Influence of Branching on the Conformational Space: Case Study of

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

The Influence of Branching on the Conformational Space: A Case Study of Tri-secondary-Butyl Phosphate Using Matrix Isolation Infrared Spectroscopy and DFT Computations Nagarajan Ramanathan, Shubhra Sarkar, Kalyanasundaram Sundararajan, Aditi Chandrasekar, Kannan Sankaran, and Ammath Suresh J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b08157 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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The Journal of Physical Chemistry

The Influence of Branching on the Conformational Space: A Case Study of Tri-secondary-Butyl Phosphate using Matrix Isolation Infrared Spectroscopy and DFT Computations N. Ramanathan,* Shubhra Sarkar, K. Sundararajan,* Aditi Chandrasekar, K. Sankaran and A. Suresh Materials Chemistry & Metal Fuel Cycle Group, Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India.

ABSTRACT The conformational analysis of long chain phosphates poses a serious challenge due to the presence of rotationally flexible multiple alkyl groups. Tri-secondary-butyl phosphate (TsBP) is an interesting example, in which branching can be expected to influence the conformational landscape. To solve the conformational problem of TsBP systematically, the conformations of model dimethyl-secondary-butyl phosphate (DMsBP), a molecule possessing a single secondary butyl strand was analyzed. Based on the analysis of the energy profile of DMsBP, a few conformational bunches were eliminated. The presence of branched methyl group appears to completely influence the conformational space of TsBP and as a result, the number of conformations is drastically reduced in comparison to its structural isomer, tri-n-butyl phosphate (TBP). B3LYP level of theory in association with 6-311++G(d,p) basis set was used for computing all the conformer geometries. Experimentally, the conformations of TsBP were studied using infrared spectroscopy by trapping the molecule in N2 and Ar matrixes at low temperatures, which were correlated well with the computational results.

Corresponding authors: [email protected] (N. Ramanathan) [email protected] (K. Sundararajan)

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Introduction Tri-secondary-butyl phosphate (TsBP), an isomer of tri-n-butyl-phosphate (TBP) is a fascinating molecule in the series of tri-alkyl-phosphates as it has been extensively investigated for nuclear reprocessing applications.1-3 Nevertheless, TBP is a proven extractant in PUREX (for separation of uranium and plutonium from fission products present in the spent fuel) and THOREX (for separation of uranium and thorium) processes, the third phase formation in the extraction of tetravalent metal ions is a major limitation with TBP. Earlier studies revealed that TsBP is a potential candidate in THOREX process.3 The branching in the butyl chains in TsBP is expected to have a profound influence on the extraction characteristics to separate Th(IV) from U(VI). A systematic study has been carried out earlier on the extraction characteristics of TsBP by analyzing its physiochemical properties, with a primary focus on uranium-thorium separation.4,5 Third phase formation is a phenomenon that arises in solvent extraction under certain experimental conditions.6-8 TsBP was studied in a greater detail for its third phase formation behavior.4,5 Though third phase formation is ubiquitous with TBP, TsBP and tri-iso-butyl phosphate (TiBP) , the limiting organic concentration for third phase formation in the extraction of Th(IV) by TsBP is reported to be the highest among these phosphates. The efficiency of separation of U(VI) from Th(IV) with 1.1 M TsBP/dodecane is much better than TBP under identical conditions. Quite recently, third phase formation of TsBP system with thorium was analyzed during solvent extraction to establish a correlation between density of organic phase with thorium concentration.9 A comparison of TsBP with TBP and TiBP was also accomplished. Furthermore, the aggregation behavior of TBP/TsBP/TiBP loaded with Th(IV) was investigated by small angle neutron scattering (SANS) technique.10,11 The results indicated that among the three phosphates, TBP has higher tendency to undergo aggregation as compared to TiBP and TsBP and the third phase formation has the same trend as that of aggregation. Based on these studies, it was concluded that as far as third phase formation is concerned, TsBP is superior to the other two phosphates, investigated. Hence, the branching in the alkyl groups in TsBP has a definite role to influence third phase formation. The structure-property correlation assumes significance, as understanding at the molecular level in deriving the properties in bulk becomes indispensable. Earlier in our laboratory, a variety of systems such as acetals-ketals,12-17 silanes18-21 and carbonates22,23 were studied for conformations using matrix isolation infrared spectroscopy. These studies highlighted 2 ACS Paragon Plus Environment

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that the conformations of carbons attached to oxygens are driven through hyperconjugative charge transfer interaction and steric factor decides the conformations of rest of the carbons. In the ground state, ‘Gauche’ orientation is preferred over ‘Trans’ and rest of the carbons adopts ‘trans’ orientation due to steric effect (The notations ‘Gauche’/‘Trans’ and ‘gauche’/‘trans’ are used to describe hyperconjugative and non-hyperconjugative carbon atoms respectively). The studies on smaller phosphates (such as trimethyl phosphate (TMP)24-26, triethyl phosphate (TEP)27,28 and triallyl phosphate (TAP)29) were carried to verify this prediction. The studies on trimethyl phosphite highlighted the role played by the phosphoryl group in conformational preferences.30 It was finally extended to TBP; a tough problem to confront due to the presence of conformationally flexible twelve carbon atoms. Through the study of these model acetals/ketals/carbonates and phosphates, the conformational problem of TBP was systematically solved.31 Solving the conformational space of TsBP, a branched isomer of TBP seems to be an equally tough problem similar to TBP. It will therefore be intriguing to study as to what difference branching actually makes in comparison to its straight chain isomer. Moreover, the study at the molecular level is imperative in extrapolating the properties to bulk such as the extraction behaviour and the third phase problem encountered during solvent extraction. The conformational analysis of a few complicated and rotationally flexible molecules was studied earlier. An extensive study of conformations of 1,4-,1,3- and 1,2-butanediols with a large number of conformational minima (both global and local) using matrix isolation infrared spectroscopy and ab initio calculations was reported by Fausto, Reva and co-workers.32-34 Boeckx et al. have studied N-acetylcysteine using matrix isolation infrared spectroscopy.35 Initially, computations predicted 438 conformations for N-acetylcysteine and on the basis of energy profile; these conformations were simplified to five minima, which were experimentally demonstrated at low temperatures. Quite recently, Dubey et al. have performed a detailed study on L-threonine amino acid both computationally and experimentally and it was concluded that a definite pattern of structures are being adopted by amino acids.36 Apart from matrix isolation, the conformational landscape of phenylalanine in the gas phase was also reported using ion-dip spectroscopy.37 Following our earlier work on straight chain TBP,31 to highlight the consequence of branching on the conformational landscape, in this work, the conformations of TsBP were studied through matrix isolation infrared spectroscopy. Density Functional Theory (DFT) methodology was adopted to unravel all possible conformations. Natural Bond Orbital (NBO) 3 ACS Paragon Plus Environment


analysis was performed on TsBP to elucidate the role of stereo electronic factors in controlling the conformational preferences. The complexation studies of uranium and thorium with TsBP were carried out with particular emphasis on third phase and the lean diluent phase. A comparison with TBP was also accomplished. Experimental and Computational Details TsBP used in this work was synthesized in our laboratory and the details of the synthesis are described elsewhere.9 Before performing the experiments, freeze-pump-thaw cycles were carried out to remove all volatile impurities in TsBP. Matrix isolation experiments were performed using a closed cycle helium cryostat (RDK-408D2, Sumitomo Heavy Industries Ltd.). Both N2 (INOX, purity: 99.9995 %) and Ar (INOX, purity: 99.9995 %) were used as matrix gases. A twin-jet nozzle source was used, of which one nozzle was used for the deposition of TsBP and other nozzle was employed for the deposition of the matrix (N2/Ar separately). Typical deposition rate of 3 mmol/hr was fixed to deposit 100 torr in a span of 90 minutes. During the deposition, TsBP was dynamically expanded from room temperature. The infrared spectra of matrix isolated samples were recorded using Bruker VERTEX 70 FTIR spectrometer at a resolution of 0.5 cm-1. For complexation studies, following methodology was adopted. TsBP-thorium and TsBPuranium complexes were formed by extracting Th(IV) and U(VI) from nitric acid media by solvent extraction. For thorium experiment, metal loading was kept at maximum so that the third phase is formed. TsBP-thorium third phase, TsBP-thorium diluent rich phase and TsBP-uranium organic phase were analyzed. For comparison, analogous TBP complexes were also prepared. Since, thorium concentration was kept at the maximum during solvent extraction; the solutions were handled inside the glove box. To record the infrared spectra of the thorium and uranium samples, Bruker ALPHA FT-IR spectrometer commissioned inside a glove box was used. The infrared spectrometer has a ZnSe beam splitter, covering the range 650-4000 cm-1 and was operated at a resolution of 4 cm-1. Infrared spectra of all the metal-TsBP/TBP complexes were recorded in attenuated total reflectance mode. All computations were performed using Gaussian 09 package.38 DFT computations were performed on all conformers using B3LYP hybrid exchange-correlation functional with 6-311++G(d,p) basis set. Positive vibrational wavenumbers ensured that all the structures were minima on the potential energy surface. NBO analysis was performed on selected structures to understand the role of charge transfer delocalization interactions on conformational stability 4 ACS Paragon Plus Environment

