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The Se∓H Bond of Benzeneselenols (Arse-H): Relationship Between

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The Se∓H Bond of Benzeneselenols (Arse-H): Relationship Between Chemical Physics 415 (2013) 18–25 Contents lists available at SciVerse ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys The Se–H bond of benzeneselenols (ArSe-H): Relationship between bond dissociation enthalpy and spin density of radicals ⇑ Pham Cam Nam a, , Minh Tho Nguyen b,c a Department of Chemistry, Danang University of Technology (DUT), The University of Danang, 54 Nguyen Luong Bang, Lien Chieu, Da Nang, Viet Nam b Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium c Institute for Computational Science and Technology at HoChiMinh City (ICST), Quang Trung Software Park, HoChiMinh City, Viet Nam article info abstract Article history: Bond dissociation enthalpies (BDE) of benzeneselenol (ArSe-H) and its para and meta-substituted deriv- Received 7 December 2012 atives are calculated using the (RO)B3LYP/6-311++G(2df,2p)//(U)B3LYP/6-311G(d,p) procedure. The com- In final form 7 February 2013 puted BDE(Se–H) = 308 ± 8 kJ/mol for the parent PhSe-H is significantly smaller than the experimental Available online 14 February 2013 value of 326.4 ± 16.7 kJ/mol [Kenttamaa and coworkers, J. Phys. Chem. 100 (1996) 6608] but larger than a previous value of 280.3 kJ/mol [Newcomb et al., J. Am. Chem. Soc. 113 (1991) 949]. The substituent Keywords: effects on BDE’s are analyzed in terms of a relationship between BDE(Se–H) and Mulliken atomic spin Benzeneselenol densities at the Se radical centers of ArSe (p radicals). Good correlations between Hammett’s substituent Bond dissociation enthalpies constants with BDE(Se–H) are established. Proton affinity and ionization energy amount to PA(C H Antioxidants 6 5- Mulliken atomic spin densities SeH) = 814 ± 4 kJ/mol and IE(C6H5SeH) = 8.0 ± 0.1 eV. IEs of the substituted benzeneselenols are also Proton affinities determined. Calculated results thus suggest that 4-amino-benzeneselenol derivatives emerge as efficient Ionization energies antioxidants. Hammett’s correlations Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The homolytic Se–H bond energy of benzeneselenol, BDE(Se–H), was initially estimated to be 280.3 kJ/mol by Newcomb et al. [10]. Chalcogen-containing compounds play an important role in Subsequently, this value was again determined experimentally by chemistry and biochemistry. The selenium element which is one Kenttamaa and coworkers, who reported a value of BDE(Se– of the chalcogens and belongs to the group 16 of the Periodic Table, H) = 326.4 ± 16 kJ/mol (78 ± 4 kcal/mol) [12]. A discrepancy of occurs in four different valence oxidation states, namely Se6+,Se4+, about 46 kJ/mol (or 11 kcal/mol) between both experimental re- Se2À and Se0. Selenium compounds are often used in the glass sults for the BDE(Se–H) is in fact quite large, but it has not been industry as decolorizing agents and in the rubber industry as vul- reevaluated in the last 15 years. The lack of a reliable BDE(Se–H) canizing agents. Selenium is also an essential trace element on hu- value prompts us to reevaluate it using the current and appropriate mans and other species. Organoselenium compounds such as quantum chemical methods. ebselen (1,2-phenyl-1,2-benzisoselenazol-3(2H)-one) [1,2], methyl The structure–antioxidant activity relationships of aromatic selenol [3], dimethyl diselenide [4] and some selenium analogs [5– compounds YC6H4XH, where X = O, S, CH2, SiH2, NH,...have exten- 7] were found to exhibit significant antioxidant activities. Another sively been investigated and correlations between energies and basic selenium compound which has also attracted much interest structural parameters have clearly been established. The effects is benzeneselenol C6H5SeH. The latter is the heavier congener of of the substituents Y on the activities are mainly exerted on the phenol (C6H5OH) and thiophenol (C6H5SH) and well known as stability of the radicals generated [13–15]. Of the latter, the spin one of the most efficient radical traps both in the gas phase and characteristics of the radical center emerge as an interesting in solution, and plays a central role in radical reaction rate mea- parameter. As a matter of fact, homolytic bond dissociation corre- surements [8–12]. The rate of radical trapping by benzeneselenol sponds to the breaking of a bond producing a radical species, and is characterized by the enthalpy change associated with the hydro- the spin density at the radical center has an important role in gen atom transfer reaction. The formation and homolysis of the determining the stability of the resulting radical. The spin density selenium–hydrogen bond in C6H5SeH are thus proved to be an in part describes the electronic structure of a radical and also the important factor in many chemical and biochemical processes. inherent delocalization of the unpaired electron and related conju- gation effects. For the phenol derivatives (YC6H4OH), the changes of the BDE(O–H) with respect to various physical properties ⇑ Corresponding author. including the spin delocalization, were investigated by several E-mail address: [email protected] (P.C. Nam). 