575
J. Phys. Chem. 1993, 97,575-580
High-pressure Raman Spectra of the Acetone Carbonyl Stretch in Acetone-Methanol Mixtures Michael S. Bradley' and John H.Krech Department of Chemistry, University of Connecticut, U-60, 21 5 Glenbrook Road, Storrs, Connecticut 06269-3060 Received: September 8,I992
The isotropic and anisotropic Raman bands of the carbonyl mode in acetone do not coincide, a phenomenon caused by resonant transfer of vibrational energy coupled with an angular dependent intermolecular potential. We report on the frequency shifts, line width changes, and noncoincidence of these bands in mixtures with methanol as the density, temperature, and acetone mole fraction (XA) are independently varied. Increased density red shifts both signals; it also narrows the anisotropic band while not greatly altering the isotropic band. Increased temperature blue shifts both bands and broadens them (considerably, for the anisotropic mode). Dilution with methanol causes red shifts and broadening of both bands. The noncoincidence shift increases as the density of XA increases. In pure acetone, temperature has little effect; in dilute solutions, increased temperature causes a pronounced decrease in noncoincidence. These observations can be consistently explained through the formation of strong acetone-methanol hydrogen bonds, balanced against weaker acetone d i p o l b acetone dipole interactions.
Introductioa The mechanism of vibrational relaxation of a target molecule will be influenced by the intermolecular The environmentwill itself respond to changesin composition, kinetic energy, and/or intermolecular spacing within the sample. These phenomena are effectivelyprobed by extracting IR and/or Raman data at constant density over several temperatures; additional insight is gained if the composition is varied.+" In a previous paper, we reported an exhaustive density-temperaturbmole fraction study of the behavior of the acetone C-C vibration in binary mixtures with methanol.12 Acetone and methanol were chosen for their miscibility, their well-resolved vibrationalmodes, and their range of possible interactions, including hydrogen bonds and dipoldipole alignment. We found the C-C vibration blue shifted with increasing density and with decreasing acetone mole fraction, XA, due to increased collisions. The line broadened as the temperature rose, with a slope which depended on the mole fraction, apparently because the acetone-methanol interactions augmented thevibrationalrelaxation. Acetonebecamelessmobile as the temperature was lowered and as XA decreased, as indicated by the percent Lorentzian character of the vibration. The carbonyl of acetone can participate in strong, associative intermolecular interactions. Thevibrationalmode associatedwith the carbonyl stretch (v~)~~-specifically, the frequency and line width-will respond to these changes. v3 also exhibits a noncoincidence in the signals from the isotropic and anisotropic Raman modes.'-3J4J5 Physically, this results from resonance transfer of excitation in the presence of an angular-dependent interaction potential.23 In acetone, an angular dependence results from dipoldipole interactions. This explanation indicates that the noncoincidence should decrease as the density is lowered, due to the lessening of such interactions,' or as the mole fraction decreases, as resonant transfer becomes less viable. We first review some pertinent theoretical concepts and then describe our experiment and results. In the discussion, we show that strong interactions are the primary influences in this system, with acetonemethanol hydrogen bonding dominating the acetone-acetone dipolar interactions in the mixtures. Theoretid seetion
Knapp and Fischerl' and Schweizer and Chandlera have developed models for the frequency and line-shape behavior of 0022-3654/5S/2097-0575sO4.00/0
binary mixtures under high pressure. Knapp and Fischer assumed the profile would be influenced by fluctuations in the distribution of nearest neighbors.' Their model line shape involved summing Lorentzians from each statistically weighted configuration (no interactions other than spatial position). They predicted the maximum line width would occur at a mole fraction of 0.5, which we did not observe in our earlier work.12 The model describes the microscopic environment based only on the composition of nearest neighbors but does not account for density fluctuations or for orientationally dependent interactions. Schweizer and Chandlera (SC) developed a more extensive model for the liquid phase by accounting for interactions which influence the relaxation rate of an excited vibration. The band shape results from a convolution of vibration-rotation coupling, attractive components, and repulsive components. This yields a line shape given by the Fourier transform of
