216
YEONG-JGI CHENAND WILLIAM H. OXTTUNG
product for the butene as compared to ethylene, but also ensures a high stereoselectivity in the epoxybutane. The imp1icat)ion of the foregoing discussion is that the epoxidation reaction is concerted with a retentioii of olefin configuration. If both carbon-oxygen bonds of the oxirane ring were not formed simultaneously, then an open intermediate form, similar to that produced in the preliminary attachment of the epoxide to the surface, would be present during the reaction sequence. If such an intermediate existed, it is quite likely that a significant, and detectable, quantity of trans-2,3-epoxybutane would result from the oxidation of cis-2-butene. The cis-2,3-epoxybutane molecule is even more strained than is the cis-Zbutene; thus if free rotation mere allowed in an intermediate species, the more stable trans-epoxide would predominate over the cis isomer. However, as has been previously
stated, no trans-2,3-epoxybutane is detected as the product of the oxidation of cis-%butene. Finally, it must be concluded that the reaction which the epoxide undergoes to give the olefin is not the reverse of the epoxidation reaction. The oxygen species which results from the dissociation of the epoxide cannot be the same one which is responsible for the formation of the epoxide. The latter species may be an electrophilic peroxy oxygen as a concerted epoxidation reaction would be consistent with a mechanism similar to that proposed for the epoxidation of olefins by organic peracids. Aclcnozdeclgments. I am pleased to acknowledge the assistance of A h . J. E. Lentz in obtaining the infrared spectra and for many useful discussions. The infrared spectra of trans- and cis-dideuterioethylene oxide from Dr. Spector's thesis were generously provided by Dr. Charles C. Price of the University of Pennsylvania.
Temperature Dependence of the Kerr Constant of Water'
by Yeong-jgi Chen2 and William H. Orttung" Department of Chemistry, University of California, Riverside, California 99609 (Received August 9, 1971) Publication costs borne completely by The Journal of Physical Chemistry
The Kerr constant of liquid water was studied from 5 to 81" a t a wavelength of 365 mp. The data suggest a 1/T2dependence a t the high arid low ends of the temperature range, with a broad iiiflectioii point of approximately zero slope near 35'. Less complete results were obtained for DzO from 5 to 50". Its Kerr constant is about 10% less than that of water at 25' and 365 mp. The inflection point could not be predicted from calculations based on temperature-dependent angular correlation of neighboring molecules unless allowance was made for a variable optical anisotropy. It was shown that hyperpolarizability interaction with the strong electric field of neighboring dipoles could account for the temperature dependence of both the mean polarizability and the optical anisotropy. The effect of hydrogen bonding on the symmetry of the molecule was also considered as a factor in the temperature dependence and in the difference between E120 and DzO.
Introduction The refractive index anisotropy of a sample subjected to an electric field was first observed in 1875 and is known as the electrooptic Kerr effect. I n liquids, the effect depends on three factors: the optical and dielectric parameters of the individual molecules; the interactions between neighboring molecules ; and the structure of the liquid. For many liquids, the molecular parameters may be evaluated from vapor data,; the interactions are relatively weak, and the structure is conveniently chaotic. In liquid water, however, strong interactions perturb the molecular properties and induce considerable ice-like structure. The Journal of Physical Chemistry, Vol. 76, No. 8, 1978
The optical anisotropy of the isolated water molecule can only be deduced from Kerr effect studies of water vapor; but in the liquid, the optical anisotropy must be smalL3 As measured by the Kerr effect, the optical anisotropy of the water molecule is proportional to az- l/z(a, a,),where at is a polarizability, x is perpendicular to the H-0-H plane, and z is the dipole
+
(1) This investigation was supported in part by Public Health Service Research Grant GM11683 from the Division of General Medical Sciences. (2) Based on a thesis submitted by Yeong-jgi Chen in partial fulfillment of the requirements for the Ph.D. degree, June 1970. (3) W. H. Orttung and J. A. Meyers, J. Phys. Chem., 67, 1905 (1963).
