THE STABILIZING ROLE OF HALIDE IONS IN ENDOHEDRAL [20

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C: Physical Processes in Nanomaterials and Nanostructures

THE STABILIZING ROLE OF HALIDE IONS IN ENDOHEDRAL [20]SILAFULLERANES: INSIGHTS FROM DFT CALCULATIONS TOWARDS SILICON NANOCAGES Miguel Ponce-Vargas, and Alvaro Muñoz-Castro J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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The Journal of Physical Chemistry

THE STABILIZING ROLE OF HALIDE IONS IN ENDOHEDRAL [20]SILAFULLERANES: INSIGHTS FROM DFT CALCULATIONS TOWARDS SILICON NANOCAGES

Miguel Ponce-Vargasa* and Alvaro Muñoz-Castrob* a

Université Paris-Est, Laboratoire Modélisation et Simulation, Multi Echelle, MSME UMR

8208 CNRS, 5 Bd Descartes, 77454 Marne-la-Vallée, France. E-mail: [email protected] b

Grupo de Química Inorgánica y Materiales Moleculares, Facultad de Ingeniería, Universidad

Autónoma de Chile, El Llano Subercaseaux 2801, Santiago, Chile. E-mail: [email protected]

ABSTRACT

As miniaturization of electronic devices is rapidly approaching the nanoscale, a deeper understanding of the electronic and structural properties of silicon nanoclusters, called to be the

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next-generation materials for circuit design, becomes of paramount importance. Herein, a detailed DFT study of the binding forces between [20]Silafullerenes frameworks and their central halide ions is conducted, prompted by the recent synthesis of the first discrete Si20 dodecahedra stabilized by an endohedral chloride and valence saturation, [Cl@Si32Cl44]-, as well as the fabrication of the first electron transistor device based on a single silicon cluster. Although more intense stabilizing forces are obtained in the chloride-containing system, a small energetic difference with respect to bromide-centered one is found (4.76 kcal mol-1) suggesting the synthetically accessibility of the latter. An energy decomposition analysis is conducted revealing that in all cases the electrostatic term is the major contributor to the binding forces, representing about of 71%. Additionally, the higher order electrostatic terms become more relevant as the halide volume increases, and this effect is quantified through a local multipole analysis. This methodology also enables us to state that those silicon atoms directly linked to peripheral chlorines play a more relevant role into the guest encapsulation than those attached to trichlorosilyl. It is also evidenced, that the presence of such peripheral groups deeply influences the charge of the inner cluster cavity making it more positive and suitable for the encapsulation of the halide anions. We expect that this research will allow a better understanding of the driven forces of these novel structures, also contributing to experimental teams searching for novel building blocks for nanoscale transistors.


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INTRODUCTION

The continuous search for miniaturization in microelectronics industry has led to the development of new silicon structures able to be integrated in smaller and more power-efficient devices.1-3 In this context, silicon clusters have been extensively studied by multiple experimental and theoretical teams searching for stable frameworks at the nanoscale size.4-7 Recently, Song and coworkers have reported the fabrication of a single electron transistor device based on a silicon cluster connected to a gold break junction with a nanometer scale separation.8 Also, a field effect transistor based on undoped silicon nanowires have been successfully fabricated opening up new possibilities for the combination of one-dimensional transport and silicon technology,9 thus, positioning silicon nanostructures as promising molecular devices materials for practical applications. Unlike carbon, sp2 hybridization is unfavorable for silicon10, leading to asymmetric and reactive clusters11, however, there exists a wide consensus with respect to the main structural modifications that could contribute to the stability of the silicon assemblies, i.e., silica coating12, hydrogen capping13, phosphorus14, carbon15, and metal doping16-19, as well as the incorporation of a central metal20-22 or halide ion.23 In the case of the silicon counterpart of graphene, silicene, the functionalization with OH radicals produces the largest structural modifications due to the presence of intralayer hydrogen bonds leading to further stability.24 The recent synthesis of the first [20]Silafullerane, by Wagner and coworkers25, represents a major milestone in the field, where the key features leading to a stable cluster are the appended trichlorosilyl and chlorine groups, the endohedral chloride ion, and the overall Si-sp3 backbone. However, the nature of the related forces ensuring the encapsulation of the endohedral anion and the silicon cage has not been studied in detail.26


