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摘要
The interest in porphyrin metal complexes is connected with the fact that metalloporphyrins have application as medicinal substances, analytical reagents, semiconductors and models for biological processes studies. There is a possibility to employ the metal etioporphyrins in the capacity of gas sensors. The metalloporphyrin complexes possess a high thermal stability and are not destructed at sublimation. However, the information about thermodynamics of cobalt, nickel, copper, zinc etioporphyrins-II vaporization is not available. This work is devoted to study of the vapor composition and to determination of the sublimation enthalpy of these complexes.
The Knudsen effusion method with mass spectrometric vapor monitoring was applied. Magnetic mass spectrometer MI 1201 modified for thermodynamic studies was used. The resolution of the instrument allowed the determination of the mass numbers for ions with an accuracy of +/- 2 amu at a range of 2-800 amu and an accelerating voltage of 5 kV.
The samples were evaporated from an effusion cell made of molybdenum, for which the ratio between the area ofevaporation and the area of the effusion aperture was similar to 1000. The cell was heated in a resistance furnace, and the temperature was measured using a tungsten-rhenium thermocouple WRe-5/20. Saturated vapors of the CoEP-II, NiEP-II, CuEP-II, ZnEP-II were studied in the temperature ranges of 465-528, 455-520, 460-528 and 475-536 K, respectively The energy of ionizing electrons was 40 eV.
The every mass spectrum consists of two groups of peaks corresponding to single and double charged parent ion and ions formed by consecutive removal of CH3, C2H5, C2H4 groups by an electron impact. The most intensive peaks in mass-spectrums belonged to singly charged parent ions. The dissociative ionization of porphyrin cycle can not be observed. Metal etioporphyrins-II in the gas phase are stable at least up to 510-520 K.
The temperature dependences In(IT)=f(1000/T) of ion currents for the [MEP-II](+) are well approximated by straight line. It is typical for the vaporization process proceeding in small temperature range without aggregate state changes. The equilibrium states in these systems were established quickly Hysteresis phenomena is not observed. The volatilities of MEP-II (M=Co, Ni, Cu, Zn) are comparable values. Experimental data treated by the second law of thermodynamics yielded the following sublimation enthalpies (kJ/mol): CoEP-II: 191 +/- 4, NiEP-II: 189 +/- 4, CuEP-II: 188 +/- 4, ZnEP-II: 191 +/- 4. It was concluded that the influence of the metal nature on the character of fragmentation and vaporization processes of MEP-II is negligible.
- 出版日期2012
