NOTES
1647
zyl acetate. The line width ratios of acetate methyl to phenyl are also consistent with proposed geometry of the solvation shell. The directions of the net dipole moments of p-tolualdehyde and p-methylacetophenone are dominated by the carbonyl group. As with the acetate derivatives, the proton nearest the carbonyl group is preferentially oriented under the influence of an anion. The following nitrile solvents were employed: ortho-, meta-, and para-substituted methylbenzyl cyanide. Under the influence of an anion, the methylene protons are preferentially oriented. The relative values observed for ortho, meta, and para substitution vary in a systematic fashion. These results clearly show that neutral molecules in the solvation shell of = t 3 charged solutes show melldefined intramolecular orientation effects. Although an ion dipole model does a good job in accounting for the experimental results, other interactions can selectively explain some of the results.2 For example, hydrogen bonding between the ligand hydrogens of Cr(H20)63+and the phosphoryl oxygen produces an orientation which is comparable to an ion dipole interaction. Acknowledgment. We gratefully acknowledge the support of the Directorate of Chemical Science, Air Force Office of Scientific Research, under Grant AFOSR 2 12-65.
The Quenching of Mercury
("1)
Resonance
Radiation by Aromatic Molecules
30 years ago. Since a complete ,understanding of the quenching mechanisms requires knowledge of the effects of structure on the quenching efficiencies of aromatic as well as saturated hydrocarbon systems, we have undertaken the measurements reported herein. We have measured the quenching cross section for benzene, benzene-&, and eight substituted benzene compounds. The apparatus used was essentially that described by Yang.6 A collimated beam of 2537-A resonance radiation from a low-pressure mercury lamp [Oriel Optics Corp. , mercury-argon lamp] was introduced into a quartz sample cell through an Infrasil window. The intensity of mercury fluorescence was measured using an 1P28 photomultiplier tube located 90" from the exciting beam in the horizontal plane. The mercury reservoir, a quartz tube sealed immediately below the sample cell, was maintained a t 0" using an ice slush bath. Fluorescence measurements, which constituted the determination of the photocurrent, Q, in arbitrary units, were made randomly a t high and low pressures of quenching gas, M, to avoid systematic errors. All chemicals used in this work were either spectrographic or research grade as specified by the supplier. Purity was confirmed by gas chromatographic analysis. The isotopic C& was obtained from Stohler Chemical Co. and certified by them as 99.5% isotopically pure. It was used as received. The data were found to be consistent with the modified Stern-Volmer formula developed by Yange6 Typical plots are shown in Figure 1 for CsH6 and C6D6,and in Figure 2 for the three isomers of xylene. The straight lines were drawn according to a least-squares fit of the data to the modified Stern-Volmer formula, i.e. 11 - Q/QoI-'
by Gilbert J. Mainsla and Mendel TrachtmanIb Department of Physical Sciences, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 19144 (Received September 1 6 , 1969)
The mechanisms by which molecules quench the
3P1 excited electronic state of atomic mercury have fascinated and, to a certain extent, eluded photochemists ever since Stuart's original study in 1925.2 Darwent3 first suggested that quenching diameters of contributing groups are additive; a suggestion which has been confirmed (within limits) by careful studies of Gunning and coworkers and Cvetanovic and coworkers in recent years. Reviews by these two inv e s t i g a t o r ~represent ~~~ the best compilations of quenching cross sections currently available. It is noteworthy that while extensive studies have been made of the quenching efficiencies of saturated and unsaturated hydrocarbons, the only quenching cross section for an aromatic hydrocarbon, benzene, was reported over
=
a
+ @[MI-'
(1)
where Q is the fluorescent photocurrent in the presence of [MI mol/l. of quenching gas, M, and Qo is the fluorescent photocurrent in the absence of quenching gas. a and 0 are constants related to the quenching rate , the mean lifetime of 3P1Hg atoms in constant, k ~ and the sample cell, t, by the equation p = kQta. Thus, the ratio of the slope to the intercept in Figure 1 and Figure 2 gives values for kQt. t may be calculated from the equation derived by Yang for a similar geometry,
ie. t
=
to(i
+ 0.25 x 104p9
(2)
(1) (a) Department of Chemistry, University of Detroit, Detroit, Mich. 48221; (b) author to whom reprint requests and correspondence should be directed. (2) H. A. Stuart, 2. P h y s i k , 32, 262 (1925). (3) B . deB. Darwent, J. Chem. Phys., 18, 1532 (1950). (4) H. E. Gunning and 0. P. Strausz, A d v a n . Photochem., 1, 209 (1963). (5) R. J. Cvetanovic, Progr. React. Kinet., 2, 39 (1964). (6) K. Yang, J. A m e r . Chem. Soc., 88, 4575 (1966). V o l u m e 74, N u m b e r 7
A p r i l 8 , 1970
1648
NOTES -
Table I : tkQ and d Values in the Quenching of Hg(aP1) Atoms by Various Aromatic hlolecules at 25’ 3
Q,
0,-Q 2
I
0 0
2
4
6
8
10
Figure 1. Quenching data of CeHa ( 0 )and CBDB(0).
