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J. Phys. Chem. 1982,86,319-321
Raman Line Width Study of Rotational Motion of Cyclohexane in Aqueous Solution Kazutoshl Tanabe Natbnal Chemical Laboratory for Industry, Tsukuba, Ibarakl305, Japan (Received: October 27, 1981; I n Flnal Form: December 1, 198 1)
Line widths of isotropic and anisotropic Raman bands of cyclohexane at 802 cm-' have been measured in HzO, CH,OH, CH3CN, and CH3SOCH3solutions and in the neat liquid. Rotational diffusion constants have been evaluated from the line widths, and the influence of iceberg structures on the rotational motion of cyclohexane molecules in aqueous solution is discussed.
Introduction It is now recognized that, when hydrocarbon molecules are dissolved into water, a structure like an "iceberg" may be formed around the solute molecules.' The structure resembles that of a gas hydrate where the gas molecule is encaged into a nearly spherical space formed by hydrogen-bonded water molecules. Structures of various gas hydrates have so far been extensively investigated with X-ray diffraction techniques," but a study of the iceberg structures formed in solution is very difficult because of the quite low solubility of hydrocarbons in water. For this reason, spectra of hydrocarbon molecules in aqueous solution have never been reported, although experimental and theoretical investigations on such systems, from a thermodynamic viewpoint, have frequently been presented in the literature. In this study, we report the observation of the Raman spectrum of cyclohexane in aqueous solution. Cyclohexane was selected because of ita high solubility in water, relative to other saturated hydrocarbons, and because of ita strong Raman line at 802 cm-'. It is expected that the iceberg structures formed around cyclohexane molecules in aqueous solution might affect the rotational method of cyclohexane molecules. Thus the rotational diffusion constants have been determined from line widths of the isotropic and anisotropicamponent spectra, and the effect of the iceberg structures on the rotational motion of cyclohexane molecules is discussed from a comparison with the rotational diffusion constants in various solutions. Experimental Sectioq Raman spectra were obtained with a Japan Spectroscopic Co. Model RSOO Raman spectrophotometer and a Coherent Radiation Model CR8 argon ion laser operating at 514.5 nm. Raman bands of cyclohexane at 802 cm-' were observed in HzO, CH30H, CH3CN, and CH3SOCH3 solutions, and in the neat liquid at room temperature. The saturated concentration (0.0055 w t % N 0.0012 mol %) was used for the aqueous solution, and solutions of the same concentration were prepared with the other solvents. In solutions of such low concentrations, the parallel polarized components of the Raman bands could be observed rather easily, while the perpendicular polarized component spectra were obtained after accumulating 50 or more scans because of the extremely low Raman line intensities. The (1) H.S.Frank and W. Mans,J. Chem. Phys., 13, 507 (1945). (2) M. von Stackelberg and H. R. Muller, Z . Elektrochem., 58, 26 (1954). ( 3 ) D. W. Davidson, "Water, A Comprehensive Treatise", Vol. 2, F. Franks, Ed.,Plenum Press, New York, 1973, p 115. (4) D. W. Davidson and J. A. Ripmeester, J. Glacial., 21, 33 (1978). 0022-3654/82/2086-0319$01.25/0
TABLE I: Vibrational Frequency v , Isotropic and Anisotropic Raman Line Widths 6 , and 6 , and Rotational Diffusion Constant D, of Cycfohexane in Various Solvents solvent
H2O
neat CH,OH CHiCN CH,SOCH,
v,
cm-' b , , cm-'
801 802 800 801 801
d8,
cm-'
1.6 * 0.1 5.4 i 0.4 1.6 t 0.1 9.6 i: 0.3 0.9 t 0.1 9.7 i: 0.3 1.2 i 0.1 10.6 + 0.3 1.9 i: 0.1 8.6 i 0.4
D,, ps-' 0.060 f 0.126 i 0.138 i 0.148 i: 0.105 i
0.006 0.005 0.005 0.005 0.006
observed spectra of parallel and perpendicular polarized components in aqueous solution are shown in Figures 1 and 2. In Figure 2, there seems to be a shoulder at 797 cm-', but it is not real. The whole shape of the line with a maximum at 802 cm-' can well be approximated by a single Lorentzian function, and the line width was evaluated through a curve-fitting technique. Parallel and perpendicular polarized component spectra I,,(v)and I,(v) were recorded with a Glan-Thomson prism as an analyzer, and isotropic and anisotropic component spectra I J v ) and I&v) were obtained from
I&)
= ILb)
(2)
The slit width effect on observed Raman line widths was corrected by use of
= 6a[1 - (s/6a)21
(3)
where at is the true (corrected) Raman line width (fwhh), 6, the apparent (observed) Raman line width, and S the spectral slit width.5 Results and Discussion Line widths of isotropic and anisotropic components of cyclohexane at 802 cm-l in various solutions are given in Table I. It is noteworthy that the line widths of the anisotropic components are highly dependent on the solvent while the vibrational frequencies and the isotropic line widths are rather independent. This indicates that only the rotational part of the Raman line width determined from a difference of isotropic and anisotropic line widths is influenced by the medium and suggests that any specific interactions such as hydrogen bonding do not have (5) K.Tanabe and J. Hiraishi, Spectrochirn. Acta, Part A , 36, 341 (1980).