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Herein we provide a systematic theoretical study of the nature of the cluster-anion interactions through reliable density functional theory (DFT) tools that will allow a better description of this recently synthesized [Cl@Si32Cl44]- structure. In addition, we extend our scope to similar systems containing bromide and iodide guests, also evaluating the effect of the external trichlorosilyl groups and chlorine atoms in the inner cavity charge. First, we conduct an NCI analysis27,28 in order to visualize the regions where more intense noncovalent forces arises, next we carry out an Energy Decomposition Analysis29-30 to clearly established the nature of the forces between the central ions and the silicon atoms. In parallel we carried out a Multipole Analysis31 to quantify the electronic cloud distortion experienced by the silicon atoms with the incorporation of the halide, also plotting the dipole vectors and quadrupole tensors to have a complete picture of the electronic effects caused by the endohedral halide. Finally, the 29Si-NMR properties were evaluated, in order to provide valuable and reliable information to experimental teams devoted to the synthesis of these fascinating structures.


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COMPUTATIONAL DETAILS

Relativistic density functional theory32 calculations were carried out by using the ADF code33, via the scalar ZORA Hamiltonian. An all electron triple-ξ Slater basis set including a polarization function (STO-TZP) was employed within the generalized gradient approximation (GGA) of Becke and Perdew (BP86).34-35 The choice of this functional is dictated by its good performance in the study of vibrational spectra of neutral36 and metal-doped silicon clusters.7,37,38 Moreover, the good performance of BP86 has also been tested for the calculation of the vertical dettachment energy of Gallium clusters39 and the optical spectra of icosahedral silver-nickel nanoclusters.40 Geometry optimizations were conducted via the analytical energy gradient method implemented by Verluis and Ziegler.41 To properly account for weak London forces, the D3 Grimme dispersion correction was incorporated.42 In D3, the dispersion coefficients are geometry-dependent as they are adjusted on the basis of local geometry. Calculations of the nuclear shielding tensors were conducted within the GIAO formalism, employing the OPBE functional along with an all-electron STO-TZ2P basis set, which were referenced to trimethylsilane (TMS) in order to account for the 29Si-NMR chemical shifts.43,44 A Noncovalent Interactions Analysis (NCI), as proposed by Yang and coworkers, was conducted by using the NCIPLOT-3.027 and NCImilano28 codes, the later using the electron density from ADF calculations. Furthermore, an Energy Decomposition Analysis was carried out following the Morokuma-Ziegler scheme.29,30 Also, the ion-multipole contribution in the studied complexes has been estimated according to a methodology based on a comparison with the hypothetical noble gas centered counterparts45,46 which has been employed in anion-host interactions studies.47,48 Finally, the Atomic Multipole Moment Analysis proposed by Swart and coworkers31, was used to study in deep the local electronic cloud variation at the silicon atoms.


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For all isosurfaces and figures the software packages Chemcraft49 and Visual Molecular Dynamics (VMD)50 were employed.

RESULTS AND DISCUSSION

The synthesis of [Cl@Si32Cl44]-, has been recently reported by Wagner and coworkers25 and it involves a chloride-induced disproportionation reaction of hexachlorodisilane, where the chloride ion plays a structure-determining role. Some relevant structural parameters concerning [Cl@Si32Cl44]- and the related hypothetical frameworks [Br@Si32Cl44]- and [I@Si32Cl44]- are presented in Table 1 and Figure 1. The backbone silicon atoms directly linked to chloride are labelled as Si1, those bonded to trichlorosilyl as Si2, and the trichlorosilyl ones as Si3 (see Figure 1). Remarkably, a good agreement between crystallographic data and theoretical results is found for [Cl@Si32Cl44]-. Making a comparison between the hollow silicon cluster and the halide-encapsulating cages, it seems that the incorporation of the halide ion does not significantly alter the silicon cluster, since [Br@Si32Cl44]- and [I@Si32Cl44]- geometries exhibit similar bond distances, whereas in [Cl@Si32Cl44]- all Si-Si bonds are slightly reduced indicating a marginal cluster contraction. This denotes that the Si32Cl44 cage is rigid enough to overcome any further deformation upon halide encapsulation. For all structures we carry out a NCI Analysis27,28 (see Figure 2) in order to determine which atoms contributing the most in keeping the anion in the center of the cavity, This analysis is based on a 2D plot of the reduced density gradient s, and the electron density ρ, where s can be expressed as


