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29Si magnetic isotope effects in the photolysis of silyl ketones
CBEMICAL
Volume 144, number 56
PHYSICS LETTERS
11 March 1988
29Si MAGNETIC ISOTOPE EFFECTS IN THE PHOTOLYSIS OF SILYL KETONES E.N. STEP, V.F. TARASOV and A.L. BUCHACHENKO Institute of Chemical Physics, USSR Academy of Sciences, Kosygin Street 4, I I7334 Moscow, USSR Received 11 November 1987
Silicon isotope selection induced by magnetic isotope effects has been discovered in the photolysis of a silyl-containing ketone, PhCH,COSi( CH,),Ph. Silicon and carbon atom isotope selection occurs most effkiently when the photolysis is directed via the triplet radical-pair mechanism. This conclusion is supported by CIDNP data.
1. Introduction
is given in ref. [ 31 and that of ketone III,
The magnetic isotope effect (MIE), i.e. the dependence of the rate of chemical interaction between radicals on the magnetic moment and the spin of their nuclei, results in a redistribution of isotopes among the initial reagents and products of a chemical reaction. The effect was discovered and studied for carbon and oxygen isotopes [ 1,2]. It has been confirmed experimentally that the highest efficiency of selection is to be expected in reactions involving triplet geminate radical pairs when the cage effect is large. These requirements are best satisfied by the photolysis of alkylaromatic ketones in micelles [ 21. With heavier nuclei the isotope selection caused by MIE is considered to be problematic due to a strong spin-orbit coupling which is believed to compete successfully with hyperfine interaction (hfi) in inducing singlet-triplet intersystem crossing of radical pairs. In our view, however, the main problem is to adapt the chemistry in such a way as to create the most favourable conditions for MIE. Our experience in detecting MIE with *‘Si isotopes supports this assumption.
2. Experimental The synthesis of ketones I and II, 0
0 Ph&(CH&Ph I
Ph!Si(CHj)IPh,
0 PhCH,!Si(CH,),Ph, III
in ref. [ 41, as well as the photolysis conditions and the technique of chromatographic analysis of the products. For carbon isotope analysis the ketone isolated after the photolysis by HPLC was burned to COz [ 31. The isotope ratios were measured using a Varian MAT-250 mass spectrometer. For silicon isotope analysis the ketone was converted into SiOs by treating it with oleum and annealing at 800°C. SiOZ was then fluorinated and the isotope ratios of SiF, were measured with the mass spectrometer.
3. Results and discussion MIE and C and Si isotope selection were studied during the photolysis of ketones I and II in the micelle Triton X-100. Photolysis of I occurs via a Norrish type-1 mechanism from the triplet excited state, yielding a triplet radical pair 31PhCO C( CH,) zPh I. The intersystem crossing in this pair is induced mainly by the hfl on H atoms of the cumyl radical methyl groups and the 13Catom of the benzoyl radical carbonyl group. The initial ketone is thus enriched by r3C isotope with a selection coefficient (Y= 1.18 [ 31. This value, although smaller than that observed during the pho-
II
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[ 1,2], still exceeds considerably the kinetic isotope effect. The study of isotope selection with 13C and 29Si during the photolysis of II reveals that substitution of C by Si is not enough to guarantee MIE. In fact the variation of the 13Cisotope content in the initial ketone (a= 1.033) does not exceed the values expected from the kinetic isotope effect. The Si isotope abundance in ketone II before and after the photolysis did not change [ 31. This can be explained by the fact that in the photolysis of II the main condition for efficient MIE-induced isotope selection is not fulfilled, i.e. the reaction does not occur via radical pairs (RP). The absence of chemically induced dynamic nuclear polarization (CIDNP) and the structure of products suggest that similarly to other silyl ketones [ 51 the photolysis of II occurs through isomerization into siloxycarbene tolysis of dibenzyl ketone in similar conditions