(1) where 00 is the gas phase transition frequency, (n) is the instantaneous shift, and the three 7 values are the rates of relaxation due to the three mechanisms. The vibration-rotation coupling gives a small contribution, so the line width is primarily determined by the interplay of rapid repulsions and slowlyvarying attractions. These two components are given by
(3) where Aq can be calculated from the cubic potential, reduced mass, and harmonic frequency (eq 3.7b in ref 8). re is the solute bond length, uavis the average solutbsolvent hard-sphere diameter, X = (1 - r,/217)~/2[1 (re/2u)], m is the particle mass, T E is the (Enskog) collision rate, ($2,) is the frequency shift due to attractions, x is the compressibility,and& is an adjustable parameter. The compressibility factor was interpreted as a measure of the distribution of local environments due to solvent number fluctuations. Equation 1 has shown good fits to available data.*J6
+
Q 1993 American Chemical Society
576
Bradley and Krech
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
In our previous paper, we discussed the differences between Gaussian and Lorentzian line shapes and some of the mechanisms which relax vibrational energy.12 There is an additional complexity present in carbonyl vibrations. Raman spectra consist of two components-one due to the isotropic portion of the polarizability tensor, and the other due to the anisotropicportion. Experimentally,these can be separated by acquiring spectra with the polarization of the excitation vertical to the experimental axes, and with the collection of either the vertically (Ivv) or horizontally (IvH) polarized scattered light. The VH signal consists of the anisotropic component (and depolarized portions of the isotropic component); VV contains both components. The isotropic component may be gotten by combining these, IISO = IVV - 4/31VH, where the 4/3 factor depends on the experimental geometry." Previous studies of carbonyl stretches showed that IISOand IVHexhibit a noncoincidence &,,4,1-3"3-'5 wherein, for a solute molecule A, the maximum of the isotropic Raman spectrumis shifted from the maximum of the anisotropic spectrum (or the IR spectrum). Noncoincidence has been studied by Mirone et a1.,18-20and D6ge,l5 with theoretical developments by Wang and McHale,21M ~ H a l e ,and ~~,~~ Physically, noncoincidence results from resonance transfer of vibrational excitation in the presence of strong interactions between molecules (such as caused by dipole-dipole coupling), which cause intermolecular interactions to be orientationally dependent. The VH (and IR) spectrum is sensitive to the orientational dependence, while the isotropic spectrum is not. assumed In developinga model for noncoincidence, that the many-body effects of liquid structure could be accounted for by using a low-density limit and by screening the dipoledipole and transition dipoledipole interactions. Mironeextended this to include a continuum dielectric However, neither model accounts for the detailed structure of the liquid. Logan developed a bath Hamiltonian, based on the mean sphericalmodel of a dipolar liquid, which was responsive to the structure of the From an analysis of the first moment of the isotropic and anisotropic (or IR) spectra, Logan found the noncoincidence of a solute A in solvent B (which may be A), &,A, due to dipolar resonant transfer was given by
(and XB = 1 - XA), p, and Tare the mole fraction of A (and B), density, and temperature, respectively. yo is found from
XA
YO= xA€A.o(P,T) + XB€B,O(P,T) with ~ A , O(and, with subscript change, ~ B , o )given by €A,O(P,T)
=P P ~ * P / ~ ~ c ,
(5)
(6)
/3 = l/kT, PA is the permanent dipole moment, and €0 is the permittivity of a vacuum. T a ,is~ found from a combination of molecular properties:
(7) where ma,A is the reduced mass of the oscillator, and UA is the hard-spherediameter. y a , ~is2 the second derivativeof the dipole with regard to the normal mode (transition dipole), and U,,A is the harmonic oscillator frequency. Jonas3and Loganz4have shown eqs 4-7 agree very well with experiment. Unfortunately, the mole fraction dependence built into the theory ( q 4) is not well developed in cases of strong intermolecular interactions between solute and solvent, such as seen in hydrogen-bonding situations.
