Supporting information for

Supporting information for

Charge-Sensitive Cluster−π Interactions Cause

Altered Reactivity of Al

Clusters with Benzene:

Enhanced Stability of Al

Bz

Mengzhou Yang,a,b,ξ _{Hanyu Zhang,}a,b,ξ _{Yuhan Jia,}a,b _{Baoqi Yin}a,b _{and Zhixun Luo}a,b,*

a._{Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for}

Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences. Beijing 100190, China

b. _{University of Chinese Academy of Sciences, Beijing 100049, China} ξ. _{These authors contributed equally to this work.}

Experimental and Theoretical Methods

The home-made reflection time-of-flight mass spectrometry (Re-TOFMS) combing

deep ultraviolet (DUV) ionization laser1 was utilized to conduct the gas phase

experiments of Aln±,0 clusters reacting with benzene (Bz). A brief description of the

apparatus is given here since it has been successfully applied in the studies of the

reactivity of cationic vanadium (Vn+) and aluminum (Aln+) clusters towards oxygen2-3

and the discovery of the stable metal centered cluster of Al+(Bz)13 with a closed Bzn

shell enclosing Al atom4. The aluminum clusters (Aln±,0) were generated in the cluster

formation channel by laser ablation of Al disk (99.999%) with He (99.999%, 1.0 MPa) as the buffer gas. After generation, benzene vapor (1%, seeded in He) at a pressure of 0.1 MPa was injected into the system a pulsed general valve (Parker, Serial 9) to react

with Aln±,0 clusters in the reaction tube. The amount of benzene was controlled by

varying the on-time pulse width of the reaction gas. The molecular beam was then skimmed into a differential chamber for mass spectrometry analysis.

The measurement of neutral clusters is associated with a laser ionization process. Prior to the laser ionization process, the charged species are deflected by an electric field (200V, DC) located downstream of the fast-flow reactor. The neutral molecular beam then enters into the second chamber through a Փ2mm skimmer. In this chamber, the beam of neutral species is photoionized by the pulsed DUV laser (177nm, pulse

duration of 15ps with pulse energy of ~20μJ)5 _{from its coaxial direction. As soon as the}

photoionization occurs, the neutral species are accelerated and detected by TOF-MS. DFT calculations were performed with the gradient-corrected PBE0

functional6 using the Gaussian 09 program.7 The geometries of the species were

fully optimized using the 6-311+g(d,p) basic set. Vibrational frequency calculations are performed on each of the optimized geometries. Zero-point vibrational corrections (ZPVEs) are performed for all the energy calculations.

Quantum theory of atoms in molecules (AIM),8 electron localization function (ELF)

analysis and noncovalent interaction plots based on independent gradient model (IGM),

localized molecular orbital energy decomposition analysis (LMO-EDA)9 _{and natural}

bond orbital (NBO)10-12 _{analysis are conducted to fully demonstrate the bond nature}

S1. Aln- react with benzene

Previous studies have revealed that the electronic configuration and geometrical structure often govern the “magic” stability or reactivity of metal clusters with defined

sizes and charge states,18-20 _{which can be generally explained by the nearly free-electron}

gas theory within a jellium model where the valence electrons of metal atoms interact and fill in the electronic shells of the sphere-like cluster,21-23 _{giving rise to likely}

superatom characteristic.24-27 _{Because of its closed-shell electron configuration and}

spherical-like structure, Al13– cluster is considered as “double magic” superatom,

showing special stability towards the reaction of oxygen, water or alcohol.28-30

Unexpectedly, the double magic character does not influence its reactivity towards

benzene. In our latest experiment, as shown in Figure S1, anionic aluminum clusters

with different size show little difference in the reactivity towards benzene. The Aln–Bz

products, in which the anionic Al cluster and the C6H6 are associated by typical Van

der Waals interaction, appear as nearly normal distribution. However, the cationic 13-atoms aluminum cluster shows special reactivity during the reaction with benzene

comparing to the Aln+ cluster with other size. It is a significative issue to determine why

Figure S1. Mass spectra of (a) anionic aluminum clusters and (b/c/d) Aln– reacting with

different amounts of 1% Benzene/He, in which the black numbers indicate pure Aln–

clusters, red numbers/arrows indicate the Aln–(Bz) series, respectively. The small peaks

in (a) are assigned to O* attachments due to trace amount of background air contamination.

