Why Is Methylene a Ground State Triplet while Silylene Is a Ground State Singlet?

Abstract

The singlet-triplet energy difference in CH 2 and SiH 2 was calculated at the CASSCF level of theory using large Gaussian basis sets that included f-type functions. The total energy was separated into nuclear repulsion and electronic energy, and the latter was further decomposed into the contributions coming from the two electrons highest in energy (the "frontier" electrons, denoted by "f") and from all the other electrons (denoted by "c" for "core"). The contribution of the frontier electrons was further decomposed into the following terms: E (f) , which is the sum of the kinetic energy and the attraction energy to the nucleus of the two frontier electrons and their repulsion energy from all other electrons, and E ee (f) , the repulsion energy between the two frontier electrons. The results are used to explain why CH 2 is a ground state triplet (lying ca. 10 kcal/mol lower in energy than the singlet) while SiH 2 is a ground state singlet (with the triplet lying ca. 20 kcal/mol higher in energy). The major conclusions are as follows: (1) In addition to the frontier electrons, the "core" electrons and the nuclear repulsion energy also affect the singlet-triplet gap. (2) About 60% of the singlet-triplet energy difference between CH 2 and SiH 2 of 29 kcal/mol may be attributed to the E ee (f) term. (3) The remaining 40% of the energy difference can be interpreted as resulting from a balance between E (f) , the energy of the "core" electrons (E (c) ), and the nuclear repulsion energy (E nuc ); these terms may be related to the HOMO-LUMO energy difference, which is often used in qualitative discussions of singlet-triplet energy gaps. A detailed discussion of the results is presented. † This paper is dedicated to Prof. Helmut Schwarz, a stimulating scientist and a wonderful human being, on the occasion of his 60th birthday.

The Versatile Chemistry of Disilenides: Disila Analogues of Vinyl Anions as Synthons in Low-valent Silicon Chemistry

“…Following first principles (Bent's rule) electropositive substituents are one way to favor the triplet state and therefore lithiosilylenes could be expected to exhibit a small singlettriplet gap if not a triplet ground state. This assertion had been quantified by Apeloig et al 18 and was experimentally verified by the synthesis of a lithiosilylene that has a thermally accessible triplet state according to EPR. 29 In view of these considerations, the planar structure of 10 is not unsurprising.…”

“…Molecular orbital considerations based on orbital overlap and orbital mixing are thus the simplest way to rationalize many distinct electronic features of low-valent silicon compounds: the pronounced preference for the singlet state in silylenes, 18 as well as the relative weakness of SiSi ³-bonding 19 and thus the conformational flexibility of many disilenes. 20 Intriguingly, compounds with Si=Si bond can adopt different conformations of the double bond as a function of relatively weak energetic parameters such as steric requirements of the substituents or crystal packing forces.…”

New Approaches for ab initio Calculations of Molecules with Strong Electron Correlation

“…114 A qualitative explanation for this observation could be based on the magnitude of the HOMO-LUMO gap which increases from CH 2 to SiH 2 to GeH 2 , but as discussed in detail in Ref. 115 other electronic and steric effects need to be taken into account to arrive at a quantitative understanding.…”

Detection and characterization of singly deuterated silylene, SiHD, via optical spectroscopy

“…Modelling the role of SiH 2 in the fabrication of amorphous silicon thin films and polycrystalline silicon has also attracted significant interest. [1][2][3][4][5][6] Motivated primarily by the desire to garner a fundamental understanding of the properties of this simplest of silicon containing polyatomic molecules, and in part by the desire to develop a real time, in situ SiH 2 monitoring scheme, there have been numerous reported experimental and theoretical [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] studies of gas-phase SiH 2 , and to a lesser extent SiD 2 . In addition, SiH 2 is predicted to be abundant in circumstellar envelopes of carbon rich stars 49 and has been tentatively identified 50,51 via the detection of the 1 11 -0 00 pure rotational transition.…”

“…28 The spectra were interpreted as quasilinear behavior in theB 1 A 1 state with a very small barrier ( 125 cm −1 ) to linearity. Now turning to the theoretical studies, there have been numerous ab initio predictions of the properties of SiH 2 , with particular emphasis on understanding the state ordering [29][30][31][32][33][34][35][39][40][41][42]47 relative to that of CH 2 , to gain insight into the unimolecular dynamics, 36,46,48 and aid in the interpretation of the spectra. 37,[43][44][45][46] In addition to these electronic structure prediction, Duxbury et al 38 reanalyzed the originally recorded X 1 A 1 →à 1 B 1 absorption spectra of SiH 2 7,8 and rationalized the strong local perturbations and observed anomalous radiative lifetimes 14 43 In that study, ab initio calculations of the potential energy surfaces (PESs), the electric dipole moments, and the electric dipole transition moment surfaces for theX 1 A 1 andà 1 B 1 were performed.…”