Thomas C. Farrar

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.

Nuclear Magnetic Relaxation Studies of Internal Rotations and Phase Transitions in Borohydrides of Lithium, Sodium, and Potassium

Proton spin–lattice relaxation times, T1, have been measured as a function of temperature for KBH4, NaBH4, and LiBH4. For NaBH4 and KBH4, 23Na and 11B relaxation measurements were also made. In all cases, the magnetization recovery is approximately exponential. Correlation times, τc, derived from the T1 data were used to calculate activation energies, V, for BH4− ion reorientations. For the cubic phase of KBH4, V = 14.8 ± 0.4 kJ/mole (3.55 ± 0.1 kcal/mole) (± always refers to rms error) from measurements on proton and 11B. For NaBH4, V was found to be 11.2 ± 0.5 and 14.8 ± 0.7 kJ/mole (2.7 ± 0.1 and 3.5 ± 0.2 kcal/mole) for the high‐ (cubic) and low‐temperature (tetragonal) phases; an anomaly in τc was observed at temperatures slightly below the phase transition, and may be interpreted as a relatively sudden change in V associated with the phase transition. In LiBH4, a rather broad minimum was observed for the proton T1 vs temperature; this has been interpreted as due to two inequivalent BH4− tetrahedra w...

The driven equilibrium fourier transform NMR technique: An experimental study☆

Abstract The Driven Equilibrium Fourier Transform (DEFT) technique for signal enhancement in pulsed 13C magnetic resonance spectroscopy has been investigated for several small 60%-enriched molecules. The experimental results demonstrate that enhancements in signal/noise over the conventional repetitive single-pulse method are indeed obtainable but are much lower than originally predicted. The principal reason is the fact that T2

NMR STUDY OF BAFPO3: 31P AND 19F CHEMICAL-SHIFT ANISOTROPIES AND THE ABSOLUTE SIGN OF THE F-P COUPLING CONSTANT.

The FPO3= ion in polycrystalline BaFPO3 is investigated from the point of view that it is a very slightly distorted, axially symmetric two‐spin system. NMR line‐shape analysis for the 31P spectra as a function of magnetic field yielded the following information: (1) the chemical‐shift anisotropy, (σ‖ − σ⊥), is − 145 ± 20 ppm for phosphorus; (2) the absolute sign of the F–P indirect dipolar coupling constant is negative; (3) the internuclear F–P distance is 1.63 ± 0.035 A. Investigation of the 19F spectra was carried out through a second‐moment analysis as a function of magnetic field. It was found that the fluorine chemical‐shift anisotropy is + 182 ± 22 ppm, where the sign is deduced from the sense of the line‐shape asymmetry. Further‐more, an estimate of the intermolecular dipolar contribution to the second moment was obtained by extrapolating the total second moment to low field and then subtracting the intramolecular dipolar contribution which is known from the 31P spectra. The intermolecular dipolar ...

13C magnetic relaxation rate studies of chloroform

Abstract The temperature dependence of 13 C spin-lattice ( R 1 ) and spin-spin ( R 2 ) relaxation rates has been studied for 60% enriched chloroform. R 1 is dominated by the intra -molecular dipole-dipole interaction with the proton, and R 2 by scalar coupling to the chlorine nuclei. The activation energies associated with the anisotropic molecular motion, and the rotational diffusion constant, D ⊥ , were obtained from the 13 C relaxation data and found to agree well with those obtained from deuterium and chlorine relaxation studies (1). The carbon-chlorine scalar coupling constant, J cc1 = 23.3 ± 0.8 Hz, was obtained from the difference between R 2 and R 1 .

Proton Magnetic Resonance and Hindered Rotation in Phosphonium Halides and Ammonium Iodide

Proton spin–lattice relaxation times T1 and second moments have been measured as a function of temperature for the phosphonium halides and ammonium iodide. Correlation times τc derived from the relaxation data were used to obtain activation energies for the reorientation of the phosphonium and ammonium ions. The activation energies for PH4Cl, PH4Br, and PH4I are (35 ± 1) × 103, (29.4 ± 0.6) × 103, and (30.7 ± 0.8) × 103 J/mole, (8.3 ± 0.2, 7.1 ± 0.1, 7.3 ± 0.2 kcal/mole), respectively, indicating little change in barrier with halide ion. The frequency factors, however, do appear to vary significantly from crystal to crystal. These results are in marked contrast with previous results for the ammonium halides and suggest that nonelectrostatic repulsive forces are important in the phosphonium salts. The activation energy for NH4+ ion reorientation in the tetragonal phase (Phase III) of NH4I is found to be (13.4 ± 0.4) × 103 J/mole (3.2 ± 0.1 kcal/mole). The present results are compared with previous spectros...

Magnetic Nonequivalence in the High‐Resolution NMR Spectra of Diborane

Proton and boron‐11 nuclear magnetic resonance spectra of 11B‐enriched neat diborane have been measured over the temperature range from −7° to −60°C. The terminal‐proton resonance and the 11B spectrum exhibit partially resolved fine structure which arises from the magnetic nonequivalence of the terminal protons and of the boron nuclei due to long‐range spin coupling. Spectra were calculated which agree quite well with the observed spectra and result in a reasonably accurate determination of the magnitudes and most of the relative signs of the various coupling constants. These values are: JBB = ∓5 Hz, JBHb = +46.2 Hz, JBHt = +133 Hz, J′BHt = +4 Hz, | JHtHb | = 7.2 Hz, JHtHb (trans or cis) = ±14 Hz, JHtHt (cis or trans) = ±6 Hz, | JHtHtgem | < 3 Hz, δ(Ht − Hb) = −4.50 ppm.

Proton and Fluorine NMR Spectra of HBF2

Proton and fluorine NMR spectra of HBF2 were recorded in the temperature range 140°K to 230°K. The values for the spin‐coupling constants JHF, J11B—H and J11B—F are 108±1 Hz, 211±2 Hz, and 84±1 Hz, respectively. JHF is temperature independent. The apparent values of J11B—H and J11B—F decrease below 165°K due to the interaction of rapidly fluctuating electric field gradients with the electric quadrupole moment of boron. The center of the proton multiplet is about 0.12 ppm downfield from the center of the terminal proton multiplet in diborane. The fluorine multiplet is about 60 ppm downfield from the fluorine resonance in BF3, which in this case was a single structureless line. The dependence of 11B–H coupling constants on the orbital hybridization of boron is discussed.

Internal Reorientations in K2ReH9 via Wide‐Line and Pulsed Proton Resonance Studies

Proton spin–lattice relaxation times (T1) and second moments (M2) have been measured as a function of temperature for K2ReH9. The activation energies for internal reorientation of ReH9= ions, and their rms errors, are 9.9 ± 0.4 and 25.0 ± 0.8 kJ/mole (2.4 ± 0.1 and 6.0 ± 0.2 kcal/mole), respectively, for type a and d sites. Our results suggest that the barriers are determined by nearest‐neighbor ReH9=–ReH9= interactions.