Ab Initio Computation of 77Se NMR Chemical Shifts with the IGLO-SCF, the GIAO-SCF, and the GIAO-MP2 Methods

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).

Computational simulation of 1 H NMR profiles of complex biofluid analyte mixtures at differential operating frequencies: Applications to low-field benchtop spectra

Estimations of accurate and reliable NMR chemical shift values, coupling patterns and constants within a reasonable timeframe remain significantly challenging, and the unavailability of reliable software strategies for the prediction of low-field (e.g., 60 MHz) spectra from those acquired at higher operating frequencies hampers their direct comparison. Hence, this study explored the applications of accessible software options for predicting these parameters in the ¹ H NMR profiles of analytes as a function of magnetic field strength; this was performed for individual analytes and also for complex biofluid matrices featured in metabolomics investigations. For this purpose, results from the very first successful experimental acquisition and simulation of the ¹ H NMR profiles of intact human salivary supernatant samples on a 60 MHz benchtop spectrometer were evaluated. Using salivary metabolite concentrations determined at 400 MHz, it was demonstrated that simulation of the low-field spectra of five biomolecules with the most prominent ¹ H resonances detectable allowed multiple component fits to be applied to experimental spectra. Hence, these salivary ¹ H NMR profiles could be successfully predicted throughout the 45-600 MHz operating frequency range. With the exception of propionate resonance multiplets, which revealed more complex coupling patterns at low field and required more astute computational and fitting options, valuable quantitative metabolomics data on salivary acetate, formate, methanol and glycine could be attained from low-field spectrometres. These studies are both timely and pertinent in view of the recent advancement of low-field benchtop NMR facilities for diagnostically significant biomarker tracking in biofluids. Experiments performed with added ammonium chloride to facilitate the release of salivary metabolites from biopolymer binding sites provided evidence that a small but nevertheless significant proportion of propionate, but not lactate, was bound to such sites, an observation of much relevance to biomolecule quantification in salivary metabolomics investigations.

Quantum chemical calculations of 77 Se and 125 Te nuclear magnetic resonance spectral parameters and their structural applications

An accurate quantum chemical (QC) modeling of ⁷⁷ Se and ¹²⁵ Te nuclear magnetic resonance (NMR) spectra is deeply involved in the NMR structural assignment for selenium and tellurium compounds that are of utmost importance both in organic and inorganic chemistry nowadays due to their huge application potential in many fields, like biology, medicine, and metallurgy. The main interest of this review is focused on the progress in QC computations of ⁷⁷ Se and ¹²⁵ Te NMR chemical shifts and indirect spin-spin coupling constants involving these nuclei. Different computational methodologies that have been used to simulate the NMR spectra of selenium and tellurium compounds since the middle of the 1990s are discussed with a strong emphasis on their accuracy. A special accent is placed on the calculations resorting to the relativistic methodologies, because taking into account the relativistic effects appreciably influences the precision of NMR calculations of selenium and, especially, tellurium compounds. Stereochemical applications of quantum chemical calculations of ⁷⁷ Se and ¹²⁵ Te NMR parameters are discussed so as to exemplify the importance of integrated approach of experimental and computational NMR techniques.

Recent advances in computational liquid-phase 77Se NMR

This review aims to highlight significant progress in the calculation of 77Se NMR chemical shifts and spin – spin coupling constants involving selenium substantiated with a vast amount of experimental data. The material is arranged in two basic sections: the first one dealing with the calculation of 77Se NMR chemical shifts and the second one dealing with the computation of spin – spin coupling constants involving 77Se nucleus, namely 77Se–1H, 77Se–13C and 77Se–77Se together with some more exotic types of couplings, 77Se – 15N, 77Se–19F, 77Se–29Si and 77Se–31P. A special attention is focused on the stereoelectronic effects involving selenium atom and their manifestation in the 77Se NMR spectra of organoselenium compounds studied with the aid of the modern calculation of 77Se NMR parametres in combination with experimental results.

The bibliography includes 114 references.

