Rotational spectra and gas phase structure of the maleimide - Formic acid doubly hydrogen bonded dimer

Rotational transitions were measured for the maleimide - formic acid doubly hydrogen bonded dimer using a Flygare-Balle type pulsed-beam Fourier transform microwave spectrometer. No splittings caused by possible concerted double proton tunneling motion were observed. Experimental rotational constants (MHz), quadrupole coupling constants (MHz), and centrifugal distortion constants (kHz) were determined for the parent and three deuterium substituted isotopologues. The values for the parent are A = 2415.0297(10), B = 784.37494(38), C = 592.44190(33), D_J = 0.0616(64), D_JK = -0.118(35), D_K = -1.38(15), 1.5χ_aa = 2.083(14), and 0.25(χ_bb-χ_cc) = 1.1565(29). The hydrogen bond lengths were determined using a nonlinear least squares structure fitting program. Rotational constants for this complex are consistent with a planar structure, with an inertial defect of Δ = -0.528 amu Ų. The B3LYP calculation yielded rotational constants within 0.1% of the experimental values.