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using NBO 6.0 version.39 All donor-acceptor orbitals involved in hyperconjugation were generated and viewed using Chemcraft package.40 The simulated infrared spectra were drawn using SYNSPEC package.41 Onsager model calculations were performed to understand the role of matrixes on the energetics of conformations. Results and Discussion Computational Methodology Model Dimethyl-secondary-Butyl Phosphate to Unravel the Conformations of TsBP. Before venturing into the actual conformational landscape of TsBP, the conformations of model dimethyl-secondary-butyl phosphate (DMsBP), which possesses a single secondary-butyl strand was first investigated. The computational analysis yields an interesting trend in contrast to its straight chain isomer, TBP. In TBP, the carbon attached to oxygen, which is referred to as hyperconjugative carbon, is restricted to have either Gauche (G±)/Trans orientation. The base orientation of TBP is therefore G±(xyz)G±(xyz)G±(xyz) and T(xyz)G±(xyz)G±(xyz) where ‘xyz’ corresponds to the orientation of the non hyperconjugative carbon atoms connected to the hyperconjugative carbon.31 This base orientation of TBP was elucidated from the computational analysis of a lower homologue, TMP. The non hyperconjugative carbon atoms can assume ‘gauche(+)’, ‘gauche(-)’ and ‘trans’ orientations, which resulted in G±(xyz)G±(xyz)G±(xyz) orientation of TBP to have 27x27x27 (=19683) possibilities, which is a massive number to work with. The number 27 was arrived at by analyzing single butyl strand of prototypical dimethyl butyl phosphate, which resulted in 3x3x3 orientations. In case of DMsBP, the conformational analysis becomes simple. In G±(xyz)G±G± cluster of DMsBP, by fixing the hyperconjugative carbon in Gauche (G±) orientation, the second branched carbon (in TBP, the chain is linear) can adopt either ‘gauche(+)’, ‘gauche(-)’ and ‘trans’ orientations. What is interesting is the orientation of the third carbon, which is entirely decided by the second carbon. While all combinations are allowed for TBP, in a single secondary-butyl strand of DMsBP, only certain combinations are plausible. If the second branched carbon is ‘gauche(+)’, the third carbon should only be ‘gauche(-)’ and the two other ‘gauche(+)’ and ‘trans’ orientations are not allowed. Similarly, if the second carbon has ‘gauche(-)’ and ‘trans’ orientations, the third carbon is forced to orient only ‘trans’ and ‘gauche(+)’ respectively. Since, hyperconjugative carbons have the flexibility of orienting both ‘Gauche(+)’ and ‘Gauche(-)’ (i.e ‘Gauche(±)’), all said orientations can be reversed. To sum up, 5 ACS Paragon Plus Environment


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if second carbon is ‘gauche(±)’, the third carbon should be ‘gauche(m)’, if second is ‘gauche(m)’ and third should be ‘trans’ and if second is ‘trans’, third should be ‘gauche(±)’. The total number of conformations of DMsBP is simplified to 3x1x3 (9) unlike the 27 orientations for the butyl strand of TBP. Two representative conformers both from ‘Gauche’ and ‘Trans’ family are given in Figure 1. If the methodology of DMsBP is expanded to TsBP, the total number of conformations worked out to be only 9x9x9 (729) for G±(xyz)G±(xyz)G±(xyz) cluster. If same approach is extended for T(xyz)G±G± cluster of DMsBP, the total number of conformers will again be 729 for T(xyz)G±(xyz)G±(xyz) cluster of TsBP and in total, 729+729 conformations only should contribute to the population at a given temperature. This number for TsBP is small in

comparison

to

19683+19683

conformations

for

G±(xyz)G±(xyz)G±(xyz)

and

T(xyz)G±(xyz)G±(xyz) clusters for linear isomeric TBP. Subsequently, the energies of the 9 conformers of DMsBP were evaluated. At the outset, of the nine conformers, the conformer G±(tg±gm)G±G± did not optimize to a minimum in the potential energy surface possibly due to the steric hindrance with the adjacent methyl groups and this conformer was interconverted to G±(gmtgm)G±G±. Of the eight conformers, G±(gmtgm)G±G± conformer corresponds to the lowest energy structure and all other possible conformers can be arranged in the energy ladder relative to G±(gmtgm)G±G± conformer (Table 1). A closer look at the energies reveals that three out of eight conformers have energies greater than 2 kcal/mol and these conformers cannot assume any experimental significance in TsBP. Since in these long chain phosphates, the energies are likely additive which would place these three conformers above 6 kcal/mol with the negligibly small population, these three conformers of DMsBP can be classified as ‘experimentally unimportant’. The remaining five conformers whose energies are equal to or less than 1 kcal/mol are presented (Table 1) in increasing energy order. Following G±(xyz)G±G± cluster, the T(xyz)G±G± cluster was examined. Analogous to the interconversion of G±(tg±gm)G±G± conformer, the T(tg±gm)G±G± conformer was also interconverted to T(gmtgm)G±G± conformer on optimization. Furthermore, the two T(tg±t)G±G± and T(tg±g±)G±G± conformers were interconverted to T(gmtt)G±G± and T(gmtgm)G±G± conformers respectively during optimization. Effectively, only six conformers should be evaluated for energy in T(xyz)G±G± cluster. Since the energies of three T(g±gmgm)G±G±, T(g±gmt)G±G± and T(g±gmg±)G±G± conformers were well above 2 kcal/mol, these three conformers were not further considered. The energies of all T(xyz)G±G± cluster of conformations relative to 6 ACS Paragon Plus Environment

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G±(gmtgm)G±G± conformer are presented in Table 1. The analysis of DMsBP prototype resulted that in G±(xyz)G±G±/T(xyz)G±G± clusters, only 5 and 3 conformers should be expanded to derive the conformations of TsBP respectively. The Number of Conformations of TsBP. Since only 5 conformers assume experimental significance based on the energetics for G±(xyz)G±G± group of conformations, the total number of possible conformations for TsBP turned out to be 5x5x5 (125) for G±(xyz)G±(xyz)G±(xyz) cluster. As three conformers alone are important in T(xyz)G±G± cluster of conformations, the total number of conformers can be calculated to be 3x5x5 (75) conformations in T(xyz)G±(xyz)G±(xyz)

cluster

in

TsBP.

All

possible

125/75

conformations

in

G±(xyz)G±(xyz)G±(xyz)/T(xyz)G±(xyz)G±(xyz) clusters respectively are given in Table S1 of supporting information. In the G±(xyz)G±(xyz)G±(xyz) cluster, by fixing the first G±(xyz) strand in a single orientation, 1x5x5 (25) conformations can be generated. Of the 25, as a rule, 10 conformations will be exactly identical to the existing 15 conformations (see Table S1 of supporting information); therefore, 15 conformations only have to be considered out of 25. Consequently, in a total of 5x5x5 (125) conformations, 50 conformations will be part of this identical group of conformations. Effectively, the total 125 conformations are reduced to 75 conformations in the G±(xyz)G±(xyz)G±(xyz) cluster. Interestingly, of the 75 conformations, 37 are identical to the remaining 38 conformers (Table S2 of supporting information). The elimination of a large number of conformers in G±(xyz)G±(xyz)G±(xyz) cluster is due to this degeneracy originating from the symmetry of the corresponding cluster. These 38 conformations which were simplified, were optimized on the potential energy surface using B3LYP level of theory in conjunction with 6-311++G(d,p) basis set. During the optimization, not all 38 conformers were optimized to be minima on the potential energy surface and 17 out of 38 conformers interconvert to any of the remaining stable 21 conformers. All possible 38 conformers along with the interconversion of 17 conformers to form stable conformations are presented in Table S3. The interconversion is a clear manifestation of steric overcrowding as a result of multiple secondary butyl chains in TsBP. Overall, through our systematic approach, the 125 conformers of G±(xyz)G±(xyz)G±(xyz) cluster were streamlined to a simple set of 21 conformers. Computed structures of a few conformers from G±(xyz)G±(xyz)G±(xyz) cluster of TsBP are presented in Figure 2.



