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. 2024 Mar 20;146(13):9302–9310. doi: 10.1021/jacs.4c01068

Tailoring Titanium Carbide Clusters for New Materials: from Met-Cars to Carbon-Doped Superatoms

Chaonan Cui †, Hanyu Zhang †, Yuming Gu ‡, Lijun Geng †, Yuhan Jia †,§, Shiquan Lin †,§, Jing Ma ‡,*, Zhixun Luo †,§,*
PMCID: PMC10996009  PMID: 38506150

Abstract

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Tailoring materials with prescribed properties and regular structures is a critical and challenging research topic. Early transition metals were found to form supermagic M8C12 metallocarbohedrenes (Met-Cars); however, stable metal carbides are not limited to this common stoichiometry. Utilizing self-developed deep-ultraviolet laser ionization mass spectrometry, here, we report a strategy to generate new titanium carbides by reacting pure Tin clusters with acetylene. Interestingly, two products corresponding to Ti17C2 and Ti19C10 exhibit superior abundances in addition to the Ti8C12 Met-Cars. Using global-minimum search, the structures of Ti17C2 and Ti19C10 are determined to be an ellipsoidal D4d and a rod-shaped D5h geometry, respectively, both with carbon-capped Ti4C moieties and superatomic features. We illustrate the electronic structures and bonding nature in these carbon-doped superatoms concerning their enhanced stability and local aromaticity, shedding light on a new class of metal-carbide nanomaterials with atomic precision.

Introduction

Transition metal carbides (TMCs) constitute diverse materials and coatings with extreme wear resistance and wide applications.1,2 Numerous efforts have been made to improve the performance of TMCs by doping heteroatoms,3 tuning the ratio of components,4 or controlling surface morphology and structures.5 Fundamental insights into the building blocks are important for understanding the structure–function relationship of the TMC materials. For example, early transition metals (Groups 4 and 5) readily form monocarbides (namely, TiC, ZrC, HfC, VC, NbC, and TaC), giving rise to face-centered-cubic (fcc) crystal structures akin to sodium chloride, with a mixture of ionic, covalent, and metallic bonds. Recently, Ti3C2 MXenes6 have become a new class of frontier materials with superior conductivity and tunable bandgaps, showing high efficiency of photocatalysis and electrocatalysis in diverse reactions including N2 fixation.7,8

Another interesting research topic of TMCs pertains to the finding of Ti8C12 as a class of metallocarbohedrenes (Met-Cars)9−11 with a common stoichiometry of M8C12 (M = Ti, V, Cr, Zr, Hf, Nb, Mo, etc.),12−17 which was repeatedly observed during dehydrogenation reactions of hydrocarbons with metal atoms/ions and small clusters in the plasma atmosphere.18−29 Met-Cars are attractive in the application potentials for hydrogen storage and catalytic reactions including the novel C1 chemistry.30−33 Recently, by downstream reactions of pure Con– and Nin– clusters with acetylene, stable M12C12– (M = Fe, Co, Ni) clusters have also been identified.34,35 However, it is still unclear if super magic TMCs are limited to M8C12 and M12C12. Further insights into the chemical reactivity, the bonding nature, and the structure–property relationship still need to be further explored.17,36 Also, it is anticipated that stable TMCs can be tailored with tunable structures and properties at atomic precision37−40 as well as novel quantum size effects and superatomic features.41−44

Gas-phase clusters that contain a few to a dozen atoms provide an ideal strategy for exploring functional materials with highly controllable properties. Herein, we report a comprehensive study on the neutral Tin clusters by utilizing a self-developed reflection time-of-flight mass spectrometer combined with a homemade 177.3 nm deep ultraviolet (DUV) laser system.45 Two fancy neutral TMCs, Ti17C2 and Ti19C10, are discovered by reacting pure Tin (n = 1–38) clusters with C2H2. We fully unveil the different electronic structures, chemical bonding, and formation mechanisms of ellipsoidal Ti17C2 and rod-shaped Ti19C10. Interestingly, the Ti17C2 and Ti19C10 clusters both exhibit vivid superatomic features and local aromaticity, which accounts for their reasonable stability.

