Paper
NJC
Table 4 Gibbs free energy for conformers I and II and calculated bond geometries for conformer I of molecules 2, 3, 5 and 6
Gibbs free energy of molecules in conformers (kJ molꢃ1
)
Distance dXꢀ ꢀ ꢀHN (Å)
Angle XHN (1)
I
II
I
I
Molecule
HB type (Xꢀ ꢀ ꢀHN)
1
2
3
5
6
(Hꢀ ꢀ ꢀHN)
ꢃ1905104.9415973
ꢃ2165724.3073241
ꢃ3111825.21609953
ꢃ2205874.4711383
ꢃ2790244.90184566
—
—
—
(Fꢀ ꢀ ꢀHN)
ꢃ2165716.7224903
1.925
2.693
1.863
2.218
132.6
107.2
134.1
126.8
(Clꢀ ꢀ ꢀHN)
(MeOꢀ ꢀ ꢀHN)
(CF3ꢀ ꢀ ꢀHN)
ꢃ3111826.69948089
—
ꢃ2790244.5760736
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provided in the ESI.† The optimized spatial structures considering
the possible electron resonance of all the conformers of molecules
1–6 are reported in Fig. 11. The extracted Gibbs free energies of
both the optimised conformations of the studied molecules along
with the bond geometries, specifically the distances and angles for
conformer I (no intramolecular HB in conformer II) of molecules 2,
3, 5 and 6 are compiled in Table 4.
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The NMR investigations on the series of synthesized molecules
containing the azomethine group (–NQCz) with different substitu-
tions on the benzoyl ring of N0-benzylidenebenzohydrazide revealed
several interesting results; (a) the correlated cross peaks in the 2D
NOESY spectrum confirm that the molecules exist as only E isomers
with respect to the CQN bond, (b) most of them exist in trans/cis
conformations around the N–C(O) bond; (c) single intramolecular
HB between different substituents on the benzoyl ring including
organic fluorine, and the proton of the amide group (C–XꢀꢀꢀH–N) is
controlling the conformational rigidity by establishing the intra-
molecular HB; and (d) the conformer populations are strongly
dependent on the strength of the HB. We envisage that the strength
of the intramolecular HB can be manipulated for the architectural
design of the molecule to a single conformation instead of possible
multiple conformations or conformers of the desired ratio. The
participation of organic fluorine in the HB has been demonstrated
by detection of 1hJHF and its disappearance upon decoupling with 19
F
and also in a high polarity solvent.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
NA is grateful to the Indian Institute of Science for the SRF.
SKM thank the Indian Institute of Science for the RA position.
NS gratefully acknowledges the generous financial support by
the Science and Engineering Research Board (SERB), New Delhi
(Grant Number: CRG/2018/002006).
References
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New J. Chem.
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