Enthalpies of trimerization

The enthalpies of trimers and tetramers were also calculated as part of this study. At the trimer level - important because this is the initial product of the reaction [dimer + zinc alkyl + aldehyde] - the most stable structures are macrocyclic. At the tetramer level there are two species of comparable enthalpy. One is the well-described cubic tetramer, and the other a barrel-like species that is conceptually related to two N - Zn lined dimers. The specific value derived from these calculations is only apparent when compared with the later computational work on the real diisopropylzinc-derived species. This highlights the importance of steric effects involving the more bulky isopropyl groups as a defining feature of asymmetric autocatalysis [80]. 

Table 3. ACPF stabilization energies and enthalpies of trimers composed of HCN and HF molecules. All values in kcal7mol.

For future reference, note that the term 7i(x3/3)ln (X3/3) in Eq. (5.4.22b) represents the negentropy of trimers assembled from n ebbs. This identification follows from a comparison with Eq. (2.5.10) for the entropy of mixtures more fundamentally, it is the standard formulation for the entropy of X3/3 units provided by statistical thermodynamics, as shown in Chapter 10. As a result, the remaining term G3 in Eq. (5.4.22b) is equivalent to the enthalpy for nl3 trimers generated from n ebb units. On multiplying by three, one obtains the molar enthalpy of trimers = 3m, a result used later. 

Let us return to the thermal decomposition of Fe(CO)(l,3-C4H6)2. Once the calibration constant is known, the enthalpy of the net process 9.10 can be calculated as the product of s and the area (A + B). The next step is to correct this value to 298.15 K by using heat capacity data. This exercise is, however, complicated by the cyclobutadiene polymerization. Brown et al. analyzed the reaction products by mass spectrometry and found several oligomers, in particular the dimer (C4H6)2 and the trimer (C4H6)3 [163]. With such a mixture, it is difficult to ascribe the observed enthalpy change to a well-defined chemical reaction. This is discussed in the paper by Brown and colleagues, who were nevertheless able to recommend a value for the standard enthalpy of formation of the iron-olefin... 

The first species we will discuss is l,3,5-trimethylhexahydro-l,3,5-triazine (33, X = NMe). The gas-phase trimerization enthalpy of A-methylmethylenimine (34), reaction 41, has been evaluated53 to be —178 kJmol-1. 

From gas-phase ion-molecule reactions we obtain54 the necessary enthalpy of formation of 34, 44 8 kJmol-1, and so derive A//f(33, g) as equal to ca —46 kJmol-1. Is this number plausible We start with the trimerization enthalpy. Intuitively, this number for C=N bonded species should interpolate the trimerization enthalpy of species with C=C and C=0 bonds. More precisely, since it is N and not C that is methylated, the trimerization enthalpy of 34 should be close to that of CH2NH. As such, this reaction enthalpy should interpolate that of CH20 and CH2CH2. Our thermochemical archive includes the enthalpies of formation of these latter two substances and of their trimers, 1,3,5-trioxane (33, X = O), and cyclohexane (alternatively identified as 33, X = CH2 and 2, n = 6), from which derive the desired reaction enthalpies —140.1 and —276.0 kJmol-1 respectively. The reaction enthalpy for the X = NMe case interpolates that of X = CH2 and X = O as expected from size and electronegativity reasoning. 

A-(dihydroxybenzoyl)serine linear trimer (257) and its dimer analogue are 10" and 10 , respectively. Enthalpies of formation for the Fe complexes of the trimer and of enterobactin are reported. Linear hexapeptides and decapeptides bearing catechol units derived from dopa (dihydrox ghenylalanine) form stable complexes with Fe +, with stability constants in the range... 

Studies of the association of ethanol molecules in carbon tetrachloride solution have been made by infrared spectroscopy, permitting the evaluation of the enthalpies of formation of the dimer, trimer, and tetramer.76 The value for the tetramer, 22.56 kcal/mole, corresponds to 5.64 kcal/mole for the energy of the hydrogen bond (no correction for van der Waals attraction is made because the interaction with the solvent counteracts it). This value agrees moderately well with the value obtained above from the enthalpy of sublimation. The values for the dimer and the trimer, 5.09 and 10.18 kcal/mole, presumably correspond to one and two hydrogen bonds, respectively. 

