77Se Chemical Shifts Research Articles

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.

Relativistic DFT Calculations of Cadmium and Selenium Solid-State NMR Spectra of CdSe Nanocrystal Surfaces.

Solid-state NMR spectra have been used to probe the structure of CdSe nanocrystals and propose detailed models of their surface structures. Density functional theory (DFT)-optimized cluster models that represent probable molecular structures of carboxylate-coordinated surface sites have been proposed. However, to the best of our knowledge, 113Cd and 77Se chemical shifts have not been calculated for these surface models. We performed relativistic DFT calculations of cadmium and selenium magnetic shielding tensors on model compounds with previously measured solid-state NMR spectra with (i) the four-component Dirac-Kohn-Sham (DKS) Hamiltonian and (ii) the scalar and (iii) spin-orbit levels within the ZORA Hamiltonian. Molecular clusters with Cd and Se sites in varying bonding environments were used to model CdSe (100) and CdSe(111) surfaces capped with carboxylic acid ligands. Our calculations identify the observed 113Cd isotropic chemical shifts δ(iso) of -465, -318, and -146 ppm arising from CdSeO3, CdSe2O2, and CdSe3O surface groups, respectively, with very good agreement with experimental measurements. The 113Cd chemical shifts linearly decrease with the number of O-neighbors. The calculated spans (δ11 - δ33) encompass the experimental values for CdSe3O and CdSe2O2 clusters but are slightly larger than the measured value for CdSeO3 clusters. Relativistic DFT calculations predicted a one-bond 113Cd-77Se scalar coupling of 258 Hz, which is in good agreement with the experimental values of 250 Hz. With a dense coverage of carboxylic acid ligands, the CdSe (100) surface shows a distribution of Cd-Se bond lengths and J-couplings. Relativistic DFT simulations thus aid in interpretation of NMR spectra of CdSe nanocrystals and related nanomaterials.

Chlorinated polyhedral selenaboranes revisited by joint experimental/computational efforts: the formation of closo-1-SeB9Cl9 and the crystal structure of closo-SeB11Cl11.

The recent success in the formation of chlorinated telluraboranes and the reactivities of pnictogenaboranes prompted us to re-examine the vacuum co-pyrolysis of B2Cl4 with Se2Cl2 at various molar ratios and temperatures in order to search for the generation of other polyhedral selenaboranes than closo-SeB5Cl5 (1a) and closo-SeB11Cl11 (1b), the latter being observed earlier. Interestingly, a new compound with the elemental composition SeB9Cl9 (2) was detected, this time by high- and low-resolution mass spectrometry. Further characterization by 1- and 2-D 11B-NMR spectroscopy suggests that 2 should adopt a closed bicapped square-antiprismatic geometry with selenium at the apical position. Moreover, vacuum sublimation gave suitable crystals of 1b, which were subjected to single-crystal X-ray structure determination. Crystallographic data analysis confirmed that 1b, consistent with its 26 skeletal electron count, adopts a distorted icosahedral structure close to the symmetry of C5v. Computations at the DFT-D3 level have revealed that 33% of the total computed binding motifs in the grown 1b crystals are due to the very strong chalcogen bonding. Moreover, SAPT decomposition has shown that the bonding motifs in the crystals are stabilized mainly by dispersion and electrostatic terms. Homodecoupling and high resolution 11B NMR and 77Se NMR experiments have resolved both coupling constants 1J(11B11B) and 1J(77Se11B) as well as the 77Se chemical shift of 1a and 1b, which are in reasonable agreement with the corresponding computed values. The computed 11B chemical shifts of 2 were determined by the well-established DFT/GIAO/NMR structural tool based on its B3LYP/6-311+G** internal coordinates. They agree well with the experimental values and provide a good representation of the molecular structure of 2 in solution. The extraordinary downfield 11B NMR chemical shift of B(10) in 2 has been ascribed to the intensive paramagnetic contribution to the shielding tensor in this bicapped square-antiprismatic motif. Calculations of the synproportionation free energies of smaller (n - 1) closo-selenaboranes with larger-sized (n + 1) ones support the extraordinary stability of octahedral, bicapped square-antiprismatic and icosahedral closo motifs in the SeBnCln family (n = 4-12).

