Light Absorption Studies: Part Iv. The Effects of Sterically Hindered Conformations on the Electronic Spectra (B-Bands) of Conjugated Systems

Abstract

The equation= cos2(B~-02) recently proposed by Braude and co-workers is developed t o account more rigorously for the spectra of unsymmetrical molecules. The results obtained from the revised equation, e/eo = + [ C O~~( B~-B~) + ~o s~( e~-O~) ] , support the interference value of hydrogen obtained independently from spectral data, and suggest a re-examination of the reported theories accounting for the spectra of alicyclic enones and dienones, and for steric effects of type I. INTRODUCTIONIn a recent series of papers, Braude and his collaborators (1, 3, 4, 6) have *developed the equation 6 / 6 0 = cos2(&-02) t o obtain the value of the interplanar angle from spectral data in sterically hindered compounds. The literature cited in one of these papers (I) reflects the uncertainty which currently prevails concerning the correct assignment of interplanar angles in even diphenyl itself, the spectrum of which has been discussed by a large number of workers. A single equation which could be used to give the roughly correct interplanar angle for a variety of conjugated molecules would therefore be of great significance, and the series of papers by Braude and co-workers in fact provide such an equation. We are in almost complete agreement with the proposed equation, but have also independently arrived a t two conclusions, which lead us t o suggest a further development of the above-mentioned equation. These conclusions are that, firstly, our value obtained for the interference value of the hydrogen atom is somewhat larger than that assumed by Braude and co-workers, who accept a value of 0.60 A for hydrogen. Our own value for hydrogen, the justification of which will be reported in a separate communication, is 0.95 A, which suggests larger interplanar angles for certain steric conformations than have hitherto been supposed. Our second conclusion concerns our discussion of the spectra of 2-methylacetophenone and 2-methylpropiophenone (7). in which we have considered 2-methylacetophenone, for example, to exist in two different conformations,* IA and IB, which by analogy with the cyclohexenones (see below), we shall refer to as the s-cis and s-trans forms. We then (7) tentatively concluded that the absorption of 2-methylacetophei~one and similar compounds is due t o transitions involving chiefly the s-cis form (IA). By combining this suggestion with the original formula we are now able to put forward a more complete explanation concerning the mechanism of absorption in sterically hindered conjugated compounds.'Manuscript

Light Absorption Studies: Part Xii. Ultraviolet Absorption Spectra of Benzaldehydes

“…An explanation of these observations may be provided if i t is remembered that steric effects in the B-band are predominantly caused by interactions between vicinal atoms in the electronic excited state (cf. 2,16,17), and consequently the observed opposite wavelength displacements may represent a previously discussed type of steric effect in which ground and excited states are displaced in opposite directions compared with a non-hindered reference compou~ld (cf. Fig.…”

“…This hypothesis also correlates with the known lesser susceptibility of benzaldehydes to steric interactions (cf. 1,16). On the other hand, the carbonyl stretching frequency of benzaldehyde lies a t 1709 cm-I (mean integrated absorptioil intensity, A = 2.56 mole-'.…”

Light Absorption Studies: Part V. The Relation of Mesomeric Effects and Ultraviolet Light Absorption Spectra

“…Further there is evidence (cf. 12,13,14) that values of transition probabilities as measured by the absorption intensity (E) are more susceptible to small steric effects, and other secondorder interactions.…”