Theoretical Modeling of 99Tc NMR Chemical Shifts - Inorganic

Article pubs.acs.org/IC

Theoretical Modeling of †

99

Tc NMR Chemical Shifts

‡

Gabriel B. Hall, Amity Andersen, Nancy M. Washton,‡ Sayandev Chatterjee,† and Tatiana G. Levitskaia*,† †

Energy and Environment Directorate and ‡Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *

ABSTRACT: Technetium-99 (Tc) displays a rich chemistry due to its wide range of accessible oxidation states (from −I to +VII) and ability to form coordination compounds. Determination of Tc speciation in complex mixtures is a major challenge, and 99Tc nuclear magnetic resonance (NMR) spectroscopy is widely used to probe chemical environments of Tc in odd oxidation states. However, interpretation of 99Tc NMR data is hindered by the lack of reference compounds. Density functional theory (DFT) calculations can help to fill this gap, but to date few computational studies have focused on 99Tc NMR of compounds and complexes. This work evaluates the effectiveness of both pure generalized gradient approximation and their corresponding hybrid functionals, both with and without the inclusion of scalar relativistic effects, to model the 99Tc NMR spectra of Tc(I) carbonyl compounds. With the exception of BLYP, which performed exceptionally well overall, hybrid functionals with inclusion of scalar relativistic effects are found to be necessary to accurately calculate 99Tc NMR spectra. The computational method developed was used to tentatively assign an experimentally observed 99 Tc NMR peak at −1204 ppm to fac-Tc(CO)3(OH)32−. This study examines the effectiveness of DFT computations for interpretation of the 99Tc NMR spectra of Tc(I) coordination compounds in high salt alkaline solutions.

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INTRODUCTION Over the last two decades, Tc(I) carbonyl compounds and, in particular, stable cationic d6 fac-Tc(CO)3(OH2)3+ complexes have been extensively investigated from the fundamental coordination chemistry standpoint and for their applications in nuclear medicine.1,2 The recent surprising discovery that facTc(CO)3+ and/or fac-Tc(CO)2(NO)2+ derivatives are potentially present in radioactive nuclear tank wastes, such as those stored at the U.S. Department of Energy Hanford Site,3 cannot be currently explained. In highly alkaline solutions typifying Hanford tank wastes, hydrolysis of the carbonyl carbon to form carboxylate4,5 followed by destruction of the stable tricarbonyl moiety and rapid oxidation of Tc(I) are expected.6 The persistence of the fac-Tc(CO)3+ compounds under these conditions highlights a need for understanding coordination properties and reactivity of the fac-Tc(CO)3+ compounds in solutions of variable ionic strengths and in the presence of small organic complexing substrates found in some nuclear waste supernatants. In experimental studies where Tc speciation is unknown, one current gap in the literature is the absence of an adequate number of Tc compounds that can serve as reference points when identifying the nature of the auxiliary ligands coordinated to the fac-Tc(CO)3+ metal core based on the spectroscopic signatures of the resulting complexes. This knowledge is essential for the rational design of the methodologies for the management of radioactive 99Tc contamination, including its selective separation from nuclear waste streams and immobilization for long-term storage. One avenue being pursued to understand the rich chemistry of Tc complexes is the development of a spectral library, including 99Tc nuclear magnetic resonance (NMR), to aid in © XXXX American Chemical Society

the assignment of novel Tc compounds, as well as any presently unidentified components/reactants present in nuclear wastes. Without additional isotopic labeling, technetium compounds of the form fac-Tc(CO)3+ often do not contain heteroatoms with a nuclear spin in close enough proximity to the 99Tc nucleus to observe J-coupling at the line widths commonly observed in 99 Tc NMR. This presents a challenge in interpreting 99Tc NMR spectra; however, 99Tc does have a large spectral window with known compounds spanning a range of at least 9000 ppm.7 A recent review describes application of 99Tc NMR for evaluation of the reactivity of fac-Tc(CO)3(OH2)3+, including ligand exchange and complex formation reactions.8 The rarity of J couplings combined with a large spectral window over which signals can occur provides a prime opportunity for theoretical calculations to be a large asset in the assignment of 99 Tc NMR signals. Recent advances in computational chemistry have been broadly applied to Tc research.9−21 These studies have focused on important aspects of fundamental technetium chemistry, from structure prediction, prediction of pertechnetate reduction products, thermodynamic properties, interpretation of extended X-ray absorption fine structure and electron paramagnetic resonance spectra, and prediction of reaction mechanisms; however, to the best of our knowledge, few reports exist focusing on validation of 99Tc NMR computational methods against empirically obtained solution-phase measurements.19 Many of the aforementioned studies have used the B3LYP functional, which is popular for transition metals, and they obtained good results for their purposes; Received: March 1, 2016

