A B S T R A C T Using Density Functional Theory (DFT) calculations, we studied the electrochemis... more A B S T R A C T Using Density Functional Theory (DFT) calculations, we studied the electrochemistry of polyoxome-talates (POMs), specifically the redox properties of Mn in tri-Mn-substituted W-based Keggin ions. For direct comparison with recent cyclic voltammetry results [J. Friedl et al. Electrochim. Acta, 141 (2014) 357], we estimated the reversible half-wave potentials of proton-and cation-coupled electron transfer for Mn(IV/III) and Mn(III/II), respectively. The calculated reversible potentials agree well with experiment, reproducing the trend with pH for Mn(IV/III). For adequate DFT energies, it is crucial to apply a reliable description of the electrolyte environment of the POM, accounting also for their rather high charges, up to À7 e. To this end, we included the Li + counterions, required for charge neutralization, directly in the quantum chemical models which were embedded in a polarizable continuum. We explored various arrangements of the Li + ions around the POMs and their effect on both structural parameters and electrochemical properties of the POMs. Hybrid functionals (TPSSh, B3LYP, PBE0) overestimate the experimental reduction potentials: the larger the exact-exchange contribution, the larger the resulting reduction potential. The best agreement with experiment is achieved with the PBE approach, likely due to fortuitous error cancellation. The results of the present work indicate that a more sophisticated (atomistic) representation of the electrolyte environment will be beneficial for predicting redox potentials in better agreement with experiment.
Journal of Molecular and Engineering Materials, 2014
Many interesting properties of polyoxometalates (POMs) are related to their ability to reversibly... more Many interesting properties of polyoxometalates (POMs) are related to their ability to reversibly accept and release electrons. The correct computational description of metal electrons in POMs is central to fully understand the implications and mechanisms related to reduced structures. We performed DFT calculations on fully oxidized [P 2 W 18 O 62 ] 6À and 1 = 2 -[P 2 MW 17 O 62 ] nÀ (M ¼ V, Mo) anions, and their one-electron reduced partners. Atomic spin populations of the reduced forms reveal that pure GGA functionals, which overdelocalize the electron density by nature, cannot correctly describe the extra electron especially when competition between the localized and delocalized situations is very close in energy, namely for 2 -[P 2 MoW 17 O 62 ] 7À . On the other hand, hybrid functionals like B3LYP or M05 can localize the electron at the correct site. Within these \well-behaved" density functionals, only the B3LYP gives the correct ordering and relative reduction energies (REs) with respect to electrochemical measurements, although the absolute values are always less exothermic than expected. At present, this is the best density functional for computing redox properties of POMs.
The first members of a promising new family of hybrid amino acid-polyoxometalates have emerged fr... more The first members of a promising new family of hybrid amino acid-polyoxometalates have emerged from a search for modular functional molecules. Incorporation of glycine (Gly) or norleucine (Nle) ligands into an yttrium-tungstoarsenate structural backbone, followed by crystallization with p-methylbenzylammonium (p-MeBzNH 3 + ) cations, affords (p-MeBzNH 3 ) 6 K 2 (GlyH)[As III 4 (Y III W VI 3 )W VI 44 Y III 4 O 159 (Gly) 8 -(H 2 O) 14 ]·47 H 2 O (1) and enantiomorphs (p-MeBzNH 3 ) 15 (NleH) 3 [As III 4 (Mo V 2 Mo VI 2 )W VI 44 Y III 4 O 160 (Nle) 9 (H 2 O) 11 ][As III 4 (Mo VI 2 W VI 2 )-W VI 44 Y III 4 O 160 (Nle) 9 (H 2 O) 11 ] (generically designated 2: l-Nle, 2 a; d-Nle, 2 b
Polyoxometalates (POMs) are inorganic entities featuring extensive and sometimes unusual redox pr... more Polyoxometalates (POMs) are inorganic entities featuring extensive and sometimes unusual redox properties. In this work, several experimental techniques as well as density functional theory (DFT) calculations have been applied to identify and assess the relevance of factors influencing the redox potentials of POMs. First, the position of the Mo substituent atom in the Wells-Dawson structure, α1- or α2-P2W17Mo, determines the potential of the first 1e(-) reduction wave. For P2W(18-x)Mox systems containing more than one Mo atom, reduction takes place at successively more positive potentials. We attribute this fact to the higher electron delocalization when some Mo oxidizing atoms are connected. After having analyzed the experimental and theoretical data for the monosubstituted α1- and α2-P2W17Mo anions, some relevant