SERGEN. VINOGRADOV AND HARRY E. GUNNIKG
1962
The Shape and Position of the 2537 A. Absorption Contour
of Mercury in Nonaqueous Solvents
by Serge N. Vinogradov and Harry E. Gunning Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada
(Received February BO, 1064)
The 2537 8. absorption envelope of mercury in 28 organic solvents comprising a number of alkanes, alcohols, and chloro- and perfluoroalkanes has been examined. The separation of the two absorption maxima of the envelope was found to be constant for a given class of conipounds. Beer’s law is followed over a fourfold dilution range by both of the overlapping bands. The origin of the band splitting is discussed.
Introduction In 1930 Bonhoeffer and Reichardt observed two absorption bands in the 2537 8. region i,n mercurysaturated n-hexane (at 2545 and 2570 A. a t 40°), methanol (at 2530 and 2570 8. at S O o ) , and water (at 2510-2520 and 2600 8. a t 120-150°).1 These authors suggested that the two bands were due to the splitting of the triply-degenerate 3P1 level of mercury into a doubly-degenerate u-component and a single n-component by the electrical force field of the molecular environment in the solution. The existence of two bands iii mercury-saturated hydrocarbon solvents has been also observed by other investigators. Phibbsa measured the absorption spectra of mercury in n-hexane, n-decane, and n-octadecane and plotted his results as (nz - l)/(2n2 1) vs. Av in order to test the applicability of the theory of Bayliss4 on the effect of solvent 011 electronic transitions to this case. Although straight-line plots were obtained for both bands, the ordinate intercept was not zero as required by the theory. Recentlyd Robinson6 has also observed a doublet in the 2537 A. absorption of mercury in liquid 2,2-dimethylpropane a t 20’ with a scparation of 26 and proposed that if clusters of solvent molecules containing mercury atoms exist and are not very large, then the number of mercury atoms on the cluster interfaces may be of the same order as that of the mercury atoms in the cluster interiors; the absorption will then be due to mercury atoms in essentially two different environments.
+
w.,
The Journal of Physical Chemistry
Roncin and Damany-Astoin6 have investigated the absorption spectra of mercury encaged in argon and 2-methylbutane matrices a t 2OOK. in the region 16502600 8.; in the 2-methylbutane matrix the band at 2S60 8. had a shoulder at 2527 8. The position and shape of the 2537 A. band of mercury in the vapor phase is very sensitive to change in pressure. The shift and broadening of the 2537 A. line (R band) and the appearance of “blue” or “red” satellite lines (S bands) on either side of the R band with increase in pressure of foreign gas have been widely investigated. The literature on this subject has been thoroughly reviewed.7 More recent work by Michels and his collaborators8 on the effect of helium, argon, neon, and krypton, of Robing on the effect of (1) H. Reichardt and K. F. Bonhoeffer, Naturwiss., 17, 933 (1929); Z . Elektrochem., 36, 753 (1930); Z . P h y s i k , 67, 780 (1931). (2) M. K. Phibbs and B. deB. Darwent, J . Chem. P h y s . , 18, 679 (1950); R. R. Kuntz and G. J. Mains, J . Phys. Chem., 67, 2219 (1963); 68, 408 (1964); P. Warrick, Jr., E. M. Wewerka. and M. M. Kreevoy, J . Am. Chem. Soc., 85, 1909 (1983). (3) M.K. Phibbs, J . Chem. Phys., 18, 1679 (1950). (4) N. S. Bayliss, ibid., 18, 292 (1950). (5) G. W. Robinson, M o l . Phys., 3 , 301 (1960). (8) J. Y. Roncin and N. Damany-Astoin, Compt. rend., 253, 835 (1961). (7) H. Margenau and W. W. Watson, Rev. M o d . Phys., 8 , 22 (1936) ; I. I. Sobel’man, Uspekhi Fiz. >Vauk,54, 551 (1954); S. Robin and S. Robin, J . phys. r a d i u m , 17, 143 (1958); S. Ch’en and M . Takeo, Rev. M o d . Phys., 29, 20 (1957). ( 8 ) A. Michels and H. de Kluiver. Physica, 22, 919 (1956); A. Alichels, H. de Kluiver. and B. Castle, ibid., 23, 1131 (1957); A. Michels, H. de Kluiver, and D. Middlekoop, ibid., 2 5 , 163 (1959). (9) J. Robin, These Doctorat Bs Sciences Physiques, UniversitB de Paris, 1958.