A

DOI: 10.1021/acs.inorgchem.6b00458 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry however, the recent study by Buhl and Golubnychiy19 suggests that B3LYP may not be the functional of choice for magnetic parameters for the center of the transition metal series where Tc lies. To this end, this study seeks to further validate computational methods against Tc(I) species bearing CO and NO ligands for which 99Tc NMR chemical shifts are known;

Relativistic effects have been shown to affect density functional theory (DFT)-computed NMR chemical shifts;23,40 thus, the effects of scalar relativistic and spin−orbit effects are evaluated for each functional within this report. The focus of this work is a threefold evaluation of computational methods for 99Tc NMR shift prediction: evaluating the effectiveness of the BLYP, PW91, and PBE exchange-correlation functionals, evaluating the inclusion scalar relativistic and spin−orbit effects, and evaluating hybrid functionals against the pure GGA counterpart.

Chart 1. Technetium Compounds with Known Literature Values of the 99Tc Nuclear Magnetic Resonance Chemical Shifts Used to Benchmark Density Functional Theory Computational Methods

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METHODS

Computational Methods. The ORCA software program version 3.0.345 was chosen due to the emphasis on magnetic spectroscopies during software development. Where appropriate, relativistic effects were accounted for with the scalar relativistic zeroth order regular approximation (ZORA) Hamiltonian,46 and spin−orbit effects were considered in all calculations of the magnetic shielding via the spin− orbit mean-field approximation (SOMF) to the Breit−Pauli (BP) operator. For all ZORA calculations, picture-change corrections were applied. Prior to calculation of NMR parameters, all structures were first optimized with the respective functional, basis set, and inclusion of relativistic effects employed for computation of the magnetic parameters. Because of the aqueous nature of the solutions of interest, all calculations were performed in a polarized continuous medium with a dielectric constant corresponding to water of 80.4 using the COnductor-like Screening MOdel (COSMO).47 Seven Tc compounds with established 99Tc NMR chemical shifts were used to benchmark DFT computational methods. The uncontracted def-2 TZVPP26,27 basis set was employed in all of the GGA, PBE,28 BLYP,29,30 PW91,31−33 B3LYP,29,34−36 PBE0,37−39 and B3PW9131−33 DFT calculations. For relativistic calculations, we employ the recontracted version of def2-TZVPP adapted to the ZORA Hamiltonians. Because of this study being primarily concerned with molecules of the general formula Tc(CO)3(OH2)3−x(OH)x1−x, using a molecule with similar size, bond lengths, and bond types, such as Tc(CO)3(OH2)3+, as a reference allows for a reduction of errors. For this reason, all 99Tc NMR chemical shifts presented in the main manuscript are reported against the average literature value of −869 ppm.41,42 Chemical shifts referenced against selected other molecules in this study can be found in the Supporting Information. Materials. Radiation Safety Disclaimer. Caution! All known isotopes of Tc are radioactive. Technetium-99 (99Tc) used in this work has a half-life of 2.12 x105 years and emits a low-energy (0.292 MeV) β particle. All handling of 99Tc should be performed in a laboratory approved for the handing of radioactive materials. Radiation safety procedures must be used at all times to prevent contamination. For preparation of (Et4N)2[fac-Tc(CO)3Cl3], in-house NH4TcO4 stock available at the Radiochemical Processing Laboratory at Pacific Northwest National Laboratory was used. Diglyme, diethyl ether, dichloromethane, and borane-tetrahydrofuran (BH3/THF) were obtained from Sigma-Aldrich and used without further purification. Gaseous CO of 99.9% purity and argon of 99.998% purity used in the synthesis was obtained from Matheson Tri-Gas. Solid (Et4N)2[facTc(CO)3Cl3] compound was prepared, via a modified procedure

see Chart 1. These results will assist in the interpretation of 99 Tc NMR signals, which we are experimentally measuring. Of particular interest is correlation of the 99Tc NMR signals with the structure of the fac-Tc(CO)3+ coordination complexes containing multidentate organic ligands. Previous work has shown accurate prediction of NMR constants to be dependent upon the exchange correlation functional22−24 and particularly sensitive to hybridization with Hartree−Fock (HF) exact exchange for transition metal complexes.25 Consequently, for this work we chose a popular large triple-ζ basis set, def-2 TZVPP,26,27 to test three popular pure generalized gradient approximation (GGA) functionals, PBE,28 BLYP,29,30 and PW9131−33 against three popular hybrid functionals, B3LYP,29,34−36 PBE0,37−39 and B3PW91.31−33

