SCS-MP2 Level Research Articles

Particularly strong C-H···π interactions between benzene and all-cis 1,2,3,4,5,6-hexafluorocyclohexane.

We present the first high-level ab initio benchmark study of the interaction energy between fluorocyclohexanes and benzene. These compounds form CH···π interactions with aromatic solvents which causes notable shielding of the axial cyclohexane protons. For the recently synthesised all-cis 1,2,3,4,5,6-hexafluorocyclohexane the interaction energy with benzene amounts to -7.9 kcal mol(-1) and -6.4 kcal mol(-1) at the MP2 and SCS-MP2 levels, respectively (extrapolated to the complete basis set limit), which according to dispersion-corrected density functional calculations, is largely due to dispersion.

A theoretical study of luminescent vapochromic compounds including an AuCu2(NHC)2 core

Vapochromic complexes [Au(im(CH(2)py)(2))(2)(Cu(MeCN)(2))(2)](3+) 1, [Au(im(CH(2)py)(2))(2)(Cu(MeOH))(2)](3+) 2 and [Au(im(CH(2)py)(2))(2)(Cu(H(2)O))(2)](3+) 3 were theoretically investigated. The Au–Cu distances of 1 and 2 (4.631 Å and 2.767 Å, respectively) optimized by the SCS-MP2 method in this work agree with the literature experimental values (4.591 Å and 2.792 Å). Their structural features are explained by computational results: (i) in 1, two MeCN molecules coordinate with the Cu center, because of the strong coordination ability of MeCN, to afford a four-coordinate tetrahedral-like Cu center. This geometry needs a long Au–Cu distance. (ii) In 2 and 3, only one MeOH or H(2)O molecule coordinates with the Cu center because of their weak coordination abilities, to afford a three-coordinate planar Cu center. Because the three-coordinate Cu center is flexible, the Au–Cu distance becomes short due to the Au–Cu metallophilic interaction, the strength of which is 5.3 kcal mol(−1) at the SCS-MP2 level. The emission energies of 1, 2 and 3 (2.62, 2.40 and 2.38 eV, respectively) calculated here by the B3PW91 agree with their literature experimental values (2.68, 2.47, and 2.39 eV). The lowest energy triplet excited state (T(1)) is assigned as the excitation from the Cu d to the pyridine π* orbital in 1 and that from the Au–Cu 5d–3d anti-bonding MO to the Au–Cu 6p–4sp bonding MO in 2 and 3. As a result, the emission energy from the T(1) to the ground state is different between these compounds. The difference in Au–Cu distance is one of the important factors for the differences in emission energy and assignment between 1 and others (2 and 3). The vapochromism of these compounds arises from the difference in Au–Cu distance which is determined by the balance between the strengths of the coordination of a gas molecule and the Au–Cu metallophilic interaction; in other words, the Au–Cu heterometallophilic interaction is important for the vapochromic activity of the complex.

A 3D-RISM-SCF method with dual solvent boxes for a highly polarized system: application to 1,6-anhydrosugar formation reaction of phenyl α- and β-d-glucosides under basic conditions

One of the difficulties in application of the usual reference interaction site model self-consistent field (RISM-SCF) method to a highly polarized and bulky system arises from the approximate evaluation of electrostatic potential (ESP) with pure point charges. To improve this ESP evaluation, the ESP near a solute is directly calculated with a solute electronic wavefunction, that distant from a solute is approximately calculated with solute point charges, and they are connected with a switching function. To evaluate the fine solvation structure near the solute by incorporating the long-range solute-solvent Coulombic interaction with low computational cost, we introduced the dual solvent box protocol; one small box with the fine spacing is employed for the first and the second solvation shells and the other large box with the normal spacing is employed for long-range solute-solvent interaction. The levoglucosan formation from phenyl α- and β-d-glucosides under basic conditions is successfully inspected by this 3D-RISM-SCF method at the MP2 and SCS-MP2 levels, though the 1D-RISM-SCF could not be applied to this reaction due to the presence of highly polarized and bulky species. This 3D-RISM-SCF calculation reproduces the experimentally reported higher reactivity of the β-anomer. The 3D-RISM-SCF-calculated activation free energy for the β-anomer is closer to the experimental value than the PCM-calculated one. Interestingly, the solvation effect increases the difference in reactivity between these two anomers. The reason is successfully elucidated with 3D-RISM-SCF-calculated microscopic solvation structure and decomposition analysis of solute-solvent interaction.

