Why Are SiX5â and GeX5â (X = F, Cl) Stable but Not CF5â and CCl5â?

Article pubs.acs.org/JPCA

Why Are SiX5− and GeX5− (X = F, Cl) Stable but Not CF5− and CCl5−? Marzena Marchaj, Sylwia Freza, and Piotr Skurski* Department of Chemistry, University of Gdańsk, Sobieskiego 18, 80-952 Gdańsk, Poland ABSTRACT: The possible existence of the CF5−, CCl5−, SiF5−, SiCl5−, GeF5−, and GeCl5− anions has been investigated using ab initio methods. The species containing Si and Ge as central atoms were found to adopt the D3h-symmetry trigonal bipyramidal equilibrium structures whose thermodynamic stabilities were confirmed by examining the most probable fragmentation channels. The ab initio re-examination of the electronic stabilities of the SiF5−, SiCl5−, GeF5−, and GeCl5− anions [using the OVGF(full) method with the 6-311+G(3df) basis set] led to the very large vertical electron detachment (VDE) energies of 9.316 eV (SiF5−) and 9.742 eV (GeF5−), whereas smaller VDEs of 6.196 and 6.452 eV were predicted for the SiCl5− and GeCl5− species, respectively. By contrast, the high-symmetry and structurally compact anionic CF5− and CCl5− systems cannot exist due to the strongly repulsive potential predicted for the X− (F− or Cl−) approaching the CX4 (CF4 or CCl4). The formation of weakly bound CX4···X− (CF4···F− and CCl4···Cl−) anionic complexes (consisting of pseudotetrahedral neutral CX4 with the weakly tethered X−) might be expected at low temperatures (approaching 0 K), whereas neither CX5− (CF5−, CCl5−) systems nor CX4···X− (CF4···F− and CCl4···Cl−) complexes can exist in the elevated temperatures (above 0K) due to their susceptibility to the fragmentation (leading to the X− loss).

1. INTRODUCTION 1.1. Superhalogens and Their Daughter Anions. The existence of superhalogens was predicted in 1981 by Boldyrev and Gutsev, who proposed a simple MXk+1 formula to define these species.1 According to their findings, practically any system containing the central atom M (a main group or transition metal atom) decorated with k + 1 halogens (where k is the maximal formal valence of the atom M) should be capable of forming stable anionic system having relatively large excess electron binding energy. In particular, the electronic stability of such anions is expected to exceed the electron affinity of chlorine atom (3.62 eV).2 As it turned out, the superhalogen anions described in the literature thus far exhibit very large electron binding energies spanning the 3.6−14 eV range.1,3,4 Since the existence of these species was postulated in the early 1980s, many theoretical efforts have been undertaken to estimate the vertical electron detachment energies (VDE) of various anions having superhalogens as their neutral parents (see refs 5−13 and references cited therein). In addition, the first experimental photoelectron spectra of such species (MX2− , where M = Li, Na, and X = Cl, Br, I) were measured by the Wang group in 1999.14 This experimental confirmation of superhalogen existence was a milestone achievement and resulted in bringing more attention to this class of compounds. As a consequence, a large number of novel superhalogen anions have been proposed, examined experimentally, and characterized theoretically.15−20 In 2009, it has been pointed out that the presence of halogen atoms in superhalogen species is not obligatory, since the alternative ligands might be applied instead. Hence, it was demonstrated that the halogen ligands might be replaced with © 2012 American Chemical Society

halogenoids (e.g., CN, SCN, OCN) and the electronic stabilities of the resulting anions may even exceed those obtained with the main group VII elements.21 According to recent findings, other alternative functional groups might also be exploited as ligands in superhalogen anions. Namely, the systems utilizing electrophilic substituents (i.e., NO2, CF3, CCl3, SHO3, and COOH)22 and acidic functional groups (i.e., ClO4, ClO3, ClO2, ClO, NO3, PO3, H2PO4, HSO4, HCO3, SH)23 as ligands were proposed and studied. The main purpose of the efforts undertaken to explore various novel superhalogen species is to provide reliable predictions considering the possible use of such compounds as electron acceptors (oxidizing agents) in the production of organic superconductors, as well as the role they can play in synthesis [e.g., in oxidation of counterpart systems with high ionization potentials (IP)].24,25 Our contribution to these studies covers the explanation of the ability of selected moderately reactive molecules to form stable systems with superhalogens. As we recently demonstrated, even the molecules possessing high ionization potentials (such as SiO2, NH3, CHCl3, CCl2F2) should form stable and strongly bound ionic compounds when combined with properly chosen superhalogen system (acting as an oxidizing agent).26 In addition, we showed that the competition between the electron binding energy of the superhalogen system and the IP of the molecule the superhalogen is combined with is a key factor for predicting the stability of certain species. Received: January 9, 2012 Published: February 8, 2012 1966

