- Email: [email protected]
Kinetics and intramolecular dynamics of radicals produced by photoreduction of glyoxal, an ESR, MESR and INDO study
CHEMICAL PHYSICS LETTERS
Volume 46, numbcr 1
KìNETlCS AND ~T~MOLE~ULAR
PRODUCED BY PHOTORED~~~O~
15 February
1977
D~AMlCS OF RADICALS OF GLYOXAL, AN ESR, MESR AND INDO STUDY
M. RUDIN, K. LOTH, F. GRAF and Hs.H. GÜNTHARD Luboratom far Physìcal Chemìstry, Swìss Federal Institute of Technology, CH-NI06 Zuràch, SwitzerIand Received 1 I September
1976
Resuits of a study of kmetics and intramolecuiar dynamics of interconversion of conformations of radicaìs produced photochemically from giyoxa1 by ESR and modulatcd cxcitation ESR (ME%) are reported. A scheme is presented which shows structural features of the radicals involved.
In this letter we wish to report results about an investigation of radicals produced by UV irradiation of gtyoxal in aprotic solvents. Photolytic reduction of aromatic and olefinic ketones and díketones has been reported on a number of occasions [ 1-31, whereby the following reaction scheme was established
Usually alcohols are used as hydrogen atom donator RH, e.g. isopropanol. This study aimes at the determination of the structural and dynamica1 properties of the intramolecular hydrogen bridge formed by photoreduction of an a-diketone. in order to minimize possible interactions between radical and solvent toluene was taken as the hydrogen donor. Glyoxal was chosen, since it may be expected to produce the simplest radical with f.he structura! fragment p
---__.
O\ ,=-c/ \ Much werk has been devoted to the investigation of electronic spectra and ab initio studies of both ground and excited state of glyoxal [4-91. Monqmeric glyoxal was produced by heating a 1: 1 mixture of polymerie giyoxaI with phosphorous pentoxide to about 150°C. The escaping gaseous glyoxal was trapped at Iiquid nïtrogen temperature and then
dissolved in toluene [ 10] _Owing to the fact that monome~~ glyoxal in solutions readily polymerizes, solutions have to be stored at a suffïcîently low temperature before and during use. Dilute solutions of glyoxal circulating in a flow apparatus were directly photolyzed in the mîcrowave cavity by focusing the light from a high pressure mercury lamp (Osram HE0 500) onto the wîndow of the cavity by means of a quartz lens system [ 111. The experimental set up used for modulated excitation ESR measurements (MESR) was basically the same as aheady published elsewhere [12,13]. The ESR signal obtained during the photolysis of glyoxal in toluene ccnsrsts of the superposîtion of several components, Temperature variatîon reveaIs that three different spectra, each with a giveng-vahre and a set of three inequivalent coupling constants, are indeed observed (cf. fig. 1). In the following the 3 radicals as well as the ESR spectra associated with them wil1 be denoted by A, B, and C. Spectra B and C proved to be strongly temperature dependent, but not A. Furthermore, when the photolysis was carried out in benzene, only spectrum A was detected. in table 1 observed couphng constants and g-values of the 3 radicals are collected. MESR experiments yielded amplitude-phasc relations between the modulatîon frequency (1-200 Hz) of the exciting Iight and the ESR signal. ín fig. 2 Bode plots for the first and the second harrnonic, as we% as for the u~odulated part of the MESR signal % are 29
=
15 February
CHEMICAL PHYSICS LETTERS
Volume 46, number 1
B
AA
AA
B
B
B
1977
Fig. 1. ESR spectra of a solution of glyoxal in toluene (5.0 X 10-* mol/liter) during photolysis. At T= -102°C the solvent mixture CCiaF/toluene (9: 1) was used instead of pure toluene. Marker pips: proton magnetic resonance magnetometer. Lines belonging to the radicals A, B and C are labeled with A, B, C.
