HSAB principle applied to the time evolution of chemical reactions

Time evolution of various reactivity parameters such as electronegativity, hardness, and polarizability associated with a collision process between a proton and an X- atom/ion (X = He, Li<SUP>+</SUP>, Be<SUP>2+</SUP>, B<SUP>3+</SUP>, C<SUP>4+</SUP>) in its ground (<SUP>1</SUP>S) and excited(<SUP>1</SUP>P,<SUP>1</SUP>D,<SUP>1</SUP>F) electronic states as well as various complexions of a two-state ensemble is studied using time-dependent and excited-state density functional theory. This collision process may be considered to be a model mimicking the actual chemical reaction between an X-atom/ion and a proton to give rise to an XH<SUP>+</SUP> molecule. A favorable dynamical process is characterized by maximum hardness and minimum polarizability values according to the dynamical variants of the principles of maximum hardness and minimum polarizability. An electronic excitation or an increase in the excited-state contribution in a two-state ensemble makes the system softer and more polarizable, and the proton, being a hard acid, gradually prefers less to interact with X as has been discerned through the drop in maximum hardness value and the increase in the minimum polarizability value when the actual chemical process occurs. Among the noble gas elements, Xe is the most reactive. During the reaction: H<SUB>2</SUB> + H<SUP>+</SUP> &#8594; H<SUB>3</SUB><SUP>+</SUP> hardness maximizes and polarizability minimizes and H<SUB>2</SUB> is more reactive in its excited state. Regioselectivity of proton attack in the O-site of CO is clearly delineated wherein HOC<SUP>+</SUP> may eventually rearrange itself to go to the thermodynamically more stable HCO<SUP>+</SUP>

Philicity: a unified treatment of chemical reactivity and selectivity

A generalized concept of philicity is introduced through a resolution of identity, encompassing electrophilic, nucleophilic, and radial reactions. Locally, a particular molecular site may be more prone to electrophilic attack or another may be more prone to nucleophilic attack, but the overall philicity of the whole molecule remains conserved. Local philicity is by far the most powerful concept of reactivity and selectivity when compared to the global electrophilicity index, Fukui function, local softness, or global softness because it contains information about almost all of the known global and local descriptors of chemical reactivity and selectivity

H \u3csup\u3e+\u3c/sup\u3e + NO(v i = 0) → H \u3csup\u3e+\u3c/sup\u3e + NO(v f = 0-2) at E Lab = 30 eV with canonical and Morse coherent states

H + + NO(v i = 0) = H + + NO(v f = 0-2) at E Lab = 30 eV is investigated with the simplest-level electron nuclear dynamics (SLEND) method. In a direct, time-dependent, variational, and non-adiabatic framework, SLEND adopts nuclear classical mechanics and an electronic single-determinantal wavefunction. A coherent-states (CS) procedure recovers quantum vibrational properties from classical mechanics. Besides canonical CS, SU(1,1), SU(2), and Gazeau-Klauder Morse CS are innovatively introduced to treat anharmonicity. SLEND vibrational differential cross, rainbow scattering angles, and H + energy loss spectra compare well with experimental data and with vibrational close-coupling rotational infinite-order sudden approximation results obtained at a higher computational cost. © 2012 Elsevier B.V. All rights reserved

Specific and Nonspecific Effects of Glycosylation

Glycosylation regulates vital cellular processes and dramatically influences protein folding and stability. In particular, experiments have demonstrated that asparagine (N)-linked disaccharides drive a “conformational switch” in a model peptide. The present work investigates this conformational switch via extensive atomically detailed replica exchange molecular dynamics simulations in explicit solvent. To distinguish the effects of specific and nonspecific interactions upon the peptide conformational ensemble, these simulations considered model peptides that were N-linked to a disaccharide and to a steric crowder of the same shape. The simulations are remarkably consistent with experiment and provide detailed insight into the peptide structure ensemble. They suggest that steric crowding by N-linked disaccharides excludes extended conformations, but does not significantly impact the tetrahedral structure of the surrounding solvent or otherwise alter the peptide free energy surface. However, the combination of steric crowding with specific hydrogen bonds and hydrophobic stacking interactions more dramatically impacts the peptide ensemble and stabilizes new structures