dx.doi.org/10.1021/jp300251t | J. Phys. Chem. A 2012, 116, 1966−1973

The Journal of Physical Chemistry A

Article

1.27 eV, respectively. The CCl5− anion was found to be thermodynamically stable (i.e., not susceptible to any fragmentation process considered). In addition, the excess electron binding energy of CCl5− (3.23 eV) was predicted to be similar to the electron affinity of the isolated chlorine atom.38 1.3. The SiX5− (X = F, Cl) Anions. In 1969, Beattie and Livingston reported the spectroscopic evidence for the formation of the SiCl5− anion in solution (nitromethane) suggesting its trigonal bipyramidal structure.39 The high stability of this anion was confirmed by Wilhite and Spialter40 and later by Larson and McMahon.41 To the best of our knowledge, the very first theoretical prediction of the stability and substantial electron affinities of the SiF5 and SiCl5 was provided by Gutsev and Boldyrev in 1981 (e.g., the EAs of SiF5 and SiCl5 were estimated as equal to 6.4 and 4.2 eV, respectively, which suggested the large electronic stability of their corresponding anionic daughters).1,42 The HF/6-31+G* calculations on the SiF5− anion performed by Deiters and Holmes led to the D3h-symmetry trigonal bipyramaidal minimum energy structure. Also, the enhanced reactivity of the pentacoordinated anionic silicon species was predicted (relative to tetracoordinated systems).43 According to the density functional investigation (local spin density approximation) undertaken by Gutsev, the minimum energy structures of SiF5− and SiCl5− exhibit D3h symmetry and both correspond to trigonal bipyramids, whereas their vertical excess electron binding energies were estimated as equal to 6.47 eV (SiF5−)38,44 and 4.21 eV (SiCl5−).38,45 The most recent theoretical studies (employing the density functional method) on the structure and electronic stability of SiF5− anion were performed by the Schaefer group.46 According to their results (obtained using the BHLYP functional with the double-ζ basis set including polarization functions), the vertical electron binding energy of the SiF5− equals to 8.54 eV for the BHLYP functional and 7.63 eV when the B3LYP functional is employed. The minimum energy structure of its anionic daughter (SiF5−) corresponds to D3h-symmetry trigonal bipyramid with the axial Si−F and equatorial Si−F bond lenghts of 1.663 and 1.624 Å, respectively. However, the neutral ground state SiF5 was reported as geometrically unstable and susceptible to the fragmentation, leading to the fluorine atom detachment.46 1.4. The GeX5− (X = F, Cl) Anions. Even though the properties of germanium fluorides in general attract attention as the systems important for the fine processing of semiconductors, the theoretical data describing the geometrical and electronic stability of the GeF5 molecule and its corresponding anion are hardly available in the literature. However, the Schaefer group published in 1999 the systematic study on the structure, thermochemistry, and electron affinities of the GeFn/GeFn− (n = 1−5) species.47 The theory level applied (density functional approach with the BHLYP functional and double-ζ quality basis set including polarization and diffuse functions) allowed for predicting the minimum energy geometrical structures and thermochemical quantities with satisfactory accuracy. In particular, the GeF5− anion was found to adopt the D3h-symmetry trigonal bipyramidal equilibrium structure with the axial and equatorial Ge−F bond lengths equal to 1.771 and 1.742 Å, respectively, which was consistent with the earlier experimental reports.48,49 The thermodynamic stability of GeF5− was also confirmed with respect to all most likely fragmentation processes, including the F− detachment for which the predicted dissociation energy of

Even though the present knowledge considering superhalogens and their daughter anions seems substantial, the available description of some of their relatively structurally simple representatives is not satisfactory. One striking example of such a system is the hypothetical CF5− anion for which the reports published in the literature thus far are contradictory (see the next section). Also, we believe that the issue of the existence and stability of the CCl5− anion has not been properly addressed in the past. As the properties of both CF5− and CCl5− species are expected to be related to those of the corresponding SiX5− and GeX5− (X = F, Cl) anions, we decided to take a closer look at these negatively charged systems. However, before presenting the aim of the present work, we offer a brief description of the previous experimental and theoretical efforts undertaken to investigate the possible existence and stability of the CX5−, SiX5−, and GeX5− (X = F, Cl) species. 1.2. The CX5− (X = F, Cl) Anions. The possible existence of the pentacoordinated carbon compounds involving halogens was postulated in 1966 by McDaniel and Deiters, who supposed the formation of stable CCl5− anions.27 This early work was followed by the experimental reports suggesting the presence of such species in solution.28−30 The X-ray diffraction measurements performed by Lindner et al. led to the observation of bromide ions coordinated to the CBr 4 tetrahedron in the solid state.31 In addition, the indications of stable CCl5− anions in the gas phase were predicted by the use of high-pressure mass spectrometry.32,33 The first theoretical studies on the issue of the possible existence and stability of pentahalocarbonate anions were performed by Wang [employing the Hartree−Fock (HF) method with minimal basis set]34 and by Carrion and Dewar (who used the semiempirical MNDO method).35 Although those reports predicted the stability of such anions in the gas phase, the results were not conclusive due to the method and basis set deficiencies these calculations suffered from. In 1986, Vetter and Zülicke employed the HF method together with much more reliable basis set of double-ζ quality and concluded that the bipyramidal (D3h) structures of CX5− (X = F, Cl, Br) were highly unstable, whereas the lower-symmetry (C3v) systems were found to be only slightly stable (2−10 kcal/mol).36 The investigation employing the density functional theory (DFT) method on the CF5− anion using a rather modest basis set was performed in the early 1990s by Gutsev and resulted in predicting the D3hsymmetry (trigonal bipyramid) lowest energy structure.37 However, the theoretical treatment employed led to the rather elongated C−F bonds in the D3h equilibrium structure of CF5− (approaching 1.7 Å). The other isomer of CF5− anion possessed C4v-symmetry structure and its energy was larger by 0.4−0.5 eV than that of the lowest energy D3h structure. Even though the CF5 neutral parent molecule matched the MXk+1 superhalogen formula, the adiabatic electron affinity estimated for this compound seemed surprisingly small (3.2− 3.5 eV) and similar to that of the fluorine or chlorine atom. It is also important to point out that the CF5− anion was found to be thermodynamically stable with respect to any fragmentation.37 The extension of this study covering also the CCl5− anion revealed the ground state minimum energy structure exhibiting C3v symmetry with one extremely elongated C−Cl bond (3.776 Å) and the quasitetrahedral CCl4 fragment.38 The two higher energy isomers found possessed D3h and C4v symmetries, and their relative energies (with respect to the lowest energy C3v structure) were estimated to read 0.57 and 1967



