1986
J. Phys. Chem. 1990, 94, 1986-1991
Surface-Enhanced Raman Spectroscopy of Bipyridines and Phenylpyrldines Thomas C. Strekas* and Panos S. Diamandopoulos Department of Chemistry, City University of New York-Queens College, Flushing, New York 1 1 367 (Received: June 16, 1989)
Raman and SERS spectra in the region 700-3150 cm-l are reported for 2,2’-, 3,3’-, 4,4’-, 2,3’-, and 2,4’-bipyridines and for 2-, 3-, and 4-phenylpyridines. Spectra for 4,4’-dimethyl-2,2’-bipyridine, l,lO-phenanthroline, and N-methyl-4,4’-bipyridinium are also reported. Comparison of the Raman and SERS spectra, and, in particular, analysis of the C-H stretching frequencies and intensities, indicates that in all cases except 2-phenylpyridine the molecules sit perpendicular to the sol surface consistent with u donation from the less sterically hindered nitrogen donor atom and an all-planar structure.
solution have involved the use of colloidal silver or silver electrode surfaces, with an adsorbate molecule possessing one or more u-donor atoms (usually N or 0).Many of these molecules also possess a potential a-donor system, one or more double bonds or an aromatic ring, which could compete2’ for interaction with the surface. Several groups have s t ~ d i e d the ~ ~SERS - ~ ~ of 2,2’-bipyridine, and a study26of the SERS of 4,4’-bipyridine on silver electrodes has appeared. The isomeric bipyridines present a series of molecules with systematic variation in nitrogen a-donor capability (electronic and steric), while a t the same time possessing an additional a-donor capability of the aromatic pyridyl rings. At one extreme, 2,2’-bipyridine presents a potential chelating pair, while at the other extreme, 4,4’-bipyridine presents a pair of single N-donor sites with no steric interference to interaction with the metal surface. We present here Raman and SERS spectra (700-3 150 cm-I) for the 2,2’-, 3,3’-, 4,4’-, 2,4‘-, and 2,3’-bipyridines on colloidal silver (citrate reduced). Also included are results for the 2-, 3-, and 4-phenylpyridines and for 1,lO-phenanthroline, 4,4’-dimethyL2,2’-bipyridine, and N-methyl-4,4’-bipyridinium. The frequencies and relative intensities in the C-H stretching region are of special significance in interpreting these data with respect to surface orientation of these molecules.
Introduction
Surface-enhanced Raman spectroscopy (SERS) remains an active area’-4 of investigation. An important aspect of S E R S is its potential for probing the interaction between various adsorbates and metallic surfaces. Elucidation of the surface orientation of adsorbed molecules with donor atoms or groups of atoms by analysis of the SERS spectrum has been the subject of numerous A successful basis for analyzing S E R S spectra with regard to orientation of adsorbates has been worked out in the form of “surface selection rules” by Moskovits and c o - w ~ r k e r s ~ and -’~ other^.'^-'^'^' These rules are based on the electromagnetic theory of SERS intensity which says that, via resonance interaction with surface plasmons of the metal, incident light increases the electromagnetic field at the surface of small metallic particles or surface features, which in turn amplifies both the Raman excitation intensity and the scattered intensity. Since the local fields are highest normal to the surface, normal modes of the adsorbed molecule involving changes in molecular polarizability with a component normal to the surface are subject to greatest enhancement. This same type of analysis has been successfully a ~ p l i e d ’ ~by - ’ ~others. Since the original S E R S workZoinvolving pyridine adsorbed on silver sols, the majority of reports of SERS spectra in aqueous
Experimental Section
