MASS SPECTRAL
FRAGMENTATION O F 1,3-BUTADIESE
1409
The Mass Spectral Fragmentation of 1,%Butadiene and
1,3-Butadiene-l,l,4,4-d4
by A. Bruce King Gulf Research & Deaelopment Company, Pittsburgh, Pennsulvania
(Received D e c e m b e r ik, 1963)
The mass spectral patterns of 1,3-butadiene and 1,3-butadiene-1,1,4,4-d4 are compared. The appearance potentials of the principal ions and the metastable transitions that are observed are reported. Complete equilibration of the hydrogen atoms in the partially deuterated co;mpound takes pla,ce before fragmentation occurs. It is suggested that this may arise from isomerization to 1,2-butadiene. A fragmentation mechanism is proposed, and the principal features are discussed.
Introduction I n recent years, the mass spectra of several lower molecular weight hydrocarbons have been investigated using partially deuterated compounds. These studies denionstrate the extent of exchange between hydrogen atoms on the different skeletal atoms prior to skeletal fragmentation. I n the simple paraffins, the degree of exchange is generally not large, as exemplified by the reported spectra of partially deuterated ethane, l , * p r ~ p a n e , l , ~n-butane,'~~and isobutane.'~~As suggested by Stevenson and Wagner,l the olefinic compounds exhibit a higher degree of exchange before the skeletal break occurs. The point is illustrated by the exchange observed in the ionic fragment formed by loss of a methyl radical from partially deuterated propylene6 and 1-butene.' The purpose of this paper is to supplement the existing information on hydrogen atom exchange prior to fragmentation by presenting similar data obtained in our laboratory for l13-butadiene-l,l,4.4-d4. Experimental The mass spectra and appearance potential measurements were made on a C.E.C. 21-103C equipped with a room temperature inlet system. The ion source was maintained a t 250'. Appearance potential measurements were made using krypton as an internal standard. The ion intensity scales of the ionization efficiency curves of the ion under study and the calibrating ion were adjusted to make the slopes of the linear regions equal. The ionization efficiencies were then plotted
on a semilog scale and the ionization potential determined from the displacement of the curves.8 For the fragment ions the curves were not identical in shape; and hence, the niiniinum threshold was estimated from the displacement of the interpolated semilog plots of the ionization efficiencies at ion currents slightly above the noise level. This procedure essentially gives results equivalent to the vanishing current method. Positive identification of some of the minor metastable transitions was made possible with spectra obtained on a C.E.C. 21-102 (modified) that was equipped with a metastable suppressor t o give comparison spectra with and without the metastable contributions. The 1,3-butadiene used was Phillips research grade. The 1,3-butadiene-l,1 ,4,4-d4was prepared by Dr. Dale E. Van Sickle of Stanford Research Institute by repetitive exchange of 2,4-dihydrothiophene 1,l-dioxide with deuterium oxide following the procedure of Cope, et aL9 The low voltage mass spectrum indicated the (1) D. P. Stevenson and C. D. Wagner, J . Chem. Phys., 19, 11 (1951). (2) D. 0. Schissler, S. 0. Thompson, and J. Turkevitch, Discussions Faraday SOC., 10, 46 (1951). (3) F. E. Condon, J . Am. Chem. Soc., 73,4675 (1951). (4) F. E. Condon, H. L. -McMurray, and V. Thornton, .J. Chem. Phys., 19, 1010 (1951). (5) W. H. McFadden and A. L. Wahrhaftig, J . Am. Chem. Soc., 78, 1572 (1956). (6) W. H. McFadden, J . Phys. Chem., 67, 1077 (1963). (7) W. A. Bryce and P. Kebarle, Can. J . Chem., 34, 1249 (1956). (8) J. W. Warren, Nature, 165,810 (1950). (9) 4.C. Cope, G. A. Berchtold. and D. L. Ross, J . Am. Chem. Soc., 83, 3849 (1961).
Volume 68, Number 6
June, 1964
1410
,4.BRUCEKING
Table I : Fragmentation Patterns of 1,3-Butadiene
Table I1 : Metastable Transitions
and 1,3-Butadiene-1,1,4,4-& a t 70 e.v. Total ionization (Cia cor.), %--C Hac HC HCH? CDxCHCHCDz
7--
mle
2 3 4
0.100 0.006
0.501 0.040 0,023
12 13 14 14 5 15 16 17 18 19 19 5 20
0.591 0.550 0.817
0.495 0.116 0.523 0.012 0.154 1.037 0.391 0.109 0.004 0,019 0.009
24 25 25 26 26 27 27 28 28 29 29 30 31 32 36 37 38 39 40 41 42 48 40
50 51 52 53 54 55 56 57
