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Carbon-13 CIDNP from biradicals in the photolysis of cyclic ketones
Volume 26, number 1
CHEMICAL PHYSICS LETTERS
CARBON-13
1 May
1974
CIDNP FROM BIRADICALS
IN THE PHOTOLYSIS OF CYCLIC KETONES R. KAPI’EINI,
R. FREEMAN+?
and H.D.W. HILL
Varian Associates, Instrument Division. Palo Alto, California 94303, USA
Received 11 February 1974
The observation of chemically induced dynamic nuclear polarization (CIDNP) in 13C Fourier transform NMR spectra taken during the photolysis of cyclic ketones is direct evidence for the intermediacy of biradicals. Cycloheptanone showed the strongest emission effects- Polariition from 1,5-bimdicals has been observed in the photolysis of camphor and fendtone.
1. Introduction The photolysis of cyclic ketones in solution yields unsaturated atdehydes and ketenes [I]. Mechanistic studies of this reaction have directed the attention to the nature of the excited states involved and to the question whether the products are formed in a concerted process or via intermediate biradicals. From quenching studies [2] it is known that an n, x* triplet state is involved, which is completely quenchable in many cases. Although a concerted reaction has been proposed [3], most authors now favour a biradical mechanism, This has been concluded from the kinetics of product formation and ketone consumption [4], and from the product distributions of methyland deuterium-substituted cyclic alkanones [5]. The observation of CIDNP provides direct evidence for the intermediacy of short-lived biradicals, which are not easily detected by other physical techniques. Proton CIDNP in similar cyclic ketones has been reported by Gloss [6]. There are two kinds ofCIDNP from biradicals 17, 8]_ One is similar to CIDNP from freely diffusing radical pairs and is based upon mixing of the electronic L r Present address: KonMclijke/SheU-Ioratorium, dam, The Netherlands. tt Present address: PhysicalChemistry .University, ?xford,‘UK_
Arnster-
Laboratory, Oxford
singlet state with the triplet state, which has GT,>= 0 (S-T, mixing), induced by the hyperfine interaction of nuclei with electrons. The spin-selective nature of this mechanism requires a reaction path competitive with the self-reaction of the biradical [7]. The other type is due to S-T_ mixing (T_: triplet state with cS,> = -1) 181. This mechanism may become effective when S and T_ states are (nearly) degenerate, i-e., when the average exchange interaction is equal to the Zeeman interaction 2v) =gpHO. When the biradical is produced from a triplet precursor and products are formed solely from the singlet state (after intersystem crossing). there is a net change in electron and nuclear spin angular momenta: the selection rule for T_ + S transitions induced by the hyperfme term AS- Zis &Ifs = +l, MT = -1. Thus, this process gives a net flow of nuclear spins from the /tit> state to the I-$> state, which results in purely emissive NMR spectra of the reaction products. This contrasts with the S-T0 mechanism, where both enhanced absorption (A) and emission (E) is to be expected. We have studied the 13C photo-CIDNP spectra of some cyclic alkanones by the Fourier transform method. 13C CIDNP spectra gen&ally contain more information than proton spectra, not only because of the greater chemical shift dispeision of CMR but also because.thti reactions of the carbon skeleton.are: ,monitored in a mdre direct way. in spite of the low natural -‘aburidance ( 121@ and the low sensitivity of the car-
Volume
1 May 1974
CHEMICAL PHYSICS LETTERS
26. number 1
Cycloheptnnone
in CDCI,
aldehyde
I
200
150
50
GO
Fig. 1. Proton-decoupled 13C FT NMR spectrum of cycloheptanone in CDC13 taken during UV irradiation (upper spectrum). comparison the undecoupled (middle) and the normal (lower) spectrum before irradiation are ako shown.
bon- 13 nucleus, I3C photo-CIDNP experiments are feasible and often give enhancements that are larger by an order of magnitude than those of the correspending proton spectra, owing to the higher hyperfiie coupling constants and the longer spin-lattice relaxation times.
