Chemistry Reference
In-Depth Information
used in wavefunction fitting. As a consequence, not only the accuracy but also
the computational effort for providing an answer in each particular case is
increasing.
7. Dipole moments from cluster calculations consistently predict a substantial
in-crystal enhancement. Experimental results, both using the multipole model
and wavefunction fitting, suggest a less pronounced enhancement for the amino
acids.
7 Conclusion
Seven measurements of high-resolution Bragg data on amino acids published
earlier were re-evaluated for a determination of their dipole moments with the
Hansen/Coppens multipole model and by a basis-set representation as used in
Hirshfeld-atom refinement/wavefunction fitting.
Initially, the general ability of the multiple model to reproduce dipole moments
of isolated-molecular calculations was studied by a projection of twenty-two small-
molecule electron densities with simulated structure factors. Theoretical dipole
moments are usually reproduced within
20% of the theoretical result, but can
deviate by more than 70% when heavier elements are involved. For the zwitterionic
amino acids a systematic underestimation of the dipole moment is seen in the
multipole projection. Choices in the treatment of the radial screening parameters
k
k
0
as well as the hydrogen-atom scattering are relevant for obtaining a reasonable
estimate. Invariom modelling applied on the theoretical geometries - which is also
based on the multipole model - equally allows reproducing the dipole moment
within a similar range. Here, amino-acid dipole moments are overestimated with
respect to the gas phase. On the positive side, the computational effort to obtain
dipole moments from database density parameters is minimal. Molecular dipole
moments could and should therefore be a routine result of accurate structure
determinations. Design choices in the invariom database have been chosen to
enable reliable estimates as far as possible.
Refinement of multipole parameters with experimental data allows obtaining
the dipole moment of a molecule as part of the crystal. Based on refinements of the
seven data sets mentioned, we made suggestions how to make experimental
determinations more reliable. Hybrid scattering factors for H-atoms from database
approaches and inclusion of accurate optimized X-H bond distances increase the
reliability of the determination.
Comparing the experimental dipole and the theoretically predicted invariom
moment (or the single-point values) allows assessing dipole-moment enhancements
in the bulk, although model inaccuracies limit the reliability of the results. A similar
comparison of isolated-molecular calculations and wavefunction fitting using
a basis-set representation yields more accurate and consistent results. A density
functional theory treatment (BLYP functional) with the DZP basis was performed
for that purpose.
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