Silver sols were prepared according to the citrate reduction method35of Hildebrandt and Stockburger. These sols were found to be stable to storage and to give reproducible SERS spectra for a period of several months. Absorption spectra of the sols consistently showed a broad peak centered at 430-440 nm. The nominal silver concentration of the sols was 2.4 X M. All bipyridines and phenylpyridines, as well as 1,lOphenanthroline, were purchased from Aldrich Chemical Co. Solids were purified by recrystallization from diethyl ether. Liquids were vacuum distilled. N-Methyl-4,4’-bipyridinium was synthesized by Robert Morgan of our department. Raman spectra were run in 1-mm4.d. glass capillary cells using transverse excitation and 90’ scattering. A Princeton Instruments OSMA (photodiode array) system was used to detect scattering. This was mounted on the first half of a SPEX 14018 1. O m double monochromator to give maximum spectral coverage. For 514.5-nm laser excitation, a 514-nm spike filter was mounted before the entrance slit to the monochromator. All spectra are displayed in a normalized format with the most intense peak assigned a value of 100 intensity units. Samples were prepared by mixing 5 volumes of colloid with 1 volume of aqueous solution for the bipyridines. Total volume prepared at any time was less than 1 mL. Ethanol/water mixtures (40/60) were used for preparing stock solutions of the phenylpyridines because of low water solubility. These were then added to the usual sol in the volume ratio mentioned above. The final sol concentrations of the molecules studied here fall in the range 6.8 X to 3.1 X M. Dilution studies were conducted to check for qualitative changes in the SERS spectra, and none were
( 1 ) Garrell, R. L. Anal. Chem. 1989, 61, 401A. (2) Garrell, R. L. Spectrochim. Acta 1988, 438, 617. (3) Vo-Dinh, T.; Alak, A.; Moody, R. L. Spectrochim. Acta 1988, 43B, 605. (4) Chang,
R. K., Furtak, T. E., Eds. Surface-Enhanced Raman Scattering; Plenum Press: New York, 1982. (5) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1988, 92, 6327. (6) Moskovits, M.; DiLella, D. P.; Maynard, K. J. Langmuir 1988, 4, 67. (7) Suh, J. S.; Moskovits, M. J . Am. Chem. SOC.1986, 108, 4711. (8) Moskovits, M.; Suh, J. S. J . Am. Chem. SOC.1985, 107, 6826. (9) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1984, 88, 5526. (IO) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1984, 88, 1293. ( 1 1 ) Suh, J. S.; DiLella, D. P.; Moskovits, M. J . Phys. Chem. 1983, 87,
1540. ( 1 2) Moskovits, M. J . Chem. Phys. 1982, 77, 4408. (13) Greenler, R . G.:Snider. D. R.; Witt, D.; Sorbello, R. S. Surf. Sci. 1982, 118, 415. (14) Creighton, J A. Surf. Sci. 1983, 124, 209. (15) Allen, C. S.; van Duyne, R. P. Chem. Phys. Lett. 1979, 63, 455. (16) Muniz-Miranda, M.; Neto, N.; Sbrana, G. J . Phys. Chem. 1988,92, 954. (17) Ni, F.; Cotton, T. M. J . Raman Spectrosc. 1988, 19, 429. (18) Clavijo, R. E.; Mutus, B.; Aroca, R.; Dimmock, J. R.; Phillips, 0. A. J . Raman Spectrosc. 1988, 19, 541. (19) Lee, H. 1.; Suh, S. W.; Kim, M. S. J . Raman Specrrosc. 1988, 19, 49. 1.
(20) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. B 1974, 26, 163. (21) Gao, P.; Weaver, M. J. J . Phys. Chem. 1985, 89, 5040. (22) Kim, M.; Itoh, K. J . Phys. Chem. 1987, 91, 126. (23) Kim, M.; Itoh, K. J . Elecrroanal. Chem. 1985, 188, 137. (24) Kim, M.; Itoh, K. Chem. Lett. 1984, 357. (25) Cooney, R. P.; Mahoney, M. R.; Howard, M. W.; Spink, J. A. Langmuir 1985, I , 273. (26) Cotton, T. M.; Kaddi, D.; Iorga, D. J . Am. Chem. SOC.1983, 105, 7462.
0022-3654/90/20941986$02.50/0 0 1990 American Chemical Societv , , I
-
& a > d b b E
-.
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
S E R S of Bipyridines and Phenylpyridines
>
1282
4,4’-bpy
1I
I ’ ,
h
996
1591
I
1609
P 300
1603
756
765 W - b p ~
1002
1295
1001
1987
1448 1574 I
1220
756 ---L.-
700
-
~
#
900
, 1100
1300
1500
1700 700
A WAVENUMBER
900
1100
1300
1500
1700
1500
1700
A WAVENUMBER 1293
700
900
1100
1300
1603
1500
1700
A WAVENUMBER
700
Figure 1. (top) Raman spectra of the solids 4-phenylpyridine, 2,4’-bipyridine, and 4,4’-bipyridine. Excitation wavelength 457.9 nm. (bottom) Same molecules in the presence of silver sol. Excitation wavelength 514.5 nm. 1293
z W z
a
P 700
900
1100
1300
1500
1700
A WAVENUMBER
5 + z-
1033
700
900
1100
1300
1500
1100
1300
A WAVENUMBER
Figure 3. (top) Raman spectra of liquid 2-phenylpyridine, excitation wavelength 514.5 nm, and solid 2,2’-bipyridine, excitation wavelength 457.9 nm. (bottom) Same molecules in the presence of silver sol. Excitation wavelength 514.5 nm.
