Fundamental Properties and Nature of CH··O Interactions in Crystals on the Basis of Experimental and Theoretical Charge Densities. The Case of 3,4-Bis(dimethylamino)-3-cyclobutene-1,2-dione (DMACB) Crystal

Abstract

The CH··O contacts in the 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione (DMACB) crystal have been characterized through a topological analysis of its experimental and theoretical densities, derived from a multipole refinement of X-ray diffraction data and from periodic Hartree−Fock calculations, respectively. The existence or the lack of an H··O bond critical pointthat is a point through the two nuclei where the gradient of the electron density vanishesallows us to distinguish between bonded and non bonded CH··O contacts, regardless of the value of their H··O separation. The 23 unique bonded contacts in DMACB are characterized by a large and nearly constant (∼140°) C−H−O angle, denoting the importance of the electrostatic energy contribution to such interactions. Instead, the nonbonded ones (four unique for H··O separations up to 3.0 Å) are more bent and may even be folded down to 90°, since their dominant van der Waals contribution to the interaction energy is independent of the C−H−O angle. The CH··O angular distribution observed for H··O separations greater than 2.7 Å is only apparently isotropic, since such isotropy clearly disappears when the bonded and nonbonded contacts are identified and their angular distributions separately analyzed. The Koch and Popelier criteria (J. Phys. Chem. 1995, 99, 9747) to establish H-bonds are, for the first time, applied in their entirety to a large set of CH··O contacts in a crystalline phase. The criteria are always satisfied by all of the bonded intermolecular CH··O contacts, with a single exception concerning one long bond and one of the six criteria only. The expressions proposed by Espinosa et al. (Chem. Phys. Lett. 1998, 285, 170), relating the potential energy densities at the critical point to the H-bond strengths, fail when applied to the weak CH··O interactions present in the DMACB crystal. The reasons for such a failure are outlined and new relationships are proposed. The importance of the promolecular charge distributions in defining topological properties of interest to the CH··O bonds is investigated. The criticism raised by Spackman (Chem. Phys. Lett. 1999, 301, 425) as to the lack of additional information provided by the experimental results to the description of such weak interactions is discussed. It is shown that the promolecular model yields significantly different electron density values at the critical point and in some instances even different topologies, compared to the corresponding multipole or theoretical densities. On the other hand, when the electron density topologies are the same, the values obtained from either electron density for the potential or kinetic energy density at the critical point, are very much alike.

Atoms in Molecules Theory for Exploring the Nature of the Active Sites on Surfaces

“…9.6a) in the middle of each pair of neighboring MoS 2 sheets. These bond CPs are weaker interactions than the MoaS bonds, as evidenced by the values of r (r MoaS ¼ 0:0908 au and r SaS ¼ 0:0112 au) at r c and are characteristic of van der Waals interactions [1,10,[58][59][60]. Similar results have been published for many hydrogen-bonded systems [59,60] and van der Waals molecules [58].…”

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

“…Spackmans calculation raises the question of whether or not the bond paths from the experimental studies are providing anything more than noise about the trend determined by the promolecule or pro-crystal electron distribution. This question has been critically analyzed by Gatti et al [38] who conclude that for CÀH···O interactions, even although charge rearrangements may occur within the atomic basins (thus changing their atomic properties), no significant information is added by experimental or theoretical studies to that contained in the pro-molecule densities for large H···O distances. Nevertheless, in several analyses of experimental electron densities in crystals, intermolecular bond paths are emphasized more or less irrespective of the size and significance of 1(r BCP ) and its Laplacian.…”

On atom–atom `short contact' bonding interactions in crystals

“…Consequently, if one seeks more than just an approximate set of nuclear coordinates (geometrical structure), as obtained in routine Xray determinations for which a promolecule model may be sufficient, additional crucial information must be determined from the experiment if a physically meaningful description of hydrogen bonding is required. Other studies involving C-HÁ Á ÁO hydrogen bonding interactions reached a similar conclusion (Gatti et al, 2002), that promolecular densities can differ from experimental determination or theoretical calculation of (r) in significant ways with regards to electron densities at BCPs and even, in some cases, the topology of the density itself.…”