The supramolecular organization of liquid water is discussed in connection with both the spectral profile of the OH stretching Raman signal measured in pure water and the distribution of tetrahedral order computed by molecular dynamics simulations. Both curves show common features and a similar temperature dependence never pointed out before. Energetic information extracted from the Raman profiles using a recently proposed integration method is also discussed. Overall results confirm that thermally induced structural variations of water in the interval of temperature ranging from 10 to 75 degrees C involve a change of the local tetrahedral order associated with a redistribution energy of 1.6-1.7 kcal/mol.

We have investigated the radicality and the vertical singlet-triplet energy gap of [n]cyclacenes (cyclic polyacenes) as a function of the system size for n even, from 6 to 22. The calculations are performed using the complete active space self-consistent field method and second-order n-electron valence perturbation theory. We present a systematic way for the selection of the active space in order to have a balanced description of the wave function as the size of the system increases. Moreover, we provide didactic insight into the failure of an approach based on a minimal active space. We find that the ground state is an open-shell singlet and its multireference character increases progressively with n. The singlet-triplet gap decreases as a function of the system size and approaches a finite positive value for the limit n → ∞. Finally, an analysis based on the one-particle reduced density matrix suggests a polyradical character for the largest cyclacenes.

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Graphynes are porous derivatives of graphene that can be considered as ideal 2D nanofilters. Here, we investigate by theoretical methods graphtriyne multilayers, proposing them as membranes featuring pores of subnanometer size suitable for CO 2 /N 2 separation and CO 2 uptake. The potential energy surfaces, representing the intermolecular interactions within the CO 2 /N 2 gaseous mixtures and between the graphtriyne layers and the molecules, have been formulated in an internally consistent way, by adopting potential models far more accurate than the traditional Lennard-Jones functions, routinely used to predict static and dynamical properties of matter. The new force fields so obtained and tested on accurate ab initio calculations have been used to perform extensive molecular dynamics simulations of membrane selectivity and adsorption. The accuracy of the potentials granted a quantitative description of the interactions and realistic results for the dynamics under a wide range of conditions of applied interest, indicating a single-layer permeation ratio CO 2 /N 2 of 4.25 (meaning that permeations of CO 2 are typically 4.25 times those of N 2). At low pressure, graphtriyne bilayer membranes exhibit good performances as a molecular sieving candidate for postcombustion CO 2 separation because of a high permeance and a relatively good selectivity. On the other hand, graphtriyne trilayer membranes present a relatively high interlayer adsorption selectivity and a high CO 2 uptake. Such properties make these graphyne nanostructures versatile materials competitive with other carbonbased adsorbing membranes suitable to cope with post-combustion CO 2 emissions. Moreover, guidelines for the extension of the proposed methodology to carbon nanostructures and other gaseous mixtures of relevance for atmosphere and combustion are also provided.