2402
NOTES
'/aH + x
The distance between these groups was obtained using molecular models assuming an orientation which gave maximum separation of the amino groups. A plot of l / R z us. k ~ where , R is the distance between the amino groups, is essentially linear. This correlation is a t least in part explained by simple charge-dipole repulsion between the protonated amino group on the one end and the dipole of the N . . .HOH which is broken during exchange at the other end. Such forces predict a 1/B2dependence. I n a recent review article, Grunwald and Ralph reported a study of the protolysis kinetics of N,N,N N'-t etraet hyl- 1,2-diaminoet hane. Two processes were detected which were attributed to exchange from trans and gauche isomers with kH values of 3 X lo9 and 3 X 107 sec-1, respectively. Our methods only detected the faster process. The agreement of the larger k H between the tetraethyl and tetramethyl compounds is quite good.
lo-'
Figure 1. 1/r us. l / a H + for
N,N,A~',Xf-tetramethyl-1,4-diaminobutane.
The middle and low acidity region of the curve is easily fitted by variation of IC, in eq 9, assuming a value of 1 X 1O'O for k-&. The results for all coppounds studied are shown in Table I. I n the case of I, 11, and 111, only the linear portion of the 1 / us. ~ l / a ~ was + available. Therefore IC, and Lacould not be determined. A statistical correction has been applied to account for the two exchangeable sites in each molecule.20
(20) J. E. Lemer and E . Grunwald, "Rates and Equilibria of Organic Reactions," Wiley, New York, N. Y., 1963, p 133. (21) E. Grunwald and E. K. Ralph, Accounts Chem. Res., 4, 107 (1971).
Shock-Tube Study of CZH2-02 Reaction. Acceleration of Reaction in the Presence
of Trace Amounts of Cr(C0)e Table I: Summary of Kinetic Parameters Compd
PKa,
PKa,
I I1 I11 IV
5.75 5.92 7.85 8.77
9.17 9.80 10.30
kH x lo-@
4.52 7.47 0.145 0.085
by Shimpei Matsuda* k-a
ka
x
10-10
Department of Chemistry, Harvard University, Cambridge, Massachusetts 03158
and David Gutman 17
1.0
Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616 (Received March 4* 1971) Publication costs assisted by the Petroleum Research Fund
Considering the values obtained for I, 111, and XV, we note a sharp increase in the magnitude of ICH for I, compared with the other two compounds. Apparently the interaction between the two amino groups increases in importance as the separating chain length is decreased. The interaction between amino groups in these diamines is also apparent in the difference between the first and second stepwise protonation conis 3.56 for I, while stant in each compound, A p k ~which , only 1.95 for 111, and 1.56 for IV. This interaction between amino groups has also been noted for primary aliphatic diamines, for which it was found that the difference in the successive enthalpies for stepwise protonation decreases appreciably as the distance between the amino groups increases.lS2 The dissociation of the N . . .HOH moiety does not involve charged species. However, a correlation was found between the value of kH and the separation between the amino groups. T h e Journal of Physical Chemistry, Vol. 76, N o . 16, 1971
The effect of metallic additives on the combustion of hydrocarbons, in particular the antiknocking effect of tetraethyl lead, has been extensively studied by kinetic spectroscopy in a flash photolysis system.' It was found that hydrocarbon combustions were retarded in the presence of Pb(C2H5)4and Te(CH3)z.2 I n the case of Pb(C2H5)4this effect was explained by assuming a chain-breaking process involving PbO. Erhard3 studied the effect of metal carbonyls on the flash-initiated combustion of C2H2-O2 mixtures. Contrary to the case of Pb(C2H5)4,the combustion was greatly accel(1) For a review see, R. G. W. Norrish, Symp. (Int.) Combust. [Proc.] l o t h , 1 (1965). (2) K. H. L. Erhard and R . G. W. Norrish, Proc. Roy. SOC.,Ser. A , 2 5 9 , 297 (1960); 234, 178 (1956); A. B. Callear and R. G. W. Norrish, ibid., 259, 304 (1960). (3) K. Erhard, 2. P h y s . Chem. (Frankfurt a m M a i n ) , 36, 126 (1963).
