Intramolecular charge-transfer absorption spectra of formamide and acrolein

Abstract

To test further Nagakura and Tanaka's conception of intramolecular charge-transfer absorption, an improved calculation based on Longuet-Higgins and Murrell's method was carried out for the π-electron structures of formamide and acrolein. This shows that the 1717 å band of formamide and 1935 å band of acrolein can be regarded as intramolecular charge-transfer absorption bands in the sense that the excited state wave functions for these two bands are mainly constructed of the charge-transfer configuration, and that the transitions corresponding to these bands are accompanied by a large amount of electron transfer from electron donor towards acceptor. The 1717 å band of formamide can not be interpreted as a shifted 1560 å formaldehyde band. The latter band must shift to shorter wavelengths as the result of interaction with non-bonding electrons of the nitrogen atom. Similar blue shifts occur in acrolein for the C = C and C = O π → π* bands relative to the corresponding bands of ethylene and formaldehyde. Similar causes operate in the blue shifts of the non-charge-transfer bands of donor-acceptor molecular complexes.