Inter-Ring Torsions in N-Phenylmaleimide and Its o-Halo Derivatives: An Experimental and Computational Study

Abstract

Structures of N-phenylmaleimide and its o-halophenyl derivatives have been determined in the solid state and show the angle between the phenyl and pyrolinyl ring planes to vary from 49.5° to 83.9° with increasing values for compounds with the larger ortho halophenyl substituents (H < F ≲ Cl ≲ Br < I). Experimental torsions and trends in the series are supported by semiempirical AM1 and ab initio SCF, DFT, and MP2 calculations. Calculations (AM1) on N-phenylmaleimide modeling the torsional deformation between the rings show that the barrier to planarity has a lower energy than that through a perpendicular conformation. In its o-halo derivatives, molecular planarity is not possible, and torsional deformation proceeds through the perpendicular conformation with diminishing, possibly vanishing, barriers with increasing halogen size. For chloro, bromo, and iodo derivatives, twisted ground-state molecular conformations reside in broad minima essentially centered around the perpendicular conformations. The unusually strong, longer wavelength electronic bands observed in the solution spectra of the series were modeled by Zindo/S CIS computations at the optimum AM1 molecular geometries. The observed lower energy (285−305 nm) band for the parent through the o-bromo derivative appears to arise from a {n⊥(O,N); π (phenyl)} → π*(maleimide) transition. The next higher energy (250−285 nm) band appears to be essentially a phenyl π → π* transition. In the o-iodo derivative, a phenyl π → σ* (C−I) transition appears to contribute to the longer wavelength band. Trends in the observed electronic spectra in acetonitrile within the series of compounds accord roughly with the results of the computations.