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Resonance-Assisted Intramolecular Chalcogen-Chalcogen Interactions?

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Resonance-Assisted Intramolecular Chalcogen-Chalcogen Interactions? FULL PAPER Resonance-Assisted Intramolecular Chalcogen ± Chalcogen Interactions? Pablo Sanz, Manuel Ya¬nƒ ez, and Otilia Mo¬*[a] Abstract: High-level B3LYP/6- independently of the nature of X and Y. nonbonding interactions involving 311G(3df,2p) density functional calcu- For X O, S, Se and Y S, Se the most chalcogen atoms, mainly Se and Te, are lations have been carried out for a series stable conformer b is the one exhibiting not always strongly stabilizing. This of saturated chalcogenoaldehydes: aYÀH ¥¥¥X IHB. Conversely when Y conclusion is in agreement with the fact CH(X)-CH2-CH2YH (X, Y O, S, Se, Te, the chelated conformer d, stabilized that intermolecular interactions be- Te). Our results indicate that in CH(X)- through aX ¥¥¥YH chalcogen ± chalco- tween Se and Te containing systems CH2-CH2YH (X Y O, S, Se) the gen interaction is the global minimum of with bases bearing dative groups are XÀH ¥¥¥X intramolecular hydrogen the potential energy surface. Systemati- very weak. We have also shown that bond (IHB) competes in strength with cally the IHB and the chalcogen ± these interactions are enhanced for un- the X ¥¥¥XH chalcogen ± chalcogen in- chalcogen interactions observed for sa- saturated compounds, through an in- teraction, while the opposite is found for turated compounds are much weaker crease of the charge delocalization with- the corresponding tellurium-containing than those found for their unsaturated in the system, in a mechanism rather analogues. For those derivatives in analogues. This result implies that the similar to the so call Resonance Assisted which X=Y, X being the more electro- Hydrogen Bonds (RAHB). The chalc- negative atom, the situation is more ogen ± chalcogen interactions will be Keywords: density functional calcu- complicated due to the existence of two also large, due to the enhancement of lations ¥ chalcogen ± chalcogen in- non-equivalent XÀH and YÀH tauto- the X ! Y dative bond, if the molecular teractions ¥ hydrogen bonds ¥ reso- mers. The YÀH tautomer is found to be environment forces the interacting nance assisted interactions lower in energy than the XÀH tautomer, atoms X and Y to be close each other. Introduction basicity and/or acidity of the system. Chalcogen ± chalcogen attractive forces also play an important role as far as the Weak interactions play a crucial role in the structural preferred conformation and reactivity of some chalcogen organization of chemical and biochemical systems and in derivatives is concerned, as it has been shown many years ago molecular recognition. Many of these weak interactions are by Adcock, Angyan et al. and B¸rgi and D¸nitz,[14±16] and directly associated with the formation of inter- or intra- more recently by Minyaev and Minkin.[17, 18] Also recently, molecular hydrogen bonds (IHBs),[1±3] while others are due to Komatsu et al. [19] showed that 17Oand77Se NMR spectro- nonbonding interactions between heavy atoms. Among the scopic data provide strong evidence for intramolecular non- latter chalcogen ± chalcogen interactions have received a bonded interaction between Se and O in hydroxy-selenenyl particular attention. These weak bonds are responsible for compounds. These interactions seem to play also an important the enhanced stability of chelated structures with respect to role in some reaction mechanisms.[20] open ones, as it is the case for instance in malonaldehyde[4±8] Very recently, using b-chalcogenovinylaldehydes as suitable and in thiomalonaldehyde (TMA),[9, 10] and modulate their model compounds, we have investigated[21] the competition intrinsic reactivity. Tropolone,[11] resorcinol,[12] and acid anhy- between XÀH ¥¥¥Y (or X ¥¥¥H-Y) IHBs (structures a and b in drides[13] are paradigmatic examples of systems where the Scheme 1) and Y! X (or X ! Y) chalcogen ± chalcogen existence or the formation of IHBs affects the intrinsic nonbonded interactions (structures c and d, respectively in Scheme 1) on the stability of these systems. A possible mixture of rapidly interconverting tautomers a [a] Prof. Dr. O. Mo¬, P. Sanz, Prof. Dr. M. Ya¬nƒ ez and b through the IHB is another important characteristic of Departamento de Quimica, C-9 this kind of compounds. As a matter of fact, for thiomalo- Universidad Auto¬ nomade Madrid naldehyde high level ab initio calculations predict both Cantoblanco, 28049 Madrid (Spain) tautomers to be very close in energy,[9] and thiomalonalde- Fax : ( 34)91-3975238 E-mail: [email protected] hyde and its derivatives have been successfully used as model Supporting information for this article is available on the WWW under systems to investigate ultrafast hydrogen transfer through [22±24] http://www.chemeurj.org/ or from the author. pulsed lasers. Also, the existence of both forms a and b 4548 ¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/chem.200304891 Chem. Eur. J. 2003, 9, 4548 ± 4555 4548±4555 6-31G* basis set expansion was used for the geometry opti- mization and harmonic vibra- tional frequency calculations for all compounds, except for those containing Te. For tellu- rium derivatives, we have em- Scheme 1. Tautomers for X O, S and Y Se, Te. ployed amixed base, which includes aSKBJ relativistic po- tential of Stevens et al.