Matrix-Isolation Infrared Spectroscopy of Organic Phosphates

Transcription

1 Matrix-Isolation Infrared Spectroscopy of Organic Phosphates LISA GEORGE, K. SANKARAN, K. S. VISWANATHAN, and C. K. MATHEWS* Chemical Group, Inclira Gandhi Centre for Atomic Research, Kalpakkam , India Matrix-isolation infrared spectra of trimethyl phosphate (TMP), triethyl phosphate (TEP), and tri-n-butyl phosphate (TBP), in argon and nitrogen matrices, are reported for the first time. The peak widths of the sharpest features in our matrix-isolated spectra are typically 2 cm-', compared with peak widths of 40 cm ~ seen in liquids for these compounds. Comparison with the vapor-phase spectrum of TMP reported earlier indicates that TMP is trapped in two different conformations in these matrices. Similar spectra were also obtained for TEP. Our matrixisolated spectra indicate that the intramolecular hydrogen bonding (which is believed to be responsible for the lowering of the P=O frequency in the C3~ conformer relative to the Cs conformer in these compounds) is stronger in TEP than in TMP. In the case of TBP, the peak widths were larger (8-10 cm -z) than those obtained for TMP and TEP. This observation is probably due to a distribution of conformers that may be trapped in the matrix, as a result of the increased alkyl chain length in TBP. Index Headings: Matrix-isolation; Infrared; FT-IR; Organic phosphates; Trimethyl phosphate; Triethyl phosphate; Tri-n-butyl phosphate; Conformations. INTRODUCTION Organic phosphates, particularly tri-n-butyl phosphate (TBP), find extensive applications as an extractant for actinides in nuclear fuel reprocessing.l They are generally used together with an organic diluent, such as dodecane, which tailors the physical properties, e.g., viscosity and density, of the organic phase (containing the phosphate and the diluent), to facilitate the solvent extraction process. However, the addition of the diluent also alters the extraction efficiency of the phosphate. It is not clearly understood exactly how the diluent affects the extraction behavior of the phosphate. No firm correlation between any of the physical properties of the diluent (such as dipole moment or dielectric constant) and the extraction behavior of the phosphate/diluent system has been established. 2 It is believed that the variation in the extraction properties of the phosphate/diluent system could result from the interaction of the phosphate and the diluent. 3-s A diluent that interacts strongly with the phosphoryl group of the phosphate leaves a lower concentration of the free extractant, resulting in a lower extraction efficiency of the phosphate/diluent system. However, a systematic correlation between the phosphate/diluent interaction and extraction efficiency has not yet been reported. We have therefore started a program to study the phosphate/diluent interactions, using matrix-isolation infrared spectroscopy. However, before taking up the work on the interactions, we thought it necessary to study the matrixisolation infrared spectroscopy of the pure organic phosphates (without the diluent). Furthermore, organo phosphorus compounds such as the phosphates and phosphonates are known to exist in Received 11 May 1993; revision received 9 August * Author to whom correspondence should be sent. different conformations. 6-1 These can be ideally studied with the use of matrix-isolation infrared spectroscopy. Hence we have taken up studies on the matrix-isolation infrared spectroscopy of organic phosphates, to be followed later with studies on the phosphate/diluent interaction. In this paper, we report, for the first time, the infrared spectra of matrix-isolated trimethyl phosphate, triethyl phosphate, and tri-n-butyl phosphate. EXPERIMENTAL The matrix-isolation experiments were carried out with the use ofa Leybold AG refrigerator-cooled cryostat, RD 210, which makes use of a closed-cycle helium