Presentation on theme: "Lecture 8. The energy is sufficient to promote or excite a molecular electron to a higher energy orbital. Consequently, sometimes called "electronic spectroscopy"."— Presentation transcript:
The energy is sufficient to promote or excite a molecular electron to a higher energy orbital. Consequently, sometimes called "electronic spectroscopy". The three types of electron transitions that give rise to UV or visible are : sigma ; pi π and nonbonding n orbitals (those with unshared pairs of electrons). As a rule, energetically favored electron promotion will be from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and the resulting species is called an excited state.
When sample molecules are exposed to light having an energy that matches a possible electronic transition within the molecule, some of the light energy will be absorbed by valence (outer) electrons. these electrons are promoted from their normal (ground) states to higher energy (excited) states (as the electron is promoted to a higher energy orbital. An optical spectrometer records the wavelengths at which absorption occurs, together with the degree of absorption at each wavelength. The resulting spectrum is presented as a graph of absorbance (A) versus wavelength. The most useful region of the UV spectrum is at wavelength longer than 200 nm. 100- 200 nm π π* for isolated double bond and * for ordinary carbon-carbon bond. The useful transitions 200-400 nm are for compounds with conjugated double bonds and some n * and n π* transitions
Because the absorbance of a sample will be proportional to the number of absorbing molecules in the spectrometer light beam (e.g. their molar concentration in the sample tube), it is necessary to correct the absorbance value The corrected absorption value is called "molar absorptivity", and is particularly useful when comparing the spectra of different compounds and determining the relative strength of light absorbing functions (chromophores). Molar absorptivity (ε) is defined as: Molar Absorptivity, ε = A / c l (where A= absorbance, c = sample concentration in moles/liter & l = length of light path through the sample in cm.) Beer’s Law. states that the light absorbed is proportional to the number of absorbing molecules – ie to the concentration of absorbing molecules.
A second law – Lambert’s law – tells us that the fraction of radiation absorbed is independent of the intensity of the radiation. Combining these two laws gives the Beer–Lambert law: log 10 I 0 /I = εlc I O = the intensity of the incident radiation I = the intensity of the transmitted radiation ε = the molar absorption coefficient l = the path length of the absorbing solution (cm) c = the concentration of the absorbing species in mol dm -3
Two useful pieces of information are the - molar absorption coefficient, ε - and λmax which is the wavelength at which maximum absorption occurs. These two pieces of information are used to identify a substance besides other spectroscopic techniques. However, if ε and λmax are known for a compound the concentration of the solution can be calculated. This is the most common application.
Absorption of polyenes Ethene contains a simple isolated carbon-carbon double bond, but the other two have conjugated double bonds. In these cases, there is delocalisation of the pi bonding orbitals over the whole molecule. c = E = h E = (hc)/ E 1/ E = energy; c = speed of light; = wavelength; = frequency; h = Planck’s constant
Less energy is required to promote pi electron of 1,3- butadiene than is needed to promote a pi electron of ethene. The reason is that the energy difference between the HOMO (highest occupied molecular orbital) and the LUMO for conjugated double bonds less than the energy difference for an isolated double bond. Because less energy is needed for pi excitation of 1,3- butadiene, this absorbs UV radiation for longer wavewlength than does ethylene.
As more conjugated double bonds are added to a molecule, the energy required to the first excited state decreases. Sufficient conjugation shifts the absorption to wavelengths that reach into visible region of the spectrum.
Absorption by aromatic systems Benzene and other aromatic compounds exhibit more complex spectra than can be explained by simple π π* transitions. A value 260 nm is often reported as the λmax for benzene because this is the position of strongest absorption above 200 nm. The absorption of UV radiation by aromatic compounds composed of fused benzene rings is shifted to longer wavelengths as the number of rings is increased because of increasing conjugation and greater resonance stabilization of the excited state.
Absorption arising from transitions of n electrons Compounds that contain N, O, S P or one of the halogens all have unshared n electrons. If the structure contain no π bonds, these n electrons can undergo only n * transitions. Because the n electrons are of higher energy than either π or electrons, so less energy is required to promote an n electron and the transition occur at longer wavelengths than * transitions.
The π* orbital is of lower energy than the * orbital. Consequently, n π* transitions require less energy than n * transitions.
Sample should be dissolved in suitable solvent that does not itself absorb light in the region under investigation. Most commonly used solvent is 95% ethanol, 1,4 dioxane, and cyclohexane Solution must be placed in suitable container that is transparent to light which is made of quartz or fused silica
Important definitions Chromophore: a covalently unsaturated group responsible for the electronic absorption e.g. C=C, C=O, NO2 Auxochrome: a saturated group with non bonded electron which when attached to a chromophore alter both the wavelength and the intensity of the absorption e.g. OH, NH2, Cl Bathochromic shift: the shift to longer wavelength due to the effect of substitution or solvent effect (red shift)
Flavonoids UV is major technique for the structural analysis of flavonoids why? Small amount of pure material is required The amount of structural information gained from UV spectrum is considerably enhanced by the use of specific reagents (NaOMe, NaOAc, H3BO 3, AlCl 3 and AlCl 3 /HCl React with one or more functional groups on the flavonoid nucleus Addition of each of these reagents separately to an alcoholic solution induce shift in UV Application of UV in natural product
Band I is considered to be associated with the absorption due to the B-ring (cinnamoyl system) and band II with the absorption involving the A-ring, benzoyl system
1- significance of sodium acetate NaOAc addition to methanol Spectrum induce bathochromic shift of 10-20 nm in band II indicate the presence of free OH at C-7 Change in band I indicates the presence of 4`-OH
Boric acid induced shift When boric acid is added to sodium acetate spectrum of the compound if it gives bathochromic shift 5 to 20 nm indicate the presence of ortho-dihydroxy groups No shift.............. No ortho-dihydroxy groups.
Significance of AlCl 3 Reagent for placing OH at C5 or C3 or ortho dihydroxy group AlCl 3 complex with flavonoid has OH at C5 or C3 or orthodihydroxy In this case bathochromic shift of 40-65 nm is observed in Band I
AlCl 3 has different stability to HCl, with the addition of 5-10 drops (10% HCl) on AlCl 3 spectrum show
Significance of Sodium methoxide shift Addition of NaOMe to MeOH spectrum increase shift in band I by 80 nm (C-4`-OH) It is important to decide if OH at C-4` occupied by sugar or free or OCH 3
MeOH 268, 296 sh, 333 AlCl 3 290, 318 sh, 393 AlCl 3 +HCl 291, 319 sh, 394 AlCl 3 shift 40- 65 nm in band I indicate the presence of OH at 5 position Flavone: 250- 280 II, 300- 350 I Flavonol: 250- 265 II, 350- 385 I Flavanone: 270- 295 II, 300- 330 I
MeOH: 267, 295sh, 336 NaoAc: 284, 301, 376 AlCl 3 : 276, 301 sh, 346, 384 AlCl 3 +HCl: 276, 299, 340, 381 AlCl 3 shift 40- 65 nm in band I indicate the presence of OH at 5 position NaoMe: 275, 324, 392 NaOMe to MeOH spectrum increase shift in band I by 80 nm (C- 4`-OH) NaoAc bathochromic shift of 10-20 nm in band II indicate the presence of free OH at C-7 Change in band I indicates the presence of 4`-OH AlCl 3 shift 40- 65 nm in band I indicate the presence of OH at 5 position