1000
j I
I
\
0 1720 1700 1680 Roman Shift ( l / c m ) Figure 1. Raman spectra for the acetone C - 0 stretch, at XA = 0.2,50 OC, and density = 0.83 g/cm3. Top: IWand PI + P2 + P4. Center: I ~ ( = I w - ' / ~ IandP1. v H ) Bottom: I ~ ~ a n d P Seefittingprocedure 4. section for details. 1740
Experimental Section Mixtures of acetone (Aldrich, dried over Drierite and filtered) and methanol (Baker photrex grade) were prepared at acetone mole fractions X A = 1.0, 0.8,0.6,0.4, and 0.2. These solutions are identified throughout by X A . Density measurements, made in a calibrated high pressure were taken for each mole fraction at 0, 25, 50, 75, and 100 OC, for pressures up to 4 kbar. The P-T-p data were fit to a quadratic, and the pressures required to obtain the desired densities at each temperature were determined. These measurements were reproducible to within 5% and were within 5% of availableliterature values. The P-T-p data are recorded in Table I of ref 12. Spectra were taken in a high-pressure cell based on designs published previou~ly.~~ Spectra were collected for three mixtures with X A = 0.10.0.05, and 0.01 in a quartz cuvette. Polarized and depolarized Raman scattering, excited by 500 mW of 488.6-nm argon laser light, were focused throughout a Glan-Thompson polarization analyzer onto a Spex 1403 double monochromator. A cryogenically cooled CCD (Princeton Instruments, EEV 1152 X 298 chip) was used as the detector, giving a spectral window of about 220 cm-I. The integration times were between 30 and 120 s. Spectra were collected with the monochromator centered at 1700cm-I; spectral calibration has been discussed e1sewhere.M The spectra were analyzed using SpectraCalc. Fittiag Procedure. Figure 1 displays the VV (top), VH (bottom), and ISO(tropic) spectra for one XA-T-~point. W clearly shows two components, due to the noncoincident isotropic and anisotropic scattering. Further, the isotropic signal shows a marked assymmetrydue to a polarized combination mode.' We denote the components which may be present in these spectra as follows: P1, polarized isotropic mode; P2, depolarized isotropic mode; P3, polarized anisotropicmode (intensity = 0, ignore); P4, depolarized anisotropic mode; P5, polarized combination mode; P6, depolarized combination mode (intensity 0, ignore). Thus, I S 0 = P1 P5, VV = P1 P2 + P4 + PS, and VH = P2 P4. Our aim was to extract frequency and line-width data for both P1 and P4 and to determine &,A. We first analyzed the spectra using the procedure of Jonas.' Briefly, a parabolic fit over the central region of the isotropic peak was used to determine the frequency of the isotropic maximum, P1. The VV spectrum was then fit to the sum of two Lorentzians, with the position of P1 fixed. At X A = 1.0, our fits were very good and in agreement with ref 1. As a check, the fitted P4 was subtracted from VH, and the residualwas compared to P1 multiplied by a constant (see below). In good fits, the frequency and width of the residual agreed well with P1. Below XA = 0.6, the agreement was poor; as P4 and P1 become less separated, the two components of VV could not be resolved easily. Further, P5 increasingly interfered with the reliability of the fits at low XA.
+
+
-
+
The Acetone Carbonyl Stretch
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 577
TABLE I: R a m Fnquencies (y~,U A ) , Liae Widths (Am, AuA),rad Nonroincidence Shift (Observed and Theoretical) for the Carboayl Modd of Pure Acetone (%A = 1.0)' ~
temp density ("C) (g/cm3) 0.83 0 0.86 0.89 0.92 0.83 25 0.86 0.89 0.92 0.80 50 0.83 0.86 0.89 0.92 0.80 75 0.83 0.86 0.89 0.77 100 0.80 0.83 0.86 0.89 0
&,A
UI
(cm-1) 1706.8 1705.6 1704.9 1704.1 1707.0 1706.1 1705.5 1704.9 1708.3 1707.8 1706.2 1706.2 1705.5 1708.5 1708.0 1707.3 1706.6 1709.7 1708.9 1708.2 1707.5 1707.0
AUI (cm-I)
UA
(cm-I)
AUA (cm-I)
&
(cm-I)
&,A
(cm-I)
~
8.0 7.8 8.0 8.1 8.8 8.6 8.4 8.5 9.2 9.6 9.4 9.4 9.3 9.9 9.9 9.6 9.3 10.9 10.6 10.7 10.2 10.0
1714.0 1713.2 1712.7 1712.3 1714.0 1713.5 1713.3 1712.7 1715.0 1714.6 1713.5 1713.5 1712.1 1715.3 1715.2 1714.7 1714.3 1716.3 1715.7 1715.3 1714.6 1714.6
18.6 17.4 17.1 17.6 20.0 18.8 17.9 18.4 21.6 20.7 19.9 19.9 19.5 24.6 22.8 21.1 20.5 28.0 26.4 25.1 22.6 21.4
7.2 7.6 7.8 8.2 7.0 7.4 7.8 7.8 6.7 6.8 7.3 7.3 6.6 6.8 7.2 7.4 7.7 6.6 6.8 7.1 7.1 7.6
9.1 9.2 9.3 9.4 8.7 8.9 9.0 9.1 8.3 8.5 8.7 8.7 8.8 8.0 8.2 8.3 8.4 7.6 7.8 7.9 8.0 8.2
calculated using eq 4.
