535
IONIC SOLVATION IN PYRIDINE
Spectroscopic Studies of Ionic Solvation in Pyridine Solutions' by William J. McKinney and Alexander I. Popov2 Department of Chemistry, Michigan State University, East Lansing, Michigan 48823 (Received July $1, 1969)
Infrared spectra of a number of lithium, ammonium, and sodium salts were obtained in the 2000-100-~m-~ spectral region in pyridine solutions. In all cases a low-frequency band was observed whose position was dependent on the nature of the cation but, with the exception of NaI, not of the anion. The band maxima fall at -418,199, and 180 cm-1 for Li, "4, and N a salts,respectively. Substitution of ND*+forNHd+ ion shifts the band by -17 cm-1. Examination of the skeletal vibrations of pyridine upon addition of alkali metal salts produces a shift to higher frequencies. The evidence indicates that the alkali cations are solvated in pyridine solutions and that the far-infrared bands can be assigned to the vibrations of the cations in the solvent cage.
Introduction During the past few years, several investigators have reported far-infrared studies of alkali metal or tetraalkylammonium salts in various solvents.a-6 I n all cases a band was observed whose frequency was dependent on the mass of the cation. I n polar solvents, such as dimethyl sulfoxideboor l-methyl-2-pyrr01idone,~~ the frequency was independent of the anion and the bands were ascribed to the vibration of cations in a solvent cage. I n nonpolar solvents such as benzeneS or tetrahydrofuran4 the band frequencies were anion dependent. These bands are probably due either to the vibration of unsolvated ion pair3 or a t least to the vibration of the cation in a cage consisting of both the solvent molecules and the anion. French and Wood6 recently measured the far-infrared spectra of sodium tetraphenylborate in pyridine, 1,4dioxane, piperidine, and tetrahydrofuran solutions. They report a band at 175 cm-' in all four solvents and assign the band to the interionic vibration of an unsolvated ion pair. They claim that the isotope shift of the 198-cm-' band for NH*+BPhd- solution to 183 cm-l for ND4+BPh4- also fits a bare ion model. It should be noted that no mention was made of the influence of anion substitution on the frequency of the observed bands. Present work represents a part of a detailed spectroscopic study of ionic solvation carried out in this laboratory.
water content of the pyridine was approximately 3 mM as determined by a Karl Fisher titration. Tetrahydrofuran and 1,Cdioxane were reagent grade chemicals dried over calcium sulfate. Piperidine was fractionally distilled from granulated barium oxide. Since both the solvents and the salts were hydroscopic, care was taken to prepare them in as nearly anhydrous conditions as possible. Exposure of the solvents and solutions to the atmosphere during preparation or transfer was minimized by performing these operations with syringes. Attempts to obtain the far-infrared spectra of potassium, rubidium, and cesium salts were unsuccessful owing to their low solubility in pyridine. Infrared spectra were obtained on two instruments, a Perkin-Elmer Rhdel225 spectrometer with a range of 4000-200 cm-l and Perkin-Elmer Model 301 spectrometer with a range of 666-20 cm-l. Normally the Model 225 instrument was used for spectra a t 4000600 cm-l and the Model 301 was used in the 666-50cm-l region. In cases in which duplicate measurements were made, they always agreed within experimental error. The frequency scale of the 301 spectrometer was calibrated using the rotational spectrum of water vapor. Standard infrared solution cells with NaCl windows were used from 4000 to 600 em-', and polyethylene cells with 0.1 or 0.2-mm path lengths, purchased from Barnes Engineering Co., were used below 600 cm-'. Concentration of solutions varied
Experimental Part All alkali metal salts were analytical grade reagents dried by standard procedures except for the ammonium and potassium tetraphenylborates which were prepared by adding a slight excess of an aqueous solution of ammonium or potassium chloride to an aqueous solution of sodium tetraphenylborate. The precipitate was then washed with water and driedunder vacuum. Purification of pyridine has been described previously.' The purified pyridine was stored in an amber glass bottle over molecular sieves (Fisher Type 4A). The
(1) Abstracted in part from the Ph.D. Thesis of W. J. McKinney Michigan State University, 1969. (2) To whom correspondence should be addressed. (3) J. C. Evans and R. Y. Lo, J. Phys. Chem., 69,3223 (1965). (4) (a) W.F. Edgell, J. Lyford, R. Wright, and W. Risen, J. Amer. Chem. Soc., 88, 181 (1966); (b) personal communication, in press. (6) (a) E.W. Maxey and A. I. Popov, J. Amer. Chem. Soc., 89, 3223 (1967); (b) ibid., 90, 4470 (1968); (e) ibid., 91, 20 (1969); (d) J. L. Wuepper and A. I. Popov, ibid., 91,4352 (1969). (6) M. J. French and J. L. Wood, J . Chem. Phys., 49,2358 (1968). (7) W. J. MoKinney, M. K. Wong, and A. I. Popov, Inorg. Chem., 7. 1001 (1968).
