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The influence of scavenging on CIDNP field dependences in biradicals during the photolysis of large-ring cycloalkanones
Chemical Physics
L= ELSEVIER
Chemical Physics 197 (1995) 157-166
The influence of scavenging on CIDNP field dependences in biradicals during the photolysis of large-ring cycloalkanones A.V. Yurkovskaya a.*, O.B. Morozova a, R.Z. Sagdeev a, S.V. Dvinskih b, G. Buntkowsky c, H.-M. Vieth h a International Tomography Center, lustitutskaya 3a, 630090 Novosibirsk 90, Russian Federation b Institute of Experimental Physics, Institute of Organic Chemistry, Free University of Berlin, D-14195 Berlin, Germany c Institute of Organic Chemistry, Free University of Berlin, D-14195 Berlin, Germany Received 12 January 1995
Abstract Investigation of the CIDNP field dependences of acyl-alkyl biradicals formed by Norrish type-I reaction during the photolysis of cycloundecanone and eyclododecanone in the presence of the scavenger CBrCI3 has been performed at low (up to 0.08 T) magnetic fields. In addition to the emissive polarization with the main maxima caused by the S-T_ mechanism we observed an absorptive polarization with maxima at low fields (< 0.01 T) for ot-CH 2 protons of initial ketones. A quantitative analysis of CIDNP amplitude dependences on scavenger concentration allows the estimation of the kinetics of biradical geminate recombination near the emissive maxima. It is demonstrated that the biradical scavenging rate constant can be determined using the kinetic data for high magnetic fields. The scavenging rate constant of CBrCI3 estimated by this method is (2.3 + 0.4) × 109 M- 1 s- 1. Two competitive channels of singlet-triplet conversion have been revealed and the qualitative picture of their contribution to the kinetics of the low-field CIDNP is presented
1. Introduction The phenomenon of magnetic spin polarization formed during spin-selective singlet-triplet conversion in intermediate radical pairs (RPs) has been a subject of major interest the last two decades (for reviews see Refs. [1,2]). Investigation of chemically induced dynamic nuclear polarization (CIDNP) could provide detailed information on the kinetics and mechanisms of these reactions as well as on the magnetic resonance parameters of the intermediates,
* Corresponding author,
Flexible biradicals are of considerable interest from the standpoint of the spin polarization formation mechanism. These species could be considered as a specific class of radical pairs which might be characterized by two features. The first peculiarity is the absence of diffusive separation of radical centers. This provides a relatively long time of the geminate evolution of such radical pairs compared with that of ordinary radical pairs. Second, the presence of a chain of molecular bonds between radical centers determines the non-zero inter-center exchange interaction during the biradical lifetime. As a result of the exchange interaction in the RP, the singlet and triplet terms are split by 21JI. The application of an external magnetic field leads to the intersection of the T_
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SSDI 0 3 0 1 - 0 1 0 4 ( 9 5 ) 0 0 1 2 5 - 5
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A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
and S (for negative J value) levels at the magnetic field H m a x = 21JI/fie ge. In this case, the T_t~-S/3 transition (where a and /3 denote nuclear-spin projections and T_ and S stand for the triplet and singlet spin states, respectively) is assumed to be the main channel of singlet-triplet conversion. This transition is accompanied by a flip-flop of one nuclear and one electron spin. At the above fields the CIDNP caused by the T_-S transition exhibits a typical maximum. In this transition the total electron-nuclear spin is conserved and for the given nucleus the sign of nuclear polarization is independent of the sign of its hyperfine interaction constant. The exchange interaction in flexible biradicals is modulated by fast conformational motion of molecular bonds. Therefore, the field dependence of steady-state CIDNP accounts for the exchange interaction value modulated by the above motion during the biradical lifetime. The chain length of molecular bonds between radical centers determines the value of exchange interaction 2J during the biradical lifetime and the position of emission maxima of CIDNP, caused by the T_-S transition. The CIDNP field dependences of biradicals of different lengths and chemical structures have been investigated earlier [3-7]. The previous experimental studies were restricted to the investigation of the field dependences of nuclear polarization for one nucleus with a long relaxation time. Either carbonyl 13C atoms of various products [5-7] or aldehyde protons [3,4] were studied. The CIDNP theory for flexible biradicals at low magnetic fields, developed by de Kanter with coauthors in Refs. [7,8], also allows to take into account only one magnetic nucleus, while the number of magnetic nuclei in real biradicals is more than one. During a considerably long (up to hundreds of nanoseconds) time scale of the geminate evolution the spin dynamics of S-T 0 transitions is essentially averaged, therefore the nuclear polarization of biradical recombination products is formed only due to the S-T_ transitions. For reactions involving radical pairs it is known that the field dependence of CIDNP is strongly determined by radical pair lifetime [ 1 ] . Corresponding data for biradicals are, however, lacking. The biradical lifetime can be manipulated by
using an effective scavenger which traps all biradicals irrespective of nuclear-spin projection and electron-spin state. Accordingly the investigation of CIDNP field dependences under a systematic change of scavenger concentration, which is the aim of this work, yields information on the kinetics of nuclear polarization formation in biradicals at low fields. In this work, the CIDNP of acyl-alkyl biradicals arising in the photolysis of large-ring cycloalkanones (cycloundecanone and cyclododecanone) are studied as function of external magnetic field. This approach makes it possible to follow the manifestation of different channels of the intersystem crossing in biradicals at low magnetic fields using the field dependences of CIDNP measured for protons of different hfi constants.
2. Experimental The field dependences of CIDNP at different scavenger concentrations were investigated using two field cycling techniques: in the first case the sample was transferred between two magnets in a movable NMR probe, while in the second case we used a flow system for sample transportation. In the first case, a sample in a cylindrical pyrex ampoule (outer diameter 5 mm) was inserted in the movable probe of a custom-made NMR spectrometer (proton resonance frequency 300 MHz) and in low field was subject to irradiation by an excimer laser (A = 308 nm, repetition rate of 20-40 Hz) for 3-10 s. After irradiation the sample was transferred to the detection field of the spectrometer magnet where NMR spectra of polarized products were detected. The probe motion and sample irradiation were controlled by the spectrometer computer. We used the mechanical apparatus previously designed for the investigation of optical nuclear polarization (ONP) of molecular crystals (for details see Ref. [9]). The low magnetic field was controlled by electric current passing through a pair of Helmholz coils located under the cryostat of the superconducting spectrometer magnet exactly along its cylindrical axis and was measured by a Hall probe. Prior to irradiation the samples were bubbled with argon or helium gas for 3 min. The optical density of solutions was below 0.5,
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with ketone concentrations ranging between 0.03 and 0.05 M. Experiments on studying the field dependences of 1H CIDNP were carded out on a setup containing a flow system and a special cell inserted into an electromagnet, where the solution was irradiated by a DRSh-500 high-pressure mercury lamp. An MSL-300 NMR spectrometer (Bruker) was also employed. The apparatus has been described in details elsewhere [10]. For the determination of the scavenging rate constant for alkyl-acyl biradicals a sample located in the probe of the MSL-300 NMR spectrometer was irradiated by 50 pulses of an ELI-94 excimer laser (A = 308 nm, pulse energy up to 30 mJ, pulse duration 20 ns) at a frequency of 20 Hz. After irradiation an NMR spectrum was observed. The light was supplied to the sample through a prism with a spherical surface (prism lens), a cylindrical lens, and an 8 mm light guide inserted into the probe, The 1H polarization formed during the photolysis of cycloundecanone and cyclododecanone protons (CIDNP spectrum) was determined as the difference of corresponding spectral lines detected after irradiation and without irradiation under identical field cycling conditions. Cycloundecanone, cyclododecanone, the solvent CDCI3, and the scavenger CBrC13 were purchased from Aldrich and used without additional purification. In experiments with the flow-system CDC13 (99% enriched)from "Isotope", Russia, was used as received,
3. Results and discussion The main objective of the investigation described above was the analysis of the mechanism of nuclear polarization formation in biradicals. All the experiments could be divided into 3 groups: (1) Determining the biradical scavenging rate constant necessary for a quantitative analysis of CIDNP amplitude dependence on scavenger concentration, (2) Investigating the CIDNP field dependences in the presence of CBrC13 scavenger at different concentrations,
o
/ cI NN~ a2~ TH2 • HEC CUE \(CHE)n/ I hv to. clL H2C// X~.n 2 H2C| rCH2 \(CH2)n/
Scheme1 ~ s / CI" H2~ ~H2 HEC CH2 \(CH2)n/ COst_ ----o---'r II "~t /C. t,- H2C CH2 \(CH2)n/
O II C--H H2~ 'C'H2 ~ H2C ~H \(CH2)n/
#tic H2C CH2 \(CH2)a/ m n=6,7
Scheme 1.
