Rigid-link constraints and rigid-body molecules

Didisheim, J.-J.; Schwarzenbach, D.

doi:10.1107/s0108767387099525

Cited by 15 publications

(8 citation statements)

References 2 publications

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“…The MSDA tensors corresponding to the internal vibrational modes (U int ) were transformed from the inertial system of the optimized molecule to the crystal system. In the course of subsequent refinements the shifts in the displacement parameters were restricted by rigid-link type constraints [48] to fulfill the rigid-body motion requirement. This was achieved by invoking 6N À 20 (N 7 is the number of atoms) independent rigid-link constraints, that is by keeping the MSDAs along links equal for a necessary number of atom pairs (DMSDA 0).…”

Section: Density Models and Multipole Refinementsmentioning

confidence: 99%

Topological Analysis of the Experimental Electron Density of Diisocyanomethane at 115 K

Koritsánszky¹,

Buschmann²,

Lentz³

et al. 1999

Chem. Eur. J.

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The structure and charge density of diisocyanomethane, derived from low-temperature X-ray diffraction data and by ab initio calculations are reported. Different refinement models are tested to judge the physical significance of the density parametrization. The experimental distances of the formal triple bonds are significantly shorter than those obtained by energy optimi-zation. In terms of topological indices the former method gives a higher bond order than the latter one, even if the calculation is performed at the exper-imental geometry, indicating that crystal field effects may not be negligible when the results are compared. These effects, although caused by weak close contacts, can directly be revealed by comparing the electrostatic potential extracted from the diffraction data with that derived from the wave function of the isolated molecule.Keywords: ab initio calculationsb ond-topological indices´electronic structure´electrostatic potentialḿ ultipole refinement[a] Prof.

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Section: Density Models and Multipole Refinementsmentioning

confidence: 99%

Topological Analysis of the Experimental Electron Density of Diisocyanomethane at 115 K

Koritsánszky¹,

Buschmann²,

Lentz³

et al. 1999

Chem. Eur. J.

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“…Another sensible assumption, albeit of somewhat less general applicability, is that atoms which are close to one another would move in similar directions with approximately similar amplitudes (20). Thus, atoms closer to one another than say 1.7 Å can be restrained to have the same U ij components within a standard uncertainty of, for example, 0.04 Å 2 .…”

mentioning

confidence: 99%

Practical suggestions for better crystal structures

Müller

2009

Crystallography Reviews

232

169

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“…A model for the orientational disorder of the C 60 molecule at 293 K was determined by trial and error. The shape and dimension of C 60 , as determined by Bu È rgi et al (1992a,b), was ®xed by distance and angle restraints, and molecular rigidbody displacements (Schomaker & Trueblood, 1968) were imposed by rigid-link restraints on the ADPs (atomic displacement parameters) of C (Didisheim & Schwarzenbach, 1987). The restrained atomic parameters of C 60 were re®ned by leastsquares, together with the parameters of the hydroquinone (HQ) molecule, starting with various molecular site symmetries and orientations.…”

Section: Structure Determinationmentioning

confidence: 99%

Orientational disorder as a function of temperature in the clathrate structure of hydroquinone and C₆₀

Blanc

Restori

Schwarzenbach

et al. 2000

Acta Crystallogr Sect B

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In the compound [C(6)H(6)O(2)](3)C(60), hydroquinone (C(6)H(6)O(2)) forms a three-dimensional hydrogen-bonded network enclosing roughly spherical cages with point symmetry 3;m and a diameter of 13.2 A at 100 K. Although C(60) fits tightly into these cages, it shows threefold orientational disorder, the molecular site symmetry being 2/m. The disorder has been studied with single-crystal Mo Kalpha X-ray data at four temperatures, 100, 200, 293 and 373 K. In the refinement, C(60) was restrained to the icosahedral molecular symmetry m3;5; and to rigid-body translational and librational displacements including third- and fourth-order cumulants to account for curvilinear atomic movements, R(|F|) = 3.2-4.7%. At 100 K, bond lengths in C(60) refine to the expected values 1.450 (1) and 1.388 (1) A. The ratio of these values increases with increasing temperature, but the radius of the molecule remains constant at 3. 537 (2) A. The r.m.s. libration amplitudes of C(60) are relatively small (5.5 degrees at 373 K) and the probability function of orientations of C(60) inside the cage shows large values only at the refined positions, indicating that the energy barrier of reorientation is large. Refinement of an ordered twinned structure was unsuccessful; the orientations of neighboring C(60) appear to be uncorrelated.

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Rigid-link constraints and rigid-body molecules

Cited by 15 publications

References 2 publications

Topological Analysis of the Experimental Electron Density of Diisocyanomethane at 115 K

Topological Analysis of the Experimental Electron Density of Diisocyanomethane at 115 K

Practical suggestions for better crystal structures

Orientational disorder as a function of temperature in the clathrate structure of hydroquinone and C₆₀

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Rigid-link constraints and rigid-body molecules

Cited by 15 publications

References 2 publications

Topological Analysis of the Experimental Electron Density of Diisocyanomethane at 115 K

Topological Analysis of the Experimental Electron Density of Diisocyanomethane at 115 K

Practical suggestions for better crystal structures

Orientational disorder as a function of temperature in the clathrate structure of hydroquinone and C60

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Orientational disorder as a function of temperature in the clathrate structure of hydroquinone and C₆₀