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The Journal of Physical Chemistty, Vol. 86, No. 3, 1982
Letters
.I 5
-
I
VI
a
v
x
n
.lo
CH350CH3
I
.O 5
805
800
795
d J ,/"
800
790 IJ (crn-')
Figure 2. Original drawing of the observed spectrum of the perpendicular component of cyclohexane in aqueous solution: laser power, 1.8 W;spectral slit width, 6.0 cm-'; sensltkrlty, X 4; accumulation, X 128.
an important role in intermolecular systems between cyclohexane and solvent molecules.6 For the analysis of the effect of iceberg structures on the rotational motion of cyclohexane molecules in aqueous solution, the rotational diffusion constant has been evaluated from the experimental isotropic and anisotropic Raman line widths (fwhh) 6, and 6,, by using D, = (~/6)(49- 6,) (4) D, determined for each solution is given in Table I. The symmetry of a cyclohexane molecule is DU,and the symmetry species of the Raman band of cyclohexane at 802 cm-l measured in thii study is aw Therefore the rotational diffusion constant determined from the Raman line widths is for the rotational motion around the C2axis ( x axis) (6) K. Tanabe and J. Hiraishi Adu. Mol. Relaxation Interac. Processes, 16, 281 (1980).
2
1
I/?
L, (cm-')
Flgure 1. Original drawing of the observed spectrum of the parallel component of cyclohexane in aqueous solution: laser power, 1.8 W; spectral SIR width, 2.5 cm-'; sensitivity, X 1; accumulation, X 8.
810
0
H20
3 (cP-')
Flgure 3. Plot of the experlmental rotational diffusion constant in solution vs. the fluidity of the solvent. Ordinate bars denote the experimental uncertainty. The dotted line denotes the correlation of the data in four solvents.
perpendicular to the molecular symmetry axis (C, or z axis).' D, obtained for aqueous solution is clearly smaller than those in other solvents, which indicates that the rotational motion of cyclohexane molecules in aqueous solution is hindered more than in the other solutions. It is known that various factors affect the rotational motion of a molecule; the viscosity of the medium is considered to be the most significant factor. In fact, a good correlation of D, or 7-l with the inverse viscosity has been established for a number of molecule systems.6 Thus, in order to separate the viscosity effect of the experimental rotational diffusion constants, a correlation of D, with the inverse viscosities, i.e., the fluidities, of the solvents was examined; a plot is shown in Figure 3. D,in CH30H, CH3CN, and CH3SOCH3solutions and in the neat liquid shows a good correlation with the fluidities of solvents, whereas D, in aqueous solution is remarkably smaller than the correlation line. This indicates that the rotational motion of cyclohexane molecules in aqueous solution is hindered more than would be predicted from the viscosity effect. It is clear that other factors such as the dielectric constant cannot explain the lowering of a, in aqueous solution. All solvents used in this study are highly polar and similar to each other in solvent effects on the spectroscopic properties. These facts suggest that the hindering of the rotational motion in aqueous solution could be attributed to the iceberg structures formed around cyclohexane molecules. I t is now known that various molecules form gas hydrates, and structures of those gas hydrates have been investigated with X-ray diffraction technique^.^-^ A gas hydrate of cyclohexane is known to exist: but its structure has not yet been determined from X-ray diffraction studies. Cyclopentane is known to form a gas hydrate of a type C5Hlo-17Hz0,where a cyclopentane molecule is encaged in a nearly spherical space of mean free diameter (7) F. J. Bartoli and T. A. Litovitz, J. Chem. Phys., 66, 413 (1972). (8) G. D. J. Phillies and D. Kivelson J.Chem. Phys., 71, 2575 (1979). (9) D. N. Glew, H. D. Mak, and N. S. Rath, "Hydrogen-Bonded Sol-
vent Systems",A. K. Covington and P. Jones, Ed., Taylor and Francis, London 1968, p 195.
