J. Phys. Chem. 1981, 85, 3727-3729
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Measurement of Infrared Intensites for Fundamental Vibrations of Gaseous Acetone J. D. Rogers,+ B. Rub, S. Goldman, and W. B. Person” Depedment of Chemisfry, Universw of Florida, Gainesvliie, Florida 326 I 1 (Received: Aprll22, 198 1; In Final Form: July 13, 198 1)
The absolute infrared intensities of the vibrational modes of gas-phase acetone have been remeasured with a Nicolet FT IR. In general, these values confirm the values reported in 1968 by Sverdlov and co-workers from measurements made with a prism spectrometer. We believe the absolute intensites reported for the overlapped infrared absorption bands of acetone are accurate to about 10%.
Introduction Recent attempts’ to test the ability to predict infrared intensities for molecules using atomic polar tensors2 transferred from other molecules to the molecule of interest3 require accurate experimental values for the absolute intensities of these molecules. Because of the experimental difficulties involved in measuring infrared intensities: we believe it is still important to remeasure the absolute intensities for any organic molecule whose intensities have not been thoroughly investigated experimentally. One such molecule of interest to us is the acetone molecule, whose intensities have only once previously been reported in a study, using a prism spectrometer,made by Vakhlueva, Finkel, Sverdlov,and Zaitseva.6 It is clearly important to repeat these measurements in order to obtain some calibration of the accuracy of these earlier studies, and to pin down the values for the absolute intensities in this, the simplest organic ketone. Experimental Section The absolute intensity A of the ith vibrational mode is defined4 as Ai = (1/100CI)Jln (Zo/Z) dv Here the factor of 1/100 converts to units of km mol-l for the absolute intensity when the concentration C is expressed in mol L-’ and the pathlength I is in cm. The integration of the absorbance (base e) extends over the wavenumber (cm-’) range of the entire absorption band. We have used the Wilson-Wells-Penner-Weber technique6 to measure the infrared intensities for gas-phase acetone. Our procedure was described in an earlier paper reporting our measurements of the absolute intensities of carbon dioxide,’ so we shall only briefly sketch the procedure here. The evacuated brass cell was filled in the gas-handling system to the desired pressure of acetone (Mallinckrodt, spectrophotometric grade); nitrogen gas (Airco, prepurified) was then admitted rapidly to the sample cell in order to broaden the rotational lines in the vibrational bands. Pressures of acetone were measured both with a Wallace and Tiernan Series 300 absolute pressure gauge (0-100-torr range) and with an MKS Instruments baratron type 221 capacitance manometer (0100-torr range). (Note that 7.502 torr = 1e a . ) Pressures of acetone (vapor pressure at 298 K = 27 kPa)* ranged from 1.3 kPa to 4.0 kPa. Nitrogen was added to increase the total pressure to 1.1MPa (or 10 atm, controlled with a Matheson Model 2 two-stage regulator) in the initial measurements, but we later found that a Beer’s law plot of the data measured at 0.1 MPa total pressure agrees t Infrared and Radio Astronomy Branch, Goddard Space Flight Center, Greenbelt, MD 20771.
satisfactorily with the points measured with 1.1MPa total pressure. We believe that one advantage of the lower total pressure is that it reduces the chance that the acetone vapor may condense on the cell walls. Spectra were measured on a Nicolet Model 7199 interferometric infrared spectrometer of the pressurized cell containing the acetone sample and also of the cell containing only nitrogen to provide the I and Io curves, respectively. The resolution was 0.5 cm-l for the samples pressurized to 1.1 MPa and 0.24 cm-l for the samples pressurized to only 0.1 MPa. The interferograms were transformed by means of a triangular apodization function and one level of zero filling. One hundred scans of I and Io were averaged for each sample concentration; the absorbance (base 10) as a function of wavenumber was then calculated by the standard Nicolet program. Using the Nicolet program, the absorption bands of acetone were integrated numerically by computer to obtain their areas, and these values were multiplied by 2.303 to obtain base e integrated absorbances. A baseline correction was made manually by using an absorbance plot of the spectra. A line was drawn connecting the absorption band wings where the absorbance was expected to be zero, and the resulting baseline absorbance correction was then subtracted form the total band area to obtain the true integrated absorbance. Apparent nonzero absorbances in regions of expected zero absorbance are usually caused by differences in the cell position in the optical compartment of the spectrometer when the I and the Io spectra are s ~ a n n e d . ~
Results The several fundamentals appearing in each spectral region overlap badly to produce a single broad absorption feature that cannot be resolved. For example, five fundamentals absorb in the C-H stretching region (3050-2910 (1)See, for example, J. D. Rogers, Ph.D. Dissertation, University of Florida, 1980. (2) J. F. Biarge, J. Herranz, and J. Morcillo, An. R. SOC. ESP.Fis. Quim., Ser. A, 57,81 (1961). See also W. B. Person and J. H. Newton, J. Chem. Phys., 61, 1040 (1974). (3) W. B. Person and J. H. Newton, J. Mol. Struct., 46, 105 (1978). (4)J. Overend in “Infrared Spectroscopy and Molecular Structure”, M. Davies, Ed., Elsevier, Amsterdam, 1963,Chapter 10. (5)V. I. Vakhlueva, A. G. Finkel, L. M. Sverdlov, and L. A. Zaitseva, Opt. Spectrosc., 25, 160 (1968). (6)E.B.Wilson, Jr., and A. J. Wells, J. Chen. Phys, 14, 678 (1946); S. S. Penner and D. Weber, J. Chem. Phys., 19, 807 (1951). (7)J. D. Rogers, B. Rub, S. Goldman, and W. B. Person, submitted to Appl. Spectrosc. (8)“Handbook of Chemistry and Physics,” 52nd ed, Chemical Rubber Co., Cleveland, 1971. (9)K. Scanlon, L. Laux, and J. Overend, Appl. Spectrosc., 33, 346 (1979).