The procedure used to obtain the values reported here was based on the fact that the intensity P2 was equal to the intensity of PI times a constant (the depolarization ratio a), but their frequenciesand widths were the same. The fit procedureof Jonas described above' allowed us to estimate 6 for XA = 1.0 and 0.8; a value of 0.10 f 0.05 was found. We began by baseline correcting a set of VV and VH spectra and generating an I S 0 spectrum from them. The low-frequency side of I S 0 was then fit to one peak (a combined Lorentzian4aussian), yielding P1. A copy of P1 was synthesized using the final fit parameters. This peak was interactively scaled and subtracted from VH until a smooth profile was observed; the scaling factor (6) was kept within the range listed above. The remaining VH signal (=P4)was fit to obtain its frequency (and, hence, &A) and line width. The smooth curves in Figure 1 show the componentsobtained in this way. The quality of fits was uniformly good, and they agreed with the available data in ref 1. We feel the underlying assumption of a constantvalue for 6 was thus appropriate. This procedure required fewer fits, and no multipeak fits, to obtain the desired data. On the basis of the calibration procedure and the number of fits, we estimate the error in the reported isotropic frequencies and linewidths of f0.2 and f0.3 cm-', respectively; for the anisotropic, the errors expected are f0.5 (frequency) and fl.O cm-I (line width).
Results and JXscussion The signal-to-noise ratio of the Raman spectra improved as the mole fraction increased; thus, Figure 1 (for XA = 0.2)shows oneof thenoisiest spectra. Thesmall peakduetothecombination mode is manifestedin the high-frequency shoulderof the isotropic peak. Data for the frequencies and line widths of bath the isotropic and anisotropic bands, obtained from the fitting procedures outlined above, and their noncoincidence, for all xA-T-p points are recorded in Tables I-V. Table VI contains data for samples in cuvette. Some relational plots are shown in Figures 2-6. Frequency Shifts. According to the SC model, the frequency (and width) of a vibration results from a competition between its attractive (red shifting) and repulsive (blue shifting) components. Changes in the environment will shift vibrational peaks blue or red depending upon which component is favored. An isothermal increaseindensitycausesbothcomponentsoftheacetonecarbonyl vibration (at all XA) to red shift (Figure 2). Similarly, lowered
TABLE Ik Raman Data at XA = 0.8 (See Table I for D e w ) temp density ("C) (g/cm3) 0 0.86 0.89 0.92 25 0.83 0.86 0.89 0.92 50 0.83 0.86 0.89 0.92 75 0.83 0.86 0.89 100 0.77 0.80 0.83 0.86 0.89
9
Aur
UA
(cm-I)
(cd)
(cm-I)
AUA (cm-I)
1705.3 1704.5 1704.0 1707.2 1706.3 1705.6 1705.1 1707.0 1706.0 1705.6 1705.1 1708.0 1707.3 1706.8 1709.7 1708.9 1708.0 1707.7 1707.0
10.9 10.7 11.6 11.3 11.0 11.4 11.7 11.7 10.9 11.7 11.9 11.8 11.8 11.9 12.4 12.0 12.8 12.4 12.7
1713.1 1712.6 1712.2 1714.4 1714.2 1713.6 1713.1 1714.3 1713.8 1713.3 1712.9 1715.0 1714.7 1714.3 1715.7 1715.3 1714.5 1714.1 1713.7
18.0 18.1 17.3 20.5 20.0 19.5 18.7 22.2 20.0 20.4 20.2 22.9 22.1 20.9 27.4 27.2 26.5 25.2 25.4
&
&,A
( c d ) (an-') 7.8 8.1 8.2 7.2 7.9 8.0 8.0 7.3 7.8 7.7 7.8 7.0 7.4 7.5 6.0 6.4 6.5 6.4 6.7
8.3 8.4 8.5 7.9 8.0 8.1 8.2 7.6 7.7 7.9 8.0 7.4 7.5 7.6 6.8 7.0 7.1 7.2 7.4
TABLE IIk Raman Data at %A = 0.6 (See Table I for Det.ils)