Mechanistic Insight into the Oxidation of Organic Phenylselenides by H2 O2

The oxidation of organic phenylselenides by H₂ O₂ is investigated in model compounds, namely, n-butyl phenyl selenide (PhSe(nBu)), bis(phenylselanyl)methane (PhSeMeSePh), diphenyl diselenide (PhSeSePh), and 1,2-bis(phenylselanyl)ethane (PhSeEtSePh). Through a combined experimental (¹ H and ⁷⁷ Se NMR) and computational approach, we characterize the direct oxidation of monoselenide to selenoxide, the stepwise double oxidation of PhSeMeSePh that leads to different diastereomeric diselenoxides, the complete oxidation of the diphenyldiselenide that leads to selenium-selenium bond cleavage, and the subsequent formation of the phenylseleninic product. The oxidation of PhSeEtSePh also results in the formation of phenylseleninic acid along with 1-(vinylseleninyl)benzene, which is derived from a side elimination reaction. The evidence of a direct mechanism, in addition to an autocatalytic mechanism that emerges from kinetic studies, is discussed. By considering our observations of diselenides with chalcogen atoms that are separated by alkyl spacers of different length, a rationale for the advantage of diselenide versus monoselenide catalysts is presented.

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).

Probing halogen bonds with solid-state NMR spectroscopy: observation and interpretation of J(77Se,31P) coupling in halogen-bonded PSe⋯I motifs

Intra-halogen bond J couplings measured via NMR spectroscopy and interpreted using natural localized molecular orbitals offer novel insights into this class of non-covalent interaction.

Inhibition of lactoperoxidase-catalyzed oxidation by imidazole-based thiones and selones: a mechanistic study

Herein, we describe the synthesis and biomimetic activity of a series of N,N-disubstituted thiones and selones that contain an imidazole pharmacophore. The N,N-disubstituted thiones do not show any inhibitory activity towards LPO-catalyzed oxidation reactions, but their corresponding N,N-disubstituted selones exhibit inhibitory activity towards LPO-catalyzed oxidation reactions. Substituents on the N atom of the imidazole ring appear to have a significant effect on the inhibition of LPO-catalyzed oxidation and iodination reactions. Selones 16, 17, and 19, which contain methyl, ethyl, and benzyl substituents, exhibit similar inhibition activities towards LPO-catalyzed oxidation reactions with IC50 values of 24.4, 22.5, and 22.5 μM, respectively. However, their activities are almost three-fold lower than that of the commonly used anti-thyroid drug methimazole (MMI). In contrast, selone 21, which contains a N-CH2CH2OH substituent, exhibits high inhibitory activity, with an IC50 value of 7.2 μM, which is similar to that of MMI. The inhibitory activity of these selones towards LPO-catalyzed oxidation/iodination reactions is due to their ability to decrease the concentrations of the co-substrates (H2O2 and I2), either by catalytically reducing H2O2 (anti-oxidant activity) or by forming stable charge-transfer complexes with oxidized iodide species. The inhibition of LPO-catalyzed oxidation/iodination reactions by N,N-disubstituted selones can be reversed by increasing the concentration of H2O2. Interestingly, all of the N,N-disubstituted selones exhibit high anti-oxidant activities and their glutathione peroxidase (GPx)-like activity is 4-12-fold higher than that of the well-known GPx-mimic ebselen. These experimental and theoretical studies suggest that the selones exist as zwitterions, in which the imidazole ring contains a positive charge and the selenium atom carries a large negative charge. Therefore, the selenium moieties of these selones possess highly nucleophilic character. The (77)Se NMR chemical shifts for the selones show large upfield shift, thus confirming the zwitterionic structure in solution.

