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

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 (D4, [(CH3)2–Si–O]4) and decamethylcyclopentasiloxane (D5, [(CH3)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, [(CH3)3–Si–O1/2]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 [(CH3)2–Si–O]n (with 3≤n≤83≤n≤8) and linear (CH3)3–Si–O–[(CH3)2–Si–O]n–Si–(CH3)3 (with 0≤n≤50≤n≤5) siloxanes over a wide range of temperatures were determined with the ab initio method.