NMR spectral properties of 16 synthetic bacteriochlorins with site-specific 13C or 15N substitution

Abstract

The 1 H , 13 C , and 15 N nuclear magnetic resonance (NMR) spectral properties have been examined of a family of synthetic bacteriochlorins wherein each member incorporates a pair of 13 C or 15 N atoms. The atom locations span the inner core of the macrocycle: (1) 15 N at the 21,23- or 22,24-positions; (2) 13 C at the meso- (5,15- or 10,20-) positions; (3) 13 C at the pyrrole α-positions (1,11- or 4,14-positions); and (4) 13 C at the pyrroline α-positions (6,16- or 9,19-positions). Each bacteriochlorin lacks peripheral substituents other than a geminal dimethyl group at the 8- and 18-positions to preclude adventitious dehydrogenation. In total, eight free base and eight zinc bacteriochlorin isotopologs were examined to directly assign 1 H , 13 C and 15 N resonances of the macrocycle skeleton. Complete and unambiguous assignments, including those for all tertiary and quaternary carbons, were accomplished chiefly by direct inspection of 1D NMR spectra of each isotopolog. Coupling constants (1 H –1 H , 13 C –1 H , 15 N –1 H , 13 C –13 C and 15 N –13 C ), which are rarely reported for tetrapyrroles, also were extracted. The 1 H and 13 C chemical shifts were then compared to those of unsaturated analogs (chlorin, porphyrin) and natural bacteriochlorophylls. The comprehensive set of NMR spectroscopic properties of sparsely substituted bacteriochlorins provides valuable information for understanding substitution effects and aromaticity in structurally more elaborate counterparts.