2H-NM R Spectroscopy of Tunneling Ammonium Ion: General Site Symmetry Zdzislaw T. Lalowicz Institut für Physikalische Chemie der Universität, D-4400 Münster* and Institute of Nuclear Physics, Krakow Z. Naturforsch. 43a, 895-907 (1988); received July 19, 1988 2H-NMR powder spectra of tunneling ammonium-d4 ions are computed. A representation of the tunneling Hamiltonian is worked out in the basis of simple product spin wavefunctions. Secular parts of quadrupole and dipole Hamiltonians are taken into account. Examples of spectra are given for tunneling about one C2 or C3 axis, as well as for overall rotations in potentials of higher symmetry. Ranges of tunneling frequencies measurable from the spectra are given for each case. Characteristic shapes of the spectra allow recognition of various ground torsional level structures. Possible further applications and available data are discussed. PACS number 76.60 I. Introduction The NMR spectrum of a two proton system was calculated by G. E. Pake as early as 1948 [2], It is We consider systems of protons (I =1/2) or deute­ commonly called Pake's doublet, in which the separa­ rons (/ = 1) in solid, diamagnetic nonconducting com­ tion vd defines the dipole-dipole coupling constant pounds. In evaluating the spectra we restrict our­ Cd: vd = 3Cd = | f i y 2r -3 . The dipole-dipole inter­ selves to dipole-dipole or quadrupole interactions. The action is not modulated by reorientation jumps over n. dipole-dipole interaction of nuclear dipole moments Alternatively, jumps by | or rotation reduce the dou­ couples all spins of a system. The interaction of a blet splitting by The 2H-NMR spectrum of a rigid nuclear quadrupole moment with an electric field gra­ pair of deuterons resembles a Pake's doublet with a dient (EFG) has single particle character. Comparison separation 5 = | CQ, scaled up due to a bigger cou­ of protonated and deuterated systems allows for mak­ pling constant CQ = e2qQ h~l by two orders of ing use of the differences. Rigid systems of N protons magnitude. The spectrum is sensitive to jumps by n for show characteristic features in the NMR spectra for an inpressively wide range of exchange rates. The re­ N — 2, 3 and 4, whereas for higher N these features are sulting temperature dependence of the correlation lost [1]. On the other hand, any number of deuterons time was measured for heavy water molecules in in a rigid system gives a single doublet spectrum, pro­ Li2S04 D20 [3], vided no differences in the EFG exist. Andrew and Bersohn [4] supplied spectra for a three The study of molecular reorientation is based on its 7 = 1/2 triangular spin system. They consist of a cen­ specific averaging effect on the spectra. Inclusion of tral line flanked by two side components. The separa­ tunneling requires quantum mechanical treatment in tion of the sidepeaks is reduced by j on reorientation terms of space coordinates. The symmetrised wave- about the C3 symmetry axis. The methyl group CH3 functions of a multispin molecule may be labelled with has often been studied. Slow tunneling frequencies, the irreducible representations of the molecular sym­ vt < 0.1 MHz, are obtained from the spectra. Fast tun­ metry group. Except for diatomic molecules, states of neling rotation was found to give spectra identical to different symmetry are coupled by molecular hyper­ those of fast reorientation [5]. On the other hand, new fine interactions. This is of basic importance not only features were found recently in the NMR spectra of for spin conversion and relaxation processes, but also CD3, which allow identification of the type of motion for the spectra. [6], * Alexander von Humboldt Foundation Fellow 1985-87. The four spins of an ammonium ion form a tetrahe­ Reprint requests to Dr. Z. T. Lalowicz, Institute of Nuclear dron. The proton NMR lineshapes were treated theo­ Physics, ul. Radzikowskiego 152, 31-342 Krakow, Poland. retically for rigid NH4 by Bersohn and Gutowsky [7], 0932-0784 / 88 / 1000-907 $ 01.30/0. - Please order a reprint rather than making your own copy. 896 Z. T. Lalowicz • 2H-NMR Spectroscopy of Tunneling Ammonium Ion and the existence of spin isomers was pointed out by forms. Subsequent comparison with experimental Tomita [8]. Three motional cases may be distin­ spectra should then allow an identification of the GTL guished in a classical analysis of the NMR spectra: structure and in some cases also a determination of rigid ammonium ion, reorientation about its C3 or C2 the splittings. axes and isotropic reorientation [9], Experimental An analysis of the NMR spectra of ND4 tunneling work of Richards and Schaefer [10] revealed a surpris­ in a tetrahedral potential was already presented ing variety of proton spectra of ammonium com­ for single crystals [19] and powder samples [20], pounds at 20 K. These were later explained by tunnel­ Methods to measure tunnelling frequencies in the ing. Formal theoretical