Cocrystallization with Acetylene. The 1 : 1 Complex with Benzene: Crystal Growth, X‐Ray Diffraction and Molecular Simulations

Abstract

Dedicated to Professor Jack D. Dunitz on the occasion of his 80th birthdayThe crystal structure of an unusual 1 : 1 molecular complex between benzene and acetylene, two very small and apolar molecules, has been determined by X-ray-analysis of crystals grown by first mixing the two liquids under conditions of low temperature and high pressure in a capillary, followed by repeated zone melting to form crystals directly on the goniometer head of a diffractometer. Each acetylene molecule is clamped between two parallel benzene rings, with its molecular axis apparently perpendicular to the benzene planes. Closer inspection of thermal-motion data from the diffraction experiment suggests that the acetylene molecule undergoes a wobbling, or precession, motion between the two rings so that it is perpendicular to them only in a timeaveraged sense. The results of quantum-chemical calculations on isolated molecular dimers and trimers support this conclusion. In addition, the calculation of separate coulombic, dispersion, polarization and repulsion contributions to intermolecular bonding reveals that the CÀH ¥¥¥ p-bond interaction between acetylene and benzene in a T-shaped dimer consists of a mixture of coulombic and polarization interactions. In the benzeneÀacetylene cocrystal, its magnitude is quantitatively comparable with that of other dispersive interactions. 5.4 ns Molecular-dynamics simulations of the liquid mixture reveal that the two components are persistently miscible, a possible key to the formation of the cocrystal. No structure is, however, observed in the solution during the relatively short simulation time.

Molecular Recognition in Organic Crystals: Directed Intermolecular Bonds or Nonlocalized Bonding?

“…The results for the T-1 and T-2 dimers in Table 3 show that in addition to the already noted translational floppiness of all benzene dimers, there is also a relatively large freedom in the mutual orientation of the two rings, so a presumed CÀH···p bond would have the unusual property of allowing one of the partners, the H atom, to be almost anywhere. The status of a "bond" seems more justified for the CÀH···p interaction in the crystalline benzene-acetylene complex, [52] where the acetylenic CÀH group points directly at the center of the benzene ring, thereby producing a symmetric and highly efficient packing of the rod-like and disk-like component molecules. The optimal CÀH···p bond, involving the best CÀH donor and the best p acceptor, should occur in the T-shaped acetylene dimer.…”

Isomerenselektive IR‐Spektroskopie von Benzol‐Acetylen‐Clustern: Vergleich mit der Struktur des Benzol‐Acetylen‐Cokristalls

“…Verschiedene Cokristalle mit molaren Verhältnissen von 1:1 und 1:2 konnten auf diese Weise hergestellt und analysiert werden. [1,2] Die grundlegenden Strukturmotive der Elementarzellen könnten im Prinzip mit der Struktur von Nanokristallen verglichen werden, die bei der adiabatischen Abkühlung in Überschallexpansionen entstehen. Allerdings können kooperative Effekte zu isomeren Kristallstrukturen führen, die nicht notwendigerweise den globalen Minimumsstrukturen der Cluster entsprechen.…”

“…[4][5][6] Ein typisches Beispiel für solch eine starke C À H···p-Wechselwirkung ist das T-förmige Benzol-Acetylen(BA)-Dimer. Wir haben uns speziell für die Untersuchung kleiner Benzol-AcetylenCluster entschieden, weil sich diese direkt mit dem 1:1-Cokristall [1] vergleichen lassen. Hier präsentieren wir die IRSpektren von BA 2 , BA 3 und B 2 A.…”

“…[8] NMR-Messungen deuten auf einen engen CÀH···p-Kontakt. [11] Das Packungsmotiv des 1:1-Cokristalls [1] besteht aus einer Anordnung von T-förmigen BAEinheiten (Abbildung 1).…”

“…Abbildung 1. Ausschnitt aus der Festkörperstruktur des Benzol-Acetylen-Cokristalls [1] mit T-förmigen BA-Einheiten, bestimmt durch Röntgen-strukturanalyse. Die Elementarzelle wird aus drei BA-Dimeren aufgebaut.…”