Both, the toluene-insoluble (A1) and toluene-soluble (A2) asphaltene fractions, isolated using the PNP method, have been characterized by elemental analysis, molecular weight (VPO, SEC, and LDMS), and NMR (1H and 13C). The most prominent results of the analysis were the differences in hydrogen aromaticity f H, high content in both fractions of hydrogen bonded to aliphatic carbons joined to aromatics (f α), and differences in carbon aromaticity f C. Thus, low f H and high f α in A1 were consistent with a single, rigid, and flat core formed by fusion of polycyclic aromatic and naphthenic units (a single and large PANU), whereas for A2, high f H and high f αwere consistent with a more flexible structure where several smaller PANU are joined by aliphatic chains. Using a MM program, models for A1 and A2 were built and the solubility parameters calculated were found in keeping with solubility difference suggesting that the above structural differences account for the solubility difference. Similar molecular weight and heteroatom content found for these fractions suggest that these play a minor or insignificant role in solubility. A dispersion mechanism of A1 by A2, relevant to solubility of asphaltene in organic solvents, is proposed.

Herein we report on the theoretical-experimental analysis of the one-and two-photon absorption and circular dichroism spectra of two intrinsically chiral aromatic molecules -hexahelicene derivatives -with helical chirality and intramolecular charge transfer (ICT). The primary outcomes of our investigation demonstrate that the TPA cross-section and the amplitude of the TPCD signal of this type of helicenes are strongly affected by the strength of the ICT and the nature of the extension of the electronic delocalization, i.e. beyond (EXO-ICT) or within (ENDO-ICT) the helicene core. These results were corroborated through the comparative theoretical analysis of the corresponding contributions of the magnetic dipole transition moment and the electric quadrupole transition moment to the TPA rotatory strength on a series of five similar helicene derivatives with different molecular electron delocalization disposition.Two-photon absorption (TPA) and two-photon circular dichroism (TPCD) spectra were obtained using the double L-scan technique over a broad spectral range (400 nm -900 nm) using 90 fs pulses at a low repetition rate (2-50 Hz) produced by an amplified femtosecond system.The theoretical simulations were performed using modern analytical response theory within the Time-Dependent Density Functional Theory (TD-DFT) approach using B3LYP and CAM-B3LYP, and the aug-cc-pVDZ and 6-311++G(d,p) basis sets.

Herein, we present an experience-based learning approach that uses the COVID-19 pandemics knowledge about virus spread and epidemics to establish an analogy between a simple epidemics modelthe SIR model (susceptible–infected–removed), and a second-order autocatalytic reaction with subsequent catalyst deactivation. Our approach provides a simple and engaging way for students to learn chemical kinetics from a current situation (the pandemic) while presenting them with programming tools to numerically solve any system of differential equations that may result from more complex chemical kinetic schemes. We include the option to fit experimental data to extract meaningful rate constants. Following Kolb’s cycle, we first establish the theoretical background of chemical kinetics and the SIR model and describe the analogy. Then, we propose the use of MATLAB and Python to numerically solve the system of differential equations associated with this model and plot the results showing the behavior of the system when changing the constants that describe the process, making the analogy with chemical kinetic constants. Finally, we fitted experimental data from the COVID-19 pandemic (up to early June 2020) to test the model and discuss the goodness of the fitting, the fitting parameters, and the utility of real kinetic data.

Herein we report on the conformational study of a chiral Salen ligand, AFX-155, in THF solution using two-photon absorption (TPA) and, even more importantly, two-photon circular dichroism (TPCD). The fragment-recombination approach (FRA), employed to perform computations on half-AXF-155 (AXF-169') and the center (C-AXF-155), allowed us to overcome the current computational limitations found in calculations of the TPCD spectra of large molecules. The comparative experimental-theoretical analysis of AXF-155 showed that its TPA is mainly determined by AXF-169' and validated, through TPCD, the presence of the two dominant Trans_R-Intra(NHB)//Trans_R-Extra(HB) and Trans_R-Intra(NHB)//Trans_R-Extra(NHB) structure in THF with a ΔΔ propeller chirality on the diphenylamine moieties at the end of each AXF-169'. The application of FRA for the analysis of the TPCD spectra of large chiral molecules has been proven to be effective.

Herein, we report on the theoretical-experimental analysis of the two-photon absorption (TPA) and two-photon circular-linear dichroism (TPCLD) spectra of a highly conjugated, rigid, and centrosymmetric molecule in solution, that is, perylene/CH2Cl2. We show how a three-energy-level diagram, under the sum-over-essential states approach, assists in the determination of the magnitude of transition electric dipole moments and the angle between them for the main TPA transitions. We demonstrate the potential of TPCLD to reveal the symmetry of excited states and the angles between their transition electric dipole moments and that of the ground state. By means of TPCLD, we explain how the overwhelming contribution of certain TPA transitions can mask important spectral features in regions where the transition electric dipole moments are perpendicular. TPCLD is expected to enhance the understanding of the photophysical properties of materials that are not accessible using conventional linear and two-photon spectroscopy. TPA and TPCLD measurements were performed employing the open-aperture Z-scan technique using an amplified femtosecond system. Time-dependent density functional theory (TD-DFT) calculations were carried out using response theory at the B3LYP level with the aug-cc-pVDZ basis set. Solvent effects were included through the polarizable continuum model (PCM).

In this article we report on the study of the polarization dependent two-photon absorption (TPA) of (S)-(+)-(1,1'-binaphthalene-2,2'-diyl)bis(diphenylphosphine) (S-BINAP) in solution, and the theoretical-experimental analysis of its two-photon circular dichroism (TPCD) spectrum. The comparative examination of the following two correlation functionals, using the 6-31G* basis set, showed that the Coulomb attenuated method variant of the Becke's three-parameter exchange and the Lee-Yang-Parr (CAM-B3LYP) is more reliable than B3LYP in molecules such as S-BINAP, a heteroaromatic diphosphine chiral ligand with strong intramolecular charge transfer. To access the theoretical TPCD spectra, we employed time dependent density functional theory (TD-DFT) at the mentioned level of theory and over the first 40 electronic excited states including solvent effects by means of the polarizable continuum model (PCM). The extended calculation on twice as many electronic excited states in vacuo proved to be crucial for the correct assignment of the experimental bands. TPA measurements were performed in the femtosecond regime and over a broad spectral range using the double L-scan technique.

We consider the calculation of non-Born-Oppenheimer, nBO, one-particle densities for both electrons and nuclei. We show that the nBO one-particle densities evaluated in terms of translationally invariant coordinates are independent of the wavefunction describing the motion of center of mass of the whole system. We show that they depend, however, on an arbitrary reference point from which the positions of the vectors labeling the particles are determined. We examine the effect that this arbitrary choice has on the topology of the one-particle density by selecting the Hooke-Calogero model of a three-body system for which expressions for the one-particle densities can be readily obtained in analytic form. We extend this analysis to the one-particle densities obtained from full Coulomb interaction wavefunctions for three-body systems. We conclude, in view of the fact that there is a close link between the choice of the reference point and the topology of one-particle densities that the molecular structure inferred from the topology of these densities is not unique. We analyze the behavior of one-particle densities for the Hooke-Calogero Born-Oppenheimer, BO, wavefunction and show that topological transitions are also present in this case for a particular mass value of the light particles even though in the BO regime the nuclear masses are infinite. In this vein, we argue that the change in topology caused by variation of the mass ratio between light and heavy particles does not constitute a true indication in the nBO regime of the emergence of molecular structure.

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