Disruption enthalpy

They are formally isoelectronic with the (ArH)Cr(CO)3 series, and are derived from Co4(CO)12. The thermal decomposition of three representatives of the series has been studied by microcalorimetry84) and the results are shown in Table 16. Once again heats of sublimation have had to be estimated by comparison with the chromium analogues. The enthalpy disruption can be divided by taking T = 134 kJ mol-1 (Table 1) so that the b.e.c of the [Co4(CO)9] fragment in Co4(CO)i2 is 1722 kJ mol-1. The (ArHCo) bond enthalpy contribution is then obtained in the usual way the results are shown in Table 16. It is clear that as in the chromium series, the b.e.c (ArH-Co) increases along the series benzene < mesitylene < hexamethylbenzene. 

The enthalpy of the H-bonds among the majority of the organic compounds is relatively low (usually within the range of about 20 kJ per one mol of hydrogen bonds) and therefore they can easily be disrupted. In order to demonstrate the presence of lateral interactions in chromatographic system, low-activity adsorbents are most advisable (i.e., those having relatively low specific surface area, low density of active sites on its surface, and low energy of intermolecular analyte-adsorbent interactions, which obviously compete with lateral interactions). For the same reason, the most convenient experimental demonstration of lateral interactions can be achieved in presence of the low-polar solvents (basically those from the class N e.g., n-hexane, decalin, 1,4-dioxane, etc.) as mobile phases. 

Bonds between many neutral ligands and transition metal atoms in low oxidation states, have mean bond disruption enthalpies in the range 80-200 kJ mol 1 (9). Bonds to charged ligands, e.g. 

The enthalpy of the R02 + RH reaction is determined by the strengths of disrupted and newly formed bonds AH= Z>R H—Droo—h- For the values of O—H BDEs in hydroperoxides, see the earlier discussion on page 41. The dissociation energies of the C—H bonds of hydrocarbons depend on their structure and vary in the range 300 - 440 kJ mol-1 (see Chapter 7). The approximate linear dependence (Polany-Semenov relationship) between activation energy E and enthalpy of reaction AH was observed with different E0 values for hydrogen atom abstraction from aliphatic (R1 ), olefinic (R2H), and alkylaromatic (R3H) hydrocarbons [119] ... 

The lattice enthalpy, Aiatt//m, is the molar enthalpy change accompanying the formation of a gas of ions from the solid. Since the reaction involves lattice disruption the lattice enthalpy is always large and positive. Aatom//m and Adiss//m are the enthalpies of atomization (or sublimation) of the solid, M(s), and the enthalpy of dissociation (or atomization) of the gaseous element, X2(g). The enthalpy of ionization is termed electron gain enthalpy, Aeg//m, for the anion and ionization enthalpy, Ajon//m, for the cation. 

This assumption is crucial to the development of a body of thermochemical information on compounds of the transition metals. The assumption is made that b.e.c values are transferable from one compound to another in which the formal oxidation state of the metal is unchanged. This means that, for example, the value of Z (Cr-CO) in Cr(CO)6 is assumed unchanged11 in [Cr(r -C6H6)(CO)3], or that Z)(Cr-C6H6) in the same compound is assumed unchanged from its value in Cr(77-C6H6)2. In this example, A.Hf [Cr(T -C6H6)(CO)3, q] = —352.3 kJ mol-1, from which the enthalpy of disruption, A, for the process... 

Table 1. Standard enthalpy of formation of metal carbonyls [Mm(CO) ] in the gas phase. Bond description and bond enthalpy contributions (T, M and B) to the enthalpy of disruption, AHq...

For a mononuclear carbonyl, M(CO) , the enthalpy of disruption AHD refers to the process... 

Table 5. Contributions to the enthalpy of disruption to valence state, AH kJ mol-1 of the M-CObond...

T - mean enthalpy of disruption to ground state products AHcq - valence reorganization energy of CO ... 

Table 12a. Standard enthalpy of formation, A/ff(g), enthalpy of disruption, AHjy, and metal-halogen bond enthalpy contribution, (M-X), in metal carbonyl halides (kJ mol-1)...

