Although the comparison of the HOMO-LUMO energy gap in different media is
based only on the DFT results (Table 7), the trend-identification has proved reliable
for other molecules [28] and is, therefore, considered realistic. For conformers not
containing the H23⋯O41 IHB, the gap decreases slightly for chloroform and
acetonitrile, with respect to in vacuo, and increase slightly in water. For conformers
containing the H23⋯O41 IHB, the slight decrease in solution is steady with
increasing solvent polarity and water solution corresponds to the smallest value.
3.3 Adducts with Explicit Water Molecules
Three adducts with explicit water molecules were calculated for conformer 1-c: an
adduct with six water molecules (1-c-6aq), an adduct with nine water molecules
(1-c-9aq), and an adduct with 13 water molecules (1-c-13aq). Figure 7 shows their
geometries and their molecule-water interaction energies and Table 9 shows the
lengths of the H-bonds between water molecules and the MUCH-B molecule and
between different water molecules. The shorter solute-water distances correspond to
the cases when an OH of the xanthone moiety is donor to a water molecule, which
is made possible because the H23⋯O17 IHB weakens, and H23 bonds simultaneously to a water molecule.
For both 1-c-6aq and 1-c-9aq, no water molecules were placed in the region of
the cooperative H25⋯O19 and H26⋯O19 IHB in the inputs. The increase in the
solute-water interaction energy from 1-c-6aq to 1-c-9aq is consistent with the
greater stabilization of an adduct when two water molecules directly H-bonded to
the solute molecule are bridged by a third water molecule.
Water molecules forming pentagonal rings of O atoms [10] around the
H25⋯O19 and around the H25⋯O19 IHBs were placed in the input of 1-c-13aq
adding them to the optimised geometry of 1-c-9aq (the pentagonal rings including
O18, O19 and three water molecules, or O18, O20 and three water molecules). The
optimisation confirms the hydrophobic character of the IHBs. Although one water
molecule remains bonded to O19, no water molecule remains bonded to O18 or
O20: water molecules bonded to each other “keep away” from the two IHBs, and
one water molecule bonds to O17 or O21, on either side of the two strong IHBs.
This is consistent with the results obtained for other molecules in which an sp
2 O
forms an IHB with an OH attached to an aromatic ring [10].
Table 10 compares the parameters of the IHBs in the isolated MUCH-B molecule and in the three adducts. Remarkable changes occur for the H23⋯O17 IHB, as
H23 rotates off plane enough to be able to simultaneously H-bond a water molecule:
the length of H23⋯O17 increases, and the OĤO angle decreases.
The counterpoise BSSE corrections (kcal/mol) were 12.43 for 1-c-6aq, 19.15 for
1-c-9aq and 25.07 for 1-c-13aq.
110
L. Mammino et al.
based only on the DFT results (Table 7), the trend-identification has proved reliable
for other molecules [28] and is, therefore, considered realistic. For conformers not
containing the H23⋯O41 IHB, the gap decreases slightly for chloroform and
acetonitrile, with respect to in vacuo, and increase slightly in water. For conformers
containing the H23⋯O41 IHB, the slight decrease in solution is steady with
increasing solvent polarity and water solution corresponds to the smallest value.
3.3 Adducts with Explicit Water Molecules
Three adducts with explicit water molecules were calculated for conformer 1-c: an
adduct with six water molecules (1-c-6aq), an adduct with nine water molecules
(1-c-9aq), and an adduct with 13 water molecules (1-c-13aq). Figure 7 shows their
geometries and their molecule-water interaction energies and Table 9 shows the
lengths of the H-bonds between water molecules and the MUCH-B molecule and
between different water molecules. The shorter solute-water distances correspond to
the cases when an OH of the xanthone moiety is donor to a water molecule, which
is made possible because the H23⋯O17 IHB weakens, and H23 bonds simultaneously to a water molecule.
For both 1-c-6aq and 1-c-9aq, no water molecules were placed in the region of
the cooperative H25⋯O19 and H26⋯O19 IHB in the inputs. The increase in the
solute-water interaction energy from 1-c-6aq to 1-c-9aq is consistent with the
greater stabilization of an adduct when two water molecules directly H-bonded to
the solute molecule are bridged by a third water molecule.
Water molecules forming pentagonal rings of O atoms [10] around the
H25⋯O19 and around the H25⋯O19 IHBs were placed in the input of 1-c-13aq
adding them to the optimised geometry of 1-c-9aq (the pentagonal rings including
O18, O19 and three water molecules, or O18, O20 and three water molecules). The
optimisation confirms the hydrophobic character of the IHBs. Although one water
molecule remains bonded to O19, no water molecule remains bonded to O18 or
O20: water molecules bonded to each other “keep away” from the two IHBs, and
one water molecule bonds to O17 or O21, on either side of the two strong IHBs.
This is consistent with the results obtained for other molecules in which an sp
2 O
forms an IHB with an OH attached to an aromatic ring [10].
Table 10 compares the parameters of the IHBs in the isolated MUCH-B molecule and in the three adducts. Remarkable changes occur for the H23⋯O17 IHB, as
H23 rotates off plane enough to be able to simultaneously H-bond a water molecule:
the length of H23⋯O17 increases, and the OĤO angle decreases.
The counterpoise BSSE corrections (kcal/mol) were 12.43 for 1-c-6aq, 19.15 for
1-c-9aq and 25.07 for 1-c-13aq.
110
L. Mammino et al.