Isabel López-Tocón

Vibrational spectrum of 4-fluoraniline

The Raman spectrum of 4-fluoraniline (4FA) has been recorded, the quadratic force field has been calculated at RHF/631 1 G p level of theory and then scaled to reproduce the experimental frequencies, by using Pulay’s scaled quantum mechanical force field (SQMFF) methodology. Likewise, DFT force field has been calculated at the B3LYP/6-31 1 G p level. On the basis of all these results, a general assignment of the vibrational spectra of 4FA has been proposed and compared with that of aniline in order to determine the substitution effect of the fluorine atom. q 2001 Elsevier Science B.V. All rights reserved.

Potential-energy surfaces related to the thermal decomposition of ethyl azide: The role of intersystem crossings

The potential-energy surfaces of ethyl azide relevant to its thermal decomposition have been studied theoretically. The geometries of minima and transition states on the S0 surfaces, as well as the lowest energy points in the seam of crossing of the triplet and singlet surfaces, have been optimized with the complete active space self-consistent field (CAS-SCF) method, and their energies, re-calculated with second-order multireference perturbation (CAS/MP2) theory and corrected by the zero-point energy (ZPE). The reaction mechanism is described by the following steps: (1) CH3CH2N3→CH3CH2N+N2, (2a) CH3CH2N→H2+CH3CN; (2b) CH3CH2N→CH3CHNH. The CN–N2 fission of ethyl azide is the rate limiting step (1), leading to ethylnitrene either along a spin-allowed path (1a) or along an alternative spin-forbidden one (1b). Both 1a and 1b channels show barriers of similar heights for CN–N2 bond fission, ΔE=42 kcal/mol, ΔE being the energy difference between the minimum of the ground singlet state potential-energy surface ...

NH2 inversion potential in the S0 and S1 electronic states of aniline: fit to the (ro-)vibrational data and comparison with ab initio and density functional results

Abstract An effective one-dimensional Hamiltonian has been determined for the NH 2 inversion motion in aniline. The anharmonic potential is represented by a quartic polynomial, fitted to the experimental vibrational transitions in the electronic states S 0 and S 1 . In the S 0 state the barrier height is about 580 cm −1 and the equilibrium angle between the NH 2 and the ring planes is 44°, while in the S 1 state the potential well is extremely flat without a well-defined equilibrium angle. These results are compared with those obtained by ab initio methods. The long-standing discrepancy between the rotational and vibrational results is partially clarified.

The aniline–argon van der Waals complex: ab initio second-order Møller–Plesset study of the potential energy surface in the ground electronic state

Abstract The potential energy surface of the aniline–argon (An–Ar) complex in the ground electronic state has been investigated by ab initio calculations using second-order Moller–Plesset (MP2) theory. The basis sets for the aromatic molecule and for the argon atom were cc-pvdz and aug-cc-pvtz, respectively. The structure in which the argon atom is opposite to the amino hydrogens ( anti conformer) is found to be more stable than the structure in which the argon atom and the amino hydrogens are on the same side of the ring ( syn conformer). The calculated binding energies for the two conformers are 407 and 393 cm −1 , respectively. The introduction of diffuse orbitals for the aniline molecule using an aug-cc-pvdz basis set does not affect the relative stability of the two conformers. The calculated intermolecular distance, structure and rotational constants of the An–Ar complex reproduce satisfactorily the experimental data.

Trace detection of triphenylene by surface enhanced Raman spectroscopy using functionalized silver nanoparticles with bis-acridinium lucigenine.

Surface enhanced raman scattering (SERS) of triphenylene (TP) has been recorded on Ag nanoparticles functionalized with the molecular assembler bis-acridinium lucigenine dication (LG) which approaches the adsorbate to the metal surface allowing for its detection. Structural information on the host and the analyte can be extracted from the SERS spectra of LG and LG/TP complex. The acridinium planes in LG are staggered, so cavities into which hydrophobic TP can be allocated are created. Moreover, the orientation of LG with respect to the metal surface changes from tilted to perpendicular when concentration of TP increases. However, perpendicular orientation of TP with respect to the metal surface is preferred according to the in-plane enhanced bands recorded in the SERS spectrum. The dependence of the Raman signal of TP on LG concentration has been checked, and trace concentrations of TP have been detected by this technique which therefore can be used as a chemical sensor of organic pollutants.

