Does Al₄H ^—₁₄ cluster anion exist? High-level ab initio study

Abstract

A comprehensive ab initio investigation using coupled cluster theory with the aug-cc-pVnZ, n = D,T basis sets is carried out to identify distinct structures of the Al₄H ^—₁₄ cluster anion and to evaluate its fragmentation stability. Both thermodynamic and mechanistic aspects of the fragmentation reactions are studied. The observation of this so far the most hydrogenated aluminum tetramer was reported in the recent mass spectrometry study of Li et al. (2010) J Chem Phys 132:241103–241104. The four Al₄H ^—₁₄ anion structures found are chain-like with the multiple-coordinate Al center and can be viewed approximately as comprising Al₂H ^—₇ and Al₂H₇ moieties. Locating computationally some of the Al₄H ^—₁₄ minima on the correlated ab initio potential energy surfaces required the triple-zeta quality basis set to describe adequately the Al multi-coordinate bonding. For the two most stable Al₄H ^—₁₄ isomers, the mechanism of their low-barrier interconversion is described. The dissociation of Al₄H ^—₁₄ into the Al₂H ^—₇ and Al₂H₇ units is predicted to require 20-22 (10-13) kcal mol^-1 in terms of ΔH (ΔG) estimated at T = 298.15 K and p = 1 atm. However, Al₄H ^—₁₄ is found to be a metastable species in the gas phase: the H₂ loss from the radical moiety of its most favorable isomer is exothermic by 18 kcal mol^-1 in terms of ΔH (298.15 K) and by 25 kcal mol^-1 in terms of ΔG(298.15 K), with the enthalpic/free energy barrier involved being less than 1 kcal mol^-1. By contrast with alane Al₄H ^—₁₄ , only a weakly bound complex between Ga₄H ^—₁₂ and H₂ has been identified for the gallium analogue using the relativistic effective core potential.

Introduction

Aluminum-hydrides, or alanes, have been proposed to be potential hydrogen storage media, building blocks for new cluster assemblies and high-energy-density materials [1–6]. During the last three years, several novel alanes, both neutral and anionic, have been made and characterized. The emerging structural and electronic properties of alanes and their derivatives involving a small Al₄ cluster core have been notably the focus of the widespread interest [1, 2, 5, 6].

The new hydrogenated Al₄H₆ neutral cluster was reported to be a stable species in the gas phase based on the results of the photoelectron study of the corresponding anion Al₄H ^—₆ and density functional theory (DFT) electronic structure calculations for the neutral [1]. Furthermore, the Al₄H₆ cluster was suggested to be a high-energy density molecule, with the estimated heat of combustion (giving the Al₂O₃ and water products) to be about 2.5 times greater than that for methane [1]. Various Al₄R₆ and Al₄R₅X derivatives of Al₄H₆ with the hydrogen atoms substituted by the bulky R groups and halogens X (R = P^tBu₂; X = Br,Cl) were subsequently prepared and structurally characterized [5].

The whole series of novel aluminum-hydride cluster anions Al₄H ^—_n were generated using the laser-induced plasma technique and characterized by mass spectrometry [2]. The most hydrogenated anionic aluminum tetramer reported in Ref. [2] was Al₄H ^—₁₃ . In the related study published recently, the new gas phase Al_nH ^—_m clusters were produced with a pulsed arc discharge source and identified using mass spectrometry [6]. The latter investigation reported the aluminum hydride clusters having an extraordinary hydrogen content, with the hydrogen atom number (m)-aluminum atom number (n) ratio exceeding 3: for n = 5-8, the clusters with m = 3n + 1 hydrogen atoms were detected. Most notably, for n = 4, the cluster having m = 3n + 2 hydrogens, or Al₄H ^—₁₄ , was observed according to the authors [6], although the actual structure of this cluster anion with an open electronic shell was not reported.

