Theoretical Search for Alternative Nine-Electron Ligands Suitable for

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Theoretical Search for Alternative Nine-Electron Ligands Suitable for Superhalogen Anions Celina Sikorska, Sylwia Freza, Piotr Skurski, and Iwona Anusiewicz* Department of Chemistry, University of Gda nsk, Sobieskiego 18, 80-952 Gdansk, Poland ABSTRACT: The calculations performed at the OVGF/6311þþG(3df,3pd)//MP2/6-311þþG(d,p) level for the representative NaX2- and AlX4- anions matching the MXkþ1superhalogen formula and utilizing 9-electron systems (i.e., consisting of various possible combinations of atoms containing nine electrons when brought together) revealed that the OH, Li2H3, and NH2 groups might be considered as alternative ligands X due to their thermodynamic stability and large values of electron binding energy (approaching or even exceeding 6 eV in some cases). All aluminum-containing AlX4- anions (excluding Al(HBLi)4-) were predicted to be thermodynamically stable, whereas the NaX2- anions for X = CH3, HBLi, CLi, BeB, and H2BeLi were found to be susceptible to the fragmentations leading to Na- loss. Among the MXkþ1- (M = Na, Al; X = Li2H3, OH, H2BeLi, BeB, NH2, HBLi, CH3, Be2H, CLi) anions utilizing systems containing 9 electrons (and thus isoelectronic with the F atom) the largest vertical electron detachment energy of 6.38 eV was obtained for Al(OH)4-.

1. INTRODUCTION Superhalogens are extraordinary compounds exhibiting enormously high electron affinities (spanning the 3.6-14 eV range).1-3 The existence of such species was predicted in 1981 by Boldyrev and Gutsev, who introduced a simple formula for one class of superhalogens, MXkþ1, where M is a main group or transition metal atom, X is a halogen atom, and k is the maximal formal valence of the atom M.1 Since the early 1980s, many theoretical efforts have been undertaken to estimate the vertical electron detachment energies (VDEs) of various anions having superhalogens as their neutral parents (see refs 4-12 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 Wang’s group in 1999.13 This experimental confirmation of the 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, e.g., MX3- (M = Be, Mg, Ca; X = Cl, Br),14 the oxygen-based BO2-,15 MCl4- (M = Sc, Y, La),16,17 and even larger species, such as NaxClxþ1- (x = 1-4).18 More recently, Pradhan, Gutsev, and Jena studied negatively charged MXn clusters formed by the transition metal atom M (M = Sc, Ti, V) and containing up to seven halogen atoms X (X = F, Cl, Br).19 Exploring various new superhalogen species is primarily focused on designing relatively large molecular clusters which are capable of forming strongly bound anions. The main purpose of these efforts is to provide reliable data and predictions r 2011 American Chemical Society

considering the possible use of such compounds as electron acceptors 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).20,21 However, we believe that designing new superhalogen species, albeit useful, should be more systematic. Therefore, we redirected our research into exploring more unusual superhalogens in which the central atom and/or halogen ligands are replaced with a hydrogen atom and various functional groups, respectively. First, we reported our findings for HF2-, HCl2-, HBr2-, and H2F3- and proved that the hydrogen atom may play a central atom role in superhalogen anions.22 The extension of this study covering also larger HnFnþ1- species (n = 3-5, 7, 9, 12) resulted in proposing the enormously strongly bound anionic system H12F13- whose vertical electron binding energy approaches 14 eV.3 Recently, we pointed out that the presence of halogen atoms in superhalogen species is not obligatory since the alternative ligands might be applied instead. Hence, we demonstrated that the halogen ligands (F, Cl, Br, I) might be replaced with halogenoids (e.g., CN, SCN, OCN) and the electronic stabilities of the resulting anions may even exceed those obtained with the VII main group elements.23 According to our 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)24 and acidic functional groups (i.e., ClO4, ClO3, ClO2, ClO, NO3, PO3, Received: January 3, 2011 Revised: January 24, 2011 Published: February 22, 2011 2077

dx.doi.org/10.1021/jp2000392 | J. Phys. Chem. A 2011, 115, 2077–2085

The Journal of Physical Chemistry A

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H2PO4, HSO4, HCO3, SH)25 as ligands were proposed and studied. Even though many polyatomic species have been proposed to play the ligand role in superhalogens, the fluorine atom remains the simplest system that might be employed (as it contains only 9 electrons). Therefore, we decided to perform a theoretical search for alternative 9-electron (i.e., isoelectronic with F atom) species that may possibly serve as ligands in superhalogen anions. We considered various combinations of atoms containing nine electrons when brought together, namely, Li2H3, CH3, H2BeLi, NH2, Be2H, HBLi, OH, BeB, and CLi. Using these substituents we initially designed the corresponding anions MXkþ1- with the sodium and aluminum as central atoms M (i.e., NaX2- and AlX4-). Our choice of Na and Al central atoms was dictated by the following prerogatives: (i) we planned to test the electronic and thermodynamic stabilities of the anions having central atoms exhibiting various valences and (ii) the central atoms chosen were not to be the components of the constituents tested; thus, the first-row atoms (such as Li or B) were excluded as containing less than 9 electrons. Hence, our main goal was to verify if any of these species lead to strongly bound anions (when assembled with a representative metal atom M to fit the MXkþ1- formula). We believe that the suggestions considering the alternative 9-electron ligands and the estimates of the vertical electron binding energies (characterizing the resulting superhalogen anions) that we provide in this work might be found useful for experimental chemists, especially those who design new materials in which the strong electron acceptors are involved (see also a recent comprehensive review on molecular anions26).

