Stephen F. Mason

Optical activity in the biaryl series

Abstract The relationship between the CD spectra of the chiral biaryls and their stereochemical configuration, as a function of the dihedral angle between the molecular planes of the aromatic moieties, has been investigated for biphenyl, 1,1′-binaphthyl, 1,1′-bianthryl and 9,9′-bianthryl in the exciton approximation and, for the 1,1′-binaphthyls, in the π-SCF approximation. Both methods provide unambiguous assignments of absolute configuration except for biaryls with a critical dihedral angle of π/2 in those with effective D data2 chromophoric symmetry, or 100–110° in the case of the 1,1′-binaphthyls.

Dynamic coupling contributions to the intensity of hypersensitive lanthanide transitions

Abstract The intensities of the pseudoquadrupolar hypersensitive f-electron transitions in Ln(III) complexes are found to arise principally from the coulombic correlation of induced electric dipoles in the ligands by the transition quadrupole moment of the metal ion. The correlation gives a non-zero resultant electric dipole transition moment in complexes devoid of inversion symmetry or S p axes for p > 5. The theory quantitatively reproduces the observed intensities of the 4 I 9/2 → 4 G 5/2 transition near 17 kK of gaseous NdBr 3 and NdI 3 .

The Electroweak Origin of Biomolecular Handedness

The parity-violating weak neutral current perturbation of the ground-state electronic energy has been calculated by ab initio methods for glycine over a range of chiral conformations, for L-alanine, L-α-aminopropionitrile, and for the peptide residue of polypeptides in the α-helix and the β-sheet conformation. It is found that L-alanine in its preferred conformation in aqueous solution and the L-peptides in the α-helix and the β-sheet conformation, have a lower ground-state energy than the corresponding D-enantiomers, because of the electroweak interaction. The enantiomer energy difference is small, of the order of 10-14 J mol-1, corresponding to an enantiomeric excess of 106 molecules of L-alanine or the L-peptide in one mole of the corresponding racemic mixture in thermodynamic equilibrium at ambient temperature. The significance of the energy difference between enantiomers arising from the electroweak interaction for the transition from racemic geochemistry to homochiral biochemistry in terrestrial evolution is discussed.

Simultaneous monitoring of ligh-absorption and optical activity in the liquid chromatography of chiral substance

Abstract A detection system for the simultaneous monitoring of the light-obsorption (absorbance, A ) and the optical activity as circular dichroism (differential absorbance for left- and right-circularly polarized light, Δ A ue5fb A R ) of the eluent from a liquid-chromatograpy column is described. The optical resolutio of pavine by liquid chromatography on a triacetyl cellulose column is reported.

The parity-violating energy difference between enantiomeric molecules

Abstract The parity-violating weak neutral current perturbation of the electronic energy has been calculated, by ab initio methods, for the (S)-isomer of hydrogen peroxide over a range of gaussian basis sets, and for L-alanine with a STO-6-31G basis. The WNC energy shift is found to be dependent in sign and magnitude upon the particular conformation adopted by the enantiomer, and to vanish in the case of isotopically enantiomeric molecules.

Anisotropic contributions in the ligand polarization model for the f-f transition probabilities of Eu(III) complexes

Abstract The intensities of the Δ M = ± 1 component of the 7 f o → 5 D 2 transition in D 3 complexes containing the [ EuO 9 ] cluster, and of other d-d or f-f transitions in D 3 complexes with the yz - and xz -components of the electric quadrupole as the leading moment, are found to be dependent upon the polarizability anisotropy of the ligands. The expectation is supported by determinations of the single-crystal absorption intensities of Na 3 [Eu(diglycollate) 3 ]·2NaClO 4 ·6H 2 O and [Eu(H 2 O) 9 ] (C 2 H 5 SO 4 ) 3 .