Previously, using density functional and correlated ab initio methods we compared the structures, thermodynamic stabilities and bonding of the multiply hydrogenated aluminum and gallium neutral clusters M₃H₉ and M₄H₁₂ (M = Al,Ga) [7]. The distinct difference found between the corresponding clusters with M = Al and Ga was the stability of the open type structures called by us “hypervalent”, involving the five- or six-coordinate M atom (in terms of the M-H bonds). We showed specifically that, for M = Al, such a structure was relatively stable, lying within 2 kcal mol^-1 of the cyclic global minimum, whereas for M = Ga, the “hypervalent” isomer appeared to be destabilized relative to the lowest energy cyclic species, especially for M₄H₁₂ [7].

Herein we present extensive high-level ab initio calculations of the structures and fragmentation stability of Al₄H ^—₁₄ as well as of the reaction paths for interconversion and decomposition of the anion. Notably, for this novel species, we have carefully studied the dependence of the calculated results on the level of theory used. As described below, we have identified four different chain-like structures of Al₄H ^—₁₄ which involve the multiple-coordinate Al center, reminiscent of the structural motif seen in the “hypervalent” isomer of the Al₄H₁₂ cluster [7]. In order to assist in the future assignment of the actual isomer of the Al₄H ^—₁₄ anion present in the experimental mass spectrum [6], we have calculated vertical electron detachment energies (VDEs) for the distinct structures. We also show that by contrast to the aluminum hydride Al₄H ^—₁₄ , only a weakly bound complex between the Ga₄H ^—₁₂ anion and H₂ has been identified for the gallium counterpart Ga₄H ^—₁₄ .

Computational methods

Coupled cluster theory with single and double excitations (CCSD) [8] and second-order Møller-Plesset perturbation theory (MP2) [9] were used in conjunction with the augmented correlation consistent aug-cc-pVnZ, n = D,T basis sets [10, 11]. All the geometries were optimized and characterized as minima or relevant transition states at each computational level. The MP2 vibrational frequencies were calculated using the analytical Hessians, whereas the CCSD frequencies were computed with the analytical gradients and numerical Hessians. For comparison, optimizations and frequency calculations were also carried out using the same basis sets with the B3LYP density functional [12, 13]. Spin-unrestricted (U) version of each method was used for the open-shell species. To establish accurate energetics, single-point energy calculations were performed at the MP2/aug-cc-pVTZ geometries using CCSD with perturbatively included triples (CCSD(T)) [8] with the aug-cc-pVTZ basis set [10, 11]. In addition, the structures and stability of Al₄H ^—₁₄ were studied using the multi-level G4 scheme [14]. Enthalpies and Gibbs free energies were calculated at T = 298.15 K and p = 1 atm. For the gallium analogue, Ga₄H ^—₁₄ , the energy-consistent effective core potential (ECP), ECP10MDF [15], which replaces the 1s²2s²2p⁶ core of Ga by the pseudopotential was used (in addition to the all-electron calculations) to take into account relativistic effects of this atom. The explicitly treated Ga electrons (3s²3p⁶3d¹⁰4s²4p¹) were described by the associated (11s12p10d2f)/[6s6p5d2f] basis set [16], augmented by diffuse functions (aug-cc-pVTZ-PP). The latter basis was employed in conjunction with the all-electron aug-cc-pVTZ basis sets for H [10, 11, 17]. The relevant computational methods using the ECP10MDF will be referred to as MP2/ECP and B3LYP/ECP. Calculations in the current work were performed employing Gaussian 09 program [18].

