Abstract:

 The overall protonation constants of three bridged crowned porphyrins and two cholesterol-porphyrins were determined by spectrometric methods and the quantum chemistry computation with HMO were made by 486 type microcomputer. The influence of electron effect and steric hindrance of substituting group on overall protonation constants was discussed.Experimental data and theoretical discussion showed that simple portonated cation (PH3+ ) has been formed before the formation of double protonated cation (PH42+ ), but simple protonated cation (PH32+) is unstable. Degree of porphyrins protonation depend on electron and steric effect. Protonation constants are increased by the donor groups through conjugate effect, while steric hindrance of substituting group reduces the constants decrease.

Key words: Porphyrin, Apparent protonation constants, Conjugate effect, Steric hindrance, UV-VIS spectra