Infrared Spectroscopy of Deuterated Compounds An Undergraduate Experiment Patrick MacCarthy Colorado School of Mines, Golden, CO 80401
I n a recent issue of THIS JOURNAL a procedure for preparing mulls of solid compounds, deuterated a t active hy. drogen sites, in a form suitable for infrared spectroscopy was discussed (1). ~h~ suitability of the procedure for an undergraduate laboratory was "'lined' This paper describes a corresponding procedure On the KBr pressed-pellet method, which, for many of the same reasons, is also suitable for use in an undergraduate laboratory. T h e pressed-pellet method, as conventionally used, is described in detail elsewhere (2-7). . . T h e mull techniaue and the pressed-pellet technique are the only two methods that are generally applicable to a wide variety of solid samples for infrared spectroscopy. With t h e development of the present variation both methods have now been modified so as to he a ~ d i c a h l eto compounds deuterated a t active hydrogen sites: The simplicity of the present technique betrays the enormity of the kchn~calprol~lcmswhich it c&cumven&;these prohlems have been discussed (8).In this paper deuteration refers specifirally to deuterium-hydrogen exchange a t active hydrogen sites in the molecule. Experimental
Chemicals In addition to the eomoounds reauired for deuteration. the following rhcmicals are re;uircd: D A ~199.8 at&), ( : H . ~ ~ 199.5 D aumW CHsOH, and hlgh.quality KBr.
s to remove atmospheric moisture and to seal the desiccator. After about 30 min, vacuum is applied in order to evaporate the sample to dmess. If the sample tends to spatter in the early stage of evaporation, the desiccator stopcock is closed for a few minutes and then opened again; this is repeated until the tendency for spattering has stopped, and the sample is then brought to dryness. The complete process is repeated with a second aliquot of deuterating solvent.For the final drying, the desiccator is evacuated for about 90 min when D20 is used and for about 30 min when CH30Dis used. The vacuum is then released and the second bolt is applied to the mini-die, the sample is compressed, and its spectrum recorded.
and &USSl,n In the figure, (a) shows the spectrum of the hydrogen form of 2-hydroxybewl aclohol, the same compound as i l l u s h k l in the previous paper (1) involving the modified mull techspe&r& of the deucrated compound is illustrated in the figure (b). T h e two O-H stretching hands a t -3420 cm-I and -3140 em-' in the hydrogen f i r m of the compound have disappeared essentially completely from the deuterated compound, and the corresponding O-D stretching hands have appeared a t -2550 cm-' and -2350 cm-', respectively. The wavenumber ratios VOH/FODfor the two pairs of hands have a value of 1.34 compared to the theoretical value of 1.41. The aliphatic C-H stretching bands that are virtually concealed beneath the broad O-Hstretching band centered a t -3140 em-' in the hydrogen form of the compound (fig., (a)) are clearly revealed in the spectrum of the deuterated
nique.- he
Equipment The apparatus consists of a vacuum desiccator connected to a vacuum line by means of heavy-duty Tygon tubing. The vacuum line has two vapor traps placed in Dewar flasks containing dry ice. A three-way valve, positioned between the traps and the vacuum pump, allows the desiccator to he connected tothe vacuum or released to the atmosphere. A mini-die (Barnes Engineering), with one holt and bushing attached, is placed inside the desiccator. Other items required are: mortar and pestle or Wig'l'hug vibrator with plastic vials and halls; syringes (3-ml capacity) and needles (22 gauge). Spectra were measured an a model 700 Perkin-Elmer speetrophotometer. Caution: This experiment involves extensive use of a vacuum line and an evacuated desiccator; protective goggles should be worn at all times. The desiccator should he partially sealed by strong adhesive tape or completely sealed with transparent tape. Procedure The comoound-KBrmixture. in the aonroximete hv . ~ ~~~~~~. F ~ ratio of 130 -, weight is finely ground and mkcd using n mortar and ppatle or a \Vig'l'bui: vibrator. About RO mgof themixture is placed on the polished surface of the mini-die bolt within the desiccator and spread by means of a microspatula. A mini-funnel is used in transferring the powdered mixture to the die to prevent it from becoming entrapped in the threadine of the mini-die. The nowder is then damoened sliehtlv of Dq0 or CH?OD 1-10 uL totali from ,. . hv. aool& . . , ,. drool& , as).ringcto different pointriof the powder. This is a most critical step in the procedure-only u w ) shghr dampening should he pruducea, as best judged by observing the darkened appearance of thedampened powder. Care should be taken not to wash any of the sampledown between the holt and the barrel of the die, and it is imperative that neither the eomnound under studvnor the KBr be allowed todissolve. A few drops of the deuterating sblvent are applied to the inner wall of the desiccator to orovide -~ or- CH.C)n-pntnmtpd .-..--.--.atmo.nhero .....r..-.~ a~ DnOand to compensate for any hydroxy groups on the glass surface. The lid is then applied to the desiccator and it is evacuated for about 30 ~~~
