A.M. Mansour, O.R. Shehab / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 263–271
265
occurs with the aliphatic part of the Mcm molecule. Generally, the
carboxylate anion has two strongly couple (CAO) bonds with bond
strengths intermediate between C@O and CAO bonds [25]. The
carboxylate ion gives rise to two bands: a strong asymmetric
stretching band near 1650–1550 cmꢁ1 and a weaker symmetrical
stretching one near 1400 cmꢁ1. The IR spectrum of CuL2 exhibited
Results and discussion
Vibrational assignments
The IR spectrum of Mcm is characterized by a band at
3445 cmꢁ1 assigned to the
m
(OH) mode, which is in agreement
with the scaled value (3453 cmꢁ1) as calculated by HF/6-31G(d).
The bands at 3328, and 3196 cmꢁ1 are attributed to the
ass(NH2)
and ss(NH2) modes, where the theoretical values are established
a broad band at 1650 cmꢁ1 assigned to the
mass(CAO) mode, whereas
the symmetric type is observed at 1393 cmꢁ1 as a weak band. It can
see also that the NH2 group is not participated in the complex
m
m
at 3519, 3404 cmꢁ1 and 3386, 3282 cmꢁ1 for DFT/B3LYP and HF,
respectively. This inconsistency may be explained on the basis that
the calculations were performed in the gas phase, whereas packing
molecules with intermolecular H-bonds (Fig. 1) are treated in the
experimental measurements [8]. The stretching mode of the
carbonyl group is observed at 1681 cmꢁ1, which is lower than
the normal C@O due to the resonance effect and H-bond [14].
formation, since the mass(NH2) and mss(NH2) modes are observed at
higher wave-numbers [16], 3409, and 3202 cmꢁ1 comparing with
those of the free Mcm, while the scaled values are found at 3384,
and 3273 cmꢁ1, respectively, as calculated by DFT method. The lib-
erational modes [8] of the coordinated water molecule are observed
at 870, 652, and 612 cmꢁ1 corresponding to the rocking (
ing (qt) and wagging (q
q
r), twist-
w), respectively. Besides, the new band at
440 cmꢁ1 is attributed to the
(CuAO) mode. Theoretically, the IR
spectrum of the CuL2 shows the ass(H2O), ss(H2O), and ds(H2O)
m
The calculated value of the
m(C@O) mode by HF method shows
deviation of 18 cmꢁ1 (Table S1, Supplementary materials) from
the experimental value due to the negligence of the presence of
C@Oꢂ ꢂ ꢂHAN H-bond. Moreover, the vibrational modes at 1604,
m
m
modes of the coordinated water molecule at 3666, 3555, and
1658 cmꢁ1. The latter vibrations are allocated for the hydrated mol-
ecules at 3638, 3444, and 1661 cmꢁ1, respectively.
1395, and 1253 cmꢁ1 are assigned to
m(C@C) + ds(NH2), m(CAN),
and
m(CAO), respectively. The asymmetrical and symmetrical
1H NMR spectral analysis
stretching modes of CAOAC are observed at 1225, and
1024 cmꢁ1, respectively.
The 1H NMR spectrum of Mcm in d6-DMSO exhibited a sharp
singlet signal at 5.16 ppm (Fig. 3) assigned to OH proton, which
disappears on deuteration reflecting its ionizing nature. The singlet
signal at 3.75 ppm is allocated to the OCH3 group, while the broad
signal at 6.46 ppm is assigned to the ANH2 protons. In addition, the
multiplet signal in the 3.91–4.02 range, with integration equiva-
lent to five protons, is attributed to both the methylene and
methine protons, which is in consistency with the previously re-
ported measurement [5]. Furthermore, in order to present a clear
assignment of the 1H NMR spectrum of the studied drug, a series
of NMR calculations at DFT/B3LYP/6-311 + G(2d,p) and HF/6-
31G(d) level of theory was performed by applying GIAO approach
[18]. The values for the 1H isotropic were referenced to TMS, which
was calculated at the same level of theory. The default PCM model
provided by Gaussain 03 was also tested in order to describe the
influence exerted by solvent (DMSO) on the NMR spectrum of
the given compound. As shown in Table 1, the 1H chemical shifts
of all the protons of Mcm, except the OH and NH2 protons, are in
a good agreement with the theoretically computed values. Results
of linear regression fit between experimental and calculated 1H
chemical shifts performed for the structure tested are also included
in Table 1. The regression coefficient between the calculated and
experimental chemical shifts is improved in presence of DMSO as
a solvent and as calculated by DFT/B3LYP/6-311 + G(2d,p) rather
than HF/6-31G(d) method. However, the calculated values of OH
and NH2 (H28-30) protons remain unacceptable apart from the
experimental values, being obviously that the chemical shifts asso-
ciated with these protons are not correctly described by continuum
model. Therefore, the combination of GIAO method and PCM mod-
el is not sufficient in reproducing the experimental findings for the
hydrogen-bonded protons (H28-30) and specific solute–solvent
interactions are expected to completely explain the NMR spectra
of the studied compound [17,18]. Finally, it is worthy to mention
that the H atom is the smallest of all atoms and mostly localized
on the periphery of molecules and thus its chemical shifts would
be more susceptible to intermolecular interactions in the aqueous
solution comparing with those for other heavier atoms.
For assignments of the CH3 group frequencies theoretically,
nine fundamentals can be associated to each CH3 group [14]. The
bands observed at 2917, and 2860 cmꢁ1 are ascribed to the asym-
metric stretching mode, while that at 2808 cmꢁ1 is attributed to
the symmetric mode as calculated at HF/6-31G(d) level of theory.
Infrared bands established at 1519, 1504, 1488, 1227, 1184, and
221 cmꢁ1 are assigned to the CHo3pb; CHi3pb; CHs3b; CHo3pr; CH3ipr, and
s
CH3 vibrations, respectively. In general, six fundamentals [24]
can be associated to each CH2 group in the studied drug though
the values of the two groups are too close to each other. The bands
observed at 2924, and 2852 cmꢁ1 are ascribed to
m
ass(CH2), while
ass(CH2) of C20,
and C14 atoms, respectively. In addition, the calculated bands at
1530, 1403, 1315, and 1160 cmꢁ1 are assigned to dsCH2,
CH2,
CH2, and CH2 modes, respectively, of both groups.
The comparison between the IR spectra of Mcm and CuL2 (Fig. 2)
those at 2872, and 2789 cmꢁ1 are attributed to
m
x
s
q
revealed the absence of the C@C stretching bands for aromatic ring
in the 1600 and 1450 cmꢁ1 region supporting that the complexation
Electronic spectra, and magnetic susceptibility
The UV–Vis. absorption spectrum of Mcm in acetonitrile shows
Fig. 1. Intermolecular hydrogen bonding stabilizing the crystal packing of
methocarbamol.
two bands at 235, and 275 nm as well as a shoulder at 220 nm. The