AN OBSERVATION OF ONE-DIMENSIONAL REORIENTATION AND TUNNEL
SPLITTING OF THE GROUND AND FIRST EXCITED STATE IN A LOW
BARRIER SYSTEM: SOLID NITROMETHANE

S. F. Trevino

(Energetic Materials Division, LCWSL, ARRADCOM, Dover, NJ)

and

(National Bureau of Standards
Washington, DC)

In this communication, we report the complete characterization of the structural and dynamical properties of a simple one dimensional rotor with the lowest known barrier to rotation in the solid state.

1

The molecule of nitromethane (CH,NO2) has, in the gas phase, one of the lowest known barriers (6 cal/mole) to reorientation of the methyl group about the C-N bond. The barrier to reorientation in the solid will, therefore, be dominated by the crystalline forces. Quantum mechanical effects exhibited as tunnel splitting of the torsional levels are extremely sensitive to the barrier height, decreasing rapidly as the barrier height increases. A model compound in which the barrier is low would thus be very desirable for the study of such phenomena.

The crystal structure of nitromethane has been determined2 to be orthorhombic with space group P212121 and containing one molecule in the asymmetric unit. The positions of the carbon, nitrogen and oxygen atoms were determined by a single crystal X-ray diffraction measurement and the position and anisotropic temperature factors of the deuterons by a neutron powder diffraction study of CD,NO, CD3N02° The three deuterons are related by three-fold symmetry about the C-N bond and the major axis of their thermal ellipsoid corresponds to rotation about the three-fold axis.

Quasielastic neutron scattering has been used to investigate the reorientation of the methyl group at temperatures comparable to the barrier height. Under these conditions it has been shown that the methyl group motion can be very well described by a jump reorientation model in which

3

it reorients between equilibrium positions after a mean residence time
T. The structure of the solid suggests that the reorientations occur by
120° jumps about the C-N bond. This model predicts the following in-
coherent neutron scattering function from a polycrystalline sample:

u2

3/2T

4

Qd (3/2 T)2 +w

2

where d is the jump distance, u is the mean squared amplitude of vibration of the H atoms; Q is the momentum transfer, and w is the frequency transfer in the scattering. A triple axis spectrometer was used in the

quasielastic and inelastic scattering measurements. In figure 1 is presented one measurement of the quasielastic scattering at 78 K and a Q of 2.88 A-1. The fit is obtained with a τ of 2.15 x 10 -12 sec. The Q dependence of the relative contributions of delta function and Lorentzian over a Q

•-1

range of 1 to 4 A are correctly predicted. The temperature dependence

of T from 50K to 150K is well described by an Arrhenius relation with an activation energy of 230 cal/mole. The tunnel splitting of the ground state of a one-dimensional three-fold rotor with such a low barrier is

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expected to be a large fraction of 100 μev. An attempt was made to measure the transition between the tunnel split ground state with a resolution of 55 μeV. Figure 2 presents the results of this measurement at a temperature of 4.2K. The upper figure presents the data and the instrumental resolution. There is clear evidence of broadening. This additional scattering cannot be attributed to quasielastic scattering due to thermally activated reorientation, since the broadening would be much smaller than the instrumental resolution. The lower figure presents a fit of the data with three gaussians, one centered at zero energy transfer and two side bands centered at +45 μeV energy transfer. We believe these result from transitions between the tunnel split ground state. This conclusion will be confirmed when a higher resolution instrument becomes available in the near future. A three-fold cosine potential which predicts a ground state tunnel splitting of 45 μeV produces one bound tunnel

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Figure 1.

The measured and calculated quasielastic neutron scattering
spectrum for solid nitromethane at 78K and Q = 2.88 Å1. The

resolution of the instrument is .44 meV. The solid line is
a fit obtained to the data as described in the test and the
dashed line is the Lorentzian contribution.

Figure 2.

2.

P1 3.7 A

In elastic scattering spectrum of CH,NO, at 4.2K and Q =
The upper figure presents the data and the measured gaussian
resolution of 55 μeV. The lower figure presents the fit to
the data obtained with three gaussians (dashed curves) as
discussed in the text.

a

split (by .75 meV) excited state whose splitting substantially narrows and whose average energy shifts down by a factor of 1.38 upon deuteration. Features in an inelastic spectrum due to librations are expected to be the most intense, for these produce the largest amplitude motions. Figure 3 presents inelastic scattering data on both CH3NO2 and CD NO

CD3NO2 (at 4.2K).

The libration spectrum of the hydrogenous sample can be fit by two very broad gaussian peaks whose average energy is 7.4 meV and which are split by .8 meV; the spectrum of the deuterated sample shows a narrow peak at 5.3 meV. The broadening of the peaks observed in the CH,NO2 reflects life 2 time effects due to the fact that the levels are very close to the top of the potential well.

We have thus observed a solid state system exhibit behavior, as a function of temperature, from the quantum mechanical to the classical thermally activated regimes.

Figure 3.

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The inelastic scattering spectra of CD,NO, (upper figure) and
CH,NO2 (lower figure) obtained at 4.2K. The triangles correspond
to the instrumental resolution. The solid lines correspond
to the overall fit and the dashed lines in the lower figure to
the individual components.

1.

2.

3.

4.

5.

E. Tannenbaum, R. J. Myers and W. D. Gevinn, J. Chem. Phys. 25, 42 (1965).

S. F. Trevino, C. Hubbard and E. Prince, J. Chem. Phys. (to be published).

C. T. Chudley and R. J. Elliot, Proc. Phys. Soc. London 77, 353 (1961) K. Skold, J. Chem. Phys. 49, 2443 (1968).

C. Steenbergen and J. J. Rush, J. Chem. Phys. 70, 50 (1979).

James E. Wollrah, Rotational Spectra and Molecular Structure, Ernest M. Loebl Ed. (Academic Press, New York, 1967) Appendix 12.

NEUTRON POWDER DIFFRACTION STUDY OF THE STRUCTURES

OF CeTaO4, CeNaO AND Nd Ta04

4'

A. Santoro

M. Marezio

(Laboratoire de Cristallographie, C.N.R.S., Grenoble-Cedex, France)

and

R. S. Roth and D. Minor

(Ceramics, Glass and Solid State Science Division)

1. Experimental

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The compounds were prepared by mixing the appropriate amounts of tantalum, niobium, cerium and neodimium oxides and by heating the mixtures overnight at 1000°C in platinum crucibles. After cooling to room temperature, the products of this heat treatment were ground, heated again at 1400°C for CeNb04, 1500°C for CeTa0 and 1600°C for NdTa04, over a period of 16-20 h, quenched in water, filtered and dried. The only single crystals obtained for these materials were too small for neutron diffraction and many of them were twinned, especially in the case of CeTa04. It was therefore decided to analyze the materials with the powder technique and to refine the structures with the Rietveld method.