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4 Radical—Radical Transformations During Diffusive Excursions

4 Radical—Radical Transformations During Diffusive Excursions

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192



CHEMICAL TRANSFORMATIONS WITHIN THE PARAMAGNETIC WORLD

2



2

c 2S



|bÒ:

Dg > 0



only RP1



1



1



a=0

2



2

c 2S



|:

1



1



1

c 2S



|bỊ:

2



Dg = 0



only RP2



a>0

2

c 2S



|:

1



2

2



|bỊ:

Dg > 0,

a=0



RP1 Ỉ RP2



1

2



Dg = 0,

a>0



1



c 2S



2

c 2S



|aÒ:

1



1



FIGURE 9.2 Vector models (projections) illustrating pair substitution. The labels “1” and

“2” denote the electron spin of the first and the second radical of the pairs. The observed proton

is contained in the first radical. Its spin state, |ai or |bi, is displayed at the respective leftmost

projection. The radical pairs are born in the triplet state, and the product is formed from

the singlet state; c2S gives the singlet character. First radical pair RP1, positive g-value difference,

zero hyperfine coupling constant; second radical pair RP2, equal g values, positive hyperfine

coupling constant. For the situations without pair substitution, the spin evolutions under the

influence of the Zeeman and the hyperfine interaction have been separated for clarity. Further

explanation, see text.



The example presented in the following serves to illustrate the effects of pair

substitution. However, the experiments were originally carried out with quite a

different purpose in mind, namely, to prove that for the reaction under study the

route giving rise to the polarizations is the only, or at least the predominant, pathway to

the observed products. This addresses a controvertial issue when CIDNP is applied to

the study of reaction mechanisms: Although it is universally accepted that CIDNP

arises only from radical pairs or biradicals, the question is often raised whether the

observation of CIDNP allows a definitive conclusion as to the reaction mechanism, the

counter argument being that, because by their nature polarizations are amplified

signals, the CIDNP experiment might have captured a minor side reaction while the

greater part of the reaction proceeds via nonradical intermediates. This objection can

be invalidated by preparing the CIDNP-active intermediates by an indirect route that



193



RADICAL—RADICAL TRANSFORMATIONS DURING DIFFUSIVE EXCURSIONS



bypasses any other intermediates potentially capable of leading to the observed

products; if the CIDNP-active intermediates so obtained yield the same products with

the same polarization intensities, other reaction routes are insignificant or

nonexistent.12

The described problem was encountered in investigations of cis–trans isomerizations and cycloadditions of donor olefins D and acceptor olefins A in acetonitrile.24,25

All polarizations, both of the cycloadducts and of the starting and isomerized olefins,

could be traced to radical ion pairs D. ỵ A. formed by photoinduced electron transfer.

As, however, exciplexes are frequently discussed as percursors to the products in such

systems,26–28 and CIDNP does not respond to exciplexes because no diffusive

separation is possible, the question as to the relative contributions of the radical

ion and exciplex pathways arose. To answer it, we employed photoinduced electron

transfer sensitization (PET-sensitization).29

PET-sensitization means that the desired radical pair D. ỵ A. is produced

indirectly by rst generating another radical pair D. ỵ X. using an auxiliary

sensitizer X and then exchanging X. À for A. À by a thermal electron transfer. X is

chosen such that the photophysical parameters and redox potentials bar all other

pathways except the PET-sensitized one. Of particular significance for the above

mechanistic question is that neither D nor A are excited; hence, an exciplex (D Á Á Á A)Ã

cannot be formed. Chart 9.2 juxtaposes the direct and the PET-sensitized formation of

D. ỵ A. À .

(D· · ·A)*

A

D



hn



D



D • + X •−



A



X*



D*

A

D • + A• −



hn



Direct reaction



X



D • + A• −



PET-sensitized reaction



CHART 9.2



The rate of the exchange of the radical anions X. À and A. À depends on the

concentration of A. If that exchange is slow, it can only involve the long-lived free

radicals X. À; in that case, CIDNP generation has long come to a close, and PET

sensitization has no influence on the polarizations in D and in the products of D, which

only stem from the pair D. ỵ X. . In contrast, if the exchange is fast on the CIDNP

timescale, the intermediacy of the pair D. ỵ X. does not reveal itself in the CIDNP

spectrum because that pair is too short lived for polarization generation; in that

situation, all polarizations arise from the second pair D. ỵ A. even though this is not

formed directly.