2. METHODS We present the ab initio results for the lowest energy isomers of the NaX2- and AlX4- anions (where X = Li2H3, CH3, H2BeLi, NH2, Be2H, HBLi, OH, BeB, and CLi), whereas the relative energies of the corresponding less stable isomers are only briefly discussed. In each case we describe the structure of the most stable anion by providing the values of the arbitrarily chosen important geometrical parameters (the detailed geometrical structures of the systems described as well as the corresponding harmonic vibrational frequencies are available upon request). The equilibrium geometries of the anions studied were obtained by applying the second-order Møller-Plesset (MP2) perturbational method with the 6-311þG(d) basis set for all of the species but those with the hydrogen atom for which the 6-311þþG(d,p) basis set27,28 was chosen. Since providing reliable vertical electron detachment energies of the superhalogen anions requires using more accurate treatment we performed direct calculations of the electron binding energies of these species. A direct scheme we applied was the outer valence Green function (OVGF) method (B approximation).29-37 The OVGF approximation remains valid only for outer valence ionizations for which the pole strengths (PSs) are greater than 0.80-0.85.38 For most states studied here, the PSs are sufficiently large to justify the use of the OVGF method (the smallest PS found for the states described in this work is 0.822). However, for some cases in which the PSs were not satisfactory, we employed the CCSD(T) (coupled cluster with single, double, and noniterative triple excitations)39 method instead and the resulting VDEs were calculated by subtracting the energies of the anions from those of the corresponding neutral species. We applied the 6-311þG(3df) or 6-311þþG(3df,3pd) (for anions containing H atoms)

Figure 1. Equilibrium structures of the isolated anionic 9-electron systems (obtained at the MP2/6-311þþG(d,p) level).

basis sets27,28 while estimating the vertical electron binding energies since analogous basis sets have been used by others for superhalogen anions and provided an excellent agreement between such calculated and experimentally measured VDEs.3,9,11-13,18 All calculations were performed with the Gaussian03 program.40

3. RESULTS 3.1. Disposition of 9-Electron Ligands To Bind an Excess Electron. Nine chemically different neutral molecules contain-

ing 9 electrons might be designed, namely, Li2H3, CH3, H2BeLi, NH2, Be2H, HBLi, OH, BeB, and CLi. Since each of these systems is isoelectronic with the fluorine atom we expect them to be capable of forming stable anionic states. Indeed, earlier experimental (in the case of OH, NH2, and CH3) and theoretical (in the case of CLi) reports confirmed the electronic stability and existence of the anions whose corresponding neutral parents contain 9 electrons. However, to the best of our knowledge, the ability of forming anions has not been investigated for less common compounds, i.e., Li2H3, H2BeLi, Be2H, HBLi, and BeB. Hence, we decided to verify the vertical and adiabatic electronic stability of these systems (by employing ab initio methods) because such information we consider as crucial for estimating their usefulness as possible ligands in novel superhalogen anions. 2078

dx.doi.org/10.1021/jp2000392 |J. Phys. Chem. A 2011, 115, 2077–2085


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Table 1. Adiabatic Electron Affinities (EA) of the 9-Electron Systems and Vertical Electron Detachment Energies (VDE) of Their Anionic Daughtersa 9-electron system (symmetry)

VDE

EA

Li2H3 (D¥h)

4.04

4.00

OH (C¥v) H2BeLi (C2v)

2.86 1.02

1.74 (1.827653 ( 0.000004)41 0.88

BeB (D¥h)

0.76

0.77

NH2 (C2v)

1.01

0.67 (0.771 ( 0.005)42

HBLi (Cs)

0.447

0.27

CH3 (C3v)

0.34

0.07145 (0.080 ( 0.030)46

Be2H (Cs)

0.57

0.38

CLi (C¥v)

0.43453 (0.52)47

0.26

a

The VDEs and EAs are calculated using the OVGF and CCSD(T) methods, respectively (the pole strengths for the OVGF calculations are in the 0.850.99 range). The experimental EAs (if available) are given in parentheses for comparison. All values are given in eV.

The most stable geometrical structures of the anions based on 9-electron molecules are depicted in Figure 1 (the plots of diatomic OH-, BeB-, and CLi- systems were omitted as obvious). Predictably, the NH2- anion adopts a C2v-symmetry structure (as isoelectronic with H2O neutral molecule), while CH3- exhibits a nonplanar pyramidal C3v-symmetry structure (as isoelectronic with the neutral NH3). The HBLi- and Be2Htriatomic anions are bent and possess Cs symmetry, whereas the four-atomic H2BeLi- adopts a Y-shaped C2v-symmetry planar structure with the two H atoms connected to Be (see Figure 1). Finally, the Li2H3- system is linear (D¥h symmetry) with the Li and H atoms localized alternately. The adiabatic electron affinities (EA) of the neutral parent 9-electron molecules (obtained with the CCSD(T) method) and the vertical electron detachment energies of the daughter 10electron anions (calculated using either the OVGF or the CCSD(T) method) are positive for all the systems considered, which means that the resulting anions are electronically stable. The estimated EA values span the 0.07-4.00 eV range, while the VDEs are in the 0.34-4.04 eV range, see Table 1. Assuming that the use of electronegative ligands should lead to the strongly bound anions (due to the so-called ‘collective effects’)1,4 we considered the Li2H3 (EA = 4.00 eV) and OH (EA = 1.74 eV) groups as the most promising. The electron binding energies calculated are in satisfactory agreement with the earlier experimental results (see Table 1 where the available experimental EAs are provided). Namely, our EA(OH) = 1.74 eV agrees well with the experimental 1.827653 eV value reported by Lineberger et al.41 and the electron affinity we estimated for the NH2 system (0.67 eV) seems to be slightly underestimated in comparison to the measured 0.771 eV value.42 In the case of the CH3 radical, however, the calculation level we employed in this work did not lead to a positive adiabatic electron affinity value, which is related to the fact that the methyl radical is a well-known example of species whose EA is difficult to determine theoretically, likely due to its small absolute value43-45 (as shown by Dixon and Feller, use of the CCSD(T)/aug-ccpVxZ approach (x = D, T, Q) and extrapolation to the complete basis set limit is required to reproduce the precise value).45 Hence, in Table 1 we provide our calculated VDE value (0.34 eV) and the EA of 0.071 eV obtained theoretically by Dixon and Feller (the latter value differs from the experimental EA by 0.008 eV45,46). As far as the CLi radical is concerned, we compared our VDE result of the corresponding daughter anion (0.43 eV) with another theoretical estimate of 0.52 eV47 since the experimental

