ACKNOWLEDGMENT
The authors would like to thank the Rohm and Haas Company for supplying the resin used for this work.
Received for review November 2 , 1973. Accepted January 31, 1974. This paper was presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, in March 1973.
Apparent Integrated Intensities and Absorption Coefficients for the 1125 cm-' to 1230 cm-I Infrared Region of Phenol in the Vapor Phase H. William Wilson Department of Chemistry, Western Washingon State College, Bellingham, Wash. 98225
One of the major deterrents to the more wide-spread use of gas phase infrared spectrophotometry in analytical work has been the inherent experimental difficulty involved in measuring apparent integrated band intensities of common, relatively high molecular weight substances such as phenols or middle range hydrocarbons. Much of the uncertainty has been due to the inadequacies of high temperature infrared gas phase cells in which average sample temperatures or pressures are unknown or uncontrollable to any acceptable degree of precision. The problem appears to have been overcome to some extent by an evacuable, double-beam hot cell assembly in which sample temperatures are known and homogeneous to within about 1 "C ( I ) . The cell is so designed than an excess of sample will yield, when the cell is fully equilibrated a t any temperature up to 300 "C or 350 "C, a vapor phase medium whose concentration is as precisely known as its vapor pressure a t the temperature in question. In addition, the double-beam construction of the hot cell also aids in compensating for emission problems and some reflection losses. The apparatus has been successful in yielding not only well-resolved spectra but, more importantly, reproducible base lines. This note reports the results of measuring the apparent integrated intensity of the strong 1125 cm-I to 1230 cm-I absorption region in the vapor phase spectrum of phenol. The absorption coefficient for a Q branch found a t 1177 cm- is also reported. EXPERIMENTAL The hot cell has been modified slightly from the form originally reported ( I ) . The oven was reconstructed from Yz-in. Marinite 36 asbestos board (Johns-Manville, Greenwood Plaza, Denver, Colo.), and allowance was made for the connection of a Teflon stopcock and sidearm through the cover and into the sample cell. Thus, the sample can be pumped-off a t any time and renewed in the sample cell in a short time as further volatilization occurs. It has been noted that removal of the air from the cell tends to enhance the band contours of many substances (Figure 1). Finally, the heating elements have been rewired into the cover of the oven. Reagent grade phenol was used throughout the study without further purification since the spectra of original and sublimed samples were identical and the melting point of the samples agreed with the values quoted in the literature ( 2 ) . All spectra were recorded on a Perkin-Elmer Model 621 infrared spectrophotometer. The instrument settings were such than an
effective spectral slit width of 0.6 cm- and a scanning rate of 0.8 cm- min were achieved ( 3 ) .The sample cells were 6-cm long and had 6-mm thick NaCl windows. The apparent integrated intensities (B) of the absorption area were determined by a Wilson-Wells type of integration ( 4 ) in which definitions and procedures have been discussed as Method I1 by Ramsay ( 5 ) . Repeated measurements indicated a precision of about f1015% in the results, a range which is to be expected in this type of work ( 6 ) . At least 5 different runs were made a t each temperature. Temperatures used in this study were limited to less than about 90 "C by the fact that the phenol bands became totally absorbing in a 6-cm cell a t about that temperature (Table I).
DISCUSSION
Survey spectra of the 1125 cm-1 to 1230 cm-' region of phenol vapor are illustrated a t three different temperatures in Figure 1 and the results of the measurements in this area are tabulated in Table I. Also listed, for comparison purposes, are the log 10 To/T values, where TOand T are the apparent incident and transmitted intensities, of the strong Q branch at 1177 cm-l. In addition to the difficulties normally encountered in measuring absorption data even under the best of experimental conditions (7), the hot cell has been found to have a t least one other important nuance in its operation. Six-mm thick windows have been found to be difficult to warm thoroughly in short periods of time. By rescanning the phenol region every 15-30 minutes, it could be shown that anywhere from 2-6 hours was often required to fully volatilize thin, virtually invisible liquid films from the interiors of the windows. The strong absorption characteristics of the condensed phase not only partially occluded the fine gas phase Q branch structure of the bands, but more importantly the base lines were unreliable. Once the film was gone, the base lines became fully reproducible. The three spectral scans traced in Figure 1 all resulted from entirely different samples. In each case, the sample was removed from the cell and replaced before reheating occurred. Once the gas phase experimental factors were brought under reasonable control, there were still a number of fac(3) Perkin Elmer Corporation, "Operating Instructions, Model 621 Spectrophotometer," Norwalk, Conn., 1965, Section 5 . Table 5-2. E . B Wilson and A J Wells. J. Chem. Phys., 1 4 , 578 (1946) D A.
