Advances in Molecular Relaxation Processes最新文献

The Mori continued fraction approximation to the orientational memory function is used to evaluate the corresponding orientational correlation function C(t) of dipolar and induced-dipolar absorptions in liquids. In the former case the exponential part of C(t) is the Fourier transform of the relaxational Debye absorption, the short time (non-exponential) part of which is the Poley absorption characteristic of dipolar liquids in the far infra-red. In the latter case, the corresponding C(t) is one of both molecular orientation and interaction, having a correlation time typically of much less than 1 ps. This is approximately the inverse width at half peak height of the broad induced-dipolar absorptions which characterise $\text{non}$-dipolar liquids in the far infra-red (1–250 cm⁻¹). This correlation function never becomes exponential, and is damped to zero in approximately the same interval of time as its memory function - meaning that the temporary dipole absorbs only during its short lifetime. The memory function is the correlation function of the random, fluctuating torque forces experienced by any molecule in the system, and is not a pure exponential. Therefore, this model is non-Markovian.

Dielectric and ultrasonic measurements on binary mixtures of benzene with hexafluorobenzene are reported. The dielectric measurements cover the submillimetre and microwave spectral regions and the ultrasonic absorption experiments were performed at 1.5 MHz. Excess functions computed from these data are found to exhibit a maximum towards the benzene-rich mixtures. The significance of these observations is discussed in terms of the existence of small concentrations of weakly interacting species in these mixtures.

On the basis of the results obtained in a series of structure investigations and a series of theoretical studies the effect of hydrogen bonding on the peptide linkage (the NCO moiety) is discussed. The structure determinations., using X-ray diffraction methods, comprise eight molecules which contain the NCO moiety. The theoretical studies have been carried out using ab initio methods with hydrogen bonded complexes of formamide as model systems. The effect of hydrogen bonding on the peptide linkage, i.e. the shortening of the CN bond and the lengthening of the CO bond, is well reflected by the theoretical calculations. These calculations indicate that the readily delocalized π-electronic system, which can counteract perturbations in the σ-electronic system, is one of the major factors in the ability of the peptide linkage to form hydrogen bonds. On the basis of the structural parameters obtained in the crystal structure determinations and those found in the gaseous state for some simple amides, it is found that the formation of one intermolecular NH… O hydrogen bond, i.e. such that both the nitrogen and oxygen atoms of the NCO moiety are involved, leads to a shortening of the CN bond by ∼ 0.025 Å and a lengthening of the CO bond by ∼ 0.014 Å

A theoretical study of the intermolecular bond N … ClCl has been carried out in the following complexes: amine-chlorine, pyridine-chlorine, nitrile-chlorine. and molecular nitrogen-chlorine. Calculations were performed by Roothaan's method using the computer program GAUSSIAN-70 with an STO-3G basis set. The theoretical results are used to show the existence of stable complexes and to define their characteristics. Heats of formation are generally small. Moreover the charge transfer towards halogen is very weak compared with the polarization of the chlorine molecule. The theoretical results are in good agreement with available experimental results.

Dielectric relaxation times have been determined by Time Domain Spectroscopy at 20 °C for solutions of 1-propanol, 1-butanol, 1-hexanol, 1-octanol and 1-decanol in cyclohexane and for 1-butanol in benzene and carbon tetrachloride. 1-Butanol and 1-propanol solutions in cyclohexane show maxima in their principal relaxation times τ₁ which shift to higher concentrations and longer relaxation times as the alkanol chain lengthens; for the longer alkanols the relaxation times lengthen progressively with concentration. Curves of the reduced relaxation time τ₁/η against mole fraction for all the alkanols in cyclohexane lie, however, near a single `master' curve, which rises from small τ₁/η values for the dilute solutions to a maximum at about 0.4 mole fraction alcohol and then falls to an intermediate value for the pure alkanols.

Dielectric, acoustic, NMR and viscoelastic relaxation measurements are reported on four cyclic carbonates. These liquids are known to possess anomalies in their physicochemical properties and two of the molecules exhibit a marked tendency to supercool. Relaxation times are reported for certain of the systems and comparison between techniques suggests that anisotropy of rotation appears at the same temperature as the observed anomalies.

The standard theories of spin relaxation have not sufficiently emphasised the connection of this phenomenon to other linear irreversible processes and the role of entropy production has also not been clearly discussed.

In order to contribute to this problem, we start here with the linear relation between the generalised fluxes J_i and generalised forces X_i and we show the verification of the bilinear relation in fluxes and forces for the dissipation function Ψ_d In this connection a general expression for the diffusion-like equation is also calculated.

Furthermore, the phenomenological relaxation equation is developed, which shows the proportionalityof therelaxation rate 1/T₁ to the phenomenological coefficient L^E. The quantum-statistical treatment of relaxation starts also from the entropy production principle and one obtains a quantum-statistical alternative of the phenomenological coefficient L^E, which yields finally the molecular expression for the relaxation rate 1/T₁ . This result has been used for the treatment of spin-lattice relaxation by dipole-dipole interaction and one has obtained the same relation for 1/T₁, relaxation rate, as by other methods, using different starting points.

A theoretical study of the intermolecular complexes H₂S … Cl₂ and NH₃ … SO₂, where the sulphur is a “donor” or “acceptor”, respectively, has been carried out. All calculations were performed by Roothaan's method using the computer program GAUSSIAN-70 with an STO-3G basis set. The theoretical results are used to show the existence of stable complexes and to define their characteristics. Charge transfer towards the “acceptor” is weak (Cl₂) or zero (SO₂) but the polarization of the compound is important.