EA for the diatomic CLi is not available. Hence, we conclude that our estimates of the electron binding energies of 9-electron systems (provided in Table 1) are reliable. In addition, our goal was to verify the usefulness of these systems as ligands in novel anionic species designed rather than calculating their electron affinities with very high accuracy. Among all 9-electron systems considered, the Li2H3 radical seems to be the most promising because it exhibits a much larger excess electron binding energy than the remaining molecules. Namely, its VDE (4.04 eV) is larger by 1.2 eV than that of the OH- anion (2.86 eV), see Table 1. Such a significant difference can easily be explained by realizing that the Li2H3 is a superhalogen molecule itself (more precisely it is a polynuclear (multicenter) superhalogen as it fits the MnXn 3 kþ1 formula). As pointed out by Boldyrev and von Niessen48 and also by Boldyrev and Simons,49 the LiH2 exhibits a superhalogen nature and its VDE was predicted to be 3.1248 or 3.35 eV49 (depending on the theory level applied). Hence, the hydrogen atoms are capable of playing a ligand role in such systems, as described in ref 48. Therefore, the Li2H3 might be treated as a superhalogen molecule containing two central atoms (Li) and three ligands (H), and one may expect this system to possess an even larger excess electron binding energy than the corresponding onecenter LiH2. Indeed, the VDE of Li2H3- is larger by ca. 0.7-0.9 eV than that of LiH2-. In addition, we would like to mention that employing superhalogen ligands in designing novel anions has been very recently proposed by Jena and co-workers.50 Hence, we feel confident while testing the usefulness of such ligands in assembling new strong electron acceptors. 3.2. Structure, Thermodynamic, and Electronic Stability of the NaX2- and AlX4- Anions (X = Li2H3, CH3, H2BeLi, NH2, Be2H, HBLi, OH, BeB, CLi). Having discussed the structures of the isolated anionic systems (isoelectronic with F-) and their adiabatic and vertical electronic stabilities, we now move on to the anions consisting of these systems and Na or Al atom (playing central atom role). While describing the properties of the resulting NaX2- and AlX4- anions (where X stands for a 9-electron ligand) we focus on their lowest energy isomers, whereas the less stable anionic species (if they exist) are only briefly characterized. 3.2.1. Thermodynamic Stability of the NaX2- and AlX4Anions. First, we present the results verifying the thermodynamic stability of the anions studied (i.e., their susceptibility to isomerization or fragmentation). As mentioned above, only the lowest energy isomers are presented in this contribution, which 2079


The Journal of Physical Chemistry A automatically means that the free enthalpies (ΔGr) of any isomerization processes considered are positive. Since the positive ΔGr values indicate the unfavorable processes, we do not discuss the higher energy isomers in this section (the proper data are provided in the following sections where the particular anions are described). On the other hand, the most likely fragmentation paths are discussed and the corresponding free energies of these reactions (ΔHr), the accompanying entropy changes (ΔSr), and the free enthalpies of the fragmentation reactions (ΔGr) are collected in Table 2 (as calculated for the temperature T = 298.15 K). We considered the three most probable fragmentation channels for the NaX2- and AlX4- anions (where X stands for a 9-electron ligand). Namely, (i) neutral ligand detachment NaX2- f NaX- þ X or AlX4- f AlX3- þ X, (ii) anionic ligand detachment NaX2- f NaX þ X- or AlX4- f AlX3 þ X-, and (iii) anionic metal atom loss NaX2- f Na- þ X2 or AlX4- f Al- þ 2X2. However, in some cases (i.e., for certain ligands X) the NaX- or AlX3- anions were found to be electronically unstable, and thus, the corresponding fragmentation paths (NaX2- f NaX- þ X or AlX4- f AlX3- þ X) are not presented in Table 2. The list of the unstable NaX- or AlX3anions contains NaBe2H-, NaLi2H3-, Al(CH3)3-, Al(NH2)3-, Al(OH)3-, Al(HBLi)3-, and Al(CLi)3-. As it can be seen, this set of electronically unstable anions is dominated by the species containing the aluminum atom which is not surprising due to the fact that the neutral AlX3 systems are expected to be nonpolar and valence-saturated molecules, so an excess electron binding is not likely. Our investigation revealed also one geometrically unstable neutral fragmentation product: (Li2H3)2. As it turned out, the Li4H4 and H2 molecules are formed instead; hence, such products are presented in Table 2 when the NaX2- f Na- þ X2 or AlX4- f Al- þ 2X2 reaction is discussed. The Na(OH)2- and Al(OH)4- anions were found to be thermodynamically stable, which means these species are not susceptible to any fragmentation process, including (i) detachment of the OH- anion and forming NaOH or Al(OH)3, (ii) Na- or Al- loss and forming hydrogen peroxide, and (iii) forming a water molecule and the remaining Al(OH)2O- anion, see Table 2, where positive ΔGr values are reported for these reactions. The analogous conclusions can be formulated while discussing the thermodynamic stability of the Na(NH2)2- and Al(NH2)4- anions. Indeed, the positive ΔGr values were obtained for all fragmentation processes considered, including those leading to Na- (or Al-) and neutral hydrazine molecule(s). The Na(CH3)2- anion is stable with respect to fragmentations, leading to (NaCH3- þ CH3) or (NaCH3 þ CH3-), whereas it was found to be thermodynamically unstable when Na- loss is considered (the ΔGr calculated for the Na(CH3)2f Na- þ C2H6 reaction reads -27.33 kcal/mol). On the other hand, the Al(CH3)4- anion was found to be not susceptible to any fragmentation process, see Table 2. The instability of both Na(HBLi)2- and Al(HBLi)4- anions with respect to the negatively charged metal atom loss was confirmed by the negative ΔGr values calculated for the Na(HBLi)2- f Na- þ (HBLi)2 (ΔGr = -114.87 kcal/mol) and Al(HBLi)4- f Al- þ 2(HBLi)2 (ΔGr = -284.24 kcal/mol) reactions. In addition, Al(HBLi)4was also found to be susceptible to the Al(HBLi)4- f Al(HBLi)3 þ HBLi- fragmentation with the corresponding 238.85 kcal/mol free enthalpy estimated for this reactions. The Al(BeB)4- anion was found to be thermodynamically stable, whereas fragmentation of Na(BeB)2- leading to BeB

