Hideaki Chihara

Note on the Theory of Nuclear Spin Relaxation Exact Formulae in the Weak Collision Limit

A general relation exists between the relative magnitude of the power spectra J ( m ) (ω) that appear in the expressions of nuclear spin relaxation times T 1 , T 2 and T 1ρ of powdered specimen, i.e. J (0) (ω): J (1) (ω): J (2) (ω)=6:1:4. the relation holds exactly in the weak collision case irrespective of the modes of motion responsible for the relaxation. The proof is based on the symmetry properties of the spherical harmonics of rank two and therefore applicable not only to dipole-dipole but also to quadrupole coupling interaction and other problems. The reduction factor of the second moment of the nuclear magnetic resonance that results from a type of molecular motion also bears a simple general relation to the minimum value of T 1 due to the motion.

Heat capacity of solid tetrachloro‐p ‐benzoquinone (chloranil) between 11 and 300 °K. Phase transition at 92 °K

Molar heat capacity of solid chloranil was measured between 11 and 300 °K with particular attention to the phase transition at 92 °K. The heat and entropy of the transition were 38 J mol−1 and 0.41 J °K−1 · mol−1. The transition is of a lambda shape starting at 70 °K and ends at 100 °K. The anomalous heat capacity curve was explained by the soft librational mode theory. There is another large transition at about 570 °K. The shape and magnitude of the transitions are very sensitive to the long‐range regularity of the structure and depend on the method of preparation.

Rotational states of NH4+ in KBr crystal. A complementary study by inelastic neutron scattering and calorimetric measurements

Rotational tunneling energy states of the NH 4 + ion doped in KBr crystal have been reinvestigated so as to obtain decisive evidence on which symmetry is the most likely one for the orientation of the ions in the lattice. The inelastic neutron scattering experiments at 1.8 K uncovered the existence of two characteristic bands in the energy range e<8 meV, which leads to the conclusion that the model of energy states for C 3 v symmetry is exclusively acceptable for this system. The Q -dependence of the scattering features was also shown to be reasonably consistent with the model. Moreover, both the “equilibrium” and “instantaneous” heat capacities obtained between 4 and 10 K yielded the confirmation of the C 3 v symmetry. The rate of conversion between the nuclear spin symmetry species of NH 4 + was examined.

Molar heat capacity and thermodynamic properties of p-quaterphenyl☆

Abstract The molar heat capacity of p -quaterphenyl was measured by adiabatic calorimetry between 3 and 300 K. The phase transition associated with a molecular-conformation change is observed as a very broad anomaly with a maximum at (233.0±0.5) K. The molar enthalpy and entropy of transition are (414±20) J·mol −1 and (1.82±0.10) J·K −1 ·mol −1 , respectively. A comparison is made with the properties of biphenyl and p -terphenyl.

Structure and molecular motion of p-chlorobenzotrichloride as studied by nuclear quadrupole resonance

Abstract In an attempt to study interactions between the π system and the substituents, pure quadrupole resonance of Cl · C6H4 · CCl3 was studied of its Zeeman effect (77 K), the temperature dependence of the three resonance lines (20–279 K, mp), and the relaxation times T1, T2 and T 2 ∗ (77–278 K). The relative orientation of the CCl3 group with respect to the benzene ring is such that one of the Cl atoms (Cl2) is placed above this plane and the other two below the plane of the ring. The asymmetry parameter η was 0.07 for Cl directly attached to the ring and 0.04 for the Cl atoms in the CCl3 group. The temperature dependence of the resonance frequencies gives 9.2 ± 0.4 kcal·mole−1 for the activation energy for reorientation of CCl3 group, which agrees with the value of 10.9 ± 0.3 kcal · mole−1 derived from the T1 result. There is some evidence for the charge migration interaction between the chlorine atom in CCl3 group and the π system of the benzene ring mediated by the localized σ orbital of the α-carbon atom.

