Greatly Enhanced Electron Affinities of Au2n Cl Clusters (n = 1-4): Effects of Chlorine Doping.

Au2n Cl- (n = 1-4) clusters are investigated by both laser ablation mass spectrometry and theoretical calculations. It is interesting to find that the electron affinities of neutral Au2n Cl (n = 1-4) clusters are much larger than those of corresponding pure Au2n clusters. Among them, the electron affinity of Au2Cl is 4.02 eV, which can be defined as a very unique superhalogen that is quite different from classical ones of M n X m (M = metal, X = halogen, and n < m). Natural bond orbital and highest occupied molecular orbital analyses indicate that the extra electron is predominantly delocalized over the positively charged metal moiety in these anionic Au2n Cl- clusters, which is the main reason for the large electron affinities of the corresponding neutral species.

Introduction

It is well-known that the electron affinity (EA) is a significant property related to chemical stabilities and reactivities of corresponding molecules. For example, superhalogens with high electron affinities exhibit superior chemical reactivity and strong oxidation properties.[1−3] Several methods have been used to measure EAs experimentally, and the most effective one is based on the photoelectron detachment.[4,5] Besides, theoretical calculations have been successfully applied to predict EAs for many species.[6−11] A number of theoretical methods such as ab initio methods and density functional theory have been widely used in such kinds of studies.[12−14] In particular, EA can be used as an important indicator to identify superhalogens, which are defined as species with higher EAs than that of chlorine atom.[15−19] Since Boldyrev and Gutsev first proposed the superhalogen examples of MX in the early 1980s, there are many kinds of superhalogens that were identified by now, such as MX3 (M = Be and Mg, Ca; X = Cl and Br), Mg2X5 (X = F and Cl), and so on.[18,20−28] The EA value can be as high as 9.7 eV for Gd(BF4)4.[29] Besides, superhalogen properties can be manifested by the vertical detachment energies (VDEs) of the corresponding anions. It has been confirmed that many superhalogens have high VDEs.[30,31] The largest VDE value is 13.871 eV for H12F13–.[31] The potential applications of superhalogens in the development of organic superconductors, biocatalysts, high energy density materials, and nonlinear optical materials have attracted more and more interest in recent years.[32−35]

As the most electronegative metal atom in the periodic table, Au has attracted great attention in the field of superhalogens.[36−48] Chemical properties including structures, superhalogen properties, and catalytic mechanisms of gold-containing clusters have been widely investigated recently.[37−42] For example, Jena and co-workers performed a systematic study of Au(BO2) clusters where n = 1–5, and the results showed that Au(BO2) clusters possessed larger EAs than those of pure Au clusters because of the delocalization of the extra electron over the Au cluster.[42] Because of their tunable chemical relativities, other element-doped gold clusters have been extensively studied.[43−48] Pyykkö et al. reported the highly stable doped gold cluster W@Au12 theoretically.[43,44] Wang et al. reported the single-doped SiAu4.[46] Their results show that [SiAu4] has a large energy gap of 2.4 eV, thus indicating an extremely chemically stable molecule. [SiAu] (n = 2–4) are structurally and electronically similar to SiH, so the gold atom can exhibit chemistry analogous to the hydrogen atom, and the Au/H analogy may allow new auro analogues of hydride molecules to be designed. In this work, we used a combination of Fourier transform ion cyclotron resonance (FT ICR) mass spectrometry and theoretical calculations to examine the polynuclear clusters of Au2Cl– (n = 1–4) comprehensively. The effects of halogen doping on the EAs of the clusters are discussed in detail.

Results and Discussions

AuCl– (n = 1–4) ions were previously studied by Ma et al. on the same instrument using similar experimental conditions.[49] Different from the previous experiments performed by the same group, the concentration of the sample of AuCl4H solution applied here was reduced from 20 to 2 mg/mL, and the applied laser energy was also reduced by 10% in order to optimize the experimental conditions for the anions reported here. Figure shows one typical laser ablation mass spectrum obtained under such conditions. Besides previously observed anions of Au2Cl3– and Au3Cl2–, cluster anions of Au2Cl– (n = 1–4) and Au2– (n = 1–4) can be observed in the spectrum. In addition, the most prominent peak in the mass spectrum of Au2Cl– clusters occurred at n = 2.

