Wyniki wyszukiwania - BazTech

Ograniczanie wyników

Znaleziono wyników: 1

Liczba wyników na stronie

Wyniki wyszukiwania

Wyszukiwano:
w słowach kluczowych: niekonwencjonalne wiązanie wodorowe (NKWW)

Sortuj według:

Ogranicz wyniki do:

Niekonwencjonalne wiązania wodorowe [Cz.] 1. Przegląd wyników teoretycznych

Jabłoński M.

Wiadomości Chemiczne

2004

[Z] 58, 7-8

511-534

The hydrogen bond, X-HźźźY, is usually characterized as a relatively weak interaction involving the X-H bond (where X is an element with the electronegativity greater than that of H) and a region with high electron density (e.g. lone electron pairs of highly electronegative element Y or a ?-electron system) [2-5, 8, 17-21, 27-52]. The interaction is believed to have predominantly electrostatic origin and is possibly accompanied by some charge transfer between X-H and Y. In the conventional picture of hydrogen bonding, there is attractive interaction between the region of high electron density (Y) and the polarized proton donor X-H bond that causes an elongation of the X-H bond and a concomitant decrease of its stretching frequency (red shift). This effect can be also reinforced by electron density transfer from the proton acceptor (Y) to the proton donor molecule. The direction of this transfer is primarily to the ?* antibonding orbital of the X-H bond and consequently causes a weakening of this bond. The X-H bond in the X-HźźźY system undergoes elongation and its stretching frequency is lowered. The red shift of the X-H stretching frequency is seen as the most important manifestation of the hydrogen bonding. However, in some cases it occurs that on complexation the X-H bond shortening and a concomitant stretching frequency shift to higher values (blue shift) may also take place. There are many experimental and theoretical evidences that this unconventional behaviour is more widespread than originally thought [75-79, 84-86, 96-101, 106-109]. Since for this kind of interaction the three main features of the classical hydrogen bond (elongation of X-H, red shift, and IR intensity increase) are all in opposite direction, this new type of bonding (interaction) is commonly referred to as improper, blue-shifting H-bond. In this review several examples of the existence of the improper, blue-shifting hydrogen bonds are presented and the magnitude of the complex interaction energies, changes of bond lengths, and blue shifts are examined. To facilitate the understanding and meaning of diverse theoretical results, references to computational methods in use, are added. Particular attention is given to methods and theoretical ideas used in calculations for hydrogen-bonded systems.