Noboru Hirota

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.

Translational diffusion of a transient radical studied by the transient grating method, pyrazinyl radical in 2‐propanol

A translational diffusion coefficient (Ddiff) of a transient radical created by a photochemical reaction of pyrazine in 2‐propanol is measured by the transient grating technique. The rise and decay curves of the transient grating signal on the millisecond time scale are attributed to the diffusion of the pyrazinyl radical and the parent molecule (pyrazine). Ddiff of both species are determined. The measured Ddiff of the radical is four times smaller than that of pyrazine even though the molecular size is similar to each other. The small Ddiff suggests that the radical–solvent interaction is stronger than the parent molecule–solvent interaction.

Measurement of the quantum yield of triplet formation and short triplet lifetimes by the transient grating technique

A transient grating method for measuring the quantum yield of triplet formation (Φisc) and lifetimes of short‐lived triplet states is proposed. The method is based on a measurement of time dependence of a thermal grating signal with a nanosecond pulsed laser and a cw probe laser. The validity of the method is demonstrated by determining Φisc of a number of molecules whose Φisc are known. Advantages and limitations of this method are discussed in detail.

Temperature dependence of the CIDEP spectrum of acetone

The CIDEP spectrum of acetone has been studied by means of time‐resolved EPR over a wide range of temperature. It was found that the spectrum varied remarkably depending on temperature indicating that different CIDEP mechanisms are involved at different temperatures. The CIDEP mechanisms operating in each temperature region are determined quantitatively mainly from the simulations of the spectra and their dependence on the diffusion constant of the solvent; from room temperature to −90 °C, the radical pair mechanism involving S–T0 mixing (ST0M)is always dominant, but the triplet mechanism and the radical pair mechanism involving S–T−1 mixing (ST−1M)become important above and below −45 °C, respectively. The relative importance of ST−1M as compared with ST0M determined below −60 °C is in good agreement with the prediction of the theory. We have also found two interesting phenomena that relatively strong E/A peaks appeared in the center portion of the complete E/A spectrum around −45 °C, and that the line sh...

An ab initio calculation on proton transfer in the benzoic acid dimer

Abstract We have made an ab initio calculation of the barriers for proton transfer in the hydrogen-bonded dimers of benzoic acid and acetic acid. Geometrical optimization values which are closer to the experiment one.

Quantities, terminology, and symbols in photothermal and related spectroscopies (IUPAC Recommendations 2004)

This paper presents quantities, terminology, and symbols of terms related to photothermal phenomena and used in photothermal and related spectroscopies. The terms used in the literature to describe photothermal phenomena and methods are reviewed, and a glossary of terms is given. The origins of photothermal phenomena, as well as the relations among various photothermal effects, are summarized. The listed terms cover the terminology in transient grating, transient lens, photoacoustic spectroscopy, photothermal radiometry, calorimetry, interferometry, deflection, reflection, and other related spectroscopies, which use or are related to photothermal effects.

A chemically induced dynamic electron polarization study on the acetone ketyl radical and radical pair in an alcohol solution

A quantitative study of the chemically induced dynamic electron polarization (CIDEP) spectra of acetone and deuterated acetone ketyl radicals in i‐propanol over a wide temperature range has been done to understand the properties of the radical pairs in solution, e.g., the interactions between the radicals, the diffusional and rotational motions of the radicals of the geminate radical pairs, and the microscopic solvent structure. The spin polarization of the separated radical is analyzed in terms of the stochastic‐Liouville equation. It is shown that the observed polarization cannot be explained on the basis of the normal diffusion of the radicals in the pair. The modified diffusion models which presuppose slow diffusion of the radicals in a microscopic solvent structure can account for the observed polarization. The radical pair electron paramagnetic resonance (EPR) spectra observed at very low temperatures (<∼−70 °C) are explained by assuming that a considerable fraction of the radicals are held together...

Translational diffusion of transient radicals created by the photoinduced hydrogen abstraction reaction in solution: Anomalous size dependence in the radical diffusion

Diffusion coefficients (D) of various radicals created by the photoinduced hydrogen abstraction reactions from alcohols (ethanol and 2‐propanol) are investigated by using the transient grating (TG) method. In all the reaction systems, D’s of the transient radicals, as well as those of the parent molecules, can be measured simultaneously. The results clearly show slower diffusive motions of the radicals, at least of the π radicals in the hydrogen abstraction reaction systems, compared with those of the parent molecules. D’s of the parent molecules usually agree well with the calculated values based on the Spernol and Wirtz modification of the Stokes–Einstein (SE) relation. Although the measured D’s of the radicals are closer to the values predicted by the simple SE equation, the agreements and the ratio of D between the radicals and its parent molecules depend on the molecular size. The ratio becomes closer to unity as the molecular size becomes large. Possible origins of this dependence are discussed.

Time-resolved EPR investigation of the triplet states of C60 and C70

Abstract The triplet states of C 60 and C 70 (fullerenes) in solid phase at 3 and 77 K are detected by the time-resolved EPR technique. The observed time-resolved EPR spectra and the time profiles of the transient signals indicate that the geometries of C 60 and C 70 are distorted from the soccerball geometry (I h symmetry) and D 5h symmetry, respectively, in the excited states. Remarkable time dependences of the signals observed at 3 and 77 K are analyzed in terms of the anisotropic sublevel decay rates (3 K) and of the anisotropic spin—lattice relaxation rates (77 K).