Structural phase transition in polyphenyls. IV. Double-well potential in the disordered phase of p-terphenyl from neutron (200 K) and X-ray (room-temperature) diffraction data

Abstract

New neutron-diffraction data collected at 200 K with deuterated p-terphenyl, and published X-ray data collected at room temperature with protonated p-terphenyl, have been used to determine the characteristics of the double-well potential for the librations of the central ring about the long molecular axis, in the disordered phase. A picture of the double well is obtained simply by halving the atoms outside the molecular axis and imposing constraints on their geometries and thermal parameters. The R values compared with that obtained with the simple-well model, are significantly improved. The deuterated benzene rings, compared to the protonated ones, show a systematic and hard-to-explain distortion: all the internuclear C-C bond lengths parallel to the long axis are shortened and the others lengthened, the mean bond lengths being the same in the two compounds. The double-well potential-barrier height is found to be about 0.6 kcal mo1-1 at 200 K and at room temperature. This value is in good agreement with that given by potential-energy calculations. The rotation angle between the two wells is about 26 °. Only for the internal g mode is the thermal energy kT sufficient to bring the rings near the top of the barrier. For the central-ring librations the doubly peaked distribution function is an argument for an order-disorder regime. However, the smallness of the double-well barrier height suggests that the p-terphenyl structural phase transition is near the boundary between the order-disorder and displacive regimes.

Structural phase transition in polyphenyls. X. Potential barrier heights in crystalline polyphenyls and in gaseous biphenyl determined uniquely from diffraction data

“…1), allowing the resolution of two components in the overall libration: a process of jumps between two sites at ~o = + 13 ° on either side of the average molecular plane and genuine libration on each site [(02) "--52 deg 2 at 300 K]. By considering a linear variation of {02) with temperature T, very good agreement is observed between X-ray diffraction results at room temperature: q~= +13.3 °, {02) = 52.5 deg 2 (o = 7.8 deg2), neutron diffraction results at T =200K: q~=_13.2 °, (02)=36.2deg 2 (or= 4.0 deg 2) (Baudour, Cailleau & Yelon, 1977) and X-ray diffraction results at T= 200 K: ¢, = _+ 13.3 ~, (02) = 35.1 deg 2 (o = 6.6 deg 2) (Baudour, Toupet, D61ugeard & Gh6mid, 1986). Thus, these concordant estimates of q~ and {02) can be considered to be very reliable.…”

“…It must be noted that if, as in a previous study (Baudour et al, 1977), we take the crude value (02) = 52.5deg 2 given by diffraction data instead of (2/3)(02) which corresponds to the torsional g mode, we obtain a significantly inferior barrier height V(0) = 2.47 kJ mol -~, which does not agree with the NM R results.…”

“…From the similarity of the p-terphenyl and pquaterphenyl structures, we can take 2q~ = 13.3 °, the value obtained by resolving the disorder in the hightemperature phase of p-terphenyl (Baudour et al, 1977(Baudour et al, , 1986). Thus we obtain the torsion angle 0o between two phenyl rings at equilibrium, when intramolecular interactions only are considered: 0o = 46.55 ~.…”

“…It is clear that the only mode which permits the molecule to jump from the (~o/2, -~p, ~o/2) conformation to the opposite one (-~0/2, ~, -~p/2) is the g-torsional mode. From the doubly peaked probability density function resolved from diffraction data, we know the amplitude of the jumps for the pterphenyl central ring: 2~0 = 26-6 ~~ (Baudour et al, 1977). Now, to determine completely the characteristics of the double potential well associated with this reorientational process, it is necessary to separate in the overall mean-square amplitude of libration (02) = 52.5 deg 2, the contribution due to the external librational mode (which has no relation with the reorientational jumps) from that due to the torsional g mode.…”

“…The probability density function describing the reorientational jumps of the central p-terphenyl ring in the model used for interpreting diffraction data (Baudour et al, 1977) is the sum of two Gaussian functions written as:…”