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2UltravioletSpectroscopy2.I MECHANICS OF MEASUTETENIIn order to obtain useful information fiom the ultraviolet or visiblespectrum of a compound, the wavelength of maximum absorption (tr-o)and the intensity of absorption must be measured accurately. The com-pound should be dissolved in some suitable solvent that does not itselfabsorb light in the region under investigation. The most commonly usedsolvent for ultraviolet spectral determinations is 957o ethyl alcohol. Waterand hexane are also commonly used. The positions of the absorptionpeaksof a compound may be shifted somewhat if different solvents are used.The tr-",. for nonpolar compounds is generally the same in alcohol andhexane; the tr-",. for polar compounds is usually shifted.The solution must be placed in some suitable container that is trans-parent to fight in the region being studied. Although ordinary glass issatisfactory for work in the visible region, glass absorbs ultraviolet lightstrongly; hence, quaftz cells must be used. The most conmonly used cellshave 1.0 cm path length. Modern spectrophotometers provide a plot ofthe intensity of transmitted or absorbed light versus wavelength. The mostsuitable sourcs of light in the ultraviolet region (180-400 mp) is thehydrogen-discharge lamp. A tungsten-filament lamp is usually used forthe visible region (400-800 mp) of the spectrum. Most spectrophotome-ters are double-beam instruments. The primary source of light is split intotwo beams, one of which passes through a cell containing the sample solu-tion and the other of which passes through a cell containing the referencesolvent. The spectrophotometer electronically subtracts the absorption ofthe solvent in the reference beam from the absorption of the solution inthe sample beam. Thus, effects owing to absorption of light by the solventare minimized.The ordinary visible-ultraviolet spectrophotometer provides accePt-able spectra over the range220-800 mp. Better instruments have mechani-cal improvements that extend the short-wavelength range to about 185 mp.The major limitation in this short-wavelength rdnge is the presence of airin the instrument. Oxygen absorbs strongly at about 200 mp and blow;frequently the range of the instrument can be extended by flushing the4

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20 UTTRAYIOTET SPECTROSCOPYfor crs and trans isomers quite generally shown + T* transition of the trans isomer occurshas a larger extinction coefficient than thatTable 2-6 demonstrate this effect.Table 2{ULTRAvToLET ABsoRprroN oF soME cli nNo lrans IsoMERs

Compound cls-Isomer Jtr-*, Dp max

Chop. 2that absorption due to theat longer wavelength andof the cis isomer. Data in

trans-lsomertr*, Dp rmaxCinnamic acidStilbene

268 10,700279 9,350 272 15,900294 24,0002.8 PROBIEmSl. The followin1a,B-unsaturated ketones have tr-* at24l mp (e 4,700),254mp"(e 9,550), and 259 mp, (e 10,790) in ethanol. Which is which?

2. The following dienes245 mp, (e 18,000), 265 mpwhich?

(2a)

(2d)3. The following polyenes have l,or.,, atand 355 mp (e 19,700) in ethanol. Which is

o(lb)have tr** at 231

(e 6,4@), and 282 mp (121,000), 236 mp (e 12,000),mp (e I I,900) in ethanol. Which is

(2c)

(2e)306 mp (e 14,500), 315 mp (e 19,800),which?

#l rec-(2b)(3a) AcO