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Table 2. Free Energies (ΔHr in kcal/mol), Entropies (ΔSr in cal/(mol 3 K)), and Free Enthalpies (ΔHr in kcal/mol) of the Fragmentation Reactions (for T = 298.15 K) Considered in This Worka fragmentation path Na(CH3)2- f NaCH3 þ CH3Na(CH3)2- f NaCH3- þ CH3 Na(CH3)2- f Na- þ C2H6 Al(CH3)4- f Al(CH3)3 þ CH3Al(CH3)4- f Al- þ 2C2H6 Na(NH2)2- f NaNH2 þ NH2Na(NH2)2- f NaNH2- þ NH2 Na(NH2)2- f Na- þ N2H4 Al(NH2)4- f Al(NH2)3 þ NH2Al(NH2)4- f Al- þ 2N2H4 Na(HBLi)2- f NaHBLi þ HBLiNa(HBLi)2- f NaHBLi- þ HBLi Na(HBLi)2- f Na- þ (HBLi)2 Al(HBLi)4- f Al(HBLi)3 þ HBLiAl(HBLi)4- f Al- þ 2(HBLi)2 Na(OH)2- f NaOH þ OHNa(OH)2- f NaOH- þ OH Na(OH)2- f Na- þ H2O2 Al(OH)4- f Al(OH)3 þ OHAl(OH)4- f Al(OH)2O- þ H2O Al(OH)4- f Al- þ 2H2O2 Na(BeB)2- f NaBeB þ BeBNa(BeB)2- f NaBeB- þ BeB Na(BeB)2- f Na- þ (BeB)2 Al(BeB)4- f Al(BeB)3 þ BeBAl(BeB)4- f Al(BeB)3- þ BeB Al(BeB)4- f Al- þ 2(BeB)2 Na(CLi)2- f NaCLi þ CLiNa(CLi)2- f NaCLi- þ Cli Na(CLi)2- f Na- þ (CLi)2 Al(CLi)4- f Al(CLi)3 þ CLiAl(CLi)4- f Al- þ 2(CLi)2 Na(Be2H)2- f NaBe2H þ Be2HNa(Be2H)2- f Na- þ (Be2H)2 Al(Be2H)4- f Al(Be2H)3 þ Be2HAl(Be2H)4- f Al(Be2H)3- þ Be2H Al(Be2H)4- f Al- þ 2(Be2H)2 Na(H2BeLi)2- f NaH2BeLi þ H2BeLiNa(H2BeLi)2- f NaH2BeLi- þ H2BeLi Na(H2BeLi)2- f Na- þ (H2BeLi)2 Al(H2BeLi)4- f Al(H2BeLi)3 þ H2BeLiAl(H2BeLi)4- f Al(H2BeLi)3- þ H2BeLi Al(H2BeLi)4- f Al- þ 2(H2BeLi)2 Na(Li2H3)2- f NaLi2H3 þ Li2H3Na(Li2H3)2- f Na- þ Li4H4 þ H2 Al(Li2H3)4- f Al(Li2H3)3 þ Li2H3Al(Li2H3)4- f Al- þ 2Li4H4 þ 2H2 a

ΔHr

ΔSr

ΔGr

50.32

23.01

33.51

23.80

43.46 26.41

-25.55

5.96

-27.33

81.09

45.50

67.68

75.67

51.02

60.46

56.71

25.82

49.02

59.39 45.36

29.01 11.44

50.74 41.95

75.21

41.13

62.98

215.10

60.69

196.92

31.47

25.89

23.75

15.71

25.05

8.24

-111.22

12.24 -114.87

-236.79

6.93 -238.85

-270.52 56.57

45.91 -284.24 19.79 50.67

87.63

25.67

79.97

120.71

13.12

116.80

93.01

33.57

83.17

64.82

32.69

55.07

359.68

62.11

341.20

63.46

30.42

54.39

71.04 -89.31

33.43 13.44

61.08 -93.32

502.77

47.88

488.50

460.60

47.74

446.37

184.64

61.76

166.23

48.31

23.71

41.24

9.95

29.17

1.26

-232.33 216.44 79.28

13.12 -236.24 34.24 68.80

206.24 58.78

194.77

35.88

184.08

52.68

19.15

46.97

188.37

54.60

172.10

336.22

29.38

327.46

160.52

53.70

144.52

23.67

29.21

14.96

37.44 -24.12

28.82 6.66

28.85 -26.11

83.17

54.65

66.88

79.47

53.50

63.53

131.67

76.77

108.79

24.05

17.96

18.70

13.36

27.95

5.03

224.15

37.33

213.04

125.22 109.31

92.63

All values calculated at the MP2/6-311þþG(d,p) level.

neutral dimer and Na- seems feasible, as the ΔGr value for this process reads -93.32 kcal/mol. Analogous behavior was observed for the anions utilizing CLi ligands. Namely, Al(CLi)42080



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Table 3. Anionic MP2/6-311þþG(d,p) Equilibrium Structures and Vertical Electron Detachment Energies (VDE, in eV) Calculated with the OVGF (or CCSD(T)) Method Using the 6-311þþ(3df,3pd) Basis Seta

a

The pole strength values are given in parentheses. The superscript 1 shows the CCSD(T)/6-311þþG(3df,3pd) result. 2081


