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On alternating phase effects in individual spin multiplets in electron spin-polarized (CIPED) free-radical spectra
Volume 115, number 1
22 March 1985
CHEMICAL PHYSICS LETTERS
COMMENT ON ~TE~A~G IN ELECTRON
PHASE EFFECTS IN INDIVIDUAL SPIN SPIN-POLARIZED
K.A. McLAUCHLAN Physiml
Reaxved
(%lPED)
FREE-RADICAL
MULTIPLETS
SPECTRA
and D.G. STEVENS
&?zemtitry Laborutov,
South Parks Road. Oxford
OX1 2Q.Z. UK
12 November 1984; KI final form 14 December 1984
Each spm-multrplet of a spectrum obtained from a secondary radrcal, created by reaction of a radxal pau mechamsmpolarized prunary radical, may show lccal mulhpiet polantatlon if a hyperfme couplmg 1s correlated between the two. The recent resufts of Sakaguchi, Hay&xi, Muras zmd i’H8ya are discussed in thzs hght. The relative signs of the correlated hyperfme couplmg constank in the two radxals can be obtamed
In a recent paper m this Journal
Elf a spm-polarized
electron spm resonance spectrum appeared which was described as being incompatible with any known prediction of polarization theory. It was said to display
an E/A pattern (emission at low field, absorption at high) on each hyper~me line. This is an obvious imposability for as single transition whose intensity reflects any possible absorptive or emissive contributions. We presume that the statement should have referred to a spin multiplet, of small splitting. centred at each “line” position due to a dorn~~t, larger, hyperfiie coupling. This interpretation 1s however inconsistent with the stick diagram
&own;
this will be discussed
below.
It
is our mzun purpose here to demonstrate that CIDEP patterns showing lines in alternate phase in mdividual spm multipl~ts are to be expected Ln the spectra of secondary radicals formed by reaction of radical part mechamsm (RPM)-polarized primary ones. An essential requirement is that the hyperfme couphng to a specific proton or set of protons should be correlated between the two radicals. This concept has been discussed previously in relation to polarization theory in ii different context [2] ; the effect that this may have upon the appearance of a spectrum is dlscussed here for the first time. To illustrate the effect we consider citify a secondary radical formed by addition of a hydrogen atom. spin-polarized by the radical pair mechanism, to a double bond. We shall assume that the atom em108
an&es from a symmetric radical pair which is triplet spin-correlated so that the primary species has the spectrum shown in fii. 1 a, with equal mtensities in emission and absorption. The low-field l.Ine arises from those hydrogen atoms with nuclei in the ar spin state and the ~~-~eid one from those with nuclei in the 6 state; this assignment Is based upon the positive sign of the hyperfiie coupling constant. The populatrons of the spin levels in the polarked atom are also shown in fig. la. When this atom adds to the double bond to form the radical shown in fig. 1 the nucleus remains coupled to the electron, now w&h a &coupling which is also positive, a.nd the spin states of the pnrnary and secondary species correlate: the spin populations m the secondary species are as shown in the energy level diagram. The spectrum predicted from this is shown in fig. 1 b also. ft is seen that each line of the doublet due to coupling to the p proton which was not Involved in the reaction (H’) Is itself spht Into a doublet which exhibits E/A polarization. The actual appearance of the spectrum depends upon the relative magrdtudes of the two p-couplings and also, interestingly, on the sign of the hyperfiie coupling to the transferred I-I atom relative to that in the H atom itself. In this example the occurrence of two approxlmately equal cou@ings somewhat obscures the effect. It becomes more apparent if any carbon_centred ra@xl with an Q or fl hydrogen coupling, CHXY or C-Cl-KY, adds to a double bond. In the resulting 0 009-2614~85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Pubhshing Division)
Volume 115. number 1
CHEMICAL PHYSICS LETTERS
The formation of polarized secondary radicals by reaction of a polarized primary within its spin-lattice relaxation time is a common event due to the high reactivity of radicals. It can lead to a wide range of intens&y distributions in their spectra. Most commonly, primary RPM polarization arises between dissimil;zr pairs of radicals which leaves each with an excess of a or p electron spin, although the overall pokui.zation is zero; this is the “net” effect. The excess electron spin is associated with spec%c nuclear states and therefore leads to secondary radicals in which each spm multlplet, in a radical formed with correla!lon of hyperfine
A
I 8-
I-+ E
H-
+
\
c=c’
\
/’ c-c\‘”
-
I
couplings, has an intensity dktribu tion which reflects this original inequality. In general therefore a symmet-
H-C, \
Fig 1_ (a) The energy level populations of a H ratom polarized ylil the radxal pau mechanism from a mpletuxrelated identlCalradical pair, xxi the resulting spectrum. (II) The uxrespondmg &agram for the secondary radtcal formed by addition of this atom to a double bond.
