Entropies and Heat Capacities of Gaseous Selenium Molecules

The presumably cyclic molecules Ses, Se6 and Se; are the main constituents of saturated selenium vapor at temperatures up to 400 °C as well as of the vapor above freely subliming trigonal selenium, and cyclic Seg molecules can be prepared as various crystalline modifications [1], Small selenium rings are also discussed as constituents of liquid and amorphous selenium obtained by quenching the liquid or the vapor [1]. To determine the equi­ librium concentrations of the various SeM molecules (h > 2) in the liquid and gaseous state, their en­ tropies and heat capacities are needed, but no experimentally determined values have been pub­ lished so far. Drowart and Smoes [2] as well as Keller et al. [3] calculated the entropies for n = 3 ... 8 using the well known statistically-thermodynamic equations [4] but no information was given concerning the molecular structures and vibrational frequencies used, which had to be as­ sumed since only the molecular structure [5] and vibrational spectra [6—10] of solid Ses are known. The vibrational spectra of Ses have recently been assigned and force constants have been calculated [11]. Provided neither the fundamental frequencies nor the structural parameters change on vaporiza­ tion of the Seg molecule, the thermodynamic func­ tions given in Table 1 can be calculated assuming rigid rotator, harmonic oscillator and ideal gas behavior. The obtained entropies differ from the estimated literature data [2] by less than 0.2°o. The