The Journal of Physical Chemistry A was predicted to be thermodynamically stable, whereas the Na(CLi)2- anion was found to be vulnerable for Na- loss (with the corresponding ΔGr of -236.24 kcal/mol, see Table 2). Interestingly, the NaX2- f Na- þ X2 fragmentation channel was found open also for another anion. The free enthalpy calculated for the Na(H2BeLi)2- fragmentation leading to Na- loss reads -26.11 kcal/mol, whereas the corresponding Al(H2BeLi)4- anion is expected to be thermodynamically stable (as indicated by the positive ΔGr values obtained for all fragmentation reactions). The BeBeH 9-electron ligands were found to form stable anions when assembled with the Na or Al central atoms. Indeed, the fragmentation channels for the resulting Na(Be2H)2- and Al(Be2H)4- anions seem closed as the corresponding free enthalpies for these reaction are positive. Finally, the Na(Li2H3)2- and Al(Li2H3)4- anions were proven thermodynamically stable and not susceptible to any fragmentation process, as indicated for positive ΔGr values calculated for all reactions considered, see Table 2. Analysis of the ΔGr values calculated for the most likely fragmentation reactions leads to the following conclusions: (i) one 9-electron ligand (i.e., HBLi) forms thermodynamically unstable NaX2- and AlX4- anions; (ii) four ligands (i.e., OH, NH2, Be2H, and Li2H3) are capable of forming thermodynamically stable NaX2- and AlX4- anions; (iii) four remaining 9-electron ligands (i.e., CH3, BeB, CLi, and H2BeLi) lead to thermodynamically stable AlX4- and unstable NaX2- species. Albeit special attention should be paid to the systems which are not susceptible to spontaneous fragmentations (because their future experimental applications are much more likely), we decided to discuss the structure and electronic stability of all the anions, including the thermodynamically unstable negatively charged systems. The main reason why we do this is the completeness of the results provided. In addition, one should also keep in mind that even the thermodynamically unstable species discussed are both electronically and kinetically (i.e., geometrically) stable and any fragmentation process would likely require overcoming certain kinetic barriers (corresponding to bond breaking and structure reorganization). 3.2.2. Structure and Electronic Stability of the Anions Containing CLi and Be2H Ligands. We begin with the Na(CLi)2and Al(CLi)4- systems involving diatomic CLi groups. According to our findings, the CLi ligands connect to the sodium or aluminum central atom through their carbon atoms, and the resulting equilibrium structures are depicted in Table 3. The two CLi groups in the C2-symmetry structure of the Na(CLi)2- are almost perpendicular to each other (the corresponding Li-CC-Li dihedral angle reads 93.2), whereas the C-Na-C fragment is almost linear (174.5). The Al(CLi)4- anion, however, adopts a more complicated boat-like structure with two pairs of eclipsed (i.e., localized in plane) CLi ligands, see Table 3. The aluminum atom is placed almost in the Li-Li-Li-Li plane (the deviation from planarity is 5.7), and the resulting symmetry is C2v with the C2 axis containing the Al atom and perpendicular to both Li-Li-Li-Li and C-C-C-C planes. In addition, we found another C2v-symmetry isomer of Al(CLi)4-, but its energy is higher than that of the depicted structure by 8.53 kcal/mol. The vertical electron detachment energies of the Na(CLi)2and Al(CLi)4- anions were calculated to be 1.07 and 1.30 eV, respectively. These relatively small VDEs (with respect to the other VDE values provided in this work) are likely caused by the small electron affinity of the CLi system (0.26 eV). However, one

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could expect a larger VDE value for the Al(CLi)4- anion since four CLi ligands are involved in this system, and thus, the collective effects were expected to play a larger role. Recalling the Na(CLi)2- anion instability with respect to Na- loss, only the Al(CLi)4- system might be of experimental interest, although its electronic stability seems modest. The anions assembled with the Na and Al atoms and the proper number of BeBeH ligands adopt relatively symmetrical structures, see Table 3. Na(Be2H)2- resembles a pyramid having the sodium atom on the top of it and the quadrilateral base (formed by the four Be atoms). The fact that the pyramid base is slightly deformed from planarity (Be-Be-Be-Be dihedral angle reads 2.7) as well as the presence of two H atoms (linked to the Be atoms) lead to the final C2v-symmetry structure depicted in Table 3. Interestingly, the BeBeH ligands seem to preserve their original geometry when assembled to form Na(Be2H)2- anion (the valence angles in the isolated and incorporated BeBeH- differ by only 13, see Figure 1). This is similar to the situation we found for the Al(Be2H)4- anion in which the Be-Be-H valence angles differ by less than 14 from that in the isolated BeBeH ligand. The equilibrium Al(Be2H)4- structure possesses S4 symmetry and contains four bent BeBeH groups connected through terminal Be atoms to the aluminum central atom (see Table 3). In addition, the Al-Be1-Be2 angles read 73.1, which makes the resulting structure rather compact. The vertical electron detachment energies found for the Na(Be2H)2- and Al(Be2H)4- anions are larger than those calculated for Na(CLi)2- and Al(CLi)4-. The VDE of Na(Be2H)2- approaches 2 eV (1.92 eV), while the corresponding value for Al(Be2H)4- reads 3.33 eV. Even though both presented values are smaller than the electron affinity of the chlorine atom (3.61 eV) they seem substantial taking into account the small EA of the BeBeH building blocks (0.38 eV, see Table 1). Hence, we conclude that the BeBeH ligands assembled together to fit the MXkþ1- formula lead to a significant increase of the electronic stability of the resulting anion. In addition, both Na(Be2H)2and Al(Be2H)4- anions were predicted to be stable with respect to any fragmentation (see section 3.2.1). 3.2.3. Structure and Electronic Stability of Anions Containing CH3 and HBLi Ligands. The equilibrium structures of the Na(CH3)2- and Al(CH3)4- anions are intuitive since the former adopts a D3d-symmetry staggered conformation resembling an ethane-like moiety which allows for maximum separations of the hydrogen atoms whereas the latter mimics a well-known Tdsymmetry neopentane structure. The methyl groups preserve their pyramidal structure (the CH3- is C3v-symmetry pyramidal) when assembled into the Na(CH3)2- and Al(CH3)4- anions, and the corresponding H-C-H-H dihedral angles are equal to 113.77 and 113.89, respectively (see Table 3 where both anionic structures are shown). The VDE calculated for Na(CH3)2- reads 2.36 eV, while the Al(CH3)4- anion was found to be vertically electronically bound by 4.46 eV, see Table 3. These electronic stabilities of the anions containing methyl groups seem surprisingly large in the context of the very small adiabatic electron affinity of the D3h-symmetry planar methyl radical (0.080 ( 0.030 eV46). However, we would like to point out that both (i) forming the fourth bond by the carbon atom (when linked to the central Na or Al atom) and (ii) providing additional electron density (coming from the excess electron distributed over the CH3 substituents) in the Na(CH3)2- and Al(CH3)4- anions cause the methyl groups to be pyramidal (instead of planar, as it is in the neutral CH3 radical) 2082