compressor. The minimum temperature attainable with this system was 12 K at the cryotip, with a thermal stability better than 0.2 K. A KBr substrate was mounted on the cryotip, and the sample, together with an inert gas, was deposited on this substrate. The deposition rate was typically 3 to 5 mmol h -1, as measured with the use of a mass flow sensor (Brooks 5860). The duration of deposition ranged from 30 rain to 2 h. The temperature at the KBr substrate was monitored with a carbon resistor. The substrate could be heated to any desired temperature, with the use of a heater mounted on the cryostat. While the substrate was heating, its temperature was controlled with a Leybold- Heraeus temperature controller (Variotemp HR 1). The cryostat was housed in a vacuum system, pumped with an Edwards Diffstak MK2 series 100/300 diffusion pump, having a pumping speed of about 300 L s -t. This device was backed by a rotary pump (Hind Hivac ED 12), with a pumping speed of 200 L min -~. The base pressure in the vacuum system was 1 x 10-6 mbar, as measured by a Penning Gauge (Hind Hivac). Nitrogen and argon (IOLAR Speciality gases, Indian Oxygen Limited) were used as matrix gases. Though the levels of impurities were extremely low in these gases (e.g., moisture less than 4 ppm), they were still passed through a column of molecular sieves (13X). A Leybold- Heraeus precision leak valve regulated the flow of the matrix gases during the deposition. A Hastings vacuum gauge (Model EDNNV-800), incorporated in the gas line, was used to measure the pressure of the matrix gas. Trimethyl phosphate (Merck), triethyl phosphate (Tokyo Kasei), and tri-n-butyl phosphate (BDH) were all obtained commercially. All the phosphates were purified further by distillation under reduced pressure (0.03 mbar). Each sample was usually distilled at least three times before use, and treated with anhydrous sodium sulphate to remove any trace of moisture. The purified samples were then transferred to a glass sample container in an inert atmosphere glove bag, to prevent any moisture uptake by the sample. Before the sample was loaded, the empty sample container was degassed thoroughly in vacuum (10 6 mbar) for at least 24 h, to remove any moisture Volume 48, Number 1, /94/ /0 APPLIED SPECTROSCOPY Society for Applied Spectroscopy

2 W CA fe C V B J cm-1 FiG. 1. Infrared spectra oftmp: (A) liquid; (B) vapor; (C) in a nitrogen matrix (M:S ratio, I000:1); and (D) in an argon matrix (M:S ratio, 1000: 1). adsorbed to the walls of the glass container. To hasten this process, we intermittently heated the sample container using a hot air gun. After degassing, the sample container was stoppered with the use ofa greaseless high- vacuum glass stop-cock, and taken into the glove bag for sample transfer. After the sample was transferred to the sample container, it was connected to the vacuum system through homemade "Veeco-type" joints. The sample was then subjected to a number of freeze-thaw cycles before use. Such laborious techniques for sample handling were adopted because, otherwise, infrared bands due to OH impurities were seen in the spectra. Even after using all these procedures, we could only cause a significant reduction in the intensity of the OH bands, not remove them completely! The required matrix-to-sample ratio (M:S) was obtained by adjusting the temperature of the phosphates (and therefore its vapor pressure), by using slush baths of organic compounds. The temperatures of the slush baths were measured with the use of a platinum resistance thermometer. The matrix gas and the vapors of the organic phosphate streamed out of two separate nozzles (twin jet mode), where they mixed before being deposited on the KBr substrate. The tips of the nozzles were located at a distance of 28 mm from the KBr substrate. We also used a single-jet mode in some of our experiments, where the matrix gas was passed through the sample cell