Electronic structures and spectra of halogenated purines and pyrimidines III.σ-Electronic structure and some comments on the mechanism of the xanthine oxidase reaction of 2-halopurines and related analogs

The extended HMO (EHMO) and Pople-Segal SCF-MO-CNDO /2 calculations on purine, 8-oxopurine, 2-oxopurine, 2-fluoropurine, and 2-chloropurine indicate significant polarizations of the σ cores. It is shown that the polarizations of both σ and π frameworks are mutually opposing in some cases. The results from the two methods are compared for these complex biomolecules. It is found that the EHMO calculations tend to over-polarize the σ and π frameworks. However, the CNDO shows anomalous π-electron densities on N₍₇₎ and C₍₈₎ of the purine rings, and the reason for this anomaly is not certain. The application of the results to the xanthine oxidase system indicates that the substrate molecules are subject to a specific orientation on the enzyme surface to counteract the electronic reactivity, in support of the previous prediction based on the π-electron calculations. The CNDO results appear to be more satisfactory than the EHMO in this respect.     

Electronic structures of the π-π type charge-transfer complexes of pyridine with boron trihalides

The electronic structures of the π-π type complexes of pyridine with boron trihalides have been studied by means of IEHMO calculation. The results indicate that BX₃(X = F, Cl, Br, I) tends to react with C₅H₅N in a planar configurations against the plane of C₅H₅N. The most stable configurations of complexes are at 60° of orientation angle φ for X = Cl, Br, I, but at 0° for X = F. A linear relationship between In E_b, the logarithm of rotation potential barriers, and the radii of halogen atoms r₀ has been observed, and has been deduced from Morse potential function. In the complex, the donating properties of BX₃ have an increase from X = F to I, and BF₃ functions as an acceptor, but the others as donors. It has been shown that every energy level of the complex is corresponding to that of the donor or the acceptor, which we have discussed by the perturbation theory. The bonds between D and A appear essentially as π-π type but not pure.     

Electronic structure of benzaldehyde: A comparative study of the lowest excited singlet π* ← π and π* ← n states

VE-PPP, CNDO/2, and CNDO/s-CI methods have been used to investigate the electronic spectrum and structure of benzaldehyde. Electronic charge distributions and bond orders in the ground and lowest excited singlet π* ← π and π* ← n states of the molecule have been studied. The molecule has been shown to be nonplanar in the lowest π* ← n excited singlet state, in agreement with the conclusions drawn from the study of vibrational spectra. Dipole moments in both excited states have been shown to be larger than the ground-state value. Thus, the ambiguity in the experimental result for the π* ← π n excited singlet state dipole moment has been resolved. It has been shown that the n orbital is mainly localized on the CHO group. Furthermore, charge distributions, dipole moments, and molecular geometries are shown to be very different in the excited singlet π* ← π and π* ← n states.     

NMR spectra and structures of oridonin derivatives complexes with β‐cyclodextrin

Complexations between three oridonin derivatives and β‐cyclodextrin (βCD) were studied by nuclear magnetic resonance (NMR) method. Job's plots for complexes were depicted by ¹H NMR spectra chemical shifts, which proved the 1:1 stoichiometry inclusion complex formation between each derivative and βCD. Two‐dimensional rotating frame overhauser effect spectroscopy (2D ROESY) support the above conclusion and also proved that ring A of each oridonin derivative deeply enters into hydrophobic cavity from the wider rim and the other parts are outside the cavity. Apparent formation constants (K_a) of complexes between three oridonin derivatives and two CDs are calculated according to Scott's equation. Copyright © 2011 John Wiley & Sons, Ltd.     

Mass spectra of π-cyclopentadienyl-diene cobalt complexes

Many reports on the mass spectra of organotransition-metal complexes have appeared in recent years,¹ whilst there have only been a few reports on the mass spectra of transition metal olefin complexes, some metal carbonyl olefin complexes²³⁴ and π-cyclooctenyl-π-cyclooctadienyl cobalt.⁵ Recently fragmentation paths of π-cyclopentadienyl-cyclooctadiene rhodium were elucidated by King.⁶ The present authors found metastable ions in the mass spectra of π-cyclopentadienyl-diene cobalt complexes as well as in the mass spectra of π-cyclopentadienyl-diene rhodium complexes.⁷. In the present paper the authors wish to report the mass spectra of several π-cyclopentadienyl diene cobalt complexes.     