facts arise that may help to rationalize the redox behavior of POMs in general. Three aspects concern the stability of systems: (i) the favorable electron delocalization, (ii) the unfavorable e(-)-e(-) electrostatic repulsion, and (iii) the favorable electron pairing. They explain trends such as the second reduction wave occurring at more positive potentials in α1- than in α2-P2W17Mo, and also the third electron reduction taking place at a less negative potential in the case of α2, reversing the observed behavior for the first and the second waves. In P2W17V derivatives, the nature of the first "d" electron is more localized because of the stronger oxidant character of V(V). Thus, the reduction potentials as well as the computed reduction energies (REs) for the second reduction of either isomer are closer to each other than in Mo-substituted POMs. This may be explained by the lack of electron delocalization in monoreduced P2W17V(IV) systems.
The radioactive element technetium-99 ( 99 Tc, half-life = 2.1 × 10 5 years, β − of 253 keV), is ... more The radioactive element technetium-99 ( 99 Tc, half-life = 2.1 × 10 5 years, β − of 253 keV), is a major byproduct of 235 U fission in the nuclear fuel cycle. 99 Tc is also found in radioactive waste tanks and in the environment at National Lab sites and fuel reprocessing centers. Separation and storage of the long-lived 99 Tc in an appropriate and stable waste-form is an important issue that needs to be addressed. Considering metal oxide solid-state materials as potential storage matrixes for Tc, we are examining the redox speciation of Tc on the molecular level using polyoxometalates (POMs) as models. In this study we investigate the electrochemistry of Tc complexes of the monovacant Wells−Dawson isomers, α 1 -P 2 W 17 O 61 10− (α1) and α 2 -P 2 W 17 O 61 10− (α2) to identify features of metal oxide materials that can stabilize the immobile Tc(IV) oxidation state accessed from the synthesized Tc(V)O species and to interrogate other possible oxidation states available to Tc within these materials. The experimental results are consistent with density functional theory (DFT) calculations. Electrochemistry of K 7−n H n [Tc V O(α 1 -P 2 W 17 O 61 )] (Tc V O-α1), K 7−n H n [Tc V O(α 2 -P 2 W 17 O 61 )] (Tc V O-α2) and their rhenium analogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues. Both Tc and Re are reduced more readily in the lacunary α1 site as compared to the α2 site. The DFT calculations elucidate that the highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation state for Tc is VI. The M V → M IV reduction processes for Tc V O-α1 are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the M V O-α2 (M = 99 Tc, Re) and Re V O-α1 where M V/IV reduction process must occur hand in hand with protonation of the terminal MO to make the π*(MO) orbitals accessible to the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data that reveal that the Tc V is "pulled" into the -α1 framework and that may facilitate the reduction of Tc V O-α1 and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, and their impact on the chemical properties of the Tc substituted in these positions.
The unusual redox behavior displayed by the two isomers of the Wells−Dawson phosphotungstate anio... more The unusual redox behavior displayed by the two isomers of the Wells−Dawson phosphotungstate anion [Fe(H 2 O)P 2 W 17 O 61 ] 7− is presented. The electrochemical measurements have been performed in aqueous media at different pH values from 0.5 up to 8.0. The cyclic voltammetry has also been carried out in organic media to get additional experimental data to establish the effect of the protonation on the redox properties of both isomers. At high pH values (pH ≥ 6) or in an organic medium, the reduction of the Fe center is easier in the case of the alpha-1 isomer, whereas for the alpha-2 isomer such reduction takes place at more negative potentials, as expected. In contrast, at lower pH values (pH ≤ 5), an inversion of this trend is observed, and the reduction of the Fe center becomes easier for the alpha-2 isomer compared to the alpha-1. We were able to highlight the influence of the pH and the pK a of the electrolyte on POM-based redox potentials given the pK a of the latter. A complementary theoretical study has also been performed to explain the experimental data obtained. In this sense, the results obtained from the DFT study are in good agreement with the experimental data mentioned above and have provided additional information for the electrochemical behavior of both isomers according to their different molecular orbital energies. We have also shown the influence of protonation state of the iron derivative on the relative reduction potentials of both isomers.
