Theoretical Study on the Vapochromic Ni(II)-Quinonoid Complex: One-Dimensional Stacking Structure-Based Color Switching.

Vapochromic crystals of Ni(II)-quinonoid complexes were theoretically investigated using density functional theory (DFT) calculations. Kato et al. previously reported that the purple crystals of a four-coordinate Ni(II)-quinonoid complex (1P) exhibited vapochromic characteristics upon exposure to methanol gas, resulting in orange crystals of the six-coordinate methanol-bound complex (1O) [Angew. Chem., Int. Ed.2017, 56, 2345-2349]. However, the authors did not characterize the crystal structure of 1P. In the present study, we computationally predicted the crystal structure of 1P by performing a crystal structure search with classical force-field computations followed by optimization using DFT calculations. The simulated powder X-ray diffraction pattern of the DFT-optimized structure agreed with experimental observations, indicating that our predicted crystal structure is reliable. Investigation of the optimized crystal structure of 1P revealed that its color change arose from changes in its 1D-band structure, which consists of Ni 3d orbitals and quinonoid π-orbitals. Intermolecular interactions were weakened upon the binding of methanol to the Ni(II) center in 1O. Consequently, the intermolecular 3d-π interaction in 1P lowered the band gap and induced the red-shifting of the monomeric four-coordinate Ni(II)-quinonoid complex. Meanwhile, the obtained absorption spectrum of 1O closely corresponded to that of the monomeric six-coordinate Ni(II)-quinonoid complex. Our study provides a new strategy for accurately predicting molecular crystal structures and reveals a new insight into vapochromism based on band structure color switching.

Electronic processes in NO dimerization on Ag and Cu clusters: DFT and MRMP2 studies.

Experimentally observed NO dimerization on Cu and Ag surfaces is surprising because binding energy of NO dimer is very small in gas phase. MRMP2, MP2 to MP4, CCSD(T), and DFT studies of NO dimerization on Ag2 and Cu2 clusters disclosed that the CCSD(T) method could be applied to this reaction on Ag2 and Cu2 unlike NO dimerization in gas phase which exhibits significantly large nondynamical electron correlation effect. Charge-transfer (CT) from Ag2 and Cu2 to NO moieties plays important role in NN bond formation between two NO molecules. This CT considerably decreases nondynamical correlation effect. Also, the DFT method could be applied to this NO dimerization, if appropriate DFT functional is used; all pure functionals examined here and most of the hybrid functionals underestimated the activation barrier (Ea ), while only ωB97X provided Ea similar to CCSD(T)-calculated value. NO dimerization on similar Cu2 and Cu5 needs moderately larger Ea than those on Ag2 and Ag5 , because frontier orbital participating in the CT exists at lower energy in Cu2 and Cu5 than in Ag2 and Ag5 . The Ea decreases in the order Ag2 >> Ag38 > Ag7 ∼ Ag5 and the reaction energy (ΔE) is positive (endothermic) in Ag2 but significantly negative in Ag38 , Ag7 , and Ag5 , indicating that various Ag clusters could be effective for NO dimerization except for Ag2 . The decreasing order of Ea and increasing order of exothermicity are attributed to increasing order of the frontier orbital energy of Ag2 < Ag38 < Ag7 ∼ Ag5 . © 2018 Wiley Periodicals, Inc.

Development of dissociative force field for all-atomistic molecular dynamics calculation of fracture of polymers.

A dissociative force field for all-atomistic molecular dynamics calculations has been developed to investigate impact fracture of polymers accompanying dissociation of chemical bonds of polymer main chain. Energy of dimer molecules was evaluated as a function of both bond-length b and bond-angle θ by CASPT2 calculations, whose quality is enough to describe dissociation of chemical bonds. Because we found that the bond dissociation energy D decreases with increasing bond-angle, we employed the Morse-type function VBond (b, θ) = {D - VAngle (θ)}[1 - exp{-α(b - b0 ) - ß(b - b0 )2 }] where a quartic function VAngle (θ) = k1 (θ - θ0 ) + k2 (θ - θ0 )2 + k3 (θ - θ0 )3 + k4 (θ - θ0 )4 . This function reproduced well the CASPT2 potential energy surface in a wide range of b and θ. The parameters have been obtained for four popular glassy polymers, polyethylene, poly(methyl methacrylate), poly(styrene), and polycarbonate. © 2019 Wiley Periodicals, Inc.

