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Borderless Science Publishing 226
Canadian Chemical Transactions Year 2016 | Volume 4 | Issue 2 | Page 226-243
Research Article DOI:10.13179/canchemtrans.2016.04.02.0290
Molecular Structure, Spectroscopic (UV-Vis, FT-IR and
NMR), Conformational Aspects of Some 3t-pentyl-2r,6c-
diphenyl/di(thiophen-2-yl)piperidin-4-ones and their
Oximes: A Comprehensive Experimental and DFT Study
Mariadoss Arockia doss, Govindasamy Rajarajan*, Venugopal Thanikachalam
Department of chemistry, Annamalai University, Annamalainagar 608 002, India
*Corresponding Author, Email: rajarajang70@gmail.com
Received: March 7, 2016 Revised: May 5, 2016 Accepted: May 18, 2016 Published: May 24, 2016
Abstract: The geometries and relative energies of 3t-pentyl-2r,6c-diphenyl/di(thiophen-2-yl)piperidin-4-
ones (PIPs) and their oxime derivatives (PIPOXIs) have been investigated. The structural and
spectroscopic analyses of PIPs and PIPOXIs were made by using B3LYP level with
6-311G(d,p) basis set. The optimized parameters show that the piperidi-4-one ring adopts chair
conformation. Observed chemical shifts were correlated with calculated values using Gauge-independent
atomic orbital (GAIO) density functional theory B3LYP including 6-311+G(2d,p) level theory. Results
from the optimized parameters and NMR chemical shifts show that the syn conformations of 2a and 2b
are thermodynamically more stable with the oxime group anti to pentyl group. The B3LYP infrared
spectra were also computed for the PIPs and PIPOXIs and compared with the experimental spectra. The
NBO analysis helps to discover the charge delocalization and E(2)
energies confirm the occurrence of
intra-molecular charge transfer within the molecule. The electronic transitions states were investigated
computationally by applying TD-DFT/B3LYP method using 6-311G(d,p) level theory and show good
agreement with the experimental data. In addition, HOMO-LUMO and Non-liner optical property were
evaluated by the B3LYP/6-311G(d,p) level theory.
Keywords: PIPs and PIPOXISs, FT-IR, GAIO, hyperpolarizability, NBO, HOMO –LUMO.
1. INTRODUCTION
Piperidin-4-ones make an interesting group of heterocyclic molecules. The compounds of this
family exhibit a broad spectrum of pharmacological properties such as antitumor, antibacterial, antiviral,
antimalarial and antiprotozoal activities [1-4]. Besides these, such compounds have drawn the attention of
photoscientists because of their huge potential in non- linear optical fields [4,5]. Therefore, the biological
importance of piperidin-4-one and its oxime has strongly stimulated the investigation of computational
properties available for these compounds. DFT calculations provide accurate results on systems such as
large organic molecules [6]. Following our studies on thiosemicarbazone and semicarbazone group in
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3t-pentyl-2r,6c-diphenylpiperidin-4-one [7,8], we thought it could be of interest to extend the study to 3t-
pentyl-2r,6c-diphenylpiperidin-4-one (1a), 3t-pentyl-2r,6c-di(thiophen-2-yl)piperidin-4-one (1b), 3t-pentyl-
2r,6c-diphenylpiperidin-4-one oxime (2a), 3t-pentyl-2r,6c-di(thiophen-2-yl)piperidin-4-one oxime (2b), with
the aim of characterizing them from the UV-Vis, IR, NMR spectra and to study their preferred
conformation(s) in gas phase by means of a computational approach. In the present study, DFT/ 6-311G
(d,p) level theory was used to determine the optimized geometry, vibrational wavenumbers in the ground
state, non-linear optical properties, HOMO–LUMO energies and Mulliken charges of the molecules.
Furthermore, NBO analysis of PIPs and PIPOXIs were performed in the same level of theories to
determine the second order perturbation energy in terms of delocalization energy E(2)
. In addition, NMR
chemical shifts were calculated on the optimized geometries using GIAO method at the 6-311+G(2d, p)
level theory.
2. EXPERIMENTAL
2.1. Synthesis of 3t-pentyl-2r,6c-diphenylpiperidin-4-one (1a)
The compound 1a was prepared according to the procedure given in literature with a little
modification [9] in Fig. 1. A mixture of ammonium acetate (0.05 mol), benzaldehyde (0.1mol) and 2-
octanone (0.05 mol) in ethanol were heated to boiling. After cooling, the viscous liquid obtained was
dissolved in ether (250 ml) and shaken with 10 mL concentrated hydrochloric acid, the precipitated
hydrochloride of 3t-pentyl-2r,6c-diphenylpiperidin-4-one was removed by filtration and washed first with
a mixture of ethanol and ether (1:1) and then with ether to remove most of the coloured impurities. The
base was liberated from an alcoholic solution by adding aqueous ammonia and then diluted with water.