(3) Measuring in detail the effect of the CBrC13 scavenger on the amplitude and the sign of CIDNP formed at different magnetic fields. On the photolysis of these compounds, the CIDNP effects of intermediate alkyl-acyl biradicals evolve according to Scheme 1. 3.1. Field dependences of CIDNP in the absence of CBrCl 3 scavenger Typical NMR spectra obtained before irradiation and CIDNP spectrum of CllH2oO at the magnetic field 25.0 mT are shown in Fig. la and Fig. lb, respectively. The spectral lines are attributed and numbered as follows: for the initial ketone under study the lines at 2.49 (1), 1.76 (2), 1.43 (3), and 1.30 ppm (4) correspond to tx-CH2, /3-CH2, "y-CH2 and unresolved 8,~-CH2 protons, respectively. For 10-undecenal (II) the lines at 4.96 (5), 5.82 (6), and 9.76 ppm (7) correspond to vinyl (CH 2 =), methyn (=CH-), and aldehyde (HCO-) protons, respectively. The photolysis reaction proceeding via formation of biradical intermediates is highly reversible, and the most intense lines of the CIDNP spectra correspond to the protons of the initial compound. In acyl-alkyl biradicals, only a-and fl-CH 2 protons of the alkyl end have significant hfi constants (A,~ = - 2 . 2 mT and At3 = 2.8 mT for monoradicals of similar structure [11]).
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A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
33[ 4
x2 b
--~ -W-'6 5
- ~ 7
~: -"~ ("~1 ~ "~ ~ 1
c --~ . . . . . . . . . . . . . . .....
I
i
I
9.0
I
i
Io
I
Q'~-Qr~O ~--~
i
5 0 8, p p m
i
I
3.0
L
I
,~ X4
h
I
! .0
Fig. 1. (a) "Dark" 1H NMR spectrum of CllH200 in CDC13.(b) 1H CIDNP spectrum during the photolysisof CllH200 in CDCI3 at the magnetic field 25 roT. (c) 1 H CIDNP spectrum during the photolysis of CllH200 in CDCI3 at the magnetic field 25 mT in the presence of 0.016M CBrC13.
Qualitatively, the CIDNP field dependences obtained in our experiments are in agreement with the results reported by Closs and Doubleday [3,4] for the CIDNP field dependences of aldehyde protons obtained during the photolysis of cycloundecanone, The field dependences of the CIDNP formed during the photolysis of cyclododecanone were first obtained by the nuclear spin-echo technique [12]. The polarization (emission) formed by the S - T _ mechanism has its main maximum at 25 and 15 mT for cycloundecanone and cyclododecanone, respectively, In addition to the main emissive maximum the field dependences of CIDNP of eyelotmdecanone and cyclododecanone exhibit the presence of a second, weaker maximum with absorptive phase for ot-CH 2 protons of the initial ketone at low ( < 10.0 mT) magnetic fields (Fig. 4a and Fig. 4c). In contrast, the polarization of fl-, 3'- and 8,e-CH 2 protons of both the initial ketones always corresponds to emission
and shows only one maximum. At the main maximum of CIDNP field dependences the relative intensities of the polarized lines of the initial ketones practically correspond to the "dark" spectrum intensities and relate to the number of protons contributing to each line of the spectrum. It is seen in Fig. lb, wherethethe 8, e-CHcIDNp2 protons of cycloundecanone have strongest signals. The same is valid for the methyn and vinyl aldehyde protons: at the maximum of the CIDNP field dependence the intensity of - C H = proton signal is 50% lower than that of the vinyl (CH 2 =) protons. The most intense polarization of the y- and 8,ec a 2 protons with the hfc less than 0.12 mT [11] at low magnetic fields may be caused by the polarization transfer from polarized nuclei (namely, fl-CH 2 protons) to the initially unpolarized ones in a diamagnetic molecule [13]. De Kanter and Kaptein [13] consider such mechanisms of polarization transfer as the dipolar cross-relaxation and nuclear spin-spin coupling. However, this is beyond the scope of the present paper. 3.2. The photolysis o f cycloketones in the presence o f CBrCl 3 . A n important role of the presence of scavenger molecules for the formation of geminate CIDNP lies in the removal of biradicals, disregarding their nuclear-and electron-spin projections, from geminate recombination or disproportionation. The reaction of alkyl-acyl biradicals with CBrC13 molecules leads to the attachment of a Br atom to one of the biradical ends, preferentially to the acyl terminal. This reaction "kills" biradicals as intermediates, moreover closes any opportunity for their recombination by cyclization into initial ketone molecules. As a result of Br atom attachment to the one of the biradical ends two types of radical pairs with uncorrelated electron spins appear. A detailed analysis of the NMR spectra of reaction products has shown the preferential attachment of Br atom to the acyl end of the biradical. Therefore only this pathway of biradical scavenging is shown on Scheme 2. We have investigated CIDNP effects in the presence of CBrC13 at high and low magnetic fields. With CBrCI 3 absent, no stationary CIDNP is observed during the photolysis of I in CDCI 3 solution
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
x /c. ~2~
~n2
H2C (~H2 \(CH2)n/
s~he=,2 ---f--s /c. . u2~ ~a2 H2C OH2 \(CH2)n/
..
~tepro~m 1,n. m
ca,c~X'xx ~ ~cB~c~ /c-B~ H2~ ~H2 H2C (~H2 ÷ ~Cb \(CH2)d O=CBr--(CH2)n+4--CH2 + CBtCI3~
O=CBr--(CH2)tr~--CH2Br + C C I 3 1V
O=CBr--(CH2)n+4--CH2 + CCI3"'~O~CBr'--(CH2)n+3--CH=CH2+ V
CHCI3
,=6.7 Scheme2. at high field. This indicates that the reaction with this solvent is of minor importance and does not play the role of a competitive decay channel for biradicals, Recently it has been reported [14] that no stationary CIDNP effects have been detected during the photolysis of a-phenyl-substituted cycloalkanones in deuterated acetonitrile, while the photolysis of the same compounds in C C l 4 exhibits strong CIDNP effects for many products of reactions with the solvent. This fact was interpreted as a result of competition between the scavenging reaction and the geminate reactions of the biradicals. For CC14, the scavenging rate constant of the acyl-benzyl biradicals, formed during the photolysis of a-phenyl-substituted cycloalkanones was estimated to be 1.5 × 106 M -1 s -1 [14]. The absence of stationary CIDNP on the photolysis of cycloundecanone and cyclododecanone at high fields suggests that corresponding rate constant of alkyl-acyl biradicals in CDCI 3 is smaller, The addition of CBrC13 scavenger leads to the cornpetition of the bromination reaction with the geminate reactions (recombination and disproportion) of biradicals and results in the decreased yield of polarized geminate products I, 11 and m (Scheme 1). 3.3. Determination of biradical scavenging rate constant
At high magnetic fields, where the S-T 0 mixing is the dominant pathway of triplet-singlet conver-
161
sion, the kinetic window for the S-T 0 transitions in the reactions of a-phenyl-substituted cycloalkanone biradicals has recently been determined from the dependence of the geminate CIDNP amplitude on the scavenger concentration [14-16]. The result of the scavenging influence on CIDNP is the removal of biradicals with different nuclear-spin projections from geminate recombination and disproportionation reactions. The maximum CIDNP effects are observed for the quencher concentrations at which the rate of scavenging of one biradical end is closely matched to the maximum rate of S-T 0 conversion. It can be understood in the frames of a simple model described by Turro in Ref. [15]. The triplet biradicals arising from the photochemical reaction are divided into nuclear-spin subensembles with different rates of singlet-triplet conversion. Since the total separation of radical centers by diffusion is impossible and only geminate processes are important, the fast S-T 0 transitions are followed by slow ones and eventually all subensembles of the T O term get singlet character and yield diamagnetic products. Accordingly, the CIDNP evolution must pass through a maximum and in the long time go to zero. The addition of scavenger sorts the products into the geminate ones and the products of scavenging. Since the net polarization for all products formed by the S-T 0 mechanism is zero, the nuclear polarization of the geminate products is equal in magnitude and opposite in sign to the nuclear polarization of corresponding protons of the products of the scavenging reactions. Fig. 2 shows the 1H CIDNP spectrum obtained during the photolysis of cycloundecanone in the presence of 0.012M CBrC13 in CDCI 3 at the magnetic field 7 T. The spectral lines are attributed and numbered as follows: the lines at 3.41 (8), 2.99 (9) and 1.83 ppm (10) correspond to the protons of ll-CH 2, 2-CH 2 and 10-CH 2 groups of l l bromoundecanoyl bromide (product IV in Scheme 2, n = 6), respectively. The signs of the nuclear polarization of the initial compound -emission of cz-CH2 protons (signal 1) and enhanced absorption of/3-CH 2 protons (signal 2) -are in accordance with that predicted by Kaptein's rules for the products of geminate recombination at high magnetic fields [1]. The corresponding protons of the products of the reaction with scavenger have the polarization of the opposite
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A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
therefore allows the determination of scavenging rate constant from the available CIDNP kinetics data. We consider the scavenging as a pseudo-first-order reaction. It allows us to assume that the scavenging reaction can be described by an exponential decay with the characteristic time ~'s = ks 1C~ 1, where k s is the rate constant of scavenging, C s is the concentration of the scavenger molecules. We believe that the scavenger reacts only with biradicals, but not with triplet molecules. In this case the functional dependence I(~'s) of the stationary CIDNP amplitude is
2
._J
9 ~
i ~
I( rs) = z(1L( Po( t),
10 1
3.0
,
1
l
2.0 6, ppm
i
1
1.0
Fig. 2. 1H CIDNP spectrum during the photolysis of Cl1H2oO at the magnetic field 7 T in the presence of 0.012M CBrC13
sign: the enhanced absorption of signal 8 and the emission of signal 10 are attributed to the a-CH 2 and fl-CH 2 protons, respectively in the product of Br atom attachment to the alkyl end of the biradical, A small contribution to the emission of signal 1 is made by the a-CH 2 protons which correspond to the acyl end of intermediate biradical and have a hfc of about 0.5 mT [11]. The cyclization of the biradical makes a-CH 2 protons of the alkyl and the acyl ends equivalent, and their contributions to emissive signal 1 cannot be distinguished in the spectrum. However, in the product of the scavenging reaction, the chemical shifts of these protons are different, and the CIDNP spectrum contains absorptive signal 9 corresponding to the a-CH 2 protons of the acyl end, with the Br atom attached to it. At high fields, the 7,& and e-CH 2 protons of the initial ketone, which hfi constants are rather small, are unpolarized and the CIDNP spectrum exhibits only residual signals at 1.2-1.5 ppm. Since the multiplicity of the biradical precursor molecule is triplet, the geminate products are enriched with nuclear-spin configurations with faster singiet-triplet transitions. Increasing the scavenging rate therefore was expected first to increase and at higher concentration to decrease the CIDNP amplitude. In this way it reflects the CIDNP kinetics and
'Ts-1),
(1)
where Po(t) is the nuclear polarization of the recombination products of biradicals on the assumption of simultaneous formation of all biradicals and
L(Po(t), r~ 1) =
~
co
exp(
-t/rs)Po(t) dt
So
(2)
is the Laplace transform of the function Po(t). We have studied [17,18] the kinetics of CIDNP, arising in biradicals, during the photolytic reaction of cycloundecanone and cyclododecanone. In those works we used the flash-CIDNP technique, based on pulsed laser excitation followed by pulsed detection of the NMR spectra with variable time delay between the laser pulse and the pulse for NMR detection. The measured value was the CIDNP formed during this time delay. In geminate recombination of biradicals, CIDNP exhibits a maximum near 200 ns and slowly decays at longer times. The position of the maximum is determined by relatively long lifetime of triplet precursor molecules rt, which shifts the maximum of CIDNP evolution Po(t) towards longer times as follows: ~t 1 , P ( t ) = J0 ~ e x p ( - t / ~ ' t ) P o ( t -
t')dt'.