All mixtures of colloid and substrate molecule became unstable and led to the precipitation of large aggregates with deterioration of S E R S signal after a variable period of time. This time was on the order of only 10 min or so for the 4,4’-bipyridine and the N-methyl-4,4’-bipyridinium. For all other molecules investigated here, the colloid-substrate mixture was stable for at least 1 h, as judged by the ability to obtain SERS spectra. Optimal S E R S signals were obtained after an initial time lag ranging from several minutes to about 1 h, even with the most stable colloid-substrate mixture.
1502
z
900
1700
A WAVENUMBER
Figure 2. (top) Raman spectra of liquid 3-phenylpyridine, excitation wavelength 514.5 nm, liquid 2,3’-bipyridine, and solid 3,3’-bipyridine, excitation wavelength 457.9 nm. (bottom) Same molecules in the presence of silver sol. Excitation wavelength 514.5 nm. found for the molecules reported here. Spectra were not observed at lower concentrations.
Results The Raman spectra of 4-phenylpyridine, 4,4’-bipyridine, and 2,4’-bipyridine in the 700-1700-~m-~region are shown in Figure 1 (top); the SERS spectra of these same molecules on silver sols are presented in Figure 1 (bottom). In Figure 2, the Raman and S E R S spectra (700-1700 cm-l) for 3-phenylpyridine, 3,3’-bipyridine, and 2,3’-bipyridine are presented. In Figure 3, the Raman and SERS spectra (700-1700 cm-’) for 2-phenylpyridine and 2,2’-bipyridine are presented. In Figure 4, the Raman and S E R S spectra (700-1700 cm-I) for 1, IO-phenanthroline and 4,4’-dimethyl-4,4’-bipyridine are presented along with the SERS spectrum only of N-methyl-4,4’-bipyridinium. In Figure 5, the SERS spectra for the C-H stretching region (2900-3150 cm-l) for all of the above molecules are presented. Discussion The 700-1 700-cm-’ Spectral Region. The most prominent Raman-active modes in this region are in-plane motions assignable as C-C and C-N bond stretching, with some contribution from in-plane C-H wagging. For the sake of simplicity, coplanarity of both rings will be assumed in assigning molecular symmetries. Surface axes will be referred to when discussing polarizability components which contribute to enhancement. Upon adsorption onto the sol surface, one of the molecular axes (see Figure 6) would become the z axis (or normal) with respect to the surface. Ad-
Strekas and Diamandopoulos
1988 The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 4,4’dimethyl-Z,2’-bpy 993
TABLE I: In-Phase and Out-of-Phase Pairs of Biphenyl (in cm-’)’ A. B,,, B,. B2-1
1561
1612 1507 1285 1190 1030 1003 742
1597 1482 1 I76 1040 1008 965 609
1595 1452 1376 1316 1156 1090 608
1570 1432 1383 1283 1156 1074 626
From ref 28. Symmetries refer to DZhpoint group of biphenyl
700
900
1100
1300
1500
1700
A WAVENUMBER
Kmethyl+‘-bpy (sol)
...
0
I
_ _/ A-f-----L 700
900
1100 1300 A WAVENUMBER
1561
1381
---
1500
1700
Figure 4. (top) Raman spectra of the solids 4,4‘-dimethyL2,2’-bipyridine and 1 ,IO-phenanthroline. Excitation wavelength 457.9 nm. (bottom) Same molecules, and N-methyl-4,4’-bipyridinium, in the presence of silver sol. Excitation wavelength 514.5 nm.
sorption via u donation to the silver sol surface would mean that both aromatic rings are oriented perpendicular to the surface. As a result, normal modes that involve in-plane motion of C-C and/or C-N bonds (or C-H wagging) perpendicular to the sol surface s h o ~ l d ~give - ’ ~ rise to the most intense S E R S signals. For an aromatic molecule perpendicular to the surface, greatest S E R S intensity would be expected for totally symmetric modes that derive significant intensity from azZ,where z is the coordinate normal to the sol surface. (This could be either ax,, ayy,or azzin the molecular coordinate system.) Some out-of-plane (molecular) motions as well as a set of nontotally symmetric in-plane motions would then involve the a,, or ayzpolarizability components, respectively, and would be subject to a lesser degree of surface enhancement. Another type of out-of-plane motion would involve axyfor which no surface enhancement is expected. All out-of-plane motions would be expected to be especially weak for this type of orientation because such modes are Raman-inactive for many of the most symmetric molecules discussed here. In addition, they fallzsin the frequency region below 1000 cm-l. If adsorption were to occur via donation of x system electrons, the plane of the aromatic rings would lie parallel to the surface, and the totally symmetric in-plane stretching modes would be expected to be subject to little or no enhancement, with certain out-of-plane modes now showing enhancement, as symmetry allows. Consider the Raman and SERS spectra of the most symmetric species (see Figure 6) 4,4’-bipyridine (Dzh,with xy defining2*the molecular plane) and 4-phenylpyridine (C,,, with y z defining34 the molecular plane). Reference to the workzs on biphenyl, which also possesses DZhsymmetry, reveals that its solid-state Raman spectrum is nearly identical with that which we observe for solid 4,4’-bipyridine, except for modest frequency differences (Figure (27) Lu. T.: Cotton, T M.; Birke, R. L.; Lombardi, J. R. Langmuir 1989, 5 , 406.
