Pergamon Spectrochimica Acta, Vol. 51A, No. 4, pp. 587-593, 1995

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved

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A matrix-isolation infrared study of the adducts of triethyl phosphate with CCI4, CHCIa and CH2C! 2

LISA GEORGE,t K. SANKARAN, K. S. VISWANATHAN and C. K. MATHEWS

Chemical Group, IGCAR, Kalpakkam 603 102, India

(Received 21 April 1994; in final form and accepted 30 June 1994)

Abstract--An infrared spectroscopic study is reported of the adducts of triethyl phosphate (TEP) with CCI4, CHCI 3 and CH2CI2, isolated in a nitrogen matrix. The adduct formation is identified from a red shift in the phosphoryl (P=O) stretching vibration of triethyl phosphate. The experimental data are explained on the basis of adduct formation involving the interaction between the phosphoryl oxygen of the triethyl phosphate and the chlorine of the chloromethanes. The shift in the P=O frequencies as a result of the complex formation with the chloromethanes follows the order: CCI4<CHCI3~CHzCI 2, which is in keeping with the Lewis acidities of chlorine in these compounds. These complexes are similar to the van der Waais or anti-hydrogen-bonded complexes seen in the case of HF adducts with some bases. No evidence is observed for the formation of any hydrogen-bonded adduct of TEP with CHCI 3 and CH2CI 2.

INTRODUCTION

ORGANIC p h o s p h a t e s , pa r t i cu la r ly t r i -n-buty l p h o s p h a t e find extens ive app l i ca t ion , as an ac t in ide ex t r ac t an t , in the reprocess ing of spen t nuc lea r fuel. The p h o s p h a t e s a re gene ra l ly used t o g e t h e r with an organic d i luen t , l ike d o d e c a n e , to ta i lor the physical p r o p e r t i e s , such as viscosi ty and dens i ty , of the organic phase , in o r d e r to a id the so lvent ex t r ac t ion process . H o w e v e r , the d i luen t , can also a l t e r the ex t rac t ion efficiency of the p h o s p h a t e - d i l u e n t sys tem, p r o b a b l y resul t ing f rom a weak in te rac t ion be tw e e n the p h o s p h a t e and the d i luen t [1, 2]. In ou r l a b o r a t o r y , t he re is a p r o g r a m m e to s tudy the p h o s p h a t e - d i l u e n t in te rac t ions using mat r ix - i so la t ion in f ra red spec t roscopy . In an ear- l ier work , we had s tud ied the h y d r o g e n - b o n d e d adduc t s of t r ie thyl p h o s p h a t e ( T E P ) with wa te r and m e t h a n o l [3].

In this p a p e r , ev idence for the fo rma t ion of weak ly b o u n d mo lecu l a r complexes of T E P with CC14, C H C I 3 and CH2C12 is p r e s e n t e d , for the first t ime. The mat r ix - i so la t ion t echn ique , due to its high r e so lu t ion capab i l i ty , is idea l ly su i ted for the s tudy of h y d r o g e n - b o n d e d and w e a k l y b o u n d van de r W a a l s complexes [4-8] . ANDREWS and co- w o r k e r s have ca r r i ed out severa l s tudies on mo lecu l a r complexes of H F with var ious bases [9-15] . In a r ecen t work , t hey also s tud ied H F adduc t s with ch lo r ina ted m e t h a n e s [15]. O u r resul ts will be c o m p a r e d with these s tudies .

EXPERIMENTAL

The matrix-isolation experiments were carried out by using a commercial Leybold A G refrigerator-cooled cryostat, RD 210, which uses a closed-cycle helium compressor. The details of the vacuum and cryogenic system have been described previously [16]. The matrix gas and vapours of the sample streamed out of three separate nozzles, and were deposited on a KBr substrate, maintained at 12 + 0.5 K. The deposition rate was about 3 mmoi h-1, as measured by using a mass flow sensor (Brooks 5860), and the duration of deposition was 30 min.

TEP (Tokyo Kasei) was dried over anhydrous sodium sulfate and distilled under reduced pressure before use. Extreme precautions were taken in the purification and loading of TEP in the sample cell, to minimize the moisture content in the phosphate. The details of the sample handling procedure have been discussed previously [16, 17]. Carbon tetrachloride (Merck), CHCI3 (BDH) and CH2C12 (S.D. Fine Chemicals, India), all of spectroscopic grade, were used without any further purification. All the samples were subjected to several freeze-thaw cycles before use. High purity nitrogen ( IOLAR speciality gases, Indian Oxygen Limited) was used as the matrix gas, after passing it through molecular sieves. Nitrogen was chosen as the matrix in preference to argon,

"t Present address: RSIC, Indian Institute of Technology, Madras 600 036, India.