Results and discussion

Al₄H ^—₁₄ structures - method dependence

Figure 1 illustrates the optimization level dependence of the four distinct structures of the Al₄H ^—₁₄ cluster anion, S1(²A), S2(²A), S3(²A′) and S3′(²A). These doublet structures are chain-like with the bridging hydrogens. Attempts to locate a cyclic Al₄H ^—₁₄ isomer were unsuccessful. Computations show that S1 exists on the potential energy surfaces calculated at the five levels (with all positive force constants predicted at each level). By contrast, the identification of S2 appeared to be method dependent: this structure has been located on the UB3LYP/aug-cc-pVDZ, UB3LYP/aug-cc-pVTZ and UMP2/aug-cc-pVTZ potential energy surfaces, but not on the UMP2/aug-cc-pVDZ and UCCSD/aug-cc-pVDZ surfaces. In turn, S3 and S3′ structures are potential minima at all the levels, but UMP2/aug-cc-pVDZ. These results indicate that with the correlated methods, the triple-zeta flexibility basis set is required to describe adequately the bonding situation for Al₄H ^—₁₄ .

Fig. 1

S1, S2, S3 and S3′ structures of the Al₄H ^—₁₄ anion (distances in Ångstroms) calculated with the correlated ab initio and density functional methods using the aug-cc-pVnZ, n = D,T basis sets. Note that S2 minimum has not been located on the UCCSD/aug-cc-pVDZ and UMP2/aug-cc-pVDZ potential energy surfaces, whereas S3(S3′) minima have not been identified on the UMP2/aug-cc-pVDZ energy surface (see the text)

As seen in Fig. 1, the unique features of the Al₄H ^—₁₄ species are the involvement of the multi-coordinate (Al1 or Al2) atom, the presence of not symmetrical Al-H-Al bridges with the H atoms not shared equally with the Al atoms as well as the presence of the significantly elongated terminal Al1(Al2)-H bonds. It is relevant to note here that multi-coordinate Al centers (participating in the six Al-H-Al bridges) were first observed in the solid state structure of AlH₃ polymer in 1969 [19].

Figure 1 also shows that the use of the correlated methods, especially in conjunction with the aug-cc-pVTZ basis set, causes a significant shortening of most of the Al-Al distances (up to 0.3 Å) along with a major decrease of the Al2-H bridging bond of S1 (relative to the UB3LYP results). As the energy calculations indicate (see Table 1), the four Al₄H ^—₁₄ species are of similar stability, lying within 2.2 and 2.4 kcal mol^-1 at the most comprehensive UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ and G4 (0 K) levels, respectively, with S3 being the most stable of the four, followed by S3′.

Table 1 Relative energies (kcal mol^-1) of the S1, S2, S3 and S3′ structures of Al₄H ^—₁₄ from the various theoretical methods^a

We suggest that the simplest description of the Al₄H ^—₁₄ anion structure would involve the ‘Al₂H ^—₇ ‘ and ‘Al₂H₇’ units. To support this model we report in Fig. 2 the optimized geometries of the isolated Al₂H ^—₇ anion and Al₂H₇ radical (both structures in Fig. 2 show positive force constants at each computational level). We find further support from the Mulliken spin populations, which indicate that the ‘Al₂H₇’ fragment of S1 and S3(S3′) is a sole carrier of the spin density. For S2, although its neutral fragment differs most from the isolated Al₂H₇ geometry (the former matches rather the higher energy Al₂H₇ isomer [20]), similar to the cases of S1 and S3(S3′), this moiety holds the spin density.

Fig. 2

Structures of the isolated Al₂H ^—₇ anion and Al₂H₇ neutral (distances in Ångstroms) calculated with the correlated ab initio and density functional methods using the aug-cc-pVnZ, n = D,T basis sets. Note that Al₂H₇ minimum has not been located on the UMP2/aug-cc-pVDZ potential energy surface