temp density ("C) (g/cm3) 0 0.83 0.86 0.89 25 0.83 0.86 0.89 50 0.83 0.86 0.89 0.92 75 0.80 0.83 0.86 0.89 100 0.77 0.80 0.83 0.86 0.89
UI
bur
UA
(cm-1) 1705.3 1704.7 1704.0 1705.6 1704.9 1704.3 1706.1 1705.1 1704.6 1704.1 1707.7 1706.8 1706.2 1705.5 1708.2 1707.7 1706.6 1706.2 1705.8
(cm-l)
(cm-I)
10.9 11.6 11.8 12.0 12.1 12.3 12.7 12.5 12.8 13.2 13.4 12.7 12.8 13.1 13.0 13.4 13.1 13.7 14.3
1713.6 1713.2 1713.2 1713.6 1713.2 1713.0 1713.7 1713.3 1713.1 1712.8 1714.6 1714.3 1713.9 1713.7 1714.3 1714.3 1714.0 1713.8 1713.1
AUA (cm-I) 18.7 18.2 17.6 20.1 20.3 18.7 22.2 20.4 19.6 19.2 24.6 22.6 22.9 21.2 26.2 25.4 23.8 23.6 23.6
&, (an-') 8.3 8.5 9.2 8.0 8.3 8.7 7.6 8.2 8.5 8.7 6.9 7.5 7.7 8.2 6.1 6.6 7.4 7.6 7.3
&A (cm-1)
7.0 7.1 7.2 6.8 6.9 7.0 6.5 6.6 6.7 6.8 6.2 6.3 6.4 6.5 5.9 6.0 6.1 6.2 6.3
TABLE IW Raman Data at %A = 0.4 (See Table I for D e w ) temp density ("C) (g/cm3) 0 0.86 0.89 0.92 25 0.83 0.86 0.89 0.92 50 0.83 0.86 0.89 0.92 75 0.80 0.83 0.86 0.89 100 0.80 0.83 0.86
vl Aui (cm-I) (an-') 1704.1 11.7 1703.7 12.7 1703.0 13.0 1706.0 13.0 1705.1 13.0 12.9 1704.3 1703.9 13.4 1706.0 13.5 1705.2 13.6 1704.7 14.0 1704.2 14.4 1706.7 13.6 1706.0 13.8 1705.3 13.8 1704.9 14.1 1706.9 14.6 1706.5 14.7 1706.1 15.5
(cm-I)
AUA ("I)
1712.3 1712.0 1711.8 1712.9 1712.7 1712.5 1711.8 1712.9 1712.5 1712.2 1712.0 1712.9 1712.6 1712.3 1712.1 1712.4 1712.4 1712.0
21.7 21.0 20.3 23.5 22.6 21.9 21.9 24.5 23.4 23.3 22.8 26.7 25.4 24.8 24.6 27.5 28.1 26.8
UA
& & (an-') (an-!) 8.2 8.3 8.8 6.9 7.6 8.2 7.9 6.9 7.3 7.5 7.8 6.2 6.6 7.0 7.2 5.5 5.9 5.9
5.6 5.7 5.7
5.3 5.4 5.5 5.5 5.1 5.2 5.3 5.3 4.8 4.9 5.0 5.1 4.6 4.7 4.8
temperature also red shifts the vibration, dramatically at high less so at lower xA. In the mixture, the acetone carbonyl may interact via dipolar interactions with other acetone carbonyls or via hydrogen bonding with surroundingmethanol molecules, both of which are attractive intermolecularinteractions. The stronger %A,
Bradley and Krech
578 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
::::t n 1716
?
-------
1714
1716 1714 (d 1712 1710
1704
fi
1
1716 1714 1712 17101
-
OoC
,
,
I
0.83
0.86
,
0.74 0.77 0.80
,
,
fi
1706
17101 1708 1706 1704 1710 1708
II
Detnils)
0.74
Rnmnn Dntn at XA = 0.2 (See Table I for
50
75
100
0.83 0.86 0.89 0.92
0.83 0.86 0.89 0.92 0.80 0.83 0.86 0.89 0.77 0.80 0.83 0.86 0.89
1706.2 1705.4 1705.2 1704.1 1706.6 1705.5 1705.4 1704.7 1707.3 1707.2 1706.3 1705.3 1708.7 1707.8 1707.2 1706.6 1705.9
13.0 13.5 15.2 14.5 15.8 14.5 15.3 15.3 13.5 15.6 14.4 14.9 15.6 15.1 16.4 16.3 16.2
1711.5 1711.1 1710.9 1710.6 1711.0 1710.7 1710.5 1710.1 1711.1 1710.4 1709.8 1709.0 1711.2 1711.1 1710.6 1710.0 1709.7
23.8 23.4 23.6 22.9 25.5 24.1 24.4 24.2 25.6 24.8 25.1 23.6 27.0 27.6 26.5 26.5 25.4
5.3 5.7 5.7 6.5 4.4 5.2 5.1 5.4 3.8 3.2
3.5 3.7 2.5 3.3 3.4 3.4 3.8
3.2 3.2 3.3 3.3 3.1 3.1 3.2 3.2 2.9 2.9 3.0 3.0 2.7 2.8 2.8 2.9 2.9
TABLE VI: Rnmnn Dntn for Vnrious Mole Frictions of Acetone in a Cuvette, nt Room Temperature (See Tnble I for
Det.ils)
mole fraction X A 0.20 0.10 0.05 0.01
Y
0.77 0.80 0.83 0.86 0.89
(cm-I) 1708.5 1709.1 1709.7 1710.3
Avr (cm-1) 13.3 12.8 12.2 11.6
VA
(cm-I) 1713.4 1712.9 1712.8 1712.9
AVA (cm-I) 24.4 23.8 22.5 22.0
0.95
Figure 3. Raman shift of anisotropic signal versus density, with the temperature constant within each subplot. X A for each symbol as in Figure 2. 28 26 24 22 20 18 28
~~
18
c
P==-l
&t.
