Differentiation of isomeric allylic alkenyl methyl ethers by Raman spectroscopy Original Research Article

The Raman spectra of several pairs of alkenyl methyl ethers of general structure R1R2Cdouble bond; length as m-dashCR5C(R3R4)OCH3 and R1R2C(OCH3)C(R5)double bond; length as m-dashCR3R4 (R1, R2, R3, R4, R5 = H or CnH2n+1, n = 1–3) are reported and discussed, with a view to establishing whether Raman spectroscopy offers a viable means of distinguishing between these isomeric unsaturated species. Key bands associated with the ν(sp2Csingle bondH) and ν(Cdouble bond; length as m-dashC) stretching modes are found to be particularly useful in this connection: R1R2Cdouble bond; length as m-dashCHCH2OCH3 and R1R2C(OCH3)CHdouble bond; length as m-dashCH2 ethers (R1, R2 = CH3, C2H5) are easily distinguished on this basis. Differentiation of their lower homologues, R1CHdouble bond; length as m-dashCHCH2OCH3 and R1CH(OCH3)CHdouble bond; length as m-dashCH2 (R1 = CH3, C2H5, C3H7), by similar means is also quite straightforward, even in cases where cis and trans isomers are possible. Pairs of isomeric ethers, such as CH3CHdouble bond; length as m-dashC(CH3)CH2OCH3 and CH3CH(OCH3)C(CH3)double bond; length as m-dashCH2, in which the structural differences are more subtle, may also be distinguished with care. Deductions based on bands ascribed to the stretching vibrations are usually confirmed by consideration of the signals associated with the corresponding δ(sp2Csingle bondH) deformation vibrations. Even C2H5CHdouble bond; length as m-dashCHCH(C3H7)OCH3 and C3H7CHdouble bond; length as m-dashCHCH(C2H5)OCH3 are found to have distinctive Raman spectra, but differentiation of these closely related isomers requires additional consideration of the low wavenumber region.