Spectroscopic and quantum chemical electronic structure investigations of 3,4-dihydrocoumarin and 3-methylcoumarin

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515

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Spectroscopic and quantum chemical electronic structure investigations of 3,4-dihydrocoumarin and 3-methylcoumarin M. Arivazhagan a,⇑, R. Kavitha b, V.P. Subhasini c a

Department of Physics, A.A. Government Arts College, Pulivalam Road, Musiri 621 201, India Department of Physics, Saranathan College of Engineering, Trichy 620 012, India c Department of Physics, Jeppiaar Engineering College, Chennai 600 119, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Ab initio and DFT calculation are

compared.  HOMO and LUMO analysis has been

done. 1 13 C chemical shift calculations have been calculated.

 H and

a r t i c l e

i n f o

Article history: Received 10 February 2014 Received in revised form 29 March 2014 Accepted 1 April 2014 Available online 13 April 2014 Keywords: Vibrational spectra HOMO LUMO NMR DHC 3MC

a b s t r a c t A complete vibrational analysis of 3,4-dihydrocoumarin and 3-methylcoumarin have been performed according to SQM force field method based on ab initio and DFT calculation 6-311++G(d,p) basis set and their frequencies are compared. The influences of carbon–oxygen bond and methyl group to the vibrational frequencies of the title compounds have been discussed. The pronounced decrease of the lone pair orbital occupancy and the molecular stabilization energy show the hyperconjugation interaction from the NBO analysis. Calculations of molecular orbital geometry show that the visible absorption maxima of DHC and 3MC correspond to the electron transition between frontier orbitals such as translation from HOMO to LUMO. Gauge-including atomic orbital (GIAO) 1H and 13C chemical shift calculations have been calculated. Area of high, neutral and low electrostatic potential is determined for DHC and 3MC. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Coumarins are found in a large number of natural products. They occur widely as secondary plant metabolites and are known to exhibit numerous interesting biological properties. More than 1800 different natural coumarins have been discovered and ⇑ Corresponding author. Tel.: +91 04312701667. E-mail address: [email protected] (M. Arivazhagan). http://dx.doi.org/10.1016/j.saa.2014.04.001 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

described to date. Most of these coumarins are mono- or deoxygenated on the aromatic ring [1]. Recently, more attention has been drawn towards the less common tri- and tetraoxygenated coumarins, which are shown to exhibit very interesting pharmacological properties such as antibacterial activity [2], antiplatelet aggregation [3] and antileukemia activity [4]. Many natural 5,6,7-trioxygenated coumarins have the capacity to induce cell differentiation in human leukemia U-937 cells, which make them potential lead compounds in the search for differentiation therapeutics [4].

M. Arivazhagan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515

503

Ramoji et al. [5] have extensively studied the vibrational assignments and ab initio studies of 3-acetyl-6-bromocoumarin and 3-acetyl-6-methylcoumarin. Tonannavar et al. [6] have elucidated halogen effect and isotope effect of 6-chloro and 7-chloro-4-bromomethylcoumarin. DFT analysis has been made by Arivazhagan et al. [7] for 7-acetoxy-4-methylcoumarin. Literature survey reveals that to the best of our knowledge no ab initio HF/DFT with 6-311++G(d,p) basis set calculations of 3,4-dihydrocoumarin (DHC) and 3-methylcoumarin (3MC) have been reported so far. It is, therefore thought worthwhile to make a comprehensive vibrational analysis using both experimentally observed IR and Raman wavenumbers. Fig. 3. FT-IR spectrum of 3,4-dihydrocoumarin.

Experimental details The pure sample of DHC and 3MC were obtained from Lancaster chemical company, UK and used as such without any further purification to record FT-IR and FT-Raman spectra. The room temperature Fourier Transform IR spectra of DHC and 3MC were measured in the 4000–400 cm1 region at a resolution of ±cm1 using BRUKER IFS-66V Fourier transform spectrometer equipped with an MCT detector, a KBr beam splitter and globar arc source. The FT-Raman spectrum was recorded on a BRUKER IFS-66V model interferometer equipped with an FRA-106 FT-Raman accessory. The FT-Raman spectrum was recorded in the 3500–50 cm1 stokes region using the 1064 nm line of Nd:YAG laser for the excitation operating at 200 mW power. The reported wave numbers were expected to be accurate within ±1 cm1.

Computational methods The density functional (DFT/B3LYP) at the 6-311++G(d,p) basis set level is adopted to calculate the properties of the DHC and 3MC in this work. All the calculations are performed using GAUSSIAN 09W program package [8] with the default convergence criteria without any constraint on the geometry [9]. Multiple scaling of the force field has been performed by scaled quantum mechanical (SQM) procedure [10,11] the systematic error caused by basis set incompleteness, neglect of electron correlation and vibrational anharmonicity [12]. Normal co-ordinate analysis has been performed to obtain full description of the molecular motion pertaining to the normal modes with MOLVIB program version 7.0 written by Sundius. The natural bonding orbitals (NBOs) calculations are performed using NBO 5.0 program as implemented in the Gaussian 09W [8] package at the DFT/B3LYP/6-311++G(d,p) level in order to understand various second order interactions between the filled orbitals of one subsystem and vacant orbitals of another subsystem, which is a measure of the intermolecular delocalization or

Fig. 1. Optimized molecular structure of 3,4-dihydrocoumarin.

Fig. 4. FT-Raman spectrum of 3,4-dihydrocoumarin.

Fig. 2. Optimized molecular structure of 3-methylcoumarin.

Fig. 5. FT-IR spectrum of 3-methylcoumarin.

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constants and f is a suitably chosen common normalization factor for all the peak intensities.

Results and discussion Molecular geometry

Fig. 6. FT-Raman spectrum of 3-methylcoumarin.

hyper conjugation. Finally the geometry of the title compounds, together with that of deuterated chloroform (CDCl3) is fully optimized. 1H and 13C NMR chemical shifts are calculated with GIAO approach [13,14] by applying B3LYP/6-311G++(d,p) method. Prediction of Raman intensities The Raman activities (Si) calculated with the GAUSSIAN 09 program are subsequently converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering [15–17].

Ii ¼

mi

h

f ðm0  mi Þ4 Si  i mi 1  exp  hc K T b

where m0 is the exciting frequency in cm1, mi the vibrational wave number of the ith normal mode, h, c and kb are the fundamental

The optimized molecular structures of 3,4-dihydrocoumarin (DHC) and 3-methylcoumarin (3MC) are shown in Figs. 1 and 2 respectively. The FT-IR and FT-Raman spectra of DHC and 3MC are shown in Figs. 3–6. The optimized structural parameters bond length, bond angle for DHC and 3MC determined by HF/6311++G(d,p) and DFT/6-311++G(d,p) methods are presented in Tables 1 and 2. From the structural data given in the tables it is observed that the various benzene ring CAC bond distances and the CAH bond lengths of DHC and 3MC are found to be almost same at all levels of calculations. The influence of the substituents on the skeletal molecular parameters seems to be negligibly small. The bond lengths determined from B3LYP method is slightly higher than that obtained from HF method but it yields bond angles in excellent agreement with the HF method and also with the experimental values. The CAC bond distance of the ring varies in the narrow range 1.389–1.540 Å and the largest C@C bond length is 1.540 Å for DHC. Whereas in the case of 3MC the CAC bond length varies from 1.359 to 1.502 and the C3AC12 bond length is 1.502 Å. This is larger than the CAC bond distances of the benzene ring. The bond angle of H14AC4AH15 for DHC and H13AC12AH14 for 3MC is the lowest and is equal to 106.72° and 106.38° respectively. This variation in bond angle depends on the electronegativity of the central atom. If the electronegativity of the central atom decreases, bond angle also decreases. The bond length and bond angle differences between

