1,721,056 research outputs found
Virtual eyes for technology and cultural heritage: towards computational strategy for new and old indigo-based dyes
Full text linkA cost-effective, robust and reliable computational strategy is applied to simulate peak positions and band shapes of UV–Vis spectra together with the dye colours perceived by human eyes. The features of our virtual multifrequency spectrometer (VMS) relevant to this topic are sketched with special focus on the selection of density functional, vibronic model and solvent description. Furthermore, the new VMS-draw graphical user interface is employed for user-friendly pre- and post-processing of the computed data. The family of indigo dyes is used as case study in view of their continued use in the field of cultural heritage, together with new promising applications for photonics and sustainable energy. After assessment of different simplified models employed in previous studies, the role of several substituents and of dimerization in tuning the colour and spectral features are analysed in detail by means of both accurate computations and interpretative models. The results are in remarkable agreement with experiment and allow to rationalize the behaviour of this class of dyes
CYANOMETHANIMINE ISOMERS IN COLD INTERSTELLAR CLOUDS: INSIGHTS FROM ELECTRONIC STRUCTURE AND KINETIC CALCULATIONS
Full text linkNew insights into the formation of interstellar cyanomethanimine, a species of great relevance in prebiotic chemistry, are provided by electronic structure and kinetic calculations for the reaction CN + CH2=NH. This reaction is a facile formation route of Z,E-C-cyanomethanimine, even under the extreme conditions of density and temperature typical of cold interstellar clouds. E-C-cyanomethanimine has been recently identified in Sgr B2(N) in the Green Bank Telescope (GBT) PRIMOS survey by P. Zaleski et al. and no efficient formation routes have been envisaged so far. The rate coefficient expression for the reaction channel leading to the observed isomer E-C-cyanomethanimine is 3.15 × 10-10 × (T/300)0.152 × e(−0.0948/T). According to the present study, the more stable Z-C-cyanomethanimine isomer is formed with a slightly larger yield (4.59 × 10−10 × (T/300)0.153 × e(−0.0871/T). As
the detection of E-isomer is favored due to its larger dipole moment, the missing detection of the Z-isomer can be due to the sensitivity limit of the GBT PRIMOS survey and the detection of the Z-isomer should be attempted with more sensitive instrumentation. The CN + CH2 = NH reaction can also play a role in the chemistry of the upper atmosphere of Titan where the cyanomethanimine products can contribute to the buildup of the observed nitrogenrich organic aerosols that cover the moon
High-Accuracy Vibrational Computations for Transition-Metal Complexes Including Anharmonic Corrections: Ferrocene, Ruthenocene, and Osmocene as Test Cases
No full textDensity functional theory calculations of infrared spectra at harmonic and anharmonic levels of theory have been carried out in order to define a reliable yet feasible strategy to perform accurate computations on metal complexes starting from metallocenes. We present different possibilities to compute with unprecedented accuracy either the ligand vibrations or vibrations where the metal atom is involved or even to obtain the entire spectrum without invoking any scaling factor. Anharmonic calculations employing second-order vibrational perturbation theory provide very good results when performed using the B3PW91 hybrid functional associated with an extended basis set and are able to reproduce quantitatively the entire spectrum of ferrocene, including the presence of overtones at ∼1700 cm-1. Furthermore, our results confirm that B3LYP is the best functional to reproduce ligand vibrations, but, unfortunately, it provides unreliable results for vibrations involving the metal atom. Conversely, the PBE0 functional gives accurate results for metal-ligand vibrational frequencies, but it is quite far from the experiment for intraligand ones
TD-DFT Benchmark on Inorganic Pt(II) and Ir(III) Complexes
No full textWe
report in the present paper a comprehensive investigation of
representative Pt(II) and Ir(III) complexes with special reference
to their one-photon absorption spectra employing methods rooted in
density functional theory and its time dependent extension. We have
compared nine different functionals ranging from generalized gradient
approximation (GGA) to global or range-separated hybrids, and two
different basis sets, including pseudopotentials for 4 iridium and
7 platinum complexes. It turns out that hybrid functionals with the
same exchange part give comparable results irrespective of the specific
