Reinaldo Pis Diez

Density functional study of small molybdenum clusters

Small molybdenum clusters up to the tetramer are investigated within the framework of the density functional theory. Both the geometry and the spin state are optimized for the dimer, trimer, and the tetramer. Moreover, all those calculations are followed by a vibrational analysis to discriminate between real minima and saddle points on the potential energy surfaces. Several low-lying excited states are found to be stable after the vibrational analysis. Equilibrium geometries, electronic configurations, binding energies, magnetic moments, and harmonic frequencies of the stable conformers are reported.xa0© 1999 John Wiley & Sons, Inc. Int J Quant Chem 76: 105–112, 2000

Quantitative structure-property relationship analysis for the retention index of fragrance-like compounds on a polar stationary phase.

A quantitative structure-property relationship (QSPR) was developed for modeling the retention index of 1184 flavor and fragrance compounds measured using a Carbowax 20M glass capillary gas chromatography column. The 4885 molecular descriptors were calculated using Dragon software, and then were simultaneously analyzed through multivariable linear regression analysis using the replacement method (RM) variable subset selection technique. We proceeded in three steps, the first one by considering all descriptor blocks, the second one by excluding conformational descriptor blocks, and the last one by analyzing only 3D-descriptor families. The models were validated through an external test set of compounds. Cross-validation methods such as leave-one-out and leave-many-out were applied, together with Y-randomization and applicability domain analysis. The developed model was used to estimate the I of a set of 22 molecules. The results clearly suggest that 3D-descriptors do not offer relevant information for modeling the retention index, while a topological index such as the Randić-like index from reciprocal squared distance matrix has a high relevance for this purpose.

Observation of Zr22+, Cd22+, Hf22+, W22+, and Pt22+ in the gas phase

Five homonuclear diatomic dications Zr(2)(2+), Cd(2)(2+), Hf(2)(2+), W(2)(2+), and Pt(2)(2+) have been observed in the gas phase by mass spectrometry. These exotic doubly positively charged molecules were produced indirectly in the ion extraction region of a secondary ion mass spectrometer during sputtering of zirconium, cadmium, hafnium, tungsten, and platinum metal foils, respectively, by energetic high-current Ar(+) ion surface bombardment. They were detected in positive ion mass spectra at half-integer mz values for ion flight times of the order of approximately 10(-5) s. To our knowledge, these species had not been observed before. This experimental work confirms two theoretical investigations that had predicted that W(2)(2+) and Cd(2)(2+) are long-lived metastable species in the gas phase, but contradicts two theoretical studies that had suggested that Pt(2)(2+) should be unstable with respect to fragmentation. Therefore an advanced theoretical investigation of the ground state of Pt(2)(2+) was also performed. Our calculation shows that the ground state of Pt(2)(2+) is metastable with an internuclear equilibrium distance of 2.36 A, a dissociation energy (with respect to the top of the barrier) of 2.32 eV, and an ionization potential of Pt(2)(+) of about 15.8 eV. The latter theoretical result strongly suggests that Pt(2)(2+) dication formation in our experiment may have taken place via the resonant electron transfer process Pt(2)(+) + Ar(+) --> Pt(2)(2+) + Ar.

Density functional calculations of Ni5 and Ni6 clusters

Abstract Density functional calculations on the electronic structure and magnetic properties of Ni5 and Ni6 clusters are presented in this work. The geometry and spin state of clusters are optimized for several starting symmetries. Moreover, those calculations are followed by a vibrational analysis to discriminate between real minima and saddle-points on the potential energy surface of clusters. Equilibrium geometries, electronic configurations, binding energies, magnetic moments, and harmonic frequencies of stable Ni5 and Ni6 clusters are reported.

A density functional study of some physical properties of carnosine (N-β-alanyl-l-histidine)

Abstract Equilibrium geometries, harmonic vibrational frequencies, electronic transition energies, and NMR chemical shifts are calculated for different structures of carnosine (N-β-alanyl- l -histidine) as a further step in the physical characterization of that molecule. The calculations are performed using the generalized gradient approximation to the density functional theory. Solvent effects are implicitly accounted for. Although total energy results indicate that zwitterionic species are more stable than neutral ones, the comparison of calculated properties with experimental geometric parameters and chemical shifts seems to favor the neutral species. The calculated electronic transition energies and harmonic vibrational frequencies suggest that both the electronic and vibrational spectra could be used to determine the nature of the carnosine species under study.

