Infrared spectroscopy and ab initio computation in conformer determination of keto ester and diketo triphenylphosphonium ylides

For stabilized triphenylphosphonium keto ester ylides acyl stretching frequencies of the anti ester group from HF/6-31G(d) calculations fit experimental values with Scale Factor, SF = 0.866, as estimated earlier for the diester ylides, but Scale Factor = 0.834 has to be used to fit data for the syn-keto group and for syn–anti diketo ylides. The DFT functional BLYP/6-31G(d), with the literature Scale Factor = 0.9945, generally gives good fits for both keto ester and diketo ylides. For both methods agreement between observed and predicted frequencies is lowest for the syn–anti di-t-butyl keto ester ylide, although, unlike the situation for diester ylides, one bulky alkyl group does not significantly degrade the fits. Comparison of predicted and observed acyl stretching frequencies is useful in establishing conformations of these stabilized phosphonium ylides.