Blue-shifted hydrogen bonds—examples

The colour of indigo depends dramatically upon its physical state and environment for example, the vapour is red but the colour on the fibre is blue. The marked solvatochromism of indigo (Table 6.4) is attributable mainly to hydrogen bonding. A progressive bathochromic shift of the visible absorption band is observed as the solvent polarity... 

The concept of polarity covers all types of solute-solvent interactions (including hydrogen bonding). Therefore, polarity cannot be characterized by a single parameter. Erroneous interpretation may arise from misunderstandings of basic phenomena. For example, a polarity-dependent probe does not unequivocally indicate a hydrophobic environment whenever a blue-shift of the fluorescence spectrum is observed. It should be emphasized again that solvent (or microenvironment) relaxation should be completed during the lifetime of the excited state for a correct interpretation of the shift in the fluorescence spectrum in terms of polarity. 

Effects of Hydrogen Bonding. In a solvatochromic plot of transition energy hv versus solvent polarity [e.g. the f D) function], the effect of hydrogen bonding between the solute and the solvent is an anomalous blue shift or red shift, of which an example is shown in Figure 3.53. This is the... 

Overall, the band shifts experimentally observed for all kinds of absorptions are the net results of three, partly counteracting contributions electrostatic (dipole/dipole dipole/induced dipole blue shift), dispersion ( red shift), and specific hydrogen-bonding blue shift). Which of these solute/solvent interactions are dominant for the solute under study depends on the solvents used. For example, the results obtained for pyridazine, as shown in Fig. 6-5, clearly implicate hydrogen-bonding as the principle cause of the observed hypsochromic band shift that occurs when the HBD solvent ethanol is added to solutions of pyridazine in nonpolar -hexane [98]. The intensity of n n absorption bands is usually very low because they correspond to symmetry-forbidden transitions, which are made weakly allowed by vibronic interactions cf. Fig. 6-5). 

Most of hydrogen-bonded molecules exhibit to a larger or lesser extent all of the properties described in section VI. There exist cases, however, when some of these features are not present or even the trend is opposite. An example are complexes in which the X-H vibrations are blue shifted (see Hobza et al, 1999, and Gu et al, 1999). Some authors questioned if such complexes should be considered hydrogen bonded. According to the definition adopted in this article, the hydrogen-bonded classification will be appropriate if the equilibrium geometry has a nearly linear X-H- -Y shape, and the bond is reasonably strong (i.e., not much weaker than about 1 kcal/mol). 

A molecular geometry which allows optimal p-orbital interaction to yield a 2a a bond between the two sulfur atoms is shown on the left hand side of Figure 3. In reality such a configuration seems most closely approached in the radical cation obtained upon one-electron oxidation of 1,5-dithiacyclooctane, 7, and was first described in ESR work by Musker and co-workers [103, 105, 106]. It exhibits an optical absorption with 400 nm and constitutes the most blue-shifted example known (except for the all-hydrogen substituted (H2S SH2) with Amax at 370 nm). Stabilization of 7 is facilitated by the establishment of two five-membered rings on either side of the transannular S S bridge. 

The following is an example of a [1,5] sigmatropic rearrangement, also sometimes referred to as a [1,5] hydrogen shift. In the transition state, the bond that is breaking and the bond that is forming are separated by two different pathways one is comprised of five atoms (labeled in red), and the other is comprised of only one atom (labeled in blue). 

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