Reorientation Of Ammonium Ion Research Articles

Deuteron NMR spectra and relaxation in fully and partly deuterated [formula omitted

Deuteron NMR spectra and relaxation were studied at the resonance frequency of 46 MHz in polycrystalline fully and partly deuterated ( NH 4 ) 2 ZnCl 4 between 300 and 5 K. Spectral components confirm existence of ammonium positions with different potential symmetry, resulting in two- and threefold reorientation of ammonium ions. The temperature dependence of the spin–lattice relaxation rate discloses two time constants in the whole range. The fitting procedure allows the separation into contributions from subsystems of ions in respective potentials. Two relaxation rate maxima are attributed to ions performing threefold uniaxial reorientation at low temperatures. The lower-temperature maximum is observed at T ⩽ 36 K . With increasing temperature reorientations about remaining axes start to contribute leading to the other maximum near 100 K. The other category of ammonium ions gives rise to the maximum at about 50 K. Below this temperature the dominant motion seems to be 180 ∘ reorientations about one twofold axis according to observed spectra. Consistent picture of ion mobility is accomplished for 5%, 30%, 70% and 100% deuterated compounds.

Ammonium ion dynamics in NH4Br at high pressure - type of reorientation

Proton NMR relaxation measurements were performed for ammonium bromide at pressures up to 800 MPa. The correlation functions for ammonium ion reorientation were calculated by Monte Carlo method. Knowledge of these functions allow calculation of the NMR relaxation times T1. Comparison between experimental and calculated values of relaxation times gives some insight into the model of ammonium ion reorientation.

Formula omitted] NMR study of natural and partially deuterated ammonium hexachlorotellurate

The proton spin-lattice relaxation times in natural and partially deuterated ammonium hexachlorotellurate have been measured. At high temperature (above 50 K) the relaxation curve reveals the classical, hindered reorientation of the ammonium ion with the same activation energy for all studied samples E a=5.6 kJ/mol. Below 50 K, the minima which appear in the T 1 temperature dependencies are related to the level-crossing. The tunnelling splitting of the ground torsional level in isotopic pure ammonium hexachlorotellurate is equal to 55 MHz and increases with increasing deuteron concentration, due to the phase transitions occurring in partially deuterated crystal. The lowest temperature minimum in isotopic pure (NH 4) 2TeCl 6 can be interpreted as a result of the rigid ammonium ion's reorientation about the threefold crystallographic axis. The limited jumps of NH 4 + are characterised by the angle Δ=4° between the NH bond and [111] direction.

ChemInform Abstract: The Thermochemistry of Ammonium Compounds and the Energy Barrier Opposing Ammonium Ion Reorientation.

AbstractA good linear correlation between the energy barrier opposing cation reorientation in a compound (NH4)nX and the quantity (Xn‐) = 1/n(ΔHf°((NH4)nX,s) ‐ ΔHf°(RbnX,s)) is discussed and used to predict thermodynamic properties of both known and unknown compounds.

The thermochemistry of ammonium compounds and the energy barrier opposing ammonium ion reorientation

There is a good linear correlation between the energy barrier opposing cation reorientation in a compound (NH4)nX and the quantity Δ(Xn–)=1//n{ΔH⊖f[(NH4)nX, s]–ΔH⊖f(RbnX, s)}. At zero barrier height, Δ(Xn–)= 143 ± 3 kJ mol–1. There is a sense in which depressions below this threshold are a measure of cation–anion hydrogen bonding but this sense is an unconventional one. The estimated value of ΔH⊖f(NH4+, g) derived from the threshold is 638 ± 9 kJ mol–1. The correlation is also used to improve literature thermodynamic data on ammonium and alkali-metal perrhenates, e.g., ΔH⊖f(NH4ReO4, s)=–965 ± 5 kJ mol–1. Unknown enthalpies of formation of some ammonium and rubidium salts are also predicted, and the procedure is used to explore the thermodynamic stability of non-existent solid ammonium compounds. Both metallic ammonium and ammonium hydroxide are unstable at normal pressures. The former is most unlikely to be stabilized at high pressure, but it may be possible to synthesize the hydroxide in this way. The problem of the ionic radius of NH4+ is also discussed.

Nuclear Magnetic Relaxation of 35CI in NH4Cl and ND4Cl

Temperature dependence of the 35 Cl spin-lattice relaxation time T 1 (5.85 and 3.5 MHz) and that of T 1ρ in the rotating frame have been measured in ND 4 Cl and NH 4 Cl between 330 K and 77 K. The 90°-flip motion of ammonium ions is investigated in detail. The spin-lattice relaxation times of 35 Cl may be interpreted to be a typical one governed by the reorientation of ammonium ions in the extremely asymmetric potential well.

Temperature dependence of 35Cl NQR in (NH 4) 2SnCl 6 and (NH 4) 2PbCl 6

The temperature dependences of 35Cl NQR frequencies in antifluorite ammonium salts (NH 4) 2SnCl 6 and (NH4) 2PbCl 6 are reported. These data are not new, but experiments were repeated because of conflicting reports in the literature for the first of these salts. For both salts, the quadrupole frequency is a smooth continuous function of temperature; inflections are found near 150 K, and at higher temperatures the slope ( ∂ν/ ∂T) p becomes less negative, which is unusual. This behavior can be accounted for by assuming that the anion torsional oscillations take place in a highly anharmonic potential. The direct involvement of ammonium ion reorientation in the NQR temperature dependence seems unlikely.

