ANÁLISIS Y RESUMEN MEDIANTE EL MODELO 5W PARA LOS CLIENTES

The technique of Chemically Induced Dynamic Nuclear Polarisation^^’^^ (hereafter referred to as CIDNP) is a means of probing the mechanism of reactions which are

believed to involve free radicals. The product(s) of the reaction are observed during or

shortly after their formation which contrasts with electron paramagnetic resonance (EPR) spectroscopy in which the radical intermediates are observed directly. The advantages of CIDNP over EPR ar e that CIDNP does not require the introduction of an additional species into the system to tiap the radicals, so the technique is non-

invasive in that sense. Furthermore, with CIDNP, the spectral changes over time of

several species can be monitored simultaneously without the deconvolution necessary with EPR. The teclmique can provide several different classes of mechanistic information ranging from simply evidence for radical precursors to the reaction products through to the identity and characteristics of the precursors. In addition, further information can be gleaned in the form of the pair formation, polarisation

identity and radical lifetimes/separations. However a drawback is that whilst EPR is a quantitative technique, the fact that dynamic signals are displayed in CIDNP NMR precludes estimation of the proportion of the products formed via a radical process without some knowledge of the kinetics of the reactions being studied. The products can only arise from two radicals paired in a singlet multiplicity and the formation of

this multiplicity is dependent on the nuclear spin state. When two radical precursors come together with a triplet multiplicity, then one radical must undergo spin inversion before reaction can occur. Those precursors with nuclear spin states which do not favour spin inversion will exhibit a sufficiently long lifetime in order to escape the cage.^^ The sign of the polarisation in the CIDNP effect observed will be opposite for products derived from singlet precursors than from those derived from triplet

2.1.3.1 Qualitative sign rules

Kaptein^^ formulated two simple qualitative sign rules by which the direction of the

net effect shown in eqn. (3) or multiplet effect [eqn. (4)] can be predicted. Conversely,

empirical observations may be used to derive one of the parameters if the others are known.

r „ = /ix £ x a , xAg (3)

The following sign convention is employed:

{À 4- if the radical pair is formed from a triplet precursor or by an encounter of free radicals.

- if the radical pair is formed from a singlet precursor.

s + for products formed by coupling or disproportionation (‘in cage’). - for products formed by cage-escape reactions.

üij + if the nucleus under consideration has a positive hyperfine coupling constant. - if the nucleus under consideration has a negative hyperfine coupling

constant.

Ag + if the observed radical has the higher g factor. - if the observed radical has the lower g factor.

Jij + if nuclei i and j have a positive spin-spin coupling constant. - if nuclei i and j have a negative spin-spin coupling constant. (Tij + when nuclei i and j belong to the same radical.

- when nuclei i and j belong to different radicals. Fne + enhanced absorption, A.

- the low-field signal(s) of a multiplet shows A, whereas the high-field

signal(s) shows E. (A/E).

Table 2.1 Magnetic properties of nitrogen nuclei. The Varian \]mtyplus spectrometer used in this work operates at 11.74 T bringing protons into resonance at 500 MHz.

% i^N

Natural abundance/% 99.64 0.36

Spin quantum number 1 i/2

NMR frequency (11.74 T/MHz) 36.12 50.66

Sensitivity relative to ^H 0 . 0 0 1 0 1 0.00194

Sensitivity relative to ^^C at natural isotopic abundance 17.22 0.0214

Before undertaking a CIDNP study using [‘^N] peroxynitrite, it was considered appropriate to undertake preliminary work with H^^NO] in order to gain experience in

the teclmique and assess how easy it would be to detect CIDNP enhancement effects with L-tyrosine as the aromatic substrate.

2.1.3.2 General problems

The extraction of meaningful results fi'om these systems is hampered by several general problems, some specific to the chemical systems and some due to the NMR

operation. The fact that chemically unstable intermediates are involved in the chemical system causes problems with timing of the experiments. Many of the

reactions about which information is required are non-stoichiometric. This factor in conjunction with the relative insensitivity of ^^N NMR compaied with ^H NMR or EPR, say, makes observation of low-yield products and any nuclear polarisation

induced in them difficult. Irreproducibility between runs on the same chemical system is deleterious and commonplace. On several occasions, particularly where long acquisition times were required, extraneous noise was also observed.