1 2 APPLIED SPECTROSCOP~

Solvent Effects on the Infrared Spectra of Organophosphorus Compounds*?

John R. Ferraro

Argonne National Laboratory, Argonne, Illinois

Abstract Solvent effects on the P "'"-> O and [ P - - O ] G absorptmns in or-

ganophosphorus compounds of the type (GO)=P ~ O, (GO)2G'P O, [(GO) G'PO(OH)].,, [(GO)=PO(OH)]2, and GaP---~O are de- scribed, where G is an alkyl or aryl group. The unassocmted phos- phoryl dipoles are affected by solvents m a manner s,radar to the carbonyl dipole The consequences of the solvent effects on the phys*- cal properties of these compounds are discussed.

Introduction

Interaction of the P ~ O dipole in organophosphorus compounds with alcohols and phenols using infrared methods of investigation have been reported (1-7). Bel- lamy (8-11) has observed the solvent effects on the infra- red spectra of the compounds POCI3 and (CH~)eHP--> O. The importance of organophosphorus compounds as me- talhc extractants, where they are used with an organic diluent, motivated further studies as to the effect of these diluents. In the use of compounds of the type** (GO) 3P ~ O, GaP --> O, (GO) 2G'P ~ O, [ (GO) G'PO (OH) ]2, and [ (GO)oPO(OH) ]2 as metallic extractants, it was apparent that the diluent or solvent used had a depressing effect on the distribution coefficients of the metal extracted (12, 1.3). The solvent effects on the P - - 4 0 bond in these compounds appeared therefore to be appreciable. Studies of the effects of the various diluents on the phosphoryl stretching vibration of the compounds tri-n-octyl phosphine oxide, tri-n-butyl phosphate, di- (2-ethvlhexyl) phosphoric acid, 2-ethylhexyl hydrogen 2-ethvlhexylphosphonate, and dibutyl butyl phosphonate would be an approach to evaluating the magnitude of these effects. This paper reports such an infrared investi- gation.

Experimental The infrared studies were made with a Beckman in-

frared spectrophotometer model IR-4 and a Perkin-Elmer model 221 infrared spectrophotometer in matched cells ranging from 0.0125 mm thickness for di-(2-ethylhexyl) phosphoric acid and 0.025 mm for the other compounds. '/'he solution spectra were solvent compensated, and con- centrations of 0.05 M were used. The use of hi~her con- centrations was precluded because of possible solute inter- actions, and the use of longer cell thickness presented the problem of increased losses in energy because of solvent absorot;on. The solvents used were of the following puri- tv; n-hexane--b.pt. 68-69°C, Matheson, Coleman and Bell Co.; cyclohexane--Spectroscopic Grade, Matheson, Coleman and Bell Co.; benzene--Spectroscopic Grade, Eastman Organic Chemicals; carbon tetrachloride--Spec- troscop,c Grade, Eastman Organic Chem,cals; chloro- form:P--Reagent Grade, Merck and Co.; acetone--Re-

Based on work performed under the auspices of the U S Atom*c Energy Commission.

"G *s alkyl or aryl group. yPresented at the 13th Mid-America Spectroscopy Symposium, Chi-

cago, I l l , May, 1962. ~The chloroform was freed from its stabdlzer using the method of Halpern (5).

agent Grade, Fisher Scientific Co., (dried over anhydrous CaCI2 before use); and absolute methanol--Anal. Re- agent Grade, - - Malhnckrodt Chemical Works. The tri- n-octyl phosphine ox,de (I) was obtained from Eastman Organic Chemicals and was used without further puri- fication. The tri-n-butyl phosphate (Ill (Eastman Or- ganic Chemicals) was purified as previously reported (14). The purification of di(2-ethylhexyl) phosphoric acid (111) and 2-ethylhexyl hydrogen 2-ethylhexylphosphonate (IV) were also previously reported (15,16). Dlbutyl butyl phosphonate (V) was from Virginia-Carolina Chem- ical Corp. and was purified in a manner simdar to that of tri-n-butyl phosphate (14).

