Search results for "conical"
showing 10 items of 109 documents
Pore structure and function of synthetic nanopores with fixed charges: tip shape and rectification properties
2011
We present a complete theoretical study of the relationship between the structure (tip shape and dimensions) and function (selectivity and rectification) of asymmetric nanopores on the basis of previous experimental studies. The theoretical model uses a continuum approach based on the Nernst-Planck equations. According to our results, the nanopore transport properties, such as current-voltage (I-V) characteristics, conductance, rectification ratio, and selectivity, are dictated mainly by the shape of the pore tip (we have distinguished bullet-like, conical, trumpet-like, and hybrid shapes) and the concentration of pore surface charges. As a consequence, the nanopore performance in practical…
Unified model for the ultrafast decay of pyrimidine nucleobases.
2006
Ultrafast decay processes detected after absorption of UV radiation in gas-phase pyrimidine nucleobases uracil, thymine, and cytosine are ascribed to the barrierless character of the pathway along the low-lying 1(pipi*) hypersurface connecting the Franck-Condon region with an out-of-plane distorted ethene-like conical intersection with the ground state. Longer lifetime decays and low quantum yield emission are on the other hand related to the presence of a 1(pipi*) state planar minimum on the S1 surface and the barriers to access other conical intersections. A unified model for the three systems is established on the basis of accurate multiconfigurational CASPT2 calculations, whereas the ef…
Singlet-Triplet States Interaction Regions in DNA/RNA Nucleobase Hypersurfaces.
2010
The present study provides new insight into the intrinsic mechanisms for the population of the triplet manifold in DNA nucleobases by determining, at the multiconfigurational CASSCF/CASPT2 level, the singlet-triplet states crossing regions and the main decay paths for their lowest singlet and triplet states after near-UV irradiation. The studied singlet-triplet interacting regions are accessible along the minimum energy path of the initially populated singlet bright (1)ππ* state. In particular, all five natural DNA/RNA nucleobases have, at the end of the main minimum energy path and near a conical intersection of the ground and (1)ππ* states, a low-energy, easily accessible, singlet-triplet…
Fluoride-Induced Negative Differential Resistance in Nanopores: Experimental and Theoretical Characterization
2021
We describe experimentally and theoretically the fluoride-induced negative differential resistance (NDR) phenomena observed in conical nanopores operating in aqueous electrolyte solutions. The threshold voltage switching occurs around 1 V and leads to sharp current drops in the nA range with a peak-to-valley ratio close to 10. The experimental characterization of the NDR effect with single pore and multipore samples concern different pore radii, charge concentrations, scan rates, salt concentrations, solvents, and cations. The experimental fact that the effective radius of the pore tip zone is of the same order of magnitude as the Debye length for the low salt concentrations used here is su…
Computational determination of the dominant triplet population mechanism in photoexcited benzophenone
2014
In benzophenone, intersystem crossing occurs efficiently between the S-1(n pi(star)) state and the T-1 state of dominant n pi(star) character, leading to excited triplet states after photoexcitation. The transition mechanism between S-1(n pi(star)) and T-1 is still a matter of debate, despite several experimental studies. Quantum mechanical calculations have been performed in order to assess the relative efficiencies of previously proposed mechanisms, in particular, the direct S-1 -> T-1 and indirect S-1 -> T-2(pi pi(star)) -> T-1 ones. Multiconfigurational wave function based methods are used to discuss the nature of the relevant states and also to determine minimum energy paths a…
Nonsymmetric conical upper density and $k$-porosity
2017
We study how the Hausdorff measure is distributed in nonsymmetric narrow cones in R n \mathbb {R}^n . As an application, we find an upper bound close to n − k n-k for the Hausdorff dimension of sets with large k k -porosity. With k k -porous sets we mean sets which have holes in k k different directions on every small scale.
Testing jet geometries and disc-jet coupling in the neutron star LMXB 4U 0614 + 091 with the internal shocks model
2020
Multi-wavelength spectral energy distributions of Low Mass X-ray Binaries in the hard state are determined by the emission from a jet, for frequencies up to mid-infrared, and emission from the accretion flow in the optical to X-ray range. In the last years, the flat radio-to-mid-IR spectra of Black Hole (BH) X-ray binaries was described using the internal shocks model, which assumes that the fluctuations in the velocity of the ejecta along the jet are driven by the fluctuations in the accretion flow, described by the X-ray Power Density Spectrum (PDS). In this work we attempt to apply this model for the first time to a Neutron Star (NS) LMXB, i.e. 4U 0614+091. We used the multi-wavelength d…
Ultrafast ring-opening/closing and deactivation channels for a model spiropyran-merocyanine system.
2011
The photochemistry of a model merocyanine-spiropyran system was analyzed theoretically at the MS-CASPT2//SA-CASSCF(14,12) level. Several excited singlet states were studied in both the closed spiropyran and open merocyanine forms, and the paths to the different S(1)/S(0) conical intersections found were analyzed. After absorption of UV light from the spiropyran form, there are two possible ultrafast routes to efficient conversion to the ground state; one involves the rupture of the C(spiro)-O bond leading to the open form and the other involves the lengthening of the C(spiro)-N bond with no photoreaction. From the merocyanine side the excited state can reach a very broad S(1)/S(0) conical i…
Weak intermolecular interactions promote blue luminescence of protonated 2,2′-dipyridylamine salts
2014
In this work we demonstrate that protonation and π-stacking can be exploited to convert non-luminescent 2,2′-dipyridylamine into blue-emitting derivatives. We have synthesized a series of luminescent 2,2′-dipyridylamine (dpa) salts, i.e., (dpaH)X·nSolv (dpa = 2,2′-dipyridylamine, X = HF2, n = 0.5, Solv = H2O 1; X = Cl, n = 2, Solv = H2O 2; X = Br, n = 2, Solv = H2O 3; X = I n = 1, Solv = H2O 4a; X = I n = 1, Solv = CHCl34b), (dpaH)2[SiF6]·H2O 5 and (dpaH)X (X = I36; SbF67; BF48) and characterized their emission properties, both in the solid-state and in solution. To rationalize our observations and relate the luminescence properties to the structure in the solid state and in solution, we ha…
On the use of reduced active space in CASSCF calculations
2014
Abstract The complete active space self-consistent field (CASSCF) method is one of the most used approaches to explore theoretically non-adiabatic photochemical processes in medium-size molecular systems. It is also frequently utilized in direct dynamics studies because this method offers a reliable description of the electronic state mixing in the regions of surface crossings (conical intersections). In this context, however, the large computational demand required to perform such on-the-fly molecular dynamics simulations using CASSCF as the electronic structure method means that only relatively small active space can be handled (typically not more than 10 electrons distributed in 10 orbit…