ORIGIN OF DEPRESSED DIPOLE-MOMENTS IN 5-MEMBERED, UNSATURATED HETEROCYCLES

The smaller molecular dipole moments of cyclopentadiene, pyrrole, fura n, and thiophene, relative to their saturated analogs, are investigate d using the HF/6-311++G*//HF/6-311++G** level of theory. Investigatio n of the atomic origins of the molecular dipole moments demonstrates t hat the decreased dipole moments of the unsaturated heterocycles resul t from simple perturbations of the butadiene fragment, rather than a d elocalization of hetero-atom lone pairs into the pi system. Comparison with the origins of the molecular dipole moments of the saturated ana logs shows that a special delocalization mechanism is not necessary.