Complexes of Furonewguinone B with a Cu 2+ Ion. A DFT Study
169
the strongest IHB. It may be recalled that, in FNGB, the molecular context has all
the features which enhance the strength of the H15···O14 first IHB in ACPLs, as it
forms on the same side as a bulky substituent (FUR) and the OH at C6 is replaced
by a keto O [27]. As in all ACPLs, the first IHB is also strengthened by its closing
a 6-member ring bordering with a benzene ring and comprising the C7=O14 double
bond, which responds to what used to be termed “resonance assisted H-bond” [54–
56]. The other clearly identifiable red shifts are those associated with the O–H···π
IHBs: 80–157 cm
−1 for H16···π1 and 192 cm
−1 for the only case in which H19···π1
is present. The red shifts for the other O–H···O IHBs are much smaller—actually
so small that they may be comparable with frequency fluctuations related to other
features of the molecular context. Nevertheless, they compare among themselves in
the same way in which the IHB lengths compare, i.e., smaller red shifts correspond
to greater bond lengths (Table 2). The fact that the red shifts are so small may be
related to the poor directionality of these IHBs, for which the vibration of the donor
does not bring the H close to the acceptor O along the O···O direction, but occurs
towards a rather different or largely different direction. This also suggests that these
IHBs are considerably weaker than what could be inferred from their bond lengths,
and that the poor directionality has a dominant weakening role.
The orientation of the phenol OHs has non-negligible influence on the energy of
the conformers of ACPLs having free OHs [26, 57]. This is not the case for FNGB,
because its phenol OHs are always engaged in some IHBs; therefore, the analysis of
the conformers’ energy in terms of IHB patterns automatically entails the consideration of the orientations of the phenol OHs. The orientation of the prenyl chain does
not appear to influence the conformers’ energy significantly, likely because the IHB
patterns have dominant roles.
3.3 Complexes of Furonewguinone B with a Cu 2+ Ion.
Results in Vacuo
Complexes were calculated considering all the possible sites of FNGB to which the
ion can bind: simultaneously to three O atoms, or to two O atoms, or to an O atom
and π1, and individually to each O atom or to π1. In a number of cases, different
inputs optimised to the same complex, which supports the reliability of the identified
binding and geometry preferences. Figure 5 shows the geometries of representative
complexes and fig. S2 shows those of all the calculated complexes. Table 3 reports the
relative energy, the MIA, and the charge and spin density on the ion, for representative
complexes (including the ten lowest energy ones); table S6 reports these quantities
for all the calculated complexes and table S7 analyses them in terms of the binding
sites of the ion. Fig. S3 shows diagrams highlighting the trends of these quantities for
all the complexes (part A) and in terms of the binding sites of the ion (part B). Fig.
S4 shows diagrams illustrating the ranges of these quantities for different binding
sites of the ion.
169
the strongest IHB. It may be recalled that, in FNGB, the molecular context has all
the features which enhance the strength of the H15···O14 first IHB in ACPLs, as it
forms on the same side as a bulky substituent (FUR) and the OH at C6 is replaced
by a keto O [27]. As in all ACPLs, the first IHB is also strengthened by its closing
a 6-member ring bordering with a benzene ring and comprising the C7=O14 double
bond, which responds to what used to be termed “resonance assisted H-bond” [54–
56]. The other clearly identifiable red shifts are those associated with the O–H···π
IHBs: 80–157 cm
−1 for H16···π1 and 192 cm
−1 for the only case in which H19···π1
is present. The red shifts for the other O–H···O IHBs are much smaller—actually
so small that they may be comparable with frequency fluctuations related to other
features of the molecular context. Nevertheless, they compare among themselves in
the same way in which the IHB lengths compare, i.e., smaller red shifts correspond
to greater bond lengths (Table 2). The fact that the red shifts are so small may be
related to the poor directionality of these IHBs, for which the vibration of the donor
does not bring the H close to the acceptor O along the O···O direction, but occurs
towards a rather different or largely different direction. This also suggests that these
IHBs are considerably weaker than what could be inferred from their bond lengths,
and that the poor directionality has a dominant weakening role.
The orientation of the phenol OHs has non-negligible influence on the energy of
the conformers of ACPLs having free OHs [26, 57]. This is not the case for FNGB,
because its phenol OHs are always engaged in some IHBs; therefore, the analysis of
the conformers’ energy in terms of IHB patterns automatically entails the consideration of the orientations of the phenol OHs. The orientation of the prenyl chain does
not appear to influence the conformers’ energy significantly, likely because the IHB
patterns have dominant roles.
3.3 Complexes of Furonewguinone B with a Cu 2+ Ion.
Results in Vacuo
Complexes were calculated considering all the possible sites of FNGB to which the
ion can bind: simultaneously to three O atoms, or to two O atoms, or to an O atom
and π1, and individually to each O atom or to π1. In a number of cases, different
inputs optimised to the same complex, which supports the reliability of the identified
binding and geometry preferences. Figure 5 shows the geometries of representative
complexes and fig. S2 shows those of all the calculated complexes. Table 3 reports the
relative energy, the MIA, and the charge and spin density on the ion, for representative
complexes (including the ten lowest energy ones); table S6 reports these quantities
for all the calculated complexes and table S7 analyses them in terms of the binding
sites of the ion. Fig. S3 shows diagrams highlighting the trends of these quantities for
all the complexes (part A) and in terms of the binding sites of the ion (part B). Fig.
S4 shows diagrams illustrating the ranges of these quantities for different binding
sites of the ion.