Volume 74,Number 8 Fehruury 6, 19YO
536
WILLIAMJ. MCKINNEY AND ALEXANDER I. POPOV
between 0.05 and 1.5 A[. In general the band intensities were proportional to concentrations.
the vibration of the ammonium ions with the interacting species,F 18 is the mass of the ammonium ion, 22 is the mass of the &-ammonium ion, and S is the Results and Discussion mass of the solvent molecule or anion interacting with The frequencies of the infrared bands of lithium, the ammonium ion. For most values of X the term ammonium, and sodium salts in pyridine are shown in under the radical reduces to 15/22. It is significantly Table I. These bands are similar in shape and are different from this value only for very small values of found in approximately the same spectral region as S. French and Woode also carried out measurements those reported for these salts in dimethyl s ~ l f o x i d e , ~ ~of band shifts upon substitution of the ND4+ cation for It is tetrahydr~furan,~ and l-methyl-2-p~~rrolidone.~~ the NH4+ cation. Their value of 183 k 3 om-' for the seen that within experimental error the frequencies of d*-ammonium tetraphenylborate band in pyridine the bands for lithium and ammonium salts are indeagrees with the value obtained in this investigation for pendent of the anion. With sodium salts the perthe d4-ammonium iodide. The above authors, however, chlorate, thiocyanate, and tetraphenylborate bands interpreted their results as an added evidence for their show approximately the same frequency at 180 cm-l assignment of the band to an inner ion-pair vibration. but the band for sodium iodide is 10 wave numbers It is seen that an equally plausible assignment of the lower in frequency. If the vibrations were due to band to a cation-solvent vibration can be made. unsolvated ion pairs, much greater dependence on the Following the example of French and Wood,6 an mass of the anion would be observed. The position of attempt was also made to obtain the infrared spectra ol sodium tetraphenylborate in 1,4-dioxane, tetrahydrofuran, and piperidine. I n the last solvent as well as in Table I: Frequencies of the Vibrational Bands of Alkali Metal pyridine our results agree with the above authors Salts in Pyridine (176 f 4 cm-I and 179 f 3 cm-l). We were unsucCompound vmam om-' cessful with 1,4-dioxane solutions since the concentrations of the saturated solutions were too low for meaLiCl 416 f 4 surements. In tetrahydrofuran, however, the band 418 f 4 LiBr 419 f 4 LiI was observed at 194 f 4 cm-l instead of 179 cm-' as 419 i 4 LiClO, reported by French and Wood. Our results, however, 420 i 4 LiNOa agree with those of Edgell, et u Z . , ~ ~who report a band NHiI 196 f 3 at 198 i 3 cm-1 for sodium tetraphenylborate solutions 197 rt 3 NHiClO4 in tetrahydrofuran. 201 f 3 NH~NOI 199 =k 3 NHiSCN Additional evidence for the solvation of alkali metal 198 f 3 NHdBFr ions by pyridine comes from the study of the shifts of 199 rt 3 NH4BPhd the pyridine skeletal vibrations in the presence of 183 f 3 NDJ alkali salts. Takahashi, et d . , 8 have shown that in the 170 f 3 Nd presence of hydrogen-bonding solvents such as H20, 182 f 3 NaClO4 180 i 3 NaSCN CH30H, CzH50H, and CHCI, these bandss hift to 179 f 3 NaBPh4 higher frequencies, instead of lower frequencies as expected from the increase in mass. The observed shifts indicate that pyridine-hydrogen bonding solvent the band for the d4-ammonium iodide salt in solution is interactions change the electron distribution of the shifted to lower energy by approximately 17 cm-l. A pyridine ring and strengthen the chemical bonds within simple calculation, based on a "&+-pyridine "dithe ring. Similar shifts have been observed by us upon atomic" model predicts a shift of 15 cm-'. The good addition of alkali metal salts to pyridine. The results agreement between theory and experiment can be used are shown in Table 11. Spectral shifts are in the same as evidence that a cation-solvent vibration is involved direction and of the same magnitude as those reported in the appearance of these bands; however, it must be by Takahashi, et uL9 For example, these authors noted that in a calcul.ation of this type in which moleobserve a shift of +11-12 cm-' for v g s band, +9-10 cules and ions are being treated as point masses, the cm-l for vl and 11 cm-l for v ~ ~It.seems reasonable predicted shift is a result of the change of mass of the to assume that in this work the shifts of the skeletal ammonium ion. It is not sensitive to the mass of the vibrations are due to the same phenomena as in the solvent or anion as can be seen in the following eqnawork of Takahashi, et aE.,namely to the interactions of tion pyridine with the alkali metal cations. At least four
+
(8) H,Takahashi, K. Mamola, and E, K. Plylm, J . Mol. Spsctrosc.,
I n this equation,
VND,+
and
The Journal of Physical Chemirrtry
VNH,+
are the frequencies of
21, 217 (1966). (9) Reference 8,Table I.
