Enthalpies of dimerisation

However at elevated temperatures (T2 > Tj, Figure 9) the increased entropy (TAS) associated with an open shell structure overcomes the ti —ti enthalpy of dimerisation associated with these distorted Ti-stacked structures and they undergo a solid-solid phase transition (Figure 9) The high temperature phase is typically associated with a Ti-stack of regularly spaced radicals which exhibit longer inter-radical S- S contacts (ca. 3.7 A). This process was first observed by Oakley60 in the DTA radical thiadiazolopyrazine-l,3,2-dithiazolyl 26, and a number of other derivatives have subsequently been identified which exhibit similar behaviour. These are compiled in Table 1. 

Fig. 45. The enthalpies of dimerisation at 298.15 K of the lanthanide tribromides as derived from the work of Gietmann et al. (1996).

In absence of more detailed information we suggest therefore that for practical reasons the enthalpies of dimerisation are treated constant in the different lanthanide halides series -233 kJ-mol 1 for the trifluorides, —213 kJ mol-1 for the trichlorides, —185 kJmol-1 for the tribromides, and —165 kJmol-1 for the triiodides. 

The mechanism of trimerisation of butadiene by a mixed cobalt(ii) chloride-aluminium triethyl catalyst has been inferred from the natures of the three products characterised. The determination of the enthalpy of dimerisation of aluminium triethyl provides a useful piece of thermochemical data for quantitative discussion of the role and energetics of aluminium triethyl in this type of reaction. Polymerisation of isoprene in the presence of Fe(acac)3-aluminium triethyl-pyridine derivatives mixtures has a negative apparent activation enthalpy, which can be attributed to the instability of the catalytic complex at elevated temperatures. Bis-cyclo-octatetraeneiron(o) is an effective oligomerisation catalyst. The composition of products accessible only by hydrogen migration indicates an oxidative addition-reductive elimination mechanism rather than insertion. 

The authors also evaluated the enthalpy of the dimerisation reaction 2SnSe(g) Sn2Sc2(g) to be A,// (298.15 K) = -(194.6 + 22.0) kJ-mol in a third law analysis of measurements made in the temperature range 936 to 991 K.. No evaluation was made according to the second law because of the short temperature range. The third law evaluation employed Gibbs energy functions calculated from parameters estimated by comparison with the isoelectronic molecules As4(g) and Sb4(g). In the absence of better values of the molecular parameters, no recalculation was made by the review. 

Theoretical calculations on the dithiazolyl radical 4 (R=CF3) have recently shown that n -n dimerisation was unfavourable but association of two such dimers via electrostatic interactions generated a thermodynamically stable tetramer consistent with single crystal X-ray studies. Thus while the value of [AE-P ] may favour (or disfavour) dimer formation, the van der Waals, dipole contributions and electrostatic interactions to the lattice enthalpy should not be underestimated in assessing the thermodynamic stability or instability of these... 

Section 12.1.2.4). The majority of derivatives of 2 and all reported derivatives of 11 tend to associate as dimers in the solid state (Figure 2). Theoretical calculations14 and solution JJV/vis and EPR measurements15 all point towards greater dimerisation enthalpies for the Se analogues. 

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