Article

3.75 eV agreed well with the experimentally measured 4.34 ± 0.3 eV value.50 The neutral parent molecule GeF5, however, was found to be geometrically unstable, although the calculations employing certain less reliable density functionals (BP86 and BLYP) led to the stable structures. Thus, the adiabatic electron affinity of germanium pentafluoride could not be calculated with the BHLYP or B3LYP method, as the equilibrium neutral geometries were not available. As far as the vertical electron detachment energy of the GeF5− is concerned, the best estimates provided by Li et al. were equal to 9.37 eV (BHLYP) and 8.66 eV (B3LYP).47 1.5. The Aim of the Present Contribution. Since the previous reports describing the possible existence of CF5− and CCl5− anions were contradictory, we decided to reexamine the geometrical stability of these species by using properly chosen ab initio methods together with sufficiently large basis sets. Also, we provide the free enthalpies of the most probable fragmentation reactions that the CF5− and CCl5− anions might be susceptible to. In particular, we demonstrate that such negatively charged systems cannot exist in the gas phase in their assumed trigonal bipyramidal structures. Instead, temporary CX4···X− (X = F, Cl) adducts might be formed (consisting of the halogen X− anion tethered weakly to the slightly perturbed neutral CX4 tetrahedral moiety). Such adducts, however, are predicted to be vulnerable to the X− loss. As far as the SiX5− and GeX5− (X = F, Cl) anions are concerned, we reexamine their equilibrium structures and we offer the reliable estimations of their excess electron binding energies as the values available in the literature were obtained using the DFT-based methods only and exhibited strong dependence on the density functional chosen [e.g., the VDE of the SiF5− anion was calculated (by employing various DFT functionals) to span the wide 6.47−8.54 eV range].38,44,46 We believe that providing conclusive ab initio results for such basic superhalogen compounds as CF5−, CCl5−, SiF5−, SiCl5−, GeF5−, and GeCl5− might be useful for chemists designing novel compounds and undertaking the actual synthesis.

the default direct SCF calculations, the keyword SCF=NoVarAcc was used and the two-electron integrals were evaluated (without prescreening) to a tolerance of 10−20 au. The optimizations of the geometries were performed using relatively tight convergence thresholds (i.e., 10−5 hartree/bohr (or radian) for the root-mean-square first derivative).

3. RESULTS 3.1. Equilibrium Structures of the SiF5−, SiCl5−, GeF5−, and GeCl5− Anions and Thermodynamic Stability Thereof. The trigonal bipyramidal D3h-symmetry structures were found to be the equilibrium structures for the SiX5− and GeX5− anions (X = F, Cl); see Figure 1. The axial (ax.) and equatorial

Figure 1. The equilibrium D3h-symmetry structures of the SiF5−, SiCl5−, GeF5−, and GeCl5− anions obtained at the MP2/6-311+G(d) level (the bond lengths r are given in Å).

2. METHODS The equilibrium geometrical structures of the species studied and the corresponding harmonic vibrational frequencies were calculated by applying the second-order Møller−Plesset (MP2) perturbational method with the 6-311+G(d) basis set.51,52 The final values of the vertical electron binding energies were obtained by employing the outer valence Green function OVGF method (B approximation).53−61 As a matter of fact, some of the earliest references59 called the approach being developed “equations of motion (EOM)” rather than Green function (GF). Since the OVGF approximation remains valid only for outer valence ionizations for which the pole strengths (PS) are greater than 0.80−0.85,62 we verified that the PS values were sufficiently large to justify the use of the OVGF method for all states studied here (the smallest PS found for the states examined in this work was 0.893). All electrons were included while the OVGF calculations were performed (i.e., core electrons were not omitted). We applied the 6-311+G(3df) basis while the vertical electron binding energies were estimated since analogous basis sets have been used for superhalogen anions and provided an excellent agreement between such calculated and experimentally measured VDEs.10,12−15,21−23 All calculations were performed with the Gaussian 03 software package.63 In order to avoid erroneous results from

