Methine carbon

In this simplified example of phenylalanine, in the first iteration the methyl groups arc given a value of I in the first classification step because they contain a primaiy C-atom, The methylene group obtains a value of 2, and the methine carbon atom a value of 3. In the second step, the carbon atom of the methyl group on the left-hand side obtains an extended connectivity (EC) value of 2 because its neighboring atom had a value of 2 in the first classification step. 

The methine carbon atom also obtains an EC value of 4 (= 1 -h 1 + 2) in the second iteration. This process is repeated iteratively until the number of different EC values (c) is lower than or equal to the number of EC values in the previous iteration. Then the relaxation procc.ss is terminated. Next, the EC numbers of the previous iteration are taken for a canonical numbering and for the determination of constitutional symmetry (Figure 2-44),... 

The three methine carbons of the chain can be provided by 1.3.3-triethoxypropene (method C) or /S-anilinoacroleinanil, vinylog of diphenyl formamidine issued from the condensation of aniline on tetraal-koxypropane (method A). 

Knott s rule concerns the importance of the place of the nitrogen atom replacing a methine carbon in the conjugated chain when the atom is separated from the active auxochromic atoms by an odd number of conjugated atoms, the shift is bathochromic. It is hypsochromic when there is an even number, Tne importance of the shift could establish a measure of M effect of various heterocyclic nuclei (79. 124). Many papers have been published, and examples have been given to verify these rules (79-84). 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

Reactions with Parting of Radicals. The one-electron oxidation of cationic dyes yields a corresponding radical dication. The stabihty of the radicals depends on the molecular stmcture and concentration of the radical particles. They are susceptible to radical—radical dimerization at unsubstituted, even-membered methine carbon atoms (77) (Fig. 6). 

Acidic Heterocycles. A similar classification is made for the acidic electron-accepting terminal groups used in dipolar (merocyanine) chromophores. The unsymmetrical dyes again incorporate the -dimethylarninophenyl group, coimected to the acidic group (Fig. 3) by one or three methine carbon atoms as in the merocyanine(9), n = 0 [23517-90-0]-, n = 1 [42906-02-5]-, n = 2 [66037-49-8]-, n = 3 [66037-48-7]. 

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

Optical properties of cyanines can be usefiil for both chiral substituents/environments and also third-order nonlinear optical properties in polymer films. Methine-chain substituted die arbo cyanines have been prepared from a chiral dialdehyde (S)-(+)-2-j -butylmalonaldehyde [127473-57-8] (79), where the chiral properties are introduced via the chiral j -butyl group on the central methine carbon of the pentamethine (die arbo cyanine) chromophore. For a nonchiral oxadicarbocyanine, the dimeric aggregate form of the dye shows circular dichroism when trapped in y-cyclodextrin (80). Attempts to prepare polymers with carbocyanine repeat units (linked by flexible chains) gave oligomers with only two or three repeat units (81). However, these materials... 

Another alternative makes use of the condensation of 5,5 -dimethyidipyrryimethenes8and 5,5 -dibromodipyrrylmethenes 9 in organic melts. In this case, the method allows the synthesis of more diversely substituted porphyrins 10. To avoid constitutionally isomeric porphyrins it is neccessary to start with one dipyrrylmcthene which is symmetrically substituted about the methine carbon. 

Proton noise decoupled 13C-NMR spectra of equimolar mixtures of the cyclic hexamer and metal thiocyanates showed that the signals of the carbonyl carbon and two methine carbons gave downfield shifts upon the addition of metal thiocyanates, while those of the three methylene carbons of the tetrahydropyran ring gave upfield... 

For quaternary carbons (C) For methine carbons (CH) For methylene carbons (CH2) For methyl carbons (CHj) ... 

With T set at V2J, the quaternary carbons generally appear with greater intensity than the other carbons, which will be of near-zero intensities, thereby allowing them to be distinguished, particularly from the CH2 carbons, as compared to the normal APT spectrum, in which both CH2 and quaternary carbons appear with positive amplitudes. A difference APT spectrum, in which an APT spectrum recorded with t set at %/is subtracted from another APT spectrum recorded with t set at /sj, can provide useful information. The methyl carbons will then appear with reduced intensities in the difference spectrum as compared to the methine carbons, allowing us to distinguish between them. 

The GASPE spectrum of vasicinone is shown. The peak at 8 126.5 is a cluster of three peaks at 8 126.3 and 126.7 representing methine carbons. Similarly, the signal at 8 160 on the positive phase of the spectrum represents two close singlets at 8 160.4 and 160.5. Predict the chemical shift values of various protonated and quaternary carbons in the structure. 

The broad-band decoupled C-NMR spectrum of ethyl acrylate shows five carbon resonances the DEPT (6 = 135°) spectrum displays only four signals i.e., only the protonated carbons appear, since the quaternary carbonyl carbon signal does not appear in the DEPT spectrum. The CH and CH3 carbons appear with positive amplitudes, and the CHj carbons appear with negative amplitudes. The DEPT (6 = 90°) spectrum displays only the methine carbons. It is therefore possible to distinguish between CH3 carbons from CH carbons. Since the broadband decoupled C spectrum contains all carbons (including quaternary carbons), whereas the DEPT spectra do not show the quaternary carbons, it is possible to differentiate between quaternary carbons from CH, CHj, and CH3 carbons by examining the additional peaks in the broad-band spectrum versus DEPT spectra. The chemical shifts assigned to the various carbons are presented around the structure. 

For instance, the COSY interactions between the proton geminal to the oxygen at 8 5.10 (attached to the methine carbon at 8 72.2) with the protons at 8 2.20 and 2.70 (methylenic protons at the C-4... 

The formation of color from triheteroarylmethanes differs from the methodology employed for triphenylmethane leuco dyes40 (Scheme 4). Dyes are initially formed by alkylation of the pyridyl nitrogen, followed by deprotonation at the central methine carbon. Thus, treatment of the colorless 3,3 -diindolyl-4-pyridylmethane 22 with excess methyl iodide produces colorless compound 23. Subsequent treatment of 23 with hydroxide... 

Moreover it has been shown that PV0CC1 prepared by free-radical polymerization of vinyl chloroformate (V0CC1) is an atactic polymer having a Bernouillian statistical distribution as expected (J[9). In order to extend our studies on the chemical modification of PV0CC1, the stereoselective character of the nucleophilic substitution of the chloroformate units with phenol has been examined by the study of the 13c-NMR spectra of partly modified polymers in the region of the aliphatic methine carbon atoms. The results obtained in this field are presented here. 

In the case of partly modified polymers the spectra B, C and D show more complicated structures which can be presumably due to significant neighbouring-groups effects between phenyl carbonate groups and modified or unmodified groups linked to the next aliphatic methine carbon atoms. 

KINETIC RESULTS FOR DCP AND TCH. The portion of the 50.13 MHz 13C NMR spectra containing the methylene and methine carbon resonances of DCP and the resultant products of its (n-Bu)3SnH reduction are presented in Figure 2 at several degrees of reduction. Comparison of the intensities of resonances possessing similar T, relaxation times (see above) permits a quantitative accounting of the amounts of each species (D,M,P) present at any degree of reduction. 

Figure 5. Methine carbon region of the 50.31 MHz 13C NMR spectra of TCH at 0 and 43% reduction with (n-Bu)3SnH.

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