Table 1. Comparison of the Chemical Shifts of the fac-Tc(CO)3+ Species Obtained in This Work with Literature Values chemical shift from this work, ppm matrix species +

fac-Tc(CO)3(OH2) fac-Tc(CO)3(OH2)2(OH) fac-Tc(CO)3(OH2)(OH)2− [fac-Tc(CO)3(OH)]4

literature chemical shift, ppm

[NaOH], M

5 M NaNO3

H2O

−869 −105541 −113941 −585,41 −56543

0 0.1−3.0 6 pH = 6.5−11

−869 −1069 −1146 −583

−868 −1062 −1139 −585

41,42

B


Article

Inorganic Chemistry reported elsewhere,43,48 by a two-step reduction procedure involving (i) reduction of TcO4− to Tc(V) and isolation as (Bu4N)[TcOCl4] solid and (ii) subsequent reduction of Tc(V) in the presence of CO to Tc(I) in the chemical form of fac-Tc(CO)3Cl3−2. The (Et4N)2[facTc(CO)3Cl3] product was separated by precipitation. Synthesis. To obtain 99Tc NMR spectra of the aqua fac-Tc(CO)3+ compounds in high ionic strength solutions mimicking nuclear tank wastes, all samples were prepared in a matrix containing 5 M NaNO3 and variable NaOH concentrations. (Et4N)2[fac-Tc(CO)3Cl3] was dissolved in 5 M NaNO3 maintained at pH = 5 to generate a 19 mM solution of fac-Tc(CO)3+. It is known that fac-Tc(CO)3Cl3−2 readily exchanges Cl− ligands with water to generate fac-Tc(CO)3(OH2)3+ at neutral and weakly acidic pH values.49 The resulting solution of facTc(CO)3(OH2)3+ in 5 M NaNO3 was analyzed by 99Tc NMR spectroscopy and showed a single fac-Tc(CO)3+ resonance at −869 ppm. To obtain the various subsequent hydrolysis products of the general formula fac-Tc(CO)3(OH2)3‑x(OH)x1−x (x = 1−3), the solution of fac-Tc(CO)3(OH2)3+ in 5 M NaNO3 was added to an equal volume of a solution containing a given concentration of NaOH and 5 M NaNO3 to generate the various species as shown in Table 1. To prepare a fac-Tc(CO)3+ species in 10 M NaOH solution, the solution of fac-Tc(CO)3(OH2)3+ in 5 M NaNO3 was added to an equal volume of 20 M NaOH solution, and a 99Tc NMR spectrum was recorded immediately. The NMR spectrum showed a single facTc(CO)3+ resonance at −1204 ppm, indicating a single fac-Tc(CO)3+ chemical environment. The chemical shift is significantly moved from the chemical shifts observed for fac-Tc(CO)3(OH2)3+ (−869 ppm), fac-Tc(CO)3(OH2)2(OH) (−1069 ppm), and fac-Tc(CO)3(OH2) (OH)2− (−1146 ppm). The value of the chemical shift, coupled with the magnitude and direction of the shift compared to the facTc(CO)3(OH2)3−x(OH)x1−x species, suggests that it is presumably due to fac-Tc(CO)3(OH)3−2. However, as described in a later section, this resonance can also be attributed to a dimeric species. The tetrameric compound [fac-Tc(CO)3(OH)]4 was prepared according to the modified literature procedure43 by dissolution of facTc(CO)3Cl3−2 in 0.1 M NaOH solution, extraction into diethyl ether, and subsequent crystallization from dichloromethane. The NMR spectrum of the tetrameric species was recorded in 5 M NaNO3 by making a saturated solution of the species in the above solution and showed a single resonance at −583 ppm. Note that the chemical shifts of the various fac-Tc(CO)3+ species in water when no NaNO3 is present are very similar to the shifts obtained in the presence of 5 M NaNO3, as shown in Table 1. Instrumentation. The 99Tc NMR sample solutions were placed in capped fluorinated ethylene propylene copolymer sleeves (Wilmad Lab Glass, Vineland, NJ), which were then inserted into 5 or 10 mm glass NMR tubes to provide secondary containment for the radioactive liquid. Technetium-99 NMR data were routinely collected at 67.565 MHz on a Tecmag Discovery spectrometer equipped with a 10 mm broadband Nalorac probe, at ambient laboratory temperature (21−25 °C) as described previously.20 A solution containing 10 mM NH4TcO4 was used as a 99Tc chemical shift reference, and all chemical shift data are quoted relative to TcO4−.50

