{"id":1811,"date":"2017-10-10T15:19:00","date_gmt":"2017-10-10T15:19:00","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&p=1811"},"modified":"2018-10-05T19:50:35","modified_gmt":"2018-10-05T19:50:35","slug":"interpreting-uv-spectra","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/interpreting-uv-spectra\/","title":{"raw":"Interpreting UV Spectra","rendered":"Interpreting UV Spectra"},"content":{"raw":"
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Objectives<\/h3>\r\nAfter completing this section, you should be able to use data from ultraviolet spectra to assist in the elucidation of unknown molecular structures.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
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Study Notes<\/h3>\r\nIt is important that you recognize that the ultraviolet absorption maximum of a conjugated molecule is dependent upon the extent of conjugation in the molecule.\r\n\r\n<\/div>\r\n
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\r\n\r\nWhen a double-bonded molecule such as ethene (common name ethylene) absorbs light, it undergoes a \u03c0<\/strong> - <\/strong>\u03c0<\/strong>* transition.\u00a0 <\/strong>Because \u03c0- \u03c0* energy gaps are narrower than \u03c3 -<\/strong> \u03c3* <\/strong>gaps, ethene absorbs light at 165 nm - a longer wavelength than molecular hydrogen.\r\n\r\n\"image026.png\"\r\n\r\nThe electronic transitions of both molecular hydrogen and ethene are too energetic to be accurately recorded by standard UV spectrophotometers, which generally have a range of 220 \u2013 700 nm.\u00a0 Where UV-vis spectroscopy becomes useful to most organic and biological chemists is in the study of molecules with conjugated pi systems.\u00a0 In these groups, the energy gap for \u03c0 -\u03c0* transitions is smaller than for isolated double bonds, and thus the wavelength absorbed is longer.\u00a0 Molecules or parts of molecules that absorb light strongly in the UV-vis region are called chromophores<\/strong>.\r\n\r\nLet\u2019s revisit the MO picture for 1,3-butadiene, the simplest conjugated system (see section 2.1B<\/a>).\u00a0 Recall that we can draw a diagram showing the four pi MO\u2019s that result from combining the four 2pz<\/sub> atomic orbitals. The lower two orbitals are bonding, while the upper two are antibonding.\r\n\r\n\"image028.png\"\r\n\r\nComparing this MO picture to that of ethene, our isolated pi-bond example, we see that the HOMO-LUMO energy gap is indeed smaller for the conjugated system. 1,3-butadiene absorbs UV light with a wavelength of 217 nm.\r\n\r\nAs conjugated pi systems become larger, the\u00a0 energy gap for a \u03c0 - \u03c0* transition becomes increasingly narrow, and the wavelength of light absorbed correspondingly becomes longer.\u00a0\u00a0 The absorbance due to the \u03c0 - \u03c0* transition in 1,3,5-hexatriene, for example, occurs at 258 nm, corresponding to a \u0394<\/strong>E of 111 kcal\/mol.\r\n\r\n\"image030.png\"\r\n\r\nIn molecules with extended pi systems, the HOMO-LUMO energy gap becomes so small that absorption occurs in the visible rather then the UV region of the electromagnetic spectrum.\u00a0 Beta-carotene, with its system of 11 conjugated double bonds,\u00a0 absorbs light with wavelengths in the blue region of the visible spectrum while allowing other visible wavelengths \u2013 mainly those in the red-yellow region - to be transmitted. This is why carrots are orange.\r\n\r\n\"image032.png\"\r\n\r\nThe conjugated pi system in 4-methyl-3-penten-2-one gives rise to a strong UV absorbance at 236 nm due to a \u03c0 - \u03c0* transition.\u00a0 However, this molecule also absorbs at 314 nm.\u00a0 This second absorbance is due to the transition of a non-bonding (lone pair) electron on the oxygen up to a \u03c0* antibonding MO:\r\n\r\n\"image034.png\"\r\n\r\nThis is referred to as an n<\/strong> - \u03c0<\/strong>* transition<\/strong>.\u00a0 The nonbonding (n) MO\u2019s are higher in energy than the highest bonding p orbitals, so the energy gap for an n - \u03c0* transition is smaller that that of a \u03c0 - \u03c0* transition \u2013 and thus the n - \u03c0* peak is at a longer wavelength. In general, n - \u03c0* transitions are weaker (less light absorbed) than those due to \u03c0 - \u03c0* transitions.\r\n
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Looking at UV-vis spectra<\/h3>\r\nWe have been talking in general terms about how molecules absorb UV and visible light \u2013 now let's look at some actual examples of data from a UV-vis absorbance spectrophotometer. The basic setup is the same as for IR spectroscopy: radiation with a range of wavelengths is directed through a sample of interest, and a detector records which wavelengths were absorbed and to what extent the absorption occurred.\u00a0 Below is the absorbance spectrum of an important biological molecule called nicotinamide adenine dinucleotide, abbreviated NAD+<\/sup>. This compound absorbs light in the UV range due to the presence of conjugated pi-bonding systems.\r\n\r\n\"image038.png\"\r\n\r\nYou\u2019ll notice that this UV spectrum is much simpler than the IR spectra we saw earlier: this one has only one peak, although many molecules have more than one.\u00a0 Notice also that the convention in\u00a0 UV-vis spectroscopy is to show the\u00a0 baseline at the bottom of the graph with the peaks pointing up.\u00a0 Wavelength values on the x-axis are generally measured in nanometers (nm) rather than in cm-1<\/sup> as is the convention in IR spectroscopy.\r\n\r\nPeaks in UV spectra tend to be quite broad, often spanning well over 20 nm at half-maximal height.\u00a0 Typically, there are two things that we look for and record from a UV-Vis spectrum..\u00a0 The first is \u03bb<\/strong>max,<\/sub> which is the wavelength at maximal light absorbance.\u00a0 As you can see, NAD+<\/sup> has \u03bb<\/strong>max<\/sub>,<\/sub> = 260 nm.\u00a0 We also want to record how much light is absorbed at \u03bb<\/strong>max<\/sub>. <\/sub>\u00a0Here we use a unitless number called absorbance<\/strong>, abbreviated 'A'.\u00a0 This contains the same information as the 'percent transmittance' number used in IR spectroscopy, just expressed in slightly different terms.\u00a0 To calculate absorbance at a given wavelength, the computer in the spectrophotometer simply takes the intensity of light at that wavelength before<\/em> it passes through the sample (I0<\/sub>), divides this value by the intensity of the same wavelength after<\/em> it passes through the sample (I), then takes the log10<\/sub> of that number:\r\n\r\nA = log I0<\/sub>\/I\r\n\r\nYou can see that the absorbance value at 260 nm (A260<\/sub>) is about 1.0 in this spectrum.\r\n
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Example<\/h3>\r\nExpress A = 1.0 in terms of percent transmittance (%T, the unit usually used in IR spectroscopy (and sometimes in UV-vis as well).\r\n