The Journal of Physical Chemistry A which in turn increases the electronic stability of the resulting anions. Despite large electronic stabilities of both Na(CH3)2and Al(CH3)4- anions, only the latter may be treated as a stable molecule since the former was found to be thermodynamically unstable (due to its susceptibility to Na(CH3)2- f Na- þ C2H6(ethane) fragmentation, as described in section 3.2.1). The HBLi radical molecules link to the Na and Al central atoms through the boron atoms, as depicted in Table 3. As described in the preceding section, section 3.1, the equilibrium geometry of the isolated HBLi- anion is bent (see also Figure 1). The HBLi groups preserve their bent structure when connected to the sodium atom in Na(HBLi)2-, and the resulting anion possesses C2 symmetry with two HBLi substituents in the anti location, see Table 3. When assembled into Al(HBLi)4-, however, the HBLi moieties adopt a perfectly linear structure, which results in the high-symmetry tetrahedral (Td) equilibrium geometry of this anion. In addition, we would like to briefly mention the two different isomeric structures (having C2v and D¥h symmetries) that were found for Na(HBLi)2- anion. Since these two isomers are much higher in energy (by more than 60 kcal/ mol) in comparison to the C2-symmetry global minimum structure of Na(HBLi)2- we do not expect them to be competitive. One should keep in mind, however, that even the most stable isomer of the Na(HBLi)2- anion and the tetrahedral Al(HBLi)4- were identified as thermodynamically unstable (their fragmentations leading to Na- or Al- and HLiB-BLiH molecules are likely, as described in the section 3.2.1), and thus, the possibility of forming any Na(HBLi)2- or Al(HBLi)4species is questionable (see Table 2). The vertical electron detachment energies of the Na(HBLi)2and Al(HBLi)4- anions were calculated to be 1.51 and 5.43 eV, respectively (see Table 3). Although the latter value seems significant the HBLi groups cannot be treated as suitable ligands (i.e., ligands leading to strongly bound and stable anions) because of the Na(HBLi)2- and Al(HBLi)4- thermodynamic instability (see Table 2). 3.2.4. Structure and Electronic Stability of Anions Containing NH2 and BeB Ligands. The Na(NH2)2- anion adopts a D2d symmetry structure with the main C2 axis containing two N atoms and one Na atom and with the two planar NH2 groups localized perpendicularly to each other. On the other hand, the NH2 substituents become pyramidal when assembled into the Al(NH2)4- anion (the H-H-N-Al dihedral angles read 136.8), and the resulting equilibrium structure is of D2d symmetry with the four nitrogen atoms connected to the central Al in a tetrahedral manner, see Table 3. The vertical electron detachment energies for the anions utilizing NH2 ligands were calculated to be rather large. Namely, the VDE of the Na(NH2)2- anion was predicted to be 2.90 eV, whereas that of Al(NH2)4- was estimated to be equal to 3.99 eV (Table 3). Taking into account the EA of the radical NH2 molecule (0.67 eV) one may conclude that utilizing such ligands for assembling anions according to the M(NH2)kþ1- formula should result in obtaining thermodynamically stable (see Table 2) and relatively strongly bound (see Table 3) negatively charged species. The BeB diatomic radicals link with the Na or Al central atom through the boron atoms, which results in forming W-shaped Na(BeB)2- and cyclic Al(BeB)4- anions, see Table 3. The BeB-Na and B-Na-B valence angles in Na(BeB)2- are equal to 75.8 and 81.7, respectively, and the whole structure is planar and possesses C2v symmetry. The Be-B-Al angles in

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Al(BeB)4- are identical with one another and read 76.3, and the resulting equilibrium anionic structure is almost planar (the deviations from planarity do not exceed 1). In addition to the four B-Al bonds, the two B-B and two Be-Be bonding interactions are present in the Al(BeB)4- anion due to the mutual proximity of the four BeB ligands (see Table 3). As a result, a cyclic quasi-planar C2-symmetry structure is formed. The alternative D2d-symmetry isomer of Al(BeB)4- is much higher in energy (by 90.5 kcal/mol) than the C2-symmetry cyclic global minimum, likely because the stabilizing ligand-ligand interactions are absent. The Al(BeB)4- anion is predicted to be both electronically and thermodynamically stable (see section 3.2.1). Na(BeB)2-, however, was found to be unstable with respect to Na- anion loss (the free enthalpy calculated for the Na(BBe)2- f Na- þ (BBe)2 reaction was negative and read -93.32 kcal/mol, see Table 2), which indicates the limited usefulness of the BeB 9-electron systems as ligands in designing strongly bound anions. In spite of the fact that the Na(BeB)2- is predicted to be only kinetically stable, we calculated its VDE and found a value of 2.54 eV, see Table 3. Interestingly, an increase of the VDE is unexpectedly small when the number of BeB ligands grows from 2 to 4 (i.e., when the VDE of the Al(BeB)4- anion is compared to that of Na(BeB)2-) as the final estimate of the Al(BeB)4vertical electron binding energy is 2.64 eV. 3.2.5. Structure and Electronic Stability of Anions Containing H2BeLi, OH, and Li2H3 Ligands. The Y-shaped H2BeLi groups qualitatively preserve their structures when assembled into the Na(H2BeLi)2- anion, which results in forming a C2-symmetry structure with the two substituents bound to the central Na via their Be atoms (see Table 3). Each of the H2BeLi groups remains almost planar, while their relative orientation in Na(H2BeLi)2- is quasi-perpendicular (e.g., the Li-Be-Be-Li dihedral angle reads 80). Due to the large number of constituting atoms, we found up to eight different isomers in the case of the Al(H2BeLi)4- anion, each of which corresponds to a local minimum (the existence of other high-energy minima also seems likely). The most stable isomer we found corresponds to the relatively compact structure shown in Table 3. In this C1symmetry global minimum structure of Al(H2BeLi)4- one can distinguish four Y-shaped HBeH groups linked to the central Al through the beryllium atoms, whereas the four remaining Li atoms are located between these HBeH groups (forming “bridges” between HBeH substituents and therefore additionally stabilizing the whole structure), see Table 3. As we showed in Table 2 (section 3.2.1), the Na(H2BeLi)2anion is expected to be thermodynamically unstable with respect to fragmentation, leading to Na- anion and H2LiBe-BeLiH2 neutral molecule (the corresponding free enthalpy of this reaction was estimated to be -26.11 kcal/mol), whereas the Al(H2BeLi)4- anion is not susceptible to any fragmentation process. Therefore, the potential usefulness of the H2BeLi system as a ligand X in forming MXkþ1- anions should be considered in the Al(H2BeLi)4- case only. The Na(H2BeLi)2anion, albeit electronically stable (its VDE was predicted to be equal to 2.00 eV, see Table 3), is expected to be unstable with respect to Na- loss. On the other hand, the Al(H2BeLi)4- anion should exist in the gas phase, and its vertical electron detachment energy was estimated to be equal to 2.14 eV. The OH radical systems lead to electronically, geometrically, and thermodynamically stable species when assembled into Na(OH)2- or Al(OH)4-. The two hydroxyl groups in 2083