[45] for has been well established by means of UV,[25] UV photo- Te, which accounts for the most important relativistic effects, electron,[26] IR,[25] and 1H NMR spectroscopy.[27, 28] However, together with the [4,1]d basis described in ref. [21] and a our results indicated that, although selenovinylaldehyde and 6-31G(d) basis set for the remaining atoms of the system. selenothiovinylaldehyde resemble closely thiomalonalde- Hereafter, for the sake of simplicity this mixed basis set, will hyde, in the sense that the O-H ¥¥¥Se and the S-H ¥¥¥Se be named for extension 6-31G(d). intramolecular hydrogen bonds compete in strength with the Final energies were obtained on single point B3LYP/6- O ¥¥¥Se and the S ¥¥¥Se interactions, the opposite is found for 311G(3df,2p) calculations on the aforementioned optimized the corresponding tellurium-containing analogues. These geometries. It must be indicated that for Se the dissimilarities are due to dramatic differences between Se 6-311G(3df,2p) basis set corresponds to that reported by and Te as far as the X ¥¥¥Y(X O, S ; Y Se, Te) interactions Curtiss et al.[46] are concerned, which for Se derivatives are rather small, while For Te-containing compounds, again a mixed basis set was for Te compounds are very strong. As a consequence, while used. It combines an all-electron 6-311G(3df,2p) basis set for selenium containing compounds the global minima of the expansion for all atoms except Te, for which the SKBJ potential energy surface (PES) correspond to a-type tauto- effective core potential is combined with the (6s,6p,3d,1f) mers, for Te containing systems the global minima correspond basis set developed in a previous paper.[21] Again for the sake to d-type tautomers. In a recent paper,[29] we have also shown of simplicity this mixed basis set will be named hereafter as that these weak interactions play a crucial role in the intrinsic 6-311G(3df,2p) basis. In a previous paper[21] we have shown, basicity and acidity of these systems. for the set of b-chalcogenovinylaldehydes that this approach The enhanced stability of the IHB in thiomalonaldehyde yields results in good agreement with those obtained by has been often associated with the so-called Resonance means of high-level G2(MP2) ab initio calculations, but at a Assisted Hydrogen Bond (RAHB) phenomenon,[1, 30±32] that much lower cost. is, the stabilization resulting from the charge delocalization The bonding characteristics of the different tautomers were triggered by the formation of the IHB. An open question, is analyzed by using two alternative procedures, namely the whether similar resonance effects may be responsible for the atoms in molecules (AIM) theory of Bader[47] and the Natural enhanced stability of d-type tautomers in the case of Se- and Bond Order (NBO) analysis of Weinhold et al.[48] The first Te-containing compounds, that is, whether chalcogen ± chalc- method is based on a topological analysis of the electron ogen interactions observed in b-chalcogenovinylaldehydes charge density and its Laplacian. Within this approach we containing Se and Te are enhanced by a possible charge have located for each bond the bond critical points (bcp) the delocalization effect. In order to analyze this possibility we charge density of which is a good indication of the strength of will compare along this paper the structure, stability and the linkage. bonding characteristics previously reported in the literature[21] The NBO analysis will allow us to obtain reliable charge for b-chalcogenovinylaldehydes, with those of the corre- distributions, as well as to evaluate quantitatively the intra- sponding saturated analogues, namely CH(X)-CH2-CH2YH molecular attractive orbital interactions which would be (X, Y O, S, Se, Te), where delocalization effects are absent. responsible for the stability of c- and d-type structures (see Scheme 1). These analyses will be complemented with those carried out in terms of the lengthening or shortening of the Computational Methods bond lengths and in terms of the shifting of the corresponding stretching frequencies. We have carried out also a Natural [49±51] The geometries of the different conformers of CH(X)-CH2- Resonance Theory (NRT) analysis to estimate the weight CH2YH (X, Y O, S, Se, Te) species were optimized using the of the most important resonance structures that may contrib- B3LYP density functional theory (DFT) approach, as it is ute to the enhancement of the strength of the IHB or that of implemented in the Gaussian 98 suite of programs.[33]The the chalcogen ± chalcogen interaction.
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