J. Phys. Chem. 1882, 86, 321-322
6.6 A formed by hydrogen-bonded water molecule^.^ We attempted to observe Raman spectra of cyclopentane in aqueous solution. Even though the solubility of cyclopentane in water is about three times higher than that of cyclohexane, the perpendicular component spectrum of cyclopentane could not be observed because of its low Raman line intensity. The largest free diameter of a hydrate cage ever reported is 6.6 A as in the above cyclopentane hydrate; a hydrate cage with a larger diameter is not known at present. The largest diameter of a cyclohexane molecule is about 6.9 A as in benzene, and benzene is the largest molecule which
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forms a gas hydrate. Recently, we succeeded in observing the Raman spectrum of benzene in aqueous solution and found as well that the rotational motion of benzene molecules in aqueous solution is hindered more than in other solvents.lo These facts suggest that cyclohexane or benzene molecules dissolved in water are packed more closely in iceberg structures similar to the hydrate cages than in other solvents, and that the close packing may hinder the rotational motion of solute molecules in aqueous solution. (10) K. Tanabe, Spectrochim. Acta, in press.
Heat of Formation of Hydrogen Isocyanide by Ion Cyclotron Double Resonance Spectroscopy Chln-Fong Pau and Warren J. Hehre’ Department of Chemistry, University of California, Iwine, California 92717 (Recelvd: November 3, 1981)
The threshold for deuteron abstraction from protonated DCN has been determined by pulsed ion cyclotron double resonance spectroscopy to be 14.8 i 2 kcal mol-’ higher in enthalpy than the corresponding threshold for proton abstraction (i.e., proton affinity of HCN). Ignoring small effects arising from differences in isotopic substitution, this difference is a direct measure of the relative thermochemical stabilities of hydrogen cyanide and hydrogen isocyanide.
The hydrogen isocyanide molecule has been the subject of considerable attention both from experimentalists and theorists alike. First observed in the laboratory as a product of photolysis of CH3N3in an argon matrix at 4 K,’ the molecule was later suggested and then confirmed2 to be present in interstellar space. Precise knowledge of the J = 1 0 rotational line from radioastronomy prompted attempts to detect and to further characterize the species by microwave spectroscopy3 and by high-resolution infrared spectrometrp in the laboratory. Although the equilibrium geometrical structure of HNC has now been firmly established by a combination of experimental methods: the thermochemical stability of the species, relative to its more stable isomer, hydrogen cyanide, is less certain. An early unsuccessful search for the J = 1 0 microwave absorption of HNC in a normal sample of HCN at room temperature led Brown and his co-workers6 to
