UV- and visible spectra

You have just seen that the energy difference between the HOMO and LUMO for butadiene is less than that for ethene. We would therefore expect butadiene to absorb light of longer 

It is no coincidence that these and many other highly conjugated compounds are coloured. All dyes and pigments based on organic compounds are highly conjugated. 

The table below shows the approximate wavelengths of light absorbed by a polyene conjugated system containing various numbers n of double bonds. Note that the colour absorbed is complementary to the colour transmitted—a red compound must absorb blue and green light to appear red. 

Absorbed frequency, nm Colour absorbed Colour transmitted R(CH=CH) R, n = 

Fewer than about eight conjugated double bonds, and the compound absorbs only in the UV. With more than eight conjugated double bonds, the absorption creeps into the visible and, by the time it reaches 11, the compound is red. Blue or green polyenes are rare, and dyes of these colours rely on more elaborate conjugated systems. 

Electrons can be promoted from any filled orbital to any empty orbital. The smallest energy difference between a full and empty molecular orbital Is between the HOMO and the LUMo. The smallerthis difference, the less energy will be needed to promote an electron from the HOMO to the LUMO the smaller the amount of energy needed, the longer the wavelength of light needed since AE= hv. Therefore, an important measurement is the wavelength 

We can get a good estimate of the absolute energies of molecular orbitals from photoelectron spectroscopy and electron transmission spectroscopy (see Chapter 2). Such experiments suggest energies for the HOMO and LUMO of butadiene to be -9.03 and +0.62 eV. respectively, whilst for ethene they are -10.51 and +1.78 eV, respectively. 

HOMO to LUMO excitation smaller gap absorption in nearer UV at 215 nm 

You can sec now that it is no coincidence that this compound and the two other highly conjugated compounds we met earlier, chlorophyll and p-carotene, are all highly coloured natural pigments. In fact, all dyes and pigments are highly conjugated compounds. 

The similarities between lycopene and JJ-carotene are-easier to see if the structure of lycopene is twisted. Lycopene is a precursor of carotene so, when a cell makes carotene, it makes lycopene en route. 

In going from X -phosphorins to X -phosphorins, profound changes in the absorption spectra are observed new bands appear in the visible region. Thus, all X -phosphorins are colored, ranging from yellow to red. Many also show fluorescence. 

The steric influence of the P substituents appears to play a more pronounced role, as can be seen by comparing 144, and 148 (Table 25) the spiro compound absorbs at much shorter wavelength. 

The bathochromic influence of substituents at positions 2,4 and 6 is quite distinct in both-X -phosphorin and X -phosphorin compounds (e. g. 118 vs. 144,  

1-Carbo-phosphorins are relatively basic. They are protonated by aqueous hydrogen chloride, thereby losing their color. According to Markl, the UV spectrum of the salt is similar to that of 1,2-dihydro-X -phosphorins (Fig. 25). However, it cannot be concluded on the basis of the UV spectrum alone that protonation takes place at position C—2. The C-4 position can also be protonated, as has been shown by NMR spectroscopy to be the case for 1, l-dimethoxy-2.4.6-triphenyl-X -phos-phorin. The influence of acid addition on the UV spectra is similar in both classes of compounds (compare Fig. 25 with Fig. 26). However, it should be noted that aqueous acids cannot be used to protonate l,l-dimethoxy-2.4.6-triphenyl-X -phos- 

More extensive studies have been made on 1, l-hetero-X -phosphorins, especially on those in which the hetero atoms are oxygen Here again a sizable bathochro-mic shift in the absorption spectra is observed in going from X -phosphorins to X -phosphorins. Replacement of aliphatic groups by aromatic substituents in the 

The wavelength of light is inversely related to its energy (E), according to the equation  

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It is also very sol in dimethylformamide and its UV and visible spectra have been measured in this solv (Ref 10). Its thermal decompn by differential thermal analysis has been studied and shows that the dehydration occurs in two steps at 93 and 282° followed immediately by a strong exotherm at 300-400° (Ref 11)... 

Treatment of 5-ethoxy-l,2,3,4-thiatriazolium tetrafluoroborates (161) with aromatic amines in boiling benzene affords yellow crystalline 5-arylamino-l,2,3,4-thiatriazolium salts (166) in good yields (60-70%) (Scheme 35). These are deprotonated with warm ethanolic aqueous sodium hydroxide to give the red crystalline mesoionic l,2,3,4-thiatriazolium-5-aminides (20). The mesoionic compounds show a strong IR absorption in the region 1580-1590 cm which can be assigned to the exocyclic (7=N stretching vibration. UV and visible spectra are reported e.g. (20a) 279 (e... 

