Harry Pearson

π–π Aggregation in metalloporphyrins: causative factors

The strength of aggregation in PdII, NiII, ZnII, and CdII metalloporphyrins is shown by n.m.r. spectroscopy to depend upon interaction of the metal ion with the porphyrin π-system; within a selected series of 2,4-disbustituted deuteroporphyrin-IX dimethyl esters complexed with a constant metal ion the aggregation is further enhanced by the presence of electron-withdrawing substituents on the porphyrin periphery.

Nuclear magnetic resonance spectroscopy. Concentration dependence of the T1 relaxation time for 13C in dioxane-D2O. Some experimental problems with T1 measurements

The ^(13)C T_1 relaxation times determined for dioxane-D_2O solutions by the progressive saturation method have been found to range from 5 sec for 1:1 (v/v) dioxane-D_2O to 12 sec for neat dioxane. The concentration dependence roughly parallels the inverse of the viscosity and is likely to be associated with changes in correlation times because of hydrogen bonding. A 2:1 (v/v) dioxane-D_2O mixture with a T_1 of 6.2 ± 0.6 sec at 30°C has been found to be a useful standard for checking instrument performance in the determination of T_1 values. A number of experimental problems associated with measuring T_1 are discussed.

Conformational equilibria and barriers to rotation in monohalogenobutanes with methyl substituents. Methyl–halogen interactions. The size of a halogen atom

An n.m.r. study of barriers to rotation and conformational equilibria in 2-halogeno-2, 3, 3-trimethylbutanes and 2-halogeno-2, 3-dimethylbutanes is reported. Results are discussed in terms of the steric requirement of halogen atoms, and the interaction of these atoms with adjacent methyl groups.

The NMR spectra of porphyrins—12: 13C and proton NMR spectra of the zinc(ii)coproporphyrin isomers2

Abstract The 13 C and proton NMR spectra of the zinc(II) complexes of the tetramethyl esters of the four coproporphyrin type isomers are reported and assigned. Effects of aggregation phenomena on these shifts are discussed and a method involving addition of a slight excess of pyrrolidine is proposed for measurement of the spectra of the “monomeric” species; spectra obtained under these conditions are capable of simple, straight-forward interpretation and assignment in terms of molecular symmetry. Thus, a facile distinction between the type isomers is obtained. The “monomer” chemical shifts so derived allow consistent SCS parameters to be derived. The C β -Me SCS are shown to be related to the bond order of the C β -C β bond in the porphyrin ring, and are thus quite different from the corresponding SCS in pyrroles. Aggregation shifts in the 13 C and proton spectra are shown to be consistent with the presence of “stacked” aggregates with the ring current of one molecule affecting the other, together with an additional effect on the chemical shifts of the meso carbons, which is probably steric in origin.

Mechanisms of aggregation in metalloporphyrins: demonstration of a mechanistic dichotomy

N.m.r. spectroscopy is used to demonstrate that the predominant interaction in aggregations of magnesium(II) porphyrins [e.g.(1)] involves the metal atom in one molecule and the propionate carbonyl group in another; in contradistinction, the similarly strong aggregation observed for the first time in zinc(II) porphyrins [e.g.(2)] is shown to be independent of a metal to side-chain interaction.

Electronic and steric effects on hindered rotation about phenyl–carbon bonds

The effect of substituents on rotation about the phenyl–carbon bond in subtituents toluenes is reported and discussed in terms of steric and electronic interactions

Carbon-13 nuclear magnetic resonance spectra of porphin and some related compounds

Carbon-13 NMR studies of the pyrrole-derived pigments continue to be of biological and theoretical interest. For example, the nature of the rearrangement process central to the biogenesis of the type-III porphyrinogen skeleton has been delineated using this technique, the critically important meso carbon assignments in protoporphyrin-IX being established using carbon-13 labelled compounds from an impressive synthetic programme. On the theoretical side, carbon-13 NMR arguments for the 16-membered π-delocalisation pathway, 4 against which we have argued, continue to be advanced. In this letter we report the carbon-13 NMR spectra of porphin (l) (the basic porphyrin skeleton, free from extraneous substituent and some derivatives (Figure); the spectrum of porphin in particular is of fundamental importance in our longer term aim to obtain a unique and unambiguous method assignment of all carbon atoms in porphyrins, and as a basis for any future theoretical developments. Moreover, these shifts provide yet more evidence against the 16-membered pathway for π-electron delocalisation in the porphyrin nucleus.

Conformations of a phenyl group, and its effect on the barrier to rotation in substituted ethanes

Barriers to rotation about the central butane bond of 2,2-dimethyl-3-phenylbutanes are unexpectedly low. This is discussed in terms of the conformation about the phenyl–Cα and Cα–Cβ bonds. The low barrier may be due to a ground state interaction between an ortho-hydrogen and the 1-methyl group of the butane. Other evidence of this interaction is adduced.

Effect of sp2 and sp hybridised atoms on barriers to rotation in highly substituted ethanes. Steric acceleration of conformational processes

Barriers to rotation about the central bond of the 1,2,2-trimethylbutyl (triptyl) group, i.e. barriers to rotation of the t-butyl group, are reported for a series of triptyl compounds But–C(CH3)2X, where the atom in X bonded to the triptyl group is an sp2 or sp hybridised carbon. Barriers in some similar compounds are also reported. Results are discussed in terms of the interactions arising during rotation, and of steric acceleration of conformational processes.

Conformational analysis. A nuclear magnetic resonance study of the conformations of ortho-disubstituted benzenes

Results pertinent to the determination of the conformation of the side chain R1R2R3C attached to a benzene ring with or without an ortho-methyl substituent are reported and discussed. Results comprise dynamic n.m.r., carbon-13 chemical shift, and lanthanide-shift reagent data. When R1 is particularly large, the preferred conformation is concluded to be close to R1-perpendicular. In two cases two such conformations with either R2 or R3syn to the ortho-substituent are observed.