Juan F. Arenas

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.

Complete analysis of the surface-enhanced Raman scattering of pyrazine on the silver electrode on the basis of a resonant charge transfer mechanism involving three states

A new general procedure to interpret surface-enhanced Raman scattering (SERS) spectra has been developed in order to clarify the controversy concerning the relevant enhancement mechanism of this type of spectra. The analysis consists of detecting the presence of the charge transfer (CT) enhancement mechanism by correlating the most enhanced SERS bands with the ab initio calculated geometries (ΔQ) and vibrational frequencies (Δv) of the isolated molecule and its radical anion. This CT mechanism is assumed to be identical to that of resonance Raman between the electronic ground state of the metal–adsorbate complex and charge transfer excited states. We consider that these excited states arise when one electron is transferred from the metal to pyrazine. For this reason, they have been labeled from the point of view of pyrazine on the basis on the symmetry of the doublet states of its radical anion. The SERS spectra of pyrazine recorded on silver surface at several electrode potentials have been analyzed on t...

The ground and excited state potential energy surfaces of nitromethane related to its dissociation dynamics after excitation at 193 nm

The relevant low-lying singlet and triplet potential energy surfaces in the photolysis of nitromethane have been studied by using the multistate extension of the multiconfigurational second-order perturbation theory in conjunction with large atomic natural orbital-type basis sets. The proposed mechanism for the photolytic decomposition of CH3NO2 provides a consistent and reinterpreted picture of the available experimental results. Two reaction paths are found in the photolysis of nitromethane after excitation at 193 nm: (1) Major Channel, CH3NO2(1A′)+hν(193 nm)→CH3NO2(2A″)→ lim  ICCH3NO2(2A′)→CH3(1A1′)+NO2(1 2B1)→ lim −hν′ICCH3(1A1′)+NO2(1 2A1)→ lim 193 nmhνCH3(1A1′)+NO(A 2Σ+)+αO(3P)+βO(1D). (2) Minor Channel, CH3NO2(1A′)+hν(193 nm)→CH3NO2(2A″)→CH3(1A1′)+NO2(1 2A2)→CH3(1A1′)+NO(X 2Π)+αO(3P)+βO(1D), being α and β fractional numbers. No ionic species are found in any dissociation path. Additionally, the respective low-lying Rydberg states of nitromethane and nitrogen dioxide have been studied too.

Vibrational spectra of methylpyridines

The vibrational assignment of the monosubstituted methylderivatives of pyridine: 2-methyl, 3-methyl and 4-methyl, and the disubstituted derivatives: 2,6-dimethyl and 3,5-dimethyl with C 2v symmetry, as well as the trisubstituted derivative 2,4,6trimethypyridine also with C2v symmetry has been carried out. The Scaled Quantum Mechanical Force Field (SQMFF) methodology has been used, which allows for the refinement of the force field calculated at the RHF/3-21G level via a set of scale factors directly transferred from related molecules. In our case the scale factors related with the ring coordinates have been transferred from the pyridine force field, whilst those of the methyl group and those related with the ring-substituent CX bond, have been transferred from 2-methylpyridine. The good agreement between the a priori calculated frequencies and the experimental ones has allowed to propose a complete assignment of the vibrational IR and Raman spectra of these methylderivatives. q 1999 Elsevier Science B.V. All rights reserved.

Vibrational spectrum and internal rotation in 2-methylpyrazine

Infrared and Raman spectra of 2-methylpyrazine have been recorded and assigned on the basis of Cs symmetry. The MINDO/3 optimized geometry and the potential-energy barrier for internal rotation of the methyl group have been computed, together with the energy levels and the most intense transitions for the i.r. spectrum of internal rotation. Some u.v. bands related to internal rotation are reported. Thermodynamic functions have been also calculated.

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 ...

Vibrational spectra of [1H4]pyrazine and [2H4]pyrazine

Infrared and Raman spectra of [1H4]pyrazine and [2H4]pyrazine have been reinvestigated and a general assignment of all the observed bands is proposed which modifies some previous assignments of fundamental vibrations. The present assignment satisfies the isotopic product rule for i.r.- and Raman-active fundamentals. On this basis thermodynamic functions have been computed.

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.