Communications to the Editor
1950 CRC Project CAPA-5, June 1973;NTlS No. PB 230 993/AS).Identification was based on mass spectral datq(R. T.M. Fraser and N. C. Paul, J. Chem. SOC.6,659 (1968))from a Finnigan combined gas chromatograph-mass spectrometer and comparison of retention times with those of authentic samples. (6)J. N. Pitts, Jr., P. J. Bekowiss, A. M. Winer, J. M. McAfee, and G. J. Doyle, to be submitted to Environ. Sci. Technol. (7)J. H. Beauchene, P. J. Bekowies, J. M. McAfee, A.M. Winer, L. Zafonte, and J. N. Pitts, Jr., Paper No. 66,Proceedings of Seventh Conference on Space Simulation, NASA Special Publication No. 336, Nov 12-14,
1973. (8) W. P. L.Carter, K. R. Darnall, A. C. Lloyd, A. M. Winer, and J. N. Pltts, Jr., Chem. Phys. Lett., in press. (9)C. W. Spicer, A. Villa, H. A. Wiebe, and J. Heicklen, J. Am. Chem. SOC.,
95, 13 (1973). (IO) C.T. Pate, B. J. Finlayson, and J. N. Pitts, Jr., J. Am. Chem. SOC., 96,6554 (1974). (11)R. Simonaitis and J. Heicklen, J. Phys. Chem., 78,653 (1974). (12)P. Robinson and K. Holbrook, “Unimolecular Reactions”, Wiley, New York, N.Y., 1972. (13)C. H. Wu, S . M. Japar, and H. Niki, J. Environ. Sci. Health-Environ. Sci. Eng., . A l l ( P ) , 191 (1976). (14)W. P. L. Carter, A. C. Lloyd, J. L. Sprung, and J. N. Pitts, Jr.. manuscript in preparation. (15) W. P. L. Carter, K. R. Darnali, A. C. Lloyd, A. M. Winer, and J. N. Pitts, Jr., manuscript in preparation. (16)N. R. Greiner, J. Chem. Phys., 53, 1070 (1970).
TABLE I: Valence Banda
Material Water MgCb CaC12 CaBi-2
Concn, M 1.9 3.8 2.2 4.4 2.0 4.0
CUClZ NaCN
2.15 4.30 4.0 8.0
cm-l
h(v0)
cm-1
r,
Sh(v) dv, cm-1
3385 3400 3390 3420 3420 3420 3460 3390 3370 3440 3440
0.28 0.31
370 380 370 380 320 360 280 410 500 420 400
112 122 121 108
v0t
0.33
0.29 0.30 0.29 0.31 0.27 0.22 0.22 0.18
100
106 98 122 114 85 76
Values of vo and r are stated to the nearest 10 cm-l; values of h ( v 0 ) and J h ( v ) dv are accurate to &IO%.
TABLE 11: Librational Banda Statewide Air Pollution Research Center University of California Riverside, California 92502
Karen R. Darnall William P. L. Carter Arthur M. Wlner Alan C. Lloyd James N. Pitts. Jr.”
Material
__
vo, cm-l
k(u0)
v’, cm-l
570 560 540 540 530 540 540 520 440b
0.43 0.43 0.45 0.48 0.46 0.46 0.48 0.42 0.40 0.46 0.49
830 820 800 760 760 760 720 820
_
Water MgCh
Received May 24, 1976
CaClZ CaBrz CUClZ
NaCN
Infrared Optical Constants of Aqueous Solutions of Electrolytes. Further Studies of Salts
Concn, M 1.9 3.8 2.2
4.4 2.0 4.0 2.15 4.30 4.0 8.0
500
450b
_
800
790 760
Values of vg and v‘ are stated to the nearest 10 cm-l. The value of h(v g) are accurate to &lo%. Indication of splitting.