Results and Discussion

The experiments conducted in this work are based on our homemade laser vaporization (LaVa) source combined with a reflection time-of-flight mass spectrometer (Re-TOFMS) and a self-developed picosecond-pulsed 177.3 nm laser system. Figure 1 presents the mass spectra of the Tin neutral clusters in the absence and presence of acetylene (C2H2, 10% in He). The as-prepared neutral Tin clusters exhibit a near-Gaussian distribution. In the presence of C2H2, all of the Tin clusters show decreased mass abundances, along with the production of diverse titanium carbides. The dehydrogenation of C2H2 to form carbides is consistent with the previously reported study of Ti+ cations, which also showed high dehydrogenation reactivity toward the chemicals involving C–H, O–H, and N–H groups.46 Among the TinCm clusters produced by the Tin clusters in reacting with C2H2, two new clusters (Ti17C2 and Ti19C10) appear as magic numbers in the mass spectra (details shown in Figures S1–S5).

Figure 1.

Figure 1

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Mass spectra of Tin neutral clusters reacting with different amounts of C2H2 (10% C2H2 in He, 1.0 atm). The gas flow is controlled by a pulsed valve, with small and large doses of the C2H2 reactant corresponding to pulse widths of 190 and 220 μs, respectively. The peak marked with an asterisk corresponds to m/z 1389.4. The inset on the top right shows the instrumentation.

As a comparison, we also prepared neutral titanium carbide clusters (TinCm) by using a TiC target (99.99% purity) and observed their reactions with different amounts of acetylene (C2H2, 10% in He) (Figure S6). The original mass spectrum displays two dominant peaks corresponding to Ti8C12 and Ti14C13. This is reasonable since the laser ablation of a TiC target could be conducive to producing metal carbides at a 1:1 ratio of Ti and C atoms (Figure S7). In the presence of gradually increased reaction gas C2H2, the two carbide clusters both endured the gas-collisional reactions and Ti8C12 survived the reactions with a large flow of C2H2. It is supposed that the Ti8C12 cluster could potentially handle the higher inertness over Ti14C13 in this condition; also, the dehydrogenation of C2H2 on the cluster could result in a C:Ti ratio larger than 1:1, which is beneficial to the formation of a carbon-rich Ti8C12 cluster. This observation of Ti8C12 and Ti14C13 is consistent with previous studies on photoelectron spectroscopy and photodissociation of the anionic and cationic counterparts.47,48 Notably, the vertical ionization energies (VIEs) of the Tin (n = 1–20)49,50 and TinCm clusters (Table S1) are less than 7 eV; thus, the ps-pulsed 177.3 nm laser is suitable for single-photon ionization of these clusters without being subjected to photoinduced fragmentation.51

The structures of the four titanium carbide clusters (i.e., Ti8C12, Ti14C13, Ti17C2, and Ti19C10) are determined by using the first-principles evolutionary algorithm as implemented in the USPEX code52 combined with the Vienna ab initio simulation package (VASP).53 Low-lying isomers of each cluster are optimized at the BLYP/SDD/6-311G* and PBE/def2svp/6-31G* levels of theory (Tables S2 and S3 and Figures S8–S14). The energetics of Tin (Figures S15 and S16) and optimized structures of Ti8C12 and Ti14C13 are consistent with previously reported studies.54,55 Specifically, Ti8C12 bears a distorted tetrahedral cage structure (quasi-Td symmetry, similar to a Reuleaux tetrahedron),56 which can be viewed as the fusion of four cambered Ti4C6 moieties. The bond length of each C2 unit is approximately 1.35 Å, which is comparable to the bond length of a normal C=C double bond (1.34 Å) but with significant extension compared to that of the nascent C≡C triple bond (1.20 Å) of C2H2. Notably, the distances between the titanium atoms are in the range of 2.81–3.03 Å, indicating negligible Ti–Ti metallic bonding in the Met-Car Ti8C12 cluster. The optimized Ti14C13 has a cubic structure of Oh symmetry akin to the crystal structure of sodium chloride, corresponding to a distorted 3 × 3 × 3 fcc lattice unit.57 The Ti–C bond lengths of Ti14C13 are in the range of 2.00–2.13 Å, with a larger Mayer bond order than that of Ti8C12 (Tables S4 and S5). The Ti–Ti distances of Ti14C13 are averaged at 2.82–2.94 Å, indicative of similar weak Ti–Ti bonds in this cluster.

