Article pubs.acs.org/JPCA
Conformational Landscape of Tri‑n‑butyl Phosphate: Matrix Isolation Infrared Spectroscopy and Systematic Computational Analysis N. Ramanathan,† K. Sundararajan,*,† and K. S. Viswanathan*,‡ †
Materials Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India ‡ Department of Chemical Sciences, Indian Institute of Science Education & Research, Sector 81, Mohali 140306, Punjab, India S Supporting Information *
ABSTRACT: The conformations of tri-n-butyl phosphate (TBP) were studied using matrix isolation infrared spectroscopy and density functional theory (DFT) calculations. TBP was trapped in a N2 matrix using both effusive and supersonic sources, and its infrared spectra were recorded. The computational exploration of TBP is a very demanding problem to confront, due to the presence of a large multitude of conformations in TBP. To simplify the problem, computations were done on model compounds, dimethyl butyl phosphate (DMBP) and dibutyl methyl phosphate (DBMP), to systematically arrive at the conformations of TBP that are expected to contribute to its chemistry at room temperature. Some predictive rules seem to simplify this complex conformational landscape problem. The predictive rules that were formulated enabled us to search the relevant portion of the conformational topography of this molecule. The computations were performed at the B3LYP level of theory using the 6-31++G(d,p) basis set. Vibrational wavenumber calculations were also performed for the various conformers to assign the infrared features of TBP, trapped in solid N2 matrix. recovery of uranium and plutonium from fission products. For simultaneous large scale recovery of plutonium in the spent fuel, initially, extractants like methyl isobutyl ketone (MIBK) and dibutylcarbitol (DBC) were used.22 These early extractants have now been replaced by TBP, diluted in an aliphatic hydrocarbon or a mixture of hydrocarbons, following the discovery by Warf.23 Today, TBP has become the work horse of nuclear reprocessing industry because it has all the desirable properties of an extractant.24 Third phase formation is an interesting though annoying complication encountered in solvent extraction processes in nuclear industry. The term “third phase formation” in solvent extraction refers to a phenomenon in which the organic phase splits into two phases.25−27 One of the two phases is diluent rich, whereas the other is rich in extractant and also contains the metal solvate. TBP forms a third phase during the extraction of tetravalent metal ions such as Pu(IV) and Th(IV). Major factors influencing the third phase formation are organic/aqueous phase compositions, nature of diluents, temperature, etc. Generally, it is believed that the third phase is formed due to the incompatibility of polar metal-solvate with nonpolar
1. INTRODUCTION The conformational analysis of tri-n-butyl phosphate (TBP) used as an extractant in nuclear fuel reprocessing,1,2 is a complex problem, because TBP possesses 12 carbon atoms, which results in a large number of conformations at a given temperature. It is therefore imperative to screen out conformers that are expected to have large energies, relative to the ground state conformer. On the basis of our earlier work on trimethyl phosphate (TMP),3−6 triethyl phosphate (TEP),7,8 acetals/ ketals,9−14 and silanes15−18 (all of which have a structural similarity with TBP), it has now become clear that a combination of hyperconjugative (the effect arises due to the delocalization interactions) and steric interactions decide the conformational preferences in these molecules. The study of trimethyl phosphite (TMPhite) which lacks a PO group, has highlighted the importance of the PO group in conformational preferences.19 In the back end of the nuclear fuel cycle, liquid−liquid extraction is the standard separation method adapted for the separation of metals from spent fuel.20,21 Furthermore, solvent extraction is the preferred method for the purification of natural uranium, separation of, zirconium from hafnium, thorium from rare earths and fissile, and fertile and fission products from spent reactor fuel. The continuous separation scheme of spent nuclear fuels through solvent extraction is unique to reprocessing because of the requirement of nearly quantitative © XXXX American Chemical Society
Received: May 24, 2017 Revised: July 19, 2017 Published: July 19, 2017 A
DOI: 10.1021/acs.jpca.7b05006 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Choi and Tedder by studying the TBP-diluent mixtures using nuclear magnetic resonance spectroscopy have reported the occurrence of both solvation and aggregation in TBP mixtures.31 Baaden et al. have reported molecular dynamics simulation of the uranyl extraction by TBP and have addressed questions concerning the extent and rate of phase separation, the distribution of TBP, and the possible complexation of uranyl by TBP.32 In another study, Ye et al. have carried out the atomistic simulations for a multicomponent two-phase system (aqueous and organic phases) to investigate the interfacial molecular mechanisms leading to uranyl extraction from the aqueous to organic phase.33 Motokawa et al. have reported the mechanism of third phase formation in biphasic liquid−liquid solvent extraction of heavy metal ions.29 Recently, Baldwin et al. have proposed the sizes of TBP aggregates containing varying concentrations of uranium or zirconium and HNO3 in an n-dodecane diluent that were obtained using diffusion NMR spectroscopy.34 A few molecular dynamics simulation studies also have been reported on TBP system to understand its aggregation behavior in bulk.35−37 None of the above work specifically addressed the issue of multiple conformations of TBP. Because the rich conformational landscape of TBP is present due to the rotational degrees of freedom of the butyl group, it is first essential to understand the conformational space of the TBP molecule as the macroscopic properties of molecules are derived only from the molecular level. Gaining insight at the conformational level for TBP in the isolated gas phase is a prerequisite before exploring it to the condensed phase. The analysis of the different conformations of TBP can eventually be expected to throw light on the bulk phase properties of TBP, such as mass density, self-diffusion coefficient, dipole moment, complexing ability with the metal ion, and so on. Hence, the reason for studying the conformations of TBP is manifold. Matrix isolation infrared spectroscopy was used as a tool to experimentally study the conformations of TBP. The DFT calculations were also used to arrive at the various possible conformations of TBP and also to corroborate our experimental results.
