ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
In the light of recent experimental interest in the spectroscopy and photochemistry of matrix isolated oxycarbenes, ab initio calculations have been employed to investigate two prototypical oxycarbenes, CH3COH and CH3OCH, derived from acetaldehyde. Singlet state methylhydroxymethylene, CH3COH, exists as trans and cis isomers lying at 61.8 and 64.2 kcal mol−1 above ground state acetaldehyde. Partial CO double bond character as evidenced by a barrier to internal rotation from trans to cis of 27.3 kcal mol−1 accounts for the existence of geometric isomers. CH3COH (S0) faces a barrier of 27.7 kcal mol−1 to the Woodward–Hoffmann "forbidden'' 1,2-hydrogen shift leading to ground state acetaldehyde. Trans and cis singlet methoxycarbenes, CH3OCH, are 73.6 and 77.3 kcal mol−1 less stable than S0 acetaldehyde. The large trans to cis rotational barrier of 30.4 kcal mol−1 is again evidence for a partial CO double bond. The barrier to the 1,2-methyl shift from trans-methoxycarbene to CH3CHO is 28.3 kcal mol−1. These carbenes, substituted with electron donating groups, OH or OCH3, are singlet stable. The singlet–triplet gaps are 23.0 kcal mol−1 for CH3COH and 21.3 kcal mol−1 for CH3OCH. Triplet CH3COH lies at 84.8 kcal mol−1 relative to ground state acetaldehyde which is 8.0 kcal mol−1 greater than the triplet acetaldehyde origin. The barrier for the triplet state 1,2-hydrogen shift from CH3COH to CH3CHO is calculated to be 33.3 kcal mol−1. Although all these methylene isomers lie high in energy in comparison with acetaldehyde, the transition state for the rearrangement to singlet CH3COH is lower than the energies of ∼95 kcal mol−1 employed in many gas phase studies of acetaldehyde photochemistry. It should be possible toisolate the trans- and cis-CH3COH species in an inert matrix following acetaldehyde photolysis and to characterize these methylenes by their vibrational spectra as predicted herein. Ion cyclotron double resonance experiments are suggested that could afford an experimental measure of the relative thermochemical stability of CH3COH with respect to CH3CHO which is calculated to be 61.8 kcal mol−1.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.450919
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