A. A. Maryott

35Cl and 19F NMR Spin–Lattice Relaxation Time Measurements and Rotational Diffusion in Liquid ClO3F

The NMR spin–lattice relaxation times of 35Cl and been 19F have measured by pulse techniques over the entire liquid range of ClO3F (130–368°K). The chlorine relaxation which is due solely to the nuclear quadrupole interaction can be used together with the known quadrupole coupling constant to determine the correlation time for molecular orientation, τθ,2. The fluorine relaxation is dominated by the spin–rotation interaction with only a small intermolecular dipole contribution at the lowest temperatures. In order to obtain the angular momentum correlation time, τJ, an independent estimate of the spin–rotation tensor was made by combining gas‐phase measurements of T1(19F) with previous data on the chemical shift and gas‐phase dielectric relaxation. The results for this quasispherical molecule are in accord with rotational diffusion theory and Hubbards relation, τθ,2τJ = I / 6kT, at the lowest temperatures and agree over the entire range with the extended treatment of McClung.

NMR Relaxation Study of Liquid CCl3F. Reorientational and Angular Momentum Correlation Times and Rotational Diffusion

Using pulsed NMR techniques, values of the self‐diffusion constant Ds and the 19F spin‐lattice and rotating frame relaxation times, T1F and T1ρF have been obtained for CCl3F over its entire liquid range. (∼150–450°K). The dependence of T1ρF on the rotating field strength ω1 has been used to derive temperature‐dependent values of the 35Cl spin‐lattice relaxation time T1Cl and the chlorine to fluorine spin‐spin coupling constant J19 F–35Cl(=11.9± 0.4 Hz, independent of temperature). Except at low temperatures where the intermolecular dipole‐dipole relaxation mechanism is important, T1F is dominated by the spin‐rotation interaction (T1F)sr. Using Ds data to separate the dipole‐dipole contribution from T1srF allows us to estimate values of the angular momentum correlation time τJ over a 300° temperature range. Over the same temperature range, values of T1Cl give the correlation times for molecular reorientation τθ,2. Although possible anisotropy in molecular motion and in the spin‐rotation interaction preclud...

Collision‐Induced Microwave Absorption in Compressed Gases. II. Molecular Electric Quadrupole Moments

A search for dielectric loss in the nondipolar gases, He, Ar, CH4, SF6, H2, C2H6, N2, and C2H4 was made at a frequency near 24 kMc and for densities not exceeding 100 amagat. For the first six gases, the loss factor was immeasurably small, less than 2×10—7, at the maximum density. The loss observed in nitrogen and ethylene is attributed to transient dipoles induced by the molecular quadrupole fields during binary collisions. From the data on the quadrupolar gases, including previous results on CO2, values of the quadrupole moments (or upper limits where no loss was observed) are derived which are in reasonable agreement with other estimates.

Collision‐Induced Microwave Absorption in Compressed Gases. I. Dependence on Density, Temperature, and Frequency in CO2

Measurements of the dielectric loss by a resonant cavity technique at 9 and 24 kMc/sec are reported for CO2 for densities up to 100 amagat and 25°C. Some additional data are reported at elevated temperatures. The loss, which increases in proportion to the frequency and very nearly as the square of the density, is attributed to transient dipoles induced by the molecular electric quadrupole fields during molecular collisions. A theoretical analysis including the line shape is made to relate the loss to the pertinent molecular parameters and to permit an intercomparison with precise data available on the second dielectric virial coefficient. It is concluded that the microwave loss in quadrupolar gases may provide a sensitive method of getting information on the molecular quadrupole moments.

Microwave Absorption in Compressed Oxygen

Precise measurements of the absorption in oxygen resulting from the small magnetic dipole moment are reported at frequencies near 2, 3, 9, and 23 kMc and at pressures in the range from 3 to 70 atmos. Up to 10 atmospheres the resonant contribution agrees with the Van Vleck‐Weisskopf theory and the line width increases in proportion to the pressure. The line width (relaxation frequency) for the nonresonant contribution also increases in proportion to the pressure, but is only about one‐third as large as the resonant line width. Above 20 atmospheres the resonant absorption shows anomalous behavior resembling that previously noted in the case of the inversion spectra of NH3 and ND3. In particular, the resonant frequency appears to decrease rapidly while the line width changes much less rapidly than the pressure.