Figure 2A presents the lowest energy structure of Ti17C2, which shows a lantern-structured ellipsoid with D4d symmetry. It can be viewed as a ferrocene-like Ti4C–Ti–Ti4C moiety embraced by a crown-shaped Ti8 ring (akin to S8)58 on the waist. For the sandwiched Ti4C–Ti–Ti4C moiety, the two Ti4C units are nonoverlapped with a twist angle of 45° (akin to staggered ferrocene). For each Ti4C unit, the carbon atom is bound with four Ti atoms at the square ring with a bond length of 2.04 Å and is slightly out-of-plane with a Ti–C–Ti angle of 171°. All Ti–Ti bonds of the Ti8 ring are identical with a bond distance of 2.47 Å, showing dominant metal–metal bonding. Notably, the bond lengths between the central Tic atom (No. 17) and the two capped carbon atoms (No. 18 or 19) are both at 2.25 Å (with a bond order close to zero, Table S6). Additionally, the central Tic atom displays almost no interaction with the 16 shell-Ti atoms, with a large Tic-Ti bond length up to 2.91 Å. This result is consistent with the charge decomposition analysis based on fragment-bonding principles (Figures S17). The stability of Ti17C2 is rationalized by a core–shell Ti@Ti16C2 with metal–metal coordination and electrostatic interactions.59

Figure 2.

Figure 2

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DFT-optimized structures of the (A) D4d Ti17C2 (Mm = 3) and (B) D5h Ti19C10 (Mm = 5) clusters and their selected fragment analysis. The bond lengths are given in Å. Atomic charges of the Ti17C2 and Ti19C10 clusters are labeled based on natural population analysis (NPA).

The lowest energy structure of Ti19C10 has a rod-like D5h symmetry with ten Ti4C units on the waist. As shown in Figure 2B, the Ti19 skeleton in the Ti19C10 cluster can be viewed as the stacking and fusing of three D5h Ti7 moieties in a double pentagonal pyramid. The three Ti7 moieties overlap and are surrounded by 10 carbon atoms to form the Ti4C units, with Ti–C bond lengths of 2.01–2.12 Å and Ti–Ti bond lengths ranging from 2.67 to 2.86 Å. In contrast to the D4d Ti17C2 with two Ti4C units on the top and bottom sides, the Ti19C10 cluster with ten Ti4C units on the waist shows a prolate rod-like structure. Notably, the distances between neighboring carbon atoms in Ti19C10 range from 2.76 to 3.04 Å, indicating full activation of the C≡C bond of acetylene on the larger titanium clusters (analysis of Mayer bond order is given in Table S7). The Ti4C unit (where the capped carbon atom functions as a tetradentate ligand) differs from the Td-symmetric CH4 within sp3 hybridization because the repulsion of bonding electron pairs is relatively weak and thus yields Ti–Ti metallic bonding. The Ti4C units in the Ti17C2 and Ti19C10 clusters also differ from the C2 units in Ti8C12, where the carbon atom adopts quasi-sp2 hybridization. The carbon-doped superatom clusters Ti17C2 and Ti19C10, with unique structures different from those of Ti8C12 Met-Cars, fcc Ti14C13, and Ti3C2 MXenes, breed a new class of metal carbides.