TEMPERATURE DEPENDEWE OF
THE
KERRCONSTANT OF WATER
axis. The small observed anisotropy therefore implies that az g 1 / 2 ( a x a y ) ,and not that ax Si ay S az. Molecular orbital calculations4 suggest that q, > az > a,, as might also be predicted from the shape of thc molecule. The dielectric parameters of watcr in the solid, liquid, and vapor states have been extensively studied and a n a l y ~ e d ,but ~ ' ~ not all of the mysteries of this phenomenon have been resolved.' It is hoped that Kerr cffect studies may lead to a better understanding of the problems of dielectric theory in its application to liquid water. When the molecular properties are independent of temperature and interactions may be neglected, the Kerr constant will be expressible in the form, AO AI/T A2/T2. The A z term is usually dominant if dipole orientation of optically anisotropic molecules occurs; but if the dipole moment or optical anisotropy is small, the contribution of molecular distortion, i.e., field-induced anisotropy, to the A. and AI terms may be observed.* If the molecular properties or interactions change with temperature, a more complex relationship will be observed. Buckingham and Raab9 developed a statistical mechanical theory that included temperaturedependent interactions between molecules. For water, they assumed a temperature-independent optical anisotropy and used the bendable hydrogen bond model of Poplelo to calculate an approximately 1/T1,8 dependence for the Kerr constant. The results presented in the following sections show that the experimental temperature dependence is considerably more complex, and suggest that a temperature dependent molecular polarizability resulting from a changing environment may also have to be considered in a satisfactory interpretation. Both the results and interpretation have been considerably extended and improved since an early publication several years ago."
+
+
+
217
lMalinckrodt NMR-quality material. The specific conductances of the water and DzO at 25" were generally about 2 X low6and 3 X 10-6 ohm-I cm-l, respectively. The treatment of data followed earlier methods,a except that analyzer rotations replaced neutral filter readings in determining 10. Corrections for residual birefringence werc usually required. The residual effects were apparently caused by the frequent changes of cell temperature, Generally eight measurements were made with gradually increasing fields, and eight with decreasing fields, for each samplc at a given temperaturc. Results Most of the data were taken at a wavelength of 365 mp because of the greatcr light intensity and near-maximum Iierr constant at this ~ a v e 1 e n g t h . l ~Some results were obtained a t 436 mp. All of the results are expressed as a ratio of the Kerr constant, B, to that of water a t 25" and the same wavelength. B is defined in eq 2 below. Water. Four series of experiments were carried out, all a t 365 mp. The temperature ranges covered were: (1) 5-54"; (2) 25-81'; (3) 25-79'; and (4) 25-50". The data of the first scriesll have been recalculated for improved elimination of residual effects. Thc results are shown in Figure 1. The points shown for series 1-3 correspond to single experiments, each consisting of S to 16 measurements at different fields. The remaining points in the range 30-55" correspond to averages of many experiments, calculated by the following equation for each temperature
where the weighting factors, wi,were assigned values n
(l/ABj)2. The uncertainty, AB?,for a
of (1/ABJ2/ j=1
