John H. Roberts and Bertin J. De Bettignies
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Raman Spectroscopic Study of Binary Systems. 1. Molecular Association in the Ammonia Hexadeuteriobenzene Liquid System John H, Roberts" Laboratoire des Metaux Alcalins dans I'AmmoniacLiquide, Equipe de Recherche Assocse au CNRS, Facuffe Libre des Sciences et Haotes Etudes Industrielle,59046 Lille, France
and Bertin J. De Bettignies Laboratoire de Spectroscopie Raman, C.5, Universitsdes Sciences et Techniquesde Lille, 59650 VilleneuveD 'Ascq, France (Reaeived May 22, 7974) Publication costs assisted by the Centre de Spectrochimie
The Raman spectra of solutions of ammonia and hexadeuteriobenzene have been measured at ambient temperature. Although 'I and v3 of ammonia decrease in intensity relative to v1 and 2 ~ 4 these , bands have been observed in very dilute solutions by means of repetitive scanning of the spectrum with data accumulation. The results show there are no specific hydrogen-bonded interactions between ammonia molecules and hexadeuteriobenzene molecules, and the hydrogen-bonded structure of liquid ammonia breaks up as the ammonia is diluted with benzene. The results support the four-band resolution of the N-H stretching region of liquid ammonia and the application of the mixture model to liquid ammonia.
Introduction There has been considerable interest in the structure of binary solvent systems.' Interest in systems with liquid ammonia has arisen because of the general problem of the structure of hydrogen-bonded liquids,Z-* radiation chemistry of the solvated electron in mixed system^,^^^ nmr studies of the structure of ammonia-containing system^,^ and the interpretation of the N-H stretching region of the Raman spectrum of 1iquid a m m ~ n i a . ~ , ~ ~ ~ The ammonia-benzene system has been used as a medium for many organic reactions of great synthetic utility.ll This is due to the formation of the solvated electron upon dissolution of alkali metals in liquid ammonia and the enhancement of the solubility of organic substrates by the addition of benzene to the system. In view of the interest in these reactions a strrictural study of the ammonia-benzene system seemed warranted. Hydrogen-bonded liquid structures and the problem of mixture modeis us. continuum models for the liquid state have been discussed in detai1.l Recent spectroscopic studies of ammonia-containing systems have been interpreted in terms of mixture i r n o d e l ~ . ~ However, ~ . ~ ~ ~ Jthe ~ interpretation of details of thie N-H stretching region of the Raman spectrum of liquid ammonia has led to some controversy, ere i s generally good agreement in the experimental results from different laboratories.3,*-10J3J5 The aesoluLion of the spectrum in this region into four bands has been accomplished mathematically by use of a computer programRand by use of a Du Pont 310 curve analyzer.9J3J6As in aqueous systems17more bands are expected in the liquid state than in the gas due to the formation of various h y ~ ~ ~ g e n - b o n ds epde c i e ~ . ~ , ~ The , ~ , ~spectrum J6 of liquid is further complicated by Fermi resonance between u l s the symmetric stretching mode of ammonia mole, harcules with CsUsymmetry, and a component of 2 ~ 4 the monic of the a $ ~ m ~ e i tbending r~c mode.8JOJ4J5 Recent interest in the n-donor properties of benzene in hydrogen- bo^^^^ systems provides further reason to examThe Journalof Phys:cal Chemistry, Vol. 78, No. 21, 1974
ine the ammonia-benzene system.18J9 Whereas water is miscible with ammonia but relatively insoluble in benzene, ammonia is miscible with both water and benzene. For this reason one could not a priori predict the relative strengths of the possible hydrogen-bond interactions in the ammonia-benzene system. The results of this study contribute to a better understanding of all of the above-mentioned phenomena.
