Advances in Chemistry
Volume 2014 (2014), Article ID 490358, 5 pages
http://dx.doi.org/10.1155/2014/490358

Research Article

Structural Conformational Study of Eugenol Derivatives Using Semiempirical Methods

Radia Mahboub

Department of Chemistry, Faculty of Sciences, University of Tlemcen, BP 119, 13000 Tlemcen, Algeria

Received 3 June 2014; Accepted 29 September 2014; Published 20 October 2014

Academic Editor: Maria Roca

Copyright © 2014 Radia Mahboub. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

We investigated the conformational structure of eugenol and eugenyl acetate under torsional angle effect by performing semiempirical calculations using AM1 and PM3 methods. From these calculations, we have evaluated the strain energy of conformational interconversion. To provide a better estimate of stable conformations, we have plotted the strain energy versus dihedral angle. So, we have determined five geometries of eugenol (three energy minima and two transition states) and three geometries of eugenyl acetate (two energy minima and one transition state). From the molecular orbital calculations, we deduce that the optimized trans form by AM1 method is more reactive than under PM3 method. We can conclude that both methods are efficient. The AM1 method allows us to determine the reactivity and PM3 method to verify the stability.

1. Introduction

Eugenol (4-allyl-2-methoxyphenol) is a phenylpropene, an allyl chain-substituted guaiacol. It is the main phenolic compound extracted from certain essential oils especially from clove oil, nutmeg, cinnamon, basil, and bay leaf [1–14]. Eugenol is a phenol derivative used in many areas such as perfumes, flavorings agent, and dental materials. It is used as an antiseptic, analgesic, fungicide, bactericide, insecticide, anticarcinogenic, antiallergic, antioxidant, anti-inflammatory, and so forth [15–17]. As derivative, the eugenyl acetate was characterized and its structural properties have investigated by Dos Santos et al. [18, 19].

To our knowledge, a study of the conformational structure as a function of the dihedral angle was not reported. In the present paper, we investigated the conformational structure of eugenol and eugenyl acetate under torsional angle effect by performing semiempirical calculations using AM1 and PM3 methods. From these calculations, we have evaluated the strain energy of conformational interconversion to provide a better estimate of stable conformations. These results can be used to make future applications possible (Figure 1).

Figure 1: Conformation structures of cis-eugenol and cis-eugenyl acetate.

2. Methodology

Molecular modeling of the optimized eugenol and eugenyl acetate was carried out with the use of an efficient program for molecular mechanics (MM). Calculations are performed for all optimized geometries using AM1 and PM3 methods. The main molecular properties to characterize the geometry structures and the molecular orbital of the eugenyl acetate were calculated and compared. For each method, the geometry of the compound was optimized by using the Polak-Ribiere conjugate gradient algorithm with a gradient of 0.01 Kcal/mol (RMS). The following quantum chemical results are considered: heat of formation (Δ), total energy (), minimum energy of conformation (), strain energy of conformational interconversion (), energy of highest occupied molecular orbital (HOMO), energy of lowest unoccupied molecular orbital (LUMO), and HOMO-LUMO energy gaps (EG).

3. Results and Discussion

Molecular geometries of eugenol and eugenyl acetate were optimized by semiempirical molecular orbital method (AM1 and PM3). The semiempirical simulations results for structure optimization of eugenol and eugenyl acetate are given in Table 1. The conformational interconversion energy-minimum of eugenyl acetate and eugenol was investigated in detail by changing different torsional angles. From these studies, we have determined five geometries of eugenol (three energy minima and two transition states) and three geometries of eugenyl acetate (two energy minima and one transition state). These geometries are important in the description of the conformational properties of our systems (Figure 2).

Table 1: Main calculated properties of eugenol and eugenyl acetate with semiempirical methods.

Entry
Eugenol

Properties
AM1
PM3

Trans
Eclipsed
Gauche
Cis
Trans
Eclipsed
Gauche
Cis

1

−43.562
−45.560
−45.540
−38.360
−44.474
−45.230
−45.214
−40.236

2

−48090.984
−48092.980
−48092.960
−48085.781
−45111.215
−45111.973
−45111.957
−45106.976