The dramatic influence of solvent effects on the UV and visible spectra of certain pyridine compounds, known generally as a solvatochromic effect, has been much utilized in the expression of solvents effects. The polarity parameter. Z or ET is defined (58JA3253, B-68MI204002) from the longest wavelength charge transfer band of 1-ethyl-4-methoxycar-bonylpyridinium iodide (equation 3). 

Diazoxine, a red product which accompanies the formation of 8-methoxy-quinoline from 8-hydroxyquinoline and diazomethane, was first encountered by Caronna and Sansone in 1939. Later, it was suggested that the properties of this product were consistent with structure 189 (R = Me, R = R = H). ° Subsequently, this product has been prepared by treatment of the iodide 190 with potassium carbonate, and the betaine structure has been confirmed by spectroscopic and chemical studies. Compound 189 (R = Me, R = R = H) is isolated as hydrated violet-red needles. The UV and visible spectra are strongly dependent on the nature of the solvent the colors of solutions vary from yellow to blue. Bromination gives the 5,7-dibromo derivative (189 R = Me, R = R = Br), which is also obtained from... 

Structural surprises are not confined to 2,6-DAP derivatives though zinc and cadmium complexes have been obtained with the quadridentate thioiminato Schiff base N,N -ethylenebis(monothioacetonimine) 347 uv and visible spectra indicate planar geometry, rather than the tetrahedral geometry observed for the salicylaldimine complexes. 

Ultraviolet (UV) and visible spectra, also known as electronic spectra, involve transitions between different electronic states. The accessible regions are 200-400 nm for UV and 400-750 nm for visible spectra. The groups giving rise to the electronic transitions in the accessible regions is termed chromophores, which include aromatic amino acid residues in proteins, nucleic acid bases, NAD(P)H, flavins, hemes, and some transition metal ions. Two parameters characterize an absorption band, namely the position of peak absorption Wmax) and the extinction coefficient (e), which is related to concentrations of the sample by the Beer-Lambert law ... 

UV and Visible Spectra of Various a-(l,2-DiTHioL-3-YLiDENE)CARBONYL Compounds... 

The ultraviolet (UV) and visible spectra of some phenothiazines were recorded for analytical purposes (characterization, identification, and dosage). Spectrophotometric studies on the colored... 

Spectroscopic studies on C-substituted S+ species obtained by oxidation with H2SO4 have been undertaken by Beckett and Curry. The UV and visible spectra showed that 3,7-dimethylphenothiazine is oxidized by strong electron acceptors to the corresponding S " species. ... 

HCIO4. The radical character was demonstrated by ESR. Its UV and visible spectra in solutions are identical to those of sulfuric acid solutions of chlorpromazine, which ESR showed to contain radical species. 

The first evidence for the existence of a stable iodine cation was obtained by Masson (2) in 1938. He postulated the presence of Ig" and Ig in solutions of iodine and iodic acid in sulfuric acid in order to explain the stoichiometry of the reaction of such solutions with chlorobenzene to form both iodo and iodoso derivatives. Later, Symons and co-workers (6) gave conductometric evidence for I3 formed from iodic acid and iodine in 100% sulfuric acid and suggested that Ig" " may be formed on the basis of changes in the UV and visible spectra when iodine is added to la solutions. Gillespie and co-workers (7) on the basis of detailed conductometric and cryoscopic measurements confirmed that Ig is formed from HlOg and Ig in 100% sulfuric acid according to Eq. (1). The Ig cation may also be prepared in fluoro-... 

The dyes formed when imidazoles couple with diazonium salts (Section 4.07.1.4.8) in alkaline medium could exist as the usually accepted azo form (95 R = H) or the hydrazone form (96). Comparison of the UV and visible spectra of the dyes with a non-tautomeric model (95 R = Me) demonstrated unequivocally that if such an equilibrium as shown in Scheme 32 exists, then the azo structure predominates (79BAP249). 

Ultraviolet and Visible Spectroscopy Spectroscopic examination of the electronic transitions of H bonded substances offers the novel prospect of revealing the H bonding properties of excited states. This is, in a sense, a means of determining chemical properties of excited molecules. Of course, UV and visible spectra have already shown usefulness in determining thermodynamics of H bond equilibria. 

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