Designing arylcations for direct observation in solution: ab initio MO calculations of UV spectra

Abstract

A detailed study of the vertical UV/visible spectra of the phenyl cation and some derivatives has been undertaken to identify an aryl cation which, when produced by dediazoniation, will be observable by laser flash photolysis. Calculations on the phenyl cation itself confirmed that it has no strongly allowed transition in the desired region of the spectrum. It was found that the primary effect of heteroatom substitution external to the ring upon the spectrum arose from the alteration of existing orbital energies. Particularly important was the effect of the heterosubstituent on the energy of the empty σ orbital, located on the dehydro carbon. Also important was the polarisation of π orbitals by the substituent and, in some cases, the configurational mixing of states of lower symmetry. Several externally substituted systems showed promise in terms of being potentially observable; however, in the cases where the transitions were in the right region of the spectrum, the intensities were rather modest and, when transitions were calculated to possess large intensities, they were not usually in the most convenient UV spectral range. Internal heteroatom substitution leading to heterocyclic aryl cations was also investigated. As well as the alteration of orbital energies, polarisation of the π space, and configurational mixing, this type of substitution perturbed the spectrum of the phenyl cation in an additional way: new transitions involving the excitation of non-bonding electrons appeared. In the majority of cases, however, the heterocyclic cations did not exhibit particularly strong transitions in the desired spectral region; an exception was the pyrimidin-5-yl cation which was calculated to possess a particularly promising electronic state (1-¹A₁) but its diazonium ion precursor is synthetically inaccessible. Externally substituted pyrimidin-5-yl cations were calculated to show a persistently strong n–σ transition in the 270–290 nm wavelength range. In the 2-amino-4-hydroxypyrimidin-5-yl cation, the intensity of this transition was particularly high, and the precursor arenediazonium ion appears to be synthetically accessible.