A thermochemical study of 5,10,15,20-tetraphenylporphine zinc(II) by rotating bomb combustion...

doi:10.1006/jcht.2001.0911Available online at http://www.idealibrary.com on

J. Chem. Thermodynamics2002, 34, 193–204

A thermochemical study of5,10,15,20-tetraphenylporphine zinc(II) byrotating bomb combustion calorimetry and byKnudsen effusion experiments

Rodrigo Patino, Myriam Campos, and Luis Alfonso Torresa

Departamento de Quımica, Centro de Investigacion y de EstudiosAvanzados del IPN, Apartado Postal 14-740, 07000 Mexico D.F., Mexico

The standard molar enthalpies of combustion{1cHom(cr)} and formation{1fH

om(cr)} at

T = 298.15 K (po= 0.1 MPa) for the 5,10,15,20-tetraphenyl-21H,23H-porphine zinc

(ZnTPP, Chemical Abstracts registry number 14074-80-7), were determined by rotatingbomb combustion calorimetry. To dissolve the zinc oxides formed during the combustionreaction, an amount of aqueous solution of nitric acid was put into the bomb at thebeginning of each experiment; in this way, when the bomb is rotated, a well defined finalstate is achieved. The corresponding enthalpy of sublimation(1

gcrH

om) at T = 559 K

was determined by the Knudsen effusion method using a quartz-crystal microbalance.The derived enthalpies of formation{1fH

om(g)} and atomization{1atHo

m(g)} for gaseousZnTPP were computed from the experimental measurements. A summary of results (T =

298.15 K; po= 0.1 MPa) is shown below:

ZnTPP

1cHom(cr)/(kJ · mol−1) −22021± 7

1fHom(cr)/(kJ · mol−1) 354± 9

1gcrH

om/(kJ · mol−1) 196± 3

1fHom(g)/(kJ · mol−1) 550± 9

1atHom(g)/(kJ · mol−1) 39109± 22

With the previously reported enthalpy of formation value for the free porphyrin, the meandissociation enthalpy of the N–Zn co-ordination bond was determined asD(N–Zn) =

(185±6) kJ · mol−1 in the studied compound.c© 2002 Published by Elsevier Science Ltd. All rights reserved.

KEYWORDS: energy of combustion; enthalpy of formation; enthalpy of sublimation;porphyrins; N–Zn bond energy

aTo whom correspondence should be addressed (E-mail: [email protected]).

0021–9614/02 c© 2002 Published by Elsevier Science Ltd. All rights reserved.

194 R. Patino, M. Campos, and L. A. Torres

1. Introduction

Porphyrinic metallic complexes are an enormous source of research, mainly to developnew materials and to clear many biological systems, involving the high redox activityof metalloporphyrins.(1) As catalysts, halogenated iron- and zinc-porphyrins have shownexceptional stability and efficiency for epoxidation and hydroxylation.(2) Additionally,since porphyrins are highly sensitive chromogenic reagents, they are used as ligands in theseparation and determination of metal ions by chromatography.(3) Due to their capabilityto co-ordinate one or two more axial ligands, zinc porphyrins have been proposed asmodel compounds to mimic the behaviour of the metal centre in heme proteins.(4) Theability of these metallic porphyrins to co-ordinate solvent molecules has been foundin some thermodynamic studies.(5–7) Moreover, zinc porphyrins have been proposed asselective diamagnetic shift reagents in nuclear magnetic resonance.(8) They have beenproved too in the inhibition of acetylcholinesterase enzyme in the nervous system(9) andin photodynamic therapies to detect and treat tumours.(10)

Zinc porphyrins show ability as photocurrent(11) and photoionizing(12) compounds, andtheir nitrogen–ligand complexes are studied as photochemical materials.(13,14) They areinvolved in materials with magnetic(15) and semiconductor(16) properties, and are proposedas oxygen(17) and nitrogen dioxide(18) sensors. Zinc porphyrins combined with fullereneshave a promotion of photoinduced electron transference in electrochemical cells.(19,20)

Dimers(21) and other polymer arrays(22,23) of these porphyrins are used as models ofphotosynthetic light harvesting and energy transference systems. Another multiporphyrinsystems can be designed in order to reach specific necessities.(24,25)

In contrast with the great importance of metallic porphyrins, its fundamental thermo-dynamic properties as the enthalpy of formation in the condensed and gas states have notbeen determined as of now. A major research project in this laboratory is the systematicthermochemical study of these molecules, leading to the experimental determination ofenthalpies of atomization, which was started some time ago and has produced some resultsfor the free ligands.(26,27) The knowledge of these quantities will permit the calculation ofthe mean dissociation enthalpy for the four N–metal co-ordination bonds.

