NOTES
3296
The Enthalpy of Formation of Porphin' by Frederick R. Longo, John D. Finarelli,2*gb Edwin Schmalzbach, Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104
and Alan D. Adler N e w England Institute, Ridgefield, Connecticut (Received March 6, 1970)
0687'7
As a result of recent mechanistic and synthetic investigation^^"-^ it has become possible to prepare porphin and the ms-tetraphenylporphins in relatively large amount and in a high state of purity. Therefore, oxygen bomb calorimetry with these materials is now feasible. Because of the unusual structure and bonding in the porphyrin nucleus it would be very useful to have the enthalpy of formation of porphin, itself, as a standard for comparison. Any unusual bonding in the porphyrin would manifest itself in the AHf" value for porphin. In addit'ion, it would then be possible to make a study of the effects of substituents on the porphin nucleus. The heats of combustion at 15" for several natural porphyrins were determined using a microcalorimeter by Stern and Klebs4. Unfortunately, the experimental data lack sufficient detail for the calculation of acceptable values of AHf0268.
Experimental Method Porphin. The porphin was prepared and purified by the method of Beitchman and Adler5 and stored in vacuo. Anal. Calcd for C20H14N4: C, 77.40; H, 4.547; N, 18.05. Found: C, 77.31; H, 4.56; N, 17.98. The electronic absorption spectrum as measured with the Cary 14 recording spectrophotometer agreed with the literatures6 The mass spectrogram (Hitachi PerkinElmer RMU-6 mass spectrometer) showed a grouping corresponding to the parent, with a major peak at m/e = 309, (290", 60 eV), as expected.' The most intense reflections noted in the powder diffraction pattern for this material occurred a t 28 values of 7.55, 8.35, 15.45, and 17.15", confirming that our samples were in the monoclinic form having the cell dimensions reported by Webb and F l e i s ~ h e r . ~ The Adiabatic Calorimeter. We used a modified Parr adiabatic bomb calorimeter, Model 1200. The novel aspect of our calorimeter is its floating base, rapid response, proportional temperature controller which we designed to maintain the adiabatic jacket at the same temperature as the water surrounding the bomb. Figure 1 is the circuit diagram for this controller. Note that there are perfectly matched thermistors, T, and T b , in two arms of a Wheatstone bridge, 2. The Journal of Physical Chemistrg, Vol. 74, No. 17,1970
Thermister T b is positioned in the water surrounding the combustion bomb; T, is in the outer oil jacket of the calorimeter. When T b is at a higher temperature than T,, a heating circuit is energized and the oil temperature follows the temperature of the water surrounding the combustion bomb. Area 3, Figure 1, represents an operational amplifier which is used to step up the error signal generated by the Wheatstone bridge imbalance. The amplifier output is led to a pulse oscillator 4,which
5 PK
0.0 K
Figure 1. Diagram of temperature control circuit.
generates short, rapid pulses and triggers the siliconcontrolled rectifiers (SCR) which are connceted in inverse-parallel with the transformer secondary and in series with the heating element. The length of time that the signal is applied to the oscillator circuit determines what portion of the ac cycle passes through the transformer windings, d and e, to the SCR's and ultimately to the heater; it is this aspect of the circuit which makes the device a proportional controller. Our tests with this circuit show that it is capable of preventing the temperature difference between the water and the oil from exceeding 0.001" under static conditions and 0.02" in the rapid heating period during combustion. The other important modifications (1) This research is supported by the National Institutes of Health, Grant GM15019. (2) (a) NSF Fellow, 1965-1968; (b) Taken in part from a thesis presented by J. D. F. in partial fulfillment for the Ph.D. degree, Drexel University, 1969. (3) (a) A. D. Adler, F. R. Longo, and W. Shergalis, J. Amer. Chem. Soc., 86, 3145 (1964); (b) A. D. Adler, F. R. Longo, J. Goldmacher, J. Assour, and L. Korsakoff, J. Org. Chem., 32, 476 (1967); (0) A. D. Adler, L. Sklar, F. R. Longo, J. D. Finarelli, and M. G. Finarelli, J . Heterocyclic Chem., 5 , 669 (1968); (d) F. R. Longo, M . G. Finarelli, and J. B. Kim, {bid., 6 , 927 (1969). (4) A. Stern and G. Klebs, Ann., 505, 295 (1933). (5) S. Beitchman, Ph.D. Dissertation, Chemistry Department, University of Pennsylvania, 1967 (Advisor, A. D. Adler). (6) C. Rimington, S. F. Mason, and 0. Kennard, Spectrochim. Acta, 12, 65 (1958). (7) A. D . Adler, J. H. Green, and M.Mautner, Org. Mass Spectrosc., 2 , 849 (1969). (8) L. E. Webb and E. B. Fleischer, J . Chem. Phys., 43, 3100 (1965).