The results in Fig. 2 further show that an appreciable bending occurs for isolated Al₂H ^—₇ at the correlated levels compared to the UB3LYP results. Earlier computational studies of Al₂H ^—₇ established this anion to be thermodynamically stable with respect to both the AlH₃ + AlH ^—₄ [21] and Al₂H₆ + H^— [22, 23] fragmentations. For isolated Al₂H₇(²A), we predicted the lowest energy structure to be double-bridged, in agreement with the recent density functional work [20] (note that with UB3LYP/aug-cc-pVTZ, Al₂H₇ has higher C_s (²A") symmetry). This radical species also features two lengthened terminal Al-H bonds, to 1.66 and 1.76 Å, with a relatively short Al-Al distance of 2.62 Å (the UMP2/aug-cc-pVTZ results), and it bears resemblance with the radical moiety of Al₄H ^—₁₄ , especially for S1. The adiabatic electron detachment energy of the Al₂H ^—₇ anion, or the adiabatic electron affinity of the Al₂H₇ neutral was computed to be relatively high at 4.14 (4.20) eV with the UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ (G4(0 K)) calculations, pointing to the appreciable electronic stability of the anion.

Interconversion of Al₄H ^—₁₄ isomers

We found that the lowest-energy isomers of Al₄H ^—₁₄ , S3 and S3′, are connected by the transition state S3-S3′ TS, shown in Fig. 3. During this interconversion, H_a and H_b of S3 move in the opposite directions, breaking C_s symmetry (see Fig. 4). Past the S3-S3′ TS, H_b keeps moving toward H_c, whereas H_a moves away from Al2. In the resulting S3′ structure of very similar stability to S3 (Table 1), the new longest terminal Al1-H links are those involving H_b and H_c of 1.67 and 1.87 Å (the UMP2/aug-cc-pVTZ results), with H_a forming “normal” terminal bond, similar to H_d. The S3 → S3-S3′ TS → S3′ reaction path is confirmed by the intrinsic reaction coordinate (IRC) computation. At the UCCSD(T)/aug-cc-pVTZ level, only a tiny barrier for the S3 → S3′ rearrangement is predicted of 0.4 kcal mol^-1 (note that in the reverse direction, the barrier does not exist at this level, see the upper left panel in Fig. 4).

Fig. 3

A structure of the transition state S3-S3′ TS (distances in Ångstroms) calculated with the correlated ab initio and density functional methods using the aug-cc-pVnZ, n = D,T basis sets. S3-S3′ TS is a saddle point for the Al₄H ^—₁₄ (S3) → Al₄H ^—₁₄ (S3′) interconversion (cf. Fig. 4); the associated imaginary frequency is given at each computational level

Fig. 4

Illustration of the Al₄H ^—₁₄ (S3) → Al₄H ^—₁₄ (S3′) interconversion occurring through the transition state S3-S3′ TS. The interconversion barrier values shown have been corrected for the zero-point vibrational energies; the value before (after) slash is for the S3→S3′ (S3′→S3) rearrangement

Fragmentation stability of Al₄H ^—₁₄

We next assessed the fragmentation stability of the Al₄H ^—₁₄ anion. The computed energies of the fragmentation reactions of the four isomers of Al₄H ^—₁₄ are reported in Table 2, while the corresponding thermodynamic values are given in Table 3. We have considered three unimolecular decomposition channels: (1) Al₄H ^—₁₄ → Al₂H ^—₇  + Al₂H₇, (2) Al₄H ^—₁₄ → Al₄H ^—₁₃  + H and (3) Al₄H ^—₁₄ → Al₄H ^—₁₂  + H₂. The choice of decomposition (1) is justified by the Al₄H ^—₁₄ anion “bonding model” proposed above. The H loss reaction (2) affords the closed-shell anion Al₄H ^—₁₃ , observed in the gas phase by two research groups [2, 6], which is indicative of a relatively stable species. With the fragmentation channel (3) we examine if Al₄H ^—₁₄ is stable with respect to loss of H₂ molecule of significant stability.

