4 pillars of the big bang theory

2Ultraviolet-Visible Spectroscopy2.1 Revision The following questions cover the important concepts that you should have understood in the first year instrumentation subject. 1. What are the wavelength ranges for the ultraviolet and visible regions of the spectrum? 2. What molecular or structural features give rise to absorption of ultraviolet/visible (UV/VIS) radiation in organic species? Give an example of an organic compound that would not absorb UV/VIS radiation. 3. What molecular or structural features give rise to absorption of ultraviolet/visible (UV/VIS) radiation in ionic species? Give an example of an ionic species that would not absorb UV/VIS radiation. 4. What solvent and cell materials would be suitable for the following scan? Analyte Region Solvent Cell (a) copper sulfate visible (b) copper sulfate UV/visible (c) methylbenzene UV (d) non-polar yellow dye visible
2. UV/VIS spectroscopy Adv. Spectr/Chrom 2.2 2.2 Absorbing species Organic compounds As you are aware, double & triple bonds and non-bonded electrons on atoms such as N, O and the halogens are the structural features that cause absorption in the UV/VIS region. Actually, this statement isn’t strictly true! In fact, all compounds absorb UV radiation, it’s just that the compounds without the multiple bonds and non-bonded electrons absorb below the “dividing line” of 200 nm. Why that figure you might ask? A purely practical one: measurements in the far UV (as < 200 nm is known) require a vacuum, since the components of air absorb strongly. This makes for a less convenient, more complex device and lots of interferences for quantitative analysis. So when we talk about UV spectra, we are actually referring to near UV. While we are on the topic of air absorbing UV, it raises a seeming contradiction to our general rule about what absorbs in the (near) UV: after all, what are the structural features of nitrogen and oxygen gases? Double (O=O) or triple (N≡N) bond and lots of non-bonded electrons (4 on each oxygen, 2 on each nitrogen). So, really air should absorb well above 200 nm, but fortunately it doesn’t (and don’t ask me why, because I don’t know). For a compound to absorb strongly above 200 nm, it requires a number of multiple bonds in conjugation (alternating single-multiple) and/or a number of non-bonded electron atoms. EXAMPLE 2.1 The following compounds show the effect of the presence or absence of absorbing groups (number given is the wavelength of maximum absorption). < 200 210256282312260265270280300These are all obviously absorbing in the UV region only. For an organic compound to absorb in the visible region (even if the peak isn’t past 400 nm), it will need numerous conjugated double bonds and non-bonded electrons. Figure 2.1 shows the structure of methyl orange to illustrate this. FIGURE 2.1 The structure of methyl orange (the yellow form in neutral solution) CH3ClOHOHOHCOOHOHNNN(CH3)2HO3S