(cm-I) 4.9 3.8 3.1 2.6
temperature dependence of the pure acetone data relative to XA = 0.2 implies the dipolar forces are weaker than the hydrogen bonding (less kinetic energy being needed to alter them). The methanol carbon-oxygen stretch shifts slightly blue when diluted with acetone (shifting about 1.5 cm-l to higher frequency from neat methanol toa mole fraction of 0.2),31implying the methanolmethanol hydrogen bonding interaction is stronger than the acetonemethanol interaction. Figures 2 and 3 display the data for the frequency shifts of the isotropic and anisotropic signals, respectively. The anisotropic mode does not red shift as rapidly as the isotropic (the slopes of the lines are greater in Figure 3 than in Figure 2). The only
I
ii 7 22 20
0.74 0.77 0.80 0.83 0.86 0.89
VI
0.92
Density (g/cc)
~~
25
0%
i
0.89 0.92 0.95
Density (g/cc)
TABLE V
25'C
1704 1706 I
Figure 2. Raman shift of isotropic signal versus density, with the temperature constant within each subplot. XA for each symbol: 0 = 1.0 (pure acetone), V = 0.8, 'I= 0.6, 0 = 0.4, B = 0.2.
~
1 75%
1714 1712 0 1710
fi
,y
I
____o
0.92 0.95
Density ( g / c c ) Figure4. Temperature dependence of isotropic linewidth versu~density, with X A constant within each subplot. Temperature for each symbol: 0 = ooc,V = 25 'c, V = 50 'c, 0 = 75 'C,B = 100 'c.
physical difference in the two modes involved the response of the anisotropic mode to the orientationally dependent potential; the slower decrease implies the orientational dependence adds a blue shiftingcomponent to thestrong red shiftingdipolar (red shifting) force. This combination causes the noncoincidence to increase with density, as we discuss below. Liae Width#. Generally, the line widths do not depend upon density and tend to increase as the temperature is increased (Figures 4 and 5 ) , at all XA. The lines broaden extensively upon dilution-the subplots in Figures 4 and 5 creep up as the mole fraction drops. Both lints broaden as the temperature is increased, regardless of XA, as previously observed in pure acetone.' The compress-
The Acetone Carbonyl Stretch
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 579
16 1
0.8
I
I
0
w
16
b 16 7
7
10 U L
16 14 12
--
I
0
r
I
P
m
0.83
0.86
5
0.4
I I
0.2
l Ro t ~
0.74
0.77
0.80
0.89
0.92
0.95
Density ( g / c c ) Figures. Temperaturedependenceofanisotropiclinewidthvenusdensity, with X A constant within each subplot. Temperatures for each symbol as
in Figure 4.
41
0
41
W
0.74
0.77
0.80 0.83
0.86
0.89 0.92
0.95
Density ( g / c c ) Figure 6. Noncoincidence of the Itso and IVH signal maxima versus density, with XA constant in each subplot. Symbols same as for Figure 4. Solid lines are calculated using q 4.
