Theoretical investigation on carbon-centered tri-s-tetrazine and its 10 derivatives

A novel species, carbon-centered tri-s-tetrazine (C₄N₉H₃), and its 10 derivatives (C₄N₉{single bond}R₃, where R = OH, F, CN, N₃, NH₂, NO₂, N{double bond, long}NH, N₂H₃, C{triple bond, long}CH, and CH{double bond, long}CH₂) have been studied computationally. Density functional theory (DFT) has been used to study the geometries, electronic structure, harmonic vibrational frequencies, ionization energies of the 11 compounds at the restricted (for neutrals) and the unrestricted (for cations) B3LYP/cc-pVDZ level of theory. Atoms in molecule (AIM) and natural bond orbital (NBO) analyses have been used to obtain the bonding properties. Valence bond (VB) theory is applied to explain the unusual pyramidal structure around the carbon-center and electron arrangements of orbitals. We found: (1) All the species possess novel bonding features and geometrical structures. The atoms on the periphery of each species are sp² hybridized. Each of these atoms offers an orbital to form an extensive conjugation system ₁₂π¹⁵ (a π system consisting of 12 centers and 15 electrons). The central carbon atom C13 is sp³ hybridized, which makes the non-planar molecule shape like a straw-hat. Atom C13 also participates in the conjugated π system with its sp³ hybridized orbital, thus forming an extensive ₁₃π¹⁶ conjugate π system covering the whole C₄N₉ framework. (2) The change of charge on C13 is the largest among all the atoms when the species is ionized and the atomic charges are redistributed. In other words, C13 is the attack center for electrophilic agents. Thus, the species is carbanion-like. (3) All the species have low ionization energies (IEs). The electron ionized mainly comes from C13. They may have wide applications in organic chemistry, in organometallic chemistry and in alkyl lithium chemistry once they are synthesized. © 2007 Elsevier Inc. All rights reserved.