3226
IZUMI MOTOYAMA AND
CHARLES
H. JARBOE
Hydroxyl Group Stretching Frequency and Extinction Coefficient Studies on Aliphatic Alcohols
by Izumi Motoyama and Charles H. Jarboe’ Department of Phannacology, School of Medicine, University of Louisville, Louisville, Kentucky (Received March 84, 1966)
Apparent molar absorptivities (E’) and A,, are reported for the hydroxyl group stretching frequencies of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, and neopentyl alcohols. Variation of ea over a limited temperature range is a straight line. Extrapolation of ea os. T plots to the boiling points of the alcohols predicts a common E’ at that are discussed. temperature. Substitution effects on A,,
In recent years near-infrared spectroscopy has been one of the most dependable and frequently used techniques for studying hydrogen bonding reactions between alcohols and proton-acceptor bases of many varieties.2 Despite the impressive volume of work on this topic there is a paucity of thermodynamic data on hydrogen-bonding reactions, especially on systems involving simple alcohols and ethers with progressively more bulky and complicated substituents adjacent to the functional groups. This steric aspect of hydrogenbonding chemistry has been our interest, and it has been germane to our objectives that we examine the molar absorptivities of several alcohols with variously complicated structures. At this time we wish to record the apparent molar absorptivities (ea)a for the hydroxyl stretch vibrations of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, &butyl, and neopentyl alcohols. In a related study, these values of ea were compared with the corresponding apparent integrated molar absorption coefficients (BO). Using experimental conditions as reported in the present work, the properties E‘ and Bo were found to be proportional. These studies utilized concentrations prohibiting self-association. The temperature range was from 21.7 to 48.1’.
Experimental Section Equipment. All spectra were recorded using a Perkin-Elmer Model 137-G Infracord grating-type spectrometer. The machine used in this work was modified to record frequency linearly, and the measurement accuracy was estimated to be *2 cm-’. The The Journal of Physical Chemistry
scan speed was 24 min/drum revolution. To facilitate precision in peak height measurements it was connected to a Sargent No. S-72180-20 slave recorder. The precision of percentage transmittance measurements was approximately =k0.5%in the region 30-500/, on the recorder; the 0 and 100% lines were adjusted just prior to each measurement. The spectral slit width of the spectrometer was programmed to vary from 10 cm-’ at 4081 cm-’ (2.45 p ) to 5 cm-1 at 3333 cm-l (3.00 p ) . The instrument, including the sample chamber, was purged with dry nitrogen. The sample chamber was isolated from the atmosphere with a polyethylene cover. This prevented cell window sweating a t low temperatures and avoided energy loss due to environmental moisture. The cells consisted of 2.499 X 1.9 cm cylindrical Pyrex bodies equipped with 10/13 standard taper joints at the middle and sodium chloride end windows. These were surrounded with a close-fitting hollow brass shell, threaded on each end and fitted (1) To whom inquiries should be directed. (2) (a) M.R. Basila, E. L. Saier, and L. R. Cousins, J . Am. Chem. SOC.,87, 1665 (1965); (b) E. D.Becker, Spectrochim. Acta, 17, 436 (1961); ( c ) T.J. V. Findley and A. D. Kidman, Australian J . Chcm., 18, 521 (1965); (d) T. Gramstad, Acta Chem. S c a d . , 15, 1337 (1961); (e) T. Gramstad, ibid., 16, 807 (1962); (f) M.Hoeke and A. L. Koevoet, Rec. Trau. Chim., 82, 17 (1963); ( 9 ) U. Liddel and E. D. Becker, Spectrochim. Acta, 10,70 (1957); (h) G. C. Pimental and A. L. McClellan, “The Hydrogen Bond,” Freeman and Co., San Francisco, Calif., 1960; (i) J. H. Walkup, J. Lyford, G. Marquardt, and G. W. Robinson, Trans. Kentucky Acad. Sci., 24, 101 (1963). (3) (a) K. Nakanishi, “Infrared Absorption Spectroscopy-Practical,” Holden-Day, Inc., San Francisco, Calif., 1962, p 17; (b) A. D. Cross, “Introduction to Practical Infra-Red Spectroscopy,” Butterworth Inc., Washington, D. C., 1964, p 38.