2. Results and discussion The spectra were recorded on a Varian XL-100 FT NMR spectrometer modified in order to irradiate the sample in the probe with UV light from a 1000 W PETS high-pressure mercury arc. The light was filtered through an aqueous NISO, solution. Details of the experimental set-up will be presented elsewhere [9]. 33C Ff spectra were taken during an irradiation period of about 4.5 minutes. Data accumulation was started after a pre-irradiation time of 20 seconds. Typical conditions were: pulse repetition time 2 set, flip angle 33’. 128 transients. CDCl, solutions of the following cyclic ketones were studied:
For
The proton-decoupled 25.1 MHz 13C IDNP spectrum taken during irradiation of cycloheptanone I7 in CDCI, is shown in fig. I. For comparison the normal spectrum before irradiation is also shown. Clearly, the strongest emission lines originate from the parent ketone (carbonyl carbon at 6 = 216.7, a-carbon at 6 = 41.3 ppm). The undecoupled spectrum shows that the E line at 203.7 ppm should be assigned to the carbony1 carbon of the alkenal product (doublet splitting +H = 173 Hz). The E line at 175.2 ppm most probably belongs to pentylketene (carbonyl carbon, no protons attached). The ketene was not observed in the proton CIDNP spectrum [6] _The E lines at 46.6 and 115.3 ppm have not been assigned. The latter, however, has the correct chemical shift for the terminal vinylic carbon of the alkenal product_ Scheme 1 gives a summary of the photoreactions of cycloheptanone. After initial excitation of the ketone to the singlet n, P* state rapid intersystem crossing occurs to the triplet state, from which a triplet biradical is formed through ol-cleavage. In order to react internally to give
ketone, alkenal and ketene products the triplet biradical must first cross over t? the singlet state. The strong CIDNP effects show that the hypefine coupkg with nuclear.spins contributes substantially to this intersystem crossing in the biradical. At present nothing is -known
about>he.6fficiency of other mechanisms sych co&pl.ing for these la&e biiadicak ._ ) ’
as spin-orbit
Volume 16,
number 1
CHEMICAL
0 =y
. c
hv
A( f3C). For a freely diffusing radical pair the func-
\
/
tiol’ral dependence may range from A u2 to A2, depending on the lifetime of the radicai pair and the presence of other nuclei. Although for biradicals a quantitative theory does not yet exist, there seems to be little justification for a square-root dependence, since this arises purely from the r3/* time dependence for the re-encounter probability of a diffusing radical pair [ 10, I I]. Perturbation treatments [ 1Z] would yield an AZ dependence. An estimate of theA(13C)‘s for the acyl-aIky1 biradical can be made by anaIogy with known acyl [13] and alkyl [14] radicals:
.
=O
1 May I974
LETTERS
field of 23 kG. Among other factors the polarization depends on the magnitude of the hyperfme coupling constant
=03
-c
PHYSICS
=O
Scheme 1
In the series of cyclic alkanones I, cycloheptanone I7 showed by far the largest polarization. The 4 and S-membered ring ketones 14 and I5 did not show any effects. A small emission for the carbonyl C atom was observed in the case of cycIohexanone Ig. The effects decreased in the series I, to I,,. This maximum in the polarization far I7 may be partly ascribed to differences in quantum yields for the or-cleavage reaction (for instance, the photochemistry of cyclobutanone deviates from that of other cycIic alkanones), but it is mainly due to the fact that for I7 the condition 2<&=&Hl-, is best satisfied, i.e., Zeeman and exchange energies are most ciosely matched in the spectrometer
A(13C):
52G
T’
I
153G
(-)
13.6 G
39.1 G
The largest effect is expected (and observed) for the carbonyl carbon, which has a particularly large hy perfine coupiing constant, viz., of the order of 1.50 G. Both the alkyl and the acyl part of the biradical contribute to the polarization of C,. In spite of acouphng constant of about 14 G for Cs in the alkyl radical part, no polarization for Ca in the ketone is observed (see fig. 1). An A2 dependence would account for this fact: the ratio of the enhancement factors for C, and 9
CAMPHIM IN
CH,-CH,.
R-Cff2-C$
0
CDCL,
1’.
old
zoo
150
loo
I 0..
50
: Fig. 2..“C ET plioto-ClDNP spectrum of camphor in dDCla with nornralspeetrum (b&tom)_ The cared. .. : :. ‘. ‘, 106. :. -<. --. .
‘.