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Corollary to G±(xyz)G±(xyz)G±(xyz) cluster, in the T(xyz)G±(xyz)G±(xyz) cluster too, of the 3x5x5 (75) conformations, in every 25 conformational bunch, 10 conformers will be identical with respect to the remaining 15, therefore, a total of 30 conformations can be eliminated out of 75 conformers. This results in 45 conformations in gross for T(xyz)G±(xyz)G±(xyz) cluster. The asymmetric nature of T(xyz)G±(xyz)G±(xyz) cluster (unlike the symmetry present in G±(xyz)G±(xyz)G±(xyz) cluster) ensured that these conformations cannot be reduced any further. Therefore, 45 conformations were optimized on the potential energy surface using B3LYP level of theory using 6-311++G(d,p) basis set. During optimization, 13 conformers were interconverted to one of the existing 32 conformers and therefore, effectively only 32 conformers alone contributed to the overall energetics of TsBP. The experimentally relevant 21 conformers in the G±(xyz)G±(xyz)G±(xyz) cluster and 32 conformers in T(xyz)G±(xyz)G±(xyz) cluster (in total 53 conformers) along with their ZPE corrected absolute and relative energies are presented in Table 2. The geometries of a few conformers are given in Figure 3. The calculated MaxwellBoltzmann populations of all these 53 conformers at 298 K using relative energies are also given. Even

the

energetically

dominant

member

of

T(xyz)G±(xyz)G±(xyz)

cluster

(T(gmtgm)G±(gmtgm)G±(gmtt) conformer) has the relative energy of 1.79 kcal/mol with respect to the ground

state

G±(gmtgm)G±(gmtgm)G±(gmtt)

conformer,

the

overall

population

of

T(xyz)G±(xyz)G±(xyz) cluster therefore turned out to be only 8 % with the majority of the population (92 %), originating only from the G±(xyz)G±(xyz)G±(xyz) cluster. Population calculation was also arrived at using relative free energy of the different conformers at 298 K and is presented in Table S4 of supporting information. The variation in the population using free energy (G±(xyz)G±(xyz)G±(xyz) cluster: 91 % and T(xyz)G±(xyz)G±(xyz) cluster: 9 %) was found to be very minimal with respect to relative energy. It can be recalled that while deriving the conformers of TsBP from DMsBP, in ±

G (xyz)G±G± cluster of DMsBP, based on the energetics, three conformers were omitted. To justify these omissions, a sample G±(g±gmgm)G±(g±gmgm)G±(g±gmgm) conformer of TsBP was calculated. The relative energy of G±(g±gmgm)G±G± conformer in DMsBP was calculated to be 2.24 kcal/mol with respect to the ground state conformer. When three of the G±(g±gmgm)strands are integrated to form G±(g±gmgm)G±(g±gmgm)G±(g±gmgm) in TsBP, the relative energy was calculated to be 11.2 kcal/mol with respect to the ground state conformer of TsBP. This relative energy is much larger than when the relative energies of 2.24 kcal/mol are additively added (6.72 8 ACS Paragon Plus Environment

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kcal/mol) to formulate the G±(g±gmgm)G±(g±gmgm)G±(g±gmgm) conformer. The higher relative energy than the expected energy based on additivity implies a strong destabilizing inter-chain interaction. It is clear therefore that branching has a profound influence on the conformational space of a given molecule. For example, in our earlier work, for the straight chain isomer TBP, at the outset the 27x27x27 (19683) conformers were simplified to 5832 conformers in the G±(xyz)G±(xyz)G±(xyz) cluster based on the elimination of certain orientations of the different conformers.31 For the T(xyz)G±(xyz)G±(xyz) cluster, another 5832 conformers were considered. To simplify the problem, predictive rules were formulated through model compounds and by keeping an upper relative energy limit of 1.7 kcal/mol, a total of around 1200 conformations were predicted to contribute to the room temperature population. The problem of arriving at this number of conformations was through a highly complex approach. Contrarily, branching removes the complexity in the conformational space of the molecule and resulted in only a limited number of conformations. A closer look reveals that even if methyl group of TsBP is substituted with a hydrogen atom to generate tri-n-propyl phosphate (TPP), the conformational analysis is not simple like TsBP. Since the removal of branching removes the restriction of the conformational orientation of the carbon to which it is attached and the rotational degrees of freedom of the carbon becomes free in TPP, which eventually introduces intricacy in unraveling the conformational landscape. For TPP, for G±(xy)G±(xy)G±(xy) cluster itself, the number of possible conformations would be 729, of which the majority of the conformers will energetically contribute to the population. Again, another 729 conformers should be considered from T(xy)G±(xy)G±(xy) cluster. Overall, branching of methyl group removes the complexity of conformational space and in turn, the number of possible conformations gets drastically reduced in comparison to even a system with less number of carbon atoms (such as TPP). In total, therefore, only 53 conformers alone contribute to the effective population at room temperature for TsBP. Surprisingly, for the simplest triethyl phosphate (TEP), the number of possible conformations can be calculated to be 27 each for ‘Gauche’ and ‘Trans’ clusters (with a total of 54 conformations). This number is comparable to the 53 conformers of TsBP. As a whole, the branching influences the number of conformations and as a result, the problem becomes as simple as if two carbons were being eliminated in a given butyl chain. 9 ACS Paragon Plus Environment


To rationalize further on branching, the position of branching is varied from first hyperconjugative carbon to different non-hyperconjugative carbon atoms. If the methyl group branching moves to the second non-hyperconjugative carbon, tri-iso-butyl phosphate (TiBP) can be generated. The methyl group substitution on the third non-hyperconjugative carbon leads to the formation of a linear butyl chain and therefore generates TBP. Thus, branching of methyl group can either create TsBP or TiBP and the branching analysis eventually converges only to TiBP to have a comparison with TsBP. To understand the conformations of TiBP, the model dimethyl-iso-butyl phosphate (DMiBP) was examined. In the single iso-butyl strand of DMiBP, the branched carbon (third carbon) restricts the fourth carbon to have certain orientations like TsBP. In addition, the second carbon since it is flanked by two methyl groups (third and fourth carbons) becomes conformationally rigid and therefore assumes only ‘trans’ orientation. For the second carbon, hence, ‘gauche(+)’ and ‘gauche(-)’ orientations are excluded. This results only 1x1x3x1 (3) conformations for G±(xyz)G±G± cluster. These conformers occur at a relative energy difference of 0.5 kcal/mol with respect to each other. For G±(xyz)G±(xyz)G±(xyz) cluster, the total number of conformations turned out to be 3x3x3 (27) and the inclusion of T(xyz)G±(xyz)G±(xyz) cluster will result in a total of 54 conformers. Since, the relative energies are small; all 54 conformers can be expected to contribute to the population of TiBP. It can be recalled that in case of TsBP, many conformations were eliminated based on degeneracy with the existing conformers and the exclusion was also based on their energies. But interestingly, in TiBP, the omission is purely on the conformational rigidity of the two carbons in comparison to a single carbon in TsBP. The effect of branching on the total number of conformations in TsBP and TiBP is therefore obvious. Experimental Results of TsBP in N2 and Ar Matrixes. Figure 4 (trace ‘b’ and ‘c’) shows the infrared spectra of TsBP in N2 and Ar matrixes respectively. The grid ‘A’ corresponds to CH3 symmetric and antisymmetric stretching regions of TsBP, while grid ‘B’ corresponds to the P=O stretching, CH3 bending, P-O stretching and C-O stretching regions in N2 and Ar matrixes. Figure 4, trace ‘a’ shows the simulated infrared spectrum of TsBP, where all 53 conformers were included by correcting for their intensities and population. In this exercise, the fraction corresponding to population was multiplied with the calculated intensities to obtain the corrected intensities. A common scaling factor of 1.017 was used to correct the vibrational wavenumbers obtained at B3LYP level of theory using 6-311++G(d,p) basis set of all 53 conformers. A good match of computational wavenumbers of all 53 conformers with the experimental wavenumbers 10 ACS Paragon Plus Environment

