THE LOWER EXCITED STATES OF SOME SIMPLE MOLECULES

1958 ◽  
Vol 36 (1) ◽  
pp. 10-23 ◽  
Author(s):  
R. S. Mulliken
Keyword(s):  
Absorption Spectra ◽  
Excited States ◽  
Strong Evidence ◽  
Electronic States ◽  
Ultraviolet Absorption ◽  
Mo Calculations ◽  
New Interpretation ◽  
Simple Molecules ◽  
Scf Calculations ◽  
Lcao Mo

The present and prospective rapidly increasing usefulness of good LCAO-MO calculations on the electronic states of simple molecules in the interpretation of observed excited states is pointed out. As examples, the observed and predicted states of, in particular, the π3π configurations of N2 and CO are compared with those of C2H2 and HCN and with those of CO2 and CS2. Further, the results of LCAO-SCF calculations on CO2 and on O3 are surveyed, and it is shown how these can be helpful in interpreting and understanding the ground and excited states of AB2 (expecially AO2) molecules in general. A new interpretation of the so-called d3П state of CO as a case b πu3πg, 3Δu state is proposed. Tentative interpretations of some of the ultraviolet absorption spectra of C2H2, HCN, and of a number of AB2 molecules are reviewed or suggested, including some discussion of the shapes of excited states. The AB2 discussion is a revision of one given earlier. Finally, following up a suggestion of Coon, it is pointed out that there exists strong evidence for slightly unequal A—O distances in certain excited states of C1O2 and SO2.

ULTRAVIOLET ABSORPTION SPECTRA OF HCN AND DCN: I. THE α—X AND β—X SYSTEMS

1957 ◽  
Vol 35 (8) ◽  
pp. 842-879 ◽  
Author(s):  
G. Herzberg ◽  
K. K. Innes
Keyword(s):  
Fine Structure ◽  
Absorption Spectra ◽  
Excited States ◽  
Linear Structure ◽  
Ultraviolet Absorption ◽  
Geometrical Parameters ◽  
Rotational Analysis ◽  
Long Wavelength ◽  
Non Linear ◽  
The Asymmetry

The ultraviolet absorption spectra of HCN and DCN have been investigated below 2000 Å. Four band systems have been found of which two, α—X and β—X, are described and analyzed in the present paper. At the long wavelength end the α—X system in both HCN and DCN consists of simple progressions of sharp bands whose fine structure has been measured. At shorter wavelengths diffuseness (predissociation) sets in gradually but at somewhat different energies in HCN and DCN. Both the vibrational and rotational analyses lead independently and unambiguously to the conclusion that in the upper state, α, unlike in the ground state, the molecule is non-linear. The geometrical parameters obtained from the rotational fine structure are r0(CH) = 1.140 Å, r0(CN) = 1.297 Å, and [Formula: see text]H—C—N = 125.0°. Two vibrational frequencies in the α state are established: for HCN, ν1 = 1506, ν2 = 949; for DCN, ν1 = 1506, ν2 = 735 cm−1. The CH stretching frequency has not been found.The β—X system has been clearly observed only for DCN. In HCN it is apparently so strongly predissociated that its presence among the diffuse α—X bands is difficult to establish. In the β state, as in the α state, the molecule is bent. The somewhat fragmentary rotational analysis leads to r0(CN) = 1.334 Å and [Formula: see text]D—C—N = 114.5° when r0(CD) = 1.140 Å is assumed. Only one vibrational frequency, ν2 = 731 cm−1, has been established in the β state of DCN. The vibrational quantum number v2 is uncertain since no isotope effect is available to establish the position of the 0–0 band.In the rotational analysis, particularly of the α—X system, the effects of the asymmetry of the molecule in the upper state are clearly demonstrated by the doubling of the levels with K = 1 and K = 2 and the large contributions of the asymmetry to the terms in J2(J + 1)2. From the sign of the K-type doubling for K = 1 it follows unambiguously that both the α and the β state belong to the species A″, i.e. the electronic eigenfunctions are antisymmetric with respect to the plane of the molecule. It appears probable that both transitions α—X and β—X correspond to forbidden transitions (1Δ—1Σ+ and 1Σ−—1Σ+) of the linear case. The non-linear structure of the excited states is briefly discussed in terms of electron configurations.A number of interesting features of the predissociation in the α state are pointed out. It is shown that the dissociation products are in all probability H + CN(2Π).

A theoretical study of the Curtius rearrangement. The electronic structures and interconversions of the CHNO species

1977 ◽  
Vol 55 (9) ◽  
pp. 1498-1510 ◽  
Author(s):  
Arvi Rauk ◽  
Paul F. Alewood
Keyword(s):  
Electronic Structures ◽  
Theoretical Study ◽  
Electronic States ◽  
Decomposition Products ◽  
Curtius Rearrangement ◽  
Nitrile Oxides ◽  
Photochemical Decomposition ◽  
Degenerate States ◽  
Scf Calculations ◽  
Lcao Mo

The thermal and photochemical decomposition of formyl azide in the Curtius rearrangement has been studied by means of RHF–LCAO–MO–SCF calculations augmented by CI. The lower electronic states of carbonyl azides, carbonyl nitrenes, isocyanates, cyanates, oxazirenes, nitrile oxides (fulminates), and isofulminates are discussed on the basis of the results for formyl azide and the corresponding isomers of formula CHNO. The discovery of two nearly degenerate states of A′′ symmetry for formyl azide which may channel decomposition products directly to both isocyanates and carbonyl nitrenes explains some puzzling features of the photolytic Curtius rearrangement. The interconvertibility of the CHNO isomers (and the RCNO isomers) is discussed in terms of the energetics and intended correlations of the accessible electronic states of these species.

LCAO-MO Calculations on Boron Compounds. I. LCAO-MO-SCF Calculations on Borazines

1965 ◽  
Vol 87 (3) ◽  
pp. 399-404 ◽  
Author(s):  
Odilon Chalvet ◽  
Raymond Daudel ◽  
Joyce J. Kaufman
Keyword(s):  
Mo Calculations ◽  
Boron Compounds ◽  
Scf Calculations ◽  
Lcao Mo

Electronic absorption, fluorescence and polarisation spectra of α,β-diamino-9,10-anthraquinones and their interpretation by the method of configuration analysis

1982 ◽  
Vol 47 (10) ◽  
pp. 2604-2614 ◽  
Author(s):  
Miloš Nepraš ◽  
Miloš Titz ◽  
Jürgen Fabian ◽  
Bohuslav Gaš
Keyword(s):  
Polyvinyl Alcohol ◽  
Absorption Spectra ◽  
Excited States ◽  
Wavelength Range ◽  
Electronic Absorption ◽  
Frequency Dependent ◽  
Absorption Frequency

On the basis of measurement of absorption frequency-dependent polarisation spectra of fluorescence (APF) and dichroism on oriented polyvinyl alcohol sheets, interpretation has been carried out of absorption spectra of α,β-diamino-9,10-anthraquinones in the wavelength range 250 to 600 nm. Character of the excited states has been investigated by the method of configuration analysis (PPP-CA).

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