322
The Journal of Physical Chemistry, Vol. 82, No.
S. Nagai, K. Matsuda, and M. Hatada
3, 1978
Application of the Spin Trapping Technique to the Study of Radiation Effects on Gaseous Mixtures of Carbon Monoxide and Hydrogen Sir0 Nagal," Kojl Matsuda, and Motoyoshi Hatada Osaka Laboratory for Radiation Chemistry, JAERI, Mii-minami 25- 1, Neyaga wa, Osaka 572, Japan (Received September 12, 1977) Publication costs assisted by the Japan Atomic Energy Research Institute
The spin trapping technique has been applied to the radiation chemistry of gaseous mixtures of carbon monoxide and hydrogen. A mixture of CO and H2 has been irradiated with electron beams in the absence or presence of phenyl-N-tert-butylnitroneused as a trapping agent of radicals. Similar experiments have been carried out on hydrogen, helium, and a mixture of CO and deuterium. ESR spectra of the products provide evidence for the formation of H atom and methyl radical from a mixture of CO and H2.
Introduction Extensive studies carried out recently in our laboratory of radiation effects on mixtures of carbon monoxide (CO) and hydrogen (H2) indicate that irradiation of the mixture produces a great number of products: COz, H20, hydrocarbons, alcohols, aldehydes, esters, and polymers of unknown structure.l These products were found to show complicated variations in their yields with experimental conditions such as composition and flow rate of the reactant gases, indicating an intricate mechanism for the formation of the individual products. Mechanism producing C02,3H20,C3H4,C23H4,3HC3H0, and a white polymer in the self-radiolysis of mixtures of CO and tritium (3H2) has been discussed by Beattie.2 According to him, a mechanism consistent with the initial rates for the formation of products involves a reaction between excited 3H2 and CO to produce CH and OH radicals. Gas phase atoms and radicals have been successfully detected by the spin trapping te~hnique.~ We have applied the technique to the radiation chemistry of CO-H2 mixtures with the principal purpose of detecting radicals formed in the gas phase. This report describes our successful detection of H atom and methyl radical (.CH3) produced by irradiation of a mixture of CO and H2 with electron beams. Experimental Section The present study was carried out with the same apparatus as employed by our studies on radiation effects of gaseous mixtures of CO and Hz. The flow reactor shown in Figure 1is made of stainless steel with an inner volume of 500 X 20 X 2 mm. A titanium foil window (30-1m thick) was attached at the top of the reactor, through which electron beams could penetrate into the reactor. The bottom of the reactor is cooled with running water in order to avoid a temperature rise due to irradiation with electron beams. Temperatures in the reactor were recorded on a multichannel plotting recorder through the use of an alumel-chrome1 thermocouple. The highest temperature recorded during irradiation was 70 "C. Phenyl-N-tert-butylnitrone(PBN) was used as a trapping agent for the radicals. In the first series of experiments, PBN powder dispersed on glass wool was placed near the outlet of the reactor as shown in Figure 1. Care was taken to shield PBN from incident electron beams by placing a P b block over PBN. A mixture of CO and Hz gases with a ratio HZ/CO of approximately 6 was introduced at flow rates of 142, 175, or 350 mL/min to the reactor through needle valves and 0022-3654/78/2082-0322$0 1.OO/O
stainless steel tubing of 3 mm i.d. and 10 m length from the gas containers. Irradiation was carried out with electron beams of 600 keV and 3 or 5 mA from a rectified transformer type accelerator (Nisshin-High Voltage Co. Ltd.). The irradiated gases pass over the glass wool dispersed with PBN and a liquid nitrogen trap attached to the outlet of the reactor. After irradiation for 30 min, the PBN powder and products in the reactor and cold trap were dissolved in benzene, outgassed, and subjected to inspection by ESR. A second series of experiments was carried out with the same apparatus and methods as the first, except that PBN powder dispersed on glass wool was placed in the irradiation zone (see legend to Table 11). PBN was irradiated in a flow of a mixture of CO and H2 for 1min. The results were compared with those obtained in a similar experiment where PBN was irradiated in a flow of He gas. Further, radiolysis of PBN in a flow of a mixture of CO and was studied. deuterium (Dz)
Results and Discussion Results obtained from the experiments where the gaseous mixture of CO and Hz was irradiated in the absence of PBN and thereafter brought into contact with PBN are described first. When the flow rate of the mixture was 175 or 142 mL/min, at least five types of nitroxides were detected in the products. The ESR spectrum observed from the products collected in the cold trap consists of a triplet with 15.5-G spacing, accompanied by two satellite lines separated 2.1 G from each component of the triplet. This spectrum is assigned to di-tert-butyl nitroxide (V).4 The products in the reactor were divided into three parts, A, B, and C as shown in insert I of Figure 1, and the ESR spectra of the individual parts were recorded. The spectrum of A together with the analysis is shown in Figure 2 . The predominant component is a triplet of doublets with aN = 14.4 G and uH = 2.2 G, which is tentatively assigned to the C(CHJ3 adduct (111) of PBN. Other contributors to the spectrum are the H atom adduct (I) and the .CH3 adduct (11) of PBN. The hyperfine splitting values for the former are uN = 15.0 G and aH= 7.5 G, and those for the latter are uN = 15.0 G and aH = 3.75 G. Nitroxides I and I11 were also seen in the spectra of B and C. The spectrum of B, in addition, indicated the presence of a triplet with 8.0-G spacing which is ascribed to benzoyl-tert-butylnitroxide (VI).5 Table I summarizes the nitroxides assigned from the observed spectra. When the flow rate of CO-H2 was as high as 350 mL/ min, the products collected in the cold trap gave ESR spectra exclusively due to the H atom adduct (I). The 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 323
Radiation Effects on Gaseous Mixtures of CO and H2
TABLE I: Nitroxides Detected in the Products of Irradiation of a Mixture of CO and H, Followed by Contact with PBNa Flow rates A B C D H,. 150 mLlmin:
0.