In the 1960s, Pope and co-workers found a linear dependence between the redox potentials of a fam... more In the 1960s, Pope and co-workers found a linear dependence between the redox potentials of a family of isostructural α-[XW 12 O 40 ] n-Keggin anions and their molecular charge with a slope of -0.18 V per unit charge. That finding was restricted to variations in the internal atom or heteroatom of the molecule (X = P V , Si IV , Al III , etc.). However, other ways to change the charge of Keggin anions are commonplace, such as previous reduction processes or chemical changes in the external positions of the W 12 O 36 cage, and also lead to variations in their oxidising power. In the present theoretical work, we analyse the oxidising power, computed
An innovative study on the electrochemical behaviour of mixed d metal-iron containing Wells-Dawso... more An innovative study on the electrochemical behaviour of mixed d metal-iron containing Wells-Dawson sandwich-type complexes [(FeOH 2 ) 2 M 2 (X 2 W 15 O 56 ) 2 ] 14− and [(MOH 2 ) 2 Fe 2 (X 2 W 15 O 56 ) 2 ] 14− (with M = Cr III , Mn II , Mn III , Co II , Ni II , Cu II or Zn II and X = P V or As V ) was carried out. These complexes have a four-centre equatorial metal cluster constituted of two Fe atoms and two atoms of another metal. The Fe III centres are either in an external position, [(FeOH 2 ) 2 M 2 ], or in an internal position, [M 2 (OH 2 ) 2 Fe 2 ]. Experimental methods (cyclic voltammetry and controlled potential coulometry) and theoretical calculations (density functional theory) allowed us to determine and analyse the redox potential values associated with the reduction of the Fe III centres in these species. The influence of the position of the Fe III centres, the nature of the metal centre M and the electron density distribution in the tetranuclear cluster (either [(FeOH 2 ) 2 M 2 ] or [(MOH 2 ) 2 Fe 2 ]) have been studied and rationalised in order to account for the observed behaviours. The data suggest that the most stable isomers are those where Fe III centres are internally-located, [(MOH 2 ) 2 Fe 2 ]. Consequently, their reduction is more difficult than those having externally-located Fe III isomers, [(FeOH 2 ) 2 M 2 ]. Some experimental results revealed a few exceptions to this rule which have not been rationalised yet.
In this contribution, a design of a synthetic calibration genetic circuit to characterize the rel... more In this contribution, a design of a synthetic calibration genetic circuit to characterize the relative strength of different sensing promoters is proposed and its specifications and performance are analyzed via an effective mathematical model. Our calibrator device possesses certain novel and useful features like modularity (and thus the possibility of being used in many different biological contexts), simplicity, being based on a single cell, high sensitivity and fast response. To uncover the critical model parameters and the corresponding parameter domain at which the calibrator performance will be optimal, a sensitivity analysis of the model parameters was carried out over a given range of sensing protein concentrations (acting as input). Our analysis suggests that the half saturation constants for repression, sensing and difference in binding cooperativity (Hill coefficients) for repression are the key to the performance of the proposed device. They furthermore are determinant for the sensing speed of the device, showing that it is possible to produce de-tectable differences in the repression protein concentrations and in turn in the corresponding fluorescence in less than two hours. This analysis paves the way for the design, experimental construction and validation of a new family of functional genetic circuits for the purpose of calibrating promoters.
