Structural and Bonding Properties of AlnC4 (n=2−4) Clusters: Anion Photoelectron Spectroscopy and Theoretical Calculations†

We measured the photoelectron spectra of AlnC4 − (n=2−4) clusters by using size-selected anion photoelectron spectroscopy. The structures of AlnC4 −/0 (n=2−4) clusters were explored with quantum chemistry calculations and were determined by comparing the theoretical results with the experimental spectra. It is found that the most stable structure of Al2C4 − anion is a C2v symmetry planar structure with two Al atoms interacting with two C2 units. In addition, Al2C4 − anion also has a D∞h symmetry linear structure with two Al atoms located at the two ends of a C4 chain, which is slightly higher in energy than the planar structure. The most stable structure of neutral Al2C4 has a D∞h symmetry linear structure. The most stable structure of Al3C4 − anion is a planar structure with three Al atoms interacting with two C2 units. Whereas neutral Al3C4 cluster has a C2v symmetric V-shaped bent structure. The global minima structures of both Al4C4 − and neutral Al4C4 are C2h symmetry planar structures with four Al atoms interacting with the ends of two C2 units. Adaptive natural density partitioning analyses of AlnC4 − (n=2−4) clusters show that the interactions between the Al atoms and C2 units have both σ and π characters.

* Authors to whom correspondence should be addressed. E-mail: xlxu@iccas.ac.cn, zhengwj@iccas.ac.cn [16], and endohedrally doped cage structures [17][18][19][20][21]. Quantum chemistry studies on small aluminum-carbon clusters predicted that the dominated structures of those clusters are flat and the aluminum-carbon bonds in those clusters are covalent bonds [22][23][24][25][26]. High-level ab initio computations and quantum molecular dynamics simulations suggested that CAl 5 + has a ppC structure [27]. Theoretical studies on C 2 E 4 (E=Al, Ga, In, and Tl) clusters suggested that C 2 Al 4 is a double planar tetracoordinate carbon structure, and subsequent calculations predicted that structure could be used to design new molecular chains [28][29][30]. There are also many experimental studies on aluminum-carbon clusters. The structures of Al n C −/0 (n=3−5) and Al n C 2 −/0 (n=1−3) clusters have been studied via combining anion photoelectron spectroscopy with ab initio calculations [31][32][33][34][35][36]. The structures of Al 12 C −/0 were also investigated in detail with photoelectron spectroscopy and theoretical calculations [37][38][39][40]. Photoelectron spec-troscopy and theoretical study of Al n C 2 −/0 (n=5− 13) clusters suggested that those clusters can be classified into two types of structures distinguished by dissociation or undissociation of carbon-carbon bond [41]. The structures of Al n C 5 −/0 (n=1−5) clusters were investigated with anion photoelectron spectroscopy and theoretical calculations [42]. The studies suggested that the most stable structures of Al 5 C 5 −/0 clusters contain a ptC unit and the star-like isomer of Al 5 C 5 − may also exist in the experiments. Mass spectrometry and theoretical calculations indicated that Al 7 C − cluster is a stable cage-like structure with resistance to oxidation [43][44][45]. Mass spectrometry studies of Al m C n H x clusters suggested that those clusters may have potential application of hydrogen storage materials [46]. Understanding the structures and properties of aluminum-carbon clusters will be helpful for the designing of aluminum-based materials. To provide more information about the structures and bonding properties of small aluminum-carbon clusters, in this work, we investigate the structural evolution and chemical bonds of Al n C 4 −/0 (n=2−4) clusters by utilizing anion photoelectron spectroscopy and theoretical calculations.