given experiment was determined by a least-squares analysis of the a f 2 / E 2us. E 2 plot.3 The number of experiments at 30, 35, 40, 45, 50, and 55" was 3, 3, 5, 13, A description of the general methods and procedures used to obtain the Kerr effect data has been p u b l i ~ h e d , ~ 8, and 5 , respectively. The difficulty in achieving reproducibility in the range 30-55" may have been and the apparatus has been described.I2 The method of pulse measurement was modified slightly by the substitution of a Tektronix Type W plug-in unit for the (4) R. Moccia, Theor. Chim. Acta, 8 , 192 (1967). Type Z unit, and a Tektronix P6013 high voltage probe (5) D. Eisenberg and W. Kauzmann, "The Structure and Properties of Water," Oxford University Press, New York, N. Y . , 1969. (1000: 1) for the homemade divider used earlier. The (6) N. H. Fletcher, "The Chemical Physics of Ice," Cambridge voltage compensation feature of the Type W was not University Press, Cambridge, New York, N. Y., 1970. used. Measurement of the pulsed fields did not re(7) N. E. Hill, Proc. Phys. Soc., London (Solid State Phys.), 3, 238 quire absolute calibration since all data are reported as (1970). (8) A. D. Buckingham and J. A. Pople, Proc. Phys. Soc., London, ratios to the water results at 25'. At the higher temSect. A , 6 8 , 905 (1955). peratures, it was necessary to substitute Dow-Corning (9) A . D. Buckingham and R. E. Raab, J . Chem. Soc., 2341 (1957). Glass and Ceramic Cement for the Apiezon W that (10) J. A. Pople, Proc. Roy. SOC., Ser. A , 205, 163 (1951). had been used to seal the windows to the cell a t lower (11) Y. Chen and W. H. Orttung, J . Phys. Chem., 72, 3069 (1968). temperatures. The water and D 2 0 samples were (12) W. H. Orttung, ibid., 73, 2908 (1969). slowly redistilled, the latter from 99.S% isotopic purity (13) J. Waibel, 2. Naturforsch. A , 21, 186 (1966). Experimental Section
The Journal of Physical Chemistry, Vol. 76,No.2,1972
YEONG-JGI CHENAND WILLIAMH. OIZTTUNG
218
t
t.
c
089
08-
07 -
0#8 T"C
Figure 1. The temperature dependence of the Kerr constant of water a t 365 inp: 0, first series; 0 , second series; A, third series; 0, weighted averages of the four series, as described in the text. The dashed line is suggested by the data and is compared with 1/T aiid 1/T2 curves.
I 5
I
I 15
I
I 25
I
I
35
I
I 45
I
I 55
I
I
To C
Figure 3. The temperature depeiidence of the Kerr coiistaiit of 1120 at 436 inb.
where nl I and nl are the rcfractive indices of the sample parallcl and perpendicular to E, the average applied field in the medium in esu, at wavelength X in air. For N interacting molecules per em3, each having an optical polarizability tensor, r a p ,the Kerr constant may be expressed in terms of the following molecular average
c
I
0,Qt
0,s 1
1
5
1
1
15
1
1
25
1
1
35
1
1
45
1
I 55
l
l
To C
Figure 2. The temperature dependence of the Kerr constant of DzO a t 365 mp: 0, first series; A, second series.
due to thermal strain caused by the window cement, which may not have been warm enough to relax quickly. In any case, reproducibility was more easily achieved above 55". The 3% uncertainty shown in Figure 1 is suggested by the data and is consistent with anticipated uncertainties. Deuterium Oxide. The conductance of D20 increased too rapidly to allow experiments above 50". Two series of experiments were carried out a t 365 mp, and the results are shown in Figure 2. At 25" and 365 mp) the I h r r constant of D20 was 90% f 4% that of H20. The results for one series of measurements at 436 mp are shown in Figure 3. The Kerr constant of D20 increased from 436 to 365 mp, suggesting that its dispersion is not unlike that of water in this wavelength range. Mixtures. The Kerr constant of a 50% Hz0-50% D20 mixture (by volume) was approximately halfway between the Kerr constants of pure water and D2O a t 25" and 365 mp.
Interpretation General Expressions. The Kerr constant is defined as B = (nil - 7tl)/XE2 (2) The Journal of Physical Chemistry, Vol. 76, N o . 2 , 1972
where ea'l and e', are unit vectors parallel and perpendicular to the applied field. (Summation over a Greek subscript is implied when it appears twice in a symbol or product of symbols.) The average is carried out a t applied field strength, E. The factor, (n2 2)2/6n, in eq 3 is a consequence of assuming the validity at optical frequencies of the Lorentz internal field
+
where i denotes parallel or perpendicular polarization. If rapis written as an expansion in powers of the total field, F,, at a molecule nap = a,p
+ PorpyFy+ '//z~,p~sF~Fa + ..