Evidence of coupled rotational tunnel states of methyl groups of hexamethylbenzene by nuclear magnetic resonance

Abstract Two tunneling frequencies were identified for methyl groups in hexamethylbenzene from 4.2 up to 30.9 K by means of the N.M.R. field cycling method. The frequencies were 9.6 ±- 0.4 MHz (A) and 7.9 ±- 0.4 MHz (B) at 4.2 K. The frequency A was almost independent of the temperature whereas the frequency B decreased as the temperature increased, the decrease being a linear function of T3. These facts indicate that the six methyl groups in hexamethylbenzene molecule are not independent but are coupled to one another rather strongly.

Heat capacity anomalies due to successive phase transitions in 1,1′-biphenyl☆

Abstract Precision heat capacity measurements on 1,1′-biphenyl revealed two broad anomalies at 11.0 K and 40.4 K. The excess enthalpies and entropies are 0.28 ± 0.02 J mol−1 and 0.026 ± 0.002 J K−1mol−1 for the anomaly extending from 7.5 to 14.0 K, and 5.02 ± 0.08 J mol−1 and 0.129 ± 0.003 J K−1mol−1 for the anomaly extending from 30.0 to 47.0 K, respectively. A possible molecular mechanism is suggested.

Rb2ZnCl4: three heat-capacity anomalies due to soft-mode and commensurate-incommensurate transitions☆

Abstract There are three phase transitions for Rb 2 ZnCl 4 . The transition temperatures, the enthalpies of transition, and the entropies of transition are (74.6±0.15) K, (30±1) J·mol −1 , (0.42±0.01) J·K −1 ·mol −1 ; (195.2±0.05) K, (6.2±0.9) J·mol −1 , (0.032±0.005) J·K −1 ·mol −1 ; (303.2±0.3) K, (222±7) J·mol −1 , (0.66±0.14) J·K −1 ·mol −1 . The second transition shows a very sharp heat-capacity peak but the other two are much broader, being consistent with the soft-mode mechanism. Thermodynamic functions of Rb 2 ZnCl 4 are tabulated. A comparison is made with the behaviour of ferroelectrics of the same type: K 2 SeO 4 and Rb 2 ZnBr 4 .

Nuclear Quadrupole Resonance Study of Molecular Motion and Intermolecular Forces in Solid Chlorine

Nuclear quadrupole resonance frequencies of 35 Cl and 37 Cl in solid chlorine were measured between 20°K and its triple point 172°K. The spectrum consists of a doublet at low temperatures because of isotopic splitting. The results were analyzed in terms of temperature-dependent libration frequencies in a quasi-harmonic approximation. The quadrupole coupling constant for 35 Cl was found to be 108.975±0.003 Mc/sec at 0°K and 109.872±0.010 Mc/sec for the static lattice. The librational frequency was 90.1±3.0 cm -1 at 0°K decreasing to 74.3 cm -1 at 160°K. The lower limit to the asymmetry of the electric field gradient was 8.5% in comparison to bromine (20%) and iodine (17.5%), showing the existence of some covalent character in the intermolecular forces. Some problems concerning the intermolecular potential in solid and gaseous chlorine are pointed out in connection with the re-analysis of the second virial coefficient.

Solid neopentane C(CH3)4 as studied by nuclear magnetic resonance A detailed examination of methyl and molecular reorientation in the low temperature phase

The molecular motions in solid neopentane were investigated by the continuous-wave and the pulsed N.M.R. methods from 4·2 K to its melting point. A single broad minimum of the spin-lattice relaxation time T 1 at 100 K is caused by reorientation of the methyl groups and of the entire molecule. An attempt was made to interpret the T 1 data in the low-temperature phase by a detailed treatment, that is the triangular structure of methyl protons was taken into the calculation of intermethyl dipole interactions. The jumping rate (angular frequency) of methyl reorientation about its C 3 symmetry axis is ω 3 0=1·65×1012 exp (−8·0 kJ mol−1/RT) s−1 and the overall molecular reorientation about its C 2 or C 3 symmetry axis occurs in an isotropic (ω 2=ω 3=2·49×1O15 exp (−18·5 kJ mol−1/RT) s−1) or slightly anisotropic manner (ω 2≠ω 3). The potential barrier hindering the rotation of methyl groups is not a simple sinusoidal function and the tunnelling frequency of the methyl group is of the same magnitude as the local ...