Miroslav Rubeš

Investigation of the benzene-dimer potential energy surface: DFT/CCSD(T) correction scheme.

A novel method, designated as the density functional theory/coupled-cluster with single and double and perturbative triple excitation [DFT/CCSD(T)] correction scheme, was developed for precise calculations of weakly interacting sp(2) hydrocarbon molecules and applied to the benzene dimer. The DFT/CCSD(T) interaction energies are in excellent agreement with the estimated CCSD(T)/complete basis set interaction energies. The tilted T-shaped structure having C(s) symmetry was determined to be a global minimum on the benzene-dimer potential energy surface (PES), approximately 0.1 kcal/mol more stable than the parallel-displaced structure. A fully optimized set of ten stationary points on the benzene-dimer PES is proposed for the evaluation of the reliability of methods for the description of weakly interacting systems.

Accurate description of argon and water adsorption on surfaces of graphene-based carbon allotropes.

Accurate interaction energies of nonpolar (argon) and polar (water) adsorbates with graphene-based carbon allotropes were calculated by means of a combined density functional theory (DFT)-ab initio computational scheme. The calculated interaction energy of argon with graphite (-9.7 kJ mol(-1)) is in excellent agreement with the available experimental data. The calculated interaction energy of water with graphene and graphite is -12.8 and -14.6 kJ mol(-1), respectively. The accuracy of combined DFT-ab initio methods is discussed in detail based on a comparison with the highly precise interaction energies of argon and water with coronene obtained at the coupled-cluster CCSD(T) level extrapolated to the complete basis set (CBS) limit. A new strategy for a reliable estimate of the CBS limit is proposed for systems where numerical instabilities occur owing to basis-set near-linear dependence. The most accurate estimate of the argon and water interaction with coronene (-8.1 and -14.0 kJ mol(-1), respectively) is compared with the results of other methods used for the accurate description of weak intermolecular interactions.

Catalysis by Dynamically Formed Defects in a Metal–Organic Framework Structure: Knoevenagel Reaction Catalyzed by Copper Benzene-1,3,5-tricarboxylate

The high catalytic activity and selectivity of the metal–organic framework (MOF) copper benzene‐1,3,5‐tricarboxylate (CuBTC) that are observed experimentally in the Knoevenagel reaction are explained on the basis of computational investigations by employing a periodic model and density functional theory. Three factors are responsible for the unusually high activity of CuBTC: One, CuBTC can act as a base, and the active methylene reactant is deprotonated, whereas a temporary defect in the framework is formed; two, the thus‐formed defect, a Brønsted acid site, simultaneously activates the aldehyde; three, the reaction takes place on two adjacent Cu2+ sites (Lewis acid sites) that are separated by 8.2 Å. The results reported herein show the great versatility of the CuBTC MOF catalyst, including its amphiphilic character and the concerted effect of nearby framework metal cations.

Investigation of the Benzene–Naphthalene and Naphthalene–Naphthalene Potential Energy Surfaces: DFT/CCSD(T) Correction Scheme

The potential energy surfaces of the naphthalene dimer and benzene-naphthalene complexes are investigated using the recently developed DFT/CCSD(T) correction scheme [J. Chem. Phys. 2008, 128, 114 102]. One and three minima are located on the PES of the benzene-naphthalene and the naphthalene dimer complexes, respectively, all of which are of the parallel-displaced type. The stabilities of benzene-naphthalene and the naphthalene dimer are -4.2 and -6.2 kcal mol(-1), respectively. Unlike the benzene dimer, where the T-shaped complex is the global minimum, the lowest-energy T-shaped structure is about 0.2 and 1.6 kcal mol(-1) above the global minimum on the benzene-naphthalene and the naphthalene dimer potential energy surfaces, respectively.

DFT/CCSD(T) investigation of the interaction of molecular hydrogen with carbon nanostructures.

The interaction of molecular hydrogen with carbon nanostructures is investigated within the DFT/CC correction scheme. The DFT/CC results are compared with the benchmark calculations at the CCSD(T) level of theory for benzene and naphthalene, and at the MP2 level for the more extended systems. The DFT/CC method offers a reliable alternative to the highly correlated ab initio calculations at a cost comparable to the standard DFT method. The results for H(2) adsorbed on graphene as well as single-wall carbon nanotubes (SWCNT) are presented. The DFT/CC binding energy on graphene of 5.4 kJ mol(-1) is in good agreement with experiment (5.00+/-0.05 kJ mol(-1)). For (10,10)-SWCNT, the H(2) molecule is mostly stabilized inside the tube with an estimated binding energy of 7.2 kJ mol(-1).

Combined theoretical and experimental investigation of CO adsorption on coordinatively unsaturated sites in CuBTC MOF.

