SCIENCE-17

Matrix Isolation Infrared Spectroscopy: A Window to Understanding the Chemistry of Solvent Extraction n EXECUTIVE SUMMARY

Matrix isolation infrared spectroscopy has been used, to study molecular conformations and weak intermolecular interactions. Using this technique, which was essentially a home-built set up, we have studied the conformations of organic phosphates, such as trimethyl phosphate, triethyl phosphate and tri-n-butyl phosphate. We also showed, for the first time, that the use of a supersonic jet source together with matrix isolation is a powerful tool in studies on molecular conformations. Together with experiments, we also perform ab initio computations to corroborate our experiments. As a result of our work, we have been to able to unravel the conformational picture in these molecules unambiguously. Results of this work have provided new insights in understanding some of the chemical phenomena observed in fuel reprocessing. n OUTLINE

We have used the technique of matrix isolation infrared spectroscopy to study the conformations of organic phosphates. The first member in this series is trimethyl phosphate (TMP), whose conformations were uncertain when we undertook the study. Our matrix isolation studies clearly showed that this molecule existed in two conformational states. This result was corroborated byab initio computations, which

indicated the two structures to have a C31 and C symmetry, which are shown in the Figure 1 as conformers 'U' and 'K' respectively. While the two conformers were identified, experimental evidence was still lacking to determine the ground state conformer. This evidence was K unambiguously provided in our experiments using a supersonic jet O C source coupled to the matrix isolation spectrometer, which showed the P ground state conformer to have the 'U' structure. Our work was the O O first to show the utility of the supersonic jet source to determine the C O energy ordering of conformational states in molecules. C While our work led to an understanding of the conformations of TMP, B we quickly realized that experiments and computations on larger phosphates such as triethylphosphate (TEP) and tri-n-butylphosphate O (TBP) would not be easy, given the large number of conformational C T P C states that these molecules can possibly have. It was realized that it O O was not sufficient to determine molecular conformations, but also to C O understand basic factors that determine the conformational stability of A U molecules. Such an understanding would help us narrow our search for conformations in molecules with a large number of possible structures. Towards this end, we studied model compounds such methoxymethanes and silanes, which have a similar backbone structure as the phosphates. These studies clearly highlighted the role of anomeric interactions in deciding the conformational stability of

these molecules. The anomeric interactions basically arise from the 1330 1280 1230 delocalization of the non-bonding electrons on the electronegative cm-1 atoms into the adjacent antibonding orbitals. These interactions direct the molecule to adopt certain structures, which would facilitate the anomeric delocalizations. Fig. 1 : A comparison of the infrared spectra of liquid TMP With the lessons learnt from these studies, it became possible to (Trace A) with that obtained for TMP isolated in a N matrix (Trace B). Features of the two determine the conformations of TEP. These studies also highlighted 2 conformers, whose structures are shown, are the role of theπ -electrons of the phosphoryl group in the organic clearly evident in the matrix isolation spectra phosphates in determining the conformations of these molecules. This has far reaching implications in reprocessing, as metal complexation is likely to alter theπ -electron density in the P=O group, which in turn can alter the conformational stabilities in the phosphates. This work is expected to lead to an understanding if this alteration of conformational stabilities in the phosphates has a bearing on the solvent extraction process.

n ADDITIONAL INFORMATION

We have also devised alternate sampling methods for matrix isolation, such as laser ablation, which allows for the study of high temperature chemistry. The matrix isolation technique can also be used in conjunction with other detection schemes such as fluorescence spectroscopy and electron spin resonance. However, since our interests are in studying molecular structure and bonding, we have used Fourier transform infrared spectroscopy to probe the trapped molecules. We have also used matrix isolation spectroscopy to study hydrogen bonded systems and van der Waals interactions, again supported byab initio computations.

34 SCIENCE-17

n WHAT IS MATRIX ISOLATION SPECTROSCOPY?

Infrared spectroscopy is an extremely powerful tool to unravel molecular structure and bonding information. However, the linewidths of infrared spectral features of solid and liquid samples are far too large to provide unambiguous information. MATRIX ISOLATION IR SET UP Matrix isolation provides a means to narrow down the spectral profiles. In this technique, the molecules of interest are trapped in a solid inert CRYOSTAT

gas matrix, such as Ar or N2 , at temperatures of ~12 K. Furthermore, the concentration of the sample is kept sufficiently low (inert gas: sample=1000:1) to ensure that the molecules of interest are surrounded only by the inert gas FTIR atoms/molecules, resulting in molecular isolation. EFFUSIVE NOZZLE At the low temperatures and in the confines of a rigid solid cage, the isolated molecules are devoid A of collisions, rotations and spectral congestion, which results in linewidths that are almost an order of magnitude smaller than that observed in the usual condensed phase sampling methods, as MIXING can be seen in Fig. 1. CHAMBER It is this spectral sharpening that allows one to study molecular conformations, as features due to the different conformations can now be resolved. The small linewidths also lets one to study observe small perturbations in the infrared lines, which usually result from weak intermolecular interactions; thereby opening up the possibility of studying hydrogen bonded and van der Waals interactions. Fig. 2 : Matrix isolation spectrometer, where the cryostat, FTIR, deposition nozzle and the mixing chamber are shown Fig. 2 shows the matrix isolation set up, consisting of the cryostat, FTIR and the nozzle for deposition. The mixing chamber is used to prepare the sample mixture in the inert gas, of the desired ratio. This equipment is essentially a home-built set up. n BRIEF DESCRIPTION OF THEORETICAL BACKGROUND

Conformations of molecules and weak intermolecular interactions have been known to influence chemical processes, such as physico-chemical properties and reactivities of molecules. A number of interesting phenomena in fuel reprocessing such as the influence of seemingly inert diluents on the extraction process and third phase formation are likely to be influenced by the molecular conformations of the extractants and the weak interactions between the diluent and the extractants. However, such studies call for specifically designed experiments, which is achieved using the technique of matrix isolation infrared spectroscopy. These studies have also important implications in the study of biological molecules, where conformations are known to play an important role in many biological phenomena. n ACHIEVEMENT The conformations of organic phosphates, have been studied, both experimentally and computationally, leading to a better understanding of the structure of these molecules. Factors deciding conformational stability have also been examined. As a result of these studies, important insights into the role of conformations in solvent extraction have been obtained. Studies of weak intermolecular interactions such as hydrogen bonded and van der Waals interactions have also been done. n PUBLICATIONS ARISING OUT OF THIS STUDY AND RELATED WORK

1. K. Sankaran, V. Venkatesan, K. Sundararajan and K. S. Viswanathan,J. Ind. Inst. Sci., 85 (2005)403 . 2. V. Kavitha, K. Sankaran and K. S. Viswanathan,J. Mol. Struct .791 (2006)165 .

Further inquiries: Dr. K. S. Viswanathan, Materials Chemistry Division Chemistry Group, IGCAR, e-mail: [email protected]

35