S2. Neutral Aln react with benzene

Figure S2. Mass spectra of (a) neutral aluminum clusters and (b-d) Aln reacting with

different amount of 2.5% Benzene/He, in which the black numbers indicate pure Aln

-clusters and the red marks indicate Bz+_{, AlBz}+ _{and Al}

S3. Cationic Aln+ react with benzene

Figure S3. Mass spectra of (a) cationic aluminum clusters and (b-d) Aln+ reacting with

different amount of Benzene/He, in which the black numbers indicate pure Aln+ clusters.

The red and blue marks indicate Aln+(Bz)1-2 series, respectively. The small peaks in (a)

S4. Calculation Details

Figure S4. The optimized ground-state structures of Al13±,0, Al15+, Al13±,0Bz and Al15+Bz

clusters at the PBE0/6-311+G(d,p) level of theory. Binding energies, defined as B.E.=

E(Aln±,0) + E(Bz) - E(Aln±,0Bz), are listed under the geometry of the addition products

with the energy unit in eV. The distances of Al-C are given in Å.

Figure S5. The isomers of Al13±,0, Al13±,0Bz and Al15+Bz clusters at the

Geometry optimization calculations are performed at PBE0/6-311+G(d,p) level of

theory. The global minima geometries (GM) of Al13+ cluster is a cap-like

low-symmetrical structure, with a hexatomic ring at the bottom, a pentagonal ring at the middle and a monatomic vertex at the top, and 0.55 eV higher in energy than the

dodecahedral isomers, which is consistent with previous studies.31-33

Figure S6. The optimized ground-state structures of Al+_Bz

1-3, Al2+Bz1-2 and Al3+Bz2

clusters at the PBE0/6-311+G(d,p) level of theory.

Figure S7. HOMO and LUMO orbitals of Al13±,0, Al15+, Al13±,0Bz and Al15+Bz clusters.

Figure S8. (Upper) Topological graphs of the Al13±,0Bz and Al15+Bz clusters based on

AIM analysis. The circle critical points (CCPs) and bond critical points (BCPs) and are indicated with red and blue dots, in which the BCPs between the Al-C are enlarged for

clarity. The bond paths for all the interactions are drawn in silver. (Below) IGM plot

iso-surfaces of the Al13±,0Bz and Al15+Bz clusters. The iso-value is set at 0.01, and the

iso-surface colored according to BGR scheme over the ED range −0.1 ≤ sign(λ2)ρ ≤ 0.1.

To visualized the noncovalent interactions between Al clusters and benzene, iso-surface plots basing on independent density gradient model (IGM) are

performed and shown in Figure S7.34 The IGM plot iso-surfaces of the four

addition products mapped by the values of sign(λ2)ρ with a volume cut-off of

δginter _{= 0.01 is addressed. From the volume of the interacting region which can} be taken as an indication of the extent of interactions, it is clearly shown that

relatively strong attractive interactions emerge at large areas in Al13+Bz and

Al15+Bz clusters. On the contrary, in Al13Bz and Al13–Bz clusters, the ranges and

Figure S9. Selected NBO orbital overlap and charge-transfer interactions of Al13±,0Bz

and Al15+Bz clusters. The second-order perturbation energies are given in kcal mol-1.

Figure S10. Natural charge population of Al+_{, Bz, Al}

13+ and Al13+ Bz. The charges are

given in au and the positive and negative charges are marked by black and green numbers, respectively.

Figure S10 shows the NPA charge distribution of Al+_{, Al}

13+, benzene molecule

Al+Bz and Al13+Bz clusters. The cluster-π interaction makes the charge more

minor electron transfer from C6H6 to Al clusters. Specifically, there is 0.202 |e|

transferred form C6H6 to Al13+ in Al13+Bz cluster, which is slightly larger than

Al15+Bz (0.190 |e| form C6H6 to Al15+) and distinctly larger than the neutral and

anionic Al13-Bz clusters (0.080 and 0.035 |e|, respectively).

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