(3)
The time resolution of the flash-CIDNP setup [17,18] was 50 ns, and the experiments were conducted at the magnetic fields 4.7 and 5.2 T. The relatively low time resolution of the method and the difference in magnetic field strengths, used previously [18] and in the present work, makes it impossible to employ the available data of Ref. [17,18] in calculating I(~-s) by formula (1). We have developed [17] an approach for the calculation the kinetics of
A. Yurkovskayaet al./ ChemicalPhysics 197 (1995) 157-166 CIDNP of biradicals in the frames of the model, proposed by de Kanter et al. [8], which is based on the numerical solution of the stochastic Liouville equation. The stochastic Liouville equation was solved [17] for the Fourier transform of the spin density matrix whose subsequent inverse Fourier transformation yielded the time dependent density matrix. We have found [17] a set of parameters reproducing the experimental kinetics of CIDNP of biradicals P(t), and used them to calculate Po(t) at the magnetic field 7 T in the present work. The details of the calculation are described elsewhere [17] and are beyond the scope of this article. The only difference in the calculations was the division of the biradical end-to-end distribution function into 400 segments of equal areas in the present paper instead of 20 segments in the previous one [17]. This was necessary for the improvement of the accuracy of Po(t) used for the l(~'s) calculation since the scavenging reaction affects the processes, occurring during tens of nanoseconds, In order to determine k s we numerically calculated I(~-s) according to Eq. (1) by using the appropriate dependence Po(t). The CIDNP spectra were recorded at different scavenger concentrations and the stationary CIDNP amplitudes lex(Cs) were also measured experimentally. The values of k s and the amplitude scaling factor were varied until the calculated I(~"s) curve showed the best fit to the experimental points lox(ksC s) (Fig. 3), which allowed us to determine the scavenging rate constant k s = (2.3 + 0.4) )< 10 9 M-1 s-1. The experimental data were
1.0 ~
.
0.a 0.e j rj o.4 o.2 °'°0
~-'--
I
100
I
I
I
200 a00 400 50o xs, ns Fig. 3. CIDNP amplitude as a function of ~'s at k~= 2.3 X 109 M-t s-l; II for ll-CH 2 protonsof IV (n = 6); O for 12-CH2 protons of IV (n = 7). Solid line is the calculationby Eq. (1).
163
obtained only for the scavenger concentrations down to C s = 0.0015M which corresponded to ~'s = 270 ns since, under our experimental conditions, for lower concentrations the scavenging could not be considered a pseudo-first-order reaction.
3.4. CIDNP field dependences in the presence of CBrCI 3 scavenger Fig. lc shows a CIDNP spectrum obtained during the photolysis of cycloundecanone in the presence of 0.016M CBrC13 at 25.0 mT. In comparison with Fig. lb the CIDNP amplitude of the initial ketone (lines 1,2,3,4) is decreased. Additional emissive signals 8 and 9, attributed to the products of the scavenging reaction, appear in the spectrum (for the description of these signals see Section 3.3 and Scheme 2). The reaction with scavenger competes with the formation of unsaturated aldehyde during the geminate disproportionation of biradicals and at the scavenger concentrations higher than 0.016M the latter is almost completely suppressed; thus the CIDNP signals of aldehyde protons can hardly be seen in the spectrum. The field dependences of CIDNP for cycloundecanone and cyclododecanone protons at different bromotrichloromethane concentrations are shown in Fig. 4a-4d, respectively. It is seen that for a-CH 2 protons of both ketones the addition of scavenger increases the absorption maximum at low ( < 10.0 mT) magnetic fields, decreases the amplitude of the emission maximum and slightly shifts its position, and changes the sign from emission to absorption in the high-field region of CIDNP field dependence. The magnetic field where the polarization of a-CH 2 protons changes its sign decreases with increasing scavenger concentration. For •-CH2, as well as for 31- and ~,e-CH 2 protons, the polarization exhibits an emissive phase for all studied magnetic fields. In all cases the field dependences of CIDNP of all products of the reactions with scavenger show emission maxima within 20.0-25.0 and 12.5-15.0 mT during cycloundecanone and cyclododecanone photolysis, respectively. The presence and coincidence of emission maxima for all reaction products can readily be accounted for in terms of the S - T _ mechanism of CIDNP formation. The S - T _ intersystem crossing involves simultaneous flips of electron and one of
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A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
the nuclear spins from a to fl spin states ( T _ a states transfer to S/3 irrespective of the sign of hfi constant). In this case, singlet biradicals are formed with fl nuclear-spin projections, while triplet biradicals still remain with excess of fl projections. The singlet biradicals can recombine from the polymethylene chain conformation at a sufficiently small distance between radical centers to yield the initial compound. Both singlet and triplet biradicals enriched during the T _ - S transition w i t h / 3 projections are involved in the scavenging reaction. Hence, after the reaction with scavenger the radicals also exhibit a negative nuclear polarization. It is obvious that the CIDNP kinetics as well as the ISC is strongly dependent on the external mag-
°.2t~ o.o~+-~e~ o. .~ g ;. ,~ -0.2~ . ~* -0.41 -0.6 • " -0. . • -1. '~" o.00 0.to 0.04 o.0s Magnetic field, T
0.0
:+ -0.2 <+
. ,^,.,, . . . . .
0.0 ~ o [] o :o II ~ -o.2-'*"*,~ ~• ;- -0.¢ " . " ¢~ -0.s • " -0.8 "
,,~,=,,*,,
0.00
°a+xaaaaa'~ . _ _
"
"'''
--0.4
rO -0.6
"0.10 ~e," -0.15/
0.oo o.04 o.= ra,mm¢~da.x
o.o8
++
.
....
•
"m•.~ 0.0~
0.04
0.06
0.08
Magnetic field, T
.'''" ,
"0.051 ~
c
-1.0 0.2 ~IA#~A
O.lS~ : o.,o!=+~o.," =, ~"" ~ o= .= 0"
d Fig. 4 (continued).