3 Do
o,-a 2 1,4 dimethylbenzene -0- 1,3 dimethylbenzene
I
-& 1,2 dimefhylbenzene
Figure 2. Quenching data of the various xylenes.
where to is the mean radiative lifetime of an isolated aP,Hg atom 11.08 X sec], p is the pressure of mercury vapor [1.85 X mm], and T is the distance from the irradiated slab of Hg vapor to the emerging surface of the cell [3.55 cm in this study]. Hence, t was taken as 2.85 X sec for all the experiments reported here. The quenching cross section, a g 2 , is readily calculated from k~ using the familiar equation from elementary collision theory.6 In Table I we report k g t , k g , and UQ’ for the ten aromatic compounds studied. Also reported in Table I for comparison are values for uv2,the Hg-M collision cross section calculated by assuming the diameter of Hg to be 2.38 8 and estimating the diameter of XI from the “b” constant in van der Waals’ equation of state. Except for benzene and p-xylene, the quenching cross sections are all considerably larger than the collision cross section estimated from van der Waals radii; in many instances, the quenching cross sections are more than 50% larger than the estimates based on van der Waals radii. It would appear that the energy-transfer processes involved in physical quenching of 3 P Hg ~ atoms occur over rather long internuclear distances, say 10 A, and T h e Journal of Physical Chemistry
Benzene CeHa Toluene p-Xylene m-Xylene 0-X ylene Chlorobenzene Fluorobenzene Bromobenzene Ethylbenzene Benzene CBD6 a
3.83 5.41 3.62 5.34 5.74 6.08 5.67 4.41 5.96 6.18
24.9 35.3 23.6 34.8 37.9 39.6 37.0 28.8 38.9 40.4
39.4 59.1 41.5 61.2 65.7 71.0 62.8 56.9 68.3 65.4
36.3 41.4 45.5 45.1 44.9 41.2 39.0 42.3 43.9 (36.3)
Units: 1. mol-’ sec-1.
that Gunning’s suggestion’ that 3P1Hg atoms exhibit electrophilic character and interact with 9-electron systems preferentially is supported by these observations. Note should also be taken that the quenching cross section for benzene, 39.4 i2, while low in comparison with the quenching cross section of the other aromatic compounds, is in excellent agreement with the value reported by Bates.8 This agreement supports the quenching cross section measurements reported here and suggests that a better understanding of the quenching act could be obtained if we understood why benzene and p-xylene exhibit such relatively small quenching cross sections. Xeglecting, for the moment, benzene and p-xylene it should be noted that no simple correlation exists for the trends in quenching rate constants for the other aromatic molecules reported in Table I. Attempts by us to explain substituent effects in terms of dipole moments, or polarizabilities, or even Hammett a-p correlations have not been successful. This is not particularly surprising in view of our fragmented knowledge of energy-transfer proces~es,~ although Metteelo has reported some success in correlation of polarizabilities with quenching probabilities for S02. However, in view of the very large isotope effect observed in this study for the C6H6-C6D6 system, such a correlation must be ruled out for the quenching of 3P1Hg atoms by aromatic compounds. The remarkable effect of deuteration on the quenching cross section of benzene suggests that vibronic factors may be rate determining in the quenching mechanism. The marked effect of deuteration on singlettriplet intersystem crossing probabilities seems well (7) Y. Rousseau, 0. P. Strause, and H. E. Gunning, J. Chem. Phys., 39, 962 (1963). (8) J. R. Bates, J . Amer. Chem. SOC.,54, 669 (1932).
(9) A. B. Callear, “Photochemistry and Reaction Kinetics,” P. G. Ashmore, F. S. Dainton, and T. M. Sugden, Ed., Cambridge University Press, Cambridge, 1967, p 133. (10) H. D. Mettee, J. P h y s . Chem., 73, 1071 (1969).
1649
NOTES
established. Thus, a mechanism involving such a crossing may be postulated as
+ hu = Hg* Hg* = Hg + hu Hg* + Ar = (Hg*-Ar) Hg
(Hg*-Ar) = (Hg-Ar*)
+ Ar = Hg + Ar*
Rate
I, kf [Hg*1 ko[Hg*][Ar]
ki [Hg*-Ar]
(Hg*-Ar) = Hg*
kd[Hg*-Ar]
(Hg-Ar*)
k*[Hg-Ar*]
Ar* = Ar
k,[Ar*I
This mechanism is similar in some respects to that suggested by Yangl' and, by application of the usual steady-state approximations, leads to the following relationship between k~ and the rate constants defined above
k~
ke[ki/(ki
+ kd)]
(3) If one assumes a loose complex, we might place an upper limit of, say, 50 X 1O1O < k, < 75 X 1Olo 1. mol-' sec-l for formation of the complex. Variations in k~ could then be attributed to the relative magnitudes of the rates of intersystem crossing, ki, and decomposition of the complex without energy transfer, led. =
It is also possible that the increase in quenching cross section upon deuteration is the result of more efficient quenching to the Hg *Postate by CsDe than CeHe. Such an effect has been observed in saturated hydrocarbon systems.12 Inclusion of such a step in the mechanism, i e . , (Hg*-Ar) = Hg'(aPo) Ar' (vibrated, rotated, excited), rate = Ki'(Hg*-Ar), results in the addition of Ki' to the numerator and denominator of the bracket term in eq 3 and requires ki' to be much larger for C6Dsthan for C&&. Since the number of compounds studied is limited to those reported in Table I, it would not be prudent to attempt further justification of the proposed mechanism or to discuss in detail the factors which determine the magnitudes of ki and hi'. Additional work is in progress involving partial deuteration and the effects of multiple substituents on the quenching cross section. It is hoped that these further studies will yield insight necessary for a more general interpretation. Acknowledgment. C. Shelhaimer and W. Hufford participated in some of the experiments reported here. We acknowledge their help with thanks.
+
(11) K.Yang, J. Amer. Chem. SOC.,87, 5294 (1965). (12) 5.Penzes, A. J. Yarwood, 0. P. Stransz, and H. E. Gunning, J . Chem. Phys., 43, 4534 (1965).
Volume 74, Number 7 April Z?, 1970