The standard enthalpies of formation of the gaseous compounds and the enthalpy of disruption derived therefrom are given in Table 13. An interesting problem arises as to how these results are to be evaluated. If the value of AHf [M(CO)s, g] derived15,1 ) from electron impact measurements on M2(CO)io (M = Mn, Re) is used, then as outlined earlier this will be expected to give an upper limit to the value of D(M-M). It has been shown16) that for all values of Z)(M-M) below specified upper limits the following relation holds... 

Table 13. Enthalpy of formation, AHf (g) and disruption, AH (kj mol x) of alkyl and acyl derivatives of manganese and rhenium carbonyls...

The benzoyl derivative, PhCOMn(CO)5 and related acyl derivatives have intrinsic interest because they can be prepared by alkyl group migration to coordinated CO, a formal internal nucleophilic attack. This reaction, which is often referred to as carbonyl insertion86), is reversible in certain instances to give the metal alkyl. The enthalpy of disruption for PhCOMn(CO)s (Table 13) can be divided up to give the b.e.c of the Mn-COPh bond if the values of T (Mn-CO) shown in Table 13 are used, then the Mn-COPh enthalpy lies in the range 105 10 kJ mol-1, the lower value being preferred as outlined earlier. 

Table 14. Standard enthalpies of sublimation, formation and disruption and bond enthalpy contributions, iT(W-N) kJ mol-1, for N-donor complexes of tungsten W(CO)6 nLn]...

Table 15. Enthalpy of sublimation, formation and disruption (kJ mol-1) for (jr-arene M(CO>3] compounds and arene-M bond enthalpy contributions IT(ArH-M) kJ mol-1...

Table 16. Enthalpy of formation, A//f (c) and disruption, AHjy and bond enthalpy contribution, i (Co-L) in LCo4(CO>9. All values in kJ mol-1 (Ref.84))...

Table 17. Enthalpy of sublimation/vaporization, A//sub/vac, enthalpy of disruption, A/fo, and olefin-iron bond enthalpy contribution for [Fe(CO)n(olefin),j] compounds. All values are in kJ mol-1...

In the previous exercise (whose outcome was not very successful), we used a new concept to assess the thermodynamic stability of chemical bonds the mean bond dissociation enthalpy (also known as mean bond disruption enthalpy). It is represented by DH° or by (DH°) (the symbol we adopt). As indicated, for... 

There are alternative ways of viewing the previous problem that are closer to the idealized concept of chemical bond strength. Consider reaction 5.20, where all the chromium-ligand bonds are cleaved simultaneously. The enthalpy of this disruption reaction at 298.15 K, calculated as 497.9 10.3 kJ mol-1 by using enthalpy of formation data [15-17,31], can be given as a sum of three chromium-carbonyl and one chromium-benzene bond enthalpy contributions (equation 5.21). 

Let us concentrate on the thermochemical cycle of figure 5.6 that involves the disruption of the complex Cr(CO)3(C6H6). The enthalpy of this reaction, previously calculated as 497.9 10.3 kJ mol-1 from standard enthalpy of formation data, can be related (equation 5.24) to the bond enthalpy contributions EsfCr-CO ) andE s(Cr (V.He) through the reorganization energies ER(C() ) and ER(C(tHf )- Two asterisks indicate that the fragment has the same structure as... 

Taking for instance a metal carbonyl cluster Mm(CO) a disruption enthalpy can be defined (Connor 1977, 1981, Mingos and Wales 1990) according to the following reaction ... 

Often, it is difficult to distinguish definitely between inner sphere and outer sphere complexes in the same system. Based on the preceding discussion of the thermodynamic parameters, AH and AS values can be used, with cation, to obtain insight into the outer vs. inner sphere nature of metal complexes. For inner sphere complexation, the hydration sphere is disrupted more extensively and the net entropy and enthalpy changes are usually positive. In outer sphere complexes, the dehydration sphere is less disrupted. The net enthalpy and entropy changes are negative due to the complexation with its decrease in randomness without a compensatory disruption of the hydration spheres. 

These results indicate that the enthalpy associated with air (and also steam) has an effect on the resulting droplet size. A larger droplet size with preheated air than steam reveals that there must be effects other than just the enthalpy associated with steam. Some of the possible factors include viscosity and density differences between the gases, and that water contained in steam may become miscible under these conditions. In this case, the large differences in the boiling points between the two fluids (water and kerosene) may lead to disruptive breakup of the liquid fuel, even at 10 mm, via rapid heat transfer from the flame. 

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