(5)
we obtain the definitions of the low field polarizabiIity a,@, and of the first and second hyperpolarizability tensors P a p r and ~ ~ p ~ ~ . ~ The average of eq 3 was evaluated by Buckingham and Raab,Qwho considered a sample in the shape of a sphere surrounded by vacuum. Using classical statistics, the average was expanded in powers of the uniform vacuum or "external" applied field, Eo = E ( € 2)/3, where B is the static dielectric constant of the sample. The first nonzero term, proportional to Eo2, led to a result of the form
+
219
TEMPERATURE DEPENDENCE OF THE KERRCONSTANT OF WATER where V , is the molar volume, N A is Avogadro’s number, and a2
1 13
= 7(37rapM,MP
- ?~,,M&,)o
(7)
in which M , is the dipole moment of the spherical sample in a given configuration, and the average is carried out a t zero applied field. E q 7 contains r a prather than a,, (see eq 5 ) because EO= 0 does not imply F, = 0. Our analysis suggests that this consideration is of major importance in the temperature dependence of the Kerr effect in liquid water. The result in eq 7 may be simplified if the traceless tensor, T‘,, = rap- ?,6i, is used1* where = ?r,,/3 is the mean polarizability, and ,,6 = 1 if a = 0, or 0 if a # p. Thus, eq 7 becomes
+
Simplifications for Water. The coefficients a0 and al of eq 6 depend directly on a,,, POBY,and y,p,a. We need not discuss their explicit form because their contribution to the Kerr constant of liquid water is probably at least an order of magnitude smaller than that of a2. There are two reasons for this conclusion. First, the B vs. T plot of Figure 1 approaches a 1/T2 dependence a t higher temperatures. Second, the data of Buckingham and Orr15 on CH2F2suggest that the al term is about 1/4 the a2 term. If a similar ratio holds in water vapor, the dipole correlation effect in the liquid would decrease the ratio by a factor of about 1/2.5 to the order of 1/10, The molecular axes, (1,2,3) = (x,y,z), of the water molecule are taken according to the usual definition, with z perpendicular to the H-O-H plane, y parallel to the H-H line, and z along the H-O-H angle dissector to form a right-handed system. Since these axes are also principal axes of the polarizability tensor, with corresponding principal traceless polarizabilities denoted by di, eq 8 may be written
+ T’ZMZ~+ ?r’3M32)0
5 a ~= (n’1M1~
(9)
which is algebraically equivalent to 5az = 3/z(r’a(M32-
+
‘/3M2))0
l/z((~’~
- n’z)(Mi2 - M2’))o
(10)
where
M2 = Ma2 = Mi2 + M22
+ Ma2
Temperature-Dependent Angular Correlation. Buckingham and Raab9 approximated T,, by a,@ and assumed that the second term of eq 10 was zero. Their expression for 5az then reduced to 5 a ~= 3/2a’a(Ma2- ‘/3M2)o
(11) because a f a Bis independent of the configuration of neighboring molecules, according to eq 5 . The aver-
age was worked out for the bendable hydrogen bond model of Pople,lO using the approach to dielectric theory proposed by Buckingham. l6 The temperature dependence of B was found to be approximately l/TIJ’ (corresponding to a2 N T o . 2 ) . A glance at Figure 1 shows that this result cannot explain the data. We have also performed calculations of the type required by eq 10 (with d,, replaced by a’,,) for rigid as well as for bendable hydrogen b0nds.l’ In both cases, the second term of eq 10 was found to be zero, verifying the assumption of Buckingham and Raab. The calculations considered first (and sometimes second) neighbors, and did not employ the dielectric theory of Buckingham. For bendable hydrogen bonds, the temperature dependence of B was always found to be greater than 1/T2, again in disagreement with the data. It was therefore decided to investigate the temperature dependence of d 3 . Temperature-Dependent Polarizability. Analyses of the refractive index of liquid water have suggested a decreasing mean molecular polarizability