The rotational spectrum, internal rotation, and structure of NH ₃-CO₂

The radio frequency and microwave spectra of NH₃-CO₂ have been measured using the molecular beam electric resonance technique. The spectrum is characteristic of an asymmetric top in which the NH₃ subunit exhibits effectively free internal rotation. The spectroscopic constants obtained for the ground internal rotor state are presented below: B+C/2(MHz) 3756.178(3), B-C/2(MHz) 597.4(2), A-B+C/2(MHz) 8035(8), Δ_J(MHz) 0.0240(4), δ_K(MHz) 0.20(2), Δ_JK(MHz) 0.23(5), δ_J(MHz) 0.007(1), eQq_aa(MHz) -3.175(4), eQq _bb(MHz) 1.557(9), eQq_cc(MHz) 1.617(11), μ(D) 1.7684(14). The N-CO₂ framework of the complex has C_2v symmetry with a N-C weak bond length of 2.9875(4) Å. The average bending angle of the NH₃ subunit is 22.71(5)° with a difference in amplitude of 1.0(4)° between the in plane and out of plane excursions. The weak bond stretching force constant is 0.070(1) mdyn/Å and the induced dipole moment is 0.411(2)D.(B+C)/2 for the first excited internal rotor state (|m|=1) is 3753.008(4) MHz and the quadrupole coupling constant eQq _aa^|m|=1=-3.176(9) MHz is identical with that measured for the ground internal rotor state.