Daniel Peláez

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.

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.

Surface-enhanced Raman scattering of hydroxybenzoic acids adsorbed on silver nanoparticles.

Surface-enhanced Raman scattering (SERS) of hydroxybenzoic acids has been studied on silver sols in H(2)O and D(2)O solutions. The adsorption behavior of 4-hydroxybenzoic acid (4HBA) is different from that of salicylic (2HBA) and 3-hydroxybenzoic (3HBA) acids. It was concluded that 4HBA is adsorbed on silver nanoparticles (Ag(n)) as either oxidobenzoate (A(2-)) or hydroxybenzoate (A(-)), depending on the pH of the solution, given rise to a flat orientation. Both 2HBA and 3HBA acids are always adsorbed as hydroxybenzoates anions (A(-)) at pH >or=5 and link to the metal through the carboxylate group (Ag(n)--A(-)), standing more or less perpendicular to the metal surface. In the case of these monoanions, the selective enhancement of the bands is due mainly to a resonant electron or charge transfer process (ET or CT) from the metallic nanoparticles to the adsorbates, yielding the transient formation of the respective radical dianions (Ag(+)(n)--A(2-)). It is found that the enhanced bands, and especially the mode 8a;nu(ring), are related to the difference between the equilibrium structures of the adsorbate in its ground (A(-)) and CT-excited (A(2-)) states. In the SERS spectrum of 4HBA dianion, the contribution of CT mechanism is not observed.

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.

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.

Photochemistry of protonated nitrosamine: chemical inertia of NH2NOH+ versus reactivity of NH3NO+.

The photochemical behavior of the protonated simplest nitrosamine [NH2NO-H](+) has been addressed by means of the CASPT2//CASSCF methodology in conjunction with the ANO-L basis sets. The relative stability of the different tautomers, namely, (1) NH2NOH(+), (2) NH3NO(+), and (3) NH2NHO(+), has been considered, and the corresponding tautomerization transition states have been characterized. With respect to the most chemically relevant species, it has been found that NH2NOH(+) corresponds to a bound structure, while NH3NO(+) corresponds to an adduct between NH3 and NO(+) at both CASSCF and CASPT2 levels of theory. Vertical transition calculations and linear interpolations on the homolytic dissociation of NH3NO(+) in combination with previous results on neutral nitrosamine [J. Chem. Phys. 2006, 125, 164311] and neutral N,N-dimethylnitrosamine [J. Org. Chem. 2007, 72, 4741] indicate that, in acidic diluted solutions, the protonation of nitrosamine takes place on the excited surface. The N-N dissociation channels have been studied both in ground and first excited singlet state. An S1/S0 conical intersection is found to be responsible for the photostability of NH2NOH(+). On the contrary, NH3NO(+) is photochemically unstable as its first excited state is purely dissociative. The latter species is characterized by a twofold reactivity: the formation of nitrosyl cation (NO(+)) in the ground state and the photorelease of physiologically relevant nitric oxide radical (NO) in its first excited state.

Entangled Photonic-Nuclear Molecular Dynamics of LiF in Quantum Optical Cavities

The quantum photodynamics of a simple diatomic molecule with a permanent dipole immersed within an optical cavity containing a quantized radiation field is studied in detail. The chosen molecule under study, lithium fluoride (LiF), is characterized by the presence of an avoided crossing between the two lowest 1Σ potential energy curves (covalent-ionic diabatic crossing). Without field, after prompt excitation from the ground state 1 1Σ, the excited nuclear wave packet moves back and forth in the upper 2 1Σ state, but in the proximity of the avoided crossing, the nonadiabatic coupling transfers part of the nuclear wave packet to the lower 1 1Σ state, which eventually leads to dissociation. The quantized field of a cavity also induces an additional light crossing in the modified dressed potential energy curves with similar transfer properties. To understand the entangled photonic-nuclear dynamics, we solve the time-dependent Schrödinger equation by using the multiconfigurational time-dependent Hartree method (MCTDH). The single mode quantized field of the cavity is represented in the coordinate space instead of in the Fock space, which allows us to deal with the field as an additional vibrational mode within the MCTDH procedure on equal footing. We prepare the cavity with different quantum states of light, namely, Fock states, coherent states, and squeezed coherent states. Our results reveal pure quantum light effects on the molecular photodynamics and the dissociation yields of LiF, which are quite different from the light-undressed case and which cannot be described in general by a semiclassical approach using classical electromagnetic fields.