John D. Worley and Daniel A. Garieiz University of Cincinnati Cincinnati, Ohio 45221
I I
Generation of Deuterated Materials in
The near infrared spectra of solutes capable of self association through hydrogen bond formation are often deceivingly complex. Absorption bands characteristic of the groups -OH, -NH, -CH, various hydrogen bonded modes, overtones and combination bands are found in the range 2500 to 3500 mp. Such an array of bands often leads to confusion and frustration on the part of the student or teacher who is trying to understand them. Such an undertaking is greatly facilitated by working in nonpolar, nonabsorbing solvents a t low concentrations so that bands arising from polymeric species are partially diluted out (with the polymers), and overtones and combination bands are reduced to negligible absorptions. Such conditions are generally optimum (with the added stipulation that a good spectrophotometer be somewhere nearby) for identifying bands and for taking data for determining the thermodynamic parameters for the associative process.' Even under such supremely happy conditions, however, all does not turn out to be sugar and roses. Quite often two bands will overlap and each will contribute to the absorbance of the other. An equally unsatisfactory situation, from the quantitative standpoint, arises when two bands sit on top of one another. Further exploration of the complexities of the spectra of hydrogen bonding solutes often becomes expensive and/or time consuming. For example, one could take the ir spectrum of a solute in its fully or partially deuterated form. Such an experiment would certainly identify bands associated with an X-H stretching mode of vibration since deuterium substitution results in a shift to longer wavelengths. An approximation that is often used as a rule of thumb guide is va = d 2 v o where v's are frequencies of absorption for X-H and X--D respectively in cm-l. To obtain spectra of deuterated materials one must either synthesize them directly or purchase them. Both courses of action are usually expensive. A less expensive and time consuming method appears to be desirable. The remainder of this article describes such a method. At the outset i t is i~nportantto emphasize that the techniques described herein do not result in a fully deuterated material. For example, if methyl alcohol is being studied, the product of this method is CHsOD and not CD,OD. The basis of the method is the iso-
Figure 1.
(I
Nonpolar Solvent
Equilibrmtor for hydmgen deuterium exchange.
piestic solute transfer experiments first described by Christian, Affsprung, and coworl~ers.~ The apparatus required is shown in Figure 1. It consists of a l-cm optical cell with NIR silica faces and a 10/30 Pyrex inner joint which has been bent and has a small bulb on the end. A carbon tetrachloride solution (3 ml) containing known concentrations of either phenyl acetic acid, N-metbylacetamide (NMA) or benzyl alcohol was added to the cell. The spectrophotometer used was a Beckman DIG1 A equipped with a germanium filter to reduce stray light effects above 1800 mp. The cell was capped with a conventional stopper and the ir spectrum taken from 3500 mp to 2500 mp against a reference of dry CCl,. This was followed by insertion of the equilibrator which had been charged with 2 ml of 99.5 mole O/o D,O. The section between the cell neck and the inner joint was wrapped with a piece of masking tape to make as tight a seal as possible. Scans were made a t selected time intervals. Figures 2, 3, and 4 show the results of these experiments for three compounds with different degrees of
hl."l
'PIMENTEL, G. C., AND MCCLELLAN, A. L., "The Hydrogen Bond," W. H. Freeman and Company, Ban Francisco, California, 1960.
CHRISTIAN, S., AFFSPRUNG, H. E., JOHNSON, J. E., AND WOREDUC., 4 0 , 4 1 9 (1963). LEY, J. D., J. CHEM.
608 / Journal of Chemical Education
Figure 2. Phenylacetic acid in CClr; concentration 0.00776 Mi band assignments, 2 8 2 5 mp IO-HIa, - 3 2 0 0 IO-H---01, others are C-H stretch, for phenyl and methylene CH's IWillord. H. H., Merritt, L L., and Dem, J. A,, "Instrumental Methods of Anolyris." (3rd Ed.) D. Van Nostrond Co., New York, 1958, pp. 144-451. Curves 1-7 correspond to the time intervals shown in Figure 5.
Figure 3. NMA in CClr; concentration -0.00876 Mi band oxrignmentl, 2 8 7 7 mp IN-HI., 2 9 6 7 mp IN-H---Oh and 3 3 8 7 mp (C-HIa. Curve 1-fully protonoted, Curve 2-partially devterated, Cuwe 3-after 2 4 hours.
association. Contained in Figure 2 are spectra of phenylacetic acid taken a t various time intervals during the exchange process. Figures 3 and 4 are reproductions of the spectra for NMA and benzyl alcohol and show only an intermediate scan. The phenylacetic acid spectra a t various stages of deuteration reveal the strong infringement of the hydrogen bonded mode on the C-H bands. It appears that the dimer hand has an absorption maximum almost directly under the C-H stretch for the phenyl protons. Comparison of Figures 2 and 4 shows the marked similarities between the C-H spectrum for phenylacetic acid and benzyl alcohol. Such similarities might have been predicted on the basis of the molecular structures of the two compounds, but could not have been established from a comparison of curves 1 and 1 in Figures 2 and 4, respectively. Figures 3 and 4 show that the C-H absorption remains nearly constant over a period of a t least 24 hr. The C-H absorption band for NMA does show significant decreases in intensity. Whether this is due to C-H exchange or, more likely, phase separation, awaits further experimentation. The kinetics of exchange for the phenyl acetic acid system have been followed and are given in Figure 5. The half life for the exchange process through the vapor phase is observed to he -97 min. It is
Figure 4. Benryl olcohol in CC4; concentration 0.00876 M; bond osignmentr 2 7 6 0 mp (0-HI., d l other. (C-HI, for phenyl and methylene C-H's IWillord, H. H., Merritt, L L., and Dean, J. A., "Instrumental Methods of Analysis." (3rd Ed.) D. Von Nostrand Co., New York. 1958, pp. 144-45). Curve 1-fully protonoted, Curve 2-parliolly deuterated. Cuwe 3-afler 2 4 hours.
interesting to speculate on the mechanism of exchange. Before speculating, i t is necessary to add two additional pieces of information which have not been included in the text to this point. First, i t has been observed that a carbon tetrachloride solution saturated with DzO and to which benzyl alcohol is added will exchange so rapidly that the decrease in absorption cannot be followed. Second, the spectra in Figures 2, 3, and 4 do not contain the absorption band for the HOD molecule which is a product of reaction (2) and which has an absorption maximum a t 2725 mv. I n 1-crn cells this band is of very weak intensity. Because of this, the behavior of the band is difficult to depict and has not been included in Figures 2, 3, and 4. When cells with path lengths of 5.0 cm are used the HOD absorption is strong (0.3) and the passage of the absorbance of the band through a maximum is very clear. A quantitative treatment of the HOD absorption in CCla is currently being prepared for publication. The mechanism we postulate, based on Figure 5 and the additional information previously given, is depicted schematically below for the phenylacetic acid case. Our picture assumes formation of dimers by the acid, but this is not a restrictive assumption. k,
(DaOh* e (D20)d"
(1)
kn k.
(D*0),,1,
Figvre 5. Plur'r ore the abrorbancer of 3 2 6 5 mp bond for phenylocelic acid and Rlled circles ore obrorbonces of the free 0 - H of 2 8 2 5 mp. See 3pectra 1-7 in Figure 2.
+ 4-CHn-COOH sk. +CHzCOOD + HOD
(2)
Based on the exchange behavior of solutes in carbon tetrachloride which had been previously saturated with D,O, it seems likely that the diffusion of DzO into the CCL solution is the rate limiting step for the forward reaction in eqn. (2). Therefore, we postulate that kl
