Nitro compounds absorption spectra

On the Theory of Dyes.—In carbon compounds absorption in the visible part of the spectrum, i.e. subjective colour, is conditioned by the presence of a so-called chromophoric group in the molecule. The nitroso-group is strongly chromophoric, the nitro-group much less so, whilst the azo-group is quite considerably chromophoric, but only in aromatic systems. Azomethane is colourless. 

In order to measure the absorption spectra, the radical anions were generated electrochemically in the optical path of a spectrophotometer. The absorption spectrum of 3,5-dinitroanisole radical anion (Figure 11, curve c) is very similar to that of the 550-570 nm species produced photochemically. So we believe this species to be the radical anion formed by electron transfer from the nucleophile to the excited 3,5-dinitroanisole and decaying by interaction with its surroundings including the nucleophile radical cation. The behaviour described seems to be rather general for aromatic nitro-compounds since it is observed with a series of these compounds with various nucleophilic reagents. 

The influence of a solvent on the absorption spectra of nitro compounds has been examined by several authors. When studying the spectrum of nitromethane, Bayliss and Brackenridge [14] stated that under the influence of a polar solvent the nitro group band underwent a hypsochromic shift (Table 21). 

It should however be borne in mind that positions 9 and 10 in anthracene are not typically aromatic. They are manifested ]>y a higher reactivity than positions Ur and 0 as established by MO calculation [50]. In addition 9-nitroanthracene shows a non-planar structure with the nitro group out of plane [51] as pointed out by Cerfontain and Telder [48]. This is very similar to the position of the nitro group in o-dinitrobenzene and all derivatives of benzene with two ortho nitro groups. It is well known that the nitro groups in o-dinitrobenzene are not planar and there is no conjugation of double bonds in this compound. Tlie fact is also reflected in ultraviolet-absorption spectrum of o-dinitrobenzene which deviates from those of m- and p-dinitrobenzenes (Vol. I, p. 169, Table 20). 

A portion of the organic by-products was dissolved in carbon tetrachloride or chloroform (spectroscopic grade) for infra-red cuialysis. The absorption peaks corresponded to nitrophenols, nitrocresols, dinitrophenols, dinitrocresols, traces of trinitro conpounds euid nitro-hydroxy carboxylic acids. The presence of individual compounds wais confirmed by comparison with the absorption spectrum of the pure compounds euid by thin-layer chromotography. Quantitative cuialyses of the concentrations of several components of the by-products were made. If a component did not have an absorption bcuid free from interference by euiother constituent of the mixture, queuititative cUialysis could still be achieved by mectsurement of the extinction coefficients of the interfering components at severcLL different frequencies, together with the absorbance of the mixture at these frequencies. Characteristic absorption bands used are shown in Tetble I. 

The amide CFj-CO NMej shows hindered rotation and two distinct H-F coupling constants (1.60 and 0.80 Hz at 35 °C) lineshape analysis of the temperature-dependent spectrum yields a value for Fa of 83.2 + 1.3 kJ mol (4% in CHClj-CHClj). Long-range H-F couplings are also seen in the amides CFa-CO-NHj and CFj CO NHMe. For the imines (5 X = H, Cl, F, OMe, Me, or NOj), lineshape analysis of the coalescence of the CF absorptions yields values for AG = = which, with the exception of that of the nitro-compound, yield a good Hammett plot using cr+ values. This was interpreted in terms of an in-plane inversion about the imine nitrogen for the nitro-compound, and rotation about the C—N bond for the remainder. 

The detection of some molecular products using IR spectra confirms the free-radical processes taking place in p-benzoquinones under the action of NO [86]. Two new bands at 1560 cm and 1340 cm" are due to the asymmetric stretches of NO groups in nitro compounds observed in the IR spectrum of TBQ exposed to NO. The formation of nitroso groups is also traced by the band at 1120 cm", indicating the presence of the nitroso trans-(X mtrs [92]. Evidence for the conversion of oxyaminoxyl radicals according to (Equations 5.144-5.146) is provided by the appearance of the absorption band at 1704 cm", which can be assigned to the carbonyl stretch in a, 3-unsaturated aldehydes. 

Aromatic nitro compounds have been used as qualitative and quantitative indices for metallic ions. It has been known that l-nitro-2-naphthol forms a stable complex with Co ions by the cooperative effect of nitro and hydroxyl groups. We have evaluated the interaction between EGT-N and transition metal ions at pH 10.5 where the nitric acid-treated Trp starts dissociating using the absorption intensity of die UV spectrum. There are no spectral changes when metallic ions are added to the uiurelated EGT. On the contrary, when the Co ions are added to EGT-N, die absorption intensity decreased and a green precipitate resulted. 

To clarify the effect of substituents we will discuss the spectrum of 4-nitro-benzenol, even though the compound has no value as a dye. It is a pale yellow compound ( raax 320 nm) with an ultraviolet absorption band tailing into the visible, as in Figure 28-8. Its close relatives are benzene, benzenol, and nitrobenzene ... 

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