AROMATIC NITRO COMPOUNDS 221are dissolved in solution to a considerable extent. All this would be impossible
if they were formed by covalent bonds.
(4) Measurable, although small, conductivities have been observed for some
molecular addition compounds (picric acid-naphthalene and 1,3,5-trinitrobenzene-
-naphthalene) in liquid sulphur dioxide, according to Weiss [118]. He suggests
that the complex molecule is essentially ionic in character, being formed by electron
transfer from the hydrocarbon (donor A) to the polynitro compound (acceptor B),
according to the diagrammatic reaction (“charge transfer compounds”):
A number of authors have studied the emission spectra characteristics of many
complexes of sym-trinitrobenzene (TNB) with aromatics (Reid [119]; Bier and
Ketelaar [120]; Bier [121]; Czekalla, Briegleb et al. [122]; McGlynn and Boggus
[123]). These experiments led to the conclusion that in most complexes the emission
is a charge transfer (E -> N) emission.
McGlynn and Boggus describe the phenomenon thus: absorption in the charge
transfer band is followed either by the converse emission or by intersystem crossing
(according to Kasha [124]) to a dissociative level of the complex which yields the
aromatic in its first excited triplet state. The aromatic hydrocarbon then phos-
phoresces.
(5) It is now recognized that electron transfer from one component to another
is responsible for the change of colour noticed in some molecular compounds
(cf. Orgel [125]).
Brackman [126] showed that it was possible to arrange the components of the
molecular compounds in such a way that if the colours of the molecular compounds
were plotted against the nitro compound, a series of roughly parallel straight lines
was obtained (Fig. 47†). It will be noted that an increase in the number or power
of the electron-attracting groups in the nitrobenzene nucleus produces a batho-
chromic effect (a shift towards longer waves), while an increase in the number
of electron-repelling groups has the opposite effect (hypsochromic effect).
(6) The magnetic susceptibility of some addition compounds, such as naphtha-
lene picrate, is nearly additive, i.e. equal to the sum of the components (Le
Fèvre [127], Baddar and Mikhail [128]). This would suggest that the molecules
are held by weak electrostatic forces.
(7) The ultra-violet absorption spectra of the addition compounds have been
found to be additive, i.e. equal to the sum of the spectra of components, according
to Friedel and Orchin [129]. This would also support the theory of ionic bond
between the components.
(8) The stability of an addition compound (A
+
B- ) is affected by the Pres-
ence of strong donor and acceptor groups. Thus electron-donor groups in mole-
cule A and/or electron-acceptor groups in B lead to a greater binding energy be-
† Here not only aromatic nitro compounds are given, but also aliphatic such as nitro-
methane, tetranitromethane and also chloranil, benzoquinone, maleic and phthalic anhydrides.