Table 1 Optimized geometrical parameters of 3,4-dihydrocoumarin obtained by HF and DFT methods with 6-311++G(d,p) basis set.

a

Bond length (Å)

HF/6-311++G(d,p)

DFT/6-311++G(d,p)

Experimental dataa

Bond angle (°)

HF/6-311++G(d,p)

DFT/6-311++G(d,p)

Experimental dataa

O1AC2 O1AC9 C2AC3 C2AO11 C3AC4 C3AH12 C3AH13 C4AH10 C4AH14 C4AH15 C5AC6 C5AC10 C5AH16 C6AC7 C6AH17 C7AC8 C7AH18 C8AC9 C8AH19 C9AC10

1.360 1.392 1.500 1.201 1.532 1.076 1.084 1.507 1.080 1.085 1.386 1.389 1.072 1.389 1.070 1.385 1.070 1.37 1.069 1.384

1.399 1.418 1.508 1.222 1.540 1.087 1.095 1.510 1.090 1.096 1.397 1.400 1.083 1.399 1.081 1.396 1.081 1.391 1.080 1.398

1.402 – – 1.249 1.485 1.080 – 1.511 – – – 1.394 – – 1.060 – – – 1.080 –

C2AO1AC9 O1AC2AC3 O1AC2AO11 C3AC2AO11 C2AC3AC4 C2AC3AH12 C4AC3AH13 C4AC3AH12 C4AC3AH13 H12AC3AH13 C3AC4AC10 C3AC4AH14 C3AC4AH15 C10AC4AH14 C10AC4AH15 H14AC4AH15 C6AC5AC10 C6AC5AH16 C10AC5AH16 C5AC6AC7 C5AC6AH17 C7AC6AH17 C6AC7AC8 C6AC7AH18 C8AC7AH18 C7AC8AC9 C7AC8AH19 C9AC8AH19 C1AC9AC8 C1AC9AC10 C8AC9AC10 C4AC10AC5 C4AC10AC9 C5AC10AC9

123.35 115.89 119.14 124.94 111.41 107.89 107.63 112.39 109.59 107.71 109.79 109.84 109.60 110.61 110.02 106.90 120.82 119.90 119.27 119.92 119.92 120.14 120.09 120.20 119.70 118.80 122.04 119.15 116.97 120.50 122.51 124.04 118.12 117.83

121.75 115.88 118.34 125.76 111.89 107.8 107.74 112.30 109.41 107.47 109.91 109.94 109.41 110.70 110.07 106.72 120.87 120.00 119.11 119.97 119.90 120.12 120.10 120.23 119.66 118.79 122.002 119.20 116.52 120.94 122.51 124.05 118.22 117.72

– – – – – – – 112.38 – – – – – – 111.87 – 120.40 – – 120.10 – – – – – 117.00 – – – – 121.30 – 118.60 –

Exp. Ref. [5].

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M. Arivazhagan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515 Table 2 Optimized geometrical parameters of 3-methylcoumarin obtained by HF and DFT methods with 6-311++G(d,p) basis set.

a

Bond length (Å)

HF/6-311++G(d,p)

DFT/6-311++G(d,p)

Experimental dataa

Bond angle (°)

HF/6-311++G(d,p)

DFT/6-311++G(d,p)

Experimental dataa

O1AC2 O1AC9 C2AC3 C2AO11 C3AC4 C3AC12 C4AC10 C4AH16 C5AC6 C5AC10 C5AH17 C6AC7 C6AH18 C7AC8 C7AH19 C8AC9 C8AH20 C9AC10 C12AH13 C12AH14 C12AH15

1.364 1.373 1.470 1.206 1.337 1.499 1.452 1.072 1.380 1.396 1.071 1.395 1.070 1.382 1.070 1.382 1.068 1.385 1.081 1.081 1.080

1.416 1.391 1.464 1.229 1.359 1.502 1.442 1.084 1.389 1.410 1.082 1.406 1.081 1.393 1.081 1.393 1.080 1.406 1.092 1.092 1.089

1.310 – 1.145 – 1.392 – 1.422 – – – 1.075 – – – – 1.406 – – – 1.074 –

C2AO1AC9 O1AC2AC3 O1AC2AO11 C3AC2AO11 C2AC3AC4 C2AC3AC12 C4AC3AC12 C3AC4AC10 C3AC4AH16 C10AC4AH16 C6AC5AC10 C6AC5AH17 C10AC5AH17 C5AC6AC7 C5AC6AH18 C7AC6AH18 C6AC7AC8 C6AC7AH19 C8AC7AH19 C7AC8AC9 C7AC8AH20 C9AC8AH20 O1AC9AC8 O1AC9AC10 C8AC9AC10 C4AC10AC5 C4AC10AC9 C5AC10AC9 C3AC12AH13 C3AC12AH14 C3AC12AH15 H13AC12AH14 H13AC12AH15 H14AC12AH15

124.44 116.86 118.67 124.46 119.16 116.25 124.57 122.31 119.82 117.86 120.36 120.30 119.33 119.94 120.06 119.98 120.55 119.87 119.57 118.53 122.21 119.25 118.13 119.59 122.27 124.05 117.62 118.31 110.97 110.96 110.58 106.96 108.61 108.61

122.85 116.77 117.44 125.78 119.82 116.27 123.89 122.52 119.46 118.01 120.50 120.44 119.05 120.06 120.03 119.90 120.57 119.88 119.54 118.69 122.11 119.18 117.82 120.06 122.11 123.99 117.95 118.04 111.00 111.00 110.90 106.38 108.69 108.69

122.14 – – – 119.21 – – 122.14 119.54 – 120.51 120.45 119.95 120.02 119.47 – – – – – – – 119.99 120.40 – – 117.00 – 111.87 – – – – –

Exp. Ref. [7].

HF/6-311++G(d,p) DFT/6-311++G(d,p)

1.6

HF/6-311++G(d,p) DFT/6-311++G(d,p) 1.5

1.5

1.4

Values (Å)

Value (Å)

1.4

1.3

1.3

1.2

1.2

1.1

1.1

1.0

1.0 C3 – C4

C4 – H15

C6 – H17

C9 – C10

Fig. 7. Bond length differences between theoretical (HF and DFT) approaches of 3,4dihydrocoumarin.

theoretical (HF and DFT) approaches of DHC and 3MC are shown in Figs. 7–10. Comparative graph of computed frequencies (HF and DFT) with experimental frequencies of DHC and 3MC are shown in Figs. 11 and 12 respectively (see Tables 3–6). Vibrational spectral analysis CAH vibrations The hetero aromatic structure shows the presence of CAH stretching vibration in the region 3100–3000 cm1 which is the

C3 – C4

C5 – C10

C7 – H19

C12 – H14

Fig. 8. Bond length differences between theoretical (HF and DFT) approaches of 3methylcoumarin.

characteristic region for the ready identification of CAH stretching vibration [18]. In the FT-IR spectrum of 3,4-dihydrocoumarin and 3-methylcoumarin the bands observed at 3090, 3030, 3000, 2980, 2920, 2890 cm1 and 3100, 3084, 3005, 2950 cm1 respectively are assigned to CAH stretching vibrations of aromatic ring and the counter part of the Raman bands are observed at 3070, 2980, 2890, 2820 cm1 and 3070, 3000, 2950 cm1 respectively [19]. The TED corresponding to these vibrations is a pure mode with the contribution of 98%. The CAH in-plane bending

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M. Arivazhagan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515

HF/6-311++G(d,p) DFT/6-311++G(d,p)

HF/6-311+G(d,p) DFT/6-311+G(d,p)

128

3500 126

Computed Frequency (cm-1)

124 122

Value (Å)

120 118 116 114 112

3000

2500

2000

1500

1000

110 500 108 106

0 0 C2–C3–C4 H12–C3-H13 C10–C4–H15 C5–C6–C7

500

1000

1500

2000

2500

3000

3500

C8–C7–H18 C1–C9–C10

Experimental Frequency (cm-1) Fig. 9. Bond angle differences between theoretical (HF and DFT) approaches of 3,4dihydrocoumarin.