ibility, which we noted above measures the distribution of local environments due to solvent number fluctuations, increases as the temperature rises, which (from eq 3) results in a decrease in the lifetime of the slowly varying attractive forces, broadening the line. There is little or no dependence of the line width on density, which was surprising as previous work on hydrogenbonded systems did show a density dependence.32 Isothermal dilution with methanol causes the line to broaden, probably due to the development of a relaxation pathway across the acetonemethanol hydrogen bond. This occurs in regular intervals with the isotropic mode, but we detect a sizable jump in the anisotropic width between XA = 0.6 and 0.4. The size of the jump is less at higher temperatures. This is likely due to
statistical variations in the composition of nearest neighbors as themole fraction reaches 0.5, as predicted by Knappand FisCher.l1 At very low X A (Table VI), both Raman signals show a narrowing. No additional effect due to hydrogen bonding is expected at these low mole fractions,as all of the hydrogen bonding sites should be occupied. This may also be due to a Knapp and Fischerl' statistical mechanism, where the distribution of possible nearest neighbors is narrowing. Noncoincidence. In Tables I-VI and in Figure 6, we report values for A,,& (noncoincidence shifts). The data for X A = 1.0 compare very well with similar data from Table I11 of ref 2. On the basis of this comparison, our reported error in frequency assignment, and the number of steps in the fitting procedure, we estimate an error of f 1.O cm-I in our calculated values for A,+, which is slightly larger than the symbol size in Figure 6. We note, in agreement with ref 2, that A a ,decreases ~ as the density is lowered but does not depend on temperature. The error in the data is too large to determine if the slope of the density dependence changes with mole fraction. The noncoincidence, over the range studied at high pressure, decreases as XA decreases, which causes the VV signal to become less asymmetric. This is seen in Figure 6, where all subplots have the same y axis. Spectra of more dilute solutions (down to X A = 0.01) in a cuvette showed the noncoincidence continued to decrease at lower concentrations. As noncoincidenceoccurswhen resonance energy exchange is efficient, these trends are as expected. The calculated values for the noncoincidence based on eq 4 appear in the last column of Tables I-V and are shown as solid lines in Figure 6. We found (as previusly noted by Jonas3) that use of the reported value of 2.7 X 1W2C2kg-l for the transition dipole r2/m of acetone24in eq 7 yields &,A values which differ by a factor of 2 from our observations. Jonas3 suggested a value of 5.4 X 1@12 C2 kg-l for the transition dipole, following work by Sheibe for methyl ethyl ketone33and calculations by Ra0.3~ This gave much better quantitative agreement; accordingly, we have used this value to calculate our &,A values. The temperature and density trends agree very well between the theory and experiment. As the temperature increases, the moleculesreorient faster, averaging out some of the affects which cause the noncoincidence, and A a , becomes ~ smaller. Similarly, as the density decreases, the molecules are freer to reorient (more gaslike). Experimentally, the density dependence is quite pronounced, while temperature does not have a large effect until XA is below 0.6. As noted earlier, eq 4 was not developed for use in systems with strong hydrogen bonding. The predicted and observed behavior of the noncoincidence as a function of mole fraction show differences which reflect this. As clearly seen in Figure 6, the observed noncoincidence is lower than the theoretical for XA = 1.0 and 0.8, but is greater at the other XA values. The theory predicts the temperature dependence of the noncoincidence will decrease as X A decreases;we observe the opposite. Experimentally, we see little change in A,,& until XA = 0.6, below which the noncoincidence decreases precipitously. This indicates the methanol-methanol interactions are strong compared to the methanolacetone interactions. Otherwise, the Raman signals would follow a statistical trend (asassumed ineq4) asacetonenearest neighbors become substituted with methanols. The theoretical values for the noncoincidence values decrease slowly with temperature at X A = 0.2, while the experimental values vary over a wider range. At low X A , the theory effectively assumes the solute molecules are in a gaslike state, and unable to associate. At the highest temperature at X A = 0.2, our data agrees. However, as the sample cools, there are apparently regions within thesample where acetone molecules can undergo resonance energy exchange. Many of the trends we observed, including those in the previous paper12 and in results regarding the acetone breathing mode and
580 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
the methanol C-0 stretch,” can be explained through the preference of methanol molecules for self-interactions (rather than with acetone),and the resultant inhomogeneous distribution of acetone in the solutions. There is, however, no hard evidence for acetone dimers or clusters. We are presently working with acetone diluted with non-hydrogen-bonding solvents, which are better adapted to the model of and which will give a wholly new set of insights into the structure of liquid acetone.