77Se and 125Te solid-state NMR and X-ray diffraction structural study of chalcogen-bonded 3,4-dicyano-1,2,5-chalcogenodiazole cocrystals.

Three novel chalcogen-bonded cocrystals featuring 3,4-dicyano-1,2,5-selenodiazole (C4N4Se) or 3,4-dicyano-1,2,5-tellurodiazole (C4N4Te) as chalcogen-bond donors and hydroquinone (C6H6O2), tetraphenylphosphonium chloride (C24H20P+·Cl-) or tetraethylphosphonium chloride (C8H20P+·Cl-) as chalcogen-bond acceptors have been prepared and characterized by single-crystal X-ray diffraction (XRD), powder X-ray diffraction and 77Se/125Te magic-angle spinning solid-state NMR spectroscopy. The single-crystal XRD results show that the chalcogenodiazole molecules interact with the electron donors through two σ-holes on each of the chalcogen atoms, which results in highly directional and moderately strong chalcogen bonds. Powder XRD confirms that the crystalline phases are preserved upon moderate grinding of the samples for solid-state NMR experiments. Measurement of 77Se and 125Te chemical shift tensors via magic-angle spinning solid-state NMR spectroscopy confirms the number of magnetically unique chalcogen sites in each asymmetric unit and reveals the impact of chalcogen-bond formation on the local electronic structure. These NMR data are further assessed in the context of analogous data for a wider range of crystalline chalcogen-bonded systems.

Predictability of Chalcogen-Bond-Driven Crystal Engineering: An X-ray Diffraction and Selenium-77 Solid-State NMR Investigation of Benzylic Selenocyanate Cocrystals.

We describe a series of new chalcogen-bonded cocrystals featuring 1,2-bis(selenocyanatomethyl)benzene (DSN) and 1,2,4,5-tetrakis(selenocyanatomethyl)-benzene (TSN) as the donor moieties and a variety of Lewis bases such as onium halides, N-oxides, and pyridine-containing heterocycles as the acceptors. Single-crystal X-ray diffraction demonstrates that, in every case, the selenocyanates consistently interact with the acceptor molecules through strong and directional Se···X chalcogen-bonds (ChBs) (X = halides, oxygen, and nitrogen). 77Se solid-state nuclear magnetic resonance spectroscopy was applied to measure selenium chemical shift tensor magnitudes and to explore potential correlations between these tensor elements and the local ChB geometry. In every case, the isotropic 77Se chemical shift decreases, and the chemical shift tensor span increases upon cocrystallization of DSN with the various ChB acceptors. This work contributes to a growing body of knowledge concerning the predictability and robustness of chalcogen bonds in crystal engineering as well as the NMR response to the establishment of chalcogen bonds. In particular, among the systems studied here, highly linear chalcogen bonds are formed exclusively at the stronger σ-hole of each and every selenium atom regardless of the size, charge, or denticity of the electron donor moiety.

Further Investigation of the 2‐Azido‐phenylselenylation of Glycals

AbstractDerivatives of 2‐azido‐2‐deoxysugars are widely applied as precursor of 2‐amino‐2‐deoxysugars in the synthesis of various oligosaccharides of bacterial, fungal and mammalian origin. Heterogeneous or homogeneous azidophenylselenylation (APS) of glycals, i. e., reaction of glycals with Ph2Se2, PhI(OAc)2 and NaN3 or TMSN3 as azide radical donors, is a straightforward way to phenyl 2‐azido‐2‐deoxy‐1‐selenoglycosides that can be directly used as glycosyl donors. However, heterogeneous APS is characterized by insufficient reproducibility and scalability. We have studied the effect of reaction conditions on the product distribution in heterogeneous APS of 3,4,6‐tri‐O‐acetyl‐d‐galactal and found the conditions that enabled reliable preparation of crystalline phenyl 2‐azido‐2‐deoxy‐1‐seleno‐α‐d‐galactopyranoside triacetate in yield of 58 % on the 3.7 mmol scale. APS of 3,4,6‐tri‐O‐acetyl‐d‐glucal under those conditions produced a ∼1 : 1 mixture of phenyl 2‐azido‐2‐deoxy‐1‐seleno‐α‐d‐gluco‐ and mannopyranosides in total yield of 78 %. Acetoxyphenylselenylation of differently protected galactals and 3,4,6‐tri‐O‐acetyl‐d‐glucal under the action of Ph2Se2 and PhI(OAc)2 has been shown to be a convenient method for the synthesis of 1‐O‐acetyl‐2‐seleno‐2‐deoxy derivatives, valuable intermediates in chemistry of 2‐deoxysugars. 2‐Seleno‐2‐deoxy sugars were characterized in detail by NMR data including 77Se chemical shifts and nJSe−H coupling constant values.

