J . Phys. Chem. 1984, 88, 1187-1190
1187
preexponential factors for the two channels. Calculated A factors are 1014 s-' for ( l a ) and 10'3.7s-' for (lb), respectively, at 1300 K, where A = ( k T / h ) ( Q * / Q ) . In the calculation of the partition function, Q, for glyoxal, both trans and cis forms were considered. Considering the differences in the A factors and in the calculated (ab initio) potential energies for the transition states (81.2 kcal mol-' for (la) and 102.3 kcal mol-' for (1b)),s it is suggested that the rate of (la) is much faster than that of (lb). This is consistent with the present experimental results, and, encouragingly, the value of A = 1014 s-' for ( l a ) is in close agreement with the experimental value. Comparison with the Photodecomposition. It is important to compare the results of the thermal decomposition and the photodeconiposition in order to discuss the reaction path. There have been reported many experimental results for photochemical processes in glyoxal since the early study by Norrish and Griffiths using a mercury arc lightz2 Product analyses in some experiments show that formaldehyde and carbon monoxide are the main products of the photolysis; on the other hand, the hydrogen molecule is only a minor p r o d ~ c t , ~The , ~ route of the photodecomposition using wavelengths longer than ca. 300 nm has _been considered as shown in Figure 10.4v5323That is, the excited A'A, state molecules transfer to the B3A, state via path 11, and a decomposition occurs to produce CHzO and CO, path 111. In part, the vibrationally excited ground state is formed by intersystem crossing from the B state, path IV, and dissociates to Hz and 2CO via path V, Le., channel la. Since the threshold energy, Eo = 55.1 kcal mol-', for channel l a is close to the lowest 5 st_ate,54.9 kcal mol-', the population of the vibrationally excited X state above Eo is very low due to the rapid vibrational relaxation. This is the reason for the small amount of Hz production in the photodissociation. In the thermal reaction, vice versa, most of the molecules that are thermally excited above Eo decompose to Hz and 2CO via path V before they transfer to the B state. In this sense, the result that the Eo value is close to the lowest B state is thought to be very reasonable to explain the experimental results involving both the thermal and the photodissociation studies.
'
'
/
/
2
'Ag
/
ICH012
Figure 10. Schematic explanation of the photodissociation routes. from Figure 9 that these two results evaluated in different temperature regions are well connected with a straight line (as shown by a dotted line), although the Arrhenius parameters in each work are slightly different (in Lowden's work, A = 1013l 4 s-' and E , = 52.6 kcal mol-'). Thus, so far as the overall decomposition of glyoxal is concerned, these experimental results are in close agreement with each other, regardless of different experimental methods. Recently, ab initio calculations have been performed for several unimolecular reactions.20 Schaefer and his c o - ~ o r k e r spaid ~~~' attention to the interesting channels in the photodissociation of glyoxal4and tried to calculate probable transition states to examine especially the possibility of the concerted reaction, Le., channel la. Their calculated results are of great interest, which provide valuable information when experimental results are examined. Thus, we tried to evaluate rate constants by using the calculated results on the basis of the transition-state theory. In the calculations, they used four different basis sets. However, there was no serious difference in the geometries of the probable transition states obtained with these basis sets. As the 3-21G basis set was used for both channels considered here, we used the data calculated with this basis set for the purpose of the comparison between the (20) See,for example, D. G. Truhlar, Ed., "Potential Energy Surfaces and
Dynamics Calculations", Plenum Press, New York, 1981. (21) Y. Osamura and H. F. Schaefer 111, J. Chem. Phys., 74,4576 (1981).
Acknowledgment. We are grateful to H. Kuroda and H. Munechika for their support in the experiments. Registry No. Glyoxal, 107-22-2. (22) R. G. W. Norrish and J. G. A. Griffiths, J . Chem. SOC.,2289 (1928). (23) J. G. Calvert and J. N. Pitts, Jr., "Photochemistry", Wiley, New York, 1966, p 376. (24) R. C. Reid, J. M. Parusnitz, and T. K. Sherwood, "The Properties of Gases and Liquids", 3rd ed., McGraw-Hill, New York, 1977.
