# Last edited on 2014-10-13 15:18:20 by stolfilocal (), ''[ .]'' volume , pages . {{doi|}} ---------------------------------------------------------------------- # From http://pubs.acs.org/doi/pdf/10.1021/jp0357976 > > Hua Hou, James T. Muckerman, Ping Liu, and José A. > Rodriguez (2003), ''[http://pubs.acs.org/doi/pdf/10.1021/jp0357976 > computational study of the geometry and properties of the metcars > {{chem|Ti|8|C|12}} and {{chem|Mo|8|C|12}}.]'' Journal of Physical > Chemistry, series A, volume 107, pages 9344--9356. > {{doi|10.1021/jp0357976}} > > Huo and others investigated {{chem|Ti|8|C|12}} and {{chem|Mo|8|C|12}}, > as well as ions like {{chem|Ti|8|C|12|4+}} and {{chem|Ti|8|C|12|2+}}, > in 2003. Their computations indicated slight displacement of 2 of the > C-C pairs in {{chem|Ti|8|C|12}} to give D_{2d} symmetry, > instead of T_d. They proposed that the four outer > titanium atoms are in the formal +1 (I) state, the four inner ones in > +2 (II) state, with the carbon atoms negatively charged. They > predicted strong reactions with {{chem|H|2|O}} and atomic chlorine. > They found that the cluster could bind to 4 carbonyls (at the outer > titanium only. Similar results were obtained for > {{chem|Mo|8|C|12}}. > > --- > "The Mo 8 C 12 + ion was determined to be a magic number cluster by Pilgrim and Duncan, [32] and the neutral Mo 8 C 12 cluster has also recently been observed experimentally to be a magic number species. [33]" > > (32) Pilgrim, J. S.; Duncan, M. A. J. Am. Chem. Soc. 1993, 115, 6958. (33) Lightstone, J. M.; Mann, H.; Wu, M.; Johnson, P. M.; White, M. G. J. Phys. Chem. B 2003, 107, 10359. ---------------------------------------------------------------------- # From http://pubs.acs.org/doi/pdf/10.1021/jp045460j > Ping Liu, José A. Rodriguez, and James T. Muckerman > (2004), ''[http://pubs.acs.org/doi/pdf/10.1021/jp045460j The > {{chem|Ti|8|C|12}} metcar: A new model catalyst for > hydrodesulfurization.]'' Journal of Physical Chemistry, series B, volume > 108, pages 18796--18798. {{doi|10.1021/jp045460j}} > > define o-Ti, i-Ti > > Investigated theoretically the use of {{chem|Ti|8|C|12}} as a catalyst > for the decomposition of [[thiophene]] {{chem|C|4|H|5|S}} by three > [[hydrogen]] molecules to [[2-butene]] {{chem|C|4|H|8}} and [[hydrogen > dysulfide]] {{chem|H|2|S}}, an important step in the removal of > [[sulfur]] from [[petroleum|oil]]. They predicted that the first > {{chem|H|2}} molecule would spontaneously dissociate in contact with the > {{chem|C|2}} pairs, and each H atom would then migrate to the adjacent > outer titanium atom. The thiophene would then react [[exothermal]]ly > with each H atom in turn, yielding a [[butadiene]] attached to an o-Ti > and the sulfur atom attached at the nearby i-Ti atom. A second > {{chem|H|2}} molecule would then dissociate at the o-Ti site and turn > butadiene into 2-butene. A third {{chem|H|2}} would dissociate at an > o-Ti site, and the two atoms would migrate to the i-Ti atom bearing the > sulfur atom, and convert it into {{chem|H|2|S}}. > ---------------------------------------------------------------------- # from http://www.sciencedirect.com/science/article/pii/S0009261404014459 > J. I. Martínez, A. Castro, A. Rubio, J. M. Poblet, > J. A. Alonso (2004), > ''[http://www.sciencedirect.com/science/article/pii/S0009261404014459 > Calculation of the optical spectrum of the {{chem|Ti|8|C|12}} and > {{chem|V|8|C|12}} met-cars.]'' Chemical Physics Letters, volume 398, > issues 4--6, pages 292–296. {{doi|10.1016/j.cplett.2004.09.058}} > > Martínez and others computed the optica absorption spectrum of > {{chem|Ti|8|C|12}} and {{chem|V|8|C|12}}. They predicted a broad > spectrum for both, with high absorption starting at about 8 eV and > centered around 12–14 eV. > > ---- > > "Some exciting properties of these clusters are, in addition to the > mentioned stability and symmetry, a relatively low ionization potential, > a delayed ionization signal [3], and a potentially interesting magnetic > behaviour due to the presence of transition metal elements. In > particular, a very appealing feature in this kind of clusters is the > important role that the d electrons of the transition metal atoms play > in the stabilization of the cluster. The Met-Cars are expected to > exhibit rich chemical and physical properties that may find applications > in electronics and catalysis. A detailed review on the subject may be > found [4]." > > > [4] M.M. Rohmer, M.S. Bénard, J.M. Poblet Chem. Rev., 100 (2000), p. 495 ---------------------------------------------------------------------- # From http://pubs.acs.org/doi/abs/10.1021/cr9803885 > Marie-Madeleine