Chalcogen-Bonded Cocrystals of Substituted Pyridine N-Oxides and Chalcogenodiazoles: An X-ray Diffraction and Solid-State NMR Investigation

We introduce methyl, methoxy, and phenyl substituents at the para-, meta-, and ortho- positions of pyridine N-oxide to investigate the effect of chemical substitution on the resulting nine chalcogen-bonded structures formed upon cocrystallization with 3,4-dicyano-1,2,5-selenodiazole and 3,4-dicyano-1,2,5-telluradiazole. Single-crystal X-ray diffraction studies reveal the presence of double chalcogen bonding interactions in the cocrystals and demonstrate the impact of the substitution on the geometric features of the chalcogen bonds. 77Se and 125Te solid-state NMR spectroscopy is employed to measure selenium and tellurium chemical shift tensors of the products, and various trends are described. The smallest component of the 77Se chemical shift tensor (δ33) provides the strongest correlation with the chalcogen bond distance. Solution NMR provides qualitative evidence for the persistence of the chalcogen bonds in solution. Finally, 1J(77Se,14N) coupling constants in 3,4-dicyano-1,2,5-selenodiazole and its chalcogen-bonded cocrystals are measured after accounting for residual dipolar coupling between 77Se and 14N; however, changes in 1J(77Se,14N) attributable to chalcogen bonding upon cocrystallization are comparable to the experimental uncertainties. This systematic study of chalcogen-bonded cocrystals demonstrates the potential utility of the substitution effect for applications of chalcogen bonds in crystal engineering and demonstrates the value of solid-state NMR in characterizing such systems.

Chemical order in binary Se-Te glasses: Results from high-resolution 2D 77Se and 125Te MATPASS NMR spectroscopy

The structure of binary TexSe100-x (0 ≤ x ≤ 35) glasses is studied using high-resolution two-dimensional magic-angle-turning phase-adjusted spinning sidebands (2D MATPASS) 77Se and 125Te nuclear magnetic resonance (NMR) spectroscopy. The 77Se (125Te) isotropic NMR spectra are consistent with the presence of three different kinds of Se (Te) sites, which correspond to Se–Se (Te)–Se, Te–Se (Te)–Se, and Te–Se (Te)–Te environments. The compositional variation of the relative fractions of the three Se sites indicates that the Se and Te atoms are randomly distributed to form [Se, Te]n copolymer chains. The 77Se and 125Te NMR isotropic chemical shifts for Se and Te atoms in isostructural environments are found to display a linear correlation. The systematic compositional variation of the CSA of Se–Se–Se sites can be related to the appearance of Te atoms as next-nearest neighbors of Se atoms in the [Se, Te]n chains.

Characterization of (Boc-Cys/Sec-NHMe)2 and (Boc-Cys/Sec-OMe)2 : Evidence of local conformational difference between disulfide and diselenide.

