Dipole Moments of Excited-State Molecules

Molecules in an electronically excited state have chemical and physical properties that differ from those of ground-state molecules. As a result of 

In solution, even centrosymmetric molecules can have large dipole moments in the excited state, as was first demonstrated in the case of 9,9 -bianthryl (7) (Beens and Weller, 1968). The symmetrical compound 8 is an example in which electronic excitation is localized almost entirely in one of the polar aromatic end groups, due to solvent-induced local site perturbation (Liptay et al., 1988a). In solution, polyenes may also show unsymmetrical charge distributions with nonvanishing dipole moments (Liptay et al., 1988b see also Section 2.1.2). 

2 Dipole Moments of Excited-State Molecules Molecules in an electronically excited state have chemical and physical properties that differ from those of ground-state molecules. As a result of Table 1.5 Dipole Moments of Ground and Excited States, all Values in D, Calculated Data in Parentheses  

Changes in the electron distribution on excitation are connected with changes in basicity and acidity. This becomes evident if one considers the indicator equilibrium 

When numerical values for constants are inserted, the following relation between the change in the pK value and the shift Ai of the absorption maxi- 

Near 300 nm a spectral shift of 30 nm corresponds approximately to a change in wave number of Ai = 3,300 cm . From Equation (1.53) this results in a p/C value change of 7 units. Since shifts of this extent are quite common on protonation, changes in dissociation constants of protonated compounds by 6-10 orders of magnitude on excitation are often observed. 

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Bond Angles and Dipole Moments of Excited State Molecules... 

The intrinsic value for 3x1 is obtained by then accounting for dipole-dipole interactions between the solute and solvent. The dominant effect is a shift in the excitation energy -ft ung of the solute molecule caused by changes in the ground and excited state dipole moments of the solute molecule... 

This important parameter can provide information on the excited states. For instance, when the dipole moment of a fluorescent molecule is higher in the excited state than in the ground state, the Stokes shift increases with solvent polarity. The consequences of this in the estimation of polarity using fluorescent polarity probes is discussed in Chapter 7. 

In a preceeding series of papers [1-8], was investigated the influence of the solvents on the spectral bands of benzene, chlorobenzene, bromobenzene, toluene, anisole, benzaldehyde, nitrobenzene, and benzonitrile with the aim to establish the behaviour differences between monosubstituted benzene molecules with the first and second order susbstituents in solutions, i.e. to contribute to the elucidation of the interactions in the solute-solvent systems determining the spectral displacements, and to evaluate the dipole moments of the solute molecules in their excited states as indices of reactivity. In the series of monosubstituted benzenes the aniline molecule has a place apart due to the presence of the amino group, and it is the object of this investigation. 

As the spectral shifts in hydrocarbons represent a susbstantial part as compared with the other solvents (excepting water and alcohols) we consider that the dispersion forces of the London [25] type have an important contribution to the solvation energies, and then to the red shiftj because the polarizability of solute molecule in the excited state increases [26], and an instantaneous redistribution of the electric charge will take place. From the McRae s [27] theory results a formula giving the spectral shift under the solvent influence (in terms of solute polarizability and dipole moment of the solute molecule and in terms of the refractive index and dielectric constant of the solvent), which, for nonpolar solvents, reduces to ... 

Vibrational Feshbach resonances (VFRs) in a vibrational Feshbach resonance, the interaction of a slow electron takes the form of a virtual excitation of a vibrational level of the neutral molecule with capture of the electron (ffotop et al. 2003 Dessent et al. 2000). For the zero point vibration, the maximum probability of interaction of the electron with parent molecule occurs at zero energy, if the dipole moment of the neutral molecule exceeds the critical value of approximately 2 Debye, the impinging electron maybe trapped into the diffuse bound state, which provides a much longer timescale for the electron to stay near the molecule (fiotop et al. 2003 Dessent et al. 2000 lllenberger 1992), and VFRs may appear as shown in O Fig. 34-5. 

The active space used for both systems in these calculations is sufficiently large to incorporate important core-core, core-valence, and valence-valence electron correlation, and hence should be capable of providing a reliable estimate of Wj- In addition to the P,T-odd interaction constant Wd, we also compute ground to excited state transition energies, the ionization potential, dipole moment (pe), ground state equilibrium bond length and vibrational frequency (ov) for the YbF and pe for the BaF molecule. 

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