How to understand very weak Cr-Cr double bonds and negative spin populations in trinuclear Cr complexes: theoretical insight.

Trinuclear Cr(ii) complex [Cr3(dpa)4Cl2] 1 (Hdpa = dipyridylamine) has two Cr-Cr double bonds linked with each other. DMRG-CASPT2 calculations reproduced its symmetrical structure. The Cr-Cr effective bond order (EBO) was evaluated to be only 0.59 based on the density matrix based on localized orbitals from DMRG-CASSCF orbitals. The CASCI calculations showed a significantly large α-spin population on the terminal Cr atoms as expected but a significantly large ß-spin population on the central Cr atom against expectations. The very small EBO and the presence of a large ß-spin population are not consistent with the simple understanding that 1 has two Cr-Cr double bonds and a quintet ground state, which requests correct understanding of 1 from the viewpoint of chemical bond theory. Comparison of 1 with the allene molecule and allyl radical disclosed that the linked Cr-Cr bonds of 1 resembled the C-C bond of the allyl radical but completely differed from the linked C-C double bonds of allene despite the similar molecular structure. Its N3 analogue [Cr3(dpa)4(N3)2] 2 has non-symmetrical structure with shorter Cr1-Cr2 and longer Cr2-Cr3 bonds unlike 1, indicating that 2 is a valence tautomer of 1. DMRG-CASPT2 could reproduce its non-symmetrical structure but DFT/B3PW91 could not. In 2, the EBO is 0.95 for the shorter Cr1-Cr2 bond and 0.47 for the longer Cr2-Cr3 one. The terminal Cr3 has a very large α spin population, and the other terminal Cr1 has a somewhat large α spin population, but the central Cr2 has a considerably large ß spin population. These results indicate that the Cr1-Cr2 bond conjugates with the Cr2-Cr3 bond, which is inconsistent with the simple understanding that 2 has a quadruple bond between Cr1 and Cr2 and no bond between Cr2 and Cr3. The symmetrical structure has a stronger Cr-X coordinate bond (X = Cl or N3) but less stable Cr3 core than does the non-symmetrical one. The relative stabilities of the symmetrical and the non-symmetrical structures are determined by the balance between stabilization energies from the Cr3 core and the Cr-X coordinate bond. All these findings show that electronic structures and Cr-Cr bonds of 1 and 2 are interesting from the viewpoint of molecular science.

Theoretical study of one-electron-oxidized salen complexes of group 7 (Mn(iii), Tc(iii), and Re(iii)) and group 10 metals (Ni(ii), Pd(ii), and Pt(ii)) with the 3D-RISM-GMC-QDPT method: localized vs. delocalized ground and excited states in solution.

One-electron oxidized salen complexes of Mn(iii) and Ni(ii) were recently reported to be unique mixed-valence compounds. Their electronic structures are sensitive to the salen ligand and solvation. We systematically investigated a series of one-electron oxidized salen complexes of group 7 metals (Mn(iii), Tc(iii), and Re(iii)) and their group 10 analogues (Ni(ii), Pd(ii), and Pt(ii)) using the general multi-configuration reference quasi-degenerate perturbation theory (GMC-QDPT) which was combined with the three-dimensional reference interaction site model self-consistent field theory (3D-RISM-SCF) to incorporate the solvation effect. The calculated absorption spectra and electronic structures agree with the experimental observation. The one-electron oxidized salen complexes of group 10 metals with a symmetrical salen ligand have a delocalized electronic structure belonging to class III (Robin-Day classification) in weakly polar solvents. The tendency for taking a delocalized electronic structure increases in the order Pd(ii) < Ni(ii) < Pt(ii). When the salen ligand is asymmetrical, the one-electron oxidized complexes have a localized electronic structure belonging to class II. The group 7 analogues of Mn(iii) and Tc(iii) have a localized electronic structure belonging to class II even in weakly polar solvents and even with a symmetrical salen ligand. However, the one-electron oxidized Re(iii) complex has no mixed-valence nature because one-electron oxidation occurs on the Re center. Theoretical study shows that the solvation effect plays a crucial role in determining the mixed-valence character, class II or III, and thereby its incorporation in the calculation is indispensable for correctly describing geometries, electronic structures, and the inter-valence absorption spectra of these complexes. The d orbital energy is one of the most important factors for determining the localization/delocalization electronic structures in these complexes. Detailed discussion of these factors is presented.