BAA
shown. Simïlar plots could also be obtaïned for the other two signals A and C. Experiments show the intensity of the steady state ESR signal to be approximately proportional to the square root of the light input. Discussion of the reaction mechanism will be based on the following photochemical equations:
C2H202 +
C2H20;,
í2)
c2H2O2
C,H,O,H-
* R-H
C2H20;
+ R-,
(3)
+= 2 CHO-,
CHO- + C,H,O,
(4a)
+ C,H,OJ.
(4b)
From the fact that photolysis of glyoxal in benzene produces only spectrum A, one can infer that radical A is probably not generated by hydrogen abstraction, but by reaction of a formyl radical CHO- with glyoxal, as outlined in eq. (4). Since spectrum A is found essentially temperature independent and indicates 3 inequivalent protons we tentatively assign this spectrum to a radical with the following constitution:
O\ r”>_
p
Hl
“3
“2
Table 1 Experimental TeO
in gauss and g-values for the radicals produced
Radiaal A
Radiaal
by photolysis
of glyoxal in toluene Kadical
B
=I
=2
=3
=1
=2
=3
-1.5 -34 -85 -102
17.7 17.7 17.7 17.8 17.8
8.3 8.4 8.5 8.8 8.9
0.9 0.9 0.9 0.9 0.9
18.0 17.9 17.9 17.9
3.0 3.0 3.0 3.1
1.1 1.2 1.2 1.3
g-vahres
2.00434 + 0.00002
20
30
couphng constants
2.00396 2 0.00002
Radical A
\
C
=l
a2
=3
14.9 14.9 14.9 14.9
3.9 4.0 4.0 4.0
2.9 2.9 3.0 3.0
2.00495 + 0.00003
Volume 46, number 1
CHEMICAL PHYSICS LETTERS
a
15 February L977
C
-1 , 1
b
f 2
4
8
1216
25324865100
2QO
4 Phasc
d
-9o”-
Phase
-330°
I
165
1
?OO
J
a
c 2
200
4
8
16
25
Fig. 2. Bode plots of signal B. A = amplitude of the MESR signal, phase = phase of the MESR signal,f= modulation frequency of the exciting UV-light m (Hz). Lifetime of the radical B = 2.9 ms. (a) Frequency dependence of the fundamental amplitude (dok), frequency dependence of the unmodulated amplitude (squares). (b) Frequency dependence of the fundamental phase. (c) Frequcncy dependence of the first harmonie amplitude. (d) Frequcncy dependencc of the fïrst harmonie phase.
Since spectra B and C are only obtained in toluene, they have to be traced back to radicals formed upon hydrogen abstraction according to eq. (3). Two of the splittings may be associated with the olefinic protons and the third splitting can be naturally ascribed to the hydroxylic proton, since it has been found in recent work [14,15] that splittings from this group are usually resolved in aprotic solvents. Kinetic effects as observed in spectra B and C can arise from different mechanisms, e.g. intramolecular proton transfer [16] , rotation of the hydroxylic group [ 161 or rotation around the C-C bond. Furthermore, intermolecular chemical exchange mechanisms may play a role, though they can be suppressed Dy appropriate choice of solvents and IOW concentratiom
,p $-
&
-
%?\ 5
/c-c
/o \ H2
L-cis
d-cis
k&
IJ
Ir t %,o&
HH3
0
\ /-c,
lH2
z 0
H1 C-trans
How to distinguish
between inter- and intramolecular kinetic effects arising from hydrogen bond dynamics by appropriate experimental conditions has been recently discussed for the o-semiquinone radicals [ 16]_ Possible intramolecular dynamic processes involving different conformers of the ketyl radical are shown in reaction schemes (5) and (6).