same structures calculated by the other computational methods as well as experimentally determined solid-state structures. While comparing solution-state structures to solution-state calculated geometries is imperfect, we are not currently aware of experimentally measured solution-state bond lengths for this set of molecules. Key bond lengths of all exchange correlations are compared to crystal structure data for TcO4− 44 and [facTc(CO)3(OH)]4 43 in Table 2. In general we observe that the Table 2. Comparison of Solid-State Crystal Structure Bond Lengths (Å) to Density Functional Theory-Computed Values TcO4 -44 B3LYP BLYP B3PW91 PW91 PBE0 PBE crystal structure

SOMF ZORA+SOMF SOMF ZORA+SOMF SOMF ZORA+SOMF SOMF ZORA+SOMF SOMF ZORA+SOMF SOMF ZORA+SOMF

[fac-Tc(CO)3(OH)]4 43

Tc−O

Tc−C

Tc−OH

1.72 1.71 1.75 1.74 1.71 1.71 1.74 1.73 1.71 1.70 1.74 1.73 1.72

1.92 1.91 1.93 1.91 1.90 1.89 1.90 1.89 1.90 1.88 1.90 1.89 1.90

2.24 2.22 2.26 2.25 2.22 2.20 2.23 2.22 2.21 2.20 2.20 2.22 2.18

hybrid functionals (B3LYP, B3PW91, and PBE0) provide decreased bond lengths in comparison with their pure GGA counterparts. These results are in agreement with the general consensus that GGA functionals tend to overestimate bond lengths.52 This trend is also observed going from SOMF-only calculations to ZORA + SOMF calculations, with bond lengths decreasing by a couple of picometers. Validation of Computational Methods. Since relativistic effects have been noted to be nontrivial for 4d transition metals,40 we chose to compare the chemical shieldings with and without ZORA for each exchange-correlation functional. To most accurately model the molecules of interest to this study, technetium carbonyl compounds, a compound of this general structure, fac-Tc(CO)3(OH2)3+, is used as a reference compound. As can be seen by comparing Table 2 and Table S4 in the Supporting Information, this decreased the overall absolute mean standard deviation for some functionals as compared to referencing against TcO4−. Figure 1 displays calculated chemical shifts for the BLYP and B3LYP functionals versus empirically measured values with an idealized line with a slope of 1 and intercept of 0 for comparison. Exact values can be seen in Table 3 with parameters relating to analysis by linear regression found in Table 4. Figure 1 shows that the ZORA + SOMF calculation for BLYP is in good agreement between DFT-computed and empirical values. The mean absolute deviation from experiment is 86 ppm (Table 4). Inclusion of ZORA into the BLYP functional shows less of a deviation than for other wellperforming functionals tested but does decrease the mean absolute deviation to 66 ppm. The drop in mean absolute deviation is attributable in large part to the more accurate prediction of the chemical shift for [fac-Tc(CO)3(OH)]4. This is true for the other functionals tested as well and is easily visualized by examining Figure 1, where the data points

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RESULTS AND DISCUSSION Effects of Ionic Strength on 99Tc Chemical Shift. To better compare the strongly alkaline conditions typical of Hanford Tank waste to literature values for fac-Tc(CO)3+ species, a study of the effect of ionic strength on 99Tc NMR chemical shift was undertaken. As viewed in Table 1, a maximum shift of 7 ppm was observed for the compounds examined with many compounds shifting less than the error of experiment. Consequently, all DFT calculations were performed with a dielectric constant of 80.4, corresponding to that of water. Comparison of Density Functional Theory Versus Xray Bond Lengths. Initial validation was performed by comparing calculated structures, in COSMO solvation, to the C


Article

Inorganic Chemistry

Figure 1. DFT-computed 99Tc NMR chemical shifts plotted vs empirically measured values for the pure GGA exchange correlation BLYP and the hybrid B3LYP. An ideal line with a slope of 1 is shown for reference. (blue ◆) SOMF calculations without ZORA. (purple ×) Calculations incorporating ZORA.

