Antonio Largo

A theoretical study of the C2H, C2F and C2Cl radicals and their positive ions

Abstract An ab initio study at the Hartree-Fock level of the low-lying states of the C 2 H, C 2 F and C 2 Cl radicals and their positive ions has been carried out. It is found, in agreement with the experimental evidence, that the ethynyl radical has a 2 Σ + ground state. In the case of the ethynyl cation a 3 Π ground state is theoretically predicted. Although the lowest-lying state at the Hartree-Fock level for the C 2 F and C 2 Cl radicals is also 2 Σ + , inclusion of correlation effects favour a 2 Π state by a very small amount, whi predicted to be the ground state. The A 2 Σ + ←X 2 Π transition is predicted to have its origin at a very low frequency, less than 90 cm −1 for both C 2 F and C 2 Cl. In the cases of both C 2 F + and C 2 Cl + the 3 Π state also lies lowest at the Hartree-Fock l correlated calculations show a 3 Σ − state to be the ground state. Ionization potentials are also computed for the neutral molecules.

Small Carbon Clusters Doped with Vanadium Metal: A Density Functional Study of VCn (n = 1-8).

A theoretical study of the different isomers of neutral VCn (n = 1-8) clusters has been carried out. Predictions for their electronic energies, rotational constants, dipole moments, and vibrational frequencies have been made using the B3LYP method with different basis sets. For linear and cyclic VCn clusters, the lowest-lying states correspond to quartet states, with the only exception being VC, which has a (2)Δ ground state. In the fan-type structures, the electronic ground state is found to be a quartet state for even n values, whereas for odd n values, the ground state is a doublet for VC3 and VC5 and a quartet for VC7. From the incremental binding energies, we can deduce an even-odd parity effect, with n-even clusters being more stable than n-odd ones in the linear and fan clusters. It was also found that neutral VCn clusters with n ≤ 6 prefer fan structures over linear and cyclic isomers, whereas cyclic ground states are predicted for the clusters with n > 6.

Low-lying states of AlC2 and SiC2+: Competition between linear and cyclic configurations

Abstract An ab initio study has been carried out for the isoelectronic systems SiC 2 + and AlC 2 . In both cases a linear state is found to lie the lowest in energy at the Hartree-Fock level. However, correlation effects tend to favour cyclic structures. Although for SiC 2 + Moller-Plesset and configuration interaction calculations provide different energy orders, the cyclic state is likely to be the ground state. In the case of AlC 2 all correlated calculations show a cyclic structure to be the ground state. The electronic structure of the low-lying states of both systems is discussed, with special attention to the differences between Al and Si compounds.

Structure and Stability of Small ZnCn Clusters

A theoretical study of the small ZnCn (n = 1-8) clusters has been carried out at the B3LYP/6-311+G(d) level. Different molecular properties for open-chain and cyclic species have been calculated. The computed properties include electronic energies, vibrational frequencies, dipole moments, and rotational constants for individual species. In addition, the relative stability of the different clusters is also discussed in terms of the incremental binding energy. In the case of open-chain clusters, the lowest-lying states correspond to triplet states with the exception of ZnC2, whereas the electronic ground state is found to be a singlet state for cyclic clusters. The incremental binding energy graph shows a smooth even-odd parity effect in the incremental binding energy, n-even species being, in general, more stable than the adjacent odd-numbered ones. It was also found that the first members in the series, excepting ZnC2, prefer open-chain structures, whereas when n ≥ 6 cyclic ground states are favored.

GAS-PHASE SYNTHESIS OF PRECURSORS OF INTERSTELLAR GLYCINE: A COMPUTATIONAL STUDY OF THE REACTIONS OF ACETIC ACID WITH HYDROXYLAMINE AND ITS IONIZED AND PROTONATED DERIVATIVES

A computational study of the reactions of hydroxylamine and its ionized and protonated derivatives with acetic acid is provided. The reaction of neutral hydroxylamine with acetic acid, despite being clearly exothermic, involves a very large energy barrier. The reaction of ionized hydroxylamine with acetic acid is also clearly exothermic, but again a significant energy barrier is found (around 24 kcal mol–1 at the CCSD(T) level). The reaction of the most stable protonated isomer of hydroxylamine, NH3OH+, with acetic acid also involves a high barrier (more than 27 kcal mol–1 at the CCSD(T) level). Only the higher energy isomer, NH2OH+ 2, leads to a sensibly lower energy barrier (about 2.3 kcal mol–1 at the CCSD(T) level). Nevertheless, an estimate of the reaction coefficient at low temperatures such as those reigning in the interstellar medium gives very low values. Therefore, it seems that precursors of interstellar glycine could not be efficiently produced from the reactions of hydroxylamine-derived ions with acetic acid.