k: d >
0NH3
0
JH3 \
%pi
lH2
,c-c\
O
‘4 &
,L
oC-trans
/o
H1 4.-cis
r-cis
CHEMICAL PHYSICS LET%RS
Volume 46, number 1
Conformations featuring open internal H-bridges are denoted by o, whereas conformations resulting from rotation of the C-C bond are denoted as cis or trans according to the arrangement of the OCCO frame. For cis type radicals, labels r and Q denote configurations obtained by intramolecular transfer of the proton H,. This last type of motion has been assumed in analogy to what has been found for the 2-hydroxy phenoxy radical which also contains the fragment HOCCO j15]. The overall photochemical equation describing the photolysis of glyoxal in a hydrogen donating solvent R--H may be described by scheme (7), in which the radicals B and C are assigned to spectra B and C. B
-Exit
B
C
-Exit
C
k-H
l
(7)
CzHzOz
,’
2CHO-
* A -Exit
A
C2’V’z
assignment of spectra B and C to the species interconverted in schemes (5) and (6) requires further discussion. Conceming the intensities of the spectra B and C fig. 1 shows immediately that at low temperatures [B]/[C] < 1 and at room temperature [B] /[Cl > 1. This fact can be rationalized keeping in mind that one is not working at equilibrium but under steady state conditions, where the relative concentrations depend not only on the common input but also on the individual output paths labeled as exïts A, B and C in scheme (7) (kinetically controlled system). In order to get approxïmate values for the coupling constants of the relevant structures as we11 as for the relative molecular energies and activation energies involved, INDO calculations were carried out. The results are shown in table 2, standard structural parameters according to Pople et al. 1171 having been used throughout. In earher papers it has been found that INDO calculations for radicals may serve only as a guideline both for energies and coupling constants, in particular energies should be subject to optimization of structural parameters. Optimized parameters
ne
32
15 February 1977
Volume 46, number 1
CHEMICAL PHYSICS LETTERS
may lead to large changes in energy in comparison to standard parameters [IS]. Assignment of spectra was based on the following arguments: (0 Model chemical exchange calculations based on intramolecular process (6) (jump of the proton between isometrie potential minima of the Hbond) yield ESR spectra with a temperature dependence not observed experimentally. We therefore assume process (6) as nonrelevant for the kinetics underlying spectra B and C. INDO results indicate the potential barrier for (4 internal rotation of the OH group to be one order of magnitude smaller than the barrier to C-C rotation (cis-trans interconversion) as well as the barrier for proton jump. (iii) According to observed spectra the members of the pairs of rotamers Q-cis,o!2-cisand !2-trans, oQ-transhave practically equal g-values but the g-values of the pairs oQ-cis,oQ-transand !&cis, Q-transare different. In order to interpret the observed temperature dependence of line shapes a formula expressing the linewidth in the fast exchange limit has been used [ 181. Starting with the signs of the coupling constants as predicted by INDO, it tumed out possible to assign the observed transitions to individual nuclear spin function of zeroth order. For simulatlon of the spectra the rate constants k: _“, and the relative concentrak”oQ Q' OP' Q tions we;e considered as adjustable parameters. Model chemical exchange calculations based on scheme (5) indicate that the selective line broadenings observed in spectra B and C are to be mainly traced back to the rotation of the OH-group whereas the rotation around the C-C bond presumably plays a role in determining the relative concentrations. Fig. 3 shows that the spectra B and C may be simulated in a satisfactory manner and table 2 shows the magnetic constants of the radicals Q-cis,oQ-cis,Qtrans and og--trans as derived from the evaluation of the formula in ref. [ 181 cited above. Assignment to conformations is tentative at the present time and it should be recalled that this analysis does not yield the absolute coupling constants associated with the re structures of the radical conformers but the signs of the differences between the coupling constants of the re structures involved. The simulated spectra
k’9 %Q
L,“L
15 Februxry
1977
Radlcal B
R.dL‘I
C
Fig. 3. Slmulated spectra for intr3molecular dynamics of thc radicals B and C (rotation of the OH-group around thc CObond). The data used for this simulation
are given in table 2.
