?&It
$PECrP13A OF‘ D
X-
alters the potential energy barrier to the outward diffuslon of the metal atoms; on oxidized Fe and Pd the barrier is likely to be lowered and thus increasing the coalcentration oi t h e rcacting sites on the surface.. If ~ %assume e that 0.4 nm of oxide IS equivalent to one oxide layer ( X = I j, then the thickness of the oxide phase in any cane did not exceed 3.0 nm. Up to an aide thickness o; -4.0 nm, the presence of the super~ n p o ~electric il .held w‘as assumed14t o have a marked ir Buence on th:: observed kinetics of the tarnishing rractiun on metals. Behavioi QJ the “bTu*o-Metul”Fdm. A solid solution or an alloy, with a superlattice structure, may be formedt6*18 011 a siow cooling or anealing at low temperatures of two 11- eltals together. The tendency to form such structures irtcreases M ith increasing dirference in the atomic diampret,erit work, each of the binary rd Pd-Nj) was exposed Lo the followi i ) the heat of condensation of Pb on Psl (or Ni) (ii) thcs effect of sintering of the tmo-metal film, and (iii)the gradual heating of the film during the expernnml. with AsIl,. Thcse factors may enhance the mobility of tho metal atoms and consequently hcilitrate ihe diffusion of Pd (or S i ) atoms into P b lattice rc>sidtiiigin the formation of a solid solution or an alloy. Siuce the difference in Ihe atomic diameter between P b (1” 0.1’15 rim> anjld Si (Y 0.424 nm) is greater than I
2857
AND TRIMETHYLAMINIUM RADICALS
=L
the difference between P b and Pd (1 -- 0.134 nm), we would expect a more stable solid solution or alloy to be formed by the system Pb-Xi tharr from Pb-Pd, ‘The surface potential of the cheniisorbed gas (ASK,) may lower the potential energy barrier l o the outn aird movement of Pd (or Ni) atoms Lhrougb the Pb l a l t i ~ e ~ ’ ~ Such an outward difluion of metal atoms is likely to be greater in the IPSS stable system, that is, Pb Pd. Tbis may account for the increased reactivity of the latter system toward As&. The introduction of ti anetal into another metal disturbs Ihe ease with which the electron and t!he defect can migrate through the solid The lattice spacing in an alloy almost invariirbly lies b ~ t een x the values of the two component metals. There are also important changes in magnetic properlies & S S U C I U W ~ with the filling of the d band. These and other factors probably operate together to form the charm1~rastic propdies of a solid solution or an alioj(14) M, W. Roberts, Quart. Rev., Chem. Eoc., Irj, 71 (1962). (15) W. Hume-R.othery and H. iM.Powell, Z . Kristallogr., 9 1 , 23 (1935). (16) H. Lipson, Progr. Metal Phys., 2, 1 (1950). (17) 6. C. Bond, “Catalysis by Meta.Is,” A-cademie Press, New 1962, pp 24-28 and 484-487. York, N. (18) Y. .M. Dadiza and J. M . Salch, J @hem. SOL,Fnradag T r a m 1, 68, 1.513 (1973). (19) N. Cabrera and N. F. Mott, R e p . Progr. Phys., 12, 1 6 3 (1948)
Electron Spin Resonance Spectra of Di- and Trimethylaminium Radicals’ tfchard W. Fessenden* and P. Neta Eadiation Research Laboratories, Center for Special Studies and Department of Chemistry, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 16213 (Received April 4, 1972) Publacation costa assisted by the Carnegie-Mellon University and the U . S. Atomic Energy Commission
The esr spectra obtained during radiolysis of acid solutions of di- and trimethylamine are shown to be due to [,he radicals (CH&NH. + and (CH,),N.+ formed by OH attack on the respective ammonium ions. The parameters which give the minimum rms error between observed and calculated positions are the following: for (C11&NH. +, aN = 19.23 G, aWaH = 21.96 G, UC&I = 33.61 G, and g = 2.00354 and for (CI13),N.+,U N = 20.55 G,aCEIH = 28.56 G, and g = 2.00357. The radical (CHa)ZNH.+ and its conjugate base (CII3)gN. were also produced by the reaction of eps- with (CH&NCl. The pK for the dissociation of (CHsjZNH. is estimated to be 6.5-7.5. -+
A previous paper2 reported esr spectra obtained during the steady-state radiolysis of acid solutions of di- and trimethylamine. However, problems were encountered in attempting t o assign the observed spectra t o radiCak reslllting directly from Raction of and 0swith the Sohles. As a result the observed Spectra
were attributed t o radicals arising in secondary processes. The assignments given did not seem wholly satisfactory on a chemical basis and the need for fur(1) Supported in part by the U. S. Atomic Energy Commission. ( 2 ) P.Neta and R. w. Fessenden, J . P h w . C h m , 75, 738 (1971).