Solution<\/h3>\r\n[reveal-answer q=\"828267\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"828267\"]We use the formula: image125.png recall from your high school algebra that if y = log(x), then x = 10y , so: image126.png . . . so the intensity of light entering the sample (I0) is 10 times the intensity of the light (at a particular wavelength) that emerges from the sample and is detected (I). Because %T is simply the reciprocal of this ratio multiplied by 100, we find that A = 1.0 corresponds to 10% transmission. E4.6: Using \u03b5 = A\/c, we plug in our values for \u03b5 and A and find that c = 3.27 x 10-5<\/sup>M, or 32.7 mM.[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nHere is the absorbance spectrum of the common food coloring Red #3:\r\n\r\n\"image040.png\"\r\n\r\nHere, we see that the extended system of conjugated pi bonds causes the molecule to absorb light in the visible range.\u00a0 Because the\u00a0 \u03bb<\/strong>max <\/sub>of 524 nm falls within the green region of the spectrum, the compound appears red to our eyes.\r\n\r\nNow, take a look at the spectrum of another food coloring, Blue #1:\r\n\r\n\"image042.png\"\r\n\r\nHere, maximum absorbance is at 630 nm, in the orange range of the visible spectrum, and the compound appears blue.\r\n
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Example 2<\/h3>\r\nHow large is the \u03c0 - \u03c0* transition in 4-methyl-3-penten-2-one?\r\n

Solution<\/h3>\r\n[reveal-answer q=\"751229\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"751229\"]We find that photons of light with wavelength of 470 nm have energy of 255 kJ\/mol.[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n
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Example 3<\/h3>\r\nWhich of the following molecules would you expect absorb at a longer wavelength in the UV region of the electromagnetic spectrum? Explain your answer.\r\n\r\n\"image036.png\"\r\n

Solution<\/h3>\r\n[reveal-answer q=\"994065\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"994065\"]Molecule A has a longer system of conjugated pi bonds, and thus will absorb at a longer wavelength. Notice that there is an sp3-hybridized carbon in molecule B which isolates two of the pi bonds from the other three.[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
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Exercise<\/h3>\r\n
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Question<\/h4>\r\nWhich of the following would show UV absorptions in the 200-300 nm range?\r\n\r\n\"\"\r\n

Solution<\/h3>\r\n[reveal-answer q=\"329904\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"329904\"]B would be the only one to show in that range.[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

Contributors<\/h3>\r\n<\/div>\r\n
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