containing the phosphate, before it reached the nozzle. The spectra obtained by using both these deposition modes were basically identical. However, in the case of TBP, it was found that the single-jet mode yielded spectra with a better signal-to-noise ratio. Hence, for TBP alone, a single-jet mode was used, whereas the twin-jet mode was used for deposition of TMP and TEP. Spectra were recorded with a Digilab FTS 15/90 FT- IR instrument, at a resolution of 1 cm-'. Typically, 128 scans were coadded, to obtain good signal-to-noise ratio. After a spectrum was recorded, the matrix was warmed to 35 K, maintained for 15 min at this temperature, and then cooled back to 12 K. Spectra of the matrix, thus annealed, were then recorded. All matrix-isolation spectra shown in this work were those recorded after the matrix was annealed. All spectra were recorded over the region 4000 to 900 cm '; however, only the region 1330 to 950 cm -~, which encompasses the P=O and P-O-C vibrations and which is relevant for our discussions, has been displayed. Liquid (neat) spectra of the phosphates were recorded by taking a thin film of the sample between nse windows. Vapor-phase spectra were recorded with the use of a commercial gas cell, fitted with KBr windows. The liquid and vapor phase were also recorded with the use of the Digilab FTS 15/90 spectrometer at resolutions of 1 cm -~, for comparison with matrix-isolated spectra. RESULTS AND DISCUSSION Trimethyl Phosphate (TMP). The infrared matrix-isolation spectra of TMP, in argon and nitrogen, are shown in Fig. 1. In these experiments, the typical matrix-tosample ratio was 1000:1. Also shown in the figure are the liquid- and vapor-phase spectra, which agree well with those reported in the literature. 1 -'2 In the vapor phase spectrum, the two bands centered around 1316 and 1291 cm-' have been assigned by Herail to the ~,P=O corresponding to two rotational isomers. Herail assigned the 1316-cm-l band (peak a) to the conformer with the C3 8 Volume 48, Number 1, 1994

3 symmetry and the one at 1291 cm-~ (peak b) to the conformer with C3v symmetry. (The structures of the two conformers are shown in Fig. 2.) In our matrix-isolation experiments, using a nitrogen matrix, we observed two peaks corresponding to the P=O absorption: one at 1287 cm -~ (peak c) and the other near 1305 cm -~ (peaks d), which appears as a doublet. Since the vapor-phase composition of the conformers is expected to be retained in the matrix, the two peaks seen in our matrix-isolation spectrum can be assigned to the two conformers of TMP, seen in the vapor phase. The peak at 1287 cm -t can be assigned to the C3v conformer, and the doublet near 1305 cm-~ to the C3 conformer. As can be seen, the C3v conformer has a lower P=O frequency than the C3 conformer, both in the gas phase and in the matrix. Intramolecular hydrogen bonding between the alkyl hydrogens and the phosphoryl oxygen probably leads to a lowering of the P=O frequency in the case of the C3~ conformer. 13-~5 In the argon matrix, the peak near 1285 cm -t appears as a multiplet (peaks e), and the one near 1307 cm -~ as a doublet (peaks JO. This structure of the band was seen in all our spectra recorded at various matrix-to-sample ratios, with the ratios varying from 1000:1 to 30,000:1. This consideration rules out the possibility that these multiplets could be due to aggregation. On annealing the matrix, the relative intensity of the peaks in the multiplet structure changed, but the structure did not significantly simplify. We believe that this observation could be due to multiplet site splitting in the argon matrix, where the TMP could be trapped in different stable sites. In contrast, the spectra in the nitrogen matrix are much simpler, with the 1287-cm -~ peak appearing as a single sharp peak. As reported in several earlier studies, argon is known to display multiplet site splitting, whereas nitrogen generally