EXTINCTION COEFFICIENT STUDIES ON ALIPHATICALCOHOLS
Table I: Frequencies and
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for Alcohol OH Stretch Vibrations a t Various Temperatures
-WavelengthAlcohol
Methyl Ethyl n-Propyl Isopropyl n-Butyl Isobutyl t-Butyl Neopent yl
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3227
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I
(om-')
2.745 f 2 (3643 i 3 ) 2.752 f 2 (3634 i 3 ) 2.750&2(3636&3) 2.757 i 2(3627 f 3) 2.748 i l(3639 f 2) 2.747 f l(3640 f 2) 2.766 f l(3615 f 2) 2 , 7 4 4 f l(36445C2)
21.7'
60.6 f 2.8"(60.4)* 56.9 f 1 . 6 (56.7) 58.35C 1 . 9 ( 5 8 . 1 ) 59.7 f 2 . 1 ( 5 9 . 6 ) 59.5 f 1 . 1(59.4) 6 4 . 1 f 1 . 4 (63.9) 66.7 i 2 . 5 (66.5) 72.13=2,8(71.6)
a Calculated using the density of carbon tetrachloride at 24'. with temperature.
with open window brass screw caps to secure the assembly. Teflon gaskets prevented the end caps from damaging the salt windows. The brass shells were provided with tubular inlets and outlets placed a t opposite ends and at 180". Both sample and reference cells were temperature regulated with a Haake Series F circulator-heater which was coupled with a Brinkman Thermo-Cool heat pump. This system controlled cell temperature to 3t0.1'. In changing temperature a l b m i n equilibration period was used. Materials. The solvent used was carbon tetrachloride. Ordinary spectroscopic grade carbon tetrachloride contains too much water for use with longpath cells. To eliminate error due to hydrogen bonding between water and alcohol and to prevent loss of energy in the critical spectral region, the solvent was rendered completely anhydrous (no absorbance above base line in the 4000-3000 em-l range). To obtain this level of desiccation the carbon tetrachloride was shaken with Woelm neutral alumina, decanted into a phosphorus pentoxide containing distilling flask, and distilled i2to phosphorus pentoxide. All further solvent transfers were carried out in a phosphorus pentoxide dried glove box. Just prior to use, it was passed through an 8 X 2 cm column of Woelm neutral alumina and collected in a flask closed with a hollow, phosphorus pentoxide containing stopper. The solute alcohols were purified by ordinary method^.^ Additionally, they were distilled from sodium in a desiccated, all-glass system, just prior to use. To avoid adsorption of atmospheric water, all transfers were carried out in the drybox. Alcohol solutions were prepared gravimetrically by using samples sealed in melting point capillaries. The capillaries were crushed in suitably sized volumetric flasks containing a small volume of carbon tetrachloride. The samples were made to volume at 24". Each value of ea appearing in Table I represents eight individual samples. The con-
30.2O
56.4 f 3 . 0 (56.8) 53 8 f l 1 . 4 ( 5 4 . 3 ) 5 6 . 4 5 ~1 . 7 ( 5 6 . 8 ) 56.2 f 2 . 4 ( 5 6 . 6 ) 56.8 f 1 . 2 ( 5 7 . 3 ) 6 1 . 1 4 ~1 , 5 ( 6 1 , 5 ) 66.7 f 2 . 5 (63.3) 6 9 . 1 f 2.7 (69.4)
39.1'
48.1°
52.3 =k 2 . 8 (53.4) 50.9&1.5(51.9) 5 3 . 8 i 1.6(54.6) 52.6 f 2 . 4 ( 5 3 . 5 ) 54.7 5C 1 . 0 ( 5 5 . 8 ) 58.5i2.1(59.6) 59.1 i 1 . 3 (60.2) 66.2 i 2.7 (67.2)
4 8 . 3 f 2 . 8 (49.8) 4 8 . 6 f 1.8(50.1) 51.6 i 1 . 3 (53.3) 4 9 . 3 f 2 . 0 (50.9) 52.8 f 1 . 1(54.4) 56.4 f 2 . 1 (58.2) 56.2 f 1 . 7 (58.0) 63.4 f 2 . 0 (65.1)
* Values in parentheses are corrected for changes in solution density
centrations of alcohols ranged from slightly over 0.002
M to just under 0.004 M . At each temperature three to five separate spectra were made for each sample. In each case the recorded curves were symmetrical and very sharp. The values of ea were calculated using the usual relationships. Values of Bo were calculated from gravimetrically measured integrated absorbancies and approximate integrated absorbancies derived from area measurements based on triangles.
Results and Discussion Table I summarizes the
ea data for the eight alcohols. Figure 1 contains plots of the average ea values as a function of temperature. At the concentrations used (>0.002 and
