Showing papers on "HOMO/LUMO published in 1978"
TL;DR: The mechanism of the protecting action of sulfur compounds against ionizing radiation from the electronic structural point of view is clarified and the interesting fact that sulfur groups act as sinks of irradiation energy might be explained by the extremely low values of LUMO of sulfur compound.
Abstract: NAGATA, C., AND YAMAGUCHI, T. Electronic Structure of Sulfur Compounds and Their Protecting Action Against Ionizing Radiation. Radiat. Res. 73, 430-439 (1978). We have attempted to clarify the mechanism of the protecting action of sulfur compounds against ionizing radiation from the electronic structural point of view. The extremely high reactivity of sulfur compounds with hydroxyl radicals, which has been considered to be related to their indirect protecting action, is explained by the significantly low energy of the lowest unoccupied molecular orbital (LUMO) and the relatively high energy of the highest occupied molecular orbital (HOMO) of these compounds in comparison with others. Intermolecular and intramolecular energy transfers which have been considered to be intimately correlated with the direct protecting action of sulfur compounds are explained on the basis of electronic structure, and the interesting fact that sulfur groups act as sinks of irradiation energy might be explained by the extremely low values of LUMO of sulfur compounds. Besides the special feature of the energy values of HOMO and LUMO, sulfur compounds are characterized by having large electron density at the p, orbital which is concerned with chemical reactivity and energy transfer. Further, the contribution of d-orbitals is considered to make sulfur compounds more reactive than other compounds which lack contributions from d-orbitals.
18 citations
TL;DR: The ground electronic state of uracil was characterized by the ab initio molecular fragment floating spherical Gaussian orbital method as mentioned in this paper, and a linear relation was found between the calculated orbital energies and the one-electron ionization potentials measured by uv photoelectron spectroscopy.
Abstract: The ground electronic state of uracil was characterized by the ab initio molecular fragment floating spherical Gaussian orbital method. Among the properties studied were ionization potentials, permanent diople moment, the electronic charge distributions of the higher occupied and lowest unoccupied molecular orbitals, the populations and bond orders which describe the total ..pi..-electronic charge distribution, and the dipole binding energies of the three hydrogen bonding sites of uracil that are available for base pairing in RNA. A linear relation was found between the calculated orbital energies and the one-electron ionization potentials measured by uv photoelectron spectroscopy. At the x-ray crystallographic geometry, the permanent dipole moment was calculated to have a length of 2.27 D and to form an angle of 27/sup 0/ with the C/sub 4/ ..-->.. N/sub 1/ direction. When the unusually short C-H and N-H bonds in the x-ray structure of uracil were lengthened to 1.10 and 1.00 A, respectively, the calculated dipole moment changed to 3.55 D and formed an angle of 38/sup 0/ with the C/sub 4/ ..-->.. N/sub 1/ direction. The lowest unoccupied molecular orbital is a ..pi..-antibonding molecular orbital while the highest occupied molecular orbital is ..pi.. nonbonding. The lowest unoccupied molecular orbital has amore » very high virtual electron density at the C/sub 6/ atom and there are nodes between the C/sub 6/ atom and the N/sub 1/ and C/sub 5/ atoms; these two factors combine to make the C/sub 6/ atom highly susceptible to ''focused'' nucleophilic attack. No single resonance structure can adequately represent the total ..pi..-electron charge distribution which has a net bond order of approximately 4.4 ..pi.. bonds. The oxygen atom attached to position 4 of the ring is a better hydrogen bonding site than the oxygen attached to position 2.« less
17 citations
TL;DR: In this article, it was shown that donor substituents are more effective in raising an HOMO than an attractor in lowering a LUMO of a conjugated substrate.
13 citations
Journal Article•
TL;DR: Qualitative models based on both regiospecific lipophilicity or electron densities and also metabolic conversion to reactive intermediates are presented, indicating that the 2-X-substituted-4,5-dimethoxyphenylisopropylamines are unusually hydrophilic.