Hetero-dinuclear complexes of 3d metals with a bridging dinitrogen ligand: theoretical prediction of the characteristic features of geometry and spin multiplicity.

Spin multiplicities and coordination structures of dinitrogen-bridged hetero-dinuclear complexes of 3d metals, (µ-N2)[M(1)(AIP)][M(2)(AIP)] (AIPH = (Z)-1-amino-3-imino-prop-1-ene; M(1), M(2) = V(i) to Co(i)), were investigated using the CASPT2 method. (µ-N2)[V(AIP)][Cr(AIP)] has a low spin doublet ((2)B2) ground state with an η(2)-side-on dinitrogen coordination structure but (µ-N2)[Mn(AIP)][Fe(AIP)] has a high spin octet ((8)A2) ground state with an η(1)-end-on coordination structure. These results are similar to those of the homo-dinuclear Cr and Fe analogues, respectively. In (µ-N2)[Cr(AIP)][M(AIP)] (M = Mn(i), Fe(i), or Co(i)) consisting of an early 3d metal (Cr) and a late one (Mn to Co), on the other hand, we found characteristic features in the geometry and the ground state electronic structure which are different from those of homo-dinuclear analogues. The Cr-Mn complex has a high spin decet ((10)B1) ground state with an η(2)-side-on structure. This decet state has the highest spin multiplicity in the dinuclear transition metal complexes, to our knowledge. The A2 state with a doublet spin multiplicity is moderately less stable than the (10)B1 state. The optimized structures and the molecular orbitals indicate that the Cr atom strongly interacts with the N2 moiety in the (10)B1 state but the Mn atom strongly interacts with the N2 moiety in the (2)A2 state. The Cr-Fe complex has a high spin nonet ((9)B1) ground state with an η(2)-side-on structure like the Cr-Mn complex, but only the Cr-Co complex has a medium spin quartet (4)A2 ground state with an η(2)-side-on structure. The different ground electronic state of the Cr-Co complex arises from the presence of 3d orbitals at low energy. Based on these results, it is concluded that the geometry is determined by the Cr center but the electronic structure and the spin multiplicity are determined by the combination of early and late 3d metals in the dinitrogen-bridged hetero-dinuclear chelates of 3d metals.

CASPT2 study of inverse sandwich-type dinuclear 3d transition metal complexes of ethylene and dinitrogen molecules: similarities and differences in geometry, electronic structure, and spin multiplicity.

The spin multiplicities and coordination structures of inverse sandwich-type complexes (ISTCs) of ethylene and dinitrogen molecules with 3d transition metal elements (Sc to Ni), (µ-C2H4)[M(AIP)]2 and (µ-N2)[M(AIP)]2 (AIPH = (Z)-1-amino-3-iminoprop-1-ene; M = Sc to Ni) were investigated by the CASPT2 method. In both ethylene and dinitrogen ISTCs of the early 3d transition metals (Sc to Cr), sandwiched ethylene and dinitrogen ligands coordinate with two metal atoms in an η(2)-side-on form and their ground states have an open-shell singlet spin multiplicity. The η(1)-end-on coordination structure of dinitrogen ISTCs is considerably less stable than the η(2)-side-on form in these metals. For the late 3d transition metals (Mn to Ni), ethylene and dinitrogen ISTCs exhibit interesting similarities and differences in spin multiplicity and structure as follows: in ethylene ISTCs of Mn to Ni, the ground state has an open-shell singlet spin multiplicity like those of the ISTCs of early transition metals. However, the ethylene ligand is considerably distorted, in which the ethylene carbon atoms have a tetrahedral-like structure similar to sp(3) carbon and each of them coordinates with one metal in a µ-η(1):η(1) structure. These geometrical features are completely different from those of ISTCs of the early transition metals. In dinitrogen ISTCs of Mn to Ni, on the other hand, the ground state has a high spin multiplicity from nonet (Mn) to triplet (Ni). The η(2)-side-on coordination structure of the dinitrogen ligand is as stable as the η(1)-end-on form in the Mn complex but the η(1)-end-on structure is more stable than the η(2)-side-on form in the Fe to Ni complexes. All these interesting similarities and differences between ethylene and dinitrogen ISTCs and between the early and late transition metal elements arise from the occupation of several important molecular orbitals.

