Chemistry Reference
In-Depth Information
7.2.3 Density Functional Theory Calculations
All of the density functional theory (DFT) calculations results presented here utilized
the Gaussian program suite.
49
The complete geometry optimization and vibrational
frequency calculations were performed analytically with the BPW91 method and the
cc-PVDZ basis set for the chemical intermediate or compound being studied. A
Lorentzian function with a 20 cm
1
bandwidth was employed in conjunction with
the computed Raman vibrational frequencies and relative intensities to produce the
computed BPW91/cc-PVDZ Raman spectra shown here.
7.3 TIME-RESOLVED RESONANCE RAMAN (TR
3
) SPECTROSCOPY
OF ARYL NITRENIUM IONS
In this section, a brief overview of the TR
3
spectroscopy results obtained for a variety
of aryl nitrenium ions is given. The structure, properties, and chemical reactivity of
the aryl nitrenium ions investigated by TR
3
spectroscopy are discussed.
7.3.1 Formation of Aryl Nitrenium Ions in Aqueous Solutions
For most of the TR
3
studies of aryl nitrenium ions reported in this chapter, the aryl
nitrenium ions were produced using a photochemical method that employs photoly-
sis of aryl azides to produce a singlet aryl nitrene that then reacts with water in the
solvent system to very quickly generate a singlet aryl nitrenium ion.
14,22-30,50
A
picosecond transient absorption experiment found that the 2-fluorenyl nitrenium ion
formed using this method appears approximately 100 ps after photolysis of
2-fluorenyl azide in a mixed aqueous solvent.
28
This photochemical method can
be used only for aryl nitrenes that react very fast with water to make aryl nitrenium
ions and thus can only be applied to produce a limited subset of aryl nitrenium ions
efficiently. Nevertheless, this photochemical method using photolysis of aryl azides in
aqueous solvents can be utilized to study a number of different aryl nitrenium ions such
as
para
-phenyl-substituted phenyl nitrenium ions and alkoxyphenyl nitrenium
ions that are discussed in this chapter. Picosecond Kerr gated time-resolved
resonance Raman (ps-KTR
3
) experiments were done to directly observe how
the photolysis of aryl azides in aqueous solutions produces the corresponding aryl
nitrenium ion for typical
para
-phenyl-substituted phenyl nitrenium and alkox-
yphenyl nitrenium ions.
Figure 7.4 shows an overview of selected ps-KTR
3
spectra acquired after 267 nm
photolysis of 2-fluorenyl azide in a water-acetonitrile (50:50) solvent using 342 nm
and 400 nm probe excitation wavelengths.
43
A 341.5 nm ns-TR
3
spectrum of the singlet 2-fluorenyl nitrene in neat acetonitrile
at 10 ns after 266 nm photolysis of 2-fluorenyl azide was acquired under conditions
very similar to a previous 416 nm observation of the singlet 2-fluorenyl nitrene and
its subsequent ring expansion reaction.
39,40,43
This 341.5 nm ns-TR
3
spectrum is
displayed in Figure 7.5a and compared to the ps-KTR
3
spectrum at 10 ps observed