treatment and measurements 0.1 MHz < v, < 20 MHz range were outlined for at low temperatures followed [11-13]. The systemat- (ND4)2SnCl6 as an example, but the methods are also ics of the spectra according to the ground torsional applicable to other cubic compounds. These are char­ level (GTL) structure and numerous fits to the low acterised by the (A, 3T, E) GTL structure, and the temperature spectra were then supplied [9,14,15]. tunneling frequency is defined by A to 3T splitting. For one-dimensional rotation the torsional energy Most ammonium salts have a crystal symmetry lower levels may be calculated from the Mathieu equation than cubic. Lower symmetry barriers lead to other with a potential Vn = ^(1 — cos n a). In the case of GTL structures at low temperatures and to much ammonium ions a similar calculation may be applied more complex reorientation at higher temperatures. only to one-dimensional rotations about one of its Representative examples of both 2H-NMR single four C3 or three C2 symmetry axes. In general, howev­ crystal and powder spectra are given here for vari­ er, ammonium ions may experience potential barriers ous GTL structures. The sensitivity to vt is visualised of various symmetries and thus perform a more com­ and the identification of various GTL structures is plex motion, both in the tunneling rotation and re­ confirmed. orientation ranges. A very thorough discussion of a tetrahedron in a cubic crystalline field may be found II. Theoretical Principles in the paper by King and Hornig [16]. Smith devel­ oped numerical methods to obtain GTL structures for 1. Basic Definitions lower symmetry potentials [17], The complexity and The calculation of the NMR spectra is based in our convergence problems in the calculations, as well as treatment on the numerical solution of the eigenvalue idealized forms of potentials lead to poor agreement problem for a general Hamiltonian. In case of the with experimental data. NMR spectra of deuterated ammonium ions we can There is a need for more data on tunneling frequen­ limit ourselves to Zeeman, tunneling, quadrupole and cies and crystalline potentials. Except for a few cases dipole-dipole contributions: accessible to the high resolution inelastic neutron scattering (INS) [18], in all remaining cases the GTL je = jfz + + JTq 4- #eD. (i) structure can be determined for NH4 from the NMR Zeeman and tunneling Hamiltonians represent zeroth spectra only. NMR remains the only method in the order energies in spin and space coordinates, respec­ ND4 case, as tunneling splitting is always reduced tively. Quadrupole and dipole interactions, bilinear in beyond the resolution of INS methods. both coordinates, are acting as perturbations. We are going to apply here a group theoretical We take as starting representation (21 +1)4 = 81 approach to the GTL structure of ND4 . Expressions simple product wavefunctions | /1/I^z^z)- Here we for GTL energy sublevels are written in terms of pa­ will use the notation I[z = a, 0, ß for </^> = 1 ,0 ,- 1, rameters representing tunneling frequenices about respectively, and the superscript /' will be recognized separate C3 and C2 symmetry axes. The tunneling from the position. This is the main difference with parameters depend on the respective cross sections of respect to a previous paper [21], where eigenfunctions a three-dimensional potential. Neither potentials nor of I = Y,Ui\ and were used as a basis from the tunneling splittings are known, so we use free tunnel­ i ing parameters. This makes the problem of the NMR beginning. spectra of tunneling ND4 ions tractable. The evolu­ We may write the Zeeman Hamiltonian as tion of the spectra on the splittings within the libra­ Jfz = - y DhB0 I I'z. (2) tional ground state multiplet supplies the master i = 1 897 897 Z. T. Lalowicz • 2H-NMR Spectroscopy of Tunneling Ammonium Ion fills the diagonal submatrices of the general Hamilto­ The magnetic quantum number M = X <7z> labels i = 1 nian with offdiagonal terms. As the contribution of the the Zeeman energy levels, and the wavefunctions may Hamiltonian (6) to the eigenenergies of the system is be grouped into respective manifolds. A matrix of relatively small and spoils the clarity of the quadru­ the Hamiltonian (1) can be structured accordingly, pole Hamiltonian symmetry, we will henceforth ne­ and we get diagonal submatrices of the dimensions glect it in the discussion. In the numerical calculations l x l (M = +4), 4x4(M = ±3), 10xl0(M = ±2), presented further on, the complete Hamiltonian has 16x16(M = ± 1), 19 x 19(M = 0)(compareTable 1 in always been used. [22]). The quadrupole interaction Hamiltonian describ­ 2. Tunneling Hamiltonian Representation ing the coupling between a non-spherical nuclear charge distribution and the EFG at the nucleus may The ammonium ion has forms a regular tetrahe­ be written as a product of a Cartesian quadrupole dron with the nitrogen in the center.