(eq) Si−F bond lengths in SiF5− read 1.682 and 1.648 Å, respectively, which is in good agreement with those predicted earlier by Deiters and Holmes (ax., 1.661 Å; eq, 1.624 Å),43 Gutsev (ax., 1.645 Å, eq, 1.603 Å),44 and King et al. (ax., 1.663 Å; eq, 1.624 Å).46 In addition, the estimated silicon−fluorine bond lengths are similar to the experimental values derived from the X-ray structure and corrected for thermal motions (ax., 1.660 Å; eq, 1.622 Å).64 The lengths of the Si−Cl bonds in the SiCl5− anion were calculated as equal to 2.223 Å (ax.) and 2.105 Å (eq); see Figure 1. In fact, these distances are slightly shorter than those predicted by Gutsev [2.254 Å (ax.) and 2.119 Å (eq)].38,45 The minimum energy structures of the GeF5− and GeCl5− were also found to exhibit D3h-symmetry with the bond lenghts of 1.806 Å (ax.)/1.782 Å (eq) for GeF5− and 2.295 Å (ax.)/2.198 Å (eq) for GeCl5−; see Figure 1. The germanium−fluorine bond lengths reported in this contribution are slightly longer (by 0.03−0.04 Å) than the corresponding distances predicted earlier by Li et al.47 As far as the thermodynamic stability of the SiF5−, SiCl5−, GeF5−, and GeCl5− anions is concerned, we found these species to be stable and not susceptible to any fragmentation. In particular, we calculated the free energies (ΔHr298), entropies (ΔSr298), and free enthalpies (ΔGr298) for the reactions (for T = 1968



Article

298.15 K) corresponding to the most likely fragmentation channels (see Table 1, where the ΔHr298, ΔSr298, and ΔGr298 are

Table 2. The Vertical Electron Detachment Energies (in eV) of Superhalogen Anions Calculated with the OVGF(full) Method Using the 6-311+G(3df) Basis Seta

Table 1. Free Energies (ΔHr298 in kcal/mol), Entropies [ΔSr298 in cal/(mol·K)], and Free Enthalpies (ΔGr298 in kcal/mol) of the Fragmentation Reactions (for T = 298.15 K) Considered in This Work, Obtained at the MP2/6311+G(d) Level fragmentation channel SiF5− SiF5− SiF5− SiF5− SiF5−

→ → → → →

F− + SiF4 F2− + SiF3 F3− + SiF2 F + SiF4− F2 + SiF3−

SiCl5− SiCl5− SiCl5− SiCl5− SiCl5−

→ → → → →

Cl− + SiCl4 Cl2− + SiCl3 Cl3− + SiCl2 Cl + SiCl4− Cl2 + SiCl3−

GeF5− GeF5− GeF5− GeF5− GeF5−

→ → → → →

F− + GeF4 F2− + GeF3 F3− + GeF2 F + GeF4− F2 + GeF3−

GeCl5− GeCl5− GeCl5− GeCl5− GeCl5−

→ → → → →

Cl− + GeCl4 Cl2− + GeCl3 Cl3− + GeCl2 Cl + GeCl4− Cl2 + GeCl3−

CF4...F−→ CF4 + F− CCl4...Cl−→ CCl4 + Cl−

ΔHr298 SiF5− 72.95 203.50 273.72 170.82 220.81 SiCl5− 21.08 99.83 122.56 86.71 95.20 GeF5− 87.54 184.06 222.35 129.39 172.43 GeCl5− 33.58 98.16 105.20 76.23 75.54 CF4...F− 0.009 CCl4...Cl− 0.006

ΔSr298

ΔGr298

27.39 43.99 44.34 35.10 40.18

64.78 193.03 260.50 160.36 208.83

24.98 42.79 43.26 35.77 39.78

13.63 87.07 109.67 76.04 83.34

27.23 42.78 42.36 35.30 39.02

79.42 171.30 209.72 118.87 160.79

25.35 42.47 42.19 37.82 39.26

26.02 85.50 92.62 64.95 63.83

19.63

−5.84

16.40

−4.88

species

VDE

PSb

SiF5− SiCl5− GeF5− GeCl5−

9.316 6.196 9.742 6.452

0.924 0.897 0.945 0.893

a

The equilibrium geometries of the anions studied were obtained by applying the MP2 method with the 6-311+G(d) basis set. bPS stands for the pole strength values obtained for the respective OVGF calculations.

9.316 eV, whereas the VDE predicted for the GeF5− anion approaches 10 eV (9.742 eV; see Table 2). Due to the theory level employed in this contribution, we believe that our vertical electron detachment energies are much more accurate than those reported in the literature thus far (i.e., 6.49,38 6.47,44 and 8.54 eV46 for SiF5−; 4.21 eV38,45 for SiCl5−, and 9.37 eV47 for GeF5−). In fact, all the previously estimated vertical electronic stabilities for these anions seem underestimated, which was likely caused by the use of the indirect DFT-based methods (rather than direct OVGF approach). The highest occupied molecular orbitals (HOMO) holding the excess electron in the SiX5− and GeX5− anions (X = F, Cl) are depicted in Figure 2. For each anion, the HOMO is

