Shigeyuki Urano

Tri-s-triazine: synthesis, chemical behavior, and spectroscopic and theoretical probes of valence orbital structure

Synthese du tri-s-triazine, et determination de sa structure par diffraction RX; etude de ses proprietes chimiques, spectroscopiques et physiques. On utilise la spectrometrie photoelectronique ainsi que des calculs ab initio et HAM3

UV Photoelectron Spectroscopy and Ab Initio Characterization of Valence Orbital Structures and Conformations of Neutral Phosphate Esters

The HeI UV photoelectron spectrum of trimethyl phosphate (TMP) has been measured and interpreted with the aid of SCF molecular orbital calculations carried out with STO-3G, STO-3G* and 4-31G basis functions. The photoelectron spectrum of TMP is more accurately reproduced by results from 4-31G calculations than by results from STO-3G or STO-3G* calculations. However, all three basis sets yield results which predict the same assignment of the photoelectron spectrum. Results at the 4-31G level indicate that whether calculations are based on crystallographic bond angles and bond lengths or on STO-3G optimized geometries has little effect on the energetic ordering of the upper occupied orbitals. The energetic ordering of orbitals is also found to be only weakly dependent upon the torsional angle phi, describing rotation of ester groups about P-O bonds and upon the torsional angle psi, describing rotation of methyl groups about C-O bonds. For trimethyl phosphate, with C3 symmetry, the vertical ionization potentials of the upper occupied orbitals are 10.81 eV (8e), 11.4 eV (9a), 11.93 eV (7e), 12.6-12.9 eV (8a and 6e), 14.4 eV (7a) and 15.0-16.0 eV (5e and 6a). Calculations at the 4-31G level indicate that many of the highest occupied orbitals in neutral dimethyl phosphate and methyl phosphate have energies and electron distributions similar to orbitals in TMP. For TMP, a search for optimized values of phi and psi has been carried out at the STO-3G*level. In agreement with previous NMR studies and with classical potential calculations, the STO-3G* results indicate that both the gauche (phi = 53.1 degrees) and anticlinal (phi = 141.9 degrees) conformations are thermally accessible. Also in agreement with the classical potential calculations, the STO-3G* results predict that in the all gauche conformation energy is minimized when the methyl groups assume a staggered geometry (psi = 60 degrees to 80 degrees) and that an energy maximum occurs for an eclipsed geometry (phi = 0 degrees to 20 degrees). A study of the dependence of optimized values of O-P-O ester bond angles on the torsional angles, phi, was carried out at the STO-3G, STO-3G* and 4-31G levels. The results demonstrate that for C3 symmetry, the coupling of O-P-O angles to phi is influence by repulsive steric interactions.

Through space interactions of double bonds by photoelectron spectroscopy

Steric effects between double bonds and remote polar substituents, previously manifested in nuclear magnetic resonance (NMR) spectra, are also evident in the ultraviolet photoelectron spectra (UPS). An ether functionality was introduced at the 3-axial position of exo-methylenecyclohexane by means of acetal groups (dimethyl and ethylene). The 3-axial ether group destabilizes the 7r orbital on the double bond by 0.1-0.2 eV by a through space interaction (the 3-equatorial ether group by itself has little or no effect). This interaction apparently is responsible for the decreased proportion of 3-axial methoxyl observed by NMR spectroscopy. In contrast, 4-axial ether functionalities in cyclohexene show a slight stabilizing of the n orbital by through bond electron withdrawal. These results also agree with the NMR observations, since the endocyclic double bond of cyclohexene permits a much larger proportion of 4-axial methoxyl. Ab initio calculations support the observations by paralleling the observed r-orbital energies and by providing electron densities. Whereas 3-axial methoxyl clearly polarizes the double bond in methylenecyclohexane, 4-axial methoxyl has little or no effect on the electron densities of cyclohexene, even though methoxyl is closer to the endocyclic than to the exocyclic double bond. The NMR, UPS, and ab initio results provide an initial understanding of the three dimensionality of the n-electron steric effects. Nuclear magnetic resonance studies have shown that the double bond interacts in a complex fashion with remote polar substituents.,-* A 3-axial substituent in a six-membered ring is more repulsive with an exocyclic double bond than with a saturated CH2 group at the same position (eq I).,., Thus in a nonpolar solvent

Influences of 7-alkyl substitution on the reversible binding of the proximate carcinogen trans-3,4-dihydroxy-3,4-dihydrobenz[a]anthracene to DNA

The effects of 7-alkyl substitution on the reversible intercalation of the proximate carcinogen trans-3,4-dihydroxy-3,4-dihydrobenz[a]anthracene (BAD) to calf thymus DNA have been examined using time-resolved fluorescence spectroscopy. The results indicate that in 10(-3) M sodium cacodylate the binding constant of BAD is 1.8 x 10(3) M-1. 7-Ethyl substitution decreases the binding constant 1.6 times, while 7-methyl substitution increases the binding constant 1.7 times. UV Photoelectron data and results from ab initio molecular orbital calculations suggest that an increase in polarizability contributes to the increased binding accompanying methyl substitution. The decreased binding accompanying ethyl substitution arises from steric inhibition. The physical binding data correlates with the decrease in carcinogenic activity which occurs with 7-ethyl substitution of benz[a]anthracene metabolites.

Stereoelectronic Effects on the Nucleophilic Addition of Phosphite to the Carbonyl Double Bond: Ab Initio Molecular Orbital Calculations on Reaction Surfaces and the α-Effect

Abstract Ah initio molecular orbital calculations have been carried out on adducts of trihydroxy phosphine, P(OH)3, and formaldehyde, H2C=0. Stationary points were located and a reaction surface calculated. One stationary point exists as a stable pentacovalent phosphorane, and the other as a 1,3-dipolar transition state. Calculations differing in the conformation about the P-OH bonds of the phosphite reveal that an antiperiplanar (app) lone pair on oxygen to the phosphorus lone pair (acyclic analogue) raises the energy of the molecule by 1.7 kcal/mol relative to a phosphite conformation with no app lone pairs to the phosphorus lone pair (bicyclic analogue). In the transition state, the relative energy between the two conformations reverses with the acyclic analogue transition state 5 kcal/mol lower energy than the bicyclic analogue transition states. The lower energy for the acyclic analogue in the transition state is attributed to the mixing of the app lone pairs on the oxygens of the phosphite mixing with the σ orbital of the newly formed bond between phosphorus and carbon. This kinetic Stereoelectronic effect can explain why acyclic phosphites react much faster in nucleophilic reactions than bicyclic phosphites. This phenomenon suggests that the origin of the α-effect, the enhanced nucleophilicity of a base possessing a heteroatom with an adjacent unshared electron pair arises from the stereoelectronic effect.