C-H- X interactions

CBiiH12 and derivatives Cation M4 (solvate, solvate-carborane C-H X interaction) Fragment M+ in molecule or polymer (M-carborane interactions, solvate, solvate-carborane C-H X interaction)... 

On one end of the spectrum of H-bonds are those that are very weak, as in the case of numerous C—H---X interactions. On the other end of this continuum are very strong interactions. HX molecules are quite acidic. If paired with a proton acceptor that has a negative charge, the interaction can become very strong indeed. In fact, the H-bond can become so short that the proton loses its association with one molecule versus the other. That is, the proton can be drawn into the midpoint of the X—X axis. Such a H-bond is referred to as centrosymmetric. 

Cage C-H X interactions in solid-state structures of icosahedral carboranes 04CCR(248)457. 

Abstract The different C-H- X interactions are analyzed in this chapter. They are compared with the other types of hydrogen bonds, especially O-H- O ones. Theoretical tools are presented, among them the atoms in molecules theory (AIM). The H-bond motifs existing in crystals are presented. The evidences that C-H- -X interactions may be often classified as H-bonds are discussed. Etter s graph set analysis is also used to describe the structure of complex aggregates. 

Keywords Hydrogen bonds van der Waals interactions C-H- - -X interactions ab initio calcu-... 

The hydrogen bond nature of the C-H- - -X interactions has been a controversial issue in the literature in old and more recent times. Therefore, to properly address such a point, we will begin by reviewing the current knowledge about what a hydrogen bond is. This will drive us into the nature of a bond. 

Table 8. Theoretical data available on the literature about C-H- X interactions. The complex, method and basis set, H- O optimum distance, and non-BSSE corrected (E) and BSSE-corrected (Ecp) formation energies are given for each complex, together with the reference from which the data are extracted (the references are given at the end of this table ordered from less to more recent)...

REF30 Rryachko, E. S. and Zeegers-Huyskens, T. (2002) Theoretical study of the C-H- - -X interaction of fiuoromethanes and chloromethanes with fluoride, chloride and hydroxide anions, J. Phys. Chem. A, 106, 6832-6838... 

The concepts described above for the C-H- O interactions can be extended to other C-H- - -X interactions. Consequently, we will not carry out a detailed study of these interactions, but just will give the most relevant details. 

Besides the C H- O interactions, the most relevant C-H- X interactions are the C H- N, C H- S and C-H- TT interactions (see Table 8 for a complete set of results). The trends for the C H- -N contacts found in crystals were first analyzed by Taylor and Kennard [72] in their seminal paper, where they also concluded that there was enough evidence to consider them as hydrogen bonds. These authors also attributed a hydrogen bond nature to the C H- -N and C H- -S contacts. 

H. Maeda, Y. Kusunose, Dipyrrolyldiketone difluoroboron complexes novel anion sensors with C-H—X interactions. Chem. Eur. J. 11, 5661-5666 (2005)... 

It is probably pertinent at this stage to cite a comment by Dunitz and Gavezzotti to the effect that most organic molecules feature hydrogen atoms at the periphery so that C-H X interactions are inevitable a similar observation applies to many metalorganic molecules. The question then arises actually how important are these interactions... 

Upon the addition of anions as tetrabutylammonium (TBA+) salts, N-H X- and bridging C-H X" interactions can be identified from the H NMR chemical shifts, as demonstrated for a series of anion receptors. UV/vis absorption and emission... 

---