Ultraviolet Spectroscopy

Transcription

1 Ultraviolet Spectroscopy The wavelength of UV and visible light are substantially shorter than the wavelength of infrared radiation. The UV spectrum ranges from 100 to 400 nm. A UV-Vis spectrophotometer measures the amount of light absorbed at each wavelength of the UV and visible regions of the electromagnetic spectrum. A UV or visible spectrophotometer has the same basic design as an infrared spectrophotometer. In a standard UV-Vis spectrophotometer, a beam of light is split; one half of the beam (the sample beam) is directed through a transparent cell containing a solution of the compound being analyzed, and one half (the reference beam) is directed through an identical cell that does not contain the compound but contains the solvent. 1Solvents are chosen to be transparent in the region of the spectrum being used for analysis.

2 The instrument is designed so that it can make a comparison of the intensities of the two beams as it scans over the desired region of the wavelengths. If the compound absorbs light at a particular wavelength, the intensity of the sample beam (I S ) will be less than that of the reference beam (I R ). Absorption of radiation by a sample is measured at various wavelengths and plotted by a recorder to give the spectrum which is a plot of the wavelength of the entire region versus the absorption (A) of light at each wavelength. 2

3 A broad absorption band in the region between 210 and 260 nm. The absorption is at a maximum at nm. It is this wavelength that is usually reported in the chemical literature λ max. 3

4 Expressions Used in Ultraviolet Spectrometry The spectrum shows that the scan is from nm. Because absorption by atmospheric carbon dioxide becomes significant below 200 nm, the nm region is usually not scanned unless special air-free techniques are employed. 4

5 The wavelength of absorption is usually reported as λ max which represents the wavelength at the highest point of the curve. The absorption of energy is reported as absorbance (not transmittance as in infrared spectra). The absorbance at a particular wavelength is defined by the equation: The absorbance by a compound at a particular wavelength increases with an increasing number of molecules undergoing transitions. 5

6 Therefore, the absorbance depends on the electronic structure of the compound and also upon the concentration of the sample and the length of the sample cell. Usually, energy absorption is reported as molar absorptivity ε (also called molar extinction coefficient) rather as the actual absorbance. The molar absorptivity (usually reported at λ max ) is a reproducible value that makes into account concentration and cell length. It is simply the proportionality constant that relates the observed absorbance (A) at a particular wavelength (λ) to the molar concentration (c) of the sample and length (l) (in centimeter) of the path of the light beam through the sample cell. 6

7 Absorption of infrared radiation by a molecule leads to increased vibrations of covalent bonds. Molecular transitions from the ground state to an excited state requires about 2-15 kcal/mol. Both UV and visible radiation are of higher energy than IR radiation. Absorption of UV or visible light results in electronic transitions; electrons are promoted from low-energy ground state or orbitals to higher-energy excited-state orbitals. These transitions require about kcal/mol. The energy absorbed is subsequently dissipated as heat, as light (e.g., fluorescence), or in chemical reactions (such as isomerization or free-radical reactions) or in dissociation or ionization of the molecule. 7 The structural unit associated with an electronic transition in the UV-Vis spectroscopy is called a chromophore.

8 The wavelength of UV or visible light absorbed depends on the ease of electron promotion. Molecules that require more energy for electron promotion absorb at shorter wavelengths. Compounds that absorb light in the visible region (that is colored compounds) have more-easily promoted electrons than compounds that absorb at shorter UV wavelengths. 8 colorless

9 Although the energy absorption by a molecule is quantized, a UV or visible spectrum consists of not a spectrum of lines or sharp peaks but rather of broad absorption bands over a wide range of wavelength. The reason for the broad absorption is that the energy levels of both the ground state and the excited state of a molecule are subdivided into rotational and vibrational sublevels. Electronic transitions may occur from any of the sublevels of the ground state to any one of the sublevels of an excited state. That is, a discrete line is not obtained since electronic absorption is superimposed on rotational and vibrational sublevels. Since these various transitions differ slightly in energy, their wavelengths of absorption also differ slightly and give rise to the broad band observed in the spectrum. 9

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12 Types of Electron Transitions The ground state of an organic molecule contains valence electrons in three principal types of molecular orbitals: sigma (σ) orbitals; pi (π) orbitals; and filled but nonbonded orbitals (n). Both σ and π orbitals are formed from the overlap of two atomic or hybrid orbitals. Each of these molecular orbitals therefore has an antibonding σ* or π* orbital associated with it. An orbital containing n electrons does not have an antibonding orbital (because it was not formed from two orbitals). 12

13 Electron transitions involve the promotion of an electron from one of the three ground states (σ, π, or n) to one of the two excited states (σ, or π ). There are six possible transitions; the four important transitions and their relative energies are: 13

14 The most useful region of the UV spectrum is at wavelengths longer than 200 nm. The following transitions give rise to absorption in the nonuseful nm range: π π* for an isolated double bond, and σ σ* for an ordinary carbon-carbon bond. The useful transitions (200 nm-400 nm) are π π* for compounds with conjugated double bonds, and some n σ* and some n π* transitions. Alkenes and nonconjugated dienes usually have absorption maxima below 200 nm. Example: Ethene gives an absorption maximum at 171 nm, 1,4-pentadiene gives an absorption maximum at 178 nm. 14

15 Absorption by Polyenes Compounds whose molecules contain conjugated multiple bonds have absorption maxima at wavelengths longer than 200 nm. For example, less energy is required to promote a π electron of 1,3- butadiene than is needed to promote a π electron of ethylene. The reason is that the energy gap between the HOMO and the LUMO for conjugated double bonds is less than the energy difference for an isolated double bond. Resonance-stabilization of the excited state of a conjugated diene is one factor that decreases the energy of the excited state. λ max at 171 nm λ max at 217 nm Because less energy is needed for a π π* transition of 1,3-butadiene, this diene absorbs UV radiation of longer wavelengths than does ethylene. 15

