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 d escription of binary interactions in gaseous tetramethylmethane, C(CH3)(4), and tetramethylsilane, Si(CH3)(4). The potential parameters of C(CH3)(4) a nd 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 obt ained 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 temperat ure dependence of the effective excited-state enlargement, calculated via t he vibrational partition function, The calculated complete sets of normal v ibrational Frequencies for C(CH3)(4) and Si(CH3)(4) are consistent with the available experimental data. In addition. good agreement is observed betwe en the calculations and new: Raman spectroscopic measurements on C(CH3)(4). Table for recommended thermophysical properties (B, eta, and self-diffusio n pD) and effective potential parameters (r(m) and epsilon) of the two glob ular gases are given for the temperature range between 250 and 800 K.