587

588 L. GEORGE et al.

C

8

[- B

o'3

f*3

o f-q

1330 1240 cm-l

Fig. 1. Matrix-isolation infrared spectra in the region 1330-1240 cm-~ of: (A) TEP/N 2 (1:1000); (B) TEP/CCI4/N2 (1:1:1000); (C) TEP/CCI4/N2 (1:6:1000).

since the peak widths of the infrared bands of TEP were smaller in the nitrogen matrix than in argon [16]. For example, the peakwidth (FWHM) of the P = O stretch in TEP, which occurs near 1270 cm - l , is - 2 cm -l in the nitrogen matrix, whereas it is - 4 cm -l in argon [17]. Furthermore, the frequency of the P = O stretch differed by only 2 cm -~ in the two matrices [16], indicating that any interaction of TEP with the matrix was similar in both argon and nitrogen.

Sample-to-matrix ratios were varied over the range 1:0.3:1000 to 1:7.5:1000 (TEP:Y:Nz; Y = C C L , CHCI3 or CH2Cl2). All spectra were recorded using a Digilab FTS 15/90 FFIR spectrometer, at a resolution of I cm -~, in the region 4000-700 cm -t. Typically 128 scans were coadded to obtain good signal-to-noise ratio. After recording a spectrum, the matrix was warmed to 35 K, maintained at this temperature for 15 min, and then recooled to 12 K. Spectra of the matrix, thus annealed, were then recorded. All spectra shown in this paper are those recorded after annealing the matrix.

RESULTS AND DISCUSSION

Interaction of TEP with CCl 4

A s r e p o r t e d ear l ie r , the P = O s t re tching v ib ra t ion of T E P was o b s e r v e d at 1270.6 and 1305.3 cm -~ [16]. These two f requenc ies c o r r e s p o n d to the P = O stetch o f the C3o and C3 con fo rmer s of T E P , respec t ive ly (Fig. I ( A ) ) . The doub le t nea r 1290 c m - i is due to the CH2 twist ing m o d e in T E P [18].

Matrix-isolation IR study of the adducts of TEP 589

When TEP was codeposited with CCI4 in a nitrogen matrix (1 TEP/1 CCi4/1000 N2), and the matrix then annealed, a new peak appeared at 1266.3cm -~ (Fig. I(B)). On increasing the CCI4 concentration (1TEP/6CC14/1000N2), the intensity of the 1266.3 cm -1 peak increased (Fig. 1(C)). Since the peak at 1266.3 cm -1 appeared only when TEP was codeposited with CCL and showed an intensity dependence on CCl4 concentration, this peak must be due to the interaction of TEP with CCL. The peak at 1266.3 cm -l can, therefore, be assigned to the red shifted P=O stretching vibration of the C3v conformer of TEP, in the TEP. . CCI4 adduct. Since the peak due to the adduct appeared even at low concentrations of TEP and CCI4, it is reasonable to assume that the adduct is a 1"1 complex of TEP and CCI4.

Since the P--O stretching frequency is shifted by only 4.3 cm -I in the adduct, the interaction is probably of the van der Waals type, involving the lone pair of electrons on phosphoryl oxygen and the lowest unfilled molecular orbital of CCI4. These type of complexes are stabilized due to the overlap of the highest occupied molecular orbital (HOMO) of the base with the lowest unfilled molecular orbital (LUMO) of the weak Lewis acid [19]. The relative frequency shifts (AP/P) of the P=O stretching mode in the TEP. . CC14 adduct are comparable to that reported for the HF stretching mode in the van der Waals complex of HF. . CICCI3 [15, 20]. It, therefore, seems reasonable to believe that the type of complex formed is similar in both (OEt)3-P--O.. CICCI 3 and HF. . CICCI3 complexes. However, we did not observe any perturbation in the CCI4 modes.

It must be mentioned, however, that we could not identify any peak due to adduct formation involving the C3 conformer of TEP. We do not find this observation surprising, since the intensity of the P--O stretch of the Ca conformer, at 1305.3 cm -1, is itself very small, and hence the peak, due to adduct formation with this conformer, is not likely to be discernible from the background. Even in our earlier studies on the hydrogen-bonded complexes of TEP, we noticed that even though the peak due to the hydrogen-bonded adduct of the C3v conformer could be clearly observed, we could not discern the peak due to that of the (?3 conformer [3].

