T.A Scott

Anisotropy of the chemical shift tensor for solid carbon monoxide

Abstract The chemical shift anisotropy Δσ = σ|| − σ⊥ of the CO molecule has been measured by 13C NMR in the solid α phase of carbon monoxide. A polycrystalline sample, enriched to 93.7% in 13C, was used and Δσ was deduced from computer simulation of the experimental powder-pattern lineshape. A small amount of anomalous structure appears in the lineshape, which may be due either to preferred orientation of crystallites or to slow reorientational motion that is possibly associated with molecular ordering in the solid. The measured chemical shift anisotropy is 365 ± 20 ppm at 4.2 K, decreasing to 335 ± 20 ppm at 46 K. Rapid librational motion of the molecules produces motional averaging responsible for the temperature dependence. The averaging factor was estimated in the harmonic approximation using published Raman frequencies to be 0.90 ± 0.02 at 4.2 K and 0.83 ± 0.02 at 46 K, in excellent accord with the observed temperature dependence of Δσ. The chemical shift anisotropy for a static molecule is 406 ± 30 ppm. This value is in good agreement with theoretical estimations based on a molecular beam measurement of the spin-rotational interaction and ab initio calculations of the diamagnetic shielding.

A nuclear magnetic resonance study of molecular motion in liquid and solid ammonia

Abstract Proton spin-lattice relaxation times have been measured in polycrystalline solid ammonia (purity 99.999 %) in the temperature interval from 62.5 to 195.4 K and at resonance frequencies of 10.8, 16.5, 20.8, and 35.0 MHz using spin-echo apparatus. Experimental results are compared with Hilt-Hubbard theory. Relaxation is dominated by thermally activated molecular rotation about the C3 axis of the molecule. A T1 minimum is observed. The activation energy for hindered rotation is 2.30 ± 0.04 kcal/mole and the temperature dependence of the correlation time is given by τc = (1.32 ± 0.43)10−13 exp (1155/T) sec. Proton lineshapes were recorded by means of continuous wave NMR for temperatures from 2 to 195.4 K. A transition occurs in linewidth (between points of maximum slope) at about 65 K but is not detected in the second moment M2. The experimental M2 at 2 K is 17.5 G2 whereas the theoretical static lattice value is approximately 48 G2. The reduction is attributed to quantum mechanical tunneling of protons and the linewidth transition at 65 K is believed to be due to onset of thermally activated rotation. A second linewidth transition, which is associated with a change in M2, begins at about 160 K and continues to the melting point. A small gradual decrease in M2 over the entire temperature range is attributed to thermal expansion. Spin-lattice relaxation of 14N was measured in the liquid between 195.4 and 239.8 K and analyzed to obtain the correlation time τQ for molecular reorientation. The associated activation energy is 1.6 ± 0.3 kcal/mole. Comparison with other work implies a significant degree of hydrogen bonding in the liquid.

Anisotropy of the chemical shift tensor for solid nitrogen

Abstract The chemical shift anisotropy of the nitrogen molecule in solid nitrogen at 4.2 K has been measured by 15N NMR of a sample enriched to 99.5% 15N. The NMR powder pattern spectrum consists of a Pake doublet, due to the intramolecular dipolar interaction, superposed on the chemical shift. The spectrum was studied as a function of magnetic field and it is pointed out that the reversal of one-half of the Pake doublet, which occurs with increasing field, provides a simple and sensitive way to measure the chemical shift anisotropy. Computer simulation of the lineshape yields 〈σ∥ − σ⊥〉 = 520 ± 20 ppm, as averaged by zero-point motion in the crystal. The chemical shift anisotropy of a static molecule is given by 〈σ∥ − σ ⊥ 〉 ζ where ζ, the orientational order parameter, has the value 0.866 ± 0.009 at 0 K for 15N2. The static result is σ∥ − σ⊥ = 603 ± 28 ppm, which is in good agreement with an estimated value based on a molecular beam measurement of the spin-rotational interaction and ab initio calculations.

Pressure dependence of the melting and ?-? phase transition temperatures of carbon monoxide

The pressure dependences of the melting temperatureTm and crystallographic phase transition temperatureTαβ of carbon monoxide have been measured as a function of hydrostatic pressure to 1.75 kbar by using associated changes in the nuclear spin-spin relaxation timeT2 of13C as an indicator of the transitions. From the Clausius-Clapeyron equation the volume increments at zero pressure are deduced to beΔVm=2.50±0.07 andΔVαβ=0.92±0.02 cm3/mole. The CO phase diagram is compared with that of N2.

