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LeBreton et al. / Photoelectron Studies of Biological Pyrimidines 2303 particles of this size. Also, it is impossible to align "B" phase samples using microscope cover slips, as for D phase samples. This also indicates the presence of many defects in the samples (Le., small crystallite size). Where it was possible to measure A values for the B phase, the values are similar to those in the D phase and do not reflect the presence of a first-order phase change. Since similar NMR behavior was observed for systems containing "B" phase (sodium octanoate, sodium octyl sulfate) and the one without a B phase (sodium octyl sulfonate) it seems likely that "B" phase is simply a continuation of D phase at high water content, and that no first-order B/D phase change exists. In the original paper,16 Ekwall et al. concluded that the x-ray evidence for the coexistence of B + D phases was not unequivocal. The boundaries shown in Figure 4 were obtained by analysis of separated samples after prolonged centrifugation. It is possible that the gravity gradient along the centrifuge tubes caused the separation observed. Certainly it is hard to find a physical reason why lamellar phase samples with -64 A water layer should separate from samples with -72 A water layers as was observed. There appears to be a case for the reexamination of low-angle x-ray scattering on samples in the B + D two-phase region.Abstract: UV photoelectron spectroscopy and C N D O j S molecular orbital calculations have been employed to investigate the electronic structure of cytosine (I), 1-methylcytosine (II), N,1-dimethylcytosine (III), N,N,l-trimethylcytosine (IV), 3-methylcytosine (V), 1,5-dimethylcytosine (VI), 1,6-dimethylcytosine (VII), 5-methylcytosine (VIII), and 6-methylcytosine (IX). The resolution of the spectra obtained for different members of this series of molecules varies markedly. Of all the molecules investigated the photoelectron bands arising from the five uppermost orbitals are well resolved only for N , I-dimethylcytosine. The variation in the resolution arises partially from the overlapping of bands. Furthermore, spectra obtained for molecules in which labile H atoms are replaced by methyl groups exhibit much better resolution than spectra for other molecules. This observation is probably related to hydrogen bonding effects. For cytosine the spacing of bands occurring in the spectrum is accurately reproduced in the results of C N D O / S calculations carried out on the 1(H) aminooxo tautomeric form of the molecule. In compounds 11-IV and VI-IX the spacing of bands and the shifts observed in the spectra are also well predicted by calculations carried out on the aminooxo tautomers. However, for 3-methylcytosine the results indicate that an imino tautomeric form is most stable. For all compounds the C N D O / S calculations indicate that three of the five uppermost orbitals are A orbitals and that two are lone-pair orbitals. In cytosine the first and fifth bands arise from A orbitals while the fourth band arises from a lone-pair orbital. The se...

Publication costs assisted by the University of Illinois at Chicago CircleUV photoelectron spectroscopy was employed to study the ground-state electronic structure of a series of aryloxiranes, of phenanthrene g,lO-oxide, and of two metabolites of benzo[a]pyrene including the highly carcinogenic diol epoxide, trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene. The photoelectron spectra of the aryloxiranes and the benzo[a]pyrene metabolites were assigned by comparison with the spectra of the parent polycyclic arenes and ethylene oxide. Spectral assignments were confirmed by comparison of experimental ionization potentials with energy levels predicted by semiempirical molecular orbital calculations based on the CNDO/S 3 method. The photoelectron spectrum of the diol epoxide of benzo[a]pyrene is approximated by the combined spectra of pyrene and ethylene oxide. However, electronic coupling of the saturated six-membered ring with its substituent groups to the A system of the pyrene moiety does occur. In the diol epoxide of benzo[a]pyrene all five of the uppermost A orbitals are destabilized compared to those in pyrene. This destabilization, which in the second highest occupied R orbital has a magnitude greater than 0.4 eV, is expected to aid in the stabilization of intermediates associated with biologically important electrophilic alkylation reactions.

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