Advances in Molecular Relaxation Processes最新文献

The anisotropic optical absorption of macromolecules can be studied, when the molecules are in dilute solution, by subjecting the system to pulsed electric fields. Measurements were made on solutions of six amylose samples of varying molecular weights (M) in the range 1.4 × 10⁵ to 1 × 10⁶, after the biopolymer had been complexed with iodine. The rates of molecular orientation enabled values of τ, the rotary relaxation time, to be obtained. When analysed, these indicated molecular flexibility. The “weakly-bending rod” model appeared to satisfy the experimental data, although a single value of the persistence length (q) did not describe the behaviour. As found in previous studies on other helical polymers, q varied continuously with M. The electrical properties and the degree of orientation in the field indicated that only induced and not permanent dipole moments were present. These induced dipole moments were analysed in terms of an anisotropy of the molecular polarisability. This was consistent with the model of a flexible helix. A value of 28 × 10⁻²⁴ F m was evaluated for the excess polarisability difference per helical pitch. Values of the molecular extinction coefficients for light of wavelength 625 nm when polarised parallel to and perpendicular to the individual helix axes, indicated that the relevant electronic transition is not confined to the long helical axis as had been previously assumed.

The individual ¹³C spin-lattice relaxation times in three molecules of different molecular size - codeine, brucine and reserpine - have been determined. In all three molecules, T₁ for methine carbons, being part of the isotropically reorientating skeleton, ranges from 0.15 to 0.23 sec corresponding to reorientational correlation times of 3 × 10⁻¹⁰ and 2 × 10⁻¹⁰ sec. All carbons, including the quaternaries, ar shown to be relaxed through dipole-dipole interaction. The relaxation rate of non-proton-bearing carbons is found to be governed by geminal and vicinal protons thus allowing reliable assignments to be made that are unavailable from other techniques. The peak heights in completely relaxed ¹³C Fourier transform spectra of medium-sized molecules are shown to reflect spin-spin relaxation, thus permitting a qualitative estimate of this parameter. As an example, the spectrum of 3-cholestanone is discussed.

A theoretical analysis is presented of the lumped capacitance method and its small capacitance approximation using a Debye dielectric permittivity relation and low frequency conductivity. The influence of fringing fields are discussed. The results of the small capacitance method are compared with those for the thin cell method.

The decay of the amplitude for a Carr-Purcell train of echoes in a system with two exchanging sites is investigated. Closed expressions are derived which are valid for all values of the chemical shift, the two transverse relaxation times, and the two average lifetimes. Previously published solutions to this problem considering pulsed and continuous-wave NMR methods emerge as special cases of the treatment presented in this paper. The analysis shows that high-pulse rates eliminate the effect of the chemical shift on the NMR relaxation but that the effect of the chemical exchange disappears only in special cases. A computer program (FORTRAN V)¹ is available for the calculation of spin-echo decay times with a given set of parameters and for the reverse task of evaluating parameters from the experimental data.