with increasing t e m p e r a t ~ r e . ’ ~Orttung’s ~~~ calculation suggested a possible decorease in the mean polarizability, ?i, of about 0.0018 A3 between 0 and 50”, the value remaining constant at hi6her temperatures. Since 7r’3 = (a3 _ T- ) -- 0.013 A3 at 25°,3 it is clear that a small change in ~ ‘ 3 , of the magnitude suggested by the change in $ with temperature, could easily explain the results in Figure 1, in which T2Bincreases about 35% from 0 to 60”. A possible source of a temperature-dependent polarizability is the interaction of the first and second hyperpolarizabilities with the electrostatic field along the x axis of a molecule. This field arises mainly from the permanent dipoles of the neighboring molecules, and is strong enough in ice to increase the vapor dipole moment, PO = 1.84 D, to p = 2.60 D.20 The ratio p/po for the liquid was estimated for the bendable hydrogen bond model by Pople.lo Since p = k 0 a3F, Pople’s result can be inverted to yield estimates of the field along the x axis as a function of temperature. The results, using PO = 1.84 debyes and a3 = 1.59 A3,21are shown in Table I for the temperatures considered by Pople. The distinct nonzero hyperpolarizabilities for a
+
(14) P. J. Flory and R. L. Jernigan, J . Chem. Phys., 48,3823 (1968). (15) A. D. Buckingham and B. J. Orr, Trans. Faraday Soc., 6 5 , 673 (1969). (16) A. D. Buckingham, Proc. Roy. Soc., Xer. A, 238, 235 (1956). (17) Y. Chen, “Temperature Dependence of the Kerr Constants of Water and Heavy Water,” Ph.D. Dissertation, University of California, Riverside, June 1970. (18) W. H. Orttung, J . Phys. Chem., 67, 503 (1963). (19) E. Reissler and H. Eisenberg, J . Chem. Phys., 43, 3875 (1966). (20) C. A. Coulson and D. Eisenberg, Proc. Roy. Soc., Xer. A , 291, 245 (1966). (21) N. E. Dorsey, “Properties of Ordinary Water-Substance,” Reinhold, New York, N. Y., 1940.
The Journal of Physical Chemistry, Vol. 76, No. 2, 1972
YEONG-JGICHENAND WILLIAMH. ORTTUNG
220 molecule with CzV symmetry are 6 1 1 3 , 6 2 2 3 , P 3 3 3 ; ~ 1 1 1 1 , YZZZZ, 7 3 3 3 3 , 7 1 1 2 2 , 7 2 2 3 3 , and 7 1 1 3 3 . 2 2 Since the hyperpolarizability tensors are symmetric to interchange of all subscripts, other nonzero coefficients have one of the values listed above. For tetrahedral symmetry, 0 1 1 3 = 6 2 2 3 = P 3 3 3 ; 7 1 1 1 1 = YZZZZ = 7 3 3 3 3 ; and 7 1 1 2 2 = 7 2 2 3 3 = 7ii33.23
The mean polarizability of a water molecule subjected to a field, F3, is obtained from eq 5 as i? =
1/3a,,
+
l/3Paa3F3
+
(12)
1/67aa33F32
Then since ~ 3 = 3 a33
+
P333Fa
4- ' / z ~ 3 3 3 3 F a ~
(13)
it follows that ?r3s
-n
=
(a33
- l/3aaa)
'/3Paa3)F3
+
+(Pa33
-
'/z(~araa-
1/3~aa33)F32
(14)
E q 12 and 14 give the desired expressions for ? and d 3 in terms of the first and second hyperpolarizabilities and the temperature-dependent field along the symmetry axis of the molecule. To make numerical predictions, estimates of Pap,, and Y , ~ , , ~are needed for the water molecule. No such data are yet available, but Buckingham and O r P have studied the Kerr effect of CH2Fz and similar molecules. The dipole moments and optical anisotropies of water and CH2F2 are very similar ( P O = 1.84 lis. 1.97 D; d 3 3 = 0.013 vs. a'33 = -0.0135 A3, respectively), so that it is not unreasonable to use the hyperpolarizabilities obtained for CH2F2 to calculate rough estimates for water. Buckingham and Orr reported loao Pan3 = -0.18 0.04 esu and loasY~~~~ = 5.5 esu. yaap8 cannot be used directly in eq 12 unless we make the approximation, yaapp = which is valid
*
Table I : Temperature Dependence of the Local Field and the Mean Polarizability of Water
lO-sFa, esu
t, O C
0 25
62 83 a
PlPO
2.002 1.go9
1.173 1.165 1.145 1.135
Relative to t
=
1.678 1.562
?---loqji-d),Q Hyperpolrtrieability effect
0.00 -0.05 -0.17 -0.21
+
+
Discussion
'43---
Data analysis18
0.00 -0.13 -0.18
...