collected). In each case, the smallest ΔGr298 value was predicted for the X− detachment (either F− or Cl−); however, all these free enthalpies were found to be positive (64.8, 13.6, 79.4, and 26.0 kcal/mol, for SiF5−, SiCl5−, GeF5−, and GeCl5−, respectively), which confirms the thermodynamic stability of the SiX5− and GeX5− anions (X = F, Cl). Our ΔGr298 values calculated for the SiF5− → SiF4 + F− (64.8 kcal/mol) and GeF5− → GeF4 + F− (79.4 kcal/mol) fragmentations agree well with the corresponding experimental dissociation energies (60.0 ± 3.9 kcal/mol and for SiF5− 41 and 100.1 ± 6.9 kcal/ mol and for GeF5− 50). According to the results gathered in Table 1, the fragmentations leading to the X2−, X3−, X, or X2 are even less likely as the calculated ΔGr298 values span the 63.8− 260.5 kcal/mol range. Therefore, we conclude that the SiF5−, SiCl5−, GeF5−, and GeCl5− anions are not susceptible to any fragmentations. 3.2. Electronic Stabilities of the SiF5−, SiCl5−, GeF5−, and GeCl5− Anions. The vertical electron detachment energies calculated at the OVGF(full)/6-311+G(3df) level for the SiX5− and GeX5− anions (X = F, Cl) are collected in Table 2. The VDEs for the SiCl5− and GeCl5− species (6.196 and 6.452 eV, respectively) are significantly smaller than those calculated for the anions containing fluorine atoms as ligands. Indeed, the vertical electronic stability of the SiF5− system reads

Figure 2. The highest doubly occupied molecular orbitals (HOMO) in the SiF5−, SiCl5−, GeF5−, and GeCl5− anions (with the corresponding orbital eigenvalues ε given in eV).

delocalized only among the halogen ligands, whereas the contributions from the central atom AOs vanish. As a consequence, the HOMOs reveal the nonbonding character with respect to the ligand−central atom interactions. Such a situation is typical for the superhalogen anions studied in the past and enables the large stability of those systems.1,5,6,12−14,21 The corresponding highly negative HOMO eigenvalues of −10.8, −7.1, −11.2, and −7.4 eV for SiF5−, SiCl5−, GeF5−, and GeCl5−, respectively, reflect the large electronic stabilities of these anions (see Figure 2). 1969



Article

The VDEs of 9.316 eV (for SiF5−) and 9.742 eV (for GeF5−) seem significant even for the negatively charged systems exhibiting superhalogen nature. Indeed, hardly any superhalogen anion containing but one central metal atom possesses a vertical electron binding energy exceeding 9 eV. Thus, both the SiF5− and GeF5− anions and their parent neutral systems are expected to be of special interest for chemists searching for strong oxidizing agents. 3.3. The Issue of the Possible Existence of the CF5− and CCl5− Anions. As described in the previous sections, the earlier calculations predicted the CF5− and CCl5− anions to be both electronically and thermodynamically stable.37,38 In particular, the CF5− system was found to adopt the bipyramidal D3h-symmetry configuration,37 whereas the C3v-symmetry minimum energy structure was predicted in the case of the CCl5− anion.38 The electron binding energies for these species were estimated to span the 3.2−3.5 eV range. Most importantly, the CF5− and CCl5− anions were characterized as not susceptible to any fragmentation process (such as the negative ion X− detachment). According to our findings, however, neither CF5− nor CCl5− anion adopts the D3h-symmetry (trigonal bipyramid) structure. Our attempts to locate such minima on the MP2 potential energy surfaces of CF5− and CCl5− failed, as the stationary point structures we found were characterized by one or two imaginary frequencies. Since the minimum energy structures of the analogous SiX5− and GeX5− (X = F, Cl) anions correspond to the D3h-symmetry trigonal bipyramids (see the preceding sections), we decided to verify our results for the CF5− and CCl5− species by employing even more accurate treatment (quadratic configuration-interaction method including single and double substitutions, QCISD). However, the QCISD calculations confirmed the geometrical instability of D3hsymmetry trigonal bipyramidal CF5− and CCl5− structures. Therefore, we felt confident that the minimum energy structures of these anions (if they existed) had to correspond to the lower symmetries. Indeed, the only true minima for the CF5− and CCl5− species were found to possess C3v-symmetries. Namely, each CX5− (X = F, Cl) anionic structure consists of a pseudotetrahedral CX4 fragment with the X atom tethered to it (see Figure 3). The separation between the outermost X atom (labeled either F3 or Cl3 in Figure 3) and the carbon central atom is very large [r(F3C) = 2.900 Å; r(Cl3C) = 3.920 Å], whereas the remaining halogen atoms form rather typical C−F or C−Cl bonds whose lengths are approximately 2 times smaller than the corresponding r(F3C) and r(Cl3C) separations (see Figure 3 for the detailed distances). In addition, the pseudotetrahedral configuration of the CF4 and CCl4 fragments is confirmed by the values of the γ(F1CF2F2) and γ(Cl1CCl2Cl2) dihedral angles approaching 120° (116.5° and 118.9°, respectively; see Figure 3). These observations suggest that one should treat the pentahalocarbonate anions as the (CX4···X)− complexes rather than as CX5− systems. The analysis of the HOMO orbitals for the (CX4···X)− species (depicted in Figure 4) reveals that the excess electron density is localized solely in the vicinity of the outermost X atom. Moreover, the HOMO angular character reflects the psymmetry atomic orbital of halogen atom X (F or Cl). This observation is consistent with the vertical electron detachment energies estimated for the (CX4···X)− complexes [4.912 eV for (CF4···F)− and 3.832 eV for (CCl4···Cl)−], the values of which are substantially smaller than those obtained for the analogous

Figure 3. The equilibrium C3v-symmetry structures of the CF4···F− and CCl4···Cl− anionic complexes obtained at the MP2/6-311+G(d) level (the bond lengths r in Å, valence angles α, and dihedral angles γ in degrees).

