Nistgov a snapshot of part of the nist database for

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Unformatted text preview: mpilations is the NIST database, freely available on line ( A snapshot of part of the NIST database for closed-shell neutral organic molecules has been provided with Spartan Student, and heats of formation where available are displayed in the Molecule Properties dialog. No attempt has been made to identify suspect experimental values. While care has been taken to ensure the integrity of this collection, because the data derive from a variety of experimental techniques and has been assembled over many decades, heats of formation for individual entries vary widely in quality. The most egregious source of error is that the structure is incorrect, meaning that the reported heat does not correspond to the reported structure. Because most of the compounds in the NIST database are fairly simple and readily available commercially, it is likely that only a very few structures are incorrectly assigned. More likely sources of error include incomplete combustion and poorly characterized combustion products. Hydrocarbons and oxycarbons present fewest problems as their combustion leads only to carbon dioxide and water, the amounts of which may easily be determined. However, combustion of molecules with other elements may give rise to a complex mixture of products and greater uncertainty. Despite their importance, heats of formation are not routinely determined (or at least are not routinely reported) for new compounds. While combustion experiments are straightforward and the results easily interpreted, accurate measurements require considerable diligence and may need to be repeated several times to establish useful error limits. More relevant, combustion experiments may require (and destroy) significant quantities of compound. Very few synthetic chemists are willing to part with hundreds of mg (a huge amount by modern standards) of a compound that they have just spent weeks or months preparing. One alternative source of experimental thermochemical data follows from measurement of equilibrium constants. The best examples of this are for ionmolecule reactions carried out in the gas phase using ion cyclotron resonance spectroscopy and related techniques. Most important among these 6 are protonation/deprotonation reactions, leading to gas-phase basicities and acidities. For example, it is possible to accurately establish the energy of protonation of a base, B, relative to that of a standard base, Bstandard, by determining the amounts of the two protonated bases present at equilibrium. BH+ + Bstandard B + BstandardH+ Equilibrium measurements require that the proton affinities of the two bases be similar (within ~10 kJ/mol). In practice, a proton affinity “ladder” of base strengths (involving hundreds of individual compounds) has been constructed, allowing an appropriate standard to be selected over a range of several hundred kJ/mol. The advantages to this kind of approach are that it is quite accurate and requires only miniscule amounts of materials. The obvious disadvantage is that it applies to ion-molecule reactions only. The NIST database contains a large collection of heats of formation for both negative and positive ions based on such measurements. Another source of experimental thermochemical data on positive ions derives from measurements of ionization potentials and electron affinities. M + e- M+ + 2eM + e- MThe former are widely available whereas the latter are less common. 7 Reaction Energies and Boltzmann Distributions As commented previously, the energy (heat) of a balanced chemical reaction may be obtained using either heats of formation or total energies. ∆E(reaction) = Eproduct 1 + Eproduct 2 + … - Ereactant 1 - Ereactant 2 - … A negative ∆E indicates an exothermic (energetically favorable) reaction, while a positive ∆E indicates an endothermic (unfavorable) reaction. An important special case is where there is only one reactant molecule and one product molecule. Here, the reactants and products are isomers, and the reaction energy accounts directly for their difference in their energies. ∆E(reaction) = ∆E(isomer) = Eisomer 2 – Eisomer 1 A negative ∆E(reaction) means that isomer 2 is more stable than isomer 1, and that the reaction will proceed as written (it is exothermic). The equilibrium composition of a mixture of isomers is given by the Boltzmann equation. Isomer 1 Isomer 2 % Isomer i = Isomer 3 ... 100 exp (-EIsomer i /kT) ! exp (-1060 E Isomer j ) j ΔEisomer i is the energy of isomer i in hartrees relative to the energy of the lowest-energy isomer. T is the temperature (in K) and k is the Boltzmann constant. In the case where the equilibrium is between two isomers, Isomer 1 Isomer 2 and assuming room temperature (298 K) the expression becomes. [ Isomer 1 ] = exp [-1060 (Eisomer1 – Eisomer2 )] [ Isomer 2 ] HCN vs. HNC: Under “normal” (laboratory) conditions, hydrogen isocyanide (HNC) is in equilibrium with its more stable isomer, hydrogen cyanide (HCN). According to the HF/6-31G* model, what is the room-temperature Boltzmann distribution of isomers? Assuming that 5% as the...
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This note was uploaded on 02/22/2010 for the course CHEM N/A taught by Professor Head-gordon during the Spring '09 term at University of California, Berkeley.

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