ЖУРНАЛ СТРУКТУРНОЙ ХИМИИ Том 51, 2 Март апрель С

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1 ЖУРНАЛ СТРУКТУРНОЙ ХИМИИ Том 51, 2 Март апрель С UDC : THEORETICAL INSIGHTS INTO THE PROPERTIES OF THE X X M n+ COMPLEXES (X = H, F, Cl; = C, Si; M = ALKALINE AND ALKALINE EARTH METALS) 2010 R. Ghiasi* Chemical Society of East Branch, East Tehran Branch, Islamic Azad University, Tehran, Iran Received August, 7, 2008 The character of alkaline (M + = Li, Na, K) and alkaline earth metal ions (M 2+ = Be, Mg, Ca) interactions with disilyne and acetylene has been studied by using high-level ab initio computations. The interaction energies were calculated at MP2/ G(2d,2p) level. These calculations show that the size and charge of cation are two significant factors that affect the character of interaction. AIM and NBO analyses of M n+ X 2 2 interactions specify that the variation of densities and the extent of charge transfers upon complexation correlate well with the obtained interaction energies. Keywords: disilyne, disilyne complexes, interaction energy, NBO, AIM. 1. INTRODUCTION A variety of new compounds containing triple bonded silicon have been synthesized and studied for their chemistry [ 1 ]. The first successful synthesis was conducted in 1994; HSi N was prepared by matrix isolation and identified by UV and IR spectroscopy [ 2 ]. While the formation of adducts of the form RSi SiR (R = Me [ 3 ] and Tip 2 C 6 H 3 [ 4 ]; Tip is 2,4,6-triisopropylphenyl)) was suggested, no conclusive evidence has been presented in their support. Only Karni et al. have demonstrated the existence of stable FSi CH and ClSi CH under gas-phase conditions [ 5 ]. This success followed previous theoretical predictions [ 6 ]. The computational studies [ 7 9 ] have shown, however, that HSi CH and HSi SiH possess a trans-bent geometry in contrast to the linear geometry of acetylene. This paper reports studies on the complex formed upon interaction between X X (X = H, F, Cl; = C, Si) and alkaline or alkaline earth metal ions. The optimized structures, the interaction energies and the natural population analysis (NPA) are discussed. For comparison, the properties of the M + 2 X 2 and M 2+ 2 X 2 complexes are calculated. The atom in molecules (AIM) methodology of Bader is also applied to study the properties of the bond critical point of metals and 2 X 2 contacts to analyze dependencies between topological, energetic, and geometrical parameters of the complexes. 2. COMPUTATIONAL METHODS The structures of the X X M n+ complexes (X = H, F, Cl; = C, Si; M = alkaline and alkaline earth metals) and the corresponding monomers were optimized by the MP2 method, and G(2d,2p) was chosen as a basis set. The optimization was done along with a frequency calculation for each complex to verify that the geometry was a real minimum without any imaginary frequency. The interaction energy, IE, can be evaluated from the difference between the energy of the complex and the sum of the energies of the 2 X 2 and metal ions: IE = E(complex) [E(X 2 2 ) + E(M n+ )]. * rezaghiasi1353@yahoo.com

2 THEORETICAL INSIGHTS INTO THE PROPERTIES OF THE X X M n+ COMPLEXES 219 The calculated interaction energies were corrected for basis set superposition errors (BSSE), which were computed for all calculations using the counterpoise correction method of Boys and Bernardi [10]. All electronic structure calculations were performed using the Gaussian 2003 program [ 11 ]. The population analysis has also been performed by the natural bond orbital method [ 12 ] at MP2/ G(2d,2p) level of theory using NBO program [ 13 ] under Gaussian 2003 program package. The AIM methodology was applied to analyze the electron density and its corresponding Laplacian at the critical point M n+ X 2 2 contact from the optimized structures of M n+ X 2 2 at the MP2/ G(2d,2p) level. The topological analyses have been performed with the AIM 2000 program [ 14 ] using the MP2/ G(2d,p) wave functions as input. 3. RESULTS AND DISCUSSION 3.1. Energetics. Table 1 presents the computed interaction energies for the X X M n+ complexes (X = H, F, Cl; = C, Si; M = alkaline and alkaline earth metals). These values show that for a given complex IE is greater for less electropositive alkaline and alkaline earth metals. On the other hand, IE values increase in halogenated complexes and they are greater in chlorinated complexes rather than fluorinated. Table 1 Calculated energy E(Hartree), interaction energy IE (kcal/mol), BSSE (kcal/mol) and corrected interaction energy IE corrected (kcal/mol) for the 2 X 2 M n+ complexes by the method MP2 with G(2d,2p) basis set M n+ E(Hartree) IE BSSE IE corrected Si C Si C Si C Si C H + H F * Cl * Li + H F Cl Na + H F Cl K + H F Cl Be 2+ H F * Cl Mg 2+ H F Cl Ca 2+ H F Cl * These complexes are not stable.