The products were recrystallized from alcohol.
2.2. 3t-pentyl-2r,6c-diphenylpiperidin-4-one oxime (2a).
The compound 2a was prepared according to the procedure given in literature with a little
modification [9]. 3t-pentyl-2r,6c-diarylpiperidin-4-one (0.05 mol) and sodium acetate trihydrate (0.15
mol) were dissolved in boiling ethanol and hydroxylamine hydrochloride (0.06 mol) was added. The
mixture was heated to 40ºC and stirred for 3-4 h and then poured into crushed ice. The separated solid
was filtered off and recrystallized from ethanol.
2.3. Synthesis of 3t-pentyl-2r,6c-di(thiophen-2-yl)piperidin-4-one(1b)
The compound 1b was prepared according to the procedure given in literature with a little
modification [9]. Dry ammonium acetate (0.05 mol) was dissolved in 50 mL ethanol and the solution was
mixed with thiophene-2-carboxaldehyde (0.1mol) and 2-octanone (0.05mol) to give a homogenous
mixture. Then the mixture was heated to boiling for about 30 minutes. After cooling, the viscous liquid
was dissolved in ether (300 mL) and shaken with 10 mL concentrated hydrochloric acid and the
hydrochloride of 3t-pentyl-2r,6c-di(thiophen-2-yl)piperidin-4-one obtained was separated by filtration
and washed with a mixture of ethanol and ether (1:1) to remove most of the coloured impurities. The
product was liberated from an alcoholic solution by adding aqueous ammonia and then diluted with water.
The crude sample was recrystalized from ethanol. Yield 75%; m.p.: 138-140 (ºC); MF: C18H23NOS2;
Elemental analysis: Calcd (%): C, 64.82; H, 6.95; N, 4.20; S, 19.23; Found (%):C, 64.91; H, 6.99; N,
4.31; S, 19.30. Mass (m/z): 334 (M+), 336, 335, 334.
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Figure 1. Numbering Pattern of PIPs and PIPOXIs.
2.4. Synthesis of 3t-pentyl-2r,6c-di(thiophen-2-yl)piperidin-4-one oxime (2b)
The compound 2b was prepared according to the procedure given in literature with a little
modification [9]. 3t-pentyl-2r,6c-di(thiophen-2-yl)piperidin-4-one (0.05 mol) and sodium acetate
trihydrate (0.15 mol) were dissolved in boiling ethanol and hydroxylamine hydrochloride
(0.06 mol) was added. The mixture was kept warm at 40ºC, stirred for 3-4 h and then poured into crushed
ice. The separated solid was filtered off and recrystallized from ethanol. Yield 79%; m.p.: 116-118 (ºC);
MF: C18H24N2OS2; Elemental analysis: Calcd (%): C, 62.03; H, 6.94; N, 8.04; S, 18.40; Found (%): C,
61.91; H, 6.96; N, 8.00; S, 18.35. Mass (m/z): 348 (M+), 351, 350, 349, 77.
2.5. Spectral measurements
The UV–Visible spectra of the compounds were recorded in SHIMADZU UV-1800 UV–Visible
Spectrophotometer at room temperature. The FT-IR spectra of the synthesized piperidone and their oxime
were taken in the range 4000-400 cm-1
on an AVATAR-330 FT-IR spectrometer (Thermo Nicolet) using
KBr (pellet form). 1H NMR spectra were recorded at 400 MHz and
13C NMR spectra at 100MHz on a
BRUKER model using CDCl3 as solvent. Tetramethylsilane (TMS) was used as internal reference for all
NMR spectra, with chemical shifts reported in δ units (parts per million) relative to the standard.
2.6. Theoretical background
All calculations were carried out by density functional theory (DFT) on a personal computer
using Gaussian 03W program package [10].The calculations were done with the B3LYP level and the
basis set 6-311G(d,p) [8] was used in the present study to investigate the molecular and vibrational
frequency of molecules in the ground state in order to support and explain the experimental observations.
Mulliken, frontier molecular orbital and Non-linear optics (NLO) were calculated from optimized
geometry of the molecule. The natural population analysis of the compounds has been made by
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performing the NBO analysis at the same level of theory [8]. NMR chemical shifts were calculated on the
optimized geometries using GIAO method at the 6-311+G(2d,p) level theory.
Table 1. Selected bond lengths, bond angles and dihedral angles of PIPs and PIPOXIs.