Table 2 Energies of unimolecular fragmentation reactions (kcal mol^-1) of Al₄H ^—₁₄ from the various theoretical methods^a

Table 3 Thermodynamic values of unimolecular fragmentation reactions (kcal mol^-1) of Al₄H ^—₁₄ calculated at the G4 level^a

The last column of Table 2 which compares our most extended zero-point energy corrected UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ reaction energies with the G4(0 K) values (the latter are given in parentheses) shows a very good agreement between both kinds of results. Therefore, the enthalpies ΔH (298.15 K) and free energies ΔG(298.15 K) presented in Table 3 and discussed in this section were estimated at the G4 level. Regarding decomposition (1), Table 3 indicates that this reaction is always endothermic, by 19.6-22.3 (10.1-13.0) kcal mol^-1 with ΔH (298.15 K) (ΔG(298.15 K)). With respect to the H loss reaction (2) and in terms of ΔH (298.15 K), we found it to be slightly endothermic for S3, essentially thermoneutral for S1 and S3′ and insignificantly exothermic for S2. With ΔG(298.15 K), this reaction is moderately exothermic for all the Al₄H ^—₁₄ species, by 4.4-7.3 kcal mol^-1. Note that the calculated chain-like structure of the Al₄H ^—₁₃ decomposition product for reaction (2) is reported in the upper panel in Fig. 5. The alternative Al₄H₁₃ + H^— dissociation process is energetically much less favorable, by about 90 kcal mol^-1. This is explained by the considerably smaller electron affinity of H compared to Al₄H₁₃, 0.9 vs. 4.8 eV.

Fig. 5

Structures of the Al₄H ^—₁₃ and Al₄H ^—₁₂ anions (distances in Ångstroms), being the products of H and H₂ loss from Al₄H ^—₁₄ , respectively, calculated with the correlated ab initio and density functional methods using the aug-cc-pVnZ, n = D,T basis sets

By contrast with the two former fragmentations, the reaction of H₂ elimination from Al₄H ^—₁₄ (decomposition (3)) is found to be significantly exothermic, by 17.8-20.5 kcal mol^-1 in terms of ΔH(298.15 K) and by 24.8-27.7 kcal mol^-1 in terms of ΔG(298.15 K) (Table 3). These results indicate that the Al₄H ^—₁₄ anion is thermodynamically disfavored. The structure of the corresponding dehydrogenation product, Al₄H ^—₁₂ , is shown in the lower panel in Fig. 5. In the next section we are reporting the related dehydrogenation pathway.

Reaction pathway for H₂ elimination from Al₄H ^—₁₄ (S3)

The results of the previous section showed clearly that H₂ elimination from Al₄H ^—₁₄ is by far the most thermodynamically favorable unimolecular decomposition of this anion. To assess kinetic stability of Al₄H ^—₁₄ , we have calculated the reaction pathway for H₂ loss occurring from the lowest energy S3 isomer. Due to the proximity of H_a and H_b in Al₄H ^—₁₄ (S3) (cf. Fig. 4), it is anticipated that the H₂ elimination will occur relatively easily from its radical moiety. Computations confirm this assumption. The Al₄H ^—₁₄ (S3) → Al₄H ^—₁₂  + H₂ reaction has an early and asynchronic transition state, H ₂ el. TS (see Fig. 6). At the TS, only Al1-H_b increases moderately relative to that in the reactant, by 0.14-0.16 Å, whereas Al1-H_a actually decreases somewhat, with a hydrogen molecule (H_a-H_b) formed as described by H_a-H_b distance of 1.10-1.18 Å and corresponding imaginary frequency values of 644-751i cm-¹.

Fig. 6

A structure of the transition state H ₂ el. TS for H₂ elimination from Al₄H ^—₁₄ (S ₃ ) (distances in Ångstroms), calculated with the correlated ab initio and density functional methods using the aug-cc-pVnZ, n = D,T basis sets; the associated imaginary frequency is given at each computational level

Figure 7 summarizes the profiles of ΔH(298.15 K) for the three unimolecular fragmentations of Al₄H ^—₁₄ (S3) considered, with the corresponding ΔG(298.15 K) values given therein in parentheses. The most significant feature of these profiles is that there is only a tiny enthalpic/free energy barrier for H₂ loss, being 0.5/0.1 kcal mol^-1, suggesting that this elimination is remarkably facile (see also Table 4). This result, combined with the exothermicity of the process predicted for S3 by 18 kcal mol^-1 in terms of ΔH(298.15 K) and by 25 kcal mol^-1 in terms of ΔG(298.15 K) suggests the Al₄H ^—₁₄ cluster to be best described as a metastable species in the gas phase.