0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 24, 1981
Rogers et al.
TABLE I: Measured Absolute Intensities for the Vibrational Modes of Acetone Ai, km mol-'
band
vi,('
cm-'
this workb
1 l4
3020 3020 2973 2926 2926 1738 6(CH)- v21
1438
V,(CC)
1218 1093 1067
VI?
1
6 (CCH) v m V6
6 (CCH) V 1 8
V,(CC)
44.7
2.6
30
140 f 3
1
best estd
44
150
64.9 i 1.1
896 779
V?
f
1 1
Vakhlueva et al.c
44.4 t 0.4
145f 5
68
1
4.3 f 0.5 7.3 i 0.5 1.44 t 0.15 e e
66.5 f 1.5
1
4.6 7.8 2.31
4.5 i 0.2 7.6 t 0.3 1.9 f 0.4
1 l7
17
e 0.70 0.70 T(CH1) vz4 105 e a Assignments and frequencies taken from R. H. Mann and W. B. Dixon, J. Chem. Phys., 57, 792 (1972). The uncertainties quoted represent the 20 scatter about the least-squares lines and probably do not represent a true measure of the actual error, because of the neglect of any systematic error. From Vakhlueva, Finkel, Sverdlov, and Zaitseva, ref 4. Note that Ai (km mol-') = ( N / c X 105)A (cm' molecule-' s - ' ) = 2.009 X 10*A (cm' molecule-' s-'). Averages of values from this work and from Vakhlueva e t aL4 The uncertainties are chosen such that the best estimates overlap the two sets of measurements. e These vibrational modes were not studied here because of wavelength limitations in our work. 0
joo1 ,,/1620-1920
cm"
0
9 40 o
36b0
3Eb0
eabo
znbo
?ob0
16bO
12bo
sbo
rbo
WRVENUMBERS
Figure 1. Survey infrared absorption spectrum of acetone vapor (1.29 kPa pressure) in a 10-cm cell, measured at 0.24-cm-' resolution wlth the Nicolet FT IR spectrometer.
cm-l). These cannot be resolved and only the total integrated absorption intensity over the entire region is reported in Table I. Some indication of the magnitude of this problem of overlapping bands can be obtained by comparing the list of the fundamentals in Table I with the survey spectrum shown in Figure 1. Beer's law plots of the integrated absorbance vs. Cl for the spectral regions measured here are presented in Figures 2-4. The limited spectral range of the KBr windows in our cell and the beam splitter and detector in the interferometer prevented our measurement of intensities for the four lower frequency vibrational modes of acetone (below 550 cm-l). We see from Figures 2 and 3 that the data are sufficiently linear and also that the scatter about the least-squares fit to the data is quite small for the spectral regions of larger absolute intensity. The scatter is, however, much larger for the regions of low intensity (Figure 4). The absolute intensities calculated from the least-squares fits to the data are given in Table I along with the 20 scatter in the data about the least-squares lines. The absolute intensities reported by Vakhlueva, Finkel,
I
0
5
10
15
20
CI x IOa (mole I-'cm)
Figure 2. Beer's law plots of the integrated absorbance vs. the product of concentration times pathlength for the two strongest absorption regions (CO stretch at 1738 cm-' and 6(CH) bends near 1450-1350 cm-I).