(28) Zerbi, G.; Sandroni, S. Spectrochim. Acta 1968, 24A, 483, 511. (29) Strukl, J S.; Walter. J. L. Spectrochim. Acta 1971, 27A. 209.
4, ref 28). Thus, the four strongest Raman bands in the spectrum of solid 4,4’-bipyridine at 1609, 1623, 1300, and 1002 cm-I correlate with the biphenyl bands (see Table I) at 1612 (A,), 1595 (Big), 1285 (A,), and 1003 cm-’ (A,), respectively. Likewise, the same four bands dominate the spectrum of 4-phenylpyridine (at 1602, 1590, 1282, and 1004 cm-I), despite the now lower symmetry (C2J due to the single N at the 4-position. Note that in 4,4‘bipyridine the highest frequency mode in this region is assigned as B1.g. It IS notable that, in the SERS spectrum of 4,4’-bipyridine, the 1623-cm-I band is no longer evident. This is consistent with its assignment as a vibration of B,, symmetry, since totally symmetric modes are expected to be subject to greatest surface enhancement while modes involving axy(where the molecular x is now normal to the surface) would be less enhanced. The same argument can be made for the 1590-cm-l band of 4-phenylpyridine, which is also not observed in the SERS spectrum. In the SERS spectrum of 4,4’-bipyridine, the 1516- and 1219-cm-’ bands, which correlate with biphenyl A, modes at 1507 and 1190 c d , are relatively more enhanced and the 1010-cm-I band has shifted upward by 8 cm-l. For these two molecules, these observations are consistent with a model in which adsorption onto the surface occurs via u donation by the nitrogen at the 4-position. Although the symmetry for 4,4’-bipyridine could be lowered to C,, in this case, the Raman spectrum of 4-phenylpyridine indicates that this could be expected to have little effect on the observed Raman spectral pattern. Our solution SERS spectra of 4,4’-bipyridine are essentially identical with previously r e p ~ r t e d ~spectra ~ , ~ ’ for this molecule on a silver electrode at pH values above 6.4. These previous results were also consistent with end-on adsorption via the 4-position nitrogen. For solid 2,4’-bipyridine, the major Raman intensity is again observed for bands at 1600, 1302, and 1000 cm-I. These bands once again correlate with the three most intense Raman-active A, bands in the spectrum of biphenyl, despite the considerably lowered symmetry of 2,4’-bipyridine ( Cs).Additional bands of moderate intensity occur at 1579 and 1071 cm-’. This spectrum may be more favorably compared to that of 2-phenylpyridine (Figure 3), indicating that a major contribution to new Raman intensity is due to the N substitution at the 2-position. On the silver sol, the SERS spectrum of 2,4’-bipyridine is more complex than observed for the solid. A pair of bands is now evident where only a single band is observed in the solid spectrum: at 994, 1012 and 1579, 1589 cm-l; the 1600-cm-I peak is shifted upward to 161 1 cm-I, and new moderately intense peaks appear at 11 29 and 1378 cm-I along with several less intense yet clearly evident peaks as labeled in the figure. These observations may be rationalized as follows. The in-plane modes of biphenyl can be g r o ~ p e dby ~ .symmetry ~~ into two sets involving normal modes described as in-phase and out-of-phase combinations of the same phenyl-based mode (Ag and B3,,; Bi4 and B2J. Observed frequency differences (Table I) are small in several cases (e.g., A , B3,, pairs at 1612, 1597 and 1003, 965 cm-l). When two pyridyl groups or a phenyl and a pyridyl group are joined, and the symmetry is appropriately lowered (C, or lower), additional modes become totally symmetric (see Figure 6) and may appear as closely spaced bands in the Raman spectrum. The frequency spacing is of course affected by the position of the nitrogen substitution and could be greater or less than observed for biphenyl. The relative Raman intensity of a selected pair of totally symmetric modes will depend on the degree to which all diagonal elements (and others in lower sym-
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 1989
S E R S of Bipyridines and Phenylpyridines 3067
%
t ln
I
\
1 U
e
31 00
3000
2900
I I 2,Z-bpy(Sol)
3000
2900
A WAVENUMBER
31 00
A WAVENUMBER
3063
3077
2928
4,4'-dlM+Z,Z'-bpy (Sol) 3056
2900
3000
31 00
2900
A WAVENUMBER
3000
3100
A WAVENUMBER