Nuclear spin-lattice relaxation in solid and liquid 15N2 and 14N2

Abstract The nuclear spin-lattice relaxation time T1 of 15N has been measured in pure liquid nitrogen for the temperature interval 63.1 I = 1 2 ), and the angular momentum correlation time τsr, may be deduced. The temperature-dependence graph of τQ in nitrogen exhibits a discontinuity at the triple point which implies that molecules reorient more slowly in the liquid than in the solid. The Hubbard relation τQτsr = I0/6kT (I0 = molecular moment of inertia) developed for rotational diffusion of spherical molecules in a liquid is approached in liquid nitrogen for T

Pressure and temperature dependence of the nuclear quadrupole coupling constant of 23Na in single crystal sodium nitrate

Abstract NMR of 23 Na in single crystal sodium nitrate has been observed in the temperature interval from 25 to 295°C. The nuclear quadrupole coupling constant ν Q was measured as a function of temperature at atmospheric pressure and as a function of hydrostatic pressure at eleven fixed temperatures. ( ∂ν Q / ∂P )| 1 atm is negative at 25°C, but decreases in magnitude with increasing temperature and is zero at 205°C, and takes on increasingly larger positive values at higher temperatures. At the lambda transition (276°C) this slope abruptly drops to a much smaller value. The unusual negative value of ( ∂ν Q / ∂P ) at low temperatures is attributed to the anisotropic compressibility of NaNO 3 and the consequence that this has on the contribution to the electric field gradient that is produced by the six nearest neighbor oxygens. A strong temperature dependence of ν Q above 150°C is associated with the phase transformation and is viewed as a consequence of increasing dynamic disorder involving hindered reorientation of nitrate groups.

Lineshape and temperature dependence of the nuclear quadrupole resonance of 14N2 in β-quinone clathrate

Abstract Nuclear quadrupole resonance of 14 N 2 molecules entrapped in β hydroquinone clathrate has been studied using Fourier transform spectroscopy. The spectrum at 4.2 K spans the frequency range 3.577 to 3.596 MHz and consists of three well-solved major components, each showing partially resolved fine structure. A consistent interpretation of intensities and frequencies is given by postulating that an N 2 molecule introduced into a clathrate cavity produces a slight lattice distortion extending to nearest-neighbor cavities arrayed along the crystallographic c axis, thereby affecting the electric field gradient. A one-to-one correlation is established among the three major components of the spectrum and the possible configurations for occupancy of nearest-neighbor cavities. Fine structure of the low- and high-frequency components is due to intramolecular dipolar interaction, whereas the middle component additionally exhibits a Stark splitting due to an asymmetric cavity environment. From the temperature dependence of the resonance frequency the molecular libration frequency in the cavity is deduced to be 50.7 cm −1 at 0 K, the zero-point rms amplitude is 11.3°, and the static resonance frequency is 4.025 MHz. The libration frequency decreases by about 4% on warming to 80 K.

Temperature dependence of dipolar spin-echo envelope modulation in solid α-nitrogen

Abstract The temperature dependence of the intramolecular dipolar splitting of the nuclear quadrupole resonance of 14N has been studied in solid αnitrogen from 4.2 to 35.6°K by observing the modulation of the envelope of the spin echoes. Over this temperature range the modulation period increases by 20% and this is ascribed to progressive motional averaging of the dipolar interaction by the molecular librations. The averaging factor is identical within 2 % with that for the quadrupole resonance frequency and is satisfactorily explained theoretically using published Raman data on the libration frequencies.

Lithium-7 NMR in Paramagnetic LiCuCl3 · 2H2O☆

Nuclear magnetic resonance of 7Li in polycrystalline LiCUCl3·2H2O has been studied as a function of temperature for 77 ⪯ T ⪯ 300 K. In accordance with known crystal structure, a standard spin-32 quadrupolar-perturbed powder pattern is observed, consisting of a central component 12 → −12 transition) flanked by broad satellites. The linewidth of the central component is dominated by dipolar interaction with neighboring protons, and in the temperature interval 165⪅ T ⪅ 190 a fourfold narrowing occurs on warming which is attributed to onset of 180° flipping motion of the H2O molecules. At temperatures above this motional narrowing transition the quadrupole coupling constant and asymmetry parameter were deduced to be 30 ± 2 kHz and 0.60 ± 0.03, respectively, on the basis of resolved structure in the satellite spectrum.