Experimental Section The ammonia used in these experiments was doubly distilled from a sodium-ammonia solution and was degassed by freezing and pumping on the solid. Spectroscopy grade hexadeuteriobenzene (Merck) was used directly or was used after distillation from sodium. The samples were prepared by placing a weighed quantity of hexadeuteriobenzene in a 6-mm 0.d. Pyrex tube of 1-mm wall thickness. The sample was frozen and degassed on a vacuum line before ammonia was distilled into the tube, which was then sealed off and weighed. The Raman spectra were recorded on an instrument comprising a Coderg double monochromator equipped with a rapid-scanning system and a system for data accumulationsZ0using the 5145-A line or the 4880-A line of an argon ion laser, Spectraphysics Model 164 AC, as an excitation source. The system was calibrated with carbon tetrachloride and benzene, and the plasma lines of the laser and the spectra were recorded at ambient temperature. Resolution of the experimental spectra into individual bands was accomplished by means of program RESQL~or with a Du Pont 310 curve analyzer. Results Figure 1 shows the resolved Raman spectrum of the N-H stretching region of ammonia in a solution with a 9:l mole ratio of "3 to C6D6. In addition to the experimental envelope (squares) and the calculated bands (circles), an error curve is included which represents the difference between
Raman Spectroscopic Study of Binary Systems
r--3129
21 07
.
3169
I
3209
I
I
1
3249
3289
3329
3369
I
3409
I
3449
7
3489
WFlVENUMBER Figure 1. Resolved Raman spectrum of the N-H stretching region of a solution with a 9:l mole ratio of NH3 to C6D6at 25'. See text for description of symbols.
TABLE I: Raman Stretching Frequencies" of Ammonia ___I__
CsDs
"3:
mole ratio
9:l 4.6:l 3:l 1.5:l 1:1.3 1:2.5 1:3.6 1:5.6 a
cd , 3200
d
3300
3406
Figure 2, Experiimental spectrum of the N-H stretching region at different concentrations: (a) pure NH3; (b) 9 NH3:I C6D6; (c) 3 NH3:1 C6D6; (d) 1.5 "3:f C&; (e) 1.3 C6D6:l "3; (f) 2.5 C8DF:l "3; (9) 5.6 C6Ds:l "3; for (e),(f), and (9)the sensitivity is multiplied by 2.
the experimental curve and the calculated intensities. Previous workers have assigned the bands as follows: 2v4 at 3212 cm-l, v' a t 3265 cm-l, V I at 3298 cm-l, and v 3 at 3388 cm-x.alo Y' is the J ~ Iband of ammonia molecules with C,
2 ~ 4 ~ Y'(P) ) ~ 3212 3265 3210 3265 3210 3264 3211 3255 3213 3213 3210 3208 3210
In cm -1.
rl(p)
3298 3299 3300 3301 3301 3302 3303 3305 3303
-v3(dpY 3388 3393 3402 3409
Mol % NH,
100 90 82 75
60 43 29 22 15
p = polarized. dp = depolarized.
symmetry and has been labeled in this way to avoid confusion with v1 of ammonia molecules with Csv symmetry. The spectrum of hexadeuteriobenzene has no bands in the 3100-3500-~m-~region, which permits observation of the ammonia bands without interference. In none of the experiments performed on solutions of NH3 and CeDs was there any evidence that proton-deuterium exchange had occurred, as is observed for D20-NHB mixture^.^ Figure 2 shows the Raman spectrum of pure ammonia and of ammonia in several ammonia-hexadeuteriobenzene solutions. The evolution of the spectrum with decreasing ammonia concentration is dominated by the decrease in intensity of 'v and v3 relative to V I and 2v4, which are in Fermi resonance. The frequency shifts, Table ISare small but regular as can be seen from Figure 3. Positions for d and u3 are not given for the more dilute solutions because the band The Journal of Physical Chemistry, Vol. 78, NO. 21. 1974
John H. Roberts and Bertin J. De Bettignies
2108
cm.’
