Geometrical distortions complexes

In order to examine the nature of the unusual geometric distortions observed in the X-ray structures of complexes 3 and 4, we have performed a series of density functional theory (DFT) and combined quantum mechanics and molecular mechanics (QM/MM) calculations [22-27], Although the computational resources to wholly treat both 3 and 4 at the DFT level are available, we have employed the combined QM/MM method to unravel the... 

The intensity distributions of well resolved vibronic spectra recorded in absorption and emission at low temperature are used to determine the geometric distortions of the electronically excited states of coordination compounds. In particular for complexes of lower symmetry, band analysis is necessary leading to results with which bond distance changes can be calculated. For spectra exhibiting no vibrational fine structure, a new technique is proposed which uses time resolved methods, considering deviations from the Poisson distribution of photons by recording time intervals between two successively emitted photons. 

The solvent-induced geometrical distortions can be very important to understand the properties and the reactivity of systems in solution for example, SN2 reactions on halomethanes, which proceed by a simple activated mechanism in the gas phase, are described by a double well mechanism in polar solutions, where stable precursor complexes appear [1], In addition, for some systems important regions of the conformational space become accessible only if solute-solvent interactions are taken into account. An outstanding example is provided by (poly)peptides whose conformations are often described in terms of the two backbone dihedral angles <.f> and tp (see Figure 3.1). 

Figure 55. Geometric distortions of M(dithiolene)3 complexes that result in a change of symmetry from trigonal-prismatic Dy,.

The silicon atom in the crystal of cationic complex [Ph3Si(bipy)] I is pentacoordinated but the bond lengths are not determined Geometrical distortion from the TBP to the SPY configuration expresses from the sum of dihedral angle method desaibed in Refs. Two crystallo-... 

The shape of the minimum in the surface is experimentally probed by vibrational spectroscopy. It is here that the computations can make direct coimection with experimental information. Formation of the H-bond from a pair of isolated molecules converts three translational and three rotational degrees of freedom of the formerly free pair of molecules into six new vibrations within the complex. The frequencies of these modes are indicative of the functional dependence of the energy upon the corresponding geometrical distortions. But rather than consisting of a simple motion, for example, H-bond stretch, the normal modes are composed of a mixture of symmetry-related atomic motions, complicating their analysis in terms of the simpler motions. In addition to these new intermoleeular modes, the intramolecular vibrations within each of the subunits are perturbed by the formation of the H-bond. The nature of each perturbation opens a window into the effects of the H-bond upon the molecules involved. The intensities of the various vibrations carry valuable information about the electron density within the complex and the perturbations induced by the formation of the H-bond. 

One way in which the fundamental forces responsible for the formation of a H-bond can be probed is by examining of the force field that restores the equilibrium geometry after small geometrical distortions. This field is directly manifested by the normal vibrational modes that exhibit themselves in the vibrational spectrum of the complex. Chapter 3 is hence devoted to a discussion of the vibrational spectra of H-bonded complexes, and what can be learned from their calculation by quantum chemical methods. While the vibrational frequencies are directly related to the forces on the various atoms, the intensities offer a window into the electronic redistributions that accompany the displacement of each atom away from its equilibrium position, so vibrational intensities are also examined in some detail. Of particular interest are relationships between the vibrational spectra and the energetic and geometric properties of these complexes. 

A, and very similar to those of the =Si-0-Cu4 surface complex (NBO). This suggests that the geometrical distortions within the metal cluster due to the bonding with different substrate defects disappear quite rapidly as the cluster size increases. 

Intramolecular "twisting" mechanisms may Involve the decay of a distorted excited state complex. Thermally equilibrated ligand field excited state complexes are expected in many cases to show geometrical distortions (Section llI-A-1), and may therefore be suspected as precursors in these reactions. 

Large geometrical distortion in the lowest excited states of Pt(gly)2 is also suggested by spectroscopic studies, which have indicated that similar square planar complexes have pseudotetra-hedral excited states (93a), by semi-empirical molecular orbital calculations (56), and also by recent quenching and sensitization studies of the cis trans reaction (Section lII-D-1). ... 

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