"
N
-0.8
M
-1.0
=t j= N ,
0.00
,
0.02
,
,
0.04
0.06
0.08
Magnetic field, T
a
0.0 ~.,,,,,, , ~ + , , + , , ~~ _i ~m~ ~ eA&~m~, • a, ,, •.. • • -0.4 • -0.6 .+ +,, -0.8 • ,,
-0.2
netic field. Since there are no direct methods for measuring the kinetics of nuclear polarization formation for low and intermediate (up to 100 mT) magnetic fields, the kinetics of CIDNP formation in biradicals near the maxima of its field dependences has not yet been studied for the above range of magnetic fields. The investigation of the CIDNP amplitude as a function of scavenger concentration yields information on biradical lifetime. One can assume that the CIDNP formation near its emissive maxima is based on the exponential time law
lu •
-1.0 . . . . . . . . 0.00
0.02
0.04
0.06
0.08
Magnetic field, T b
Fig. 4. (a) CIDNP field dependences for a-CH 2 protons of CnH200; ~r in the absence of CBrCI3; • i n the presence of 0.008M CBrCI3; zx in the presence of 0.016M CBrCl 3. (b) CIDNP field dependences for/3-CH 2 protons of CnH200; ~ in the absence of CBrC13; • in the presence of 0.008M CBrCI3; zx in the presence of 0.016M CBrCI 3. (c) CIDNP field dependences for a-CH 2 protons of C12H220; Sr in the absence of CBrCI3; • in the presence of 0.01M CBrCI3; zx in the presence of 0.04M CBrC13; [] in the presence of 0.08M CBrCI 3. (d) CIDNP field dependences for ot-CH2 protons of C12H220; ~ in the absence of CBrCI3; • in the presence of 0.01M CBrCI3; t, in the presence of 0.04M CBrC3; [] in the presence of 0.08M CBrCI 3.
I ( t ) = Io[1 - exp( - t/z)]
(4)
where ~" is the characteristic lifetime of biradical. I 0 is the stationary nuclear polarization. This assumption is reasonable because for similar system it was experimentally found that the CIDNP evolution really c a n b e d e s c r i b e d f o r dominating S - T _ mechanism in g o o d approximation by an exponential runetion [19]. In the presence of the biradical s c a v e n g e r the CIDNP amplitude can b e described by the Stern-Volmer dependence: 1o
+"
-/s(';'s) - = I + - - = I +"rsT k + C
s, I o=I(C+=O). (5)
165
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
In the frames of this model the CIDNP amplitude dependence on scavenger concentration in geminate recombination could be used to obtain information about biradical lifetimes at a low magnetic field from Stern-Volmer plot. Experimental values presented in Fig. 5a show significant deviation from linear behavior. Only near the emissive maxima the dependence can be approximated by a linear fit (Fig. 5b). The times corresponding to the linear approximation are 120 _ 24 ns for cycloundecanone and 45 + 9 ns for cyclododecanone. At the higher scavenger concentrations C s the deviation of the curve from the linear Stern-Volmer is more pronounced, in particular for a-CH 2 protons at the highest concentration at the magnetic field higher than 30 mT the polarization changes its sign (see Fig. 4a and Fig. 4c) to an absorptive phase and becomes opposite to the polar-
6 5
*
w~
•
3 - 2
~
-
°~ 0.0
10-
~
• ~ '~
~ ~ , 5.~0-3 c~, M a
/
. 1.~0-2
_~
s
0 0.0
4.0x102 8.0~0"2 E,, M b Fig. 5. (a) Stern-Volmer dependence for the CIDNP amplitude for CllH20 O at the magnetic field 32.4 mT; • for ot-CH 2 protons; r, for T-CH2 protons. (b)Stem-Volmer dependencefor the CIDNP amplitude; zx for /3-CH2 protons of CllH2oO at the magnetic field 25 mT; [] for y-CH2 protons of C12H220 at the magnetic field 15 mT.
0.5 ~ s'T°s'T° \~__
\\
~ 0.0 ~ ~ -0.5 s-x_/ ~ -1.0 0
~
200
i
400 time, ns
i
600
Fig. 6. Schematic presentation of CIDNP kinetics formed by S-To and S-T_ mechanismsin biradicals at low magnetic fields, and the resulting curve for nuclei with negativehfi constant.
ization of /3-CH 2 protons. These results are consistent with the assumption that in the above range the magnetic field the relative contribution of the S - T 0 transition becomes more important as the scavenger concentration increases. We find positive polarization for a-CH 2 protons and negative for fl-CH 2 protons and only the S - T 0 contribution into intersystem crossing depends on the sign of the hypeffine cou pli ng constants. As it has been shown [19], the kinetics of CIDNP formed by the S - T 0 transitions goes through a maximum (emissive for the protons with positive hyperfine coupling constant and absorptive for those with a negative constant), and decreases for a longer time, while the S - T _ contribution asymptotically reaches its maximum (emissive) value, as schematically presented in Fig. 6. If we take for S - T o and for S - T _ CIDNP kinetics the curves shown in Fig. 6, then the influence of the scavenger is more pronounced for the S - T _ contribution. The field dependences of the CH2Br protons of compound IV (Scheme 2) support unambiguously the assumption about the two channels of singlet-triplet conversion contributing to the CIDNP pattern. These species correspond to the a-protons of the alkyl biradical end. In full accordance with the consideration described in Section 3.3, the polarization of scavenging products by the S - T 0 channel is opposite to the polarization of corresponding protons of the geminate products. Thus, the CIDNP of product IV has an emissive phase for all magnetic fields, while the polarization of cycloundecanone and cyclododecane a-CH 2 protons changes its sign twice.
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
166
However, in the case of nuclear polarization formation only by the S - T _ channel, the sign of polarization with increasing scavenger concentration would coincide for both products and remain unchanged. A kinematic approximation has recently been proposed [20] for the description of magnetic and spin effects in chemical reactions. This approximation has been used to describe the CIDNP kinetics for biradicals under strong magnetic fields. Now this approach
is being extended to the description of the CIDNP of biradicals under weak magnetic fields. Numerical calculations performed in terms of this theoretical approach are to be published in a forthcoming paper.
4. Conclusion Investigation of the CIDNP field dependences formed during photolysis of cycloundecanone and cyclododecanone in the presence of the scavenger C B r C I 3 a l l o w s the analysis of the kinetics of biradical geminate recombination. It is demonstrated that the biradical scavenging rate constant can be determined using t h e k i n e t i c data f o r high magnetic fields. The scavenging rate constant of CBrC13 estimated by this method is (2.3 + 0 . 4 ) × 109 M -1 s -1. Two channels of singlet-triplet conversion have been revealed and the qualitative picture of their contribution to the kinetics of the CIDNP at low magnetic fields has been presented.
Acknowledgements
Deutsche Forschungsgemeinschaft, (Sfb 337), for financial support.
References [1] K.M. Salikhov, Yu.N. Molin, R.Z. Sagdeev and A.L. Buchachenko, Spin polarization and magnetic effects in radical reactions, ed. Yu.N. Molin (Elsevier, Amsterdam, 1984).
[2] U.E. Steinerand H.-J. Wolff, in: Photochemistryand photophysics, Vol. 4, eds. J.F. Rabek and G.W. Scott (CRC Press,
Boca Raton, 1991)p. 1. [3] G.L. Closs and C.E. Doubleday, J. Am. Chem. Soc. 94 (1972) 9248. [4] G.L. Closs and C.E. Doubleday, J. Am. Chem. Soc. 95 (1973) 2735. [5] C. Doubleday Jr., Chem. Phys. Letters 77 (1981) 131. [6] C. Doubleday Jr., Chem. Phys. Letters 81 (1981) 164. [7] F.J.J. de Kanter and R. Kaptein, J. Am. Chem. Soc. 104 (1982) 4759. [8] F.J.J. de Kanter, J.A. den Hollander, A.H. Huiser and R. Kaptein, Mol. Phys. 34 (1977) 857. [9] J. Allgeier, G. Buntkowsky, S. Hentrich, M. Nack and H.-M. Vieth, Ber. Bunsenges. Physik. Chem. 93 (1989) 1281. [10] E.G. Bagryanskaya and R.Z. Sagdeev, Progr. Reaction Kinet. 18 (1993) 63. [11] H. Fischer, ed., Magnetic properties of free radicals, Landolt-B6rnstein New Series, Voi. II 9b (Springer, Berlin,
1980). [12] A.V. Dushkin, Yu.A. Grishin and R.Z. Sagdeev, Chem. Phys. Letters55 (1978) 174. [13] F.J.J. de Kanter and R. Kaptein, J. Am. Chem. Soc. 94 (1972) 6269. [14] Kuo Chu Hwang, N.J. Turro and C. Doubleday Jr., J. Am. Chem. Soc. 113 (1991) 2850. [15] N.J. Turro, Kuo Chu Hwang, V. Pushkara Rao and C. Doubleday Jr., J. Phys. Chem. 95 (1991) 1872. [16] N. Han, Kuo Chu Hwang, Xuegong Lei and N.J. Turro, J. Photochem. Photobiol. A 61 (1991) 35.
The authors are grateful to Yu Grishin, M. Benkert and A. PfivaIov for technical assistance in the modi-
fication of the experimental setup in Berlin, to N. Lukzen for fruitful discussion, to E. Bagryanskaya for her assistance with flow system operation. The authors thank the Russian Foundation for Fundamental R e s e a r c h (Project number 93-03-18593) a n d
[17] A.V. Yurkovskaya, Yu.P. Tsentalovich, N.N. Lukzen and R.Z. Sagdeev, Res. Chem. Intermed. 17 (1992) 145. [18] Yu.P. Tsentalovich, A.V. Yurkovskaya, R.Z. Sagdeev, A.A. Obynochny, P.A. Purtov and A. Shargorodsky, Chem. Phys. 139 (1989) 307. [19] G.I. Closs and O.D. Redwine, J. Am. Chem Soc. 107 (1985) 6131. [20] P.A. Purtov and A.B. Doktorov, Chem. Phys. 178 (1993) 47.
