy. Beyond this orbital, there are
several low lying antibonding ␲ molecular orbitals with size-
able populations. Fig. 2 also gives a pictorial description of the
lowest unoccupied molecular orbital (LUMO), and it is clear
that almost all the atoms are involved. Table 3 and Fig. 2 reveal
that the oxygen lone pairs are much lower in energy and hence
most of the properties of thiohydroxamates are dependent on the
presence of sulfur. This accounts for the difference in properties
between hydroxamates and thiohydroxamates.
NBO analysis of the Z conformer of N-(p-ethylphenyl)thiobenzohydroxamic
acid
Energy (eV)
Occupancy
Orbital
−0.326
−0.317
−0.314
−0.286
−0.265
−0.256
−0.252
−0.250
−0.250
−0.249
−0.212
0.019
0.028
0.028
0.031
0.033
0.034
0.299
0.328
0.424
1.93
1.73
1.73
1.78
1.66
1.67
1.65
1.65
1.64
1.66
1.83
0.39
0.31
0.37
0.33
0.34
0.31
0.24
0.23
0.34
n2(O)
␲(C–S)
␲(C–S)
␲(C–N)
␲(C17–C18
␲(C19–C22
␲(C6–C8)
␲(C11–C13
␲(C20–C24
␲(C7–C9)
n2(S)
)
)
)
)
␲*(C17–C18
␲*(C19–C22
␲*(C6–C8)
␲*(C11–C13
␲*(C20–C24
␲*(C7–C9)
␲*(C–S)
)
)
)
)
␲*(C–S)
␲*(C–N)
Table 4
Wiberg bond indices for selected bonds in the Z structure
Bond
Wiberg bond index
A clearer picture emerges from the calculated electrostatic
potential map (mapped on to the total electron density). This is
shown in Fig. 3. It is obvious that the molecule has electron-rich
centers only at the sulfur atoms (red), and to a lesser extent,
the oxygen atom, where a positive charge would be stabilized,
thus facilitating chelation with metal ions. Deprotonation of
O–H should further decrease the negative charge on oxygen and
increase the negative potential in its vicinity.
C–N
C–S
C1–C6
N–O
S–N
H5–O
S–H5
N–C17
1.292
1.465
1.025
1.002
0.195
0.630
0.126
0.977
The C–S ␲ orbitals are degenerate, with low occupancies.
Moreover, the corresponding ␲ orbitals are also occupied,
4.3.3. Vibrational frequencies
*
The calculated vibrational frequencies for the two rotamers
and their assignment are given in Table 5. Because of the
large number, we have included only the important ones.
The schematic spectra for Z and E are plotted in Fig. 4. As
seen from this figure, there is a crowding of frequencies in
the low frequency region (upto ∼1600 cm−1), after which a
few absorptions (notably the O–H stretch) occur in the high
reducing the bond order. The ␲ orbitals are actually centered on
the sulfur (∼65%). Consequently, the corresponding antibond-
ing orbitals are centered on the carbon, and their occupation
shifts the electron density away from the sulfur atom, reducing
the polarity of the C S bond. Table 4 gives the calculated Wiberg
bond orders for selected bonds. As seen from the NBOs, due to
Fig. 2. Pictorial description of the HOMO and LUMO of the Z form of N-(p-ethylphenyl)thiobenzohydroxamic acid.

1368
R. Kakkar et al. / Spectrochimica Acta Part A 68 (2007) 1362–1369
Fig. 3. Electrostatic potential map of N-(p-ethylphenyl)thiobenzohydroxamic
acid. The range of plotted values is −4.26 × 10−2 au (red) to 4.26 × 10−2 au
(violet). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of the article.)
frequency region. Schematically, the two calculated spectra are
in close correspondence to the experimental spectrum, given in
the Electronic Supplementary Information. The notable point
is that neither the two calculated spectra nor the experimental
spectrum depict a band at ∼1450 cm−1 due to the C S stretch.
As we have seen in the last section, this bond is considerably
weakened due to hydrogen bonding and should thus shift to
lower frequencies (∼800 cm−1).
Comparison of the calculated vibrational frequencies for the
Z and E forms with the experimentally observed frequencies in
Table 5 shows that the two observed bands that can be unambigu-
ously assigned to the Z conformer are the (NOH bend + CNC
stretch) and C–S bands at 1599 and 837 cm−1, respectively. The
latter, in particular, is expected at a much higher wavenumber
for the E conformer due to the absence of hydrogen bonding,
which weakens the C–S bond of the Z form. The experimental IR
data, therefore, agrees with the theoretical prediction that Z is the
stable conformer. The other bands cannot be as unambiguously
assigned, partly because of the uncertainty of the empirically
scaled B3LYP/6-31G(d) harmonic frequencies. Differences
between the theoretical and experimental frequencies may also
be attributed to the fact that the calculated values are for the gas
phase, while the experimental values pertain to the solid state.
Fig. 4. Calculated infrared spectrum of the Z and E stereoisomers of N-(p-
ethylphenyl)thiobenzohydroxamic acid.
Nonetheless, the correspondence between theoretical and exper-
imental values is good, and infrared spectroscopy may be gain-
fully employed to distinguish between the two stereoisomers.
Moreover, theoretical calculations indicate that the strong
band at 1599 cm−1 has a strong contribution from the NOH
bend. This agrees with Nagata and Mizukami’s [46] view that
this is a thioamide band.
5. Conclusions
We studied the properties of a prepared thiohydroxamic
acid, N-(p-ethylphenyl)thiobenzohydroxamic acid. These stud-
ies indicate that the stable form in the gas phase is the thione Z
form, because of the existence of intramolecular hydrogen bond-
ing. The infrared spectrum of the prepared thiohydroxamic acid
is in close agreement with the calculated spectrum for the Z form.
Thusbothexperimentalandcomputationalstudiesagreethatthis
thiohydroxamic acid exists predominantly in the Z form. This
study should be helpful in assigning the correct structure to thio-
hydroxamic acids stabilized by resonance with two phenyl rings.
Table 5
Calculated vibrational frequencies (cm−1) for the two formsa
Mode
Observed
frequency
Calculated
Z
E
O–H stretch
NOH bend + CNC stretch
C–N stretch
3270
1599
1348
961
3073(119)
1514(176)
1362(330)
950(50)
3530(26)
1374(172)
1310(103)
976(76)
N–O stretch
C–S stretch
NCS bend
837
683
835(20)
681(21)
1224(79)
681(17)
Appendix A. Supplementary data
a
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.saa.2007.02.021.
See Fig. 1. The numbers in parentheses are the calculated intensities in
km/mol.

R. Kakkar et al. / Spectrochimica Acta Part A 68 (2007) 1362–1369
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