ABSORPTIOX CONTOUR, OF MERCURY I N NONAQUEOUS SOLVENTS
nitrogen, helium, hydrogen, and argon, and of Vodar and his collaborators1‘) on the effect of hydrogen and deuterium on the position and shape of the R band a t pressures up to 6000 kg. cm.-2 has not resulted in any significant clarification of the phenomenon of the appearance of S bands. In cases where no Etatellite bands appear, as in the presence of hydrogen or deuterium, the observed shift and broadening of the R band has been treated theoretically with some degree of success.11r12 On the other hand, only semiquantitative rationalizations of the appearance of S bands13 have been offered so far. The earliest one, that of Oldenberg and Kuhn and of Preston,14 was based on the variation in the relative shapes of the potential curves for the upper and lower states concerned in the transition. Another explanation has been proposed by RobineJ6based on the concept of an optically active atom in a cage defined by a large number of foreign perturbing molecules. The satellite bands would be due to a simultaneous transition involving the combination of the electronic transition V R and the intermolecula,r vibration frequency VIM of the atom vibrating in the potential well of its cage; the satellite frequency is then YS = 2/11
+
VIM.
The existence of a large number of lines on the high 8. line ranging from about 2540 to 3000 A. in the absorption and fluorescence spectra of mercury vapor alone has been consistently ascribed to the formation of Hgz molecules.1e KO detailed work seems to have been done on the effect of hydrocarbons on the absorption and fluorescence spectra of mercury vapor although the effect of hydrocarbons ranging from propane to dodecane on the .absorption spectra of rubidium and cesium vapor has been carefully investigated by Ch’en and Jefimenko.I7 Glockler and Martin1* investigated the fluorescence of a mixture of mercury vapor and methane (1 atm.) a t 20’. They found several red bands and assigned them to quantized states of a mercurymethane complex. We have examined the absorption in the 2537 A. region of mercury dissolved in a number of alcohols, normal and branched alkanes and cycloalkanes, chloroalkanes, and two perfluorinated solvents. X side of the 2537
Experimental All spectra were obtained a t 26 3t l o , using a Cary Model 14 spectrophotometer. The wave length calibration was made with a National Bureau of Standards holmium oxide standard transmission glass. Fused quartz cells 10, 5, and 2 cm. in length were used. Unless otherwise indicated, each spectrum was obtained
1963
by first running the pure solvent uncompensated and then the solvent saturated with mercury, again uncompensated. The organic solvents were saturated with mercury by shaking at room temperature for at least 120 hr. in a sealed glass ampoule.19 Triple-distilled mercury (Mallinckrodt) was used without further purification. The hydrocarbons were purified by a method described previously.20 The alcohols were distilled over sodium metal and the corresponding dialkyl phthalate. The chloroalkanes were purified by shaking with concentrated sulfuric acid, washing with distilled water, and passing through a silica gel column. The chemical FC-75, a mixture of isomeric CsFleO ethers (Minnesota Mining and Rlfg. Co.), and the perfluorodimethylcyclobutaneZ1were purified in the same manner as the hydrocarbons.