Abstract: The potency of a series of 2,4,5-ring substituted phenylisopropylamines was examined using the rabbit hyperthermia assay. An excellent correlation (r = 0.99) was found between the rabbit hyperthermic and human psychotomimetic potencies. In the hyperthermic model, the 4-X-substituted-2,5-dimethoxyphenylisopropylamines were found to be one to two orders of magnitude more potent than the 2-X- or 5-X-substituted positional isomers (X = -H, -CH3, -SCH3, -Br). Conformational perturbations induced by substituents ortho to the ethylamine side chain were studied with the PCILO and ab initio molecular orbital methods. The variations in the biological activities could not be rationalized in terms of the ability of the ortho substituents to stabilize conformations which mimic LSD. The electronic structures of the positional isomers were examined in the corresponding toluene analogues using the CNDO/2 method. A reasonable correlation (r = 0.98) was found between the Highest Occupied Molecular Orbital (HOMO) energy and the ionization potentials reported from photoelectron spectroscopy studies. In the case of the positional isomers, the HOMO energies were ordered as follows: 4-X greater than 5-X greater than 2-X. However, the regression analysis of the relationship between these orbital energies and Log Biological Activity (B.A.) was not impressive. Examination of the partition coefficients (octanol/water) of the positional isomers indicated that the 4-X- and 5-X-substituted compounds have almost equivalent Log P's, but that the 2-X-substituted-4,5-dimethoxyphenylisopropylamines are unusually hydrophilic. The regression of Log H.P. to the HOMO energies resulted in a marginally significant relationship; addition of the Log P's resulted in no significant improvement. Qualitative models based on both regiospecific lipophilicity or electron densities and also metabolic conversion to reactive intermediates are presented.
6 citations
TL;DR: In this article, the nucleophilic substitution of polyene-M(CO) 3 complexes is correlated with the frontier electron density parameter, the partial valence-inactive population (P LUMO A ).
5 citations
TL;DR: In this paper, extended-basis SCF calculations indicate that BeF may exist as a metastable species, and the orbital occupied by the electron of BeF is well approximated by the lowest unoccupied molecular orbital of neutral BeF2.
Abstract: Results of extended-basis SCF calculations indicate that BeF may exist as a metastable species. Comparison of results obtained from SCF calculations on neutral BeF2 with those of BeF shows that the orbital occupied by the electron of BeF is well approximated by the lowest unoccupied molecular orbital of neutral BeF2.
4 citations
TL;DR: In this article, the perturbation potential has been defined and shown to have the symmetry of the direct product of the pertinent substrate and catalyst orbitals, and the (HOMO) α − (LUMO) β energy gap is decreased by the second order perturbations energy.
3 citations
TL;DR: In this paper, a connection is drawn between two recent models for molecular shapes, namely, those based on the second-order Jahn-Teller (SOJT) effect and the highest occupied molecular orbital (HOMO) postulate respectively.
Abstract: For a closed-shell molecule, a connection is drawn between two recent models for molecular shapes, namely, those based on the second-order Jahn-Teller (SOJT) effect and the highest occupied molecular orbital (HOMO) postulate respectively. Two necessary and sufficient conditions are derived within the molecular orbital framework for the approximation inherent in the SOJT model to be valid. One of these conditions is akin to the HOMO postulate.
2 citations
TL;DR: In this paper, the orbital interaction of electrophile′s LUMO and olefin′s HOMO was interpreted as the result of the cyclohexene double bond protonation.
Abstract: Reaction of tricyclo[4.2.2.02,5]deca-3,7-diene derivatives 1 or 2 with DMSO-conc.HCl gave transannular methylthiolactonization product 3 and 3-chloro-4-methylthiotricyclo[4.2.2.02,5]dec-7-ene derivatives 4. Reaction of 1 or 2 with HBF4 in DMSO resulted in the protonation of the cyclohexene double bond. These results have been interpreted as the result of the orbital interaction of electrophile′s LUMO and olefin′s HOMO.
1 citations
TL;DR: In this article, it was shown that the LUMO interaction is involved in the D*A-D+*A− or DA*D+A−* interaction, the frontier orbital interaction being the same as one in usual thermal reactions.
Abstract: Photochemical reactions between powerful electron donors and acceptors are likely controlled by the (HOMO)D–(LUMO)A interaction involved in the D*A–D+*A− or DA*–D+A−* interaction, the frontier orbital interaction being the same as one in usual thermal reactions.
1 citations
TL;DR: In this article, a series of linear alkynes and their analogs are compared, and the total electronic energies are partitioned into bond contributions, which can be attributed primarily to alterations in the π-overlap population.
Abstract: Ab initio calculations have been applied to a series of linear alkynes and their lithioand cyano-derivatives. The calculations predict that the C-C and CC internuclear distances will depend on the length of the molecule, the position of the particular bond in the molecule, and the presence or absence of substituents. Although small changes in the σ-overlap population are observed, the variations in bond lengths call be attributed primarily to alterations in the π-overlap population. The HOMO and LUMO energies for the various related molecules are compared, and the total electronic energies are partitioned into bond contributions.