Taro Udagawa

Theoretical Study of H/D Isotope Effects on Nuclear Magnetic Shieldings Using an ab initio Multi-Component Molecular Orbital Method

We have theoretically analyzed the nuclear quantum effect on the nuclear magnetic shieldings for the intramolecular hydrogen-bonded systems of σ-hydroxy acyl aromatic species using the gauge-including atomic orbital technique combined with our multi-component density functional theory. The effect of H/D quantum nature for geometry and nuclear magnetic shielding changes are analyzed. Our study clearly demonstrated that the geometrical changes of hydrogen-bonds induced by H/D isotope effect (called geometrical isotope effect: GIE) is the dominant factor of deuterium isotope effect on 13C chemical shift.

H/D isotope effect on charge‐inverted hydrogen‐bonded systems: Systematic classification of three different types in H3XH…YH3 (X = C, Si, or Ge, and Y = B, Al, or Ga) with multicomponent calculation

Three different H/D isotope effect in nine H3XH(D)…YH3 (X = C, Si, or Ge, and Y = B, Al, or Ga) hydrogen‐bonded (HB) systems are classified using MP2 level of multicomponent molecular orbital method, which can take account of the nuclear quantum nature of proton and deuteron. First, in the case of H3CH(D)…YH3 (Y = B, Al, or Ge) HB systems, the deuterium (D) substitution induces the usual H/D geometrical isotope effect such as the contraction of covalent R(CH(D)) bonds and the elongation of intermolecular R(H(D)…Y) and R(C…Y) distances. Second, in the case of H3XH(D)…YH3 (X = Si or Ge, and Y = Al or Ge) HB systems, where H atom is negatively charged called as charge‐inverted hydrogen‐bonded (CIHB) systems, the D substitution leads to the contraction of intermolecular R(H(D)…Y) and R(X…Y) distances. Finally, in the case of H3XH(D)…BH3 (X = Si or Ge) HB systems, these intermolecular R(H(D)…Y) and R(X…Y) distances also contract with the D substitution, in which the origin of the contraction is not the same as that in CIHB systems. The H/D isotope effect on interaction energies and spatial distribution of nuclear wavefunctions are also analyzed.

Multicomponent Molecular Orbital–Climbing Image–Nudged Elastic Band Method to Analyze Chemical Reactions Including Nuclear Quantum Effect

To analyze the H/D isotope effects on hydrogen transfer reactions in XHCHCHCHY↔XCHCHCHYH (X, Y=O, NH, or CH2 ) including the nuclear quantum effect of proton and deuteron, we propose a multicomponent molecular orbital-climbing image-nudged elastic band (MC_MO-CI-NEB) method. We obtain not only transition state structures but also minimum-energy paths (MEPs) on the MC_MO effective potential energy surface by using MC_MO-CI-NEB method. We find that nuclear quantum effect affects not only stationary-point geometries but also MEPs and electronic structures in the reactions. We clearly demonstrate the importance of including nuclear quantum effects for H/D isotope effect on rate constants (kH /kD ).

Why is N···Be distance of NH3H(+)···DBeH shorter than that of NH3D(+)···HBeH? Paradoxical geometrical isotope effects for partially isotope-substituted dihydrogen-bonded isotopomers.

The partial isotope substitution for the change of geometrical parameters, interaction energies, and nuclear magnetic shielding tensors (σ) of dihydrogen‐bonded NH3X+···YBeH (X, Y = H, D, and T) systems is analyzed. Based on the theoretical calculation, the distance between heavy atoms RN···Be of NH3H+···DBeH is clearly found to be shorter than that in NH3D+···HBeH. Such apparently paradoxical geometrical isotope effect (GIE) on RN···Be is revealed by the cooperative effect of two kinds of (1) primary covalent‐bonded GIE and (2) secondary dihydrogen‐bonded one. We have demonstrated that (1) the covalent bond lengths become shorter by heavier isotope‐substitution and (2) the dihydrogen‐bonded distance RX···Y becomes shorter by heavier Y and lighter X isotope‐substitution due to the difference of electronic structure reflected by the nuclear distribution. We have also found that interaction energy of NH3H+···DBeH is stronger than that of NH3D+···HBeH and isotopic deshielding effect of magnetic shielding becomes large in lighter isotope.

