Abstract
The maximum wavelength of strong absorption band of methylol melamine (MM) near 200 nm depends on the number of the substituted methylol group and its band position shifts linearly to the longer wavelength side against the number of methylol group. Such linearity between the maximum wavelength and the number of methylol group of MM is useful to an analysis of the molecular species of MM homologues.
In this paper the spectroscopic property of the absorption bands near 200 nm of melamine and four MM (1MM, 2MM, 3MM, and 4MM) and relationship between the maximum of absorption band of MM and the number of methylol group were studied by the CNDO/2 method with the singly-excited configuration interaction concerning with π-electron.
The absorption spectrum of melamine near 200 nm is consisted of the π-π*transitions from the ground 1A1′ state to the excited 1A2′, 1A1′, and 1E′ states under the molecular symmetry of D3h. The first two transitions are symmetry-forbidden and the other transition is doubly-degenerate in energy and symmetry-allowed.
The absorption spectrum of MM which appears in 200-260 nm region is consisted of the four different electronic transitions which correspond to the π-π* transitions from the ground 1A1′ state to the excited 1A2′, 1A1′ and 1E′ states in melamine. The maximum wavelength corresponding to each calculated electronic transition does not change linearly against the number of methylol group, while that of the absorption curve synthesized from these calculated absorption bands varries linearly. The above calculated result is in good agreement with the observed behavior of the maximum wavelength of the absorption spectrum band of MM against the number of methylol group.