HF/6-311++G(d,p) DFT/6-311+G(d,p)

128

Fig. 12. Comparative graph of computed frequencies (HF and DFT) with experimental frequencies of 3-methylcoumarin.

Table 3 Definition of internal coordinates of 3,4-dihydrocoumarin.

126

No. (i)

Symbol

Type

Definition

124

1–8

Ri

CAH

9–11 12–20

Si

li

CAO CAC

21–26

bi

Ring1

27–32

bI

Ring2

33–48

ai

CACAH

49, 50

ti

OACAO

C3AH12, C3AH13, C4AH14, C4AH15, C5AH16, C6AH17, C7AH18, C8AH19 C9AO1, C2AO1, C2AO11 C2AC3, C3AC4, C4AC10, C10AC5, C5AC6, C6AC7, C7AC8, C8AC9, C9AC10 O1AC2AC3, C2AC3AC4, C3AC4AC10, C4AC10AC9, C10AC9AO1, C9AO1AC2 C9AC10AC5, C10AC5AC6, C5AC6AC7, C6AC7AC8, C7AC8AC9, C8AC9AC10 C2AC3AH12, C2AC3AH13, C4AC3AH12, C4AC3AH13, C3AC4AH14, C3AC4AH15, C10AC4AH14, C10AC4AH15, C10AC5AH16, C6AC5AH16, C5AC6AH17, C7AC6AH17, C6AC7AH18, C8AC7AH18, C7AC8AH19, C9AC8AH19 O1AC2AO11, C3AC2AO11

122 120

Value (Å)

118 116 114 112 110 108 106 104 C2–C3–C4 C10–C4-H16 C5–C6–H18

C7–C8–C9 C8–C9–C10 C3–C12–H14

Fig. 10. Bond angle differences between theoretical (HF and DFT) approaches of 3methylcoumarin.

Out-of-plane bending 51–58 pi

CACAH

Computed Frequency (cm-1)

HF/6-311++G(d,p) DFT/6-311++G(d,p) 3500

59

xi

OACAO

3000

Torsion 60–71

ti

sRing

72, 73

ti

Butterfly

2500 2000 1500

C2AC4AC3AH12, C2AC4AC3AH13, C3AC10AC4AH14, C3AC10AC4AH15, C10AC6AC5AH16, C5AC7AC6AH17, C6AC8AC7AH18, C7AC9AC8AH19 O1AC3AC2AO11 O1AC2AC3AC4, C2AC3AC4AC10, C3AC4AC10AC9, C4AC10AC9AO1, C10AC9AO1AC2, C9AO1AC2AC3, C9AC10AC5AC6, C10AC5AC6AC7, C5AC6AC7AC8, C6AC7AC8AC9, C7AC8AC9AC10, C8AC9AC10AC5 C8AC9AC10AC4, C5AC10AC9AO1

For numbering of atom refer Fig. 1. 1000 500 0 0

500

1000

1500

2000

2500

3000

3500

Experimental Frequency (cm-1) Fig. 11. Comparative graph of computed frequencies (HF and DFT) with experimental frequencies of 3,4-dihydrocoumarin.

frequencies appear in the range 1000–1300 cm1 and are very useful for characterization purpose [20]. The CAH in-plane bending vibrations appear as a weak to medium strong bands in FT-IR spectrum at 1190, 1140, 1100, 1010, 990 cm1 for DHC and 1270, 1195, 1180, 1090 cm1 for 3MC as a medium to weak band. Similarly the Raman bands are 1230, 1210, 1190, 1160, 1100, 1020, 990 cm1 and 1260, 1215, 1195, 1080 cm1 for DHC and 3MC respectively and show good agreement with computed wavenumber by B3LYP/6-311++G(d,p) method at 1246–1046 cm1 and

M. Arivazhagan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515 Table 4 Definition of internal coordinates of 3-methylcoumarin.

507

Table 5 Definition of local symmetry coordinates of 3,4-dihydrocoumarin.

No. (i)

Symbol

Type

Definition

No. (i)

Symbola

Definitionb

1–5

Ri

CAH

6–8 9–18

Si

CAO CAC

C4AH16, C5AH17, C6AH18, C7AH19, C8AH20 C2AO1, C9AO1, C2AO11 C2AC3, C3AC4, C4AC10, C10AC9, C10AC5, C5AC6, C6AC7, C7AC8, C8AC9, C3AC12 C12AH13, C12AH14, C12AH15

1–8 9–11 12–20 21

CAH CAO CAC R1 tridg

R1, R2, R3, R4, R5, R6, R7, R8 S9, S10, S11 l12, l13, l14, l15, l16, l17, l18, l19, l20 pffiffiffi (b21  b22  b23  b24 + b25  b26)/ 6 pffiffiffiffiffiffi (b21  b22 + 2b23  b24  b25 + 2b26)/ 12 (b21  b22 + b24  b25)/2 pffiffiffi (b27  b28 + b29  b30 + b31  b32)/ 6 pffiffiffiffiffiffi (b27  b28 + 2b29  b30  b31 + 2b32)/ 12 (b27  b28 + b30  b31)/2 pffiffiffi pffiffiffi pffiffiffi (a33  a34)/ 2, (a35  a36)/ 2, (a37  a38)/ 2, pffiffiffi pffiffiffi pffiffiffi (a39  a40)/ 2, (a41  a42)/ 2, (a43  a44)/ 2, pffiffiffi pffiffiffi (a45  a46)/ 2, (a47  a48)/ 2 pffiffiffi (m49  m50)/ 2

19–21

li

Wi

CH (methyl)

In-plane bending 22–27 bi

Ring1

28–33

Ring2

34–43

44, 45 46, 47 48–50 51–53

bI

ai

ti ci di hi

Out-of-plane bending 54 xi 55–59 pi

CACAH

OACAO CACAC CACAH (methyl) HACAH (methyl)

CACAC CACAH

ri

CACAO

Torsion 61–72

ti

sRing

ti ti

sCH3 Butterfly

C2AC4AC3AC12 C3AC10AC4AH16, C10AC6AC5AH17, C5AC7AC6AH18, C6AC8AC7AH19, C7AC9AC8AH20 C3AO1AC2AO11 O1AC2AC3AC4, C2AC3AC4AC10, C3AC4AC10AC9, C4AC10AC9AO1, C10AC9AO1AC2, C9AO1AC2AC3, C5AC6AC7AC8, C6AC7AC8AC9, C7AC8AC9AC10, C8AC9AC10AC5, C9AC10AC5AC6, C10AC5AC6AC7 C3AC12A(H14AH13AH15) C8AC9AC10AC4, C5AC10AC9AO1

For numbering of atom refer Fig. 2.