The sole representative of this class of compounds for which there is an experimentally measured enthalpy of formation is species 63, alternatively called 1-azacyclopentene and A pyrroline147. The literature value, 64.0+ 1.3 kJmol-1, was obtained by measurement of the enthalpy of hydrogenation of its trimer, 64a, to form pyrrolidine 64b... 

Accepting the enthalpies of formation given by Jackman and Packam150 for all three imines results in a value of 40 14 kJ mol 1, considerably lower than experiment148. In principle, clarification of this discrepancy could be achieved by related studies on other 1-azacycloalkenes. However, except for an unsuccessful attempt148 for the corresponding 1-azacyclohexene (alternatively, A1-piperideine or 3,4,5,6-tetrahydropyridine) 65—its trimer fails to monomerize—we do not know of any such study. 

In the gas phase, monomeric SO3 has a >3h planar structure with bond length S-0 142 pm, as shown in Fig. 16.6.2(a). The cyclic trimer (803)3 occurs in colorless orthorhombic y-S03 (mp 16.9°C), and its structure is shown in Fig. 16.6.2(b). The helical chain structure of P-SO3 is shown in Fig. 16.6.2(c). A third and still more stable form, a-S03 (mp 62°C), involves cross-linkage between the chains to give a complex layer structure. The standard enthalpies of formation of the four forms of SO3 at 298 K are listed below ... 

The enthalpy of formation of crystalline imidazole is 14.6 0.8 kcal mole-1 its heat of sublimation is 16.0 1.0 kcal mole-1, and the heat of formation of gaseous imidazole is 30.6 1.8 kcal mole-1.291 From these data the resonance energy of imidazole has been calculated as 14.2 kcal mole-1.291,292 From an infrared examination of the association of 4-methylimidazole in carbon tetrachloride and in 1,1,2,2-tetrachloroethane,293 the concentration dependence of the maximum extinction coefficient of the free NH valency band allowed determination of the monomer content, the mean viscosity, and the equilibrium constants K12 and Kl >. From the temperature dependence of these values the mean heat of addition (8.2 0.5 kcal mole-1) and the heats of formation of the dimers and trimers (10.2 and 8.1 kcal mole-1, respectively) were obtained. Although the mean heat of addition was found to be about 3 kcal mole-1 higher than that of Zimmermann203 this is not contradictory to the assumption that the imidazoles form chainlike associations with angled structures. 

Two compartment Knudsen cells were used by Gesenhues and Wendt [456] as well as Schafer and Florke [457] to study the vapor over AlCl3(s). The electron impact ionization mass spectra of AlCl3(g) and (AlCl3)2(g) as well as the trimerization enthalpy of AlCl3(g) resulted. 

Binary Systems and Related Compounds.—Halides. The vapourization characteristics of ReBr3 have been studied by effusion techniques and the enthalpy and entropy of sublimation determined as 200 kJ mol and 227 e.u. (per mole of trimer), respectively.The X-ray characteristics of rhenium(iii) iodide, prepared from HRe04 and HI in MeOH at 80-90 °C, have been reported. 

Table 1.4 compares the experimental trimer enthalpies of formation to mDC, mPWPW91, and PM3ep. mDC is in much better agreement with the experimental results than the other methods. 

ACPF stabilization energies and enthalpies of these five more stable trimers are shown in table 3. Corresponding experimental values are not available yet. We note, however, that in the case of (HCN) 3 cyclic and linear complexes are now very close in energy, differing by only 0.5 kcal7mol in aHq. In table 4 computed rotational constants are confronted with experimental data. Generally, the computed values are too low by a few percent. Dipole moments of the open chain trimers are given in table 5. 

Similarly, enthalpy and entropy of trimer formation, etc. can be defined. For water, the experimentally determined A/i and As are available. 

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