ATOM B3LYP/6-311G(d,p)
XRDa
B3LYP/6-311G(d,p)
XRDb Bond
length (Å) 1a 1b 2a’ 2a 2b’ 2b
N1-H1 1.015 1.016 0.911 1.015 1.01 1.014 1.013 0.970
N1-C2 1.465 1.468 1.471 1.470 1.475 1.471 1.476 1.486
C2-C3 1.576 1.572 1.549 1.573 1.565 1.574 1.553 1.544
C3-C4 1.527 1.531 1.526 1.527 1.514 1.527 1.520 1.505
C4-C5 1.514 1.518 1.506 1.509 1.515 1.509 1.519 1.503
C5-C6 1.558 1.550 1.532 1.549 1.542 1.550 1.545 1.529
C4-X4 1.213 1.211 1.211 1.280 1.279 1.281 1.278 1.284
Bond angle (
°)
C3-C2-N1 114.20 113.87 109.32 107.74 112.84 113.07 107.93 109.70
C5-C6-N1 112.71 112.43 107.47 108.60 108.60 111.13 108.84 107.30
C3-C4-X12 123.01 122.72 122.01 132.68 116.82 132.73 116.87 118.00
C5-C4-X12 122.09 121.60 121.93 114.17 125.05 114.19 125.72 126.20
C3-C4-C5 114.74 115.61 116.06 112.41 118.06 112.33 117.26 115.60
Dihedral(° )
N1-C2-C3-C7 -175.16 -174.02 -
178.04 -168.90 -180.45 61.09 -179.01
-
178.50
C7-C3-C4-C5 179.72 177.12 177.84 165.15 178.10 -92.77 174.28 177.80
C7-C3-C4-
X12 4.06 0.032 -7.74 25.48 101.20 26.42 141.18 178.50
H3-C3-C4-
X12 124.71 120.22 - -143.86 -15.83 -144.93 -28.04 -51.90
N1-C2-C3-C4 -50.19 -48.68 -51.99 59.18 -46.87 49.77 -53.26 -51.90
N1-C6-C5-C4 52.16 52.99 54.81 55.09 47.68 54.70 52.58 52.01
C3-C4-X12-O13 - - - 1.68 179.24 2.00 177.85 179.10
C5-C4-X12-O13 - - - 170.89 2.12 171.14 2.19 5.60
Energy
(kcal/mol)
-
617992.6219
-
1020556.871
-
652693.1693
-
652691.3095
-
1055264.173
-
1055263.027
a,b- values are taken from Ref. 11and 12, X- O for compounds 1a, 1b and N for 2a, 2a’, 2b and 2b’.
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3. RESULTS AND DISCUSSION
3.1 Geometry optimization
The optimized bond lengths, bond and dihedral angles of PIPs and PIPOXIs were calculated by
B3LYP method with 6-311G (d,p) basis set level theory and the results are listed in Table 1, in
accordance with atom numbering scheme as shown in Fig.1. The optimized structure of compounds is
shown in Fig.2.
1a 1b
2a 2a’
2b 2b’
Figure 2. The optimized structure of compounds
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Table 2. 1H Chemical shift values of PIPs and PIPOXIs.
Atom 1a 1b 2a’ 2a
Expta 2b’ 2b
Expt. DFT Expta. DFT Expt. DFT DFT
H1 1.71 - 1.57 - 1.94 1.55 - 2.23 1.80 -
H6 4.87 4.06 4.36 4.39 5.17 4.17 3.90 4.43 4.20 4.21
H2 4.52 3.70 4.00 4.06 5.09 4.02 3.65 4.64 4.31 3.99
H3 3.68 2.58-
2.65 2.61 2.70 2.63 2.44 2.49 2.15 2.17 2.39
H5A 3.67 2.73 2.95 2.80 3.50 2.29 2.03 2.64 2.26 2.08
H5B 3.18 2.58-
2.65 2.44 2.56 2.32 3.61 3.65 2.43 3.17 3.73
Ar.C-H 8.19-
7.45
7.46-
7.26
7.53-
7.36
7.25-
6.93
8.47-
7.12
8.31-
6.14
7.47-
7.23
7.75-
7.16
7.66-
6.95
7.27-
6.94
-CH2- 2.30-
1.45
1.65-
0.93
1.55-
1.20
1.66-
1.10
1.45-
1.10
1.70-
0.90
1.63-
0.83
1.56-
1.42
1.63-
1.29
1.65-
0.80
-CH3 1.25 0.73 0.70 0.81 0.73 0.73 0.77 1.22 1.02 0.80
H13 - - - - 8.11 8.14 8.25 6.94 7.16 7.79
a- values are taken from Ref. 9.
Signals of aromatic carbons were observed in the range 142.83-126.53 ppm. The upfield signal at
13.95 ppm is assigned to methyl group and other upfield signals in the region 31.90-22.36 ppm are
assigned to four methylene carbons of pentyl side chain at C3. The downfield signal at 209.33 ppm is
assigned to C4. The shifting of signal towards downfield is due to neighboring electronegative oxygen
(O4) [9]. The signal around 67.24 and 61.88 ppm are due to benzylic carbons at C2 and C6 and the
remaining signals at 57.13 and 51.59 ppm are due to C3 and C5, respectively. As can be seen from Figs.
3 and 4, 1a & 1b have same splitting pattern.
Table 3 13
C Chemical shift values of PIPs and PIPOXIs.
