n→π*‐Übergänge in Dicarbonylverbindungen Der Einfluss der n, n‐ und π*, π*‐Wechselwirkung

Abstract

Summavy. By CNDO-CI calculations we have found that dicarbonyl compounds exhibit only two n --f n* transitions in the visible or near UV. region, instead of four as expected from simpler MO-models. The dominant features of the long-wavelength electronic spectra may be characterized by the relative energy of the two n and the two lowest n* orbitals. In general we distinguish between threc cases: i\. Large splitting (about 2 eV) both between the n and the n* orbitals. The longest-wavelength w+-n* transition is shifted to the red. The higher transition remains roughly where it would appear in the monocarbonyl compound.B. No or very small splitting between either the two n, or the two n* orbitals, or both. Both n --f n* transitions merge, roughly, in the wavelength region of the corresponding monocarbonyl compound.C. Intermediate (0.5-1.0 eV) splitting between the two n orbitals on one hand, the two n* orbitals on the other. Spectral properties between A. and B.The splitting between the two n orbitals and the two n* orbitals depends not only on the distance between the two carbonyl groups, but also very strongly on their mutual position and on the nature of the molecular fragment connecting them. As one would expect, in a-dicarbonyl compounds the splitting between the n orbitals is large, namely 1.8 eV. In some y-diketones such as p-quinone, one still finds, however, a value of 0.7 eV. The splitting between the n orbitals gets significantly lowered if the two carbonyl groups are coaxial, but their Zp, nodal planes are perpendicular to each other. In planar p-diketones the splitting between the n* orbitals is small, but may increase to an effective value of 0.4 eV when they are skewed. Our calculations agree well with measured electronic spectra and CD. data. An electrochromism experiment on camphorquinone convincingly supports our conclusions on a-dicarbonyl compounds.

Singly and doubly excited states of butadiene, acrolein, and glyoxal: Geometries and electronic spectra

“…67͒ for this state. This state was also reported by other theoretical calculations, 18 however, the estimated values range widely as 3.47, 18 3.96, 68 4.97, 19 and 4.36 eV. 36 In higher energy region, we obtained the Rydberg transition 1 1 B u ͑n + → 3p x ͒ at 7.83 eV which agrees with the observed state at 7.9 eV by Verhaart and Brongersma, 7 but, their assignment was different as → * transition.…”

Photoelectron spectroscopy of cyclic .beta.-diketones and their enolone tautomers

“…Semiempirical calculations indicate that the throughbond interaction generally has a larger magnitude than the through-space interaction.2-5 Extended Hiickel and CNDO calculations indicate that 1,3-cyclobutanedione is an exception and has the n orbitals arranged in the "inverted" order. 3,5 Although magnitudes of the splitting of n orbital levels observed in the photoelectron spectrum agree with those of the calculated splittings,5 6no experimental evidence supporting either the "natural" order or the "inverted" order of orbitals in /3-diketones has been obtained. This study of a series of cyclic /3-diketones was undertaken in an attempt to seek this experimental support.…”

Ultraviolet photoelectron spectra of 2-norbornanone, 2,5-norbornanedione, their alkyl derivatives and thio-analogues. An investigation of transannular interactions by photoelectron spectroscopy

“…The observed splitting between the two oxygen non-bonding orbitals of 5 (0.16eV) is due to a through-bond interaction (7,8), i.e. a mixing of the no orbitals with the skeletal o orbitals.…”

“…For most dicarbonyl compounds, interest arises in the mechanism of the interaction between the two carbonyl groups, and whether it occurs throughspace or through-bond (2,3,(7)(8)(9). A typical case, particularly germane to the compounds studied in this work, is 1,4-cyclohexanedione which can exist in three conformations: boat (C,,), chair (Clh), and skew (or twist) (D,).…”

“…The monosubstituted ketones in question are: 2-norbornanone (1); 6,6-dimethyl-2-norbornanone (2); 3,3-dimethyl-2-norbornanone (3), and 1,3,3-trimethyl-2-norbornanone (4). The diones are 2,5-norbornanedione (5); 3,3-dimethyl-2,5-norbornanedione (6); 1,3,3-trimethyl-2,5-norbornanedione (7), and 3,3,6,6,-tetramethyl-2,5-norbornanedione (8). Also studied w e i i 1,3,3-trimethyl-2-norbornanethione (thiofenchone) (9); 3,3,6,6-tetramethyl-5-0x0-2-norbornanethione (lo), and 3,3,6,6-tetramethyl-2,5-norbornanedithione (11).…”