Fig. 7

Schematic profiles of ΔH (298.15 K) for the three fragmentation channels of the Al₄H ^—₁₄ (S3) anion (in kcal mol^-1); the corresponding ΔG (298.15 K) values are given in parentheses (G4 results). Note that the Al₄H ^—₁₂ …H₂ post-reaction complex (shown in Fig. 8) has been omitted in the H₂ elimination path

Table 4 Barrier height (kcal mol^-1) for the reaction of H₂ elimination from Al₄H ^—₁₄ (S3) calculated at the various computational levels

The post-reaction van der Waals complex between Al₄H ^—₁₂ and H₂ has also been identified as reported in Fig. 8. This complex, denoted Al₄H ^—₁₂ …H₂, was optimized at all five computational levels. Figure 8 shows that both the actual position of the H₂ subsystem within the complex and interaction distance(s) are method dependent. With UMP2/aug-cc-pVTZ, describing best long-range dispersion (at the optimization level), the complex has a structure in which the hydride fragment (Al₂-H) of Al₄H ^—₁₂ interacts with the slightly polarized H-H bond (based on the Mulliken charges). Furthermore, the UMP2/aug-cc-pVTZ calculated interacting distance of 2.53 Å is distinctly reduced relative to those predicted with the other methods. The UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ binding energy (D_e) of Al₄H ^—₁₂ …H₂ amounts to 0.7 kcal mol^-1. Note that this value coincides with the UMP2/aug-cc-pVTZ result indicating the adequate description of dispersion already at the latter level (for comparison of the six D_e values of Al₄H ^—₁₂ …H₂, see lower panel in Fig. 8).

Fig. 8

A structure (distances in Ångstroms) and binding energy (D_e, kcal mol^-1) of the complex Al₄H ^—₁₂ …H₂; D_e has been calculated as [E(Al₄H ^—₁₂ ) + E(H₂) – E(Al₄H ^—₁₂ …H₂)]

VDE energies

The VDE energies of the four hydride structures of the Al₄H ^—₁₄ anion computed at the UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ level are compared in Table 5. For the S1, S2, S3 and S3′ species the respective values are 4.83, 5.23, 4.72 and 4.77 eV. These VDEs are found to be even larger than the (adiabatic) electron detachment energy of the Al₂H ^—₇ anion discussed above. On the other hand, except for the S2 result, the remaining three VDEs are quite similar and might not help to assist in the future assignment of the actual isomer of the Al₄H ^—₁₄ anion .

Table 5 The VDE energies (eV) of the S1, S2, S3 and S3′ structures of Al₄H ^—₁₄ calculated a the UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ level^a

Does Ga₄H ^—₁₄ galane exist?

Following the earlier work on comparing the structures, stabilities and bonding of the hydrogenated aluminum and gallium clusters [7], we next studied computationally Ga₄H ^—₁₄ cluster anion. Although at the all-electron DFT UB3LYP/aug-cc-pVTZ level we located the Ga₄H ^—₁₄ minimum analogue of Al₄H ^—₁₄ (S3) (Fig. 9), neither our UB3LYP/ECP optimization nor that using the correlated UMP2/ECP method confirmed this result (the last two approaches employed the relativistic effective core potential ECP10MDF, see the footnote under Fig. 9). Similarly, our attempts to optimize the Ga₄H ^—₁₄ counterparts of S1 and S2 led instead to the van der Waals complex Ga₄H ^—₁₂ …H₂, depicted in Fig. 10. As in Al₄H ^—₁₂ …H₂, the location of H₂ unit within Ga₄H ^—₁₂ …H₂ and intermolecular separation are method dependent. Expectedly, the shortest separation between Ga₄H ^—₁₂ and H₂ is calculated with UMP2/aug-cc-pVDZ and UMP2/ECP (the highest correlated optimization levels used for the complex). At the former geometry, the UCCSD(T)/aug-cc-pVTZ binding energy (D_e) of Ga₄H ^—₁₂ …H₂ is 1.3 kcal mol^-1 (see the lower panel in Fig. 10). As Fig. 10 shows additionally, the use of the relativistic ECP resulted in a significant shortening of the terminal Ga-H bonds compared to the all-electron results (this is actually a joint ECP/basis set effect).