Synthesis and reactivity of 4,5-[1,2-dicarba-closo-dodecaborano(12)]-1,3-diselenacyclopentane: opening of the icosahedron to give a zwitterionic intermediate and conversion into 7,8-dicarba-nido-undecaborate(1-)

The reaction of the 1,2-diselenolato-1,2-dicarba-closo-dodecaborane(12) dianion [1,2-(1,2-C(2)B(10)H(10))Se(2)](2-) with dichloromethane (CH(2)Cl(2) or CD(2)Cl(2)) in the presence of donor solvents gave 4,5-[1,2-dicarba-closo-dodecaborano(12)]-1,3-diselenacyclopentane, the title compound, which was characterized by X-ray structural analysis and NMR spectroscopy ((1)H, (11)B, (13)C, and (77)Se). In the presence of pyridine, opening of the icosahedron took place, and a zwitterionic intermediate was isolated and fully characterized in the solid state by X-ray diffraction and in solution by multinuclear magnetic resonance techniques. Although such types of intermediates, prior to deboronation of the ortho-carborane cage, have been proposed several times, this is first example for which the structure has been confirmed unambiguously. This intermediate possesses a nido structure and contains a 7,8-dicarba-nido-undecaborate(1-) anion and a boronium cation, the latter with two pyridine rings linked to the boron atom, which has been extruded from the cage. It was shown that this process is reversible as long as the deboronation is not complete. The formation of the intermediate is accompanied by deboronation, which leads to the 7,8-dicarba-nido-undecaborate(1-) anion. The latter was prepared independently by conventional routes from the title compound, isolated as crystalline material as the tetrabutyl ammonium salt, and characterized by X-ray structural analysis and multinuclear magnetic resonance spectroscopy ((1)H, (11)B, (13)C, and (77)Se).

Synthetic and structural studies of 1,8-chalcogen naphthalene derivatives

Four novel 1,8-disubstituted naphthalene derivatives 4-7 that contain chalcogen atoms occupying the peri positions have been prepared and fully characterised by using X-ray crystallography, multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular distortion due to noncovalent substituent interactions was studied as a function of the bulk of the interacting chalcogen atoms and the size and nature of the alkyl group attached to them. X-ray data for 4-7 was compared to the series of known 1,8-bis(phenylchalcogeno)naphthalenes 1-3, which were themselves prepared from novel synthetic routes. A general increase in the E...E' distance was observed for molecules containing bulkier atoms at the peri positions. The decreased S...S distance from phenyl-1 and ethyl-4 analogues is ascribed to a weaker chalcogen lone pair-lone pair repulsion acting in the ethyl analogue due to the presence of two equatorial S(naphthyl) ring conformations. Two novel peri-substituted naphthalene sulfoxides of 1, Nap(O=SPh)(SPh) 8 and Nap(O=SPh)(2) 9, which contain different valence states of sulfur, were prepared and fully characterised by using X-ray crystallography and multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular structures were analysed by using naphthalene ring torsions, peri-atom displacement, splay angle magnitude, S...S interactions, aromatic ring orientations and quasi-linear O=S...S arrangements. The axial S(naphthyl) rings in 8 and 9 are unfavourable for S...S contacts due to stronger chalcogen lone pair-lone pair repulsion. Although quasi-linear O=S...S alignments suggest attractive interaction is conceivable, analysis of the B3LYP wavefunctions affords no evidence for direct bonding interactions between the S atoms.

Experimental and theoretical investigations of selenium nuclear magnetic shielding tensors in Se–N heterocycles

A preliminary study involving solid-state 77Se NMR spectroscopy and first principles calculations of 77Se NMR parameters in Se–N heterocycles is reported. 77Se CP/MAS NMR spectra of the ring systems reveal expansive selenium chemical shift (CS) tensors, which are extremely sensitive to molecular geometry, symmetry, ligand substitution, and intermolecular contacts. For systems with known crystal structures, hybrid density functional theory (DFT) calculations of selenium nuclear magnetic shielding (NMS) tensors were carried out, and tensor orientations in the molecular frames examined. Additional DFT calculations of selenium NMS tensors are presented, along with a detailed analysis of pairs of occupied and virtual molecular orbitals that give rise to the Se NMS tensors. A new naturalized local molecular orbital (NLMO) analysis under the same DFT framework is also discussed. Collectively, the NMR data and first principles calculations provide understanding of the influences of electronic structure, bonding, and intermolecular interactions on the selenium NMS tensors, allowing for (i) prediction of unknown molecular structures and (ii) insight into the positions of the stereochemically active selenium lone pairs.