3 220 R. GHIASI Table 2 Electron densities (e/a 3 0 ) and Laplacians 2 (e/a 5 0 ) at bond critical points of X 2 2 M n+ interaction and variation of electron density at the bond critical point of, calculated at the MP2/ G(2d,2p) level within the AIM theory M n+ (H 2 Si 2 M n+ ) 2 (SiSi) ( SiSi) Si C Si C Si C Si C H F Cl H + H F Cl Li + H F Cl Na + H F Cl K + H F Cl Be 2+ H F Cl Mg 2+ H F Cl Ca 2+ H F Cl Considering the magnitudes of IE corrected, it can be found that the size and charge of the cation are important factors determining the strength of the binding in these systems Atoms in molecules analysis. The analysis of electron density by means of the Bader s methodology (AIM) provides useful tools to confirm the presence of H 2 Si 2 M n+ interactions. Table 2 lists the values of and 2 of the complexes and X 2 2 at the MP2/ G(2d,2p) level. It contains the electron densities at the bond critical points of M n+ X 2 2 contacts and bonds. Several features of the electron densities analysis obtained in the AIM framework are summarized as follows: 1. All the BCPs of M n+ 2 X 2 interactions are characterized by small (r) values and positive Laplacian of the electron density. The sign of the Laplacian is determined by the positive curvature of (r) along the interaction line, as the Pauli Exclusion Principle leads to a relative depletion of charge density in the interatomic surface. These interactions are dominated by the contraction of charge density away from interatomic surface toward each of interacting species. It has been confirmed that the electron density at the bond critical point can be used as a measure of the binding strength. Figure 1 represents the plot of calculated interaction energies of M n+ 2 X 2 complexes versus their electron density at the BCP(SiSi).

4 THEORETICAL INSIGHTS INTO THE PROPERTIES OF THE X X M n+ COMPLEXES 221 Fig. 1. Correlation between the interaction energies and the density at the BCP of 2 X 2 M n+ complexes 2. Among all complexes, the obtained values for electron density are lower for the interaction of M + with 2 X 2 rather than M 2+. Also, theses values are greater in C 2 X 2 M n+ than in Si 2 X 2 M n+ complexes. The decrease of these values in halogenated complexes is compatible with their electronegativity. 3. An interesting feature of H + Si 2 H 2, Be 2+ Si 2 H 2, Mg 2+ Si 2 F 2, Be 2+ Si 2 Cl 2, and Mg 2+ Si 2 Cl 2 interactions is the formation of a ring critical point (RCP, (3,+1)) along the line connecting the cation to the center of SiSi upon complexation (see Fig. 2). 4. The variation of electron density at the Si Si bond critical points in different complexes can also present another indicator for the strength of M n+ H 2 Si 2 interactions. The computed variations of electron density of the Si Si bond critical points ( (SiSi)) upon complexation are given in Table 2. These values of (SiSi) show that the Si Si strength increases little upon the complex formation, except for Si 2 H 2 H +, Si 2 H 2 Be 2+, Si 2 F 2 Mg 2+, Si 2 Cl 2 Be 2+, Si 2 Cl 2 Mg 2+. It can be seen that the maximum reduction corresponds to the case of Si 2 Cl 2 Be 2+ interaction. The calculated variations of electron density of the C C bond critical points ( (CC)) upon complexation are given in Table 2. These values of (CC) show that the C C strength decreases little upon the complex formation, except for C 2 F 2 H +, C 2 Cl 2 H +, C 2 H 2 Be 2+, C 2 H 2 Mg 2+, C 2 Cl 2 Ca 2+, C 2 H 2 Ca Natural population analysis. Natural bond orbital analysis emphasizes the role of intermolecular orbital interaction in the complex, particularly charge transfer. The analysis is carried out by considering all possible interactions between filled donor and empty acceptor NBOs and estimating their energetic importance by second-order perturbation theory. For each donor NBO (i) and acceptor NBO (j), the stabilization energy E(2) associated with the electron delocalization between donor and acceptor Fig. 2. The formation of a ring critical point (RCP, (3,+1)) along the line connecting the cation to the center of SiSi upon complexation in H + Si 2 H 2, Be 2+ Si 2 H 2, Mg 2+ Si 2 F 2, Be 2+ Si 2 Cl 2, and Mg 2+ Si 2 Cl 2 complexes