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was ascertained in N2 and Ar matrixes. Of the different vibrational regions, for the conformational analysis, the P=O stretching region (1320-1200 cm-1 as highlighted in Figure 4 for clarity) was chosen as this region can be clearly discerned for different conformational clusters. Normal mode analysis using Chemcraft package reveals that the spectral feature corresponding to P=O stretching is a mixed mode originating from mixing of P=O stretching with CH2 twisting mode as a similar mixing was also observed for TBP.31 Figure 5 shows the comparison of simulated infrared spectrum in the P=O stretching + CH2 twisting mode of TsBP with that of the spectra obtained in N2 and Ar matrixes. The broad features that occur as a triplet at 1274.8, 1267.7 and 1261.5 cm-1 are due to the G±(xyz)G±(xyz)G±(xyz) cluster of TsBP in N2 matrix. A similar triplet at 1275.3, 1269.1 and 1262.2

cm-1

was

observed

in

Ar

matrix.

The

observation

of

triplet

for

the

G±(xyz)G±(xyz)G±(xyz) cluster should be due to the multiple site-effect as site-splitting is very common in matrix isolation experiments.30,42-48 For T(xyz)G±(xyz)G±(xyz) cluster, a broad singlet at 1289.3 and 1292.6 cm-1 in N2 and Ar matrixes, respectively was observed. The feature observed at 1247.5 cm-1 in N2 matrix and at 1248.5 cm-1 in Ar matrix should be due G±(xyz)G±(xyz)G±(xyz) cluster-H2O adduct as H2O is always present as an impurity in matrix isolation experiments. A population calculation was accomplished experimentally in N2 and Ar matrixes to test the approach of our arriving at the conformations computationally. The area under the infrared peak for G±(xyz)G±(xyz)G±(xyz) and T(xyz)G±(xyz)G±(xyz) clusters was calculated in both N2 and Ar matrixes and from the total area, fraction of the individual cluster was computed. It turned out that the population corresponding to G±(xyz)G±(xyz)G±(xyz) cluster was ∼ 93 % in both Ar and N2 matrixes leaving behind the minor composition for T(xyz)G±(xyz)G±(xyz) cluster (∼6-7 %). This number is reasonably close to the 92%/8% population calculated for G±(xyz)G±(xyz)G±(xyz)/T(xyz)G±(xyz)G±(xyz) clusters computationally. Thus, there is a good match between the computed and experimental population, which indeed demonstrates that our methodology of arriving at the conformational landscape of TsBP is precise. It is important to point out that the straight chain isomer, TBP has a calculated population of 21% for T(xyz)G±(xyz)G±(xyz) cluster in comparison to 8% for its TsBP counterpart. The smaller population for T(xyz)G±(xyz)G±(xyz) cluster in TsBP should be a direct consequence of branching, which likely increases the steric strain in adjacent secondary-butyl groups. 11 ACS Paragon Plus Environment


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The annealing experiments of TsBP in N2 and Ar matrixes are interesting. In addition to increasing the intensity of the feature characteristic of TsBP-H2O adduct, notably, annealing also decreases the intensity of the features characteristic of T(xyz)G±(xyz)G±(xyz) cluster of TsBP in both N2 and Ar matrixes. It is clear from Figure 5, grid ‘B’ that annealing the matrix at 25 K decreases the feature of T(xyz)G±(xyz)G±(xyz) cluster and a further decrease was observed at a higher annealing temperature of 30 K. A similar trend was observed in Ar matrix, where a higher annealing temperature of 35 K is accessible and thus, almost complete interconversion of T(xyz)G±(xyz)G±(xyz) cluster was noticed. At the outset, it looks unusual that the conformer interconversion happens for a molecule such as TsBP with so much of structural complexity (the molecule has three long chain secondary butyl groups with branching) since for its straight chain isomer, TBP, no such interconversion was noticed at low temperatures.31 To start with, the influence of a low temperature matrix on the conformations of TsBP was calculated using Onsager model. Two representative G±(gmtgm)G±(gmtgm)G±(gmtt) and T(gmtgm)G±(gmtgm)G±(gmtt) conformers were considered and the effect of a low temperature matrix (with the dielectric constant of 2.00) was investigated. An upper limit of a dielectric constant of 2.00 would model both N2 and Ar matrixes. Even though the dipole moment of T(gmtgm)G±(gmtgm)G±(gmtt)conformer

is

3

times

more

than

the

ground

state

G±(gmtgm)G±(gmtgm)G±(gmtt) isomer, the extent of stabilization of both the conformers was found to be similar and the gas phase relative energy profile is maintained in the matrixes too. Since, the energy difference between the G±(xyz)G±(xyz)G±(xyz) and T(xyz)G±(xyz)G±(xyz) clusters was calculated to be in excess of 1.5 kcal/mol, the calculated population of conformers of TsBP indicate a negligible composition of T(xyz)G±(xyz)G±(xyz) cluster in comparison to G±(xyz)G±(xyz)G±(xyz) cluster. Onsager model therefore supports the conformer interconversion on the basis of energetics form T(xyz)G±(xyz)G±(xyz) cluster to G±(xyz)G±(xyz)G±(xyz) cluster as the G±(xyz)G±(xyz)G±(xyz) cluster is thermodynamically more stable at low temperatures. To rationalize the conformer interconversion behaviour of TsBP, the barrier connecting a few of the T(xyz)G±(xyz)G±(xyz) to G±(xyz)G±(xyz)G±(xyz) cluster was calculated. Keeping the ‘xyz’ orientations constant in reactant/product, the barrier for hyperconjugative carbon rotation from ‘Trans’ to ‘Gauche’ was calculated to be ∼ 1 kcal/mol. The additional change of ‘xyz’ orientation increases the barrier roughly to ∼ 1.5 kcal/mol. It is surprising to notice a facile conformer interconversion of T(xyz)G±(xyz)G±(xyz) cluster to G±(xyz)G±(xyz)G±(xyz) cluster in 12 ACS Paragon Plus Environment

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spite of their large barriers experimentally in N2 and Ar matrixes. These barriers are free molecular barriers in the gas phase and in the matrix, these barriers can be anticipated to increase/decrease further due to the influence of the matrix.49,50 The decrease of barrier in the low temperature matrixes can be a possibility arising due to the effect of branching. More work is indeed necessary to understand if fall of the barrier as a result of branching is responsible for conformer interconversion. NBO Analysis of TsBP. NBO analysis was performed on the representative conformers from both G±(xyz)G±(xyz)G±(xyz) and T(xyz)G±(xyz)G±(xyz) families to investigate the nature of charge transfer interactions present in TsBP. For this investigation, the illustrative G±(gmtgm)G±(gmtgm)G±(gmtt) and T(gmtgm)G±(gmtgm)G±(gmtt) conformers of TsBP were taken. As with phosphates,30 both four centered vicinal and three centered geminal interactions are important in determining the energy profile of all conformers of TsBP. Based on the deletion of vicinal and geminal interactions independently in TsBP, it has been estimated that around 60/70 % contribution to stability arises from vicinal interaction with remaining 40/30 % for geminal stabilization in the G±(xyz)G±(xyz)G±(xyz)/ T(xyz)G±(xyz)G±(xyz) clusters, respectively. The variation in energy as a function of deletion of vicinal and geminal donor/acceptor orbitals is presented as a bar graph in Figure 6. In the same figure, the effect of deleting all hyperconjugative interactions (including remote interactions) is also shown. The vicinal stabilization

is

marginally

more

in

T(xyz)G±(xyz)G±(xyz)

