Effect of selected anions and solvents on the electronic absorption, nuclear magnetic resonance, and infrared spectra of the N-retinylidenebutylammonium cation
The specific conteranion and the solvent have been shown to regulate the electronic excitation energy of the N-retinylidene-n-butylammonium cation. Halogenated hydrocarbon solvents which can hydrogen bond with the anion shift the lambda-max to longer wavelengths, whereas the solvent dipole, acting as a bulk effect, shifts the wavelength-max to shorter wavelength. Here solvents which can donate two hydrogens for hydrogen bonding, such as cis- and trans-1,2-dichloroethylene and cis- and trans-1,2-dichlorocyclohexane, are used as solvents for the Cl-, Br-, and I- salts. As expected the cis solvents allow longer wavelengths than do the trans solvents. Results of nuclear magnetic resonance spectroscopy are shown to be in agreement with electronic absorption spectroscopy. The C-11 proton and the C-13 and C-9 methyl protons show a considerable downfield shift in the salts with respect to the Schiff base. Furthermore the same protons show a continuing downfield shift as the anions are exchanged from Cl-, Br- to I-. This is an agreement with the interpretation of greater positive charge delocalization as the anions are changed in the above manner. The infrared absorptions of the C - N group in the Schiff base and the protonated form are shown to be almost similar. This is rationalized by showing that the force constant can remain constant as the highly related factors bond order, bond distance, and the effective electronegativity are changed in a self-compensating manner.