3

−2496.805
−2498.802
−2498.782
−2491.602
−2497.716
−2498.472
−2498.456
−2493.478

4

5.202
7.200
7.180
0.000
4.238
4.994
4.978
0.000

5
HOMO
−8.599
−8.614
−8.606
−8.592
−8.670
−8.703
−8.701
−8.666

6
LUMO
0.327
0.332
0.342
0.338
0.254
0.250
0.260
0.263

7
EG
8.926
8.946
8.948
8.930
8.924
8.953
8.961
8.929

Entry
Eugenyl acetate

Properties
AM1
PM3

Trans
Eclipsed
Gauche
Cis
Trans
Eclipsed
Gauche
Cis

8

−68.176
−77.344
−77.356
−77.356
−77.728
−83.323
−83.330
−83.328

9

−62008.605
−62017.772
−62017.785
−62017.785
−58055.633
−58061.226
−58061.234
−58061.230

10

−3026.961
−3036.129
−3036.140
−3036.140
−3036.514
−3042.108
−3042.115
−3042.113

11

9.180
0.012
0.000
0.000
5.600
0.005
−0.002
0.000

12
HOMO
−8.852
−9.215
−9.205
−9.205
−8.925
−9.302
−9.297
−9.299

13
LUMO
0.001
−0.134
−0.130
−0.130
−0.044
−0.191
−0.188
−0.189

14
EG
8.852
9.081
9.075
9.075
8.881
9.101
9.109
9.110

The strain energy () for each geometry of a molecule is defined as the difference between the minimum energy of conformation for that geometry and the most stable conformation of the molecule.

(a)

(b)

Figure 2: Calculated strain energy for conformational interconversion with semiempirical AM1 and PM3 methods. (a) Eugenol and (b) eugenyl acetate.

We have obtained the curves plotted in Figure 2 from the simulated data using the nonlinear fitting process based on the Levenberg-Marquardt algorithm implemented in the Origin v. 6.0. Software [20].

The dihedral angle for rotation about C₄–C₁₀ bond in eugenol has several stationary points. A/A′, C, C′, and E/E′ are minima and B, B′ and D, D′ are maxima. Only the structures at the minima represent stable species and of these, the syn conformation is more stable than the anti. The gauche and the eclipsed represent the transition states. In eugenyl acetate, the stationary points A/A′, B/B′, C/C′, E/E′, F/F′, and G/G′ are minima and D, D′ are maxima. The anti conformation represents the transition state while the syn, gauche, and eclipsed conformations are stable species. So, the deformation around C₄–C₁₃ in eugenyl acetate remains unchanged and is not influenced by torsional angle effect.

To provide a better estimate of conformations, we should search the conformational space in reasonable computing time. So, we run the simulations; then we run a geometry optimization on each structure. Thus, we have grouped the resulting structures in Figure 3. First, we observe that all geometries from cis conformations obtained after optimization present deformation mainly on branching allyl. The torsional angle value varies around 136°. This situation is due to methylene group (sp³ hybridization) which gives a noncoplanar final geometry. Then, the -bond of branching allyl is situated in the same side that acetate group. Second, we note that the geometry in anti conformations stays unchanged after optimization. All the substituents of aromatic ring, acetate and methoxy groups then branching allyl, are situated in the same plane (trans: 180°).

Figure 3: Estimated conformation structures of eugenol and eugenyl acetate; (a1) and (a2) cis forms, (b1) and (b2) trans forms. Molecule orientation was chosen around x-axis. Hydrogen atoms are omitted for clarity.

From our molecular orbital calculations, we want to deduce the structure-reactivity relationship depending on different conformations. First, AM1 and PM3 calculations show that the cis forms are favored (Table 1, entry 7, EG: 9.075, 9.110 ev). On the other hand, the same calculations show that the most active site of the nucleophilic reaction is located on the oxygens of the acetate group and methoxy, and the most active site of the electrophilic reaction is C8 position of eugenyl acetate.

The AM1 and PM3 calculations show that the trans form is most active (Table 1, entry 7, EG: 8.852; 8.881 ev). Thus, these results reveal that the instability is caused by the high activity of methylene group in the strand allyl which provides to the aromatic ring another nucleophilic reaction site. This one is stabilized by resonance with the double bonds of the aromatic ring and the allylic radical (Figure 4). So, we note that the value of the strain energy obtained with AM1 method (Table 1, entry 4, Es: 9.180 ev) is higher than that obtained by PM3 calculation (Table 1, entry 4, Es: 5.600 ev). Consequently, we deduce that the optimized trans form by AM1 method is more reactive than under PM3 method. We can conclude that the efficient method for the eugenyl acetate is the semiempirical method AM1.

Figure 4: Molecular orbital calculated for eugenol and eugenyl acetate by semiempirical methods (AM1 and PM3). Contour values: 0.05 Å⁻³. Blue lines represent positive contours. Red lines represent negative contours. Hydrogen atoms are omitted for clarity.