Simulated T ← S spectra of benzaldehydes as a function of the energy gap between the ππ and nπ levels

The T_1,2 ← S₀ spectra of benzaldehydes have been studied as a function of the energy separation between the vibrationless levels. It is shown that the spectra are very complicated in the region of ΔE[T₂⁰(nπ^*)-T₁⁰(ππ^*)] = 250–400 cm⁻¹, reflecting effective vibronic interferences between T₂⁰(0-0) and each of the ν₃₆-ν₃₃ out-of-plane vibrational levels of T₁⁰(ππ^*). The simulated spectra correspond to the observed spectra. In the case of T₁⁰ = ³nπ^* and T₂⁰ = ³ππ^* the spectral change is not so drastic as in the reverse case loc. cit. because the optical intensity generally concentrates in the longest wavelength band, i.e., the origin band of the T₁(nπ^*) ← S₀ transition. The simulation spectra are useful for interpretation of the absorption spectra in similar electronic structure systems of substituted benzaldehydes.     

Electronic structures and spectra of cyanoethylene derivatives

Results of two different types of molecular orbital calculations of severalπ-orbital systems containing nitrile groups are reported and compared. The bidentate ligandbis-(methylmercapto) maleonitrile and related molecules are included. Electronic spectra of these molecules are reported and assigned in terms of the calculated energies.     

Vibrational spectra of π-olefin-transition metal complexes : (π-maleic anhydride)iron tetracarbonyl

The infrared and Raman spectra of solutions and solid samples of (π-maleic anhydride)iron tetracarbonyl have been studied. An assignment of the modes is suggested and the ligand vibrations in the complex are compared the data for maleic and succinic anhydrides. The C=C stretching frequency maleic anhydride shifts from 1595 to 1352 cm⁻¹ after coordination with the The essential decrease of the IR intensities of out-of-plane CH modes is for the complex and explained by the lowering of the effective charge on olefinic protons due to back-donation from metal to ligand.     

Electronic structure and electrical properties of interfaces between metals and π‐conjugated molecular films

The field of organic thin films and devices is progressing at an extremely rapid pace. Organic–metal and organic–organic interfaces play crucial roles in charge injection into, and transport through, these devices. Their electronic structure, chemical properties, and electrical behavior must be fully characterized and understood if the engineering and control of organic devices are to reach the levels obtained for inorganic semiconductor devices. This article provides an extensive, although admittedly nonexhaustive, review of experimental work done in our group on the electronic structure and electrical properties of interfaces between films of π‐conjugated molecular films and metals. It introduces several mechanisms currently believed to affect the formation of metal–organic interface barriers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2529–2548, 2003     

Theoretical investigation of the π -electronic structure and spectra of protonated aromatic carbonyl compounds

The π-electronic structure and spectra of the protonated aromatic carboxylic acids, aldehydes and ketones have been calculated by the Pariser–Parr–Pople method. An essential modification was that the positive charge has been considered as delocalized within the substituent group. The best agreement for the cation of benzoic acid was obtained using a symmetric carboxy model with equal charges on both oxygen atoms. This model gave equally good results for the protonated fluoro-, chloro- and methylbenzoic acids, as well. The delocalized charge model was successfully applied in the calculation of aromatic aldehydes and ketones. The methyl group was treated both as a one- and as a two-centre substituent.     

Stable π-electron systems and new aromatic structures

Aromatic character, as expressed in the reactions of azulene and its derivatives, is correlated with the development, in the appropriate transition state, of a stable π-electron system (sextet) in the five- or the seven-membered ring.

The recognition that considerable stability is associated with the ions and radical derived from perinaphthene leads to the formulation of several new types of hydrocarbons which may be expected to show aromatic character. A brief theoretical consideration of these hydrocarbons is given.

The synthesis of carbocyclic and heterocyclic derivatives of one of these hydrocarbons, cyclopenta[a]perinaphthene, is outlined, and a detailed account of their properties is given.