Effect of meta Electron-Donating Groups on the Electronic Structure of Substituted Phenyl Nitrenium Ions

Abstract

Density functional theory (UB3LYP/6-31G(d,p)) was used to determine substituent effects on the singlet-triplet-state energy gap for 21 meta-substituted phenylnitrenium ions. It was found that strongly electron-donating substituents stabilize the triplet state relative to the singlet state. With sufficiently strong meta electron donors (e.g., m,m'-diaminophenylnitrenium ion) the triplet is predicted to be the ground state. Analysis of equilibrium geometries, Kohn-Sham orbital distributions, and Mulliken spin densities for the triplet states of this series of nitrenium ions leads to the conclusion that there are two spatially distinct types of low-energy triplet states. Simple arylnitrenium ions such as phenylnitrenium ions as well as those having electron-withdrawing or weakly donating meta substituents have lowest-energy triplet states that are n,pi in nature. That is, one singly occupied molecular orbital is orthogonal to the plane of the phenyl ring and one is coplanar. These n,pi triplets are generally characterized by large ArNH bond angles (ca. 130-132 degrees ) and an NH bond that is perpendicular to the plane of the phenyl ring. In contrast, meta donor arylnitrenium ions have a lowest-energy triplet state best described as pi,pi. That is, both singly occupied molecular orbitals are orthogonal to the aromatic ring. Such pi,pi states are characterized by NH bonds that are coplanar with the phenyl ring and have ArNH bond angles that are more acute (ca. 110-111 degrees ). These triplet nitrenium ions have electronic structures analogous to those of meta-benzoquinodimethane derivatives.

Electronic Properties of Nitrenium Ions

“…Strongly electron-donating substituents in the meta position increase the value of the DE st , and with sufficiently strong donors, ground state triplets are predicted. 35 The small energy gaps that arise with meta donors can be traced to a change in the electronic configuration of the triplet state nitrenium ions. When the meta substituent becomes sufficiently electron donating, the triplet state can be described as a diradical cation formally resulting from an internal electron transfer from the substituent orbital to the nitrenium ion center (Fig.…”

Ab Initio MO Analysis of the Excited Electronic States of High‐Spin Quintet 2‐Methylphenylene‐1,3‐dinitrene

“…[7] Winter, Falvey, and Cramer reported that strongly electron-donating substituents at the m positions result in a (p,p*)-type triplet ground state in phenylnitrenium ions. [8] Our preliminary calculations have shown that OCA predicts correct trends of the band shifts for derivatives of phenylnitrene (2) with electronically weak substituents, but the OCA-based model is broken when a robust substituent such as a nitro group is introduced, as will be reported in a forthcoming publication. .…”

“…[2d, [6][7][8] Excited electronic states of high-spin nitrenes and carbenes have been documented for only a few systems, and all of the theoretical studies deal with triplet-state mononitrenes [9,10] or monocarbenes. [11] Disclosing the nature of the excited electronic states of high-spin molecular entities seems an important issue from the viewpoint of materials science.…”

Aryl Nitrenium Ions in Aqueous Solution

“…Although both singlet (S 0 ) and triplet (T 1 ) states of 1 exist, experiments and calculations [40][41][42][43][44][45][46][47] agree that the singlet is, with few exceptions, 37,38,46,47 the ground state due to donation of electron density from the p-orbitals of the aromatic ring into the empty p-orbital of the presumably planar nitrenium center. The triplet is accessible via triplet-sensitized photolysis, 33,37,38 but most chemistry that has been examined is due to singlet 1.…”