MOLECULAR-GEOMETRY IN FULLERENE C-60 - A DIRECT DETERMINATION OF THE BOND-LENGTH DIFFERENCE DC-C-DC=C

The molecular structure factor of fullerene C60 has recently been dete rmined with a high accuracy using neutron scattering over a large mome ntum-transfer range. The two bond lengths d(C-C) and d(C=C) have been measured with a precision of 10(-3) angstrom. As the temperature is lo wered-from 295 K down to 4 K, the single bond is elongated and the dou ble bond shortened, but the radius of the sphere remains almost unchan ged. Unfortunately, the covariance matrix obtained from the fit shows that the two lengths are strongly correlated; this means that any erro r made on the determination of one bond length is compensated by an op posite error on the other bond length without significantly increasing the chi2 and, thus, the exactness of the reported temperature effect could be argued against. We present in this paper an alternative way o f processing the data which allows us to remove almost completely the coupling, using an appropriate change of variables. The temperature ef fect is confirmed and resolved with a better accuracy.