In-Situ Raman Spectroscopy Studies of Room-Temperature and Hydrothermal Reactions
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In-Situ Raman Spectroscopy Studies of Room-Temperature and Hydrothermal Reactions Brendan T. McGrail1, Laurent J. Jouffret1, Eric M. Villa1, and Peter C. Burns1 Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN, 46556, USA
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ABSTRACT By contracting with the Parr Instrument Company and Bruker Optical Systems, we have developed a system for continuous monitoring of hydrothermal and room temperature reactions by Raman spectroscopy. Using the uranyl peroxide cage cluster {[UO2(O2)(OH)]16[UO2(O2)2]4}24- (denoted {U20R}) and a coordination polymer made from uranyl ions and 4,4’-biphenyldicarboxylate as model systems, we demonstrate the spectroscopically observable changes associated with reaction progress and crystallization. INTRODUCTION The formation mechanisms of complex inorganic compounds, such as polyoxometalates (POMs), binary and ternary metal oxides and sulfides, zeolites, and metal-organic frameworks (MOFs) remain poorly understood despite their ubiquity and importance to both technology and fundamental science. Mechanistic understanding coupled with structure-property and structurereactivity relationships underlie the ability of organic, organometallic, and polymer chemists to “tailor-make” molecules for specific applications in high yield with little waste. The largest impediment to understanding the progress of reactions of interest in allinorganic synthesis is the lack of in situ methods capable of replicating the success of NMR methods in the study of organic and organometallic reaction mechanisms. In situ synchrotron Xray diffraction has been used by Cahill, [1, 2] O’Hare, [3-6] and others [7, 8] to study the crystallization process in some reactions, and Kanatzidis has applied high temperature 31P MASNMR to the synthesis of chalcophosphates, [9] but synchrotron X-ray diffraction and high temperature NMR require specialized instrumentation and facilities at a minimum, and, in the case of NMR methods, a fully diamagnetic system with a nucleus of interest with nonzero spin, etc. To overcome these limitations, we have, by contracting with the Parr Instrument Company and Bruker Optical Systems, developed a system for the use of vibrational spectroscopy for the in situ monitoring of hydrothermal and room temperature reactions (the system is shown schematically in Figure 1). Vibrational spectra are well suited to following inorganic reactions for a variety of reasons: a simple experimental setup, a large pre-existing body of literature on the positions of vibrational bands for a variety of functional groups, [10-16] and sensitivity to all bonds, regardless of the phase of the material. Furthermore, the large number of synthetic uranyl peroxide cage clusters [17-26] and extended solids[27-35] containing uranium enable us to build an extensive library of correlations between crystallographically determined bond lengths and angles and the positions of Raman bands corresponding to vibrational modes of those bonds. In addition, the experimental configuration allows fo
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