Juan Soto

The role of charge-transfer states of the metal-adsorbate complex in surface-enhanced Raman scattering

Surfaced-enhanced Ramon scattering (SERS) spectra of pyrazine are analyzed on the basis of the properties of the electronic states of the metal-adsorbate surface complex. Ab initio CIS calculations have been carried out for the Ag2-pyrazine complex, which have enabled us to find two excited singlets, namely CT0;1B1 and CT1;1A2, with properties quite similar to those of the pyrazine radical anion in its electronic 2B3u and 2Au states, respectively, and with energies falling in the range of the exciting photons usually employed in Raman spectroscopy. SERS spectra of pyrazine are compatible with a resonance Raman enhancement mechanism involving electronic transitions between the ground state S0;1A1 and both CT levels of the surface complex.

A MULTICONFIGURATIONAL SELF-CONSISTENT FIELD STUDY OF THE THERMAL DECOMPOSITION OF METHYL AZIDE

Thermal decomposition of methyl azide has been studied computationally by using the complete active space self-consistent field (CASSCF) method and Moller–Plesset theory using the CASSCF wave function as the zeroth-order wave function (CAS/MP2). The calculations have been performed in conjunction with the 6-31G* basis set. The reaction is predicted to occur in two steps via nitrene intermediate: (1) CH3N3→CH3N+N2; (2a) CH3N→H2+HCN, (2b) CH3N→H2CNH. The rate-limiting step is the N2 extrusion (1), being a competitive mechanism between a spin-forbidden path and a spin-allowed one. The calculated energy barrier height for both processes is found to be isoenergetic, ΔE=41 kcal/mol, where ΔE represents the difference between the energy at the minimum on the singlet state surface of methyl azide and the energy at the minimum energy crossing structure (ISC1) or the singlet transition state (TS1) for the spin-forbidden path and the spin-allowed one, respectively. The nitrene intermediate formed in step (1) can und...

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 ...

Multiconfigurational second-order perturbation study of the decomposition of the radical anion of nitromethane

The doublet potential energy surfaces involved in the decomposition of the nitromethane radical anion (CH(3)NO(2) (-)) have been studied by using the multistate extension of the multiconfigurational second-order perturbation method (MS-CASPT2) in conjunction with large atomic natural orbital-type basis sets. A very low energy barrier is found for the decomposition reaction: CH(3)NO(2) (-)-->[CH(3)NO(2)](-)-->CH(3)+NO(2) (-). No evidence has been obtained on the existence of an isomerization channel leading to the initial formation of the methylnitrite anion (CH(3)ONO(-)) which, in a subsequent reaction, would yield nitric oxide (NO). In contrast, it is suggested that NO is formed through the bimolecular reaction: CH(3)+NO(2) (-)-->[CH(3)O-N-O](-)-->CH(3)O(-)+NO. In particular, the CASSCF/MS-CASPT2 results indicate that the methylnitrite radical anion CH(3)ONO(-) does not represent a minimum energy structure, as concluded by using density functional theory (DFT) methodologies. The inverse symmetry breaking effect present in DFT is demonstrated to be responsible for such erroneous prediction.

How a resonant charge transfer mechanism determines the relative intensities in the SERS spectra of 4-methylpyridine

Abstract The surface-enhanced Raman scattering (SERS) spectra of 4-methylpyridine (4-MP) have been recorded at different electrode potential versus saturated Ag/AgCl/KCl reference electrode. By comparing the relative intensity of the SERS with the Raman spectrum of the aqueous solution it is possible to determine that 12, 6a, δ(CH) and especially 8a modes are strongly enhanced. In this work, it is demonstrated that these vibrations are closely related to Franck–Condon factors of a resonant photoinduced charge transfer (CT) mechanism similar to a resonance Raman (RR) process. The theoretically calculated ΔQ displacements represent the differences between geometries of the potential energy minima of the states involved in the resonant process, allowing to calculate the relative intensities in RR from the Peticolas’ equation. These calculated intensities are in a perfect agreement with the experimental behavior.

