Data di Pubblicazione:
2011
Citazione:
Non-covalent Interactions descriptor using experimental electron density / G. Saleh, C. Gatti, L. Lo Presti, J. Contreras Garcia. - In: ACTA CRYSTALLOGRAPHICA. SECTION A, FOUNDATIONS OF CRYSTALLOGRAPHY. - ISSN 0108-7673. - 67:(2011), pp. C448-C449. ((Intervento presentato al 22. convegno Congress and General Assembly of the International Union of Crystallography tenutosi a Madrid nel 2011.
Abstract:
Non covalent interactions (NCI) play a crucial role in biology
(protein-drug recognition) and in the design of new materials (selfassembly).
A novel electron density (ED) based descriptor of non
covalent interaction was proposed in 2010 by Johnson et al.[1]. It
exploits a key quantity in DFT, the reduced density gradient s∝|∇ρ|/
ρ4/3, and reveals NCI in terms of low s-value isosurfaces, defi ned in
low-ED regions and on which the ED is mapped with a colour related
to the sign of the local density curvature along the second largest
variation direction and to the magnitude of the ED itself. Though
directly obtainable from experimental EDs, up to now such descriptor
has been applied only to theoretical EDs or to independent atom model
(IAM) densities.
In this work, we explore the application of this new descriptor to
NCI in the bulk, using X-ray derived EDs. In particular, molecular
crystals represent ideal supramolecular entities for studying non
covalent interactions and the global effect that the crystal fi eld has on
them. Austdiol [2], benzene [3] and the two polymorphs of the antiulcer
drug famotidine [4] were chosen as representative case studies.
Atom-centred multipole expansions (XD2006 package [5]) were
adopted in the refi nement against the experimental structure factors
and the grid fi les for implementing the NCI descriptor were obtained
with an ad-hoc code. The NCI isosurfaces obtained from experiment
have been also compared with those calculated using ab-initio periodic
wavefunctions and the IAM densities.
On the basis of the experience gained on the investigated systems,
it turns out that when applied to experimental EDs the NCI descriptor
gives essentially similar information to that found for theoretical EDs.
For instance, as shown in the fi gure below for benzene crystal, we fi nd
that interactions having a presumably delocalized nature, such as π-π
stackings or C-H⋅⋅⋅π contacts, are indeed seen as extended surfaces,
at variance with the localized and discontinuous picture unavoidably
provided by the bond path analysis. Moreover, we confi rm that
mapping the ED on the s isosurface highlights the strength of the
various interactions, while the sign of the curvature helps to distinguish
true stabilizing interactions from simple steric repulsions. The visible
difference between the NCI isosurfaces portrait found in benzene
crystal on passing from the multipolar to the IAM density corroborates
the stabilizing and not simply steric nature of the π-π stacking and CH⋅⋅⋅
π interactions. [1] E.R. Johnson et al., JACS 2010, 132, 6498-6506. [2] L. Lo Presti et al. J.
Chem Phys. B 2006, 110, 6405-6414 [3] H.-B. Bürgi et al. Chem. Eur. J. 2002,
8, 3512-3521 [4] J. Overgaard et al. Acta Cryst. 2004, A60, 480-487 [5] http://
xd.chem.buffalo.edu/
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
Non-covalent interactions ; electron density
Elenco autori:
G. Saleh, C. Gatti, L. Lo Presti, J. Contreras Garcia
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