Iv assignmedt of
ihe l&s is in+
Volume
CHEMICAL
26, number 1
PHYSICS
Cp would then be Vo: VP = (~52~ + 392) : 142 = 22 : 1, and the polarization of C, would be too small to be
detected_ In the case of the six-membered ring ketones II and III the polarization was found to be considerably larger than that of cyclohexanone. For methyl-substituted cyclohexanones and cyclopentanones enhanced absorption has been observed in the proton photoCIDNP spectrum of the aldehyde [ 151. The origin of_ this effect is not clear; it may arise from intermolecular reactions. No such complications were encountered in the 13C spectra. Although no emission was observed during irradiation of cyclopentanone I,, rather large effects were found in the case of camphor, which also gives a 1 Sbiradical. Fig. 2 shows the 13C photo-CIDNP spectrum of camphor with an assignment of the lines [ 161. Again the
largest polarization occurs for the ketone
itself. Carbons 1,7,2 and 3 (a and I3carbons in alkyl and acyl radical parts) show emission, compatible with a-cleavage of the Cl -C2 bond, resulting in the most stable alkyl radical (see scheme 2).
LETTERS
1 May 1974
work shows that the 13C FT NMR technique can be successfully applied to the study of the photochemistry of cyclic ketones. The CIDNP effects constitute the most direct evidence for the intermediacy of biradicals. They also show that a-cleavage and reclosure of the biradical is a major energy-loss mechanism of triplet-excited-state ketones. It should be noted, however, that the observation of photo-CIDNP does not exclude singlet-state reaction paths leading to biradicals, since these would probably not give rise to polarization_
References
11I
J.D. Coyle and H.A.J. Carless, Chem. Sot. Rev. 1 (1972)
465: NJ. Turro, J.C. Dalton, K. Dawes. C. Farrington, R. Hautola, D. Morton. hf. Niemczyk and N. Schore, Accounts Chem. Res. 5 (1972) 92. 121 P.J. Wagner and R.W. Spoerke, J. Am. Chem. Sot. 91 (1969) 4437; J.M. Beard and R.H. Eastman. Tetr&edmn Letters (1970)
3029.
[3] R. Srinivasan, Advan. Photochem. 1 (1953) 83. (41 J.A. Barltrop and J.D. Coyle, Chem. Commun. (1969) 1081. [5] J.D. Coyle, J. Chem. Sac. B (1971) 1736; WC. Agosta and W.L. Schreiber, J. Am. Chem. Sot. 93 (1971) 3947. [6] G.L. Gloss and C.E. Doubleday, J. Am. Chem. Sot. 94
.
PL
Scheme 2
[7]
Observation of the polarization of C, (emission just cancelling the normal absorption effect) is facilitated by the long T1 of this carbon atom. The alkenal VI gives rise to the E line at 202.3 ppm and may also be responsible for the emissions at 44.8 and 148.0 ppm; the E line appearing at 170.2 ppm might point to the formation of a ketene. In the case of fenchone V similarly E effects were observed for the C=O carbons of the ketone and alkem$ product, although the intensities were lower. than in the previous case. These observations indicate that, apart from ring
size, steric factors and flexibility strongly affect the polarizations from biriidicals, presumably through their effects on the exchange interaction. The present
(1972) 9248;95 (1973) 2735. R Kaptein. M. Friter-Schroder
and L.J. Oosterhoff, Chem. Phys. Letters 12 (1971) 16. [S] G.L. Gloss, J. Am. Chem. Sot. 93 (1971) 1546; Ind. Chim. Beige 36 (1971) 1064. [9 J R. Kaptrin. H.D.W. Hill and R. Freeman, to be published. [lo] F.J. Adrian, J. Chem. Phys. 53 (1970) 3374~54 (1971) 3912. [ll] R. Kaptein, J. Am. Chem. Sot. 94 (1972) 6251. [ 121 S.H. Glarum, in: Chemically induced magnetic polarization. eds. A-R. Lepley and G.L. Gloss (Whey, New York, 1973) ch. 1. 1131 RC. McCaiIeyand A-L. Kwiram, J. Am. Chem. Sot. 92 (1970) 1441. [ 141 R-W. Fessenden, J. Phys. Chem. 71(1967) 74: [15] A.M. Trozzolo, private communication; R Kaptein and J.A. den Hollander, unpublished results. [ 161 LF. Johnson and WC. Jankowski, Carbon-13 NMR spectra. (Wiley.New York, 1972).
.’
-. :..
_;-
;:
..
._ ..
-:
.;
..
1
‘.... .:
::.
..
.~ . .
.
.
.-..
.~
:: . .
.
:. .-
:
.:.
107 y ,.
