Ideal-gas heat capacities of dimethylsiloxanes from speed-of-sound measurements and ab initio calculations

Abstract

A two-pronged approach has been used to obtain accurate ideal-gas heat capacities of cyclic and linear dimethylsiloxanes that are useful for thermodynamic modeling of several processes involving these compounds. Acoustic resonance measurements were made on gas-phase octamethylcyclotetrasiloxane (D 4, [(CH 3) 2–Si–O] 4) and decamethylcyclopentasiloxane (D 5, [(CH 3) 2–Si–O] 5) over the temperature range 450–510 K. These new data, along with previously published molecular vibrational frequency data for hexamethyldisiloxane (MM, [(CH 3) 3–Si–O 1/2] 2), were used to develop an appropriate frequency scaling factor that can be used with ab initio frequency calculations to produce reliable ideal-gas heat capacities as a function of temperature. Ideal-gas heat capacities for both cyclic [(CH 3) 2–Si–O] n (with 3 ≤ n ≤ 8 ) and linear (CH 3) 3–Si–O–[(CH 3) 2–Si–O] n –Si–(CH 3) 3 (with 0 ≤ n ≤ 5 ) siloxanes over a wide range of temperatures were determined with the ab initio method.