These observations remain the same for eugenol, except for the energy gaps. From Figure 2(a), we note that the trans form (C or C′) is near the transition states B and D, respectively, B′ and D′. This situation is clearer when the eugenol is optimized by PM3 (Table 1, entries 5 and 6, PM3: HOMO −8.599, LUMO 0.327 ev; AM1: HOMO −8.670, LUMO 0.254 ev). So, this trans form (C or C′) corresponds to the reaction intermediate present in eugenol and absent in eugenyl acetate (AM1: HOMO −8.606, LUMO 0.343; PM3: HOMO −8.702, LUMO 0.260 ev).

Furthermore, we also see that the HOMO is located at the oxygen sites whereas for the molecule the Homo is distributed along the aromatic cycle site (Figure 4, Table 2). This clearly shows the high reactivity of eugenol compared to its corresponding acetate. This reactivity is due to mobility of hydrogen and the nucleophilicity on the aromatic ring.

Table 2: Mulliken charges of the optimized structures of eugenol and eugenyl acetate.

Compound
State
Method
Type charge

Eugenol
Transition
AM1
C₁ 0.065, C₂ 0.058, C₃ −0.148, C₄  −0.078, C₅  −0.103, C₆  −0.165, O₇  −0.208, O₈  −0.166,
C₉ 0.047, C₁₀  −0.019, C₁₁  −0.134, C₁₂  −0.168.

PM3
C₁ 0.063, C₂ 0.063, C₃  −0.164, C₄  −0.072, C₅  −0.095, C₆  −0.169, O₇  −0.208, O₈  −0.166,
C₉ 0.047, C₁₀  −0.017, C₁₁  −0.143, C₁₂  −0.170.

Intermediate
AM1
C₁ 0.054, C₂ 0.058, C₃  −0.170, C₄  −0.067, C₅  −0.124, C₆  −0.185, O₇  −0.230, O₈  −0.187,
C₉ 0.079, C₁₀  −0.096, C₁₁  −0.156, C₁₂  −0.224.

PM3
C₁ 0.065, C₂ 0.058, C₃  −0.147, C₄  −0.078, C₅  −0.095, C₆  −0.169, O₇  −0.208, O₈  −0.166,
C₉ 0.047, C₁₀  −0.019, C₁₁  −0.134, C₁₂  −0.168.

Eugenyl acetate
Transition
AM1
C₁ 0.057, C₂ 0.079, C₃  −0.197, C₄  −0.043, C₅  −0.136, C₆  −0.168, O₇  −0.190, C₈ 0.306,
O₉  −0.291, C₁₀  −0.253, O₁₁  −0.178, C₁₂  −0.081, C₁₃  −0.098, C₁₄  −0.157, C₁₅  −0.226.

PM3
C₁ 0.042, C₂ 0.081, C₃  −0.174, C₄  −0.054, C₅  −0.113, C₆  −0.161, O₇  −0.156, C₈ 0.351,
O₉  −0.327, C₁₀  −0.139, O₁₁  −0.160, C₁₂ 0.046, C₁₃  −0.020, C₁₄  −0.145, C₁₅  −0.166.

From calculating wave functions, we observe that the charge distributions are mainly located on electrowithdrawing oxygen atoms in each molecule. They also are situated on aromatic ring and the strand allyl. The charge density is much higher under AM1 than under PM3. So, these results are in accordance with their energy properties (see Table 1) and the electronic properties of each substituent. The acetate group is an electron withdrawing type which reduces the aromatic cycle charge while hydroxyl group is an electron donor type that can provide its charge to the aromatic ring and thus increases its nucleophilic effect (Table 2).

4. Conclusion

In the present work, we have studied the conformational structure of eugenol and eugenyl acetate under torsional angle effect by performing semiempirical calculations using AM1 and PM3 methods. From quantum calculations, we have evaluated the strain energy of conformational interconversion. To provide a better estimate of stable conformations, we have plotted the strain energy versus dihedral angle. So, we have determined five geometries of eugenol (three energy minima and two transition states) and three geometries of eugenyl acetate (two energy minima and one transition state). We have verified the presence of the intermediate form of eugenol which corresponds to the trans form (C or C′).

From the molecular orbital calculations, we deduce that the optimized trans form by AM1 method is more reactive than under PM3 method. We note that the charge distributions are mainly located on the aromatic ring and the strand allyl in each molecule. We can conclude that both methods are efficient. The AM1 method allows us to determine the reactivity and PM3 method to verify the stability.

Conflict of Interests

The author declares that there is no conflict of interest regarding the publication of this paper.

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