Photochemical Generation and Characterization of -Aminophenyl-Nitrilimines: Insights on Their Bond-Shift Isomers by Matrix-Isolation IR Spectroscopy and Density Functional Theory Calculations.

The intriguing ability of -phenyl-nitrilimine to co-exist as allenic and propargylic bond-shift isomers motivated us to investigate how substituents in the phenyl ring influence this behavior. Building on our previous work on the - and -OH substitution, here we extended this investigation to explore the effect of the NH substitution. For this purpose, -(4-aminophenyl)- and -(3-aminophenyl)-nitrilimines were photogenerated in an argon matrix at 15 K by narrowband UV-light irradiation (λ = 230 nm) of 5-(4-aminophenyl)- and 5-(3-aminophenyl)-tetrazole, respectively. The produced nitrilimines were further photoisomerized to carbodiimides via 1-diazirines by irradiations at longer wavelengths (λ = 380 or 330 nm). Combining IR spectroscopic measurements and DFT calculations, it was found that the -NH-substituted nitrilimine exists as a single isomeric structure with a predominant allenic character. In contrast, the -NH-substituted nitrilimine co-exists as two bond-shift isomers characterized by propargylic and allenic structures. To gain further understanding of the effects of phenyl substitution on the bond-shift isomerism of the nitrilimine fragment, we compared geometric and charge distribution data derived from theoretical calculations performed for -phenyl-nitrilimine with those performed for the derivatives resulting from NH (electron-donating group) and NO (electron-withdrawing group) phenyl substitutions.