Gauge-Origin-Independent Coupled Cluster Singles and Doubles Calculation of Magnetic Circular Dichroism of Azabenzenes and Phosphabenzene Using London Orbitals

A computational study of the Faraday B term of magnetic circular dichroism at the coupled cluster singles and doubles level is presented for pyridine, pyrazine, pyrimidine, and phosphabenzene. Gauge-origin independence is obtained by expressing the B term as a total derivative of the one-photon dipole transition strength and using London orbitals. The high quality of the coupled cluster singles and doubles (CCSD) B terms makes these useful for the assignment of experimental spectra. Previous assignments of the experimental spectra based on the qualitative perimeter model are confirmed by the CCSD results for the three azines, while a reassignment is proposed for phosphabenzene. For non-overlapping bands, the B terms calculated at the equilibrium geometries are in good agreement with the experimental values. For overlapping bands, large deviations occur. Attributing a line width to the calculated equilibrium B terms leads to a large cancellation of positive and negative contributions. This cancellation may result in a large displacement of the band center maximum, leading to a large uncertainty in the assignments of "vertical experimental excitation energies" (pyridine). Bands may also completely vanish due to such cancellation (phosphabenzene). Explicit consideration of the cancellation yields simulated theoretical spectra that are in good agreement with experiment once the theoretical spectra are parallel displaced. A major contribution for this parallel displacement is the shift in the excitation energies due to correlation beyond CCSD, as seen when comparing vertical CCSD and CC3 equilibrium-geometry excitation energies.

The chromophore structural changes during the photocycle of phytochrome: a combined resonance Raman and quantum chemical approach

Phytochromes are sensory photoreceptors that, upon light irradiation, can be transformed between an inactive and an active state. The conversion is initiated by the photoisomerization of the cofactor, a linear methine-bridged tetrapyrrole, followed by conformational relaxations of the chromophore and the protein matrix that finally leads to the formation of the signaling state. To elucidate the underlying molecular processes, resonance Raman spectroscopy combined with quantum chemical calculations constitutes a powerful approach since it allows determination of the chromophore structure in the various states of phytochrome. On the basis of these studies, a molecular model for the photoinduced reaction cycle is derived.

1-Cyclohepta-2,4,6-trienyl-selanes--a 77Se NMR study: indirect nuclear 77Se--13C spin-spin coupling constants and application of density functional theory (DFT) calculations

1-Cyclohepta-2,4,6-trienyl-selanes Se(C(7)H(7))(2) (2c), R--Se--C(7)H(7) with R = Bu, (t)Bu, Ph, 4-F--C(6)H(4) (12a,b,c,d) were prepared by the reaction of the corresponding silanes, Si(SeMe(3))(2) and R--Se--SiMe(3), respectively, with tropylium bromide C(7)H(7)Br. In spite of the low stability of the selanes even in dilute solutions and at low temperature, they could be characterised by their (1)H, (13)C and (77)Se NMR parameters. Coupling constants (1)J((77)Se,(13)C) were measured and calculated by DFT methods at the B3LYP/6-311+G(d,p) level of theory. The comparison of experimental and calculated coupling constants (1)J((77)Se,(13)C) included numerous selenium carbon compounds with largely different Se--C bonds, revealing a satisfactory agreement. Both the spin-dipole (SD) and the paramagnetic spin-orbital (PSO) terms contributed significantly to the spin-spin coupling interaction, in addition to the Fermi contact (FC) term.

A Combined Experimental and Quantum Chemistry Study of Selenium Chemical Shift Tensors

A comprehensive investigation of selenium chemical shift tensors is presented. Experimentally determined chemical shift tensors were obtained from solid-state 77Se NMR spectra for several organic, organometallic, or inorganic selenium-containing compounds. The first reported indirect spin-spin coupling between selenium and chlorine is observed for Ph(2)SeCl(2) where 1J(77Se,35Cl)iso is 110 Hz. Selenium magnetic shielding tensors were calculated for all of the molecules investigated using zeroth-order regular approximation density functional theory, ZORA DFT. The computations provide the orientations of the chemical shift tensors, as well as a test of the theory for calculating the magnetic shielding interaction for heavier elements. The ZORA DFT calculations were performed with nonrelativistic, scalar relativistic, and scalar with spin-orbit relativistic levels of theory. Relativistic contributions to the magnetic shielding tensor were found to be significant for (NH4)2WSe4 and of less importance for organoselenium, organophosphine selenide, and inorganic selenium compounds containing lighter elements.