To fully understand the stability mechanism of the four titanium carbide clusters, we analyzed the natural charge population (NPA) and projected densities of states (PDOS). Notably, the NPA charge distributions on the Ti17C2 and Ti19C10 clusters display large differences from that on Ti8C12 and Ti14C13. The Ti and C atoms of the Ti4C units in Ti17C2 are positively and negatively charged by 0.32 and −0.87 |e|, respectively, and the NPA charge of the crown-shaped Ti8 moiety is close to zero (insets in Figure 2). Notably, the central Ti atom of Ti17C2 is negatively charged at −0.99 |e|, showing electrostatic repulsion interactions with the two separated carbon atoms (−0.87 |e|) of lantern-structured Ti17C2 (Figure S18 and Table S8). For Ti19C10, the 10 C atoms each have a negative charge of −0.57 |e|, while the 15 Ti atoms in the 10 Ti4C units display a positive charge of 0.34–0.45 |e|, with rare NPA charges on the four Ti atoms along the molecular major axis. For Ti8C12 and Ti14C13, all carbon atoms display a negative NPA charge, while the Ti atoms have positive charges (Figure S19). Specifically, the C2 dimers in Ti8C12 have a negative charge of −0.56 ∼ −0.59 |e|, while the Ti atoms have similar positive charges ranging from 0.32 to 0.47 |e|. Although analogous to the TiC crystal structure, the Ti14C13 cluster shows an altered charge distribution.60 The NPA charges on the six Ti atoms of the face center of Ti14C13 are close to zero, but the other Ti atoms show a positive charge of approximately 0.70 |e|; the 12 carbon atoms on the edge are negatively charged by −0.49 |e|, but the charge on the central carbon is −0.35 |e|.

Figure 3 presents the PDOS patterns and frontier orbitals of the Ti17C2 and Ti19C10 clusters (that of Ti8C12, Ti14C13, and Ti17 clusters given in Figures S20–S23). Notably, the gap between α-HOMO (highest occupied α-orbital) and α-LUMO (lowest unoccupied α-orbital) follows a trend of Ti17C2 (1.33 eV) > Ti19C10 (1.15 eV) > Ti8C12 (1.08 eV) > Ti14C13 (0.87 eV), which could be associated with their stability. For these clusters, the orbitals located at lower energy (e.g., −20 eV for Ti8C12 and −14 eV for Ti14C13; Ti17C2 and Ti19C10 at −13 ∼ −14 eV) are dominantly contributed by the C(2s) electrons, while the C(2p) orbitals hybridize with Ti(3d/4s) at higher energy levels. The PDOS compositions of Ti17C2 and Ti19C10 exhibit comparable patterns, with many hybridized orbitals above −8 eV showing vivid superatomic features.61,62 Notably, the superatomic 1S and 1Pz orbitals of Ti17C2 are mainly composed of the 2s orbitals of the carbon atoms, while the superatomic 1Px, 1Py, 2S, 2P, and 1D orbitals are caused by hybridization of the valence C(2p) orbitals and Ti(3d/4s) orbitals. This is analogous but partly different from the superatomic feature of nascent Ti17 (Figure S23). Regardless of pure metal clusters or doped superatoms, the diffused superatomic orbitals largely balance the electron–nucleus interactions, which accounts for the enhanced stability of the Ti17C2 and Ti19C10 clusters.

Figure 3.

Figure 3

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Electronic structure analysis. Partial densities of states (PDOS) and selected α-orbitals of the (A) Ti17C2 and (B) Ti19C10 clusters. The light red area corresponds to the total density of states (TDOS), while the red, green, blue, and orange curves indicate the PDOS of C2s, C2p, Ti3d, and Ti4s, respectively. Iso values are set to 0.02 unless otherwise stated.

Figure 4A,B shows the chemical bonding and local aromaticity in the Ti17C2 and Ti19C10 clusters by adaptive natural density partitioning (AdNDP)63 analysis (details in Figures S24 and S25). Electron delocalization in Ti17C2 results in remarkable multicenter bonds. There are 24 3c-1e bonds and 8 4c-1e bonds in Ti17C2 embodying partially delocalized electrons for the Ti–Ti–Ti and Ti–Ti–Ti-C moieties, respectively, as well as two 6c-1e bonds with a π character corresponding to delocalized electrons within the Ti4C–Tic moieties. While the α and β electrons have similar chemical bonding patterns, the spin-polarized configuration of Ti17C2 (M = 3) results in five 19c-1e bonds for the α electrons but with only three 19c-1e bonds for the β electrons. AdNDP analysis of Ti19C10 also shows electron delocalization and multicenter bonds (Figure 4B), which concurs with its superatomic feature and local aromaticity.

