208
N. Tshilande and L. Mammino
three molecules highlight similarity of trends for most of the molecular properties
considered, and frequent closeness of their individual values. The results in solution show narrowing of the energy gaps between conformers and increase of the
conformers’ dipole moment as the medium polarity increases.
2 Computational Details
The overall study involved three sets of calculation: preliminary calculation of a
model structure, focused on the conformational preferences of the system comprising
all the rings and performed at the Hartree-Fock (HF) and density Functional Theory
(DFT) levels; calculation of conformers of the three molecules in which a linear
geometry of the R chain is associated in turn with each identified stable geometry of
the rings-system, performed at the HF level; and calculation of conformers in which
different geometries of R are associated with the lowest-energy identified geometry of
the rings-system, performed at the HF, DFT and Møller-Plesset Perturbation Theory
(MP2) levels. All the calculations were performed with fully relaxed geometry. Each
method utilised the same basis set as in previous studies of ACPLs [10–21] to enable
meaningful comparisons; thus, HF and MP2 calculations utilised the 6-31G(d,p)
basis set, and DFT calculations were performed with the B3LYP functional [25–27]
and the 6-31+G(d,p) basis set.
Previous studies [10–21] had shown that HF/6-31G(d,p) calculations can provide a
good overview of conformational preferences, and of the factors influencing them, for
ACPLs, at a significantly lower computational cost. Therefore, they were performed
as initial calculations for all the conformers investigated.
MP2 takes into account both the electron correlation and dispersion interactions
[28–30] and, therefore, can provide good descriptions of IHBs. On the other hand,
MP2 calculations are computationally highly demanding for molecules of this size.
Therefore, they were added for the case when the influence of the factor under
investigation (the geometry of the R chain) is expected to be small, to enable better
detection of its effects; these conformers turned out to comprise most of the lower
energy conformers.
Previous studies [10–21] had also shown that the presence of diffuse functions on
the heavy atoms is important for the quality of DFT/B3LYP results for ACPLs. The
use of diffuse and polarization functions is also important for a better description
of IHBs [29]. Since DFT/B3LYP/6-31+G(d,p) calculations are considerably more
expensive (computationally) than HF/6-31G(d,p), they were performed only for the
50 lower energy conformers of the model structure and for the set of calculations
meant to check the influence of the geometry of R.
Frequency calculations (harmonic approximation) were performed only at the
HF/6–31G(d,p) level and only for the lower energy conformers of each of the
three molecules; the results were scaled by 0.8992, as recommended for this [31]
level (DFT frequency calculations proved computationally highly expensive for
molecules of this size).
N. Tshilande and L. Mammino
three molecules highlight similarity of trends for most of the molecular properties
considered, and frequent closeness of their individual values. The results in solution show narrowing of the energy gaps between conformers and increase of the
conformers’ dipole moment as the medium polarity increases.
2 Computational Details
The overall study involved three sets of calculation: preliminary calculation of a
model structure, focused on the conformational preferences of the system comprising
all the rings and performed at the Hartree-Fock (HF) and density Functional Theory
(DFT) levels; calculation of conformers of the three molecules in which a linear
geometry of the R chain is associated in turn with each identified stable geometry of
the rings-system, performed at the HF level; and calculation of conformers in which
different geometries of R are associated with the lowest-energy identified geometry of
the rings-system, performed at the HF, DFT and Møller-Plesset Perturbation Theory
(MP2) levels. All the calculations were performed with fully relaxed geometry. Each
method utilised the same basis set as in previous studies of ACPLs [10–21] to enable
meaningful comparisons; thus, HF and MP2 calculations utilised the 6-31G(d,p)
basis set, and DFT calculations were performed with the B3LYP functional [25–27]
and the 6-31+G(d,p) basis set.
Previous studies [10–21] had shown that HF/6-31G(d,p) calculations can provide a
good overview of conformational preferences, and of the factors influencing them, for
ACPLs, at a significantly lower computational cost. Therefore, they were performed
as initial calculations for all the conformers investigated.
MP2 takes into account both the electron correlation and dispersion interactions
[28–30] and, therefore, can provide good descriptions of IHBs. On the other hand,
MP2 calculations are computationally highly demanding for molecules of this size.
Therefore, they were added for the case when the influence of the factor under
investigation (the geometry of the R chain) is expected to be small, to enable better
detection of its effects; these conformers turned out to comprise most of the lower
energy conformers.
Previous studies [10–21] had also shown that the presence of diffuse functions on
the heavy atoms is important for the quality of DFT/B3LYP results for ACPLs. The
use of diffuse and polarization functions is also important for a better description
of IHBs [29]. Since DFT/B3LYP/6-31+G(d,p) calculations are considerably more
expensive (computationally) than HF/6-31G(d,p), they were performed only for the
50 lower energy conformers of the model structure and for the set of calculations
meant to check the influence of the geometry of R.
Frequency calculations (harmonic approximation) were performed only at the
HF/6–31G(d,p) level and only for the lower energy conformers of each of the
three molecules; the results were scaled by 0.8992, as recommended for this [31]
level (DFT frequency calculations proved computationally highly expensive for
molecules of this size).