Potential coupling of intramolecular to intermolecular modes: an ab initio study of the amino inversion and van der Waals motions in the aniline–argon complex

Abstract In this paper, we present a description of the vibrating aniline–argon (An–Ar) van der Waals complex in its electronic ground state by using second order Moller–Plesset (MP2) theory to elucidate the key factors which govern the configuration of this complex. The interaction between the intramolecular mode associated to the inversion motion of the amino group and the van der Waals bending mode involving the motion of the Ar atom in the direction toward the nitrogen atom, has been modeled. The ab initio results indicate not only that there is a significant coupling between the two motions, in agreement with experimental findings on vibrational predissociation, but also that the interaction between the Ar atom and the nitrogen lone pair governs the complex topology of this four-dimensional potential energy surface.

Raman study of the rigidity of penta-p-phenylene derivatives used as legs in molecular tripods.

Molecular planarity of penta-p-phenylene (P5P) and several substituted derivatives with four side chains of various lengths, including deca(ethylene glycol) groups, is discussed by considering the changes in the intensity ratio between the Raman bands recorded at 1280 and 1220 cm(-1). The intensity ratio between both bands I(1280)/I(1220) shows a small increase with the size of the substituent, indicating a high rigidity for all these compounds, even those with long oligo(ethylene glycol) side chains. This result is important given that these phenylene derivatives are versatile building blocks for the construction of nanometric tripod-shaped adsorbates for biological applications since the side chains should prevent the nonspecific interaction with proteins.

Rotationally resolved electronic spectroscopy of aniline excited vibronic levels

Abstract Experiments of molecular beam high resolution electronic spectroscopy were performed on several vibronic bands of the S1←S0 electronic transition of aniline. Here we report on the properties of the excited states with a vibrational energy up to 1300 cm−1 above the fundamental vibrational level of S1. The spectroscopic parameters of these levels, such as band centers and rotational constants, have been determined with high accuracy from the fit of more than 300 ro-vibronic transitions for each band. Each single ro-vibronic transition was found to be homogeneously broadened by about 18 MHz and no evidence of spectral perturbation was detected.

Large amplitude motions in the electronic ground state of 4-fluoroaniline

The rotational spectra of the ground and several vibrational states of 4-fluoroaniline, and of the ground vibrational states of the NHD and ND2 isotopomers have been measured. Most of the measured lines were split into two (0+ and 0−) components, due to amino group inversion. The potential energy surface and the associated structural relaxation have been determined for the three lowest in energy vibrations by combining these results with ab initio and suitable flexible model quantum calculations.

Structure and Dynamics of van der Waals Complexes by High Resolution Spectroscopy

Molecular beams represent an extraordinary tool for the study of molecular properties [1]. The expansion of a gas through a small nozzle into a vacuum chamber has a number of consequences which have been extensively exploited to setup different experiments. The adiabatic expansion process results in a very efficient cooling of the sample. An experiment carried out on a sample cooled in a supersonic expansion allows to observe molecular aggregates bound by weak interactions, or metastable species. It is well known that, increasing the distance from the nozzle, the frequency of molecular collisions enormously decreases: the mean free path, especially in a molecular beam experiment with differentially pumped vacuum chambers, can be as large as a few meters. Therefore, if during the first moments of the expansion some molecular aggregate bound by weak forces is formed, it can survive long enough to be observed. In a conventional gas cell a similar aggregate would be immediately broken by collisions. The sample preparation in a supersonic molecular beam makes easier its spectroscopic study. The spectrum can be highly simplified because most of the molecules are in their ground vibrational state and then the hot bands are very weak. Also only a few rotational states are significantly populated and the rotational contour becomes much simpler. The experimental spectral resolution can