On the molecular structure of uranium dicarbide: T-shape versus linear isomers.

A theoretical study of the molecular structure of uranium dicarbide has been carried out employing DFT, coupled cluster, and multiconfigurational methods. A triangular species, corresponding to a (5)A(2) electronic state, has been found to be the most stable UC(2) species. A triplet linear CUC species, which has been observed in recent infrared spectroscopy experiments, lies much higher in energy. A topological analysis of the electronic density has also been carried out. The triangular species is shown to be in fact a T-shape structure with a U-C interaction which can be considered to be a closed-shell interaction.

Structure and bonding in third-row main group dicarbides C2X (X=K―Br)

The molecular structures of third-row main group dicarbides C(2)X (X=K-Br) have been studied by theoretical methods. It is found that K, Ca, and Ga favor C(2v)-symmetric (T-shape) ground states, whereas As, Se, and Br have linear or quasilinear ground states. In the case of germanium. a very flat potential energy surface is found and an L-shape structure seems to be the ground state. Dissociation energies into X+C(2) are relatively high. The main features of these compounds, in particular, the preference for linear or cyclic structures, have been rationalized in terms of the most relevant interactions between the third-row atom and dicarbon with the help of an energy decomposition analysis.

Theoretical study of AlC3: linear or cyclic ground state?

Abstract A theoretical study of the AlC 3 species has been carried out. There are three species that lie quite close in energy: a rhombic four-membered ring, 1 ( 2 A 1 ) , a T-shape structure, 2 ( 2 B 2 ) , and a linear species, 3 ( 2 Π ) . The linear isomer is predicted to be lower in energy, but 1 ( 2 A 1 ) lies only about 1–2 kcal mol −1 higher. In any case, it seems that both isomers could be accesible to experimental detection. Our conclusion is in contrast with a previous theoretical study on this system that predicted a quartet state as the global minimum.

Peptide bond formation through gas-phase reactions in the interstellar medium: formamide and acetamide as prototypes

A theoretical study of the reactions of with formaldehyde and with formamide is carried out. The viability of these gas-phase ion-molecule reactions as possible sources of formamide and acetamide under the conditions of interstellar medium is evaluated. We report a theoretical estimation of the reaction enthalpies and an analysis of their potential energy surfaces. Formation of protonated formamide from the reaction between ammonium cation and formaldehyde is an exothermic process, but all the channels located on the potential energy surface leading to this product present net activation energies. For the reaction between methanium and formamide, different products are possible from a thermodynamic point of view. An analysis of its potential energy surface showed that formation of protonated acetamide and amino acetaldehyde takes place through barrier-free paths. Therefore, this reaction could be a feasible source of acetamide and amino acetaldehyde in space.

Theoretical study of the C3Cl radical and its cation

Abstract A theoretical study of C 3 Cl and C 3 Cl + isomers has been carried out. The global minimum for C 3 Cl is a cyclic C 2V species (a three-membered ring with an exocyclic chlorine atom). However, a quasi-linear CCCCl structure is predicted to lie only 3-5 kcal mol −1 higher. This quasi-linear structure is floppy, since the linear arrangement lies only 2-3 kcal mol −1 higher in energy. The cyclic and open-chain isomers have dipole moments of 1.986 and 3.363 D, respectively. In C 3 Cl + the global minimum is a linear singlet species, the singlet cyclic isomer lying about 19 kcal mol −1 higher. The ionization potentials of cyclic and open-chain C 3 Cl are estimated to be 9.17 and 8.21 eV, respectively, suggesting that these species should be easily ionized if present in the interstellar medium.