Use of the CNDO Method in Spectroscopy. II. Five‐Membered Rings
01 May 1968-Journal of Chemical Physics (American Institute of Physics)-Vol. 48, Iss: 9, pp 4050-4055
TL;DR: In this paper, the modified CNDO method was used to calculate the electronic spectra of cyclopentadiene, the Cyclopentadienide ion, pyrrole, furan, pyrazole, imidazole, 2-pyrrolecarboxaldehyde, and furfural.
Abstract: The modified CNDO method previously reported has been used to calculate the electronic spectra of cyclopentadiene, the cyclopentadienide ion, pyrrole, furan, pyrazole, imidazole, 2‐pyrrolecarboxaldehyde, and furfural. In general, the results obtained agree quite well with experimental data. Because the CNDO method treats explicitly all σ and π valence electrons of a molecule, the results of the calculations are used to discuss some of the σ–π interactions which previously could not be treated. The calculations are successful in reflecting changes in the electronic spectra of compounds as a result of extending conjugation or addition of a substituent.
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TL;DR: In this paper, the modified CNDO method was applied to the calculation of a series of monosubstituted benzenes and pyridines and the calculated values of the transition energies and corresponding oscillator strengths were in good agreement with available experimental data on the electronic spectra of these compounds.
Abstract: The modified CNDO method previously reported has been applied to the calculation of a series of monosubstituted benzenes and pyridines, namely, aniline, anilinium ion, benzonitrile, nitrosobenzene, phenol, phenoxide ion, toluene, pyridinium ion, pyridine‐N‐oxide, 1‐hydroxy‐pyridinium ion, the aminopyridines, and the cyanopyridines. The calculated values of the transition energies and corresponding oscillator strengths are in good agreement with available experimental data on the electronic spectra of these compounds. An analysis of the lower‐energy excited states by means of the calculated results suggests that there is an astonishingly small amount of charge‐transfer character associated with the observed ultraviolet bands. The calculations indicate that the changes in charge densities in going from the ground state to the lowest energy π → π* excited state can be related to the Hammett σ's for the substituents.
333 citations
TL;DR: In this article, the basic principles and concepts of quantum chemical CD calculations for the configurational assignment of chiral compounds with stereogenic centers and/or elements of axial or planar chirality are described.
292 citations
TL;DR: In this paper, the origin of the unusually long UV-vis absorption maximum, λmax = 398 nm, of 3,3′, 4,4,4′-tetraphenyl-2,2′-bisilole was investigated.
Abstract: The purpose of this work is the theoretical elucidation of the origin of the unusually long UV-vis absorption maximum, λmax = 398 nm, of 3,3′,4,4′-tetraphenyl-2,2′-bisilole, which we have observed ...
217 citations
05 Jan 2007
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TL;DR: In this article, the electronic structure and spectra of the icosahedral C 60, "follene-60" were examined by use of the CNDO/S method.
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References
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TL;DR: In this article, the CNDO method was modified by substitution of semi-empirical Coulomb integrals similar to those used in the Pariser-Parr-Pople method, and by introducing a new empirical parameter κ to differentiate resonance integrals between σ orbitals from those between π orbitals.
Abstract: The CNDO method has been modified by substitution of semiempirical Coulomb integrals similar to those used in the Pariser‐Parr‐Pople method, and by the introduction of a new empirical parameter κ to differentiate resonance integrals between σ orbitals from those between π orbitals. The CNDO method with this change in parameterization is extended to the calculation of electronic spectra and applied to the isoelectronic compounds benzene, pyridine, pyridazine, pyrimidine, and pyrazine. The results obtained were refined by a limited CI calculation and compared with the best available experimental data. It was found that the agreement was quite satisfactory for both n→π* and π→π* singlet‐singlet transitions. The relative energies of the pi and lone‐pair orbitals in pyridine and the diazines are compared and an explanation proposed for the observed orders. Also, the nature of the “lone pairs” in these compounds is discussed.
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01 Jan 1965
TL;DR: In this article, the basic principles of organic chemistry are discussed and discussed in the context of basic principles in organic chemistry, including basic principles for organic chemistry and its application in the field of biology.
Abstract: Basic principles of organic chemistry , Basic principles of organic chemistry , کتابخانه مرکزی دانشگاه علوم پزشکی تهران
897 citations
TL;DR: The semi-empirical SCF-LCAO-MO procedures described in previous papers have now been extended to compounds containing heteroatoms, in particular nitrogen and oxygen as mentioned in this paper.
Abstract: The semiempirical SCF—MO procedures described in previous papers of this series have now been extended to compounds containing heteroatoms, in particular nitrogen and oxygen. This is a treatment based on the Huckel σ/π approximation, in which the total energy of the σ electrons is approximated by assuming additivity of bond energies (allowance being made for variations of these with length by using Morse functions) and in which the π energy is calculated by a Pople‐type SCF—LCAO—MO treatment with neglect of differential overlap and penetration integrals. The problem of estimating the remaining integrals is discussed in detail; valence‐state ionization potentials and one‐center repulsion integrals are determined empirically by standard methods from spectroscopic data, but a new procedure is used for estimating effective nuclear charges. Two sets of values are used for the remaining repulsion integrals, one closely similar to that recommended by Pariser and Parr, the other corresponding to a modified SPO tr...
108 citations