The adsorption of CO in metal-organic framework CuBTC material is investigated by a combination of theoretical and experimental approaches. The adsorption enthalpy of CO on CuBTC determined experimentally to be -29 kJ mol(-1) at the zero-coverage limit is in very good agreement with the adsorption enthalpy calculated at the combined DFT-ab initio level with the periodic model. Calculations show that polycarbonyl complexes cannot be formed on regular coordinatively unsaturated sites in CuBTC. Experimental IR spectra of the CO probe molecule adsorbed in CuBTC are interpreted based on calculated CO stretching frequencies. Calculations show that long-range interactions are insignificant for the CO/CuBTC system and that this system can be accurately modeled with just a Cu(2)(HCOO)(4) cluster model of the paddle wheel. The reliability of various methods for the description of CO interaction with the Cu(2+) site in CuBTC is discussed based on the experimental results and accurate coupled-cluster calculations. It is shown that standard exchange-correlation functionals do not provide a reliable description of CO interaction with coordinatively unsaturated Cu(2+) sites in CuBTC.

Accurate Ab Initio Description of Adsorption on Coordinatively Unsaturated Cu2+ and Fe3+ Sites in MOFs

The performance of different exchange-correlation functionals was evaluated for the description of the interaction of small molecules with (i) cluster models containing Cu(2+) and Fe(3+) coordinatively unsaturated metal sites and (ii) HKUST-1 metal organic framework (MOF). Adsorbates forming dispersion-bound complexes (CH4), complexes with important dispersion and electrostatic contributions (H2, N2, CO2), and complexes stabilized also by a partial dative bond (CO, H2O, and NH3) were considered. The interaction with coordinatively unsaturated sites was evaluated with respect to the coupled-cluster calculations for Cu(2+) and Fe(3+) centers represented by cluster models. The adsorption on dispersion-stabilized sites was examined for the cage-window and the cage-center sites in HKUST-1 with respect to the experimental and DFT/CC results. None of the functionals considered can accurately describe the interaction of all seven adsorbates with Cu(2+) and Fe(3+) sites and with dispersion-dominated adsorption sites. The interaction with coordinatively unsaturated sites was frequently underestimated, for adsorbates forming a partial dative bond in particular, while the adsorption at dispersion-stabilized sites was overestimated. Consequently, interaction energies calculated for different adsorption sites were often in qualitatively incorrect order. The optimal exchange-correlation functional for a particular adsorbate/MOF can thus be found by comparing the performance of various functionals with respect to highly accurate calculations on smaller cluster models as a good representative of MOF structural building blocks.

Computational investigation of the Lewis acidity in three-dimensional and corresponding two-dimensional zeolites: UTL vs IPC-1P.

The adsorption and catalytic properties of three-dimensional zeolite UTL were investigated computationally along with properties of its two-dimensional analogue IPC-1P that can be obtained from UTL by a removal of D4R units. Adsorption properties and Lewis acidity of extra-framework Li(+) sites were investigated for both two- and three-dimensional forms of UTL using the carbon monoxide as a probe molecule. The CO adsorption enthalpies, calculated with various dispersion-corrected DFT methods, including DFT/CC, DFT-D2, and vdW-DF2, and the CO stretching frequencies obtained with the νCO/rCO correlation method are compared for corresponding Li(+) sites in 3D and 2D UTL zeolite. For the majority of framework Al positions the Li(+) cation is preferably located in one of the channel wall sites and such sites remains unchanged upon the 3D → 2D UTL transformation; consequently, the adsorption enthalpies become only slightly smaller in 2D UTL (less than 3 kJ mol(-1)) due to the missing part of dispersion interactions and νCO becomes also only up to 5 cm(-1) smaller in 2D UTL due to the missing repulsion with framework oxygen atoms from the opposite site of the zeolite channel (effect from the top). However, when Li(+) is located in the intersection site in 3D UTL (about 20% probability), its coordination with the framework is significantly increased in 2D UTL and that is accompanied by significant decrease of both νCO (about 20 cm(-1)) and adsorption enthalpy (about 20 kJ mol(-1)). Because the intersection sites in 3D UTL are the most active adsorption and catalytic Lewis sites, the results reported herein suggest that the 3D → 2D transformation of UTL zeolite is connected with partial decrease of zeolite activity in processes driven by Lewis acid sites.

A Computationally Feasible DFT/CCSD(T) Correction Scheme for the Description of Weakly Interacting Systems

A computationally feasible DFT/CCSD(T) correction scheme is proposed for precise calculations (close to the CCSD(T) accuracy) of weakly interacting molecular clusters. This approach formally falls within the DFTD class of methods (empirically corrected DFT methods), however, there are several important differences between the DFT/CCSD(T) scheme proposed here and a standard DFTD approach: (i) it is parameter free, (ii) it does not use any damping functions, and (iii) the error of DFT is assumed to be anisotropic in general. In addition, the proposed DFT/CCSD(T) correction scheme allows the analysis of assumptions commonly used in the DFTD calculations. Applica- tion of this method on the ethylene-benzene and benzene-benzene complexes leads to the conclusion that interaction ener- gies obtained with the DFT/CCSD(T) correction scheme can be obtained with a near reference level accuracy with an er- ror not exceeding 0.1 kcal/mol. A proper choice of a reference set is shown to be more important than the anisotropy of the DFT/CCSD(T) correction.