Chemistry Reference
In-Depth Information
The preferred geometry of the hydrogen bond is typically close to a linear confor-
mation, with the hydrogen atom positioned along the line connecting the hetero-
atoms, although there are many examples of nonlinear hydrogen bonding
(Lommerse et al. 1997). A dominant property of hydrogen bonds is their direction-
ality and requirement for closeness. The strength of individual hydrogen bonds
ranges from 1 to 10 kcal/mol (Coleman et al. 1991), which leads to longer,
weaker bonds compared to covalent bonds (70-110 kcal/mol). Thus, when hydro-
gen bonds serve as the primary secondary interaction, the resultant structures are typi-
cally closer to thermodynamic equilibrium and are quite dynamic with respect to
changes in the environment. This is due to the simple fact that the thermal energy
(kT) more closely matches the bond strengths in these systems. For instance, an associ-
ation energy of 3 kcal/mol leads to a 1% level of dissociation among units in a single
component bulk liquid bound with this energy at ambient temperature (Coleman et al.
1991). This figure obviously represents a time average; the typical lifetime of a hydro-
gen bond in water, for example, is roughly 10
211
s (Conde and Teixerira 1983).
The acidity of the donor and the Lewis basicity of the acceptor play important roles
in the strength of the interaction (Beijer et al. 1996; Kawakami and Kato 1998).
Phenols, which possess stronger acidity than aliphatic alcohols, produce stronger
hydrogen bonds with acceptors. These properties, which are measured in terms of
pK
a
or pK
b
, are founded in the electronic structure of the donors and acceptors and
their ability to stabilize negative or positive charges, respectively. Electron withdraw-
ing substituents such as fluorine increase the acidity of the hydrogen bond donors, as
in the case of the hexafluoroisopropanol group (Liu et al. 2000). In the extreme case,
mixing a strongly acidic hydrogen bond donor with a strongly basic hydrogen bond
acceptor results in an acid-base neutralization reaction and the formation of an ion
pair. Sedlak et al. (2003) discussed the fact that hydrogen bonding interactions are
indeed founded in electrostatic attractive and repulsive interactions.
hydrogen bonding interaction
A-H
þ
BO
A-H22B
(4
:
1)
formation of ion pairs
A
þ
BH
þ
A-H
þ
BO
The common measure of the strength of association or complexation is the association
constant (K
a
), defined in terms of the equilibrium between associated and dissociated
units [Eq. (4.2)]:
A
þ
B
OA
B K
a
¼
[A
B]
[A][B]
(4
:
2)
The K
a
value is a key measurement of the strength of the hydrogen bonding associ-
ation and determines the dynamics of systems incorporating the hydrogen bonding
groups. Thus, for weaker hydrogen bonding interactions, the lifetimes of associated
species are shorter than for systems with higher association constants. This has a
dramatic effect on material properties, such as relaxation rates, creep, modulus, melt
