Centro symmetrical molecules

In order to determine the structural factors maximizing 2PA cross section values, we analyze (8) from Sect. 1.2.1. For all cyanine-like molecules, symmetrical and asymmetrical, several distinct 2PA bands can be measured. First, the less intensive 2PA band is always connected with two-photon excitation into the main absorption band. The character of this 2PA band involves at least two dipole moments, /

symmetry forbidden for centro-symmetrical molecules, such as squaraines with C, symmetry due to A/t = 0, and only slightly allowed for polymethine dyes with C2V symmetry (A/t is small and oriented nearly perpendicular to /t01). It is important to note that a change in the permanent dipole moment under two-photon excitation into the linear absorption peak, even for asymmetrical D-a-A molecules, typically does not lead to the appearance of a 2PA band. 2PA bands under the main absorption peak are typically observed only for strongly asymmetrical molecules, for example, Styryl 1 [83], whose S0 —> Si transitions are considerably different from the corresponding transitions in symmetrical dyes and represent much broader, less intense, and blue-shifted bands. Thus, for typical cyanine-like molecules, both symmetrical and asymmetrical, with strong and relatively narrow, S (I > S) transitions, we observe... 

In sections 3 and 4, we have considered non-rigid molecules with a solid reference frame. In order to achieve the comparison between the NRG s and the Longuet-Higgins and Altmann s groups, let us consider some very symmetric systems, such as linear molecules with equivalent rotors, in which the reference frame is only a rotational axis, or centro-symmetric molecules with permutational rearrangements around a central point, in which the reference frame is a single atom. 

In the case of centro-symmetric molecules like phosphorus pentafluoride, the restricted NRG cannot be written out, furthermore the full NRG is not easy to determine. This should be expressed in terms of double rotation operations, Bi, and two- and threefold overall rotations. 

It may be concluded that, when the external and internal motions may be considered separely, the local full NRG is isomorphic to the direct product of the restricted NRG by the symmetry point group of the molecule in its most symmetric configuration (20). This conclusion holds in the case of synunetric linear molecules, in which the reference frame is an axis, except that double valued groups have to be used, but it does not hold in the case of centro-symmetric molecules in whidi the reference frame is a single atom. 

Centro symmetric molecule Only n -symmetry modes active Only -symmetry modes active... 

The fluoride, B2F4 is an explosive gas. The structure of the planar, centro-symmetrical molecule, (a), has been determined in the crystal at — 120°C. The chloride B2CI4 is formed by passing BCI3 vapour through an electric discharge... 

Two calix[4]arenes may be connected also by a spiro-linker derived from pentaerythritol (D with X = — (CH2)20(CH2)20CH2—) ° or by two tetravalent atoms (E, X = Si, Ti). In the latter case, centro-symmetric molecules with two open cavities pointing in opposite directions (koilands) are obtained (SiCLt, NaH, THF, 52%) and may form one-dimensional networks in the crystalline state (koilates) when a suitable connector (e.g. hexadiyne) is included in their cavities. ... 

Verdet constants have been investigated in [74] and [38] for a few centro-symmetric molecules using coupled-cluster response-theory techniques. For atoms, corresponding results can be found in [149]. 

Therefore, it has at most five independent components, and fewer if the molecule has some symmetry. Symmetric top molecules have only one independent component of , and, in such cases, the axial component is often referred to as the quadrupole moment. A quadrupolar distribution can be created from four charges of the same magnitude, two positive and two negative, by arranging them in the form of two dipole moments parallel to each other but pointing in opposite directions. Centro-symmetric molecules, like CO2, have a zero dipole moment but a non-zero quadrupole moment. 



Figure 3. The centro-symmetric molecule [Me2Si(NtBu)2Sn] [Fe(CO)4]2 (9) from an X-ray structure determination. For further details see caption to Fig. 1.

In a centro-synunetric molecule the polarization must reverse sign under a reversal of the electric field. This means that all even powers of the electric field vanish in eqn (8.1). It therefore follows that in a centro-symmetric molecule vanishes for all the even ns. Since conjugated polymers are often centro-symmetric. 

Thus to account for the far i.r absorption of rare gas mixtures it is necessary to go outside the no-overlap model. For the important case of linear centro-symmetric molecules y, and 6 vanish(5) so that ... 



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