Fumio Imashiro

An NMR relaxation study on the proton transfer in the hydrogen bonded carboxylic acid dimers

We have studied the proton spin‐lattice relaxation times (T1) of a series of benzoic acid (BAC) derivatives and decanoic acid (DAC) over a wide range of temperature and analyzed the results in terms of the double proton switching along the hydrogen bonds. The proton T1 in the high temperature region are analyzed using the classical jump model and the barrier heights for the proton transfer are determined. The thermodynamic parameters for the equilibria between the two configurations in the solid state are also determined by the FT–IR measurements. It is shown that the energetics and dynamics of the proton transfer in DAC and the para‐ and meta‐substituted BAC are all similar, but they are very different in the ortho‐substituted ones. It is suggested that the low temperature behavior of the proton T1 of the dimers of carboxylic acid is due to the tunneling and the asymmetry of the potential brings in a small activation energy.

A study on the proton transfer in the benzoic acid dimer by 13C high-resolution solid-state NMR and proton T1 measurements

Abstract By a solid-state 13 C and proton NMR study of the hydrogen bonded dimer of benzoic acid, the rate of proton transfer, the height of the potential barrier for the transfer and the relative population of two different configurations were determined.

Study on conformations of n‐alkanes in urea inclusion compounds using CH dipolar switching‐angle sample spinning nuclear magnetic resonance and MM2 molecular mechanics calculations

The CH dipolar spectra of n‐heptane, n‐octane, n‐nonane, and n‐decane in urea inclusion compounds were separately obtained for the α‐, β‐, and inner‐CH2 and CH3 groups using the SASS (switching‐angle sample spinning) NMR method. The CH dipolar coupling constant for the CH2 group decreases with the increasing distance from the center of the molecule and with the decreasing chain length. MM2 molecular mechanics calculations suggested that the included molecule can have various conformations, and provided their geometries and populations. The experimental results could be reproduced by using the calculated parameters and assuming the occurrence of dynamical disorder of various conformations in addition to rotation of the included molecule around the long axis. This is the first evidence for the existence of conformations other than all‐anti in the included n‐alkane molecule; it has long been taken to be in the all‐anti conformation. Previous studies which were explained by the all‐anti conformation are reint...

A study of hydrogen-bond dynamics in carboxylic acids by NMR T1 measurements: isotope effects and hydrogen-bond length dependence

Abstract Proton (deuteron) transfer of hydrogen bonds in benzoic, glutaric and p-formylbenzoic acids was studied by proton (deuteron) T1 measurements. Deuteration of carboxylic protons was found to increase the barriers to classical proton jumping as well as quantum-mechanical tunneling. The former barriers increase as the hydrogen-bond distance increases.

Comment: Proton transfer in solid benzoic acid. Reply to the comment by K. Furić

Abstract In response to the Comment of Furic, attention is drawn to four points: the dinstinction between the reaction and normal coordinates, zero-point energy, an alternative of the NMR results, and ab initio calculations.

1H and 13C NMR study on rotation of congested methyl groups in methyl substituted phenanthrenes, fluorenes, and fluorenones

We have investigated methyl rotation in 4‐methyl and 4,5‐dimethyl substituted phenanthrenes, fluorenes, and fluorenones by the temperature dependence of the 1H spin‐lattice relaxation times and high‐resolution solid‐state 13C NMR spectra. Distinctly unsymmetric curves of the relaxation times with temperature are ascribed to the existence of inequivalent methyl groups in the unit cell. The inequivalence is considered to be brought about by distortion about the central C4a–C4b bonds and anisotropic intermolecular forces in the crystalline state. The steric effect of the protruding 5‐hydrogen atom in a planar molecular structure gives rise to a remarkably high barrier (5.05±0.13 kcal mol−1) to rotation of the methyl group in 4‐methylphenanthrene (1).

`Guest-Free' Dianin's Compound

The existence of empty hour-glass-shaped cavities is confirmed for the crystal of the title compound, 4-(p-hydroxyphenyl)-2,2,4-trimethylchroman [systematic name: 4-(2,2,4-trimethylchroman-4-yl)phenol, C 18 H 20 O 2 ]. The structure of the cavities is essentially invariant when guest molecules are intercalated. The effects of the cage structure on 13 C NMR chemical shifts of the methyl C atoms are elucidated.

A 13C NMR study of the dynamic structure of the thiourea-ferrocene inclusion compound

Abstract 13 C NMR anisotropic chemical shielding spectra of the thiourea-ferrocene inclusion compound were measured between 125 and 294 K. The orientations and dynamic behaviour of ferrocene molecules trapped in the channels are discussed.

NMR study of molecular motions in perisubstituted naphthalenes

Molecular motions in solid 8‐substituted 1‐methylnaphthalenes with the substituents H, CH3, Cl, and Br have been investigated by the temperature dependence of the 1H spin‐lattice relaxation times and of the 1H second moments and high‐resolution solid‐state 13C NMR spectra with or without magic‐angle sample spinning. The observed barriers to rotation of the perimethyl groups increase in the order of H (2.30 kcal/mol)≪CH3 (3.22)<Cl (3.61)<Br (4.37), which is in line with the sequence of the van der Waals radii. The halogenated methylnaphthalenes are found to undergo in‐plane molecular rotation with relatively low activation energies (4.5–4.7 kcal/mol). Molecular motions in 1,4,5,8‐tetramethylnaphthalene are also discussed.

Electronic structures and charge-transfer spectra of tris[2.2.2]paraxylylene and related compounds

Abstract Electronic spectra of tris [2.2.2]paraxylylene (3°-PX) and some related compounds were measured in order to investigate the electronic interaction (pπ-pπ and pσ-pσ type) between “normal” (undistorted) benzene rings. 3°-PX showed a large hypochromic effect, i.e., its oscillator strength was observed to be 0·61 times of that of the corresponding linear polyparaxylylene. Charge-transfer (CT) spectra between these compounds and some electron acceptors were also studied. MO calculations (the VI/1, CNDO/2 and CI methods) were carried out in order to elucidate the electronic structure of 3°-PX and also the CT interactions between tetracyanoethylene and 3°-PX, benzene, toluene or p -xylene.