Fig. 9

A structure of the Ga₄H ^—₁₄ analogue of Al₄H ^—₁₄ (S3) (distances in Ångstroms) calculated at the UB3LYP/aug-cc-pVTZ and UB3LYP/ECP levels. This structure appeared to be a minimum only with the all electron UB3LYP/aug-cc-pVTZ. No Ga₄H ^—₁₄ minimum has been identified with either UB3LYP/ECP (the first-order saddle point with the imaginary frequency of 40i cm^-1 was obtained at this level with the stringent optimization criteria and ‘ultrafine’ grid) or UMP2/ECP (H dissociation occurred from Ga1 during optimization)

Fig. 10

A structure (distances in Ångstroms) and binding energy (D_e, kcal mol^-1) of the complex Ga₄H ^—₁₂ …H₂ calculated at the all-electron and ECP levels; D_e has been calculated as [E(Ga₄H ^—₁₂ ) + E(H₂) – E(Ga₄H ^—₁₂ …H₂)] - note that the UCCSD(T)/aug-cc-pVTZ value of D_e has been computed at the UMP2/aug-cc-pVDZ optimized structures

The differences noticed here between the Al₄H ^—₁₄ and Ga₄H ^—₁₄ cluster anions are consistent with our recent symmetry-adapted perturbation theory (SAPT) analysis of the aluminum and gallium species pointing out to much stronger “hydride” character of the former [24]. The larger propensity of Al atom for hypercoordinate bonding situations compared to Ga [25] is a relevant qualitative explanation.

Conclusions

For the first time, distinct minima structures of the hydrogen-rich alane Al₄H ^—₁₄ , experimental observation of which was recently reported [6], have been identified at the correlated ab initio levels, and thermodynamic and kinetic stability of this species was assessed at T = 0 K and T = 298.15 K. The structures found are chain-like, contain the multiple-coordinate Al center and approximately comprise the Al₂H ^—₇ and Al₂H₇ moieties. Locating computationally some of the Al₄H ^—₁₄ minima on the UCCSD and UMP2 potential energy surfaces required the triple-zeta quality basis set to describe adequately the Al multi-coordinate bonding. The dissociation of Al₄H ^—₁₄ into the Al₂H ^—₇ and Al₂H₇ units is predicted to require 20-22 (10-13) kcal mol^-1 with ΔH(298.15 K) (ΔG(298.15 K)). However, Al₄H ^—₁₄ is predicted to be metastable, because H₂ loss from its most favorable S3 isomer is exothermic by 18 kcal mol^-1 in terms of ΔH(298.15 K) and by 25 kcal mol^-1 in terms of ΔG(298.15 K), with the enthalpic/free energy barrier involved being less than 1 kcal mol^-1. The global minimum on the Al₄H ^—₁₄ anion energy surface corresponds to the weakly bound complex Al₄H ^—₁₂ …H₂. This kind of complex appeared to be the only minimum structure identified for the gallium counterpart Ga₄H ^—₁₄ , when Ga relativistic effective core potential was used.

In the original experimental study, Li et al. [6] reported that they could observe strong Al_nH ^—_m intensities for m > 3n “under some source conditions”. Our current thermodynamic and kinetic (fragmentation reaction barrier) considerations suggest that due to the predicted Al₄H ^—₁₄ metastability in the gas phase, this ion would in fact be detected only under the special experimental conditions.