The Journal
of
Physical Chemistry, Vol. 76, N o . $0. 1971
Table 1: Esr 'Parameters of J X axid "~imet~~yiaminium RBdicals ------iCU-' \
,--.-..".--(Clti[.'
"12
Prr:sent work a I
g factor ax aNHtl CLCHlH
2.00354 19.23 (-)2[.96 33.61
Nli.+..Dmon rand
P~~etii,
~ , i c ~ r c ~ , . n ~ work" ~
2.83036 19.28 2%. '73 34.2'7
-'q
*13.
'TenCtl'~ (solid phnsg)
L'. .00::*57 20 55
38.0
28.66
26.7
a The irradiated dubions contained 0.34.5 M of the amine at p1.X 1 or 3 arid were saturated wibh NzO. The hyperfine constants are given in Gauss and are accurate to ~t-0.030. The g factors are measured relative to the .peak from t,he silica celi and are accurate LO &trO.O01P03. Parameters were calculated using 53 lines. The rms error between observed a.nd "dcuht,ed @peelra was 0.02 G. Th.e xnaximuin change in line position upon ehanging aN$ from io is lese tham the mx errov so the sign cannot be determined. c .Reference 3 . ci Parameters were cslculated using 65 lines. The rms error was 0.tX (2. * Relsr~enee7.
-+-
for aqueous solution. The formation of aminiim radicals can be wlrit ten as
(C>Ha)JVH?' -1- "OR --+ (CJ33)aNIEJ.. +
+ IeZO
(8)
and regarded as either an oxidation by electron transfer or €1 abstraction from the 1VH position. The faat that the spectrum observed a i p B 3 in the prewnce of K20 JG(OW) = 5.6) was more antense than that at p R P (G(0H) = 2.8, G(H) -= 3.6) shows that 011 i 4 the main contributor to the radicxl loa:mation ixiidw these experimental condil,ions. The rate for this reaction can be estimated from the fact that 3 rnAd fer!-bury1 alcohol caused -50% drop in the signal of (@&)dTI -+ with 0.5 M j @ R g ) ~ N H ~ +'The rate constant for re~ietion 1 $0 obtained is -lo6 M-l see Because the overall rate constant for reaction of OH T amines is -1V J4-l sec-l in acid solutnone R major. fraction of the OR must react by another path. This path has been shown by bolth ilia produet analysisq and eqr spectra2 to be the abstraction of hydrogen from methyl groups to form carbon-centered radicals. In acid solutions the csr spectra vi' these c u b o n centered radicals are not evident, apparently 8 8 R result of line broadening by exchange of the ammoniiim pro1 om. (3) W. 6 . Damn and R , C. Rickard, J . Amel. Chem,. Sot,, 94, 3254 (1972). (4) Even with this misassignment the parameters obtained by us2 are accurate because the positions of the observed linea in the secondorder groups are the same with either four or six equivalent protons. ( 5 ) D. Behar and R. W. Fessendon, Y. Phys. C'hem., 76, 1710 (1972). (6) R. W. Fessenden, J . Magn. Resonance, 1, 2'57 (1969). ( 9 ) A. J. Tench, J . Chem. Phps., 38, 593 (1963). (8) N. Getoff and F. Schworer, Int. i.Eadiat. Pkys. Chem., 5 , 429 (1971). (9) See the review by TISr.M. Garrison in "Current Topics in Radiation Research," X. Ebert and A. Howard, Ed,, North-Holland Publishing Co., Amsterdam, 1968, p 43.