In addition to the spectral features discussed above, we also observed a peak near 1254cm -l, which appeared even when TEP was deposited alone (Fig. I(A)). In our earlier studies on the hydrogen-bonded adducts of TEP, we have shown that this peak is due to the red-shifted P=O frequency in the T E P . . H 2 0 adduct [3]. The T E P . . H 2 0 adduct was observed in the present experiments also, since water is an inevitable impurity in any matrix-isolation experiment.

Interaction of TEP with CHCI3

When TEP was codeposited with CHCI3 in a nitrogen matrix (1 TEP/1 CHCI3/1000N/), and the matrix then annealed, a new peak appeared at 1264.3cm -1, as shown in Fig. 2(B). On increasing the chloroform concentration (1 TEP/7.5 CHCI3/1000 N2), an increase in the intensity of the 1264.3 cm -1 peak was noticed (Fig. 2(C)). The shift in the P=O frequency of TEP, due to complex formation with CHCI3, was 6.3 cm-1; slightly higher than what was observed with CCL.

The adduct formation in the case of TEP/CHCI3 can be explained on the basis of the formation of a weak TEP. . CHCI3 bond, similar to that seen for TEP. . CC14. Two points should be noted for the TEP. . CHCI3 adducts. The shift in the P=O frequency for the CHCI3 adduct is comparable to that observed with CC14, and the peak shapes in both cases are similar. We therefore believe that the nature of interaction must be the same in both the TEP. . CC14 and TEP. . CHCI3 complexes. Since in the TEP. . CCI 4 complex, the interaction must involve the chlorine of CC14 and the phosphoryl oxygen of TEP, it was concluded that in the TEP..CHCI3 complex also, the adduct is of the type (OEt)3-P= O. . CICHCI2, involving a chlorine-oxygen interaction. Since no perturbation either in the C - H stretching (at 3067 era-l) or bending (at - 1230 era-1) modes of CHC13 was observed, we do not see any evidence for an interaction of the type (OEt)3-P=O. . HCCI3. The failure to observe such an interaction was a little surprising

590 L. GEORGE et al.

to us, since, generally a H . . O interaction would be expected to be more stable than a van der Waals type C I . . O interaction. However, several theoretical [20-22] and experimental [14, 15, 23-35] studies have reported such van der Waals complexes involving HF and a number of bases like C12, CIF, CC14 and CHCI 3. These van der Waals complexes have also been referred to as anti-hydrogen-bonded complexes. In some cases it was only the anti-hydrogen-bonded complexes that were observed [23-25], which seems to be true in our T E P . . CHC13 system, also. It may also be possible that, we could identify only the dominant adduct featuring the O . . CI interaction, and the peak due to the O . . H interaction was too weak in intensity to be identified. The O . . CI adduct could be the dominant adduct, because statistically the chlorine-oxygen interaction is more likely than a hydrogen-oxygen interaction in CHCI 3. Even if the hydrogen-bonded adduct were to be thermodynamically more stable than the van der Waals adduct, the rearrangement to the more stable form may be hindered in the matrix.

Interaction o f TEP with CH2C12

When TEP and CH2C12 were codeposited, and the matrix then annealed, a new peak appeared at 1259.0cm -~. Even at very low concentrations of CH2C12 (1 TEP/0.3 CHECIffl000 N2) this peak could be observed (Fig. 3(C)). When the concen- tration of CH2C12 was increased, an increase in the intensity of the 1259.0 cm -~ peak was observed (Fig. 3(D)). As in the T E P . . CCI 4 and T E P . . CHC13 adducts, we believe that CH2C12 also interacts with TEP to give rise to a 1:1 adduct. We therefore assign the peak

C

1330 cm-1

• A

¢,q

1240

Fig. 2. Matrix-isolation infrared spectra in the region 1330-1240 cm J of: (A) TEP/N 2 (1:1000); (B) TEP/CHCI3/N2 (1:1:1000); (C) TEP/CHCIffN 2 (1:7.5:1000).

Matrix-isolation IR study of the adducts of TEP 591

[-,

,f D

~_ B

O

t 'q

t ~

1330 1240 cm-1

Fig. 3. Matrix-isolation infrared spectra in the region 1330-1241)cm -1 of: (A) TEP/N2 (1:10~); (B) CH2CI2/N2 (1.5:1000); (C) TEP/CHeCIJN2 (1:0.3:1000); (D) TEP/CH2C|JN 2 (1:5:101)0).

at 1259.0 cm -1 to the P=O stretching vibration of the C3v conformer of TEP in the TEP. . CH2C12 adduct.

