Substituent Effect on the Aromaticity of Phosphazene

Important information about the possible reactions and other propeties of a molecule can be determined by the aromaticity. Therefore, measurement of aromaticity is very important. Although there are a few applications for the determination of aromaticity, Nucleus Independent Chemical Shift (NICS) calculations provide the easiest computation and best approach to the discussion. Phosphazene is a nonaromatic material with structural resemblance to benzene. Phosphazenes nonaromatic character can be modified by subtitution of the hydrogen atoms on phosphorous atoms with electron donating or withdrawing groups.


Introduction
Aromaticity has still been considered as an actively investigated area of chemistry.There are a few criterion for monocyclic compounds to be promoted as aromatic; they possess uncut conjugation in the main circle with the proper number of π-electrons (i.e., the Hückel rule).
While this criteria work sufficiently to predict the aromaticity of neutral and charged ring systems, it can not be an obvious indicator of aromaticity for more complex structures.
Aromaticity is described by a combination of properties in cyclic conjugated compounds.Generally, aromaticity has been expressed in terms of energetic, structural and magnetic criteria (Minkin, Glukhovtsev & Simkin, 1994;Schleyer & Jiao, 1996;Glukhovtsev, 1997;Krygowski, Cyranski, Czarnocki, Hafelinger & Katritzky, 2000;Schleyer, 2001;Cyranski, Krygowski, Katritzky & Schleyer, 2002).In 1996, Schleyer introduced an easy but an efficient way for definition aromaticity: Nucleus-independent chemical shift (NICS) (Schleyer, Maerker, Dransfeld, Jiao & Hommes, 1996), which is the calculated value of the negative magnetic shielding at some selected point in space, usually, at center of a ring or cage.Negative NICS data indicate aromaticity (-11.4 for naphthalene) and positive NICS data mean antiaromaticity (28.8 for cyclobutadiene) while small values of NICS are accepted as an indicator for nonaromaticity (-3.1 for 1,3-cyclopentadiene). NICS might be a useful indicator of aromaticity that often correlates well with the other energetic, structural and magnetic criteria for aromaticity (Jiao & Schleyer, 1998;Schleyer, Kiran, Simion & Sorensen, 2000;Quinonero, et al. 2002;Patchkovskii & Thiel, 2002).Resonance energies and magnetic susceptibilities can be used to define the overall aromaticity of a polycyclic compound, but do not provide information about the individual rings.However, NICS is an effective probe for local aromaticity of separate rings of polycycles.
In order to enlighten the effect of substitution on the phosphazene skeleton, we have applied NICS calculations using Density Functional Theory.The computed NICS values have been used to compare the stabilities of the structures.
The BLYP method shows a better improvement over the SCF-HF results.Its predictions are in quite good agreement with experiment (Scuseria, 1992;Sosa & Lee, 1993;Wilson, Amos & Handy, 2000).
The normal mode analysis for each structure did not yield any imaginary frequencies for the 3N-6 vibrational degrees of freedom, where N is the number of points in the system.This shows that the structure of each optimized structure corresponds to at least a local minimum on the potential energy surface.
The ground state geometry optimizations and NICS computations of all the compounds have been performed by the use of Gaussian 09 package program (Frisch et al., 2010).

Results and Discussion
Investigation of the effect of substitution of an electron donating or electron withdrawing groups with the hydrogens connected to the parent circle of well-known aromatic or nonaromatic compounds has been attractive scientists for both theoretical and experimental studies.In this work, phosphazenes aromaticity has been tried to increase with substitution with NO 2 , Ph and OMe groups theoretically, by the application of B3LYP/6-31+G(d,p) level of theory in order to judge their ground state stabilities and aromaticities.
The first step of the current study was obtaining the ground state geometries of all the compounds using B3LYP/6-31+G(d,p) level of theory.The geometry optimized structures of phosphazenes can be seen in Figure 1.  the phosphazene system almost an aromatic compound by enhancing the NICS data up to -5.64 ppm.Phenyl substitution (P-Ph) on the other hand, had no effect on the aromaticity of the parent compound.The nonplanar alignment of the phenyl groups has been thoughout to be responsible for this behaviour.Finally, a very strong electron withdrawing NO 2 groups were substituted by the hydrogens by the idea that they could pull the electrons towards themselves so that electrons located on nitrogens have been gained by the π-system of the ring.P-NO 2 molecule showed an enhanced aromaticity as expected (Table 1).