._
.
:.~ I
J.
-. ; : ;’
CHEMICAL PHYSICS LETTERS
CARBON-13
1 May
1974
CIDNP FROM BIRADICALS
IN THE PHOTOLYSIS OF CYCLIC KETONES R. KAPI’EINI,
R. FREEMAN+?
and H.D.W. HILL
Varian Associates, Instrument Division. Palo Alto, California 94303, USA
Received 11 February 1974
The observation of chemically induced dynamic nuclear polarization (CIDNP) in 13C Fourier transform NMR spectra taken during the photolysis of cyclic ketones is direct evidence for the intermediacy of biradicals. Cycloheptanone showed the strongest emission effects- Polariition from 1,5-bimdicals has been observed in the photolysis of camphor and fendtone.
1. Introduction The photolysis of cyclic ketones in solution yields unsaturated atdehydes and ketenes [I]. Mechanistic studies of this reaction have directed the attention to the nature of the excited states involved and to the question whether the products are formed in a concerted process or via intermediate biradicals. From quenching studies [2] it is known that an n, x* triplet state is involved, which is completely quenchable in many cases. Although a concerted reaction has been proposed [3], most authors now favour a biradical mechanism, This has been concluded from the kinetics of product formation and ketone consumption [4], and from the product distributions of methyland deuterium-substituted cyclic alkanones [5]. The observation of CIDNP provides direct evidence for the intermediacy of short-lived biradicals, which are not easily detected by other physical techniques. Proton CIDNP in similar cyclic ketones has been reported by Gloss [6]. There are two kinds ofCIDNP from biradicals 17, 8]_ One is similar to CIDNP from freely diffusing radical pairs and is based upon mixing of the electronic L r Present address: KonMclijke/SheU-Ioratorium, dam, The Netherlands. tt Present address: PhysicalChemistry .University, ?xford,‘UK_
Arnster-
Laboratory, Oxford
singlet state with the triplet state, which has GT,>= 0 (S-T, mixing), induced by the hyperfine interaction of nuclei with electrons. The spin-selective nature of this mechanism requires a reaction path competitive with the self-reaction of the biradical [7]. The other type is due to S-T_ mixing (T_: triplet state with cS,> = -1) 181. This mechanism may become effective when S and T_ states are (nearly) degenerate, i-e., when the average exchange interaction is equal to the Zeeman interaction 2v) =gpHO. When the biradical is produced from a triplet precursor and products are formed solely from the singlet state (after intersystem crossing). there is a net change in electron and nuclear spin angular momenta: the selection rule for T_ + S transitions induced by the hyperfme term AS- Zis &Ifs = +l, MT = -1. Thus, this process gives a net flow of nuclear spins from the /tit> state to the I-$> state, which results in purely emissive NMR spectra of the reaction products. This contrasts with the S-T0 mechanism, where both enhanced absorption (A) and emission (E) is to be expected. We have studied the 13C photo-CIDNP spectra of some cyclic alkanones by the Fourier transform method. 13C CIDNP spectra gen&ally contain more information than proton spectra, not only because of the greater chemical shift dispeision of CMR but also because.thti reactions of the carbon skeleton.are: ,monitored in a mdre direct way. in spite of the low natural -‘aburidance ( 121@ and the low sensitivity of the car-
Volume
1 May 1974
CHEMICAL PHYSICS LETTERS
26. number 1
Cycloheptnnone
in CDCI,
aldehyde
I
200
150
50
GO
Fig. 1. Proton-decoupled 13C FT NMR spectrum of cycloheptanone in CDC13 taken during UV irradiation (upper spectrum). comparison the undecoupled (middle) and the normal (lower) spectrum before irradiation are ako shown.
bon- 13 nucleus, I3C photo-CIDNP experiments are feasible and often give enhancements that are larger by an order of magnitude than those of the correspending proton spectra, owing to the higher hyperfiie coupling constants and the longer spin-lattice relaxation times.
