Intensity Behaviour of the Long Wavelength UV Transition of the Pyridine Chromophore Under Substitution in the Para Position to the Nitrogen Atom

When pyridine is para substituted to the nitrogen atom, the intensity of the long wavelength electronic transition is strongly decreased compared to the ortho and meta derivatives. N belonging to the ring, plays the part of a donating center when a strong electron withdrawing substituent is in the para position, and the part of an attracting center when faced to a donating substituent. the local transition moment vector of N, is in both cases opposed to the vector of the substituent. Furthermore, the π HOMO is priviledged (instead of the two highest π MO in benzene), and this HOMO does not allow a strong interaction with the substituent in the para position.