Figure 4.

Figure 4

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Chemical bonding pattern and antiaromaticity. Multicenter bonding patterns of α-electrons in (A) Ti17C2 and (B) Ti19C10. ON refers to the occupation number. (C) LOL analysis of the π MO (molecular orbitals) of Ti17C2 and Ti19C10. The iso-chemical shielding surface (ICSS) with an external magnetic field along the ZZ axes (ICSS_ZZ) and an orientation averaged (ICSSiso) of (D) Ti17C2 and (E) Ti19C10. The isovalue is set to ±10 ppm. Blue: shielding; yellow: deshielding surfaces. (F) Color-filled map on the plane 1 Å above the center of the Ti4C unit of Ti17C2 ICSS_ZZ. The positive (negative) values of the color bar refer to the shielding (deshielding) regions.

The different geometric structures and dramatic NPA charges on these titanium clusters result in different electron cloud distributions. Figure 4C presents the localized-orbital locator (LOL) analysis of the molecular π orbitals of Ti17C2 and Ti19C10 clusters; the LOL of Ti14C13 is provided in Figure S26. Ti8C12 does not have comparable π orbitals. The LOL (based on the kinetic energy density) reflects electron-delocalized shell structures, which is consistent with the analysis based on the electron localization function (ELF, Figures S27 and S28). Additionally, the contour of deformation density contributions of the two strongest electron pairs by natural orbitals for chemical valence (NOCV) was also used to describe the formation of carbon bonding in the Ti17C2 and Ti19C10 clusters (details in Figures S29 and S30). It is shown that the alpha electrons act as a donation from the capped C to nearby Ti atoms, contributed by both the s and p orbitals of the two C atoms. The sum of the NOCV pairs shows a delocalized electron density concentration pertaining to 3c-2e Ti–Ti–C bonds in the two clusters. This contrasts with the dominant interaction of π back-donation from Ti to the C=C fragment in Ti8C12 as well as σ donation from Ti to C in Ti14C13 (Figures S31 and S32, and Tables S9–S13).

Apart from the different electronic configurations and bonding nature, the superatomic Ti17C2 and Ti19C10 clusters exhibit magnetic responses. Figure 4D,E displays the patterns of the iso-chemical shielding surface (ICSS), which is known as a real space function extending the nucleus-independent chemical shifts (NICS) to three-dimensional space (details in Figure S33). The distinctive deshielding patterns for the three specific orientations of the applied field (e.g., ICSS_ZZ) indicate an overall antiaromatic surface of Ti17C2 at the ground state.64,65 Therefore, the Ti17C2 cluster is overall antiaromatic but with significant local aromaticity.66,67 This result is consistent with fullerene C60, which was also found to be antiaromatic, consisting of 20 hexagonal units of local aromaticity.68−70 To verify the local aromaticity and overall antiaromaticity of the Ti17C2 cluster, Figure 4F shows a color-filled map of the ICSS_ZZ on the plane of 1 Å above the center of a Ti4C unit. There are four shielding areas (in red) corresponding to four 3c-2e Ti–Ti–C bonds, surrounded by a square deshielding area (in yellow) outside of the unit (Figure S34). The Ti19C10 cluster exhibits a similar feature for the ten Ti4C units (Figure S35), with comparable deshielding responses. The local aromaticity and overall antiaromaticity are caused by the unique structures of the Ti17C2 and Ti19C10 clusters and, in return, contribute to their chemical stability.

Interestingly, both Ti17C2 and Ti19C10 contain carbon-capped Ti4C moieties, indicating that reactant C2H2 is subject to both dehydrogenation and C–C bond dissociation on the pure titanium clusters. In view of this, we performed calculations on the reaction dynamics of Ti17 with C2H2, as shown in Figure 5 (more details in Figures S36–S38). Notably, C2H2 adsorption on the Ti17 cluster results in a large energy decrease of 2.45 eV, which facilitates C–H bond dissociation and H atom transfer (HAT) to the adjacent hollow site by overcoming a transition state of the 0.79 eV energy barrier. Following that process, two likely pathways were considered: one path (blue curve) corresponding to an initial C–C bond dissociation and another path (in green) undergoing a second H atom transfer. The energy profiles indicated that the two pathways could be competitive during the reaction process. After H2 release, intermediate I7 causes further C–C bond cleavage and separates the two C atoms on the neighboring Ti sites, along with spin crossing from the singlet to the triplet state. It is worth mentioning that the energy cost for H2 release is much smaller than that for cluster dissociation or C2/TiC2 removal (Figure S37).

