Abstract
In view of the fact that the phosphorus atom in its low co-ordination state (coordination numbers 1 and 2) has been termed as the carbon copy, there have been attempts to investigate, theoretically as well as experimentally, the effect of the exchange(s) of CH- moiety with phosphorus atom(s) (CH/P) on the structural and other aspects of the classical carbocyclic and heterocyclic systems. Tropylium ion is a well-known non-benzenoid aromatic system and has been studied extensively for its aromatic character. We have now investigated the effect of mono- and poly-CH/P exchange(s) on the aromaticity of the tropylium ion. For this purpose, the parameters based on the geometry and magnetic properties, namely bond equalization, aromatic stabilization energies (ASE), Nucleus-Independent Chemical Shift (NICS) values, (NICS(0), NICS(1), NICS(1)zz), proton nucleus magnetic resonance (1H-NMR) chemical shifts, magnetic susceptibility exaltation and magnetic anisotropic values of mono-, di-, tri- and tetra-phosphatropylium ions have been determined at the Density Functional Theory (DFT) (B3LYP/6-31+G(d)) level. Geometry optimization reveals bond length equalization. ASEs range from −46.3 kcal/mol to −6.2 kcal/mol in mono- and diphospha-analogues which are planar. However, the ions having three and four phosphorus atoms lose planarity and their ASE values approach the values typical for non-aromatic structures. Of the three NICS values, the NICS(1)zz is consistently negative showing aromatic character of all the systems studied. It is also supported by the magnetic susceptibility exaltations and magnetic anisotropic values. Furthermore, 1H-NMR chemical shifts also fall in the aromatic region. The conclusion that mono-, di-, tri- and tetra-phosphatropylium ions are aromatic in nature has been further corroborated by determining the energy gap between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) (HOMO − LUMO gap), which falls in the range, ca. 3 × 10−19–9 × 10−19 J. The systems having more than four phosphorus atoms are not able to sustain their monocyclic structure.
Highlights
The concept of aromaticity is of fundamental importance for explaining the structure, stability and reactivity of many molecules, due to which it continues to attract attention of many eminent theoretical chemists and forms the basis of many interesting monographs [1,2,3], research papers [4,5,6,7,8,9]and thematic issues [10,11,12]
Lowest Unoccupied Molecular Orbital (LUMO) (HOMO − LUMO gap), which falls in the range, ca. 3 × 10−19 –9 × 10−19 J
Hückel’s predictions based on the molecular orbital theory that conjugated monocyclic planar systems containing (4n + 2) π electrons should be aromatic, i.e., conjugatively stabilized [13], was modified by Platt [14], who broadened the scope of the postulation by including neutral as well as charged polycyclic systems in this category
Summary
The concept of aromaticity is of fundamental importance for explaining the structure, stability and reactivity of many molecules, due to which it continues to attract attention of many eminent theoretical chemists and forms the basis of many interesting monographs [1,2,3], research papers [4,5,6,7,8,9]. Schleyer et al [20] have reported improved results by calculating isomerization stabilization energies (ISE) instead of ASEs. The most reliable and commonly used indicators for validating aromaticity of different systems including the transition states are the magnetic criteria. ASEs, 1H-NMR chemical shifts, NICS(0), NICS(1), NICS(1)zz, magnetic susceptibility exaltation and occupied molecular orbital and lowest unoccupied molecular orbital (HOMO − LUMO energy gap) magnetic susceptibility anisotropy values and the energy difference of the highest occupied molecular for the tropylium ion and its mono-, di-, tri-orbital and tetra-phospha orbital and lowest unoccupied molecular (HOMO − LUMOanalogues energy gap)at forthe theDensity tropyliumFunctional ion Theory (DFT). As sustain discussedtheir later,monocyclic phosphatropylium ions having five or more phosphorus more phosphorus atoms level
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