Results The results are presented in Table I. In all the solutions examined, the absorption by the dissolved mercury was a doublet of strongly overlapping bands, the R band nearest to 2537 8. and the S band on the high X side. The absorption maximum of the S band was always appreciably broader than that of the R band. The positions of the two bands could be affected by the overlap, but we have not attempted any correction for this possibility. Columns 3 and 5 (10) R. Granier, F. Schuller, and B. Vodar, Compt. rend., 252, 3216 (1961). (11) H. Margenau and L. Klein, J . Chem. Phys., 30, 1556 (1959). (12) F. Schuller and B. Vodar, Compt. rend., 251, 1877, 1997 (1960); R. Granier, J. Granier, and E. DeCroutle, ibid., 256, 3622 (1963). (13) H. Margenau and L. Klein in “PropriBtBs Optiques et Acoustique des Fluides ComprimBs.” Centre National de la Recherche Scientifique, Paris, 1959, p. 195. (14) 0. Oldenberg, 2 . P h y s i k , 47, 184 (1928); 55, 1 (1929); H. Kuhn and 0. Oldenberg, P h y s . Rev.,41, 72 (1932); W. M . Preston, ibid., 51, 298 (1937). (15) J. Robin, R. Bergeon, I,. Galatry, and B. Vodar, Discussions Faraday Soc., 22, 30 (1956). (16) I. Agirbiceanu and A. Ichimescu, Bull. I n s t . Politechn. Bucuresti, 21, 41 (1959); E. Koernicke, Z . P h y s i k , 33, 219 (1925); H. Kuhn and K. Freudenberg, ibid., 76, 38 (1932); H. Kuhn, Proc. Roy. SOC. (London), A158, 212, 230 (1937); S. LMrozowski, 2 . P h y s i k , 50, 657 (1929); 55, 338 (1929); Phys. Rev., 36, 1168 (1930); J. G. Winans. ibid., 37, 897 (1931); J. G. Winans and M. P. Heitz, ibid., 65, 65 (1944). (17) S. Y. Ch’en and 0. Jefimenko, J . Chem. Phys., 26, 256, 913 (1957); S. Y. Ch’en in “PropriBtBs Optiques et Acoustique des Fluides ComprimBs.” Centre National de la Recherche Scientifique, Paris, 1959, p. 207. (18) G. Glockler and F. N. Martin, J . Chem. P h y s . , 2, 46 (1934). (19) Although the reported data were obtained without deaeration of solutions, check experiments were performed on mercury solutions, carefully degassed and sealed under vacuum before saturation. There was no change in the observed spectra. (20) S.N. Vinogradov, Can. J . Chem., 40, 2170 (1962). (21) The perfluorodimethylcyclobutane was a gift from Dr. H. A. Eperati, Polychemicals Division, E. I. du Pont de Nemours and Co., Wilmington, Del.
Volume 68, N u m b e r 7
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SERGEN. VINOGRADOV AXD HARRY E. GUNNING
1964
Table I : Absorption Due to Dissolved Mercury a t 26 f lo,in
8.
Solvent
Methanol Ethanol 1-Propanol" 2-Propanol 1-Butanol C yclopentane Cyclohexane Methylcyclohexane Ethylcyclohexane Cycloheptane Cyclooctane n-Pentane 2-Methylbutane n-Hexane 3-Methylpentane n-Heptane n-Octane 2,2,4-Trimethylpentane n-Decane n-Dodecane n-Te tradecane 2-Chloropropane" 1-Chlorobutane" Dichloromethane" 1,2-Dichloroethanea 1,2,3-Trichloropropane" Perfluorodimethylcyclobutane" C8Fl60 ether (FC75)"
R
2525 2531 2535 2532 2538 2544 2544 2546 2548 2549 2551 2538 2539 2540 2539 2541 2543
f3 f2 f3 f3 f3 f2 f2 f2 i3 f2 i2 f2 f2 i2 f2 f2 i2
2542 2544 2545 2551 2533 2539 2541 2544
f2 f2 f2 f3 f3 f2 i3 f3
S
AXs
AXS-R
f3 f2 f3 f3 f4 f2 f2 f2 f4 f2 f2 f2 f2 f. 2 f3 f2 f2
27 33 32 31 38 41 41 41 43 46 48 26 27 32 28 36 38
39 f 6 39f4 34 f 6 36 f 6 37 f 7 34 i 4 34 f 4 32 f 4 32 f 7 34 f 4 34 f 4 25 f 4 25 f 4 29 f 4 26 f 5 32 f 4 32 i 4
5 2574 f 2 7 2575 f 2 8 2578 f 2 14 2584 f 4 -4 2571 zt 3 2 2576 f 2 4 2603 f 5 7 2595 f 5
37 38 41 47 34 39 66 58
32 f 4 31 f 4 33 f 4 33 f 7 38 f 6 37 f 4 62 f 8 51 f 8
A ~ R
-12 -6 -2
-5 +1 7 7 9 11 12 14 1 2 3 2 4 6
2529 f 5
-8
2500 f 5
_-til
2564 2570 2569 2568 2575 2578 2578 2578 2580 2583 2585 2563 2564 2569 2565 2573 2575
2583 f 5 46
54 f 10
2515 f 10 -22
a The absorption spectra of mercury in these solvents were obtained with a matched cell of appropriate length and filled with the pure solvent, placed in the reference beam of the spectrophotometer.