0-
C, H,CH-N-C(CH, I
),
I
C(CH,), a
,1,
,
C, H C-N-C(CH
I C,H,CH-N--C(CH,),
0.
c,H,cH-N-c(cH,), I
I
C,H,CH-N--C(CH,), I C(CH,),
PBN dispersed on glass wool was placed in A, B, and C in the reactor as shown in insert I of Figure 1. Scheme I
0I
C6H5CH=N-C(CH,),
I
PiN
I
H
0 I
C,H,CH,-N-C(CH,), I
\\y
0.
L ~ , ~ , ~ ~ - N - ~ ( ~ ~ , ) , 11
0. I
C, H, CH-N-C( I
I
CH, ),
C(CH313 I11
O= N-C( CH, ),
0.
Irradiation sone
Flgure 1. Schematic of flow reactor.
t-BuNo
CH,-N-C( CH, ), Iv
\C(CH,),
/
0 0I
/
C,H,C-N-C(CH,), VI
Figure 2. ESR spectrum of products A (see legend to Table I) resulting from irradiation of a mixture of CO and H2 followed by bringing into contact with phenyl-N-terf-butylnitrone (PBN). The spectrum indicates the presence of at least four types of nitroxides, three of which are assigned to the C(CH,), adduct (111), the H atom adduct (I), and the GH, adduct (11) of PBN. Lines denoted by arrows are not identified.
spectrum of the products in the reactor was due to nitroxides 11, 111, and VI. In a similar experiment with H2 instead of a mixture of CO and H2, a t a flow rate of 150 mL/min, nitroxides I, 111, and VI were detected from the products in the reactor and nitroxide V from the products in the cold trap. Reactions producing nitroxides 1-111, V, and VI detected in the first series of experiments are shown in Scheme I. Nitroxides 111, V, and VI evidently arise from recombination of the decomposition products of PBN. The formation of nitroxide VI by UV irradiation of PBN in benzene has been reported by Bluhm and Mrein~tein.~ These authors have discussed possible mechanisms for the formation of nitroxide VI, one of which assumes the
formation of 2-methyl-2-nitrosopropane (tert-nitrosobutane, t-BuNO, O=NC(CH,),) by decomposition of PBN through phenyl-N-tert-butyloxazirane.Our observation of nitroxides V and VI, together with nitroxide IV to be described later, indicates the presence of t-BuNO as an intermediate in our system, although the reaction mechanism producing t-BuNO from PBN is not known. Although H atom and .CH3 were successfully detected in the experiments described above, the formation of nitroxides 111, V, and VI indicates appreciable decomposition of PBN, most likely due to scattered electron beams and secondary x rays. Thus there is some doubt whether the H atom and the C H 3trapped by PBN would be produced from a mixture of CO and Hz. Instead they might have come from PBN. The above problem was investigated in the second series of experiments where PBN was irradiated in a flow of either a mixture of CO and Hz or He. Nitroxides observed from the products were I, 111-VI as shown in Table 11, irrespective of different irradiation atmospheres of PBN. The relative concentration of the nitroxides, however, was quite dependent on the atmosphere. Figure 3a,b shows the ESR spectra of the products collected in the cold trap after irradiation of PBN in a flow of a mixture of CO and Hz, and of He, respectively. Both spectra are due to nitroxides IV and V. The hyperfine splitting values for nitroxide IV, the C H 3 adduct of tBuNO, are uN = 15.7 G and uH = 11.7 G in agreement with reported value^.^ As described before, nitroxide V arises
324
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
S. Nagai, K. Matsuda, and M. Hatada
TABLE 11: Nitroxides Detected in the Products of Irradiation of PBN in the Flow of a Mixture of CO and H, and of Hea Flow rates A B C H,, 126 mL/min; 0. 0 0. 0. I I iI I CO, 23 mL/min C,H,CH,-N-C(CH3),
C, H,C-N-C(CH
),
CH,-N-C(CH,
),
0.