537
IR MATRIXISOLATION SPECTRUM OF CHa RADICAL salts of each cation were examined and no anion effect was observed. Infrared spectra of the tetraalkylammonium perchlorate likewise did not indicate any shift for the three bands. The magnitude of the shift is Table 11: Frequency Shifts of Three Pyridine Skeletal Vibrations Produced by Alkali Metal Salts
Pyridine Na+ solutions NHI+ solution Li+ solution
1581 ., . 1590 $9 1592 $11 1597' >$12
992 . . . 996 $4 999 $7 1003 '$11
603 . . . 611 +8 614 $11 620 $17
"Assignments of L. Corrsin, B. J. Fax, and R. C. Lord, J . Chem. Phys., 21,1170 (1953). 'The shifted 1581-cm-' band lies too close to the 1598-cm-1pyridine band to be resolved; consequently, only a single band at 1597 cm-1 is observed.
N a + < "49 < Li+ and should give an indication of the relative bond strength of the three ions with the pyridine molecules. Finally, the interaction of pyridine with alkali metal salts has also been demonstrated by the formation of solid solvates with lithium chloride. lo Likewise, the
low conductances of alkali metal salts in pyridine have been explained as being due to an interaction between the alkali metal ions and pyridine. l1 On the basis of the above evidence it seems reasonable to conclude that the alkali metal cations are solvated in pyridine solutions and that the infrared bands listed in Table I are due to the vibrations of the cations in the solvent cage. This observation does not preclude the formation of solvent-separated ion pairs in pyridine solutions as demonstrated by conductometric measurements.11112 In most cases, however, electrostatic interaction between solvated ions does not seem to influence noticeably the frequencies of the solventmetal ion vibrations. It is possible that in the case of sodium iodide, the anion may enter into the primary solvation shell of the cation.
Acknowledgment. The authors gratefully acknowledge the support of this work by a grant from the National Science Foundation. (10) H. Brusset and S. Halut-Desportes, Bull. SOC.Chim. Fr., 469 (1967). (11) D. S. Burgess and C. A, Kraus, J. Amer. Chem. Soc., 70, 706 (1948). (12) H. C.Mandel, W. M. McNabb, and J. F. Hazel, J . Electrochem. Soc,, 102,263 (1955).
Infrared Matrix Isolation Spectrum of the Methyl Radical Produced by Pyrolysis of Methyl Iodide and Dimethyl Mercury by Alan Snelson IIT Research Institute, Chicago, Illinois 60616 (Received March 12, 2969)
The matrix isolation technique has been used t o trap methyl radicals formed by the gas-phase pyrolysis of methyl iodide, &-methyl iodide, and dimethyl mercury. The three observed infrared-active vibration frequencies of the CH, radical assuming a planar structure, point group DBh, are v 2 = 617 cm-l, ~3 = 3162 om-', and ~4 = 1396 om-'. The corresponding frequencies for CDa are vz = 463 cm-', Y8 = 2381 cm-', and ~4 = 1026 cm-l. The infrared-inactive frequency V I is calculated a t 3044 and 2153 om-' for CH, and CD,, respectively, using the UBFF potential for an analysis of the vibrational modes. Some observations on the mechanisms of decomposition of methyl iodide and dimethyl mercury under the present experimental conditions are made, and a tentative vibrational assignment for the radical HgCH, is given.
Introduction The results of three investigations aimed a t trapping methyl radicals in rare gas matrices have recently been published. I n the first of these, Pimentel and Andrews' attempted to form methyl Edicals by reaction of either methyl. iodide or bromide with lithium
atoms during the deposition of the matrix.2 Two absorption bands were Observed in an argon matrix a t 730.3 and 1383 em-' which were assigned to the (1) L.Andrews and G. C. Pimentel, J. Chem. Phys., 47,3637 (1967). (2) w. L.8. Andrews and G. c. PimenteI, ibid., 44,2527 (1966).
Volume 7d8Number 9 February 6, 1970