Evidences for the contribution of a resonant charge transfer process to the surface-enhanced Raman scattering of 2,6-dimethylpyridine

The surface-enhanced Raman scattering spectra of 2,6-dimethylpyridine have been recorded on silver at different electrode potentials. The main feature of these spectra is the strong enhancement of the band corresponding to the 8a mode when the electrode potential becomes more negative. This enhancement has been discussed on the basis of a resonant charge transfer (CT) mechanism by using a methodology based on the analysis of the ab initio optimized equilibrium structures of the electronic states involved in the CT resonant process namely: the ground electronic states of the neutral molecule and its radical anion.

Comment on "Multiconfigurational perturbation theory can predict a false ground state" by C. Camacho, R. Cimiraglia and H. A. Witek, Phys. Chem. Chem. Phys., 2010, 12, 5058.

Prediction of the true ground state of Sc(2) with multiconfigurational perturbation theory requires a balanced active space in building the reference wave function.

Surface-enhanced Raman scattering of picolinamide, nicotinamide, and isonicotinamide: unusual carboxamide deprotonation under adsorption on silver nanoparticles.

Surface-Enhanced Raman Scattering (SERS) of picolinamide, nicotinamide, and isonicotinamide has been studied on silver colloids at pH⩾7. The wavenumbers of the SERS bands assigned to 1; νring and ν(C-X) vibrational modes show important blue-shifts (ca. +50cm(-1)) with respect to the Raman spectra, whereas the Amide III bands undergo red-shifts up to -50cm(-1). We demonstrate that these shifts are originated by the deprotonation of the carboxamide groups which link to the metal through the nitrogen and oxygen atoms of the respective azanion groups. In order to support this conclusion, theoretical DFT force field calculations have been carried out, confirming that the pyridinecarboxamides interact with the metallic surface in their deprotonated forms as benzamide does.

Photodissociation mechanism of methyl nitrate. A study with the multistate second-order multiconfigurational perturbation theory

The photodissociation reactions of methyl nitrate CH(3)ONO(2) starting at the 193 and 248 nm photolytic wavelengths have been studied with the second-order multiconfigurational perturbation theory (CASPT2) by computation of numerical energy gradients for stationary points. In addition, energy profiles of reaction paths and vertical excitations have been investigated with the multistate extension of the multiconfigurational second-order perturbation theory (MS-CASPT2). It is found that excitation at 193 nm yields three reaction paths: (i) the so-called slow channel CH(3)ONO(2)--> CH(3)O + NO(2)--> CH(3)O + NO + O; (ii) the fast channel CH(3)ONO(2)--> CH(3)O + NO(2); and (iii) CH(3)ONO(2)--> CH(3)ONO + O. The slow channel starts at the S(4) surface, in contrast, the population of the S(3) state can lead to the fast channel or to direct atomic oxygen extrusion. The rather high relative yield of the channel leading to oxygen extrusion from methyl nitrate is explained on the basis of an S(3)/S(2) conical intersection that transfers the initial excitation localized in the npi* S(3) state to the sigmapi* S(2) state with a consequent weakening of the N-O bond. With respect to photolysis at 248 nm, it was not possible to unambiguously distinguish between S(1) and S(2) as the populated state, however, the S(2) state is suggested as mainly responsible for dissociation at this excitation energy.

A method to improve the agreement between calculated and observed vibrational frequencies after scaling of a quantum mechanical force field

A systematic method to fit calculated to observed vibrational frequencies has been developed and implemented in a computer program. The procedure consists of the refinement of a scaled quantum mechanical force field (SQMFF) previously obtained according to Pulay’s method. The key step in the process is the generation of an intermediate matrix, CΛCT, which is then refined. The above step produces only small corrections to the scaled force constants, yielding a considerable improvement of the fitted frequencies. This scheme of refinement can be carried out using any kind of coordinates. To show the reliability and performance of the proposed method, the force fields of two very different systems, as benzene and tetranitromethane, have been chosen as example tests.