Thermophysical properties of tetramethylmethane and tetramethylsilane gas calculated by means of an isotropic temperature-dependent potential

An isotropic temperature-dependent potential (ITDP) is calculated For the description of binary interactions in gaseous tetramethylmethane, C(CH3)(4), and tetramethylsilane, Si(CH3)(4). The potential parameters of C(CH3)(4) and Si(CH3)(4) are determined bq solving an inverse problem of minimization of the sum of weighted squared relative deviations between experimental and calculated pure gas viscosity (eta), second (pVT)-virial coefficient (B), and second acoustic virial coefficient (beta) data. At T = 0 K they are obtained for C(CH3)(4) and Si(CH3)(4), respectively, as repulsive parameter n = 28.02(12) and 20.79(11), equilibrium distance r(m) = 5.7790(30) x 10(-10) and 5.9051(36) x 10(-10) m, potential well depth epsilon /k(B) = 586.32(42) and 674.75(91) K and the first excited-level enlargement delta = 0.0141(3) x 10(-10) and 0.0188(3) x 10(-10) m, The influence of the temperature on the potential parameters r(m)(T) and epsilon (T) is implied in the temperature dependence of the effective excited-state enlargement, calculated via the vibrational partition function, The calculated complete sets of normal vibrational Frequencies for C(CH3)(4) and Si(CH3)(4) are consistent with the available experimental data. In addition. good agreement is observed between the calculations and new: Raman spectroscopic measurements on C(CH3)(4). Table for recommended thermophysical properties (B, eta, and self-diffusion pD) and effective potential parameters (r(m) and epsilon) of the two globular gases are given for the temperature range between 250 and 800 K.