Carbonyl compounds solvatochromism

The situation is quite different for n- r transitions. The lone electron pair is particularly well stabilized by polar and particularly by protic solvents so it becomes energetically more difficult to excite. Figure 2.45 shows the spectrum of N-nitrosodimethylamine in different solvents. Results of calculations indicate that the negative solvatochromism of carbonyl compounds can be explained on the basis of the structural changes due to the formation of hydrogen bonds (Taylor, 1982). Molecular dynamics simulations, however, indicate that the net blue shift is primarily due to electrostatic interactions (Blair et al., 1989). A large number of water molecules around the entire formaldehyde are responsible for the total blue shift the first solvation shell only accounts for one-third of the full shift. 

The H-bond formation may affect the energies of various excited states in different ways [1, 2, 68]. It is well established that the specific H-bond interactions strongly influence the n tt transition of carbonyl compounds [1,2, 68, 69, 75, 76]. In this case, the H-bondlng Interactions (Aw ) stabilize the ground state better than the less-dipolar excited state. Hence, the H-bonds donating solvents exhibit the solvent-induced blue shifts (negative solvatochromism). On the other hand, the presence of H-bonds can also strongly influence the intensity of the CT absorption band (tt TT ) as it is observed in tire case of the p-nitroaniline (PNA) molecule [77]. 

An integral equation formalism (lEF) has been developed as particularly suitable for the description of solvent effects on spectral transition energies within the PCM model. The respective theoretical equations have been applied for the calculation of solvatochromic shifts of several carbonyl-group containing molecules at the self-consistent field (SCF), configuration interaction (Cl) and multiconfiguration self-consistent (MC SCF) field level of theory. The calculated spectral shifts accompanying the transfer of a solvatochromic compound from the gas phase to water were comparable with the experimental data. In Table 11.1.4, the results of calculations are presented for three carbonyl compounds, formaldehyde, acetaldehyde and acetone. 

Table 11.1.4. The calculated and experimental solvatochromic shifts (from the gas phase to water) in the spectra of some carbonyl compounds (cm ) ...

S. E. DeBolt and P. A. Kollman,/. Am. Chem. Soc., 112, 7515 (1990). Theoretical Examination of Solvatochromism and Solute-Solvent Structuring in Simple Alkyl Carbonyl Compounds. Simulations Using Statistical Mechanical Free Energy Perturbation Methods. 

The group VI compounds M(CO)4(DAB) were first prepared by Tom Dieck and his coworkers in the 1960s and their strong solvatochromism arising from the antiparallel dipole of the excited state was reported shortly thereafter. Resonance Raman studies established the relationship between the MLCT absorption and several molecular vibrations, most notably the carbonyl stretching frequencies. The resonance Raman spectra of this class of compounds has been reviewed. Vlcek has published a detailed review of the photophysics and photochemistry of the group VI diimine compounds. ... 

A variety of compounds having the thiaazulenone structures 187 and 188 have been prepared,70-403 404 and these include the parent molecules.403 Infrared data, e.g., for 187 (R = Me),70 show carbonyl bands at 6.36 and 6.21/i and the low frequencies imply a degree of single-bond character for the carbonyl function. However, positive solvatochromism observed by Seitz and Monnighoff405 for 189 was interpreted by these authors in terms of a large contribution from the apolar form 189a, and the lower dipole moments of methyl derivatives of 187 and 188 relative to 4,5-... 

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