1263–1146 cm1 for DHC and 3MC respectively. The CAH out-ofplane bending vibrations are strongly coupled vibrations and they are listed in Tables 7 and 8. CAO vibrations The carbonyl (C@O) stretching vibration is highly characteristic which is intense absorption and is also calculated to be intense by the two levels. The wavelength at 1890 cm1 in both the spectra for DHC and IR band observed at 1970 cm1 for 3MC are assigned to C@O stretching vibration [21]. In comparison with the C@O stretching mode, the CAO stretching mode is less characteristic in nature and is sensitive to substitution. The assignment in the present work is rendered difficult owing to its overlap with CAH in-plane bending and CH3 wagging vibrations [22,23]. In DHC the IR and Raman band observed at 1310, 1290 cm1 and 1300, 1280 cm1 are assigned to CAO stretching vibrations. Whereas in

R1 symd R1 asymd R2 tridg

25

R2 symd

26 27–34

R2 asymd Bch

35

Bco

Out-of-plane bending xCH 36–43 44 xCO Torsion 45

50

s Ring1 trigd s Ring1 symd s Ring1 asymd s Ring2 trigd s Ring2 symd s Ring2 asymd

51

Butterfly

46 47 48 49

a

59

73 74, 75

O1AC2AC3, C2AC3AC4, C3AC4AC10, C4AC10AC9, C10AC9AO1, C9AO1AC2 C9AC10AC5, C10AC5AC6, C5AC6AC7, C6AC7AC8, C7AC8AC9, C8AC9AC10 C3AC4AH16, C10AC4AH16, C10AC5AH17, C6AC5AH17, C5AC6AH18, C7AC6AH18, C6AC7AH19, C8AC7AH19, C7AC8AH20, C9AC8AH20 C3AC2AO11, O1AC2AO11 C2AC3AC12, C4AC3AC12 C3AC12AH14, C3AC12AH13, C3AC12AH15 H14AC12AH14, C14AC12AH15, H13AC12AH15

22 23 24

b

p51, p52, p53, p54, p55, p56, p57, p58 x59 pffiffiffi (s60  s61 + s62  s63 + s64  s65)/ 6 (s60  s61 + s63  s64)/2

pffiffiffiffiffiffi (s60 + 2s61  s62  s63 + 2s64  s65)/ 12 pffiffiffi (s66  s67 + s68  s69 + s70  s71)/ 6 pffiffiffi (s66  s67 + s69  s70)/ 2 pffiffiffiffiffiffi (s66 + 2s67  s68  s69 + 2s70  s71)/ 12 pffiffiffi (s72  s73)/ 2

These symbols are used for description of normal modes by TED. The internal coordinates used here are defined in Table 3.

the case of 3MC, the Raman band at 1325 cm1 and IR band at 1310 cm1 show contribution from CAO stretching vibration along with contributions from other bond oscillators. These assignments are in agreement with the assignments in 6-chloro-4-bromomethylcoumarin [21]. The IR band observed at 770 cm1 for DHC and the Raman band observed at 750 cm1 for 3MC are assigned to C@O in-plane bending vibrations. The out-of-plane bending mode is observed at 110 cm1 in the Raman spectrum for DHC and 3MC. Methyl group vibrations The title compound 3MC possesses a CH3 group in the third position. For the assignments of CH3 group frequencies, basically nine fundamentals can be associated with each CH3 group namely, CH3 ass: asymmetric stretch; CH3 ips: in-plane stretch (i.e., inplane hydrogen stretching modes); CH3 ipb: in-plane-bending (i.e., hydrogen deformation modes); CH3 sb: symmetric bending; CH3 ipr: in-plane rocking; CH3 opr: out-of-plane rocking and tCH3: twisting hydrogen bending modes. In addition to that, CH3 ops: out-of-plane stretch and CH3 opb: out-of-plane bending modes of the CH3 group would also be expected to be depolarized. The stretching in CH3 occurs at lower frequencies than those of aromatic ring (3000–3100 cm1). The CH3, stretching is expected around 2900–3000 cm1, the in-plane deformations around 1370–1450 cm1 and the rocking around 990–1040 cm1 [24]. For 3MC asymmetric stretching frequency is observed at 2900 cm1 in both the spectra. The in-plane and out-of-plane stretching vibrations are observed at 2870 and 2810 cm1 in IR spectrum. For the methyl substituted benzene derivatives the inplane and symmetric bending deformation vibrations of methyl groups normally appear in the region 1440–1465 cm1 and 1370–1390 cm1, respectively [25,26]. In the work carried by Tahir Gulluoglu et al. [27] on 2-and 5-methylbenzimidazole, 3-acetyl-6methylcoumarin by Ramoji et al. [5] the frequency of 974 and 1041 cm1 in FT-Raman are assigned to the rocking modes of CH3. The rocking vibrations of the CH3 group in 3MC appear as independent vibrations in 940 cm1 in IR (in plane) and

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Table 6 Definition of local symmetry coordinates of 3-methylcoumarin.

a b

No. (i)

Symbola

Definitionb

1–5 6–8 9–18 19

CAH CAO CAC CH3 ss

R1, R2, R3, R4, R5 S6, S7, S8 l9, l10, l11, l12, l13, l14, l15, l16, l17, l18 pffiffiffi (l19 + l20 + l21)/ 3 pffiffiffi (2l19 + l20 + l21)/ 6 pffiffiffi (l20  l21)/ 2 pffiffiffi (b22  b23 + b24  b25 + b26  b27)/ 6 pffiffiffiffiffiffi (b22  b23 + 2b24  b25  b26 + 2b27)/ 12 (b22  b23 + b25  b26)/2 pffiffiffi (b28  b29 + b30  b31 + b32  b33)/ 6 pffiffiffiffiffiffi (b28  b29 + 2b30  b31  b32 + 2b33)/ 12 (b28  b29 + b31  b32)/2 pffiffiffi pffiffiffi pffiffiffi (a34  a35)/ 2, (a36  a37)/ 2, (a38  a39)/ 2, pffiffiffi pffiffiffi (a40  a41)/ 2, (a42  a43)/ 2 pffiffiffi (m44  m45)/ 2 pffiffiffi (c46  c47)/ 2 pffiffiffi (d48  d49  d50 + h51 + h52 + h53)/ 6 pffiffiffi (h51  h52  2h53)/ 6 pffiffiffi (h52  h53)/ 2 pffiffiffi (2d48  d49  d50)/ 6 pffiffiffi (d49  d50)/ 2

20

CH3 ips

21

CH3 ops

22

R1 tridg

23

R1 symd

24 25

R1 asymd R2 tridg

26

R2 symd

27 28–32

R1 asymd bCH

33

bCO

34

bCC

35

CH3 sb

36

CH3 ipb

37

CH3 opb

38

CH3 ipb

39

CH3 opb

40 41–45 46 47

xCC xCH xCO t1 trigd

48

t1 Rsymd

49

t1 Rasymd

50

t2 trigd

51

t2 Rsymd

52

t2 Rasymd

53 54

tCH3 Butterfly

x54 p55, p56, p57, p58, p59 r60 pffiffiffi (s61  s62 + s63  s64 + s65  s66)/ 6 pffiffiffi (s61  s62 + s64  s65)/ 2 pffiffiffiffiffiffi (s61 + 2s62  s63  s64 + 2s65  s66)/ 12 pffiffiffi (s67  s68 + s69  s70 + s71  s72)/ 6 pffiffiffi (s67  s68 + s70  s71)/ 2 pffiffiffiffiffiffi (s67 + 2s68  s69  s70 + 2s71  s72)/ 12 s73 pffiffiffi (s74 + s75)/ 2

These symbols are used for description of normal modes by TED. The internal coordinates used here are defined in Table 4.