Conformations of disulfide and diselenide were compared in (Boc-Cys/Sec-NHMe)2 and (Boc-Cys/Sec-OMe)2 using X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, density functional theory (DFT), and circular dichroism (CD) spectroscopy. Conformations of disulfide/diselenide in polypeptides are defined based on the sign of side chain torsion angle χ3 (-CH2 -S/Se-S/Se-CH2 -); negative indicates left-handed and positive indicates right-handed orientation. In the crystals of (Boc-Cys-OMe)2 and (Boc-Sec-OMe)2 , the disulfide exhibits a left-handed and the diselenide a right-handed orientation. Characterization of cystine and selenocystine derivatives in solution using 1 H-NMR, natural abundant 77 Se NMR, 2D-ROESY, and chemical shift analysis coupled to DMSO titration has indicated the symmetrical nature and antiparallel orientation of Cys/Sec residues about the disulfide/diselenide bridges. Structural calculations of cystine and selenocystine derivatives using DFT further support the antiparallel orientation of Cys/Sec residues about disulfide/diselenide. The far-ultraviolet (UV) region CD spectra of cystine and selenocystine derivatives have exhibited the negative Cotton effect (CE) for disulfide and positive for diselenide confirming the difference in the conformational preference of disulfide and diselenide. In the previously reported polymorphic structure of (Boc-Sec-OMe)2 , the diselenide has right-handed orientation. In the X-ray structures of disulfide and diselenide analogues of Escherichia coli protein encoded by curli specific gene C (CgsC) retrieved from Protein Databank (PDB), disulfide has left-handed and the diselenide right-handed orientation. The current report provides the evidence for the local conformational difference between a disulfide and a diselenide group under unconstrained conditions, which may be useful for the rational replacement of disulfide by diselenide in polypeptide chains.

77Se NMR Probes the Protein Environment of Selenomethionine.

Sulfur is critical for the correct structure and proper function of proteins. Yet, lacking a sensitive enough isotope, nuclear magnetic resonance (NMR) experiments are unable to deliver for sulfur in proteins the usual wealth of chemical, dynamic, and structural information. This limitation can be circumvented by substituting sulfur with selenium, which has similar physicochemical properties and minimal impact on protein structures but possesses an NMR compatible isotope (77Se). Here we exploit the sensitivity of 77Se NMR to the nucleus' chemical milieu and use selenomethionine as a probe for its proteinaceous environment. However, such selenium NMR spectra of proteins currently resist a reliable interpretation because systematic connections between variations of system variables and changes in 77Se NMR parameters are still lacking. To start narrowing this knowledge gap, we report here on a biological 77Se magnetic resonance data bank based on a systematically designed library of GB1 variants in which a single selenomethionine was introduced at different locations within the protein. We recorded the resulting isotropic 77Se chemical shifts and relaxation times for six GB1 variants by solution-state 77Se NMR. For four of the GB1 variants we were also able to determine the chemical shift anisotropy tensor of SeM by solid-state 77Se NMR. To enable interpretation of the NMR data, the structures of five of the GB1 variants were solved by X-ray crystallography to a resolution of 1.2 Å, allowing us to unambiguously determine the conformation of the selenomethionine. Finally, we combine our solution- and solid-state NMR data with the structural information to arrive at general insights regarding the execution and interpretation of 77Se NMR experiments that exploit selenomethionine to probe proteins.

Conformation dynamics of cyclic disulfides and selenosulfides in CXXC(U) (X=Gly, Ala) tetrapeptide redox motifs.

Thioredoxin fold proteins often contain a Cys-(Xxx)n -Cys(Sec) or CXn C(U) motif, where the active cysteine (C) or selenocysteine (U) is bridged by X residues, which vary with protein function. The effect of the X residues on the conformation space of the oxidized disulfide and selenosulfide forms of the CXXC(U) motif has been investigated using molecular dynamics (MD) and density functional theory. Multi-microsecond-length MD simulations of the CGGC, CGAC, and CAGC cyclic peptides show that CGGC rings readily exchange between several conformations over the course of the simulation, but steric interactions with the methyl group of Ala limit the conformation space available to the cyclic peptide, especially for CGAC. The potential for the motif to be reduced, as measured by the energy of the lowest unoccupied molecular orbitals, is dependent upon the ring conformation. These results suggest that control of available conformations by the bridging residues and the protein tertiary structure may be important for defining the function of the CXXC motif. Theoretical 77 Se chemical shifts of the selenosulfide moiety are dependent upon the conformation and/or intramolecular Se···O interactions with the backbone carbonyl group of the C-terminal U residue.