0 _&_Si< + _COSi< rather than via photodissociation yielding RP. To divert the reaction route from the carbene mechanism and direct it through radical pairs one should modify the chemical structure of ketone II. For this purpose we replaced the benzoyl group by phenacyl counting on the predominance of the dissociation reaction over the isomerization into siloxycarbene. We synthesized benzyl (dimethylphenylsilyl) ketone (III) and studied its photolysis. The photolysis products (see table 1a) indicate that indeed the main path of photodecomposition of III is Norrish type-1 dissociation. This conclusion is confirmed by CIDNP of ‘H and *3C observed during the photolysis of III in benzene, cyclohexane and methanol. A strong polarization is exhibited by the protons of the CHI group (A) and of the 13Cnucleus of CHI groups (E), the phenyl ring of the benzyl group (A) and the carbonyl carbon (A) of the starting ketone. In the photolysis products nuclear polarization appears in PhCH,Si( CH3)zPh ( CH2 protons are A polarized) and 13C in carbon monoxide (E). The methyl protons at Si are practically non-polarized both in III and in PhCH2SiCH3)zPh [ 61. The CIDNP picture and the composition of the products show unambiguously that direct photolysis occurs through the singlet excited state to yield a sin524
11 March 1988
glet geminate RP via dissociation of the C-C bond: 0 PhCH2/!Si(CH3)ZPh hu +’
I
PhCH* CSi( CH3)2Ph 1 -+ products . b
The main photolysis products are formed in the cage reactions of decarbonylation and recombination. The CIDNP of the initial ketone and the photolysis products are also generated in the geminate pair [6]. The kinetic isotope effect in the bond rupture reaction is known to enrich ketone III in isotopes 13C, 29Siand 3oSi.Table 2 (direct photolysis) shows that with the carbon the efficiency of MIE in singlet RP is so insignificant that the kinetic isotope effect prevails leading in the course of photolysis to the enrichment of III by 13C with a= 1.032. The 3oSi content is also increased slightly due to the kinetic isotope effect arising during the rupture of the Si-CO bond in the COSi(CHS)zPh radical in secondary decarbonylation cage reactions. It is only with 29Si (table 3, direct photolysis) that MIE predominates over the kinetic isotope effect and depletes III in 29Si. In this manner, through modifying the structure of the silyl ketone, we directed its photodecomposition via the radical route. However, it turned out that this path starts from the singlet excited state of the ketone to yield singlet RP which are unfavourable for isotope selection induced by MIE [ 11. To make the reaction follow the triplet channel we used triplet sensitizers (acetophenone and triphenylene). The sensitized photolysis of III exhibits a complete inversion of the CIDNP signs for the initial ketone [ 61, indicating that we managed to direct the photolysis along the triplet channel. There is a considerable difference in the composition of the products of the sensitized and direct photolyses of III (see table 1). With the direct photolysis the radical mechanism is almost the only one, the main products being formed in cage RP reactions. With the sensitized photolysis in micelles of sodium dodecylsulfate (SDS) a considerable fraction of the products are siloxy compounds Ph( CHs),SiOH and Ph( CH3),SiOSi( CH3)?Ph -
11 March 1988
CHEMICAL PHYSICS LETTERS
Volume 144, number $6
Table 1 The product yield during the photolysis of III. (a) Direct photolysis; (b) photolysis sensitized by acetophenone Yield (%)
Product
(a)
benzene
methanol
SDS micelles
PhCH&(CH&Ph PhCH,CH,Ph Ph(CH&SiPh Ph(CH3)$iOH
43 9 12
55 2
84
Product
Yield (%)
6
PhCH,Si( CH,),Ph PhCHzCH2Ph Ph(CH&SiPh Ph(CH,),SiOH Ph( CH&SiOSi( CH,),Ph ‘) Sensitizer triphenylene.
benzene a)
methanol
12 16
6 13 17
10
4
b) 5
38 7
‘) The yield was not measured.
