13.6 Experimental Molecular Study of Chemical Reaction Mechanisms 609
Detection of Reactive Intermediates
If a reactive intermediate included in a proposed mechanism can be detected in the
experimental system, that mechanism becomes more plausible, and if the intermedi-
ate’s concentration can be determined as a function of time, individual rate constants
for elementary steps can sometimes be evaluated. The most direct technique for detect-
ing reactive intermediates is spectroscopy, since spectrophotometers with very rapid
response times can be constructed. An early example of spectroscopic detection of a
reactive intermediate was a study of the decomposition of N 2 O 5.^36 According to the
mechanism of Example 12.13, the first step is the formation of NO 2 and NO 3 from
N 2 O 5. Schott and Davidson carried out shock tube studies, using the reaction vessel
as a spectrophotometer cell. They monitored the absorption of light at 546 nm and
652 nm, at which wavelengths NO 3 absorbs much more strongly than NO 2 ; at 366 nm,
at which wavelength NO 2 absorbs more strongly than does NO 3 ; and at 436 nm, at
which wavelength NO 2 and NO 3 absorb nearly equally. They were able to determine
the concentration of NO 3 as a function of time and to calculate values for the elementary
rate constants in the mechanism of Example 12.13. They found that the preexponential
factor in the Arrhenius expression fork 2 is equal to 1. 66 × 108 L mol−^1 s−^1 and that
the activation energy fork 2 is approximately equal to 16 kJ mol−^1 over the temperature
range from 300 K to 820 K.Exercise 13.24
Find the value ofk 2 at 300 K and at 800 K.Another technique for the detection of reactive intermediates ismass spectrometry.
In a mass spectrometer molecules are converted into positive ions, often undergo-
ing fragmentation in the process. The resulting ions are accelerated by an electrical
field, attaining a speed depending on their charge/mass ratio. They are then passed
through electrical and magnetic fields (or other analyzing devices) and separated, so
that the number of ions with each charge/mass ratio can be determined. The iden-
tity and structure of a substance can sometimes be deduced from its molecular mass
and from its fragmentation pattern. To detect reactive intermediates, one carries out a
gas-phase reaction in a vessel that adjoins the ionization chamber of a mass spectrom-
eter. A small aperture allows the mixture of reacting gases to pass into the ionization
chamber of the mass spectrometer. The low pressure in both chambers lowers the col-
lision rate of reactive intermediates, prolonging their lifetimes. The mass spectrum of
a reactive intermediate can sometimes be found in the mass spectrum of the reacting
mixture.
It is also possible to infer the presence of certain kinds of reactive intermediates
from their chemical effects. In themirror techniquea reacting gas is passed through a
tube with a metallic mirror deposited on its inner surface. If free radicals are present
they can combine with the metal to form volatile products that can be trapped at low
temperature and analyzed. For example, a lead mirror will combine with methyl radicals
to form tetramethyl lead, Pb(CH 3 ) 4 , a stable substance that can be condensed in a cold(^36) G. Schott and N. Davidson,J. Am. Chem. Soc., 80 , 1841 (1958). See also H. Sun and F. Heinz,J. Phys.
Chem.,B101, 705 (1997).