These limiting situations can be seen in Fig. 9.3. The system consisted of 9cyanophenanthrene as the sensitizer X, trans-anethole as the donor D, and diethylfumarate as the acceptor A. The excitation wavelength, 357 nm, was chosen such that



194



CHEMICAL TRANSFORMATIONS WITHIN THE PARAMAGNETIC WORLD



t

t



c



c F



(d) c



t



t



M



S



t

c

c



(c)



(b)



c

c



t



t



t

(a) c



c



c

t

t



7.5



7



6.5



6



5.5 4



3.5 2



1.5



ppm

FIGURE 9.3 Photo-CIDNP spectra in the system 9-cyanophenanthrene (X, 3 mM), transanethole (D, 20 mM), and diethylfumarate (A, 110 mM), excitation at 357 nm. Adjacent signals

groups “t” and “c” belong to the same proton in the starting and the isomerized anethole. The

labels “F” and “M” denote the olefinic protons in A and its isomerized form, and “S” is an

imperfectly suppressed solvent signal. Trace (a), only X and D; trace (b), only D and A; trace (c),

only X and A; trace (d), all three components. Further explanation, see text. Reproduced from

Ref. 29 with permission, copyright (C) 2006 the PCCP Owner Societies.



D and A do not absorb. The analysis most conveniently focuses on the CIDNP signals

of the starting and isomerized donor olefin, which all result from geminate reactions of

the radical pairs. Their polarization pattern reflects the hyperfine coupling constants of

the trans-anethole radical cation. The precursor multiplicity is singlet (m ¼ À1), the

starting olefin is recovered by reverse electron transfer of singlet pairs (e ẳ ỵ 1), and

the isomerized olen is formed via reverse electron transfer of triplet pairs (e ¼ À1) to

give the anethole triplet, which then decays to both isomers. The reason why the

CIDNP intensities of the starting and the isomerized anethole are identical despite

different chemical yields is well understood.25



RADICAL—RADICAL TRANSFORMATIONS DURING DIFFUSIVE EXCURSIONS



195



Trace (a) shows the outcome of an experiment without the acceptor olefin, i.e., for

the first of the two limiting cases discussed above. The polarizations necessarily stem

from the pair D. ỵ X. , which has a positive Dg, that is, g (D. ỵ ) > g (X. ).

Trace (b) represents a control experiment on a mixture D and A, without the

sensitizer X but all other conditions identical. The absence of any CIDNP signals,

together with the observation that the same mixture produces strong polarizations at

other excitation wavelengths (e.g., 308 nm), proves that the first step of any photoreaction at 357 nm is the excitation of X.

Trace (c) belongs to yet another control experiment: X in the presence of only A (at

the same concentration as in traces (b) and (d)) yields small CIDNP signals of the

starting and isomerized acceptor olefin, which stem from radical pairs X. ỵ A. .

However, Stern-Volmer experiments show that this reaction channel is blocked in the

presence of D, which is a much better quencher of XÃ .

The PET-sensitization experiment uses a very high concentration of A to make the

exchange of X. À for A. À fast on the CIDNP timescale, that is, realizes the second of the

two above-mentioned limiting cases. It is displayed as trace (d). The polarizations of

the donor olefin are seen to be a mirror images of those in trace (a), which is simply due

to the fact that the polarizations now arise from the pair D. ỵ A. , for which Dg is

negative. The phase inversion is thus a spectacular demonstration of the radical

exchange in the three-component system.

The PET-sensitized experiment of trace (d) can be compared to the direct

photoreaction of D and A at 308 nm, that is, without X. When the amounts of light

absorbed are matched in these two cases, the absolute CIDNP intensities are found to

be very similar, not only for the olefins but also for the cycloaddition products. Hence,

the pathway via radical ion pairs is the predominant pathway to these products.

It is obvious that in between the two extremes of no radical-anion exchange (trace

(a)) and very fast exchange (trace (d)) there must be a range where the exchange rate

falls within the timescale of CIDNP generation, and for which, therefore, pronounced

pair substitution effects are expected. This can be conveniently explored by varying

the concentration of the acceptor olefin A. Best suited for a quantitative analysis is the

strongly polarized b proton of the anethole (see Fig. 9.3; trans, about 6.2 ppm; cis,

about 5.7 ppm). The normalized integrals over these signals have been displayed in

Fig. 9.4 as functions of the concentration of A.

