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on structure–activity relationships. Apart from other organic com-
pounds, semicarbazones and thiosemicarbazones are also known
to have antiviral, antibacterial and antifungal effects in the field
of medicine, pest control and are used as drugs to cure diseases
[3–7]. In quantitative structure activity relationship (QSAR) stud-
ies, surface area and molecular volume are found to play an impor-
tant role in the overall activity of the biologically active molecule
[8]. Several semicarbazones and its derivatives have proved the
efficiency and efficacy in combating various diseases [9]. Vibra-
tional spectroscopy is a valuable tool for the elucidation of mole-
cular structure and gives a dynamical picture of the molecule.
Vibrational spectroscopy has contributed significantly to the
growth of polymer chemistry, catalysis, fast reaction dynamics,
etc., [10]. The philosophy of computational methods of vibrational
spectroscopy changed significantly after the introduction of Scaled
Quantum Mechanical calculations (SQM).
The present research work mainly focused on the synthesis of
EMDPS and its FT-IR, FT-Raman vibrational spectra characteriza-
tion. To support our experimental investigation the theoretical cal-
culation such as conformational, vibrational and electronic
excitation analysis were studied using B3LYP/6-311++G(d,p). For
the accurate prediction of vibrational assignments the total energy
distribution analysis was carried out. In addition the intra-molecu-
lar charge transfer and non-linear optical activity of the title
molecule also studied.
3. Computational details
The quantum chemical calculations of EMDPS have been per-
formed using the B3LYP/6-311++G(d,p) basis set, using the Gauss-
ian 03 program. The entire calculations were performed at DFT
levels on a Pentium 1 V/3.02 GHz personal computer using Gauss-
ian 03W [11] program package, invoking gradient geometry opti-
mization [11,12]. Initial geometry generated from standard
geometrical parameters was minimized without any constraint in
the potential energy surface at DFT level, adopting the standard
6-311++G(d,p) basis set. The optimized structural parameters were
used in the vibrational frequency calculations at the DFT level to
characterize all stationary points as minima. Then, vibrational
averaged nuclear positions of EMDPS were used for harmonic
vibrational frequency calculations resulting in IR and Raman fre-
quencies together with intensities and Raman depolarization
ratios. In this study, the DFT method (B3LYP/6-311++G(d,p)) was
used for the computation of molecular structure, vibrational fre-
quencies and energies of optimized structures. The vibrational
modes were assigned on the basis of TED analysis using SQM pro-
gram [13]. It should be noted that the Gaussian 03W package able
to calculate the Raman activity. The Raman activities were trans-
formed into Raman intensities using Raint program [14] by the
expression:
1
mi
4
Ii ¼ 10ꢁ12 ꢂ ðm0
ꢁ
miÞ ꢂ ꢂ RAi
ð1Þ
2. Experimental details
where Ii is the Raman intensity, Ai is the Raman scattering activities,
mi is the wavenumber of the normal modes and m0 denotes the
wavenumber of the excitation laser [15]. In order to establish the
stable possible conformations, the conformational space of title
compound was scanned with molecular mechanic simulations. This
calculation was performed with the Spartan 10 program [16]. For
meeting the requirements of both accuracy and computing econ-
omy, theoretical methods and basis sets should be considered.
Density functional theory (DFT) has been proved to be extremely use-
ful in treating electronic structure of molecules. The basis set 6-
311++G(d,p) was used as an effective and economical level to study.
After the most stable conformer of the title compound determined,
geometry optimizations of this conformer have been performed.
2.1. Synthesis of (E)-1-(3-methyl-2,6-diphenylpiperidin-4-
ylidene)semicarbazide[EMDPS]
Dry ammonium acetate (0.1 mol) has been dissolved in 50 ml
ethanol and the solution was mixed with aromatic aldehyde
(0.2 mol) and ethyl methyl ketone (0.1 mol) so as to make a homo-
geneous mixture. Then the mixture was heated to simmering care-
fully and allowed to stand at room temperature overnight. Dry
ether (150 ml) was added followed by concentrated hydrochloric
acid (5 ml) and the precipitated hydrochloride salt was collected
and washed repeatedly with ethanol and ether (1:5) mixture. A
suspension of the hydrochloride salt in acetone was treated with
strong liquid ammonia solution and the free base was obtained
by pouring water. The precipitated base was filtered, dried and
recrystallized from absolute ethanol.
4. Results and discussion
Semicarbazone derivative of 3-methyl-2,6-diphenyl-4-piperi-
done were synthesized by the reaction of 3-methyl-2,6-diphenyl-
piperidin-4-one with semicarbazide hydrochloride. A mixture of
semicarbazide hydrochloride (0.01 mol) and 3-methyl-2,6-diphe-
nyl piperidin-4-ones (0.01 mol) in ethanol (30 ml) was refluxed
for 3 h with continuous stirring. Then the contents were cooled.
The product was obtained by pouring it in ice water then the prod-
uct was filtered, washed with water, vacuum dried and recrystal-
lized from absolute ethanol. The yield of the compound was 80%.
4.1. Conformational stability
The chair conformer of piperidine derivatives are most stable
conformer. Therefore, we neglected other conformations (boat,
envelope or twist boat) for the further calculation. Moreover, it
has two possible chair conformations, which differ in the axial
(A) or equatorial (E) positions of the NAH group [17–19]. Piperi-
dine molecule shows the equatorial form of NAH of chair con-
former as the most stable. Piperidine derivative adopts the NAH
equatorial position of the chair conformer. To find the stable con-
formers, a meticulous conformational analysis was carried out for
the title compound. Rotating bond around the free rotation up to
360° with 10° interval, conformational space of the title compound
was scanned by molecular mechanic simulations and then full
geometry optimizations of these structures were performed by
B3LYP/6-311++G(d,p) method. Results of geometry optimizations
were indicated that the title compound may have at least 27 con-
formers as shown in Fig. S1 (Supporting information). Ground state
energies, zero point corrected energies (Eelect. + ZPE), relative
energies and dipole moments of conformers were presented in
Table 1. From the calculated energies of 27 conformers the
conformer one is the most stable. The optimized structure of
EMDPS is shown in Fig. 1.
2.2. FT-IR spectra and FT-Raman spectra
The FT-IR spectrum of the synthesized piperidone was mea-
sured in the 4000–400 cmꢁ1 region at the spectral resolution of
4 cmꢁ1 using on SHIMADZU FT-IR affinity Spectrophotometer
(KBr pellet technique) made in Japan. The FT-IR spectrum was
recorded in Faculty of Marine Biology, Annamalai University,
parangipettai and only noteworthy absorption levels are listed.
The FT-Raman spectrum of the title compound was recorded on
BRUKER: RFS27 spectrometer operating at laser 100 mW in the
spectral range of 4000–50 cmꢁ1. FT-Raman spectral measurements
were carried out from Sophisticated Analytical Instrument Facility
(SAIF), Indian Institute of Technology (IIT), Chennai.
Dhandapani
