ey Oona ae Organic | ald ’ Sy a Y.R. SHARMA2 Ultra-violet and Visible Spectroscopy 2.1, INTRODUCTION ‘The alternate title for this technique is Electronic Spectroscopy since it in- volves the promotion of electrons (6, ™, n* electrons) from the ground state to the higher energy state. It is very useful to measure the number of conju. gated double bonds and also aromatic conjugation within the various mole. cules, It also distinguishes between conjugated and non-conjugated systems; a, B-Unsaturated carbonyl compounds from B, y-analogues; homoannular and Heteroannular conjugated dienes etc. For visible and ultra-violet spec- trum, electronic excitations occur in the range 200-800 mu and involves the promotion of electrons to the higher energy molecular orbital. Since the energy levels of a molecule are quantised, the energy re- quired to bring about the excitation is a fixed quantity. Thus, the elec- tromagnetic radiation with only a particular value of frequency will be’ able to cause excitation. Clearly, if the substance is exposed to radiation of some different value of frequency, energy will not be absorbed and thus, light or radiation will not suffer any loss in intensity. If radiation of a desired or correct frequency is passed or made to fall on the sample of the substance, energy will be absorbed and electrons will be promoted to the higher energy states. Thus,.light radiation on leaving the sample _ after absorption will be either less intense or its intensity may be com- pletely lost. Substances absorbing in the visible range will appear coloured to the human eye (For visible range — See Fig. 2.1). The wavelength of particular radiation absorbed can also be expressed in terms of frequency or energy in kcal mole“. Ultraviolet Visible v|l |B\Gly jojA 1500 2000 4000 ——> Wavelength ~ Fig. 2.1. The range of UV-visible Spectra. 1p = 104 cm *Non-bonding electrons. Ultr 280ah uta-violet and Visible Spectroscopy of loga- Imjt = 1 nm* = 10°? em = 10A Let us calculate the energy associated with radiations having wavele: et us ca 280 mH. 1 = 280 mp = 280 x 10-7 cm We know that E = hy =h i (h = 6.62 x 10°?” ergs sec.) Avogadro's number N = 6.023 x 10 4.18 x 107 ergs = 1 calorie 6.62 x 1077 x 3 x 10! x 6,023 x 10% eat pce 280 x 10-7 x 4.18 x 107 x 10° = 100 kcal mole™!, Note. It is not advisable to keep the compounds in ultra-violet radiations ex- cept for taking the spectrum, A record of the amount of light absorbed by the sample as a function of the wavelength of light in mj or nm units is called absorption spectrum which generally consists of absorption bands, The far ultra-violet region (below 200 my) is not much studied due to absorption by oxygen and nitrogen. Moreover, studies in these regions re- quire vacuum instruments, Problem P21: Calculate the energy associated with radiations having wave- length 400 nm. Calculate the answer in keals mole”! 2.2, THE ABSORPTION LAWS. There are two laws which govern the absorption of light by the molecules. These are : ; (@ Lambert's law and — (ii) Beer's law (@) Lambert’s Law : It states that : . When a beam of monochromatic radiation passes through a homoge- neous absorbing medium, the rate of decrease of intensity of radiation with thickness of absorbing medium is proportional to the inensity of the incident radiation. Mathematically, the law is expressed as ae ae 5 where J = medium, sno = infinitesimally small decrease in the intensity of radiation on pass- ing through infinitesimally small thickness, dx of the medium. | intensity of radiation after passing through a thickness x, of the tate of decrease of intensity of radiation with thickness of the absorbing medium, > eee : *m means nanometers. :_ Elementary Organic i proportionality constant or absorption coefficient. Its value epg the nature of the absorbing medium. i Let [y be the intensity of radiation before entering the absorbing f x= 0). rt : : Then /, the intensity of radiation after passing through any thig say x of the medium can be calculated as : ‘A wax H __fear nf x=0 Uf: In bog -ke or 0 | l=he*™ The intensity of the radiation absorbed, /,,, is given by : Tat = fy 1 = Io(l - €*) ‘The above Lambert's law equation can also be written by changing he natural logarithm to the base 10. 