By the other hand, combustion calorimetry of organometallic and co-ordinationcompounds has been poorly tested. This is related with the problems to achieve a completecombustion reaction in a bomb and to produce a well defined final state. A moving bombmust be used in order to have high accuracy in the results for these sort of experiments.(28)

The results from combustion calorimetry experiments of the synthetic metalloporphyrin(figure1): 5,10,15,20-tetraphenyl-21H,23H-porphine zinc (ZnTPP, Chemical Abstractsregistry number 14074-80-7) are reported in this work. The use of an aqueous solution ofHNO3 into a rotating-bomb has permitted one to obtain a reproducible final state because,after combustion, Zn oxides are dissolved in this solution and in the same calorimetricmeasurement. Additionally, the enthalpy of sublimation was determined by the Knudseneffusion method. The sensibility of the sublimation equipment permits precise andreliable results. With the experimental results the mean dissociation enthalpy of the N–Znco-ordination bond was calculated for ZnTPP.

Experimental thermodynamics of a zinc porphyrin 195

N

N

NN Zn

FIGURE 1. 5,10,15,20-tetraphenyl-21H,23H-porphine zinc (ZnTPP).

2. Experimental

The ZnTPP was obtained from Aldrich Chemical Co. As provided, the sample wascontaminated with the corresponding chlorin (a 7,8-dihydropophyrin) which was verifiedby 1H n.m.r. There is a characteristic signal (δ = 4.15) attributed to chlorin, that isnot found when sample is purified. The chlorin elimination was achieved as previouslydescribed.(29)

From trial experiments, quantification of zinc after the combustion reaction revealedlower quantities than expected. The visible absorption spectrophotometry analysis of thesample showed that ZnTPP was mixed with a small amount of free 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP). There is a characteristic band (λ = 650 nm) for TPP, butnot for ZnTPP (figure2). Then, as described by Abrahamet al.,(30) the sample wasdissolved in boiling chloroform and refluxed with a saturated solution of zinc acetate inmethanol. Filtration afforded the pure ZnTPP as purple crystals, that were dried overnightat T = 360 K in a vacuum oven. Purity was confirmed by1H n.m.r. and by visibleabsorption spectrophotometry. Decomposition of the ZnTPP occurs during melting andd.s.c could not be used for purity determination of the sample.

Benzoic acid (SRM 39j) used as a combustion calorimetric standard and auxiliarymaterial was supplied by NIST. Oxygen (x = 0.9999) was obtained from Alphagaz(U.S.A.). A rotating-bomb combustion calorimeter (Argonne National Laboratory design)was used to perform the combustion experiments. The combustion bomb (Parr InstrumentCo., 1004C), used in the inverted position, is platinum lined with an internal volume of0.3452 dm3. All the experiments were carried out in the presence of oxygen atp =

3.0 MPa and a volume of 10 cm3 of an HNO3(aq) solution (c = 2.0 mol · dm−3) intothe bomb. Then, 200 s after ignition, the combustion bomb was rotated until the end ofeach experiment.

The temperature of the jacket was kept atT = (298.15± 1 · 10−2) K with a precisiontemperature controller (Tronac, PTC 41) and a water circular (Lauda, RM20). During each


a b

500 600 700 500 600λ /nm λ /nm

−log

(I/I 0

)

−log

(I/I 0

)

FIGURE 2. The visible spectra for: a, crude sample (dotted line) and free-chlorin sample (dashedline); and b, final purified sample;−log(I /I0) denotes the absorbance andλ is the wavelength of theradiation (in nm).

experiment, the temperature of the calorimetric vessel (with 2708 g of distilled water) wasmeasured every 10 s with a linearized thermistor connected to a multimeter (Keithley,2001) coupled to a personal computer for automatic data collection. Zoom graphics of thedata were used to determine the beginning and the end of the main period. The correctedtemperature rise was calculated using the Regnault–Pfaundler method as described byWadso.(31)

