Thermochemistry of 2-amino-3-quinoxalinecarbonitrile-1, 4-dioxide. Evaluation of the mean dissociation enthalpy of the (N-O) bond Page: 2,510
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the cyano and amino groups are known to have opposite effects
in terms of electron donation as given by the different signs in
their Hammett constants. This can be explained if one considers
a conjugation of factors. First, a cancellation of their electronic
effects may exist due to the proximity of these substituents and
also due to possible delocalization through the aromatic ring.
Second, resonance between the N-O groups and the attached
cyano and amino substituents is also expected. This may occur as
N=C-C=N+O- - -N=C-C=N+=O for the former interaction
and H2N-C=N+O- - H2N+=C-N-O- for the interaction between
adjacent NO and NH2 groups. Third, the most important factor is the
hydrogen bond found between NO and NH2. The use of the Atoms
in Molecules (AIM)22 approach shows a (3, -1) critical point for
the N-O... H-NH interaction with p = 0.024 and V2 = 0.112; these
values were found to be characteristic of a weak interaction." This
is consistent with a similar N-O bond enthalpy of dissociation
computed for N-O near CN or near NH2 substituents despite the
different N-O distances reported in the preceding section. Thus,
the removal of the oxygen atoms breaks with this stabilizing inter-
action and, therefore, N-O bond enthalpy of dissociation increases.
The comparison of the present data with data reported recently for
2-methyl-3-R-quinoxaline-1,4-dioxide, R = H, methyl, methoxy-
carbonyl, ethoxycarbonyl and benzyl derivatives7' shows that
the effect of the amino substituent in the computed (N-f) bond
dissociation enthalpy is identical to that coming from the ethoxy-
carbonyl group. The amino and ethoxycarbonyl substituents are
those that reduce more the (N-O) bond enthalpy of dissociation.
If we neglect the effects caused by the other group attached to
the quinoxaline ring, the oxidizing power varies in the following
decreasing order: methoxycarbonyl >> amino > ethoxycarbonyl >
cyano > methyl H = benzyl.
Conclusions
In the present work, a combined experimental and computational
study has been entailed to obtain the (N-O) bond enthalpy of
dissociation for 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide
compound in the gas-phase. The experimental value was obtained
from the experimental enthalpies of formation in the gas-phase of
2-amino-3-quinoxalinecarbonitrile-1,4-dioxide and atomic oxygen
and from a estimated result for the enthalpy of formation of the
2-amino-3-quinoxalinecarbonitrile species since it was not possible
for us to synthesize this compound. The result obtained compares
excellently with the N-O bond dissociation enthalpy computed
at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) level of theory.
The experimental/estimated result is 248.3 8.3 kJ mol^' while
the fully computed result is 252.2 kJ moL'. The oxygen atom
that is first removed is the one that is nearer to the amino group
(242.5 kJ mol-t) while a larger energy, 261.9 kJ mol-', is needed to
remove the second oxygen atom.0
m-) 2. NH12
N,N ON
NA N NH2
I . Q , -. .C
SN r NH2The design of new heterocyclic di-N-oxides is dependent on
the energetics of their (N-O) bonds which have particular interest
in the development of the pharmacological properties of these
compounds. They have the potential to act as oxidizing agents, and
thus, it is desirable to place them in increasing order in terms of
their capacity to transfer oxygen atoms in chemical and biochemical
conversions. The 2-amino-3-quinoxalinecarbonitrile- 1,4-dioxide
compound has an oxidizing capacity larger than those previously
reported for 2-methyl-3-R-quinoxaline-1,4-dioxide derivatives
(R = H, CH3, C(O)OC2H5, CH2C6H,), identical to that reported
for 2-acetyl-3-methylquinoxaline-1,4-dioxide and lower than that
reported for 2-methyl-3-methoxycarbonylquinoxaline-l,4-dioxide
and 2-phenyl-3-benzoylquinoxaline-1,4-dioxide.'
Experimental
Materials
2-Amino-3-quinoxalinecarbonitrile-1,4-dioxide was prepared
by slow addition of triethylamine to a N,N-dimethylformamide
solution containing dissolved benzofuroxan and malonodinitrile
reactants according to the published method of Ley et a L The
ensuing chemical reaction is very exothermic; hence the reaction
mixture was maintained at 20 C by placement in a cold-water
bath. After the addition of triethylamine was complete, the solution
was stirred an additional 90 minutes at ambient room temperature.
The crude product was collected by vacuum filtration, washed with
methanol and further purified by three crystallizations from hot
N,N-dimethylformamide. Elemental analysis was in agreement with
calculated value: mass fractions for CHN402: found C, 0.5355; H,
0.0291; N, 0.2765; calculated: C, 0.5347; H, 0.0299; N, 0.2771.
As the compound is sensitive to moisture, before the combustion
measurements, the sample was kept in an oven (T = 373 K) for
several hours, and cooled in a desiccator with a dried atmosphere.
The purity of the compound was confirmed by the average ratio of
the mass of carbon dioxide recovered after combustion experiments
to that calculated from the mass of sample: (0.9934 0.0006),
where the uncertainty is the standard deviation of the mean. The
relatively low value for carbon dioxide recovery corresponds to the
presence of a small amount of water in the sample, as it has been
checked by Karl Fisher analysis.
The thermal behaviour of the compound has been studied by
DSC. The thermogram shows that there is not transition phase
before the melting temperature (514 K, with decomposition).
Combustion calorimetry
The energy of combustion of 2-amino-3-quinoxalinecarbonitrile-
1,4-dioxide was measured in a isoperibol static bomb calorimeter,
with a twin valve bomb, of internal volume of 0.290 dm, formerly
installed in England, as is described in the literature.9" The_n0
N CN
1N ' NH,_ a /
At-mean D(N-0) = 2522
-20
Fig. I Computed first, second, and mean (N-O) bond dissociation enthalpy for 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide. Results computed at the
B3LYP/6-31 1+G(2d,2p)//B3LYP/6-31G(d) level of theory. All values are given in kJ mol-1.
Q1 rg. 8B~iomo. Chert ., 2004, 2,2507-251
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Silva, Maria D. M. C. Ribeiro da; Gomes, José R. B.; Gonçalves, Jorge M.; Sousa, Emanuel A.; Pandey, Siddharth & Acree, William E. (William Eugene). Thermochemistry of 2-amino-3-quinoxalinecarbonitrile-1, 4-dioxide. Evaluation of the mean dissociation enthalpy of the (N-O) bond, article, August 10, 2004; [Cambridge, England]. (https://digital.library.unt.edu/ark:/67531/metadc725818/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.