CIDNP Determination of the Rate of In-Cage Deprotonation of the Triethylamine Radical Cation

Abstract

CIDNP (chemically induced dynamic nuclear polarization) effects in the sensitized photoreactions of triethylamine DH (sensitizers A, 9,10-anthraquinone and 2,7-dinitrofluorenone) are investigated in a series of aprotic solvents of widely varying relative permittivity ε, including mixtures of acetonitrile and chloroform. The dependence of the polarization pattern, that is, the relative polarization intensities of the olefinic α- and β-protons in the reaction product N,N-diethylvinylamine, on ε is studied quantitatively by evaluating the polarization ratio r of these protons. The factor responsible for the change of the polarization pattern is the rate, relative to the kinetic window of CIDNP, of in-cage deprotonation of the triethylamine radical cation by the sensitizer radical anion (rate constant k dep), which converts the initially formed radical ion pair into a neutral radical pair . A theory of pair substitution taking place solely during encounters of the radicals is presented. Based on this, an expression for the dependence of r on k dep is derived, which in turn allows the extraction of k dep from the experimentally observed polarization ratio. The accessible range of k dep in these systems is from about 108 to about 1010 M-1 s-1.

Electron and hydrogen self-exchange of free radicals of sterically hindered tertiary aliphatic amines investigated by photo-CIDNP

“…Often, they are true two-step processes with an initial full charge transfer to give a radical ion pair, which then undergoes a proton transfer [1–7]; partial charge transfer (i.e., formation of an exciplex) as the first step has also been observed [8–9]. …”

Electron self-exchange activation parameters of diethyl sulfide and tetrahydrothiophene

“…On the AM1 level, the heats of formation of D and are −92 kJ/mol and −14 kJ/mol for diethyl sulfide DES as opposed to −69 kJ/mol and +85 kJ/mol for triethylamine, the quencher used in our earlier work [21–23], so for any sensitizer the radical pair containing lies about 75 kJ/mol higher above the reactants in the case of the amine. From the oxidation potentials in the literature ( DES , 1.65 V [27]; triethylamine, 0.96 V [21]; both in acetonitrile versus SCE) the opposite order, even with a numerically similar difference, is calculated for the radical pair with D • + , although observations [28–29] that for bifunctional donors comprising both an amine and a sulfide moiety photoinduced electron transfer occurs exclusively from S , not N , cast some doubt on whether these reported potentials are the true equilibrium potentials for these irreversible redox systems.…”

“…All known aliphatic radical cations are heteroatom-centered, which considerably increases the mechanistic intricacies. The main complication arises from fast deprotonation of D • + at the α carbon to give a neutral radical which can occur at the radical-pair stage in a direct reaction (base, A • − ) or even at any stage of the reaction by a relayed proton transfer (relay base, D ) [21–23] and turns the gross reaction into a hydrogen abstraction. In a previous report on two aliphatic amines [20], we have used time-resolved CIDNP experiments to distinguish between the ensuing electron and hydrogen self-exchanges of D • + and .…”

Chemical Transformations within the Paramagnetic World Investigated by Photo‐CIDNP

“…However, k dep can be changed by varying the thermodynamic driving force. With different sensitizers, 30 this can be effected only in rather large steps, but a much finer gradation can be realized by the relative permittivity of the reaction medium, 31,32 which only influences the free energy of the radical ion pair but leaves that of the pair of neutral radicals essentially unaffected. …”

“…Again, the equations can be converted to a form that contains the parameters of the two radical pairs implicitly and expresses them by the polarization patterns in the two limiting cases. 32 The use of the patterns, that is, polarization ratios, has the advantage that all factors influencing the absolute size of the polarizations cancel. Figure 9.6 shows the result for the sensitizer 9,10-anthraquinone.…”