Figure 4. The highest doubly occupied molecular orbitals (HOMO) in the CF4···F− and CCl4···Cl− anionic complexes (with the corresponding orbital eigenvalues ε given in eV).

germanium- and silicon-containing systems. In fact, the VDEs estimated for the (CF4···F)− and (CCl4···Cl)− anionic complexes seem to be related to the electron affinities of the F and Cl halogen atoms. Therefore, we postulate to consider the pentahalocarbonate anions to be the CF4···F− and CCl4···Cl− complexes consisting of the neutral pseudotetrahe1970



Article

dral CF4 (or CCl4) system perturbed by the weakly tethered F− (or Cl−) anion. In order to discuss the possible existence of the CF4···F− and CCl4···Cl− anionic complexes, we focus on their energetic stability first. The energy profiles shown in Figure 5

CCl4···Cl− anionic complexes, however, are entirely different. For the F− (or Cl−) ion approaching the CF4 (or CCl4) molecule, the potential is slightly attractive at larger distances and becomes strongly repulsive for shorter separations (when the X− enters the CX4 valence region); see Figure 5. This results in forming a shallow minimum at relatively large CX4···X− distance (2.90 Å for CF4···F− and 3.92 Å for CCl4···Cl−). Unlike for the SiF5− case, no minimum is formed for any shorter separation. The shallow minima corresponding to the CF4···F− and CCl4···Cl− anionic complexes shown in Figure 5 reflect very small dissociation energies estimated for these species. Namely, the dissociation energies calculated for CF4···F− and CCl4···Cl− are 6.4 and 4.7 kcal/mol, respectively. The insertion of the zero-point energy corrections slightly reduces these values (by ca. 0.2−0.4 kcal/mol). However, the inclusion of the thermal correction (for T = 298.15 K) and the entropy effects leads to the vanishing of these two shallow minima (which is not demonstrated in Figure 5 as the presented profiles represent the energy contributions only). Indeed, the ΔHr298, ΔSr298, and ΔGr298 values collected in Table 1 for the CF4···F− → CF4 + F− and CCl4···Cl− → CCl4 + Cl− processes indicate that the thermal corrections cause a substantial decrease of the CX4···X− species stability, which is also further reduced by the inclusion of the entropy effects. Therefore, we conclude that the CF4···F− and CCl4···Cl− species could temporarily exist as weakly bound complexes in the gas phase at T = 0 K. However, these systems should not exist in the elevated temperatures, as they are expected to spontaneously detach the X− (either F− or Cl−) anion. As a consequence, we predict neither the CF5− nor the CCl5− anion to exist as a stable compound (in detectable concentrations). 3.4. The Instability of CX5− vs the Stability of SiX5− and GeX5− (X = F, Cl). The reason why SiF5−, SiCl5−, GeF5−, and GeCl5− are very stable anionic systems exhibiting large electron binding energies whereas the corresponding CF5− and CCl5− species are not might be related to the fact that the carbon atom possesses much larger ionization potential (IP) than the Si and Ge atoms. Recently, we successfully explained the instability of the hypothetical HAlCl4 acid (whose LiAlCl4, NaAlCl4, and KAlCl4 salts are well-known, stable compounds) using similar criterion.65 Albeit the analysis is not so straightforward in the present case, we postulate that the high ionization potential of the carbon atom (in comparison to the IPs of the Si and Ge) does not allow one to form the pentacoordinated anionic system with the C as central atom and halogens as ligands, which is manifested by the strongly repulsive potential predicted for the X− approaching the CX4 (depicted in Figure 5 for the X = F, Cl). Since neither the highsymmetry CF5− nor CCl5− structure can be formed, the additional stabilization due to the forming of a superhalogen anion cannot be “turned on”. In other words, if the highsymmetry CX5− (either CF5− or CCl5−) system were geometrically stable, one would expect that this species would form a superhalogen anion (possessing large excess electron binding energy), which would further increase the depth of such a minimum and thus the stability of the system. We believe that such a situation is the case for the SiF5−, SiCl5−, GeF5−, and GeCl5− anions investigated in this contribution. In particular, the large stability of the silicon- and germaniumcontaining pentacoordinated anionic systems is likely caused by the possibility of forming high-symmetry and spatially compact MX5− (M = Si, Ge; X = F, Cl) geometrically stable structures which are further additionally stabilized by the significant excess

Figure 5. The MP2 minimum energy profiles describing the fragmentations of the SiF5−, CF4···F−, and CCl4···Cl− anionic species leading to the X− (either F− or Cl−) ion loss. Relative energies (with respect to the global energy minimum) are given in kcal/mol. The relaxed potential energy scans were performed (with no symmetry constrains) along the M−X coordinate (M = Si or C; X = F or Cl) with the 0.02 Å step.

demonstrate the minimum energy paths for the most likely dissociation channels leading to the X− loss. For comparison, we also present the SiF5− → SiF4 + F− case [as representative for all SiX5− and GeX5− (X = F, Cl) stable anions] for which the potential becomes strongly attractive when the F− approaches the SiF4 valence region. As a result, the SiF5− is expected to be stable with the dissociation energy of 73.8 kcal/ mol (72.8 kcal/mol after inclusion of the zero-point energy corrections). The inclusion of the thermal corrections (for T = 298.15 K) and entropy effects reduces this value to 64.8 kcal/ mol (see Table 1). The energy profiles for the CF4···F− and 1971



Article

electron binding energies (as capable of “becoming” superhalogen anions).