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Infraredsmcba of 2-hvdr6xvbenzvl . . . alcohol in KBr oellets la\ . . undsuteated:. lb\ .~. dewated by two successive dmpeningldryingsleps wilh D20and evacuating to dryness for 30 min in each case: (c) same pellet as that used in (b) afler Standing exposed to abnosphere for 15.5 h. The cm-' axis is linear between 800 and 2000 cm-', it is also linear between 2000 cm-' and 4000 cm-', but withe differentscale. ~
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Number 7 July 1985
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compound at -2900 cm-I and 2960 cm-' (fig., (b));the weak aromatic C-H stretching band is also visible a t 3050 cm-I in the spectrum of the deuterated compound (fig., (b)). The baseline in the figure (h) is excellent, indicating that the dampeningldrying process has not adversely affected the compound-KBr matrix. This is found not to he the case if the compound or the KBr is allowed t o dissolve during the dampening process. In the figure, (c) shows the spectrum of the same pellet as that in (b) after standing exposed to the atmosphere for 15.5 h, demonstrating that reprotonation of the sample under these conditions is not a serious problem. However, when this pellet was reground and repelletized it showed a considerable degree of reprotonation. p-Hydroxybenzaldehyde could not be deuterated to a high degree by this technique using either DzO or CHBODalone as the deuterating solvent, even after repeated dampening1 drying steps (8). However, when the sample was dampened alternately with D20 and CH30D or with a mixture of D20/ CHaOD, effective deuteration was accomplished. While the reason for his behavior is not clear a t present i t does suggest an approach for dealing with other compounds which may resist deuteration by either CHsOD or D20 alone using the modified KBr method. Conclusions A technique for preparing KBr pellets of compounds deuterated a t active hydrogen sites has been developed. The method is simple, rapid, and convenient and does not require any expensive equipment. In particular, the use of a dry-box,
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.lournal of Chemical Education
which would make the oneration more cumhersone. timeconsuming, and expensive, and would eliminate its potential as a n d n f : tool. is avoided. The deuterated oellets exhibit a rrmarkal,le resistance lo reprutonatiol~b; atmospheric moisture. This rant~rimentis suilahle h r an undereraduare laboratory in anaiytical chemistry, instrumental analysis, or oraanic chemistrv in terms of its teachine value (analoeous t o t h a t of ref. (f)),cost of equipment, and durkion. T h e method reported here is com~lementarvto the modified mull (I). t e c h n i q u e ~ r e v i o u sreported ~~ Acknowledgment This research was supported in part by a grant from the donors of the Petroleum Research Fund, administered by the American Chemical Society, and in part by an L. J. Beckham grant from the Colorado School of Mines. Literature Cited ,I t2 I:,
M a d arthi. l'.sod H n w m r ~ ~ ..I.. S 1 . W E U Fnw'.S9.~PI~1981,. St ma,", h l > I , . m ' P h a ~ e wm I n f r a r e d SIMC!U w o p ) , " V d t , (P:,l ! m S ~ , r n m < k i , II A . P I ~ ~ J ~ I ~ ~ . ~ Su t r~ . p, p111 ~ ~ lW w S ~ ~ 1'0119, !V J .JI Infrared 5pe?rr+. IT.' l..lohn U ' ~ l e y a n d S u n *Nru
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\., .L 19L1 (41 A1pert.N. L.. Kehr, W. E., and Szymmk. H. A.,"m-ThwryandPradiee of I n i d S ~ t m s w p y . " P l e n v mPress. New York,ISM. ( 5 ) Fridmann, S. A.. in "Progress in Infrared Spectroaeopy," Vol.3, (Editor: Szymanski. H.A.I.PlenumPmss. New York, 1 9 6 7 , ~1-23. ~. (61 Parker,F. S., "Applications af Infrared Spectrosmpy in Bbehemistry, Biolom and Medicine, Plenum Press. New York. 1971. (71 Rice, W. J.,in "BIxrataryMethodsin Infrared Specbcempy,"2nded..(Edilors:MiUer, R. G.J., and Staee. B. C.),Heyden and Sons. Ltd.. New York, 1972. (81 MaeCarthy, P., Colorado School oiMinta Quart.,78.9, (19831.
