Thermochemical study of 1-acetyl vinyl p-nitrobenzoate: vinyl bond enthalpy in captodative olefins.

Captodative olefins are highly reactive and selective substrates in Diels-Alder and 1,3-dipolar cycloadditions. Seeking an explanation of this fact based on molecular energetics, the thermochemical analysis of 1-acetyl vinyl p-nitrobenzoate, a captodative olefin, has been performed using semi-micro-combustion calorimetry, effusion measurements through a quartz crystal microbalance, and differential scanning calorimetry. The molar standard combustion energy and enthalpy as well as the molar standard formation enthalpy are reported along with sublimation and melting enthalpies. From these data, experimental formation enthalpy of the gas-phase is derived and compared with the theoretical value calculated through the density functional theory procedure. The olefinic bond enthalpy is also computed from experimental data, and the relevance of the results is discussed.

Experimental determination of enthalpies of solution of tetraphenyl porphyrin (TPP) and some metal derivatives, in chloroform: interpretation of the solvation processes at a molecular level.

The enthalpies of solution, Delta(sol)H(m), for 5,10,15,20-tetraphenylporphine (CA registry number 917-23-7, TPP), 5,10,15,20-tetraphenylporphine of Co(II), Ni(II), Cu(II), and Zn(II) (to be written as CoTPP, NiTPP, CuTPP, and ZnTPP) in chloroform, were calorimetrically measured at T = 298 K in the concentration ranging from 3.5 x 10(-5) to 2.8 x 10(-4) mol.kg(-1). Through the linear extrapolation of the experimental data, corresponding values at infinite dilution were determined as: Delta(sol)H(m) (ZnTPP) = (55.5 +/- 0.2) kJ x mol(-1), Delta(sol)H(m) (CoTPP) = (36.9 +/- 0.2) kJ x mol(-1), Delta(sol)H(m) (TPP) = (25.7 +/- 0.6) kJ x mol(-1), Delta(sol)H(m) (NiTPP) = (15.6 +/- 0.1) kJ x mol(-1), and Delta(sol)H(m) (CuTPP) = (15.6 +/- 0.1) kJ x mol(-1). The enthalpies of solvation for the five compounds were also determined using the previously published values for the enthalpy of sublimation, as well as complementary data from the literature. The values obtained are as follows: Delta(solv)H(m)(TPP) = -(158.3 +/- 2.1) kJ x mol(-1), Delta(solv)H(m)(CoTPP) = -(154.1 +/- 2.0) kJ x mol(-1), Delta(solv)H(m)(CuTPP) = -(149.4 +/- 5.0) kJ x mol(-1) Delta(solv)H(m)(NiTPP) = -(141.4 +/- 4.0) kJ x mol(-1), and Delta(solv)H(m)(ZnTPP) = -(140.5 +/- 3.0) kJ x mol(-1). The results are analyzed in relation to several molecular properties such as ionic radius, electronic spectra, and Connolly surface. An explanation of the observed trends for solvation enthalpies is proposed.

Strength of the Zn-N coordination bond in zinc porphyrins on the basis of experimental thermochemistry.

The compound, 5,10,15,20-tetrakis(4-methoxyphenyl)porphine zinc(II) (ZnTMPP), was prepared, and its thermochemical properties were experimentally established. The standard molar energy of combustion (Delta(c)U degrees m) was determined from oxygen rotating-bomb combustion calorimetry experiments. The standard molar enthalpies of combustion (Delta(c)H degrees m) and formation (Delta(f)H degrees m) were derived. The enthalpy of sublimation (Delta(cr)(g)H degrees m) was determined by Knudsen effusion at high temperatures. With these results, the standard molar enthalpies of formation and atomization (Delta(at)H degrees m) in the gas state were calculated. A summary of the results at T = 298.15 K (p degrees = 0.1 MPa) is shown in Table 1. Using these results and those previously obtained for the free ligand, 5,10,15,20-tetrakis(4-methoxyphenyl)porphine, the mean dissociation enthalpy for the Zn-N coordination bond is obtained as D(Zn-N) = (160 +/- 9) kJ.mol-1. This value is consistent with the results obtained using the same experimental approach in a similar system (5,10,15,20-tetraphenylporphine, TPP/ZnTPP) reported elsewhere. A discussion of the strength for the Zn-N coordination bond is made in terms of the structural and electronic features of the molecules involved.