Abstract
With the aim of recognizing the steric effects on the silylenic H~2~C~2~Si structures, ab initio and DFT calculations are carried out on 24 structures of X~2~C~2~Si (where X is hydrogen (H), methyl (Me), isopropyl (i‐pro), and tert‐butyl (tert‐Bu)). These species are at either triplet (t) or singlet (s) states. They are confined to the following three sets of structures (1~X~, 2~X~ and 3~X~). Structures 1~X~ include silacyclopropenylidenes (1~s‐H~ and 1~t‐H~) and their 2,3‐disubstituted derivatives (1~t‐Me~, 1~s‐Me~; 1~t‐i‐pro~, 1~s‐i‐pro~; 1~t‐tert‐Bu~, 1~s‐tert‐Bu~). Structures 2~X~ include vinylidenesilylenes (2~s‐H~ and 2~t‐H~) and their 3,3‐disubstituted derivatives (2~t‐Me~, 2~s‐Me~; 2~t‐i‐pro~, 2~s‐i‐pro~; 2~t‐tert‐Bu~, 2~s‐tert‐Bu~). Structures 3~X~ include ethynylsilylenes (3~s‐H~ and 3~t‐H~) and their 1,3‐disubstituted derivatives (3~t‐Me~, 3~s‐Me~; 3~t‐i‐pro~, 3~s‐i‐pro~; 3~t‐tert‐Bu~, 3~s‐tert‐Bu~). Singlet–triplet energy separations (Δ E~s‐t, X~) and relative energies for the above structures are acquired at HF/6‐31G*, B~1~LYP/6‐31G*, B3LYP/6‐31G*, MP2/6‐31G*, HF/6‐31G**, B1LYP/6‐31G**, B3LYP/6‐31G**, and MP2/6‐31G** levels of theory. The highest Δ E~s‐t, X~ is encountered for 1~X~. All singlet states of X~2~C~2~Si, are more stable than their corresponding triplet states. Linear correlations are found between the LUMO–HOMO energy gaps of the singlet 1~s‐X~ and 2~s‐X~ with their corresponding singlet–triplet energy separations calculated at B3LYP/6‐31G**. The seven structures 2~s‐Me~, 2~t‐Me~, 3~s‐Me~, 1~t‐Me~, 1~s‐Me~, 1~s‐tert‐Bu~, and 3~t‐tert‐Bu~ do not appear to be real isomers. Different stability orders are obtained as a function of the substituents (X). The order of stability for six isomers of H~2~C~2~Si is 1~s‐H~ > 2~s‐H~ > 3~s‐H~ > 2~t‐H~ > 3~t‐H~ > 1~t‐H~. Replacing hydrogen atoms by methyl group (X = Me) presents a new stability order: 1~s‐Me~ > 3~s‐Me~ > 2~s‐Me~ > 3~t‐Me~ > 2~t‐Me~ > 1~t‐Me~; and for (i‐pro)~2~C~2~Si is 1~s‐i‐pro~ > 2~s‐i‐pro~ ≈ 3~s‐i‐pro~ > 3~t‐i‐pro~ ≈ 2~t‐i‐pro~ > 1~t‐i‐pro~. Using the larger tert‐butyl group as a substituent (X), yet it offers a more different stability order for six structures of (tert‐Bu)~2~C~2~Si: 1~s‐tert‐Bu~ > 3~s‐tert‐Bu~ > 2~s‐tert‐Bu~ > 3~t‐tert‐Bu~ > 1~t‐tert‐Bu~ > 2~t‐tert‐Bu~. Among eight levels employed, B3LYP/6‐31G** appears as the method of choice. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:619–633, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20204