RESUMEN
Carbon-doped aluminum cluster anions, AlnC- (n = 6-15), were generated by laser vaporization and investigated by mass-selected anion photoelectron spectroscopy. The geometric structures of AlnC- (n = 6-15) anions were determined by the comparison of theoretical calculations with the experimental results. It is found that the most stable structure of Al6C- is a carbon endohedral triangular prism. The Al7C- anion is a magic cluster with high stability. The structures of Al7-9C- can be viewed as the additional aluminum atoms attached around the triangular prism Al6C-. Two isomers of Al10C- have been detected in the experiments. The most stable one has a planar tetracoordinate carbon structure. The second one derives from Al9C- with the carbon atom located in a pentagonal bipyramid. The Al11C- anion has a bilayer structure composed of one planar tetracoordinate carbon and one aluminum-centered hexagon, in which the major interactions between two layers are multicenter bonds. The structures of Al12-14C- can be viewed as evolving from Al11C- by adding aluminum atoms to interact with the carbon atom. In Al15C-, the carbon atom stays at the surface with a tetracoordinate structure, and an icosahedral Al13 unit can be identified as a part of the geometric structure of Al15C-.
RESUMEN
The structures and electronic properties of LaSin- (n = 2-6) anions and their neutral counterparts were investigated by anion photoelectron spectroscopy and theoretical calculations. The vertical detachment energies of the most stable structures of LaSin- (n = 2-6) were measured to be 1.28, 1.58, 2.30, 2.05, and 2.91 eV, respectively. The lowest-energy isomer of LaSi2- is an isosceles triangle with a C2v symmetry. For LaSi3-6- clusters, the most stable isomers are polyhedrons with La atom face-capping the Sin frameworks. The lowest-energy structures of neutral LaSi2,4,5 clusters are similar to their anionic counterparts. The most stable isomer of neutral LaSi3 is a planar structure with C2v symmetry, which is different from the triangular pyramid structure of LaSi3- anion. The lowest-energy isomer of LaSi6- is a C5v symmetric pentagonal bipyramid structure, while for neutral LaSi6 cluster, the C5v structure is not the most stable one. The natural population analysis showed that there is electron transfer from La atoms to Si atoms in LaSin-/0 (n = 2-6). The ZZ tensor component in isochemical shielding surfaces and the anisotropy of the induced current density analyses indicate that the most stable isomer of LaSi6- has aromaticity.
RESUMEN
The resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra of 3-fluoro-5-methylanisole (3F5MA) were recorded to explore the conformers arising from the rotation of meta-methyl group (m-CH3) and methoxy group (OCH3), namely the staggered (s)/eclipsed (e)-cis/trans 3F5MA. The theoretical calculations predicted that the stable conformer of cis 3F5MA is staggered in the S0 and S1 states, but eclipsed in the D0 state. While for trans 3F5MA, the staggered conformer is stable only in the S1 state and the eclipsed one is stable in the S0 and D0 states. The first electronic excitation energies (E1s) of cis and trans 3F5MA were determined to be 36,709⯱â¯3 and 36,615⯱â¯3â¯cm-1 by the R2PI spectroscopy. Correspondingly, by the MATI spectroscopy, the adiabatic ionization energies (IEs) were measured to be 66,908⯱â¯5 and 66,692⯱â¯5â¯cm-1. Compared with the cis 3F5MA, more low-frequency vibronic bands assigned to m-CH3 torsions are observed for the trans conformer in the R2PI spectrum, supporting the theoretically predicated e-transâ¯ââ¯s-trans isomerization upon excitation. The MATI spectra of the cis and trans conformers are similar. Most of the observed cationic bands are related to the m-CH3 torsion modes, revealing the s-cisâ¯ââ¯e-cis and s-transâ¯ââ¯e-trans isomerizations upon the D0â¯ââ¯S1 ionization. By comparing with several analogues, it is found that the s/e conformational preference in each electronic state is mainly influenced by OCH3 group instead of the F atom. The combined effects of meta-substituents on molecular conformation and transition energies are discussed in detail. A kind of additivity of meta-substituent effects on s/e preference is qualitatively true for the S0 and S1 states but false for the D0 state. This is different from that of ortho-substituent effect, which is previously reported that the additivity of ortho-substituent effects on methyl rotation barriers in 2-fluoro-6-chlorotoluene is applicable to all the three electronic states while the additivity of meta-substituent effect on cis/trans preference is found to be true in all three electronic states of 3F5MA.
RESUMEN
Multiple approaches reveal the strong effects of a positive charge introduced by protonation or ionization on the conformation of o-heteroaromatic ethers and amides. The ethers and amides containing an ortho-N heteroatom are syn-preferring while those containing an ortho-O or ortho-S heteroatom are mostly anti-preferring. However, for all the monocyclic o-heteroaromatic ethers and amides, the protonated ones are all anti-preferring while the ionized ones are all syn-preferring. Interestingly, although both the protonation and ionization introduce a positive charge, they have such different effects on molecular conformation, very informative for understanding the origin of conformational preferences. Detailed analysis shows that the population of the introduced positive charge dictates the conformational preferences via electrostatic and orbital interactions. Compared to ortho-heteroatoms, meta-heteroatoms have weaker effect on conformational preference. Achieved by complete inductive method, the regularity of conformational preferences and switching provides easy ways to modulate conformers (by pH or redox), and makes this kind of ether or amide bond a conformational hinge applicable to design of functional molecules (drugs and materials) and modulation of molecular biological processes.
RESUMEN
Conformers from the rotations of the methyl group and the methoxy or methylamino group, namely staggered (s)/eclipsed (e)-cis/trans 2-methoxypyridine (2MOP) and 2-N-methylaminopyridine (2NMP), are studied using theoretical calculations in combination with one-color resonance-enhanced two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopies. The calculations predict that, for cis 2MOP, trans 2MOP and trans 2NMP, only the s conformers are stable in the S0, S1 and D0 states. However, for cis 2NMP, the stable conformer is staggered in the S0 state but eclipsed in the S1 and D0 states, indicating an isomerization upon the excitation or ionization from the S0 state. This is experimentally supported by the 1C-R2PI and MATI spectra of 2NMP. Due to the relative instability, the number density of trans 2MOP is too low in the sample to be detected. All the bands in the 1C-R2PI and MATI spectra of 2MOP are assigned to s-cis 2MOP. The energy differences between cis and trans conformers are derived from excitation and ionization energies, indicating another conformational isomerization: stable trans 2NMP in the S0 and S1 states but stable cis 2NMP in the D0 state. For 2MOP, the so-called syn preference previously found for the S0 state is also observed in the S1 and D0 states. The conformational preference and isomerization are discussed with natural bond orbital calculations and reduced density gradient analysis. For 2MOP, the syn preferences are mainly caused by the exchange repulsion among several σ-orbitals of the OCH3 group and the pyridine ring. While the relative stabilities of the s and e conformers of cis 2MOP and cis 2NMP are simultaneously influenced by steric repulsion and orbital interactions.