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=

1 ⁄

23 ⁄

When non-covalent interactions arise, there exists a noticeable variation in s, producing density critical points that generate an isosurface. To determine the type of interaction, the second eigenvalue of the electronic density Hessian (λ2) is invoked, where intense stabilizing interactions, such as hydrogen bonds, are characterized by λ2 < 0, non-bonded interactions such as steric repulsion by λ2 > 0, and weak interactions by λ2 ≈ 0. These λ2 values are denoted over the isosurface by using a color scale. In the [X@Si32Cl44]- series, the NCI analysis evidences the rise of non-covalent forces between the silicon atoms and the central anion, irrespective of the silicon environment. As it is expected, these noncovalent forces become stronger for those silicon centers in close proximity to the halide, which is denoted by blue regions in the isosurface. The nature of the cluster∙∙∙anion interactions can be explored in deep through the Energy Decomposition Analysis (EDA) proposed by Morokuma and implemented by Ziegler29,30 which separates the total interaction energy in different terms such as electrostatic (∆Eelec), Pauli repulsion (∆EPauli), and orbital overlapping (∆Eorb). In this analysis, the interaction between the frozen charge densities is accounted by ∆Eelec, which is calculated by subtracting the Coulomb integral of the fragments from that corresponding to the overall system. The product of density fragments, which is normalized but violates the Pauli principle, is antisymmetrized and renormalized to give an intermediate state. The energy difference between the states before and after the antisymmetrization-renormalization is called Pauli repulsion, ∆EPauli, and can be related to destabilizing forces between the fragments. Finally, the fragment orbitals are relaxed to yield the final state corresponding to the inclusion system. The energy decrease caused from such orbital mixing is identified as the orbital interaction, ∆Eorb. Adding ∆Eelec, ∆EPauli, and ∆Eorb we obtain the total interaction energy ∆Eint, of the fragments. To overcome the basis set


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superposition error (BSSE), the counterpoise method proposed by Boys and Bernardi51 is employed denoting forces of about 3 kcal mol-1, which are negligible with respect to the intensity of the studied energies. The dispersion interaction is included according to the Grimme pairwise dispersion correction42 where an attractive energy term summed over all atomic pairs is incorporated, taking into account the weak forces associated to instantaneous fluctuations of fragment electronic densities. Finally, the energy difference between the isolated fragments geometries and the overall system is denoted as preparation energy, ∆Eprep. The Energy Decomposition Analysis (Table 2) evidences the major stability of the [Cl@Si32Cl44]- structure (-110.35 kcal mol-1), where the main contributing term is ∆ , representing about 71% of the stabilizing terms, denoting the mainly electrostatic character of the cavity∙∙∙anion interactions. Remarkably, ∆ significantly increases in line with the volume of the encapsulated ion, i.e. from -159.77 kcal mol-1 in [Cl@Si32Cl44]- to -278.21 kcal mol-1 in [I@Si32Cl44]-. However, this effect is counterbalanced by the increment of the Pauli repulsion which varies from -113.41 kcal mol-1 in [Cl@Si32Cl44]-, to -300.70 kcal mol-1 in [I@Si32Cl44]-. The calculated preparation energy, ∆Eprep, varies in agreement to the suggested stiffness from geometrical parameters (see above), showing a small ∆Eprep amounting to 1.77 for [Cl@Si32Cl44], 1.58 for [Br@Si32Cl44]- and 1.66 kcal mol-1 for [I@Si32Cl44]-. The effect of replacing the peripheral chlorine atoms by hydrogens can be evaluated through the [X@Si32H44]- series (see Table 3), where it is clear that a significant loss of stability occurs, with total binding forces now ranging from -68.53 kcal mol-1 in [Cl@Si32H44]- to -47.64 kcal mol-1 in [I@Si32H44]-. The ∆ term is the most affected with the absence of the peripheral trichlorosilyl and chlorines, unraveling that an inductive effect promoted by these groups, that make the inner cavity more electron-deficient occurs, also enhancing the electrostatic interactions towards the negative-charged central ion, and hence ensuring the latter encapsulation observed ACS Paragon Plus Environment