0301-0104/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemphys.2013.02.008 P.C. Nam, M.T. Nguyen / Chemical Physics 415 (2013) 18–25 19 groups [16–20]. Bond dissociation enthalpies and ionization ener- The homolytic Se–H bond dissociation enthalpy value at gies of polyphenols have been the topics of many previous papers. 298.15 K for the molecule Y-C6H4-SeH is calculated from the Eq. The polyphenols are characterized by an extended conjugation and (2): delocalization of the p-electrons involving the aromatic ring(s) and the substituents. They give rise to stable radical species upon the BDEðSe—HÞ¼Hf ðY À C6H4Se ÞþHf ðH ÞHf ðY À C6H4Se—HÞð2Þ removal of a hydrogen atom or an electron. The odd electron ap- pears to be delocalized over the entire molecule thanks to the pla- where Hf are the enthalpies of different species at 298.15 K. The en- nar geometry conformation. The stability of radicals is enhanced by ergy of hydrogen atom is taken from the experimental value of the possibility to establish internal H-bonds between the radical- À0.5 hartree. In all cases considered, the parent C6H5Se radical is ized oxygen atom and vicinal hydroxyl groups [21–24]. However, used as a reference for comparison. the behaviors of benzeneselenol and its derivatives as radical scav- The ionization energy (IE) of Y-C6H4SeH is defined as the small- engers, antioxidants and related properties have not been studied est amount of energy required to remove an electron from a deriv- yet. In this context, we set out to theoretically determine the trend ative. It is computed as the difference between the energies of its of Se–H bond strengths in a series of ArSeH derivatives, along with radical cation and the neutral molecule (Eq. (3)). For both neutral the effects of various substituents at both the para and meta posi- and radical cation species, zero-point energies (ZPE) determined tions of the aromatic ring on the BDE(Se–H) values. from harmonic vibrational frequencies obtained at the B3LYP/6- The purpose of the present study is twofold. First, we calculate 311G(d,p) level are added, without scaling factor, to the electronic energy to obtain the energy E0 at 0 K (Eq (3)). the standard BDE(Se–H) values at 298.15 K of the parent C6H5SeH and two series of 3- and 4-substituted benzeneselenols YC H SeH þ 6 4 IEðY À C6H4SeHÞ¼E0ðY À C6H4 À SeH ÞE0ðY À C6H4SeHÞð3Þ with different Y substituents. In view of the discrepancy of exper- imental results on the BDE(Se–H) of C6H5SeH mentioned above, it is of importance to reevaluate it using reliable computations. Sec- 3. Results and discussion ond, we attempt to find out if there is any relationship between the BDE(Se–H) with the spin densities at the Se centers of the 3.1. Optimized geometry of C6H5SeH and its thermochemical ArSe radicals [19,25]. This could provide us with a better under- parameters standing of the origin of the substituent effect on Se–H bond dis- sociation energy in terms of spin delocalization. To establish such The equilibrium geometry of the parent benzeneselenol is char- a relationship, we make use of the changes of the Mulliken atomic acterized by two lowest-energy conformations shown in Fig. 1a spin density (hereafter denoted as MASD) with respect to the and Fig. 1b. Contrary to phenol and thiophenol, C6H5SeH is not pla- changes of BDE(Se–H) when the substituents Y are placed at the nar. The hydrogen atom (at Se) in both optimized structures is lo- para- and meta-position of the ring. In addition, the classical cated outside the benzene ring. Some selected bond lengths, bond Hammett parameters are also employed to unravel the effects angles and dihedral angles of both conformers obtained using dif- of the electron withdrawing and donating groups on the distribu- ferent functionals are given in the Supplementary information (SI) tion of spin at the selenium center and thereby the stability of the file. radicals. The C–C bond distances within the aromatic ring and the C1– Se bond length obtained by four DFT functionals are quite similar0 2. Computational methods to each other. The length variation is about from 0.001 to 0.005 ÅA when using B3LYP, B3PW91, PBE1PBE and BMK functionals. A sig- Previous studies abundantly showed that density functional nificant difference occurs only on the bond distance of C1–Se and theory (DFT) is one efficient and economic tool to determine with the dihedral angle of C2C1SeH.
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