give the differences, XR - 2537 and XS - 2537, while column 6 gives XS - XR. Figure 1 shows some of the spectra obtained. One of the main difficulties associated with saturating the solvents with mercury was the occasional and nonreproducible formation of very fine quasi-colloidal suspensions which exhibited appreciable scattering of light and prevented quantitative measurement of the absorbance due to the mercury in solution. The disagreement between our values for the positions of the R and S bands in n-hexane and methanol and those of Reichardt and Bonhoefferl is very likely due to the fact that their measurements were performed at an appreciably higher temperature than ours. Additional results not included in Table I are summarized below. The Journal of Physical Chemistry
C
D
Figure 1. The absorption spectra of mercury dissolved in some organic solvents: solution -; solvent ---- -- - - -. (A) n-hexane, 10 cm.; (B) cyclooctane, 5 cm.; (C) ( I ) 1,2-dichloroethane, 10 cm.; ( 2 ) 1-chlorobutane, 10 cm. ; (D) (1) methanol, 10 em.; (2) 1-propanol, 10 em. The spectra shown in ( A ) and ( D ) were all obtained with a 10-cm. cell filled with solvents in reference.
I. The intensities of the R and S bands were, respectively, 0.015 f 0.005 cm.-l and 0.016 f 0.005 cm.-l in the alcohols, 0.046 f 0.005 cm.-' and 0.05 f 0.005 cm.-l in the alkanes, and 0.06 f 0.01 cm.-l and 0.07 -1: 0.01 cm.-l in the cycloalkanes. The absorbance per cm. of path length of the two bands in the chloroalkanes was intermediate between the n-alkanes and the cycloalkanes. The absorbance of mercury in the two perfluorinated solvents was more than an order of magnitude smaller than in the hydrocarbons and it was impossible to determine with any degree of certainty whether one or two bands were present. 11. I n all the n-alkanes, branched alkanes, cycloalkanes, and haloalkanes investigated, the absorbance a t the maxima of the R and S bands followed Beer's law to within 10% over a fourfold dilution range. N o shift in the X of either band was observed on dilution. In general the S band was broader than the R band.
ABSORPTION COXTOUR OF MERCURY IN NONAQUEOUS SOLVENTS
111. The proton magnetic resonance spectra of some of the n-alkanes and haloalkanes saturated with mercury were examined using a Varian Model A-60 spectrometer. A slight (about 0.5-2 c.P.s.) and not easily reproducible shift to either side of the pure solvent signal was observed. S o splitting of the signal was found in the mercury-saturated solvent. One of the most striking results in Table I is that AXS-R, the interval between the R and S bands, is constant within the estimated experimental error for a given class of compounds. The only exception seemed to be the rather distinct separation between the pentanes and hexanes on one hand and the other alkanes on the other.