which are formed from the unstable product of insertion of siloxycarbene PhCH$OSi( CH3)*Ph into the O-H bond of water [ 51. Only about a third of the triplet photodissociation of III in the SDS micelles occurs via the radical-pair mechanism, the other two thirds occurring through the carbene mechanism [ 41. The isotope analysis of carbon during the sensitized photolysis of III in SDS micelles (table 2) revealed a significant efficiency increase for i3C isotope selection (cy= 1.086). Taking into account that the radical route in the sensitized photolysis occurs only Table 2 Variation of the carbon isotope abundance in ketone III during the photolysis in micelles as compared with the isotope abundance of the initial ketone Conversion (%) 0
34.6 37.0 50.5 54.3 57.0 70.3
Direct photolysis a) d(13C) (S)
Sensitized photolysis w J(W) (%a)
0
0
1.93 0.88 1.02 1.77 _
SDS micelles
benzene
one third of the time and recalculating Q for this path one finds that (Y= 1.26 which is comparable with CY for the enrichment of ketone I and is close to the cy value for dibenzylketone [ 1,2]. Table 3 shows the variation of the Si isotope content under the sensitized photolysis of III. Here, as with the direct photolysis, one observes some enrichment of the initial ketone by 3oSi, which is due to the kinetic isotope effect. At the same time ketone III is considerably enriched in the magnetic isotope 29Si (nuclear spin l/2, magnetic moment -0.555), the enrichment being many times larger than the contribution induced by the classical isotope effect (compare with the “Si isotope). With the sensitized photolysis the (Yvalue is 1.023. Once again taking into account that the radical path only occurs one third of the time and recalculating a(29Si) for this path we obtain cr(29Si) = 1.07 which is lower than (w(“C)=1.26. The difference, in our view, stems from the fact that the HFI in 0
4.77
*!Xi(CH,J2Ph
5.04
radicals is different for 13Cand 29Si nuclei: a( “C) being 120-l 30 G as with typical acyl a-radicals, while a(29Si) is certainly to be lower. For this reason the
a) Photolysis in micelles of Triton X-100. ‘) Photolysis in SDS micclles with acetophenone as the sensitizer.
,525
11 March 1988
CHEMICAL PHYSICS LETTERS
Volume 144, number 56
Table 3 Variation of the silicon isotope abundance in ketone III during the photolysis in micelles as compared to the isotope abundance of the initial ketone Conversion (%)
50.6 56.0 61.3
Sensitized photolysis b,
Direct photolysis ‘) 6(“Si) (960)
S(“Si) (%I)
-2.2
2.5
6(2gSi) (%a)
d(‘OSi) (%a)
15.7
4.0
26.2
3.8
8’Photolysis in micelles of Triton X-l 00. b, Photolysis in SDS micelles with acetophenone as sensitizer.
magnitude of MIE and consequently the efficiency of Si isotope selection in this RP is lower than for carbon isotopes. A similar effect - different enrichment coefficient for carbon atoms in different positions of a molecule and, consequently, different hti constants in the formed RP - has been observed during the photolysis of dibenzyl ketone [ 731. Thus, we have demonstrated that during the photolysis of the !&containing alkylaromatic ketone III there is an isotope selection for both carbon and silicon. The efficiency of selection was shown to considerably exceed the kinetic isotope effect. The results obtained make it clear that at least with silicon atoms the role of spin-orbit coupling in the intersystern crossing in radical pairs is not significant. This lends credence to the possibility of selecting isotopes and detecting MIE with heavier elements.
526
Acknowledgement The authors thank Professor E.M. Galimov, Professor V.A. Grylenko and Dr. V.I. Ustinov from the Institute of Geochemistry and Analytical Chemistry of the Academy of Sciences of the USSR for carrying out the high-quality isotope analyses.
References [ 1 ] A.L. Buchachenko, Progr. Reaction Kinetics 13 (1983) 163. [2] N.J. Turro, Ann. Rev. Phys. Chem. 18 (1984) 1. [ 31 E.N. Step, V.F. Tarasov and A.L. Buchachenko, Russian .I. Gen. Chem. 55 (1985) 2348. [4] E.N. Step, V.F. Tarasov and A.L. Buchachenko, Izv. Akad. Nauk SSSR. Ser. Kbim., to be published. [ 51 J.M. Duff and A.G. Brooks, Can. J. Chem. 51 (1973) 2869. [6] E.N. Step, V.F. Tarasov and A.Z. Yankelevich, Kbim. Fiz. (1987), to be published. [7] N.J. Turro, C.-J. Chung, P.J. Layler and W.J. Smith, Tetrahedron Letters 23 (1982) 3223. [ 81 V.F. Tarasov and A.L. Buchachenko, Izv. Akad. Nauk SSSR. Ser. Kbim. (1983) 86.