1= fy 10 where a = extinction coefficient of the absorbing medium peta 2.303 Note : For ultraviolet spectrum, the region from 200 mp to 380 mu (callet Guartz region) is considered. The molecular absorption in the UV—VIS region de Pends mainly on the electronic structure of the molecule. Depending upon the pres ence of a common group, the ultraviolet spectrum of a complex compound and tht of a simple compound may be almost identical. Beer’s Law: This law states that: When a beam of monochromatic mi diation is passed through a solution of an absorbing substance, the rate of decrease of intensity of radiation with thickness of the absorbing solution ts proportional to the intensity of incident radiation as well as the concel® tration of the solution. Mathematically, this law is stated as _dl a where c = conc. of the solution in moles litre"! K’ = molar absorption coefficient and its value defends upon the nature of the absorbing substance. Suppose 1p be the intensity of the radi ation before entering the absorbing Solution, (when x= a), then the intensity of radiation, / after passing the thickness x, of the medium can be calculated : 1 rm 1 hetira-violet and Visible Spectroscopy "1 ‘The above equation can also be written by changing the nature of loga- am to the base 10. rithm T=1y. 10 Here =a’ where a’ = molar extinction coefficient of the absorbing K 2.303 solution, Beer's law can also be stated as : When a monochromatic light is passed through a solution of an absorb- ing substance, its absorption remains constant when the conc (c) and the thickness of the absorption layer (x) are changed in the inverse ratio. 23. INSTRUMENTATION A spectrophotometer is a device which detects the percentage transmittance of light radiation when light of certain intensity and frequency range is passed through the sample. Thus, the instrument compares the intensity of the transmitted light with that of the incident light. ‘The modern ultra-violet—visible spectrometers consist of light source, monochromator, detector, amplifier and the recording devices. The most suitable sources of light are : Tungsten Filament lamp and hydrogen-deu- terium discharge lamp which cover the whole of the UV-visible region. ‘Tungsten filament lamp is particularly rich in red radiations i.e., radiations with wavelength 375 mp, while the deuterium discharge lamp covers the region below it. The intensity of the deuterium discharge source falls above 360 mp. The single source is found satisfactory over the entire UV-VIS region. Ordinary spectrometers cover a range 220-800 mu. Better instru- ments cover upto a short wavelength range of 185 mpi. This spectroscopic technique is not useful below 200 my (inaccessible region) since oxygen absorbs strongly at 200 mp: and below. To study absorption below 200 mu, the whole path length is evacuated. The region below 200 mu. is called vacuum ultra violet région. The low wavelength region can be extended upto 150 my by flushing the instrument with nitrogen which absorbs below 150 mp. Most spectrophotometers are double beam instruments. The pri- mary source of light is divided into two beams of equal intensity. Before dividing it into two beams, the incident radiation is dispersed with the help of @ rotating prism. The various wavelengths of a light source are separated with a prism and then selected by slits such that the rotation of the prism causes a series of continuously increasirig wavelengths to pass through the slits for recording purposes. The selected beam is monochromatic which is then divided into two beams of equal intensity. Dispersion grating can also be employed to obtain monochromatic beam of light from poly chromatic radiation Nae radiation). As the dispersion of a single beam or grating is very small, Possible to isolate or collimate very narrow band widths. Thus, light ay the first dispersion is passed through a slit and then sent to the second = * “persion. After the second dispersion, light passes through the exit slit, The a advantage of the second dispersion is that the band width of the emergent ight increases and the light passing through the exit slit is almost monochro- . matic. Also most of the stray light is suppressed. 4 F ws12 Elementary Organic Spectre One of the beams of selected monochromatic light oS Fi id 2.2) ig passed through the sample solution and the other ae st finn intensj is passed through the reference solvent. The solvent as well as ad soluti of the sample may be contained in cells* made atl & nent which i transparent throughout the region under study. Glass et used singg it absorbs strongly in the ultra-violet region. Silica cells can be used, These must be properly stored and their optical surfaces should re be handled, Quartz cells also serve the purpose best. Glass can be used Satisfactorily in the visibie region. This type of spectrometer is called double beam spec. trophotometer. Each absorbance measurement on the solution is accom, panied by a simultaneous measurement on the pure solvent. Photocell Mirror compe Pp, Rotating sits ui prism | Mirror Reference Beam Light Pen WA. Source ‘Spectrum’ g M Electronic Ce AmGmIE Fig. 2.2. Ultra-violet spectrophotometer, Usually, samples are scanned in dilute solutions. One mg of the com: Pound under investigation (Molecular weight 100-200) is accurately Weighed and dissolved in a suitable solvent to make the solution upto 100 mil volume. A little of this solution is taken in a silica cell. The thickness