(15) Elliott, M.; Koyle, E.; Boldyrev, A. I.; Wang, X.-B.; Wang, L.-S. J. Phys. Chem. A 2005, 109, 11560−11567. (16) Ziai, H.-J.; Wang, L.-M.; Li, S.-D.; Wang, L.-S. J. Phys. Chem. A 2007, 111, 1030−1035. (17) Yang, J.; Wang, X.-B.; Xing, X.-P.; Wang, L.-S. J. Chem. Phys. 2008, 128, 201102. (18) Joseph, J.; Behera, S.; Jena, P. Chem. Phys. Lett. 2010, 498, 56− 62. (19) Alexandrova, A. N.; Boldyrev, A. I.; Fu, Y.-J.; Yang, X.; Wang, X.B.; Wang, L.-S. J. Chem. Phys. 2004, 121, 5709−5719. (20) Pradhan, K.; Gutsev, G. L.; Jena, P. J. Chem. Phys. 2010, 133, 144301. (21) Smuczyńska, S.; Skurski, P. Inorg. Chem. 2009, 48, 10231− 10238. (22) Anusiewicz, I. J. Phys. Chem. A 2009, 113, 6511−6516. (23) Anusiewicz, I. J. Phys. Chem. A 2009, 113, 11429−11434. (24) Bartlett, N. Proc. Chem. Soc. 1962, 218. (25) Wudl, F. Acc. Chem. Res. 1984, 17, 227−232. (26) Sikorska, C.; Skurski, P. Inorg. Chem. 2011, 50, 6384−6391. (27) McDaniel, D. H.; Deiters, R. M. J. Am. Chem. Soc. 1966, 88, 2607−2608. (28) Creighton, J. A.; Thomas, K. M. J. Mol. Struct. 1971, 7, 173− 178. (29) Creighton, J. A.; Thomas, K. M. J. Chem. Soc., Dalton Trans. 1972, 403−410. (30) Creighton, J. A.; Thomas, K. M. J. Chem. Soc., Dalton Trans. 1972, 2254−2257. (31) Lindner, H. J.; Kitschke-von Gross, B. Chem. Ber. 1976, 109, 314−319. (32) Kebarle, P.; Searles, S. K.; Zolla, A.; Scarborough, J.; Arshadi, M. Adv. Mass Spectrom. 1968, 4, 621. (33) Dougherty, R. C.; Dalton, J.; Roberts, J. D. Org. Mass Spectrom. 1974, 8, 77−79. (34) Wang, S.-J. Z. Naturforsch. 1973, 28a, 1832. (35) Carrion, F.; Dewar, M. J. S. J. Am. Chem. Soc. 1984, 106, 3531− 3539. (36) Vetter, R.; Zülicke, L. Chem. Phys. 1986, 101, 201−209. (37) Gutsev, G. L. Chem. Phys. 1992, 163, 59−67. (38) Gutsev, G. L. Chem. Phys. 1992, 166, 57−68. (39) Beattie, I. R.; Livingston, K. M. J. Chem. Soc. A 1969, 859−860. (40) Wilhite, D. L.; Spialter, L. J. Am. Chem. Soc. 1973, 95, 2100− 2104. (41) Larson, J. W.; McMahon, T. B. J. Am. Chem. Soc. 1985, 107, 766−773. (42) Gutsev, G. L.; Boldyrev, A. I. Chem. Phys. Lett. 1981, 84, 352− 355. (43) Deiters, J. A.; Holmes, R. R. J. Am. Chem. Soc. 1990, 112, 7197− 7202. (44) Gutsev, G. L. J. Chem. Phys. 1993, 99, 3906−3912. (45) Gutsev, G. L. J. Phys. Chem. 1994, 98, 1570−1575. (46) King, R. A.; Mastryukov, V. S.; Schaefer, H. F. III J. Chem. Phys. 1996, 105, 6880−6886. (47) Li, Q.; Li, G.; Xu, W.; Xie, Y.; Schaefer, H. F. III J. Chem. Phys. 1999, 111, 7945−7953. (48) Wharf, I.; Onyszchuk, M. Can. J. Chem. 1970, 48, 2250−2256. (49) McNair, A. M.; Ault, B. S. Inorg. Chem. 1982, 21, 2603−2605. (50) Mallouk, T. E.; Rosenthal, G. L.; Müller, G.; Brusasco, R.; Bartlett, N. Inorg. Chem. 1984, 23, 3167−3173. (51) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639− 5648. (52) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650−654. (53) Zakrzewski, V. G.; Ortiz, J. V.; Nichols, J. A.; Heryadi, D.; Yeager, D. L.; Golab, J. T. Int. J. Quantum Chem. 1996, 60, 29−36. (54) Simons, J. J. Chem. Phys. 1971, 55, 1218−1229. (55) Ortiz, J. V. J. Chem. Phys. 1988, 89, 6348−6352. (56) Rowe, D. J. Rev. Mod. Phys. 1968, 40, 153−166. (57) Cederbaum, L. S. J. Phys. B 1975, 8, 290−303. (58) Simons, J. J. Chem. Phys. 1972, 57, 3787−3792.