Publication costs assisted by the Office of Naval Research
Sir: In earlier studies of reflection spectra we have used Kramers-Kronig methods to obtain values of the real n(v)and imaginary h ( v ) parts of the complex index of refraction for aqueous solutions of the alkali halides1 and of strong acids and base^.^,^ Because our chief interest has been on the influence of these solutes on the infrared spectrum and on the structure of water itself, we have limited our studies to simple salts which do not have characteristic absorption bands of their own in the infrared. There are certain extremely intense bands in the water spectrum that cannot be studied by absorption methods without resorting to rather elaborate technique^;^^^ for this reason we have employed reflection methods. This note summarizes the results we have obtained with solutions not previously studied by reflection methods. These include solutions of several alkaline-earth halides, of cupric chloride, and of sodium cyanide. Although the CN- ion of sodium cyanide has a single characteristic vibration band in the infrared, the band is weak as compared with the major water bands and occurs in a spectral region remote from the major water bands. Our methods of investigation were the same as those employed in earlier studies and involved measurements of spectral reflectance at near-normal incidence in the range 5500-350 cm-l. Kramers-Kronig analysis provided reliable values of n(u)and k (v) in the range 5000-400 cm-l within the ranges of uncertainty specified in reports of our earlier studies. The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
The two major features of the water spectrum are a very strong absorption band occurring near 3400 cm-l and a second band near 570 cm-l in water at ambient temperatures. Although the band near 3400 cm-l involves the v3, VI, and 2vZ bands of water molecules, we shall for reasons of brevity refer to it as the ualence band. The band near 570 cm-l is the socalled librational band and is associated with the hindered rotational motion of the HzO molecule in the field of its neighbors. The results of the present study are summarized in Tables I and 11, which apply to the valence band and the librational band, respectively. In Table I we list the frequency vo a t which the maximum value of h ( v ) occurs, the value of the absorption index k ( v 0 ) for this frequency, the full width r of the band a t half maximum, and the integrated absorption Jk(v) dv for the valence band in each of the solutions studied; the concentrations of the solutes are listed, The band maximum shifts t o higher frequencies relative to vo for water in all the solutions except for the 4.3 M solution of CuC12, in which it is shifted to slightly lower frequencies. The value of k(v0) for all the alkaline-earth halides is slightly higher than h(v0) for water; in CuClZ and NaCN solutions the value h ( v 0 ) is significantly lower than k ( y o ) for water. As compared with water, the band-width parameter r shows little change for MgC12, CaC12, and 2 M CaBrz but is significantly smaller for 4 M CaBrZ; for the CuClZ and NaCN solutions r is significantly larger than for water. Except for NaCN, the vaiue of the integrated absorption f k (v)
~
Communications to the Editor
1951
solution spectra that cannot be duplicated by merely changing dv for the valence band in solution is within f12%of the value the temperature of water itself. for water; in the spectra of the NaCN solutions the value of Recent progress has been made in quantum-mechanical Jh(v) dv is strikingly smaller. calculations of the energies of interaction between small ions In Table I1 we list the frequency vo at which the maximum and water molecules and has recently been summarized by value of k (v) occurs for the librational band and the value k(v0) Schuster et aL7 These calculations also provide information noted for this frequency; also listed is the value V’ at which k (v) regarding the intermolecular configuration and electron attains half its maximum value in the high-frequency wing of densities involved in the solvation complexes. They indicate the band. The librational band for all solutions is shifted to that specific properties of the ions other than the Bernallower frequencies relative to its position in the water spectrum Fowler parameters of charge and ionic radius must be conas indicated by the listed values for both vo and v’. The values sidered. In the hope that the newer calculations will eventually of k ( v 0 ) for the solutions are, in general, equal to or greater be extended to the spectral changes produced by various ions, than k (vo); the only exceptions to this are the CuC12 solutions, plots of the reflection spectra R(v)vs. v, the values of n(v)vs. for which k(v0) is slightly smaller than that for water. v, and the values of k ( v ) vs. v are available as supplementary In our earlier study of alkali-halide solutions we noted some material (see paragraph at end of text regarding supplemenqualitative agreement with predictions based on the Bernaltary material); the plots should be of use in testing new theoFowler6 theory, but detailed analysis of the results led to retical predictions. certain discrepancies