174
L. Mammino
the ion binds to O25˛O30, O25˛O30˛O12, or O25˛O12; it decreases considerably
when the ion binds to O18˛O10, and increases considerably (by 0.34–0.49 Å) when
it binds to O14. The H19···O10 length decreases by 0.16–0.28 Å when the ion binds
to O8˛O18 and increases in the other cases. The H16···O18 length may increase or
decrease, depending on the ion binding site and on the conformer type; the largest
decreases (0.46–0.55 Å) occur with d*-β-γ-ε-e conformers. The H16···O25 length
mostly decreases, with the greatest decreases (0.19–0.38 Å) when the ion binds to
O8˛O18. The H31···O25 length decreases when the ion binds to O30 and increases
considerably in the other cases, with the greatest increase (0.45–0.50 Å) when the
ion binds to O8˛O18.
Table S11 reports the vibrational frequencies (harmonic approximation) of the
OH groups in the calculated complexes and table S12 analyses them in terms of the
binding sites of the ion. Table S13 reports the red shifts in the vibrational frequencies
of the OH groups caused by IHBs, evaluated with respect to the same references
as for the uncomplexed conformers. The changes in the red shifts induced by complexation are analysed both in terms of conformer types (table S14) and in terms of
the ion binding sites (table S15). Comparison of tables S14 and S15 shows that the
binding site of the ion is the major factor determining how the red shift changes on
complexation, although the influence of the conformer-type also appears to be significant. The changes are discussed in some details in the next paragraphs, because
they provide indications as to whether a certain IHB is weakened or strengthened by
complexation.
Like for the complexes of other ACPLs [6–8, 58], the greatest decreases in the
red shift (741–876 cm
−1 ) concern O8–H15 when the ion binds to O14. Together
with the previously-analysed increases in the bond length, this is clear indication
of considerable weakening of the first IHB when the ion binds to its acceptor O.
The red shift of O8–H15 also decreases—often considerably—for most of the other
binding sites. In complexes with the ion binding to O30, the red shift changes depend
markedly on whether H15 transfers to O14 or not, as it decreases considerably when
the proton transfer occurs and increases considerably when it does not occur; this is
consistent with the observation that the IHB length decreases for all the complexes
where no proton transfer occurs (table S16). The red shift also increases slightly for
some complexes in which the ion binds to O25˛O12 and no proton transfer occurs.
Like for the uncomplexed conformers, the red shifts for the other O–H···O IHBs
are much smaller, and their changes are often comparable with frequency fluctuations related to other features of the molecular context. Therefore, they cannot be
considered to offer unequivocal indications about the weakening or strengthening of
an IHB on complexation; better indications are provided by the changes in the IHB
parameters (table S10).
The O–H···π IHBs are practically lost on complexation: inputs in which they are
present optimise to outputs in which they are not present.
The C–H···O IHBs are mostly maintained on complexation, although their length
may vary significantly. Table S17 reports the lengths of the C–H···O IHBs for the
representative complexes considered in Table 3, comparing them with the lengths of
the corresponding IHBs in the uncomplexed conformer. The changes in the lengths
L. Mammino
the ion binds to O25˛O30, O25˛O30˛O12, or O25˛O12; it decreases considerably
when the ion binds to O18˛O10, and increases considerably (by 0.34–0.49 Å) when
it binds to O14. The H19···O10 length decreases by 0.16–0.28 Å when the ion binds
to O8˛O18 and increases in the other cases. The H16···O18 length may increase or
decrease, depending on the ion binding site and on the conformer type; the largest
decreases (0.46–0.55 Å) occur with d*-β-γ-ε-e conformers. The H16···O25 length
mostly decreases, with the greatest decreases (0.19–0.38 Å) when the ion binds to
O8˛O18. The H31···O25 length decreases when the ion binds to O30 and increases
considerably in the other cases, with the greatest increase (0.45–0.50 Å) when the
ion binds to O8˛O18.
Table S11 reports the vibrational frequencies (harmonic approximation) of the
OH groups in the calculated complexes and table S12 analyses them in terms of the
binding sites of the ion. Table S13 reports the red shifts in the vibrational frequencies
of the OH groups caused by IHBs, evaluated with respect to the same references
as for the uncomplexed conformers. The changes in the red shifts induced by complexation are analysed both in terms of conformer types (table S14) and in terms of
the ion binding sites (table S15). Comparison of tables S14 and S15 shows that the
binding site of the ion is the major factor determining how the red shift changes on
complexation, although the influence of the conformer-type also appears to be significant. The changes are discussed in some details in the next paragraphs, because
they provide indications as to whether a certain IHB is weakened or strengthened by
complexation.
Like for the complexes of other ACPLs [6–8, 58], the greatest decreases in the
red shift (741–876 cm
−1 ) concern O8–H15 when the ion binds to O14. Together
with the previously-analysed increases in the bond length, this is clear indication
of considerable weakening of the first IHB when the ion binds to its acceptor O.
The red shift of O8–H15 also decreases—often considerably—for most of the other
binding sites. In complexes with the ion binding to O30, the red shift changes depend
markedly on whether H15 transfers to O14 or not, as it decreases considerably when
the proton transfer occurs and increases considerably when it does not occur; this is
consistent with the observation that the IHB length decreases for all the complexes
where no proton transfer occurs (table S16). The red shift also increases slightly for
some complexes in which the ion binds to O25˛O12 and no proton transfer occurs.
Like for the uncomplexed conformers, the red shifts for the other O–H···O IHBs
are much smaller, and their changes are often comparable with frequency fluctuations related to other features of the molecular context. Therefore, they cannot be
considered to offer unequivocal indications about the weakening or strengthening of
an IHB on complexation; better indications are provided by the changes in the IHB
parameters (table S10).
The O–H···π IHBs are practically lost on complexation: inputs in which they are
present optimise to outputs in which they are not present.
The C–H···O IHBs are mostly maintained on complexation, although their length
may vary significantly. Table S17 reports the lengths of the C–H···O IHBs for the
representative complexes considered in Table 3, comparing them with the lengths of
the corresponding IHBs in the uncomplexed conformer. The changes in the lengths