Figure 5. SERS spectrum of molecules, as indicated, in the presence of silver sol. Excitation wavelength 514.5 nm. Y
81,
Y
XY
z
Y
82
YZ
Bl
YZ
A, x2, y2, z2
&,
B"
B2
8%
8"
A, *,y2>z2
B,
XZ
xz
Figure 6. Symmetry species of in-plane modes for representative molecules, including molecular polarizibility components and molecular axis
designations. metry groups) of the molecular polarizibility contribute to the normal mode. In the S E R S spectrum, however, the relative intensity of these same modes depends more specifically on the degree to which azzalone (where z is the normal to the surface) contributes to the Raman intensity for each. Thus, where a single band appears in the Raman spectrum, a pair of bands may appear in the SERS spectrum (or vice versa). As observed for 2,4'-bipyridine on the sol, some of the modes deriving from either the B,, or BZubiphenyl modes are nearly as intense as their A,-derived counterparts. The general pattern established by the molecules in Figure 1 is one in which the totally symmetric, in-plane, C-C and C-N vibrations observed in Raman spectra of the solid or liquid are most enhanced upon adsorption onto the silver sol surface. If significant adsorption were occurring via interaction of the A systems, such vibrations would be less intense and out-of-plane modes might appear in the spectrum. It appears to be the case that adsorption occurs via the sterically unhindered 4-position nitrogen, resulting in enhancement for normal modes involving significant C-C and C-N motion normal to the sol surface. The Raman spectra of solid 3,3'-bipyridine (Figure 2) indicate a relatively high symmetry (C2h or CzO)point group. Three
prominent bands appear in the same region as seen for the 4substituted molecules, at 1594, 1307, and 1036 cm-l. An additional band of moderate intensity at 1051 cm-l is also present. Each can be correlated with a totally symmetric vibration of biphenyl (Table I). Both liquids 2,3'-bipyridine and 3-phenylpyridine would have at best C, symmetry. All in-plane modes would then be totally symmetric and Raman-active. For 2,3'bipyridine, a new band is observed at 993 cm-I. For 3-phenylpyridine, two bands appear at 995 and 1006 cm-' and a lowfrequency shoulder appears on the 1037-cm-l band. In addition, a triplet of bands appears at 1588, 1600, and 161 1 cm-l. The bands in both spectra near 995 cm-l may be related to a B,, mode of biphenyl, now totally symmetric. Additional bands in the spectrum of 3-phenylpyridine may be due to out-of-phase combinations, B,, or B3, modes, which are totally symmetric in the lower symmetry molecule. It is notable that the strong band near 1300 cm-' shows no additional nearby bands in the Raman spectrum of any of the molecules reported here. Assignments for biphenyl showz8that the out-of-phase combination of this band (1285 cm-' for biphenyl) lies at much lower frequency, 1176 cm-I. On the silver sol, the S E R S spectra of 3,3'-bipyridine and 2,3'-bipyridine are virtually identical with the Raman spectra of the compounds. This is consistent with u donation to the sol surface through the 3-position nitrogen, which would maintain the symmetry of the 2,3'-bipyridine but lower that of the 3,3'bipyridine, albeit with no apparent effect on the Raman band pattern. In the SERS spectrum of 2,3'-bipyridine, the significant increase in relative intensity of the 1033-cm-I band ( e g , relative to the adjoining bands at 997 and 1056 cm-') is notable with regard to intensity considerations discussed above. The SERS spectrum of 3-phenylpyridine is, however, simplified, with single bands only appearing at 1601, 1032, and 1000 cm-l where multiple bands are observed for the liquid. Contrary to the case discussed above (2,4'-bipyridine and 2,3'-bipyridine), here the SERS intensity appears to be significantly lower for some out-of-phase combinations. Intensity differences between members of the 4-substituted and 3-substituted series may be related to the respective orientation which series members adopt with respect to the sol surface. For the 4-substituted series, both rings would lie
1990
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