Figure 1

Anions observed in the laser ablation mass spectrum of AuCl4H.

To gain further insight into anions of Au2Cl– (n = 1–4), theoretical calculations were performed at the MP2 level with the Stuttgart energy-consistent relativistic pseudopotentials ECP60MDF, in which the valence basis sets of MDF60 and aug-cc-pVTZ were employed for Au and for Cl, respectively. Some stable structures of the anions are shown in Figure . It is also found that the most stable isomers (1a, 2a, 3a, and 4a) all prefer singlet. Except for Au8Cl–, these lowest energy structures are all characterized by planar configurations. For Au2Cl–, the most stable geometric structure is linear and the Cl atom in the terminal position. Interestingly, this trend is maintained for all the most stable isomers of Au2Cl– (n = 2–4). The equilibrium geometries of their corresponding neutral clusters were also optimized at the same level. It is also found that all these geometries preferred doublets. As shown in Figure , there is a significant difference between the lowest energy geometries of anionic and neutral species. Neutral Au2Cl and Au4Cl clusters are characterized by their planar configurations, while Au6Cl and Au8Cl clusters are characterized by their 3D-configurations. Au2Cl has a bent structure with a C2 symmetry. Moreover, it is found that the linear isomers of 1b and 1c shown in Figure are respectively 8.1 and 16.6 kcal/mol higher in energy than the most stable isomer 1a. Obviously, the geometric structures of these clusters are dramatically altered by the extra electron in the anions. For example, the most stable structure of Au2Cl has a bent structure with a C2 symmetry, but the most stable structure of the Au2Cl– anion cluster is linear.

Figure 2

Some geometries and their relative energies (ΔE, in kcal mol–1) of Au2Cl– (n = 1–4) clusters obtained at the MP2 level.

Figure 3

Some geometries and their relative energies (ΔE, in kcal mol–1) of Au2Cl (n = 1–4) clusters obtained at the MP2 level.

To better understand the properties of these clusters, their EA values and corresponding VDEs of their corresponding anions were calculated and the results are listed in Table . Interestingly, the EA value of Au2Cl is 4.02 eV, which is higher than that of chlorine atoms, namely, 3.62 eV. The point makes Au2Cl a superhalogen that is very different from those classical superhalogens of MCl (n < m) described by Gutsev and Boldyrev of which the number of metal atoms is less than the number of halogen atoms. This superhalogen characterized by the number of metal atoms exceeds the number of halogen atoms and it can be classified as a multimetal superhalogen. For Au4Cl, Au6Cl, and Au8Cl clusters, the electron affinities are 3.43, 3.04, and 3.44 eV, respectively, which are larger than the corresponding values of pure Au4 (2.63 eV), Au6 (2.00 eV), and Au8 (2.79 eV) clusters.[48,50] In our work, the VDEs of Au2Cl– (n = 1–4) clusters are higher than the EAs of corresponding neutral clusters. The results are shown in Table .

Table 1

EAs of Au2Cl (n = 1–4) Clusters, and VDEs of Their Anions, Compared with Those Experimental VDEs of Au2– (n = 1–4)a

cluster	EA	VDEb	cluster	VDEc
Au2Cl	4.02	4.71	Au2–	1.90
Au4Cl	3.43	4.11	Au4–	2.63
Au6Cl	3.04	3.11	Au6–	2.00
Au8Cl	3.44	3.53	Au8–	2.79

All of energies are in eV.

For corresponding anions of the species shown in the first column.

Experimental values taken from ref (50).