77Se-NMR Probes the Protein Environment of Selenomethionine

77Se NMR is highly sensitive to its chemical environment and can report on the interactions and degrees of freedom available for selenium-containing amino acids in biological systems. Particularly, selenomethionine can be used as a probe for methionine environment, an amino acid which is often found in protein-protein interfaces and in ligand binding sites. Yet, for these larger and more complex biological systems, reliable data interpretation that systematically connects variations of system variables to changes in the NMR spectra is still lacking. To build a biological 77Se magnetic resonance databank, we thus constructed a library of GB1 variants with a single selenomethionine at different locations within the protein. The structure of five of such GB1 variants was solved to 1.2 Å, allowing the conformation of the selenomethionine to be unambiguously determined. We recorded the isotropic 77Se chemical shift for six GB1 variants along with their T1 relaxation times and line width. For four of the GB1 variants we were able to determine the chemical shielding tensor of SeM by 77Se solid-state NMR. Finally, we combine solution and solid-state NMR with the structural information to arrive at general recommendations for the use of selenomethionine as NMR probe in biological systems.

Titanocene Selenide Sulfides Revisited: Formation, Stabilities, and NMR Spectroscopic Properties.

[TiCp2S5] (phase A), [TiCp2Se5] (phase F), and five solid solutions of mixed titanocene selenide sulfides [TiCp2SexS5−x] (Cp = C5H5−) with the initial Se:S ranging from 1:4 to 4:1 (phases B–E) were prepared by reduction of elemental sulfur or selenium or their mixtures by lithium triethylhydridoborate in thf followed by the treatment with titanocene dichloride [TiCp2Cl2]. Their 77Se and 13C NMR spectra were recorded from the CS2 solution. The definite assignment of the 77Se NMR spectra was based on the PBE0/def2-TZVPP calculations of the 77Se chemical shifts and is supported by 13C NMR spectra of the samples. The following complexes in varying ratios were identified in the CS2 solutions of the phases B–E: [TiCp2Se5] (51), [TiCp2Se4S] (41), [TiCp2Se3S2] (31), [TiCp2SSe3S] (36), [TiCp2SSe2S2] (25), [TiCp2SSeS3] (12), and [TiCp2S5] (01). The disorder scheme in the chalcogen atom positions of the phases B–E observed upon crystal structure determinations is consistent with the spectral assignment. The enthalpies of formation calculated for all twenty [TiCp2SexS5−x] (x = 0–5) at DLPNO-CCSD(T)/CBS level including corrections for core-valence correlation and scalar relativistic, as well as spin-orbit coupling contributions indicated that within a given chemical composition, the isomers of most favourable enthalpy of formation were those, which were observed by 77Se and 13C NMR spectroscopy.

Selenium Chain Length Distribution in GexSe100-x Glasses: Insights from (77)Se NMR Spectroscopy and Quantum Chemical Calculations.

The statistics of selenium chain length distribution in GexSe100-x glasses with 5 ≤ x ≤ 20 are investigated using a combination of high-resolution, two-dimensional (77)Se nuclear magnetic resonance (NMR) spectroscopy and quantum chemical calculations. This combined approach allows for the distinction of various selenium chain environments on the basis of subtle but systematic effects of next-nearest neighbors of Se atoms in -Se-Se-Se- linkages on the (77)Se chemical shift tensor parameters. Simulation of the experimental (77)Se NMR spectral line shapes indicates that Se chain speciation in these chalcogenide glasses follows the Flory-Schulz distribution, originally developed for organic chain polymers.

Use of (77)Se and (125)Te NMR Spectroscopy to Probe Covalency of the Actinide-Chalcogen Bonding in [Th(En){N(SiMe3)2}3](-) (E = Se, Te; n = 1, 2) and Their Oxo-Uranium(VI) Congeners.

