The non-planar vibrations of some substituted ethylenes and related compounds

By applying a general normal co-ordinate treatment to the non-planar group of vibrations of related series of substituted ethylenes, it has been found possible to obtain consistent sets of force constants for the non-planar bending motions, and to correlate their variation with the observed shifts in the group frequencies and with the electro-negativities and electrondonating powers of the substituents. From calculations on twelve compounds, a linear relation between the CH2= bending constant, (ƒCH2), and fbe corresponding group frequency (δCH2) is obtained and used to fit a further nine compounds into the series. Substitution by CH3—, CH3.CO.O—, and the halogens leads to a decrease in ƒCH2 compared with its ‘neutral’ value in ethylene; the increment in CH2= CX2 is greater than in CH2=CH.X for all the examples studied, and increases with the electronegativity of the substituent X, being particularly large for fluorine. Substitution by —CN and —CO.O.CH3 leads to positive increments. These results suggest that the main effect operative is the electromeric, (resonance) effect directing aromatic substitution, the powers of conjugation of vinyl and phenyl being known to be approximately equal. CH3—, CH3. CO . O— and the halogens lead to ortho-para-substitution, and —CN and —CO. O . CH3 to meta-substitution. Thus, withdrawal of electrons from the terminal carbon atom of the vinyl group results in an increase in the forces opposing non-planar bending of the = CH2 group. The results can be extended to include related u n satu rated compounds. The relatively high value of ƒCH2 in form aldehyde can be related to the fact that this molecule is a resonance hybrid in which the ionic structure CH2+— O- is of considerable im portance. The low value of ƒCH2 in allene suggests that this is also a resonance hybrid, with contributions from ionic structures CH-2— C+= CH2. In the allyl halides, the resonance effect is inhibited by the CH2 group, and the order of the frequencies is the reverse of that in the vinyl halides. The chloride lies midway between propylene and ethylene, agreeing with the fact that in C6H5. CHCl2 there is no appreciable difference in the reactivity at the ortho-, para- and meta-carbon atoms. The resonance effect cannot operate directly on the odd = C —H bond in CH2—CHX; the bending constant / CH decreases as th e electronegativity of th e sub stitu en t X increases, — CN now lying between CH3 and Cl and the effect again being particularly large for fluorine. The values of ƒCH in acetylene and benzene (non-planar) suggest that there may be an inverse relation between the constants for C— C stretching and for bending of the adjacent C— H bonds.