The superhalogen behavior of the Au2Cl cluster can be attributed to the fact that the extra electron delocalizes over the metal moiety in the anionic clusters.[42] The charges on each of the atoms in both neutral and anionic Au2Cl clusters were calculated using natural bond orbital (NBO) analyses. As shown in Table S1, the charges on the Au2 moieties are +0.69e and −0.24e in Au2Cl and Au2Cl–, respectively, indicating that the extra electron is delocalized over the metal moiety predominantly (93%) in the anion.

For the Au4Cl cluster, the charge on the Au4 moiety is +0.68e. When an extra electron is added to the neutral Au4Cl cluster, it is expected that this charge would localize mostly on Au4. NPA charge analysis indeed confirms this and 96% of the extra electron goes to the Au4 moiety. Similarly, NPA charge analyses show that the charges on Au6 and Au8 moieties in neutral Au6Cl and Au8Cl are +0.59e and +0.61e because of the doping of chlorine atom. For Au6Cl– and Au8Cl–, the charges on the Au6 and Au8 moieties are −0.31e and −0.30e, respectively. This also indicates that the extra electron is mostly distributed over the metal moiety (90 and 91%) which is positively charged in the neutral species. It is the main reason for the larger electron affinities of Au6Cl and Au8Cl than those of pure Au6 and Au8 clusters. Interestingly, even though the EA of Au4, Au6, and Au8 clusters are not high, they can still win the extra electron from Cl in Au4Cl, Au6Cl, and Au8Cl clusters, and the change in their EAs are indeed induced by the doping of chlorine atoms.[51] In addition, we also compare the VDEs of Au2Cl– clusters with that of other gold–chlorine clusters with different compositions. Because AuCl clusters make more positively charged gold moieties, thus it is expected that AuCl clusters should have larger VDEs. Based on the structures suggested previously, the VDEs were calculated. In addition, the results do show that the VDEs of AuCl– (n = 1–7) range from 4.99 to 7.06 eV (Table S2), which are larger than the VDEs of Au2Cl– clusters.

To investigate the electron distribution of the lowest energy configurations, their highest occupied molecular orbitals (HOMOs) were also analyzed. Figure shows the HOMOs of the four most stable isomers of the Au2Cl– clusters. From the figure, it can be clearly found that the HOMOs of the Au2Cl– clusters have main contribution from the Au moieties, instead of Cl atoms. The energy gap between HOMO and lowest occupied molecular orbitals (LUMOs) is generally thought to be a good measure of the chemical inertness of a species.[52−54] In our work, the HOMO–LUMO gap values of Au2Cl– (n = 1–4) are 5.53, 5.36, 4.24, and 4.69 eV, in turn. These large energy gaps make these anionic clusters stable, and thus can be generated and detected in the experiments readily.

Figure 4

HOMOs of the clusters of Au2Cl– (n = 1–4), corresponding to the isomers of 1a, 2a, 3a, and 4a shown in Figure .

Two aspects can be further discussed here. The first one is that if the doping chlorine atom is replaced by one F or Br atom, how their structures and EAs would be changed. Thus, theoretical calculations were performed for Au2F and Au2Br (n = 1–4) clusters on the same level. The most stable structures of the Au2F–, Au2F, Au2Br–, and Au2Br (n = 1–4) are shown in Figure S1–S4. Most of these isomers have similar structures. However, Au6F and Au8Br show two different cases. The first and second most stable isomers reverse their energy order in the cases of F and Br. As shown in Figure , the EA values of Au2F, Au4F, Au6F, and Au8F are 4.08, 3.49, 3.22, and 3.46 eV, in turn, which are all slightly larger than those of Au2Cl (n = 1–4). For Au2Br (n = 1–4) clusters, the EA values are 3.93, 3.20, 2.98, and 3.43 eV, which are all slightly smaller than those of Au2Cl (n = 1–4) clusters. The results show that the species of halogen atom has little effect on the EAs of Au2X (n = 1–4, X = F, Cl, and Br), and the doping of the F atom can increase their EAs more. We also carried out the calculations of Cu2Cl and Ag2Cl (n = 1–3). The electron affinities of Cu2Cl (n = 1–3) are 2.51, 1.72, and 1.56 eV, in turn. Moreover, those for Ag2Cl are 2.51, 1.90, and 1.87 eV. The results show that the electron affinities of Cu2Cl and Ag2Cl (n = 1–3) are much lower than those of Au2Cl, and none of them should be considered as a superhalogen. The second aspect will be more difficult to answer, that is, how about doping a Cl atom into large gold clusters, such as Au20? It will be interesting to see to what extent the doping of a single halogen atom can affect the structures and properties of large-sized gold clusters, and this will be the topic of our future research.

