Charge-sensitive vibrations in p-chloranil: the strange case of the C=C antisymmetric stretching.

We have combined DFT calculations with single-crystal polarized infrared spectra to reinvestigate the assignment of the C=C antisymmetric stretching mode b(2u)nu(18) of p-chloranil (CA). The frequency of this mode indeed seems to display a nonlinear dependence on the average charge on the CA molecule (rho), at variance with the behavior of the antisymmetric C=O stretching frequency. The DFT calculations show that the origin of the problem is a drastic, 2 orders of magnitude decrease of the infrared intensity of the C=C antisymmetric stretching upon electron addition. Therefore, no infrared band can be easily associated to this mode in charge-transfer (CT) solids with rho approximately > 0.5. On the other hand, a linear relationship between rho and the b(2u)nu(18) frequency is found in quasi-neutral CT complexes of CA.