Formation of a ground state twisted-internal-charge-transfer conformer of 4-(dimethylamino)benzaldehyde

Ultraviolet absorption spectra and visible Raman spectra were collected for 4-(dimethylamino)benzaldehyde (DABA) in aqueous solution in the pH range of 9.56 to -1.08. Equilibrium arguments show that only the free base and the dimethylamino protonated species will be present in appreciable concentrations in this pH range. Protonation leads to increased sp³ hybridization about the amino nitrogen with rotation to form a twisted- internal-charge-transfer (TICT) conformer. Molar absorptivities were calculated for the 352 nm band and these values were used to calculate the angle of rotation of the dimethylamino group out of the ring plane. The Raman spectrum of DABA in concentrated hydrochloric acid solution is consistent with a TICT structure in which the amino group is rotated 90°out of the plane.