symmetrically along the surface normal, while for the 3-substituted series, the center line of one ring is skewed at an angle of approximately 60’ from the normal. This would mean that the molecular axis system polarizability components involving the normal to the surface which contribute to a given mode would vary significantly for otherwise similar normal modes of the two groups of molecules. The liquid 2-phenylpyridine and solid 2,2’-bipyridine Raman spectra (Figure 3) are quite different from one another. The Raman and infrared spectra of 2,2’-bipyridine and 2,2’-bipyridine coordinated to Pd(II), Pt(II), Zn(II), Mn(II), Co(II), Fe(II), Ru(lI), and Ag(1) have been previously r e p ~ r t e d ~ and ~ - ’ ~normal-coordinate calculation^^^^^^^^^^^^ performed. The Raman spectrum of liquid 2-phenylpyridine is generally like those previously discussed. Bands that dominate the spectrum occur at 1603, 1295. and 1001 cm-I, with a band of moderate intensity at 1585 cm-I. Lesser intensity bands are observed at 1500, 1242, 1063, and 1042 cm-I. For solid 2,2’-bipyridine, bands corresponding to those appearing in all spectra are found at 1591, 1574, 1303. and 996 cm-I. Equally prominent bands, however, also occur at 1484, 1448, and 1237 cm-’, with another less prominent band at 1046 cm-I. The trans (C2h) conformation is known for 2,2’bipyridine in the solid. Biphenyl modes of both A, and B,, symmetry in DZh are both now A, and, as observed, give rise to significant Raman intensity. The S E R S spectrum of 2-phenylpyridine is the weakest which we observe for a stable adsorbate-colloid system. This is not surprising since adsorption via u donation from the 2-substituted nitrogen would be severely restricted by steric factors. The S E R S spectrum for 2,2’-bipyridine is quite strong, as r e p ~ r t e d by ~ ~others. - ~ ~ Most notable is the almost complete loss of intensity for the 1448- and 1237-cm-’ bands seen in the solid spectrum. These bands are also weak or absent in the Raman spectrum22of bis( 2,2’-bipyridine)silver( I) and for bipyridine in 1 M HCI (monoprotonated), both of which have 2,2’-bipyridine in a cis conformation. The correspondence of the SERS spectrum of 2,2’-bipyridine under certain conditions to that of the silver(1) complex has been previously pointed out. Our sol spectra compare most closely to reportedz3S E R S spectra of 2,2’-bipyridine in 0.1 M KCI on a silver electrode a t a potential between +.05 and -0.2 V . W e feel our data are also consistent with the previous int e r ~ r e t a t i o n that ~ ~ -2,2’-bipyridine ~~ interacts uniquely with the sol surface (or adatoms on the surface) via c donation from both 2-position nitrogens, in a chelating fashion. Additional support for a chelating mode of interaction comes from the Raman and SERS spectra of 1,lO-phenanthroline (Figure 4). The similarity of the diimine chelation site, but inflexibility of the aromatic system, precludes single nitrogen atom u donation for the 1 ,IO-phenanthroline. Interaction with the surface via the T system seems unlikely, based on the similarities between the Raman and S E R S spectra. Likewise, comparison of the Raman and S E R S spectra of 4,4’-dimethyl-2,2’-bipyridine indicates a conformational transition similar to that which occurs for 2,2’bipyridine, from a C2, trans structure in the solid to a C, chelating structure on the silver sol surface. The Raman spectrum of the solid is quite comparable to that of 2,2’-bipyridine. The band pattern is similar, with substantial shifts in frequency and an additional band of moderate intensity near 1289 cm-I. The shifts may be due to redistribution of normal-mode composition due to mixing of the C-CH, stretch with ring modes. The S E R S spectrum, like that of 2,2’-bipyridine, is simplified, with the intensity of several bands now close to zero (e.g., 1237. 1289, 1427 and 1448 cm-I). (30) Strukl, J. S.; Walter, J. L. Spectrochim. Acta 1971, 27A, 223. (31) Castellucci. E.; Angeloni, L.; Neto, N.; Sbrana, G. Chem. Phys. Left. 1979, 43, 365. ( 3 2 ) Neto, N.; Muniz-Miranda, M.; Angeloni, L.; Castellucci, E. Spectrochim. Acra 1983, 39A, 97. (33) Mallick, P. K.; Danzer, G.D.; Strommen, D. P.; Kincaid, J. R. J . Phys. Chem. 1988, 92, 5628. (34) Caswell, D. S.; Spiro, T.G. Inorg. Chem. 1987, 26, 18. (35) Hildebrandt, P.; Stockburger, M. J . Phys. Chem. 1984, 88, 5935. (36) Creighton. .I. A. Adu. Spectrosc. 1988, 16, 37.