16 The UV spectrum of cis,trans-1,3-cyclooctadiene The π π* transition in cis,trans-1,3- cyclooctadiene involves excitation of an electron from the HOMO to LUMO 16

17 Sufficient conjugation shifts the absorption to wavelengths that reach into the visible region of spectrum. The compound responsible for the red color of tomatoes 17 The compound responsible for the color of carrot

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19 General Rule: General Rule: the greater the number of conjugated multiple bonds a compound contains, the longer will be the wavelength at which the compound absorbs light. 19

20 Absorption by Compounds with C=O Bonds The carbonyl groups of saturated aldehydes and ketones give a weak absorption band in the UV region between 270 and 300 nm. Aldehydes and ketones have two absorption bands in the ultraviolet region. Both involve excitation of an electron to an antibonding π* orbital (n π* and π π* ). This band is shifted to longer wavelengths ( nm) when the carbonyl group is conjugated with a double bond. 20

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22 Also, compounds in which the carbon-oxygen double bond is conjugated with a carbon-carbon double bond have absorption maxima corresponding to n π* excitation and π π* excitations. The n π* absorption maxima occur at larger wavelengths but are much weaker (i.e., smaller molar absorptivities) 22

23 Absorption by Aromatic Systems The conjugated π electrons of a benzene ring give characteristic ultraviolet absorptions that indicate the presence of a benzene ring in an unknown compound. Benzene and other aromatic compounds exhibit morecomplex spectra than can be explained by simple π π* transitions. The complexity arises from the existence of several low-lying excited states. One absorption band of moderate intensity occurs near 205 nm and another, less intense band appears in the nm range. Conjugation outside the benzene ring leads to absorptions at other wavelengths. 23

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25 Absorption Arising from Transitions of n Electrons Compounds that contain nitrogen, sulfur, phosphorous, or one of the halogens all have unshared n electrons. If the structure contains no π bonds, these n electrons can undergo only n σ* transitions. Because the n electrons are of higher energy than either σ or π electrons, less energy is required to promote an n electrons, and transitions occur at longer wavelengths than σ σ* transitions. 25

26 The π* orbital is of lower energy than the σ* orbital; consequently, n π* transitions require less energy than n σ* transitions and often are in the range of a normal instrument scan. The n electrons are in a different region of space from σ* and π* orbitals, and the probability of an n transition is low. Since molar absorptivity depends on the number of electrons undergoing transitions, ε values for n transitions are low, in the range (compared to about 10,000 for a π π* transition). A compound such as acetone that contains both a π bond and n electrons exhibits both π π* and n π* transitions. Acetone shows absorption at 187 nm (π π*) and 270 nm (n π*) 26

27 270 nm 187 nm 27

28 Absorption by Alcohols Unless the molecule has other chromophores, alcohols are transparent above about 200 nm. Example: λ max for methanol is 177 nm. Absorption by Ethers and Epoxides Simple ethers have their absorptions maximum at about 185 nm and are transparent to ultraviolet radiation above about 220 nm. 28

29 Absorption by Amines In the absence of any other chromospheres, the UV-Vis spectrum of an alkylamine is not very informative. The longest wavelength absorption involves promoting one of the unshared electrons of nitrogen to an antibonding σ* orbital (n σ*) with a λ max in relatively inaccessible region near 200 nm. In arylamines the interaction of the nitrogen lone pair with the π-electron system of the ring shifts the ring s absorptions to longer wavelengths. Tying up the lone pair by protonation causes the UV-Vis spectrum of anilinium to resemble benzene. 29

30 Absorption by Phenols An OH group affects the UV-Vis spectrum of benzene in a way similar to that of an NH 2 group, but to a smaller extent. In basic solution, in which OH is converted to O _, however, the shift to longer wavelengths exceeds that of an NH 2 group. 30

31 Absorption by Carboxylic Acid and Carboxylic Acid Derivatives In the absence of any additional chromophores, carboxylic acids absorb at a wavelength (210 nm) that is not very useful for diagnostic purposes. The following values are typical for the n π* absorption associated with C=O group of carboxylic acid derivatives. 31

32 32 Analytical Uses of UV-Vis Vis Spectroscopy In practice, ultraviolet spectrometry is limited to conjugated systems for the most part, and UV-Vis spectroscopy can be used in the structure elucidation of organic molecules to indicate whether conjugation is present in a given sample. Although conjugation in a molecule may be indicated by data from IR, NMR, or mass spectrometry, UV-Vis analysis can provide corroborating information. A more widespread use of UV-Vis spectroscopy, however, has to do with determining concentration of an unknown sample. The relationship A = εcl indicates that the amount of absorption by a sample at a certain wavelength is dependent on its concentration. Using calibration curve of λ max versus concentration of standards, concentration of an unknown sample could be determined.

33 Quantitative analysis using UV-Vis spectroscopy is routinely used in biochemical studies to measure the rates of enzymatic reactions (kinetics). The concentration of a species involved in the reaction (as related to its UV-Vis absorbance) is plotted versus time to determine the rate of reaction. UV-Vis spectroscopy is also used in environmental chemistry to determine the concentration of various metal ions (sometimes involving absorption spectra for organic complexes with the metal), as a setection method in HPLC. 33

34 There is an advantage to the selectivity of ultraviolet absorption; characteristic groups may be recognized in molecules of widely varying complexities. A large portion of a relatively complex molecule may be transparent in the ultraviolet so that we may obtain a spectrum similar to that of a much simpler molecules. The absorption results from the conjugated enone portion of the two compounds 34

35 35 THE END