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Modern crystallographic refinement methods treat each atom in a molecule as neutral with spherical electron density. Atoms, however, exhibit partial atomic charges arising from intramolecular forces via bonding. These partial charges are crucial for understanding electronic structure and bulk physical properties of molecules. Typically the polarity and polarizability of molecules are calculated using IR and Raman spectroscopy, respectively. While these techniques can be used on small molecules, fine elucidation of partial charges on individual atoms is still unrealized. Here we present crystallographic refinement developments that allow us to refine electron density around individual atoms to experimentally calculate partial atomic charges. Comparison between these experimentally calculated charges to theoretical quantum calculated charges will also be presented.
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Measurement of partial atomic charges by least-squares refinement of variable electron density crystallographic models.
Of interest in understanding electronic structure, bulk physical properties, enthalpies of phase changes, dipole moments, and numerous other properties of molecules, is the determination of realistic partial atomic charges on atoms. Atoms may take on partial positive or negative charges due to polar covalent bonds, coordinate covalent bonds, or due to formal charges imposed by Lewis structure constraints. Traditional crystallographic refinement treats each atom as a neutral, spherical atom however. We present a ongoing developments of a mode of crystallographic model refinement that permits refinement of electron density at individual atoms in order to arrive at partial atomic charges of atoms in a crystallographic model. Comparison to calculated partial charges (CHELPG, NBO, Mulliken) from quantum calculations (DFT, MP2) in both the gas phase and crystalline state will be presented.
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- Award ID(s):
- 1800105
- PAR ID:
- 10187694
- Date Published:
- Journal Name:
- American Crystallographic Association Annual Meeting 2020
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Derivation of partial charges in small and large scale molecular systems is important for modeling of various experimental and theoretical properties like dipole moments, auto-correlation functions, charge disparity, understanding of dispersion, benchmark of classical MD simulations and electrostatic potential energy surface mapping. A correspondence between theoretical calculations (based on single/small number of molecules) is usually established with macroscopic IR/Raman spectra or dipole moment measurements. Such comparisons are indirect and lack a fine mapping of electrostatic potential from theory to experiment. In a new approach developed as the experimental part of this work, partial charges are calculated from crystallographic model refinement. The experimental method exhibits a satisfactory correspondence with partial charges obtained using quantum chemistry calculations. Particularly, gas phase partial charges from CHELPG method and condensed phase Lowdin charges correlate well and validate this experimental method.more » « less
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