2403
NOTES
erated in the presence of Ni(CO),, Fe(CO)r, and Cr(CO)s. Erhard suggested the formation of reactive species of the type Cr(CO)a(CzHz)on the photodissociation of Cr(CO)aand questioned whether the acceleration of combustion reactions was also observed in thermal reaction systems.' Recently, the oxidation of carbon monoxide in the presence of trace amounts of Cr(CO)s has been studied in shock waves.' It has been observed that CO oxidation is greatly accelerated in the presence of Cr(C0)' (gas-phase homogeneous catalysis by transition metal compounds). This note describes the effect of Cr(CO)s on the shock-initiated CZHZ-OZ reaction. The shock-tube apparatus used in this study has been described previously.' Argon gas was saturated with Cr(CO)s vapor by passing Ar through the Cr(C0)spacked column.' The reaction gas mixtures were prepared by adding the Cr(C0)s saturated Ar to the CZHT Oz-Ar mixtures. The vapor pressure of Cr(C0)s was taken from the literature.' All reaction gas mixtures used in this study are listed in Table I.
CH* 314 nrn-
(200mV/div)
Table I Mi... t"r0
I I1 111 IV
C,H,. %
0,. %
0.5 0.5 0.5 0.5
1.0 1.0 1.0 1.0
Cr(CO)I.
AI.
PPm
%
10 25 50
98.5 98.5 98.5 98.5
The reaction course was followed by monitoring the emissions a t 308 nm (OH* plus continuum)* and 314 nm (CH' CZZf - X's) simultaneously by the end-on detection technique.',* Experiments were performed at about 1700"1< with a total concentration in the re flected shock zone of 1.4 X lo-' mol/l. Oscillograms from two experiments performed under nearly identical conditions, one with mixture I and the other with mixture 11, are shown in Figure 1 and the emission intensities are plotted against reaction time in Figure 2. As shown in Figure 2 the emission intensity displays an exponential growth of about one decade when the Crconcentration is 10 ppm. It can be seen in Figure 2 that the reaction is greatly accelerated in the presence of only 10pprn of Cr(CO)a. The induction time (defined as the time when the falloff from the exponential growth occurs) is reduced to almost '/z in the presence of 10 ppm of Cr(CO)s. When the reaction gas mixtures containing more than 25 ppm of Cr(CO)', ie., mixtures 111 and IV,are shock heated, the reaction proceeds extremely fast and exponential growth of the emissions is no longer observed. The emissions were over within 130 and 200 psec with mixtures IV and 111, respectively, under the same experimental conditionsas given in Figure 1. It should be noticed in Figure 2 that the exponential growth constant of the
\L""
111 ","I",
-
+
50psec
( b) Figure 1. Oscillograms obtained from two experiments. Reaction time increases imm left to right: (a) mixture 11, T = 1645*K,C = 1.40 X lo-' mol/].; (b) mixture I, T 1650'K, C = 1.40 x loFa moljl.
-
emission is not altered in the presence of Cr(CO)', and that the falloff from the exponential growth occurs a t the same emission intensity whether the reaction gas mixture contains Cr(C0)' or not. The C2HrOz reaction is one of the chain-branching reaction systems whose induction period is characterized by the exponential growth of chain carriers,) for example (OH) = (OH), exp(at). The exponential growth constant, a, is a characteristic function of the reactant concentrations and the rate constants in the chain-branching reactions and independent of initiation (4) 8 . Metsuda, T. P. J. Isod. and G. B. Kistinkowsky, submitted for publiestion in J . C h m . Phys. (5) D. Gutman,E. A. Hardwidre, F. A. Doughherty. and R. W. Luts.
aid.. 47. 4400 (1967). (6) W. L. sohakleford. Ph.D. Thesis. Cnlifornin Institute of Technology, Paendena. Cdif., 1964. (7) (a) T. D. Wilkemn. Ph.D. Thesis. University of Michiam. Ann Arbor. Mich.. 1902; (b) A. A. Boni, J . Eledmchem. Soc., 113, 1089 (1966).
(8) 1. R. Single, D. File. 5. Mstsuda. J. Marquart. and D. Gutman. submitted for publication in J . C h m . Phys. (9) (a) D. Gutman and 5. Matsudn. aid.. 52. 4122 (1970): (b) G. P. GI-. G. B. Kstiakowsky. J. V. Michael, and II. Niki. aid.. 42, 608 (1965).
