The vapor spectra of some purine and pyrimidine bases are presented. Vapor pressures of adenine and 9-methyladenine have been determined in order to obtain absolute intensities. Heats of vaporization are also determined from the spectral data. 193 (1963).
The spectra of some purine and pyrimidine bases are presented. Correlations found among the bands of these bases and some related compounds suggest that the electronic states of all the bases are simply derived from those of benzene. A classijication of the spectral bands is presented which leads to polarization directions and p H dependence consistent with experiment.
Equations for the birefringence of a macromolecular solution in a rapidly reversed electric field are derived. The equations are plotted for various values of the electrical parameters of the molecule and the advantage of the reversing pulse method for determining these parameters is discussed. The effect of a time dependent polarizability and of polydispersity is considered. A more quantitative interpretation of O'Konski, ei al.'s experimental data is made.
The energies of the lower out-of-plane transitions of nucleic acid bases and their tautomers and ions are calculated with the CNDO-CI method, which includes all valence electrons. Precise values are obtained by using the energy differences between the observable * transitions and the * transitions, instead of calculating absolute values, and by correcting them empirically for systematic deficiencies of the MO-CI model applied. The * transitions of the various bases are characterized and correlated by their transition densities. This reveals that the lowest * state bears a close relation to the structure and energy of the highest nonbonding orbital. The structure of the nonbonding orbitals of interest can also be rationalized in simple terms. From the structure of the lowest * state and the highest nonbonding orbital it is shown that the bases naturally fall into three classes with respect to their low-energy out-of-plane spectra. The first, which we call the pyrimidine class, comprises the bases without a carbonyl group. The lower * transitions are located on nitrogen atoms of the six-membered ring of the bases and have a structure similar to those of pyrimidine. The second, or carbonyl class, consists of the bases containing a carbonyl group with no heteroatom with a lone pair of electrons next to it. The lowest * transitions are confined largely to the carbonyl oxygen and strongly resemble that of cyclohexanone. The third class is intermediate to the other two, and the bases belonging to it have a nitrogen atom with a lone pair of electrons next to a carbonyl group. The two lowest * states are mainly spread out over this fragment of the molecule. Their energy is lower than for the corresponding tautomers belonging to the carbonyl class. From the structure of the * transitions the energetic shifts occurring upon structural alterations and the formation of ions can readily be understood. The introduction of an amino or hydroxyl group in a position to the location of a transition leads to a strong blue shift. For this reason, the lowest * transition in adenine is shifted in energy above the lowest * transition, whereas in purine the * transition is lowest in energy. For no neutral nucleic acid base is the * transition calculated to be lower in energy than the first * transition. Protonation of the nucleic acid bases often does not affect the structure of a * transition. In such a case a red shift rather than a blue shift occurs. For deprotonation the opposite is true. Both findings show that the generally accepted conclusions drawn from small chromophores are not necessarily valid for larger ones containing several heteroatoms.
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