Atom 1a 1b 2a’ 2a
Expta 2b’ 2b
Expt. DFT Expta. DFT Expt. DFT DFT
C4 212.31 209.33 210.46 207.53 166.76 160.66 159.54 171.13 163.30 158.24
C2 70.85 67.24 65.41 62.04 59.75 70.64 68.03 66.50 63.00 62.82
C6 65.56 61.88 61.10 58.34 73.18 64.78 60.92 64.25 55.75 56.07
C3 60.52 57.13 59.98 56.85 55.69 50.93 48.52 55.90 50.05 49.87
C5 55.01 51.59 52.53 52.00 45.37 39.20 34.13 46.73 39.18 34.73
Ar-C 149.62-
128.12
142.83-
126.53
151.08-
125.05
146.21-
123.75
153.25-
130.78
152.14-
131.28
143.67-
126.69
161.32-
127.41
154.36-
127.55
123.63-
147.11
-CH2- 37.22-
22.56
31.90-
22.36
36.63-
22.58
31.95-
22.43
32.50-
23.05
32.50-
23.01
31.96-
22.43
35.96-
23.01
35.25-
22.61
32.02-
22.51
-CH3 15.60 13.95 15.38 14.02 15.91 1.38 13.98 15.69 14.50 14.06
a- values are taken from Ref. 9.
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Figure 3. 1H NMR spectrum.
Inspection of the experimental and calculated chemical shifts and their relation with
conformations (2a, 2aꞌ, 2b & 2bꞌ) helps us to make some further considerations on the preferred
conformations. From experimental chemical shifts of 2a, the most downfield signal at 8.25 ppm
is due O-H proton in the oxime group. The aromatic protons appear in the region 7.47 -7.23 ppm.
The one doublet of doublet and a doublet observed at 3.90 and 3.65 ppm are obviously due to
benzylic protons at C6 and C2, respectively. The syn α-axial (H5A) proton is observed at
2.03 ppm and the anti α-proton (H3) at 2.49 ppm. The chemical shift of C5 axial proton is,
however, much less than that of the its equatorial proton of 2a. Hence, the syn
α-(C-H) bond is neither equatorial nor axial. The negative charge on the syn α-carbon C5 is
transmitted to syn α-axial hydrogen to some extent. Therefore, axial hydrogen at C5 is shielded
whereas equatorial hydrogen is deshielded (H5B@ 3.65 ppm) due the oximation.
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Figure 4. 13
C NMR spectrum of 1b.
From the results reported in Table 2, the computed values of 2a match with experimental values,
whereas 2aꞌ chemical shifts ruled out the trend. The multiplets around the region 1.63-0.83 ppm are
assigned to the methylene protons of the pentyl side chain at C3. The upfield triplet at 0.77 ppm is
assigned to methyl protons of the pentyl side chain.
The aromatic carbons could be easily distinguished by their characteristic absorptions around
143.67-126.69 ppm. The upfield signal at 13.98 ppm is assigned to methyl carbon of pentyl group at C3
and other upfield signals in the region 31.96-22.43 ppm are assigned to methylene carbons of pentyl
group at C3. C4 could readily be distinguished from other heterocyclic ring carbons by their characteristic
downfield signals observed around 159.54 ppm and also by their low intensities. The signals at 68.03 and
60.92 ppm are due to benzylic carbons at C2 and C6 and remaining signals at 48.52 and 34.13 ppm are
due to C3 and C5 carbons, respectively. Compound 2b has same splitting pattern of 2a. The use of
experimental chemical shits with computational studies helped us to explicitly determine and assign 2a
and 2b live in solution. Thus, the possibility of compounds 2aꞌ and 2bꞌ are ruled out for further studies.
In order to find a correlation between experimental and calculated chemical shifts, the
experimental data were plotted against calculated chemical shifts and the results shown in Figs S1 and S2.
It can be seen that the calculated chemical shifts of 1a, 1b, 2a and 2b are in accordance with the
experimental chemical shifts, whereas data of 2aꞌ and 2bꞌ show larger deviation.
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3.3 IR study
Vibrational frequency calculation was carried out on the optimized geometries of PIPs and
PIPOXIs. DFT hybrid B3LYP functional methods tend to overestimate the fundamental modes.
Therefore, a scale factor has to be used for obtaining a considerably better agreement with the
experimental data. Thus, the scale factor 0.9608 [16] has been uniformly applied to the DFT/ B3LYP
method.