920 cm1 in both the spectra (out-of-plane). Methyl out of plane bending vibration is observed at 1120 cm1 in IR and 1130 cm1 in Raman spectrum. All these vibrations coincide very well with the calculated DFT frequencies (Table 8). As expected the CH3 torsional mode is expected below 400 cm1, the calculated frequency at 113 cm1 in 3MC is assigned to this mode.

transfer or conjugative interaction in molecular systems. Some electron donor orbital, acceptor orbital and the interacting stabilization energy resulted from the second-order theory are reported [29,30]. The larger the E(2) value, the more intensive is the interaction between electron donors and electron acceptors, i.e. the more donating tendency from electron donors to electron acceptors and the greater the extent of conjugation of the whole system. Delocalization of electron density between occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied (antibond or Rydberg) non-Lewis NBO orbitals correspond to a stabilizing donor–acceptor interaction. NBO analysis has been performed on the title compounds at the B3LYP/6-311++G(d,p) level in order to elucidate the intramolecular, rehybridization and delocalization of electro density within the compounds and they are shown in Tables 9 and 10. The intramolecular interaction is formed by the orbital overlap between bonding CAC, CAO and CAH antibond orbital which results intramolecular charge transfer (ICT) causing stabilization of the system. These interactions are observed as increase in electron density (ED) in antibonding orbital that weakens the respective bonds. The importance of hyper conjugation and electron density transfer from lone electron pairs of the Y atom to the XAH antibonding orbital in the XAHY system has been reported [31]. In both the compounds the intermolecular CAOC hydrogen bonding is formed by the orbital overlap between the n(O) and r(CAO) which results intramolecular charge transfer (ICT) causing stabilization of the H-bonded systems. Hence hydrogen bonding interaction leads to an increase in electron density (ED) of CAO antibonding orbital. The increase of the population of CAO antibonding orbital weakens the CAO bond. Thus the nature and strength of the intermolecular hydrogen bonding can be explored by studying the changes in electron density in the vicinity of CO hydrogen bonds. The NBO analysis of DHC and 3MC clearly explains the evidence of the formation of strong interaction between the LP(O) and r⁄(CAO) antibonding orbitals. The stabilization energy E(2) associated with hyperconjugative interaction n2(O1) ? r⁄(C2AO11) and n2(O11) ? r⁄(O1AC2) for DHC and n2(O1) ? r⁄(C2AO11), n1(O1) ? r⁄(O1AC2) for 3MC are obtained as 155.89, 305.18 and 196.81, 77.78 kJ mol1 respectively are shown in Tables 9 and 10 which quantify the extend of intermolecular hydrogen bonding. The differences in E(2) energies are reasonably due to the fact that the accumulation of electron density in the CAO bond is not only drawn from the n(O) of hydrogen-acceptor but also from the whole compound.

NBO analysis

Electronic properties

NBO analysis provides the most accurate possible ‘natural Lewis structure’, because all orbital details are mathematically chosen to include the highest possible percentage of the electron density. A useful aspect of the NBO method is that it gives information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra- and intermolecular interactions. The second order Fock matrix was carried out to evaluate the donor–acceptor interactions in the NBO analysis [28]. The interactions result is a loss of occupancy from the localized NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the delocalization i ? j is estimated as

Ultraviolet spectra analysis of DHC and 3MC have been investigated by CIS/B3LYP/6-311++G(d,p) method. The calculated visible absorption maxima of kmax which is a function of the electron availability have been reported in Tables 11 and 12. Calculations of molecular orbital geometry show that the visible absorption maxima of these compounds correspond to the electron transition between frontier orbitals such as translation from HOMO to LUMO. As it can be seen from the UV–VIS spectra, absorption maxima values have been found to be 207 and 270 nm for DHC and 3MC, (Figs. 13 and 14) respectively. The kmax is a function of substitution, the stronger the donor character of the substitution, the more electrons are pushed into the molecule, the larger kmax. These values may be slightly shifted by solvent effects. Owing to the interaction between HOMO and LUMO orbital of a structure, transition of p–p⁄ type is observed with regard to the molecular orbital theory [32,33]. The calculated results involving the vertical excitation energies, oscillator strength (f) and wavelength are carried out. CIS/6-311++G(d,p) predict one intense electronic transition at 5.984 eV (207 nm) with an oscillator strength f = 0.0372 for DHC and 4.587 eV (270 nm) with an oscillator strength f = 0.429 for

2

Eð2Þ ¼ DEij ¼ qi

Fði; jÞ ej  ei

where qi is the donor orbital occupancy, ei and ej are diagonal elements and F(i, j) is the off diagonal NBO Fock matrix element. Natural Bond Orbital analysis provides an efficient method for studying intra- and intermolecular bonding and interaction among bonds, and also provides a convenient basis for investigating charge

Table 7 The observed (FT-IR and FT-Raman) and calculated (Unscaled and Scaled) frequencies (cm1), IR intensity (km mol1), Raman intensity (Å4 amu1) and probable assignments (characterized by TED) of 3,4-dihydrocoumarin using HF and B3LYP/6-311++G(d,p) calculations. S. no

Calculated values HF/6-311++G(d,p)

Assignments with TED% among types of coordinates

B3LYP/6-311++G(d,p)

FT-IR

FT-Raman

Unscaled frequency (cm1)

Scaled frequency (cm1)

IR intensity

Raman intensity

Unscaled frequency (cm1)

Scaled frequency (cm1)

IR intensity

Raman intensity

3090 – 3030 3000 2980 2920 2890 – 1890 1780 1610 1580 1520 – 1490 – – 1340 1310 1290 – – 1190 – 1140 1100 1010 990 905 – – 920 – 795 770 – 735 720 690 610 570 540 510 480 – – – – – – –

– 3070 – 3095 2980 – 2890 2820 1890 1780 1610 1580 – 1500 – 1410 1390 1340 1300 1280 1230 1210 1190 1160 – 1100 1020 990 – – – – 900 – – 750 – 720 690 610 570 – 520 490 420 400 320 300 210 110 –

3374 3356 3337 3321 3272 3224 3174 3153 1904 1793 1763 1653 1638 1622 1617 1518 1485 1436 1394 1364 1353 1316 1312 1282 1253 1220 1167 1135 1122 1100 1080 1002 997 992 875 865 821 795 760 665 629 597 555 523 469 447 345 312 218 145 81

3096 3075 3036 3007 2988 2927 2896 2826 1898 1788 1618 1588 1527 1507 1499 1418 1400 1347 1319 1298 1238 1217 1199 1167 1151 1112 1026 1002 916 958 927 925 911 803 778 764 739 733 704 619 582 549 534 512 435 419 329 310 224 124 111

6.063 25.614 14.903 5.383 14.106 22.574 13.4434 19.908 526.782 37.976 19.318 70.272 12.397 12.853 53.463 24.213 0.475 43.057 84.218 197.785 7.003 54.021 20.298 192.721 15.381 13.554 8.217 0.016 1.816 5.891 18.026 1.077 22.389 21.099 99.183 13.247 2.981 3.131 10.103 8.104 5.756 0.916 6.893 13.604 6.777 0.239 2.875 0.645 0.979 8.530 1.318

24.261 9.136 31.260 48.328 14.397 48.925 19.335 62.197 67.056 151.175 350.325 409.769 481.575 39.128 387.829 53.795 243.219 454.760 396.615 27.704 29.298 14.050 114.250 53.097 143.145 579.663 163.59 566.043 289.946 1831.51 70.647 94.292 177.755 223.099 28.986 22.788 126.430 117.233 699.727 565.898 1.879 0.977 4.008 1.446 468.661 1.100 61.632 1.507 4.020 8.535 3142.029

3207 3190 3174 3158 3114 3071 3018 2999 1729 1647 1618 1530 1516 1502 1497 1398 1359 1342 1328 1274 1246 1216 1206 1205 1139 1131 1065 1049 1011 1003 980 917 898 896 799 787 753 728 696 613 576 548 512 477 430 411 319 287 202 134 81

3095 3074 3034 3005 2986 2923 2894 2824 1893 1784 1615 1586 1524 1503 1495 1406 1384 1343 1316 1285 1236 1215 1197 1164 1148 1109 1018 999 912 954 928 924 899 800 776 762 737 731 701 616 550 547 531 508 431 412 327 309 219 121 99