shown in fig. 3, however, are based on the exact numerical solution of the chemical exchange,problem of two inequivalent sites. The data for the two pairs of radical conformations do not allow, however, a more detailed assignment in temts of scheme (5). ESR spectra obtained from experiments with attenuation of the light input show the radical concentration to be proportional to the square root of the exciting light power, indicating that second order processes are relevant for the radical kinetics (exits A, B and C). Fig. 2 represents an amplitude-phase diagram for the fundamental of the modulation frequency. Obviously the diagram corresponds nearly to that expected for a linear (first order) termination kinetics. As has been pointed out earlier by Hunziker [19] and more recently by Paul [20] the phase amplitude diagrams for tìrst and second order terminatron reactions cannot be expected to differ noticeably. Amplitude-phase diagrams for the first harmonic of the modulation frequency, which should not exist for Iinear weakly pumped systems, were found to exhibit a peculiar behavior, as is demonstrated by fig. 2. It shows that amplitude-phase diagrams for higher harmonics carry valuable information about the mechanism of radical reactions. Furthermore, the unmodulated part of the MESR signal proved to be dependent on the light modulation frequency, in contrast to what one predicts for linear chemical networks [21] . The radical production (2), (3) is believed to proceed through an excited triplet state: tìrstly the quan33
Volume 46, number 1
CHEMICALPHYSICS LETTERS
turn yield af the radical production is very markedly reduced by traces of oxygen and secondly ab initio caIculations predict a biradical character for the first excited triplet [4]. Direct spectroscopic evidente for the triplet as a precursor is not yet avalable.
The authors wish to express their gratitude to the Schweizerische Stiftung zur Förderung von VoIkswirtschaft durch wissenschaftliche Forschung, to Messrs. Sandoz AG, Basle, and the administration of the ETHZ for financial support of this werk. Mr. M. Andrist has given valuable technical support with muItichanne1 phase and amplitude measurements. Finaliy we wish to express our gratitude to Dr. J. Heinzer for providing the program ESRJXN used for sïmulation of exchange broadened spectra.
References [l] P.B. Ayscough and F.P. Sargent, Proc. Chem. Sec. (1363) 94. (2j‘A. Beckett and G. Porter, Trans. Faraday Sec. 59 (1963) 2039. [3] H.L.J. Bäckström and K. Sandros, J. Chem. Phys. 23 (1955) 2197.
34
15 February 1977
[4] C.E. Dykstra and H.F. Schaefer 111,J. Am. Chem. Sec. 98 (1976) 401, and references therein. [5] W. Holler and D.A. Ramsay, Can. J. Phys. 48 (1970) 1759. [6] C.N. Currie and D.A. Ramsay,Can. J. Phys. 49 (1971) 317. [7] T.X. Ha, J. Mol. Struct. 12 (1972) 171. !8] D.A. Ramsay and C. Zauli, Acts Phys. Acad. Sci. Hung. 35 (1974: 79. [9] IJ. Pincelli, B. Cadioli and D.J. David, J. Mol. Struct. 9 (1971) 173. [IO] F. Verdenme, J. Chem. Phys. 52 (1970) 719. [ 111 M. Forster, K. Loth, hl. Andrist, U.P. Fringeli and Hs.H. Giînthard, Chem. Phys. 17 (1976) 59. [121 K. Loth, Thesis No. 5540, ETH-Zurïch (1975). 1131 K. Loth, 1’. Graf and Hs.H. Günthard, Chem. Phys. Letters 29 (1074) 163. 1141 H. Zeldes and R. Livingston, J. Chem. Phys. 47 (1967) 146.5. LI51 K. Loth, ‘r‘.Graf and Hs.H. Giinthard, Chem. Phys. 13 (1976) 95_ [161 K. Loth F. Graf and Hs.H. Giínthard, Chem. Phys. Letters45 (1977) 191. (171 J.A. Pople, D.L. Beveridge and P.A. Dobosh, J. Am. Chem. Sx. 90 (1968) 4201. to mag1181 A. Carrington and A. Mclachlan, Introduction netic resxrance (Harper and Row, London, 1969) p. 208. [19i H.E. Hu xiker, IBM J. Res. Develop. 10 (1971). [201 H. PauI, Chem. Phys. 15 (1976) 115. 1211 M. Forsfer, U.P. Fríngelí and Hs.H. Giinthard, Helv. Chim. Acts 56 (1973) 389.