between observations and predictions. In comparing the plots of k ( v ) vs. v with earlier plots of abBernal and Fowler introduced the concept of a structural sorption spectra, it should be noted that absorption spectra temperature involving an intermolecular, hydrogen-bonded are usually presented as curves purporting to give fractional structure that is extensive and tightly bound at low tempertransmittance T(v)vs. v or as plots of the Lambert absorption atures and becomes less extensive and less tightly bound at coefficient a(v)vs. v, where a(v) is defined by the relation T(v) higher temperatures. According to their theory ions in aqueous = exp(-a(v)x), in which x is the thickness of the absorbing solution affect the structural temperature in a manner that layer. The absorption index k ( v ) is a dimensionless optical depends on the ratio of ionic charge to ionic radius; large singly constant related to a(v)by the expression k ( v ) = a(v)/47rvwith charged ions are supposed to produce a greatly increased a(v)and v expressed in cm-l. The separation of the k ( v ) and structural temperature, while smaller ions are supposed to be a(v) peaks becomes important only when r/vo is large; for less effective in breaking up the normal water structure and example, the librational band of water occurs at 570 cm-1 in thus to have less effect on the structural temperature. In our a k (v) vs. v plot and at 680 cm-l in a plot of a ( v ) vs. Y. studies of the alkali halide spectra we found that negative ions We note that k ( v ) is a maximum for the frequency vnm at had greater effects on the water spectrum then did positive which the matrix element I Rnml is a maximum. The spectral ions. absorptance is a maximum at the frequency for which In the case of the valence band, our earlier studies of water V , , ( R ~ ~is (a ~maximum, and spectral emission is a maximum have shown that (1) k(v0) decreases, (2) vo increases, and (3). r increases with increasing temperature. The values of these a t the frequency for which v,m41Rnm12 is a maximum. Thus, the peak in k ( v ) appears at the frequency most directly assoquantities listed in Table I are accurate to approximately f10 ciated with the matrix elements involved in the transition cm-l for vo and I? and to f10% for k ( v 0 ) and S k ( v ) dv. For the probability.8 alkaline-earth halides listed Table I, we would conclude that the combined effects of anions and cations (1) have little influence on the structural temperature as measured by k ( ~ ~ ) , (2) increase the structural temperature as measured by YO, and Acknowledgments. We wish to acknowledge the support (3) decrease the structural temperature as measured by r. No of the Office of Naval Research for this work. startlingly large effects can be attributed to multiply charged cations of the alkaline earths as compared with the singly charged cations of the alkalis studied earlier. With the regard Supplementary Material Auailable: Plots of R(v)vs. Y,the to the valence band, both CuC12 and NaCN greatly increase values of n(v)vs. v, and the values of h (v) vs. v for the materials the structural temperatures as measured by ~ ( v o and ) as shown in Tables I and 11 (40 pages). Ordering information is measured by r; CuClz greatly decreases and NaCN greatly available on any current masthead page. increases the structural temperature as measured by yo. In the case of the librational band, our earlier studies of water have shown that (1)h (vo) does not change significantly References and Notes with temperature, (2) vo decreases with increasing temperature, and (3) the band width increases with increasing tem(1) P. Rhine, D.Williams, G. M. Hale, and M. R. Querry, J. Phys. Chem., 78,238 ( 1974). perature. Except for CuC12, all solutes listed in Table I1 have (2) P. Rhine, D.Williams, G. M. Hale, and M. R. Querry, J. Phys. Chem., 78, 1405 values of k ( v 0 ) that are somewhat but not significantly higher (1974). ( 3 ) H. D.Downing and D. Williams, J. Phys. Chem., in press. than k (vo) for water. All solutes increase the structural tem(4) C.W. Robertson and D. Williams, J. Opt SOC.Am., 61,1316 (1971). perature as measured by VO. All alkaline-earth halides listed (5) J. P. Hawranek and R. N. Jones, Spectrochim. Acta, Part A, 32, 99 in Table I1 decrease the band width as estimated from v’ - vo (1976). (6) J. D. Bernal and R. H. Fowler, J. Chem. Phys., 1, 515 (1933). and thus decrease the structural temperature as measured by (7) P. Schuster, W. Jakubetz, and W. Marius, Top. Current Chem.. 60, 1 this parameter; CuClz and NaCN increase the structural 11975\ _,. (8) G. Herzberg, “Spectra of Diatomic Molecules”, Van Nostrand, Princeton, temperature as measured by the band width parameter. N.J., 1950, p 20. In view of various discrepancies between structural temperatures of solutions as measured by various spectroscopic Department of Physics Harry D. Downing parameters we can only conclude that the Bernal-Fowler Kansas State University Dudley Williams’ theory does not provide a satisfactory basis for interpreting Manhattan, Kansas 66506 the spectra of solutions. Various effects are observed in the Received April 19, 1976 ~
The Journal of Physical Chemistry. Vol. 80, No. 17, 1976