The Journal of Physical Chemistry A Na(OH)2- adopt a trans orientation with respect to the perfectly linear O-Na-O fragment, while the whole system is planar and possesses C2h symmetry, see Table 3. The equilibrium structure of the Al(OH)4- anion also exhibits relatively high S4 symmetry with the four hydroxyl groups linked to the central Al atom in a tetrahedral manner, as depicted in Table 3. However, the mutual orientation of the OH moieties allows for intramolecular hydrogen bonds among them, which additionally stabilizes the structure. The VDEs calculated for the Na(OH)2- and Al(OH)4anions are surprisingly large and read 4.89 and 6.38 eV, respectively. Such significant values of the vertical electron detachment energy are partly caused by the large EA of the OH groups (1.74 eV, see Table 1) that the Na(OH)2- and Al(OH)4- anions consist of. Interestingly, the VDEs reported approach the VDEs of the corresponding analogous NaCl2- (5.688 eV)51 and AlCl4- (7.016 eV)52 superhalogen anions. Finally, the Li2H3 systems lead to Cs-symmetry Na(Li2H3)2and C2-symmetry Al(Li2H3)4- anions when used as ligands (see Table 3 where these structures are depicted). The Na(Li2H3)2equilibrium structure contains a quasi-linear H-Li-H-LiH- fragment (deviation from linearity does not exceed 0.5) connected to the planar six-membered ring (consisting of a Na atom, 3 hydrogen atoms, and 2 lithium atoms) through the sodium atom. The Al(Li2H3)4- anionic structure is more symmetrical and contains two identical double-ring quasi-planar moieties oriented perpendicularly to each other and linked with the aluminum central atom, see Table 3. We also found two other minimum energy structures for Na(Li2H3)2- (both of them possessing C2 symmetry); however, their relative energies were estimated to be higher by 4.64 and 7.94 kcal/mol with respect to the lowest energy Cs-symmetry Na(Li2H3)2- isomer. In the case of the Al(Li2H3)4- anion we found only one additional geometrically stable structure, but its energy was calculated to be 23.02 kcal/mol higher than that of the most stable C2-symmetry configuration. Hence, we conclude that the presence of at least two isomeric Na(Li2H3)2- structures is likely at elevated temperatures, whereas the Al(Li2H3)4- anion is expected to adopt only one (C2 symmetry) structure. Recalling that both Na(Li2H3)2- and Al(Li2H3)4- anions were predicted to be stable with respect to any fragmentation process (see section 3.2.1 and Table 2), we calculated the VDE values for the lowest energy structures of these two negatively charged species. According to our results, the vertical electron detachment energy of Na(Li2H3)2- exceeds 4 eV (4.30 eV) and the VDE of Al(Li2H3)4- approaches 6 eV (5.78 eV), see Table 3.

4. CONCLUSIONS Our theoretical search performed at the MP2/6-311þþG(d, p) level (determining the equilibrium structures and verifying their thermodynamic stabilities) and at the OVGF/6-311þþG(3df,3pd) or CCSD(T)/6-311þþG(3df,3pd) level (calculating the electron binding energies) for alternative 9-electron ligands covered species containing nine electrons, that is, various combinations of atoms containing nine electrons when brought together were taken into account: Li2H3, OH, H2BeLi, BeB, NH2, HBLi, CH3, Be2H, and CLi. The electronic stabilities of the anionic 9-electron groups themselves were investigated, and their ability to play a ligand role in superhalogen anions was further