CASPT2 study of inverse sandwich-type dinuclear Cr(I) and Fe(I) complexes of the dinitrogen molecule: significant differences in spin multiplicity and coordination structure between these two complexes.

Inverse sandwich-type complexes (ISTCs), (µ-N2)[M(AIP)]2 (AIPH = (Z)-1-amino-3-imino-prop-1-ene; M = Cr and Fe), were investigated with the CASPT2 method. In the ISTC of Cr, the ground state takes a singlet spin multiplicity. However, the singlet to nonet spin states are close in energy to each other. The thermal average of effective magnetic moments (µeff) of these spin multiplicities is close to the experimental value. The η(2)-side-on coordination structure of N2 is calculated to be more stable than the η(1)-end-on coordination one. This is because the d-orbital of Cr forms a strong dπ-π* bonding interaction with the π* orbital of N2 in molecular plane. In the ISTC of Fe, on the other hand, the ground state takes a septet spin multiplicity, which agrees well with the experimentally reported µeff value. The η(1)-end-on structure of N2 is more stable than the η(2)-side-on structure. In the η(1)-end-on structure, two doubly occupied d-orbitals of Fe can form two dπ-π* bonding interactions. The negative spin density is found on the bridging N2 ligand in the Fe complex but is not in the Cr complex. All these interesting differences between ISTCs of Cr and Fe are discussed on the basis of the electronic structure and bonding nature.

An anhydrous polymorphic form of trehalose.

An anhydrous polymorphic form of alpha,alpha-trehalose was prepared from trehalose dihydrate by two different drying methods: (1) heating under vacuum; and (2) heating in hot air. Preparation of this anhydrous form by vacuum heating showed good reproducibility. This form was characterized by X-ray powder diffraction analysis and differential scanning calorimetry. This anhydrous form was converted to an amorphous phase at 127 degrees C and was found to be hygroscopic. At 43% relative humidity at 25 degrees C, this form rapidly reverted to dihydrate, while the amorphous phase remained unchanged. When an amorphous phase coexisted with this form, the rate of water adsorption to the amorphous phase was slower than that to the amorphous phase alone. These properties of this anhydrous form of alpha,alpha-trehalose may explain the effects of trehalose in dehydration tolerance of plants and insects in the desert.

Theoretical Study of One-Electron Oxidized Mn(III)- and Ni(II)-Salen Complexes: Localized vs Delocalized Ground and Excited States in Solution.

One-electron oxidized Mn(III)- and Ni(II)-salen complexes exhibit unique mixed-valence electronic structures and charge transfer (CT) absorption spectra. We theoretically investigated them to elucidate the reason why the Mn(III)-salen complex takes a localized electronic structure (class II mixed valence compound by Robin-Day classification) and the Ni(II)-analogue has a delocalized one (class III) in solution, where solvation effect was taken into consideration either by the three-dimensional reference interaction site model self-consistent field (3D-RISM-SCF) method or by the mean-field (MF) QM/MM-MD simulation. The geometries of these complexes were optimized by the 3D-RISM-SCF-U-DFT/M06. The vertical excitation energy and the oscillator strength of the first excited state were evaluated by the general multiconfiguration reference quasi-degenerate perturbation theory (GMC-QDPT), including the solvation effect based on either 3D-RISM-SCF- or MF-QM/MM-MD-optimized solvent distribution. The computational results well agree with the experimentally observed absorption spectra and the experimentally proposed electronic structures. The one-electron oxidized Mn(III)-salen complex with a symmetrical salen ligand belongs to the class II, as experimentally reported, in which the excitation from the phenolate anion to the phenoxyl radical moiety occurs. In contrast, the one-electron oxidized Ni(II)-salen complex belongs to the class III, in which the excitation occurs from the doubly occupied delocalized π1 orbital of the salen radical to the singly occupied delocalized π2 orbital; the π1 is a bonding combination of the HOMOs of two phenolate moieties and the π2 is an antibonding combination. Solvation effect is indispensable for correctly describing the mixed-valence character, the geometrical distortion, and the intervalence CT absorption spectra of these complexes. The number of d electrons and the d orbital energy level play crucial roles to provide the localization/delocalization character of these complexes.