Figure 5.

Figure 5

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(a) Reaction pathways for C2H2 dehydrogenation and dissociation on Ti17. The red line shows the reaction pathway for C atom transfer through the inner space of an I7 intermediate to the final D4d Ti17C2. All energies are corrected with zero-point vibrations. Blue arrows present the direction of the vibrational frequency at the transition states. (b) Optimized adsorption structures and relative binding energies of a H2 molecule on different sites of Ti17C2. (c/d) 16 H2 and 28 H2 molecules adsorbed on the Ti17C2, with hydrogen storage gravimetric density shown by wt %. (e,f) Isosurface pattern of charge density difference for 16 H2 and 28 H2 molecules adsorbed on the Ti17C2 cluster; QH values refer to the total charge on the H atoms.

To form the final product of D4d Ti17C2, one of the two carbon atoms struggles to cross the interior space of the cluster through three steps, with surmountable energy barriers of 0.20, 0.85, and 0.10 eV. Notably, the entire reaction path involving HAT and carbon atom transfer (CAT) is significantly exothermic, which agrees with the experimental observation of a prominent Ti17C2 product. As a comparison, we also calculated the favorable paths for hydrogen atom transfer on the Ti8 cluster (Figures S39 and S40). The dehydrogenation of C2H2 on Ti8 was found to be comparable to that on Ti17; however, C–C dissociation on the Ti8 cluster suffers from a larger activation energy barrier. This result is consistent with the experimental observation of a tendency to form Ti8C12 with a structure of six C2 units. It is worth mentioning that the reaction and dehydrogenation on the Ti8 cluster can be accelerated by multiple C2H2 molecules (Figures S41 and S42); in comparison, the formation of highly stable Ti17C2 is not conducive to subsequent reactions.

To fully unveil the chemistry of this new class of titanium carbides, we have estimated the hydrogen storage capability typically for Ti17C2 (Figure 5b–d). A comparison of the other TMCs and a modeling TiC surface is given in Figures S43–S46 and Table S14. Notably, the TMCs exhibit larger H2-binding energy than the TiC surface, and for all of them, the H2 molecules prefer to adsorb on the Ti atoms rather than on the C atoms. The adsorption of 16 H2 molecules was tested for Ti17C2 (Figure 5c). The average hydrogen binding energy, which is a general criterion to evaluate the hydrogen uptake capability, was calculated to be −0.24 eV for 16 H2 molecules adsorbed on each shell-Ti atom of the Ti17C2 cluster. Bader charge analysis showed that the electron cloud density of the adsorbed H2 molecules is polarized, and the optimized Ti–H bond lengths range from 2.10 to 2.17 Å, showing monolayer stable 16 H2 adsorption (Figure 5e). Interestingly, Ti17C2 shows comparable average H2 adsorption energy with Ti8C12 and they both allow for adsorption of 16 undissociated H2 molecules (Figure S46),71 but there is a difference. While the C atoms in Ti17C2 also do not adsorb H2, the triangular Ti3 facets allow for H2 dissociation72,73 and H atom adsorption (up to 24H), indicating a total intake of 28H2 with a mass hydrogen storage density up to 6.3%, as shown in Figure 5d/f. The additive effect of both physisorption and chemisorption is important for H2 storage.74