the

cluster

than

G±(xyz)G±(xyz)G±(xyz) cluster but the stabilization arising due to geminal interaction has a reverse trend. A much stronger stabilization from geminal interaction was noticed in G±(xyz)G±(xyz)G±(xyz) cluster than T(xyz)G±(xyz)G±(xyz) cluster. When all interactions were deleted, the energy increase of G±(xyz)G±(xyz)G±(xyz) cluster is around 50 kcal/mol higher than the T(xyz)G±(xyz)G±(xyz) cluster. By and large, the geminal interaction decides the stability of TsBP, which appears to be unique for phosphorus centric systems. The predominant geminal stabilization

arises

from

the

delocalization

of

π(P1O2)/σ(P1O2)

to

the

adjacent

σ*(P1O3)/σ*(P1O4)/σ*(P1O5) and the delocalization of σ(P1O3)/σ(P1O4)/σ(P1O5) to π*(P1O2). The vicinal delocalization is from lone pairs of oxygens to various anti-bonding orbitals such as σ*(P1O3)/σ*(P1O4)/σ*(P1O5)/σ*(P1O2)/π*(P1O2). A few of the vicinal and geminal orbitals together with donor-acceptor delocalization interactions are illustrated in Figure 7. A closer look at the hyperconjugative delocalization revealed that a new π1(P1O2) hybrid 13 ACS Paragon Plus Environment


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orbital is generated, which is perpendicular to the existing π(P1O2) molecular orbital in both G±(xyz)G±(xyz)G±(xyz) and T(xyz)G±(xyz)G±(xyz) clusters. This π1(P1O2) hybrid orbital is constructed through the delocalization of phosphoryl oxygen lone pair to the vacant ‘d’ orbital on phosphorus atom. It is affirmed that the occupancy of one of the phosphoryl oxygen lone pairs becomes zero as a result of this delocalization. The new bonding π1(P1O2) hybrid orbital for G±(xyz)G±(xyz)G±(xyz) and antibonding π1*(P1O2) orbital for T(xyz)G±(xyz)G±(xyz) cluster are shown in Figure 8. This new π1(P1O2) hybrid orbital created should thus be responsible for additional vicinal and geminal stabilizations in TsBP such as the delocalization of lone pairs of alkoxy oxygens to π1*(P1O2) and σ(P1O3)/σ(P1O4)/σ(P1O5) to π1*(P1O2) orbital. Since this kind of charge transfer interactions are assembled as a result of delocalization of one of the phosphoryl oxygen lone pairs, these delocalization interactions are referred to as ‘secondary’ interactions in contrast to the existing ‘primary’ interactions. A few of these ‘secondary’ vicinal and geminal interactions are displayed in Figure 9. As it is obvious that unlike the π1*(P1O2) orbital as an acceptor in vicinal interactions, the corresponding contribution is more from ‘secondary’ geminal than vicinal to the overall stability of conformers of TsBP. The larger contribution of ‘secondary’ geminal interactions is due to the fact that both bonding π1(P1O2) and antibonding π1*(P1O2) orbitals take part both as acceptor and donor simultaneously. Furthermore, the % ‘d’ character in the hybrid π1(P1O2) orbital of G±(xyz)G±(xyz)G±(xyz) cluster is 52 % in comparison to 45 % in T(xyz)G±(xyz)G±(xyz) cluster. This difference in ‘d’ character should certainly be responsible for larger ‘secondary’ geminal stabilization of G±(xyz)G±(xyz)G±(xyz) cluster in preference to T(xyz)G±(xyz)G±(xyz) cluster. This explains therefore

the

energetic

domination

of


cluster

over

T(xyz)G±(xyz)G±(xyz) cluster. Since a vacant ‘d’ orbital is important for facile delocalization of the oxygen lone pair, to test whether the elements close to phosphorus in the period (silicon and sulphur) also possess this property, two model dimethyl sulphite and dimethoxy silanone molecules were examined. The structures of sulphite and silanone are given in Figure S1 of supporting information. For comparative purpose, a simple phosphate (trimethyl phosphate) without branching was considered. NBO deletion analysis indicated that the contribution from geminal interaction is ∼ 10 % from both sulphite and silanone in contrast to almost ∼40-50 % stabilization from phosphate. In particular, both sulphite and silanone were not stabilized through ‘secondary’ 14 ACS Paragon Plus Environment

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interactions as no delocalization was noted from oxygen (double bonded) lone pair to vacant ‘d’ orbital of either silicon or sulphur unlike phosphate. Though a vacant ‘d’ orbital is common among all three silicon, phosphorus and sulphur atoms, phosphorus should be considered ‘exceptional’ as delocalization of oxygen lone pair invokes the geminal stabilization that has a significant role to play in the overall stabilization of the conformations. Comparison of Conformational Preferences of TsBP with TBP. While the degree of hyperconjugative interactions operating in TsBP/TBP dictates the carbons attached to oxygens in ‘Gauche’/‘Trans’

orientations

in

the

ground

state


and

T(xyz)G±(xyz)G±(xyz) clusters, the non-hyperconjugative carbons have different predilection in their orientations. Whereas the ground state orientation in both G±(xyz)G±(xyz)G±(xyz) and T(xyz)G±(xyz)G±(xyz) clusters for TBP is G±(ttt)G±(ttt)G±(ttt) and T(ttt)G±(ttt)G±(ttt) conformers, in TsBP, it is G±(gmtgm)G±(gmtgm)G±(gmtt) and T(gmtgm)G±(gmtgm)G±(gmtt)conformers. The ‘trans’ orientation for non-hyperconjugative carbons is preferred due to steric effect in TBP. At a first glance, it looks surprising that there is a deviation from the ‘trans’ orientation in the conformers of TsBP but in fact, the orientation ‘gauche(m m)’ for the second branched carbon forces the third carbon to orient in ‘trans’. It can be recalled that if the branched carbon is ‘gauche(m m)’, the third carbon should only be ‘trans’ and the branched carbon does not directly come in the way of steric crowding of the secondary butyl chain. What is interesting is the orientation of the fourth carbon. The ‘gauche(m m)’orientation of fourth carbon in TBP increases the energy of the conformer by 1 kcal/mol with respect to ‘trans’ orientation,31 and on the other hand in TsBP, the ground state geometry is ‘gauche(m m)’ in at least two of the secondary butyl strands. A steric strain was noted between the hydrogen connected to branched carbon in ‘gauche(m m)’ orientation and the hydrogen connected to fourth carbon in ‘trans’ orientation. To avoid this steric overcrowding, the fourth carbon has a ‘gauche(m m)’ preference in TsBP in its lower energy in comparison to straight chain TBP isomer. It is therefore the steric effect that directs all non-hyperconjugative carbons to orient ‘trans’ in TBP and the same effect encourages the branched and fourth carbons to adopt ‘gauche(m m)’ orientation in TsBP. Third Phase Diagnostics of Thorium-TsBP Complexes in Glove Box. Since TsBP forms third phase during Th(NO3)4 extraction, infrared spectra of Th(NO3)4-TsBP third phase (rich thoriumTsBP complex in dodecane diluent) were recorded and analyzed. To study the third phase, a high 15 ACS Paragon Plus Environment


