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This study explains the vibration and interaction of p-xylene and effect of three elements (fluorine, chlorine and bromine) of the halogen family substitution on it. Basic chemistry of four, compounds p-xylene (PX); 3,6-diflouro-p-xylene (DFPX); 3,6-dichloro-p-xylene (DCPX) and 3,6-dibromo-p-xylene (DBPX) has been explained extensively using theoretical approach. Vibrational energy distribution analysis (VEDA) software was used to study the potential energy distribution (PED) analysis, bond length, bond angles and dihedral angles of PX, DFPX, DCPX, DBPX after optimization with GAUSSIAN 09 software. The trend in chemical reactivity and stability of the studied compounds was observed to show increasing stability and decreasing reactivity moving from DBPX, DCPX, DFPX to PX and this was obtained from the calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) values. Our results show that PX is the best electron donor (best nucleophile) while DBPX is the best electron acceptor (the best electrophile). We also observed that the substituted halogen increases the value of the bond angles but the effect is reduced as the size of the halogen increases. The maximum intensity and the frequency value for the maximum intensity of the different compounds was determined using the VEDA 04 software . From our natural bond orbital (NBO) 7.0 program analysis, the studied compounds are said to show biological activities as well as the intramolecular hyperconjugative interactions responsible for stabilizing the compounds. The NBO results also revealed that the non-bonding interaction existing between the lone pair electron on the halogen atoms and the aromatic ring increases the stability of the halogen substituted para-xylene molecules. Multiwfn: A Multifunctional Wavefunction Analyzer was used for the spectroscopic plots.

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Background: The chemistry of cyclopropanes has been widely studied over the years as a result of its high reactivity which is due to its highly strained ring. The cyclopropane ring is highly significant in drug research as the 10th most frequently found ring in small molecules and as such, there is a need for research to improve further the reactivity of cyclopropanes in natural products and pharmaceuticals. In this work, we present an extremely comprehensive and detailed investigation on a variety of properties of cyclopropane including Geometrical properties, bonding nature, bond polarity index (BPI), natural bond orbitals (NBO), orbital charge analysis, Density of states (DOS), molecular electrostatic potential (MEP), UV-spectral analysis, electron ionization, affinity and accompanied process, local reactivity parameters and the effects of substitution of heteroatoms Nitrogen (N-doped C3H8), Oxygen (S-doped C3H8) and Sulphur (S-doped C3H8) each in the place of carbon atom (otherwise known as doping) and also the substitution effects of Fluorine, Chlorine and Bromine each on one hydrogen atom in cyclopropane molecule (halogenation) and investigate how it chemically affects the properties outlined above. Results: It is observed from conceptual (CDFT) results that S-doped C3S has the highest reactivity of all the molecules studied. The UV-spectral analysis also predicts doped C3S as the molecule that gives the highest Bathochromic shift of all the molecules. Some other properties studied also give us the most potential sites for electrophilic and nucleophilic attack by electrophiles and nucleophiles. The NBO investigation revealed the strongest stabilization to the cyclopropane molecule for the heteroatoms follow the trend C3S<C3N<C3O. It is also observed that for the halogens the strongest stabilization for the cyclopropane molecule follows the trend C3Br<C3Cl<C3F<C3. The strong intra-molecular hyperconjugation interaction of the 2-centered bond (BD) and CR electrons of C-C anti C-C bond in the ring leads to stabilization of the cyclopropane ring as evident from the order perturbation energy analysis. Conclusion: The results obtained from the research will help scientists further improve on the reactivity of cyclopropanes as a motif in drug discovery through substitution (halogenation) and doping with heteroatoms (O, S, N)

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