Question

From the output calculations the Homo value is (-7.513) and LUMO is (-3.335) so the calculated band gap was 4.17 ev .

According to the following equation :

Wavelength(in nm)=1240.8/Energy(in eV) = 297.55 nm

My question is why I got this small wavelength 297.55 mm while the maximum wavelength from UV-VIS was above 500 mm ??

Answer 1

The exact reason of the shift can only be analyzed once you furnish information on the reaction you are conducting. I will provide a few general explanations.

The calculations are correct; he wavelength of UV-Vis absorption for the said molecule turns out around 297 nm as you obtained.

The shift to low wavelengths is known as hyposchromic shift. Hyposchromic shift is the opposite of bathochromic shift (shift to higher wavelengths). Hyposchromic shift is often caused by changes in the environment of the reaction. A very likely cause of hyposchromic shift is the change in solvent polarity. If, due to some reason, the solvent polarity of the reaction system increases, the result is a hyposchromic shift.

Often, closely related molecules in a substitution reaction causes hyposchromic shift. For example, if the starting material and the product of a substitution reaction have almost similar skeletal framework (except the functional group), the result is a blue shift (hyposchromic shift).