TABLE I. PHOSPHORYL ABSORPTION BANDS OF SEVERAL

ORGANOPHOSPHORUS COMPOUNDS IN VARIOUS SOL- VENTSa~ CM -1

Solvent

2-ethyl- hexyl d,- ( 2-

hydrogen ethyl- trl- tri- 2-ethyl- di-n-butyl hexyl)

n-octyl n-butyl hexyl- n-butyl phos- phosphine phos- phos- phos- phoric

oxMe phate phonate phonate ac*d

Liqmd l170m 1280m 1210s,b 1255m 1233s 1260m

Sohd 1150m n-hexane 1200w 1292m 1221m 1255m 1233s

1272m Cyclohexane 1200w 1290m 1217m 1256m 1233s

1275m CCh 1170w 1278m 1216m 1250m 1233s

1270m C~H6 1184w 1280m 1209m 1252rn 1230s

1171w 1260m CHCI3 1144m 1260m,b 1209m 1248m 1233s

1210m Acetone 1280m 1240s

1235m 1220m 1218m

CH~OH 1133m 1255m,b 1200m 1220m 1230m,b I l l 0 w 1236m

a s, strong; m, medmm; w, weak; b, broad

Experimental Results The effects on the phosphoryl stretching frequency

in these compounds in such solvents as n-hexane, cyclo- hexane, benzene, carbon tetrachloride, chloroform, ace- tone, and methanol are tabulated in Table I. The solvent effects increase as the solvent becomes more polar. The ratio of absorbancies of the bands (P-O)G/(P--~.O) using the peak intensities, remains rather constant in less polar solvents. As the solvent becomes more polar the ratio for II and IV decreases, whde for V it remains rather constant in these solvents. For i l l the ratio shows no in- crease. Although the ratios in peak intensit,es used in this comparison are at best only qualitative, they do indi- cate that changes may also be occurring at the (P-O)G bond. In addition there is a tendency for the (P-O)G absorption band to broaden and to shift toward higher frequencies in the more polar solvents. This shift is in

VOL. 17, No. 1, 1963 13

TABLE II . TABULATED ~XV/V t VALUES FOR VARIOUS OR- GANOPHOSPHORUS C O M P O U N D S AND A C E T O P H E N O N E

W I T H VARIOUS SOLVENTS a

2-ethyl- hexyl

[2 w dl (2- hydro- dl- ethyl- gen 2-

~: try- n-butyl hexyl) ethyl- n-butyl n-butyl phos- hexyl phos- phos- phorm phos-

Solvent phate" phonate ac*d phonate

grl- n-octyl phos- Aceto- phme phenone oxide (8-9)

n-hexane . . . . . . Cyclo-

hexane 1 5 0 0 3.3 0 0.6 Benzene 9.3 2.4 0 9.9 13.3 4.1 CCh 10 8 4.0 0 4 1 25 3.0 CHCIa 24.8 5 6 2.4 9 9 46 7 8.3 CHoOH 28 6 15.1 2 4 20.6 55.8 4.1

10.0

'v ' ,s the P-----90 frequency m the n-hexane solunon and A. *s the &fferenee between this frequency m the n-hexane solutmn and in the other solvents.

b The column shows only the results for the P "'"-> O absorpnon at the hagher frequency.

the same direction as was found for the hydrogen bond- ing of tri-ethyl phosphate (5).

Bellamy (8, 9) has plotted &v/v for X z O dipoles vs. &v/v of acetophenone and obtained straight hnes. The frequency of the gas spectrum, v, was taken as the start- lng point. For the compounds studied in th,s work it is extremely difficult to obtain the frequency of the gas spectra, since the hqmds H-V have very high boiling points, and I is a solid. Therefore, the frequency in n- hexane, v', is used as the starting point. Table II shows the tabulated ~v/v" results, and Figure 1 shows a plot of &v/v' for the organophosphorus compounds versus zXv/v' for acetophenone. Straight lines are obtained and slopes of 0.3 and 0.2 are found for II and I respectively, while a large slope is obtained for III. For IV and V the points are better fitted with 2 straight lines; the slope of the initial line being more steep than that of the second. From Figure 1, it is observed that the P-~O bond in I is of greater polarity than in H, which is greater than in W and V. The compound HI, which is a dimer in dilute solu- tions of n-hexane, cyclohexane, benzene, and carbon tetra- chloride, (17) is extremely resistant to solvents effects, until one goes to highly polar solvents such as acetone, chloroform or methanol, which will destroy the dimer.

