The leaves of O. basilicum L. were collected in Mansoura, Egypt (latitude 31˚3'0"N, longitude 31˚23'0"E, temperature 24˚C - 30˚C, loam soil) from plants growing in the Mansoura University campus in July 2013. The taxonomic identification of plants was confirmed at the Botany Department, Faculty of Agriculture, Mansoura University, where a voucher specimen has been deposited.

The air-dried leaves (1.5 kg) of O. basilicum L. were subjected to hydrodistillation using a Clevenger-type apparatus for 4 hours. The oily layer obtained on top of the aqueous distillate was separated and dried with anhydrous sodium sulfate (Na₂SO₄). The extracted essential oil was kept in sealed air-tight glass vials and covered with aluminum foil at 4˚C until used for GC/MS analysis and biological activity tests. The yield of the essential oil was 24.25% (v ⁄w).

GC/MS data were acquired using (Trace GC-ISQ mass spectrometer, Thermoscintific, USA) equipped with A3000 autosampler and TG-5MS Capillary column of 30 m length, 0.25 mm i.d. and 0.25 μm film thickness. Temperature was programmed from 50˚C - 280˚C at a rate of 10˚C/min. Mass spectrometer in EI mode at 70 eV, source temperature, 200˚C; interface temperature, 220˚C; injector temperature, 220˚C. Diluted sample of 1 μl injected in splitless mode and mass scan, 50 - 600 amu. Helium was used as a carrier gas with 1 mL/min flow rate. The components of essential oil were identified tentatively by comparing their relative retention times and mass spectra with those of WILEY (Wiley Registry of Mass Spectral Data, 9th Edition Version 1.02) and NIST 05 (NIST/EPA/NIH mass spectral library version 2.0d) mass spectral database.

The fragmentation pathways of the main fragment ions formed from molecular ions of the studied compounds at 70 eV are interpreted through fragmentation Schemes 1-7.

Scheme 1. Fragmentation pathway of α-pinene.

Scheme 2. Fragmentation pathway of sabinene.

Scheme 3. Fragmentation pathway of β-pinene.

Scheme 4. Fragmentation pathway of d-limonene.

Scheme 5. Fragmentation pathway of eucalyptol.

Scheme 6. Fragmentation pathway of l-linalool.

Scheme 7. Fragmentation pathway of estragole.

In our study we are divide the studied compounds to two groups, the first group contain α-pinene, sabinene, β-pinene and d-limonene (isomers compounds) which have the chemical formula (C₁₀H₁₆). These four compounds represent the isomers compounds which can be readily distinguished from its mass spectra through unimolecular fragmentation processes. The second group contains l-linalool, eucalyptol (C₁₀H₁₈O) and estragole (C₁₀H₁₂O) compounds which have an oxygen atom in their structures.

The mass spectra of these compounds show, the relative intensity (R I) of the molecular ions [M]^+Ÿ at m/z = 136 for the first group are low (α-pinene R I = 10%, sabinene R I = 20%, β-pinene R I = 11% and d-limonene R I = 31%). These aliphatic compounds have the cyclic structures which hardly produce higher abundance molecular ions in comparison with relative intensities of the second group (eucalyptol m/z = 154 R I = 67%, estragole m/z = 148 R I = 100%) except linalool m/z = 154 which has linear structure compound which has no peak for the molecular ion. This is due to its low stability in the ion source.

The fragmentation processes of the first group are achieved by mean of their unimolecular dissociation of the parent ions by loss of C₃H₇ radical to produce the main fragment ion (base peak) [M-C₃H₇]⁺ at m/z = 93, which they are in stable cyclic structures (six or seven membered ring). The second fragmentation process of the molecular ion of is the formation of the fragment ion at m/z 121 which represents the ion [M-CH₃]⁺ with low relative intensities (α-pinene R I = 12%, sabinene R I = 8%, β-pinene R I = 13% and D-limonene R I = 30%). The base peak of the d-limonene in the first group is formed directly from the molecular ion by simple cleavages of C-C bond directly to form the fragment C₅H₈⁺ ion at m/z = 68 as shown in Schemes 1-4.

The fragmentation processes of the second group are achieved by mean of the loss of from the molecular ion to form the fragment ion [M-CH₃]⁺ at m/z 139 (R I = 63%) and at m/z 133 (R I = 20%) in the case of eucalyptol and estragole, respectively. On the other hand, the fragmentation process of the molecular ion of l-linalool achieved by mean of the loss of H₂O to form the fragment ion [M-H₂O]⁺ at m/z = 136 (R I = 11%) as shown in Schemes 5-7. In the case of eucalyptol, the mass spectrum show the fragment ion at m/z = 81 have represented the most intense peak (base peak) which is formed by secondary process [M-C₃H₆O-CH₃]⁺ as shown in Scheme 5.

The structure of each compound from our studied compounds was optimized using MNDO (semi-empirical molecular orbital method) to calculate thermochemical and energies data. The semi-empirical methods can be optimized for different purposes. For instance, MNDO method (see theoretical calculations paragraph and the fourth paragraph at introduction section) were used to calculate heats of formation and structures energies data of the studied compounds in the gas phase as shown in Table 3.

From these calculations, we have determined the values of heats of formation, total energies of the neutral and charged molecules and their ionization energies. These thermochemical data are important in the description of the conformational properties of the studied compounds and are not published before as our knowledge.

One can observe that the calculated values for the total energies for the first group (isomers structures) at the ground state (α-pinene = −34,707, sabinene = −34,708, β-pinene = −34,704 and d-limonene = −34,722 Kcal/mol), the calculated values of dipole moments (α-pinene = 0.113, sabinene = 0.122, β-pinene = 0.112 and

D-limonene = 0.145D) and binding energies values (α-pinene = −2525, sabinene = −2527, β-pinene = −2522 and D-limonene = −2541) have similar values, respectively.

Also, for the second group (an oxygen atom in their structures) the calculated values of total energies (eucalyptol = −42,819, linalool = −42,801 and estragole = −40,816 Kcal/mol), dipole moments (ecuolyptol = 1.385, linalool = 1.576 and estragole = 1.18 D) and binding energies values (eucalyptol = −2758, linalool = −2740 and estragole = −2402) have also similar values (Table 3).

One can calculate the ionization energies of these compounds as listed in Table 3. The lowest value is 8.0 eV for estragole compound, which is due to the phenyl group and oxygen atom in its structure. The presence of Π bonds and lone pair electron reduce the EI value. One of the important values of the chemical properties of organic compounds are the heats of formation ΔH_f(M). The theoretical values of ΔH_f(M) are calculated by semi- empirical MNDO method are 17, 16, 20 and 2 kcal/mole for the first group and −52, −34, −8 for the second group. From these values the first group is relatively less stable than the second group which has an oxygen atom in their structures.

To our knowledge, no experimental or theoretical values for ∆H_f(M) and ∆H_f(M)^+• and the energies values for the studied compounds were reported in the literature.