thorium concentration of ∼ 450g/L in 1 M HNO3 medium was extracted with 30% TsBP in dodecane. The lean diluent phase (with ∼20 g/L of thorium) formed as a result of phase splitting was examined. For comparison, the infrared spectra of neat TsBP and 30% TsBP in dodecane were also investigated. Figure 10 shows the infrared spectra of neat TsBP (trace ‘a’), 30 % TsBP in dodecane (trace ‘b’), diluent phase (trace ‘c’), third phase (trace ‘d’) along with solid Th(NO3)4 (trace ‘e’). For neat TsBP, two major spectral bands were observed at 1024.9 and 982.5 cm-1 (P-O and C-O stretching regions overlap and appear as broad features) and 1250.5 cm-1 (P=O stretching). When TsBP was diluted in dodecane (30% TsBP) and equilibrated with 1 M HNO3, the spectral features became sharper in comparison to neat TsBP (trace ‘b’ of Figure 10). The sharpening of spectral features should be a direct consequence of dilution effect. In trace ‘b’, a feature characteristic of asymmetric stretching of NO3- occur at 1462.7 cm-1 with enhanced intensity. Furthermore, the spectral features of TsBP in dodecane were blue-shifted with respect to neat TsBP. The spectral features of Th(NO3)4 were observed at 1314.7 and 1279.4 cm-1 (asymmetric stretching of NO3-), 1028.2cm-1 (symmetric stretching of NO3-) and 806.5 and 742.4cm-1 (out-of-plane bending vibration of NO3-(trace ‘e’).51 Although blue-shifted with respect to TsBP in dodecane, a considerable broadening near 1250-1300 cm-1 (near P=O stretching region) was noticed for Th(NO3)4-TsBP complex for lean diluent phase in dodecane. The broadening should be due to the overlap of spectral features characteristic of P=O stretching of TsBP and asymmetric stretching of NO3- of Th(NO3)4 in the complex in the dodecane medium. Figure S2 of supporting information shows the broadening in the P=O stretching region of Th(NO3)4-TsBP complex. The occurrence of blue-shift is likely due to the interaction of TsBP/Th(NO3)4-TsBP complex with dodecane surroundings. A detailed molecular simulation study would be required to delineate as to why the blue-shift is occurring. Of all the phases, third phase deserves more attention. What is interesting is that the spectral features that correspond to third phase is considerably broadened (see trace ‘d’ of Figure 10) with respect to lean diluent phase and even in comparison to pure TsBP. It has been estimated through acid equilibration method that the concentrations of TsBP in dodecane in the diluent-rich phase and third phase are ∼ 5% and ∼ 90% respectively.9 For diluent-rich phase (trace ‘c’ in Figure 10), it is not surprising to notice that the infrared bands are relatively sharper compared to 30% TsBP in dodecane. In comparison to 30% TsBP in dodecane in trace ‘b’, the concentration of lean diluent phase is considerably less (∼ 5%) as sharpening should be due to 16 ACS Paragon Plus Environment

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this concentration effect. A closer examination of the spectral feature of third phase centered at 1001.4 cm-1 revealed that the peaks are ∼10 cm-1 broader than pure TsBP. Furthermore, in all other vibrational modes, a considerable broadening was noticed. This anomalous broadening of spectral features cannot be explained due to concentration effect in third phase as the concentration of TsBP in dodecane was estimated to be ∼ 90 % in comparison to neat TsBP (100 %). If concentration effect is only operating, the broadening cannot be greater than that of neat TsBP. A recent SANS study on Th(NO3)4-TsBP system in dodecane from organic phase to near third phase divulged that under near third phase conditions, the aggregation of extracted species and formation of reverse micelles was facilitated.11 The effect of unusual broadening in the third phase is indicative of the fact the molecular aggregation does occur in the third phase to form reverse micelle, as was evidenced by infrared spectroscopy. Figure 10, trace ‘f’ shows the infrared spectrum of UO2(NO3)2-TsBP organic phase as organic phase alone was recorded since uranium (VI) does not form third phase. The spectrum is broader than Th(NO3)4-TsBP diluent phase and sharper than the third phase. This further supports aggregate formation in the third phase. Under identical conditions, the infrared spectra of neat TBP, 30% TBP in dodecane thorium diluents rich phase, thorium third phase and uranium organic phase were recorded (see Figure S3 of supporting information). The response of third phase of Th(NO3)4-TBP is analogues to the one observed for TsBP. Nevertheless, both TsBP and TBP form third phase with thorium, the limiting organic concentration (third phase formation limit) with TsBP is higher than that of TBP. The influence of branching cannot be ruled out for the higher limit of third phase formation with TsBP as branching reduces the number of conformations from the straight chain TBP isomer (∼1200) to 53. Smaller number of conformations is one of the possibilities in deciding the size of the aggregate; however, a detailed molecular simulation work is essential to affirm this hypothesis. Conclusions A case study of TsBP using matrix isolation infrared spectroscopy and DFT computations led to the following important conclusions. (1) TsBP is a unique system in comparison to its straight chain TBP isomer as branching reduces a large number of conformations. The sizeable number of conformations of TBP was converged to an undemanding number of conformations in TsBP as a result of branching. In addition to reducing the number of conformations, branching also dictates the orientation of non17 ACS Paragon Plus Environment


hyperconjugative carbon atoms in TsBP. While non-hyperconjugative carbon atoms have a ‘trans’ preference as directed by the straight chain TBP isomer, branching in TsBP instigates ‘gauche(m m)’ inclination for second and fourth non-hyperconjugative carbon atoms to steer clear of steric crowding. The conformational problem of TsBP was solved using model DMsBP and all possible conformations of TsBP were extrapolated based on the analysis of this single secondary-butyl strand of DMsBP. The complete conformational analysis predicted only 53 conformations for TsBP of which 21 conformations contribute from G±(xyz)G±(xyz)G±(xyz) cluster and the remaining 32 from T(xyz)G±(xyz)G±(xyz) cluster. Of the two families of clusters, G±(xyz)G±(xyz)G±(xyz) cluster does energetically dominate with the maximum population of ∼ 92 % in the gas phase. (2) Matrix isolation experiments were carried out by expanding TsBP with Ar and N2 matrixes at low temperatures. The finger print P=O stretching region was examined to ascertain the population of G±(xyz)G±(xyz)G±(xyz) and T(xyz)G±(xyz)G±(xyz) clusters. A fair agreement of the experimental with the DFT calculated population was discerned. As a result of annealing, an interconversion of T(xyz)G±(xyz)G±(xyz) cluster to G±(xyz)G±(xyz)G±(xyz) was noted in N2 and Ar matrixes and the accessibility of the higher possible temperature in Ar over N2 ensured near complete interconversion of T(xyz)G±(xyz)G±(xyz) cluster. Though the population characteristic of room temperature was deposited in the initial low temperature matrix, thermal energy provided through annealing facilitated the accomplishment of thermal equilibrium population, distinctive of such low temperatures. Howbeit, the interconversion is surprising for TsBP with the anticipated structural complexity and in view of the cage effect of the matrixes at low temperatures, the plausibility branching induced conformer interconversion cannot be ruled out. (3) The charge transfer hyperconjugative delocalization interactions present in TsBP were investigated through NBO analysis. Nonetheless, vicinal hyperconjugative interactions are common in deciding the conformational stability, the proportion of geminal interactions is equally important in TsBP and in general in phosphates. The presence of vacant ‘d’ orbital is indeed true in invoking geminal interactions, TsBP is a notable example as secondary interactions get established through the delocalization of phosphoryl oxygen lone pair. (4) Both Th(NO3)4-TsBP diluents rich and third phases and UO2(NO3)2-TsBP organic phase were explored using infrared spectroscopy in glove box. A comparison of TBP complexes 18 ACS Paragon Plus Environment

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was also achieved. The inherent broadening of infrared spectra of the third phase is suggestive of the formation of aggregates in the third phase, which agrees well with the SANS results.11 The existence of a smaller number of conformations may play a role in deciding the limit of third phase formation, which can however be contended with caution. Overall, the case study of TsBP undertaken in this work is thoroughgoing, as the problem is cascaded in all directions to untangle a methodical solution.

Supporting Information Total number of all possible conformations of TsBP after elimination of a bunch of conformers based on the analysis of DMsBP, further reduction in the total conformations of TsBP, population of TsBP calculated using relative free energy at 298 K using B3LYP/6-311++G(d,p) basis set, the structures of dimethyl silanone and dimethyl sulphite, infrared spectra of Th(NO3)4TsBP complex in dodecane, infrared spectra of Th and U-TBP system in dodecane. Acknowledgements: Shubhra Sarkar and Aditi Chandrasekar gratefully acknowledge the grant of Research Fellowship from Department of Atomic Energy, India.



Table 1. The conformational energy ordering of various conformations of DMsBP optimized at B3LYP level of theory using 6-311++G(d,p) basis set. S. No.