1b
2b
Figure 5. IR spectra compound 1b and 2b
As seen from Fig. 5, the C=O band [17] is observed at 1714 and 1717 cm-1
for compounds 1a and
1b, respectively. The band around 1715 cm-1
is clearly missing in the spectrum of the oximes and
appearance of new bands at 3435& 1605 and 3433 & 1622 cm-1
are due to the O-H and C=N of 2a and
2b, respectively. The N-H [17] stretching frequency observed at 3315, 3412, 3320 and 3322 cm-1
is due to
1a, 1b, 2a and 2b, respectively. The aromatic C-H stretching frequency [18] is usually observed in the
region 3150-3050 cm-1
and for our compounds it is seen at 3061, 3069, 3062 and 3145 cm-1
for 1a, 1b, 2a
and 2b, respectively. The methyl group [19] C-H stretching is observed at 2920-2956 cm-1
for PIPs and
PIPOXISs. The band at 1492, 1429, 1458 and 1440 cm-1
are due to C=C stretching frequencies of 1a, 1b,
Experimental
B3LYP/6-311G (d,p)
Experimental B3LYP/6-311G (d,p)
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2a and 2b, respectively. The piperidine ring C-N of compounds 1a, 1b, 2a and 2b appeared at 1027,
1039, 1113 and 1094 cm-1
, respectively. The N-O [20] stretching in oxime group in PIPOXISs is
observed around 940 cm-1
. As seen in from Table 4, calculated value of N-O is less than experimental
value. This difference is due to hyperconjucation interaction between oxygen lone pair and adjacent C=N
(Table 5) bond. Hence, calculated N-O stretching frequency is less than the experimental frequency. The
in-plane and out-of-plane bending modes of compounds are observed in the range ~ 750 and ~ 625 cm-1
,
respectively.
Table 4. Experimental and calculated wavenumbers of PIPs and PIPOXIs.
Assignment 1a 1b 2a 2b
Expta.
DFT Expt.
DFT Expt
a.
DFT Expt.
DFT
scaled Intensity scaled Intensity scaled Intensity scaled Intensity
νN-H 3315 3354 2.85 3412 3366 1.74 3320 3406 5.77 3322 3381 2.79
νO-H - - - - - - 3435 3506 7.21 3433 3590 115.72
νArC-H 3061 3065 15.63 3069 3070 8.58 3062 3067 9.79 3145 3118 0.57
νC-H 2920 2952 20.02 2949 2951 99.54 2956 2960 76.58 2934 2963 45.39
νC=O 1714 1710 219.7 1717 1714 211.51 - - - - - -
νC=N - - - - - - 1605 1611 11.31 1622 1645 13.72
νC=C 1492 1480 8.16 1429 1435 7.56 1458 1465 16.79 1440 1441 9.27
νC-N 1027 1035 11.24 1039 1058 50.53 1113 1116 54.39 1094 1098 10.91
νN-O - - - - - - 943 931 100.69 930 919 59.6
βC-H 759 783 1.28 705 745 141.18 752 752 13.39 767 770 18.41
ΓC-H 611 626 22.06 630 656 21.05 603 621 27.93 617 616 63.71
a- Values are taken from Ref. 9
ν- Stretching, β- in-plane bending, Γ-out-of-plane bending.
Additional support for the assignment of bands of the compounds comes from the correlation
between theoretical and experimental wavenumbers. Fig. S3 represents a very good linear correlation
between theoretical and experimental wavenumbers of PIPs and PIPOXIs.
3.4 NBO analysis
NBO analysis offers useful insights into the intramolecular delocalization and donor- acceptor
interactions based on the second order interactions between filled and vacant orbitals. It is better to
understand the importance of ground state stabilization interactions that make the molecules to be stable
in the ground state [21-23]. Hence, NBO analysis has been carried out
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B3LYP/6-311G(d,p) [8] level theory and the results are summarized in Table 5. This table lists the major
second order perturbation interactions along with the corresponding donor and acceptor NBOs. It is
interesting to note that in all the molecules, the lone pair on N, O atoms participate in the stabilization of
PIPs and PIPOXIs through n-σ* interactions contributing nearly 30 - 87 kJ/mol towards stabilization.
Table 5. Second order perturbation interactions obtained at B3LYP/6-311G(d,p) from NBO calculations.
X- O for compounds 1a, 1b and N for 2a and 2b.
Compd
Type Donor (i) ED/e Acceptor(j) ED/e E
2(kJ/mol) Ej-Ei(a.u.) Fi,j(a.u.)