First detection of a selenenyl fluoride ArSe-F by NMR spectroscopy: the nature of Ar2Se2/XeF2 and ArSe-SiMe3/XeF2 reagents

Arylselenenyl fluorides ArSeF are obtained from diselenides Ar2Se2 or arylselenotrimethylsilanes ArSe-SiMe3, and XeF2. They are detected by low-temperature 19F and 77Se NMR spectroscopy. Substitution in the ortho position of the aromatic ring to provide electronic or steric protection is a requirement for their formation. ArSe--F compounds decompose according to 3 ArSe-F-->[ArSe-SeF2Ar]+ArSe-F-->ArSeF3+Ar2Se2. Reaction energies for this disproportionation as well as that of the sulfur and tellurium homologues have been calculated with MP2, CCSD(T,) and B3 LYP methods. They were found to be increasingly exothermic in the sequence S<Se<Te. For selected Se-C and Se-F compounds the 77Se and 19F chemical shifts have been calculated by GIAO-MP2 and GIAO-B3 LYP methods and are in good agreement with experimental values.

Electronic Structures and Molecular Properties of Chalcogen Nitrides Se2N2 and SeSN2

The electronic structures and molecular properties of S2N2 as well as the currently unknown chalcogen nitrides Se2N2 and SeSN2 have been studied using various ab initio and density functional methods. All molecules share a qualitatively similar electronic structure and can be primarily described as 2pi-electron aromatics having minor singlet diradical character of 6-8% that can be attributed solely to the nitrogen atoms. This diradical character is manifested in the prediction of their molecular properties, in which coupled cluster and multiconfigurational approaches, as well as density functional methods, show the best performance. The conventional ab initio methods RHF and MP2 completely fail to describe these systems. Predictions for the vibrational frequencies, IR intensities, Raman activities, and 14N, 15N, and 77Se chemical shifts, as well as singlet excitation energies of Se2N2 and SeSN2, have been made. The computed high-level spectroscopic data will be of considerable value in future efforts aimed at the preparation of the conducting polymers (SeN)x and (SeNSN)x.

Synthetic Applications of (Me3SiNSN)2E (E = S, Se) in Chalcogen-Nitrogen Chemistry: Formation and Structural Characterization of Cl2TeESN2 (E = S, Se) and [PPh4]2[Pd2(μ-Se2N2S)X4] (X = Cl, Br)

The reaction of (Me3SiNSN)2S with TeCl4 in CH2Cl2 affords Cl2TeS2N2 (1) and that of (Me3SiNSN)2Se with TeCl4 produces Cl2TeSeSN2 (2) in good yields. The products were characterized by X-ray crystallography, as well as by NMR and vibrational spectroscopy and EI mass spectrometry. The Raman spectra were assigned by utilizing DFT molecular orbital calculations. The pathway of the formation of five-membered Cl2TeESN2 rings by the reactions of (Me3SiNSN)2E with TeCl4 (E = S, Se) is discussed. The reaction of (Me3SiNSN)2Se with [PPh4]2[Pd2X6] yields [PPh4]2[Pd2(mu-Se2N2S)X4] (X = Cl, 4a; Br, 4b), the first examples of complexes of the (Se2N2S)2- ligand. In both cases, this ligand bridges the two palladium centers through the selenium atoms.