I C6H5CH-N-C(CH3
I
),
C(CH,)3
0.
I
C(CH3 ),--N-C(CH,
He, 150 mL/min
0. I C6HSCH,-N-C(CH, 0. I C, H, CH-N-C(CH,
I
)j
0 0. ),
il I C6H,C-N-C(CH,),
),
C(CH3 13 0 0. /I I C,H5 C-N-C(CH,), a
PBN dispersed on glass wool was placed in A and B in the reactor as shown in insert I1 of Figure 1.
Figure 3. ESR spectra of products collected in the cold trap after irradiation of phenyl-N-ten'-butyinitronein the flow of a mixture of CO and H2 (a), and in the flow of He (b). Both spectra are due to the CH3 adduct (IV) of 2-methyl-2-nitrosopropaneand di-teet-butylnitroxide (V).
exclusively from the decomposition of PBN. Nitroxide IV, on the other hand, may be produced by addition of CH3 to t-BuNO formed by decomposition of PBN, where C H 3 could be formed both from decomposition of PBN and from the mixture of CO and H2. If one estimates the concentration of nitroxide IV relative to nitroxide V from each spectrum in Figure 3, the value from the spectrum a is more than 30 times greater than that from the spectrum b. This means that most of the .CH3 trapped by t-BuNO in the flow of CO-HZ is produced from the gaseous mixture. The ESR spectra of the products collected in the irradiation zone of the reactor showed the presence of H atom adduct (I) of PBN and nitroxide I11 in addition to a small amount of nitroxide V or VI. The concentration of the H atom adduct (I) relative to nitroxide I11 produced in the flow of CO-H2 is estimated to be about three times higher than that produced in the flow of He, which indicates that the H atoms trapped by PBN in the flow of CO-H2 are produced from both CO-H2 and PBN in comparable amounts. The successful detection of H atom and methyl radical was further confirmed by a similar experiment carried out
Figure 4. ESR spectra of products collected in the reactor (a), and in the cold trap (b), after irradiation of phenyl-N-ten'-butylnitrone (PBN) in the flow of a mixture of CO and D,. The spectrum a is due to the .C(CH& adduct (111), the H atom adduct (I), and the D atom adduct of PBN. Spectrum b is due to the CD, adduct of 2-methyl-2-nitrosopropane.
with a mixture of CO and deuterium (DJ. PBN powder was irradiated in the flow of the gaseous mixture for 30 s. The ratio of D2/C0 and flow rate of the mixture were 6 and 175 mL/min, respectively. Figure 4a,b shows the ESR spectra of the products collected in the reactor and cold trap, respectively. The spectrum in Figure 4a is due to three types of nitroxides, C(CHJ3 adduct (111),H atom adduct (I), and D atom adduct of PBN, as shown by the stick plots below the spectrum. The hyperfine splitting values for the D atom adduct are uN = 14.9 G, uH = 7.5 G, and uD = 1.1G. The ratio of the concentrations between the D atom adduct and H atom adduct (I) is roughly 2, indicating appreciable decomposition of PBN followed by recombination to produce the H atom adduct (I). The spectrum in Figure 4b is due to CD3N(O-)C(CH& for which the hyperfine splitting values are uN = 15.5 G and uD = 1.8 G. The C H 3 adduct (IV) of t-BuNO was not detected, which supports the above-described suggestion that -CH3is produced predominantly from the mixture of CO and H2. Acknowledgment, The authors thank Professor Keiji Kuwata of Osaka University for kindly supplying the
Internal Vs. External Referencing in NMR Studies
phenyl-N-tert-butylnitroneused in this work, and Dr. Ryusei Of Research for helpful discussions.
References and Notes (1) Ann. Rep. Osaka Lab. Radiat. Chem., JAERI-M, 6260,4 (1975); JAERI-M, 6702, 4 (1976); JAERI-M, 7355, 4 (1977).
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 325 ( 2 ) W. H. Beattie, Report of the Los Alamos Scientific Laboratory, University of California, LA-4658 (1971).
