Magic-angle-spinning 15N NMR of ammonium sulfate

Volume 139, number 3,4

MAGIC-ANGLE-SPINNING

CHEMICAL PHYSICS LETTERS

15N NMR OF AMMONIUM

28 August 1987

SULFATE

Marco L.H. GRUWEL, Michael S. McKINNON and Roderick E. WASYLISHEN Department of Chemutry, Dalhousie University, Ha&x,

Nova Scotia, Canada B3H 4J3

Received 4 June 1987

Magic-angle “N NMR measurements indicate that the nitrogen chemical shift difference for the two inequivalent ammonium ion sites of (NH&SO, is only 0.30 ppm in the room-temperature phase at 290 K. The implication is that the immediate environment of the two ions is similar; this is supported by T, ( “N) measurements of the two chemically shifted resonances. At temperatures immediately below the known phase transition (T=223 K) the chemical shift difference between the two sites is 1.2 ppm and the 15NT, values differ by almost an order of magnitude, implying substantially different environments for the two distinct ammonium ions in the ferroelectric phase.

The unusual ferroelectric properties of ammonium sulfate at temperatures below the solid-solid phase transition, T= 223 K, have resulted in numerous investigations of this intriguing salt [ l-l 11. Neutron-diffraction studies [ 21 indicate that there are two inequivalent ammonium ions in both the room-temperature phase and the lower-temperature ferroelectric phase. In both phases the two distinctive ammonium ions differ apparently because of the extent to which they hydrogen bond to neighbouring sulfate ions [ 2-41. In a previous variable-temperature ‘H and *H NMR study of ( NH4)*S04 O’Reilly and Tsang [ 31 were able to analyze their data at temperatures between 77 and 170 K to obtain effective reorientational correlation times for each of the two distinctive NH: ions. In the room-temperature phase the two types of ammonium ions could not be distinguished and an average value for teff was reported. There has been considerable controversy in the literature [ 5-7,121 regarding the rotational motion of the two different NH: ions in the room-temperature phase; here we present preliminary results of magicangle-spinning (MAS) “N NMR experiments which allow us to obtain separate rotational correlation times for each ion. The “N resonance of ( ‘5NH4)2S04at 230 and 215 K is shown in fig. 1. The spectra were obtained with magic-angle sample spinning on a Bruker MSG200 at 20.3 MHz. At 290 K the difference in 15Nshield-

215 K

I

7

”

I

”

”

!

-100

0

HZ Fig. I. MAS 15NNMR spectrum of solid ( “NH&SO., at 230 and 215K.

ing constants for the two distinctive ammonium ions is only 6.0 Hz (0.30 ppm); at 230 K the difference increases to 0.42 ppm. At the phase transition both of the 15Nresonances undergo a low-frequency shift; one moves by either 0.82 or 2.01 ppm while the other moves by either 1.59 or 0.39 ppm, respectively. Our 15N spin-lattice relaxation data (vide infra) suggest that one shifts by 2.01 ppm while the other shifts by

0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

321

CHEMICAL PHYSICS LE’ITERS

Volume 139, number 3,4

28 August 1987

Table I “N spin-lattice relaxation times for ammonium sulfate Sample

T(K)

T, (low freq.) (s)

7’, (high freq. ) (s)

Method

bl

296 296 296 215 200

35.0 36.5 37.0 0.76 0.426

29.5 29.8 26.3 5.7 3.79

cf c) d) d) d)

a) b) a) a)

‘) ( ‘5NH.,)2S0.,. ” Anuniabelledsample;all isotopesat naturalabundance.