First Electron Spin Resonance Identification of a Nitrogen Peroxy Radical as Intermediate in the Photooxidation of 2,2,6,6-Tetramethylpiperidine Derivatives A. Faucitano,* A. Buttafava, F. Martinotti, Dipartimento di Chimica Generale, K l e Taramelli, 12-27100 Pavia, Italy
and P. Bortolus FRAE C.N.R. Bologna, Italy (Received: May 10, 1983; In Final Form: August 19, 1983) ESR evidence has been obtained for the existence of an N-peroxyl radical intermediate in the conversion of the 2,2,6,6tetramethylpiperidinaminyl radical to the corresponding nitroxide. This observation is of interest for the understanding of the mechanism of photostabilization of polyolefines by tetramethylpiperidine derivatives. The species identified is the first example of a novel class of peroxy radicals characterized by the direct binding of the oxygen atoms to a nitrogen atom; information regarding its structure has been obtained by INDO MO calculations and by ''0 ESR measurements.
The inhibition mechanism of polypropylene photooxidation by 2,2,6,6-tetramethylpiperidine (TMP) derivatives has been the 0022-3654/84/2088-1187$01.50/0
object of numerous investigations' but several important aspects of the mechanism are still far from being fully understood. A 0 1984 American Chemical Society
1188 The Journal of Physical Chemistry, Vol. 88, No. 6, 1984
Faucitano et al.
generally accepted sequence within the overall reaction scheme is thought to involve the formation of TMP aminyl radicals which then react with oxygen yielding the T M P nitroxide radicals: R
R
H
0
Conclusive ESR evidence of the aminyl nitroxide conversion was obtained by Roberts and Ingold2 who also proposed a mechanism based on the N-peroxyl radical intermediate: R
R
I
I
J I
0 ' 0 .
0,
0
'
'0.
However, any attempt to detect this intermediate by ESR was unsuccessful.2 As no information is available from the literature pertaining to the existence and stability of N-peroxyls, it cannot be excluded that they are not formed at all and that a different mechanism applies for the aminyl nitroxide conversion. The previous measurements have been made in the liquid state or close to the matrix melting point. As a consequence it is possible, because of the fast rate of coupling, that the N-peroxyls do not attain the limit of detection. If this is so, it would be of great help to work with a solid matrix which, while not preventing the diffusion of oxygen, hinders the translational motion of radical centers. Following this line, we have succeeded in obtaining evidence for the formation of the N-peroxyl intermediate and for its conversion into the corresponding nitroxide. Films of isotactic polypropylene about 0.05 mm thick containing 3-5% of a 2,2,6,6-tetramethyl derivative (tetramethylpiperidinyl sebacate) were sealed under high vacuum in quartz tubes and exposed to the full light from a 100-W high-pressure mercury lamp at 77 K, an intense signal soon developed, consisting of a broad slightly asymmetric triplet (Figure la). This signal is unambiguously assigned to a T M P aminyl radical by comparison with the spectrum of the true 2,2,6,6-tetramethylpiperidinyl radical obtained by photolysis of TMP-benzene solutions containing cumyl peroxide (Figure 2a,b). At room temperature the characteristic TMP aminyl spectrum consists of a triplet of multiplets generated by the interaction of the unpaired electron with a nitrogen nucleus (aN = 14.5) and at least 12 y, 6 protons (aH = 0.75 G, Figure 2a); at low temperature, in the solid state, this signal takes the appearance of a broad triplet practically identical with that of Figure l a (Figure 2b). This marked temperature effect must be related to the anisotropy of the nitrogen hyperfine (hf) tensor, and the peak separation of about 30 G, observed in the rigid spectrum, is likely to correspond to the Allprincipal value. Under
-
(1) Sedlar, J.; Marchal J.; Petruj, J. Polym. Photochem. 1982, 2, 175, and other references therein. (2) Roberts, J. R.; Ingold, K. U. J. Am. Chem. SOC.1973, 95, 3228.
Figure 1. ESR spectra of UV-irradiated polypropylene containing 5% 2,2,6,6-tetramethylpiperidinyl sebacate: (a) recorded a t -1 80 OC following the UV irradiation under vacuum a t -196 OC,showing the formation of the tetramethylpiperidinyl radical; (b) the same as (a), after the admission of air, showing the formation of the N-peroxyl radical.
b
' V
m
Figure 2. ESR spectrum of the 2,2,6,6-tetramethyIpiperidinyl radical in benzene solution recorded a t room temperature (a) and a t -140 O C in the solid state (b).
vacuum, the aminyl radical is stable up to about -100 OC;above this temperature it begins to decay, presumably by abstracting hydrogen from neighboring molecules2 without generating new detectable ESR signals. Under air, the behavior of the aminyls
Photooxidation of 2,2,6,6-Tetramethylpiperidine Derivatives
The Journal of Physical Chemistry, Vol. 88, No. 6, 1984 1189
I " " 1 " " " " ' l "
V Figure 5. ESR spectra of 4-amino-2,2,6,6-tetramethylpiperidin-l-yloxy radical recorded in a polymer matrix (a) and in solution (b).