The Jmrrnd o/Phydml Chsnishy. Vol. 76, No. 16. 1971
2404
NOTES
I
100
/:;
a
I
A
I
200 300 T I M E AFTER SHOCK REFLECTION (piset)
e-
and Sadhan K. De Chemistry Department, I n d i a n Institute of Technology, Kharagpur, West Bengal, I n d i a (Receiued February 1 , 1971) Publication costs borne completely by T h e Journal of Physical Chemistry
J
400
reactions. lo The initial concentration of chain carriers, (0H)o in the above example, is linearly proportional to the initiation rate.I0 I n the present study it has been found that the exponential growth constant is not affected substantially, but the reaction time is greatly reduced in the presence of Cr(C0)G. From these observations it can be said that Cr or some species containing Cr is not involved in the main chain branching reactions but enhances the chain initiation process. It could be suggested that the Cr compound(s) form some activated complex with C2Hz which produces chain carriers either by reaction with O2 or by thermal decomposition. The activated complex of the type Cr(CO)S(C2Hz) as suggested in the flash photolysis systemj3however, is not probable a t all, because under the present experimental conditions the decomposition of Cr(C0)G is known t o be very fast.4 I n the shock-tube study of the Cr(C0)6-Qz and Cr(CO)G-CO-02 reaction systems by a time-of-flight mass spectrometer, the formation of CrO, Cr02, and Cr03 is ~bserved.~ Therefore, it seems more likely that Cr atom formed by the decomposition of Cr(C0)s is oxidized to CrO, CrOz, and CrOs in the present reaction system and the CrOB(or Cr02)reacts with CzH2to form chain carriers. Since a very low concentration of Cr(CO)Gis used in the present study, the possibility of solid particle formation (chromium oxides) is completely eliminatede4 Acknowledgments. The authors gratefully acknowledge financial support from the donors of the Petroleum Research Fund and from the National Science Foundation. The authors wish to thank Professor G. B. Kistiakowsky and Dr. T. P. J. Izod for their helpful discussions. K. Bradley, Trans. Faraday
Sac., 63, 2945 (1967).
T h e Journal of Physical Chemistry, Val. 76, N o . 16,1971
by S.Mukherjee, S. R. Palit,* Physical Chemistry Department, I n d i a n Association for the Cultivation of Science, Jadavpur, Calcutta-S%, I n d i a
Figure 2. Plots of emission intensities vs. reaction time from two oscillograms shown in Figure 1. The exponential growth constant calculated is for (a) 3.35 X 104 sec-’ and for (b) 3.20 X lo4 sec-’.
(10) J.
a Hydrogen Bonding
A
A A
N-H . .
Although hydrogen bonding of hydroxylic compounds (that is, phenols and alcohols) with P-electron systems of olefins and aromatics has been studied by various workers in the recent year^,^-^ there has been but little investigation on N-H . a-type hydrogen bondingaG This note presents the results of our investigations on the hydrogen-bonding interaction where N-H of anaphthylamine acts as proton donor and n-electron systems of aromatics act as proton acceptors. We have utilized Nagakura and Baba’s suggestions that a-a* transitions of organic molecules with suitable chromophores undergo a red shift in proton-accepting solvents due to solute-solvent hydrogen-bonding interaction and also spectral measurements at the shifted peak can be used to evaluate the equilibrium constant for the complex formation.’
-
d8
Experimental Section The a-naphthylamine (B.D.H.) mas recrystallized before use. The a bases (Eastman Kodak) were purified by standard methods9 and distilled before use. The nonhydrogen bonding solvent used was n-heptane (E. Rlerck) which showed cutoff at 220 mp. The details of spectral measurements made on a Hilger uv spectrophotometer were same as described previously.8
Results and Discussion The a-a* band of a-naphthylamine in n-heptane at 318 mp undergoes a red shift to 322 mp in the P bases. Figure 1 shows the absorption spectra of a-naphthyl amine in n-heptane and in mesitylene. (1) (a) R. West, J . A m e r . Chem. Sac., 81, 1614 (1959); (b) W. Beckering, J . P h y s . Chem., 65, 206 (1961); (c) M . Oki and H. Iwamura, J . A m e r . Chem. Sac., 89, 567 (1967). (2) P . J. Krueger and H. D. Mette, Can. J . Chem., 42, 288 (1964). (3) B. Ghosh and 9 . Basu, Trans. Faraday Soc., 61, 2097 (1965). (4) M. R. Basila, E. L. Saier, andL. R. Cousins, J . A m e r . Chem. Soc.,
87, 1665 (1965). (5) (a) Z, Yoshida and E. 0. Sawa, ibid., 87, 1467 (1965) ; 88, 4019 (1966); (b) Z. Yoshida and N. Ishibe, Bull. Chem. Sac. Jap., 42, 3254 (1969). (6) B. Chakravortv and 8. Basu, J . Chim. Phys., 64, 950 (1967). (7) (a) 6. Nagakura and H. Baba, J . A m e r . Chem. Sac., 74, 5693 (1952); (b) S. Suzuki and H. Baba, J . Chem. Phys., 35, 1118 (1961). (8) (a) S. K. De and S.R. Palit, J . P h y s . Chem., 71, 444 (1967); (b) S. Mukherjee, S. R. Palit, and S. K. De, ibid., 74, 1389 (1970). (9) “Techniques of Organic Chemistry,” A . Weissberger, Ed., Vol. VII, Interscience, New York, N. Y., 1965.
.,