Reaction of [Th(I)(NR2)3] (R = SiMe3) (1) with 1 equiv of either [K(18-crown-6)]2[Se4] or [K(18-crown-6)]2[Te2] affords the thorium dichalcogenides, [K(18-crown-6)][Th(η(2)-E2)(NR2)3] (E = Se, 2; E = Te, 3), respectively. Removal of one chalcogen atom via reaction with Et3P, or Et3P and Hg, affords the monoselenide and monotelluride complexes of thorium, [K(18-crown-6)][Th(E)(NR2)3] (E = Se, 4; E = Te, 5), respectively. Both 4 and 5 were characterized by X-ray crystallography and were found to feature the shortest known Th-Se and Th-Te bond distances. The electronic structure and nature of the actinide-chalcogen bonds were investigated with (77)Se and (125)Te NMR spectroscopy accompanied by detailed quantum-chemical analysis. We also recorded the (77)Se NMR shift for a U(VI) oxo-selenido complex, [U(O)(Se)(NR2)3](-) (δ((77)Se) = 4905 ppm), which features the highest frequency (77)Se NMR shift yet reported, and expands the known (77)Se chemical shift range for diamagnetic substances from ∼3300 ppm to almost 6000 ppm. Both (77)Se and (125)Te NMR chemical shifts of given chalcogenide ligands were identified as quantitative measures of the An-E bond covalency within an isoelectronic series and supported significant 5f-orbital participation in actinide-ligand bonding for uranium(VI) complexes in contrast to those involving thorium(IV). Moreover, X-ray diffraction studies together with NMR spectroscopic data and density functional theory (DFT) calculations provide convincing evidence for the actinide-chalcogen multiple bonding in the title complexes. Larger An-E covalency is observed in the [U(O)(E)(NR2)3](-) series, which decreases as the chalcogen atom becomes heavier.

Experimental and Computational (77)Se NMR Investigations of the Cyclic Eight-Membered Selenium Imides 1,3,5,7-Se4(NR)4 (R = Me, (t)Bu) and 1,5-Se6(NMe)2.

The cyclocondensation reaction of equimolar amounts of SeCl2 and (Me3Si)2NMe in THF affords 1,3,5,7-Se4(NMe)4 (5b) [δ((77)Se) = 1585 ppm] in excellent yield. An X-ray structural determination showed that 5b consists of cyclic, puckered crown-shaped molecules with a mean Se-N bond length of 1.841 Å typical of single bonds. A minor product of this reaction was isolated as unstable orange-red crystals, which were identified by X-ray analysis as the adduct 1,5-Se6(NMe)2·(1)/2Se8 (1b·(1)/2Se8), composed of cyclic 1,5-Se6(NMe)2 and disordered cyclo-Se8 molecules. A detailed reinvestigation of the cyclocondensation reaction of SeCl2 and (t)BuNH2 as a function of molar ratio and time by multinuclear ((1)H, (13)C, and (77)Se) NMR spectroscopy revealed that the final product exhibits one (77)Se resonance at 1486 ppm and equivalent N(t)Bu groups. The shielding tensors of 28 selenium-containing molecules, for which the (77)Se chemical shifts are unambiguously known, were calculated at the PBE0/def2-TZVPP level of theory to assist the spectral assignment of new cyclic selenium imides. The good agreement between the observed and calculated chemical shifts enabled the assignment of the resonance at 1486 ppm to 1,3,5,7-Se4(N(t)Bu)4 (5a). Those at 1028 and 399 ppm (intensity ratio 2:1) could be attributed to 1,5-Se6(NMe)2 (1b).

Determining the Ligand Properties of N‐Heterocyclic Carbenes from 77Se NMR Parameters

AbstractThe 1JCSe coupling constants for a range of NHC–selenium adducts have been measured and used to establish a correlation with the σ‐donor strength of the respective carbenes. For the subclass of amido‐carbenes, the 1JCSe values revealed a high donor capacity, very much in contrast to what the DFT‐calculated HOMO energies suggest. The 1JCH coupling constants for the C‐2 atoms in azolium‐type NHC precursors were more readily obtained and show the same trend as the 1JCSe coupling constants. In addition, the use of 77Se chemical shifts to determine π‐acidity has been extended to a broader range of derivatives, namely 1·Se–22·Se. The superior resolution of the δ(77Se) method in comparison with the Tolman electronic parameters derived from IR spectroscopy is demonstrated for the caffeine‐derived bis‐carbene 19.