341 CI
3400
4” 339C
3360 330’5
93300
Mde A ‘ ’6
3296 10
Figure 3.Change of the frequencies (in
20
30
40
’6 I
I
50
60
m
80
90
’100
cm-’) v3 and v, of ammonia with the concentration of benzene.
maxima of these weak, broad bands are very difficult to determine with accuracy. A data accumulation system was used to observe ~3 and Y’ in the dilute solutions. With 36 scans v3 was obseirved in all the solutions studied, even though it was of very low intensity. The gradual decrease in v’ continues until in the most dilute solutions there is almost no intensity in the 3265-cm-l region. Also apparent in Figure 2 is the decrease in the halfwidths of V I and 2U4 with decreasing ammonia concentration, and in the solLutions of very low ammonia concentration the spectrum resembles that of gaseous ammonia.21 Figure 4 shows the changes in half-widths of v 1 and 2v4 with decreasing airamonia concentration. The bending modes of ammonia, which occur a t lower frequencies, were obscured ,due to overlap with the bands of benzene. No significant changes were observed in the portion of the spectrum attributed to hexadeut,eriobenzene.
Discussion There are several possible types of interactions which could occur in the ammonia-benzene binary solvent system. Conceivably, idherecould be a specific interaction between an N-H of ammonia and the rr-electron cloud of benzene similar to the C-H interaction with benzene syst e m ~ . If ~ this ~ - ~were ~ the case, ?r-donor complexes of 1:1 and/or 1:2 benzene to ammonia ratios could be envisaged. Another possibility for a hydrogen-bond type interaction would be between the C-H of benzene and the lone pair on nitrogen of ammonia. Evidence for this type of association could best be obtained by observing vg, the symmetric bending mode of ammonia which is particularly sensitive to interactions affecting the lone pair.12,25Although v2 cannot ND3, be observed independently in any of the four (“3, @&s, C6Hs) binary systems due to the many intense benThe Journalof Physical Chemistry, Vol. 78. No. 21. 1974
zene bands in the 700-1100-~m-~ region, an interaction of this sort would also affect the stretching modes to some extent and this is not observed. Also, previous work has shown that a t least three halogen atoms are required on the benzene ring before the remaining C-H groups are acidic enough to participate in C-He-N hydrogen bonds,26 so this specific interaction would not be expected to occur in this system. Since there are no breaks in the plots of frequency shift of the bands as a function of concentration, Figure 3, it appears that definite ammonia-benzene complexes do not form in this system a t ambient temperature, and thus no specific hydrogen-bonded interaction occurs between ammonia and hexadeuterioberizene molecules. It has recently been recognized that the N-H stretching region of the Raman spectrum of liquid ammonia consists of four bands3y8,9J3and is further complicated by Fermi resonance.1° The fourth band, referred to here as v’, appears at a frequency intermediate between V I and 2u4, the bands which are in Fermi resonance. The features of the spectrum have been discussed in terms of the presence of two symmetry species of ammonia molecules in solution, and Y’ has been attributed to the symmetric stretch of C, ammonia molecules whose symmetry has been lowered from C3” by the formation of a hydrogen bond of the type N-H*.*N.s The results of this study show that as the concentration of ammonia in the ammonia-benzene system decreases, V’ decreases in intensity. This indicates that C, ammonia is being converted into CsU ammonia; i.e., with increasing dilution by benzene the hydrogen-bonded structure of liquid ammonia breaks down and the ammonia molecules become isolated. The decrease in the half-widths of v1 and 2v4 are a result of the increased freedom of the unbound isolated ammonia molecules.24The trends in frequency shift of the N-H stretching bands, v l and v3, to higher frequency
21 09
Raman Spectroscopic Situdy of Binary Systems
so
10
1
1
0
i
i
T I
10
20
30
40
50
60
701
I 80, t1
, 90
c 6 O6
100
Figure 4. Influence of the concentration of CBDGon the half-width (in cm-') of v i and 2u4 of ammonia.