Volume 46, numbcr 1
KìNETlCS AND ~T~MOLE~ULAR
PRODUCED BY PHOTORED~~~O~
15 February
1977
D~AMlCS OF RADICALS OF GLYOXAL, AN ESR, MESR AND INDO STUDY
M. RUDIN, K. LOTH, F. GRAF and Hs.H. GÜNTHARD Luboratom far Physìcal Chemìstry, Swìss Federal Institute of Technology, CH-NI06 Zuràch, SwitzerIand Received 1 I September
1976
Resuits of a study of kmetics and intramolecuiar dynamics of interconversion of conformations of radicaìs produced photochemically from giyoxa1 by ESR and modulatcd cxcitation ESR (ME%) are reported. A scheme is presented which shows structural features of the radicals involved.
In this letter we wish to report results about an investigation of radicals produced by UV irradiation of gtyoxal in aprotic solvents. Photolytic reduction of aromatic and olefinic ketones and díketones has been reported on a number of occasions [ 1-31, whereby the following reaction scheme was established
Usually alcohols are used as hydrogen atom donator RH, e.g. isopropanol. This study aimes at the determination of the structural and dynamica1 properties of the intramolecular hydrogen bridge formed by photoreduction of an a-diketone. in order to minimize possible interactions between radical and solvent toluene was taken as the hydrogen donor. Glyoxal was chosen, since it may be expected to produce the simplest radical with f.he structura! fragment p
---__.
O\ ,=-c/ \ Much werk has been devoted to the investigation of electronic spectra and ab initio studies of both ground and excited state of glyoxal [4-91. Monqmeric glyoxal was produced by heating a 1: 1 mixture of polymerie giyoxaI with phosphorous pentoxide to about 150°C. The escaping gaseous glyoxal was trapped at Iiquid nïtrogen temperature and then
dissolved in toluene [ 10] _Owing to the fact that monome~~ glyoxal in solutions readily polymerizes, solutions have to be stored at a suffïcîently low temperature before and during use. Dilute solutions of glyoxal circulating in a flow apparatus were directly photolyzed in the mîcrowave cavity by focusing the light from a high pressure mercury lamp (Osram HE0 500) onto the wîndow of the cavity by means of a quartz lens system [ 111. The experimental set up used for modulated excitation ESR measurements (MESR) was basically the same as aheady published elsewhere [12,13]. The ESR signal obtained during the photolysis of glyoxal in toluene ccnsrsts of the superposîtion of several components, Temperature variatîon reveaIs that three different spectra, each with a giveng-vahre and a set of three inequivalent coupling constants, are indeed observed (cf. fig. 1). In the following the 3 radicals as well as the ESR spectra associated with them wil1 be denoted by A, B, and C. Spectra B and C proved to be strongly temperature dependent, but not A. Furthermore, when the photolysis was carried out in benzene, only spectrum A was detected. in table 1 observed couphng constants and g-values of the 3 radicals are collected. MESR experiments yielded amplitude-phasc relations between the modulatîon frequency (1-200 Hz) of the exciting Iight and the ESR signal. ín fig. 2 Bode plots for the first and the second harrnonic, as we% as for the u~odulated part of the MESR signal % are 29
=
15 February
CHEMICAL PHYSICS LETTERS
Volume 46, number 1
B
AA
AA
B
B
B
1977
Fig. 1. ESR spectra of a solution of glyoxal in toluene (5.0 X 10-* mol/liter) during photolysis. At T= -102°C the solvent mixture CCiaF/toluene (9: 1) was used instead of pure toluene. Marker pips: proton magnetic resonance magnetometer. Lines belonging to the radicals A, B and C are labeled with A, B, C.