Volume 144, number 56
PHYSICS LETTERS
11 March 1988
29Si MAGNETIC ISOTOPE EFFECTS IN THE PHOTOLYSIS OF SILYL KETONES E.N. STEP, V.F. TARASOV and A.L. BUCHACHENKO Institute of Chemical Physics, USSR Academy of Sciences, Kosygin Street 4, I I7334 Moscow, USSR Received 11 November 1987
Silicon isotope selection induced by magnetic isotope effects has been discovered in the photolysis of a silyl-containing ketone, PhCH,COSi( CH,),Ph. Silicon and carbon atom isotope selection occurs most effkiently when the photolysis is directed via the triplet radical-pair mechanism. This conclusion is supported by CIDNP data.
1. Introduction
is given in ref. [ 31 and that of ketone III,
The magnetic isotope effect (MIE), i.e. the dependence of the rate of chemical interaction between radicals on the magnetic moment and the spin of their nuclei, results in a redistribution of isotopes among the initial reagents and products of a chemical reaction. The effect was discovered and studied for carbon and oxygen isotopes [ 1,2]. It has been confirmed experimentally that the highest efficiency of selection is to be expected in reactions involving triplet geminate radical pairs when the cage effect is large. These requirements are best satisfied by the photolysis of alkylaromatic ketones in micelles [ 21. With heavier nuclei the isotope selection caused by MIE is considered to be problematic due to a strong spin-orbit coupling which is believed to compete successfully with hyperfine interaction (hfi) in inducing singlet-triplet intersystem crossing of radical pairs. In our view, however, the main problem is to adapt the chemistry in such a way as to create the most favourable conditions for MIE. Our experience in detecting MIE with *‘Si isotopes supports this assumption.
2. Experimental The synthesis of ketones I and II, 0
0 Ph&(CH&Ph I
Ph!Si(CHj)IPh,
0 PhCH,!Si(CH,),Ph, III
in ref. [ 41, as well as the photolysis conditions and the technique of chromatographic analysis of the products. For carbon isotope analysis the ketone isolated after the photolysis by HPLC was burned to COz [ 31. The isotope ratios were measured using a Varian MAT-250 mass spectrometer. For silicon isotope analysis the ketone was converted into SiOs by treating it with oleum and annealing at 800°C. SiOZ was then fluorinated and the isotope ratios of SiF, were measured with the mass spectrometer.
3. Results and discussion MIE and C and Si isotope selection were studied during the photolysis of ketones I and II in the micelle Triton X-100. Photolysis of I occurs via a Norrish type-1 mechanism from the triplet excited state, yielding a triplet radical pair 31PhCO C( CH,) zPh I. The intersystem crossing in this pair is induced mainly by the hfl on H atoms of the cumyl radical methyl groups and the 13Catom of the benzoyl radical carbonyl group. The initial ketone is thus enriched by r3C isotope with a selection coefficient (Y= 1.18 [ 31. This value, although smaller than that observed during the pho-
II
03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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CHEMICALPHYSICSLETTERS
[ 1,2], still exceeds considerably the kinetic isotope effect. The study of isotope selection with 13C and 29Si during the photolysis of II reveals that substitution of C by Si is not enough to guarantee MIE. In fact the variation of the 13Cisotope content in the initial ketone (a= 1.033) does not exceed the values expected from the kinetic isotope effect. The Si isotope abundance in ketone II before and after the photolysis did not change [ 31. This can be explained by the fact that in the photolysis of II the main condition for efficient MIE-induced isotope selection is not fulfilled, i.e. the reaction does not occur via radical pairs (RP). The absence of chemically induced dynamic nuclear polarization (CIDNP) and the structure of products suggest that similarly to other silyl ketones [ 51 the photolysis of II occurs through isomerization into siloxycarbene tolysis of dibenzyl ketone in similar conditions