Theoretical evidence of bound metastable states in the doubly ionized nickel dimer Ni22

Abstract Density functional theory (DFT) with generalized gradient corrections is used to study the electronic structure of Ni 2 2+ . We predict several bound metastable states, all of them having four unpaired electrons. Those states are stabilized against dissociation by a Coulomb barrier. The stability can be explained as due mainly to the interaction between the s electrons of the constituent Ni + cations, with d 8 s 1 configurations, that pair themselves in a bonding σ molecular orbital.

A density-functional study on the formation of Mo22+

The presence of metastable states in the doubly ionized molybdenum dimer is studied using gradient-corrected scalar-relativistic density-functional theory. Seventeen metastable states are found within an energy range of less than 6.5 eV. All those states show lifetimes large enough to assure experimental detection. The calculation of the second adiabatic ionization potential of the neutral molybdenum dimer seems to confirm that the doubly ionized dimer is produced by the electron-capture process Mo2++Ar+-->Mo2(2+)+Ar, in which the ionization potentials of Ar and Mo2+ play a crucial role [K. Franzreb, R. C. Sobers, Jr., J. Lorincik, and P. Williams, J. Chem. Phys. 120, 7983 (2004)]. Moreover, the present results indicate that other species having ionization potentials between 13.01 and 15.34 eV could be used as projectiles to produce Mo(2)2+. It is also shown that Xe+ ions could not react with Mo2+ to produce double ionized dimers. A simple thermodynamic argument is also proposed that seems to increase the possibilities of forming Mo2(2+) from Mo2+ by using Ar+ as projectile ions.

Electronic metastable bound states of Mn22+ and Co22+

Abstract The doubly ionized Mn 2 2+ and Co 2 2+ dimers are investigated using the density functional theory as implemented in the A msterdam D ensity F unctional and G aussian 98 packages. Several bound metastable states are found for both dimers. Bond distances, barrier heights, and vibrational frequencies are calculated. Both codes agree well to describe the properties of the dimers when they are characterized by a sparse density of lower-energy metastable states. But when the density of lower-energy states becomes denser, the agreement is acceptable only for the lowest states. Anharmonic frequencies and lifetimes are also calculated. The lifetimes are very large, so these metastable states could be experimentally detected.

The diatomic dication CuZn2+ in the gas phase

In the present combined experimental and theoretical study we report the observation of the novel gas-phase dication CuZn(2+) and provide some theoretical insight into the electronic binding of this exotic metastable molecule and its formation mechanism. Using mass spectrometry we have detected four isotopomer signals of CuZn(2+) at half-integer m/z values for ion flight times of about 14 μs. CuZn(2+) was unambiguously identified by its isotopic abundance. High-current energetic Ar(+) ion bombardment of a brass surface was used for its production. Subsequent dication formation was found to take place in the ion extraction region of our mass spectrometer several tens of microns in front of the sputtered brass surface. The dication formation mechanism appears to be resonant electron transfer in soft gas-phase collisions between sputter-ejected singly charged CuZn(+) molecular ions and incoming Ar(+) projectiles. This conclusion is supported by our theoretical study that obtained an ionization energy of CuZn(+) of 15.75 eV, in excellent agreement with both the experimental and calculated ionization energy of Ar (15.76 and 15.67 eV, respectively). The ground state of CuZn(2+) is found to be a metastable one with a very shallow potential well at an internuclear equilibrium distance of about 2.7 Å the dissociation energy being very difficult to estimate. Interestingly, spin-orbit corrections are found to be necessary to get an adequate description of the metastable state of CuZn(2+), whereas relativistic corrections have no effects on neutral CuZn nor on CuZn(+).

A density functional study of the adsorption of pyridine, 2-vinylpyridine, and 4-vinylpyridine onto a silica surface

Abstract The interaction of pyridine, 2-vinylpyridine, and 4-vinylpyridine with silica surfaces represented by model clusters is studied within the framework of the density functional theory to shed light on the different trend towards polymerization showed by the vinyl derivatives of pyridine. It is found that pyridine interacts more strongly with silica clusters than its derivatives, whereas 2-vinylpyridine, in its turn, exhibits a larger interaction energy than 4-vinylpyridine. It is also found that the interaction pattern is dominated in all cases by the formation of a hydrogen bond between the nitrogen atom and a hydroxyl group of the silica cluster, a second, very weak hydrogen bond is formed in some of the systems, though. A natural bond orbital analysis and the calculation of harmonic vibrational frequencies allow to confirm those findings. Finally, it is proposed that the trend of not forming nanocomposites when using 2-vinylpyridine is mainly due to the involvement of its vinyl group in a weak hydrogen bond to an oxygen atom of the silica surface avoiding to polymerize and not due to a weak acid–base interaction nor steric effects as it was previously supposed.