Rohmer, Marc Bénard, and Josep-M. > Poblet (2000), ''[http://pubs.acs.org/doi/abs/10.1021/cr9803885 > Structure, reactivity, and growth pathways of metallocarbohedrenes > {{chem|M|8|C|12}} and transition metal/carbon clusters and > nanocrystals:  A challenge to computational chemistry.]'' Chemical > Reviews, volume 100, issue 2, pages 495–542. > {{doi|10.1021/cr9803885}} > > -------- > "The dis-covery, also by Wang, that the met-car peaks are far > from magic and even, under certain experimental conditions, completely > absent from the mass spectrum of titanium carbide anions[33,34]" > > "exceedingly stable, they have resisted until now all attempts of purification, either > in solution or as a solid material. Production in > macroscopic quantity in the soots generated in an arc > between two composite Ti-C electrodes has been > reported however, first, by Castleman’s group[36] and, > very recently, by Selvan and Pradeep.[37] In both > reports, the presence of met-cars in the soots could > be established either from the mass spectrum of laser > desorbed soot or from the mass spectrum of thermally > vaporized material from soot exposed to organic > solvents. Even though the met-car abundance in > some samples was estimated to ∼1%,[36] the clusters > were found extremely air-sensitive and undergo > complete degradation within a few minutes.[37] " > > > "For met-cars made of group 4 metals, the > electronic state of lowest energy is a high-spin > configuration (9Ag ) obtained by distributing the eight > metal valence electrons (an oxidation state of +3 > being assumed) into the eight combinations of radi- > ally outpointing orbitals with appropriate symmetry > (Figure 3)." "observation of association products be- tween met-car cations and polar, σ-donor molecules (NH3,[3,7a,59,60] H2O,59,60 methanol,[59-61] acetone,[62,63]...)." > "Rather than considering a limited > distortion of the dodecahedral cage, Ceulemans and > Fowler proposed an alternative structure composed > of a fully conjugated ring of 12 carbons atoms capped > by two Ti4 tetrahedra.[64]" > > "Let us note that Pauling’s “cubic” > architecture (Figure 6) is not topologically distinct > from Castleman’s pentagonal dodecahedron and was > never singled out as an energy minimum associated" > > "Khan relied on Zerner’s ZINDO UHF method[79] to > optimize for Ti8C12 a series of structures (“metal- > decorated cages” or MDC) based upon the encapsula- > tion of an icosahedral C12 [not icosahedral but cuboctahedral] cluster in a metal cage > shaped either as a distorted cube or as a bicapped > trigonal antiprism (Figure 7).[80]" > "one > reported by Dance in his first communication on the > geometric and electronic structure of Ti8C12 and > summarized in Figure 10.[13]" > > "The tetracapped tetrahedron (Td) was found more > stable than the pentagonal dodecahedron (Th) by a > considerable amount: 351 kcal‚mol-1 at the LDA > level,13 slightly reduced to ∼ 300 kcal‚mol-1 for the > neutral as for the cationic cluster when the nonlocal > corrections are accounted for (Table 1).[27]" > > "The geometries of the seven isomers of Ti8C12 [ with the C2 clusters > aligned with all possible combinations of cube diagonals ] have been > optimized at the ab initio RHF level and their energies compared at the > RHF level[15] and then at the CI level[90] by Rohmer et al." > > "Chen et al.[49] reported a work independent from > that of Dance[13] and leading to very similar conclu- > sions, except that the isomer found to be much more > stable than the pentagonal dodecahedron was the one > with D2d symmetry. This may not be surprising if it > is remembered that the D2d form was found second > lowest in energy, just after the Td isomer, from the > HF/CI calculations of Figure 12. [ .. ] For Ti8C12, the energy > difference between the D2d and the Th isomers is 314 > kcal/mol at the LDA level,[49] not so different from > the 351 kcal/mol stabilization claimed by Dance for > the Td form.[14] Moreover, it was proved[49] that the Th > dodecahedron structure is energetically unstable > against a Jahn-Teller distortion leading to the D2d > form, as it is with respect to the Td isomer.[88] It must > be noted that the enhanced stability of the Td or the > D2d form of Ti8C12 makes the formation of those > isomers exothermic with respect to metal carbide and > graphite." > "The D2d isomer of Chen et al. has been reinvesti- > gated in subsequent DFT studies and compared to > the Th form with similar conclusions.