Theoretical insight into electronic and molecular properties of halogenated (F, Cl, Br) and hetero-atom (N, O, S) doped cyclooctane
Introduction
Cycloalkanes are technologically and industrially indispensable; owing to their diverse applications in their normal or transformed state [1]. Their usage for different purposes are classified by the number of constituent carbon atoms in the ring, and the nature and size of their substituents (for the derivatives) [[1], [2], [3]]. They are applied as motor fuel, petroleum gas, natural gas and heavy fuels depending on the volatility degree, melting and boiling points and density, which are functions of their molecular sizes [1,3,4]. Cycloalkanes are grouped into; Small rings (C3–C4), common rings (C5–C7), medium rings (C8–C12) and large rings (C13) due to their ring sizes [5]. Unlike aromatic rings, in which their bonds are restrained from rotations, cycloalkanes exhibit structural conformations, and their compounds seems to settle in the most stable conformation [4,6,7]. Many cycloalkanes show conformations with significant energy differences [[6], [7], [8]]. Cyclooctane is an eight membered ring cycloalkane, with the formula, C8H16, it is a liquid at room temperature. The stability of cyclooctane has been studied through vigorous computational investigations [[8], [9], [10], [11]], and its numerous conformers differs with little or no energy difference [[9], [10], [11]]. Its main route of synthesis is the dimerization of butadiene, a process catalyzed by Ni complexes to give 1, 5-cyclooctadiene, which is hydrogenated to give cyclooctane [12]. Cyclooctane was believed not to undergo any form of reaction except those peculiar to saturated hydrocarbons, but alkane functionalization was achieved in 2009 using peroxide [[13], [14], [15]], this has exposed more of its chemistry, such that groups as phenylamino-group can be added to cycloalkanes [13,16,17].
In recent years, organic molecules have been doped in order to diverge the horizons of organic materials application. This concept of doping heteroatoms into carbon matrix can be dated back to the 1920's, and very much development have been recorded in the field of carbon material doping, exposing the transformations in the chemistry of the doped compounds [18,19]. Change in physicochemical properties; improved gas adsorption, catalytic properties, oxygen reduction in fuel cells, improved electronic properties, etc. Boron, Nitrogen, Oxygen, Phosphorus and Sulfur are heteroatoms involved in this process [20,21]. Also, the substitution of electronegative groups will introduce more reactive sites, hence diversify the reaction pathways of the compound. The halogens are well known to be the most electronegative in their respective periods, hence their presence in organic molecule, e.g. cyclooctane will distort the symmetry and electronic distribution in the normal molecule, altering their chemistry [22].
A clear picture of the reactivity of molecules can be achieved by using special and reliable computational tools for their investigations [23,24]. Chemical reactions entail the exchange of electronic charges, hence by understanding the electronic make-up, one can predict the behaviour of chemical species. The density functional theory (DFT) is a computational model which is useful in predicting the distribution of electron density around the compound under study [[24], [25], [26]]. An extension of the quantum mechanical theory based on Schrödinger's equation [27], as a function of electron density, the behaviour of complex systems can be computationally calculated. A wide spectrum of DFT functionals [23,25] have been developed in recent years (example. B3LYP and M062X), to assure accurate energy determination. This has eased the study of the properties of organic compounds and their reactions. Also, by studying the nature, length and angle of interatomic bonds of the compound, the stability and strains around the bond can be understood through vibrational and spectroscopic analyses. Many computer softwares have been developed in order to successfully study molecular properties, such as Gaussian 0.9 [28,29], vibrational energy distribution analysis (VEDA) [30] software, Multiwfn program [31], natural bond orbital analysis (NBO) program [32], etc. were developed in the last decades. Sakhaee, N., & Sakhaee, S [11], studied the pseudorotation in cyclooctane in 2020, using spherical conformational landscape (SCL) model. Their study was able to eliminate many nomenclature redundancies around its different conformational forms in literature, and also commented on the speed of phase (ϕ) distribution which minimizes torsional strains in cyclic compounds.
However, there is limited study on the reactivity of the higher membered cycloalkanes (such as cyclooctane) and most of their substituted-hetero-atom-doped derivatives. Imperatively, the electronic, reactivity and structural study of halogenated eight-membered cycloalkanes (cyclooctane) derivatives have not been reported. Hence, considering the vast application of cycloalkanes, and the limited research for these compounds, the reactivity and structural investigation of cyclooctane, bromocyclooctane, chlorocyclooctane, fluorocyclooctane, azocane, oxocane and thiocane using DFT with B3LYP functional will be exposed in the course of this study, with the aim of re-exploring an insight into the electronic and molecular properties to advance the applicability of the titled molecules in research and other fields of life. DFT calculations have been employed in this study to explain the global reactivity descriptors, condensed Fukui function, Laplacian bond order (LBO), bond polarity index (BPI), atomic charges, natural bond orbital analysis (NBO) which account for the nonbonding interactions of the titled molecules. Lastly, to validate the spectroscopic properties and vibrational specificities of the studied molecules. This will be achieved through the use of descriptive tools in the field of computational chemistry.
Section snippets
Computational details
All computations in this study was performed using the Gaussian 09 [29] software with GaussView 6.0.16 interface, Multiwfn [31], VEDA [29], and NBO version 3 [32] programs. Firstly, the ground state cyclooctane, halogenated cyclooctane, and the N, O, and S-doped cyclooctane molecules were optimized using Gaussian 09 with B3LYP functional [33], and 6-311++G(d, p) and aug-cc-pVDZ basic sets. The highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) studies,
Electrostatic potential (ESP)
The optimized structures of the titled molecules and electrostatic potential plots were rendered by means of multiwfn and Visual Molecular Dynamics (VMD) software [38] (Fig. 1). In the ESP plots, the results clearly show that strong electron density regions are associated with high negative maxima values and as such will function as potential sites of electrophilic attack and these regions are mainly localized over the halogen and hetero-atoms in the title molecules. Similarly, the positive
Conclusion
An extensive theoretical exploration on the reactivity and structural characterization of cyclooctane, bromocyclooctane, chlorocyclooctane, fluorocyclooctane, azocane, oxocane and thiocane have been carried out and reported in detail using the density functional theory (DFT) method. In this study, we reported elaborately on global reactivity descriptors, condensed fukui function, conceptual density functional theory descriptors, Laplacian bond order (LBO), bond polarity index (BPI), atomic
CRediT authorship contribution statement
John A. Agwupuye: Conceptualization, Formal analysis, Investigation, Writing – review & editing. Hitler Louis: Supervision, Conceptualization, Project administration. Obieze C. Enudi: Formal analysis, Resources, Writing – review & editing. Tomsmith O. Unimuke: Visualization, Resources, Data curation, Validation. Moses M. Edim: computer simulation of studied compounds, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research work was not funded by any external agency, however, Mr. J.A. Agwupuye is very thankful to H. Louis for their immense support and contributions and Ogar Francis for typing this manuscript.
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