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.