0 _&_Si< + _COSi< rather than via photodissociation yielding RP. To divert the reaction route from the carbene mechanism and direct it through radical pairs one should modify the chemical structure of ketone II. For this purpose we replaced the benzoyl group by phenacyl counting on the predominance of the dissociation reaction over the isomerization into siloxycarbene. We synthesized benzyl (dimethylphenylsilyl) ketone (III) and studied its photolysis. The photolysis products (see table 1a) indicate that indeed the main path of photodecomposition of III is Norrish type-1 dissociation. This conclusion is confirmed by CIDNP of ‘H and *3C observed during the photolysis of III in benzene, cyclohexane and methanol. A strong polarization is exhibited by the protons of the CHI group (A) and of the 13Cnucleus of CHI groups (E), the phenyl ring of the benzyl group (A) and the carbonyl carbon (A) of the starting ketone. In the photolysis products nuclear polarization appears in PhCH,Si( CH3)zPh ( CH2 protons are A polarized) and 13C in carbon monoxide (E). The methyl protons at Si are practically non-polarized both in III and in PhCH2SiCH3)zPh [ 61. The CIDNP picture and the composition of the products show unambiguously that direct photolysis occurs through the singlet excited state to yield a sin524
11 March 1988
glet geminate RP via dissociation of the C-C bond: 0 PhCH2/!Si(CH3)ZPh hu +’
I
PhCH* CSi( CH3)2Ph 1 -+ products . b
The main photolysis products are formed in the cage reactions of decarbonylation and recombination. The CIDNP of the initial ketone and the photolysis products are also generated in the geminate pair [6]. The kinetic isotope effect in the bond rupture reaction is known to enrich ketone III in isotopes 13C, 29Siand 3oSi.Table 2 (direct photolysis) shows that with the carbon the efficiency of MIE in singlet RP is so insignificant that the kinetic isotope effect prevails leading in the course of photolysis to the enrichment of III by 13C with a= 1.032. The 3oSi content is also increased slightly due to the kinetic isotope effect arising during the rupture of the Si-CO bond in the COSi(CHS)zPh radical in secondary decarbonylation cage reactions. It is only with 29Si (table 3, direct photolysis) that MIE predominates over the kinetic isotope effect and depletes III in 29Si. In this manner, through modifying the structure of the silyl ketone, we directed its photodecomposition via the radical route. However, it turned out that this path starts from the singlet excited state of the ketone to yield singlet RP which are unfavourable for isotope selection induced by MIE [ 11. To make the reaction follow the triplet channel we used triplet sensitizers (acetophenone and triphenylene). The sensitized photolysis of III exhibits a complete inversion of the CIDNP signs for the initial ketone [ 61, indicating that we managed to direct the photolysis along the triplet channel. There is a considerable difference in the composition of the products of the sensitized and direct photolyses of III (see table 1). With the direct photolysis the radical mechanism is almost the only one, the main products being formed in cage RP reactions. With the sensitized photolysis in micelles of sodium dodecylsulfate (SDS) a considerable fraction of the products are siloxy compounds Ph( CHs),SiOH and Ph( CH3),SiOSi( CH3)?Ph -
11 March 1988
CHEMICAL PHYSICS LETTERS
Volume 144, number $6
Table 1 The product yield during the photolysis of III. (a) Direct photolysis; (b) photolysis sensitized by acetophenone Yield (%)
Product
(a)
benzene
methanol
SDS micelles
PhCH&(CH&Ph PhCH,CH,Ph Ph(CH&SiPh Ph(CH3)$iOH
43 9 12
55 2
84
Product
Yield (%)
6
PhCH,Si( CH,),Ph PhCHzCH2Ph Ph(CH&SiPh Ph(CH,),SiOH Ph( CH&SiOSi( CH,),Ph ‘) Sensitizer triphenylene.
benzene a)
methanol
12 16
6 13 17
10
4
b) 5
38 7
‘) The yield was not measured.
