Temperature Solution Calorimetric Studies

Amett and co-workers,75 76 in a series of investigations, have determined heats of ionization (AHi) of secondary and tertiary chlorides and alcohols in SbF5-S02ClF and HS03F-SbF5-S02ClF solutions, respectively, at low temperatures. They have also measured heats of isomerizations of secondary to tertiary carbocations in the superacid media. These measured thermochemical data have been useful to determine the intrinsic thermodynamic stability of secondary and tertiary carbocations. 

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If the classical structure were correct, the 2-norbornyl cation would be a usual secondary carbocation with no additional stabilization provided by c-delocalization (such as the cyclopentyl cation). The facts, however, seem to be to the contrary. Direct experimental evidence for the unusual stability of the secondary 2-norbomyl cation comes from the low-temperature solution calorimetric studies of Arnett and Petro.75 In a series of investigations, Arnett and Hofelich76 determined the heats of ionization (AHi) of secondary and tertiary chlorides in SbF5-SC>2ClF [Eq. (3.131)] and subsequently alcohols in HS03F-SbF5-SC>2ClF solutions [Eq. (3.132)]. 

Figure 8.3 Typical temperature-time curves obtained when two calibrations are made in isoperibol reaction-solution calorimetric studies of (a) an exothermic reaction and (b) an endothermic reaction. A fore period of the first calibration experiment B main period of the first calibration experiment ...

Johnson et al. [143] used low-temperature adiabatic calorimetry and high-temperature drop calorimetry to obtain the heat capacity of both forms of mordenite as a function of the temperature. These results and the results of the reaction-solution calorimetric studies discussed herein, enabled the tabulation of the thermodynamic properties (C°, S°, Af H°, and Af G°) of mordenite from 0 K to 500 K and dehydrated mordenite from 0 K to 900 K. 

The sol—gel process can be used to obtain lamellar silica (LS) samples by using neutral amines as template molecules. Its is found that such LS samples are able to act as sequestrating agents toward transition metal cations and that the coordination of such metal cations of the three dimensional network structure of the sffica exerts remarkable effects on its nanostructure and thermal stabffity [4]. Furthermore, such metal sequestrating abffity and the consequent nanostructure modifications are observed even if the metal—sffica reaction is performed in the solid state and room temperature [5]. By performing a solution calorimetric study, it is possible to verify that the total amount of metal cations that the lamellar matrix is able to sequester as well as its affinity by the metal cations, for example, Ni > Cu > Co [6] is a consequence of the influence of main two factors the metal—nitrogen coordination enthalpies and the structural disorder provoked into the lamellar network by the metal—nitrogen coordination. 

Low melting metals (Sn and also Bi, In, Pb, and Cd) are extensively used as solvents in calorimetric studies of metallic phases [35]. Transition metals do not, however, dissolve readily in tin [43] and other solvents such as Cu and A1 have been used. An experimental probe for high-temperature solution calorimetry is shown in Figure 10.8. 

These distonic superelectrophiles (145-147) have been characterized by low-temperature H NMR (and 13C NMR in the case of 147) from FSO3H-SbFs solution. Dication 146 was also studied by calorimetric studies to determine the heat of diprotonation of 2,5-hexanedione.48 It was found that the heat of diprotonation for the y-diketones (like 2,5-hexanedione) is about 5 kcal/mol less than expected, when compared to twice the heat of protonation of acetone or other monoketones. The destabilization of dication 146 by 5 kcal/mol can be the result of electrostatic effects, and it can be considered evidence for the superelectrophilic character of such dications. When 2,6-admantanedione is reacted in FSO3H—SbFj solution, the dication 148 is formed as a persistent species, observable by and 13C NMR.12 The carboxonium carbons of 148 are observed at Z I3C 247.7, while the mono-cationic species (149) has a carboxonium carbon at 513C 267.1. These 13C NMR data were interpreted as evidence for the increasing importance of the carboxonium-type resonance structure (148a) due to electrostatic repulsive effects. Some examples of aromatic diketones (i.e., diacetylbenzenes) have also been reported to produce bis-carboxonium dications in their protonation reactions in superacids.47... 

A characteristic feature of the hydrophobic interaction is that it is dominated by entropy effects. Both the temperature dependence of alkane solubilities in water126,127) and direct calorimetric measurements128 show that Iihf is close to zero at room temperature. Some calorimetric data for heats of solution of hydrocarbons in water are shown in Table 3.2. A further noticeable feature is that Iihf is temperature dependent due to the rather large heat capacity, Cp F, associated with the hydrophobic interaction. From a systematic calorimetric study of a series of compounds with rather short alkyl chains129 it was found that... 

Calorimetric studies on embryos by DSC showed an endothermic event attributable to a glass transition between — 60 and — 65°C. Thus, at storage conditions (5°C), the systems were well above the glass-transition temperature, with most of their solutes in solution or in supercooled liquid state. At such a high proportion of water, metabolic activity and deteriorative reactions involved in the short life span of this seed are not precluded by mobility restrictions. The nonstored seeds presented the relative lowest values of freezable water and they were selected for the studies on cryoconservation. 

Oppermann et al. studied the ternary system Bi-O-Se. Six crystalline phases were prepared by high temperature synthesis from Bi203(cr) and Se02(cr). The phases were characterised by X-ray powder diffraction and IR spectroscopy. The standard enthalpies of formation and entropies of the compounds were obtained from the temperature dependence of the equilibrium constants of the decomposition reactions. The standard enthalpies of formation were also found from solution calorimetric work in which the enthalpies of dissolution of the compounds, Bi203(cr), and Se02(cr) in 4 M HCI were measured. It should benoted that the results in the paper were obtained with a correct value of the enthalpy of formation of Se02(cr) and not the erroneous value in Table II of tha paper. 

There is one thorough calorimetric study of the enthalpy of reaction of thorium with hydrogen, but at one temperature only, by Picard and Kleppa [1980P1C/KLE], who measured the partial molar enthalpy of solution (H2, 700 K), as a function of the... 

The selected value is that from the calorimetric study [1980P1C/KLE], since that has an exphcit integration of the partial molar enthalpy of solution across the whole region from H/Th = 0 to 2 and was carried out at the lowest temperature, where the effects of non-stoichiometiy are smallest. 

One phase of our most recent work has focused on calorimetric studies using Ti 6-4 powders (4). The higher surface area of the powder compared to the standard lap shear coupon permits heats of immersion to be measured. The heats of immersion of Ti 6-4 have been measured in water, in PPQ and LARC-13 primer solutions and their respective solvents. The Ti 6-4 powder was heated to different temperatures in vacuum for varying time periods prior to immersion. The heats of immersion sharply increased... 

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