A B S T R A C T Using Density Functional Theory (DFT) calculations, we studied the electrochemis... more A B S T R A C T Using Density Functional Theory (DFT) calculations, we studied the electrochemistry of polyoxome-talates (POMs), specifically the redox properties of Mn in tri-Mn-substituted W-based Keggin ions. For direct comparison with recent cyclic voltammetry results [J. Friedl et al. Electrochim. Acta, 141 (2014) 357], we estimated the reversible half-wave potentials of proton-and cation-coupled electron transfer for Mn(IV/III) and Mn(III/II), respectively. The calculated reversible potentials agree well with experiment, reproducing the trend with pH for Mn(IV/III). For adequate DFT energies, it is crucial to apply a reliable description of the electrolyte environment of the POM, accounting also for their rather high charges, up to À7 e. To this end, we included the Li + counterions, required for charge neutralization, directly in the quantum chemical models which were embedded in a polarizable continuum. We explored various arrangements of the Li + ions around the POMs and their effect on both structural parameters and electrochemical properties of the POMs. Hybrid functionals (TPSSh, B3LYP, PBE0) overestimate the experimental reduction potentials: the larger the exact-exchange contribution, the larger the resulting reduction potential. The best agreement with experiment is achieved with the PBE approach, likely due to fortuitous error cancellation. The results of the present work indicate that a more sophisticated (atomistic) representation of the electrolyte environment will be beneficial for predicting redox potentials in better agreement with experiment.
Journal of Molecular and Engineering Materials, 2014
Many interesting properties of polyoxometalates (POMs) are related to their ability to reversibly... more Many interesting properties of polyoxometalates (POMs) are related to their ability to reversibly accept and release electrons. The correct computational description of metal electrons in POMs is central to fully understand the implications and mechanisms related to reduced structures. We performed DFT calculations on fully oxidized [P 2 W 18 O 62 ] 6À and 1 = 2 -[P 2 MW 17 O 62 ] nÀ (M ¼ V, Mo) anions, and their one-electron reduced partners. Atomic spin populations of the reduced forms reveal that pure GGA functionals, which overdelocalize the electron density by nature, cannot correctly describe the extra electron especially when competition between the localized and delocalized situations is very close in energy, namely for 2 -[P 2 MoW 17 O 62 ] 7À . On the other hand, hybrid functionals like B3LYP or M05 can localize the electron at the correct site. Within these \well-behaved" density functionals, only the B3LYP gives the correct ordering and relative reduction energies (REs) with respect to electrochemical measurements, although the absolute values are always less exothermic than expected. At present, this is the best density functional for computing redox properties of POMs.
The first members of a promising new family of hybrid amino acid-polyoxometalates have emerged fr... more The first members of a promising new family of hybrid amino acid-polyoxometalates have emerged from a search for modular functional molecules. Incorporation of glycine (Gly) or norleucine (Nle) ligands into an yttrium-tungstoarsenate structural backbone, followed by crystallization with p-methylbenzylammonium (p-MeBzNH 3 + ) cations, affords (p-MeBzNH 3 ) 6 K 2 (GlyH)[As III 4 (Y III W VI 3 )W VI 44 Y III 4 O 159 (Gly) 8 -(H 2 O) 14 ]·47 H 2 O (1) and enantiomorphs (p-MeBzNH 3 ) 15 (NleH) 3 [As III 4 (Mo V 2 Mo VI 2 )W VI 44 Y III 4 O 160 (Nle) 9 (H 2 O) 11 ][As III 4 (Mo VI 2 W VI 2 )-W VI 44 Y III 4 O 160 (Nle) 9 (H 2 O) 11 ] (generically designated 2: l-Nle, 2 a; d-Nle, 2 b
Polyoxometalates (POMs) are inorganic entities featuring extensive and sometimes unusual redox pr... more Polyoxometalates (POMs) are inorganic entities featuring extensive and sometimes unusual redox properties. In this work, several experimental techniques as well as density functional theory (DFT) calculations have been applied to identify and assess the relevance of factors influencing the redox potentials of POMs. First, the position of the Mo substituent atom in the Wells-Dawson structure, α1- or α2-P2W17Mo, determines the potential of the first 1e(-) reduction wave. For P2W(18-x)Mox systems containing more than one Mo atom, reduction takes place at successively more positive potentials. We attribute this fact to the higher electron delocalization when some Mo oxidizing atoms are connected. After having analyzed the experimental and theoretical data for the monosubstituted α1- and α2-P2W17Mo anions, some relevant