A study of .pi.-electron delocalization in model compounds of visual pigment by UV and carbon-13 NMR spectra

Abstract

Carbon-13 NMR and UV absorption spectra have been obtained for a//-Z/-anj-7V-retinylidene-«-butylamine and its acid-addition salts as an analogue of the Schiffs base linkage compound in visual pigment. Carbon-13 chemical shift changes on going from aZZ-rrans-ZV-retinylidene-n-butylamine to its acid-addition salts are observed. The -electron density and -bond order of polyene chain were calculated for these compounds by CNDO/2-MO method. The change in carbon-13 chemical shift was found to correlate quite well with that in -electron density. It was deduced that the large red shift of UV absorption maximum by N protonation of a/Z-Zz-tinj-ZV-retinylidene-zi-butylamine may be mainly due to the decrease of the degree of bond alternation and the delocalization of the positive charge in conjugated polyene chain. The effect of solvent (methanol and carbon tetrachloride) on the UV bathochromic shift and carbon-13 chemical shift was also discussed briefly.

A Study of N‐Butyl‐(all‐trans‐retinylidene)amine and Its Protonated Species by 1H‐ and 13C‐NMR. Spectroscopy

“…Assignment depended on the number of protons in the meru-position and their long-range couplings (results to be published). Our results are in agreement with those made earlier [8] [9] except that assignments of C (7) and C(12) of 3 have to be inversed in [9]. Assignments of the quaternary 13Cresonances in 1 and 2 are based on previous data [7] [8] and were extended to 3 by chemical shift correlation on titration with TFA.…”

“…proves therefore that there is a remarkable decrease of the degree of bond alternation, if no collapse, in the protonated Schiff base. The finding is supported by X-ray [24] and resonanceenhanced Raman [25] spectral data and can be supposed to induce the bathochromic shift of about 90 nm observed for A , , , in the electronic absorption spectrum [9].…”

“…The resonances of even-numbered C-atoms shift to low frequency and those of odd-numbered C-atoms to high frequency. This alternation has been attributed [9] to the delocalization of the positive charge on the N-atom over the polyene chain and is explained by several limiting resonance structures [22]. In contrast to the C-atoms no alternation in the sign of the chemical shifts was found for the olefinic protons ( Table 3).…”

“…These differences fall outside the experimental error of kO.3 Hz. We have compared the changes in the coupling constants on going from BATRA to BATRAH+ with those of the n-bond orders calculated by Inoue et al [9] using the CND0/2-MO method. The coupling constants H-C (7), H-C (8) and H-C (1 l), H-C(12) decreased by 1.9 and 3.3% (n-bond orders by -1.4 and -3.7%); on the contrary, the coupling constants H-C( lo), H-C (1 1) and H-C (14), H-C (15) increased by 4.4 and 16.5% (n-bond orders by + 10 and +34.6%).…”

Protonation of a conjugated Schiff base by weak to strong acids: a 13C and 15N nuclear magnetic resonance study

“…This pattern is obtained for all acids used and in both solvents (CDC13 and CD30D). It has already been noted by Harbison et al (5b), Inoue et al (13), and by Pattaroni and Lautenvein (14), who report a similar behavior for all-trans-retinylidene n-buty lamine (NRBA) and strong acids. It can be rationalized by the fact that all odd-numbered carbon atoms have a partial positive charge while those that are even numbered are in a nonresonance position.…”