Linear conformation

The geometrically optimized model of BD had a roughly linear conformation. This spontaneous ordering was unexpected given the general orientation of dipolar molecules. Azobenzenes that have permanent dipoles parallel to the molecular axis would intuitively be expected to tend to pair with their dipole oriented in the opposite direction. The linear geometry is probably due to the... 

Figure 1. Schematic of the radial cuts of the ground- and excited-state potential energy surfaces at the linear and T-shaped orientations. Transitions of the ground-state, T-shaped complexes access the lowest lying, bound intermolecular level in the excited-state potential also with a rigid T-shaped geometry. Transitions of the linear conformer were previously believed to access the purely repulsive region of the excited-state potential and would thus give rise to a continuum signal. The results reviewed here indicate that transitions of the linear conformer can access bound excited-state levels with intermolecular vibrational excitation.

The higher energy features can indeed be associated with transitions of He lCl(K,v" = 0) ground-state complexes with rigid He I—Cl linear geometries. In contrast to the T-shaped band that is associated with transitions to the most strongly bound intermolecular vibrational level in the excited state without intermolecular vibrational excitation, n = 0, the transitions of the linear conformer access numerous excited intermolecular vibrational levels, n > 1. These levels are delocalized in the angular coordinate and resemble hindered rotor levels with the He atom delocalized about the l Cl molecule. 

In 1999, the Klemperer group published experimental results from a dispersed fluorescence study of the linear and T-shaped Ar l2(X, v = 0) conformers to obtain information about the X- and B-state potentials of the complex and to gain insight into the photodissociation dynamics in the B electronic state [65]. In carrying out their analysis, they assumed that the linear and T-shaped conformers existed in a thermodynamic equilibrium, with the abundance of the linear conformer being three times that of the T-shaped conformer. In an effort to test the validity of the thermodynamic hypothesis Bastida, et al. [66], performed a... 

Figure 10. The transfer of population between the T-shaped and linear conformers may occur via two reaction mechanisms [66], isomerization (a) and exchange (b), even at the very low temperatures, < 2K, observed in the expansion at x/d > 10. Adapted from Ref. [67].

The model [39] was developed using three assumptions the conformers are in thermodynamic equilibrium, the peak intensities of the T-shaped and linear features are proportional to the populations of the T-shaped and linear ground-state conformers, and the internal energy of the complexes is adequately represented by the monomer rotational temperature. By using these assumptions, the temperature dependence of the ratio of the intensities of the features were equated to the ratio of the quantum mechanical partition functions for the T-shaped and linear conformers (Eq. (7) of Ref. [39]). The ratio of the He l Cl T-shaped linear intensity ratios were observed to decay single exponentially. Fits of the decays yielded an approximate ground-state binding... 

As we have reviewed here, the linear region is not fully repulsive, and transitions of the ground-state, linear conformer access vibrationally excited intermolecular levels that are delocalized in the angular coordinate. As depicted in Fig. 1, however, the internuclear distance is significantly longer in the excited state at the linear geometry. Consequently, there is favorable Franck-Condon overlap of the linear conformer with the inner-repulsive wall of the excited-state potential. It is therefore possible for the linear Rg XY conformers to be promoted to the continuum of states just above each Rg - - XY B,v ) dissociation limit. 

The structure of a non-hydrolyzed dimer, cA-[(2,2 -bipyridine)palladium(II)]2(/i-l,3-N03)2 2+ is shown in Figure 45.521 The dimerized m-( 2.2-bi pyridine) Pd11 units aggregate into a dimer-to-dimer linear conformation through direct metal—metal interaction522 and tt-tt stacking.523 Notably, the dimer was first reported to be linked by double nitrato-bridges (/x-l,3-N03)2 in a cofacial... 

For small-molecule, metal-carbon monoxide complexes, the carbon monoxide ligand is almost always in a linear conformation and perpendicular to the metal. If one assumed bonding of CO to Hb or Mb in its normal linear, perpendicular mode, steric conflicts as illustrated in Figure 4.20 would occur and thus one might predict... 

The electron distribution in the Fe-O-O linear conformation follows instead the scheme ... 

The minimum of the triplet state corresponds to a larger Fe-O-O angle (131°) than the S = 1 state (121°), as we also found for a small FeP(Im)(02) system [6b]. On the other hand, an S = 0 closed-shell state is well separated in energy in the linear conformation (22 kcal mol-1 relative to the ground triplet state), but becomes very close to the ground state in the bent conformation (1.4 kcal mob1). Its electronic configuration can be shown schematically as ... 

The ability to adopt an extended configuration has been recognised for many years to be a desirable feature of substantive dyes. It helps to explain why J acid is such a popular choice as a central component in unsymmetrical disazo dyes. For example, it is much easier for aniline—>] acid—>H acid (3.61) with the 2,6-naphthylene substitution pattern to adopt a linear conformation than for the similar disazo dyes aniline—acid— H acid (3.62) and aniline—>H acid—>J acid (3.63) with 2,7- and 2,8-disubstitution respectively. 

In the absence of a suitable template, species such as 51 are long lived in solution (due to their preference for a linear conformation) reacting intermolecularly to form larger macrocycles in preference to intramolecular ring closure. The template, on the other hand, provides the hydrogen bonding acceptor groups to... 

The observed spectrum of HNO is that of a nearly symmetric top, and shows that the molecule is bent in both the upper and lower states. Figure 7 shows the geometrical data. Both states are unquestionably singlet states which, in all probability, are derived from the A state of the linear conformation. Since HNO is isoelectronic with 02, its lowest electron configuration for the linear conformation must be. .. 7r2 which, as in 02, must give rise, in addition to JA, to a 3E and a 1S+ state. As for 02, the 3S is expected to be the ground state as long as the HNO molecule is linear. But it is difficult to predict whether the... 

It is interesting to consider the variation of the bending frequency v2 in the series of linear molecules with 12, 13, 14, 15, and 16 valence electrons. In Figure 10 this variation is plotted as a function of the number of electrons in the ng orbital. It is seen that, with increasing number of ng electrons the frequency v2, that is, the rigidity of the linear conformation, increases very rapidly it is very small when no electrons are present in the ng orbital, as in C3. 

Fig. 1. Schematic representation of the mucin polymer network. Notice the tangled topology of the network, the linear conformation of the mucins and the presence of S S bonds in the apomucin backbone...

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