Debye Relaxation in Symmetric‐Top—Foreign‐Gas Mixtures; Temperature Dependence of Collision Cross Sections

The Debye relaxation spectra of three symmetric‐top gases, CH3Cl, CHF3, and SO2F2, in the pure state and in dilute mixtures with the foreign gases, He, H2, D2, Ar, N2, CH4, CO2, and C3F8, were obtained over the temperature range −20° to 145°C at a frequency of 1220 MHz. Relaxation‐rate parameters and the corresponding collision cross sections are derived. The cross sections vary with the absolute temperature as T−m, where the value of m ranges from 0.3 to 0.9 for the different systems. The cross sections are considered from a classical kinetic veiwpoint and an empirical relation is obtained which accurately relates the dielectric cross section to the effective viscosity cross section (or the corresponding Lennard‐Jones parameters), the internal‐to‐orbital‐angular‐momentum ratio, and one additional parameter related in some way to the shape (prolateness or oblateness) of the top. Deviations in the shape of these spectra from the simple Debye form, which are attributable to a distribution of relaxation rate...

Effective collision numbers for angular momentum relaxation from nuclear relaxation studies of simple liquids

Abstract Angular momentum correlation times, τJ, derived from nuclear relaxation studies of a number of liquids composed of small molecules of high symmetry are compared with hard sphere and cell model collision rates. Either model leads to the conclusion that collision efficiency is high (collision number of 1 to 2) and provides a useful and simple relationship for predicting τJ semi-quantitatively over the liquid range.

Collision‐Induced Microwave Absorption in Compressed Gases. III. CO2—Foreign‐Gas Mixtures

Measurements of the increase in dielectric loss accompanying the addition of various foreign gases (He, H2, Ar, N2, CH4, C2H6, and SF6) to CO2 have been made at a frequency near 24 GHz and a temperature of 25°C. This loss is attributed to the dipoles induced in the foreign gas by the molecular quadrupole field of CO2. Excluding mixtures with He and H2, for which the effect is very small, the value of the quadrupole moment of CO2 derived from these losses is about 4.4×10—26 esu in all cases. This is in good agreement with the directly measured value of 4.1×10—26 esu reported by Buckingham and Disch.

Nonresonant Absorption in Symmetric‐Top Gases: Dependence of Relaxation Frequency on Temperature

The nonresonant absorption spectra of CHF3, CH3F, and CClF3 were obtained in the gaseous state at various temperatures in the range 230° to 360°K. In all cases the maximum value of the dielectric loss per unit pressure varies as T—2, in accordance with the Debye equation. The variation of relaxation frequency (line width) with pressure and temperature is represented by Δν∞pT—m, where m has the following values: 1.59±0.03 for CHF3, 1.60±0.02 for CH3F, and 1.27±0.02 for CClF3, Since CHF3 and CH3F have rather large dipole moments, the predominant interaction should be of the first‐order dipole‐dipole type. On this basis Andersons theory predicts m = 1. Closer agreement with the experimental data is obtained with an expression derived essentially from dimensional considerations of a torque‐impulse model, which gives m = 1.5. In the case of CClF3, which has a comparatively small dipole moment, the data indicate that molecular reorientation is governed primarily by forces of shorter range.

Absorption in the Low‐Frequency Wing of the NH3 Inversion Spectrum

The absorption due to the low‐frequency wings of the NH3 inversion lines was measured accurately at 2800 Mc for a pressure of 10‐cm Hg. At this pressure, where the shift in inversion frequency has not set in, the absorption was about 40 percent larger than that calculated by summing the Van Vleck‐Weisskopf expression over the rotational states using known resonance frequencies and line widths. A similar discrepancy was found in comparing the theoretical absorption with previously obtained data at 9000 Mc, while at resonance, 24 000‐Mc theory and experiment were in very good agreement. To investigate various features of pressure broadening, the variation of absorption with pressure (5 to 30‐cm Hg), temperature (−12 to 100°C), and frequency (1200 to 2800 Mc) was examined. These functional variations are interpretable, for the most part, by the Van Vleck‐Weisskopf relation.