Discussion

The experimental results previously presented indicate a difference in the polarity of the phosphoryl dipole de- pending upon the nature of the molecule or the environ- ment around the dipole. The neutral organophosphorus molecules show a much more polar phosphoryl bond than the dl-(2-ethylhexyl)phosphorlc acid type. As expected i shows the most polar phosphoryl bond of the five types of compounds studied. This is in agreement with the infrared results showing a phosphoryl absorption in I at a lower frequency than in the other compounds. The P--~O dipole in II appears to behave like the dipole in I. Thus, the neutral organophosphorus compounds containing ex- posed P--~O dipoles are more easily affected by solvents. In the plots shown in Figure 1, straight hnes are obtained. The results compare favorably with those of Bellamy (9) for similar compounds, such as (CHs)2HP---~O and POC13. Compounds IV and V appear to show different be- havior and have a break in the slope line. Initially, with low polar solvents the effects are small. However, with

more polar solvent such as alcohol there is a break, and the new line approaches the slope of compounds I and II. This might be caused by a difference in behavior for these solutes in the various solvents then that observed for ace- tophenone. However, in compound W it is also possible that initially in the low polar solvents, the predominant aggregation is dimeric, and the associated P--->O dipole is thus less available for interaction with the solvent than is the unassociated P-->O dipole in I and II. Generally, these four classes of compounds (I, II, W, VI appear to have a P--3.O dipole of a polarity somewhere between that of the CO &pole in acetophenone and the SO &pole m dimethyl sulfoxide (9).

The P - s O stretching frequency in di(2-ethylhexyl) phosphoric acid is independent of the nature of the solvents. This is probably due to the strong dimers that this class of acids form. Until the ring can be ruptured, the phos- phoryl bond resists any solvent effects. Polar solvents like chloroform, acetone, or methanol are capable of breaking the ring and of interaction to form hereto intermolecular compounds, causing definite effects on the phosphoryl bond. This behavior is similar to that found for pyrrole (18) and other dimers (19) and is an indication that P-->O bond in III is only very slightly polar as long as it is part of a strong &merle ring (15). Isopiestic molecu- lar weights (17) obtained in various solvents in&cated that III was predominantly dimeric in dilute solutions of n-hexane, cyclohexane, benzene, and carbon tetrachloride, began to form monomers only in chloroform and acetone, and was monomerlc in methanol. The molecular weight results parallel the results obtained in infrared. The differ- ence in solvent behavior between III and IV is probably due to the difference in the stablhty of the dimers. The acid IV dimer, which in very dilute solutions m these sol- vents appears to breakdown (17), shows less resistance to solvent effects than I!1.

Theuse of organophosphorus compounds for the sol- vent extraction of metal cations is well known. Most of the extractions are carried out in an organic phase using a diluent or solvent such as those used in this investiga- tion. It ~s of interest to comment on the consequences of these solvent effects on the metal extraction. In the neu-

LIt/ I I I I I I ~ Z / 0 W

° /

: / I 20 -HDEHP w D B B P / I

/ TBP

0 0 I0 20 3 0 4 0 5 0 6 0

A v l v i x 103

FIG. 1. PLOT OF daY/V" OF ACETOPHENONE VS. ~iv/v t FOR T H E O R G A N O P H O S P H O R U S C O M P O U N D S

HDEHP-dx(2-ethylhexyl)phosphorm acid, DBBP-dt-n-butyl n-butyl phosphonate, HEH(EHP)-2-ethylhexyl hydrogen 2-ethylhexyl phos- phonate, TBP-tn-n-butyl phosphate, TOPO-tri-n-octyl phosphme oxide

14 APPLIED SPECTROSCOPY

tral organophosphorus compounds, the more polar phos- phoryl bond will interact with the solvent, thus remowng the cation extraction sites, and this should cause a lower distribution coefficient. An investigation in our laboratory (12, 13) revolving organophosphorus compounds of the type described in this paper substantiates this. For ex- ample extraction of promethium into an organic phase of 1,1,1,-trifluoro- 3-2'-thenoyl acetone and t r i -n-butyl phosphate is highest for cyclohexane and decreases in the order n-hexane > carbon tetrachloride > benzene > methyl lsobutyl ketone > chloroform. With the acidic type of compounds stmllar results have been obtained and the more polar the diluent the lower the distribution coefficients (20, 21).

