Steric Effects in the Electronic Spectra of Organic Compounds

Braude, E. A.; Sondheimer, Franz; Forbes, W. F.

doi:10.1038/173117a0

Cited by 85 publications

(30 citation statements)

References 6 publications

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“…Benzoic acid absorbs maximally with smaller intensity of absorption than acetophenone and this may again be ascribed to steric inhibition of resonance i n the excited state due to the orthohydrogen atoms. Both acetophenone and benzoic acid from dipole moment and X-ray crystallographic d a t a (both of which measure grouncl state contributions only) appear to be planar, or nearly planar, in the ground state (4,5,14), which corroborates the postulated hindrance in the excited state. T h e steric hindrance in benzoic acid would therefore be expected to give rise to recluced absorption intensity and a slight hypsochromic shift.…”

Section: Meta-substituted Benzoic Acidssupporting

confidence: 60%

“…Ungnade argues that although the primary band of o-methoxyacetanilide is somewhat less intense than that of the meta-isomer, the order of intensities is reversed for the secondary bands and the wave-lengths of corresponding bancls in the ortho-and meta-isomers are virtually identical. In our view, the ortho-compound exhibits a typical steric effect of a type which has been discussed fully elsewhere (3, 4,8), in which the location of maximal absorption remains approximately the same, while the intensity of absorption is reduced. This eflect becomes readily apparent if the ortho-compound is compared with the corresponding para-compound rather than with the meta-compound.…”

Section: Acetanilidesmentioning

confidence: 85%

“…This eflect becomes readily apparent if the ortho-compound is compared with the corresponding para-compound rather than with the meta-compound. This seems justified, since it has been shown in the study of acetophenones (4,8) that the former comparison has greater validity. We would propose a similar steric effect for the anisidine spectra referred to b~7 Ungnade (13).…”

Section: Acetanilidesmentioning

confidence: 99%

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Light Absorption Studies: Part Ii. Ultraviolet Absorption Spectra of Substituted Benzoic Acids and Phenyl Benzoates

Forbes¹,

Sheratte²

1955

Can. J. Chem.

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The spectra of a number of substituted benzoic acids, acetanilides, and phenyl benzoates are discussed in terms of the electronic and steric effects of substituerrts. INTRODUCTIONThe absorption band observed in the 230 mp region of the absorption spectrum, and designated by iMoser and Icohlenberg (12) as the B-band, is of interest in the orfho-substituted benzoic acids, since the usual explanation of the steric effect due to an ortho-substituent must be modified in order to account for the observed spectra.The for~nula of benzoic acid may be written in two forms, one possessing an intact carboxyl group (type IA), the other a hydrogen-bridged structure (type IB). On steric considerations alone, neither form accounts satisfactorily for all of the ortho-substituted benzoic acid spectra (see Table I). For example, supposing o-toluic acid exists as structure IA (R = Me), no unusual loss of intensity should be apparent since a similar coilformation may be shown to be relatively insensitive to steric eflects involving excited states only in o-methylacetophenone (8), where the OH group of IA is replaced b l~ a methyl group. In o-toluic acicl, however, there is observed a pronounced l~ypsocl~romic shift accompanied by almost complete loss of intensity of absorption as coinpared to p-toluic acicl. Similarly, if IB (R = Me) alone were the correct configuration, and one ortho-methyl substit~ient mas to produce a twist in the carbon-carbon linkage sufficient to accommoclate the substituent, a second methyl substituent in the other ortho-position would not be expected to bring about much further change in the observed spectrum. However, in 2,G-cliinetl~ylbei~zoic acid the band in this region has disappeared complete1 y (12).Hence it may be concl~~cled that the observed changes in the B-band of the spectra of substituted benzoic acids are primarily due to the electrical effects of substituents, which is not unexpected since an isolated C=O group is known lManuscript received Augz~st 9,1955. Contribution from the

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Section: Meta-substituted Benzoic Acidssupporting

confidence: 60%

Section: Acetanilidesmentioning

confidence: 85%

Section: Acetanilidesmentioning

confidence: 99%

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Light Absorption Studies: Part Ii. Ultraviolet Absorption Spectra of Substituted Benzoic Acids and Phenyl Benzoates

Forbes¹,

Sheratte²

1955

Can. J. Chem.

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“…Other reports of UV (Adembri, Sarti-Fantoni & Belgodere, 1966) or IR (Zverev, Stolyarov, Yakupova & Kitaev, 1975;Braude, Sondheimer & Forbes, 1954) determinations on similar compounds are in accord with the above arguments. It is also interesting that tetracyanoethylene shows better complexing properties than chloranil (Table 1): this is not surprising owing, other things being equal, to the higher electron affinity of the former (2.2 eV) with respect to the latter acceptor (1.7 eV) (Briegleb, 1964).…”

Section: Resultssupporting

confidence: 76%

Charge-transfer complexes of hydrazones. VI. Structures of six hydrazone derivatives. Infrared and structural evidence for substituent effects on charge-transfer interactions

Tosi¹,

Cardellini²,

Bocelli³

1988

Acta Crystallogr Sect B

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The problems connected with the formation of chargetransfer complexes were examined by analysis of the IR spectra of 21 phenylhydrazones and determination of the crystal structures of six of them at room temperature with CuKt~ radiation (2= 1.5418A). Benzaldehyde methyl(phenyl)hydrazone (compound 2): was 0.069 for 709 observed reflections. In phenylhydrazones the possibility of obtaining a chargetransfer interaction is related to the absence of even a slight amount of strain in the hydrazono group, which must be perfectly planar to allow conjugation of the group. Mesomeric and inductive effects of substituents contribute only to the stability and the amount of interaction.

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“…One of the aims of studying the steric effects in ultraviolet light absorption data is to seek an explanation for steric effects of type I as defined previously. One explanation for this phenomenon has been provided by assuming electronic transitions between a non-planar ground state and a near-planar excited electronic state (3,5).…”

Section: B (S-trans)mentioning

confidence: 99%

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

Forbes¹,

Mueller²

1956

Can. J. Chem.

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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

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Steric Effects in the Electronic Spectra of Organic Compounds

Cited by 85 publications

References 6 publications

Light Absorption Studies: Part Ii. Ultraviolet Absorption Spectra of Substituted Benzoic Acids and Phenyl Benzoates

Light Absorption Studies: Part Ii. Ultraviolet Absorption Spectra of Substituted Benzoic Acids and Phenyl Benzoates

Charge-transfer complexes of hydrazones. VI. Structures of six hydrazone derivatives. Infrared and structural evidence for substituent effects on charge-transfer interactions

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

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