which are formed from the unstable product of insertion of siloxycarbene PhCH$OSi( CH3)*Ph into the O-H bond of water [ 51. Only about a third of the triplet photodissociation of III in the SDS micelles occurs via the radical-pair mechanism, the other two thirds occurring through the carbene mechanism [ 41. The isotope analysis of carbon during the sensitized photolysis of III in SDS micelles (table 2) revealed a significant efficiency increase for i3C isotope selection (cy= 1.086). Taking into account that the radical route in the sensitized photolysis occurs only Table 2 Variation of the carbon isotope abundance in ketone III during the photolysis in micelles as compared with the isotope abundance of the initial ketone Conversion (%) 0
34.6 37.0 50.5 54.3 57.0 70.3
Direct photolysis a) d(13C) (S)
Sensitized photolysis w J(W) (%a)
0
0
1.93 0.88 1.02 1.77 _
SDS micelles
benzene
one third of the time and recalculating Q for this path one finds that (Y= 1.26 which is comparable with CY for the enrichment of ketone I and is close to the cy value for dibenzylketone [ 1,2]. Table 3 shows the variation of the Si isotope content under the sensitized photolysis of III. Here, as with the direct photolysis, one observes some enrichment of the initial ketone by 3oSi, which is due to the kinetic isotope effect. At the same time ketone III is considerably enriched in the magnetic isotope 29Si (nuclear spin l/2, magnetic moment -0.555), the enrichment being many times larger than the contribution induced by the classical isotope effect (compare with the “Si isotope). With the sensitized photolysis the (Yvalue is 1.023. Once again taking into account that the radical path only occurs one third of the time and recalculating a(29Si) for this path we obtain cr(29Si) = 1.07 which is lower than (w(“C)=1.26. The difference, in our view, stems from the fact that the HFI in 0
4.77
*!Xi(CH,J2Ph
5.04
radicals is different for 13Cand 29Si nuclei: a( “C) being 120-l 30 G as with typical acyl a-radicals, while a(29Si) is certainly to be lower. For this reason the
a) Photolysis in micelles of Triton X-100. ‘) Photolysis in SDS micclles with acetophenone as the sensitizer.
,525
11 March 1988
CHEMICAL PHYSICS LETTERS
Volume 144, number 56
Table 3 Variation of the silicon isotope abundance in ketone III during the photolysis in micelles as compared to the isotope abundance of the initial ketone Conversion (%)
50.6 56.0 61.3
Sensitized photolysis b,
Direct photolysis ‘) 6(“Si) (960)
S(“Si) (%I)
-2.2
2.5
6(2gSi) (%a)
d(‘OSi) (%a)
15.7
4.0
26.2
3.8
8’Photolysis in micelles of Triton X-l 00. b, Photolysis in SDS micelles with acetophenone as sensitizer.
magnitude of MIE and consequently the efficiency of Si isotope selection in this RP is lower than for carbon isotopes. A similar effect - different enrichment coefficient for carbon atoms in different positions of a molecule and, consequently, different hti constants in the formed RP - has been observed during the photolysis of dibenzyl ketone [ 731. Thus, we have demonstrated that during the photolysis of the !&containing alkylaromatic ketone III there is an isotope selection for both carbon and silicon. The efficiency of selection was shown to considerably exceed the kinetic isotope effect. The results obtained make it clear that at least with silicon atoms the role of spin-orbit coupling in the intersystern crossing in radical pairs is not significant. This lends credence to the possibility of selecting isotopes and detecting MIE with heavier elements.
526
Acknowledgement The authors thank Professor E.M. Galimov, Professor V.A. Grylenko and Dr. V.I. Ustinov from the Institute of Geochemistry and Analytical Chemistry of the Academy of Sciences of the USSR for carrying out the high-quality isotope analyses.
References [ 1 ] A.L. Buchachenko, Progr. Reaction Kinetics 13 (1983) 163. [2] N.J. Turro, Ann. Rev. Phys. Chem. 18 (1984) 1. [ 31 E.N. Step, V.F. Tarasov and A.L. Buchachenko, Russian .I. Gen. Chem. 55 (1985) 2348. [4] E.N. Step, V.F. Tarasov and A.L. Buchachenko, Izv. Akad. Nauk SSSR. Ser. Kbim., to be published. [ 51 J.M. Duff and A.G. Brooks, Can. J. Chem. 51 (1973) 2869. [6] E.N. Step, V.F. Tarasov and A.Z. Yankelevich, Kbim. Fiz. (1987), to be published. [7] N.J. Turro, C.-J. Chung, P.J. Layler and W.J. Smith, Tetrahedron Letters 23 (1982) 3223. [ 81 V.F. Tarasov and A.L. Buchachenko, Izv. Akad. Nauk SSSR. Ser. Kbim. (1983) 86.