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Matrix-Isolation Infrared Spectroscopy of Organic Phosphates

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Matrix-Isolation Infrared Spectroscopy of Organic Phosphates 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 603 102, India Matrix-isolation infrared spectra of trimethyl phosphate (TMP), triethyl different conformations.6-1° These can be ideally studied phosphate (TEP), and tri-n-butyl phosphate (TBP), in argon and nitro- with the use of matrix-isolation infrared spectroscopy. gen matrices, are reported for the first time. The peak widths of the Hence we have taken up studies on the matrix-isolation sharpest features in our matrix-isolated spectra are typically 2 cm-', infrared spectroscopy of organic phosphates, to be fol- compared with peak widths of 40 cm ~ seen in liquids for these com- pounds. Comparison with the vapor-phase spectrum of TMP reported lowed later with studies on the phosphate/diluent inter- earlier indicates that TMP is trapped in two different conformations in action. In this paper, we report, for the first time, the these matrices. Similar spectra were also obtained for TEP. Our matrix- infrared spectra of matrix-isolated trimethyl phosphate, isolated spectra indicate that the intramolecular hydrogen bonding (which triethyl phosphate, and tri-n-butyl phosphate. 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 EXPERIMENTAL 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 obser- The matrix-isolation experiments were carried out with vation is probably due to a distribution of conformers that may be trapped the use ofa Leybold AG refrigerator-cooled cryostat, RD in the matrix, as a result of the increased alkyl chain length in TBP. 210, which makes use of a closed-cycle helium compres- Index Headings: Matrix-isolation; Infrared; FT-IR; Organic phos- sor. The minimum temperature attainable with this sys- phates; Trimethyl phosphate; Triethyl phosphate; Tri-n-butyl phos- tem was 12 K at the cryotip, with a thermal stability better phate; Conformations. 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 INTRODUCTION 5 mmol h -1, as measured with the use of a mass flow sensor (Brooks 5860). The duration of deposition ranged Organic phosphates, particularly tri-n-butyl phosphate from 30 rain to 2 h. The temperature at the KBr substrate (TBP), find extensive applications as an extractant for was monitored with a carbon resistor. The substrate could actinides in nuclear fuel reprocessing.l They are generally be heated to any desired temperature, with the use of a used together with an organic diluent, such as dodecane, heater mounted on the cryostat. While the substrate was which tailors the physical properties, e.g., viscosity and heating, its temperature was controlled with a Leybold- density, of the organic phase (containing the phosphate Heraeus temperature controller (Variotemp HR 1). and the diluent), to facilitate the solvent extraction pro- The cryostat was housed in a vacuum system, pumped cess. However, the addition of the diluent also alters the with an Edwards Diffstak MK2 series 100/300 diffusion extraction efficiency of the phosphate. It is not clearly pump, having a pumping speed of about 300 L s -t. This understood exactly how the diluent affects the extraction device was backed by a rotary pump (Hind Hivac ED 12), behavior of the phosphate. No firm correlation between with a pumping speed of 200 L min -~. The base pressure any of the physical properties of the diluent (such as dipole in the vacuum system was 1 x 10 -6 mbar, as measured moment or dielectric constant) and the extraction behav- by a Penning Gauge (Hind Hivac). ior of the phosphate/diluent system has been established. 2 Nitrogen and argon (IOLAR Speciality gases, Indian It is believed that the variation in the extraction prop- Oxygen Limited) were used as matrix gases. Though the erties of the phosphate/diluent system could result from levels of impurities were extremely low in these gases the interaction of the phosphate and the diluent. 3-s A (e.g., moisture less than 4 ppm), they were still passed diluent that interacts strongly with the phosphoryl group through a column of molecular sieves (13X). A Leybold- of the phosphate leaves a lower concentration of the free Heraeus precision leak valve regulated the flow of the extractant, resulting in a lower extraction efficiency of the matrix gases during the deposition. A Hastings vacuum phosphate/diluent system. However, a systematic cor- gauge (Model EDNNV-800), incorporated in the gas line, relation between the phosphate/diluent interaction and was used to measure the pressure of the matrix gas. extraction efficiency has not yet been reported. We have Trimethyl phosphate (Merck), triethyl phosphate (To- therefore started a program to study the phosphate/dil- kyo Kasei), and tri-n-butyl phosphate (BDH) were all uent interactions, using matrix-isolation infrared spec- obtained commercially. All the phosphates were purified troscopy. However, before taking up the work on the further by distillation under reduced pressure (0.03 mbar). interactions, we thought it necessary to study the matrix- Each sample was usually distilled at least three times isolation infrared spectroscopy of the pure organic phos- before use, and treated with anhydrous sodium sulphate phates (without the diluent). to remove any trace of moisture. The purified samples Furthermore, organo phosphorus compounds such as were then transferred to a glass sample container in an the phosphates and phosphonates are known to exist in inert atmosphere glove bag, to prevent any moisture up- take by the sample. Before the sample was loaded, the Received 11 May 1993; revision received 9 August 1993. empty sample container was degassed thoroughly in vac- * Author to whom correspondence should be sent. uum (10 6 mbar) for at least 24 h, to remove any moisture Volume 48, Number 1, 1994 0003-7028/94/4801-000752.00/0 APPLIED SPECTROSCOPY 7 © 1994 Society for Applied Spectroscopy 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 fe these procedures, we could only cause a significant re- duction in the intensity of the OH bands, not remove them completely! The required matrix-to-sample ratio (M:S) was ob- tained 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 or- ganic phosphate streamed out of two separate nozzles (twin jet mode), where they mixed before being deposited C on the KBr substrate. The tips of the nozzles were located at a distance of 28 mm from the KBr substrate. We also W used a single-jet mode in some of our experiments, where CA Z the matrix gas was passed through the sample cell con- taining 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 Z B 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 J 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 spec- tra shown in this work were those recorded after the ma- trix 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 ZnSe win- dows. Vapor-phase spectra were recorded with the use of a commercial gas cell, fitted with KBr windows. The liq- uid and vapor phase were also recorded with the use of V the Digilab FTS 15/90 spectrometer at resolutions of 1 cm -~, for comparison with matrix-isolated spectra. 1330 950 RESULTS AND DISCUSSION Trimethyl Phosphate (TMP). The infrared matrix-iso- cm-1 lation spectra of TMP, in argon and nitrogen, are shown FiG. 1. Infraredspectra ofTMP: (A) liquid; (B) vapor; (C) in a nitrogen in Fig. 1. In these experiments, the typical matrix-to- matrix (M:S ratio, I000:1); and (D) in an argon matrix (M:S ratio, 1000: sample ratio was 1000:1. Also shown in the figure are the 1). liquid- and vapor-phase spectra, which agree well with those reported in the literature. 1°-'2 In the vapor phase adsorbed to the walls of the glass container. To hasten spectrum, the two bands centered around 1316 and 1291 this process, we intermittently heated the sample con- cm-' have been assigned by Herail to the ~,P=O corre- tainer using a hot air gun.
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