Strekas and Diamandopoulos 603’H
Figure 7. Models for interaction of pyridyl ring systems via u donation to sol surface. Orientation of representative C-H stretching coordinates is indicated. TABLE 11: SERS Intensity of C-H Stretch Relative to the 1300-cm-’ Band‘
relative compound 2,4-bipyridine 4-phenylpyridine N-methyl-4,4’-bipyridine
SERS intensity 0.07 0.10 0.20
3,3’-bipyridine 2,3’-bipyridine 2,2’-bipyridine 4,4’-dimethyl-2,2’-bipyridine 4,4’-dimethyl-2,2’-bipyridine
0.32
N-methyl-4,4’-bipyridine
0.55b
0.60 1 .o
0.82 1.46
“Peak height ratioed to peak height of prominent band near 1300 cm-’ for each compound. All are aromatic C-H stretch, except last two entries. *Methyl C-H stretch. The N-methyl-4,4’-bipyridinium S E R S spectrum is similar to that of 4,4’-bipyridine with a major new band a t 1641 cm-I. A similar mode has been reported34upon protonation of coordinated 4,4’-bipyridine and is likely due to an upshifted out-of-phase component, formerly near 1600 cm-I. C-H Stretching Spectral Region. In the spectral region between 2900 and 3150 cm-’ we observe (Figure 5) the C-H stretching modes. Because the C-H stretching coordinates do not mix significantly with other vibrations of the aromatic rings, their S E R S intensities provide the most specific evidence of the orientation5-15 of the adsorbing molecules with respect to the sol surface (Figure 7). S E R S intensity associated with symmetric stretching of a specific C-H bond (or group of equivalent bonds) will depend upon the angle a t which the bond lies in relation to the surface. Maximum intensity will be expected for stretching of C-H bonds normal to the surface (90’ angle from surface). As the angle is reduced to O”, the S E R S intensity will fall off rapidly. Thus, for a pyridyl ring adsorbed via nitrogen u donation, the C-H at the 4-position is normal to the surface and should contribute significant S E R S intensity in the C-H region. The other four C-H bonds lie at a 30’ angle to the surface and should produce relatively weak S E R S intensity. There should be an additional intensity factor involving the distance of the C-H bond from the sol surface. For the 3,3’-, 2,4’-, and 2,3’-bipyridines (Figure 5, top) a single band is observed in the C-H region a t 3062-3067 cm-’. This is the region where the totally symmetric C-H mode has been r e p ~ r t e d ~for * - both ~ ~ biphenyl and 2,2’-bipyridine. Both 3- and 4-phenylpyridine show a C-H mode (Figure 5, bottom left) in the same frequency range. In each of these cases, adsorption via the N which is least sterically hindered results in one or more C-H bonds normal to the sol surface. It is notable that, for 2,4’-bipyridine and 4-phenylpyridine, the C-H region intensity is significantly lower (Table 11) relative to the intensity of a prominent band that appears near 1300 cm-’ in the C-C and C-N stretching region in each spectrum. This is probably due to the increased distance to the single C-H bond which is normal to the silver sol surface, a t the remote 4’-position. 4,4’-Bipyridine shows no detectable S E R S signal (Figure 5 , top right) in the C-H region. In contrast, when the remote 4’-position is methylated, in N methyl-4,4’-bipyridinium a SERS signal (Figure 5, bottom right) is observed at 2928 cm-l. This is assigned as the symmetric methyl C-H stretch. An additional weaker signal is observed a t 3077 cm-I. On the basis of these results, we conclude that the majority of the S E R S C-H intensity is generated by symmetric C-H
J. Phys. Chem. 1990, 94, 1991-1996 stretching which is normal to the surface and that C-H motion with an approximate angle of 30’ with respect to the surface generates little S E R S intensity. The unique chelating orientation of 2,2’-bipyridine on the sol surface is further supported by the S E R S spectrum in the C-H stretching region. Of the molecules studied here, only 2,2’-bipyridine gives rise to two C-H signals assignable as aromatic C-H stretch, at 3066 and 3100 cm-l. Solid 2,2’-bipyridine shows Raman at 3075 and 3053 cm-I (as well as 3030 and 301 1 cm-l) assignable as totally symmetric C-H stretches. Note that in the chelating geometry (Figure 7), six of the C-H bonds of 2,2’-bipyridine lie at a 60° angle to the surface, with the pair at the 6- and 6’-positions just above the surface. Since this angle is much closer to the normal, S E R S intensity is increased. The SERS spectrum of 4,4’-dimethyl-2,2’-bipyridine (Figure 5, bottom right) shows a single aromatic C-H band at 3056 cm-’ and a methyl C-H stretch at 2916 cm-I. This would imply that the intensity of the higher frequency aromatic C-H band in the 2,2’-bipyridine S E R S spectrum is largely due to the 4-and 4’position C-H bonds, while the lower frequency band is due to the 6- and 6’-position (and possibly also the 3 and 3’) C-H bonds. 1,IO-Phenanthroline (spectrum not shown) shows a single C-H band at 3072 cm-I in the S E R S spectrum.