correlation functional (i.e., B3LYP is very close to B3PW91 and PBE0
is very close to MPW1PW91). More recent functionals, such as CAM-B3LYP
and M06-2X, overestimate excitation energies, whereas local functionals
(BP86 -GGA-, M06-L -Meta GGA-) strongly underestimate transition energies
with respect to experimental results. As expected, basis set effects
are weak, and the use of a triple-ζ polarized (def2-TZVP) basis
set does not significantly improve the computed excitation energies
with respect to a classical double-ζ basis set (LANL2DZ) augmented
by polarization functions, but it significantly raises the computational
effort
Accurate Infrared (IR) Spectra for Molecules Containing the C≡N Moiety by Anharmonic Computations with the Double Hybrid B2PLYP Density Functional
No full textHerein, we report a comprehensive benchmark of C≡N stretching vibrations computed at harmonic and anharmonic levels with the aim of proposing and validating a reliable computational strategy to get accurate results for this puzzling vibrational mode without any ad hoc scaling factor. Anharmonic calculations employing second-order vibrational perturbation theory provide very good results when performed using the B2PLYP double-hybrid functional, in conjunction with an extended basis set and supplemented by semiempirical dispersion contributions. For larger systems, B2PLYP harmonic frequencies, together with B3LYP anharmonic corrections, offer a very good compromise between accuracy and computational cost without the need of any empirical scaling factor
Implementation of a graphical user interface for the virtual multifrequency spectrometer: The VMS-Draw tool
No full textThis article presents the setup and implementation of a graphical user interface (VMS-Draw) for a virtual multifrequency spectrometer. Special attention is paid to ease of use, generality and robustness for a panel of spectroscopic techniques and quantum mechanical approaches. Depending on the kind of data to be analyzed, VMS-Draw produces different types of graphical representations, including two-dimensional or three-dimesional (3D) plots, bar charts, or heat maps. Among other integrated features, one may quote the convolution of stick spectra to obtain realistic line-shapes. It is also possible to analyze and visualize, together with the structure, the molecular orbitals and/or the vibrational motions of molecular systems thanks to 3D interactive tools. On these grounds, VMS-Draw could represent a useful additional tool for spectroscopic studies integrating measurements and computer simulations
Gas phase formation of the prebiotic molecule formamide: insights from new quantum computations
No full textNew insights into the formation of interstellar formamide, a species of great
relevance in prebiotic chemistry, are provided by electronic structure and
kinetic calculations for the reaction NH2 + H2CO -> NH2CHO + H. Contrarily to
what previously suggested, this reaction is essentially barrierless and can,
therefore, occur under the low temperature conditions of interstellar objects
thus providing a facile formation route of formamide. The rate coefficient
parameters for the reaction channel leading to NH2CHO + H have been calculated
to be A = 2.6x10^-12 cm^3 s^-1, beta = -2.1 and gamma = 26.9 K in the range
of temperatures 10-300 K. Including these new kinetic data in a refined
astrochemical model, we show that the proposed mechanism can well reproduce the
abundances of formamide observed in two very different interstellar objects:
the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular
shock L1157-B2. Therefore, the major conclusion of this Letter is that there is
no need to invoke grain-surface chemistry to explain the presence of formamide
provided that its precursors, NH2 and H2CO, are available in the gas-phase
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Reassessment of the Thermodynamic, Kinetic, and Spectroscopic Features of Cyanomethanimine Derivatives: A Full Anharmonic Perturbative Treatment
No full textHerein we report a full thermodynamic
and vibrational investigation
of C-cyanomethanimine isomers rooted into the Density Functional Theory
(DFT) and the second-order vibrational perturbation theory (VPT2).
We show that an anharmonic treatment affects dramatically the vibrational
behavior of the molecules, especially thanks to the inclusion of interaction
terms between the various modes. Furthermore, the equilibrium constant
between the isomers, as well as the rate constant, have been obtained
at both harmonic and anharmonic levels showing, as expected, slight
but non-negligible differences. To support our investigation, dispersion
effects have been employed
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