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for chloride anion. Furthermore, the dispersion forces contribute the least to total interaction energy in both the [X@Si32Cl44]- and [X@Si32H44]- series, representing about 2.5%. Interestingly, the obtained preparation energy, ∆Eprep, in the [X@Si32H44]- series, denotes a lower stiffness in comparison to the [X@Si32Cl44]- series, showing a larger ∆Eprep amounting to 3.24 for [Cl@Si32H44]-, 4.82 for [Br@Si32H44]- and 8.43 kcal mol-1 for [I@Si32H44]-, suggesting that the electron withdrawing -SiCl3 groups are able to induce more rigidity in comparison to -SiH3, in addition to a more favorable encapsulation energy, supporting the statement of their relevant role from the initial report on [Cl@Si32Cl44]- .25 Moreover, a small difference in the favorable encapsulation energies is found between [Cl@Si32Cl44]- and [Br@Si32Cl44]- with interaction energies of -110.35 and -105.59 kcal mol-1, respectively, suggesting that a bromine-containing silicon cluster could be stable enough to be obtained experimentally, and also [I@Si32Cl44]- despite of the lower encapsulation energy (-93.54 kcal mol-1). This exposes that the hypothetical analogous structures are suitable for further experimentation. In addition, the case of an endohedral fluorine atom is studied (see Table S2), revealing a ∆Eint of -132.65 kcal mol-1, where the ∆Eelec term contributes in 65.82%. Thus, the total interaction energy of [F@Si32Cl44]- is comparable to that observed for its Cl, Br and I counterparts. When the peripheral chlorine atoms are replaced by H, in [F@Si32H44]-, the stabilization is dramatically decreased to -3.37 kcal mol-1, highlighting the relevant effect of the peripheral chlorines in making the cluster cavity more positive, and then suitable for the incorporation of a negative-charged species. In order to acquire a deeper understanding of the inclusion forces, we decompose the main contributing term to the stabilizing forces, i.e. the ∆ term, into two groups, one encompassing ion-multipole interactions (i.e. ion-dipole, ion-quadrupole, etc.) and other covering


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higher-order interactions (i.e. dipole-dipole, dipole-quadrupole, quadrupole-quadrupole, etc.) following a protocol recently applied to inclusion compounds displaying metallic centers.45,46,52 Then, the central halide ions (Cl-, Br-, I-) are replaced by their isoelectronic noble gas counterparts (Ar, Kr, Xe), thus enabling only the rise of the higher-order forces. The comparison of the electrostatic terms for each series (1 − ∆ /∆ )% allows us to quantify the

contribution of ion-multipole electrostatic forces into ∆ . The results of the EDA for these hypothetical silicon clusters containing noble gases are presented in Table S1. The ion-multipole nature of the electrostatic forces is revealed, representing 83.14%, 77.24% and 69.67% of ∆ for [Cl@Si32Cl44]-, [Br@Si32Cl44]- and [I@Si32Cl44]-, respectively. Additionally, the increasing relevance of the higher order terms from the chloride system to the iodine one can be explained by the major electronic cloud distortion experienced by the anion as its volume increases. In turn, the orbital term (∆ ) contribution also increases with the halide volume, suggesting a more effective orbital overlap, which in turn allows a higher electron charge transfer from the cluster to the anion. These results are in good agreement with the atomic charges obtained from the multipole analysis (see Table 4), where a -0.44 |e-| charge amount is transferred from the cluster to the iodide ion, quite far from the +0.22 |e-| and -0.03 |e-| obtained for the chloride and bromide clusters, respectively. The influence of the peripheral halide atoms is evaluated along the [Cl@Si32X44]- (X=F, Cl, Br, I) series (Table S3), where it is clear that as more electronegative X, a stronger encapsulation energy is found, going from -202.71 kcal mol-1 in [Cl@Si32F44]- to -73.51 kcal mol1