It is very important then that the distribution co- efficients of metal extractions be compared only when the so-called inert diluent or solvent used is the same. To compare distribution coefficients of metallic extractions into organophosphorus compounds when the dlluents vary from n-hexane to methanol is meaningless.

Acknowledgments The author thanks the following Argonne employees:

Mr. George W. Mason for the purification of di(2-ethyl- hexyl) phosphoric acid and 2-ethylhexyl hydrogen 2- ethylhexylphosphonate and Dr. E. Phlllip Horwitz for the purification of dl-n-butyl n-butyl phosphonate.

Literature Cited

(1) W. Gordy and S. C. Stanford, J. CHEM. PHYS. 8, 170 (1940) ; 9, 204 (1941)

(2) C. S. Marvel, M. J. Copley, and E. J. Gmsburg, J. AM. CHEM. SOC. 62, 3109 (1940)

(3) L. F. Audrieth and R. J. Steinman, IBID. 63, 2115 (1941)

(4) G. M. Kosolapoff and J. F. McCullough, IBID. 73, 5392 (1951)

(5) E. Halpern, J. Bouck, H. Finegold, and Jerome Goldenson, IBID. 77, 4472 (1955)

(6) G. Aksnes and T. Gramstad, Ac'rA CVIEM. SCAND. 14, 1485 (1960)

(7) T. Gramstad, IBID. 15, 1337 (1961) (8) L. J. Bellamy and R. L. Williams, TRANS. FARADAY

SOC. 55, 14 (1959) (9) L. J. Bellamy, C. P. Conduit, R. J. Pace, and K. L.

Williams, ImD. 55, 1677 (1959) (10) L. J. Bellamy and P. E. Rogasch, SFECTROCHIM.

ACTA, 16, 30 (1960) (11) L. J. Bellamy and R. L. Williams, PROC. RoY. Soc.

(LONDON) Ser. A 255, 22 (1960) (12) T. V. Healy, J. INORG. NUCL. CHEM. 19, 314

(1961) (13) T. V. Healy, IBID. 19, 328 (1961) (14) D. F. Peppard, G. W. Mason, and J. L. Maler, IBID.

3, 215 (1956) (15) D. F. Peppard, J. R. Ferraro, and G. W. Mason,

IBID. 7, 231 (1958) (16) D. F. Peppard, J. R. Ferraro, and G. W. Mason,

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1169 (1954) (19) C. G. Cannon, MIKROCHIM. ACTA 2, 555 (1955) (20) C. A. Blake, Jr., C. F. Baes, K. B. Brown, C. F.

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Submitted Apral 9, 1962

v

The Effect of Graphite Absorption Using Vacuum Cup Electrodes by Spark Techniques'::

S. T. Bass and J. Soulati

Department of Biochemistry, Michigan State University,

East Lansing, Michigan

Abstract

It has been found after not recording the first exposure with vacuum cup electrodes, that the next eight to ten exposures on each electrode are much ~mproved m the precision of intensity ratios The results show the possxbdxty of using a single vacuum cup electrode to de- termine not only rephcas of a single sample, but that more than one sample may be determined on a single electrode These electrodes have been tested only wxth s~mdar elements present and w~th 2 ½ t~mes increase m sample concentrataon, and no effects were apparent by changmg the samples, provxdmg certain analytmal precautmns were followed

A study of the adsorptmn of grapMte electrodes was carried out m solution s~mdar to the samples. The results indicated that adsorp- tmn was dependent upon concentration, but that once an electrode had adsorbed elements, even long permds m water would not cause the loss of the adsorbed elements. Neither would less concentrated samples cause a reduction in the amount of elements adsorbed

' Pubhshed with the approval of the Dxrector of the Michigan Agricul- tural Experiment Statmn as Journal Artxcle No. 2893.

Introduction

Since the introduction of the vacuum cup electrode system by Zink (1), a number of spectrographers have tried using it with varying degrees of success. Its chief shortcomings seem to be 1) the electrodes are rather cost- ly and 2) it exerts a corrosive action on the arc stand.

The corrosiveness can be controlled by using a suitable plastic material for construction of an excitation chamber inside the arc stand, but the cost of the electrodes is an unavoidable expense. Therefore, in an effort to lessen the cost per sample analyzed with such electrodes, the authors have investigated the possibility of using the same electrode for a number of excitations. The purpose of this com- munication is to report their findings.