The energy equivalent of the calorimeterε(calor) was determined from five com-bustion experiments of benzoic acid with a certified massic energy of combustionunder experimental conditions of1cu = −(26434± 3) J · g−1. The mean value isε(calor) = (14039.2 ± 0.4) J · K−1; the quoted uncertainty represents the standard devia-tion of the mean. Due to the lack of some information, mainly the solubility and enthalpyof solution of O2 and CO2 in aqueous HNO3 containing Zn(NO3)2, comparison experi-ments were carried out in order to reduce errors on the reduction to standard states. Then,a small ZnO pellet was put in the platinum crucible near a benzoic acid pellet to be burned.After benzoic acid combustion, the bomb rotation permitted the dissolution of ZnO in theaqueous HNO3. Extreme care was taken to detect the correct final point of the main periodbecause zinc oxides dissolution is a slow process and involves a very small quantity ofenergy. Actually the comparison experiment leads to a value ofεcomp(calor), determinedunder particular conditions required by the combustion experiment and that include theenergy related to the zinc oxide reaction-solution in aqueous HNO3. Therefore, from 12comparison experiments, the mean value ofεcomp(calor) = (14044.9 ± 3.2) J · K−1, wasused to calculate the massic energy of combustion of ZnTPP. This procedure is valid if thecomparison experiments are designed in such a way to reproduce as closely as possiblethe final state obtained after combustion reaction of ZnTPP.

The energy of combustion of purified ZnTPP was determined from 11 independentexperiments. To ensure complete combustion, a pellet of around 500 mg of benzoic


TABLE 1. Physical properties atT = 298.15 K: M is the molar mass of the sample;ρ is thedensity;(δu/δp)T is the isothermal variation of the massic energy with pressure; andCp is the

massic heat capacity (estimated values are in parenthesis)

Substance Composition Ma ρ (δu/δp)T Cp

g · mol−1 g · cm−3 J · g−1· Mpa−1 J · K−1

· g−1

ZnTPP C44H28N4Zn 678.1 1.275b (−0.1) 1.046c

Zinc oxide ZnO 81.39 5.610 (−0.1) 0.519c

Benzoic acid C7H6O2 122.121 1.320(38)−0.115(38) 1.21(38)

Cotton CH1.774O0.887 27.9903 1.5(38)−0.289(38) 1.7(38)

a Molar masses are based on the 1995 IUPAC recommendation.(36) b Density determined by x-raydiffraction.(37) c Heat capacity determined by d.s.c. in this work.

acid was required as an auxiliary material. Reduction of the amount of benzoic acidleads to incomplete combustion. After combustion experiments, the amount of finalHNO3 in the aqueous solution was quantified by titration with standardized NaOH(aq)and the final Zn(II) quantity in solution was determined by titration with standardizedethylenediaminetetraacetic acid.(32) Generally, after both comparison and combustionexperiments, a thin film of gray residuals on crucible was observed with a mass corre-sponding to less than two per cent of the initial zinc quantity contained in the originalsample, experiments with bigger residuals could not reproduce energy values and werediscarded. Corrections to standard states for the experimental reactions were computedaccording to the usual procedure.(33,34) Energy values for HNO3 decomposition wereused from Wagmanet al.,(35) The physical properties summarized in table1 were usedto transform apparent mass to mass, and for calculations of the standard massic energies.The energy of combustion of the ZnTPP sample contaminated with a small amount ofTPP was determined from five experiments and the mean value(1cuo

= −33162 J· g−1),quite different from the value for the purified sample.

The enthalpy of sublimation was determined by the Knudsen effusion method. Theexperimental system, procedure and data analysis have been reported elsewhere.(27) Aspecial Knudsen effusion cell holder was constructed with glass–glass, metal–metal andmetal–glass joints, to work at high temperatures as required by the very low vapourpressure of ZnTPP. The effusion orifice diameter was 1.2 mm. Samples of around 20 mgwere placed into the effusion cell. Vacuum atp = 10−3 Pa was achieved with aturbomolecular vacuum pump (Pfeiffer, TSH 050) and measured with the IKR sensor(Balzers, 010) connected to a cold cathode gauge control (Pirani, PKG020). Heating ofthe effusion cell was made with an 80� resistance cartridge and a RTD sensor is usedto measure and control the temperature. Both elements are connected to an electronictemperature controller (Micromega, CN77000).