in [Cl@Si32I44]-. In all cases it is the ∆Eelec which contributes the most to total interaction

energy, but this effect is more pronounced in [Cl@Si32F44]- and [Cl@Si32Cl44]- (~71%) in


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comparison to [Cl@Si32Br44]- and [Cl@Si32I44]- (~57%). These results support further explorative synthesis efforts for obtaining the related F exo-bonded derivative. To quantify the extent of the electronic cloud distortion experienced by the silicon atoms with the inclusion of the halide ion, we calculate their local dipole vectors and quadrupole tensors according to the methodology proposed by Swart and coworkers31 where the local electrostatic potential is mainly determined by the charge distribution around the respective atom, or within the atomic multipole expansion, by the atomic multipoles near to that point. This charge analysis offers an accurate description of the electrostatic potential from the charge distribution in molecules by writing the total density as a sum of atomic densities expressed in terms of atomic functions, which in turn are used to define a set of atomic multipoles. This methodology enables a simple description of charge distributions in endohedral complexes, which represents a major challenge.52,53 The results of the multipole analysis are presented in Tables 5 and 6, and the corresponding graphical representations in Figures 3 and 4. The vector analysis reveals a more pronounced electronic cloud distortion in terms of dipole moments for those silicon atoms bonded to chlorine (Si1), which is expected given the influence caused by the near electronegative halide. Then, the Si1 dipole moment magnitude varies from 0.963 D in Si32Cl44. to 0.511 D in [Cl@Si32Cl44]-, 0.485 D in [Br@Si32Cl44]-, and 0.395 D in [I@Si32Cl44]-. On the other hand, the vectors associated to those silicon atoms linked to the trichlorosilyl groups (Si2) vary to a considerable lesser extent from 0.310 D in Si32Cl44 to 0.215 D in [Cl@Si32Cl44]-, 0.235 D in [Br@Si32Cl44]-, and 0.205 in [I@Si32Cl44]-. In the case of the peripheral silicon atoms (Si3), their dipole magnitude reduction can be associated with the charge transfer toward the central halide. The quadrupole anisotropy values show a significant electronic deviation in Si1 as the size of the central halide increases, especially in the case of [I@Si32Cl44]-, where a different


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electronic cloud distribution is found, with the quadrupole positive lobe pointing toward the center of the cavity (see Figure 4). In the case of Si2, although the presence of the chloride ion clearly modifies the electronic density, it is the incorporation of Br- and I-, which causes a more pronounced distortion, reversing the lobes charges, thus favoring the presence of the central anion. Notably, the small quadrupole tensors for the endohedral halide anions in all evaluated systems suggest that there is not a significant electronic cloud deviation for them, even in the iodide case. Finally, the 29Si-NMR properties were evaluated owing to the valuable data they offer for further experimental guidance. For [X@Si32Cl44]-, the three chemically different Si atoms were identified, namely, Si1, Si2 and Si3 (Table 7 and Figure 1) in a 8:12:12 ratio, which exhibits an almost spherical nuclear shielding tensor due to the sp3-hybridization of the silicon belonging to the structural backbone. For the experimentally characterized [Cl@Si32Cl44]- cluster, the calculated values for the isotropic chemical shift (δiso) amounts to 31.6, -59.9 and 13.3 ppm, for Si1, Si2 and Si3, respectively, in good agreement with the experimental