Figure 5

Electron affinities of the clusters of Au2X (n = 1–4, X = F, Cl, and Br).

Conclusions

In summary, we report a joint study with laser ablation mass spectrometry and theoretical calculation for Au2Cl– (n = 1–4) clusters. Systematic theoretical calculations of these anions and their corresponding neutral species were performed at the MP2 level to investigate their most stable geometries and electron affinities. The results show that the most stable isomers of Au2Cl– (n = 1–4) clusters have quite different structures from their corresponding neutral species. The electron affinities of Au2Cl (n = 1–4) are greatly enhanced by the doping of chlorine atoms. The most striking fact is that the EA of Au2Cl is found to be 4.02 eV, indicating that it can be identified as a unique multimetal superhalogen. For Au4Cl, Au6Cl, and Au8Cl, although their electron affinities are below than that of chlorine atoms, their VDEs increased about 60%, compared to the corresponding pure Au4, Au6, and Au8 clusters. NPA charge analyses were performed to trace the origin of their high electron affinities. The predominant delocalization of the extra electron over the previously positively charged metal moieties in anionic clusters is the main reason for their large electron affinities of Au2Cl clusters. HOMO analyses are also applied and the results are consistent. Considering the species diversity of the gold-related clusters and their structure diversities, this study also examined Au2F (n = 1–4) and Au2Br (n = 1–4) clusters, and similar results have been obtained.

Methods

Experimental Section

Experiments were performed on a 7.0 T FT ICR mass spectrometer (Varian IonSpec, Lake Forest, CA, U.S.). It is equipped with a ProMALDI ion source by using a 355 nm Nd:YAG laser (Orion, New Wave). The AuCl4H sample (Energy Chemical) was prepared in water at a concentration of 2 mg/mL just before the experiment. The graphene sample (Timesnano Company) was dispersed in acetone with a concentration of 1 mg/mL by sonication for 10 min prior to usage. Then, 1 μL dispersion of graphene was applied onto the stainless target spot first, after it dried up, 1 μL solution of AuCl4H was then added on the same spot. The target was then dried in air and placed into the source region of a FT ICR mass spectrometer. The mass spectrum in this work was measured in negative ion mode.

Computational

All calculations were performed with the Gaussian 09 program.[55] The reliability of the MP2 method for gold has been clearly proven by previous studies.[56−58] In our work, theoretical calculations were carried out at the MP2 level with the Stuttgart energy-consistent relativistic pseudopotentials ECP60MDF and the corresponding valence basis set of MDF60 for Au and aug-cc-pVTZ for F, Cl, and Br.[59−61] Spin–orbit calculations are not performed here, thus the numerical results might change slightly, but the main conclusion should not. In order to further verify the accuracy of the calculation results, single point calculations were also done with the CCSD(T) method for Au2Cl (n = 1–3). The results are shown in Table S3, and similar results have been obtained. Vibrational frequency analyses were performed for all the optimized structures on the same level to confirm that no imaginary frequencies are present. In addition, the reported electronic energies of all structures were calculated at 0 K with zero-point energy corrections. Different spin states including: singlet, triplet, and quintet spin states for even-electron systems and doublet, quartet, and sextet spin states for odd-electron systems were calculated. NBO analyses were performed to obtain charge distributions in Au2Cl– and Au2Cl clusters (n = 1–4).