Chemistry
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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
Chapters
Summary July 27th, 2022
Line shape analysis of NMR spectra collected over a range of temperatures serves as a guide for the rearrangement of inner coordination-sphere atoms at a chiral, eight-coordinate, rhenium(V) polyhydride complex, ReH5(PPh3)2(sec-butyl amine). Line shape analysis is also used to determine the activation parameters ΔH‡, ΔS‡, and ΔG‡ for those atom rearrangements.
Transcript
This method examines the dynamic behavior of the eight atoms that are bound to a central metal atom through line shape fitting of dynamic NMR spectrum. The visual nature of the line shape fitting technique allows for ready development of dynamic exchange models in comparisons of the model results with the observed spectra. Line shape fitting of NMR spectra is a method that is used to investigate a variety of dynamic molecular processes with activation energies in the range of 5 to 25 kilocalories per mole.
I would expect novice users to have questions regarding how to complete the inputs for the line shape fitting application. Practicing on the application, especially with an experienced user, is helpful. To begin, combine 0.15 grams of sodium borohydride and 0.41 grams of oxotrichlorobis triphenylphosphine rhenium-V in a two-or three-necked 100-milliliter round-bottom flask fitted with the rubber septum and gas port, or a 100-milliliter Kjeldahl flask fitted with a rubber septum.
In a fume hood, use a piece of rubber pressure tubing to connect the gas port of the reaction vessel with one of the stopcocks of a dual glass manifold for vacuum and nitrogen gas. Connect the glass vacuum manifold to a vacuum pump with rubber pressure tubing, glass nitrogen manifold to a regulated nitrogen gas cylinder, and the exit gas from the nitrogen gas manifold to a stopcock that can be used to direct the vented gas through either a two-centimeter column of mineral oil or mercury. Then, open the tap on the nitrogen cylinder and adjust the pressure on the flowing gas to 34 pounds per square inch and vent the nitrogen gas flow through the mercury bubbler.
Next, evacuate the gas inside the reaction vessel by adjusting the stopcock on the glass manifold to connect the vessel to the vacuum manifold, and fill the reaction vessel with nitrogen gas by changing the glass manifold stopcock that it connects the gas manifold with the reaction vessel. Then, add eight milliliters of deoxygenated water and eight milliliters of deoxygenated tetrahydrofuran to the solids in the reaction vessel via a syringe. Upon achieving an orange to tan color for the reaction mixture, filter the mixture through a 30-milliliters medium-centered glass funnel and wash the recovered solid three times each with 15-milliliters portions of water, methanol, and ethyl ether.
Next, fit the flask to a condenser equipped with a gas port and add a volume of eight milliliters of deoxygenated tetrahydrofuran to the reaction vessel via a syringe by cracking the joint between the round-bottomed flask and the condenser. Then, pour the reaction mixture into 25 milliliters of methanol in a 125-milliliter Erlenmeyer flask and add five milliliters of water to induce the formation of a flocculent yellow precipitate. To prepare the spectrometer, enter a flow rate of 200 liters per hour for the cooling gas and a target temperature of 290 kelvin for the probe, while allowing the spectrometer to stabilize at the target temperature for two minutes.
After shimming the sample at 290 kelvin, change the file name for each of the previously measured spectra by adding the temperature to the end of the file name and acquire a set of three spectra at 290 kelvin. Then increase the cooling gas flow rate by more than or equal to 30 liters per hour as needed to stabilize at the next temperature and decrease the target temperature by 10 kelvin. For line shape analysis of the measured spectra, click on the Edit Range button to enter the upper and lower chemical shifts for line shape fitting, and click the OK button to accept those limits.
Then, start a model for line shape fitting by clicking on the SpinSystem tab in the line shape fitting window and click on the Add button to allow for the building of a model spin system. Next, unselect LB and enter the value for line broadening manually with the mouse and the LB button on the line shape fitting toolbar. Add the first nucleus into the model by clicking on the Nucleus tab, followed by clicking on the Add button, and a set of default values will appear for nucleus one.
Then, adjust the chemical shift for nucleus one by entering a value for chemical shift in the new NuISO box or with the chemical shift tool on the line shape fitting toolbar. For nucleus one, input the number of equivalent nuclei for nucleus one with each spin half nucleus equivalent to 0.5 in counting, and enter the sum of the spins into the Pseudo Spin box in order to account for all equivalent nuclei. Using the In Molecule box, assign resonances that arise from different molecules to separate molecules using designations such as 1, 2, et cetera for different molecules, and for resonances that arise from a single molecule, assign 1 for all In Molecule values.
Next, add the second and all subsequent nuclei to the model by clicking on the Nucleus tab, followed by clicking on the Add button. Then, include spin-spin coupling between nuclei by either entering the coupling in the appropriate JM box or by adjusting the scaler coupling button on the line shape fitting toolbar. Begin the process of describing the atom exchanges by clicking on the Reaction tab and click on the checkbox.
if the rate constant for the exchange is to be varied in line shape fitting, then enter the number of nuclei to be exchanged into the Exchanges box for the first exchange in the model. Next, define the exchanges between nucleus tabs in the boxes below the Exchanges box, ensuring that exchanges are cyclic, in that if a nucleus is moved from nucleus one, another nucleus has to be moved into nucleus one. Use the exchange speed button on the line shape fitting toolbar to change the initial value of K in order to iteratively adjust the value of K, even if the checkbox is selected for the rate constant.
Add more exchanges to the model by clicking on the Reaction tab, followed by clicking on the Add button. Use the tools on the line shape fitting toolbar to adjust the starting variables and begin iterative line shape fitting by clicking on the Start the spectrum fit button on the line shape fitting toolbar. Continue iterative fitting until no change is found in the best overlap between spectrum and model or until 1, 000 iterations are reached.
If fitting stops at 1, 000 iterations, continue further iterations with the Start the spectrum fit button, and the model spectrum is displayed with the actual spectrum for comparison. The dynamic proton-decoupled phosphorus-31 NMR spectra of the rhenium complex were measured at several temperatures. The spectra show the coalescence of the two resonances that arise from the diastereotopic phosphorous atoms into a single resonance at higher temperatures.
The temperature dependence of the difference in chemical shifts between the two proton-decoupled phosphorous-31 resonances was determined. Extrapolation allows for estimation of the chemical shifts of the individual resonances at higher temperatures. The temperature dependence for the hydride resonance chemical shifts was determined.
The chemical shifts calculated from the best linear fits were used for the line shape fitting of the observed spectra. The results of line shape fitting for pairwise exchange of A site hydride ligands, turnstile exchange of three adjacent hydride ligands, and proton exchange between one proton of water and the unique hydride ligand were compared with the observed hydride region of a series of proton-decoupled phosphorous-31 NMR spectra from 225 kelvin to 240 kelvin. A comparison of the models for rearrangement of hydride ligands, with and without proton exchange, versus the proton-decoupled phosphorous-31 NMR spectrum measured at 225 kelvin.
Rate constants arising from the line shape fitting of proton-decoupled phosphorous-31 NMR spectra show a good fit for the Eyring equation. Temperature changes for the sample should not exceed 10 kelvin, and the target temperature should be maintained for at least two minutes to protect the probe of the instrument.
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