Computational methods Enthalpy

After you choose the computation method and options, you can use Start Log on the File menu to record results, such as total energies, orbital energies, dipole moments, atomic charges, enthalpies of formation (for the CNDO, INDO, MINDO/3, MNDO, AMI, PM3, ZINDO/I, and ZINDO/S methods), etc. 

Lu, N. Kofke, D. A. Woolf, T. B., Staging is more important than perturbation method for computation of enthalpy and entropy changes in complex systems, J. Phys. Chem. B 2003,107, 5598-5611... 

The freezing point depression of a solvent is proportional to the concentration of solute particles and may be used to measure the extent of ionization once the new particles have been identified qualitatively as ions. The method has the obvious disadvantage of not allowing measurements over a range of temperatures in a single solvent. It is almost certainly not worth while to compute an enthalpy of ionization from ionization constants at two different temperatures in two different solvents. Usable solvents are limited not only by the requirement that the melting point be at a convenient temperature but also by the requirement that the solvent be capable of producing ions yet not be sufficiently nucleophilic to react irreversibly with them once they are formed. For this reason most cryoscopic work has been done in sulfuric acid or methanesulfonic acid.170... 

L. A. Curtiss and K. Raghavachari in Computational Thermochemistry, K. K. Irikura and D. J. Frurip, Eds., ACS Symposium Series 677, American Chemical Society, Washington, DC, 1998, pp. 176-196. Computational Methods for Calculating Accurate Enthalpies of Formation, Ionization Potentials, and Electron Affinities. 

Therefore the main computational method for determining the values of chemical bond energy is the use of corresponding thermal-chemical values (enthalpies of formation of reaction products and initial molecule). 

Undeniably, the composite methods are best suited for prediction of bond dissociation enthalpies. However, these methods may be impractical for many molecules of interest. In these cases, extreme caution must be used with any other computational method. The best situation would be to test the method of choice with a series of related molecules, comparing these computed BDEs with either experimental or high level computed values to obtain some sort of correlation that would justify the use of the more moderate computational method. 

Comparison of the computed binding enthalpies in Table 2.50 with the experimental quantitites in the last column " confirm the ability of ab initio methods to treat such extended systems with a high degree of accuracy. The experimental enthalpies were obtained... 

These may be classified into three groups equilibrium measurements, calorimetric measurements, and computational methods. The first address surface free energy, the second surface enthalpy, and the third (usually) surface energy. 

The set of 2C simultaneous equations is nonlinear and fairly complex since it involves calculating fugacities and enthalpies, themselves nonlinear functions of the temperature, pressure, and composition. The equations may be solved simultaneously or by some iterative method, tn general, the computational methods depend on which two variables are selected as the independent variables. Although in principle any two independent variables may be fixed, the problem complexity may vary from case to case. It is found, for instance, that a solution is more readily reached if P and Trather than P and Q are the independent variables. Since most of these calculations are carried out on computers, the solution methods should be designed for speed of convergence and reliability. Several methods have been proposed for handling the different types of flash calculations, some of which are discussed herewith. 

Finally, a brief overview was presented of important experimental approaches, including GITT, EMF-temperature measurement, EIS and PCT, for investigating lithium intercalation/deintercalation. In this way, it is possible to determine - on an experimental basis - thermodynamic properties such as electrode potential, chemical potential, enthalpy and entropy, as well as kinetic parameters such as the diffusion coefficients of lithium ion in the solid electrode. The PCT technique, when aided by computational methods, represents the most powerful tool for determining the kinetics of lithium intercalation/deintercalation when lithium transport cannot be simply explained based on a conventional, diffusion-controlled model. 

All computational methods applied here agree that both trans-la (Tq) and cis-la (TpHs) structures exist as discrete minima in singlet states (Fig. 1), and that there are no additional minima (Yq, etc.) of low energy on the singlet potential energy surface for Rh(PH3)2Cl. The singlet cis-trans enthalpy difference is... 

An activation enthalpy decreasing with increasing solvent polarity is provided by the PCM method, as shown in Tables 4 and 5. The gas phase NC activation enthalpies decrease by 1.5 and 3.8 kcal/mol in benzene and water, respectively. In benzene (dielectric similar to toluene), the computed activation enthalpy is 17.6 or 1.5 kcal/mol lower than the gas phase activation enthalpy. 

For computing the enthalpy departures of pure gases and gas mixtures, the equations of state of Sec. 14-2 may be used for both the vapor and liquid phases (1) modified versions of the BWR equation of state, such as the one proposed by Starling, (2) the Soave-Redlich-Kwong, and (3) the Peng-Robinson equation of state. Another method which has been tested extensively and found to give accurate results even for polar gas mixtures is the Yen-Alexander correlation.78... 

KAR/KUL] Karlsson, R., Kullberg, L., A computer method for simultaneous calculation of equilibrium constants and enthalpy changes from calorimetric data, Chem. Scr., 9, (1976), 54-57. Cited on pages 368, 377. 

Develop a computational method for enthalpy and entropy using only PVT data and ideal gas heat capacity. 

Hence, the enthalpy change between T, and Tj. Pi niay be computed from the variation for an ideal gas plus the variation of the departure function, which accounts for non-ideality. The big advantage of the departure functions is that they can be evaluated with z PVT relationship, including the corresponding states principle. Moreover, the use of departure functions leads to a unified framework of computational methods, both for thermodynamic properties and phase equilibrium. 

Because of the scarcity of experimental data, computational methods need to be employed to predict molecular thermochemical data for the species. Important part of the development of such quantum chemical methods is their critical assessment by comparison to accurate experimental data. Nowadays, different theoretical schemes, such as the G2, CBS, or G3 theory, are able to predict enthalpies of formation for small systems at accuracy within a few kcal per mole. Unfortunately, these approaches, which involve the use of high-level ab initio methods, are limited by the size of molecule systems that can be studied and are still prohibitively costly. A recent review on the use of different theoretical methods for computing enthalpies of formation was published by Curtiss et al. [18, 19]. Density Functional Theory... 

It is difficult to determine these thermochemical parameters from experiment, because it is hard to monitor the precursor hydrocarbon radical and the formed peroxy radical. The experiment is further complicated by the presence of reactions to new products by the energized peroxy radicals which can prevent the monitoring of equilibrium. Experiments on ion methods using proton affinity or basicity, often with mass spectrometric analysis, are also utilized to determine enthalpies of formation of radicals. Our methods rely heavily on experimentally determined thermochemical data and we would like to point out that this data is very valuable to validate the computational methods. 

Enthalpies of transition state structures are determined with the above composite ab initio and DFT methods. Enthalpies of transition states structures are calculated as the difference between the calculated value of the TS structure and the value of the stable radical adduct(s) (adjacent product and reactant where both are a single species) at the corresponding levels of calculation. The computational methods for enthalpies are G3MP2B3 and G3 (whenever possible), and DFT. Zero-point energies (ZPVEs) and thermal corrections to 298.15 K are from DFT. Table... 

---