Towards Amplified Probing of Weak Intermolecular Interactions on the External Surfaces of Molecular Capsules.

We report a sensitive method for comparing weak interactions between aryl rings located on the external surfaces of equilibrating homo- and heterodimeric capsules. Two identical self-complementary resorcin[4]arene tetrabenzoate molecules and one tetramethylammonium cation form in CDCl3 hydrogen-bonded homodimeric capsules whose exteriors are decorated with four tight pairs of weakly interacting aryl rings. The pair wise mixing of six different homodimers establishes their equilibria with the corresponding heterodimeric species in which two types of aryl rings exert on each other some gentle forces. This equilibrium is significantly shifted either towards homo- or heterodimers depending on the nature and location of the substituents in the weakly interacting aryl rings. The thermodynamic favorability or disadvantage of the heterodimerization is determined by stronger or weaker aryl-aryl attractions in the hetero- or homodimeric capsules, respectively. The four-fold amplification of weak aryl-aryl interactions on the external surfaces of the equilibrating capsules is responsible for high sensitiveness of our approach.

Revised values for the X23 benchmark set of molecular crystals.

We present revised reference values for cell volumes and lattice energies for the widely used X23 benchmark set of molecular crystals by including the effect of thermal expansion. For this purpose, thermally-expanded structures were calculated via the quasi-harmonic approximation utilizing three dispersion-inclusive density-functional approximations. Experimental unit-cell volumes were back-corrected for thermal and zero-point energy effects, allowing now a direct comparison with lattice relaxations based on electronic energies. For the derivation of reference lattice energies, we utilized harmonic vibrational contributions averaged over four density-functional approximations. In addition, the new reference values also take the change in electronic and vibrational energy due to thermal expansion into account. This is accomplished by either utilizing experimentally determined cell volumes and heat capacities, or by relying on the quasi-harmonic approximation. The new X23b reference values obtained this way will enable a more accurate benchmark for the performance of computational methods for molecular crystals.

Towards hybrid density functional calculations of molecular crystals via fragment-based methods.

We introduce and employ two QM:QM schemes (a quantum mechanical method embedded into another quantum mechanical method) and report their performance for the X23 set of molecular crystals. We furthermore present the theory to calculate the stress tensors necessary for the computation of optimized cell volumes of molecular crystals and compare all results to those obtained with various density functionals and more approximate methods. Our QM:QM calculations with PBE0:PBE+D3, PBE0:PBE+MBD, and B3LYP:BLYP+D3 yield at a reduced computational cost lattice energy errors close to the ones of the parent hybrid density functional method, whereas for cell volumes, the errors of the QM:QM scheme methods are in between the generalized gradient approximation and hybrid functionals.

On the Existence of HeHe Bond in the Endohedral Fullerene Hе2 @C60.

Twenty years have already been passed since the endohedral fullerene's void ceaselessly attracts attention of both, experimentalists and theoreticians, computational chemists and physicists in particular, who direct their efforts on computer simulations of encapsulating atoms and molecules into fullerene void and on unraveling the arising bonding patterns. We review recent developments on the endohedral He2 @C60 fullerene, on its experimental observation and on related computational works. The two latter are the main concerns in the present work: on the one hand, there experimentally exists the He dimer embedded into C60 void. On the other, computational side, each He atom exhibits a negligible charge transfer to C60 resulting in that altogether, the He dimer exists as a fractionally charged (He+δ )2 . Whether there exists a bond between these two helium atoms is the key question of the present work. Since a bond is a two-body creature, we assert that it suffices to define the bond on the basis of Löwdin's postulate of a molecule which we invoke to investigate such formation of the He dimer in a given C60 void in terms of the HeHe potential energy well. It is analytically demonstrated that this well enables to maintain at least one bound (ground) state, and therefore, according to Löwdin's postulate which is naturally anticipated within quantum theory, we infer that (He+δ )2 is a molecule, a diatomic, where two heliums are bonded to each other. Using these arguments, we also propose to extend the concept of stability of endohedral fullerenes. © 2017 Wiley Periodicals, Inc.

Development of Embedded and Performance of Density Functional Methods for Molecular Crystals.

