The molecular and crystal structures of C(C6H5)4, Si(C6H5)4, Sn(C6Hs)4 and Pb(C6Hs)4 have been determined by conformational analyses and lattice energy calculations. The molecular conformations and crystal structures correspond to the potential energy minima• The structure of Sn(C6Hs)4 is in good agreement with that determined experimentally by the X-ray method•

On the basis of experimental data on the structure and elasticity tensor of naphthalene crystals a t low temperatures a more accurate fitting of the parameters of the C-C, C-H, and H-H potential curves was made. The potential curves involved have the form p = = -(A/r6) + Becar and the following numerical parameters: Acc = 421 (kcal/mol) As, Bcc = 7.16X lo4 kcal/mol, a c~ = 3.68 A-l; A~I J = 118 (kcal/mol) A 6 , BLH = 1 . 8 6~ lo4kcal/mol, ac11-3.94 8-l; A11~=29(kcdl/niol) A6, B~~= 0 . 4 9 x 1OPkca1/mol, a~~= 4 . 2 9 A -l . IntroductionThe atom-atom potential method that is widely used a t present for calculating the energy of inter-and intramolecular interactions [l] is of particular interest for hydrocarbons, as it enables us to calculate the properties of an immense number of compounds with the help of a small number of empirical parameters. The simplest way of obtaining numerical parameters of potential curves that describe interaction of the carbon and hydrogen atoms is to find them from crystal properties [a]. In doing so one can use as standards one or several crystals for which enough reliable experimental data are available.In our previous papers [3, 43 and also in J5 to 7 ) C-C, C-H, and H-H potential curves were obtained on the basis of the experimental data on the structure and sublimation heat of 10 t o 15 hydrocarbon crystals. However, these propertics are primarily determined by the positions and depths of the minima of the interatomic potential curves and are only slightly dependent on the shape (steepness) of the potentials in the vicinity of the minimum.More detailed information on the shape of potential ciirves can be obtained from elasticity moduli ( C t j ) of crystals which are determined a t absolute zero by the second derivatives of the density of the lattice energy with respect t o the lattice deformations [8]. Experimental data on the elasticity tensors as a function of temperature are available only for a few hydrocarbon crystals [9 to 111. Among these, measurements for the naphthalene crystal seem t o be the most thorough. For this crystal data on the thermal expansion tensor at low temperatures [la] are also available and various measurements of the sublimatinn heat are known [13]. That is why we have chosen the naphthalene crystal as a standard for more accurate fitting of the parameters of the C-C,

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