radical the hydrogen atom in the primary becomes either the 7 or the 6 proton in the secondary,
22 March 1985
with
small hype&me coupling constants. In the analogous radical to that discussed above, each line of the doublet
due to the @-coupling from the H atom present originally in the olefm would show multiplet polarization behaviour. Whether this was E/A or A/E would depend upon the sign of the hyperfine coupling in the secondary species relative to that in the primary one. It is simple to envisage situations where this might yield important information on the nature of the primary species if the sign of the coupling in the secondary is known from an independent experiment.
nc distortlon in each multiplet is not expected. If the secondary radical is formed with no correlation of hyperfiie coupiings with ihe primary, the excess electron spin is simply statistically distributed about alI the nuclear spin states of the radical: the spectrum appears with normal relatrve intensities of the lines. This transfer of “net” polarization has been discussed previously [3,4]. The situations in which the complex behaviour is likely to be observed include all H-atom transfer reactions, reactions involving radical additions to stable molecuies and radical rearrangements. Although to predict the qutitative effects on the appearance of spectraE straightforward, the quantitative ones require detailed analysis in order to relate the polarization which arose in the primary to that in the secondary. A preliminary analysis was made some time ago but unfortunately was concerned with the magnetisation in the field direction rather than that observed [5] _ Marc recent theory has calculated the time-dependence of the magnetisation observed from secondary radicals
formed in flash-photolysis experiments both with direct observation [6] and using time-integration spectroscopy (TIS) methods [7]. The former briefly discussed the problem of correlated hyperfme couplings but no account of the effect on observed spectra has been given. With extension of the existing theory, studies of secondary species may be expected to become quite common _ Finally we return to the interpretation of the spectrum given in fig. 4 of the paper of Sakaguchi et al. [ I] _It is not obvious to us that the behaviour observed was correctly recagnised nor whether the stick spectrum shown is relevant for it seems not to reproduce 109
Volume I1 5. number
1
CHEMlCAL
PHYSICS
the observed spectrum even at the longer time shown. It appears as though the line positions in the later spectrum correlate with some of those in the earlier one, and that their phases are consistent between the Tao. This leaves a number of fines in opposite phszz at slightly different positions. We have recently managed to reproduce! the spectra observed by Sakaguchi et al. at 1 and 3 ps after the laser flash using the technique of time-integration spectroscopy. Using a computer subtraction method it can be shown that the behaviour after I ~.lsis in fact the same as that after 3 w except for the addition of lines due to what must be a third radical showing A[E characteristics. This would presumably have resulted from the reaction of the excited singlet state of benzophenone to give a geminate singlet spin-correlated radical pair. This singlet pola.rizatlon later becomes dominated by a strong triplet one;sunilar effects have been reported in the photolysis of choline [83 _ In a system as complex as the one reported, whrch involved radical formation in a micetlar -alution made with sodium dodacyl sulphate,
110
22 March 1985
LETTERS
two tiferent not unlikely. JXS thanks
radicals from the hydrocarbon
the SERC for a m~ten~ce
cham are
award.
References [l]
Y. Sakagudu, H. HayshL H. Muti and Y II. I’Haya, Chem. Phys. Letters 110 (‘1984) 275. [2] SK. Wong,D.A. Hut&son and J.K.S. Wan, J. Chem.
Phys. 60 (1974) 2987. [3] J-B. Pedersen. FEBS Letters 97 (1979) 305. [4] CD. Buckley, A.I. Grant, K.A. McLauchlan and AJ.D.
[S) 16) [7] [8]
Ftitchie, Faraday Dlscussons Chem. Sot. 78 (1984), to be published. K.A. McLauchlan, R.C. Sealy and J-N. Wrttman, Mol. Phys. 35 (1978) 5 1. PJ. Hare and K.A. McLauc?dan. Mol. Phys 42 (1981) 533. S. Basu. K-A. McLauchian and G-R. Sea&. Mol. Ptys 52 (1984) 431. S. Basu, K.A. McLauchlan and A.Z.D. Ritchie, Chem Phys. 79 (1983) 95.
