Intermolecular interactions in a series of organoammonium tetraphenylborates

by Robertson, Katherine N.

High quality, low temperature, x-ray crystallographic data sets were collected for four organoammonium tetraphenylborates. On close scrutiny, two types of N-H ... X interactions were identified, the more "traditional" N-H. ..N hydrogen bonds, and N-H....ir(Ph) hydrogen bonds [centroid, edge or single atom type], in which the phenyl ring of the anion serves as the hydrogen bond acceptor. In addition, the four salts were found to form a large number of close C-H...X type contacts. Some of these adopted hydrogen bond type geometries while others were classified as "H...Hl1 type contacts, using criteria developed in this work. Charge density calculations were carried out on the four salts. From the results, static and dynamic electron density maps were plotted in a variety of planes to aee if evidence of the intermolecular interactions would be visible. The convcntional N-H ... N hydrogen bonds were used as the interna1 standard with which the other, weaker, interactions could be compared. Certain features were identified in the maps that could consistently be attributed to the formation of N-H....ir( Ph) hydrogen bonds. The electron density distributions calculated using the aspherical models generated in the XD refinements were used as input for topological analyses of the four salts. This allowed a more quantitative investigation of the weak intermolecular interactions. The location of a rational bond path and a (3,-1) bond critical point was characteristic of almost every contact studied. The values, at the bond critical points, of the electron density. p,(+), and the Laplacian of the electron density. v , were calculated. They were always positive, typical of closed she11 interactions, and were shown ta be correlated with the distance over which the interaction occurred, specified in this work as the H...acceptor distance. In both cases [p,(r)/v2p,(r) versus d(H.. .A)1, a simple exponential curve was found to mode1 the collected experimental data reasonably well. The experimentally derived topological data from this work were consistent with literature values reported for more traditional hydrogen bonds, which shows the N-H-..lr(Ph) hydrogen bonds behave topologically like al1 others. Finally, the majority of the C-H...phenyl interactions were identified 3s " H...Hl1 type contacts, while the remainder were found to be hydrogen bonds, behaving topologically like ather X-H....lr(Ph) hydrogen bonds. Abbreviations and Symbols B d, 0 h,kJ Mo- A x/a,y/b,z/c Be, U,,' F(OO0) Fo* Fobe Fcf Fcal, NdbS' d NVI P r R Rw, wR2 Rmerge GOF Formula weight Number of molecules in the unit ce11 Unit ce11 dimensions Angles between crystal axes Volume of unit ce11 Linear absorption coefficient Calculated density Bragg angle Miller indices of the reflecting plane Molybdenum Ka radiation Wavelength of X-radiation Fractional coordinates Isotropie temperature factor Anisotropic temperature factors Number of electrons in the unit ce11 Observed structure amplitude Calculated structure factor Number of observed reflections Number of parameters refined Number of reatraints applied R-f actor Weighted R-value Merging R-value for equivalent reflections Goodness of fit parameter W TLS rms Weight in minimized function Translation/Libration/Screw Root mean square Estirnated standard deviation Correlation coefficient Coefficient of determination Standard deviation Electron density in molecule Difierence electron density Vector to a point in real space Scale factor Vector to a point in reciprocal space Observed structure factor amplitude Calculated structure factor amplitude Core atomic density Valence density Multipolar spherical harmonic functions in real form Expansion-contraction parameters Core population parameters Valence population parameters Multipole population parameters Radial functions Normalization factor Deformation electron density Electron density at the bond critical point Laplacian of the electron density at the bond critical point Eigenvalues of the Hessian matrix, principal curvatures of the electron density at the bond critical point Ellipticity of the electron density at the bond critical point Znternuclear separation, its equilibrium value, bond path length Hydrogen bond X-H.. .r(Ph) X-H...p henyl Hydrogen bond accepted by a phenyl ring Any type of interaction involving a phenyl ring, generally not HBs but as a group can include HBs Distance characterizing the X-H...A HB Phenyl ring centroid [lOO (maximum - minimum X/H. ..Cr,- distance) ] a parameter characterizing the X-H... x(Ph) HB HO N High order Neutron X CP BP Critical point Bond path Ph TB CSD Tetraphenylborate anion Cambridge Structural Database