Figure 3. Low- and high-gain recording of the N-peroxyl radical spectrum obtained after admission of oxygen (20% enriched I7O)showing the 170hf features.
h
-
20
oc
f"
TABLE I: Experimental and Calculated (INDO MO) Hyperfine Parameters for the N-Peroxyl Radical 170p orbital AXX(170) spin density a P a P gzz gxx,gyy expt 0609 0224 95 35 2 033 2.006 calcd 0 7 5 0.25 84 9 22.8 way it formed, following the decay under oxygen of the precursors aminyl radicals and its subsequent conversion into the nitroxides signal, strongly supports its assignment to the N-peroxyl internitroxide conmediates (reaction 2). Most of the N-peroxyl version takes place in the temperature range -80 to +20 OC where segmental diffusion in the amorphous regions (where the T M P based additive is thought to be localized) is less hindered so that bimolecular encounters between radicals are possible; therefore a mechanism based on coupling with disproportionation (reaction 3) may be considered reasonable.
-
Structure of the N-Peroxy Radical
Figure 4. ESR spectra of the conversion of the N-peroxyl into the tetramethylpiperidinyl nitroxide radical above -100 O C . The peaks marked with an asterisk belong to the nitroxyl radical.
follows a completely different pattern. The onset of the decay is observed at a temperature as low as -150 OC and it is accompanied by the buildup of a new signal which has the shape expected from strongly immobilized randomly oriented peroxy radicals with a nearly axially symmetric g tensor (gzz= 2.033 and g,, gyy = 2.006, Figure lb). The identification of peroxy type radicals is further supported by the two 1 7 0 hf structures, nearly symmetrically disposed about the g, position, which consist of two sextets of 95 and 35 G (Figure 3); these 1 7 0 couplings are assigned to two magnetically different oxygen atoms in the most abundant labeled species R170160and R160170-(the third species R170170. is not detected because of its much lower concentration). When warmed above -80 "C, the peroxyls decay generating the T M P nitroxide radicals (Figures 4 a-d and 5). Although the signals of Figure 1b do not show any resolved nitrogen hf splittings, the
-
If it is assumed, as is reasonable, that the 1 7 0 hf and the g tensors share nearly the same principal axis system, then the observed I7O couplings of 95 and 35 G are assigned to the A,, principal values of a and 170.Furthermore, as no other resolved 170structures are observed, it may be inferred that A,, and A , are very small. By neglecting them, to a first approximation, one can obtain the following isotropic values from the traces of the tensors: A,,,( 170), = 3 1.6 G; Ais,(170)@= 11.6 G. By assuming the same sign for the isotropic (Also)and anisotropic (B,,) components for each of the A,, principal values, one finds that the corresponding B,, values for the a and p 170dipolar hyperfine tensors are calculated to be 63.4 and 23.3 G. These values, when compared to BFo= 104 G,3yield 60.9% and 22.4% of p orbital character for the a and p 1 7 0 nuclei, respectively. Allowance being made for the approximations adopted in their derivation, the magnitude of the isotropic a and p 170hf couplings in the Nperoxyl radical is seen to be similar to that of the couplings previously reported by Adamic, Ingold, and Morton4 for carbon-centered peroxy radicals and by Howard and Tait for organometallic 5B peroxy radical^.^ Systematic INDO M O calculations, performed on the model radical (CH3)2N02. as a function of the 0-0 and N-0 bond lengths and CNO and N O 0 bond angles, have shown that a satisfactory agreement between calculated and experimental hf parameters can be obtained (Table I) by adopting the following geometry:
(3) (a) Atkins, P. W.; Symons, M. C. R. "The Structure of Inorganic Radicals"; Elsevier: New York, 1967; p 21. (b) Che, M.; Tench, A. J. J . Chem. Phys. 1976, 64, 2370. (4) Adamic, K.; Ingold, K. U. Morton, J. R. J . Am. Chem. SOC.1970,92, 922. (5) Howard, J. A.; Tait, J. C. Can. J . Chem. 1978, 56, 2163
1190
J . Phys. Chem. 1984,88, 1190-1 194
The most sensitive parameter was found to be the N O 0 angle; the value of 144O obtained may appear too large at first glance when compared to the corresponding bond angles in peroxides (104O, ref 6) but it is in agreement with the results of a recent studies7 which have shown that values ranging from 104O to 14O0 (6) “Tables of Interatomic Distances and Configuration in molecules and Ions” Chem. SOC.Spec. Publ., 1958,No. 1 1 . (7) Suryanarayana, D.; Kevan, L.; Schlick, S. J . Am. Chem. SOC.1982, 104, 668.
may be expected for peroxy radicals.