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Si-NMR spectrum

values (31.1, -60.3 and 10.3 ppm).25 For the hypothetical hollow Si32Cl44 cluster, the respective values are 53.9, -63.1 and 7.9 ppm, denoting a high shielding for Si1 (~22 ppm), in contrast to the deshielding in a lesser extent for Si2 and Si3, of about 3 and 5 ppm. Such observation is in line with the more predominant role of the silicon atoms bonded to peripheral chlorines (Si1) into the guest encapsulation, in comparison to those bearing trichlorosilyl ligands, and appears as more sensitive to the chlorine encapsulation in the 29Si-NMR spectrum. Similarly, for the Br- and I- clusters, Si2 and Si3 remain in a narrow range in comparison to [Cl@Si32Cl44]-, unraveling the progressive shielding of the chemical shift for Si1 from 53.9 (Si32Cl44) < 31.6 ([Cl@Si32Cl44]-) < 25.7 ([Br@Si32Cl44]-) < 15.5 ppm ([I@Si32Cl44]-), further


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confirming the respective characteristic pattern in the

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Si-NMR spectrum. We also notice that

into [I@Si32Cl44]- the peaks corresponding to Si1 and Si2 are almost overlapped.

CONCLUSIONS

A series of [20]Silafullerenes exhibiting an endohedral halide ion have been studied through a detailed DFT protocol, which reveals the preference of the Si20 framework for the chloride guest, as well as the strong electrostatic character (∆Eelec of about 71%) of the noncovalent forces in all systems. Looking in detail for the ∆Eelec, the increasing relevance of the higher order terms going from [Cl@Si32Cl44]- to [I@Si32Cl44]- is manifested, which can be explained by a major electronic cloud distortion for the silicon atoms as the guest volume increases. The small difference between the total interaction energies corresponding to the recently reported [Cl@Si32Cl44]- (-110.35 kcal mol-1) and its hypothetical counterpart [Br@Si32Cl44]- (105.59 kcal mol-1) suggests that experimental efforts could be oriented to the synthesis of the latter. Conversely, the lower stability of [I@Si32Cl44]- (-93.54 kcal mol-1) can be rationalized by the sharp rise of the Pauli repulsion term (300.70 kcal mol-1), more than twice of that corresponding to [Cl@Si32Cl44]- (113.41 kcal mol-1). A comparative study between the [X@Si32Cl44]- clusters and the hypothetical silyl counterparts with -SiH3 at the position of -SiCl3, given by [X@Si32H44]- analogues, evidences that peripheral chloride atoms and trichlorosilyl groups clearly promotes the guest incorporation as they make the inner cavity more positive, and hence more suitable for accommodating the halide guest. It supports the research of new peripheral groups able to influence the cage charge through inductive effects. Thus, the role of -SiCl3 is crucial in the initial encapsulation of chlorine


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in the experimentally observed [Cl@Si32Cl44]- species, making [Br@Si32Cl44]- as a potentially synthetic target, among related [X@Si32Cl44]- structures. The electronic cloud distortion experienced by the silicon atoms with the guest inclusion can be rationalized in terms of local dipoles and quadrupole moments. This analysis reveals that silicon atoms linked to the peripheral chlorines play a more relevant role in encapsulating the endohedral guest variation in comparison to those linked to trichlorosilyl, as it is evidenced by their more pronounced multipole variation with the guest incorporation. We envisage that the stability of [Cl@Si32Cl44]- as well as the apparent synthetic accessibility to its analogues will stimulate the exploration of similar materials as an interesting approach to fabricate novel singleelectron transistors.

ACKNOWLEDGEMENTS The authors thank the financial support from FONDECYT 1180683.


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TABLES AND FIGURES

Table 1: Selected structural parameters of the studied systems (Å and degrees). [Cl@Si32Cl44]-

Si32Cl44

Exp.a

Calc.

