Computational Study of Shuangancistrotectorine A …
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of a symmetric pair have the same IHBs in either S or S’ and the parameters of these
IHBs are identical.
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 conformer’s energy. For
example, the removal of O43–H52···O42 IHB in SHA-1-a-b-g-h-i-u-q-v-x leads to
the formation of O43–H52···π (yielding SHA-8-b-c-g-h-i-u-q-v-x); the corresponding energy increase (≈5 kcal/mol) can be considered an indication of the energy
difference between the O43–H52···O42 and O43–H52···π IHBs. On the other hand,
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 individual IHB by comparison with a
conformer in which it is removed by 180° rotation of the donor [26–35].
No bond length can be defined for the O–H···π IHB, as the acceptor is a whole π
electron distribution and not an individual atom. It may be convenient to consider the
distance between the H atom of the donor OH and the closest C atom in the acceptor
aromatic system, in order to compare the O–H···π IHBs in different conformers.
The H···C distances for the O43–H52···π and O44–H54···π IHBs are very close. The
ranges of these distances (Å) are 2.216 2.228/HF and 2.184 2.198/DFT for both
O43–H54···π and O44–H55···π. The values for all the conformers are reported in
Table S2.
The ranges of the H···O distance for the C–H···O interactions/IHBs are reported
in Table 3 and the values for all the conformers are reported in Table S3. This
distance is considerably longer than the H···O distance for O–H···O IHBs, consistently
with the fact that the C–H···O IHBs are considerably weaker H-bonds. The H···O
distance is slightly shorter when the IHB involves a C–H and the O within the
same isoquinoline moiety (C–H49···O41 and C–H59···O46) than when it involves
a C–H in one naphthalene moiety and an O atom in the other naphthalene moiety
(C–H53···O44 and C–H56···O43). Corresponding trends are also observed for the
donor-acceptor C···O distance and the C ˆ
HO bond angle. Thus, the C–H···O IHBs
between naphthalene moieties are somewhat weaker than C–H···O IHBs within an
isoquinoline moiety.
Table 4 reports the calculated harmonic vibrational frequencies of the O–H bonds
present in SHA. For molecules containing IHBs, it is interesting to consider the redshift (lowering of the vibrational frequency of the donor OH) caused by the IHBs.
The red shift is evaluated with respect to the frequency of a free OH of the same type
(an OH in the same position in the molecule, but not engaged in IHBs). Since the OHs
in SHA are never free (as is also the case with other dimeric NIQ alkaloids [4, 6]),
it is necessary to resort to a model structure to evaluate the frequency of a free OH
in a similar or comparable molecular context. The model structure shown in Fig. 3
was utilised here to this purpose. It contains only a naphthalene moiety because OH
groups are attached only to the naphthalene moieties in SHA; the OH is free, the
CH 3 and OCH 3 substituents are in the same positions as in SHA, and the presence of
the other moieties is mimicked by CH 3 groups. The vibrational frequency of the OH
in the model structure is 3733.29 cm
−1 , and it is used as a reference to calculate the
191
of a symmetric pair have the same IHBs in either S or S’ and the parameters of these
IHBs are identical.
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 conformer’s energy. For
example, the removal of O43–H52···O42 IHB in SHA-1-a-b-g-h-i-u-q-v-x leads to
the formation of O43–H52···π (yielding SHA-8-b-c-g-h-i-u-q-v-x); the corresponding energy increase (≈5 kcal/mol) can be considered an indication of the energy
difference between the O43–H52···O42 and O43–H52···π IHBs. On the other hand,
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 individual IHB by comparison with a
conformer in which it is removed by 180° rotation of the donor [26–35].
No bond length can be defined for the O–H···π IHB, as the acceptor is a whole π
electron distribution and not an individual atom. It may be convenient to consider the
distance between the H atom of the donor OH and the closest C atom in the acceptor
aromatic system, in order to compare the O–H···π IHBs in different conformers.
The H···C distances for the O43–H52···π and O44–H54···π IHBs are very close. The
ranges of these distances (Å) are 2.216 2.228/HF and 2.184 2.198/DFT for both
O43–H54···π and O44–H55···π. The values for all the conformers are reported in
Table S2.
The ranges of the H···O distance for the C–H···O interactions/IHBs are reported
in Table 3 and the values for all the conformers are reported in Table S3. This
distance is considerably longer than the H···O distance for O–H···O IHBs, consistently
with the fact that the C–H···O IHBs are considerably weaker H-bonds. The H···O
distance is slightly shorter when the IHB involves a C–H and the O within the
same isoquinoline moiety (C–H49···O41 and C–H59···O46) than when it involves
a C–H in one naphthalene moiety and an O atom in the other naphthalene moiety
(C–H53···O44 and C–H56···O43). Corresponding trends are also observed for the
donor-acceptor C···O distance and the C ˆ
HO bond angle. Thus, the C–H···O IHBs
between naphthalene moieties are somewhat weaker than C–H···O IHBs within an
isoquinoline moiety.
Table 4 reports the calculated harmonic vibrational frequencies of the O–H bonds
present in SHA. For molecules containing IHBs, it is interesting to consider the redshift (lowering of the vibrational frequency of the donor OH) caused by the IHBs.
The red shift is evaluated with respect to the frequency of a free OH of the same type
(an OH in the same position in the molecule, but not engaged in IHBs). Since the OHs
in SHA are never free (as is also the case with other dimeric NIQ alkaloids [4, 6]),
it is necessary to resort to a model structure to evaluate the frequency of a free OH
in a similar or comparable molecular context. The model structure shown in Fig. 3
was utilised here to this purpose. It contains only a naphthalene moiety because OH
groups are attached only to the naphthalene moieties in SHA; the OH is free, the
CH 3 and OCH 3 substituents are in the same positions as in SHA, and the presence of
the other moieties is mimicked by CH 3 groups. The vibrational frequency of the OH
in the model structure is 3733.29 cm
−1 , and it is used as a reference to calculate the