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Steric effects on the dialkyl substituted X 2 C 2 Si silylenes: A theoretical study

Steric effects on the dialkyl substituted X 2 C 2 Si silylenes: A theoretical study With the aim of recognizing the steric effects on the silylenic H2C2Si structures, ab initio and DFT calculations are carried out on 24 structures of X2C2Si (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 (1X, 2X and 3X). Structures 1X include silacyclopropenylidenes (1s‐H and 1t‐H) and their 2,3‐disubstituted derivatives (1t‐Me, 1s‐Me; 1t‐i‐pro, 1s‐i‐pro; 1t‐tert‐Bu, 1s‐tert‐Bu). Structures 2X include vinylidenesilylenes (2s‐H and 2t‐H) and their 3,3‐disubstituted derivatives (2t‐Me, 2s‐Me; 2t‐i‐pro, 2s‐i‐pro; 2t‐tert‐Bu, 2s‐tert‐Bu). Structures 3X include ethynylsilylenes (3s‐H and 3t‐H) and their 1,3‐disubstituted derivatives (3t‐Me, 3s‐Me; 3t‐i‐pro, 3s‐i‐pro; 3t‐tert‐Bu, 3s‐tert‐Bu). Singlet–triplet energy separations (Δ Es‐t, X) and relative energies for the above structures are acquired at HF/6‐31G*, B1LYP/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 Es‐t, X is encountered for 1X. All singlet states of X2C2Si, are more stable than their corresponding triplet states. Linear correlations are found between the LUMO–HOMO energy gaps of the singlet 1s‐X and 2s‐X with their corresponding singlet–triplet energy separations calculated at B3LYP/6‐31G**. The seven structures 2s‐Me, 2t‐Me, 3s‐Me, 1t‐Me, 1s‐Me, 1s‐tert‐Bu, and 3t‐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 H2C2Si is 1s‐H > 2s‐H > 3s‐H > 2t‐H > 3t‐H > 1t‐H. Replacing hydrogen atoms by methyl group (X = Me) presents a new stability order: 1s‐Me > 3s‐Me > 2s‐Me > 3t‐Me > 2t‐Me > 1t‐Me; and for (i‐pro)2C2Si is 1s‐i‐pro > 2s‐i‐pro ≈ 3s‐i‐pro > 3t‐i‐pro ≈ 2t‐i‐pro > 1t‐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)2C2Si: 1s‐tert‐Bu > 3s‐tert‐Bu > 2s‐tert‐Bu > 3t‐tert‐Bu > 1t‐tert‐Bu > 2t‐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 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Heteroatom Chemistry Wiley

Steric effects on the dialkyl substituted X 2 C 2 Si silylenes: A theoretical study

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Publisher
Wiley
Copyright
Copyright © 2006 Wiley Periodicals, Inc.
ISSN
1042-7163
eISSN
1098-1071
DOI
10.1002/hc.20204
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Abstract

With the aim of recognizing the steric effects on the silylenic H2C2Si structures, ab initio and DFT calculations are carried out on 24 structures of X2C2Si (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 (1X, 2X and 3X). Structures 1X include silacyclopropenylidenes (1s‐H and 1t‐H) and their 2,3‐disubstituted derivatives (1t‐Me, 1s‐Me; 1t‐i‐pro, 1s‐i‐pro; 1t‐tert‐Bu, 1s‐tert‐Bu). Structures 2X include vinylidenesilylenes (2s‐H and 2t‐H) and their 3,3‐disubstituted derivatives (2t‐Me, 2s‐Me; 2t‐i‐pro, 2s‐i‐pro; 2t‐tert‐Bu, 2s‐tert‐Bu). Structures 3X include ethynylsilylenes (3s‐H and 3t‐H) and their 1,3‐disubstituted derivatives (3t‐Me, 3s‐Me; 3t‐i‐pro, 3s‐i‐pro; 3t‐tert‐Bu, 3s‐tert‐Bu). Singlet–triplet energy separations (Δ Es‐t, X) and relative energies for the above structures are acquired at HF/6‐31G*, B1LYP/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 Es‐t, X is encountered for 1X. All singlet states of X2C2Si, are more stable than their corresponding triplet states. Linear correlations are found between the LUMO–HOMO energy gaps of the singlet 1s‐X and 2s‐X with their corresponding singlet–triplet energy separations calculated at B3LYP/6‐31G**. The seven structures 2s‐Me, 2t‐Me, 3s‐Me, 1t‐Me, 1s‐Me, 1s‐tert‐Bu, and 3t‐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 H2C2Si is 1s‐H > 2s‐H > 3s‐H > 2t‐H > 3t‐H > 1t‐H. Replacing hydrogen atoms by methyl group (X = Me) presents a new stability order: 1s‐Me > 3s‐Me > 2s‐Me > 3t‐Me > 2t‐Me > 1t‐Me; and for (i‐pro)2C2Si is 1s‐i‐pro > 2s‐i‐pro ≈ 3s‐i‐pro > 3t‐i‐pro ≈ 2t‐i‐pro > 1t‐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)2C2Si: 1s‐tert‐Bu > 3s‐tert‐Bu > 2s‐tert‐Bu > 3t‐tert‐Bu > 1t‐tert‐Bu > 2t‐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

Journal

Heteroatom Chemistry – Wiley

Published: Jan 1, 2006

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