Internal dynamics in azetidine: A microwave and ab initio study

The internal dynamics of interconversion between equivalent conformations d ue to the coupling between ring puckering and NH inversion in azetidine has been investigated by rotational spectroscopy and ab initio computations. A nalysis of the rotational spectra in the 8-220 GHz region has been complete d for the ground state and first four excited states of the ring-puckering vibration. Rotational transitions exhibit a characteristic doubling origina ted by tunneling between equivalent conformations through a C-2v barrier, w hich is related to symmetric (A(1)) and antisymmetric (B-1) inversion state s. Additionally, nuclear quadrupole hyperfine structure arising from the N nucleus could be resolved for low-J transitions. Accurate rotational and ce ntrifugal distortion parameters together with the energy difference between inversion states derived from mu (c)-type inversion transitions have been derived for each ring-puckering state using a two-state Hamiltonian. An eff ective monodimensional reduced potential function for the ring-puckering vi bration V(X)=10.82(X-4+14.29X-8.93X(2)-0.28X(3)) has been found consistent with the observed experimental variation of the rotational and centrifugal distortion constants with ring-puckering. This asymmetric single minimum po tential function supports the existence of only one stable equatorial form. The barrier to interconversion between equivalent equatorial conformers, r elated to the C-2v conformation of azetidine in which the ring atoms and th e NH group are coplanar, has been estimated to range between 1900 and 2600 cm(-1). The strong dependence of the dipole moment and quadrupole coupling constants with ring-puckering vibrational state evidence structural changes that occur along the ring-puckering coordinate. (C) 2001 American Institut e of Physics.