4. CONCLUSIONS On the basis of ab initio OVGF(full)/6-311+G(3df)//MP2/6311+G(d) calculations performed for the CF5−, CCl5−, SiF5−, SiCl5−, GeF5−, and GeCl5− anions we conclude the following: (i) The SiF5−, SiCl5−, GeF5−, and GeCl5− anions are electronically, geometrically, and thermodynamically stable species and adopt the D3h-symmetry trigonal bipyramidal equilibrium structures. (ii) The MX5− (M = Si, Ge; X = F, Cl) negatively charged systems are strongly bound superhalogen anions (matching the MXk+1− formula) whose vertical electron binding energies are 9.316 eV (SiF5−), 6.196 eV (SiCl5−), 9.742 eV (GeF5−), and 6.452 eV (GeCl5−). (iii) The CF5− and CCl5− systems do not exist in the elevated temperatures (above 0 K) due to their thermodynamic instability (i.e., susceptibility to the fragmentations leading to the X− (either F− or Cl−) loss. (iv) The formation of weakly bound CX4···X− (CF4···F− and CCl4···Cl−) anionic complexes (consisting of pseudotetrahedral neutral CX4 with the weakly tethered X−) might be expected at low temperatures (approaching 0 K). (v) The high-symmetry compact anionic CF5− and CCl5− systems cannot be formed due to the strongly repulsive potential predicted for the X− (F− or Cl−) approaching the CX4 (CF4 or CCl4).

■ ■

AUTHOR INFORMATION

Corresponding Author

*E-mail:: [email protected].

ACKNOWLEDGMENTS This work was supported by the Polish National Science Centre (NCN) Grant No. DS/8371-4-0137-2 and by the Foundation for Polish Science (to S.F.). The computer time provided by the Academic Computer Center in Gdańsk (TASK) is also gratefully acknowledged.

■

REFERENCES

(1) Gutsev, G. L.; Boldyrev, A. I. Chem. Phys. 1981, 56, 277−283. (2) Hotop, H.; Lineberger, W. C. J. Phys. Chem. Ref. Data 1985, 14, 731−750. (3) Sobczyk, M.; Sawicka, A.; Skurski, P. Eur. J. Inorg. Chem. 2003, 3790−3797. (4) Freza, S.; Skurski, P. Chem. Phys. Lett. 2010, 487, 19−23. (5) Gutsev, G. L.; Boldyrev, A. I. Russ. Chem. Rev. 1987, 56, 519− 531. (6) Gutsev, G. L.; Bartlett, R. J.; Boldyrev, A. I.; Simons, J. J. Chem. Phys. 1997, 107, 3867−3875. (7) Scheller, M. K.; Cederbaum, L. S. J. Chem. Phys. 1994, 100, 8934−8942. (8) Ortiz, J. V. Chem. Phys. Lett. 1993, 214, 467−472. (9) Ortiz, J. V. J. Chem. Phys. 1993, 99, 6727−6731. (10) Anusiewicz, I.; Skurski, P. Chem. Phys. Lett. 2007, 440, 41−44. (11) Gutsev, G. L.; Jena, P.; Bartlett, R. J. Chem. Phys. Lett. 1998, 292, 289−294. (12) Anusiewicz, I.; Skurski, P. Chem. Phys. Lett. 2002, 358, 426−434. (13) Anusiewicz, I.; Sobczyk, M.; Dab̨ kowska, I.; Skurski, P. Chem. Phys. 2003, 291, 171−180. (14) Wang, X.-B.; Ding, C.-F.; Wang, L.-S.; Boldyrev, A. I.; Simons, J. J. Chem. Phys. 1999, 110, 4763−4771. 1972



Article

(59) Simons, J.; Smith, W. D. J. Chem. Phys. 1973, 58, 4899−4907. (60) Zakrzewski, V. G.; Ortiz, J. V. Int. J. Quantum Chem. 1995, 53, 583−590. (61) Zakrzewski, V. G.; Ortiz, J. V. Int. J. Quantum Chem., Quantum Chem. Symp. 1994, 28, 23−27. (62) Zakrzewski, V. G.; Dolgounitcheva, O.; Ortiz, J. V. J. Chem. Phys. 1996, 105, 5872−5877. (63) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford CT, 2004. (64) Schomburg, D.; Krebs, R. Inorg. Chem. 1984, 23, 1378−1381. (65) Sikorska, C.; Freza, S.; Skurski, P. J. Phys. Chem. A 2010, 114, 2235−2239.

1973


Why Are SiX5â and GeX5â (X = F, Cl) Stable but Not CF5â and CCl5â?

Recommend Documents

Why Are SiX5â and GeX5â (X = F, Cl) Stable but Not CF5â and CCl5â?