6.3. Natural bond orbital analysis
The NBO calculations were used to calculate the stabilization
energies E(2) due to electron delocalization processes occurred in the investigated
structures. This gives indication on the strength of the intramolecular
charge transfer (ICT) in the studiedmolecules. High E(2) value
indicates strong electron delocalization fromthe donor NBO to acceptor
one [53,54]. The most important ICT interactions and their stabilization
energies were presented in Table 5. The π → π* electron delocalization
causing stabilization of the system up to 47.68, 44.62, 45.05 and
45.70 kcal/mol for compounds 6a–i, respectively. Themaximumstabilization
in the studied systems due to the π→π* ICT is for 6a as it has the
most planar triazine ring. Strong n → π* ICT from the lone pair of the
amine nitrogen atom to the adjacent π*-orbital of the C_N bond of
the triazine ring. This ICT leads to maximum stabilization of the system
by 66.56 kcal/mol for 6a. Moreover, the stabilization of the system due
to the n → σ* ICT from the LP(O) of the carbonyl O-atom to σ*(C–O)
and σ*(C–N) anti-bonding orbitals having maximum E(2) values of
32.42 kcal/mol for 6g. It is clear that, the ICT interaction energies due
to the electron delocalization from the LP(2)O4 of the carbonyl group
to the BD*(1)N8-H28 is very small (0.72 kcal/mol) indicating the
weak N–H…O intramolecular interaction in 6a.
6.4. IR Vibrational spectra
The theoretical and experimental IR spectra of the studiedmolecules
are shown in Figs. 8 and S8 (Supplementary data). The calculated wave
numbers and intensities of 6a–i obtained from the B3LYP/6-311G(d,p)
method were collected in Table S9 (Supplementary Information).
Since the calculated frequencies are in general overestimate the experimental
data so the vibrational frequencies calculated at B3LYP/6-
311G(d,p) level of theory were scaled by 0.9670 in order to correct
the calculated harmonic frequencies [55]. Generally, the scaled infrared
vibrational frequencies showed good agreementwith the experimental
data. Fig. 9 showed good correlations between the calculated and the
experimental wave numbers (R2= 0.9961–0.9995). The IR fundamental
modes were collected and assigned in Table 6. Herein the descriptions
of the most important modes were presented.