3178
S. TAKAO, Y. HATANO, AND S.SHIDA
above: (1) absence of OCP in the X-ray and ir spectra of the materials; (2) the proton of HP042- groups in OCP is easily replaceable by deuterium exchange14 (e.g., at 60" and moderate D20 vapor pressure), while in our samples the deuterium exchange occurs only under hydrothermal conditions (Figure 1); (3) if nonstoichiometric hydroxyapatites were constituted of epitaxial intergrowths of OCP and stoichiometric apatite, there is no simple justification for the break observed a t M/P = 1.5 in the relation between HPOd2concentration and deficiency (Figure 2); (4) only the presence of water molecules replacing OH- groups in the cages of nonstoichiometiic apatites can cause the integrated intensity of the VL(OH)band to be independent of stoichiometry. Since pure OCP does not contain a librational band,23the intensity of v ~ ( 0 H ) should decrease if OCP were present in our nonstoichiometric samples. It, seems likely that, when the precipitation of hydroxyapatite is carried out by rapid mixing of the reagents, both cationic (M2+) and anionic (OH-) va-
cancies are present in the lattice. Experimental evidence has been given in this study that cationic and anionic vacancies are filled by interstitial protons and water molecules, respectively. This does not preclude the possibility that under different conditions of synthesis (e.g., in biological systems) nonstoichiometry may result from t'he presence of OCP.21 Acknowledgments. The authors are grateful to the National Research Council of Canada for an operating grant, and to the Canada Council, who granted S.J. Joris a post-doctoral fellowship. Special thanks are due to Dr. G. Y. Chao and Dr. G. B. Skippen, Geology Department, Carleton University, in whose laboratories the X-ray spectra were determined and t,he high pressure deuterium-exchange experiments were performed, respectively. The authors are indebted to D. Luk and S. Murray for technical assistance. (23) B. 0. Fowler, E. C. Moreno, and W. E. Brown, Arch. Oral Biol., 11, 477 (1966).
The Gas-Phase Radiolysis of Nitrous Oxide. The Effect of the Addition of Several Hydrocarbons by Satoshi Takao, Yoshihiko Hatano, and Shoji Shida* Laboratory of Physical Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo
(Received March 1, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
The effect of the addition of several paraffins and olefins on the gas-phase radiolysis of NzO has been examined. The total G values of oxygen-containing products at 3% hydrocarbons added were around 6.8 irrespective of the nature of hydrocarbons, while G(N2)decreased by about 2.6. Consideration of possible reaction schemes both on ionic and nonionic processes leads to the conclusion that excited oxygen atoms play a major role in the nonionic processes in the gas-phase radiolysis of NzO. On the basis of the ionic processes proposed previously and of the nonionic processes assumed here, the main features of the primary processes in the radiolysis of NzO can be depicted as NzOG+NzO+
+ o* (excited states)
%$NZ Nz
+ 0 (the ground state)
Introduction Nitrous oxide has been used as an electron scavenger to investigate the ionic Drocesses in the radiolvsis of hydrocarbons.1 Since it produces nitrogen after its electron capture, many a t t e m p t ~ z -have ~ been made t o correlate the nitrogen yields and the G values of ionizac.3
The Journal of Physical Chemistry, Vol. 76, No. 20, 1071
+ e-
G
=
3.0
G
&
4
G
< 0.5
tion, but they have failed. This is caused mainly by the fact that the mechanism of the decomposition of nitrous (1) J. M. Warman, K.-D. Asmus, and R. H. Schuler, Advan. Chem. Ser., No. 82, 25 (1968). (2) S. Sato, R. Yugeta, K. Shinsalta, and T . Terao, Bull. Chem. soc.Jap., 39, 166 (1966).
3179
GAS-PHASE RADIOLYSIS OF NITROUS OXIDE oxide in hydrocarbons as well as in itself is not yet known accurately. I n connection with these problems, the accurate determination of water and other oxygencontaining products has been AS to the mechanism of radiolytic decomposition of nitrous oxide itself, on the other hand, a few reports have recently been published,8-10 which focused mainly on the ionic processes. I n our previous papers,8 an ionic mechanism in the gas-phase radiolysis of nitrous oxide was proposed, and the importance of processes other than the ionic processes was suggested. The purpose of the present study is to reveal the main features of the nonionic processes in the radiolysis of nitrous oxide by examining the effect of the addition of hydrocarbons to nitrous oxide.
Experimental Section Kitrous oxide was used directly from the cylinder supplied by Takachiho-Shoji Co. After several freezepump-thaw cycles t o remove any noncondensable impurities, the obtained purity was 299.995%. Research grade hydrocarbons also supplied by Takachiho-Shoji Co. were degassed in a vacuum system and were used without further pu4cations. A binary vapor mixture, about 90 cm total pressure, of NzO and a hydrocarbon was irradiated at loom temperature with 6oCo y rays at a dose rate of 4.2 X 1019 eV/g hr and the highest dose used was 2.1 X lozoeV/g. All irradiations were carried out in cylindrical vessels of about, 50 cc and, prior to use, they were baked in air a t 500" and then pumped a t least for 0.5 hr down to mm. The vessels were sealed while opened to the vacuum system. Great care was taken in the analysis of the oxygen-containing products, especially in HzO. The noncondensable products, Nz and CO, were collected by a Toepler pump with a gas buret and were analyzed gas chromatographically on a 5-m column of molecular sieve 5A at 60". Quantitative analysis of water has been known to be very difficult. In this experiment, however, a good reproducibility has been attained. Water and alcohol products were analyzed gas chromatographically on a polyethylene glycol-200 column at 70" after removal of NzO through a -120" cold trap. The products were identified by their retention times on the gas chromatographic columns by seeding with authentic samples.