yields spectra free from such splittings) 6-~9 The peak near 1305 cm -~, however, appears as a doublet in both matrices, probably due, again, to multiple site effects. It should be noted that the 1305-cm -~ doublet in the nitrogen matrix is shifted by about 4 cm-~ to the red, relative to that in the argon matrix. The (P)-O-C stretch of TMP appears near 1046 cm -~. From Fig. 1, it can be seen that this band is again simpler in the nitrogen matrix than in the argon matrix. This result may also be due to the site splitting effects in the argon matrix. Triethyl Phosphate (TEP). Figure 3 shows the infrared spectra of TEP in nitrogen and argon matrices. The liquidand vapor-phase spectra are also shown for comparison. Unlike in the case of TMP, it can be seen that the matrixisolated spectra of TEP in argon do not display multiplet site splitting effects. The P=O stretch for TEP occurs at 1271 cm-~ (peak a) in the nitrogen matrix. This is about 16 cm -~ lower than what is seen in the case of TMP (1287 cm-~). The P=O frequency is consistently lower in the case of TEP, when compared with TMP, in every single case--matrixisolated, liquid, and vapor. In the case of TMP (in a nitrogen matrix) the 1287-cm -~ peak was assigned to the C3v conformer. As already mentioned, this conformer gives rise to the possibility of intramolecular hydrogen bonds between the alkyl hydrogens and the phosphoryl oxygen. If a similar conformer in TEP is responsible for the peak at 1271 cm -~ (in the nitrogen matrix), then it becomes " C P 0 C FIG H The C3,. and the C3 conformers of TMP. possible to rationalize the lower P=O frequency in the case of TEP, compared with TMP. In the case of TMP, a five-membered ring may be formed, which results in the intramolecular hydrogen bonding, 2 as shown in Fig. 4. However, in the case oftep, a six-membered ring may be formed, z This formation brings the hydrogens closer to the phosphoryl oxygen in TEP than in TMP. The hydrogen bond would, therefore, be expected to be stronger in TEP than in TMP, and hence results in a lower P=O frequency in TEP. The 1271-cm -~ peak can therefore be assigned to the P=O frequency for a conformer of TEP similar to conformer I of TMP (with a C3v symmetry). In the vapor, the corresponding peak appears at 1279 cm- (peak d in Fig. 3). As with the 1305-cm -t doublet in TMP, we also observed a similar peak at 1305 cm -~ for TEP (peak b in Fig. 3). This peak actually appears as a doublet before annealing; however, on annealing it collapses to a singlet. In analogy with TMP, this peak can be assigned to a conformer of TEP, similar to conformer II of TMP (with the C3 symmetry). Since this conformer does not involve any intramolecular interactions, the P=O frequency can be expected to be left almost unaltered in both molecules, as observed. A third peak, a doublet near 1290 cm -~ (peak c in Fig. 3), has already been assigned to the CH2 twisting mode.t~ The (P)-O-C stretch for TEP occurs near 1042 cm -~. In the case of TEP, both nitrogen and argon matrices yield similar spectra, except for two differences. The peak widths, in the nitrogen matrix, are smaller than those obtained in the argon matrix. Second, the P=O frequency in the nitrogen matrix is red shifted by about 2 cm -~ relative to that in the argon matrix (1273 cm -~, peak e in Fig. 3). However, unlike the case of TMP, where the P=O absorption appeared as a multiplet in the argon matrix, in the case of TEP the P=O absorption (for the C3v conformer) is seen to be a singlet. Tri-n-Butyl Phosphate (TBP). Figure 5 shows the infrared spectra of TBP in nitrogen and argon matrices, together with that in the liquid phase. As already mentioned, the matrix-isolation spectra of TBP were recorded with the use of the single-jet mode, where the matrix gas was passed through the sample. In the case of TBP, it was found that, in spite of the use of high dilutions (M: S, 6500:1), the peak widths were never nearly as sharp as those obtained with TMP and TEP. The P=O peak, which appears near 1279 cm -~ (peak a), was about 8-10 cm-1 broad, in both nitrogen and argon matrices. Similar APPLIED SPECTROSCOPY 9