BAA
shown. Simïlar plots could also be obtaïned for the other two signals A and C. Experiments show the intensity of the steady state ESR signal to be approximately proportional to the square root of the light input. Discussion of the reaction mechanism will be based on the following photochemical equations:
C2H202 +
C2H20;,
í2)
c2H2O2
C,H,O,H-
* R-H
C2H20;
+ R-,
(3)
+= 2 CHO-,
CHO- + C,H,O,
(4a)
+ C,H,OJ.
(4b)
From the fact that photolysis of glyoxal in benzene produces only spectrum A, one can infer that radical A is probably not generated by hydrogen abstraction, but by reaction of a formyl radical CHO- with glyoxal, as outlined in eq. (4). Since spectrum A is found essentially temperature independent and indicates 3 inequivalent protons we tentatively assign this spectrum to a radical with the following constitution:
O\ r”>_
p
Hl
“3
“2
Table 1 Experimental TeO
in gauss and g-values for the radicals produced
Radiaal A
Radiaal
by photolysis
of glyoxal in toluene Kadical
B
=I
=2
=3
=1
=2
=3
-1.5 -34 -85 -102
17.7 17.7 17.7 17.8 17.8
8.3 8.4 8.5 8.8 8.9
0.9 0.9 0.9 0.9 0.9
18.0 17.9 17.9 17.9
3.0 3.0 3.0 3.1
1.1 1.2 1.2 1.3
g-vahres
2.00434 + 0.00002
20
30
couphng constants
2.00396 2 0.00002
Radical A
\
C
=l
a2
=3
14.9 14.9 14.9 14.9
3.9 4.0 4.0 4.0
2.9 2.9 3.0 3.0
2.00495 + 0.00003
Volume 46, number 1
CHEMICAL PHYSICS LETTERS
a
15 February L977
C
-1 , 1
b
f 2
4
8
1216
25324865100
2QO
4 Phasc
d
-9o”-
Phase
-330°
I
165
1
?OO
J
a
c 2
200
4
8
16
25
Fig. 2. Bode plots of signal B. A = amplitude of the MESR signal, phase = phase of the MESR signal,f= modulation frequency of the exciting UV-light m (Hz). Lifetime of the radical B = 2.9 ms. (a) Frequency dependence of the fundamental amplitude (dok), frequency dependence of the unmodulated amplitude (squares). (b) Frequency dependence of the fundamental phase. (c) Frequcncy dependence of the first harmonie amplitude. (d) Frequcncy dependencc of the fïrst harmonie phase.
Since spectra B and C are only obtained in toluene, they have to be traced back to radicals formed upon hydrogen abstraction according to eq. (3). Two of the splittings may be associated with the olefinic protons and the third splitting can be naturally ascribed to the hydroxylic proton, since it has been found in recent work [14,15] that splittings from this group are usually resolved in aprotic solvents. Kinetic effects as observed in spectra B and C can arise from different mechanisms, e.g. intramolecular proton transfer [16] , rotation of the hydroxylic group [ 161 or rotation around the C-C bond. Furthermore, intermolecular chemical exchange mechanisms may play a role, though they can be suppressed Dy appropriate choice of solvents and IOW concentratiom
,p $-
&
-
%?\ 5
/c-c
/o \ H2
L-cis
d-cis
k&
IJ
Ir t %,o&
HH3
0
\ /-c,
lH2
z 0
H1 C-trans
How to distinguish
between inter- and intramolecular kinetic effects arising from hydrogen bond dynamics by appropriate experimental conditions has been recently discussed for the o-semiquinone radicals [ 16]_ Possible intramolecular dynamic processes involving different conformers of the ketyl radical are shown in reaction schemes (5) and (6).