Proton magnetic relaxation in NH 4IO 4

Proton relaxation times T 1 and T 1 ϱ have been measured in NH 4IO 4 between 77 and 300 K. Ammonium ion reorientation with an activation energy of 8.64 ± 0.2 kJ/mole dominates the relaxation, but above about 200 K dipolar coupling between protons and 127I nuclei, which are rapidly relaxed by quadrupolar coupling to the lattice, makes a large contribution to proton relaxation in the rotating frame.

On the anomalous temperature dependence of nor frequencies in some ammonium salts

Anomalous temperature dependence of NOR frequencies in some ammonium salts has been successfully interpreted in terms of a model which assumes that the reorientation of ammonium ion modifies the NQR frequency at a neighboring nucleus to an extent which is determined by the relative length of the jumping or transient time for the reorientation of NH 4 + ion to the residence time in one of the equilibrium orientations. Examples of the analysis are given for α-NH 4HgCl 3, NH 4ReO 4, and NH 4I 3, leading to the jumping time of 2-3 ps.

Effect of ammonium ion reorientation on the anomalous temperature dependence of nuclear quadrupole resonance frequencies

An anomalous temperature dependence of NQR frequencies in some ammonium salts has been successfully interpreted in terms of a model where the reorientational jumping time of NH + 4 was taken into account. Examples of the analysis are given for αNH 4HgCl 3 and NH 4ReO 4, leading to a jumping time of (2—3) x 10 -12s.

AnomalousH1spin-lattice relaxation in antiferroelectric ammonium dihydrogen phosphate (ADP)

$^{1}\mathrm{H}$ NMR static, and rotating-frame spin-lattice relaxation times and dipolar relaxation times were measured between 135 and 270 K for a single crystal of ammonium dihydrogen phosphate. Random fluctuation in nuclear dipolar interactions by means of ammonium-ion reorientation was found to be the dominant relaxation process. A critical fluctuation contribution to relaxation near the paraelectric-antiferroelectric phase transition, which was postulated in a previous study, was found to be unnecessary to account for the relaxation-time results.

Ammonium ion reorientation in NH 4ReO 4 by NMR

Second moments of the proton absorption lineshape and spin-lattice relaxation times have been measured for NH 4ReO 4 in the temperature range 77 to 430 K. The results indicate a large degree of freedom for the NH 4 + ionic reorientation over this range. The relaxation time results point to the existence of a low-temperature 28° reorientation mode of the ion changing to a general reorientation at higher temperatures. The second-moment results suggest that a transformation from general reorientation to isotropic tumbling of the NH 4 + ions is accompanied by a rearrangement of the lattice. A model is proposed whereby this lattice rearrangement is transmitted via the oxygen framework to the rhenium nucleus and thereby produces the anomalous temperature dependence of the 187Re NQR frequency observed in this material.

Nuclear magnetic relaxation in solid (NH 4) 2 TiF 6 and (ND 4) 2 TiF 6

Spin-lattice relaxation times T 1 for H 1 and 2H have been measured inpolycrystalline (NH 4) 2 TiF 6 and (ND 4) 2 TiF 6 between liquid-helium and room temperature at the Larmor frequency of 7.0 MHz. Activation energies of the reorientation of NH + 4 and ND + 4 ions have been reported and the reorientation of ammonium ion by classical and by tunneling rotation about one C 3 axis is discussed.

Proton Spin–Lattice Relaxation Study of NH4+ Motion in Ammonium‐Doped Ferroelectric RbH2PO4

AbstractThe temperature dependences of the proton Zeeman and dipolar spin–lattice relaxation rates have been measured in a Rb1–x(NH4)xH2PO4 mixed single crystal both in the paraelectric and the ferroelectric phase. The correlation time for ammonium ion reorientation has been determined and it is found that ultraslow reorientational motion of the H2PO4 groups exists above room temperature.

NMR observation of spin isomerism of the ammonium group protons

The shape of the 40 MHz proton magnetic resonance absorption line of powdered (NH 4) 2Ce(NO 3) 6 at liquid helium temperature is explained by proton spin isomerism as suggested by Tomita for solid methane. In the studied solid, the ammonium ion reorientation is hindered by a very weak crystal field. The presence of a meta isomer ( I=2) and a small percentage of ortho isomer ( I=1) spin states is inferred from the line shape at 4°K, indicating that the majority of the ions are in the rotational ground state ( L=0) and about 10 per cent of the ions are in the L=1 rotational state, which is about 8°K above the ground state.

A Deuteron and Proton Magnetic Resonance Study of Ammonium Oxalate Monohydrate

Abstract The quadrupole spectra of deuteron magnetic resonance were observed on single crystals of (ND4)2C2O4D2O. The quadrupole coupling tensors for stationary deuterons were determined at −110°C. eqQ⁄h and η are 163.7–172.8 kHz and 0.005–0.015 for the ammonium deuterons and 213.3 kHz and 0.090 for the water deuteron. The eqQ values and principal axis directions of EFG tensors are consistent with the structure deduced from the diffraction studies with the exception that the z principal axes of D in the ammonium ion deviate appreciably from the NH directions derived from neutron diffraction. From the change in the line-width with the temperature, the rate of the C3 reorientation of ammonium ions and that of the 180° flip of water molecules were estimated at various temperatures. The ammonium-ion reorientation is found to be quite isotropic. The temperature dependence of the proton T1 and T1ρ and the deuteron T1 were measured on (NH4)2C2O4H2O and (ND4)2C2O4D2O powers. The T1 values were then compared with the calculated values, assuming ammonium reorientation. The sharp drop of the proton T1ρ with the temperature above 50°C can be interpreted as being due to the 180° flip motion of water molecules through a relaxation mechanism of “slow motion.” The rates of motion derived from the relaxation-time measurements are consistent with those estimated from the line-width change.