On the surface of a piezoelectric quartz crystal, working as a mass detector and placedat the end of a liquid N2 cold finger, the sublimed substance is condensed in front ofthe effusion orifice. The quartz crystal is connected to an external 6 MHz oscillator(QSK/QSG) and then to a quartz thickness and rate monitor (BAL-TEC, QSG060). The


TABLE 2. Summary of ideal reactions during comparison experiments

Comparison experiments

1. 15.7 · (C7H6O2 + 7.5O2 = 7CO2 + 3H2O)

2. ZnO+ 45.2HNO3 • 1243.2H2O = Zn(NO3)2 • 43.2HNO3 • 1244.2H2O

3. a15.7C7H6O2 + 117.8O2 + 45.2HNO3 • 1243.2H2O + ZnO =

109.9CO2 + Zn(NO3)2 • 43.2HNO3 • 1291.3H2O

a Reaction 3 is the addition of reactions 1 and 2. Symbol· refers to a multiplication;symbol• refers to a chemical association.

thickness increment of the crystal is automatically collected by a personal computer. Fourseries of effusion experiments were made betweenT = 555 K andT = 567 K. The rateof sample condensing is determined at each temperature; these values are proportional totheir corresponding vapour pressure. A least-squares fitting using the Clausius–Clapeyronequation, lead to the enthalpy of sublimation1

gcrHo

m.

3. Results

The idealized chemical process occurring during the comparison experiment is displayedin table 2. Combustion of benzoic acid and dissolution of zinc oxide are representedby equations (1) and (2), respectively. Addition of these equations leads to the totalthermochemical process, represented by equation (3), in which the right-hand termincludes the water molecules produced during experiment. Because the enthalpy offormation of Zn(NO3)2 in aqueous HNO3 is unknown, the energy associated to reaction 2was considered as included into theεcomp(calor).

The idealized chemical process occurring during the combustion experiment is displayedin table3. Combustion of benzoic as an auxiliary material is represented by equation (4).Combustion of ZnTPP is represented by equation (5). Dissolution of zinc oxide producedfrom combustion of ZnTPP is represented by equation (6). Addition of these equationsleads to the total idealized process represented by equation (7), that includes the waterproduced during combustion of benzoic acid and ZnTPP.

The amount of CO2 at the final state of both, comparison and combustion experiments,as shown in equations (3) and (7), is practically the same. The slight dilution differencebetween both equations can be energetically unconsidered. Under these conditions theenthalpy of formation of ZnTPP is calculated using the equation (5). The typical massesand energies involved in the comparison and combustion experiments are presented intable 4 in order to show the scheme of calculation of the standard massic energy ofcombustion.

The results for 12 comparison experiments are summarized in table5. The mean value ofεcomp(calor) = (14044.9 ± 3.2) J · K−1 was used to calculate the standard massic energyof combustion of ZnTPP. The results of 11 combustion experiments of ZnTPP, leading to1cuo(ZnTPP, cr) = −(32453.5 ± 9.6) J · g−1 are shown in table5. The uncertainty rep-resents the standard deviation of the mean. Using anε(calor) = (14039.2 ± 0.4) J · K−1,


TABLE 3. Summary of ideal reactions during combustion experiments

Combustion experiments

4. 9.3 · (C7H6O2 + 7.5O2 = 7CO2 + 3H2O)

5. C44H28N4Zn + 51.5O2 = 44CO2 + 14H2O + ZnO+ 2N2

6. ZnO+ 45.2HNO3 • 1243.2H2O = Zn(NO3)2 • 43.2HNO3 • 1244.2H2O

7. aC44H28N4Zn + 9.3C7H6O2 + 121.3O2 + 45.2HNO3 • 1243.2H2O =

109.1CO2 + Zn(NO3)2 • 43.2HNO3 • 1286.1H2O + 2N2

aReaction 7 is the addition of reactions 4, 5 and 6. Symbol· refers to a multiplication;symbol• refers to a chemical association.

TABLE 4. Typical masses and energy quantities for the comparison andcombustion experiments atT = 298.15 K (po

= 0.1 MPa)

Comparison experiment Combustion experiment

m′(ZnTPP)· g−1 0.292415

M ′′(benzoic acid)· g−1 0.845485 0.499275

m′′′(cotton)· g−1 0.002414 0.002507

m(ZnO)· g−1 0.033390

m(Pt)· g−1 12.989781 12.990764

ni (Zn) · mol−1 0.00041 0.00043

ni (H2O) · mol−1 0.52 0.52

ni (HNO3) · mol−1 0.02 0.02

1T/K = (ti − t f + 1tcorr) · K−1−1.58979 −1.61614

ε(calor)(1T) · kJ−1−22.3270 −22.6986

ε(cont)(1T) · kJ−1−0.0808 −0.0820

1udec(HNO3) · kJ−1 0.0000 0.0232

1U (corr. To std. states)· kJ−1 0.0346 0.0330

−m′′′· 1cuo(cotton)· kJ−1 0.0409 0.0425

−m′′· 1cuo(benzoic acid)· kJ−1 22.3323 13.1877

m′· 1cuo(ZnTPP)· kJ−1

−9.4942

1cuo(ZnTPP)/(kJ · g−1) −32.4684

determined from calibration experiments, the enthalpy of reaction-solution process of ZnOin aqueous HNO3, can be estimated as1Uo(ZnO) = (21± 12) kJ · mol−1, a very lowquantity when compared with the involved combustion energy.