0".
for the case of tetrahedral symmetry. In this approximation the results for .rr - E from eq 12 are shown in Table I. The apeement with the earlier estimatels of A? = -0.0018 A3 from 0 to 50°, and no further change from 50 to 60°, is very close. The present consideraThe Journal of Phgsical Chemistry, Vol. 76, No. 2, 1979
tions therefore provide a reasonable interpretation of the earlier calculations. Eq 14 requires a knowledge of 0 3 3 3 and yaas3that is not presently available. We are thus forced to calculate only limiting cases. The four cases considered in Table I1 are explained as follows: (1) and (2) set the y term of eq 14 to zero, while ( 3 ) and (4)set the p term to zero; (1) assumes P a 3 3 = 0 and (2) assumes Pl13 0 2 2 3 = 0, while ( 3 ) assumes 7 3 3 3 3 = 0 and (4)assumes 71133 7 2 2 3 3 = 0 ; (3) and (4) also assume yaap8 = 3vaa33 as in the preceding paragraph. The four calculations are compared with the Kerr constant data in Table 11. It is readily seen that cases (2) and (3) are capable of explaining the data. The range of values of a'3 obtained as a by-product of this calculation is also of interest. The Effect of Hydrogen Bonding. Since the dielectric constants of HzO and DzO are almost the same the angular correlation and (79.39 us. 79.06) a t electric field arising from neighboring dipoles must be very similar in the two liquids. The 10% lower Kerr constant of DzO relative to HzO may therefore depend on an additional effect such as a difference in hydrogen bonding. An increase of hydrogen bond strength would be expected to modify the CZ, symmetry of the water molecule toward a more tetrahedral structure in the liquid state. The increased symmetry could then account for a reduced optical anisotropy. Since it is generally thought that the D bond is stronger than the H bond, an effect of this type could explain the lower Kerr constant of DzO. If this argument is valid, the same effect should also be of importance in the transition from vapor to liquid, or from high to low temperature in the liquid. Raising the temperature of the liquid should then change the molecular symmetry away from tetrahedral toward Czv,thereby increasing the anisotropy. We may conclude from the foregoing considerations that hyperpolarizability interaction with the dipole field of neighboring molecules is capable of explaining the temperature dependence of the Kerr effect of liquid water. The same interaction also provides a very nice explanation of the temperature dependence of the mean polarizability. The direct effect of temperaturedependent angular correlations seems to be less important, but is not negligible, on the basis of relatively simple calculations. Improved calculations of this phenomenon would be desirable. The variation of hydrogen bond strength with temperature and isotopic substitu(22) A. D. Buckingham and B. J. Orr, Quart. Rev. (London), 21, i95 (1967). (23) A. D. Buckingham, Aduan. Chem. Phys., 12, 107 (1967). (24) G . A. Vidulich, D. F. Evans, and R. L. Kay, J . Phus. Chem., 71, 656 (1967).
