Inter-ring torsions in N-phenylmaleimide and its o-halo derivatives: An experimental and computational study

Structures of N-phenylmaleimide and its o-halophenyl derivatives have been determined in the solid state and show the angle between the phenyl and pyr olinyl ring planes to vary from 49.5 degrees to 83.9 degrees with increasin g values for compounds with the larger ortho halophenyl substituents (H < F less than or similar to Cl less than or similar to Br < I). Experimental t orsions and trends in the series are supported by semiempirical AMI and ab initio SCF, DFT, and MP2 calculations. Calculations (AM1) on N-phenylmaleim ide modeling the torsional deformation between the rings show that the barr ier to planarity has a lower energy than that through a perpendicular confo rmation. In its o-halo derivatives, molecular planarity is not possible, an d torsional deformation proceeds through the perpendicular conformation wit h diminishing, possibly vanishing, barriers with increasing halogen size. F or chloro, bromo, and iodo derivatives, twisted ground-state molecular conf ormations reside in broad minima essentially centered around the perpendicu lar 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 e nergy (285-305 nm) band for the parent through the o-bromo derivative appea rs to arise from a {n perpendicular to(O,N); pi (phenyl)} --> pi*(maleimide ) transition. The next higher energy (250-285 nm) band appears to be essent ially a phenyl pi --> pi* transition. In the o-iodo derivative, a phenyl pi --> sigma* (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.