A. Experimental methods
The experiments were conducted on a home-built apparatus which has been described elsewhere [47]. Briefly, the aluminum-carbon clusters were generated in a laser ablation source, in which a rotating and translating disk target containing a mixture of aluminum and carbon powder (Al:C mole ratio 5:1) was ablated by the second harmonic (532 nm) light pulses from a nanosecond Nd:YAG laser (Continuum Surelite II-10). Simultaneously, helium carrier gas with ∼0.4 MPa backing pressure was injected into the cluster source through a pulsed valve (General Valve Series 9) to cool the formed clusters after undergoing a supersonic expansion. Aluminum-carbon clusters were mass-analyzed by a time-of-flight mass spectrometer (TOF-MS). The Al n C 4 − (n=2−4) cluster anions were each size-selected by a mass-gate and decelerated by a momentum-decelerator before being photodetached by the fourth-harmonic light pulses (266 nm) from another nanosecond Nd:YAG laser (Continuum Surelite II-10). The detached photoelectrons were energy-analyzed by a magnetic-bottle photoelectron spectrometer. The pho-toelectron spectra of Al n C 4 − (n=2−4) cluster anions were calibrated with the spectra of Bi − and Pb − anions obtained under similar experimental conditions. The resolution of the magnetic-bottle photoelectron spectrometer is about 40 meV for photoelectrons with kinetic energy of 1 eV.
No symmetry constraint was imposed during the full geometry optimizations. Harmonic vibrational frequency analyses were also carried out to confirm that the optimized structures were true local minima on the potential energy surfaces. To obtain more accurate relative energies of the low-lying isomers for clusters, the single-point energies were calculated by using the coupled-cluster methods including single, double, and perturbative contributions of connected triple excitations [CCSD(T)] [55,56] methods with the augcc-pVTZ basis set [57][58][59]. It is necessary to mention that we  Table I. The theoretical VDE of each cluster anion is calculated as the energy difference between the neutral and anion both at the equilibrium structure of the cluster anion. The theoretical ADE is calculated as the energy difference between the neutral and anion with the neutral cluster relaxed to the nearest local minimum using the anionic structure as the initial structure. The theoretical photoelectron spectra of Al n C 4 − (n=2−4) cluster anions are simulated based on the generalized Koopmans' theorem (GTK) [63,64], and compared with the experimental spectra in FIG. 4. The most stable isomer of Al 2 C 4 − anion (2A) has a C 2v symmetry planar structure that two C 2 units are connected by two Al atoms. In that structure, the C−C bond lengths are 1.26Å, the Al−C bond lengths are 2.00, 2.08, and 2.20Å, respectively. Isomer 2B has a D ∞h symmetry linear structure that two Al atoms locate at the two ends of a C 4 chain. The energy of isomer 2B is slightly higher than isomer 2A by only 0.05 eV. The theoretical VDE of isomer 2A is calculated to be 2.94 eV at the B3LYP level and 3.00 eV at the ROCCSD(T) level, in agreement with the broad peak at 2.99 eV in the experimental spectrum. The theoretical VDE of isomer 2B is calculated to be 1.39 eV at the B3LYP level and 1.05 eV at the ROCCSD(T) level, consistent with the peak at 1.05 eV in the exper-   The theoretical VDE of isomer 3A is calculated to be 2.80 eV at the B3LYP level and 3.08 eV at the ROCCSD(T) level, in agreement with the experimental value (2.99 eV). The simulated spectrum of isomer 3A matches the experimental spectrum very well. The existence of isomers 3B, 3C, and 3D can be ruled out because they are much higher in energy than isomer 3A by 0.23, 0.51, and 0.59 eV, respectively. Therefore, it is suggested that isomer 3A is the most probable one observed in our experiments. The most stable structure of neutral Al 3 C 4 cluster (3a) is a bent structure, which can be viewed as two linear Al−C≡C−Al units sharing an Al atom. Isomer 3b is slightly higher in energy than 3a by only 0.02 eV, which has a planar structure similar to the second structure of Al 3 C 4 − anion. Isomers 3c and 3d are higher than 3a in energy by 0.13 and 0.22 eV, respectively.
For Al 4 C 4 − cluster, isomers 4A, 4B, 4C, and 4D are close in energy with 4B, 4C, and 4D higher than 4A by 0.07, 0.08, and 0.10 eV, respectively. Isomer 4A is a C 2h symmetry planar structure composed of two Al−C≡C−Al chains. In that structure, the Al−Al bond lengths are 2.71Å, the C−C bond lengths are 1.26Å, and the Al−C bond lengths are 1.  0.46 eV, respectively. It is worth mentioning that the structures of neutral Al n C 4 (n=2−4) found in this work are in agreement with those reported in the literature [25,26].

IV. DISCUSSION
To probe the electron distribution and bond strength of Additionally, two 3c-2e π bonds and two 3c-2e bonds with ONs=1.96−2.00 |e| between the Al atoms and C 2 units of each cluster can be found. For the 3c-2e bond, the C−C interaction has π character while the Al−C interaction has σ character. Observation of different chem-ical bonds between the Al atoms and the C atoms is consistent with the Mayer bond index analyses. Each C 2 unit has a 2c-2e C−C σ bond and participates a 3c-2e bond and a 3c-2e π bond, leading to the C−C bond length (1.26Å) of those clusters are longer than the C≡C bond length (1.20Å) in acetylene and shorter than the C=C bond length (1.33Å) in ethylene.

V. CONCLUSION
The electronic and geometric structures of Al n C 4 −/0 (n=2−4) were studied by using size-selected anion photoelectron spectroscopy and theoretical calculations. The most stable structure of Al 2 C 4 − has a planar structure with two Al atoms interacting with two C 2 units. A linear structure with two Al atoms attached to two ends of one C 4 chain can also be detected in our experiments. Neutral Al 2 C 4 cluster has a linear structure.