1991
Although we were able to obtain a S E R S spectrum of 2phenylpyridine in the 700-1700-~m-~region, no SERS signal could be detected in the C-H region. The orientation at which 2phenylpyridine lies with respect to the silver sol surface is difficult to describe with any certainty based on these data. Given the obvious steric hindrance to u donation via the single 2-position nitrogen, one possibility would be that the molecule adsorbs through a combination of u nitrogen donation and a interaction, with the phenyl ring rotated -90’ about the C-C’ inter-ring bond. This would reduce the steric hindrance to the u donation and provide a simultaneous a donation capability. The C-H bonds of the phenyl group would lie parallel to the surface, and SERS intensity would be expected to be very low. The orientation of the C-H bonds of the pyridyl group would be an irregular one, which results in little or no component of C-H stretch normal to the surface.
Acknowledgment. We thank the PSC-CUNY Faculty Research Award Program for support of this work. Registry No. 4-Ph-py, 939-23-1; 2,4‘-bpy, 58 1-47-5; 4,4’-bpy, 55326-4; 3-Ph-py, 1008-88-4;2,3‘-bpy, 581-50-0; 3,3’-bpy, 58 1-46-4;2-Phpy, 1008-89-5;2,2‘-bpy, 366-18-7; 4,4’-Me2-2,2’-bpy,1134-35-6; 1 , l O phen, 66-11-7; N-Me-4,4’-bpy, 22906-73-6; silver, 7440-22-4.
Surface Tensions of Molten Alkali-Metal Halides Yuzuru Sato,* Tatsuhiko Ejima, Department of Metallurgy, Tohoku University, Sendai 980, Japan
Mikio Fukasawa, Ome Work, Sumitomo Metal Mining Co. Ltd., Tokyo 110, Japan
and Kenji Abe Sendai Work, Nippon Metal Steel Products, Sendai 983, Japan (Received: June 20, 1989; In Final Form: September 4 , 1989)
Surface tensions of molten alkali-metal chlorides, bromides, and iodides have been measured by the capillary-rise method. The surface tension cell, made of quartz, has two different capillaries, and the melt is sealed in it under vacuum. The surface tension is determined through the height difference of menisci in the capillaries and is highly reproducible. Surface properties of molten alkali-metal halides vary systematically with the radii of the anions and cations except for the lithium ion; i.e., lithium halides show considerably lower surface energies and surface entropies. Internal pressures of lithium halides, determined through the surface energies, show very low values. Hypothetical surface energies have been calculated by taking the internal pressures of the melts into consideration and assuming no ionic interactions other than long-range Coulombic energy. It is suggested that a considerable part of the Coulombic energy of the molten halides with small cation is canceled by short-range interactions.
Introduction
Surface tension is an essential thermodynamic property of liquids and is of practical importance in many industrial processes. The arrangement of molecules in the surface phase of a liquid is thought to be similar to that in the bulk phase for a singlecomponent liquid without surface-active species. Surface tension is one of the effective physicochemical properties to get the information on the interaction energies in liquids because surface tension or surface energy of the single-component liquid reflects the interactions among the molecules in the bulk phase and the cohesive force of the liquids. Molten alkali-metal halides are the simplest ionic melt among the molten salts because they consist of spherical and univalent cations and anions, and the principal interactions in the molten alkali-metal halides are long-range Coulombic force, although some short-range interactions between the ions may exist. Surface tension measurement is not easy for molten salts because of the experimental difficulties caused by the characteristics of salt such as high melting temperature and hygroscopic and/or 0022-3654/90/2094-1991$02.S0/0
unstable properties. Therefore, large discrepancy is frequently found in the literature values. The capillary-rise method, which is commonly used for the room temperature liquid, is adopted in the current work by considering its high reliability and precision among the methods for surface tension measurement and the capability of sealing the melt in a surface tension cell. The surface tension cell used has been newly designed so as to be suited for high-temperature molten salt. In this paper, we report the surface tension of molten alkali-metal chlorides, bromides, and iodides measured at temperatures ranging from their melting temperatures to about 1200 K, and we then discuss the interactions between the cations and anions in the molten alkali-metal halides on the basis of the results obtained. Experimental Section
Since the capillary-rise method is the most reliable and precise one, and the apparatus is the simplest among the methods of surface tension measurement, it is widely accepted at room temperature. However, it has not been used for high-temperature 0 1990 American Chemical Society