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verified. The calculations performed led to the following conclusions: (i) The OH group should be considered as a perfect candidate for playing a ligand role in superhalogen anions since the anions utilizing these groups possess the largest values of VDE (among the species studied in this contribution), i.e., 6.38 (Al(OH)4-) and 4.89 eV (Na(OH)2-). In addition, the superhalogen anions containing the hydroxyl groups are expected to be thermodynamically stable (not susceptible to any fragmentation), as indicated by the confirmed thermodynamic stability of the Al(OH)4- and Na(OH)2compounds. (ii) The Li2H3 groups lead to strongly bound anions when assembled together with the central metal atom according to MXkþ1- formula. The resulting anions are predicted to be thermodynamically stable, and their VDEs are estimated as 5.78 (Al(Li2H3)4-) and 4.30 eV (Na(Li2H3)2-). (iii) The NH2 and Be2H species are also expected to form thermodynamically stable and relatively strongly bound MXkþ1- anions whose VDEs were calculated to be 3.99, 2.90, 3.33, and 1.92 eV for Al(NH2)4-, Na(NH2)2-, Al(Be2H)4-, and Na(Be2H)2-, respectively. (iv) The BeB and H2BeLi groups lead to the MXkþ1- anions having VDEs in the 2.00-2.64 eV range; however, their invulnerability to fragmentation is questionable since the Na(BeB)2- and Na(H2BeLi)2- species were found to be unstable with respect to Na- loss. Similar conclusions might be formulated for the anions utilizing the CLi ligand, although the predicted VDE values are significantly smaller (spanning the 1.07-1.30 eV range). Again, the sodiumcontaining Na(CLi)2- species is vulnerable to the Na(CLi)2- f Na- þ LiCtCLi fragmentation process. (v) The anions utilizing HBLi groups as ligands were found to be electronically stable (with predicted VDE values of 5.43 and 1.51 eV for Al(HBLi)4- and Na(HBLi)2-, respectively) but thermodynamically unstable with respect to Na- or Al- loss. (vi) Despite the small electron binding energy of the CH3 radical (EA = 0.08 eV), the Na(CH3)2- and Al(CH3)4anions were found to be relatively strongly bound (electronically) and the estimated VDE values read 2.36 and 4.46 eV, respectively. However, the sodiumcontaining anion is expected to be susceptible to fragmentation, leading to the Na- and C2H6 (ethane) molecule, whereas the aluminum-containing Al(CH3)4species was proven thermodynamically stable. Hence, the stability of other M(CH3)kþ1- anions (i.e., with central metal atoms different than those studied in the present work) seems difficult to predict. Finally, we would like to point out that the 9-electron groups investigated, albeit promising in the chosen cases, always lead to anions whose electronic stabilities are smaller than the VDEs found for the corresponding anions possessing fluorine atoms as ligands. Therefore, we conclude that the fluorine atom remains the best choice among the 9-electron systems when the strong electron acceptors are to be designed according to the MXkþ1 formula. However, the use of OH, Li2H3, and NH2 groups seems to be a possible alternative since the anions utilizing these ligands are expected to be thermodynamically stable and their VDE values span the relatively wide 2.90-6.38 eV range. 2084



’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Polish State Committee for Scientific Research (KBN) Grant No. BW/8371-5-0447-0 and partially by the KBN Grant No. DS/8371-4-0137-1. The computer time provided by the Academic Computer Center in Gda nsk (TASK) is also gratefully acknowledged. ’ REFERENCES (1) Gutsev, G. L.; Boldyrev, A. I. Chem. Phys. 1981, 56, 277. (2) Sobczyk, M.; Sawicka, A.; Skurski, P. Eur. J. Inorg. Chem. 2003, 3790. (3) Freza, S.; Skurski, P. Chem. Phys. Lett. 2010, 487, 19. (4) Gutsev, G. L.; Boldyrev, A. I. Russ. Chem. Rev. 1987, 56, 519. (5) Gutsev, G. L.; Bartlett, R. J.; Boldyrev, A. I.; Simons, J. J. Chem. Phys. 1997, 107, 3867. (6) Scheller, M. K.; Cederbaum, L. S. J. Chem. Phys. 1994, 100, 8934. (7) Ortiz, J. V. Chem. Phys. Lett. 1993, 214, 467. (8) Ortiz, J. V. J. Chem. Phys. 1993, 99, 6727. (9) Anusiewicz, I.; Skurski, P. Chem. Phys. Lett. 2007, 440, 41. (10) Gutsev, G. L.; Jena, P.; Bartlett, R. J. Chem. Phys. Lett. 1998, 292, 289. (11) Anusiewicz, I.; Skurski, P. Chem. Phys. Lett. 2002, 358, 426. (12) Anusiewicz, I.; Sobczyk, M.; Da-bkowska, I.; Skurski, P. Chem. Phys. 2003, 291, 171. (13) Wang, X.-B.; Ding, C.-F.; Wang, L.-S.; Boldyrev, A. I.; Simons, J. J. Chem. Phys. 1999, 110, 4763. (14) Elliott, M.; Koyle, E.; Boldyrev, A. I.; Wang, X.-B.; Wang, L.-S. J. Phys. Chem. A 2005, 109, 11560. (15) Ziai, H. -J.; Wang, L.-M.; Li, S.-D.; Wang, L.-S. J. Phys. Chem. A 2007, 111, 1030. (16) Yang, J.; Wang, X.-B.; Xing, X.-P.; Wang, L.-S. J. Chem. Phys. 2008, 128, 201102. (17) Joseph, J.; Behera, S.; Jena, P. Chem. Phys. Lett. 2010, 498, 56. (18) Alexandrova, A. N.; Boldyrev, A. I.; Fu, Y.-J.; Yang, X.; Wang, X.-B.; Wang, L.-S. J. Chem. Phys. 2004, 121, 5709. (19) Pradhan, K.; Gutsev, G. L.; Jena, P. J. Chem. Phys. 2010, 133, 144301. (20) Bartlett, N. Proc. Chem. Soc. 1962, 218. (21) Wudl, F. Acc. Chem. Res. 1984, 17, 227. (22) Smuczynska, S.; Skurski, P. Chem. Phys. Lett. 2007, 443, 190. (23) Smuczynska, S.; Skurski, P. Inorg. Chem. 2009, 48, 10231. (24) Anusiewicz, I. J. Phys. Chem. A 2009, 113, 6511. (25) Anusiewicz, I. J. Phys. Chem. A 2009, 113, 11429. (26) Simons, J. J. Phys. Chem. A 2010, 112, 6401. (27) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (28) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (29) Zakrzewski, V. G.; Ortiz, J. V.; Nichols, J. A.; Heryadi, D.; Yeager, D. L.; Golab, J. T. Int. J. Quantum Chem. 1996, 60, 29. (30) Simons, J. J. Chem. Phys. 1971, 55, 1218. (31) Ortiz, J. V. J. Chem. Phys. 1988, 89, 6348. (32) Rowe, D. J. Rev. Mod. Phys. 1968, 40, 153. (33) Cederbaum, L. S. J. Phys. B 1975, 8, 290. (34) Simons, J. J. Chem. Phys. 1972, 57, 3787. (35) Simons, J. J. Chem. Phys. 1973, 58, 4899. (36) Zakrzewski, V. G.; Ortiz, J. V. Int. J. Quantum Chem. 1995, 53, 583. (37) Zakrzewski, V. G.; Ortiz, J. V. Int. J. Quantum Chem., Quantum Chem. Symp. 1994, 28, 23.

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Theoretical Search for Alternative Nine-Electron Ligands Suitable for

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