5 222 R. GHIASI The results of second-order perturbation theory analysis of the Fock matrix within the NBO basis (E 2 energy values for the donor-acceptor interactions in kcal/mol) Table 3 M n+ donor acceptor E 2 j i F i,j Si C Si C Si C Si C H + H (C C) n*(h) F (C C) n*(h) Cl (C C) n*(h) Li + H (Si Si) n*(li) (C C) n*(li) F (Si Si) n*(li) (C C) n*(li) Cl (Si Si) n*(li) (C C) n*(li) Na + H (Si Si) n*(na) (C C) n*(na) F (Si Si) n*(na) (C C) n*(na) Cl (Si Si) n*(na) (C C) n*(na) K + H (Si Si) n*(k) (C C) n*(k) F (Si Si) n*(k) (C C) n*(k) Cl (Si Si) n*(k) (C C) n*(k) Be 2+ H (C C) n*(be) F (C C) n*(be) Cl (C C) n*(be) Mg 2+ H (C C) n*(mg) F (C C) n*(mg) Cl (Si Si) n(mg) (C C) n*(mg) Ca 2+ H (Si Si) n*(ca) (C C) n*(ca) F (Si Si) n*(ca) (C C) n*(ca) Cl (Si Si) n*(ca) (C C) n*(ca) is estimated as: (2) ( Fi, j) E qi, where q i is the orbital occupancy, i, j are diagonal elements and F ij is the off-diagonal NBO Fock matrix element. The results of the second-order perturbation theory analysis of the Fock matrix at MP2/ G(2d,2p) level of theory are collected in Table 3. For the M n+ X 2 2 complexes, most charge transfer energies reported in Table 4 are related to dominant interaction which occurs between -electrons of donor species and n*(m n+ ) ( (, = C, Si) n*(m n+ )). There is no interaction of this type when M n+ = H +, Be 2+, Mg 2+. The (Si Si) n*(li) and (Si Si) n*(ca) interactions occur in Si 2 F 2 Li +, Si 2 Cl 2 Ca 2+ complexes, respectively. On the other hand, (C C) n*(h) occur in C 2 X 2 H + complexes. The interaction of H + with Si 2 H 2 has attractive aspect. The strongest interaction is between (Si2 H3) donor and n*(si2) acceptor. The hybridization coefficients for the Si2 H3 bond are: (Si2 H3):0.4903(sp 5.87 d 0.15 )Si (sp 0.02 )H. The occupancy of this bond is A comparison between the results of IEs corrected in Table 1 and E(2) in Table 4 shows that although for a given cation the interaction energy is greater when the Si 2 H 2 interacts with alkaline earth i 2 j

6 M n+ THEORETICAL INSIGHTS INTO THE PROPERTIES OF THE X X M n+ COMPLEXES 223 Table 4 The natural population analysis for the M n+ 2 X 2 M M M n+ Si C Si C Si C Si C H + H Be 2+ H F F Cl Cl Li + H Mg 2+ H F F Cl Cl Na + H Ca 2+ H F F Cl Cl K + H F Cl metals, especially for Be and Mg. Figure 3 represents the plot of calculated interaction energies of M n+ X 2 Si 2 complexes versus E(2). The natural population analysis shows that Be and Mg have the smallest charge (Table 4). This implies different nature for Si 2 X 2 Be and Si 2 X 2 Mg interaction in comparison to other interactions. It means that these interactions are more covalent. 4. CONCLUSIONS In the present theoretical study, interaction of alkaline (M + = Li, Na, K) and alkaline earth metal ions (M 2 = Be, Mg, Ca) with disilyne and acetylene have been investigated. The calculations suggest that the size and charge of cation are two significant factors that affect the nature of interaction. Fig. 3. Correlation between the E (2) stabilization energy and interaction energies of 2 X 2 M n+ complexes

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