Calculating absorption maxima aromatic molecules

Diaz et al. [599] studied the electrooxidation of various aromatic monomers and oligomers, including pyrrole and its oligomers, to identify relationships between the number of repeat units in the substrate molecule and the oxidation potential as well as the UV-vi absorption maximum of the oxidation product (soluble as well as deposited on the electrode). As can be expected based on theoretical calculations alreacfy mentioned, the oxidation potential shifts to less positive values with an increasing number of pyrrole units in the educt, whereas the absorption maximiun shifts considerably to longer wavelengths. 

The vertical (Franck-Condon) excitation energy ( V) itself is an important photochemical characteristic of azide. This calculated parameter corresponds to the experimentally measured maximum of the long-wavelength absorption band and characterizes the region of spectral sensitivity of azide. From Table 12 it is seen that monotonically decreases with an increase in the size of the azide molecule for both neutral and protonated azides. The decrease in is symbatic to the above-discussed decrease in the HOMO-LUMO gap, which is an essential part of E, and corresponds to the bathochromic shift of the absorption band observed in the spectra of aromatic compounds with an increase in the number of annelated aromatic rings [55]. 

Figure 24 shows the dependence of the frequency (wavelength) at maximum of the long-wavelength absorption band on the size of the aromatic 7t-system (munber of 7t electrons) for neutral azides A1 -A6 for comparison, the calculated spectra are matched with the known experimental spectra of the first members of the series A1 - A3 (note that hereinafter the calculations are made for bare molecule, while experimental spectra are measured in acetonitrile). It is seen that the TD B3LYP/6-31G method substantially imderestimates the energy of the So... 

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