L= ELSEVIER
Chemical Physics 197 (1995) 157-166
The influence of scavenging on CIDNP field dependences in biradicals during the photolysis of large-ring cycloalkanones A.V. Yurkovskaya a.*, O.B. Morozova a, R.Z. Sagdeev a, S.V. Dvinskih b, G. Buntkowsky c, H.-M. Vieth h a International Tomography Center, lustitutskaya 3a, 630090 Novosibirsk 90, Russian Federation b Institute of Experimental Physics, Institute of Organic Chemistry, Free University of Berlin, D-14195 Berlin, Germany c Institute of Organic Chemistry, Free University of Berlin, D-14195 Berlin, Germany Received 12 January 1995
Abstract Investigation of the CIDNP field dependences of acyl-alkyl biradicals formed by Norrish type-I reaction during the photolysis of cycloundecanone and eyclododecanone in the presence of the scavenger CBrCI3 has been performed at low (up to 0.08 T) magnetic fields. In addition to the emissive polarization with the main maxima caused by the S-T_ mechanism we observed an absorptive polarization with maxima at low fields (< 0.01 T) for ot-CH 2 protons of initial ketones. A quantitative analysis of CIDNP amplitude dependences on scavenger concentration allows the estimation of the kinetics of biradical geminate recombination near the emissive maxima. It is demonstrated that the biradical scavenging rate constant can be determined using the kinetic data for high magnetic fields. The scavenging rate constant of CBrCI3 estimated by this method is (2.3 + 0.4) × 109 M- 1 s- 1. Two competitive channels of singlet-triplet conversion have been revealed and the qualitative picture of their contribution to the kinetics of the low-field CIDNP is presented
1. Introduction The phenomenon of magnetic spin polarization formed during spin-selective singlet-triplet conversion in intermediate radical pairs (RPs) has been a subject of major interest the last two decades (for reviews see Refs. [1,2]). Investigation of chemically induced dynamic nuclear polarization (CIDNP) could provide detailed information on the kinetics and mechanisms of these reactions as well as on the magnetic resonance parameters of the intermediates,
* Corresponding author,
Flexible biradicals are of considerable interest from the standpoint of the spin polarization formation mechanism. These species could be considered as a specific class of radical pairs which might be characterized by two features. The first peculiarity is the absence of diffusive separation of radical centers. This provides a relatively long time of the geminate evolution of such radical pairs compared with that of ordinary radical pairs. Second, the presence of a chain of molecular bonds between radical centers determines the non-zero inter-center exchange interaction during the biradical lifetime. As a result of the exchange interaction in the RP, the singlet and triplet terms are split by 21JI. The application of an external magnetic field leads to the intersection of the T_
0301-0104/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved
SSDI 0 3 0 1 - 0 1 0 4 ( 9 5 ) 0 0 1 2 5 - 5
158
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
and S (for negative J value) levels at the magnetic field H m a x = 21JI/fie ge. In this case, the T_t~-S/3 transition (where a and /3 denote nuclear-spin projections and T_ and S stand for the triplet and singlet spin states, respectively) is assumed to be the main channel of singlet-triplet conversion. This transition is accompanied by a flip-flop of one nuclear and one electron spin. At the above fields the CIDNP caused by the T_-S transition exhibits a typical maximum. In this transition the total electron-nuclear spin is conserved and for the given nucleus the sign of nuclear polarization is independent of the sign of its hyperfine interaction constant. The exchange interaction in flexible biradicals is modulated by fast conformational motion of molecular bonds. Therefore, the field dependence of steady-state CIDNP accounts for the exchange interaction value modulated by the above motion during the biradical lifetime. The chain length of molecular bonds between radical centers determines the value of exchange interaction 2J during the biradical lifetime and the position of emission maxima of CIDNP, caused by the T_-S transition. The CIDNP field dependences of biradicals of different lengths and chemical structures have been investigated earlier [3-7]. The previous experimental studies were restricted to the investigation of the field dependences of nuclear polarization for one nucleus with a long relaxation time. Either carbonyl 13C atoms of various products [5-7] or aldehyde protons [3,4] were studied. The CIDNP theory for flexible biradicals at low magnetic fields, developed by de Kanter with coauthors in Refs. [7,8], also allows to take into account only one magnetic nucleus, while the number of magnetic nuclei in real biradicals is more than one. During a considerably long (up to hundreds of nanoseconds) time scale of the geminate evolution the spin dynamics of S-T 0 transitions is essentially averaged, therefore the nuclear polarization of biradical recombination products is formed only due to the S-T_ transitions. For reactions involving radical pairs it is known that the field dependence of CIDNP is strongly determined by radical pair lifetime [ 1 ] . Corresponding data for biradicals are, however, lacking. The biradical lifetime can be manipulated by
using an effective scavenger which traps all biradicals irrespective of nuclear-spin projection and electron-spin state. Accordingly the investigation of CIDNP field dependences under a systematic change of scavenger concentration, which is the aim of this work, yields information on the kinetics of nuclear polarization formation in biradicals at low fields. In this work, the CIDNP of acyl-alkyl biradicals arising in the photolysis of large-ring cycloalkanones (cycloundecanone and cyclododecanone) are studied as function of external magnetic field. This approach makes it possible to follow the manifestation of different channels of the intersystem crossing in biradicals at low magnetic fields using the field dependences of CIDNP measured for protons of different hfi constants.
2. Experimental The field dependences of CIDNP at different scavenger concentrations were investigated using two field cycling techniques: in the first case the sample was transferred between two magnets in a movable NMR probe, while in the second case we used a flow system for sample transportation. In the first case, a sample in a cylindrical pyrex ampoule (outer diameter 5 mm) was inserted in the movable probe of a custom-made NMR spectrometer (proton resonance frequency 300 MHz) and in low field was subject to irradiation by an excimer laser (A = 308 nm, repetition rate of 20-40 Hz) for 3-10 s. After irradiation the sample was transferred to the detection field of the spectrometer magnet where NMR spectra of polarized products were detected. The probe motion and sample irradiation were controlled by the spectrometer computer. We used the mechanical apparatus previously designed for the investigation of optical nuclear polarization (ONP) of molecular crystals (for details see Ref. [9]). The low magnetic field was controlled by electric current passing through a pair of Helmholz coils located under the cryostat of the superconducting spectrometer magnet exactly along its cylindrical axis and was measured by a Hall probe. Prior to irradiation the samples were bubbled with argon or helium gas for 3 min. The optical density of solutions was below 0.5,
159
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
with ketone concentrations ranging between 0.03 and 0.05 M. Experiments on studying the field dependences of 1H CIDNP were carded out on a setup containing a flow system and a special cell inserted into an electromagnet, where the solution was irradiated by a DRSh-500 high-pressure mercury lamp. An MSL-300 NMR spectrometer (Bruker) was also employed. The apparatus has been described in details elsewhere [10]. For the determination of the scavenging rate constant for alkyl-acyl biradicals a sample located in the probe of the MSL-300 NMR spectrometer was irradiated by 50 pulses of an ELI-94 excimer laser (A = 308 nm, pulse energy up to 30 mJ, pulse duration 20 ns) at a frequency of 20 Hz. After irradiation an NMR spectrum was observed. The light was supplied to the sample through a prism with a spherical surface (prism lens), a cylindrical lens, and an 8 mm light guide inserted into the probe, The 1H polarization formed during the photolysis of cycloundecanone and cyclododecanone protons (CIDNP spectrum) was determined as the difference of corresponding spectral lines detected after irradiation and without irradiation under identical field cycling conditions. Cycloundecanone, cyclododecanone, the solvent CDCI3, and the scavenger CBrC13 were purchased from Aldrich and used without additional purification. In experiments with the flow-system CDC13 (99% enriched)from "Isotope", Russia, was used as received,
3. Results and discussion The main objective of the investigation described above was the analysis of the mechanism of nuclear polarization formation in biradicals. All the experiments could be divided into 3 groups: (1) Determining the biradical scavenging rate constant necessary for a quantitative analysis of CIDNP amplitude dependence on scavenger concentration, (2) Investigating the CIDNP field dependences in the presence of CBrC13 scavenger at different concentrations,
o
/ cI NN~ a2~ TH2 • HEC CUE \(CHE)n/ I hv to. clL H2C// X~.n 2 H2C| rCH2 \(CH2)n/
Scheme1 ~ s / CI" H2~ ~H2 HEC CH2 \(CH2)n/ COst_ ----o---'r II "~t /C. t,- H2C CH2 \(CH2)n/
O II C--H H2~ 'C'H2 ~ H2C ~H \(CH2)n/
#tic H2C CH2 \(CH2)a/ m n=6,7
Scheme 1.