of the solution in the cell should be 1 cm. When the constitution of the absorbing material is unknown, the absorptivity may be sometimes ex. Pressed as E¢,, Pure solvent is also taken in an exactly similar cell (Ref erence cell). These cells are then exposed to the monochromatic beams of equal intensity in the spectrometer, After the beams pass through the sample cell as well as the reference cell, the intensities of the respective transmitted beams are then compared over the whole wavelength range of the instru- ment. The spectrometer electronically subtracts the absorption of the solvent in the reference beam from the absorption of the solution. Hence, the effects ue (0 the absorption of light by the solvent are minimised. In this way, —_—______ *Clean cells should be used. These are rinsed many times with the solvent To remove the last trace of the previous sample, the cell may be cleaned with hot nitric acid or with a detergent, eeviolet and Visible Spectroscopy ultra tron is called o*+ antibonding orbital. So 6 to o* transition takes place electro! (sigma) electron is promoted to antibonding (6) orbital. It is rep. ented 38 o* transition. sv.) When a non-bonding electron ** (n) gets promoted to an antibonding Mt orbital (o*), then it represents no* transition. ei) milarly m — 7* transition represents the promotion of 7 electrons jn antibonding 7 orbital, é., 1 orbital, (See Fig. 2.4) to ts o i ry S 3 © wa ra in ne n> o* t TK 9 o> oF Fig. 2.4. Electronic excitation energies. Similarly, when an n-electron (non-bonding) is promoted to antibonding x orbital, it represents non transition. The energy required for various transitions obey the following order : 63 of >n > O* > -T*>n > A o* Antibonding >, | m Antibonding s & 2 Glia Nonbonding x Bonding o Bonding Fig. 2.5. Various electronic energy levels. Let us now consider the various transitions involved in ultraviolet spec- troscopy. (a) 6 ~ o* transitions. It is a high energy process since o bonds are, in general, very strong. The organic compounds in which all the valence shell electrons are involved in the formation of sigma bonds do not show *tcalled sigma asterisk. **Unshared pair of electrons.Ey 16 Elementary Organic Spectroscopy absorption in the normal ultra-violet region, i.e., 180-400 mu. For satura, hydrocarbons, like methane, propane etc. absorption occurs near 150 (high energy). Consider 6 — 6* transition in a saturated hydrocarbon ; Nit tepals ot oie apa eam hac oan aehgleian a anene ve eer oo Energy c= Fig. 2.6. Various transition involved in Electronic spectroscopy. Such a transition requires radiation of very short wavelength (High en- ergy). See Fig. 2.6. The usual spectroscopic technique cannot be used below 200 mut, since oxygen (present in air) begins to absorb strongly. To study such high energy transitions (below 200 mp), the entire path length must be evacuated.* Thus, the region below 200 mut is commonly called vacuum| ultraviolet region. The excitation of sigma bond electron to o* (anti-| bonding) level occurs with net retention of electronic spin. It is called ex- cited singlet state which may, in tum, gets converted to excited triplet state, This region is less informative, (b) n — o* transition. This type of transition takes place in saturated! compounds containing one hetero atom with unshared pair of electrons (1 electrons). Some compounds undergoing this type of transitions are satu- rated halides, alcohols, ethers, aldehydes, ketones,-amines ete. Such tran- jons require comparatively less energy than that required for ¢ —> o# transitions. Water absorbs at 167 mp, methyl alcohol at 174 my. and methyl chloride absorbs at 169 mp. i In saturated alkyl halides, the energy required for such a transition de+ creases with the increase in the size of the halogen atom (or decrease in the electronegativity of the atom). i +c *Air must be excluded from the instrument so as to avoid absorption cust ie Pete une ;and Visible Spectroscopy 21 ultra-violet nine ~ 200 5000 Methanol .—NO2 rR ne ~ 274 15 at non 204 60 Methanol oH yen eae 338 25 Ethanol -CONI2 = nn 178 9500 Hexane non 220 6 H lexane When aromatic nucleus is substituted with groups which can extend the chromophore, the absorption occurs at still higher valves of extinction coef. ficients. Note. The presence of a compound or a functi by other spectroscopic techniques. "All compounds with the same functional group will absorb at the same wavelength with nearly the same extinction coefficient if the disturbing factors such as conjugation, substituents ete. are absent. Some of the chromophores with their respective absorption maxima and extinction coefficients are given in table T2-1. 