k: d >
0NH3
0
JH3 \
%pi
lH2
,c-c\
O
‘4 &
,L
oC-trans
/o
H1 4.-cis
r-cis
CHEMICAL PHYSICS LET%RS
Volume 46, number 1
Conformations featuring open internal H-bridges are denoted by o, whereas conformations resulting from rotation of the C-C bond are denoted as cis or trans according to the arrangement of the OCCO frame. For cis type radicals, labels r and Q denote configurations obtained by intramolecular transfer of the proton H,. This last type of motion has been assumed in analogy to what has been found for the 2-hydroxy phenoxy radical which also contains the fragment HOCCO j15]. The overall photochemical equation describing the photolysis of glyoxal in a hydrogen donating solvent R--H may be described by scheme (7), in which the radicals B and C are assigned to spectra B and C. B
-Exit
B
C
-Exit
C
k-H
l
(7)
CzHzOz
,’
2CHO-
* A -Exit
A
C2’V’z
assignment of spectra B and C to the species interconverted in schemes (5) and (6) requires further discussion. Conceming the intensities of the spectra B and C fig. 1 shows immediately that at low temperatures [B]/[C] < 1 and at room temperature [B] /[Cl > 1. This fact can be rationalized keeping in mind that one is not working at equilibrium but under steady state conditions, where the relative concentrations depend not only on the common input but also on the individual output paths labeled as exïts A, B and C in scheme (7) (kinetically controlled system). In order to get approxïmate values for the coupling constants of the relevant structures as we11 as for the relative molecular energies and activation energies involved, INDO calculations were carried out. The results are shown in table 2, standard structural parameters according to Pople et al. 1171 having been used throughout. In earher papers it has been found that INDO calculations for radicals may serve only as a guideline both for energies and coupling constants, in particular energies should be subject to optimization of structural parameters. Optimized parameters
ne
32
15 February 1977
Volume 46, number 1
CHEMICAL PHYSICS LETTERS
may lead to large changes in energy in comparison to standard parameters [IS]. Assignment of spectra was based on the following arguments: (0 Model chemical exchange calculations based on intramolecular process (6) (jump of the proton between isometrie potential minima of the Hbond) yield ESR spectra with a temperature dependence not observed experimentally. We therefore assume process (6) as nonrelevant for the kinetics underlying spectra B and C. INDO results indicate the potential barrier for (4 internal rotation of the OH group to be one order of magnitude smaller than the barrier to C-C rotation (cis-trans interconversion) as well as the barrier for proton jump. (iii) According to observed spectra the members of the pairs of rotamers Q-cis,o!2-cisand !2-trans, oQ-transhave practically equal g-values but the g-values of the pairs oQ-cis,oQ-transand !&cis, Q-transare different. In order to interpret the observed temperature dependence of line shapes a formula expressing the linewidth in the fast exchange limit has been used [ 181. Starting with the signs of the coupling constants as predicted by INDO, it tumed out possible to assign the observed transitions to individual nuclear spin function of zeroth order. For simulatlon of the spectra the rate constants k: _“, and the relative concentrak”oQ Q' OP' Q tions we;e considered as adjustable parameters. Model chemical exchange calculations based on scheme (5) indicate that the selective line broadenings observed in spectra B and C are to be mainly traced back to the rotation of the OH-group whereas the rotation around the C-C bond presumably plays a role in determining the relative concentrations. Fig. 3 shows that the spectra B and C may be simulated in a satisfactory manner and table 2 shows the magnetic constants of the radicals Q-cis,oQ-cis,Qtrans and og--trans as derived from the evaluation of the formula in ref. [ 181 cited above. Assignment to conformations is tentative at the present time and it should be recalled that this analysis does not yield the absolute coupling constants associated with the re structures of the radical conformers but the signs of the differences between the coupling constants of the re structures involved. The simulated spectra
k’9 %Q
L,“L
15 Februxry
1977
Radlcal B
R.dL‘I
C
Fig. 3. Slmulated spectra for intr3molecular dynamics of thc radicals B and C (rotation of the OH-group around thc CObond). The data used for this simulation
are given in table 2.