Sverdlov, and Zaitseva for acetone5are also given in Table I. Our values for the infrared intensities of acetone generally agree with the values reported by Vakhlueva et al? within 10%. Quantitative agreement is seen both for the C-H stretching region near 3000 cm-l and for the CH bending region near 1450 cm-l. Our value of 140 km/mol for the C=O stretching region near 1750 cm-' appears to be somewhat lower than the value reported by Vakhlueva et a1.,4 but the two values probably overlap within the experimental uncertainties (Vakhlueva et a1.6 estimate their probable error to be 5-109'0). Our value for the weak symmetric C-C stretching mode, v7, is, however, 60% lower than the value reported by Vakhlueva et al.5 It is still always a surprise to be able to reproduce absolute infrared intensities in the literature within the ex-
J. Phys. Chem. 1881, 85, 3729-3730 15Or
CI I 10’(mols V c m )
Figwe 3. Beer’s law plots of the Integrated absorbance w.the product of concentration times pathlength for the two moderately strong absorDtion regions (CC . stretch at 1218 cm-’ and CH stretches near 3000 cm’-’).
CI x
10’ (mole 1.’ cm)
Figure 4. Beer’s law plots of the integrated absorbance vs. the product of concentration times pathlength for the remainlng three weak absorption regions (G(CCH) bends near 1070 cm-’, the one near 900 cm-‘, and the weak CC stretch near 780 cm-‘).
perimental errors estimated by each worker. We were quite pleasantly surprised, indeed, to find such good agreement between our work and the earlier studies, particularly since we had expected the higher resolution
3728
in our present study to reduce (or eliminate) the “resolution error” that is usually expected4 to be so important in earlier studies. It seems quite clear that acetone is heavy enough that the rotational lines overlap each other sufficiently so that a relatively low pressure is needed to pressure-broaden its vibrational bands sufficiently for accurate intensity measurements. Even so, we had expected also considerable trouble with accurate measurement of the acetone pressure since we are using a brass cell, and since we expected the acetone to adsorb on the cell walls and in the stopcocks, etc., in the vacuum line! Although we tried to watch carefully for such problems in our work, we suppose that the agreement between our study and the one reported earlier by Vakhlueva et a1.16 verifies either that neither one of us had such troubles or else that we both had exactly the same absorption troubles. If we could be sure that our acetone pressure measurements were indeed accurate measures of the acetone concentrations in our studies, and if we could be absolutely sure that we had no other systematic errors (for example, in the way we chose our baseline) then the 2a scatter reported for our work in Table I could be an accurate estimate of the errors in our intensities reported there. For many bands (for example, the C-H stretching band intensity near 3000 cm-’, and the 6(CH&bending region near 1450 cm-’) the agreement between our work and the earlier work is within this 2a scatter, suggesting that our values may indeed be accurate within the 2a scatter. However, intensity studies are still sufficiently difficult (in this case primarily the questions about concentration and choice of base line) that we are content to list as out best estimate for the intensitiesgiven in Table I the average of our values and those of Vakhlueva et al.6 The indicated uncertainties are chosen such that the “best estimates” encompass both sets of measurements. These “best estimates” for the absolute intensities of acetone should provide useful data with which to compare intensities predicted by using atomic polar tensors2 transferred to acetone from other molecules.
Acknowledgment. We are grateful to the National Science Foundation (Grant No. CHE-7818940)for support of this work.
COMMENTS Comment on “Temperature Dependence of Bragg Scattering from Crystalllred Suspensions of Macroions” by J. G. Daly and I?. Hastings
Sir: The ratios of the sines of the Bragg angles for the first two reflections of cubic lattices were used to demonstrate the occurrence of a body-centered-cubic structure of macroions in aqueous suspensions and to negate the occurrence of either simple-cubic or face-centered-cubic structures in these suspensions.’ In eq 4 of the Daly-Hastings (DH) paper the ratios of the sines of the first two angles are given correctly for simple-cubicand face-centered-cubic (1) Daly, J. G.; Hastings, R. J. Phys. Chem. 1981,85, 294-300. 0022-3654/81/2085-3729$01.25/0
lattices, but the value for the bccub structure is incorrect. Instead of 31/2,the ratio should be 2lI2 (110 and 200 planes). The entries for 02(bccub)in DH Table I follow from this incorrect ratio and are therefore also incorrect. Since the values of 02(bccub)in DH Table I were interpreted as proving the existence of the bccub structure, the occurrence of bccub structure has not been demonstrated. The entries for x in the last column of DH Table I, based on O1 originating from the 110 planes of bccub structure, also appear to be incorrect. I suggest that the data reported in the DH paper support the existence of a loose face-centered-cubic lattice of mac r o i o n ~ .The ~ ~ ~criterion to be met is that the first angle (2) Bahe, L. W.J. Phys. Chem. 1972, 76, 1062-71.
0 1981 American Chemical Society