⁷⁷Se NMR spectroscopy as a sensitive probe for Hammett σ constants.

Herein we showcase the use of a combination of (1)H, (13)C, and (77)Se NMR spectroscopy as a sensitive tool for correlation analysis. A series of substituted O-aryl selenocarbamates [ArOC(Se)N(CH3)2] and Se-aryl selenocarbamates [ArSeC(O)N(CH3)2] have been investigated by means of (1)H, (13)C, and (77)Se NMR spectroscopy. We have determined the (1)H, (13)C, and (77)Se chemical shift values as well as both one- and two-bond heteronuclear (13)C-(77)Se coupling constants, and the changes in both the chemical shift values and the coupling constants were found to obey linear free energy relationships with Hammett's σ(p) and σ(p)(-) constants. For the eight studied O-aryl selenocarbamates, we observe linear free energy correlations with two of the (13)C and (77)Se chemical shift values and as well as one (13)C-(77)Se coupling constant. With the five examples of Se-aryl selenocarbamates, linear correlations are observed with three different (13)C-(77)Se coupling constants. The strong internal consistency validates the use of (77)Se NMR spectroscopy for correlation analysis.

(77)Se chemical shift tensor of L-selenocystine: experimental NMR measurements and quantum chemical investigations of structural effects.

The genetically encoded amino acid selenocysteine and its dimeric form, selenocystine, are both utilized by nature. They are found in active sites of selenoproteins, enzymes that facilitate a diverse range of reactions, including the detoxification of reactive oxygen species and regulation of redox pathways. Due to selenocysteine and selenocystine's specialized biological roles, it is of interest to examine their (77)Se NMR properties and how those can in turn be employed to study biological systems. We report the solid-state (77)Se NMR measurements of the L-selenocystine chemical shift tensor, which provides the first experimental chemical shift tensor information on selenocysteine-containing systems. Quantum chemical calculations of L-selenocystine models were performed to help understand various structural effects on (77)Se L-selenocystine's chemical shift tensor. The effects of protonation state, protein environment, and substituent of selenium-bonded carbon on the isotropic chemical shift were found to be in a range of ca. 10-20 ppm. However, the conformational effect was found to be much larger, spanning ca. 600 ppm for the C-Se-Se-C dihedral angle range of -180° to +180°. Our calculations show that around the minimum energy structure with a C-Se-Se-C dihedral angle of ca. -90°, the energy costs to alter the dihedral angle in the range from -120° to -60° are within only 2.5 kcal/mol. This makes it possible to realize these conformations in a protein or crystal environment. (77)Se NMR was found to be a sensitive probe to such changes and has an isotropic chemical shift range of 272 ± 30 ppm for this energetically favorable conformation range. The energy-minimized structures exhibited calculated isotropic shifts that lay within 3-9% of those reported in previous solution NMR studies. The experimental solid-state NMR isotropic chemical shift is near the lower bound of this calculated range for these readily accessible conformations. These results suggest that the dihedral information may be deduced for a protein with appropriate structural models. These first-time experimental and theoretical results will facilitate future NMR studies of selenium-containing compounds and proteins.

The effects of σ(C-Se)-π hyperconjugation in 1-methoxy- and 1-nitro-4-[(phenylselanyl)methyl]benzene.

The structures of 1-methoxy-4-[(phenylselanyl)methyl]benzene, C14H14OSe, (1), and 1-nitro-4-[(phenylselanyl)methyl]benzene, C13H11NO2Se, (2), were determined at 130 K. The two structures, which differ in that (1) contains an electron-rich aromatic ring and (2) contains an electron-deficient aromatic ring, both adopt conformations which allow for σ(C-Se)-π hyperconjugation. However, although there are significant differences in the (77)Se chemical shifts for these two compounds, they do not display significantly different H2C-Se or H2C-C(ar) bond lengths, suggesting that the effects of σ(C-Se)-π hyperconjugation in (1) and (2) are not strong enough to be manifested in measurable differences in the structural parameters.