[42,45,49,67] The > report by Lou and Nordlander[67] is particularly in- > teresting since it is the only one to compare the three > isomers Th, T d, and D2d with the same DFT formalism > (LDA using von Barth-Hedin exchange-correlation > potential[70]). The metal-metal distances in the D2d > form (2.79 and 2.84 Å) are found shorter by ∼ 0.10 Å > than in the Td isomer and by ∼0.20 Å with respect > to the dodecahedral form, [ ... ] question of the existence of metal-metal bonds.[92] However, > the Td form is found more stable than the D2d one by > ∼ 70 kcal‚mol-1 (0.15 eV/atom, Table 1).[67] Those > orbital diagrams suggest a still enhanced stability for > the cation or for the dication of tetrahedral Ti8C12." > "Finally, zinc is the only metal of the first row > for which the dodecahedral conformation of M8C12 > becomes lowest in energy (Figure 25)." > "this technique was used to produce a series of > Zr-substituted clusters with stoichiometries Ti8-x- > ZrxC12. Those clusters could be characterized by a > mass spectrometer as a series of peaks with a regular > evolution of the peak intensities from x ) 0 to x ) 5. > This regularity, particularly visible under 4:1 Ti:Zr > molar ratio conditions (Figure 26),[111] has been inter- > preted as arising from a purely statistical substitu- > tion of Ti by Zr.[110,111] This represents a strong > argument in favor of a structure with equivalent > metal sites. As a matter of fact, one could reasonably > expect that, in the tetracapped tetrahedron structure, > zirconium has a higher affinity for a specific metal > site, thus endowing the stoichiometry Ti4Zr4C12 with > a particular stability. The peaks of Figure 26, which > do not support this conjecture, have probably delayed > by several years the recognition of the Td form as the > most probable conformational isomer for met-cars.[110,111]" > "For the specific case of Ti4Zr4C12, localizing the four zirconium > atoms on thn rather than on THN resulted in a global energy change of > 0.5 kcal‚mol-1, which explains why the distribution of the > substitution sites for that stoichi- ometry does not appreciably > deviate from the statis- tical one." > "Ti8C12 was therefore considered as a metallic > sphere with 80 delocalized valence electrons, subject > to the spherically averaged potential created by the > tetravalent cores of titanium and carbon atoms. With > this approximation, the static polarizability, obtained > for ω ) 0, was computed to be of the same order of > magnitude as for C60, with a dominant contribution > coming from the 20 π electrons." "The earliest investiga- tions carried out in Castleman’s group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas.[59] " " Byun, Freiser and co-workers have investigated the reactivity of V8C12+, Nb8C12+, and M-C nanocrystals.[60,129,131,162] " "Ti8C12+ is very reactive toward polar molecules such as CH3OH, H2O, and NH3 (or ND3). The mass spectrum of the products arising from the reaction is composed of eight peaks corresponding to the association products Ti8C12+(P)n, 1 e n e 8.59 An increase of the reactant pressure just displaces the relative peak heights toward higher values of n. No additional peak is observed. It can be easily deduced from such a spectrum that the reaction proceeds through multistep attachment of the reactant to each metal center: Ti8C12+ + (P)n-1 + P --> Ti8C12+(P)n" "Stepwise association reactions are also observed between Ti8C12+ and molecules which do not have permanent dipole moment but do have a π-bonding system such as benzene and ethylene. At variance with polar molecules, no more than four such π-bond- ed molecules can be attached to Ti8C12+.[59,61]" "Association reactions have also been observed between Ti8C12+ and pyridine terminating at Ti8C12+- (pyridine)4.[61] With acetone, five adducts are observed, namely Ti8C12+ (acetone)1-5.[62,63]" "Adsorption of methane molecules on neutral Ti8C12 has been observed at low temperature.[132] The characterization by a mass spectrometer of those low- energy adsorbates resulting from charge induced dipole interactions rules out the hypothesis proposed by Brock and Duncan of a production of Ti8C12 by fragmentation of larger neutral metal-carbon species upon photoionization.[22] The energy involved in such a fragmentation process would then yield a desorp- tion of the methane molecules. The high stability of neutral Ti8C12 is thus inferred from its presence in large amount in the cluster beam.