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.

Effects of the charge on the structural, electronic and reactivity properties of 43 substituted N–Phenylmaleimides. A DFT study

“…For instance, at the ortho position, the calculated (experimental) torsion angles for F, Cl, and BrPM compounds are equal to: 66.8 (54.1), 88.0 (66.1), and 89.0 (71.1); Table 1. This tendency is consistent with the results of Miller et al [28] The different torsion angles of the halogen derivatives may be reflected in the different properties of the synthesized polymers from these compounds. For example, largest torsion angles may produce more rigidity of the polymeric chain [56e59].…”

“…This data allowed to know the geometry of the molecule, which was further optimized with the B3LYP/6-311þG(d,p) method, in order to obtain the electronic (molecular orbitals) and energetic (ionization energies, electron affinities and reactivity descriptors) properties. The obtained structural parameters, Table SIe1 in the Supporting Information (SI), are in good agreement with the theoretical results reported by other authors [24,26,28] as well as with the experimental values. For instance, the computed C ortho -C ipso -N-C carbonyl torsion angle, 47.1 , is near to the observed one, 49.5 .…”

“…For example, N-phenylmaleimide (PM) crystallises in the monoclinic space group with four formula units per cell [28]. This data allowed to know the geometry of the molecule, which was further optimized with the B3LYP/6-311þG(d,p) method, in order to obtain the electronic (molecular orbitals) and energetic (ionization energies, electron affinities and reactivity descriptors) properties.…”

“…Miller et al [28] have studied the effect of the size of the halogens (F, Cl, Br and I in the ortho position) on the inter-ring torsion angle. Theoretical calculations realized with the AM1 semiempirical method, revealed that the increase of the angle depends on the steric effect of the halogen.…”

nπ-ct-π* Transitions: nπ* Transitions of the second kind

“…Seliskar and McGlynn [10] decided that the intensity of the unusual band and the substantial shifts in position with changes in N-substitution were in agreement with a p-p* transition [9] and reported some semiempirical calculations to support this assignment. Two reports on N-arylmaleimides largely agreed with Matsuo, although the intensity of the unusual band was clearly a problem [11,12]. A further calculation with a rather wordy discussion agreed with these assignments [13].…”