6.200 23.891 12.596 6.329 11.536 18.303 14.318 16.079 397.897 29.879 12.362 34.627 17.987 13.288 46.807 8.962 0.779 0.295 8.395 35.330 33.738 3.400 23.912 44.516 316.496 61.844 17.750 3.608 0.043 29.048 3.484 16.909 45.959 25.229 25.037 68.581 3.796 2.034 7.366 4.726 3.577 0.246 4.506 12.078 4.451 0.569 0.944 0.705 0.831 6.548 1.057

84.787 65.710 41.862 28.267 59.192 43.188 25.384 112.195 86.018 37.049 66.010 13.225 16.527 21.322 3.025 9.805 10.091 40.864 6.606 29.729 120.176 14.974 12.034 24.296 55.372 1.474 5.805 121.200 0.220 17.167 1.306 26.297 6.802 3.698 9.869 2.417 39.129 92.848 29.493 24.228 40.905 26.961 19.524 48.024 82.630 99.742 11.994 45.411 70.715 102.972 25.123

mCH(99) mCH(97) mCH(98) mCH(97) mCH(96) mCH(96) mCH(94) mCH(94) mC@O(95) mCH(3) mCC(83) bCH(12) mCC(85) bCH(10) mCC(82) bCH(3) mCC(75) bCH(5) mCC(78) bCH(11) mCC(86) mC@O(9) mCC(86) mCO(8) mCC(85) mCC(5) mCC(84) bCH(4) mCO(86) bCH(9) mCO(85) bCH(5) bCH(75) bCH(74) bCH(73) bCH(73) bCH(72) bCH(71) bCH(70) bCH(72) R1 trigd(64) xCH(12) R1 symd(63) xCH(10) R1 asymd(62) xCH(5) R2 trigd(64) xCH(11) R2 symd(63) xCO(6) R2 asymd(62) xCH(4) bC@O(66) xCH(13) xCH(57) xCC(8) xCH(54) xCO(8) xCH(54) xCO(5) xCH(57) xCC(8) xCH(55) xCC(10) xCO(8) xCH(54) xCC(8) xCH(53) xCO(5) xCC(3) xCH(52) xCO(3) tR1 trigd(57) xCH(5) tR1 symd(56) xCO(5) tR1 asymd(55) xCH(4) tR2 trigd(57) xCC(8) tR2 symd(56) xCO(4) tR2 asymd(55) xCH(4) xCO(58) tR trigd(15) Butterfly

Abbreviations: R – ring, b – bending, m – stretching, symd – symmetric deformation, x – out-of-plane bending, asymd – asymmetric deformation, trigd – trigonal deformation, ss – symmetric stretching, rock – rocking, twist – twisting, sciss – scissoring, wag – wagging.

509

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A

Observed frequencies (cm1)

M. Arivazhagan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

Symmetry species

510

No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Symmetry species

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A

Observed frequencies (cm1)

Calculated values

FTIR

FTRaman

Unscaled frequency (cm1)

Scaled (cm1)

IR intensity

Raman intensity

Unscaled frequency (cm1)

Scaled frequency (cm1)

IR intensity

Raman intensity

3100 3084 – 3005 2950 2900 2870 2810 1970 1920 1790 1750 1710 – 1620 1600 – 1570 1490 1380 1360 – 1310 1270 – 1195 1180 1120 1090 1040 – – 1000 940 920 850 780 –

– – 3070 3000 2950 2900 – – – – – – 1700 1640 – 1605 1595 1580 1495 1390 – 1325 – 1260 1215 1195 – 1130 1080 – – – 1010 – 920 860 – 750

3379 3360 3341 3330 3325 3244 3231 3169 1864 1829 1786 1749 1647 1638 1623 1613 1581 1519 1435 1388 1359 1356 1315 1259 1232 1198 1192 1126 1124 1117 1102 1034 1011 988 922 868 811 794

3108 3089 3077 3010 2958 2910 2877 2816 1979 1927 1782 1743 1696 1636 1627 1609 1588 1578 1505 1389 1357 1331 1316 1250 1223 1206 1189 1128 1097 1051 1049 1042 1021 949 927 869 788 757

6.017 23.543 16.121 15.619 0.066 19.857 17.575 34.105 789.52 0.675 130.43 5.960 9.994 5.580 10.283 38.837 1.750 12.517 26.993 6.020 75.975 4.205 50.558 6.308 25.445 52.687 1.472 0.185 40.620 16.623 8.115 24.426 8.380 7.348 2.356 95.224 17.318 1.009

60.315 63.456 36.927 39.856 5.819 33.414 42.747 111.053 296.663 531.344 281.819 214.366 74.796 57.397 31.104 16.482 23.828 66.253 6.146 562.098 9.037 203.136 4.318 21.127 20.484 17.753 6.772 2.549 111.651 81.031 0.288 29.076 13.848 1.362 33.405 3.416 21.418 90.590

3410 3389 3371 3359 3354 3282 3261 3209 1965 1849 1781 1741 1658 1654 1650 1628 1588 1525 1449 1389 1369 1349 1297 1263 1245 1218 1199 1191 1146 1126 1120 1088 1035 1028 924 917 892 864

3104 3087 3074 3007 2956 2905 2873 2814 1975 1923 1786 1748 1704 1638 1624 1607 1590 1574 1501 1387 1359 1328 1314 1254 1220 1200 1187 1125 1095 1049 1047 1040 1019 947 925 862 784 754

2.048 13.662 7.809 7.019 2.898 13.237 10.191 17.858 581.16 29.34 57.438 12.117 6.852 17.681 8.543 39.564 1.890 10.680 38.828 25.717 31.976 3.287 66.215 22.970 22.119 3.631 43.724 1.404 10.332 18.933 32.322 2.765 28.248 12.811 39.530 4.648 58.794 50.251

49.441 62.378 32.826 36.137 4.889 28.773 46.436 79.488 115.288 349.824 218.376 162.038 34.300 113.623 55.584 6.510 71.017 47.028 6.504 345.979 42.369 176.024 61.017 41.170 18.743 17.420 23.407 2.225 6.657 95.345 14.602 39.141 3.455 26.069 48.819 24.953 1.350 54.152

HF/6-311++G(d,p)

Assignments with TED% among types of coordinates

B3LYP/6-311++G(d,p)

mCH(99) mCH(99) mCH(98) mCH(98) mCH(98) CH3 as(95) mCH(3) CH3 ips(99) CH3 ops(99) mC@O(96) mCH3 ss(3) mCC(85) bCH(10) mCC(83) bCH(12) R2 asyd(10) mCC(85) bCH(10) R2 asyd(5) mCC(82) CH3 ss(8) bCH(3) mCC(75) CH3 ipr(15) bCC(5) mCC(65) CH3 sb(23) mCC(78) bCH(11) mCC(86) mCO(9) CH3 wag(10) mCC(85) mCO(8) CH3 ipb(83) bCH(8) CH3 sbdef(84) bCH(10) mCC(75) bCH(11) mCO(86) bCH(9) CH3 wag(5) mCO(85) bCH(5) CH3 wag(4) bCH(58) CH3 twist(40) bCH(75) bCH(74) bCH(73) CH3 opb(85) bCH(72) R1 trigd(64) xCH(12) CH3 ass(11) R1 symd(63) CH3 ass(20) R1 asymd(62) Rsymd(15) R2 trigd(63) CH3 ass(13) CH3 ipr(87) CH3 opr(82) xCH(10) R2 symd(68) CH3 ass(20) R2 asymd(67) xCH(12) bC@O(66) xCH(12)

M. Arivazhagan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515

Table 8 The observed (FT-IR and FT-Raman) and calculated (Unscaled and Scaled) frequencies (cm1), IR intensity (km mol1), Raman intensity (Å4 amu1) and probable assignments (characterized by TED) of 3-methylcoumarin using HF and B3LYP/6-311++G(d,p) calculations.