Conclusions

To summarize, a comprehensive study of titanium carbide clusters was performed by using a self-developed DUV-TOFMS instrument. We observed prominent mass abundances of Ti8C12 and Ti14C13 via laser ablation of a TiC target followed by a reaction with C2H2; meanwhile, by reacting pure Tin (n = 1–38) clusters with C2H2, we produced two titanium cluster carbides of Ti17C2 and Ti19C10, which display prominent mass abundances. Using a global-minimum structure search and detailed theoretical calculations, we illustrate the structural stability of the four titanium carbide clusters, including the fcc-structured Ti14C13, Met-Cars Ti8C12, lantern-structured D4d Ti17C2, and rod-like D5h Ti19C10. Interestingly, both Ti17C2 and Ti19C10 exhibit superatomic features and delocalized electrons in the Ti4C units pertaining to carbon-doped superatoms, showing overall antiaromaticity with local σ- and π-aromatic features. Also, we demonstrate the reaction dynamics for C2H2 dehydrogenation and dissociation on Ti17. It is found that both hydrogen atoms and carbon atoms can migrate on the metal cluster to achieve proper sites for H2 evolution. This study not only provides a strategy for controllable preparation of stable titanium carbide clusters, from Met-Cars Ti8C12 to cubic Ti14C13 and to superatomic Ti17C2 and Ti19C10, but also expands the scope of titanium carbides with new structures and properties for new materials.

Materials and Methods

Experimental Section

The experiments were carried out by utilizing a self-developed reflection time-of-flight mass spectrometer (Re-TOFMS) combined with a 177.3 nm deep-ultraviolet laser vaporization (LaVa) cluster source. The Tin and TinCm clusters were generated by laser ablation of a titanium disk (99.999%) and a TiC disk (99.9%), respectively. The buffer gas (He, 99.999%) was controlled by a pulsed general valve, and another pulsed general valve was used to control the reaction gas flow (10% C2H2 seeded in He) with an on-time duration of 150–235 μs per period. A set of electrode plates was designed downstream of the reaction tube to filter out the ions for the detection of neutral clusters. The neutral clusters and reaction products were then collimated into another high-vacuum (<10–5 Pa) chamber through a skimmer (Φ= 2 mm). The neutral clusters were ionized by a 177.3 nm deep-ultraviolet laser and analyzed with a dual microchannel plate (MCP) detector.

Computational

The energy-minimum structures of the neutral Ti16–20, Ti8C12, Ti14C13, Ti17C2, and Ti19C10 clusters were determined by global minima search using the ab initio evolutionary algorithm USPEX (Universal Structure Predictor: Evolutionary Xtallography)52 combined with the Vienna ab initio simulation package (VASP) software.53 Meanwhile, structures of the other clusters are optimized based on the previously reported studies.75,76 All the isomers with lower energies were optimized using the Gaussian 16 program to confirm the global minimum structures.77 The molecular orbitals and LOL patterns were calculated by using Beck’s three-parameter hybrid exchange functional with the Lee–Yang–Parr correlation functional (denoted B3LYP). The NPA charge, AdNDP, ETS-NOCV, LOL, and ICSS analyses were carried out using the Multiwfn program.78 The VMD79 and VESTA80 softwares were used to create the figures.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 92261113 and 22272180), CAS Project for Young Scientists in Basic Research (Grant No. YSBR-050), CAS Key Research Program of Frontier Sciences (QYZDBSSW-SLH024), and Beijing Natural Science Foundation (Grant No. 2232035).

Glossary

ABBREVIATIONS

Met-Cars

metallo-carbohedrenes

LaVa

laser vaporization

HOMO

highest occupied molecular orbital

LUMO

lowest unoccupied molecular orbital

NPA

natural charge population

PDOS

projected densities of states

AdNDP

adaptive natural density partitioning

VASP

Vienna ab initio simulation package

USPEX

universal structure predictor: evolutionary crystallography

VMD

visual molecular dynamics

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c01068.

  • Details of additional experimental and theoretical methods and results (mass spectra, structure optimization, bond orders, frontier orbitals, AdNDP analysis, ELF and ETS-NOCV analysis, aromaticity and antiaromaticity analysis, reaction mechanisms and hydrogen storage calculations) (PDF)

Author Contributions

This manuscript was written through contributions of all authors.

The authors declare no competing financial interest.

Supplementary Material

ja4c01068_si_001.pdf (12.9MB, pdf)

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