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Electronic Effects and Ring Strain Influences on the Electron Uptake Electronic Effects and Ring Strain Influences on the Electron Uptake by Selenium-Containing Bonds E´ LISE DUMONT,1 PIERRE-FRANC¸ OIS LOOS,2 ADE` LE D. LAURENT,2 XAVIER ASSFELD2 1Laboratoire de Chimie, UMR 5182 CNRS E´ cole Normale Supe´rieure de Lyon, 46 alle´e d’Italie, 69364 Lyon Cedex 07, France 2E´ quipe de Chimie et Biochimie The´oriques, UMR 7565 CNRS-UHP, Institut Jean Barriol (FR CNRS 2843), Faculte´ des Sciences et Techniques, Nancy-Universite´, B.P. 239, 54506 Vandoeuvre-le`s-Nancy, France Received 24 November 2008; accepted 22 December 2008 Published online 16 June 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.22072 ABSTRACT: The gas-phase electron attachment of thiaselena and diselena derivatives is investigated on model organic systems by ab initio calculations (level of theory MP2/DZPϩϩ). Electronic contributions favor the one-electron addition on selenium- containing compounds, with adiabatic electron affinities of 0.03, 0.24, and 0.43 eV, respectively, for dimethyldisulfide, dimethylselenenylsulfide, and dimethyldiselenide. This ensures the possibility of an excess electron binding on OSeOSO and OSeOSeO linkages. The so-formed radical anionic intermediates present a three-electrons two-centers 2cO3e bond, whose nature is confirmed by Mulliken spin densities and NBO analysis. They are stable towards dissociation, with a low barrier evaluated between about 25–60 kJ/mol. Cyclization strongly enhances dichalcogen propensity to fix an excess electron. Adiabatic electron affinities of a series of 1,2-thiaselena-cycloalkanes and 1,2-diselena-cycloalkanes are positive and range from 0.24 to 1.30 eV. This can be traced back to the release of ring strain energy upon one-electron addition: this geometrical effect is nevertheless less marked than for disulfide analogs. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 513–523, 2010 Key words: thiaselenide; diselenides; disulfide; electron attachment; three-electrons two-centers bonds; ab initio calculations; ring strain energy Correspondence to: E´ . Dumont; e-mail: elise.dumont@ ens-lyon.fr Contract grant sponsor: The Jean Barriol Institute. Contract grant sponsor: Supercomputer resources of the Pole Scientifique de Modelisation Numerique (PSMN), ENS Lyon. International Journal of Quantum Chemistry, Vol 110, 513–523 (2010) © 2009 Wiley Periodicals, Inc. DUMONT ET AL. 1. Introduction elenium versatile chemistry has raised much S interest in various areas [1, 2]. There is a fun- damental interest in understanding its difference with sulfur analogs, in terms of structure and reac- tivity (spectroscopy, stereoelectronic effects). Even simple selenium-containing derivatives still need to be properly characterized [3], for instance for their environmental aspects [4]. Selenium also plays a crucial and dual role in biology [5–8]. Although its toxicity beyond certain quantities has been early recognized, evidences for its essential role in anti- oxidant protection have been reported: cancer pro- tection [9, 10], Keshan disease, etc. Selenium is most often incorporated in proteins as selenocysteine FIGURE 1. Electron attachment reaction on (a) a di- (Sec), the 21st proteinogenic amino-acid. Its redox sulfide, (b) a selenosulfide, and (c) a diseleno bonds. potential is lower than cysteine and this confers to The so-formed radical anions feature a three-electron selenoproteins an enhanced reactivity by a factor of two-center bond, with a drastic lengthening of the ini- 10, 100, or even 1,000 [11]. Much research efforts tially covalent bond. The resulting hemi-bond is repre- currently focus on enlighting the underlying mech- sented with dashed lines. anisms of antioxidant protection, where electron transfers play a determinant role [12]. A recent example is MsrB1, which has a much higher effi- hydrogen atom, or proton addition,… The central ciency towards methionine sulfoxide reduction question in this article is to gain some insights on than other classes of methionine sulfoxyde reduc- the electron affinity of thiaselena- and diselena- tases [13]. Other experimental studies have consid- linkages when compared with dithia analogs (point ered cysteine to selenocysteine mutation to design (i)). Respective dissociation barriers (point (ii)) of inhibitors with more controlled properties [14]. the corresponding 2cO3e bonds, for which no ex- Both experimental techniques (pulse radiolysis, perimental data are available, will also be briefly synchrotron X-ray,…) and ab initio calculations commented for linear systems. have ascertained the importance of transient three- Electron affinity is intrinsically delicate to deter- electrons two-centers intermediates in the biologi- mine either experimentally or by quantum mechan- cal systems [15–18]. They present a hemi-bond, usu- ics calculations [21]. Some simple linear aliphatic or ally denoted as XІY, formed by capture of a low- aromatic disulfides have been carefully studied lying energy electron. Motivations are twofold for [22–24] but no analog results on hemi-bonded an- characterizing such species, and special importance