4 o H 0 oo "" H P c e D TMP TEP FIG. 4. Illustration of the five- and the six-membered ring formation in TMP and TEP, in the type I conformer (C3,.). W I-- I-- u3 I-- d a cn,, 1 Fro. 3. Infrared spectra oftep: (A) liquid; (B) vapor; (C) in a nitrogen matrix (M:S ratio, 1000:1); and (D) in an argon matrix (M:S ratio, 1000: 1). increases in the peak widths of the carbonyl bands of ketones and esters, with an increase in the alkyl chain length, have been reported by Coleman and Gordon in their matrix-isolation experiments. 2'-23 They have attributed this peak broadening to (1) nearest-neighbor interactions and/or (2) trapping of a distribution of conformers in the matrix, as a result of an increase in the alkyl chain length. We do not believe that nearest-neighbor interactions are the cause of broadening in our TBP spectra, C B since we did not see any improvement in the peak widths, even when the matrix-to-sample ratio was increased to 6500:1. It is possible that, with an increase in the alkyl chain length, the number of conformers trapped in the matrix also increases, in which the alkyl groups are oriented only slightly differently. This pattern is particularly accurate for long alkyl chains, where the rotation about the C-C bond can give rise to a distribution of conformers between the two extreme conformations--syn and anti. z4 These different conformations would be expected to alter the P=O frequencies only marginally. This factor would, therefore, give rise to a broad peak, in which the different P=O frequencies have not been resolved. It should be noted that the P=O frequency in the nitrogen matrix (1277 cm-') again occurs about 2 cm -' to the red of that seen in the argon matrix (1279 cm-'). The (P)-O-C stretch in TBP occurs near 1030 cm -~, and the peak width is again broader than what we had observed in the cases of TMP and TEP. In addition to these two peaks, we also observed a peak at 1239 cm -~ (peak b) in the matrix-isolated spectra of TBP. In the liquid spectra, a shoulder appears in this region (c). Earlier workers have observed this shoulder, 3,15 but no assignment has been made. This shoulder in the liquid spectrum is clearly resolved as a peak in the matrixisolated spectra. It is more prominent in the argon matrix than in the nitrogen matrix. Although we do not have a definitive assignment for this peak, it is our conjecture that this 1239-cm-' peak could be the P=O stretch of a conformer of TBP (Fig. 6) in which the hydrogens on the carbon atoms, C2 and C4 of the butyl group, are both involved in a hydrogen-bonding with the phosphoryl oxygen. In fact, in this process, a six-membered ring is formed, which, being stable, probably stabilizes this conformer. Since there are three butyl groups in TBP, this process leads to a total of six hydrogens participating in the hydrogen-bonding, resulting in a significant red-shift of the P=O frequency. Such a conformer is possible only if the alkyl chain is at least four carbon atoms long, as in the case of the butyl phosphate. Interestingly, such a peak was not observed in the case of TMP and TEP, where the alkyl chains are less than four carbon atoms long. In these lower phosphates, only a maximum of three hydrogens can be involved in the hydrogen bonding (Fig. 4). CONCLUSION Matrix-isolated infrared spectra have been recorded for TMP, TEP, and TBP in nitrogen and argon matrices. The 10 Volume 48, Number 1, 1994