Results The results of the addition of various hydrocarbons to F z O are shown in Figures 1-6 and summarized in Table I. The addition of small amounts of a paraffin t o NzO caused a decrease in the nitrogen yield and then made its plateau value. As shown in Figures 1-3, on the contrary, large yields of oxygen-containing products, HSO and alcohols, were observed. In Figures 4-6 are given the effects of the addition of olefins, which have been thought to be very different from paraf-
0
5 10 C2H6 , mol,%
Figure 1. The effect of the addition of CzHe to NzO: 0, Nz; @, HzO; 0,i-CdHgOH; A, CzHhOH.
fins with respect to the reactivities with oxygen at0rns.l' The addition of olefins to NzO, however, showed nearly the same effect except for observing the formation of CO instead of alcohols. These experimental results are summarized in Table I. I n each case, the decrease of the nitrogen yield is about 2.6,12while the total yield of oxygen-containing products is around 6.8. I n the case of n-C4Hlothe total yield of oxygen-containing products was found to be somewhat less than those in the other cases. This may be due to missing higher alcohols in the gas chromatographic analysis. In the presence of hydrocarbons, NO and Oz were not detected and other oxygen-containing products, such as aldehydes, ketones, or epoxide, etc., were also not observed.
Discussion To investigate the origin of the observed decrements of the nitrogen yields in the presence of hydrocarbons, (3) J. M. Warman, J . Phys. Chem., 71, 4066 (1967). (4) R. A. Holroyd, ibid., 72, 759 (1968). (5) N. H. Sagert and A. 8. Blair, Can. J . Chem., 45, 1351 (1967); K. Takeuchi, K. Shinsaka, S. Takao, Y. Hatano, and S. Shida, Bull. Chem. SOC.Jap., 44, 2004 (1971). (6) R. 0. Koch, J. P. W. Houtman, and W. A. Cramer, J . Amer. Chem. SOC.,90, 3326 (1968). (7) A. Menger and T. Gauman, H e h . Chim. Acta, 52, 2477 (1969). (8) S. Takao, S. Shida, Y. Hatano, and H. Yamazaki, Bull. Chem. SOC.Jap., 41, 2221 (1968); S. Takao and S. Shida, ibid., 43, 2766 (1970). (9) J. T. Sears, J. Phys. Chem., 73, 1143 (1969). (10) R. W. Hummel, Chem. Commun., 995 (1969). (11) (a) G. Paraskevopoulos and R. J. Cvetanovib, J . Chem. Phyy., 50, 590 (1969); (b) R. J. Cvetanovib, "Advances in Photochemistry," Vol. 1, Interscience, New York, N. Y . , 1963, p 115. (12) According to our data reported previously,* the result of reaction 4, in the absence of other effects, would be t o increase the yield of nitrogen by G 0.9. The decrement observed of 2.6 corresponds, therefore, to an actual one by other reactions of -3.5 G units.
-
The Journal of Physical Chemistry, Vol. 75, No. 20, 1971
S.TAKAO, Y. HATANO, AND S. SHIDA
3180
Table I: The Effect of the Addition of Hydrocarbons to NzOO Q(tots1 oxygenQ(E~co- containing
Additiveb
10
CaH6 CsHs
(3
n-C4H10
CaHh CsHs cis-2-CdHs
5 a
-AQtNa)
Q(C0)
Q(He0)
hols)
produots)
2.5 2.5 2.7 2.6 2.7 2.5
0.0 0.0 0.0
5.3 5.4 4.8 4.3
1.5 1.5 0.7
6.8 6.9 5.5 6.8 6.4 7.2
2.5 0.4 0.0
Total pressure, about 90 cm.
0.0 0.0 0.0
6.0
7.2
Concentration, 3.0%.
0
5 10 C3Hg , mol.%
0
Figure 2. The effect of the addition of CaHa to NsO: 0, Na; HzO; 0 , n-CaH?OH; A, CzH60H.
Q,
10 (3
5
YZXX
I
0 0
5 C2H4, mol%
I
10
Figure 4. The effect of the addition of CtH, to NzO: 0, Nz; HzO; X, CO.
(3,
Neutralization of N20- with RH+ is followed by the hydrogen atom abstraction of OH
/&.-.-e-.-C
0
I
0
5 n-C4H,0
+ RH+ +N2 + OH + R OH + RH +HzO + R
10
N2O-
mol%
Figure 3. The effect of the addition of n-CdHlo to NzO: 0, Nz; Q, HzO; 0, n-CaH?OH.
