Pieter E. Schipper

Determination of binding geometry of DNA-adduct systems through induced circular dichroism

Abstract Many molecules (adducts) bound to DNA are postulated to intercalate between successive DNA base pairs. Linear dichroism (LD) has been used to yield information about the angular orientation of the adduct relative to the helix axis, but cannot probe the orientation within the plane perpendicular to this axis. A model is presented in this paper which predicts that the degree of alignment relative to a DNA fixed axis in this plane may be directly probed through the sign of the circular dichroism (CD) induced in an adduct transition of known polarization. Comparison with experimental data suggests that the method can complement LD studies in giving detailed information about the binding geometry.

Symmetry selection rules for reaction mechanisms

Abstract The quantum description of a potential energy surface is shown, in the context of simple transition state theory, to lead naturally to a two-stage symmetry selection procedure for reaction mechanisms which is philosophically consistent with the generalized selection rule approach recently developed for spectroscopic processes. The first stage is the determination of those reaction mechanisms that are strictly allowed by symmetry, and is related to those approaches in which spatial symmetry is used to eliminate certain transition state symmetries. The second stage, which addresses the question of the relative probabilities of the symmetry-allowed mechanisms, is related to orbital symmetry approaches to reaction mechanisms.

On the induction of optical activity in achiral molecules

It has been observed experimentally that circular dichroism bands can appear at absorption frequencies of achiral molecules when chiral species (which do not associate with the achiral molecules) are added to the solution. This paper treats those contributions to the induced activity in the achiral species which are the results of van der Waals coupling to the chiral species. Use of perturbation theory leads to the expression of the induced rotatory strength as a series in 1/r, r being used in a general sense as the effective achiral-chiral molecule separation. Averaging over all orientations of each molecule yields non-vanishing terms falling off as 1/r 6. The results suggest that the effect may be important even in completely dissociated systems.

Dispersion-induced circular dichroism (DICD): General derivation and inclusion of coupled-oscillator contributions

Abstract The circular dichroism induced in the transitions of an achiral species (A) due to dispersive coupling to a chiral species (B) for a totally random distribution of relative orientations is referred to as dispersion-induced circular dichroism (DICD). This phenomenon may be directly exploited as a spectroscopic probe of the transitions of the achiral species, particularly of its magnetic dipole allowed transitions. In this paper, a complete derivation of the DICD contributions is presented using the concept of point chirality functions to isolate potentially non-vanishing terms. All contributions having the leading inverse sixth power dependence on the AB separation are considered, including previously neglected coupled-oscillator terms which are shown to survive the averaging process. General aspects of the phenomenon are discussed.

Averaged discriminatory interactions in chiral systems

Abstract The types of interactions between orientationally uncorrelated chiral molecules leading to discrimination in d-d, d-1 interaction energies are discussed. A purely electric dispersion energy depending on the ninth inverse power of the separation of the two species is suggested as leading to a discrimination which is appreciably stronger than that arising from electric-magnetic dispersion terms at short range. Model calculations for simple helical systems suggest that such discriminations could lead to chemically significant effects.

Symmetry rules for dispersion-induced circular dichroism (DICD) in achiral complexes

Abstract A theory for the circular dichroism bands induced at dd transition frequencies of symmetric (achiral) complexes due to long-range dispersive coupling with other chiral species (all species being dissociated in an achiral solvent) is discussed. The conditions for a finite effect are interpreted as symmetry rules restricting the symmetries of the states of the achiral complex that may exhibit the effect. Some suggestions are made as to what type of systems should be studied experimentally.

Absorption anisotropy of cubic or randomly oriented chromophores in anisotropic solvents. Dispersion induced linear dichroism (DILD)

Abstract A new mechanism through which cubic or orientationally averaged solutes could gain absorption anisotropy (linear dichroism) in the presence of an anisotropic (oriented) solvent medium is proposed. Transitions of the unoriented species exhibit a dispersion induced linear dichroism (DILD) as a result of dispersive coupling to the transitions of the oriented system. The phenomenon depends on the nature of the angular distribution of solute molecules about a particular solvent species, being maximised for a cylindrical distribution around a polymer, but still yielding a measurable DILD for spherical distributions of the solute. It is also shown that the LD of non-cubic or oriented solutes in anisotropic media should be corrected for a significant DILD contribution.

Spectroscopic investigation of magnetic dipole allowed transitions through the magnetic transition moment

An experimental technique for detecting magnetic dipole allowed transitions by their dispersion-induced circular dichroism (DICD) is discussed. The DICD spectra of three nitrogen heterocycles (pyridine, pyrazine and tetraphenylporphyrin) are shown to be dominated by the n — π* transitions, as distinct from the normal absorption spectra which are dominated by the electric dipole allowed π — π* bands.

Vibronic contributions to DICD in achiral molecules

Abstract The theory of dispersion-induced circular dichroism (the CD induced in a transition of an achiral species through long-range dispersive coupling with a chiral species) is extended to include vibronic terms. Symmetry rules are deduced for DICD-active vibronic states. It is shown that the intensity distribution over DICD-active vibrations within a given electronic band of the achiral species gives both insight into the mechanism through which the DICD appears, and vibronic spectral data not accessible through direct absorption studies. Applications to the carbonyl chromophore and comparison with recent experimental studies suggest that vibronic terms may predominate in certain cases over those expected from the purely electronic case.

Generalized selection rules for circular dichroism: a symmetry-adapted perturbation model for magnetic dipole allowed transitions

Abstract The independent systems approach for the circular dichroism (CD) of a magnetic dipole allowed transition of an achiral chromophore (A) in a chiral molecule leads to a perturbation expansion in terms of the electrostatic coupling operator V connecting the A states to those of the residual chiral framework C. The concomitant multipole expansion of V reduces the form of the CD to a power series in the inverse radial separation of A and C. Using the recently developed generalized selection rule procedure, the resultant expansion is analytically symmetry adapted to the point symmetries of the A and C chromophoric systems, with the explicit results presented for a wide range of symmetries. The results confirm that symmetry (particularly of A) is the principal determinant of the dominant CD mechanism, providing a rationale for the apparent inability of a unique mechanism to describe the CD activity of the wide range of A chromophore symmetries as found, for example, in transition metal complexes. Rigorous sector rules for the dominant mechanism for each A symmetry are derived. The general agreement with experimental behaviour consolidate the relevance of the independent systems approach.