The results from four series of effusion experiments are summarized in table6. A meanvalue for the enthalpy of sublimation1g

crHom(ZnTPP) = (183± 3) kJ · mol−1 at the mean

temperature 559 K was obtained. Since heat capacity for this compound in the gas phaseis unknown and it is not possible to calculate the value from group contributions, the


TABLE 5. Summary of the calorimeter energy equivalent comparison ex-periments and of the standard massic energies of combustion of ZnTPP at

T = 298.15 K (po= 0.1 MPa)

Comparison experiments Combustion experiments

εcomp(calor)/(J · K−1) 1cuo(ZnTPP)/(J · g−1)

14044.0 −32468.4

14029.7 −32523.7

14050.6 −32447.3

14041.1 −32434.9

14061.1 −32445.8

14050.6 −32447.4

14052.9 −32429.2

14041.9 −32409.9

14054.4 −32427.8

14035.1 −32464.9

14053.2 −32489.1

14024.6

Mean value 14044.9 −32453.5

Standard deviation of the mean 3.2 9.6

standard molar enthalpy of sublimation atT = 298.15 K cannot be precisely knownfrom data atT = 559 K. Burkinshaw and Mortimer(40) have used a value of1Cp =

Cp(g) − Cp(cr) = −50 J· mol−1· K−1 for a number of different molecular structures,

as suggested by Pedley in a personal communication. This traditional value was used toinclude a correction of 13 kJ· mol−1 for the molar enthalpy of sublimation of ZnTPP,leading to a value of1g

crHom(298.15 K) = (196± 3) kJ · mol−1. It is necessary to keep in

mind that this is an approximate correction.The derived standard enthalpies of formation and atomization in gas phase at

T = 298.15 K were calculated from the standard molar enthalpies of com-bustion and sublimation at that temperature, and using the following values:(41)

1f Hom{CO2(g)} = −(393.51± 0.13) kJ · mol−1, 1f Ho

m{H2O(l)} = −(285.83± 0.04)kJ · mol−1, 1f Ho

m{ZnO(cr)} = −(350.46± 0.27) kJ · mol−1, 1f Hom{C(g)} = −(716.68±

0.45) kJ · mol−1, 1f Hom{H(g)} = (217.998± 0.006) kJ · mol−1, 1f Ho

m{N(g)} =

(472.68 ± 0.40) kJ · mol−1 and 1f Hom{Zn(g)} = −(130.40± 0.40) kJ · mol−1. Results

are given in table7. The third column in the same table contains the correspondingquantities for the free ligand TPP, previously reported.(26,27)

4. Discussion

A few works on combustion of zinc organic compounds have been reported,(42,43) andjust using static bomb calorimeters. Rotating combustion bomb experiments in this work


TABLE 6. Experimental values for determination of the molar enthalpy ofsublimation of ZnTPP using the quartz-crystal microbalance,L is the thickness ofthe sample deposited on the piezoelectric crystal. Effusion-orifice diameter: 1.2 mm

T/K 103· (dL/dt)/nm · s−1 102

· 1 T/K 103· (dL/dt)/nm · s−1 102

· 1

Series 1:1gcrH

om = (183.2 ± 2.3)a kJ · mol−1 at 〈T〉 = 560 K

555.1 1.893 −0.39 567.1 4.346 −0.19

559.1 2.531 0.62

Series 2:1gcrH

om = (178.4 ± 4.9) kJ · mol−1 at 〈T〉 = 556 K

551.1 1.396 0.58 559.1 2.435 1.23

555.1 1.812 −1.03 559.1 2.391 −0.60

Series 3:1gcrH

om = (190.8 ± 5.2) kJ · mol−1 at 〈T〉 = 559 K

555.1 1.993 0.33 563.1 3.561 0.34

559.1 2.633 −1.04

Series 4:1gcrH

om = (181.2 ± 2.9) kJ · mol−1 at 〈T〉 = 561 K

555.1 1.914 0.80 563.1 3.293 −0.03

559.1 2.479 −1.24 567.1 4.331 0.41

Mean value:〈1gcrH

om〉 = (183± 3)b kJ · mol−1 at 〈T〉 = 559 K

aUncertainty σi of individual values represents the standard deviation of the fitting.bThe weighted averageµ and the corresponding uncertaintyσs were calculated fromµ =

6(xi /σ2i )/6(1/σ2

i ) andσs = σµ · N1/2, with σµ = {6(1/σ−2i )}−1/2 and N being the

number of points, as described in reference39.

