2.8　Relationship Between Molecular Structure and UV Spectrum

2.8.1　Saturated Hydrocarbon Compounds

Alkanes contain nothing but single bonds and lack atoms with unshared electron pairs，the only possible electronic transitions are the σ→σ*type.These transitions are of such a high energy that they absorb ultraviolet energy at very short wavelengths，methane^[111] absorbs around 125 nm，ethane^[112] absorbs around 135 nm.In saturated molecules that contain atoms bearing nonbonding pairs of electrons，such as alcohols，ethers^[113]，amines^[114]，and sulfur compounds^[115]，transitions of the n→σ*type become important.They are also rather high-energy transitions，but they do absorb radiation that lies within an experimentally accessible range.Alcohols and amines absorb in the range from 175 to 200 nm，while organic thiols^[116] and sulfides absorb between 200 and 220 nm.Most of the absorptions are below the cutoff points for the common solvents，so they are not observed in solution spectra（Table 2-3）.

2.8.2　The Simplest Unsaturated Compounds

Alkenes and alkynes^[117] with unsaturated molecules，π→π*transitions become possible.These transitions are of rather high energy as well，but their positions are sensitive to the presence of substitution.Alkenes absorb around 175 nm，and alkynes absorb around 170 nm.

Unsaturated molecules that contain atoms such as oxygen or nitrogen such as carbonyl compounds may also undergo n→π* transitions.These are perhaps the most interesting and most studied transitions.These transitions are also rather sensitive to substitution on the chromophoric structure.The typical carbonyl compound undergoes an n→π*transition around 280 to 290 nm（ε=15）.Most n→π*transitions are forbidden and hence are of low intensity.Carbonyl^[118] compounds also have a π→π* transition at about 188 nm（ε=900）.Table 2-4 lists the wavelength maxima of typical carbonylcompounds.

2.8.3　The Conjugated Alkenes

Woodward and Fiesers investigated many alkene compounds，especially the terpene and steroid compounds，and an empirical formula was summarized to calculated the wavelength maxima of conjugated alkenes as following Table 2-5.

Application of Woodward-Fieser rules should be noted：

①The rule applies only to conjugated diene，triene，and tetraene.

②Selecting a longer conjugated system as a parent.

③In crossed conjugated systems^[119]，only a conjugated bond should be selected，the double bond in bifurcation^[120] is not the extending double bond of conjugation，and the conjugated system with a longer absorption should be chosen.

④This rule does not apply to an aromatic system.

⑤In conjugated system all substituents and exocyclic double bond should be taken into account.

Examples　Predict the UV maximum for each of the following substances：

　　　　　　　　　　　

2.8.4　The Conjugated Unsaturated Carbonyl Compounds

The rule for calculating the absorption maximum of the conjugated unsaturated carbonyl compounds are given in Table 2-6.

Application of Woodward-Fieser rules should be noted：

①The number of carbon atoms of conjugated unsaturated carbonyl compounds is ^δCC—^βCC—CO.

②Do not take the carbonyl in a ring as exocyclic double bond.

③When there are two conjugated unsaturated carbonyl groups，the larger wavelength of carbonyl should be chosen as parent.

④In this case，the wavelength maximum（λ_max）of the K-band of conjugated unsaturated carbonyl compounds is significantly influenced by the solvent polarity，therefore，a solvent correction^[121]（from Table 2-7）is subtracted^[122] from the calculated value to obtain the value expected for a solvent other than the standard solvent ethanol.

Examples　Predict the UV maximum for each of the following substances：

　　　　　　　　

2.8.5　Aromatic Compounds

Benzene absorbs at 184，203 and 254 nm in hexane solution，the latter band called B-band，shows vibrational fine structure.Substitution on the benzene ring can cause bathochromic and hyperchromic shifts.If the non-bonding electrons are conjugated by the π-electron system，whatever it is electron withdrawing groups^[123] and electron donating^[124] substituents，the red shift of E₂ and B-bands will occur.If it is auxochrome，the effect is related with the electron withdrawing ability.Substituents that carry nonbonding electrons（n electrons）can cause shifts in E₂ and B-bands.The nonbonding electrons can increase the length of the π system through resonance^[125].The more available these n electrons are for interaction with the π system of the aromatic ring，the greater the shifts will be.Examples of groups with n electrons are the amino，hydroxyl，and methoxy groups，as well as the halogens.Substituents may have different effects on the positions of absorption maxima，depending on whether they are electron releasing^[126]or electron withdrawing.Any substituent，regardless of its influence on the electron distribution elsewhere in the aromatic molecule，shifts E₁ and E₂ bands to longer wavelength.Electron-withdrawing groups have essentially no effect on B-band，of course，the electron-withdrawing group is also capable of acting as a chromophore.However，electron-releasing groups increase both the wavelength and the intensity of E₂-band.If chromophore is introduced，due to the conjugation of double bond of chromophore and benzene ring，a new electron transfer band will be produced，this new band may be so intense as to obscure E₂-band of the benzene system.Table 2-8 lists the absorption maximum of the single substituted benzenes^[127]（Pavia，et al.，Fourth Edition）.

In disubstituted benzenes，it is necessary to consider the effect of each of the two substituents.For para-disubstituted benzenes，two possibilities exist.If both groups are electron releasing or if they are both electron withdrawing，they exert effects similar to those observed with single substituted benzenes.The group with the stronger effect determines the extent of shifting of the primary absorption band.

If one of the groups is electron releasing while the other is electron withdrawing，such as amino and nitro^[128]，there is a pronounced red shift in the main absorption band，compared to the effect of either substituent separately，due to the extension of the chromophore from the electron donating group to the electron withdrawing through the benzene ring.Alternatively，when two groups are situated ortho or meta to each other or when the para disposed groups are not complementary，then the observed apectrum is usually closer to that of the separate chromophores.

The enhanced shifting is due to resonance interactions of the following type：

An empirical correlation of structure with the observed position of the primary absorption band has been developed by Scott（Table 2-9）.The Scott rules provide a means of estimating the position of the primary band for benzoyl derivatives^[129] within about 5 nm.

Examples　Predict the UV maximum for the following substance：