2.9. AUXOCHROME An auxochrome can be defined as any 8roup-which does not itself act as a chromophore but whose presence brings about a shift of the absorption band towards the red end of the Spectrum (longer wavel ional group can be confirmed le, benzene shows an absorption maximum at 255 mit {mar 203] whereas aniline absorbs at 280 ML [Emax 1430]. Hence, amino (—NH,) group is an auxochrome. 2.10. ABSORPTION AND INTENSITY SHIFTS (@) Bathochromic effect. It is an effect by virtue of which the absorption maximum is shifted towards longer wavelength due to the Presence of an auxochrome or by the change of solvent. (See Fig. 2.7.). Such an absorption shift towards longer wavelength is called Red shift or bathochromic shift, The n—s1* transition for carbonyl compounds experiences bathochromic shift when the polarity of the solvent is decreased, (©) Hypsochromic shift or effect. It is an effect by virtue of which theyotet and Visible Spectroscopy 21 uta n-n* ~ 200 5000 Methanol —NO2 - non ~ 274 15 =0 eT non 204 60 Methanol OH _N=N— n-1 338 toh Ethanol —CONH2 n> nt 178 9500 Hexane nan 220 63 Hexane When aromatic nucleus is substituted with groups which can extend the chromophore, the absorption occurs at still higher values of extinction coef- ficients. Note. The presence of a compound or a functional group can be confirmed by other spectroscopic techniques. All compounds with the same functional group will absorb at the same wavelength with nearly the same extinction coefficient if the disturbing factors such as conjugation, substituents etc. are absent. Some of the chromophores with their respective absorption maxima and extinction coefficients are given in table T2-1. 2.9. AUXOCHROME An auxochrome can be defined as any group-which does not itself act as a chromophore but whose presence brings about a shift of the absorption band towards the red end of the spectrum (longer wavelength). The absorption at longer wavelength is due to the combination of a chromophore and an auxo- chrome to give rise to another chromophore. An auxochromic group is called colour enhancing group. Auxochromic groups do not show charac- teristic absorption above 200 mu. Some common auxochromic groups are —OH, —OR, —NH;, —NHR, —NR,, —SH etc. The effect of the auxo- chrome is due to its ability to extend the conjugation of a chromophore by the sharing of non-bonding electrons. Thus, a new chromophore results which has a different value of the absorption maximum as well as the ex- tinction coefficient. For example, benzene shows an absorption maximum at 255 mbt {pq 203] whereas aniline absorbs at 280 mb [Emax 1430]. Hence, amino (—NH,) group is an auxochrome. 2.10. ABSORPTION AND INTENSITY SHIFTS (@) Bathochromic effect. It is an effect by virtue of which the absorption maximum is shifted towards longer wavelength due to the presence of an auxochrome or by the change of solvent. (See Fig. 2.7.). Such an absorption shift towards longer wavelength is called Red shift or bathochromic shift. The nn transition for carbonyl compounds experiences bathochromic shift when the polarity of the solvent is decreased, (6) Hypsochromic shift or effect. It is an effect by virtue of which the aie oe ¢ |2 Elementary Organic Spectroscop, Hyperchromic shift Bathochromic shift Hypsochromic een ——> Wavelength (Amex) Fig, 2.7. Absorption and intensity shifts. absorption maximum is shifted towards shorter wavelength. The absorptig shifted towards shorter wavelength is called Blue shift or hypsochromid shift. It may be caused by the removal of conjugation and also by changi the polarity of the solvent. In the case of aniline, absorption maximum occu at 280 mt because the pair of electrons on nitrogen atom is in conjugatig with the ™ bond system of the benzene ring. In its acidic solutions, a blu shift is caused and absorption occurs at shorter wavelength (~203 my). i “ ag NH, ion formed in acidic solution, the electron pair is no longd present and hence conjugation is removed. (c) Hyperchromic effect. It is an effect due to which the intensity absorption maximum increases i.¢., ;,.. increases. For example, the B-b for pyridine at 257 mt pax 2750 is shifted to 262 Mp. Epye 3560 for 2-1 Pyridine (ie., the value Of Eye, increases). The introduction of an a chrome usually increases intensity of absorption. (d) Hypochromic effect. It is defined as an effect due to which intensity of absorption maximum decreases, i.e., extinction coefficient, Eq decreases, The introduction of group which distorts the geometry of ti molecule causes hypochromic effect. For example, biphenyl absorbs at 25 MHL, Epox 19000 whereas 2-methyl biphenyl absorbs at 237 mm, E>, [Emax decreases). It is due to the distortion caused by the methyl 2-methyl biphenyl. 1 By | 2.11, TYPES OF ABSORPTION BANDS 4 Following types of bands originate as a result of the possible transitions inj compound. # (a) K*Bands. K-bands originate from_a compound containing a conji gated system. Such type of bands arise in compounds like dienes, poly pees oe *Konjugierte-German,