Spectroscopy and dynamics of excited states in maleimide and N-methyl maleimide: Ionic projection and ab initio calculations

Abstract

The state that is responsible for the strong one-photon absorption around 200 nm in the vapor absorption spectrum of maleimide and N-methyl maleimide has been investigated using excited-state photoelectron spectroscopy in combination with ab initio calculations. The projection of the wave function of the excited state on the ionic manifold done in this way reveals multiple, vibrationally resolved, ionization pathways to ground- and excited states of the radical cation, which provide direct evidence for electronic couplings with other, lower-lying states. From a comparison of the experimental intensity distribution over the ionic vibrational states with ab initio calculated Franck–Condon factors, we are able to elucidate the role of the various electronically excited states in the ionization process. The experiments also provide the first determination of adiabatic ionization energies in the two molecules. For maleimide values of 10.330 and 10.903 eV are found for D0 and D1, respectively; for N-methyl maleimide D0 is found at 9.897 or, in an alternative interpretation of the spectrum, at 9.676 eV. Calculations and experiment demonstrate that in this molecule the ground ionic state changes its character with respect to maleimide from a lone pair to a π orbital ionization.

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

“…Our calculated IE for benzene, 9.06 eV, compares also well with the experiment, 9.2459 ± 0.0002 eV [77]. Likewise, our computed adiabatic IE for Nmaleimide, 10.13 eV, is in concordance with a reported adiabatic estimation, 10.22 eV, using the TD-DFT/B3LYP/6-311þG(d,p) method [78]. The experimental IE of N-maleimide is not known.…”

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

“…A further calculation with a rather wordy discussion agreed with these assignments [13]. Ter Steege and Buma [14] have used more extensive calculations to assign the absorption as a p N -p* transition, the electron being promoted from the electron pair substantially localized on the amide nitrogen in a p-type p-orbital. Their analysis suggests that the transition is indeed a charge-transfer transition; we would now term the absorption as a np* transition of the second kind or n p -ct-p*.…”

Reaction of functionalized maleimides with versatile nucleophiles. Synthesis, electronic spectra and molecular orbital study

“…The highly accurate optimized geometries of a maleimide molecule have been reported using DFT (B3LYP)/ 6-311CG* [32] and p-CASSCF/ANO optimization [33], respectively, in which the carbonyl double bond length (C]O) embedded within the ring was examined in detail. The DFT length (1.206 Å ) is perfectly consistent with the experimental one obtained by the electron diffraction study in the gas phase [34] whereas CASSCF (1.193 Å ) slightly underestimated probably due to the lack of dynamical electron correlations.…”