+ e-
(1)
can be captured by NzO itself prior to neutralization
NzQ
+ e- -+N2O-
(2)
Charge transfer can occur between N 2 0 + and hydrocarbon R H
NzO+
+ RH +N20 + RH+
The Journal of Physical Chemistry, Val. 76,No. $0, 1971
(5)
Another possibility for H20formation via ionic processes is considered
the contribution from the ionic processes is considered, lor which the following possible processes can be assumed. Electrons produced by the primary ionization of N2O N20w-+N20 +
(4)
(3)
+ R H +N2 + OH- + R OH-+ R H f 4 H z O + R OH- + N2Of +OH + NzO
N2Oor
(6)
(7) (8)
Energetic considerations she%- all reactions except reaction 6 to be possible. The exothermicity of reaction 6 cannot be determined, because the electron affinity of IY2O is not yet known a c c ~ r a t e l y . ~ ~Reac,'~ (13) W. J. Holtslander and G. R. Freeman, Can. J . Chem., 45, 1661 (1967). (14) J . F. Paulson, J . Chem. Phys., 52, 959 (1970).
GAS-PHASE RADIOLYSIS OF NITROUS OXIDE
3181 oxygen-containing products is much larger than the decrement of the nitrogen yield. Moreover, the remainder of the total yield of oxygen-containing products minus that contributed from the ionic processes assumed above is still larger than the observed decrement of the nitrogen yield. It seems that the nonionic dissociation of NzO would be mainly as
NzO**NZ
+ 0, 0 + NzO -?(+
with small contribution from
P\TzO~--)NZ4-0*,O*
0
10
5 C3H6 mol.% 8
Figure 5. The effect of the addition of C3HBto NzO: 0, Nz; a,HzO; X, CO.
10 0
5
0
0
5 C4H8 I
10 l?lO\?/a
Figure 6. The effect of the addition of cis-2-CaH8 to NzO: 0, Nz; a, HzO.
tion 6 cannot occur if the electron affinity of XZOexceeds 1.0 eV. It is likely that the electron affinity of NZO is equal to or less than 1.0 eV.15 At present, it seems possible that this process occurs. According to the above scheme, however, as much nitrogen as water should be produced and the nitrogen yield should not be decreased by the addition of hydrocarbon, i.e., the addition of hydrocarbon to NzO should give rise to the formation of HzO without decreasing the nitrogen yield. This is not the case. The observed decrease in the nitrogen yield, then, must be due to some other processes, most probably some nonionic processes. As given in Table I, the total yield of
+ NzO +Nz,OZ,and KO
since, according to this scheme, the addition of hydrocarbon to NzO should result in the observation of the large yield of oxygen-containing products with small decrement of the nitrogen yield. In the above scheme, however, the small contribution of O* cannot explain the large yield of the nonionic formation of NO in the gas-phase radiolysis of pure Nz0.8 This may arise from the reaction of 0" with N2O. It seems, then, reasonable to assume that excited oxygen atom O* plays an important role besides the ground state oxygen atom 0. The former can decompose NzO to produce Nz but the latter cannot,16 and both react with hydrocarbons to form oxygen-containing pr0ducts.l' In previous papers,8 the effect of the addition of SFG on the gasphase radiolysis of NZOwas examined. The following results were obtained: for the ionic processes G(- NzO) = 3.0 = G(e-),I7 G(Nz) = 2.1, G(Oz) = 0.8, and G(N0) = 1.8; for the nonionic processes G(-NzO) = 9.6, G(Nz) = 8.0, G(Oe) = 3.0, and G(N0) = 3.3. The large yield of the nonionic formation of NO certainly arises from the reaction of 0" with NZO. On the basis of the observed decrement of the nitrogen yield (- AG(N2) = 2.6) in the presence of hydrocarbon, combined with the above G(S0) value, it may be concluded that O* plays a major role in the nonionic dissociation processes of NzO.Is Then, the nonionic processes in the radiolysis of N2O may be written as follows with the assigned G values 19--22 (15) J. F.Paulson, Advan. Chem. Ser., No. 58, 28 (1966). (16) M. G.Robinson and G. R. Freeman, J . Phys. Chem., 72, 1394 (1968). (17) G.G. Meisels, J . Chem. Phys., 41, 51 (1964). (18) Besides the above processes, both ionic and nonionic, the possibility of the following process for h i p formation might be considered
NzO
+ R +Nz + OR
(a)
Since, however, Nz0 has been thought to be inert with respect to reactions with free radicals,11breaction a may be eliminated. (19) The reactions of O* with NzO to give Nz 0 2 and 2 N 0 were already reported by several authors. From a combination of the activation energy difference given by Kaufman, et a1.,20 and the absolute rates a t 2000OK given by Barton, et al.,*lan estimate of the ratio of the rate constants for the two reactions at room temperature can be obtained which is in good agreement with the ratio which has been assumed to explain the present radiolysis results. Besides, the ratio of the two reactions assumed here is not inconsistent with the conclusion of Yang and Servedio.22
+
The Journal of Physical Chemistry, Vol. 76,N o . 90, 1971
3182
NOTES
+ o*
&Oq+-Lnz
+0
