Cyclopentanone absorption spectrum

The ultraviolet absorption spectrum of cyclobutanone has been reported only in heptane solution (24). The absorption is appreciably shifted towards shorter wavelengths as compared to cyclopentanone, but shows a similarly banded structure. 

The ultraviolet absorption spectrum of cyclohexanone reflects the n jt transition common to all carbonyls see figure IX-E-1. The data derived from gas-phase measurements of the cross sections for cyclohexanone from two different research groups [National Center for Atmospheric Research (NCAR) and Ford Scientific Laboratories (Ford)] are in reasonable agreement (Iwasaki et al., 2008). The cyclohexanone cross sections as measured in cyclohexane solution by Benson and Kistiakowski (1942) had indicated seemingly low values (cross sections shown here is significantly less than those observed for cyclopropanone, cyclobutanone, and cyclopentanone, and in fact, all other carbonyls considered in this work. It is not obvious why these significant differences exist in the probability for the n -> 7T transition for cyclohexanone and that of the other cyclic ketones and most other carbonyl compounds. Theoretical studies will be important in defining the reasons for these differences. 

An example of Fermi resonance in an organic structure is the doublet appearance of the C=0 stretch of cyclopentanone under sufficient resolution conditions. Figure 3.2 shows the appearance of the spectrum of cyclopentanone under the usual conditions. With adequate resolution (Fig. 3.3), Fermi resonance with an overtone or combination band of an a-methylene group shows two absorptions in the carbonyl stretch region. 

The first absorption region of cyclopentanone extends from about 3500 A. to 2300 A. The absorption spectra in the vapor state and in heptane solution appear to be identical and show definite banded structure on both sides of the region of maximum absorption (24). A detailed analysis of the spectrum has been carried out (26a). 

The details of the photochemical primary process in cyclopentanone are fairly well understood (33). Since the spectrum in the first absorption region appears to be similar to that of the simple aliphatic ketones, it has been assumed that initial excitation is to the upper singlet state and that the zero-zero band is near 3600 A. 

The ultraviolet absorption spectra of cyclohexanone in the vapor phase and in heptane solution appear to be identical and are situated in the same region of the spectrum as that of cyclopentanone, although more band structure is apparent in the latter instance (24). 

The clues to the nature of this functionality were evident from the PMR spectrum, which showed a doublet (/ = 7.6 Hz) methyl group at 1.12 ppm and the 1750 cm-1 carbonyl absorption characteristic of a cyclopentanone. At this point structure 243 was proposed. Reduction of meloscandonine with sodium borohydride gave two isomeric alcohols [244 and 249 (M+ 322)] each of which was readily acetylated (to 246 and 247, respectively). Evaluation of the PMR spectra of these compounds defined the stereochemistry at C-20. 

Section 2.13A (p. 53). Ketene is included in Figure 2.44 because it is an extreme example of an exo double-bond absorption (see p. 38). The s character in the C=0 group increases as the ring size decreases, until it reaches a maximum value that is found in the 5p-hybridized carbonyl carbon in ketene. The spectrum of cyclopentanone (Fig. 2.41) shows how ring strain increases the frequency of the carbonyl group. 

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