Ab Initio and DFT Computational Study …
221
interaction is somewhat stronger when O8–H15 is the donor. Tables S24–S26 and
Figs. S31–S33 compare the H16···C23 distance in the HF, DFT and MP2 results, for
the conformers with different geometries of R and the same geometry of ABDE. For
a given conformer, the distance is longest in the HF results, intermediate in the DFT
results and shortest in the MP2 results; this is consistent with the fact that MP2 takes
into account also dispersion contributions, which have a relevant role in O–H···π
interactions.
The red shift (lowering of the vibrational frequency of the donor) caused by an
H-bond provides indication on the relative strengths of H-bonds, as stronger H-bonds
cause greater red shifts. Tables S27–S29 report the vibrational frequencies of the OH
groups of the three molecules, and Tables S30–S32 report the red shifts of the OHs
engaged in IHBs. A red shift is calculated as difference between the frequency of
an OH engaged in an IHB and the frequency of the same OH when it is free (not
engaged in an IHB); in this work, the reference value for a free OH is taken as the
average of the vibrational frequencies of that OH in all the conformers in which it
is free. It has also to be taken into account that the red shifts calculated here are
surely smaller than the actual ones, because of the tendency of HF to underestimate
H-bonds strengths; on the other hand, they enable realistic comparisons of trends (as
proven in studies of ACPLs, where also DFT frequencies and red shifts had been
calculated). The red shift (cm
−1 ) of O8–H15 is 352.6-427.0/MYRA, 278.6-318.6/cDPBO and 237.1-319.5/t-DPBO when it forms the O8–H15···O14 first IHB, and
94.0-112.9/MYRA, 94.2-113.1/c-DPBO and 92.9-113.1/t-DPBO when it forms the
O8–H15···π IHB with the B ring; the red shift of O12–H17 is 273.1-306.6/MYRA,
277.2-306.1/c-DPBO and 277.3-306.2/t-DPBO when it forms the O12–H17···O14
first IHB; and the red shift of O10–H16 is 49.2-76.1/MYRA, 68.2-76.2./c-DPBO
and 69.8-76.4./t-DPBO when it forms the O10–H16···π IHB with the B ring. These
values are consistent with the inferences (from IHB lengths) that the H15···O14 IHB
is slightly stronger than the H17···O14 IHB, and that the H15···π IHB is slightly
stronger than the H16···π IHB.
3.3.4 Dipole Moments of the Conformers
The dipole moment of the calculated conformers of the three molecules (Tables S33–
S35) is mostly influenced by the orientation of the OHs, being greater when more
OHs are oriented in the same direction. The nature of R does not appear to influence the dipole moment significantly. Tables S36–S38 and Figs. S34–S36 compare
the results from different calculation methods for the calculated conformers with
different geometries of R and the same geometry of ABDE. The values from the
same method are very close, confirming that the geometry of R influences the dipole
moment only marginally. The HF and MP2 values are mostly very close, whereas
DFT values differ more noticeably and are often somewhat smaller than the HF and
MP2 ones.
221
interaction is somewhat stronger when O8–H15 is the donor. Tables S24–S26 and
Figs. S31–S33 compare the H16···C23 distance in the HF, DFT and MP2 results, for
the conformers with different geometries of R and the same geometry of ABDE. For
a given conformer, the distance is longest in the HF results, intermediate in the DFT
results and shortest in the MP2 results; this is consistent with the fact that MP2 takes
into account also dispersion contributions, which have a relevant role in O–H···π
interactions.
The red shift (lowering of the vibrational frequency of the donor) caused by an
H-bond provides indication on the relative strengths of H-bonds, as stronger H-bonds
cause greater red shifts. Tables S27–S29 report the vibrational frequencies of the OH
groups of the three molecules, and Tables S30–S32 report the red shifts of the OHs
engaged in IHBs. A red shift is calculated as difference between the frequency of
an OH engaged in an IHB and the frequency of the same OH when it is free (not
engaged in an IHB); in this work, the reference value for a free OH is taken as the
average of the vibrational frequencies of that OH in all the conformers in which it
is free. It has also to be taken into account that the red shifts calculated here are
surely smaller than the actual ones, because of the tendency of HF to underestimate
H-bonds strengths; on the other hand, they enable realistic comparisons of trends (as
proven in studies of ACPLs, where also DFT frequencies and red shifts had been
calculated). The red shift (cm
−1 ) of O8–H15 is 352.6-427.0/MYRA, 278.6-318.6/cDPBO and 237.1-319.5/t-DPBO when it forms the O8–H15···O14 first IHB, and
94.0-112.9/MYRA, 94.2-113.1/c-DPBO and 92.9-113.1/t-DPBO when it forms the
O8–H15···π IHB with the B ring; the red shift of O12–H17 is 273.1-306.6/MYRA,
277.2-306.1/c-DPBO and 277.3-306.2/t-DPBO when it forms the O12–H17···O14
first IHB; and the red shift of O10–H16 is 49.2-76.1/MYRA, 68.2-76.2./c-DPBO
and 69.8-76.4./t-DPBO when it forms the O10–H16···π IHB with the B ring. These
values are consistent with the inferences (from IHB lengths) that the H15···O14 IHB
is slightly stronger than the H17···O14 IHB, and that the H15···π IHB is slightly
stronger than the H16···π IHB.
3.3.4 Dipole Moments of the Conformers
The dipole moment of the calculated conformers of the three molecules (Tables S33–
S35) is mostly influenced by the orientation of the OHs, being greater when more
OHs are oriented in the same direction. The nature of R does not appear to influence the dipole moment significantly. Tables S36–S38 and Figs. S34–S36 compare
the results from different calculation methods for the calculated conformers with
different geometries of R and the same geometry of ABDE. The values from the
same method are very close, confirming that the geometry of R influences the dipole
moment only marginally. The HF and MP2 values are mostly very close, whereas
DFT values differ more noticeably and are often somewhat smaller than the HF and
MP2 ones.