22 March 1985
CHEMICAL PHYSICS LETTERS
COMMENT ON ~TE~A~G IN ELECTRON
PHASE EFFECTS IN INDIVIDUAL SPIN SPIN-POLARIZED
K.A. McLAUCHLAN Physiml
Reaxved
(%lPED)
FREE-RADICAL
MULTIPLETS
SPECTRA
and D.G. STEVENS
&?zemtitry Laborutov,
South Parks Road. Oxford
OX1 2Q.Z. UK
12 November 1984; KI final form 14 December 1984
Each spm-multrplet of a spectrum obtained from a secondary radrcal, created by reaction of a radxal pau mechamsmpolarized prunary radical, may show lccal mulhpiet polantatlon if a hyperfme couplmg 1s correlated between the two. The recent resufts of Sakaguchi, Hay&xi, Muras zmd i’H8ya are discussed in thzs hght. The relative signs of the correlated hyperfme couplmg constank in the two radxals can be obtamed
In a recent paper m this Journal
Elf a spm-polarized
electron spm resonance spectrum appeared which was described as being incompatible with any known prediction of polarization theory. It was said to display
an E/A pattern (emission at low field, absorption at high) on each hyper~me line. This is an obvious imposability for as single transition whose intensity reflects any possible absorptive or emissive contributions. We presume that the statement should have referred to a spin multiplet, of small splitting. centred at each “line” position due to a dorn~~t, larger, hyperfiie coupling. This interpretation 1s however inconsistent with the stick diagram
&own;
this will be discussed
below.
It
is our mzun purpose here to demonstrate that CIDEP patterns showing lines in alternate phase in mdividual spm multipl~ts are to be expected Ln the spectra of secondary radicals formed by reaction of radical part mechamsm (RPM)-polarized primary ones. An essential requirement is that the hyperfme couphng to a specific proton or set of protons should be correlated between the two radicals. This concept has been discussed previously in relation to polarization theory in ii different context [2] ; the effect that this may have upon the appearance of a spectrum is dlscussed here for the first time. To illustrate the effect we consider citify a secondary radical formed by addition of a hydrogen atom. spin-polarized by the radical pair mechanism, to a double bond. We shall assume that the atom em108
an&es from a symmetric radical pair which is triplet spin-correlated so that the primary species has the spectrum shown in fii. 1 a, with equal mtensities in emission and absorption. The low-field l.Ine arises from those hydrogen atoms with nuclei in the ar spin state and the ~~-~eid one from those with nuclei in the 6 state; this assignment Is based upon the positive sign of the hyperfiie coupling constant. The populatrons of the spin levels in the polarked atom are also shown in fig. la. When this atom adds to the double bond to form the radical shown in fig. 1 the nucleus remains coupled to the electron, now w&h a &coupling which is also positive, a.nd the spin states of the pnrnary and secondary species correlate: the spin populations m the secondary species are as shown in the energy level diagram. The spectrum predicted from this is shown in fig. 1 b also. ft is seen that each line of the doublet due to coupling to the p proton which was not Involved in the reaction (H’) Is itself spht Into a doublet which exhibits E/A polarization. The actual appearance of the spectrum depends upon the relative magrdtudes of the two p-couplings and also, interestingly, on the sign of the hyperfiie coupling to the transferred I-I atom relative to that in the H atom itself. In this example the occurrence of two approxlmately equal cou@ings somewhat obscures the effect. It becomes more apparent if any carbon_centred ra@xl with an Q or fl hydrogen coupling, CHXY or C-Cl-KY, adds to a double bond. In the resulting 0 009-2614~85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Pubhshing Division)
Volume 115. number 1
CHEMICAL PHYSICS LETTERS
The formation of polarized secondary radicals by reaction of a polarized primary within its spin-lattice relaxation time is a common event due to the high reactivity of radicals. It can lead to a wide range of intens&y distributions in their spectra. Most commonly, primary RPM polarization arises between dissimil;zr pairs of radicals which leaves each with an excess of a or p electron spin, although the overall pokui.zation is zero; this is the “net” effect. The excess electron spin is associated with spec%c nuclear states and therefore leads to secondary radicals in which each spm multlplet, in a radical formed with correla!lon of hyperfine
A
I 8-
I-+ E
H-
+
\
c=c’
\
/’ c-c\‘”
-
I
couplings, has an intensity dktribu tion which reflects this original inequality. In general therefore a symmet-
H-C, \
Fig 1_ (a) The energy level populations of a H ratom polarized ylil the radxal pau mechanism from a mpletuxrelated identlCalradical pair, xxi the resulting spectrum. (II) The uxrespondmg &agram for the secondary radtcal formed by addition of this atom to a double bond.