Conclllcrions The thermodynamic variables of density, temperature, and mole fraction influenced Raman spectra of the carbonyl stretch of acetone in mixtures with methanol largely by altering the acetons-acetone and acetone-methanol attractions. Shifts in the Raman signal as the acetone is isothermallyand isochorically diluted indicate the acetoncmethanol attraction is stronger than the acetone-acetone attraction. Dilution shows the solution dynamics, and induces shifts which are characteristic of strong attractions. The line broadening of the carbonyl mode as the temperature is raised is explained as a compressibility effect. The theory of Logan correctly models the behavior of the Raman mode noncoincidence as a function of temperature and density. Some discrepanciesarise when the amount of methanol present is large, due to the extensive hydrogen bonding present. Good evidenceexiststhat a mechanism involvingstatistical fluctuations in nearest neighbors, of the type prowed by Knapp and Fischer, begins to affect the carbonyl mode. Acknowledlpneat. Theauthors would like to thankT. W. Zerda and S.Efrima for helpful discussions. We are extremelygrateful to the reviewers for their suggestions. This work was supported by the University of Connecticut Research Foundation. References and Notes (1) Schindler, W.; Sharko, P. T.; Jonas, J. 1. Chem. Phys. 1982, 76, 3493.
Bradley and Krech (2) Sun, T. F.; Chan, J. B.; Wallen, S.L.; Jonas, J. J . Chem. Phys. 1991, 94, 7486. (3) Thomas, H. D.; Jonas, J. J . Chem. Phys. 1989, 90,4144. (4) Bradley, M.;Zerda, T. W.; Jonas, J. Spectrochim. Acta 1984, 40A, 1117. (5) Zerda, T. W.; Bradley, M.;Jonas, J. Chem. Phys. Lrrr. 1985,117, 566. (6) Zerda, T. W.; Thomas, H. D.; Bradley, M.; Jonas, J. J . Chem. Phys. 1987, 86, 3219. (7) Schindler, W.; Jonas, J. J . Chem. Phys. 1980, 73, 3547. (8) Schweizer, K. S.;Chandler, D. J. Chem. Phys. 1982, 76, 2296. (9) Schwartz, M.;Moradi-Araghi, A.; Koehler, W. H. J. Mol. Srrucr. 1982, 81, 245. (10) we,G . In Raman Specrroscopy, Proc. 8th Inr. Conf.; Lascombe, J., Huong, P. U., Eds.; Wiley: Chichester, 1982; p 327. (11) Knapp, E. W.; Fischer, S. F. J . Chem. Phys. 1982, 76,4730. (12) Bradley, M.S.;Krech, J. H. J. Phys. Chem. 1992, %, 75. (13) Dellepiane, G.; Overend, J. Speclrochim. Acra 1966, 22, S93. (14) hrkayastha, A.; Kumar, K.Spectrochim. Acta 1987,13A, 1269. (IS) Shiel, D.; Richter, W.; we,G . Z . Narurforsch. A, in press. (16) Ben-Amotz, D.; Lee, M.-R.; Cho, S.Y.;List, D. J. J . Chem. Phys. 1992,%, 8781. (17) Long, D. A. Ramun Specrroscopy;McGraw-Hill: New York, 1977. (18) Fini, G.;Mirone, P.; Fortunato, B. J . Chem. Soc.. Faraday Trans. 2 1973,69, 1243. (19) Fini, G.; Mirone, P. J. Chem. Soc., Chem. Trans. 2 1974,70,1776. (20) Mirone, P.; Fini, 0. J. Chem. Phys. 1979, 71, 2241. (21) Wang, C. H.; McHale, J. J. Chem. Phys. 1980, 72,4039. (22) McHale, J. H. J . Chem. Phys. 1981, 75, 30. (23) McHale, J. H. J . Chem. Phys. 1982, 77,2705. (24) Logan, D. E. Mol. Phys. 1986,58,97. (25) Logan, D. E. Chem. Phys. 1986,103,215. (26) Logan, D. E. Chem. Phys. 1989, 131, 199. (27) Mirone, P. 1.Chem. Phys. 1982, 77, 2074. (28) Baker, K.Ph.D. Dissertation, University of Illinois, 1984. (29) Bradley, M.Ph.D. Dissertation, University of Illinois, 1985. (30) Bradley, M.; Krech, J. H. Rev. Sci. Insrrum. 1991, 62, 1. (31) Bradley, M.;Krech, J. H., manuscript in preparation. (32) Bradley, M.;Zerda, T. W.; Jonas, J. J. Chem. Phys. 1985,82,4007. (33) Sheik, D. J . Raman Spectrosc. 1982, 13, 103. (34) Rao, C. N. R.; Randhawa, H. S.;Rcddy, N. V. R. Specrrochlm. Acra, Parr A 1976, 32. 685.