3297
NOTES which we made of the Parr calorimeter include improved stirring, the use of low-viscosity silicone oil instead of water in the outer jacket, insertion of a controlled, 20-ohm, flat nichrome wire heater (wound in the outer jacket and directly exposed to the silicone oil to reduce thermal lag), and the insertion of copper cooling coils in the base of the outer jacket. The heat equivalent of the calorimeter was determined by electrical calibration, using the method of Challoner, Gundry, and Meetham.9 The calibration was checked by determination of the heat of combustion of benzoic acid. We obtained an average value for five burnings of -771.48 kcal/mol with a standard deviation 0.02kcal/mol. This value differs from the defined value of - 770.908 kcal/mol by 0.074%. The temperature change caused by the combustion was measured with a Myers four-lead, platinum resistance thermometer (calibrated by Leeds and Northrup), coupled with a Leeds and Northrup Mueller bridge (Model G-2) and a MinneapolisHoneywell Magnetik null indicator, Model 2HG-1P.
Results In Table I are presented the values for the enthalpies of combustion, AH,", a t 298.15"K, for 7 burnings of porphin. All calculations are based on the stoichiometry C20H14N4(monoclinic) 23.50z(g) 20CO2(g) 97H20(1) 2N2(g) and corrections as suggested
+
+
Table I : E n t h d p y of Combustion for Porphin Sample
Mass
1
0.10196 0.09743 0.09977 0.10331 0.11036 0.09236 0.18039
2 3 4 5 6 7
AH,'
- 2094.93 - 2095.22 - 2094.84 - 2094.24
- 2094.96
-2093.84 - 2094.72
by Hubbard, Scott, and Waddington" were incorporated in the data reduction. The average value of AH," is -2094.68 kcal/mol and the standard deviation is 0.43 kcal/mol. From this average value and the standard enthalpies of formation for C02(g)and HzO(l) we calculated a value of -264.56 kcal/mol for the enthalpy of formation of porphin. The standard enthalpy of formation calculated using the reliable bond enthalpy data tabulated by COX^^^,^ and the sublimation enthalpy, 26 kcal/mol, given by Edwards, Dolphin, Gouterman, and Adler,13 is 3.154.5 kcal/mol, which is 419.0 kcal/mol higher than the experimental value, indicating that a peculiar bonding endows
the prophyrin nucleus with an unusual stability. We suggest that a "group enthalpy'' of - 39.0 kcal/mol be assigned to the porphin tetraradical (porphin minus its four methene bridge hydrogen atoms). This value of the group enthalpy was obtained using the Cox tabulations12a,band the bond lengths given by Fleischer and Webb.8 It can be used to estimate with acceptable accuracy the enthalpies of formation of ms-tetraphenylporphin and ms-tetraethylporphin, the burnings of which will be reported in a subsequent communication. To attribute 419 kcal/mol to resonance stabilization requires some justification. Theoretical calculations of delocalization energy show that the amount of delocalization stability is proportional to the number of identical units linked in conjugation. l4 Thus, the delocalization energy for a styrene is 43.6 kcal/mol, for stilbene 87.8 kcal/mol, and for tetraphenylethylene 174.9 kcal/mol. In addition, the delocalization stabilization doubles upon the cyclization of hexatriene to benzene. Also, the addition of the vinyl group to a benzene ring to form styrene theoretically should give an additional stabilization of approximately 8 kcal/mol. If we consider porphin as a cyclic tetramer of 2-vinylpyrrole with the loss of appropriate hydrogens, we can show that the resonance enthalpy should be quite high: the observed AHrO for pyrrole is 15.07 kcal/mol15 and the calculated AHr" is 46 kcal/mol.12a~bThus, the resonance energy of pyrrole is 31 kcal/mol, and 2-vinylpyrrole might be considered to have a resonance stabilization of 39 kcal/mol. Since porphin is considered as a cyclic tetramer of 2-vinylpyrrole, a resonance energy of 312 kcal/mol might be expected. This type of argument is not rigorous but it can rationalize 75% of the observed resonance stabilization of porphin. Acknowledgment. We wish to express our appreciation for the enlightening discussions and kind assistance of Drs. E. J. Prosen and E. S. Domalski of the National Bureau of Standards, Gaithersburg, Maryland, and to Mr. John J. Leonard for his assistance with the X-ray analysis.
(9) A. R. Challoner, H. A. Gundry, and A. R. Meetham, Trans. Roy. Soc., London, A247, 553 (1955). (10) (a) R. S. Jessup, J. Res. iVat. Bur. Stand., 29, 247 (1942); (b) R. S. Jessup, ibid., 36, 421 (1946). (11) W. Hubbard, D. Scott, and G. Waddington, J. Phys. Chem., 58, 152 (1954).
(12) (a) J. D. Cox, Tetrahedron, 18, 1337 (1962); (b) J. D. Cox, zbzd., 19, 1175 (1963). (13) L. Edwards, D. H. Dolphin, M. Gouterman, and A. D. Adler, J. Mol. Spectrosc., in press. (14) H. E. Zimmerman, "Quantum Mechanics Notes for Organic Chemists," American Chemical Society, 1969, p 164. (15) D. W. Scott, W. T. Berg, I. A. Hossenlopp, W. N. Hubbard, J. F. Messerly, S. S. Todd, D. R. Douslin, J . P. McCullough, and G. Waddington, J. Phys. Chem., 71, 2263 (1967).
The Journal of Physical Chemistry, Vol. 74, N o . 17, 1970