Abbreviations: R – ring, b – bending, m – stretching, symd – symmetric deformation, x – out-of-plane bending, asymd – asymmetric deformation, trigd – trigonal deformation, ss – symmetric stretching, rock – rocking, twist – twisting, sciss – scissoring, wag – wagging.

t1 trigd(57) CH3 opr(10) t1 Rsymd(56) CH3 twist(22) xCH(5) t1 Rasymd(55) CH3 twist(21) xCH(4) t2 trigd(57) CH3 opr(8) xCH(5) t2 Rsymd(55) CH3 twist(22) xCH(5) t2 Rasymd(56) CH3 twist(21) xCO(5) xCC(58) tRtrigd(15) xCO(59) tRtrigd(12) tCH3(56) xCO(5) Butterfly

39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

A A A A A A A A A A A A A A A A

720 – 680 610 595 570 440 – – – – – – – – –

– 700 680 – 590 – 450 – 395 320 260 – 210 – 110 –

777 732 669 625 605 516 490 469 427 349 322 281 266 172 113 96

728 713 668 623 612 558 465 452 404 331 274 262 224 163 122 109

24.885 0.377 11.572 6.646 0.156 3.085 8.426 2.969 0.277 5.820 0.088 1.583 0.194 0.076 0.835 3.705

37.345 27.890 21.639 14.732 0.859 5.682 172.507 123.903 65.283 15.699 2.929 3.246 12.499 41.627 39.275 44.463

802 739 675 644 630 548 500 473 447 351 350 283 282 156 118 109

725 709 671 621 600 560 462 449 402 329 271 258 219 158 119 99

0.049 0.983 10.468 0.028 4.568 5.643 8.529 2.638 0.203 0.047 6.524 0.389 1.466 0.000 1.419 2.577

66.548 20.529 21.096 2.654 7.591 14.746 133.814 114.651 100.562 29.742 15.303 28.202 1.494 53.559 67.704 104.528

bCC(60) xCH(10) xCH(57) CH3 opr(9) xCH(56) xCC(5) xCH(54) CH3 twist(8) xCC(5) xCH(52) CH3 opr(10) xCO(8) xCH(53) xCC(5)

M. Arivazhagan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515

511

3MC. Therefore, while the energy of the HOMO is directly related to the ionization potential, LUMO energy is directly related to the electron affinity. Energy difference between HOMO and LUMO orbital is called as energy gap that is an important stability for structures. In addition, 3D plots of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) for DHC and 3MC are shown in Figs. 15–18. According to B3LYP/6-311G++(d,p) calculation, the energy band gap (DE) (translation from HOMO to LUMO) of the compounds DHC and 3-MC are 0.1524 a.u. and 0.2161 a.u. respectively (Table 13). The LUMO of p nature, (i.e. benzene ring) is delocalized over the whole CAC bond. Consequently the HOMO–LUMO transition implies an electron density transfer to almost all the atoms.

13

C and 1H NMR spectral analysis

The isotropic chemical shifts are frequently used as an aid in the identification of reactive organic as well as ionic species. It is recognized that accurate predictions of molecular geometries are essential for reliable calculations of magnetic properties. Therefore, full geometry optimization of DHC and 3MC have been performed by using B3LYP/6-311++G(d,p) method. Then, Gauge-including atomic orbital (GIAO) [34] 1H and 13C chemical shift calculations of the compounds have been made by same method. GIAO procedure is somewhat superior since it exhibits a faster convergence of the calculated properties upon extension of the basis set used. Taking into account the computational cost and the effectiveness of calculation, the GIAO method seems to be preferable from many aspects at the present state of this subject. On the other hand, the density functional methodologies offer an effective alternative to the conventional correlated methods, due to their significantly lower computational cost. The 1H and 13C chemical shifts are measured in a less polar (CDCl3) solvent. The results in Tables 14 and 15 show that the range 13C NMR chemical shift of the typical organic molecule usually is >100 [35], the accuracy ensures reliable interpretation of spectroscopic parameters. The 13C NMR chemical shift of C3, C4 of DHC and C2, C5 of 3MC are observed larger than other carbons whereas their chemical shifts are lower than all other observed. This C3 and C4 chemical shifts are determined at 53.54 and 12.60 ppm for DHC and C2, C5 chemical shifts are determined at 129.64 and 124.22 ppm for 3MC. The protons of DHC and 3MC are observed at 9.28, 3.60, 2.28, 2.90 ppm and 0.74, 0.74, 0.136 ppm respectively. Another important aspect is that, hydrogen attached or nearby electron withdrawing atom or group can decrease the shielding and move the resonance of attached proton towards to a higher frequency. By contrast electron donating atom or group increases the shielding and moves the resonance towards to a lower frequency. Electronegative atoms such as nitrogen, oxygen, and halogens deshield hydrogens. The extent of deshielding is proportional to the electronegativity of the heteroatom and its proximity to the hydrogen. Electrons on an aromatic ring, double bonded atoms, and triple bonded atoms deshield attached hydrogens. The two oxygen atoms DHC and 3MC show electronegative property. The increase in chemical shift of C9 in both DHC and 3MC is due to the substitution of more electronegative atoms in the benzene ring. The presence of electronegative atom attracts all electron clouds of carbon atoms towards them, which leads to deshielding of carbon atom and net result in increase in chemical shift value (Figs. 19 and 20) deshielding of the title compounds. Also the chemical shifts obtained and calculated for the hydrogen atoms are quite low. All values are <3 ppm in methyl group of 3MC due to the shielding effect. The O atom(s) (i.e.) more electronegative property polarizes the electron distribution in its bond to adjacent carbon atom and decreases the electron density of the compounds.

512

M. Arivazhagan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 502–515

Table 9 Second order perturbation theory analysis of Fock matrix in NBO basis for 3,4-dihydrocoumarin. Donor (i) C3AH13 C5AC6 C5AC6 C7AC8 C7AC8 O1 O11 O1 O11 a b c

Type

r p p p p p p p p

ED/e

Acceptor (j)

1.96 1.97 1.72 1.97 1.73 1.6 1.85 1.85 1.85

C2AO11 C7AC8 C9AC10 C5AC6 C9AC10 C2AO11 O1AC2 C9AC10 C2AC3

Type 

p p p p p LP(2) LP(1) LP(2) LP(1)

ED/e

E(2) (kJ mol1)a

E(j)  E(i) (a.u.)b

F(i, j) (a.u.)c

0.019 0.016 0.033 0.283 0.033 0.112 0.119 0.033 0.047

42.13 155.64 145.60 142.96 155.51 155.89 305.18 127.98 105.22

0.93 0.51 0.50 0.52 0.50 0.72 0.92 0.66 1.18

0.088 0.124 0.120 0.119 0.124 0.147 0.232 0.133 0.158

ED/e

E(2) (kJ mol1)a

E(j)  E(i) (a.u.)b

F(i, j) (a.u.)c

0.402 0.424 0.282 0.424 0.268 0.424 0.402 0.424 0.074

141.08 67.27 78.07 78.45 71.67 78.91 196.81 98.78 77.78

0.22 0.28 0.29 0.27 0.29 0.27 0.28 0.34 0.53

0.080 0.062 0.065 0.065 0.064 0.066 0.106 0.083 0.089

E(2) means energy of hyper conjugative interaction (stabilization energy). Energy difference between donor and acceptor i and j NBO orbitals. F(i, j) is the Fock matrix element between i and j NBO orbitals.