Acknowledgment. Financial support by the C.N.R. (Progetti Finalizzati Chimica Fine e Secondaria) is gratefully acknowledged. Registry No. N-Peroxyl-2,2,6,6-tetramethylpiperidin-4-y1 sebacate 2,2,6,6-tetramethylpiperidinyl radical, 3895 1-81-4; radical, 88730-71-6; 4-amino-2,2,6,6-tetramethylpiperidin1 -yloxy radical, 14691-88-4; 2,2,6,6-tetramethylpiperidinyl sebacate, 57013-29-3;(dimethylamino)peroxy radical, 88730-72-7.
Hydrogen Bonding in a-Phenylethyl Hydrbperoxide
R. DanGczy, S. Holly, G . Jalsovszky, and D. Gil* Central Research Institute for Chemistry of the Hungarian Academy of Sciences, H-1525 Budapest, Hungary (Received: May 24, 1983)
The H-bonded associates of a-phenylethyl hydroperoxide (HROOH) were investigated in CC14,at various concentrations (0.788 X 10-3-960X M) and temperatures (22-75 “C), by using IR spectroscopicmethods. A very stable intramolecular OH-T bond was observed with a low enthalpy value. Above a concentration of 50 X M cyclic dimers and trimers were found, the latter with the lowest thermal stability. The corresponding thermodynamic parameters have been calculated.
Introduction We report a systematic study of the hydrogen-bonded associates of a-phenylethyl hydroperoxide (HROOH) carried out by infrared spectroscopy. Special attention has been paid to the various forms of association by means of intra- and intermolecular hydrogen bonds, and to their thermodynamic parameters, since they may affect the decomposition of H R O O H in different ways. The kinetics and mechanism of the decomposition of hydroperoxides, affected presumably by their associated complexes, are of great importance with regard to the low-temperature oxidation of hydrocarbon^.^-' These decomposition processes result in secondary initiation, thus contributing to the determination of the reaction kinetics of the process. Recently many alky1,4,5,s-15aralky1,5*15-25 and cyclic and het(1) L. Bateman and H. Hughes, Q. Reu., Chem. SOC.,8, 147 (1954);J . Chem. SOC.,4594 (1952). (2) V. Stannett, A. E. Woodward, and R. B. Mesrobian, J. Phys. Chem., 61,360 (1957). (3) R. Hiatt and K. C. Irwin, J . Org. Chem., 33, 1436 (1968). (4) J. R. Thomas and 0. L. Harle, J. Am. Chem. SOC.,77, 246 (1955). (5) 0.P. Yablonskii, V. A. Belyaev, and A. N. Vinogradov, Russ. Chem. Reu., (Engl. Transl.), 41, 565 (1972), and papers cited therein; M. S. Yablonskii, Dissertation, Institute for Chemical Technology, Ivanovo, USSR, 1979. (6) Ch. F.Wurster, L. J. Durham, and H. S. Mosher, J. Am. Chem. SOC., 80, 327 (1958). (7) Dankzy, G. Vasvlri, and D. GBI, J . Phys. Chem., 76,2785 (1972); J . Chem. SOC.,Faraday Trans. 1 , 73, 135 (1977); 75, 236 (1979). (8) Ch. Walling and D.Heaton, J . Am. Chem. SOC.,87,48 (1965). (9) H. E.Mare, J . Org. Chem., 25, 2114 (1960). (10)S. S. Ivanchev, P. I. Selivanov, and V. V. Konovalenko, Zh. Prikl. Spektrosk. 16,508 (1972);22, 877 (1975). (1 1) G. Geisseler and H. Zimmermann, Naturwissenschaften, 51, 106 (1964). (12) Sh. Fujiwara, M. Katayama, and S. Kamio, Bull. Chem. SOC.Jpn., 32, 657 (1959). (13) H . R. Williams and H. S. Mosher, Anal. Chem., 27, 517 (1955). (14) G. E.Hall and D. G. Roberts, J. Chem. SOC.B, 1100 (1966). (15) G.L. Bitman, M. E. Elyashberg, and N. P. Makove’eva, Dokl. Akad. Nauk USSR,208,631 (1973);Zh. Obshch. Khim., 43, 1777 (1973);44, 1361 (1974) ‘ (16) L.P. Mamchur, Yisn.L’uivpolitekh. Inst.,58, 14 (1971);Zh. Prikl. Spektrosk., 22, 1067 (1975). (17) R.V. Kucher, F. P. Ivanenko, and I. P. Shevchuk, Neftekhimiya, 7, 751 (1967);8,236 (1968); 10,730 (1970). (18) H . Kropf, C. R. Bernert, Fresenius’, Z . Anal. Chem., 248,35 (1969).
