Mari Kubota

Electronic structure of uracil and uridine derivatives studied by photoelectron spectroscopy

Abstract The gas phase He(I) photoelectron spectra of uracil (I) and its derivatives, 5-fluorouracil (II), ftorafur (tegafur, 5-fluoro-1-(tetrahydro-2-furyl)uracil) (III), 2′,3′-dideoxyuridine (IV) and 2′,3′-dideoxy-5-fluorouridine (V), have been studied by means of MNDO calculations. The first four photoelectron spectral bands of I and II are π1, n1, π2 and n2 from the top, respectively. In contrast to these cases it is extremely difficult to assign the photoelectron spectral bands n1, π2 and n2 of III because of their complete overlapping with the oxygen atom n orbital band of the tetrahydrofuryl group. In order to solve this problem the difference spectrum method has been used, and the exact peak positions have been successfully detected. The photoelectron spectral band shifts were reasonably analyzed in terms of the conjugative, the short-range inductive and the long-range molecular polarization effects. Although for I, II and III the keto forms are stable, it has been revealed that in the case of IV or V both a keto and an enol form (at least) coexist predominantly in the gas phase. The π1 ionization energy of the predominant enol form of IV is completely indifferent to the 5-fluorination, as is the case of the keto form of I. This unusual finding can be explained as follows: the molecular polarization and the electron-withdrawing inductive effects of the 5-fluorine atom are almost cancelled by the π conjugative effect.

The structures of citreohybridone A and B novel sesterterpenoid-type metabolites of a hybrid strain KO 0031 derived from penicillium citreo-viride B. IFO 6200 and 4692

Abstract Two new cytotoxic substances against HeLa cell, citreohybridone A and B, have been isolated from the mycelium of a hybrid strain KO 0031 derived from Penicillium citreo-viride B.IFO 6200 and 4692, and their stereostructures have also been elucidated on the basis of their spectral data coupled with an X-ray crystallographic analysis of the citreohybridone A. These metabolites are regarded as new type of sesterterpenoids.

Revision of the structure of raphanusanins, phototropism-regulating substances of radish hypocotyls

Abstract The structures of raphanusanins A and B, growth inhibitors involved in phototropism of radish hypocotyls, were determined by X-ray crystallographic structure analysis and by spectral studies to be (3R*,6R*)- and (3R*,6S*)-3-[methoxy(methylthio)methyl]-2-pyrrolidinethione, respectively. Therefore, the previously proposed structure of these compounds of 2-piperidinethione skeleton should be revised.

The photoelectron spectra of xanthone, thioxanthone, and acridone

Abstract The He I photoelectron spectra of xanthone, thioxanthone, and acridone, which are system iso-π-electronic with one another, have been measured and are comparatively discussed on the basis of the HMO, MINDO/3, MNDO, and PM3 methods.

Electronic structures of melatonin and related compounds studied by photoelectron spectroscopy

Abstract Melatonin is a hormone structurally regarded as being composed of a 5-methoxyindole group and an N -ethylacetamide group; its various physiological activities have attracted a great deal of attention recently. The gas phase He(I) photoelectron spectra of melatonin (M) and its related compounds including N -acetylserotonin have been studied with the aid of molecular orbital calculations. The first photoelectron spectral band group of compound M is ascribed to ionizations from the two π orbitals localized on the methoxyindole group. The second band group is quite complicated and is regarded as being composed of several bands. The lower energy part of the second band group is ascribed to the three orbitals relevant to the third highest occupied π orbital of 5-methoxyindole and the highest occupied π and the n CO orbitals of N -ethylacetamide. The interactions among the three orbitals have been found to operate on the basis of the molecular orbital calculations; these interactions depend strongly on the conformations. The high energy end of the second band group is relevant to the π orbital mainly localized on the 5-methoxyindole group and is ascribed to the fourth highest occupied π orbital of 5-methoxyindole.

Electronic structure of docosahexaenoic acid studied by photoelectron spectroscopy

Abstract The gas-phase He(I) photoelectron spectra of docosahexaenoic acid (I) and its ethyl ester, docosahexaenoic acid ethyl ester (II), have been studied with the aid of MNDO calculations. These molecules have six CC double bonds and a carboxyl or an ethoxycarbonyl end-group. The six CC double bonds in these molecules are not directly adjacent to one another, but are placed with great regularity; that is, there is one methylene group between each pair of double bonds adjacent to one another. The first photoelectron spectral band groups of compound I show almost the same vertical ionization energy values as those of compound II and are ascribed to the ionizations from the six CC double bond π orbitals. The six CC double bonds of both compounds seem to be isolated from one another, as judged by their molecular structure, but the occupied π orbitals have been found to be delocalized over several CC double bond groups on the basis of the photoelectron spectra and the molecular orbital calculations. In compound II an independent second band is apparently well separated from the following band group. This second band group is ascribed to the carbonyl oxygen atom n band and the ethoxycarbonyl group π band overlapping each other.

Electronic structure of some pyridylacetylenes studied by He I photoelectron spectroscopy: A weak orbital interaction between the nonbonding electron pair on the nitrogen atom and the in-plane π orbital in the ethynyl group

Abstract The He I photoelectron spectra (PES) of 2- and 4-ethynylpyridine were measured, and the origin of each ionization band was clarified. For instance, 4-ethynylpyridine shows five PES bands at 9.58, 9.9, 10.0, 10.75 and 11.84 eV in the lower energy region, the 9.9 eV and 10.75 eV bands in particular being assigned to the ionizations from the a 1 n orbital of the pyridyl group and the in-plane b 2 π orbital of the ethynyl group, respectively. Furthermore, with the aid of first order perturbation theory, the 10.59 eV band of 2-ethynylpyridine has been ascribed to the ionization from the a 2 -like π orbital, and the 10.8 eV band to that from the in-plane b 2 -like π one. Through the process of the band assignments for the ethynylpyridines, it has been revealed that there is a weak interaction between the n orbital on the orbio-substituted nitrogen atom and the in-plane π orbital localized on the ethynyl group of 2-ethynylpyridine. The photoelectron spectra of 1,4-bis(2-pyridyl)-1,3-butadiyne and 1,4-bis(4-pyridyl)-1,3-butadiyne, whose π-electron systems are regarded electronically as double ones of the corresponding ethynylpyridines, were also measured, and we have succeeded in making band assignments of these compounds on the ground of the correlation energy diagram with the ethynylpyridines.

Intramolecular orbital interactions in 9,10-disilatriptycene studied by photoelectron spectroscopy

Abstract 9,10-Disilatriptycene is of high symmetry and structurally very close to triptycene. In the latter case it is known that the through-space orbital interactions among the π electron systems of the three benzene rings predominate, the effect of the through-bond interactions being negligibly small. In this study, the vapor-phase He(I) photoelectron spectra of triptycene and 9,10-disilatriptycene were measured and studied comparatively in order to investigate the relative importance of the Jahn–Teller effects in the cationic states of these compounds, the through-space and the through-bond interactions. Simple LCBO model calculations could reproduce almost quantitatively the features of their observed spectral patterns. The spectral pattern is quite complex in 9,10-disilatriptycene in contrast to that of triptycene due to the back-donation from the benzene rings to the Si atoms in addition to the Jahn–Teller effects in the cationic states and the through-space interactions.