TABLE 7. Standard molar thermochemical functions forZnTPP atT = 298.15 K (po

= 0.1 MPa): 1cUom,

1cHom, 1f Ho

m, 1Hgcr,m and1atHo

m denote standard mo-lar energy of combustion, enthalpy of combustion, en-thalpy of formation, enthalpy of sublimation and enthalpy

of atomization

ZnTPP TPP

1cUom(cr)/(kJ · mol−1) −22007± 7 −22244± 7(26)

1cHom(cr)/(kJ · mol−1) −22021± 7 −22257± 7(26)

1f Hom(cr)/(kJ · mol−1) 354± 9 655± 9(26)

1gcrH

om/(kJ · mol−1) 196± 3 184± 2(27)

1f Hom(g)/(kJ · mol−1) 550± 9 839± 9(27)

1atHom(g)/(kJ · mol−1) 39109± 22 39125± 22(27)

were made to achieve a well defined final state and a satisfactory reproducibility for energyresults has been obtained. Previous reports for combustion energy of ZnTPP are not known.


Two very different values for the enthalpy of sublimation of ZnTPP have been reported.Edwardset al.(44) have calculated the value1g

crHom(ZnTPP) = 109 kJ· mol−1 at T =

666 K from absorption spectroscopy data in the vapour phase and in silicon oil and byapplying a SC-MO method. This value seems very low for so large a molecule. Perlovichet al.(45) use an indirect method to measure the vapour pressure of the sample in avery wide temperature range, fromT = 563 K to T = 663 K; the reported value is1

gcrHo

m(ZnTPP) = (213± 4) kJ · mol−1. The value obtained in our work is comparablewith the enthalpy of sublimation for the free ligand,1

gcrHo

m(TPP) = (171± 2) kJ · mol−1

at T = 550 K, reported elsewhere.(27)

From the values atT = 298.15 K, a stabilization of the metallic porphyrin is observedwhen its enthalpy of formation is compared with the corresponding for the free ligandTPP. In the solid phase, stabilization is slightly bigger (301 kJ· mol−1) than in the gasphase (289 kJ· mol−1). X-ray diffraction studies(37,46) have reported planar configurationfor both TPP and ZnTPP. Therefore, the small difference between their values of enthalpyof atomization reveal that one N–H covalent bond in gaseous TPP has almost the samemean dissociation enthalpy that two N–Zn co-ordination bonds. Moreover, using therecommended value of Laidler parameters(28) for the contribution to the enthalpy ofatomization by the N–H bond,D(N–H) = 377.4 kJ, the corresponding value for themean dissociation enthalpy of the N–Zn co-ordination bond is calculated asD(N–Zn) =

(185± 6) kJ in the studied molecule.In conclusion, precise thermochemical results for 5,10,15,20-tetraphenyl-21H,23H-

porphine zinc have been obtained. A new method to perform combustion calorimetryto obtain reliable values for the corresponding enthalpy of formation of zinc-organiccompounds was established. Comparison experiments have been used in order to considerthe energetic processes after combustion. The chemistry involved in the combustionprocess was studied to achieve the dissolution of zinc oxides in a nitric acid aqueoussolution in order to obtain a well defined final state in combustion rotating bomb.

The Knudsen effusion experiments have been proved as a high-sensitive method todetermine the enthalpy of sublimation of compounds with very low vapour pressures.Measurements can be made at high temperatures using a vacuum system with glass–metal joints. The quartz-crystal micro-balance is very sensitive and allows one to workin a narrow temperature range.

Comparing the enthalpies of atomization of the zinc porphyrin and of the correspondingfree porphyrin, the related mean dissociation enthalpy of the N–Zn co-ordination bondwas determined from experimental results and may be taken as reference for theoreticalcalculations.

Rodrigo Patino thanks CONACYT (Mexico) for his scholarship.

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(Received 7 December 2000; in final form 31 May 2001)

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A thermochemical study of 5,10,15,20-tetraphenylporphine zinc(II) by rotating bomb combustion...

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