“) Seeref. [ 141;CP/MASrecovery. d)Low-power‘Hdecouplingduringacquisitiononly. 0.39 ppm (i.e. the resonance frequencies of the two distinctive ions interchange at the phase transition). The increased 15N shielding observed for each ion below the phase transition is consistent with the calculations of Ratcliffe et al. [ 131 which indicate that o(N) increases as the N-H bond length decreases. When all four N-H bond lengths in an isolated ammonium ion decrease by 0.01 A the nitrogen shielding constant is predicted to increase by 2.8 ppm. Experimentally, the neutron diffraction results of Schlemper and Hamilton [2] indicate that the average N-H bond length for one type of ammonium ion, NH4( I), decreases from 1.075 to 1.052 A, while for the other, NH,(II), TNHdecreases from 1.062 to 1.048 A. Although the change in nitrogen shielding at the phase transition is much less than one might anticipate [ 131, it is important to note that a(N) will also be very dependent on the orientation of neighbouring sulfate ions as well as the various H-N-H bond angles. In addition, it is important to recognize that the error in the average reported N-H bond lengths is approximately 0.01 A [ 21. In order to characterize the reo~entational motion of each type of ammonium ion we carried out inversion-recovery 15NT, measurements with MAS in two different ways: in the first, the cross polarization (CP) experiment described by Torchia [ 141 was used, in the second, measurements were made using low-power ‘H decoupling during acquisition (the decoupler was turned off at all other times). At 296 K the relaxation measurements were carried out on both 15N labelled samples (95%) and on a sample containing “N at natural abundance (0.365%). These experiments were carried out to eliminate the possible influence of spin diffusion [ 15,161. The 322

results, table 1, indicate that within experimental error the lsN T, at 296 K is independent of the experimental technique. The average values for the enriched sample, 28.0 and 36.8 s for the high- and low-frequency signals respectively, indicate reff values of 2.2 and 1.7 ps for the two different NH: ions. Here we have assumed that the intramolecular ‘H, 15Ndipole~ipole relaxation mechanism completely dominates, thus rew=(4T,)-‘(YHYlsNjll(r~-H))-2

;

in these calculations we have used a value of TN-H= 1.06 8, [ 2,171. NOE measurements [ 171, fig. 2, confirm that the dipole-dipole mechanism completely dominates (q= -4.9 rfrO.1 for both nitrogens) and that w&r& *= 1. For comparison we also

10

I

0

1

-to

H% Fig. 2. NOE measurement on solid ( ‘5NH.,)2S0, at 296 K. (a) “N NMR spectrum with ‘H decoupling at all times. (b) “N NMR spectrum with ‘H decoupling only during acquisition.

Volume 139, number 3,4

CHEMICAL PHYSICS LETTERS

28 August 1987

carried out 2H NMR measurements on a powder sample which had been deuterium labelled (95*h). Aithough it is not possible to obtain accurate individual T, values for each of the two overlapping powder patterns [ 3,18 ] we estimate T,= 0.62 4 0.15 s and T,= 1.12 +0.25 s (the longer Tlbeing associated with the outer edge of the ‘H NMR powder spectrum which has been assigned to NH,(II) [ 31 following the notation of Schlemper and Hamilton [ 21. Using the empirical procedure of Hunt and MacKay [ 191, ;y z 178 kHz for both ammonium ions and since [20]

7,.values of 3.4+ 1.0 and 1.920.6 ps are obtained for the unique sites I and II respectively at 296 K. Although these values are slightly longer than the values obtained from the more accurate “N results, the agreement must be regarded as very good considering the uncertainty in T,(‘H) and x( 2H). Our conclusion that feff for each of the two distinctive ammonium ions differs by less than a factor of two in the room-tem~rature phase contrasts with that of an earlier 2H NMR study where free induction decays were not Fourier transformed [ 121. Also, the r__@values obtained in a quasielastic incoherent neutron scattering study [ 71 of (NH,)2S0, are approximately five to seven times longer than the r,ffvalues obtained here, however it is not clear that the NMR reorientational correlation times are directly comparable to those obtained in the neutron study, In contrast to the observations at 296 K, we find the 15Nspin-lattice relaxation times for NH,(I) and NH,(B) to differ by almost an order of magnitude at 2 15 and 200 K (see table 1 and fig. 3). Below 223 K the “N resonance with the shorter T,is the lowfrequency resonance which we assign to the more slowly rotating NH4( I) ion [ 3,121. Note that in the room-temperature phase the low-frequency resonance is assigned to NH4( II) on the basis of our ’5N and ‘H relaxation data and an earlier *H NMR single crystal study of f ND4)$04 [ 31. The T,values in table 1 indicate that reff(NH4( I)) = 10 and reff(NH,(II)) = 75 ps at 2 1.5IQ these values increase to 16 and 142 ps respectively at 200 K. At lower temperatures, “N T,measurements on a powder (without MAS) also confirm the presence of two very different ammonium ions. At 8.48 T (36.585 MHz)