A benchmark quantum chemical study of the stacking interaction between larger polycondensed aromatic hydrocarbons

Large-scale electronic structure calculations were performed for the interaction energy between coronene, C24H12 with circumcoronene, C54H18, and between two circumcoronene molecules, in order to get a picture of the interaction between larger graphene sheets. Most calculations were performed at the SCS-MP2 level but we have corrected them for higher-order correlation effects using a calculation on the coronene-circumcoronene system at the quadratic CI, QCISD(T) level. Our best estimate for the interaction energy between coronene and circumcoronene is 32.1 kcal/mol. We estimate the binding of coronene on a graphite surface to be 37.4 or 1.56 kcal/mol per carbon atom (67.5 meV/C atom). This is also our estimate for the exfoliation energy of graphite. It is higher than most previous theoretical estimates. The SCS-MP2 method which reproduces the CCSD(T) and QCISD(T) values very well for smaller aromatic hydrocarbons, e.g., for the benzene dimer, increasingly overestimates dispersion as the bandgap (the HOMO-LUMO separation) decreases. The barrier to the sliding motion of coronene on circumcoronene is 0.45 kcal/mol, and for two circumcoronene molecules 1.85 kcal/mol (0.018 and 0.034 kcal/mol per C atom, respectively). This means that larger graphenes cannot easily glide over each other.

Computational study of lanthanide(iii) hydration

Lanthanide(iii) hydration was studied by utilizing density-functional theory and second-order Møller-Plesset perturbation theory combined with scalar-relativistic 4f-in-core pseudopotentials and valence-only basis sets for the Ln(iii) ions. For [Ln(iii)(H(2)O)(h)](3+) (h = 7, 8, 9) and [Ln(iii)(H(2)O)(h-1)·H(2)O](3+) (h = 8, 9) molecular structures, binding energies, entropies and energies of hydration as well as Gibbs free energies of hydration were calculated using (8s7p6d3f2g)/[6s5p5d3f2g] basis sets for Ln(iii) and aug-cc-pV(D,T)Z basis sets for O and H in combination with the COSMO solvation model. At the generalized gradient approximation level of density-functional theory a preferred hydration number of 8 is found for La(iii)-Tm(iii) and 7 for Yb(iii)-Lu(iii), whereas hybrid density-functional theory predicts a hydration number 8 for all Ln(iii). At the SCS-MP2 level of theory the preferred hydration number is found to be 9 for La(iii)-Sm(iii) and 8 for Eu(iii)-Lu(iii) in good agreement with experimental evidence.

Noncovalent interactions in supramolecular complexes: A study on corannulene and the double concave buckycatcher

Stimulated by the recent observation of pi-pi interactions between C60 and corannulene subunits in a molecular tweezer arrangement (J Am Chem Soc 2007, 129, 3842), a density functional theory study was performed to analyze the electronic structure and properties of various noncovalent corannulene complexes. The theoretical approach is first applied to corannulene complexes with a series of benchmark molecules (CH4, NH3, and H2O) using several new-generation density functionals. The performance of nine density functionals, illustrated by computing binding energies of the corannulene complexes, demonstrates that Zhao and Truhlar's MPWB1K and M05-2X functionals provide energies similar to that obtained at the SCS-MP2 level. In contrast, most of the other popular density functionals fail to describe this noncovalent interaction or yield purely repulsive interactions. Further investigations with the M05-2X functional show that the binding energy of C60 with corannulene subunits in the relaxed molecular receptor clip geometry is -20.67 kcal/mol. The results of this calculation further support the experimental interpretation of pure pi-pi interactions between a convex fullerene and the concave surfaces of two corannulene subunits.

Structure and binding energies of the porphine dimer

Density functional theory optimizations with GGA functionals that include an empirical correction term for the dispersion energy (denoted as -D) have been performed for the van der Waals dimer of porphine. Interaction energies also have been obtained with a recently developed double-hybrid functional (B2-PLYP-D) and at the SCS-MP2 level of theory. The preferred conformer is a parallel displaced complex (1.72 Å lateral displacement) with an interplanar distance of 3.27 Å. The orientation (rotation) of the two monomers has only minor effects (<1 kcal mol− 1) on the binding energy ΔE, implying a certain flexibility of mutual movement around the monomer principal axes. The best estimate for ΔE is − 25 kcal mol− 1 with B2-PLYP-D. The also investigated T-shaped structures are much higher (about 15 kcal mol− 1) in energy. While dispersion contributions are absolutely essential for the binding of all investigated structures, the electrostatic contributions mainly determine the preferred conformations (e.g. displacement versus rotation).