Silicon Prismanes: Calculation and Comparison of Heats of Formation

Background: The objects of this work are silicon prismanes - new highenergy materials that can be used in various electronic, optoelectronic, thermoelectric, and biological fields and compete with traditional chemical energy carriers. However, only compounds with hexasilacubane and octasilacubane have been synthesized to date. Synthesis of other silapismanes, and even more so polysilaprismanes, has not yet been achieved. Therefore, both the “simplest” and the highest silaprismanes are studied mainly theoretically, including by quantum chemistry methods. Objective and Methods: To expand information about the enthalpies of formation of "simple" (n = 2) and multilayer (n = 3-7) silicon prismanes using semiempirical calculation methods; bond lengths Si-Si and Si-H; compare them with the results of ab initio calculations already available in the literature. Results: The geometry was optimized by semiempirical methods, the enthalpies of formation, Si-Si and Si-H bond lengths, and the bond angles of the bilayer (m = 3-12) and some multilayer (n = 3-8 and m = 3-9) silaprismanes were determined. Comparison with the enthalpies of formation obtained using ab initio calculations shows that the enthalpies of formation obtained by the MINDO/3 method (Modified Intermediate Neglect of Differential Overlap, version 3) are closest to the results of ab initio calculations of two-layer silicon prismanes; and for multilayers - by the AM1 method (Austin Model 1). Large differences in the lengths of Si-H bonds determined ab initio and semi-empirically by both methods are not observed. As for the Si-Si bonds, the results of semi-empirical methods indicate the presence of the so-called auxetic effect in multilayer silaprismanes. Conclusion: We believe that in the future, the calculations carried out in this work and a comparative assessment of the enthalpies of formation obtained by calculation methods will help to overcome the problems associated with the synthesis of silicon prismanes.