Fig. 3. Inversion-recovery ( ‘5NH4)2S0, at 200 K.

MAS “N NMR spectra of solid

two well separated T,minima were observed at 164 and 127 K which we assign to NH4( I) and NH,( II) respectively. In this study we have shown that magic-anglespinning expe~ments allow one to resolve completely the “N resonance signals of the two unique NH: ions in both the room-temperature and ferroelectric phases of ( NH4) 2S04. Spin-lattice relaxation measurements of these resolved resonances indicate very similar reff values for the two ions at room temperature; however, below the phase transition at 223 K the rotational dynamics of the two ions are quite different. We are grateful to NSERC for financial support.

References [ 1 ] B.T. Matthias and J.P. Remeika, Phys. Rev. 103 (1956) 262. [2] E.O. Schlemper and WC. Hamilton, J. Chem. Phys. 44 ( 1966) 4498. [ 3 ] D.E. O’Reilly and T. Tsang, J. Chem. Phys. 46 (1967) 1291. [ 4 f D.E. G’Reilly and T. Tsang, J. Chem. Phys. 46 ( 1967) 1301. [ 5] WC. Hamilton and J.A. Ibers, Hydrogen bonding in solids (Benjamin, New York, 1968) pp. 147,245. [6] N.G. Parsonage and L.A.K. Stavely, Disorder in crystals (Clarendon Press, Oxford, 1978) p. 350. [ 71 P.S. Goyal and B.A. Dasannacharya, J. Chem. Phys. 68 (1978) 2430. [ 81 K. Hasebe, J. Phys. Sot. Japan 50 (198 1) 1266. [9] T. Chiba and S. Miyajima, J. Chem. Phys. 83 (1985) 6385. [ 10 ] B. Rakvin and N.S. Dalal, J. Chem. Phys. 85 (1986) 6060.

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CHEMICAL PHYSICS LETTERS

[ 111 Y.S. Jain, P.K. Bajpai, R. Bhattacharjee and D. Chowdhury, J. Phys. Cl9 (1986) 3789. [ 121 D.W. Kydon, H.E. Petch and M. Pintar, J. Chem. Phys. 51 (1969) 487. [ 131 C.I. Ratcliffe, J.A. Ripmeester and J.S. Tse, Chem. Phys. Letters 99 (1983) 177. [ 141 D.A. Torchia, J. Magn. Reson. 30 (1978) 613. [ 151 D. Suter and R.R. Ernst, Phys. Rev. B32 (1985) 5608.

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[ 161 N.J. Clayden, J. Magn. Reson. 68 (1986) 360.

[ 171 G.C. Levy and R.L. Lichter, Nitrogen-15 nuclear magnetic resonance spectroscopy (Wiley, New York, 1979) ch. 5.

[ 181 T. Chiba, J. Chem. Phys. 36 (1962) 1122. [ 191 M.J. Hunt and A.L. MacKay, J. Magn. Reson. 15 (1974) 402.

[ 20 j A. Abragam, The principles of nuclear magnetism (Oxford Univ. Press, Oxford, 196 1) ch. 8.