The removal of the O43−H52⋯O42 IHB brings about the O43−H52⋯π
interaction and the removal of the O44−H54⋯O45 IHB brings about the O44
−H54⋯π interaction. Each of these removals causes an increase in the energy of the
conformer. Comparison of the energies of conformers JZM-a-b-e-f-g-h-p-v-x and
JZM-b-c-e-f-g-h-p-v-x (removal of O43−H52⋯O42 and formation of O43
−H52⋯π) shows an energy increase of ≈6 kcal/mol. Since the two units are
identical, the presence or the absence of a given IHB in only one of the units has the
same effects whether it occurs in unit S (O43−H52⋯O42) or in unit S′ (O44
−H54⋯O45). Therefore, the conformers JZM-a-d-e-f-g-h-p-v-x and JZM-b-c-e-fg-h-p-v-x (or the conformers JZM-a-d-e-f-g-h-q-v-y and JZM-b-c-e-f-g-h-q-v-y,
or JZM-a-d-e-f-g-h-p-t-x and JZM-b-c-e-f-g-h-p-t-x, or JZM-a-d-q-t-x and
JZM-b-c-q-t-x, or JZM-b-c-q-v-y and JZM-a-d-q-v-y, or JZM-b-c-p-t-x and
JZM-a-d-p-t-x, or JZM-b-c-p-v-x and JZM-a-d-p-v-x, or JZM-b-c-q-t-x and
JZM-a-d-q-v-x (Table S13) have the same relative energy and similar molecular
properties such as dipole moment and HOMO-LUMO energy gap, and also rather
close values of ΔG solv .
Since the removal of an O−H⋯O IHB brings about an O−H⋯π IHB and vice
versa, it is not possible to evaluate the energy of an IHB by comparison with a
conformer in which it is removed by 180° rotation of the donor [33–42]. It is only
possible to compare the strength of the O−H⋯O and O−H⋯π IHBs utilizing
conformers in which only one or the other is present and the other geometry
characteristics are identical, as in the previously considered case of
JZM-a-b-e-f-g-h-p-v-x and JZM-b-c-e-f-g-h-p-v-x. Other analogous cases can be
considered. For instance, JZM-a-b-e-f-g-h-p-v-x and JZM-c-d-e-f-g-h-p-v-x differ
only because the former has two O−H⋯O IHBs and two O−H⋯π IHBs and the
latter has four O−H⋯π IHBs. The relative energy of the latter is 11.627/HF and
11.508/DFT kcal/mol higher than that of the former, suggesting that in this case the
stabilisation brought by an O−H⋯π IHB is ∼5.8 kcal/mol smaller than that of an
O−H⋯O IHB. Conformers JZM-a-b-e-f-g-h-q-v-x and JZM-c-d-e-f-g-h-q-v-x,
JZM-a-b-e-f-g-h-q-t-x and JZM-c-d-e-f-g-h-q-t-x, JZM-a-b-e-f-g-h-q-v-y and
JZM-c-d-e-f-g-h-q-v-y, and JZM-a-b-e-f-g-h-p-t-x and JZM-c-d-e-f-g-h-p-t-x are
other pairs of this type with the relative energy of the latter in each pair ∼10 kcal/
mol higher than that of the corresponding former, with both HF and DFT.
Table 3 reports the ranges of the lengths of all the IHB types present in the
conformers. Complete data of the parameters of all the conformers are reported in
the Supplementary Information. The parameters of the O43−H52⋯O42 and O44
−H54⋯O45 IHBs in the same conformer are very close. For S/S′ symmetric pairs,
the parameters are identical whether these IHBs are in S or S′. The bond length (Å)
is close to 1.73/DFT and 1.77/HF, the O⋯O distance (Å) is close to 2.58 with both
HF and DFT and the OĤO bond angle is ∼143°/HF and ∼145°/DFT for conformers having also other types of IHBs. On the basis of their bond lengths, O⋯O
distances and bond angles, the O−H⋯O IHBs in JZM can be classified in the lower
region of moderate H-bonds [43].
