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Unformatted text preview: mpilations is the NIST database,
freely available on line (http://webbook.nist.gov).
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
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.
- Spring '09