facts arise that may help to rationalize the redox behavior of POMs in general. Three aspects concern the stability of systems: (i) the favorable electron delocalization, (ii) the unfavorable e(-)-e(-) electrostatic repulsion, and (iii) the favorable electron pairing. They explain trends such as the second reduction wave occurring at more positive potentials in α1- than in α2-P2W17Mo, and also the third electron reduction taking place at a less negative potential in the case of α2, reversing the observed behavior for the first and the second waves. In P2W17V derivatives, the nature of the first "d" electron is more localized because of the stronger oxidant character of V(V). Thus, the reduction potentials as well as the computed reduction energies (REs) for the second reduction of either isomer are closer to each other than in Mo-substituted POMs. This may be explained by the lack of electron delocalization in monoreduced P2W17V(IV) systems.
The radioactive element technetium-99 ( 99 Tc, half-life = 2.1 × 10 5 years, β − of 253 keV), is ... more The radioactive element technetium-99 ( 99 Tc, half-life = 2.1 × 10 5 years, β − of 253 keV), is a major byproduct of 235 U fission in the nuclear fuel cycle. 99 Tc is also found in radioactive waste tanks and in the environment at National Lab sites and fuel reprocessing centers. Separation and storage of the long-lived 99 Tc in an appropriate and stable waste-form is an important issue that needs to be addressed. Considering metal oxide solid-state materials as potential storage matrixes for Tc, we are examining the redox speciation of Tc on the molecular level using polyoxometalates (POMs) as models. In this study we investigate the electrochemistry of Tc complexes of the monovacant Wells−Dawson isomers, α 1 -P 2 W 17 O 61 10− (α1) and α 2 -P 2 W 17 O 61 10− (α2) to identify features of metal oxide materials that can stabilize the immobile Tc(IV) oxidation state accessed from the synthesized Tc(V)O species and to interrogate other possible oxidation states available to Tc within these materials. The experimental results are consistent with density functional theory (DFT) calculations. Electrochemistry of K 7−n H n [Tc V O(α 1 -P 2 W 17 O 61 )] (Tc V O-α1), K 7−n H n [Tc V O(α 2 -P 2 W 17 O 61 )] (Tc V O-α2) and their rhenium analogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues. Both Tc and Re are reduced more readily in the lacunary α1 site as compared to the α2 site. The DFT calculations elucidate that the highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation state for Tc is VI. The M V → M IV reduction processes for Tc V O-α1 are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the M V O-α2 (M = 99 Tc, Re) and Re V O-α1 where M V/IV reduction process must occur hand in hand with protonation of the terminal MO to make the π*(MO) orbitals accessible to the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data that reveal that the Tc V is "pulled" into the -α1 framework and that may facilitate the reduction of Tc V O-α1 and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, and their impact on the chemical properties of the Tc substituted in these positions.
The unusual redox behavior displayed by the two isomers of the Wells−Dawson phosphotungstate anio... more The unusual redox behavior displayed by the two isomers of the Wells−Dawson phosphotungstate anion [Fe(H 2 O)P 2 W 17 O 61 ] 7− is presented. The electrochemical measurements have been performed in aqueous media at different pH values from 0.5 up to 8.0. The cyclic voltammetry has also been carried out in organic media to get additional experimental data to establish the effect of the protonation on the redox properties of both isomers. At high pH values (pH ≥ 6) or in an organic medium, the reduction of the Fe center is easier in the case of the alpha-1 isomer, whereas for the alpha-2 isomer such reduction takes place at more negative potentials, as expected. In contrast, at lower pH values (pH ≤ 5), an inversion of this trend is observed, and the reduction of the Fe center becomes easier for the alpha-2 isomer compared to the alpha-1. We were able to highlight the influence of the pH and the pK a of the electrolyte on POM-based redox potentials given the pK a of the latter. A complementary theoretical study has also been performed to explain the experimental data obtained. In this sense, the results obtained from the DFT study are in good agreement with the experimental data mentioned above and have provided additional information for the electrochemical behavior of both isomers according to their different molecular orbital energies. We have also shown the influence of protonation state of the iron derivative on the relative reduction potentials of both isomers.
