A Combined Experimental and Theoretical Study of ESR Hyperfine Coupling Constants for N,N,N',N'-Tetrasubstituted p-Phenylenediamine Radical Cations

A test set of N,N,N',N'-tetrasubstituted p-phenylenediamines are experimentally explored using ESR (electron spin resonance) spectroscopy and analysed from a computational standpoint thereafter. This computational study aims to further aid structural characterisation by comparing experimental ESR hyperfine coupling constants (hfccs) with computed values calculated using ESR-optimised J-style basis sets (6-31G(d,p)-J, 6-31G(d,p)-J, 6-311++G(d,p)-J, pcJ-1, pcJ-2 and cc-pVTZ-J) and hybrid-DFT functionals (B3LYP, PBE0, TPSSh, omega B97XD) as well as MP2. PBE0/6-31g(d,p)-J with a polarised continuum solvation model (PCM) correlated best with the experiment, giving an R2 value of 0.8926. A total of 98% of couplings were deemed satisfactory, with five couplings observed as outlier results, thus degrading correlation values significantly. A higher-level electronic structure method, namely MP2, was sought to improve outlier couplings, but only a minority of couples showed improvement, whilst the remaining majority of couplings were negatively degraded.

Automated summary

A test set of N,N,N',N'-tetrasubstituted p-phenylenediamines was experimentally explored using ESR spectroscopy and analyzed computationally. The computational study aimed to aid structural characterization by comparing experimental ESR hyperfine coupling constants (hfccs) with computed values. Among the various methods and functionals tested, PBE0/6-31g(d,p)-J with a polarized continuum solvation model (PCM) showed the best correlation with the experiment, with an R2 value of 0.8926. The majority of couplings were deemed satisfactory, but there were a few outlier results that significantly degraded the correlation values. The higher-level electronic structure method MP2 was attempted to improve these outlier couplings, but only a minority showed improvement while the majority were negatively affected.