hydrocarbon diluents but the exact reason for the third phase formation is not clearly known. It is likely that the third phase formation results from the molecular conformations of TBP.28,29 TBP has a large multitude of conformations with varying dipole moments that probably decide its chemistry at a given temperature but the conformational analysis of TBP is challenging. Computations on the conformations of TMP, the lower homologue of TBP are comparatively a simple problem. The three carbons attached to oxygen in TMP can grossly adopt Gauche(+), Gauche(−), and Trans orientations with respect to the PO group of TMP. The resultant number of conformational possibilities would be 27, of which only a few would correspond to unique minima. Computations identified three minima for TMP, corresponding to conformers with C3(G±G±G±), C1(TG±G±), and Cs(TG+G−) structures, given in order of increasing energy.4,5 Taking the degeneracy into consideration, these three conformers account for 11 of the 27 possible orientations. At room temperature, only C3(G±G±G±) and C1(TG±G±) conformers were found to be important. The Cs(TG+G−) conformer with an energy of 2.1 kcal/mol with respect to the ground state C3(G±G±G±) conformer had a negligible population at room temperature. The preference for the Gauche orientation in the different conformers of TMP was attributed to the operation of both geminal and vicinal delocalization (hyperconjugative) interactions.19 The next higher member of the series, TEP has an additional carbon. On the basis of the earlier computational studies on TEP, it was concluded that while the carbons attached to oxygen prefer a Gauche orientation due to the hyperconjugative effect, the terminal carbons prefer a trans orientation due to the steric effect.8 The higher energy conformers of TEP resulted as the orientation of the hyperconjugative carbons was progressively converted to Trans from their preferred Gauche orientation. Likewise, the energies of the conformers were also raised as the terminal nonhyperconjugative carbons were converted to gauche from their preferred trans orientation. It can be noted that the terminology, Gauche/Trans and gauche/ trans are used to denote the hyperconjugative and nonhyperconjugative carbon atoms, respectively. This predictive scheme greatly simplified the search for the conformers in TEP. On the basis of the predictive scheme, it was concluded that the structure of the ground state conformation would be G±G±G±(ttt). The energy of the conformers increased by moving progressively from the ground state G±G±G±(ttt) conformation to one in which the Gauche was converted to Trans or trans converted to gauche. The problem of TBP is complex, because TBP has a longer alkyl chain than TMP or TEP, and hence systematic and meticulous conformational examination is necessary to clearly unravel the conformations of TBP. Nevertheless, numerous experimental studies were reported in the literature by exploring the fundamental properties of TBP as an extractant; theoretical studies on TBP are sparse. Beudaert et al. have studied, using molecular dynamics simulation, the degree of the self-association mechanism of TBP in a vacuum, in water and chloroform, and in the water/ chloroform interface.28 Their analysis was restricted only to six conformers of TBP where they fixed only the carbons attached to oxygen in either Gauche or Trans orientation. A force field comparative study of TBP for predicting its thermophysical properties was reported.30 Specifically, in this work, the TBP bulk liquid behavior using all-atom models was characterized.
2. EXPERIMENTAL DETAILS TBP (MERCK make with purity ≥99%) was used without any further purification. However, the sample was subjected to several freeze−pump−thaw cycles before use. Di-n-butyl phosphate (DBP;