[132]" "The partial or complete replacement of Ti by Nb strongly modifies the reactivity of the met-car cage, especially with respect to acetone. Ti7NbC12+ and Nb8C12+ react with acetone to give adducts with one and two oxygen atoms, thus implying that niobium- containing met-cars can induce carbon-oxygen bond- breaking.60,61 Ti7NbC12+ and Nb8C12+ as well as their oxygen adducts can undergo further associations with a limited number of acetone molecules (4 for Ti7Nb- C12+; 2 for Nb8C12+, Nb8C12+O, Ti7NbC12+O, and Ti7- NbC12+O2; 1 for Nb8C12+O2).60,61" "The only abstraction reaction characterized to date with Ti8C12+ occurs with methyl iodide and involves the breaking of the I-C bond to give the monoadduct Ti8C12+I: + +Ti8C12 + CH3I f Ti8C12 (I) + CH362 (10) The monoiodine adduct can further react with polar molecules such as methanol to give the distribution Ti8C12+(I)(CH3OH)1-7. [61]" "Ti8C12+ seems to be inert with respect to molecules having neither a permanent dipole moment nor a π-bonding system, such as oxygen and methane.59 However, neutral metal-carbon species produced in a laser vaporization source rearrange in the presence of oxygen to yield selectively and exclusively Ti8- C12+.133,134 " "At variance with Ti8C12+, V8C12+ reacts with oxygen to generate first V8C10+. That cluster itself reacts with oxygen to give additional metal-carbon clusters and oxidation products.129 The difference in reactivity between Ti8C12+ and V8C12+ " "The formation of the Ti8C12- (Cl)4 ligand is largely exothermic (186 kcal‚mol-1) with respect to the neutral fragments Ti8C12 + 4Cl" Relation to 3x3x3 and 3x3x4 Fcc clusters of Ti,C alternating (limiting TiC formula) "Laser-induced photodissociation of 3 × 3 × 4 and 3 × 3 × 3 nanocrystals of titanium have been reported by Pilgrim and Duncan.25,109 The photodis- sociation of Ti17C19+ generated the mass spectrum reproduced in Figure 36a where the most intense peaks are assigned to the metallocarbohedrene Ti8C12+ +and to the fcc crystallite Ti14C13 . Results obtained from the photodissociation of this latter nanocrystal are more surprising, since the most prominent dis- sociation peak does not correspond to the expected met-car cage but to Ti8C13+ (Figure 36b)." " A met-car cage is formed, whereas the central carbon remains trapped inside, yielding the encapsulation complex (C⊂Ti8C12)+ .25 V14C13+ has the same dissociation pattern, but Zr14C13+ does not seem to display any endohedral structure in its dissociation spectrum. The mass spectrum obtained by Yu et al.144 from laser-induced plasma reaction of titanium vapor with dehydrogenated CH4 shows around the domi- nant peak corresponding to Ti8C12+ two satellite peaks respectively assigned to the endohedral met- cars (C⊂Ti8C12)+ and (C2⊂Ti8C12)+. " "The original report by Castleman’s group displaying the exceptional abun- dance of Ti8C12+ (Figure 1) was obtained by detecting only the cationic species that were directly produced in the source.3 All subsequent experiments carried out on the group of cationic M/C clusters confirmed the magic, or supermagic, character of the met-car peak. Selecting the neutral species produced a dif- ferent mass spectrum with enhanced contributions from small non-met-car clusters. The mass spectra of neutral Ti/C and V/C clusters obtained by Wei et al. at various fixed laser wavelengths show that, besides M 8C12, the peaks corresponding to (1,2), (2,4), (3,6), (4,8), (5,10), (6,12), and (7,13) are local maxima for TinCm and VnCm.7" "Wang et al. applied the standard techniques de- scribed by Castleman et al. to produce and detect metal carbon cluster anions.126 The mass spectra showing the distribution of the titanium carbon cluster anions obtained either from the vaporization of pure metal with a CH4-seeded helium carrier gas or from the vaporization of a solid TiC target in pure helium are reproduced in Figure 37 and compared to the well-known mass spectra obtained " "Figure 37b shows that the expected met-car anion Ti8C12- is not produced in the laser vaporization source when using a pure metal sample. Ti8C12- is produced in significant amount by laser vaporization of solid TiC but appears in the spectrum as a totally nonmagic peak.126 " "There is indeed no mathematical evidence that the structure of tetracapped tetrahedron, lowest in en- ergy among the above 15, represents the global minimum." "Note Added in Proof. In a quite recent report, the quantum chemical investigations on the structure of E8C12 clusters have been extended to the carbides of main group elements with four valence electrons (E = Si, Ge, Sn). By means of ab initio RHF, MCSCF, MCQDPT2, and MP2 calculations, Bode and Gordon (J. Chem. Phys. 1999, 111, 8778-8784) show that the conformations preferred for the homologues of C20 are the dodecahedral form (for E = Si) and, for E = Sn and Ge, a new cage conformation with D2h symmetry and no more than 16 bonds between the E atoms and acetylenic dicarbons characterized by very short C-C bond lengths."