Chemistry Reference
In-Depth Information
spectrometrists. Each group has their own definition of a cluster (The old IUPAC
mass spectrometry definition of a cluster is as follows: An ion formed by the
combination of more ions or atoms or molecules of a chemical species often in
association with a second species: [
22
]),
1
,
2
and these definitions can be further
expanded to include the types of chemical bonding that 'holds the cluster together'
[
23
]. The most recent recommended IUPAC definition of a
cluster ion
in mass
spectrometry is an 'Ion formed by the combination via non-covalent forces of two
or more atoms or molecules of one or more chemical species with an ion' [
24
], and
thus this definition encompasses a wide range of ions formed from chemical and
biological sources and also across all the types of bonding.
Further distinctions have been made in the literature based on size and other
criteria:
1. An encyclopaedia article [
25
] has classified clusters as
(a) 'Microclusters having from 3 to 10-13 atoms. Concepts and methods of
molecular physics are applicable';
(b) 'Small clusters have from 10 to 13 to about 100 atoms. Many different
geometrical isomers exist for a given cluster size with almost the same
energies. Molecular concepts lose their applicability';
(c) 'Large clusters have from 100 to 1,000 atoms. A gradual transition is
observed to the properties of the solid state. Small nanoparticles and
nanocrystals have at least 1,000 atoms. These bodies display some of the
properties of the solids state'.
2. Kreibig and Vollmer have defined metal clusters by the number,
N
, of atoms per
cluster as being:
(a) 'very small clusters' fall in the range 2
<
N
20;
(b) 'small clusters' are those where 20
500;
(c) 'larger clusters' span a substantial range of sizes: 500
N
10
7
[
26
].
N
3. Ott and Finke have made an important distinction between transition-metal
nanoclusters and colloids: 'nanoclusters are expected to be smaller (1-10 nm)
1
The IUPAP does not appear to have a formal definition of a cluster. Johnston [
20
] suggests the
following 'physics' definition of clusters: I will take the term cluster to mean an aggregate of a
countable number (2-10n, where n can be as high as 6 or 7) of particles (i.e. atoms or molecules).
The constituent particles may be identical, leading to homo-atomic (or homo-molecular) clusters,
Aa, or they can be two or more different species - leading to hetero-atomic (hetero-molecular)
clusters AaBb.
2
The IUPAC inorganic chemistry definition of cluster is as follows: A number of metal centres
grouped close together which can have direct metal bonding interactions or interactions through a
bridging ligand, but are not necessarily held together by these interactions. See IUPAC. Compen-
dium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and
A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML online corrected version:
http://goldbook.iupac.org
(2006) created by M. Nic, J. Jirat, B. Kosata; updates compiled by
A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.
