SPECTROSCOPIC EXAMINATION OF HYDROGEN BONDING and PROTON TRANSFER IN MODEL SCHIFF BASES RELATED TO THE VISUAL PIGMENT CHROMOPHORE

By means of a variable low temperature cell holder, double beam spectra were obtained to temperatures as low as 88 K. As the temperature was lowered for a 3McP solution containing N‐2,4,6,8,10‐dodecapentaenylidene‐n‐butylamine (2) and phenol, three separate species could be detected, (i) The first spectrum taken at room temperature exhibits sharp fine structure and is the unperturbed spectrum of 2. As the temperature is lowered, a completely new spectrum with fine structure develops at slightly longer wavelength while simultaneously the old fine structure disappears. (ii) The new spectrum belongs to the H‐bonded species formed between 2 and phenol. As temperature is further lowered, the second spectrum disappears as a new band develops at about 440 nm. (iii) This is the proton transferred species. The same phenomenon is repeated when p‐nitrophenol and acetic acid are substituted for phenol. The ‐ΔH and ΔS values for the H‐bonding process between phenol and 2 in 3MeP are 3900 cal mol‐1 and 1.4 cal dgr‐1 mol‐1 respectively. Additional experiments firmly establish the process to be H‐bonding with the phenolic hydroxyl group. Alcohol also H‐bonds with 2 but does not proton transfer.