It should be noted that CH2C12 itself has an absorption near 1270 cm -1 (vs) [26]. To ensure that the new peak seen at 1259 cm -~ was not due to any complex of the type CH2C12.. H20 (since traces of water are always present in any matrix-isolation experi- ment), experiments were carried out where CH2CI: and H20 were codeposited. Since the 1259 cm-~ peak was not observed in our CH2CI:-H20 experiments, any possibility of this peak being the red shifted, v8 vibration of CH2C12, as a result of its interaction with H20 in the matrix, can be ruled out.

As in the TEP. . CHCI 3 adduct, no shift in the C-H stretching or C-H-CI bending modes of CH2C12 was observed. In the TEP. . CH2C12 complex also, the interaction seems to be between the phosphoryl oxygen of TEP and the chlorine of CH:CI2. This conclusion is based on the observation that the adduct peak was sharp, as with the CC14 and CHCI3 adducts, indicative of a van der Waals interaction. It was not surprising that the hydrogen-bonded interaction was not observed in the TEP/CH2CI2 system, since no evidence for a hydrogen-bonded interaction was seen even with CHCI3, where the hydrogen is more electropositive than in CH2C12. Though statistically the hydrogen- bonded interaction would be more probable with CH2C12 than with CHCI3, the smaller

592 L. GEORGE et aL

Table 1. Observed P=O stretching frequencies of TEP in TEP.. CC14, TEP.. CHCI 3 and TEP.. CHzCI 2 complexes

v(P=O) Shift ~ AP Relative Expt (cm -l) (cm -1) shift b

TEP/N2 1270.6 - - - - TEP/CCIJN2 1266.3 4.3 0.0034 TEP/CHCI3/N 2 1264.3 6.3 0.0050 TEP/CH2CI2/N2 1259.0 11.6 0.0091

Shift (AP) = (v(P=O) . . . . pJex~a - v(P=O)comp~x~). b Relative shift = AP/v (P= O) . . . . . plexea.

electropositive character of hydrogen in CH2CI 2 compared with that in CHCI3, probably does not favour the formation of the hydrogen-bonded adduct of TEP with CHECI 2.

It is interesting to note that the peak shift in the P=O stretching vibration of TEP in the TEP. . CH2C12 adduct was large (11.6 cm -1) compared with that of the TEP. . CCI4 and TEP.. CHCI3 adducts, indicating a greater strength of interaction in the CHECI2 adduct. This shift is almost twice that seen in the TEP. . CHCI3 adduct. The relatively larger shift in the CH2CI2 adduct could be due to the higher Lewis acidity of chlorine in CHECI2, compared with that in CCI4 and CHC13. A similar conclusion was also arrived at by HUNT and ANDREWS [15], though they did so from their intensity data. Though we do not draw any quantitative intensity information from our spectra, it is still interesting to note that the adduct in the case of TEP. . CH2CI2, was observed even at very low CH2CI 2

concentrations. It is also possible that the large shift seen in the case of the TEP.. CH2C12 adduct might

be as a result of a change in structure of the complex in going from CCI4 to CHECI 2. Such changes in structures, have indeed been reported by DEE BENE and MEITEE [20], in their theoretical studies on the adducts of HF with chloromethanes. If with CH2CI2, a

bifurcated structure involving two chlorines and the phosphoryl oxygen were involved, while in the case of CCI4 and CHCI3, the bonding in the complex were to involve only one chlorine, then the larger shift in the TEP..CH2CI 2 adduct would be possible. Calculations to determine the equilibrium geometry of these complexes would help to better understand this point.

The frequencies and the relative shifts in frequencies of the P=O stretching vibration of TEP in the TEP..CCI4, TEP..CHCI3 and TEP..CHECI 2 adducts are given in Table 1.

CONCLUSIONS

Adducts of TEP with CCl4, C H C I 3 and CH2C12 were studied by using matrix-isolation infrared spectroscopy. The P=O stretching vibration of TEP, at 1270.6 cm -~, shifted to 1266.3, 1264.3 and 1259.0 cm -1 in TEP.. CC14, TEP. . CHCI3 and TEP.. CH2C12, respec- tively. The interaction seems to be a van der Waals interaction involving the phosphoryl oxygen of TEP and the chlorine of chloromethanes. The strength of the interaction in the complex, as observed from the shifts in P = O frequency, appears to be CC14 < CHCI3 <~ CH2C12, which correlates with the order of chlorine Lewis acidities in these compounds.

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Matrix-isolation IR study of the adducts of TEP 593

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