Nuclear quantum effect and H/D isotope effect on F + (H2O)n → FH + (H2O)n−1OH (n = 1-3) reactions

Potential energy profiles for F + (H2O)n → FH + (H2O)n-1OH (n = 1-3) reactions, which are widely relevant to solvent effects on chemical reactions, have been investigated using the conventional quantum mechanical (QM) methods and our multicomponent QM (MC_QM) methods, which can take account of nuclear quantum effect of light nucleus, such as proton and deuteron. For these reactions, Li and co-workers [G. Li et al., J. Phys. Chem. A 117, 11979 (2013)] reported that (i) for F + H2O → FH + OH reaction, MPW1K density functional gave the best barrier among 49 kinds of density functionals and (ii) the energy of transition state of F + (H2O)2 → FH + (H2O)OH reaction is lower than that of the separated reactant molecules by the contribution of the second water molecule using high-accuracy CCSD(T)/cc-pVQZ calculations. We have found that ω B97XD density functional reasonably reproduces the CCSD(T) geometries well, whereas MPW1K was not suited for analyzing F + (H2O)2 → FH + (H2O)OH reaction. Our MC_QM calculations reveal that nuclear quantum nature of hydrogen nucleus lowers the activation barrier of the reactions. The H/D isotope effect on F + (H2O)n → FH + (H2O)n-1OH (n = 1-3) reactions was also investigated.

Self-organization of dipyridylcalix[4]pyrrole into a supramolecular cage for dicarboxylates

Cis- and trans-dipyridylcalix[4]pyrroles were synthesized and cis-dipyridylcalix[4]pyrrole formed a supramolecular cage upon dimerization and coordination with Pd(II). The cage molecule recognised suberate selectively by hydrogen bonding to the two calix[4]pyrroles.

Driving force of metallo (Mg–H and Mg–Cl)-ene reaction mechanisms

The potential-energy surfaces of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene were studied using ab initio molecular-orbital (MO) methods. The concerted path and the stepwise path of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene were identified and it was determined that the energy barriers on concerted paths of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene are lower than those on the stepwise paths. Furthermore, the concerted path of the metallo-ene reaction of allyl-MgCl with ethylene is more favorable than that of the allyl-MgH reaction system. The reaction mechanisms were analyzed using a CiLC method on the basis of CASSCF MOs. The driving force for the concerted path reactions arises from the migration process of the metal. The difference between the reactivity of allyl-MgH and allyl-MgCl can be explained with the reaction mechanism on the basis of the driving force.

Unusual H/D isotope effect in isomerization and keto–enol tautomerism reactions of pyruvic acid: nuclear quantum effect restricts some rotational isomerization reactions

Isomerization and keto–enol tautomerism reactions of the pyruvic acid molecule have been investigated using the multicomponent B3LYP (MC_B3LYP) methods, which can take account of the nuclear quantum effect (NQE) of a light nucleus, such as a proton and a deuteron. While the conventional harmonic zero point vibrational energy (ZPVE) correction makes the activation energies of all the reactions in this system lower, a contrasting behavior is found in our MC_B3LYP results for several rotational reactions. In such cases, the H/D isotope effect on the activation energy is also completely opposite between harmonic ZPVE-corrected B3LYP and MC_B3LYP calculations. In our MC_B3LYP calculation, the activation energies of several C–C or O–H rotational reactions of H species are slightly higher than those of D species, since the NQE of a hydrogen-bonded proton strengthens the hydrogen-bonded interaction more than that of a deuteron, and, thus, the rotational motion of H species is restricted. Such an “unusual” H/D isotope effect on the activation energies can be observed only in the MC_B3LYP results. Our MC_B3LYP calculations clearly demonstrate that direct inclusion of NQE is indispensable to analyze H/D isotope effects on activation energies of not only hydrogen transfer reactions but also C–C and O–H rotational reactions in the isomerization and keto–enol tautomerism of pyruvic acid molecule.

Nuclear quantum effect and H/D isotope effect on Cl· + (H2O)n → HCl + OH·(H2O)n−1 (n = 1–3) reactions

Cl· + (H2O)n → HCl + OH·(H2O)n−1 (n = 1–3) reactions are fundamental and important ones in atmospheric chemistry. In this study, we focused on the nuclear quantum effect (NQE) of the hydrogen nucleus on these reactions with the aid of the multicomponent quantum mechanics (MC_QM) method, which can directly take account of NQE of light nuclei. Our study reveals that the NQE of the hydrogen nucleus lowers the activation barriers of the reactions and enhances the catalytic effects of second and third water molecules. In particular, we find that (i) the NQE of the proton removes the activation barrier of the reverse reaction of HCl + OH· → Cl· + H2O, and (ii) the catalytic effect of the third water molecule appears in only our MC_QM calculation. We also analyze the H/D isotope effects on these reactions by using the MC_QM method.

Selenolactams as Synthetic Intermediates for the Synthesis of Polycyclic Amines via Seleno-Claisen Rearrangements

A highly diastereoselective α-allylation of selenolactams with allyl halides is reported. DFT analyses and experimental observations suggested that this reaction proceeds via a Se-allylation of the eneselenolates of the lactams followed by a seleno-Claisen rearrangement. The thus-obtained products could be efficiently transformed into polycyclic amines using a previously developed sequential addition of organometallic reagents and ring-closing metathesis.