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Hexamethylenetetramine at 298 K: new refinements
M Terpstra1, B M Craven, R F Stewart
1Crystallography Department, University of Pittsburgh, PA 15260.
Acta Crystallographica. Section A, Foundations of Crystallography
|September 1, 1993
Summary
New refinements of hexamethylenetetramine (HMT) reveal significant anharmonic C-H bond stretching. This study highlights challenges in multipole model applications for noncentrosymmetric crystal structures.
Area of Science:
- Crystallography
- Materials Science
- Quantum Chemistry
Background:
- Hexamethylenetetramine (HMT) is a molecule with a well-established crystal structure.
- Accurate charge density distribution analysis is crucial for understanding chemical bonding and molecular properties.
Purpose of the Study:
- To refine the crystal structure of HMT using advanced modeling techniques.
- To investigate the anharmonicity of C-H bonds and analyze the charge density distribution.
- To evaluate the applicability of the multipole model to noncentrosymmetric structures.
Main Methods:
- Refinement of crystal structure using neutron and X-ray diffraction data at 298 K.
- Inclusion of third-order Gram-Charlier coefficients to model anharmonic C-H bond stretching.
- Analysis of charge density distribution using the pseudoatom model.
- Experimental determination of the molecular octapole moment.
Main Results:
- Significant anharmonicity in C-H bond stretching was identified.
- The molecular octapole moment of HMT was determined as
= +1.0(3)/e/ ų. - The study identified strong least-squares correlations in multipole refinement for noncentrosymmetric structures.
Conclusions:
- The refined HMT structure accurately describes anharmonic C-H bond stretching.
- The multipole model exhibits deficiencies when applied to noncentrosymmetric structures like HMT.
- This work serves as a critical example for improving crystallographic refinement strategies.