Structure

1 2 3 4 5 6 7 8 9

G±(gmtgm)G±G± G±(gmtt)G±G± G±(tg±t)G±G± G±(tg±g±)G±G± G±(gmtg±)G±G± G±(g±gmgm)G±G± G±(g±gmt)G±G± G±(g±gmg±)G±G± G±(tg±gm)G±G±

10 11 12 13 14 15 16 17 18

T(gmtgm)G±G± T(gmtt)G±G± T(gmtg±)G±G± T(g±gmgm)G±G± T(g±gmt)G±G± T(g±gmg±)G±G± T(tg±t)G±G± T(tg±g±)G±G± T(tg±gm)G±G±

ZPE corrected absolute ZPE corrected relative energy (Hartrees) energy (kcal/mol) G±(xyz)G±G±cluster -879.974909 0.00 -879.973813 0.69 -879.973674 0.77 -879.973263 1.03 1.13 -879.973105 -879.971336 2.24 -879.969090 3.65 5.45 -879.966217 optimized to G±(gmtgm)G±G± T(xyz)G±G±cluster -879.972919 1.25 -879.972178 1.71 -879.971722 1.99 -879.96883 3.81 -879.966441 5.31 -879.963417 7.21 ± ± m optimized to T(g tt)G G optimized to T(gmtg±)G±G± optimized to T(gmtgm)G±G±


Page 20 of 38

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Table 2. Total number of conformations of TsBP optimized at B3LYP level of theory using 6-311++G(d,p) basis set. S. No.

Structure

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

G±(gmtgm)G±(gmtgm)G±(gmtt) G±(gmtgm)G±(gmtgm)G±(gmtgm) G±(gmtgm)G±(gmtgm)G±(tg±g±) G±(gmtgm)G±(gmtgm)G±(gmtg±) G±(gmtgm)G±(gmtt)G±(gmtt) G±(gmtgm)G±(gmtt)G±(tg±t) G±(gmtgm)G±(gmtgm)G±(tg±t) G±(gmtgm)G±(gmtt)G±(tg±g±) G±(gmtgm)G±(gmtt)G±(gmtg±) G±(gmtt)G±(gmtt)G±(tg±t) G±(gmtgm)G±(tg±g±)G±(tg±g±) G±(gmtt)G±(gmtt)G±(tg±g±) G±(tg±t)G±(tg±t)G±(gmtg±) G±(gmtgm)G±(gmtg±)G±(gmtg±) G±(gmtt)G±(tg±g±)G±(gmtg±) G±(gmtt)G±(gmtg±)G±(gmtg±) G+(gmtt)G+(tg±g±)G+(tg±g±) G±(tg±g±)G±(gmtg±)G±(gmtg±) G±(gmtt)G±(gmtt)G±(gmtt) G±(tg±g±)G±(tg±g±)G±(tg±g±) G±(tg±g±)G±(tg±g±)G±(gmtg±)

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

T(gmtgm)G±(gmtgm)G±(gmtt) T(gmtt)G±(gmtgm)G±(gmtt) T(gmtt)G±(gmtgm)G±(gmtgm) T(gmtt)G±(gmtgm)G±(tg±t) T(gmtgm)G±(gmtgm)G±(gmtgm) T(gmtgm)G±(gmtgm)G±(gmtg±) T(gmtg±)G±(gmtgm)G±(gmtt) T(gmtg±)G±(gmtgm)G±(gmtgm) T(gmtg±)G±(gmtgm)G±(tg±t) T(gmtgm)G±(gmtt)G±(gmtt) T(gmtt)G±(gmtgm)G±(gmtg±) T(gmtt)G±(gmtt)G±(gmtt) T(gmtt)G±(gmtt)G±(tg±t) T(gmtgm)G±(gmtt)G±(gmtg±) T(gmtg±)G±(gmtgm)G±(gmtg±)

ZPE corrected absolute energy (Hartrees)

ZPE corrected relative energy (kcal/mol)

G±(xyz)G±(xyz)G±(xyz) cluster -1115.772459 0.00 -1115.772424 0.02 -1115.771860 0.38 -1115.771788 0.42 -1115.771457 0.63 -1115.771429 0.65 -1115.771081 0.86 -1115.770796 1.04 -1115.770704 1.10 -1115.770413 1.28 -1115.770071 1.50 -1115.769995 1.55 -1115.769795 1.67 -1115.769354 1.95 -1115.769237 2.02 -1115.769143 2.08 -1115.768910 2.23 -1115.768459 2.51 -1115.768439 2.52 -1115.768430 2.53 -1115.7683980 2.55 T(xyz)G±(xyz)G±(xyz) cluster -1115.769602 1.79 -1115.769333 1.96 -1115.769311 1.98 -1115.769236 2.02 -1115.768828 2.28 -1115.768822 2.28 -1115.768800 2.30 -1115.768737 2.34 -1115.768737 2.34 -1115.768709 2.35 -1115.768560 2.45 -1115.768548 2.45 -1115.76850 2.48 -1115.76814 2.71 -1115.768146 2.71


Population at 298 Ka

Scaled calculated P=O stretching + CH2 rocking vibrational wavenumber (cm-1)b

19.4 18.7 10.3 9.5 6.7 6.5 4.5 3.3 3.0 2.2 1.5 1.4 1.2 0.7 0.6 0.6 0.5 0.3 0.3 0.3 0.3

1269.5 (191) 1269.1(190) 1269.6 (193) 1268.5 (200) 1269.2 (196) 1269.9 (186) 1270.1 (175) 1269.2 (194) 1270.5 (195) 1270.2 (206) 1269.5 (192) 1269.9 (201) 1270.9 (184) 1269.9 (191) 1269.8 (193) 1270.9 (199) 1269.5 (192) 1271.9 (191) 1270.4 (205) 1270.4 (205) 1271.1 (203)

0.9 0.7 0.7 0.6 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.2 0.2

1291.9 (238) 1294.5 (231) 1293.2 (232) 1295.5 (232) 1292.1 (222) 1294.7 (231) 1294.3 (239) 1293.8 (231) 1293.9 (247) 1296.1 (243) 1293.6 (235) 1295.9 (243) 1297.2 (241) 1293.9 (228) 1295.3 (244)


37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

T(gmtg±)G±(gmtt)G±(gmtt) T(gmtgm)G±(gmtt)G±(tg±t) T(gmtt)G±(gmtt)G±(gmtg±) T(gmtt)G±(tg±t)G±(gmtg±) T(gmtg±)G±(gmtt)G±(tg±t) T(gmtg±)G±(gmtgm)G±(tg±g±) T(gmtt)G±(gmtt)G±(tg±g±) T(gmtg±)G±(gmtt)G±(gmtg±) T(gmtg±)G±(tg±t)G±(gmtg±) T(gmtgm)G±(gmtg±)G±(gmtg±) T(gmtt)G±(gmtg±)G±(gmtg±) T(gmtg±)G±(gmtt)G±(tg±g±) T(gmtgm)G±(tg±g±)G±(gmtg±) T(gmtt)G±(tg±g±)G±(gmtg±) T(gmtt)G±(tg±g±)G±(tg±g±) T(gmtg±)G±(gmtg±)G±(gmtg±) T(gmtg±)G±(tg±g±)G±(tg±g±)

-1115.768104 -1115.767999 -1115.767948 -1115.767948 -1115.767946 -1115.767833 -1115.767441 -1115.767215 -1115.767215 -1115.76706 -1115.766972 -1115.766960 -1115.766846 -1115.766827 -1115.766503 -1115.766304 -1115.766211

2.73 2.80 2.83 2.83 2.83 2.90 3.15 3.29 3.29 3.39 3.44 3.45 3.52 3.53 3.74 3.86 3.92

a

Page 22 of 38

0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0

1296.4 (242) 1296.4 (242) 1295.8 (252) 1297.3 (249) 1296.6 (242) 1293.8 (241) 1295.4 (237) 1293.8 (241) 1293.6 (239) 1295.3 (238) 1297.1 (235) 1294.3 (223) 1294.7 (237) 1296.0 (234) 1296.0 (234) 1295.1 (238) 1295.6 (237)

population was calculated using Maxwell-Boltzmann equation based on the ZPE corrected relative energy with respect to the ground state structure b calculated intensity in km/mol is given in parenthesis


Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Second Non Hyperconjugative branched carbon atom First Hyperconjugative carbon atom

G±(gmtgm)G±G±

Third Non Hyperconjugative carbon atom

Fourth Non Hyperconjugative carbon atom

T(gmtgm)G±G± Figure 1. Computed geometries of G±(gmtgm)G±G± and T(gmtgm)G±G± conformers of DMsBP optimized at B3LYP level of theory using 6-311++G(d,p) basis set.




