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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: [email protected] 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

Molecular Structure, Spectroscopic (UV-Vis, FT-IR and NMR ...Molecular Structure, Spectroscopic (UV-Vis, FT-IR and NMR), Conformational Aspects of Some 3 t -pentyl-2 r ,6 c - diphenyl/di(thiophen-2-yl)piperidin-4-ones

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Page 1: Molecular Structure, Spectroscopic (UV-Vis, FT-IR and NMR ...Molecular Structure, Spectroscopic (UV-Vis, FT-IR and NMR), Conformational Aspects of Some 3 t -pentyl-2 r ,6 c - diphenyl/di(thiophen-2-yl)piperidin-4-ones

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: [email protected]

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

Page 2: Molecular Structure, Spectroscopic (UV-Vis, FT-IR and NMR ...Molecular Structure, Spectroscopic (UV-Vis, FT-IR and NMR), Conformational Aspects of Some 3 t -pentyl-2 r ,6 c - diphenyl/di(thiophen-2-yl)piperidin-4-ones

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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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