(3) Measuring in detail the effect of the CBrC13 scavenger on the amplitude and the sign of CIDNP formed at different magnetic fields. On the photolysis of these compounds, the CIDNP effects of intermediate alkyl-acyl biradicals evolve according to Scheme 1. 3.1. Field dependences of CIDNP in the absence of CBrCl 3 scavenger Typical NMR spectra obtained before irradiation and CIDNP spectrum of CllH2oO at the magnetic field 25.0 mT are shown in Fig. la and Fig. lb, respectively. The spectral lines are attributed and numbered as follows: for the initial ketone under study the lines at 2.49 (1), 1.76 (2), 1.43 (3), and 1.30 ppm (4) correspond to tx-CH2, /3-CH2, "y-CH2 and unresolved 8,~-CH2 protons, respectively. For 10-undecenal (II) the lines at 4.96 (5), 5.82 (6), and 9.76 ppm (7) correspond to vinyl (CH 2 =), methyn (=CH-), and aldehyde (HCO-) protons, respectively. The photolysis reaction proceeding via formation of biradical intermediates is highly reversible, and the most intense lines of the CIDNP spectra correspond to the protons of the initial compound. In acyl-alkyl biradicals, only a-and fl-CH 2 protons of the alkyl end have significant hfi constants (A,~ = - 2 . 2 mT and At3 = 2.8 mT for monoradicals of similar structure [11]).
160
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
33[ 4
x2 b
--~ -W-'6 5
- ~ 7
~: -"~ ("~1 ~ "~ ~ 1
c --~ . . . . . . . . . . . . . . .....
I
i
I
9.0
I
i
Io
I
Q'~-Qr~O ~--~
i
5 0 8, p p m
i
I
3.0
L
I
,~ X4
h
I
! .0
Fig. 1. (a) "Dark" 1H NMR spectrum of CllH200 in CDC13.(b) 1H CIDNP spectrum during the photolysisof CllH200 in CDCI3 at the magnetic field 25 roT. (c) 1 H CIDNP spectrum during the photolysis of CllH200 in CDCI3 at the magnetic field 25 mT in the presence of 0.016M CBrC13.
Qualitatively, the CIDNP field dependences obtained in our experiments are in agreement with the results reported by Closs and Doubleday [3,4] for the CIDNP field dependences of aldehyde protons obtained during the photolysis of cycloundecanone, The field dependences of the CIDNP formed during the photolysis of cyclododecanone were first obtained by the nuclear spin-echo technique [12]. The polarization (emission) formed by the S - T _ mechanism has its main maximum at 25 and 15 mT for cycloundecanone and cyclododecanone, respectively, In addition to the main emissive maximum the field dependences of CIDNP of eyelotmdecanone and cyclododecanone exhibit the presence of a second, weaker maximum with absorptive phase for ot-CH 2 protons of the initial ketone at low ( < 10.0 mT) magnetic fields (Fig. 4a and Fig. 4c). In contrast, the polarization of fl-, 3'- and 8,e-CH 2 protons of both the initial ketones always corresponds to emission
and shows only one maximum. At the main maximum of CIDNP field dependences the relative intensities of the polarized lines of the initial ketones practically correspond to the "dark" spectrum intensities and relate to the number of protons contributing to each line of the spectrum. It is seen in Fig. lb, wherethethe 8, e-CHcIDNp2 protons of cycloundecanone have strongest signals. The same is valid for the methyn and vinyl aldehyde protons: at the maximum of the CIDNP field dependence the intensity of - C H = proton signal is 50% lower than that of the vinyl (CH 2 =) protons. The most intense polarization of the y- and 8,ec a 2 protons with the hfc less than 0.12 mT [11] at low magnetic fields may be caused by the polarization transfer from polarized nuclei (namely, fl-CH 2 protons) to the initially unpolarized ones in a diamagnetic molecule [13]. De Kanter and Kaptein [13] consider such mechanisms of polarization transfer as the dipolar cross-relaxation and nuclear spin-spin coupling. However, this is beyond the scope of the present paper. 3.2. The photolysis o f cycloketones in the presence o f CBrCl 3 . A n important role of the presence of scavenger molecules for the formation of geminate CIDNP lies in the removal of biradicals, disregarding their nuclear-and electron-spin projections, from geminate recombination or disproportionation. The reaction of alkyl-acyl biradicals with CBrC13 molecules leads to the attachment of a Br atom to one of the biradical ends, preferentially to the acyl terminal. This reaction "kills" biradicals as intermediates, moreover closes any opportunity for their recombination by cyclization into initial ketone molecules. As a result of Br atom attachment to the one of the biradical ends two types of radical pairs with uncorrelated electron spins appear. A detailed analysis of the NMR spectra of reaction products has shown the preferential attachment of Br atom to the acyl end of the biradical. Therefore only this pathway of biradical scavenging is shown on Scheme 2. We have investigated CIDNP effects in the presence of CBrC13 at high and low magnetic fields. With CBrCI 3 absent, no stationary CIDNP is observed during the photolysis of I in CDCI 3 solution
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
x /c. ~2~
~n2
H2C (~H2 \(CH2)n/
s~he=,2 ---f--s /c. . u2~ ~a2 H2C OH2 \(CH2)n/
..
~tepro~m 1,n. m
ca,c~X'xx ~ ~cB~c~ /c-B~ H2~ ~H2 H2C (~H2 ÷ ~Cb \(CH2)d O=CBr--(CH2)n+4--CH2 + CBtCI3~
O=CBr--(CH2)tr~--CH2Br + C C I 3 1V
O=CBr--(CH2)n+4--CH2 + CCI3"'~O~CBr'--(CH2)n+3--CH=CH2+ V
CHCI3
,=6.7 Scheme2. at high field. This indicates that the reaction with this solvent is of minor importance and does not play the role of a competitive decay channel for biradicals, Recently it has been reported [14] that no stationary CIDNP effects have been detected during the photolysis of a-phenyl-substituted cycloalkanones in deuterated acetonitrile, while the photolysis of the same compounds in C C l 4 exhibits strong CIDNP effects for many products of reactions with the solvent. This fact was interpreted as a result of competition between the scavenging reaction and the geminate reactions of the biradicals. For CC14, the scavenging rate constant of the acyl-benzyl biradicals, formed during the photolysis of a-phenyl-substituted cycloalkanones was estimated to be 1.5 × 106 M -1 s -1 [14]. The absence of stationary CIDNP on the photolysis of cycloundecanone and cyclododecanone at high fields suggests that corresponding rate constant of alkyl-acyl biradicals in CDCI 3 is smaller, The addition of CBrC13 scavenger leads to the cornpetition of the bromination reaction with the geminate reactions (recombination and disproportion) of biradicals and results in the decreased yield of polarized geminate products I, 11 and m (Scheme 1). 3.3. Determination of biradical scavenging rate constant
At high magnetic fields, where the S-T 0 mixing is the dominant pathway of triplet-singlet conver-
161
sion, the kinetic window for the S-T 0 transitions in the reactions of a-phenyl-substituted cycloalkanone biradicals has recently been determined from the dependence of the geminate CIDNP amplitude on the scavenger concentration [14-16]. The result of the scavenging influence on CIDNP is the removal of biradicals with different nuclear-spin projections from geminate recombination and disproportionation reactions. The maximum CIDNP effects are observed for the quencher concentrations at which the rate of scavenging of one biradical end is closely matched to the maximum rate of S-T 0 conversion. It can be understood in the frames of a simple model described by Turro in Ref. [15]. The triplet biradicals arising from the photochemical reaction are divided into nuclear-spin subensembles with different rates of singlet-triplet conversion. Since the total separation of radical centers by diffusion is impossible and only geminate processes are important, the fast S-T 0 transitions are followed by slow ones and eventually all subensembles of the T O term get singlet character and yield diamagnetic products. Accordingly, the CIDNP evolution must pass through a maximum and in the long time go to zero. The addition of scavenger sorts the products into the geminate ones and the products of scavenging. Since the net polarization for all products formed by the S-T 0 mechanism is zero, the nuclear polarization of the geminate products is equal in magnitude and opposite in sign to the nuclear polarization of corresponding protons of the products of the scavenging reactions. Fig. 2 shows the 1H CIDNP spectrum obtained during the photolysis of cycloundecanone in the presence of 0.012M CBrC13 in CDCI 3 at the magnetic field 7 T. The spectral lines are attributed and numbered as follows: the lines at 3.41 (8), 2.99 (9) and 1.83 ppm (10) correspond to the protons of ll-CH 2, 2-CH 2 and 10-CH 2 groups of l l bromoundecanoyl bromide (product IV in Scheme 2, n = 6), respectively. The signs of the nuclear polarization of the initial compound -emission of cz-CH2 protons (signal 1) and enhanced absorption of/3-CH 2 protons (signal 2) -are in accordance with that predicted by Kaptein's rules for the products of geminate recombination at high magnetic fields [1]. The corresponding protons of the products of the reaction with scavenger have the polarization of the opposite
162
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
therefore allows the determination of scavenging rate constant from the available CIDNP kinetics data. We consider the scavenging as a pseudo-first-order reaction. It allows us to assume that the scavenging reaction can be described by an exponential decay with the characteristic time ~'s = ks 1C~ 1, where k s is the rate constant of scavenging, C s is the concentration of the scavenger molecules. We believe that the scavenger reacts only with biradicals, but not with triplet molecules. In this case the functional dependence I(~'s) of the stationary CIDNP amplitude is
2
._J
9 ~
i ~
I( rs) = z(1L( Po( t),
10 1
3.0
,
1
l
2.0 6, ppm
i
1
1.0
Fig. 2. 1H CIDNP spectrum during the photolysis of Cl1H2oO at the magnetic field 7 T in the presence of 0.012M CBrC13
sign: the enhanced absorption of signal 8 and the emission of signal 10 are attributed to the a-CH 2 and fl-CH 2 protons, respectively in the product of Br atom attachment to the alkyl end of the biradical, A small contribution to the emission of signal 1 is made by the a-CH 2 protons which correspond to the acyl end of intermediate biradical and have a hfc of about 0.5 mT [11]. The cyclization of the biradical makes a-CH 2 protons of the alkyl and the acyl ends equivalent, and their contributions to emissive signal 1 cannot be distinguished in the spectrum. However, in the product of the scavenging reaction, the chemical shifts of these protons are different, and the CIDNP spectrum contains absorptive signal 9 corresponding to the a-CH 2 protons of the acyl end, with the Br atom attached to it. At high fields, the 7,& and e-CH 2 protons of the initial ketone, which hfi constants are rather small, are unpolarized and the CIDNP spectrum exhibits only residual signals at 1.2-1.5 ppm. Since the multiplicity of the biradical precursor molecule is triplet, the geminate products are enriched with nuclear-spin configurations with faster singiet-triplet transitions. Increasing the scavenging rate therefore was expected first to increase and at higher concentration to decrease the CIDNP amplitude. In this way it reflects the CIDNP kinetics and
'Ts-1),
(1)
where Po(t) is the nuclear polarization of the recombination products of biradicals on the assumption of simultaneous formation of all biradicals and
L(Po(t), r~ 1) =
~
co
exp(
-t/rs)Po(t) dt
So
(2)
is the Laplace transform of the function Po(t). We have studied [17,18] the kinetics of CIDNP, arising in biradicals, during the photolytic reaction of cycloundecanone and cyclododecanone. In those works we used the flash-CIDNP technique, based on pulsed laser excitation followed by pulsed detection of the NMR spectra with variable time delay between the laser pulse and the pulse for NMR detection. The measured value was the CIDNP formed during this time delay. In geminate recombination of biradicals, CIDNP exhibits a maximum near 200 ns and slowly decays at longer times. The position of the maximum is determined by relatively long lifetime of triplet precursor molecules rt, which shifts the maximum of CIDNP evolution Po(t) towards longer times as follows: ~t 1 , P ( t ) = J0 ~ e x p ( - t / ~ ' t ) P o ( t -
t')dt'.