We report an alternative quantum mechanical:quantum mechanical (QM:QM) method to the currently used periodic density functional calculations including dispersion and investigate its performance with respect to main structural and energetic properties of the X23 set of molecular crystals. By setting the goal of reproducing reference periodic BLYP+D3 values and by embedding BLYP+D3 into DFTB, we obtain results similar to those of periodic BLYP+D3-typically within 1-2% in lattice energies and ∼0.4% in cell volumes. The accuracy of this QM:QM method in comparison to DFTB+D and DFT+D for the X23 set of molecular crystals is discussed.

Strain in nonclassical silicon hydrides: An insight into the "ultrastability" of sila-bi[6]prismane (Si18 H12) cluster with the endohedrally trapped silicon atom, Si19 H12.

The recently postulated concept of "ultrastability" and "electron-deficient aromaticity" (Vach, Nano Lett 2011, 11, 5477; Vach, J Chem Theory Comput 2012, 8, 2088) in a sila-bi[6]prismane having an additional entrapped silicon atom, Si19 H12 , has been disproved on the basis of a careful analysis of the energetic characteristics related to the formation of this and other silicon hydrides. The central silicon atom in Si19 H12 is weaker bound to other silicon atoms than in conventional tetrahedral silanes; moreover, Si19 H12 possesses a significant amount of strain. The role of strain in the formation of the title compounds has been further rationalized by calculating the relative energies for the transformation to a half-planar conformation in methane and in silane and by calculating the respective strain energies. The strain energy value in Si18 H12 is equal to 9.93 eV whereas the same property for Si19 H12 lies in range of 6.42-8.85 eV. Two low-energy isomers of Si19 H12 which lie by 2.77 and 3.42 eV (!) lower in energy than the originally considered sila-bi[6]prismane-based structure have been proposed.

Encapsulation of diatomic molecules in fullerene C60: implications for their main properties.

The endohedral complexes of diatomic guest molecules H2, N2, O2, F2, HF, CO, LiH, LiF, BN, and BeO with C60 have been characterized computationally by employing second-order Møller-Plesset (MP2) theory and its density-fitting local (DF-LMP2) variant. The interaction energies, equilibrium geometries, dipole moments and harmonic vibrational frequencies of these complexes have been systematically calculated. It was found that all guest molecules are stabilized inside the C60 cage, with the most pronounced stabilization effect (of about 50 kcal mol(-1)) observed for the polar covalent BeO and BN molecules. It is noteworthy that the normally short-lived BN molecule is the only guest molecule that was found to chemisorb on the inner surface of C60. When encapsulated, all guest molecules (except for BN) exhibit bond elongation (up to 0.07 Å) and, consequently, a red shift in vibrational stretching frequencies. In fact, the calculated vibrational properties of the H2@C60 complex agree well with those derived from experiment. The C60 geometry is not perturbed significantly upon encapsulation, but a subtle tendency to decrease the carbon-carbon bond alternation is observed. Polar guest molecules inside C60 are located at an off-center position and a significant decrease in their dipole moments upon encapsulation is observed. The importance of explicitly taking into account electron correlation effects, as well as full geometry relaxation, to yield a correct description of the complexes investigated is clearly demonstrated. The present results may serve as a guide for future attempts to synthesize such complexes employing the "molecular surgery" approach.

Calculations of the C2 fragmentation energies of higher fullerenes C80 and C82.

The C2 fragmentation energies of the most stable isolated-pentagon-rule (IPR) isomers of the C80 and C82 fullerenes were evaluated with second-order Møller-Plesset (MP2) theory, density-functional theory (DFT) and the semiempirical self-consistent charge density-functional tight-binding (SCC-DFTB) method. Zero-point energy, ionization energy and empirical C2 corrections were included in the calculation of fragmentation energies for comparison with experimental C2 fragmentation energies of the fullerene cations. In the case of the most probable Stone-Wales pathway of C2 fragmentation of C80, the calculated [Formula: see text] agree well with experimental data, whereas in the case of C(82) fragmentation, the calculated [Formula: see text] exceed by up to 1.2 eV the experimental ones, which suggests that other IPR isomers may be present in sufficient amounts in experimental samples. Computer-intensive MP2 calculations and DFT calculations with larger basis sets do not yield much improved C2 fragmentation energies, compared to those reported earlier with B3LYP/3-21G. On the other hand, semiempirical approaches such as SCC-DFTB, which are orders of magnitude less intensive, yield satisfactory fragmentation energies for higher fullerenes and may become a method of choice for routine calculations of fullerenes and carbon nanotubes.