Table 3. Density Functional Theory-Computed 99Tc NMR Theoretical Chemical Shiftsa compound BLYP B3LYP PW91 B3PW91 PBE PBE0 exp

SOMF ZORA+SOMF SOMF ZORA+SOMF SOMF ZORA+SOMF SOMF ZORA+SOMF SOMF ZORA+SOMF SOMF ZORA+SOMF NA

1

2

3

4

5

6

7

8

−869 −869 −869 −869 −869 −869 −869 −869 −869 −869 −869 −869 −86941,42

−23 66 −294 −144 212 270 −74 40 221 277 −79 43 0

−1058 −1046 −1070 −1053 −1043 −1032 −1063 −1046 −1038 −1028 −1061 −1040 −98142

−1089 −1065 −1166 −1131 −1033 −1016 −1115 −1085 −1029 −1013 −1125 −1090 −106041

−1177 −1132 −1326 −1259 −1099 −1061 −1243 −1186 −1091 −1057 −1261 −1198 −114641

−339 −372 −343 −382 −378 −408 −374 −410 −376 −408 −381 −414 −26851

−2176 −2156 −2293 −2244 −2116 −2105 −2234 −2195 −2108 −2099 −2250 −2205 −196142

−741 −600 −839 −681 −701 −564 −818 −666 −708 −571 −821 −667 −58541

a

The chemical shifts reported in this table have been referenced against fac-Tc(CO)3(OH2)3+. For this table compounds were relabeled as follows: 1 = fac-Tc(CO)3(OH2)3+, 2 = TcO4−, 3 = fac-Tc(CO)4(OH2)2+, 4 = fac-Tc(CO)3(OH2)2(OH), 5 = fac-Tc(CO)3(OH2)(OH)2−, 6 = facTc(CO)2(NO)(OH2)3+2, 7 = Tc(CO)6+, 8 = [fac-Tc(CO)3(OH)]4.

ppm for all computational data. BLYP ZORA + SOMF calculations yield a favorable R2 value of 0.994 for the line formed by plotting DFT-computed values against experimental values, and 0.993 for BLYP/ZORA + SOMF level calculations. The corresponding hybrid functional, B3LYP, underperforms BLYP in terms of mean absolute deviation, with values of 190 and 127 ppm for SOMF-only and ZORA + SOMF calculations, respectively. The R2 remains favorable for ZORA + SOMF at 0.993 while decreasing for SOMF only calculations to 0.978. This is the largest difference in performance seen with respect to R2 due to the inclusion ZORA for any functional tested in

corresponding to the tetranuclear species lie much closer to the idealized line in all cases. Note that the aforementioned 66 ppm absolute mean deviation corresponds to only ∼0.7% of the overall 99Tc NMR spectral window (9000 ppm). Importantly, the uncorrected accuracy of the chemical shift is the linear correlation between the computed values and the experimental values. A deviation from empirical values in an orderly fashion (R2 ≈ 1) can be corrected ex post facto. A linear regression analysis can be seen in Table 4; note that, when calculating the slope for each given data set, the value for facTc(CO)3(OH2)3+ was excluded, as it was artificially set to −869 D


Article

Inorganic Chemistry

shift, we attempted to obtain experimental 99Tc NMR spectrum of fac-Tc(CO)3(OH)3−2 and compute its chemical shift. Exposure of fac-Tc(CO)3Cl3−2 to an alkaline solution will allow substitution of chlorides with aqua ligands and hydroxides, producing fac-Tc(CO)3(OH2)2(OH) and facTc(CO)3(OH2)(OH)2− depending on solution alkalinity. Going to more alkaline conditions (Et4N)2[fac-Tc(CO)3Cl3] was dissolved in a 10 M caustic solution, and a new 99Tc NMR shift was experimentally measured at −1204 ppm. To investigate the structure responsible for this spectral signal, the calculated chemical shift from the four best performing functionals, BLYP, B3LYP, B3PW91, and PBE0, are analyzed for three possible structures seen in Chart 2.