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interaction and the removal of the O44−H54⋯O45 IHB brings about the O44
−H54⋯π interaction. Each of these removals causes an increase in the energy of the
conformer. Comparison of the energies of conformers JZM-a-b-e-f-g-h-p-v-x and
JZM-b-c-e-f-g-h-p-v-x (removal of O43−H52⋯O42 and formation of O43
−H52⋯π) shows an energy increase of ≈6 kcal/mol. Since the two units are
identical, the presence or the absence of a given IHB in only one of the units has the
same effects whether it occurs in unit S (O43−H52⋯O42) or in unit S′ (O44
−H54⋯O45). Therefore, the conformers JZM-a-d-e-f-g-h-p-v-x and JZM-b-c-e-fg-h-p-v-x (or the conformers JZM-a-d-e-f-g-h-q-v-y and JZM-b-c-e-f-g-h-q-v-y,
or JZM-a-d-e-f-g-h-p-t-x and JZM-b-c-e-f-g-h-p-t-x, or JZM-a-d-q-t-x and
JZM-b-c-q-t-x, or JZM-b-c-q-v-y and JZM-a-d-q-v-y, or JZM-b-c-p-t-x and
JZM-a-d-p-t-x, or JZM-b-c-p-v-x and JZM-a-d-p-v-x, or JZM-b-c-q-t-x and
JZM-a-d-q-v-x (Table S13) have the same relative energy and similar molecular
properties such as dipole moment and HOMO-LUMO energy gap, and also rather
close values of ΔG solv .
Since the removal of an O−H⋯O IHB brings about an O−H⋯π IHB and vice
versa, it is not possible to evaluate the energy of an IHB by comparison with a
conformer in which it is removed by 180° rotation of the donor [33–42]. It is only
possible to compare the strength of the O−H⋯O and O−H⋯π IHBs utilizing
conformers in which only one or the other is present and the other geometry
characteristics are identical, as in the previously considered case of
JZM-a-b-e-f-g-h-p-v-x and JZM-b-c-e-f-g-h-p-v-x. Other analogous cases can be
considered. For instance, JZM-a-b-e-f-g-h-p-v-x and JZM-c-d-e-f-g-h-p-v-x differ
only because the former has two O−H⋯O IHBs and two O−H⋯π IHBs and the
latter has four O−H⋯π IHBs. The relative energy of the latter is 11.627/HF and
11.508/DFT kcal/mol higher than that of the former, suggesting that in this case the
stabilisation brought by an O−H⋯π IHB is ∼5.8 kcal/mol smaller than that of an
O−H⋯O IHB. Conformers JZM-a-b-e-f-g-h-q-v-x and JZM-c-d-e-f-g-h-q-v-x,
JZM-a-b-e-f-g-h-q-t-x and JZM-c-d-e-f-g-h-q-t-x, JZM-a-b-e-f-g-h-q-v-y and
JZM-c-d-e-f-g-h-q-v-y, and JZM-a-b-e-f-g-h-p-t-x and JZM-c-d-e-f-g-h-p-t-x are
other pairs of this type with the relative energy of the latter in each pair ∼10 kcal/
mol higher than that of the corresponding former, with both HF and DFT.
Table 3 reports the ranges of the lengths of all the IHB types present in the
conformers. Complete data of the parameters of all the conformers are reported in
the Supplementary Information. The parameters of the O43−H52⋯O42 and O44
−H54⋯O45 IHBs in the same conformer are very close. For S/S′ symmetric pairs,
the parameters are identical whether these IHBs are in S or S′. The bond length (Å)
is close to 1.73/DFT and 1.77/HF, the O⋯O distance (Å) is close to 2.58 with both
HF and DFT and the OĤO bond angle is ∼143°/HF and ∼145°/DFT for conformers having also other types of IHBs. On the basis of their bond lengths, O⋯O
distances and bond angles, the O−H⋯O IHBs in JZM can be classified in the lower
region of moderate H-bonds [43].
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