Dipole strengths

The intermolecular interaction described above provides information about the magnitude of spectral shifts, but it does not explain why the absorption spectra of molecular aggregates usually have either an H- or J-band. The square of transition dipole moment (in Debye2 units) is usually termed the dipole strength and is related to the intensity of the absorption band as (van Amerongen et al. 2000)... 

The Temperature Dependence of the Angle Relation, Transition Dipole Strength Ratio, and Transition Dipole Orientation of the Chromophores in the K26.Phe-a RCs... 

Carotenoids are characterised by two low lying singlet excited states. The S2( Eu) state has a high absorption dipole strength and is very weakly fluorescent ( F 10 -10 ) [200, 201-204], The state is dipole forbidden... 

Owing to the forbidden, or at least very weak, nature of S -Si absorption, EET to carotenoids from chi is expected to be via the exchange coupling mechanism [161], which does not require donor/acceptor dipole strength. In this case the chl-car distance would need to be very short, i.e. less than the van der Waals contact distance (w 4 A). 

The intensity of a vibrational transition 1 - 0 for the fundamental of normal mode Qa is proportional to the dipole strength, D,o, defined as... 

Rotational and Dipole Strength Calculations for the CH-Stretching Vibrations of L-alanine Using the Localized Molecular Orbital, Nonlocalized Molecular Orbital, Atomic Polar Tensor, and Fixed Partial Charge Models ... 

See refs. 38, 42, 86, and 87. Reprinted with permission from ref. 38. Copyright 1984 American Chemical Society. Dipole strength (esu cm ) x 10 . 

Once the normal modes are computed, the nature or properties of the excited vibrational states can also be of use for structural studies. The most obvious of these is the dipole strength, directly proportional to the IR absorption intensity, which will depend on the environment and stereochemistry of the amide group. A key element in utilizing these calculational results is the use of intensities, through spectral simulation, to compare pre-... 

Figure 11. (Left) CD and UV spectra of 1 in methanol. (Right) Rotational and dipole strengths and polarization of transition of 1 calculated by the n-SCF-CI-DV MO method. Symbols and mean perpendicular and parallel, respectively, to the C2 axis. Reprinted with permission from J. Am. Chem. Soc. 109, 1661 (1987).

A parameter that has proven useful in the interpretation of experimental MCD spectra is the dipole strength. The dipole strength of a transition to state J, Dj, can be derived from an absorption spectrum. Dj is defined as... 

The dipole strength is a measure of the intensity of the transition to state J in the absorption spectrum and is closely related to the oscillator strength. The MCD intensity and thus MCD parameters produced by a particular transition are proportional to the absorption intensity of that transition. 

The dipole strength parameter is straightforward to obtain in a TDDFT calculation. 

Dipole strength parameter of transition to state J. Associated with the absorption intensity (Section II.C.6). 

The strength or intensity of absorption is related to the dipole strength of transition D or square of the transition moment integral M m , and is pressed in terms of oscillator strength / or integrated molar extinction jfe Jv. A transition with /= 1, is known as totally allowed transition. But the transitions between all the electronic, vibrational or rotational states are not equally permitted. Some are forbidden which can become allowed under certain conditions and then appear as weak absorption bands. The rules which govern such transitions are known as selection rules. For atomic energy levels, these selection rules have been empirically obtained from a comparison between the number of lines theoretically... 

There are difficulties associated with the use of ordinary electronic absorption spectra of lanthanide complexes in solution to provide detailed information regarding coordination number and geometry. However, difference spectra versus NdCl3 are reported for Nd3+-ligand (L) solutions for the 4/9/2— -4G5/2, 4G7,2 transitions (L = dipicolinate, oxydiacetate, iminodiacet-ate, malate, methyliminodiacetate and Ar,Ar -ethylenebis Af-(o-hydroxyphenyl)glycinate ). Hypersensitive behaviour was examined and transition dipole strengths were discussed in terms of the nature of the complex species present.431... 

Next, consider the simplest model in which jL(l) and (1(2) are perpendicular to the line joining them. The only geometric variables are the distance between the dipoles, d, and the dihedral angle between the dipoles, a. The rotational strength R and dipole strength D, for a given transition of 0 -> a with the wave-number v0a are given as ... 

The experimental dipole strength and rotational strength are obtained in c.g.s. units from the appropriate band areas by means of the following expressions ... 

Debye = 0.3934267 atomic units of dipole strength = 3.335611x10 30 C m. 

For a few systems, two empirical models are entered in the table. While the parameters may differ widely, at the spectroscopically significant separations the dipole strengths are consistent. For the isotropic overlap components, significant separations R range from a few percent below a to roughly 1.2 a. Outside of the range of validity, the empirical dipole models may be seriously in error and must not be used. 

Recent work improved earlier results and considered the effects of electron correlation and vibrational averaging [278], Especially the effects of intra-atomic correlation, which were seen to be significant for rare-gas pairs, have been studied for H2-He pairs and compared with interatomic electron correlation the contributions due to intra- and interatomic correlation are of opposite sign. Localized SCF orbitals were used again to reduce the basis set superposition error. Special care was taken to assure that the supermolecular wavefunctions separate correctly for R —> oo into a product of correlated H2 wavefunctions, and a correlated as well as polarized He wavefunction. At the Cl level, all atomic and molecular properties (polarizability, quadrupole moment) were found to be in agreement with the accurate values to within 1%. Various extensions of the basis set have resulted in variations of the induced dipole moment of less than 1% [279], Table 4.5 shows the computed dipole components, px, pz, as functions of separation, R, orientation (0°, 90°, 45° relative to the internuclear axis), and three vibrational spacings r, in 10-6 a.u. of dipole strength [279]. 

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