1a
n- σ* LP(1)N1
1.89770 C2-C3
0.04973 38.91
0.62 0.069
n- σ* C5-C6
0.4192 37.66 0.63 0.068
n- σ* LP(2)X12
1.88870 C3-C4
0.07147 86.36 0.66 0.105
n- σ* C4-C5
0.05864 82.93 0.66 0.104
1b
n- σ* LP(1)N1
1.90248 C2-C3 0.028921 34.31 0.63 0.065
n- σ* C5-C6
0.03733 34.27 0.65 0.066
n- σ* LP(2)X12
1.99974 C3-C4
0.07103 86.90 0.65 0.105
n- σ* C4-C5 0.05772 83.47 0.66 0.104
n-π* LP(2)S32
1.99916 C30-C31
0.31622 96.02 0.27 0.071
n-π* C33-C35
0.29693 91.76 0.26 0.069
n-π* LP(2)S40
1.99919 C38-C39
0.31449 93.35 0.27 0.070
n-π* C41-C43
0.29310 91.96 0.26 0.069
2a
n- σ* LP(1)N1
1.90343 C2-H1
0.02784 41.09 0.67 0.073
n- σ* LP(1)X12
1.93447 C4-C5
0.04371 49.71 0.80 0.088
n-π* LP(2)O13
1.90182 C4-N4
0.14440 79.58 0.35 0.074
2b
n- σ* LP(1)N1
1.90766 C2-H1
0.02822 30.21 0.68 0.063
n- σ* LP(1)X12
1.95234 C4-C5 0.03790 40.42 0.85 0.081
n-π* LP(2)O13
1.91992 C4-N4
0.18437 62.46 0.35 0.066
n-π* LP(2)S30
1.98229 C28-C29
0.30303 91.59 0.27 0.070
n-π* C31-C33
0.29525 88.49 0.26 0.068
n-π* LP(2)S38
1.63043 C36-C37
0.31106 93.67976 0.27 0.071
n-π* C39-C41
0.29769 89.49 0.26 0.068
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Yet the predominant stabilizing interactions in sulphur containing compounds (1b & 2b) are n-π*
interactions arising from lone pair of sulphur to the π* of adjacent C-C bond which is more dominant than
the n- σ*. Overall the results highlight the importance of the incorporation of heteroatom towards the
ground state stabilization of molecules. In addition, the sulphur atom in thiophene group (1b &2b) also
takes part in the stabilization through n-π* interactions.
3.5 Mulliken population analysis
The more positive charge on H1 and C4 atoms are due to the highly electronegative nitrogen and
oxygen attached to that hydrogen and carbon atoms. Results from Table 6 show that H5A has higher
positive value than H3 in 2a and 2b. This difference is due to the from that H5A experiences –I effect
from oxygen (O13) atom. This further confirms the anti orientation of O-H group. An increased electron
density (negative charge) can be found at N1, C3 for 1a, 1b and at N1, C3, N12, O13 for 2a, 2b.
Therefore, it can be concluded that nucleophilic and electrophilic substitutions are favored especially of
the above positions of the atoms of PIPs and PIPOXIs.
Table 6. Mulliken atomic charges of PIPs and PIPOXIs.
Atom 1a 1b 2a 2b
N1 -0.33 -0.364 -0.439 -0.441
H1 0.288 0.204 0.206 0.297
C2 0.033 0.102 0.108 0.185
H2 0.122 0.13 0.13 0.155
C3 -0.222 -0.233 -0.236 -0.261
H3 0.119 0.125 0.143 0.308
C4 0.207 0.214 0.299 0.276
X12 -0.299 -0.291 -0.113 -0.295
O13 - - -0.113 -0.103
H4 - - 0.236 0.255
C5 -0.143 -0.177 -0.19 -0.131
H5A 0.13 0.134 0.24 0.271
H5B 0.115 0.126 0.133 0.155
C6 -0.096 0.013 0.004 -0.29
H6 0.128 0.147 0.133 0.174
X- O for compounds 1a, 1b and N for 2a and 2b.
3.6 Absorption spectroscopy
The absorption spectra of the investigated PIPs and PIPOXIs in both gas and solution phases
were computed using TD-DFT/6-311G(d,p) from the optimized geometry calculated at DFT/B3LYP-6-
311G(d,p) method to rationalize the nature of electronic transitions, contributing configurations to the
transitions and charge transfer probability [24,25]. The calculated wavelengths from absorption (Table 7)
excitation energies, main transition configurations and oscillator strengths for the most relevant singlet
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Table 7. Computed and experimental absorption maxima (λmax, nm), Oscillator strength (ƒ) and
electronic excitation energies ( E, eV) of PIPs and PIPOXIs.
H-HOMO; L-LUMO
Molecule State Cal.
λmax(nm)
Expt.