ions involving selenium are known to the best of is given to X and Y being chalcogens as it corre- our knowledge. The only experimental result sponds to the biologically relevent situation. published so far is due to King and Illies: they Their structure has been in-depth explored, no- reported mass spectroscopy data that “very ticeably with topological analysis [19]. Yet, less is strongly support a two-center three-electron SeІSe ϩ known concerning their reactivity, and in particular interaction in [(CH3)2SeІSe(CH3)2] ” [25]. the ease by which such intermediates are formed. In this article, ab initio calculations are per- Far from being a prototype reaction, the one-elec- formed to investigate the one-electron addition on tron addition (see Fig. 1) is involved in many bio- selenium-containing organic compounds. The chemical redox processes involving dichalcogen methodological aspects for describing 2cO3e bonds bridges [20]. are briefly recalled in Section 2. It is crucial to The variation of redox potential for a sulfur-to- ensure the reliability of our calculations, especially selenium substitution depends among others on: (i) the basis set dependence, as discussed in Section 2. the propensity to fix an excess electron, (ii) the The reactivity of dimethydisulfide, dimethylselene- barrier for the dissociation of the transient hemi- nylsulfide, and dimethyldiselenide are compared in bonded intermediate, (iii) their reactivity toward Sub- section 3.1, to evaluate the importance of in- 514 INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY DOI 10.1002/qua VOL. 110, NO. 3 ELECTRON UPTAKE BY SELENIUM-CONTAINING BONDS TABLE 1 ______________________________________________________________________________________________ Adiabatic electron affinities AEA (in eV) for MeOXOYOMe compounds, at the MP2 level of theory and for sev- eral basis sets. Values in parenthesis correspond to the dif-ference of electron affinities with respect to disulfide analog. Although absolute values are strongly basis-set dependent, the respective order of reactivity is conserved. Basis set 6-31ϩG** 6-31ϩG**(S,Se), 6–31G*(C,H) 6-311ϩG** DZPϩϩ DMDS Ϫ0.11 Ϫ0.14 0.02 Ϫ0.16 DMSeS 0.03 (0.14) 0.03 (0.17) 0.17 (0.15) 0.01 (0.17) DMDSe 0.21 (0.33) 0.21 (0.35) 0.37 (0.31) 0.15 (0.31) trinsic electronic contributions. The variation of anion states at their respective optimized geome- electron affinity of these model systems upon dihe- tries. Vertical electron affinities (VEA) and vertical dral torsion is analyzed in Subsection 3.2. It pro- detachment energy (VDE) are defined as the differ- vides a first estimate of geometric effects upon di- ence between the total energies of the neutral and chalcogen linkages. They are further investigated in radical anion states, on the neutral and radical an- Subsection 3.3 on small-members cyclic linkages. ion optimized geometries, respectively. AEA ϭ E(neutral)ϪE(anion) (1) 2. Methodological Approach VEA ϭ E(neutral) Ϫ E(anion) at the optimized Chemical bonds with an odd number of elec- trons constitutes a well-documented pitfall case for geometry of the neutral compound (2) Hartree–Fock and DFT methods, as well-estab- lished in the literature. We have recently summa- VDE ϭ E(neutral) Ϫ E(anion) at the rized in a more detailed way the methodological reasons for such a failure [26], and we will here optimized geomtery of the anion (3) simply refer the interested reader to the original analysis [27–31]. The second-order Møller–Plesset A specific and key issue is the choice of the basis perturbation theory (MP2 [32]) ensures a proper set [21]. We performed auxiliary calculations on description of the one-electron addition on dichal- three prototype dichalcogens, at the MP2 level of cogen linkages. theory and with four different basis sets, to assess (U)MP2(fc) calculations, as well as NBO analysis, the stability of energetic quantities. Not unexpect- were performed with the Gaussian 03 series of pro- edly, AEAs of the neutral moities and dissociation 2 grams [33]. ͗ S ͘ values were never greater than 0.77 energy (Edisso) of the radical anions (reported in (the exact value is 0.75), such that no spin contam- Tables I and II) are both basis-set dependent. It lies ination will affect our results. Adiabatic electron beyond the scope of this study to explore in depth affinities (AEA) are defined as the difference be- this methodological aspect, and we would lack a tween the total energies of the neutral and radical comparison with the exact experimental values. By TABLE II ______________________________________________________________________________________________ Dissociation energies Edisso (in kJ/mol) for 2cO3e MeOXІYOMe radical anions, at the MP2 level of theory and for several basis sets. Values in parenthesis correspond to the difference with respect to disulfide analog. Although absolute values are strongly basis-set dependent, the respective order of reactivity is conserved. Basis set 6-31ϩG** 6-31ϩG**(S,Se), 6–31G*(C,H) 6-311ϩG** DZPϩϩ DMDS 28.4 26.6 26.4 28.4 DMSeS 27.9 (Ϫ0.5) 22.9 (Ϫ3.2) 26.0 (Ϫ0.4) 27.7 (Ϫ0.7) DMDSe 38.1 (ϩ9.7) 41.8 (15.2) 36.9 (ϩ10.5) 38.1 (ϩ9.7) VOL.
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