Table 10 Second order perturbation theory analysis of Fock matrix in NBO basis for 3-methylcoumarin. Donor (i) C3AC4 C3AC4 C5AC6 C5AC6 C7AC8 C7AC8 O1 O1 O1 a b c

Type

p p p p p p p p p

ED/e

Acceptor (j)

1.74 1.74 1.71 1.71 1.70 1.70 1.71 1.71 1.91

C2AO11 C9AC10 C7AC8 C9AC10 C5AC6 C9AC10 C2AO11 C9AC10 O1AC2

Type 

p p p p p LP(2) LP(2) LP(2) LP(1)

E(2) means energy of hyper conjugative interaction (stabilization energy). Energy difference between donor and acceptor i and j NBO orbitals. F(i, j) is the Fock matrix element between i and j NBO orbitals.

Table 11 The computed excitation energies, oscillator strength and electronic transition configuration of 3,4-dihydrocoumarin.

Table 12 The computed excitation energies, oscillator strength and electronic transition configuration of 3-methylcoumarin.

Excited state

EE (eV)/ wavelength (nm)

F

Configuration

CI expansion coefficient

Excited state

EE (eV)/ wavelength (nm)

F

Configuration

CI expansion coefficient

1

4.9142/252.30

0.0000

36 ? 43 36 ? 46 36 ? 47 36 ? 50 36 ? 53 36 ? 55 36 ? 61 36 ? 81

0.2135 0.5255 0.1303 0.1015 0.2588 0.1684 0.1098 0.1105

1

4.5501/272.49

0.1098

39 ? 43 39 ? 53 39 ? 54 39 ? 56 40 ? 43 42 ? 43

0.2135 0.5255 0.1303 0.1015 0.2588 0.1684

2

4.5877/270.25

0.4294

38 ? 46 38 ? 47 38 ? 50 38 ? 43 38 ? 46 38 ? 47

0.1424 0.1437 0.2199 0.3278 0.3815 0.2781

39 ? 43 40 ? 43 42 ? 43

0.2399 0.1132 0.5886

3

5.7293/216.40

0.0372

41 ? 43 41 ? 53 42 ? 51 42 ? 52 42 ? 53

0.5644 0.1095 0.2116 0.1638 0.1324

38 ? 43 38 ? 46 38 ? 47 38 ? 51 38 ? 53

0.2029 0.2370 0.1964 0.1031 0.1499

2

3

5.7880/214.21

5.9847/207.17

0.0931

0.0372

Molecular electrostatic potential Molecular electrostatic potential (MESP) at a point in the space around a molecule gives an indication of the net electrostatic effect produced at that point by the total charge distribution (electron + nuclei) of the molecule and correlates with the dipole moments, electronegativity, partial charges chemical reactivity of the molecule. It provides a visual method to understand the relative polarity of the molecule. Thus MESP serves as a useful quantity

to explain hydrogen bonding, reactivity and structure–activity relationship of molecule including biomolecules and drugs. The different values of the electrostatic potential at the surface are represented by different colors; red represents regions of most negative electrostatic potential, which corresponds to an attraction of the proton by the concentrated electron density in the molecule; blue represents regions of most positive electrostatic potential and green represents regions of zero potential. Such electrostatic potential surfaces have been plotted for DHC and 3MC in B3LYP/ 6-311G++(d,p). Projections of these surfaces along the molecular plane are depicted in Figs. 21 and 22. The value of the electrostatic potential is largely responsible for the binding of a substrate to its receptor binding sites since the

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513

207 nm

5000

Epsilon

4000

3000

2000

1000

0

100

150

200

250

300

350

Wavelength (nm)

Fig. 16. LUMO image of 3,4-dihydrocoumarin.

Fig. 13. UV–VIS absorption spectra of 3,4-dihydrocoumarin.

25000

270nm 20000

Epsilon

15000

10000

5000

0

Fig. 17. HOMO image of 3-methylcoumarin. 100

150

200

250

300

350

400

Wavelength (nm) Fig. 14. UV–VIS absorption spectra of 3-methylcoumarin.

Fig. 18. LUMO image of 3-methylcoumarin.

Fig. 15. HOMO image of 3,4-dihydrocoumarin.

receptor and the corresponding legends recognize each other at their molecular surface. Area of high, neutral and low electrostatic potential is determined for DHC and 3MC. The binding site in relevant enzyme is expected to have opposite areas of electrostatic potential.

From the MESP curve as shown in Figs. 21 and 22. The negative (red1) regions of MESP are related to electrophilic reactivity and the positive (blue) regions to nucleophilic reactivity. Both DHC and 3MC have two possible sites of electrophilic attack. The negative regions are mainly over the O11 and O1 atoms. The MESP value around O11 is more negative than that of O1. The sites corresponding to carbon and hydrogen atoms and they correspond to highly active nucleophilic region which play an important role in the activity of the compounds.

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Table 13 Calculated HOMO–LUMO energy using B3LYP/6-311++G(d,p) method. 3,4-Dihydrocoumarin

3-Methylcoumarin

HOMO energy = 0.3028 a.u. LUMO energy = 0.1504 a.u. HOMO–LUMO energy gap = 0.1524 a.u.

HOMO energy = 0.3020 a.u. LUMO energy = 0.1859 a.u. HOMO–LUMO energy gap = 0.2161 a.u.

Table 14 The calculated chemical shifts of carbon and hydrogen atoms of 3,4-dihydrocoumarin using B3LYP/6-311++G(d,p) method. Atoms

Isotropic chemical shielding tensor (r) (ppm)

Shift (ppm)

C2 C3 C4 C5 C9 H12 H13 H14 H15

51.68 128.91 169.86 56.83 22.93 22.59 28.27 29.73 28.97

130.77 53.54 12.60 125.62 159.52 9.285 3.608 2.286 2.905

Fig. 21. Molecular electrostatic potential of 3,4-dihydrocoumarin.

Table 15 The calculated chemical shifts of carbon and hydrogen atoms of 3-methylcoumarin using B3LYP/6-311++G(d,p) method. Atoms

Isotropic chemical shielding tensor (r) (ppm)

Shift (ppm)

C2 C3 C5 C9 H13 H14 H15

52.82 52.26 58.23 18.36 31.14 31.14 31.74

129.64 130.20 124.22 164.22 0.740 0.74 0.136

Fig. 22. Molecular electrostatic potential of 3-methylcoumarin.

Conclusion

Fig. 19. NMR chemical shift of 3,4-dihydrocoumarin.

A complete vibrational analysis of 3,4-dihydrocoumarin and 3methylcoumarin have been performed according to SQM force field method based on ab initio and DFT calculation 6-311++G(d,p) basis set and their frequencies are compared. In particular, the results of B3LYP/6-311++G(d,p) method indicates better fit to experimental values. The influences of carbon–oxygen bond and methyl group to the vibrational frequencies of the title compounds have been discussed. Any discrepancy noted between the observed and the calculated frequencies may be due to the fact that the calculations have been actually done on a single compound contrary to the experiment values recorded in the presence of intermolecular interactions. The pronounced decrease of the lone pair orbital occupancy and the molecular stabilization energy show the hyperconjugation interaction from the NBO analysis. Calculations of molecular orbital geometry show that the visible absorption maxima of DHC and 3MC correspond to the electron transition between frontier orbitals such as translation from HOMO to LUMO. Gauge-including atomic orbital (GIAO) 1H and 13C chemical shift calculations have been calculated. Area of high, neutral and low electrostatic potential is determined for DHC and 3MC. References

Fig. 20. NMR chemical shift of 3-methylcoumarin.

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