G±(gmtgm)G±(gmtt)G±(tg±g±)

G±(tg±g±)G±(tg±g±)G±(gmtg±)

G±(gmtgm)G±(gmtg±)G±(gmtg±)

Figure 2. Computed geometries of a few conformers from G±(xyz)G±(xyz)G±(xyz) cluster of TsBP optimized at B3LYP level of theory using 6-311++G(d,p) basis set.


Page 24 of 38

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


T(gmtg±)G±(gmtgm)G±(gmtt)

T(gmtgm)G±(gmtgm)G±(gmtt)

T(gmtg±)G±(gmtt)G±(tg±t)

T(gmtg±)G±(tg±g±)G±(tg±g±)

Figure 3. Computed geometries of a few conformers from T(xyz)G±(xyz)G±(xyz) cluster of TsBP optimized at B3LYP level of theory using 6-311++G(d,p) basis set.



3200

3000

1002.7

B

1450

1350

1250

1150

985.4 963.0

1053.5

1050

983.4

b)

955.9

1052.1 1021.6

1160.2 1133.1

1216.0

1268.9

1411.1

1295.0

994.5

1550

1437.6

1534.4

2800

c)

962.0

1001.3

1030.8 1130.7 1117.8 1100.2 1130.0 1118.8 1099.9

1051.4

1180.1 1180.5

1031.8

1292.6 1275.3 1269.5 1261.5

1289.3 1274.8 1267.7

1383.4

1467.4 1461.6

1383.4

1468.0 1458.2

2886.3 2857.1 2889.3 2861.4

2843.1

3111.1 3095.0 3086.9 3055.2 3031.9

2946.4

2983.3

2892.0

A

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 26 of 38

950

a)

850

750

-1

Wavenumber, cm

Figure 4. The computed and experimental infrared spectra of TsBP. Grid ‘A’: CH3 symmetric and antisymmetric stretching regions, Grid ‘B’: P=O stretching, CH3 bending, P-O stretching and C-O stretching regions of TsBP. a) simulated infrared spectrum of 53 conformers of TsBP in which the intensity is corrected for their population; Infrared spectrum of matrix isolated TsBP b) in N2 matrix c) in Ar matrix. The P=O stretching region of TsBP is highlighted in grid ‘B’. 26 ACS Paragon Plus Environment

1240

1200 1320

1262.2 1275.3 1269.1

*

i)

e)

h) 1248.5

1292.6

1247.5

1289.3 1280

*

1267.7

1261.5

b)

f)

1274.8

*

1247.5

c)

a) 1320

C

B

*

1269.1 1262.2 1248.5 1261.5

1274.8 1268.9 1267.7

1292.6 1289.3

A

1295.0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1275.3

Page 27 of 38

d) 1280

1240

1200 1320

g) 1280

1240

1200

Wavenumber, cm-1

Figure 5. The computed and experimental infrared spectra of TsBP in the P=O stretching + CH2 twisting mode region. Grid ‘A’: a) simulated infrared spectrum of 53 conformers of TsBP in which the intensity is corrected for their population; Infrared spectrum of matrix isolated TsBP b) in N2 matrix c) in Ar matrix. Grid ‘B’: Infrared spectrum of matrix isolated TsBP in N2 matrix d) as deposited at 12 K, e) re-cooled at 12 K after annealing at 25 K f) re-cooled at 12 K after annealing at 30 K. Grid ‘C’: Infrared spectrum of matrix isolated TsBP in Ar matrix g) as deposited at 12 K, h) re-cooled at 12 K after annealing at 30 K i) re-cooled at 12 K after annealing at 35 K. The features marked with asterisk decrease as a result of annealing.



A ll

1000

Energy after deletion (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

A ll

900 800 700

V ic in a l V ic in a l

600 500

G e m in a l 400

G e m in a l

300 200 100 0 1

2



Figure 6. A bar graph showing the effect of deletion of vicinal, geminal and all interactions on the energies in representative G±(gmtgm)G±(gmtgm)G±(gmtt) and T(gmtgm)G±(gmtgm)G±(gmtt) conformers of TsBP.


Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


G±(gmtgm)G±(gmtgm)G±(gmtt) Primary vicinal

Primary geminal

n2O5 → π*(P1-O2) E2= 3.14 kcal/mol

n2O3 → σ*(P1-O2) E2= 4.61 kcal/mol

π(P1-O2) → σ*(P1-O5) E2= 22.74 kcal/mol

σ(P1-O5) → π*(P1-O2) E2= 9.48 kcal/mol


2

σ (P1-O2) → σ*(P1-O5) n O3 → π*(P1-O2) π(P1-O2) → σ*(P1-O3) n1O2 → σ*(P1-O3) E = 9.86 kcal/mol E = 4.99 kcal/mol E2= 28.55 kcal/mol 2 2 E2= 1.03 kcal/mol Figure 7. A few important ‘primary’ vicinal and geminal hyperconjugative interactions along with their E2 energies present in the G±(gmtgm)G±(gmtgm)G±(gmtt) and T(gmtgm)G±(gmtgm)G±(gmtt) conformers of TsBP. 29 ACS Paragon Plus Environment



π1(P1O2) hybrid orbital perpendicular to the existing π(P1O2) orbital


π1*(P1O2) hybrid orbital perpendicular to the existing π*(P1O2) orbital Figure 8. The pictorial representation of the new π1(P1O2) hybrid orbital formed through the delocalization of oxygen lone pair to vacant ‘d’ orbital of phosphorus in the G±(gmtgm)G±(gmtgm)G±(gmtt). The corresponding antibonding π1*(P1O2) orbital is shown for T(gmtgm)G±(gmtgm)G±(gmtt) conformer of TsBP.


Page 30 of 38

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


G±(gmtgm)G±(gmtgm)G±(gmtt) Secondary vicinal

n2O3 → π1*(P1-O2) E2= 4.19 kcal/mol

Secondary geminal

n1O4 → π1*(P1-O2) E2= 1.00 kcal/mol

π1(P1-O2) →σ*(P1-O4) E2= 29.49 kcal/mol

σ(P1-O4) → π1*(P1-O2) E2= 27.44 kcal/mol


n1O3 → π1*(P1-O2) E2= 10.88 kcal/mol

n2O4 → π1*(P1-O2) E2= 2.55 kcal/mol

π1(P1-O2) →σ*(P1-O4) E2= 6.65 kcal/mol

σ(P1-O5) → π1*(P1-O2) E2= 9.48 kcal/mol

Figure 9. A few important ‘secondary’ vicinal and geminal hyperconjugative interactions along with their E2 energies present in the G±(gmtgm)G±(gmtgm)G±(gmtt) and T(gmtgm)G±(gmtgm)G±(gmtt) conformers of TsBP.


1300

721.2

816.9 742.4

e)

741.3

806.5 808.6

863.9

f)

741.3 720.1

c) 720.1

816.9 819.2

863.9

809.5

d)

863.9

1175.1

1024.9

1175.1

1172.8 1150.5

1166.5

1026.9 1026.9 1031.2 996.2 982.5 1016.3 1001.4 960.8 952.8

1028.2

863.9

960.8 940.7

1026.9

1188.8 1172.8

1262.5 1279.4 1300.8 1271.0 1260.2 1283.7 1250.5 1261.3

1384.8 1379.6 1378.5 1380.8

1463.9 1462.7 1462.7

1500

1293.4 1274.2

1314.7

1381.7

1463.9 1460.5

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 38

996.0


b) a)

1100 -1 Wavenumber, cm

900

700

Figure 10. The infrared spectra of Th(NO3)4-TsBP complex in dodecane. a) neat TsBP b) 30% TsBP in dodecane equilibrated with 1 M HNO3 c) Th(NO3)4-TsBP complex equilibrated with 1 M HNO3 diluent phase d) Th(NO3)4-TsBP complex equilibrated with 1 M HNO3 third phase e) solid Th(NO3)4. 5H2O f) UO2(NO3)2-TsBP complex equilibrated with 1 M HNO3 organic phase.


Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic Straight chain to branching

TsBP

TGG Cluster 32

GGG Cluster 21



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Influence of Branching on the Conformational Space: Case Study of

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