• The relative lengths of the two distinct C=O bonds in the ring are dictated by the
degree of p electronic delocalization from the nitrogen atoms, being longer
when the C=O fragment is connected to two nitrogen atoms than when connected to a single nitrogen;
• the internal angles of the ring with a nitrogen atom in the apex are much larger
(around 113°) than those with carbon atoms in the apex (*101–106°), due to
different s–p compositions of the hybrid orbitals of the N and C atoms used to
make the ring bonds;
• the p charge of the oxygen atom of the carbonyl group connected to two
nitrogen atoms is more negative than that of the oxygen atom of the second
carbonyl group (connected to a single nitrogen atom), in agreement with its
longer bond length and involvement in a more extended p mesomerism;
• the r charges of the two oxygen atoms are considerably less negative than the
corresponding p charges and have opposite relative values, showing that a larger
p bond polarization toward the oxygen atom leads to reduce the trend for the
associated r bond to be polarized in the same direction. These structural characteristics have relevant consequences for the reactivity of the hydantoin moiety,
as it will be pointed out in Sect. 7.3.2.
The parent hydantoin, as well as its studied methyl derivatives (1MH, 5MH)
have no conformational flexibility, being, by that reason, quite rigid structurally. On
the other hand, AAH bears a conformationally flexible substituent, with three
different conformationally relevant degrees of freedom, and has 13 different conformers: six forms bearing a cis carboxylic group (O=C–O–H dihedral of *0°) and
seven possessing this group in the trans configuration (*180°). The most stable
conformer of AAH is shown in Fig. 7.3, being a cis carboxylic acid form. The
remaining cis carboxylic acid conformers have relative energies within 17 kJ mol
−1
[B3LYP/6-311++G(d,p) calculated values], while all trans carboxylic acid conformers have relative energies of at least 19 kJ mol
−1 , with the highest energy
conformer (Fig. 7.3) having a relative energy of about 40 kJ mol
−1 .
The electronic structure of the studied hydantoins was investigated by several
methods of electron density analysis, starting with the simple picture provided by
atomic charges p/r partition, and going through natural bond orbital (NBO) and
Atoms in Molecules (AIM) analyses. All these analyses were found to be consistent
with the usual representation of the structure of the hydantoin moiety in terms of
different resonance structures (Fig. 7.4), with the nitrogen atoms exhibiting a
positive p charge and the oxygen atoms bearing a negative p charge. The most
positively charged nitrogen atom was found to be that showing a positive charge in
a larger number of mesomeric structures (structures III and IV in Fig. 7.4), while
the relative p charges of the oxygen atoms are also in agreement with the relative
number of resonance structures where these atoms appear negatively charged. On
the other hand, the results also showed that the polarization of the r system follows
the inverse trends compared to those characterizing the p system. For the carbonyl
groups, a higher p bond polarization toward oxygen leads to reduce the trend for the
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degree of p electronic delocalization from the nitrogen atoms, being longer
when the C=O fragment is connected to two nitrogen atoms than when connected to a single nitrogen;
• the internal angles of the ring with a nitrogen atom in the apex are much larger
(around 113°) than those with carbon atoms in the apex (*101–106°), due to
different s–p compositions of the hybrid orbitals of the N and C atoms used to
make the ring bonds;
• the p charge of the oxygen atom of the carbonyl group connected to two
nitrogen atoms is more negative than that of the oxygen atom of the second
carbonyl group (connected to a single nitrogen atom), in agreement with its
longer bond length and involvement in a more extended p mesomerism;
• the r charges of the two oxygen atoms are considerably less negative than the
corresponding p charges and have opposite relative values, showing that a larger
p bond polarization toward the oxygen atom leads to reduce the trend for the
associated r bond to be polarized in the same direction. These structural characteristics have relevant consequences for the reactivity of the hydantoin moiety,
as it will be pointed out in Sect. 7.3.2.
The parent hydantoin, as well as its studied methyl derivatives (1MH, 5MH)
have no conformational flexibility, being, by that reason, quite rigid structurally. On
the other hand, AAH bears a conformationally flexible substituent, with three
different conformationally relevant degrees of freedom, and has 13 different conformers: six forms bearing a cis carboxylic group (O=C–O–H dihedral of *0°) and
seven possessing this group in the trans configuration (*180°). The most stable
conformer of AAH is shown in Fig. 7.3, being a cis carboxylic acid form. The
remaining cis carboxylic acid conformers have relative energies within 17 kJ mol
−1
[B3LYP/6-311++G(d,p) calculated values], while all trans carboxylic acid conformers have relative energies of at least 19 kJ mol
−1 , with the highest energy
conformer (Fig. 7.3) having a relative energy of about 40 kJ mol
−1 .
The electronic structure of the studied hydantoins was investigated by several
methods of electron density analysis, starting with the simple picture provided by
atomic charges p/r partition, and going through natural bond orbital (NBO) and
Atoms in Molecules (AIM) analyses. All these analyses were found to be consistent
with the usual representation of the structure of the hydantoin moiety in terms of
different resonance structures (Fig. 7.4), with the nitrogen atoms exhibiting a
positive p charge and the oxygen atoms bearing a negative p charge. The most
positively charged nitrogen atom was found to be that showing a positive charge in
a larger number of mesomeric structures (structures III and IV in Fig. 7.4), while
the relative p charges of the oxygen atoms are also in agreement with the relative
number of resonance structures where these atoms appear negatively charged. On
the other hand, the results also showed that the polarization of the r system follows
the inverse trends compared to those characterizing the p system. For the carbonyl
groups, a higher p bond polarization toward oxygen leads to reduce the trend for the
206
R. Fausto et al.