File talk:Ethane-staggered-CRC-MW-dimensions-2D.png
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Wrong angel![edit]
There is a wrong angel given in this picture. Tetrahedral molecules have a angel of 109.47° (arccos of minus one-third), see: en:Tetrahedral molecular geometry. --LordOider (talk) 12:53, 4 August 2014 (UTC)
- It's not wrong. It's experimental data from microwave spectroscopy, as explained in the info box. A perfectly symmetrical tetrahedral molecule with the point group Td, such as methane, will have an H-C-H angle of exactly 109.47°, but in a more complicated molecule like ethane, the angle will deviate from ideality. There are a number of factors that determine the precise angle, but a simplistic explanation is the C-C bond repels the C-H bonds more than the C-H bonds repel each other.
- If you read en:Tetrahedral molecular geometry carefully, you will discover that this issue is discussed briefly: "The bond angles are cos−1(−1/3) ≈ 109.5° when all four substituents are the same, as in CH4". The text implies that when the four substituents are not the same, the bond angles are not necessarily 109.5°.
- --Ben (talk) 16:16, 5 August 2014 (UTC)
- I wasn't aware of this, thanks for the explanation. I didn't know either, that such a precise measurement is possible, as molecules aren't static shapes. --LordOider (talk) 20:02, 5 August 2014 (UTC)
- Yes, it's quite impressive. See en:Rotational spectroscopy for details. It requires a few assumptions to be made for more elaborate molecules and relies on the analyte being in the gas phase, but for accurate geometries of small molecules, it's a pretty amazing technique. You can also get a good indication of accurate molecular structure from techniques such as en:X-ray crystallography. As you say, molecules are dynamic, but you can define the average geometry over time or over a large number of individual molecules. en:Computational chemistry is an area of research that has provided many tools for modelling the structure of molecules. You can get very accurate calculated structures using the latest software and hardware. --Ben (talk) 16:05, 6 August 2014 (UTC)
- Yes, the valence angles in ethane are not perfectly tetrahedral. In fact, carbon atoms in ethane only adopt perfectly tetrahedral geometry when C-C bond is stretched to 1.7 Ang.J. Phys. Org. Chem., 2015, 147 128.186.186.8 15:20, 12 September 2022 (UTC)
