Statistical analysis reaction enthalpies

Equation 2.67 indicates that the standard enthalpy and entropy of reaction 2.64 derived from Kc data may be close to the values obtained with molality equilibrium constants. Because Ar// is calculated from the slope of In AT versus l/T, it will be similar to the value derived with Km data provided that the density of the solution remains approximately constant in the experimental temperature range. On the other hand, the error in ArSj calculated with Kc data can be roughly estimated as R In p (from equations 2.57 and 2.67). In the case of water, this is about zero for most solvents, which have p in the range of 0.7-2 kg dm-3, the corrections are smaller (from —3 to 6 J K-1 mol-1) than the usual experimental uncertainties associated with the statistical analysis of the data. 

Table 15 9 Experimental reaction enthalpies (kJ/mol). The reactions used for the statistical analysis are given in bold...

In Chapter 15, we conclude our exposition of molecular electronic structure theory by applying the standard models of quantum chemistry to the calculation of a number of molecular properties equilibrium stmctures, dipole moments, electronic energies, atomization energies, reaction enthalpies, and conformational energies. The performance of each model is examined, comparing with the other models and with experimental measurements. The large number of molecular systems considered enables us to carry out a statistical analysis of the different methods, making... 

Thermodynamic properties calculated for the current study are presented in Table 7.5. Enthalpy of formation and entropy values are reported at 298 K, as most experimental data are referenced or available at 298 K. This facilitates the use of these thermodynamic properties and the use of isodesmic reaction set. Entropies and heat capacities are calculated by statistical mechanics using the harmonic-oscillator approximation for vibrations, based on frequencies and moments of inertia of the optimized B3LYP/6-311G(d,p) structures. Torsional frequencies are not included in the contributions to entropy and heat capacities instead, they are replaced with values from a separate analysis on each internal rotor analysis (IR). 

They are similar and significantly differ from the emission due to the F + I2F reaction as shown in Fig. la. The population analysis for both systems resulted in more non-statistical distributions which are nearly identical within the error bars, as shown in Fig. 2b. If an interpretation in terms of the KaMer and Lee mechanism is again attempted here this leads to several contradictions. Firstly the different energetics of Ae two reaction systems should be reflected by the chemiluminescence spectra. Secondly, the threshold for the formation of QIF is significantly higher than for I2F (24.7 kJ mol compared with 17.6 U moT ) [2]. Thirdly, one would expect the formation of electronically excited FQ and FBr as anticipated by Kahler and Lee rather than excited IF [2]. Finally, the enthalpy for the reaction F + QIF with AHg = -182.1 kJ mol is far from sufficient to populate the B state of IF where at least 226.7 kJ mol is required. (Although the stability of BrIF is not exactly known, the argument also holds for tiie F + BrDF reaction). 

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