associated r bond to be polarized in the same direction, and the same applies to the
relative polarization of the p and r systems associated with the carbon–nitrogen
bonds.
Orbital interactions obtained from the NBO analyses are also in good agreement
with the picture obtained based on both the geometrical parameters and the atomic
charges [26, 28, 29], and the same applies when relevant parameters extracted from
the AIM analysis for electron distribution characterization are taken into account.
For example, a direct correlation between the value of the charge density at the
bond critical points of the five bonds of the hydantoin ring in 1MH and the
corresponding bond lengths was found to exist, the shorter the bond, the greater the
charge density at the associated bond critical point [28].
On the whole, the charge density analyses reveal that the main electronic effects
in the hydantoin ring are, by one side, the N-to-O p electron donation, and, by the
other side, the r system backdonation from the r lone electron pairs of the oxygen
atoms to the ring.
The infrared spectra of the investigated hydantoins isolated in argon matrices
were found to fit well the predicted infrared spectra for the calculated minimum
energy structures (see example in Fig. 7.5, concerning 1MH). For AAH, the
analysis of the experimental spectrum revealed that only the most stable conformer
of the molecule subsists in the matrix upon deposition. According to the calculated
relative conformational energies, the room temperature gas phase population of the
most stable AAH conformer prior to matrix deposition should be *93%. The
predicted room temperature gas phase populations for all other conformers but
Fig. 7.4 I: dominant canonic form of the hydantoin ring. II, III, and IV: Mesomeric structures
assuming delocalization of p electrons in the NCO fragments. R,R′ = H,H (H); CH 3 ,H (1MH); H,
CH 3 (5MH); H,CH 2 COOH (AAH)
7 Hydantoins and Mercaptoimidazoles: Vibrational …
207
relative polarization of the p and r systems associated with the carbon–nitrogen
bonds.
Orbital interactions obtained from the NBO analyses are also in good agreement
with the picture obtained based on both the geometrical parameters and the atomic
charges [26, 28, 29], and the same applies when relevant parameters extracted from
the AIM analysis for electron distribution characterization are taken into account.
For example, a direct correlation between the value of the charge density at the
bond critical points of the five bonds of the hydantoin ring in 1MH and the
corresponding bond lengths was found to exist, the shorter the bond, the greater the
charge density at the associated bond critical point [28].
On the whole, the charge density analyses reveal that the main electronic effects
in the hydantoin ring are, by one side, the N-to-O p electron donation, and, by the
other side, the r system backdonation from the r lone electron pairs of the oxygen
atoms to the ring.
The infrared spectra of the investigated hydantoins isolated in argon matrices
were found to fit well the predicted infrared spectra for the calculated minimum
energy structures (see example in Fig. 7.5, concerning 1MH). For AAH, the
analysis of the experimental spectrum revealed that only the most stable conformer
of the molecule subsists in the matrix upon deposition. According to the calculated
relative conformational energies, the room temperature gas phase population of the
most stable AAH conformer prior to matrix deposition should be *93%. The
predicted room temperature gas phase populations for all other conformers but
Fig. 7.4 I: dominant canonic form of the hydantoin ring. II, III, and IV: Mesomeric structures
assuming delocalization of p electrons in the NCO fragments. R,R′ = H,H (H); CH 3 ,H (1MH); H,
CH 3 (5MH); H,CH 2 COOH (AAH)
7 Hydantoins and Mercaptoimidazoles: Vibrational …
207