RELATIVE GROUND AND EXCITED-STATE ENERGIES OF CH3(CH=CH)5CH=NC4H9, ITS HYDROGEN-BONDED AND PROTON-TRANSFERRED SPECIES, AND CHARGE PARTITIONING AND DISTRIBUTION IN THE PROTONATED SCHIFF-BASE OF RETINAL

CH3(CH=CH)5CH=NC4H9 (compound 1) is structurally related to the Schiff base of retinal, the prosthetic group in visual pigments. Dilute solu tions of a weak acid (phenol) and 1 in a hydrocarbon solvent, when sub jected to decreasing temperature, show striking changes in electronic absorption spectra. Initially only the spectrum of compound 1 is prese nt, but as the temperature is lowered, the absorbance of 1 decreases, and the spectrum of the H-bonded form of 1 appears and increases. Cont inued temperature lowering then causes a decrease in absorption of the H-bonded form and an appearance and rise in absorption of the proton- transferred form of 1. Concentrations of the various species are measu red as a function of temperature, and by standard procedures, the ther modynamic constants for both reaction steps are computed. Values of DE LTAH0 are taken as relative energies among the three ground states, an d the lambda(max) value of each species yields relative energies among excited states. By employing data from electronic absorption spectros copy, nuclear magnetic resonance (NMR) and theoretical calculations fo r retinal Schiff base, charge partitioning between nitrogen and the po lyene chain and charge distribution among the carbon atoms of the poly ene chain are calculated.