2. Results and discussion The spectra were recorded on a Varian XL-100 FT NMR spectrometer modified in order to irradiate the sample in the probe with UV light from a 1000 W PETS high-pressure mercury arc. The light was filtered through an aqueous NISO, solution. Details of the experimental set-up will be presented elsewhere [9]. 33C Ff spectra were taken during an irradiation period of about 4.5 minutes. Data accumulation was started after a pre-irradiation time of 20 seconds. Typical conditions were: pulse repetition time 2 set, flip angle 33’. 128 transients. CDCl, solutions of the following cyclic ketones were studied:
For
The proton-decoupled 25.1 MHz 13C IDNP spectrum taken during irradiation of cycloheptanone I7 in CDCI, is shown in fig. I. For comparison the normal spectrum before irradiation is also shown. Clearly, the strongest emission lines originate from the parent ketone (carbonyl carbon at 6 = 216.7, a-carbon at 6 = 41.3 ppm). The undecoupled spectrum shows that the E line at 203.7 ppm should be assigned to the carbony1 carbon of the alkenal product (doublet splitting +H = 173 Hz). The E line at 175.2 ppm most probably belongs to pentylketene (carbonyl carbon, no protons attached). The ketene was not observed in the proton CIDNP spectrum [6] _The E lines at 46.6 and 115.3 ppm have not been assigned. The latter, however, has the correct chemical shift for the terminal vinylic carbon of the alkenal product_ Scheme 1 gives a summary of the photoreactions of cycloheptanone. After initial excitation of the ketone to the singlet n, P* state rapid intersystem crossing occurs to the triplet state, from which a triplet biradical is formed through ol-cleavage. In order to react internally to give
ketone, alkenal and ketene products the triplet biradical must first cross over t? the singlet state. The strong CIDNP effects show that the hypefine coupkg with nuclear.spins contributes substantially to this intersystem crossing in the biradical. At present nothing is -known
about>he.6fficiency of other mechanisms sych co&pl.ing for these la&e biiadicak ._ ) ’
as spin-orbit
Volume 16,
number 1
CHEMICAL
0 =y
. c
hv
A( f3C). For a freely diffusing radical pair the func-
\
/
tiol’ral dependence may range from A u2 to A2, depending on the lifetime of the radicai pair and the presence of other nuclei. Although for biradicals a quantitative theory does not yet exist, there seems to be little justification for a square-root dependence, since this arises purely from the r3/* time dependence for the re-encounter probability of a diffusing radical pair [ 10, I I]. Perturbation treatments [ 1Z] would yield an AZ dependence. An estimate of theA(13C)‘s for the acyl-aIky1 biradical can be made by anaIogy with known acyl [13] and alkyl [14] radicals:
.
=O
1 May I974
LETTERS
field of 23 kG. Among other factors the polarization depends on the magnitude of the hyperfme coupling constant
=03
-c
PHYSICS
=O
Scheme 1
In the series of cyclic alkanones I, cycloheptanone I7 showed by far the largest polarization. The 4 and S-membered ring ketones 14 and I5 did not show any effects. A small emission for the carbonyl C atom was observed in the case of cycIohexanone Ig. The effects decreased in the series I, to I,,. This maximum in the polarization far I7 may be partly ascribed to differences in quantum yields for the or-cleavage reaction (for instance, the photochemistry of cyclobutanone deviates from that of other cycIic alkanones), but it is mainly due to the fact that for I7 the condition 2<&=&Hl-, is best satisfied, i.e., Zeeman and exchange energies are most ciosely matched in the spectrometer
A(13C):
52G
T’
I
153G
(-)
13.6 G
39.1 G
The largest effect is expected (and observed) for the carbonyl carbon, which has a particularly large hy perfine coupiing constant, viz., of the order of 1.50 G. Both the alkyl and the acyl part of the biradical contribute to the polarization of C,. In spite of acouphng constant of about 14 G for Cs in the alkyl radical part, no polarization for Ca in the ketone is observed (see fig. 1). An A2 dependence would account for this fact: the ratio of the enhancement factors for C, and 9
CAMPHIM IN
CH,-CH,.
R-Cff2-C$
0
CDCL,
1’.
old
zoo
150
loo
I 0..
50
: Fig. 2..“C ET plioto-ClDNP spectrum of camphor in dDCla with nornralspeetrum (b&tom)_ The cared. .. : :. ‘. ‘, 106. :. -<. --. .
‘.