Dynamics of H\u3csup\u3e+\u3c/sup\u3e N2 at ELab 30 eV

The H+ N2 system at ELab 30 eV, relevant in astrophysics, is investigated with the simplest-level electron nuclear dynamics (SLEND) method. SLEND is a time-dependent, direct, variational, non-adiabatic method that employs a classical-mechanics description for the nuclei and a single-determinantal wavefunction for the electrons. A canonical coherent-states procedure, intrinsic to SLEND, is used to reconstruct quantum vibrational properties from the SLEND classical mechanics. Present simulations employ three basis sets: STO-3G, 6-31G, and 6-31G**, to determine their effect on the results, which include reaction visualizations, product predictions, and scattering properties. Present simulations predict non-charge-transfer scattering and N2 collision-induced dissociation as the main reactions. Average vibrational energy transfer, H+ energy-loss spectra, rainbow angle, and elastic vibrational differential cross sections at the SLEND6-31G** level agree well with available experimental data. SLEND6-31G** results are comparable to those calculated with the vibrational close-coupling rotational infinite-order sudden approximation and the quasi-classical trajectory method. © 2011 American Institute of Physics

Photoinduced Homolytic Bond Cleavage of the Central Si–C Bond in Porphyrin Macrocycles Is a Charge Polarization Driven Process

Photoinduced cleavage of the bond between the central Si atom in porphyrin macrocycles and the neighboring carbon atom of an axial alkyl ligand is investigated by both experimental and computational tools. Photolysis and electron paramagnetic resonance measurements indicate that the Si–C bond cleavage of Si–phthalocyanine occurs through a homolytic process. The homolytic process follows a low-lying electronic excitation of about 1.8 eV that destabilizes the carbide bond of similar bond dissociation energy. Using electronic structure calculations, we provide insight into the nature of the excited state and the resulting photocleavage mechanism. We explain this process by finding that the electronic excited state is of a charge transfer character from the axial ligand toward the macrocycle in the reverse direction of the ground state polarization. We find that the homolytic process yielding the radical intermediate is energetically the most stable mechanistic route. Furthermore, we demonstrate using our computational approach that changing the phthalocyanine to smaller ring system enhances the homolytic photocleavage of the Si–C bond by reducing the energetic barrier in the relevant excited states

Excitonic Interactions in Bacteriochlorin Homo-Dyads Enable Charge Transfer: A New Approach to the Artificial Photosynthetic Special Pair

Excitonically coupled bacteriochlorin (BC) dimers constitute a primary electron donor (special pair) in bacterial photosynthesis and absorbing units in light-harvesting antenna. However, the exact nature of the excited state of these dyads is still not fully understood. Here, we report a detailed spectroscopic and computational investigation of a series of symmetrical bacteriochlorin dimers, where the bacteriochlorins are connected either directly or by a phenylene bridge of variable length. The excited state of these dyads is quenched in high-dielectric solvents, which we attribute to photoinduced charge transfer. The mixing of charge transfer with the excitonic state causes accelerated (within 41 ps) decay of the excited state for the directly linked dyad, which is reduced by orders of magnitude with each additional phenyl ring separating the bacteriochlorins. These results highlight the origins of the excited-state dynamics in symmetric BC dyads and provide a new model for studying the primary processes in photosynthesis and for the development of artificial, biomimetic systems for solar energy conversion

What Is the Optoelectronic Effect of the Capsule on the Guest Molecule in Aqueous Host/Guest Complexes? A Combined Computational and Spectroscopic Perspective

Encapsulation of dye molecules is used as a means to achieve charge separation across different dielectric environments. We analyze the absorption and emission spectra of several coumarin molecules that are encapsulated within an octa-acid dimer forming a molecular capsule. The water-solvated capsule effect on the coumarin’s electronic structure and absorption spectra can be understood as due to an effective dielectric constant where the capsule partially shields electrostatically the dielectric solvent environment. Blue-shifted emission spectra are explained as resulting from a partial intermolecular charge transfer where the capsule is the acceptor, and which reduces the coumarin relaxation in the excited state