e.
0022-3654/84/2088-1190$01.50/0
e r o c y ~ l i cmonohydroperoxides ~ ~ ~ ~ , ~ ~ ~ ~ as well as di-,17O X Y - , and ~J~ keto hydroperoxide^^*^^ and azo hydroperoxide^^^ were investigated in various solvents by using 1R and N M R methods for determining the structures and thermodynamic parameters of the associates. According to literature data the following types of association are possible: (i) intermolecular self-association in the form of linear dimers, etc., cyclic dimers, and n-mers, with the participation of cyclic dimers; (ii) intramolecular association with aromatic rings, double bonds, etc. (OH..*),and with functional groups; (iii) intermolecular associations with solvent and product molecules. It was established that the structure and heat of formation of the associates affect considerably the activation energy of hydroperoxide decomposition?J7 For example, Kucher et al.17found that the activation energy of the decomposition of cumene hydroperoxide is much lower in nitrosobenzene (50 kEmol-’) than in cumene (108 kEmol-I). At the same time the monomer-dimer (19) G. J. Minkoff, Proc. R. SOC.London, Ser. A , 224, 176 (1954). (20) I. F. Franchuk and L. I. Kalinina, Zh. Prikl. Spektrosk., 15, 896 (1971); Teoc Eksp. Khim., 8,552 (1972);Zh. Fiz. Khim., 46,2771, 1673 (1972). (21) D. Bernard, K. R. Hargrave, and G. M. G. Higgins, J . Chem. SOC., 2845 (1956). (22) V. A. Terent’ev and V. L. Antonovskii, Zh. Fiz. Khim., 41, 499 (1967); 42,1880(1968)(Russ.J . Phys. Chem. (Engl. Tranrl.), 8,1968); “The Chemistry of Orgadic Peroxide Compounds and Autoxidation”, Khimiya, Moscow, 1969,pp 435, 442. (23) I. B. Rabinovich and N. A. Kozlov, “The Chemistry of Organic Peroxide Compounds and Autoxidation”. Khimiva. Moscow. 1969. D 463. (24) V. P a r h a n u , I. Hogea, and L. ChomontLany, Chim. Anal., 5 2 1397 (1970). (25) F.I. Tovstokhat’ko, V. I. Ovchinnikov, S. K. Kozlov, V. M. Potekhin, and V. P. Pozdnyakov, Zh. Prikl. Khim., 53, 1924 (1980). (26) Lee-Poy and F. V. Anthony, Diss. Absrr. In?. B., 39, 1403 (1978). (27) K. Takumi, I. Kohichi, and I. Kuniaki, Nippon Kagaku Kaishi, 252 (1976). (28) 1. I. Glazyrina, Yu. E. Shapiro, G. N. Koshel, and I. V. Shutov, Zh. Obshch. Khim., 49, 444 (1979). (29) A. I. Chirko, I. G. Tushenko, G. M. Kochovskii, and A. F. Abramov, Belorussian Bull. Acad. Sei. Chem. Serb,No. 5, 122 (1975). (30)L.A. Dement’eva and G. A. Kurkchi, Zh. Prikl. Spektrosk., 14,482 (1972). (31) Kuniaki Itoh, Nippon Kagaku Kaishi, 714 (1978). (32) G. N.Koshel, I. I. Glazyrina, 0. P. Yablonskii, and N. S. Lastochkina, Izu. Vyssh. Uchebn. Zaued., Khim. Khim. Tekhnol., 18,427(1975);Zh. Prikl. Khim. (Leningrad), 51, 10, 2325 (1978). (33) W. H. Richardson and R. F. Steed, J . Org. Chem., 32,771 (1967). (34) M. Schultz and V. Missol, J . Prakt. Chem., 322, 417 (1980).
0 1984 American Chemical Society