are also consistent with a structure-breaking effect as the References and Notes (1) A. K. Covington and P. Jones, "Hydrogen-Bonded Solvent Systems," ammonia becomes more dilute.24Indeed, the general evoluTaylor and Francis, London, 1968. tion of the spectrum from pure liquid ammonia to a spec(2) J. H. Roberts and J. J. Lagowski, "Electrons in Fluids," J. Jortner and N. trum resembling gaseous ammonia in the solutions of low R. Kestner, Ed., Sprlnger-Verlag, Berlln, 1973. (3) J. H. Roberts, A. T. Lemley, and J. J. Lagowski, Spectrosc. Lett., 5, 271 ammonia concentration supports this view. (1972). Further evidence for this interpretation comes from the (4) J. H. Roberts and J. J. Lagowski, Abstracts, 165th National Meeting of the American Chemical Society, Dallas, Tex., April 1973, No. PHYS ~,~~ Raman specitrum of IVH3 in the NH3-CC14 s y ~ t e m .No 147. hydrogen-bonded interactions between NH3 and CC14 are (5) U. Schindewolf, "Metal-Ammonia Solutions," J. J. Lagowski and M. J. detected as would be expected, and the breakup of the Sienko, Ed., Butterworths, London, 1970. (6) J. L. Dye, M. G. De Backer, and L. M. Dorfman, J. Chem. Phys., 52, structure of liquid ammonia as a function of dilution by 6251 (1970). CCl4 can easily he observed. The evolution of the spectrum (7) M. Alei, Jr., and A. E. Florin, J. Phys. Chem., 7 2 , 550 (1968). (8)A. T. Lemley, J. H. Roberts, K. R. Plowman, and J. J. Lagowski, J. Phys. in the N-H stretching region in the NH3-CC14 system is Chem., 77, 2185 (1973). analogous to that of the NH3-CcD6 system. (9) D. J. Gardiner, R . E. nester, and W. E. L. Grossman, J. Raman SpecConcE usionsi
No specific hydrogen-bonded interactions between ammonia molecules and benzene molecules are observed in this binary solvent system. As ammonia is diluted with benzene, the hydrogen-bonded structure of ammonia breaks up and the ammonia molecules become isolated. Changes in the N-H stretching region of the Raman spectrum support the resolution of this region into four bands. These results are furtlher evidence for the mixture model of liquid ammonia. Acknowledgments. We wish to thank the National Science Foundation and the Centre National de la Recherche Seientifique for the Exchange of Scientists Grant to J. IFI. R. We also wish to thank Dr. J. J. Lagowski for making program RESOL available to us and professor J. P. Mathieu for the use of' his curve analyzer.
trosc., 1, 81 (1973). (IO) B. De Bettignies and F. Wallart. C. R. Acad. Sci., Ser. 5, 271, 640
(1970). (11) H. Smith, "Organic Reactions in Liquid Ammonia," Interscience, New York, N. Y., 1963. (12) J. Corset and J. Lascombe, J. Chim. Phys. Physicochim. Bioi, 64, 665, 1707 (1966). (13) D. J. Gardiner, R . E. Mester, and W. E. L. Grossman, J. Chem. Phys., 59, 175 (1973). (14) T. Birchall and I. Drummond, J. Chem. SOC.A, 1859 (1970). (15) M. Schwartz and C. J. Wang, J. Chem. Phys., 59$5258 (1973). (16) B. De Bettignies, unpublished results. (17) G. E. Walrafen, ref 1, p 9. (18)T. S. Pang and S. Ng, Spectrochim.Acta, Part A, 29, 207 (1973). (19) A. S. Kertes and F. Grauer, J. Phys. Chern., 77, 3107 (1973). (20) F. Wallart, Thesis, Lille, 1970. (21) L. M. Lewis and W. V. Houston, Phys. Rev., 44, 903 (1933). (22) C. M. Huggins and G. C.Pimental, J. Chern. Phys., 23, 251 (1955). (23) L. W. Reeves and W. 6.Schnsider, Can. J. Chem., 35,251 (1957). (24) G. C. Pimental and A. L. McClellan, "The Hydrogen Bond," W. H. Freeman, San Francisco, Callf., 1960. a(25) K. R . Plowman and J. J. Lagowski, J. Phys. Chem., 78, 143 (1974). (26) A. Allerhand and P. Von R. Schleyer, J. Amer. Chern. SOC.,85, 1715 (1963). (27) J. H.Roberts and B. J. De Bettignies, to be submitted for publication.
The Journalof Physical Chemistry, Vo/. 78. No. 21, 1974