λmax(nm)
Osicillator
Strength (ƒ) E(eV)
Main contributing
configurations
1a
Gas phase
293.6 0.29 4.22 H-1→L(80%)
H-2→L+1 (14%)
269.1 0.15 4.60 H→L (96%)
243.9 0.10 5.08 H→L+1(93%)
Chloroform
294.5 293.0 0.47 4.30 H-1→L(74%)
H-2→L+1 (15%)
269.8 261.0 0.26 4.59 H→L (95%)
239.9 0.14 5.17 H→L+1(89%)
Methanol
296.0 296.0 0.49 4.33 H-1→L(75%)
H-2→L+1 (16%)
269.1 269.0 0.26 4.61 H→L (95%)
238.9 0.13 5.19 H→L+1(87%)
1b
Gas phase
291.9 0.07 4.25 H-3→L(53%)
H-3→L+1 (14%)
263.0 0.54 4.71 H→L (86%)
250.9 0.55 4.94 H-1→L(73%)
Chloroform
297.1 325.5 0.12 4.31 H-3→L(64%)
H-3→L+1 (16%)
267.6 267.0 0.93 4.63 H→L (90%)
253.5 0.67 4.89 H-1→L(79%)
Methanol
294.7 328.5 0.42 4.71 H-3→L(64%)
H-3→L+1 (16%)
268.6 268.5 0.63 4.78 H→L (79%)
253.1 0.02 4.81 H-1→L(80%)
2a
Gas phase
258.6 0.36 4.79 H→L(91%)
257.2 0.05 4.82 H→L+1 (89%)
254.6 0.04 4.87 H→L+2(90%)
Chloroform
260.3 285.0 0.46 4.76 H→L(96%)
257.3 243.0 0.06 4.81 H→L+1 (92%)
255.7 0.02 4.84 H→L+2(93%)
Methanol
262.9 290.0 0.42 4.71 H→L(96%)
259.3 258.5 0.06 4.78 H→L+1 (88%)
257.6 0.02 4.81 H→L+2(90%)
2b
Gas phase
281.1 0.22 4.94 H→L(69%)
H-2→L(19%)
241.2 0.32 5.14 H→L+1 (58%)
238.6 0.86 5.19 H-2→L+2(46%)
H→L(26%)
Chloroform
285.3 306.5 0.25 4.85 H→L(67%)
H-2→L(18%)
246.8 246.0 0.22 5.02 H→L+1 (63%)
239.8 0.17 5.17 H-2→L+2(48%)
H→L(25%)
Methanol
287.9 317.0 0.20 4.80 H→L(70%)
H-2→L(20%)
249.5 249.0 0.18 4.96 H→L+1 (65%)
239.8 0.12 5.17 H-2→L+2(50%)
H→L(28%)
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excited states are summarized in this section. The calculated absorption spectra of compounds are in good
agreement with the experimental results with the largest deviation of 34 nm. A comparison of the
absorption spectra in the gas phase with those in solution phase (chloroform and methanol) shows that
there is a consistent red shift in solution and this is due to solute–solvent interactions (Table 7 & Fig. S4).
In solution, the dominant absorption band of 1a is observed at 296.9 and 294.5 nm for methanol
and chloroform, respectively and if the two phenyl groups are replaced by thiophene rings at C2 and C6
positions (1b), the absorption bands were further red shifted by about ~33 nm. The above results clearly
show that the sulphur atom in thiophene ring significantly increases the wavelength. In the studied
molecules, the dominant band is associated with HOMO-1 → LUMO transition. From Table 7 it is
observed that, in 1a, the experimental band found at 261(chloroform) and 269 (methanol) nm originates
from a HOMO→ LUMO transition (~95%) with π → π* character. For 1b, the same band is observed at
267.0 and 268.50 nm for chloroform and methanol, respectively. The carbonyl group in 1a was replaced
by oxime group in 2a. The dominant absorption bands of 2a molecule is found at 285.0 (chloroform) and
290.0 nm (methanol) and it corresponds to HOMO absorption spectrum. In the studied molecules, the
dominant band is associated with HOMO-1 → LUMO (96%) transition and the small peaks observed at
243.0 and 258.5 nm for chloroform and methanol, respectively are due to the electronic transition
between HOMO → LUMO+1 orbital, while in 2b molecule, the former exhibits the maximum absorption
wavelength at 246.0 (chloroform) and 249.0 (methanol) nm. The red shift with respect to 2a, this is due to
the replacement of phenyl group by thipohene group.
Figure 6. Molecular orbitals and energies for the HOMO and LUMO in gas phase.
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3.7 HOMO-LUMO analysis
The energy gap between HOMO-LUMO characterizes the chemical reactivity and kinetic
stability and it is a critical parameter to determine the electrical transport properties of molecules [26].
Distributions and energy levels of HOMO and LUMO orbitals computed with
B3LYP/6-311G(d,p) level for PIPS and PIPOXIs are shown in Fig. 6.
From Fig. 6, it has been observed that the LUMO is spread over the whole molecule except
pentyl group for 1a and 1b, whereas the electron-cloud distribution of HOMO is largely localized on
piperidine ring for 1a and 1b. In 2a and 2b, HOMO resides over the piperidine ring, whereas LUMO is
located in aromatic group attached at C2 and C6 positions. The HOMO-LUMO gaps lie over a range of
5.40 to 5.56 eV (Fig. 6). The increasing order of HOMO- LUMO energy gap is as follows 1b < 1a < 2a <
2b.