(3) E. G. Janzen, “Creation and Detection of the Exclted State”, W. R. Ware, Ed., Marcel Dekker, New York, N.Y., 1976, Chapter 3. (4) A. R. Forrester, J. M. Hay, and R. H. Thomson, “Organic Chemistry of Stable Free Radicals”, Academic Press, New York, N.Y., 1968, Chapter 5. (5) A. L. Bluhm and J. Weinstein, J . Am. Chem. SOC.,92, 1444 (1970).
Internal Vs. External Referencing in Nuclear Magnetic Resonance Studies of Complex Formation. Complexes of Acetylenes with Benzenes, Thiophenes, and Furans Wayne C. Appleton+ and James Tyrrell” Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 1 (Received August 8, 1977)
Complex formation of benzoylacetylene and phenylacetylene with a number of methylated benzenes, furans, and thiophenes as well as their parent compounds has been investigated using NMR techniques. The relative effectiveness of internal vs. external referencing methods are investigated for the different solvent systems. Equilibrium constants obtained using the external referencing procedure are calculated. Variable temperature studies on the benzoylacetylene-toluene and benzoylacetylene-2-methylthiophenecomplexes provide estimates of the enthalpy of formation of these complexes.
Introduction The ethynyl proton in acetylenic compounds is well known to act as an electron acceptor in hydrogen bonding,’ the electron donor being, most commonly, the lone pair of a highly electronegative atom such as oxygen or nitrogen or the 7 electrons of an aromatic or unsaturated molecule. The principal physical technique utilized for the study of this form of complex formation has been nuclear magnetic resonance spectroscopy (NMR), observing the variation of the position of the ethynyl proton resonance signal, relative to some reference, as a function of the changing concentration of the electron donor solvent in some “inert” solvent.2 The “inert” solvents used have been principally chloroform, carbon tetrachloride, and cyclohexane with a preference in more recent work for cyclohexane because of evidence that chloroform and carbon tetrachloride are themselves capable of complex f ~ r m a t i o n . The ~ referencing technique used in the vast majority of studies has involved an internal reference where the reference is dissolved in the solution being studied. In particular, cyclohexane is often utilized both as an inert solvent and as the internal reference. This technique has the advantage of simplicity of operation and eliminates the need for correction of the data obtained for differences in the bulk magnetic susceptibility. The use of an internal reference presupposes that either the position of the proton resonance signal of that reference will be invariant to changing solution composition or that at worst any changes in its position will be insignificant relative to the shifts in the signal of the hydrogen under investigation. Becker4 has shown that the proton resonance signals of commonly used references such as chloroform, cyclohexane, and tetramethylsilane (TMS) show significant chemical shifts in the presence of increasing concentrations of aromatic solvents due to the changing magnetic anisotropy of the solvent surrounding the reference molecule. Though Becker4 has indicated, Present Address: Velsicol Chemical Corp., A n n Arbor, Mich.
0022-365417812082-0325$0 1.OO/O
based on his investigation, that these shifts in the position of the internal reference can cause serious error particularly in dealing with weak hydrogen bonds where the shifts may be of the order of 10 Hz, and that internal references should be avoided in systems containing aromatics at high concentration there is little evidence that this caution has been heeded by the majority of the large body of workers who have utilized NMR as a tool to study hydrogen bonding or, more generally, complex formation in solution. Most of the work done in the area of NMR studies of complex formation particularly in aromatic solvents utilizes internal references for the reasons mentioned earlier but without, in general, any indication of concern for errors introduced by the shift in the reference position with changing concentration. This is particularly true in the study of the so-called “aromatic solvent induced shift” (ASIS) which has been the subject of extensive investig a t i ~ n .There ~ have been relatively few attempts made to take account of the medium shift of the internal reference. Rummens and Krystynak6 discussed the relationship of the ASIS to the chosen internal reference and attempted to determine a modified ASIS independent of the nature of the reference. Appleton and Tyrrel17 considered the relative merits of internal vs. external references in the case of complex formation between acetylenes and anisoles where the perturbation due to the medium effect on the internal reference is substantial. The use of an external reference in the case of complex formation between the acetylenes and the anisoles allowed the results to be interpreted in terms of a 1:l complex whereas the data obtained using the internal reference did not appear to fit any meaningful model. The present work is an attempt to extend the method applied in the case of the anisoles to complexes formed between the same acetylenic compounds and a variety of benzenes, thiophenes, and furans and to again compare and contrast the use of an external vs. an internal reference. The model used to interpret, the data is based on a method devised by Landauer and McConnelP using the 0 1978 American Chemical Society
