Ion—Molecule Reactions by a Photoionization Mass Spectrometer. I. Propylene and 1,3-Butadiene

Abstract

Ion—molecule reactions in propylene and 1,3-butadiene have been studied using a mass spectrometer which utilizes photoionization as an ion source instead of electron impact. The light source used was a low-pressure hydrogen-discharge lamp with a LiF window (substantially Lyman alpha, 1216 Å). It has been shown that this method has two major advantages in the study of ion—molecule reactions compared with the electron-impact method; one is the lack of unimolecular fragmentation of the parent ion and the other the absence of hot filament, which enables one to get high pressure without suffering pyrolysis. Five reactions of C3H6+ with C3H6 and eight reactions of C4H6+ with C4H6 have been identified and the cross sections determined for these reactions. In propylene, the reaction leading to the formation of C3H7+ was found to be of the largest cross section and, although cross section was small, the reaction forming C3H3+ was found as a new reaction. It was shown that C3H6+ could not be the precursor of secondary C4H6+ in propylene, criticizing the Fuchs assignment. In 1,3-butadiene, three new reactions have been found; the reaction leading to the formation of C3H3+, the hydrogen-atom abstraction reaction just as in propylene, and the parent-minus-one reaction. The first one was of the largest cross section in all reactions in 1,3-butadiene. This reaction has been discussed in connection with the structure and the heat of formation of C3H3+.

Intracluster Polymerization Reactions of Alkene Cluster Ions

“…The reaction rate was found to fall off dramatically after the third and fourth additions, which was attributed to steric effects. Similar results obtained by various other experimental methods applied to the study of a number of different systems led the investigators to suggest that gas phase cationic polymerization is responsible for the occurrence of the high molecular weight ions. − …”

“…Similar results obtained by various other experimental methods applied to the study of a number of different systems led the investigators to suggest that gas phase cationic polymerization is responsible for the occurrence of the high molecular weight ions. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Meanwhile, considerable effort has been devoted to the investigation of these ion-molecule reactions in the cluster regime [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] as well. This has been prompted by the recognition that interrogating clusters provides a way of studying the energetics and dynamics of intermediate states of matter as cluster systems evolve from the gas toward the condensed state.…”

“…Gas phase ion−molecule reactions involving systems of unsaturated hydrocarbons and their derivatives have been the subject of many investigations, spanning several decades. Many different experimental techniques have been utilized, including photolysis, radiolysis, , high-pressure mass spectrometry, − tandem quadruple MS/MS, and ion cyclotron resonance. − In a high-pressure MS experiment in ethene, , Kebarle and co-workers observed that an increase in the concentration of ethene in the ion source led to large increases in the intensities of intermediate size C 2 n H 4 n + with n = 4−5 and the disappearance of smaller ions with n = 1−3. This effect was explained by invoking the occurrence of association reactions of ethene molecules.…”

Effect of kinetic energy on the ionic reactions in propylene and cyclopropane

“…It might be noted that reaction 2, collision-induced dissociation, is sim-ilar to the reaction responsible for formation of CH3+ in the reaction CH4 + + CH4,16 while reaction 3 involves decomposition of the C3H7+ ion produced in the initial collision process and thus requires that the excess energy be retained in the product ion. The experimental threshold for formation of C3H4+, approximately 20 kcal/mol maximum, is definitely too low to allow reaction 5 as the source of C3H4+; however, reaction 6 C3H6+ + C3H6 -> C3H5 + C3H7+ -> C3H4+ + H2 + H (AH = 129 kcal/mol) (5) c8h6+ + c8h6 -> C8Hs + C3H4+ (AH = 20 kcal/mol) (6) C3H6+ + C8H6 -> C3H6 + C3H4+ + H2 (AH = 55 kcal/mol) (7) is permitted, as is reaction 7 if we allow that some 35 kcal/mol of internal energy may be utilized. The latter appears unlikely and it is probable that reaction 6 is responsible for formation of C8H4+.…”

Mass spectrometry

“…Concurrent ionmolecule reactions of HC1 and HCN with D2 and CD4 have been studied (668). Photoionization mass spectrometers have been employed in investigations of some ion-molecule reactions (921,922,(1543)(1544)(1545).…”