Origin of depressed dipole moments in five-membered, unsaturated heterocycles

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