Suggest an explanation for the following observations:

a. Although the hydrocarbon calicene has so far defied synthesis, but it has estimated that it would have a dipole moment as large as

b. The masured dipole moment of 4-nitroaniline is larger than the value calculated using standard group dipoles

c. The dipole moments of furan and pyrrole are in opposite directions.

Calicene NO NH 4-nitroaniline pyrrole furan