A Computational and Experimental Study of the Structures and Raman and 77Se NMR Spectra of SeX3+ and SeX2 (X = Cl, Br, I): FT-Raman Spectrum of (SeI3)[AsF6]

The ability of MP2, B3PW91 and PBE0 methods to produce reliable predictions in structural and spectroscopic properties of small selenium-halogen molecules and cations has been demonstrated by using 6-311G(d) and cc-pVTZ basis sets. Optimized structures and vibrational frequencies agree closely with the experimental information, where available. Raman intensities are also well reproduced at all levels of theory. Calculated GIAO isotropic shielding tensors yield a reasonable linear correlation with the experimental chemical shift data at each level of theory. The largest deviations between calculated and experimental chemical shifts are found for selenium-iodine species. The agreement between observed and calculated chemical shifts for selenium-iodine species can be improved by inclusion of relativistic effects using the ZORA method. The best results are achieved by adding spin-orbit correction terms from ZORA calculations to nonrelativistic GIAO isotropic shielding tensors. The calculated isotropic shielding tensors can be utilized in the spectroscopic assignment of the 77Se chemical shifts of novel selenium-halogen molecules and cations. The experimental FT-Raman spectra of (SeI3)[AsF6] in the solid state and in SO2(l) solution are also reported.

The Theoretical 77Se Chemical Shift as a Probe of Selenium State in Selenoproteins and Their Mimics

Theoretical 77Se chemical shifts of a series of simple organoselenium compounds are compared to known NMR data for various derivatives of selenoenzymes and selenoamino acids. Since the theoretical data only differs from the biochemical data set by an overall approximately 15-30 ppm downfield shift, simple theoretical model studies are suggested as an additional tool for the interpretation of selenoenzyme spectra. Further studies demonstrate that model systems can be extended to incorporate the effects of intramolecular interactions (such as Se.N bonds).

Experimental investigations and ab initio studies of selenium(ii) dialkanethiolates, Se(SR)2

Selenium(II) dimethanethiolate, Se(SMe)(2), was synthesized by reaction of SeO(2) with HSMe. Basic spectroscopic data for Se(SMe)(2) and selenium(II) bis(2-methyl-2-propanethiolate), Se(S(t)Bu)(2), were recorded and interpreted with the support of ab initio calculations. Both compounds are thermodynamically unstable relatively to selenium and the corresponding disulfide. The UV/vis spectra of both compounds are qualitatively similar, the two bands being attributed to n(Se)-sigma*(Se-S) transitions. The bands at 369 and 397 cm(-1) in the IR spectra of Se(SMe)(2) and Se(S(t)Bu)(2), respectively, are assigned to nu(as)(SeS(2)). The (77)Se NMR shifts of Se(SMe)(2)(784 ppm) and Se(S(t)Bu)(2)(556 ppm) differ substantially from each other and show positive temperature gradients. Calculations at the GIAO-HF/962+(d) level reproduced the difference of the (77)Se NMR chemical shifts between Se(SMe)(2) and Se(S(t)Bu)(2). At the same level, the effect of conformational changes on (77)Se shifts were studied for Se(SMe)(2). In the solid state Se(SMe)(2) forms long intermolecular SeS contacts while Se(S(t)Bu)(2) does not. Both compounds exhibit anti-conformations of the methyl and tert-butyl groups with respect to the SeS(2) plane. MP2/LANL2DZ(d) geometry optimizations, single point energy and frequency calculations performed for Se(SMe)(2) show, that syn- (C(s)) and anti-conformers (C(2)) represent minima on the potential energy surface, the latter being by 8 kJ mol(-1) lower in energy than the former. Both conformers are stabilized by intramolecular pi-type n(S(1))-sigma*(Se-S(2)) orbital interactions. The energy of the transition state for the mutual conversion of the two conformers was calculated to be 31 kJ mol(-1) above that of the syn conformer, allowing a rapid interconversion of the two conformers at room temperature. Intermolecular interactions between Se(SMe)(2) molecules were also studied by means of calculations at the MP2/LANL2DZ(d) level. For Se(S(t)Bu)(2) MP2/LANL2DZ(d) geometry optimizations and single point energy calculations revealed a C(2)-symmetric anti- and a C(1) symmetric syn-conformer, the latter being 21 kJ mol(-1) higher in energy than the former. Se(SMe)(2) and Se(S(t)Bu)(2) exchange thiolate groups with other selenium(II) dithiolates, tellurium(II) dithiolates and with thiols, if catalytic amounts of p-CH(3)C(6)H(4)SO(3)H are added.