The absorption and circular dichroism spectra of chiral olefins

Abstract Absorption and CD spectra over the quartz and vacuum UV region down to 165 nm are reported for a range of chiral alkenes in the vapour phase and in solution from +70° to −185°C. A major couplet of oppositely-signed CD bands with comparable band areas, near 48 and 55 kK, is observed in a number of dissymmetric olefins and in some cases a weaker Rydberg CD absorption is found at lower frequency. The Rydberg CD band is characterised by its sharp vibronic structure in the vapour phase and by large blue-shifts produced on passing to the condensed phase and by a reduction in temperature. The olefin couplet of major CD bands with opposite sign is assigned to a near-complete mixing of the electric-dipole πx→πx* and the magnetic-dipole πx→πy* excitations, producing a pair of isotiopic absorption bands with the same polarisation and comparable dipole strengths associated with the CD couplet. Three mixing mechanisms are discussed; sterically-induced π-bond torsion, a first-order static field model, and a second-order dynamic-coupling model dependent, respectively, upon the effective charge and upon the mean polarisability of dissymmetrically-located substituent. The latter two models give the octant rule previously proposed empirically connecting the sign of the rotational strength of the lower- and higher-frequency member of the olefin CD couplet with the position of the substituent in the chromophore coordinate frame.

Universal dissymmetry and the origin of biomolecular chirality

Handed systems are distributed over four general domains. These span the fundamental particles, the molecular enantiomers, the crystal enantiomorphs, and the spiral galaxies. The characterisation of the molecular enantiomers followed from the identification of the crystal enantiomorphs and revealed a chiral homogeneity in the biomolecules of the organic world. The origin of the homogeneity has been variously ascribed to a universal dissymmetric force, from Pasteur, or to a chance choice of the initial enantiomer perpetuated by the stereoselection of diastereomer production with recycling, from Fischers key and lock hypothesis. The classical chiral fields identified by Curie require a particular time or location on the Earths surface for a determinate molecular enantioselection, as do the weak charged current agencies of the non-classical weak interaction. The weak neutral current of the electroweak interaction provides a constant and uniform chiral agency which favours both the L-series of amino acids and polypeptides and the parent aldotriose of the D-series of sugars. The enantiomeric bias of the electroweak interaction is small at the molecular level: it may become significant either as a trigger-perturbation guiding the transition from a metastable autocatalytic racemic process to one of the two constituent enantiomeric reaction channels, or by cumulative amplification in a large chirally-homogeneous aggregate of enantiomer units.

The circular dichroism, crystal and molecular structure, and absolute configuration, of dinaphtho[2,1-c,1',2'-e]dithiin

Abstract The structure of the title compound has been determined by single-crystal X-ray analysis and refined by least-squares methods based on diffractometer data (1388 counter intensities, R 0.035). The crystals are monoclinic, space group P2 1 , with Z=2. a = 13.360(2), b =5.755(1), c = 10.318(1) A and β = 108.32(1)°. The atoms of each naphthalene nucleus in the molecule are coplanar to within the rms value of 0.087 A, and the intramolecular dihedral angle between the mean planes of those nuclei is 56°. Both the Bijvoet X-ray method and an analysis of the circular dichroism spectrum indicate that the (+)−isomer has the (S)-configuration, providing a reference for the correlation of configuration in the series of chiral 1,1-binaphthyl-2.2-sulphur-substituted derivatives.

The ring conformation d-electron optical activity of a cyclic triamine cobalt(III) complex

Abstract The synthesis of the ligand R(-)-2-methyl-1,4,7- triazacyclononane and its cobalt(III) complex [Co (R-MeTACN)2]3+ is reported. The complex is found to exhibit in the 480 nm region the largest ring-conformation d-electron optical activity yet recorded for the [CoN6 chromophore, due to a chiral puckering, with the λ-conformation, of each of the six chelate rings. The c.d. spectrum of the complex is found to be consistent with the dynamic coupling model of d-electron optical activity, but not with the corresponding crystal field model.