A Tale of Two Tails: Rotational Spectroscopy of N -Ethyl Maleimide and N -Ethyl Succinimide.

Broadband microwave spectra of N -ethyl maleimide (NEM) and N -ethyl succinimide (NES) have been recorded using chirped pulse Fourier transform microwave spectroscopy in the K a -band (26.5-40 GHz). The spectra for both molecules were fit to a Watson A-reduced Hamiltonian in the I r representation to obtain best fit experimental rotational constants (NEM: A 0 = 2143.1988(29), B 0 = 1868.7333(22), C 0 = 1082.98458(36); NES: A 0 = 2061.47756(14), B 0 = 1791.73517(12), C 0 = 1050.31263(11)), centrifugal distortion constants, and nuclear quadrupole coupling constants. While the heavy atoms of the five-membered ring of both molecules are planar, the ethyl chain has its terminal methyl group perpendicular to the ring. Along the relaxed potential energy curve for the ethyl dihedral angle (θ 1 = C(6)-C(5)-N-C(4)), the ethyl group experiences significant steric strain when it is in the plane of the ring, associated with the interaction of the ethyl group with the two carbonyl oxygens. This leads to calculated barriers of θ 1 = 1469 cm -1 and θ 1 = 1680 cm -1 in N -ethyl maleimide and N -ethyl succinimide, respectively.