(3)
The time resolution of the flash-CIDNP setup [17,18] was 50 ns, and the experiments were conducted at the magnetic fields 4.7 and 5.2 T. The relatively low time resolution of the method and the difference in magnetic field strengths, used previously [18] and in the present work, makes it impossible to employ the available data of Ref. [17,18] in calculating I(~-s) by formula (1). We have developed [17] an approach for the calculation the kinetics of
A. Yurkovskayaet al./ ChemicalPhysics 197 (1995) 157-166 CIDNP of biradicals in the frames of the model, proposed by de Kanter et al. [8], which is based on the numerical solution of the stochastic Liouville equation. The stochastic Liouville equation was solved [17] for the Fourier transform of the spin density matrix whose subsequent inverse Fourier transformation yielded the time dependent density matrix. We have found [17] a set of parameters reproducing the experimental kinetics of CIDNP of biradicals P(t), and used them to calculate Po(t) at the magnetic field 7 T in the present work. The details of the calculation are described elsewhere [17] and are beyond the scope of this article. The only difference in the calculations was the division of the biradical end-to-end distribution function into 400 segments of equal areas in the present paper instead of 20 segments in the previous one [17]. This was necessary for the improvement of the accuracy of Po(t) used for the l(~'s) calculation since the scavenging reaction affects the processes, occurring during tens of nanoseconds, In order to determine k s we numerically calculated I(~-s) according to Eq. (1) by using the appropriate dependence Po(t). The CIDNP spectra were recorded at different scavenger concentrations and the stationary CIDNP amplitudes lex(Cs) were also measured experimentally. The values of k s and the amplitude scaling factor were varied until the calculated I(~"s) curve showed the best fit to the experimental points lox(ksC s) (Fig. 3), which allowed us to determine the scavenging rate constant k s = (2.3 + 0.4) )< 10 9 M-1 s-1. The experimental data were
1.0 ~
.
0.a 0.e j rj o.4 o.2 °'°0
~-'--
I
100
I
I
I
200 a00 400 50o xs, ns Fig. 3. CIDNP amplitude as a function of ~'s at k~= 2.3 X 109 M-t s-l; II for ll-CH 2 protonsof IV (n = 6); O for 12-CH2 protons of IV (n = 7). Solid line is the calculationby Eq. (1).
163
obtained only for the scavenger concentrations down to C s = 0.0015M which corresponded to ~'s = 270 ns since, under our experimental conditions, for lower concentrations the scavenging could not be considered a pseudo-first-order reaction.
3.4. CIDNP field dependences in the presence of CBrCI 3 scavenger Fig. lc shows a CIDNP spectrum obtained during the photolysis of cycloundecanone in the presence of 0.016M CBrC13 at 25.0 mT. In comparison with Fig. lb the CIDNP amplitude of the initial ketone (lines 1,2,3,4) is decreased. Additional emissive signals 8 and 9, attributed to the products of the scavenging reaction, appear in the spectrum (for the description of these signals see Section 3.3 and Scheme 2). The reaction with scavenger competes with the formation of unsaturated aldehyde during the geminate disproportionation of biradicals and at the scavenger concentrations higher than 0.016M the latter is almost completely suppressed; thus the CIDNP signals of aldehyde protons can hardly be seen in the spectrum. The field dependences of CIDNP for cycloundecanone and cyclododecanone protons at different bromotrichloromethane concentrations are shown in Fig. 4a-4d, respectively. It is seen that for a-CH 2 protons of both ketones the addition of scavenger increases the absorption maximum at low ( < 10.0 mT) magnetic fields, decreases the amplitude of the emission maximum and slightly shifts its position, and changes the sign from emission to absorption in the high-field region of CIDNP field dependence. The magnetic field where the polarization of a-CH 2 protons changes its sign decreases with increasing scavenger concentration. For •-CH2, as well as for 31- and ~,e-CH 2 protons, the polarization exhibits an emissive phase for all studied magnetic fields. In all cases the field dependences of CIDNP of all products of the reactions with scavenger show emission maxima within 20.0-25.0 and 12.5-15.0 mT during cycloundecanone and cyclododecanone photolysis, respectively. The presence and coincidence of emission maxima for all reaction products can readily be accounted for in terms of the S - T _ mechanism of CIDNP formation. The S - T _ intersystem crossing involves simultaneous flips of electron and one of
164
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
the nuclear spins from a to fl spin states ( T _ a states transfer to S/3 irrespective of the sign of hfi constant). In this case, singlet biradicals are formed with fl nuclear-spin projections, while triplet biradicals still remain with excess of fl projections. The singlet biradicals can recombine from the polymethylene chain conformation at a sufficiently small distance between radical centers to yield the initial compound. Both singlet and triplet biradicals enriched during the T _ - S transition w i t h / 3 projections are involved in the scavenging reaction. Hence, after the reaction with scavenger the radicals also exhibit a negative nuclear polarization. It is obvious that the CIDNP kinetics as well as the ISC is strongly dependent on the external mag-
°.2t~ o.o~+-~e~ o. .~ g ;. ,~ -0.2~ . ~* -0.41 -0.6 • " -0. . • -1. '~" o.00 0.to 0.04 o.0s Magnetic field, T
0.0
:+ -0.2 <+
. ,^,.,, . . . . .
0.0 ~ o [] o :o II ~ -o.2-'*"*,~ ~• ;- -0.¢ " . " ¢~ -0.s • " -0.8 "
,,~,=,,*,,
0.00
°a+xaaaaa'~ . _ _
"
"'''
--0.4
rO -0.6
"0.10 ~e," -0.15/
0.oo o.04 o.= ra,mm¢~da.x
o.o8
++
.
....
•
"m•.~ 0.0~
0.04
0.06
0.08
Magnetic field, T
.'''" ,
"0.051 ~
c
-1.0 0.2 ~IA#~A
O.lS~ : o.,o!=+~o.," =, ~"" ~ o= .= 0"
d Fig. 4 (continued).
"
N
-0.8
M
-1.0
=t j= N ,
0.00
,
0.02
,
,
0.04
0.06
0.08
Magnetic field, T
a
0.0 ~.,,,,,, , ~ + , , + , , ~~ _i ~m~ ~ eA&~m~, • a, ,, •.. • • -0.4 • -0.6 .+ +,, -0.8 • ,,
-0.2
netic field. Since there are no direct methods for measuring the kinetics of nuclear polarization formation for low and intermediate (up to 100 mT) magnetic fields, the kinetics of CIDNP formation in biradicals near the maxima of its field dependences has not yet been studied for the above range of magnetic fields. The investigation of the CIDNP amplitude as a function of scavenger concentration yields information on biradical lifetime. One can assume that the CIDNP formation near its emissive maxima is based on the exponential time law
lu •
-1.0 . . . . . . . . 0.00
0.02
0.04
0.06
0.08
Magnetic field, T b
Fig. 4. (a) CIDNP field dependences for a-CH 2 protons of CnH200; ~r in the absence of CBrCI3; • i n the presence of 0.008M CBrCI3; zx in the presence of 0.016M CBrCl 3. (b) CIDNP field dependences for/3-CH 2 protons of CnH200; ~ in the absence of CBrC13; • in the presence of 0.008M CBrCI3; zx in the presence of 0.016M CBrCI 3. (c) CIDNP field dependences for a-CH 2 protons of C12H220; Sr in the absence of CBrCI3; • in the presence of 0.01M CBrCI3; zx in the presence of 0.04M CBrC13; [] in the presence of 0.08M CBrCI 3. (d) CIDNP field dependences for ot-CH2 protons of C12H220; ~ in the absence of CBrCI3; • in the presence of 0.01M CBrCI3; t, in the presence of 0.04M CBrC3; [] in the presence of 0.08M CBrCI 3.