221
PHOSPHORESCENCE SPECTRA OF SOMEHALONAPHTHALENES
-
Table 11: Temperature Dependence of the Optical Anisotropy of Water Assumptions'
7
Case
Bass
ysaaa
1 2 3
Pap8
'/a~aaaa l/a~rraaa
'IaPaa3
0
4
'IaPaaa
Yaa33
Datab
0
...
...
10"a'sc
,--t - 0
0.16 3.58 2.41 -0.92
1.00 1.00 1.00 1.00 1.00
...
b Assuming that a The assumptions are described in the text. curve in Figure 1. c Assuming that n'a = 0.013 at 25".
7'8
--r'a($)/r'a(t = 0)-
----.---.---.---
t = 25
t = 62
t = 83
0.96
0.86 1.32 1.30 0.53 1.30
0.81 1.44
1.10 1.09 0.85 1.09
1.40
0.37 1.34
is proportional to P B , the values are estimated from the dashed
creases with increasing temperature and shows an inflection near 30". The plot of intensity vs. temperature is remarkably similar to the plot of Kerr constant vs. temperature. Efforts to uncover possible common molecular origins in these and other observations might well lead to further important insights in the three difficult areas involved in this problem.
tion must also be a factor in the temperature dependence, and quantum mechanical calculations of this effect would also be of interest. The need to simultaneously unravel problems involving molecular parameters, interactions, and liquid structure suggests the desirability of investigations covering many types of phenomena. For example, an infrared band of intermolecular origin has been observed a t 2100 cm-1.26 The intensity of this band de-
(25) C. Salama and D. A. I. Goring, J . Phys. Chem., 70,3838 (1966).
The External Heavy Atom Effect on the Phosphorescence Spectra of Some Halonaphthalenes by Linda G. Thompson1 and S. E. Webber* Department of Chemistry, The University of Texas at Austin, Austin, Texas 78718
(Received August 9, 1971)
Publication costs assisted by the Robert A . Welch Foundation
Phosphorescence spectra are presented for 1- and 2-halonaphthalenes (halo = chloro, bromo, and iodo) in a series of matrices (ethanol and 1-propyl chloride, bromide, and iodide). The phosphorescence spectra of these molecules are found to be sensitive to external heavy atom perturbations, with 2-halonaphthalenes being more sensitive than 1-halonaphthalenes. It is suggested that the reason for this solvent effect is t h a t the vibrationally induced portion of the TI --c SOtransition intensity is not important in heavy atom solvents.
Introduction It has been known since the early work of McClure2 and Kasha8that internal ( i e . , chemically bonded) or external heavy atoms have a striking effect on processes involving the triplet state of aromatic molecules. It is currently supposed that aromatic molecules possess this extreme sensitivity to heavy atoms as a consequence of their planarity, which symmetry reduces the magnitude of the spin-orbit coupling between singlet and triplet T-T* excited state^.^ A number of reviews are available which discuss the so-called "heavy atom effect" in general.6-7
It is the purpose of this article to present some phosphorescence spectra of various 1- and 2-halonaphthalenes in various matrices (ethanol and 1-propyl chloride, (1) Present address: Department of Chemistry, Memorial University, St. Johns, Newfoundland. (2) D. 8. McClure, J . Chem. Phys., 17,905 (1949). ( 3 ) M. Kasha, ibid., 20, 71 (1952). (4) D.8. McClure, ibid., 17, 665 (1949). (5) M. A. El-Sayed, Accounts Chem. Res., 1, 8 (1968). (6) S. K. Lower and M . A. El-Sayed, Chem. Rev., 66, 199 (1966). (7) 8. P. McGlynn, T. Azumi, and M. Kinoshita, "Molecular Spectroscopy of the Triplet State," Pi-entice-Hall, Englewood Cliffs, N. J., 1969,Chapters 5-8. The Journal of Phyeical Chemistry, Vole76, No. 8, 1978