Ramsay, J. Amer Chem. Soc..7 4 , 7 2 (1952)
E D. Schmid, F. Langenbucher. and H W Wilson, Spectrochim. ( 1 ) H . W . Wilson.App/.Specrrosc..2 4 , 6 (1970) ( 2 ) "Handbook of Chemistry and Physics," Robert C. WeaSl, Ed Chemical Rubber Publishing Co , Cleveland, Ohio, 1971. p C44.
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1974
Acta. 19, 835 ( 1 9 6 3 ) . ( 7 ) K . S. Seshadri and R (1963).
N
Jones, Spectrochim. Acta,
19,
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Table I. Absorption Data for Phenol i n the Vapor Phase at Various Temperatures B,
T, " C
Vapor pressure Torr (2)
atm-1 cm-2
log10Io/I
41 76 85
1.7 11.2 17.8
572 512 556
0.105 0.575 0.96
k, cm2/ cl, moles/cmz
5.22 X 30.6 X 48 X
lo-'
24OC
mole
424 440 450
B is the apparent integrated intensity' of absorption between 1125 cm-' and 1230 cm-1. k is the absorption coefficient for the Q branch a t 1177 cm-I.
tors regarding the reliability of the intensity measurements which had to be considered. All of them have been discussed by various authors (7-12). There is always the question of the relationship between the true and apparent intensities, particularly in the case of the Wilson-Wells method where previous difficulties have been encountered in measuring sharp spectral features similar to the Q branches observed in this work. Pressure broadening, for instance, is commonly used as a corrective procedure since it removes the finer details from the spectra (8). In this case, the heavy molecule being studied has conspicuous but relatively broad Q branches whose half-band widths of more than 3 cm-1 tend to minimize the problem. Nonetheless, the results given here were measured a t the total pressures listed in Table I, and attempts to use the data a t much higher or lower pressures should be carried out with caution. In addition, it should be noted that the data were obtained a t a spectral slit width of about 0.6 cm-I and some difficulty may be encountered when using instruments'of lower resolution ( 5 ) . It can be shown that the intensities of fundamental vibrational modes are virtually independent of temperature (13, 14), a t least over the range being considered here. Two of the three bands observed here have been assigned to fundamentals (15) in the vapor phase spectrum of phe-
( 8 ) J. Overend, M . J Youngquist, E. C. Curtis, and 8. L. Crawford. J r . , J. Chem. Phys.. 3 0 , 532 (1959) ( 9 ) T. Fujiyama, J. Herrin. and 6. L Crawford, Jr.. Appl. Spectrosc.. 24. 9 (1970) (10) L D. Kaplan and E F . Eggers. Jr , J . Chem. Phys., 25. 876 (1956) (1 1) D. Steele. "Theory of Vibrational Spectroscopy," W . B Saunders. Philadelphia, Pa., 1971, Chap. 8. (12) Reference 5. (13) J. C Breeze, C. C. Ferriso. C. 6. Ludwig, and W Malkrnus. J. Chem. Phys.. 42, 402 (1965) ( 1 4 ) G. Herzberg. "Molecular Spectra and Molecular Structure. I . Spectra of Diatomic Molecules," Van Nostrand, Princeton, N.J., 1950, pp 121-128 (15) J C Evans. Spectrochim Acta, 16, 1382 (1960)
1400
IIO0
CM" Figure 1. Survey spectra of phenol in the vapor phase at various temperatures
nol, but the third, a t 1196 cm-I, has not been previously reported. On the basis of this and other hot cell studies, we believe that the band is a fundamental rather than an overtone or combination. Some question must, of course, remain until this can be shown conclusively to be the case. Wing corrections are an important aspect of intensity measurements. If the bands here were considered to be Lorenztian in form, the wing corrections, as discussed by Ramsay ( 5 ) , would add anywhere from 6 to 12% of the measured band areas to the results. On the other hand, since the type A contours actually observed are not particularly well approximated by Lorenztian curves, we have computed the characteristics of a type A phenol band using a rigid rotor model. We find that beyond about 18 cm- from the central Q branches, the intensity of the rotational transitions falls to essentially zero values even for an instrument with a spectral slit width much greater than the 0.6 cm-I one used here. In the most restricted region in our measurements, the 1196 cm-I Q branch of the highest frequency band still lies about 34 cm-1 below the upper integration limit. Primarily on the basis of the contour calculations, we have decided against wing corrections and none have been added to the results. Received for review October 23, 1973. Accepted February 21, 1974.
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