radical the hydrogen atom in the primary becomes either the 7 or the 6 proton in the secondary,
22 March 1985
with
small hype&me coupling constants. In the analogous radical to that discussed above, each line of the doublet
due to the @-coupling from the H atom present originally in the olefm would show multiplet polarization behaviour. Whether this was E/A or A/E would depend upon the sign of the hyperfine coupling in the secondary species relative to that in the primary one. It is simple to envisage situations where this might yield important information on the nature of the primary species if the sign of the coupling in the secondary is known from an independent experiment.
nc distortlon in each multiplet is not expected. If the secondary radical is formed with no correlation of hyperfiie coupiings with ihe primary, the excess electron spin is simply statistically distributed about alI the nuclear spin states of the radical: the spectrum appears with normal relatrve intensities of the lines. This transfer of “net” polarization has been discussed previously [3,4]. The situations in which the complex behaviour is likely to be observed include all H-atom transfer reactions, reactions involving radical additions to stable molecuies and radical rearrangements. Although to predict the qutitative effects on the appearance of spectraE straightforward, the quantitative ones require detailed analysis in order to relate the polarization which arose in the primary to that in the secondary. A preliminary analysis was made some time ago but unfortunately was concerned with the magnetisation in the field direction rather than that observed [5] _ Marc recent theory has calculated the time-dependence of the magnetisation observed from secondary radicals
formed in flash-photolysis experiments both with direct observation [6] and using time-integration spectroscopy (TIS) methods [7]. The former briefly discussed the problem of correlated hyperfme couplings but no account of the effect on observed spectra has been given. With extension of the existing theory, studies of secondary species may be expected to become quite common _ Finally we return to the interpretation of the spectrum given in fig. 4 of the paper of Sakaguchi et al. [ I] _It is not obvious to us that the behaviour observed was correctly recagnised nor whether the stick spectrum shown is relevant for it seems not to reproduce 109
Volume I1 5. number
1
CHEMlCAL
PHYSICS
the observed spectrum even at the longer time shown. It appears as though the line positions in the later spectrum correlate with some of those in the earlier one, and that their phases are consistent between the Tao. This leaves a number of fines in opposite phszz at slightly different positions. We have recently managed to reproduce! the spectra observed by Sakaguchi et al. at 1 and 3 ps after the laser flash using the technique of time-integration spectroscopy. Using a computer subtraction method it can be shown that the behaviour after I ~.lsis in fact the same as that after 3 w except for the addition of lines due to what must be a third radical showing A[E characteristics. This would presumably have resulted from the reaction of the excited singlet state of benzophenone to give a geminate singlet spin-correlated radical pair. This singlet pola.rizatlon later becomes dominated by a strong triplet one;sunilar effects have been reported in the photolysis of choline [83 _ In a system as complex as the one reported, whrch involved radical formation in a micetlar -alution made with sodium dodacyl sulphate,
110
22 March 1985
LETTERS
two tiferent not unlikely. JXS thanks
radicals from the hydrocarbon
the SERC for a m~ten~ce
cham are
award.
References [l]
Y. Sakagudu, H. HayshL H. Muti and Y II. I’Haya, Chem. Phys. Letters 110 (‘1984) 275. [2] SK. Wong,D.A. Hut&son and J.K.S. Wan, J. Chem.
Phys. 60 (1974) 2987. [3] J-B. Pedersen. FEBS Letters 97 (1979) 305. [4] CD. Buckley, A.I. Grant, K.A. McLauchlan and AJ.D.
[S) 16) [7] [8]
Ftitchie, Faraday Dlscussons Chem. Sot. 78 (1984), to be published. K.A. McLauchlan, R.C. Sealy and J-N. Wrttman, Mol. Phys. 35 (1978) 5 1. PJ. Hare and K.A. McLauc?dan. Mol. Phys 42 (1981) 533. S. Basu. K-A. McLauchian and G-R. Sea&. Mol. Ptys 52 (1984) 431. S. Basu, K.A. McLauchlan and A.Z.D. Ritchie, Chem Phys. 79 (1983) 95.