Knowledge Bank

The far ultraviolet absorption spectra $(2500-1500\backslash AA)$ of a selected group of organic amines yield information of the lowest energy electronic transitions in these compounds. The substances were studied in vapor state with a fluorite prism vacuum spectrograph and Beckman spectrophotometer, and absorption curves showing values of molar extinction coefficients were prepared. A comparison of primary, secondary, and tertiary methyl and ethyl amines with ammonia shows an appreciable effect of the groups adjacent to the nitrogen atom on the energy and intensity of the transitions. The effect of cyclization is shown by the study of pyrrolidine, piperidine and piperazine. Certain interesting differences in behavior are evident when these spectra, representing in each case transitions of the nonbonding electrons of nitrogen, are compared with those involving $\pi$-electrons of carbon. In acid solution the first bands of the amines disappear and the effect of solvents is reversed from that of unsaturated hydrocarbons. From a study of base strength there is evidence, yet incomplete, that decreased reactivity of these molecules may be associated with greater ease of excitation. The compounds pyridine and pyrrole, involving both $\pi$-electrons and nonbonding nitrogen electrons, have also been investigated in this vacuum region. Their observed spectra are compared with values calculated by molecular orbital methods.