Chemistry Reference
In-Depth Information
7.1
INTRODUCTION
Aromatic amines can be found in very tiny amounts in cigarette smoke, automobile
exhaust, fermented fish, and in waste from some industrial processes.
1-12
Some of these
aromatic amine pollutants such as 2-acetyl-aminofluorene can be metabolized into
sulfate esters of the corresponding
N
-hydroxylamines and in aqueous environments the
sulfate anion will be cleaved spontaneously to form an aryl nitrenium ion.
12-22
Aryl
nitrenium ions are very reactive short-lived species that can selectively react with
guanine bases in DNA to produce adducts that in some cases may give rise to
carcinogenic mutations.
10,11,15,22
This has led to much interest in characterizing aryl
nitrenium ions to better understand their properties and chemical reactivity. It is difficult
to directly examine aryl nitreniumions in roomtemperature solutions since they arevery
short-lived reactive species. Several research groups have developed photochemical
methods to generate aryl nitrenium ions after laser flash photolysis of an appropriate
precursor and this has enabled a variety of time-resolved spectroscopic methods to be
applied to study aryl nitrenium ions in room temperature solutions.
22-37
Time-resolved
transient absorption spectroscopy experiments have been reported for a number of aryl
nitrenium ions and their reactions with other species.
22-37
A great deal of information
about the lifetimes of aryl nitrenium ions and their rate constants for reactionswith other
compounds have been learned from these experiments.
22-37
However, there have been relatively few studies that have probed the structure and
bonding of these short-lived aryl nitrenium ion and their reaction intermediates.
Time-resolved infrared absorption (TRIR) experiments have been used by Toscano
and Falvey and coworkers to acquire vibrational spectra of several aryl nitrenium
ions
36
,
37
in organic solvents and their first study on the diphenyl nitrenium ion
demonstrated that this species had an iminocyclohexadienyl cation-like structure.
36
Further TRIR investigations examined substituent effects on the symmetrical
aromatic C
C stretch mode(s) in the 1580-1628 cm
1
region for four
N
-methyl-
N
-phenyl nitrenium ions
37
in organic solvents and found that the experimental
vibrational frequencies were in excellent agreement with values calculated from
BPW91/cc-PVDZ density functional theory (DFT) calculations whose results
displayed noticeable bond length alternation in the phenyl rings, smaller C
N
bond lengths and significant positive charge delocalization into the phenyl rings.
37
There have also been reports of vibrational spectra of aryl nitrenium ions in HCl
matrices.
38
Since there is much interest in aryl nitrenium ions and their reactions in a
biological context, it would be very useful to directly study them in aqueous solutions
that are more physiologically relevant environments than organic solutions or low
temperature matrices. Time-resolved resonance Raman (TR
3
) spectroscopy is a
powerful method to study short-lived intermediates in aqueous solutions since the
Raman spectrum of water is broad and weak and the Raman signal can be greatly
increased for intermediates of interest whose electronic absorption is in resonance
with the probe wavelength used in the experiments. We have therefore applied TR
3
spectroscopy to directly examine aryl nitrenium ions and their reactions with other
species in aqueous and mixed aqueous solutions and this chapter will briefly review
the results of these studies.
39-47
