The mass spectra1 fragmentation of methyl esters of a$-and p,y-unsaturated carboxylic acids has been studied. Deuterium labelling, metastable ion analysis and high resolution mass spectrometry have been utilised to elucidate the mechanism of a number of fragmentations and to check possible double bond migration prior to fragmentation. Some breakdown modes were found to occur through double bond migration. Additional support was provided by application of the 'Metastable Ion Characteristics' method. Partial isomerisation of molecular ions must therefore be accepted.
I N T R O D U C T I O NALTHOUGH a few reports have been published on the mass spectra of acyclic unsaturated esters,l to 5 no detailed investigation of this class of compounds has appeared in the literature. In recent publication^^*^*^ the fragmentations of some c,i.s-and trans isomers were compared. The behaviour of p,y-unsaturated esters does not appear to have been discussed. It is known from literature that some double bond position isomers exhibit similar mass s p e~t r a .~* l~ This is rationalised by a preIiminary migration of the double bond to a common intermediate. Double bond migration for q h n s a t u r a t e d esters has already been proposed to explain their b e h a v i o~r .~.~ From the results of the present study, it appears that the c(,/l-and ,!?,y-unsaturated esters do isomerise to only a small extent. The mass spectra of a$-unsaturated esters and the ,!?,y-isomers show only small qualitative but strong quantitative differences. (I) R, = R, = H; R8 = Et (11) R1 = R, = H; R, = n.Pr (111) R, = R, = H; R, = i.Pr (IV) R, = R, = H; Rs = n.Bu WI) Rl = H; R, = R3 = Me (VII) R, = H; Rz = Me: R, = Et (VIII) R, = H; R, = Me; R3 = n.Pr (IX) R, = H; R, = Me; R3 = n.Bu (XI) R1 = Me; R, = H; R3 = n.Pr (V) R, = R2 = H; R3 = t.Bu (X) R, = H: Rz = R, = Et (XI) R, = R2 = H; R, = Me (XIII) R, = R2 = H; R3 = Et (XIV) R, = Rz = H; R, = n.Pr (XV) Rl = H; R2 = Me; R, = Et (XVI) Rl = H; R, = Et; R, = Me (XVII) R1 = Me; R, = H; R, = Et