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(1) (a) D. E. Magan and M. E. Jacox, J.Chem. Phys., 39,712 (1963); for later work see (b) D. E. Milligan and M. E. Jawx, ibid., 47,278 (1967). (2) (a) L. E. Snyder and D. Buhl, Bull. Am. Acad. Sci., 3,388 (1971); (b) Ann. N.Y.Acad. Sci., 194,17 (1972); (c) Bull. Am. Acad. Sci., 4,227 (1972); (d) B. Zuckerman, M. Morris, P. Palmer, and B. E. Turner, Astrophys. J. Lett., 173, L125 (1972); (e) M. Morris, B. Zuckerman, B. E. Turner, and P. Palmer, ibid., 192, L27 (1974); (f) L. E. Synder and J. M. Hollis, ibid., 204, L139 (1976); (g) R. L. Snell and H. A. Wootten, ibid., 216, L l l l (1977); (h) P. D. Godfrey, R. D. Brown, H. I. Gunn, G. L. Blackman, and J. W. V. Storey, Mon.Not. R. Astron. SOC.,186, (1977); (i) R. D. Brown, Nature (London),270,39 (1977); 6)R. L. Snell and A. Wootten, Astrophys. J., 228, 748 (1979). (3) (a) R. J. Saykally, P. G. Szanto, T. G. Anderson, and R. C. Woods, Astrophys. J.Lett., 204, L143 (1976); (b) G. L. Blackman, R. D. Brown, P. D. Godfrey, and H. I. Gunn, Nature (London),261,395 (1976); (c) R. D. Brown, P. D. Godfrey, J. W. V. Storey, and F. D. Clark, ibid., 262,672 (1976); (d) R. A. Creswell, E. F. Pearson, M. Winnewisser, and G. Winnewsser, 2.Naturwissenschaften A , 31,221 (1976); (e) E. F. Pearson, R. A. Creswell, M. Winnewisser, and G. Winnewisser, ibid., 31, 1394 (1976). (4) (a) C. A. Arrington and E. A. Ogryzlo, J. Chem. Phys., 63, 3670 (1976); (b) A. G. Maki and R. L. Sams,to be published. (5) R. A. Creswell and A. G. Robiette, Mol. Phys., 36, 869 (1978). 0022-3654/82/2086-0321$01.25/0
conclude that the difference in the thermochemical stabilities of the two molecules was no less than 10.8 kcal mol-’. More recently Maki and Sams’ successfully recorded the infrared spectrum of HNC in “pure” HCN at 1000 K. Based on analysis of their intensity data, these authors estimated the relative energies of the two forms as 10.3 kcal mol-’. Work by Ellison and co-workers8suggests that HNC lies somewhere between 17.2 and 26.3 kcal mol-’ above HCN in enthalpy based on the fact that while reaction of CN- with HI leads both to HNC and HCN, proton abstraction either from HBr or from HC1 results only in the production of the more stable isomer. Both of these experimental determinations are in serious disagreement with all high level quantum mechanical calculations which have been performed on the HCN/HNC system to date. In particular, work by Pearson, Schaefer, and Walgrengusing the extensive configuration interaction expansion yielded an energy separation of 14.6 kcal mol-’; efforts by Redman, Purvis,and Bartlettg and by Krishnan and Poplegusing many-body (Moller-Plesset)perturbation theory through fourth-order suggested differences of 15 f 2 and 15.8 kcal mol-’, respectively. In view of the importance attached to the role of HNC as a active participant in the chemistry of interstellar (6) G. L. Blackman, R. D. Brown, P. D. Godfrey, and H. I. Gunn, Chem. Phys. Lett., 34, 241 (1975). (7) A. Maki and R. Sams, J . Chem. Phys., 75,4178 (1981). (8) M. M. Maricq, M. A. Smith, S. J. S. M. Simpson, and G. B. Ellison, J . Chem. Phys., 74, 6154 (1981). (9) (a) P. K. Pearson, H. F. Schaefer, 111, and U. Wahlgren, J. Chem. Phys., 62, 350 (1975); (b) L. T. Redman, G. D. Purvis, 111, and R. J. Bartlett. ibid.. 72, 986 (1980): IC) R. Krishnan and J. A. Pode. to be published. Other theoreticd work includes (d) P. K. Pearion,’ G. L. Blackman, H. F. Schaefer, 111, B. Roos, and U. Wahlgren, Astrophys. J. Lett., 184, L19 (1973); (e) P. Botschwina, E. Nachbaur, and B. M. Rode, Chem. Phys. Lett., 41,486 (1976); (0 P. R. Taylor, G. B. Bacskay, N. S. Hush, and A. C. Hurley, J. Chem. Phys., 69, 1971 (1978).
0 1982 American Chemical Society