5 a c, 1 FIG. 6. A conformation of TBP, where the orientation of one of the butyl groups around the phosphoryl oxygen is shown. For clarity, not all the atoms have been depicted. w V ~ a H / <[ c two compounds remains almost unaltered. This observation has been explained on the basis of intramolecular hydrogen bonding, which is stronger in TEP than in TMP. The matrix-isolated spectra of the TMP and TEP are clearly sharper and simpler in the nitrogen matrix than in the argon matrix. The peak width of the sharpest features is typically 2 cm -1. However, in the case of TBP the peaks are not as sharp. This result, we believe, is due to conformational effects. The 1239-cm -1 peak in TBP has been tentatively assigned to the P=O stretch of a conformer of TBP where a total of six hydrogens participate in hydrogen bonding with the phosphoryl oxygen. Table I lists the frequencies of the P=O and P-O-C vibrations of the three organic phosphates in liquid, vapor, and matrix-isolated conditions. These studies also clearly indicate that, for our future studies on phosphate/diluent interactions, TEP is a good choice among the phosphates, and specifically in a nitrogen matrix, since it yields sharper features under these experimental conditions cm-1 Fi~. 5. Infrared spectra of TBP: (A) liquid; (B) in a nitrogen matrix (M:S ratio, 1000:1); and (C) in an argon matrix (M:S ratio, 1000:1). two conformers of TMP seen by Herail in the vapor phase 12 have also been observed in our matrix-isolated spectra. TEP also displays similar spectra. However, it was observed that the P=O stretch of TEP occurs about 16 cm-' to the red of that observed for TMP, for the C3v conformer. However, the P=O stretch of the C3 conformer for the TABLE I. Frequencies (em ') and vibrational assignments for the organic phosphates in liquid, vapor, and matrix-isolated conditions. Matrix-isolated Mode Liquid Vapor Nitrogen Argon Trimethylphosphate P=O , , , P-O--C , , Triethylphosphate P=O , P-O-C , , 1045, Tri-n-butyl phosphate P=O (sh) a P-O-C " Note: sh = shoulder APPLIED SPECTROSCOPY 11

6 1. H.A.C. McKay, in Science and Technology of Tributyl Phosphate, Vol. I, W. W. Schulz and J. D. Navratil, Eds. (CRC Press, Boca Raton, Florida, 1984), Chap D. A. Orth, R. M. Wallace, and D. G. Karraker, in Science and Technology of Tributyl Phosphate, Vol. 1, W. W. Schulz and J. D. Navratil, Eds. (CRC Press, Boca Raton, Florida, 1984), Chap L. L. Burger, in Science and Technology of Tributyl Phosphate, Vol. I, W. W. Schulz and J. D. Navratil, Eds. (CRC Press, Boca Raton, Florida, 1984), Chap P. M. Petkovich and B. A. Kezele, Proc. Int. Solvent. Extraction Conf., 1971, Vol. 2, (Soc. Chem. Ind., London), J. R. Ferraro, Appl. Spectrosc. 17, 12 (1963). 6. O. A. Raevsky, J. Mol. Struct. 19, 275 (1973). 7. F. Herail, C. R. Acad. 261, 3375 (1965). 8. J. R. Durig, J. Mol. Struct. 113, 127 (1984). 9. B. J. Van der Veken and M. A. Herman, J. Mol. Struct. 42, 161 (1977). 10. R. A. Nyquist and W. J. Potts, in Analytical Chemistry of Phosphorus Compounds, M. Halmann, Ed. (Wiley Interscience, New York, 1972), Chap F. S. Mortimer, Spectrochim. Acta 9, 270 (1957). 12. F. Herail, J. Chim. Phys. 68, ). 13. E. M. Popov, M. I. Kabachnik, and L. S. Mayants, Russian Chem. Revs. 30, 362 (1961). 14. L. S. Mayants, E. M. Popov, and M. I. Kabachnik, Opt. Spectrosc. 7, 108 (1959). 15. D. Dyrssen and Dj. Petkovic, J. Inorg. Nucl. Chem. 27, 1381 (1965). 16. E. L. Wehry and G. Mamantov, Prog. Anal. Spectrosc. 10, 507 (1987). 17. A. Givan, A. Lowenschuss, K. D. Bier, and H. J. Jodl, Chem. Phys. 106, 151 (1986). 18. G. Mamantov, A. A. Garrison, and E. L. Wehry, Appl. Spectrosc. 36, 339 (1982). 19. S. Li and Y.-S. Li, Spectrochim. Acta 47A, 201 (1991). 20. K. Ohwada, Appl. Spectrosc. 21, 332 (1967). 21. W. M. Coleman III and B. M. Gordon, Appl. Spectrosc. 41, 1163 (1987). 22. W. M. Coleman III and B. M. Gordon, Appl. Spectrosc. 42, 101 (1988). 23. W. M. Coleman III and B. M. Gordon, Appl. Spectrosc. 42, 666 (1988). 24. J. March, Advanced Organic Chemistry." Reactions, Mechanisms, and Structure (Wiley Eastern Limited, New Delhi, 1987), 3rd ed. 25. K. Nukada, K. Naito, and U. Maeda, Bull. Chem. Soc. Japan 33, 894 (1960). 12 Volume 48, Number 1, 1994