By using the HOMO and LUMO energy values, the quantum chemical reactivity descriptors like
hardness, chemical potential, electronegativity and electrophilicity index as well as local reactivity have
been defined [27]. Pauling introduced the idea of electronegativity as the power of an atom in a molecule
to attract electrons to it. Hardness ( η), chemical potential ( η) and electronegativity (χ) are defined using
Koopman’s theorem as η = (I -A)/2, μ = -(I + A)/2 and χ = (I + A)/2, where I = -EHOMO and A = -ELUMO
are the ionization potential and electron affinity of the molecule. Considering the chemical hardness, large
HOMO– LUMO energy gap suggests a hard molecule and small gap means a soft molecule. Therefore,
harder molecule is less reactive [28]. As can be seen from Table 8,
Table 8. Calculated energy values (eV) of PIPs and PIPOXIs in gas phase.
DFT/B3LYP/6-311G(d,p) 1a 1b 2a 2b
EHOMO -6.331 -6.428 -6.107 -6.493
ELUOMO -0.907 -1.019 -0.575 -0.934
ELUMO-HOMO 5.424 5.409 5.532 5.558
Electrinegativity(χ) -3.619 -3.723 -3.341 -3.713
Hardness(η) 2.712 2.704 2.766 2.779
Electrophilicity index(ψ) 2.415 2.563 2.018 2.481
Softness(s) 136.494 136.885 133.832 133.200
2b is harder and so less reactive than the other compounds. Electronic chemical potential is defined
because of the electronegativity of a molecule [28]. Physically, μ describes the escaping tendency of
electrons from an equilibrium system [28]. The trend in electronic chemical potential for the compounds
is 2a > 1a > 2b > 1b.
The greater the electronic chemical potential, the less stable or more reactive is the isomer. From
the above results we are able to conclude that 2a is more reactive than the other compounds. A
comparison of the calculated electrophilicity values indicates that compound 1b (2.563 eV) is a stronger
nucleophile than the other compounds.
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3.8 Non-linear optical studies
NLO is important property providing key for areas such as telecommunications, signal processing
and optical interactions [29,30]. Therefore, NLO is an important for current research. Some quantum
chemical descriptors which are total static dipole moment (μ), the mean polarizability (α), the anisotropy
of the polarizability (Δα) and first order hyperpolarizability (β) have been used for explaining the NLO
properties in many computational studies [7,9,17]. The quantum chemical descriptors calculated from the
Gaussian output have been explained in detail earlier work [31]. According to Table 9, all values of PIPS
and PIPOXIs are greater than the urea [32]. Therefore, our compounds have NLO properties. Results
from Table 9,
Table 9. Non-linear optical properties of PIPs and PIPOXIs calculated using B3LYP method using
6-311G(d,p) basis set.
NLO behavior 1a 1b 2a 2b
Dipole moment(μ) D 2.92 2.58 3.72 0.57
Mean polarizabilty (α) x10-23
esu 2.13 2.17 2.3 2.23
Anisotropy of the
Polarisabiltiy (Δα) x10-24
esu 2.80 2.30 4.33 1.28
First order polarizabilty (β0) x10-30
esu 0.90 0.92 0.80 1.01
The general ranking of NLO properties should be as follows: 2b > 1b > 1a > 2a. According to this
ranking, molecule 2b is the best candidate for NLO material.when it is compared with similar piperidin-4-
one compounds in the literature, the β0 value PIPS and PIPOXIs are larger than that of (E)-1-(3-methyl-
2,6-diphenyl piperidin-4-ylidene) semicarbazide (β0 = 0.6396 x10-30
esu) [17], 3t-pentyl-2r,6c-
diphenylpiperidin-4-one semicarbazone (β0 = 0.6566 x10-30
esu [8] and less than the 3t-pentyl-2r,6c-
diphenylpiperidin-4-one thiosemicarbazone (β0 = 1.2846 x10-30
esu) [7]. Form the above we concluded that
designing of NLO property of 2b using suitable group. That result may bring up 2b into NLO world.
4. CONCLUSION
A comparison of calculated and experimental geometrical parameters shows that the piperidin-4-
one ring adopts chair conformation. The calculated NMR chemical shifts are in excellent agreement with
the experimental data. Geometrical parameters and NMR chemical shifts helped us to determine the
conformations. The IR spectra were also well reproduced by the B3LYP calculations. Stability of the
molecule arising from hyper-conjugative interaction leading to charge delocalization has been analyzed
using NBO analysis. In addition, Mulliken charge analysis predicts the most reactive parts in the
molecule. The electronic transitions and states were investigated computationally and show good
agreement with the experimental data.
The calculated HOMO and LUMO energies were used to analyze the charge transfer within the
molecule. The calculated dipole moment and first order hyperpolarizability results indicate that the
molecule has a reasonably good nonlinear optical behavior.
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ACKNOWLEDGMENTS
One of the authors, Dr. G. Rajarajan is thankful to UGC [F. No. 42-343/2013 (SR)] for providing
funds to this research study. Mr. M. Arockia doss is thankful to UGC for providing fellowship. The
authors also wish to thank Dr. N. Jayachandramani, former Head, Department of Chemistry,
Pachaiyappa’s college, Chennai-30 for critical suggestions.
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The authors declare no conflict of interest
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