Chapter 7 Some Dimers of Triatomic Radicals
with 17 and 19 Valence-Shell
Electrons
A number of triatomic radicals can form dimers whose geometries have been wellcharacterized. A study of the electronic structures of these dimers can illustrate
aspects of qualitative valence-bond and molecular orbital theory for electron-rich
polyatomic molecules, and interconnections between these theories can be
demonstrated. Dinitrogen tetroxide is a molecule par excellence that may be used
for these purposes, and here we shall give primary consideration to its electronic
structure and bond properties.
7-1 The Long, Weak N-N Bond of N 2 O 4 : Lewis Valence-Bond
Theory
2
NO with 17 valence-shell electrons can form several dimers whose geometries
are displayed in Figure 7-1.
The most stable dimer is planar with a long, weak N-N bond. The N-N bondlength for this dimer is 1.78 Å
2
, which is 0.33 Å longer than the N-N single bond
of 2 4
N H
3 . The N-N bond-dissociation energy of 57 kJ mol
-1 for 2 4
N O is much
smaller than that of 250 kJ mol
-1 for
2 4
N H
4, 5 . Both molecular orbital and
valenceFigure 7-1: 2 4
N O isomers
1
Ó Springer International Publishing Switzerland 2016
R.D. Harcourt, Bonding in Electron-Rich Molecules,
Lecture Notes in Chemistry 90, DOI 10.1007/978-3-319-16676-6_7
87