4 research outputs found
Gas-Phase Enthalpies of Formation and Enthalpies of Sublimation of Amino Acids Based on Isodesmic Reaction Calculations
Accurate gas-phase enthalpies of
formation (Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>) of 20 common α-amino
acids, seven uncommon amino
acids, and three small peptides were calculated by combining G4 theory
calculations with an isodesmic reaction approach. The internal consistency
over a set of Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(g) values was achieved
by sequential adjustment of their values through the isodesmic reactions.
Four amino acids, alanine, β-alanine, sarcosine, and glycine,
with reliable internally self-consistent experimental data, were chosen
as the key reference compounds. These amino acids together with about
100 compounds with reliable experimental data (their accuracy was
supported by G4 calculations) were used to estimate the enthalpies
of formation of remaining amino acids. All of the amino acids with
the previously established enthalpies of formation were later used
as the reference species in the isodesmic reactions for the other
amino acids. A systematic comparison was made of 14 experimentally
determined enthalpies of formation with the results of calculations.
The experimental enthalpies of formation for 10 amino acids were reproduced
with good accuracy, but the experimental and calculated values for
4 compounds differed by 11–21 kJ/mol. For these species, the
theoretical Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(g) values were suggested
as more reliable than the experimental values. On the basis of theoretical
results, the recommended values for the gas-phase enthalpies of formation
were also provided for amino acids for which the experimental Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(g) were not available. The enthalpies
of sublimation were evaluated for all compounds by taking into account
the literature data on the solid-phase enthalpies of formation and
the Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(g) values recommended in our work.
A special attention was paid to the accurate prediction of enthalpies
of formation of amino acids from the atomization reactions. The problems
associated with conformational flexibility of these compounds and
harmonic treatment of low frequency torsional modes were discussed.
The surprisingly good agreement between the Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(g) values calculated from the atomization and isodesmic reactions
is largely the result of a fortuitous mutual compensation of various
corrections used in the atomization reaction procedure
Enthalpies of Formation of Hydrazine and Its Derivatives
Enthalpies
of formation, Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>, in both
the gas and condensed phase, and enthalpies of sublimation or vaporization
have been estimated for hydrazine, NH<sub>2</sub>NH<sub>2</sub>, and
its 36 various derivatives using quantum chemical calculations. The
composite G4 method has been used along with isodesmic reaction schemes
to derive a set of self-consistent high-accuracy gas-phase enthalpies
of formation. To estimate the enthalpies of sublimation and vaporization
with reasonable accuracy (5–20 kJ/mol), the method of molecular
electrostatic potential (MEP) has been used. The value of Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(NH<sub>2</sub>NH<sub>2</sub>,g) = 97.0
± 3.0 kJ/mol was determined from 75 isogyric reactions involving
about 50 reference species; for most of these species, the accurate
Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(g) values are available in Active
Thermochemical Tables (ATcT). The calculated value is in excellent
agreement with the reported results of the most accurate models based
on coupled cluster theory (97.3 kJ/mol, the average of six calculations).
Thus, the difference between the values predicted by high-level theoretical
calculations and the experimental value of Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(NH<sub>2</sub>NH<sub>2</sub>,g) = 95.55 ± 0.19 kJ/mol recommended
in the ATcT and other comprehensive reference sources is sufficiently
large and requires further investigation. Different hydrazine derivatives
have been also considered in this work. For some of them, both the
enthalpy of formation in the condensed phase and the enthalpy of sublimation
or vaporization are available; for other compounds, experimental data
for only one of these properties exist. Evidence of accuracy of experimental
data for the first group of compounds was provided by the agreement
with theoretical Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(g) value.
The unknown property for the second group of compounds was predicted
using the MEP model. This paper presents a systematic comparison of
experimentally determined enthalpies of formation and enthalpies of
sublimation or vaporization with the results of calculations. Because
of relatively large uncertainty in the estimated enthalpies of sublimation,
it was not always possible to evaluate the accuracy of the experimental
values; however, this model allowed us to detect large errors in the
experimental data, as in the case of 5,5′-hydrazinebistetrazole.
The enthalpies of formation and enthalpies of sublimation or vaporization
have been predicted for the first time for ten hydrazine derivatives
with no experimental data. A recommended set of self-consistent experimental
and calculated gas-phase enthalpies of formation of hydrazine derivatives
can be used as reference Δ<sub>f</sub><i>H</i><sub>298</sub><sup>°</sup>(g) values
to predict the enthalpies of formation of various hydrazines by means
of isodesmic reactions