Microwave spectrum and gas phase structure of maleimide

The rotational spectrum of maleimide was measured in the 5-12 GHz range using a Flygare-Balle type, pulsed-beam Fourier transform microwave spectrometer. Rotational transitions were measured for the parent, all unique singly substituted ¹³C isotopologues, and an -ND, deuterium substituted isotopologue. The parent (or normal isotopologue) rotational constants, centrifugal distortion constants, and quadrupole coupling constants are A = 6815.3251(12) MHz, B = 2361.85011(64) MHz, C = 1754.32750(64) MHz, D_J = 0.232(24) kHz, D_JK = 0.546(54) kHz, 1.5χ_aa = 2.4227(53) MHz, and 0.25(χ_bb-χ_cc) = 1.3679(15) MHz. A best fit gas phase structure was determined using the experimental rotational constants of the isotopologues and some parameters from calculations. The inertial defect is Δ = -0.054 amu Ų, indicating a planar structure for maleimide, with no large amplitude motions observed on the -NH hydrogen atom. Calculations using B3LYP/aug-cc-pVTZ provided rotational constants which are much closer (within 1-2%) to the experimental values compared to the MP2/aug-cc-pVTZ calculated values.