The enthalpy of sublimation of 5,10,15,20-tetraphenylporphine and 5,10,15,20-tetrakis(4-methoxyphenyl)porphine

Abstract

The enthalpies of sublimation of 5,10,15,20-tetraphenylporphine (CA registry number 917-23-7, TPP) and 5,10,15,20-tetrakis(4-methoxyphenyl)porphine (CA registry number 22112-78-3, TMPP), were determined by the Knudsen effusion method using a quartz-crystal microbalance in a system designed specially to work at high temperatures. The system includes stainless steel-Pyrex glass joints over one of which the effusion cell holder is welded in such a way that the heating and temperature sensor for measurement and control are accessible without venting the system. The design permits precise measurements of the actual sample temperature under low-pressure conditions. From the experimental results the derived values for this property are:ΔcrgHmo (TPP, 550 K) = ( 171 ± 2) kJ · mol −1andΔcrgHmo (TMPP, 563 K) = (213 ± 10) kJ · mol −1. Combining these values with the previously reported standard molar enthalpies of formation in the condensed phase, the corresponding standard molar enthalpies of formation in the gas state at T= 298.15 K were calculated as ΔfHmo(g) = (839 ± 9) kJ · mol −1for TPP andΔfHmo (g) = (295 ± 14) kJ · mol −1for TMPP. An estimate of these enthalpies by the second-order Benson’s group additivity approach was computed based upon the assumption of total or null aromaticity in the porphyrin macrocycle. Using the enthalpies of formation of the gaseous atoms in their ground states, the enthalpies of atomisation were derived as:ΔatHmo (g) = (39125 ± 22) kJ · mol −1for TPP andΔatHmo (g) = (45277 ± 25) kJ · mol −1for TMPP. Recommended Laidler parameters were used to predict these values, considering a correction for four pyrrole ring strain contribution.