Table 4. Linear Regression Analysis of the Functionals Used in This Studya XC BLYP B3LYP PW91 B3PW91 PBE PBE0

relativistic SOMF ZORA + SOMF ZORA + SOMF ZORA + SOMF ZORA + SOMF ZORA + SOMF ZORA +

SOMF SOMF SOMF SOMF SOMF SOMF

slope

R2

mean absolute deviation

1.06 1.08 1.03 1.06 1.10 1.11 1.06 1.08 1.10 1.11 1.07 1.09

0.994 0.993 0.978 0.993 0.980 0.974 0.990 0.992 0.978 0.973 0.990 0.992

86 66 190 127 104 108 131 90 106 107 139 94

Chart 2. Possible Reaction Products of (Et4N)2[facTc(CO)3Cl3] with 10 M NaOH/5 M NaNO3 Caustic Solution

a

Because of the chemical shift of fac-Tc(CO)3(OH2)3+ being set to −869 ppm as a reference compound it has been excluded from the below analysis.

this study. While a more modest increase in mean absolute deviation was seen for most molecules with inclusion of ZORA for B3LYP, much of the improvement in mean absolute deviation is due to more accurate prediction of TcO4− and the tetranuclear [fac-Tc(CO)3(OH)]4. The case of B3LYP ZORA + SOMF computations highlights the importance of selecting the proper reference compound for the functional being studied. Examination of Figure 1 shows that for B3LYP the magnitude of all chemical shifts is overestimated. Thus, with selection of a different reference molecule the mean absolute deviation would improve for B3LYP while perhaps becoming larger for other functionals. This can be seen by comparing the mean absolute deviation listed in Table 4 to the mean absolute deviation in Table S4, where all chemical shifts were referenced to TcO4−. With respect to the linearity of BLYP versus the linearity of B3LYP a large difference is seen for SOMF-only calculations, but calculations utilizing ZORA + SOMF perform comparably. When comparing PW91 to B3PW91, a significant improvement is seen for the hybrid functional. PW91 performs better with SOMF-only calculations than with ZORA + SOMF but is still lacking with an R2 of only 0.980. B3PW91 however performs much better and is comparable to the BLYP and B3LYP functionals with R2 values above 0.99 and mean absolute deviations that fall between those of BLYP and B3LYP. Similar results are seen for PBE and PBE0 with the PBE0 hybrid functional far outperforming the pure GGA counterpart. Overall, chemical shifts were better predicted for compounds that have a similar chemical shift to our chosen reference compound and that possess a single Tc center. Inclusion of ZORA became most necessary for two molecules in particular, TcO4− and [fac-Tc(CO)3(OH)]4. Examination of Table 3 or Figure 1 shows that there are three functionals, BLYP, B3PW91, and PBE0, that are much better than the others at predicting the chemical shift of TcO4−. This is particularly interesting for TcO4−, as it is the standard reference for experimental 99Tc NMR spectra and highlights the need to wisely choose a reference compound for DFT NMR calculations. 99 Tc Nuclear Magnetic Resonance of Trihydroxo Species. To test the ability of the best-performing computational methods against a compound of unknown experimental

Because of the large improvement seen in calculated chemical shift with inclusion of ZORA for [Tc(CO)3(OH)]4, which contains more than one Tc center, only the ZORA + SOMF level of theory is being considered in the main text with SOMFonly calculations tabulated in the Supporting Information. The chemical shift of fac-Tc(CO)3(OH)3−2 has calculated values of −1261 ppm (BLYP), −1424 ppm (B3LYP), −1336 ppm (PBE0), and −1325 ppm (B3PW91). The magnitude of the chemical shift is overestimated in all instances. This is in agreement with the results for the other Tc(CO)3(OH2)3−x(OH)x1−x species, which are generally overestimated for BLYP, B3LYP, and PBE0. While these calculations were performed at a dielectric constant of 80.4 and the experimental spectra were collected at high ionic strength, this is not believed to be a contributing factor, as Table 1, and the discussion thereof, shows that 99Tc NMR signals for these compounds varies only slightly with increasing ionic strength. While the error in chemical shift is within the range of error seen during validation of the computational method, other possible reaction products, seen in Chart 2, were considered. Literature precedence exists for Tc2μ-(OH)3(CO)6− with the Re analogue Re2μ-(OH)3(CO)6− and halide derivatives Tc2μCl3(CO)3− and Tc2μ-Br3(CO)3− being known.53−55 Technetium compounds with three strong π-backbonding ligands in a fac geometry with two bridging anionic ligands, trans-Tc2μ-Cl2(CO)4(NO)2Cl2, also exist in the literature,51 giving some precedence for trans-Tc2μ-(OH)2(CO)6(OH)2−2 as a possible structure. Calculations on these two structures were performed with unrestrained symmetry, and after averaging the chemical shift of the technetium centers, Tc2μ(OH)3(CO)6 gave chemical shifts of −1019 (BLYP), −1171 ppm (B3LYP), −1101 ppm (PBE0), and −1093 ppm (B3PW91). The chemical shifts for trans-Tc 2 μ(OH)2(CO)6(OH)2−2 were calculated as −1075 (BLYP), −1205 ppm (B3LYP), −1149 (PBE0), and −1146 (B3PW91). These values are summarized in Table 5. As expected, the 99Tc chemical shifts corresponding to the dimeric E


