文档介绍：9. (NH3)2 – a Prototype
of Hydrogen Bonding?
Hydrogen Bonding
One of the important concepts in physical chemistry is the hydrogen bond.
It is well known that there is a significant attractive interaction between
a hydrogen atom attached to an electronegative atom, such as oxygen, a
halogen or nitrogen, and another electronegative atom. The strength of this
interaction clearly exceeds the values for the normal van der Waals inter-
action. Binding energies for hydrogen bonding are typically on the order of
1000–3000 cm−1, whereas van der Waals bonds have energies in the range of
100–300 cm−1. The relevance of hydrogen bonding is demonstrated by the
fact that it is directly responsible for the secondary structure of protein chains
and for the duplication of the ic code. There is still some controversy
concerning the nature of hydrogen bonding. For a detailed discussion see, for
example, [185]. The hydrogen bond was first considered to be a pure electro-
static effect [153] and only point charges were taken into account. This model
failed in many cases, which gave rise to the conclusion that additional charge
transfer terms were an important contribution to the total binding energy of
a hydrogen bond; see, for example, [162]. It was argued then that an actual
amount of electron charge ( charge) was transferred from the electron
donor to the electron acceptor. Effective numbers for charge can be
found in [162] for various hydrogen-plexes. This charge transfer is
assumed to contribute substantially to the stabilization of the bond. Hydro-
gen bonds would therefore have some contribution from incipient chemical
bonding.
However, the failure of a simple point charge model is not surprising in
view of the previous discussion in Chap. 2. If only point dipoles are included,
as in most cases for these simple models, a linear structure is predicted for
(HF)2. But the inclusion of higher-order multipole moments leads to a correct
description of the