Iv assignmedt of
ihe l&s is in+
Volume
CHEMICAL
26, number 1
PHYSICS
Cp would then be Vo: VP = (~52~ + 392) : 142 = 22 : 1, and the polarization of C, would be too small to be
detected_ In the case of the six-membered ring ketones II and III the polarization was found to be considerably larger than that of cyclohexanone. For methyl-substituted cyclohexanones and cyclopentanones enhanced absorption has been observed in the proton photoCIDNP spectrum of the aldehyde [ 151. The origin of_ this effect is not clear; it may arise from intermolecular reactions. No such complications were encountered in the 13C spectra. Although no emission was observed during irradiation of cyclopentanone I,, rather large effects were found in the case of camphor, which also gives a 1 Sbiradical. Fig. 2 shows the 13C photo-CIDNP spectrum of camphor with an assignment of the lines [ 161. Again the
largest polarization occurs for the ketone
itself. Carbons 1,7,2 and 3 (a and I3carbons in alkyl and acyl radical parts) show emission, compatible with a-cleavage of the Cl -C2 bond, resulting in the most stable alkyl radical (see scheme 2).
LETTERS
1 May 1974
work shows that the 13C FT NMR technique can be successfully applied to the study of the photochemistry of cyclic ketones. The CIDNP effects constitute the most direct evidence for the intermediacy of biradicals. They also show that a-cleavage and reclosure of the biradical is a major energy-loss mechanism of triplet-excited-state ketones. It should be noted, however, that the observation of photo-CIDNP does not exclude singlet-state reaction paths leading to biradicals, since these would probably not give rise to polarization_
References
11I
J.D. Coyle and H.A.J. Carless, Chem. Sot. Rev. 1 (1972)
465: NJ. Turro, J.C. Dalton, K. Dawes. C. Farrington, R. Hautola, D. Morton. hf. Niemczyk and N. Schore, Accounts Chem. Res. 5 (1972) 92. 121 P.J. Wagner and R.W. Spoerke, J. Am. Chem. Sot. 91 (1969) 4437; J.M. Beard and R.H. Eastman. Tetr&edmn Letters (1970)
3029.
[3] R. Srinivasan, Advan. Photochem. 1 (1953) 83. (41 J.A. Barltrop and J.D. Coyle, Chem. Commun. (1969) 1081. [5] J.D. Coyle, J. Chem. Sac. B (1971) 1736; WC. Agosta and W.L. Schreiber, J. Am. Chem. Sot. 93 (1971) 3947. [6] G.L. Gloss and C.E. Doubleday, J. Am. Chem. Sot. 94
.
PL
Scheme 2
[7]
Observation of the polarization of C, (emission just cancelling the normal absorption effect) is facilitated by the long T1 of this carbon atom. The alkenal VI gives rise to the E line at 202.3 ppm and may also be responsible for the emissions at 44.8 and 148.0 ppm; the E line appearing at 170.2 ppm might point to the formation of a ketene. In the case of fenchone V similarly E effects were observed for the C=O carbons of the ketone and alkem$ product, although the intensities were lower. than in the previous case. These observations indicate that, apart from ring
size, steric factors and flexibility strongly affect the polarizations from biriidicals, presumably through their effects on the exchange interaction. The present
(1972) 9248;95 (1973) 2735. R Kaptein. M. Friter-Schroder
and L.J. Oosterhoff, Chem. Phys. Letters 12 (1971) 16. [S] G.L. Gloss, J. Am. Chem. Sot. 93 (1971) 1546; Ind. Chim. Beige 36 (1971) 1064. [9 J R. Kaptrin. H.D.W. Hill and R. Freeman, to be published. [lo] F.J. Adrian, J. Chem. Phys. 53 (1970) 3374~54 (1971) 3912. [ll] R. Kaptein, J. Am. Chem. Sot. 94 (1972) 6251. [ 121 S.H. Glarum, in: Chemically induced magnetic polarization. eds. A-R. Lepley and G.L. Gloss (Whey, New York, 1973) ch. 1. 1131 RC. McCaiIeyand A-L. Kwiram, J. Am. Chem. Sot. 92 (1970) 1441. [ 141 R-W. Fessenden, J. Phys. Chem. 71(1967) 74: [15] A.M. Trozzolo, private communication; R Kaptein and J.A. den Hollander, unpublished results. [ 161 LF. Johnson and WC. Jankowski, Carbon-13 NMR spectra. (Wiley.New York, 1972).
.’
-. :..
_;-
;:
..
._ ..
-:
.;
..
1
‘.... .:
::.
..
.~ . .
.
.
.-..
.~
:: . .
.
:. .-
:
.:.
107 y ,.
._
.
:.~ I
J.
-. ; : ;’