Chemical bond donor-acceptor

Chemical functionality (hydrogen bond donor, acceptor) with geometric constraint ++ +... 

Thus, the nature of chemical features might differ between two hypotheses and the most specific is generally favored. For instance, models with more features will be favored as well as models with directional features (H-bond donor/ acceptors, ring aromatics). 

Theoretically, the purine- and pyrimidine-based nucleic acid constituents and the barbiturates have the potential to occur in several tautomeric forms of the keto/ enol and amino/imino type where the aromatic character of the six-membered pyrimidine ring is fully or, as in the barbiturates, partially retained, as illustrated in Fig. 15.4. In these molecular species, which are all feasible on the basis of organic chemical considerations, the hydrogen-bonding donor/acceptor properties of the functional amino, imino, enol and keto groups vary considerably, being donor in one form and acceptor in the other. 

Supramolecular chemistry is concerned with assemblies of molecules held together by non-covalent forces, such as hydrogen bonds, donor-acceptor interactions between aromatic stacks, ion-ion, ion-dipole, dipole-dipole and van der Waals attractions [1, 2]. Molecules can also be gathered around a metal this is the realm of coordination chemistry. Last, molecules can be linked together without the need for any chemical bond the so-called physical or mechanical bond is found in catenanes (species formed of interlocked rings) and rotaxanes (Figure 1). 

Simply changing the concentration of the solute can sometimes result in significant chemical shift changes, especially for environments near a hydrogen bond donor/acceptor or r-systems with significant diamagnetic anisotropy. For example, the H resonances of acridine in CD3OD shift... 

If the solvent that is selected is polar (e.g., acetone, acetonitrile, chloroform, dimethylsulfoxide, and methanol), there are stronger dipole interactions between solvent and solute, especially if the solute molecule also contains polar bonds. The interactions between the polar solvent and a polar solute are likely to be stronger than the interactions between the solvent and tetramethylsilane (TMS, which is nonpolar), and the result is that the observed chemical shift of the molecule of interest will be shifted with respect to the observed chemical shift in a nonpolar solvent. The magnitude of this solvent-induced shift can be on the order of several tenths of a parts per million in a proton spectrum. Furthermore, simply changing the concentration of the solute can result in chemical shift changes, especially for environments near a hydrogen bond donor/acceptor or an exchangeable site. 

Interactions between hydrogen-bond donor and acceptor groups in different molecules play a pivotal role in many chemical and biological problems. Hydrogen bonds can be studied with quantum chemical calculations and empirical methods. 

Morokuma K 1977. Why Do Molecules Interact The Origin of Electron Donor-Acceptor Complexes, Hydrogen Bonding, and Proton Affinity. Accounts of Chemical Research 10 294-300. 

The chemisorptive bond is a chemical bond. The nature of this bond can be covalent or can have a strong ionic character. The formation of the chemisorptive bond in general involves either donation of electrons from the adsorbate to the metal (donation) or donation of electrons from the metal to the adsorbate (backdonation).2 In the former case the adsorbate is termed electron donor, in the latter case it is termed electron acceptor.3 In many cases both donation and backdonation of electrons is involved in chemisorptive bond formation and the adsorbate behaves both as an electron acceptor and as an electron donor. A typical example is the chemisorption of CO on transition metals where, according to the model first described by Blyholder,4 the chemisorptive bond formation involves both donation of electrons from the 7t orbitals of CO to the metal and backdonation of electrons from the metal to the antibonding n orbitals of CO. 

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