[Br@Si32Cl44]- [I@Si32Cl44]-

Si1-Si2

2.405

2.355

2.398

2.403

2.412

Si2-Si3

2.367

2.320

2.367

2.370

2.373

Si2-Si2

2.388

2.352

2.386

2.394

2.403

Si1···Si1

6.664

6.547

6.639

6.659

6.691

Si2···Si2

6.779

6.628

6.761

6.777

6.795

Si2-Si1-Si2

109.3

108.9

109.3

109.3

109.1

Si1-Si2-Si1

106.2

106.7

106.1

106.2

106.4

Si1-Si2-Si3

107.9

107.5

108.2

108.1

108.1

Si1···center

3.318

3.319

3.330

3.330

3.346

Si2···center

3.386

3.382

3.388

3.388

3.397

a. Experimental data from reference 22

Table 2: Energy Decomposition Analysis (EDA) (kcal mol-1) for the halide-substituted silicon clusters. [Cl@Si32Cl44]-

[Br@Si32Cl44]1.58

[I@Si32Cl44]-

∆Eprep

1.77

1.66

∆Eorb

-57.35

25.63%

-73.45

25.80%

-108.82

27.60%

∆Eelec

-159.77

71.40%

-203.94

71.64%

-278.21

70.57%

∆Edisp

-6.64

2.97%

-7.28

2.56%

-7.21

1.83%

∆EPauli

113.41

179.08

300.70

∆Eint

-110.35

-105.59

-93.54


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Table 3: Energy Decomposition Analysis (EDA) (kcal mol-1) for the hydrogen-substituted silicon clusters. [Cl@Si32H44]-

[Br@Si32H44]4.28

[I@Si32H44]-

∆Eprep

3.24

8.43

∆Eorb

-62.30

35.55%

-76.33

32.76%

-105.89

31.97%

∆Eelec

-106.95

61.03%

-150.37

64.54%

-219.78

66.35%

∆Edisp

-5.99

3.42%

-6.30

2.70%

-5.59

1.69%

∆EPauli

106.71

170.58

283.62

∆Eint

-68.53

-62.42

-47.64

Table 4. Charge variation for selected atoms in the studied systems. [Cl@Si32Cl44]-

[Br@Si32Cl44]-

[I@Si32Cl44]-

Si1

-0.09

-0.08

-0.04

Si2

0.08

0.11

0.14

Si3

-1.11

-1.12

-1.14

X

0.22

-0.03

-0.44

Table 5: Electronic dipole vectors for silicon and halide ions (Debyes). Si32Cl44

[Cl@Si32Cl44]- [Br@Si32Cl44]- [I@Si32Cl44]-

Si1

0.963

0.511

0.485

0.395

Si2

0.310

0.215

0.235

0.205

Si3

1.161

0.688

0.708

0.750


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Table 6. Anisotropy of the electronic quadrupole tensors for the representative silicon atoms and halide ions (Buckinghams). Si32Cl44

[Cl@Si32Cl44]- [Br@Si32Cl44]-

[I@Si32Cl44]-

Si1

-3.7305

-2.5918

-1.6536

0.3398

Si2

0.6075

0.5878

-0.9071

-1.1787

Si3

2.1675

1.3262

1.1482

0.7379

X

-

-0.0005

0.0006

0.0042

Table 7. Calculated 29Si-NMR chemical shifts (δ) for the studied series (ppm).

Si32Cl44 [Cl@Si32Cl44][Br@Si32Cl44][I@Si32Cl44]a

Si1 53.9 31.6 25.7 15.5

Si2 -63.1 -59.9 -58.8 -58.0

Si3 7.9 13.3 13.4 14.6

Absolute shielding referenced to TMS (σref=366.3 ppm)


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Figure 1. Optimized structures and some relevant distances for the inclusion compounds (distances in Å)


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Figure 2. NCI Analysis of the studied systems.

Figure 3. Graphical representation of the electronic dipole vectors of the studied systems where the blue arrowhead denotes the negative region (Debyes).


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Figure 4. Graphical representation of the electronic quadrupole tensors of the studied systems (orange lobes denote negative regions and the blue ones, positive regions).

Figure 5. Calculated 29Si-NMR spectrum for the studied systems.


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TOC GRAPHIC


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THE STABILIZING ROLE OF HALIDE IONS IN ENDOHEDRAL [20

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