Inorganic Chemistry

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Table 5. Calculated Chemical Shift for Possible Products of the Reaction of fac-Tc(CO)3Cl3−2 with 10 M Caustic Solutiona XC BLYP B3LYP B3PW91 PBE0

relativistic ZORA ZORA ZORA ZORA

+ + + +

SOMF SOMF SOMF SOMF

9

10

11

−1261 −1424 −1325 −1336

−1075 −1205 −1146 −1149

−1019 −1171 −1093 −1101

*E-mail: [email protected]. Phone: 509-375-5646. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the U.S. Department of Energy’s Office of Environmental Management and performed as part of the Technetium Management Hanford Site project at the Pacific Northwest National Laboratory (PNNL) operated by Battelle for the U.S. Department of Energy under Contract No. DE-AC05-76RL01830. Part of this research was performed at EMSL, a national scientific user facility at PNNL managed by the Department of Energy’s Office of Biological and Environmental Research. The authors would like to especially acknowledge Dr. N. P. Machara for the stewardship of this research.

For the purposes of this table, 9 = fac-Tc(CO)3(OH)3 , 10 = Tc2μ(OH)3(CO)6, and 11 = trans-Tc2μ-(OH)2(CO)6(OH)2−2.

structures have a lower magnitude chemical shift than the mononuclear species. All values calculated for trans-Tc2μ(OH)2(CO)6(OH)2−2 underestimate the experimentally observed value, while to a lesser degree Tc2μ-(OH)3(CO)6 is also underestimated in all cases other than B3LYP, which overestimates the value by a single part per million. This goes against the trend seen in Table 3 and Figure 1 in which BLYP, B3LYP, and PBE0 consistently overestimate the magnitude of chemical shift. While a definitive assignment is not possible due to the error displayed by the calculations, the trend of overestimation of fac-Tc(CO)3(OH2)3−x(OH)x1−x species by BLYP, B3LYP, and PBE0 suggest the mononuclear fac-Tc(CO)3(OH)3−2 as the most likely structure of the signal observed at −1204 ppm. Further empirical measurements will be necessary to unambiguously assign this species and are currently underway.

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CONCLUSIONS The effectiveness of the BLYP, PW91, PBE, B3LYP, B3PW91, and PBE0 exchange correlation functionals for modeling 99Tc NMR spectra has been tested. Within each functional, the inclusion of relativistic effects and a comparison of pure GGA functionals to their hybrid counterparts has been investigated. With the exception of BLYP, the hybrid functionals have been found to more accurately reproduce empirical chemical shifts, both in terms of the mean absolute deviation from empirical values, and in linearity of their linear regression line of calculated chemical shifts as a function of experimental values. Inclusion of ZORA was shown to improve accuracy of the calculations, particularly with respect to the tetranuclear species [fac-Tc(CO)3(OH)]4. The overall best functional tested was BLYP with little difference seen in R2 between SOMF-only and ZORA + SOMF calculations but a significant improvement in mean absolute deviation for ZORA + SOMF calculations, particularly with respect to polynuclear species. On the basis of the calculations, a new assignment has tentatively been made for an experimentally observed 99Tc NMR peak at −1204 ppm as fac-Tc(CO)3(OH)3−2, though more experimental work is needed to verify this assignment.

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00458. Example input files for SOMF and ZORA + SOMF calculations, chemical shifts and linear regression of studied molecules, calculated chemical shifts for possible reaction products (PDF) F


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Theoretical Modeling of 99Tc NMR Chemical Shifts - Inorganic

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