![]() The positive solvatochromism is due to the increase in stability of the excited state with increasing solvent polarity. Solvatochromism is caused by differential solvation of the ground and first excited state of the chromophore. The electronic transitions of particular interest in this respect are π→π* and n→π* as well as charge transfer absorptions. On the other hand, the absence of hydrogen bonding between ether molecules becomes apparent in the much lower boiling point of diethyl ether (34.6 ☌, source) compared to water.The solvent effect on spectra, resulting from electronic transitions, is primarily dependent on the chromophore and the nature of the (σ→σ*, n→σ*, π→π*, n→π* and charge-transfer absorption) transition. Nevertheless, the ether oxygen can act as a hydrogen bond acceptor in combination with better hydrogen bond donors like OH groups, which explains why diethyl ether is sparingly soluble in water despite its low polarity (69 g/l at 20 ☌ ( source)). Hydrogen bonding between C-H groups and oxygens, although possible in principle (see comments to your question) and, for example, found in protein structures, can be neglected in this case due to the low number of donors and the weakness of the interaction (< 17 kJ/mol). The permanent dipoles can also interact with each other, in the form of intermolecular attractive interactions between centers with $\delta $ and $\delta-$ partial charge (and likewise repulsion between centers of equal partial charge). They are responsible for the weak interactions between the alkyl chains, like in other alkanes and non-polar molecules. London dispersion forces, which result from short-lived dipoles induced by fluctuations in the electron shell of molecules, are also present. As both dipole moment vectors point roughly in the same direction, they do not cancel each other out like in the case of carbon dioxide, so the molecule has a net dipole moment and is therefore (weakly) polar. As you have already figured out, diethyl ether is a small dipole due to the electronegativity difference between carbon and oxygen, which can also be seen in this electron density plot. ![]()
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February 2023
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