2859
EBRSPECTRA OF Dx- AND TRIMETHYLAMINIUM RADICALS The large total signal intensity of the aminium radicalslO Table 11: Esr hrameters of (CH3)2N toqether wnth only ~ a r conversion ~ ~ a ~of OH imply that .the d ~ ~ a rate ~ constants p ~ for a these ~ ~species ~ are ~ ~ Present worka re1stively low. Although at plT 3 in mere found the effect of
-
strong signals of (CH&NH * wcre found e ~ ~ with~ (CH3)2NHz* ~ ~ no~signals e at, p1-l 8. This result was interpreted as the dissoeia tion (CHa)2N *
+
+ 13'
(2)
and consequent line broadening. To investigate this dissociation further zt source of radicals applicable over a wider range of p W was necessary. Following preliminary experiments with a sample of N-chlorodiisopropylaminell it \*:ab found that the reaction
a)aNG1 ----*. (CH,)2N* 4- C1-
g factor
~
a~ N
2.00440 15.65 2b. 48
s
aCHF
3
Damn, et n1.b
2.0044 14.78 27.36
The irradiated solution contained M (C!1&)2NC1 at pEI 10.0 and was deoxygenated by bubbling with pure nitrogen. The hyperfine constants are given in Gauss and are accurate to &Q.03 G. The g factor is measured relative to the peak from the silica cell and is accurate to &0.00003. Second-order corrections have been made. The unit-intensity lines both at the ends of the spectrum and in the second-order patterns were too weak to be observed. References 3 and 13. The radical was prepared by photolysis of the corresponding tetrazene in cyclopropane solution a t -90'. Q
(3)
is a source of (GyX3)2W (or of its conjugate acid (CH& KEI. +). E X J E ~ ~ I Bwith ~ I I ~(CH3)2n'cl1' S at pH 4.0 and 4 3 showed essentially the same signal intensity from (CH&YH %chile at pH 5.2 the lines were much weaker. No lines at all were detected at pH 7.2 and 8.0 but a t p B 8.7 Sines attributable to (CH&N. were found. Sorrae.vlhat higher intensities of this latter species were found a t p 9-11. The parameters found n Table I1 together with those 1 1Ei;en~ler.l~It should be noted that this radical was not found in a previous study2 with dirnetliylamine solutions although its formation was expected. The absence of lines in the region 5.2 < pH < 8.7 is taken t o mean that proton exchange (net result reaction 2 ) carma line broadening and that the pK of (CH,)'i'4N. is 6.5-7.5. 'The exchange cannot involve H+ c r OH- directly because of the near neutral conditions. Several solutes such as the -mM phosphate buffer and s
-
the (CH&NC1 are, however, present at sufficient concentration to mediate the exchange. A more quantitative study of the pK and of the exchange is precluded by the weakness of the spectra of the two forms of the radical when produced in this way.
Acknowledynzent. We are indebted to Dr. W. C. Danen for communicating his results prior to publication and for supplying a sample of N-chlorodiisopropylamine. (10) The unit-intensity lines represent 1 / 3 s r and 1/153b of the total esr intensity for (CH3)zNH + and (CH3)sN * +, respectively. (11) Kindly supplied by Dr. W. C. Danen. (12) Prepared as described by W. 8. Metcalf [ J . Chem. Soc., 148 (1942) J by mixing phosphate-buffered solutions of (CIIIB)~VH~ + and HOC1. The reaction mixture was then diluted t o a nominal 1 mM of (CH8)zNCl and used immediately after adjustment of the PH * (13) W. C. Damn and T . T. Kensler, J . Amer. Chem. Soc., 92, 5235 (1970). I
The Journal of Physical Chemistry, VoL 76, No. go, 197%