I ( t ) = Io[1 - exp( - t/z)]
(4)
where ~" is the characteristic lifetime of biradical. I 0 is the stationary nuclear polarization. This assumption is reasonable because for similar system it was experimentally found that the CIDNP evolution really c a n b e d e s c r i b e d f o r dominating S - T _ mechanism in g o o d approximation by an exponential runetion [19]. In the presence of the biradical s c a v e n g e r the CIDNP amplitude can b e described by the Stern-Volmer dependence: 1o
+"
-/s(';'s) - = I + - - = I +"rsT k + C
s, I o=I(C+=O). (5)
165
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
In the frames of this model the CIDNP amplitude dependence on scavenger concentration in geminate recombination could be used to obtain information about biradical lifetimes at a low magnetic field from Stern-Volmer plot. Experimental values presented in Fig. 5a show significant deviation from linear behavior. Only near the emissive maxima the dependence can be approximated by a linear fit (Fig. 5b). The times corresponding to the linear approximation are 120 _ 24 ns for cycloundecanone and 45 + 9 ns for cyclododecanone. At the higher scavenger concentrations C s the deviation of the curve from the linear Stern-Volmer is more pronounced, in particular for a-CH 2 protons at the highest concentration at the magnetic field higher than 30 mT the polarization changes its sign (see Fig. 4a and Fig. 4c) to an absorptive phase and becomes opposite to the polar-
6 5
*
w~
•
3 - 2
~
-
°~ 0.0
10-
~
• ~ '~
~ ~ , 5.~0-3 c~, M a
/
. 1.~0-2
_~
s
0 0.0
4.0x102 8.0~0"2 E,, M b Fig. 5. (a) Stern-Volmer dependence for the CIDNP amplitude for CllH20 O at the magnetic field 32.4 mT; • for ot-CH 2 protons; r, for T-CH2 protons. (b)Stem-Volmer dependencefor the CIDNP amplitude; zx for /3-CH2 protons of CllH2oO at the magnetic field 25 mT; [] for y-CH2 protons of C12H220 at the magnetic field 15 mT.
0.5 ~ s'T°s'T° \~__
\\
~ 0.0 ~ ~ -0.5 s-x_/ ~ -1.0 0
~
200
i
400 time, ns
i
600
Fig. 6. Schematic presentation of CIDNP kinetics formed by S-To and S-T_ mechanismsin biradicals at low magnetic fields, and the resulting curve for nuclei with negativehfi constant.
ization of /3-CH 2 protons. These results are consistent with the assumption that in the above range the magnetic field the relative contribution of the S - T 0 transition becomes more important as the scavenger concentration increases. We find positive polarization for a-CH 2 protons and negative for fl-CH 2 protons and only the S - T 0 contribution into intersystem crossing depends on the sign of the hypeffine cou pli ng constants. As it has been shown [19], the kinetics of CIDNP formed by the S - T 0 transitions goes through a maximum (emissive for the protons with positive hyperfine coupling constant and absorptive for those with a negative constant), and decreases for a longer time, while the S - T _ contribution asymptotically reaches its maximum (emissive) value, as schematically presented in Fig. 6. If we take for S - T o and for S - T _ CIDNP kinetics the curves shown in Fig. 6, then the influence of the scavenger is more pronounced for the S - T _ contribution. The field dependences of the CH2Br protons of compound IV (Scheme 2) support unambiguously the assumption about the two channels of singlet-triplet conversion contributing to the CIDNP pattern. These species correspond to the a-protons of the alkyl biradical end. In full accordance with the consideration described in Section 3.3, the polarization of scavenging products by the S - T 0 channel is opposite to the polarization of corresponding protons of the geminate products. Thus, the CIDNP of product IV has an emissive phase for all magnetic fields, while the polarization of cycloundecanone and cyclododecane a-CH 2 protons changes its sign twice.
A. Yurkovskaya et al. / Chemical Physics 197 (1995) 157-166
166
However, in the case of nuclear polarization formation only by the S - T _ channel, the sign of polarization with increasing scavenger concentration would coincide for both products and remain unchanged. A kinematic approximation has recently been proposed [20] for the description of magnetic and spin effects in chemical reactions. This approximation has been used to describe the CIDNP kinetics for biradicals under strong magnetic fields. Now this approach
is being extended to the description of the CIDNP of biradicals under weak magnetic fields. Numerical calculations performed in terms of this theoretical approach are to be published in a forthcoming paper.
4. Conclusion Investigation of the CIDNP field dependences formed during photolysis of cycloundecanone and cyclododecanone in the presence of the scavenger C B r C I 3 a l l o w s the analysis of the kinetics of biradical geminate recombination. It is demonstrated that the biradical scavenging rate constant can be determined using t h e k i n e t i c data f o r high magnetic fields. The scavenging rate constant of CBrC13 estimated by this method is (2.3 + 0 . 4 ) × 109 M -1 s -1. Two channels of singlet-triplet conversion have been revealed and the qualitative picture of their contribution to the kinetics of the CIDNP at low magnetic fields has been presented.
Acknowledgements
Deutsche Forschungsgemeinschaft, (Sfb 337), for financial support.
References [1] K.M. Salikhov, Yu.N. Molin, R.Z. Sagdeev and A.L. Buchachenko, Spin polarization and magnetic effects in radical reactions, ed. Yu.N. Molin (Elsevier, Amsterdam, 1984).
[2] U.E. Steinerand H.-J. Wolff, in: Photochemistryand photophysics, Vol. 4, eds. J.F. Rabek and G.W. Scott (CRC Press,
Boca Raton, 1991)p. 1. [3] G.L. Closs and C.E. Doubleday, J. Am. Chem. Soc. 94 (1972) 9248. [4] G.L. Closs and C.E. Doubleday, J. Am. Chem. Soc. 95 (1973) 2735. [5] C. Doubleday Jr., Chem. Phys. Letters 77 (1981) 131. [6] C. Doubleday Jr., Chem. Phys. Letters 81 (1981) 164. [7] F.J.J. de Kanter and R. Kaptein, J. Am. Chem. Soc. 104 (1982) 4759. [8] F.J.J. de Kanter, J.A. den Hollander, A.H. Huiser and R. Kaptein, Mol. Phys. 34 (1977) 857. [9] J. Allgeier, G. Buntkowsky, S. Hentrich, M. Nack and H.-M. Vieth, Ber. Bunsenges. Physik. Chem. 93 (1989) 1281. [10] E.G. Bagryanskaya and R.Z. Sagdeev, Progr. Reaction Kinet. 18 (1993) 63. [11] H. Fischer, ed., Magnetic properties of free radicals, Landolt-B6rnstein New Series, Voi. II 9b (Springer, Berlin,
1980). [12] A.V. Dushkin, Yu.A. Grishin and R.Z. Sagdeev, Chem. Phys. Letters55 (1978) 174. [13] F.J.J. de Kanter and R. Kaptein, J. Am. Chem. Soc. 94 (1972) 6269. [14] Kuo Chu Hwang, N.J. Turro and C. Doubleday Jr., J. Am. Chem. Soc. 113 (1991) 2850. [15] N.J. Turro, Kuo Chu Hwang, V. Pushkara Rao and C. Doubleday Jr., J. Phys. Chem. 95 (1991) 1872. [16] N. Han, Kuo Chu Hwang, Xuegong Lei and N.J. Turro, J. Photochem. Photobiol. A 61 (1991) 35.
The authors are grateful to Yu Grishin, M. Benkert and A. PfivaIov for technical assistance in the modi-
fication of the experimental setup in Berlin, to N. Lukzen for fruitful discussion, to E. Bagryanskaya for her assistance with flow system operation. The authors thank the Russian Foundation for Fundamental R e s e a r c h (Project number 93-03-18593) a n d
[17] A.V. Yurkovskaya, Yu.P. Tsentalovich, N.N. Lukzen and R.Z. Sagdeev, Res. Chem. Intermed. 17 (1992) 145. [18] Yu.P. Tsentalovich, A.V. Yurkovskaya, R.Z. Sagdeev, A.A. Obynochny, P.A. Purtov and A. Shargorodsky, Chem. Phys. 139 (1989) 307. [19] G.I. Closs and O.D. Redwine, J. Am. Chem Soc. 107 (1985) 6131. [20] P.A. Purtov and A.B. Doktorov, Chem. Phys. 178 (1993) 47.