shown in fig. 3, however, are based on the exact numerical solution of the chemical exchange,problem of two inequivalent sites. The data for the two pairs of radical conformations do not allow, however, a more detailed assignment in temts of scheme (5). ESR spectra obtained from experiments with attenuation of the light input show the radical concentration to be proportional to the square root of the exciting light power, indicating that second order processes are relevant for the radical kinetics (exits A, B and C). Fig. 2 represents an amplitude-phase diagram for the fundamental of the modulation frequency. Obviously the diagram corresponds nearly to that expected for a linear (first order) termination kinetics. As has been pointed out earlier by Hunziker [19] and more recently by Paul [20] the phase amplitude diagrams for tìrst and second order terminatron reactions cannot be expected to differ noticeably. Amplitude-phase diagrams for the first harmonic of the modulation frequency, which should not exist for Iinear weakly pumped systems, were found to exhibit a peculiar behavior, as is demonstrated by fig. 2. It shows that amplitude-phase diagrams for higher harmonics carry valuable information about the mechanism of radical reactions. Furthermore, the unmodulated part of the MESR signal proved to be dependent on the light modulation frequency, in contrast to what one predicts for linear chemical networks [21] . The radical production (2), (3) is believed to proceed through an excited triplet state: tìrstly the quan33
Volume 46, number 1
CHEMICALPHYSICS LETTERS
turn yield af the radical production is very markedly reduced by traces of oxygen and secondly ab initio caIculations predict a biradical character for the first excited triplet [4]. Direct spectroscopic evidente for the triplet as a precursor is not yet avalable.
The authors wish to express their gratitude to the Schweizerische Stiftung zur Förderung von VoIkswirtschaft durch wissenschaftliche Forschung, to Messrs. Sandoz AG, Basle, and the administration of the ETHZ for financial support of this werk. Mr. M. Andrist has given valuable technical support with muItichanne1 phase and amplitude measurements. Finaliy we wish to express our gratitude to Dr. J. Heinzer for providing the program ESRJXN used for sïmulation of exchange broadened spectra.
References [l] P.B. Ayscough and F.P. Sargent, Proc. Chem. Sec. (1363) 94. (2j‘A. Beckett and G. Porter, Trans. Faraday Sec. 59 (1963) 2039. [3] H.L.J. Bäckström and K. Sandros, J. Chem. Phys. 23 (1955) 2197.
34
15 February 1977
[4] C.E. Dykstra and H.F. Schaefer 111,J. Am. Chem. Sec. 98 (1976) 401, and references therein. [5] W. Holler and D.A. Ramsay, Can. J. Phys. 48 (1970) 1759. [6] C.N. Currie and D.A. Ramsay,Can. J. Phys. 49 (1971) 317. [7] T.X. Ha, J. Mol. Struct. 12 (1972) 171. !8] D.A. Ramsay and C. Zauli, Acts Phys. Acad. Sci. Hung. 35 (1974: 79. [9] IJ. Pincelli, B. Cadioli and D.J. David, J. Mol. Struct. 9 (1971) 173. [IO] F. Verdenme, J. Chem. Phys. 52 (1970) 719. [ 111 M. Forster, K. Loth, hl. Andrist, U.P. Fringeli and Hs.H. Giînthard, Chem. Phys. 17 (1976) 59. [121 K. Loth, Thesis No. 5540, ETH-Zurïch (1975). 1131 K. Loth, 1’. Graf and Hs.H. Günthard, Chem. Phys. Letters 29 (1074) 163. 1141 H. Zeldes and R. Livingston, J. Chem. Phys. 47 (1967) 146.5. LI51 K. Loth, ‘r‘.Graf and Hs.H. Giinthard, Chem. Phys. 13 (1976) 95_ [161 K. Loth F. Graf and Hs.H. Giínthard, Chem. Phys. Letters45 (1977) 191. (171 J.A. Pople, D.L. Beveridge and P.A. Dobosh, J. Am. Chem. Sx. 90 (1968) 4201. to mag1181 A. Carrington and A. Mclachlan, Introduction netic resxrance (Harper and Row, London, 1969) p. 208. [19i H.E. Hu xiker, IBM J. Res. Develop. 10 (1971). [201 H. PauI, Chem. Phys. 15 (1976) 115. 1211 M. Forsfer, U.P. Fríngelí and Hs.H. Giinthard, Helv. Chim. Acts 56 (1973) 389.