Pressure broadened lines are, however, not normally considered to be collision-induced, certainly not to that extent to which a specific line intensity may be understood in terms of an individual atomic or molecular dipole transition moment. A well-known example of such a process is the pressure broadening of allowed spectral lines. Spectroscopists have always known certain phenomena that are caused by collisions. Given the definition of the sign of P in FLCs, this also means that domains of the ShiCaPa phase with positive chirality have negative ferroelectric polarization, and vice versa. As indicated in Figure 8.25, this means that for an up field, the molecular arrows are pointing down. It may be noted that simple MOPAC AMI calculations suggest that the dipole moment of NOBOW is oriented antiparallel to the molecular arrow. In the former definition, the vector points from the positive to the negative direction, while the latter has the orientation reversed. It is important to note that the direction of the vector dipole used by chemists is defined differently in classical physics. Experimental gas phase dipole moments45 are compared to ab initio and as molecular mechanics computed values. The next section in this chapter provides a brief comparison of the dipole moment (magnitude and direction) for a set of simple alcohols. The definition of quadrupole moment is only independent of the coordinate origin when the charges sum to zero and when the electric dipole moment is zero.
Thus, whilst the absence of a permanent electric dipole in CO2 simply means that the molecule is linear, the fact that the electric quadrupole moment is negative shows that our simple chemical intuition of 0 C" 0 is correct. Īt the molecular level, electric quadrupoles can lead to useful structural information. Meyer to the brilliant idea of piezoelectric polarization. By definition, the dipole moment of the unit volume is electric polarization.
Due to the distortions, the densest packing of our pears and bananas results in some preferable ahgimient of molecular skeletons in such a way that molecular dipoles look more up than down. The new polar symmetry allows for the existence of macroscopic polarization, large or small, depending on the magnitude of the strain and molecular dipole moments shown by small arrows. The unprimed symbols are defined in accordance with eqn (4.16). The symbol, F), (and its derivative, (9F i/9H )o) is introduced here to denote provisionally a reported macroscopic nonlinearity before assessing its precise definition. It is relatively easy to identify how the microscopic parameter has been obtained and which molecular convention is being used provided the identity of the macroscopic quantity is clearly established. In calculating these values different versions of the above procedures may have been employed and, in particular, different values of the molecular dipole moment inserted into y to extract the y value. Much of the discussion in the literature deals directly with the final values of the hyperpolarizability presented in the various experimental papers. Whatever the validity of these formulae and the underlying cavity field theories it is very desirable that, before applying them, the definition and consistency of the macroscopic quantities as measured by different groups should be assessed. These are characteristic factors for many macro-molecules and give valuable information regarding structural properties such as length, shape, and mass. Thus pertinent experimental investigation permits a direct determination of the molecular dipole moments and rotational relaxation times (or rotational diffusion coefficients, respectively). Owing to their definite structures, most biomolecules have an appreciable permanent dipole moment which must lead to dielectric polarization via the rotational mechanism of preferential orientation. The analogous definition can be applied to the microscopic polarization p of a molecule with the molecular dipole moment jli and the polarizabilities a,/3, and y instead of the static polarization P (0) and the susceptibilities y-P. The dielectric constant of the vaccum c(j is included in the susceptibility definitions as SI units are used throughout this work.
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