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.

The molecular dipole moment of the 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione (DMACB) molecule and its enhancement in the crystal was evaluated by periodic RHF ab initio computations. A discrete boundary partitioning of the electronic density that allows an unambiguous partitioning of the molecular space in the condensed phase was adopted. The resulting molecular dipole in the crystal compares favorably with the experimental value obtained by a multipolar analysis of single-crystal X-ray diffraction data recorded at 20 K, using a fuzzy boundary partitioning of the derived pseudoatom densities. We show that a large and highly significant molecular dipole enhancement may occur upon crystallization, despite the lack of a strongly hydrogen bonded environment in the crystal. The 23 unique C-H...O interactions which are formed upon packing of the DMACB molecule induce an increase in the molecular dipole (over 75%) that is comparable to or greater than that found in systems which are characterized by the stronger O-H...O and N-H...O hydrogen bonds. The DMACB molecule constitutes an excellent system for the study of C-H...O interactions in the condensed phase, since no other kind of competing hydrogen bonds is present in its crystal. A simple and qualitative model for the matrix contribution to the DMACB molecular dipole enhancement in the crystal is proposed. The formation of several weak C-H...O bonds is found to yield a small (about 0.2 e) net flux of electronic charge flowing from the hydrogens of the methyl groups to the carbonyl oxygen atoms. Despite the limited increase of the intramolecular charge transfer upon crystallization, a large molecular dipole enhancement occurs because the centroids of the positive and negative induced charges are quite far apart. This work highlights a new and important role of the C-H...O bond, besides those already known in the literature.

Quantum chemical topology study on the electronic structure of cis-and trans-FONO J. Chem. Phys. 133, 034304 (2010); 10.1063/1.3460593 Electron-density topology in molecular systems: Paired and unpaired densities A combined molecular dynamics-ab initio study of H 2 adsorption on ideal, relaxed, and temperaturereconstructed MgO(111) surfaces J. Chem. Phys. 109, 7515 (1998); 10.1063/1.477409Surface reconstruction of InP(001) upon adsorption of H 2 S studied by low-energy electron diffraction, scanning tunneling microscopy, high-resolution electron energy loss, and x-ray photoelectron spectroscopies

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