Discussion
+
We have plotted AXR and Ah, us. (nz- l ) / ( 2 n z 1) using the literature data for n Z 5 D . The plots are shown in Fig. 2. It can be seen that, for each class of solvents there exists an approximately linear relationship similar to the one found by Phibbs3; the ordinate intercept was again not zero as required by the theory of Bayliss.* However, the relationships obtained contribute very little to an explanation of the existence of two bands in the absorption spectrum of dissolved mercury in the 2537 8. region. The following hypotheses can be discussed. I. The explanation of Reichardt and Bonhoefferl based on the splitting of the 3P1level of the mercury atom in a molecular force field is not in agreement with the results obtained so far. The AXS-R would be expected to vary appreciably with the dielectric constant of the solvent medium. It can be seen from
0.22
1965
Table I that not only is AXS-R quite constant within a given class of solvents ( e . g . , in alcohols where the dielectric constant (at 25') varies from 32.6 for methanol to 17.1 for 1-butanol) but also that the magnitude of AXS-R for different classes of solvents seems to be quite independent of the dielectric constant. Thus, the AXS-R of the alkanes whose dielectric constant is about 2 is equal, within experimental error, to that for the alcohols and the two monochloroalkanes, 2-chloropropane and 1-chlorobutane, whose dielectric constants are 9.52 and 7.39, respectively. 11. It is possible that the S band is the strongly shifted 2650 A. line ('So 4 3P0) forbidden in the vapor phase.Z2 It is, however, n:t clear why the solventinduced shift of the 2537 A. line should be so much smaller than that of the solvent-induced 2650 A. line. 111. One of the two observed bands, probably the S band, could be due to the existence of Hg, molecules
I
- I ; I
-4
e
+rj
0.20
G+ -
5
w m
rM
0 ALKANES 0 ALCOHaS 0 CYCLOALUANCS
CLn
I
I
I
I
I
I
I
I
I
f
I
'
0 CHLOROALKWES
-IO
o
IO
eo
I
I
I
30
40
50
eo
AX, A
+
Figure 2 . Plots of ( n z -. 1)/(2na 1 ) u.3. A i for the R and S absorption bandai of dissolved mercury.
1
Figure 3. Graphical resolution of the R and S absorption bands of dissolved mercury. (22) We are indebted to Prof. W. A . Noyes, Jr., for suggesting to us this alternative explanation.
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J u l y , 1964
1966
in solution. I n this connection it is of interest to examine closely the profiles of the two bands. Two examples of a rough graphical resolution are shown in Fig. 3. Assuming the R band to be only the solventperturbed 2537 A. band of atomic mercury, and hence probably symmetrical, it becomes obvious that the S band is strongly asymmetric, tailing toward higher A. This may be due to the fact that the S band is an envelope of several closely spaced transitions decreasing in intensity towards higher A, very much like the spectrum observed in the vapor phase and assigned to Hg, molecules.16 Our observation, however, on the validity of Beer’s law seems to contradict this hypothesis. IV. One of the two bands, again probably the S band, could be due to “complex” formation with the solvent. Two alternatives can be visualized: (A) collisional c o r plexes and (B) longer-lived complexes whose half-life would be appreciably greater than the collision time and which would be in equilibrium with single mercury atoms. Our n.m.r. results indicate that no stable complex is formed in the case of the alkanes and chloroalkanes. Moreover, in the case of the longer-lived or stable complexes, it would be expected that complexes with alcohols or the chloroalkanes would be more stable and the interaction be-
The Journal of Physical Chemistry
SERGEN. VINOGRADOV AND HARRY E. GUNNING
tween the two moieties of the complexes-onger than in the alkanes, this prediction does not seem to be borne out by the shifts obtained in this investigation. V. The hypothesis proposed by Robinson and based on the concept of comparable numbers of mercury atoms existing in two different solvent cluster env i r o n m e n t ~deserves ~ serious consideration and is not in disagreement with any of our results. VI. Finally, the explanation based on the concept of an optically active atom in a solvent cage9,l6does not seem to be too different from the foregoing hypothesis. To a good degree of approximation we can put AIM = AS - AR = AAs-R. This would then provide a readily acceptable rationalization for the observed constancy of AAs-R in a given class of solvents. I n conclusion, we feel that our results thus far militate against hypotheses 1-111 and IV(B). It is anticipated that further work, now in progress, will provide sufficient experimental data as to probe into the nature of this phenomenon.
Acknowledgment. This research was supported in part by the National Research Council under Grant No. KRC T-162, which support is gratefully acknowledged.