A description of pair substitution by a numerical solution of Equation 9.3, after

appropriate modification, is always feasible. However, that frequently employed

procedure has one major drawback: it depends on many parameters, some of which are

often not known very precisely. The usual remedy is to determine them by a

multiparameter nonlinear fit, but the uniqueness of a many-component solution vector

obtained in that way is questionable if the curves do not have very characteristic

shapes, as in Fig. 9.4. As an alternative approach, one can exploit the fact that the same

parameters are contained in the polarization intensities in the limits of no pair

substitution and of infinitely fast pair substitution. For the system of Fig. 9.3, recasting

the equations in terms of these experimental quantities leads to a closed-form

expression29 that contains most parameters implicitly and only has a single adjustable

parameter, namely, the rate constant of pair substitution divided by the intersystem



196



CHEMICAL TRANSFORMATIONS WITHIN THE PARAMAGNETIC WORLD



S norm

1

0.5

0

–0.5

–1

0



50



100



[DEF ] / mM



FIGURE 9.4 Signal integrals of the anethole b protons under the conditions of Fig. 9.3d, but

for varying concentrations [DEF] of the acceptor olefin A. Filled symbols and solid curve,

trans-D (signal at about 6.2 ppm); open symbols and dashed curve, cis-D (signal at about

6.2 ppm). All integrals were normalized to the value for trans-D in the absence of A. The curves

are fit functions of a theoretical model. Further explanation, see text. Reproduced from Ref. 29

with permission, copyright (C) 2006 the PCCP Owner Societies.



crossing frequency of the first radical pair (Eq. 9.2). The fit curves of Fig. 9.4 were

obtained in that way and allow a reliable determination of that kinetic parameter.



9.5 RADICAL—RADICAL TRANSFORMATIONS AT REENCOUNTERS

The very first use of the polarization pattern to identify the paramagnetic intermediates

was in photosensitized (sensitizer, A) reactions of tertiary aliphatic amines DH, for

example, triethylamine.17 Although the gross reaction is a one-step hydrogen abstraction

.



.



A* ỵ DH ! AH ỵ D



9:6ị



to give an a-amino alkyl radical D. , for example, >N-- CH . --CH3 , CIDNP spectra do

not always exhibit the polarization pattern of that radical but in many cases that of an

amine radical cation DH. ỵ , for example, > N . ỵ --CH2 --CH3 , instead.

In D. the hyperfine coupling constants of both the a and the b protons are large and

have opposite signs whereas in DH. ỵ only the a protons have a substantial hyperne

coupling. These distributions are translated into an up/down (or down/up) pattern for

the neutral radical, and a pattern of polarized a and unpolarized b protons for the

radical cation. Evidently, these two very distinct patterns allow the determination of

the intermediate the polarizations stem without knowledge of the other parameters

entering Equation 9.5 (i.e., Dg, m, and e). The bottom and top traces of Fig. 9.5 show

examples of these two cases in the same reaction product, N, N-diethylvinylamine V

(for the formula see below, Chart 9.3).



RADICAL—RADICAL TRANSFORMATIONS AT REENCOUNTERS



V-a



197



V-b

e = 35.9

e = 17.5



e = 16.0



e = 4.7



FIGURE 9.5 Polarity-dependent polarization patterns in photosensitized hydrogen abstractions from triethylamine DH (sensitizer A, 9,10-anthraquinone). For the formulas, see Chart

9.3. Shown are the signals of the olefinic a and b protons of the product N, N-diethylvinylamine,

V-a (6.05 ppm) and V-b (3.45 ppm), as functions of the relative permittivity e (given at the

right). Top, pure acetonitrile-d3; bottom, pure chloroform-d3; other traces, mixtures of these two

solvents. All spectra were normalized with respect of the absolute amplitude of V-a. Further

explanation, see text.



The occurrence of the radical ion pattern in a polar solvent indicates that the

photoreaction involves an electron transfer step

A* ỵ DH ! A



.







ỵ DH



.







9:7ị



under these circumstances. However, a direct reaction of the radical cation DH. ỵ to

give the vinylamine V is chemically inconceivable; the direct precursor to V must be

the neutral radical D. . Hence, a deprotonation of DH. ỵ yielding the conjugated base

D. is a key step of the electron transfer-induced formation of V.

Extensive studies of the sensitizer dependence30 and the solvent dependence31 of

the polarization patterns led to the identification of two parallel pathways of that

deprotonation. One is a proton transfer within the spin-correlated radical pairs, with

the radical anion A. À acting as the base. The other is a deprotonation of free radicals,

in which case the proton is taken up by surplus starting amine DH. Furthermore,

evidence was obtained from these experiments that even in those situations where the

polarization pattern suggests a direct hydrogen abstraction according to Equation 9.6

these reactions proceed as two-step processes, electron transfer (Eq. 9.7) followed by

deprotonation of the radical cation by either of the described two routes. The whole

mechanism is summarized by Chart 9.3 for triethylamine as the substrate. Best suited

for an analysis is the product V.



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