In the 1960s, Pope and co-workers found a linear dependence between the redox potentials of a fam... more In the 1960s, Pope and co-workers found a linear dependence between the redox potentials of a family of isostructural α-[XW 12 O 40 ] n-Keggin anions and their molecular charge with a slope of -0.18 V per unit charge. That finding was restricted to variations in the internal atom or heteroatom of the molecule (X = P V , Si IV , Al III , etc.). However, other ways to change the charge of Keggin anions are commonplace, such as previous reduction processes or chemical changes in the external positions of the W 12 O 36 cage, and also lead to variations in their oxidising power. In the present theoretical work, we analyse the oxidising power, computed
An innovative study on the electrochemical behaviour of mixed d metal-iron containing Wells-Dawso... more An innovative study on the electrochemical behaviour of mixed d metal-iron containing Wells-Dawson sandwich-type complexes [(FeOH 2 ) 2 M 2 (X 2 W 15 O 56 ) 2 ] 14− and [(MOH 2 ) 2 Fe 2 (X 2 W 15 O 56 ) 2 ] 14− (with M = Cr III , Mn II , Mn III , Co II , Ni II , Cu II or Zn II and X = P V or As V ) was carried out. These complexes have a four-centre equatorial metal cluster constituted of two Fe atoms and two atoms of another metal. The Fe III centres are either in an external position, [(FeOH 2 ) 2 M 2 ], or in an internal position, [M 2 (OH 2 ) 2 Fe 2 ]. Experimental methods (cyclic voltammetry and controlled potential coulometry) and theoretical calculations (density functional theory) allowed us to determine and analyse the redox potential values associated with the reduction of the Fe III centres in these species. The influence of the position of the Fe III centres, the nature of the metal centre M and the electron density distribution in the tetranuclear cluster (either [(FeOH 2 ) 2 M 2 ] or [(MOH 2 ) 2 Fe 2 ]) have been studied and rationalised in order to account for the observed behaviours. The data suggest that the most stable isomers are those where Fe III centres are internally-located, [(MOH 2 ) 2 Fe 2 ]. Consequently, their reduction is more difficult than those having externally-located Fe III isomers, [(FeOH 2 ) 2 M 2 ]. Some experimental results revealed a few exceptions to this rule which have not been rationalised yet.
In this contribution, a design of a synthetic calibration genetic circuit to characterize the rel... more In this contribution, a design of a synthetic calibration genetic circuit to characterize the relative strength of different sensing promoters is proposed and its specifications and performance are analyzed via an effective mathematical model. Our calibrator device possesses certain novel and useful features like modularity (and thus the possibility of being used in many different biological contexts), simplicity, being based on a single cell, high sensitivity and fast response. To uncover the critical model parameters and the corresponding parameter domain at which the calibrator performance will be optimal, a sensitivity analysis of the model parameters was carried out over a given range of sensing protein concentrations (acting as input). Our analysis suggests that the half saturation constants for repression, sensing and difference in binding cooperativity (Hill coefficients) for repression are the key to the performance of the proposed device. They furthermore are determinant for the sensing speed of the device, showing that it is possible to produce de-tectable differences in the repression protein concentrations and in turn in the corresponding fluorescence in less than two hours. This analysis paves the way for the design, experimental construction and validation of a new family of functional genetic circuits for the purpose of calibrating promoters.
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Papers by Pablo Aparicio