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
Eugenol (4allyl2methoxyphenol) is a phenylpropene, an allyl chainsubstituted guaiacol. It is the main phenolic compound extracted from certain essential oils especially from clove oil, nutmeg, cinnamon, basil, and bay leaf [
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
Conformation structures of
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 PolakRibiere conjugate gradient algorithm with a gradient of 0.01 Kcal/mol (RMS). The following quantum chemical results are considered: heat of formation (Δ
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
Main calculated properties of eugenol and eugenyl acetate with semiempirical methods.
Entry  Eugenol  

Properties  AM1  PM3  









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  











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 (
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
The dihedral angle for rotation about C_{4}–C_{10} 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
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
Estimated conformation structures of eugenol and eugenyl acetate; (a1) and (a2)
From our molecular orbital calculations, we want to deduce the structurereactivity relationship depending on different conformations. First, AM1 and PM3 calculations show that the
The AM1 and PM3 calculations show that the
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
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
Mulliken charges of the optimized structures of eugenol and eugenyl acetate.
Compound  State  Method  Type charge 

Eugenol  Transition  AM1  C_{1} 0.065, C_{2} 0.058, C_{3} −0.148, C_{4} −0.078, C_{5} −0.103, C_{6} −0.165, O_{7} −0.208, O_{8} −0.166, 
PM3  C_{1} 0.063, C_{2} 0.063, C_{3} −0.164, C_{4} −0.072, C_{5} −0.095, C_{6} −0.169, O_{7} −0.208, O_{8} −0.166,  
Intermediate  AM1  C_{1} 0.054, C_{2} 0.058, C_{3} −0.170, C_{4} −0.067, C_{5} −0.124, C_{6} −0.185, O_{7} −0.230, O_{8} −0.187,  
PM3  C_{1} 0.065, C_{2} 0.058, C_{3} −0.147, C_{4} −0.078, C_{5} −0.095, C_{6} −0.169, O_{7} −0.208, O_{8} −0.166,  


Eugenyl acetate  Transition  AM1  C_{1} 0.057, C_{2} 0.079, C_{3} −0.197, C_{4} −0.043, C_{5} −0.136, C_{6} −0.168, O_{7} −0.190, C_{8} 0.306, 
PM3  C_{1} 0.042, C_{2} 0.081, C_{3} −0.174, C_{4} −0.054, C_{5} −0.113, C_{6} −0.161, O_{7} −0.156, C_{8} 0.351, 
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
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
From the molecular orbital calculations, we deduce that the optimized
The author declares that there is no conflict of interest regarding the publication of this paper.