77Se NMR Spectroscopy As a Sensitive Probe for Hammett σ Constants

Five Hypotheses on the Origins of Temperature Dependence of 77Se NMR Shifts in Diselenides

Notable thermal shifts in diselenides have been documented in 77Se NMR for more than 50 years, but no satisfactory explanation has been found. Here, five hypotheses are considered as possible explanations for the large temperature dependence of the 77Se chemical shifts of diaryl and dialkyl diselenides compared to monoselenides and selenols. Density functional theory calculations are provided to bolster hypotheses and better understand the effects of barrier height and dipole energies. It is proposed that the temperature dependence of diselenide 77Se NMR chemical shifts is due to rotation around the Se-Se bond and sampling of twisted conformers at higher temperatures. The molecular twisting is solvent dependent; here, DMSO-d6 and toluene-d8 were evaluated. No correlation was established between para-substituents on diaryl diselenides and the magnitude of the change in the 77Se NMR shift (Δδ) with temperature.

Assessing the effects of covalent, dative and halogen bonds on the electronic structure of selenoamides

The C–NMe2 bond rotation of a selenoamide is proposed as an experimental probe to compare different chemical interactions.

Selenate-An internal chemical shift standard for aqueous 77 Se NMR spectroscopy

The ⁷⁷ Se NMR spectra of selenate were studied under various circumstances, such as concentration, pH, temperature, ionic strength, and D₂ O:H₂ O ratio, in order to examine its potential as a water-soluble internal chemical shift standard. The performance of selenate as a chemical shift reference and that of other attempted ones from the literature (dimethyl selenide, tetramethylsilane/TMS, and 3-(trimethylsilyl)propane-1-sulfonate/DSS) was also explored. The uncertainty in the resulting chemical shift relative to the effective spectral width is comparable to that of DSS. Compared to the currently prevalent water-soluble external chemical shift reference, selenic acid solution, the properties of internal selenate are much more favorable in terms of ease of use. We have also demonstrated that selenate can be used in reducing media, which is inevitable for the analysis of selenol compounds. Thus, it can be stated that sodium selenate is a robust internal chemical shift reference in aqueous media for ⁷⁷ Se NMR measurements; the chemical shift of this reference in a solution containing 5 V/V% D₂ O at 25°C and 0.15 mol·dm^-3 ionic strength is 1048.65 ppm relative to 60 V/V% dimethyl selenide in CDCl₃ and 1046.40 ppm relative to the ¹ H signal of 0.03 V/V% TMS in CDCl₃ . In summary, a water-soluble, selenium-containing internal chemical shift reference compound was introduced for ⁷⁷ Se NMR measurements for the first time in the literature, and with the aforementioned results all previous ⁷⁷ Se measurements can be converted to a unified scale defined by the International Union of Pure and Applied Chemistry.

Benzylic Thio and Seleno Newman-Kwart Rearrangements

The thermally induced O_Bn → S_Bn and O_Bn → Se_Bn migration reactions facilitate the rearrangement of O-benzyl thio- and selenocarbamates [BnOC(═X)NMe₂] (X = S or Se) into their corresponding S-benzyl thio- and Se-benzyl selenocarbamates [BnXC(═O)NMe₂] (X = S or Se). A series of substituted O-benzyl thio- and selenocarbamates were synthesized and rearranged in good yields of 33-88%. The reaction rates are higher for substrates with electron-donating groups in the 2 or 4 position of the aromatic ring, but the rearrangement also proceeds with electron-withdrawing substituents. The rearrangement follows first-order reaction kinetics and proceeds via a tight ion pair intermediate consisting of the benzylic carbocation and the thio- or selenocarbamate moiety. Computational studies support these findings.