Reactive collisions of polyatomic ions at solid surfaces

International Journal of Mass Spectrometry

and Zon Processes, 82 (1988) 131-150

131

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

REACTIW AT SOLID

COLLISIONS SURFACES

OF POLYATOMIC

IONS

T. AST Faculty of Technology and Metallurgy,

University of Belgrade, Belgrade (Yugoslavia)

Md.A. MABUD and R.G. COOKS * Department of Chemistry, Purdue University, West Lafayette, IN 47907 (U.S.A.)

(First received 20 July 1987; in final form 8 September 1987)

ABSTRACT Reactive ion/surface collisions are shown to represent a common phenomenon when polyatomic ions of 20-60 eV energy collide with metal surfaces. Abstraction of one hydrogen atom is the usual reaction, although pick-up of up to four hydrogen atoms and/or a methyl group was observed in some cases. With very few exceptions, only odd-electron ions undergo reactive ion/surface collisions, in line with the thermochemical advantage of their acquisition of the more stable even-electron configuration. Ions generated from organonitriles and heteroaromatic nitrogen compounds were most effective in hydrogen abstraction from the surface. It is suggested that hydrocarbon adsorbates on the surface provide the source of hydrogen; the essentially passive role of the metal itself is deduced from experiments in which spectra obtained on stainless steel, Pt, and Ag surfaces are compared. Experiments are also described in which the collision energy, nature of the surface adsorbate, and surface temperature are varied. INTRODUCTION

A hybrid BQ mass spectrometer was recently modified in order to allow studies on low-energy (eV range) collisions of polyatomic ions with solid surfaces [l]. This instrument permits definition of (i) the mass-to-charge ratio of ions in both the incident and emergent beams, (ii) the collision energy, (iii) the collision angle, and, to some degree, (iv) the nature of the target. The ability to scan both mass analyzers allows the results of surfaceinduced dissociation (SID) and other ion/surface processes to be recorded in the form of daughter spectra, parent spectra, and neutral loss spectra. In addition to surface-induced dissociation rn:%rn:’

+ rni + m3

* To whom correspondence 0168-1176/88/$03.50

should be addressed. 0 1988 Elsevier Science Publishers B.V.

132

and reflection (elastic scattering) [2] m,+ %rn: a number of charge-changing processes are observed upon collision of 20-100 eV polyatomic ions with metal surfaces. These include neutralization [31 m,+ %m, dissociative charge exchange [4] ml

2+

s -)m,+* + ml + m3

and chemical sputtering, adsorbed species [ 31

i.e. charge exchange accompanied

by release of

m+ s(A)>A+* + A+1 ,A;,A;... 1 Most recently, surface-induced place [6] m:-%m;*

in a modified triple quadrupole mass spectrometer [5], charge inversion reactions have also been shown to take

+ my + m3

Surface-induced dissociation is by far the most studied of the polyatomic ion/surface collision processes. This reaction has been used to differentiate between isomeric ion structures [7-91 and to rationalize fragmentation mechanisms [6,10-121. Energy deposition in SID reactions has been found to vary systematically with the collision energy. The distribution of internal energy is not broad and high average energies are accessible [13,14]. During the course of our SID studies, an additional phenomenon has been observed: under the normal experimental conditions employed, some ions undergo reactive collisions. Frequently, this involves abstraction of a hydrogen atom from the surface; the product ion may be observed as such or it can dissociate to yield characteristic fragment ions m+ -% (M + H) + * + fragments The (M + H)+ ion is formed with great efficiency in some cases; indeed, it is more abundant than fragments due to SID in certain spectra. On the other hand,‘it is present in moderate or small abundance for some of the ions studied and is absent altogether in others. In a limited number of cases, ion/surface reactions result in abstraction of up to four hydrogen atoms. In

133

other experiments, abstraction of a methyl group has also been observed. Hydrogen abstraction is a common ion/molecule reaction in the gas phase. Since the early work of Stevenson [15], who examined some of the fundamental aspects of these types of process, a large number of studies, both theoretical and experimental, have dealt with this subject. However, the observation that polyatomic organic ions can undergo a reactive collision at a surface in the tens of eV energy range is both novel and, at first sight, remarkable. In this paper, we report results obtained for collisions of a variety of different organic ions; the influence of a number of experimental parameters is also examined. EXPERIMENTAL

The experiments employed a BQ mass spectrometer [1,2] which was modified by replacing the collision quadrupole of a BQQ mass spectrometer [16] with a solid surface and repositioning the mass analyzing quadrupole at 90 o with respect to the incident beam. The method used to acquire daughter spectra due to ion/surface interactions has been described earlier [l]. Ions were generated by 70 eV electron impact in a source held at a potential of 20-60 V with respect to ground. The ion beam was accelerated to 6 keV prior to mass analysis by the magnetic sector and decelerated to ground potential prior to collision at a metal surface. The collision energy is thus defined by the potential difference between the source and the target surface. Lenses used for extraction of the emerging beam and its subsequent transfer into the quadrupole were adjusted to maximize the total ion current reaching the final detector. Mass analysis of the emerging beam was achieved using a quadrupole mass filter. All data were taken at a nominal incident angle of 25 o (estimated uncertainty f 3” ) with respect to the surface normal. The angle of deviation was 121” from the original beam direction. The pressure in the main chamber containing the target surface was maintained at 5.0 x lo-’ torr. At this pressure, the target surface is assumed to carry adsorbed gases. Heating of the surface was accomplished by a small cartridge heater mounted on the back of the surface holder. Samples were obtained commercially and used without purification. Data acquisition was carried out using a custom-built acquisition system [17]. Spectra obtained on different occasions were usually highly reproducible, in spite of the fact that experimental parameters, collision energy in particular, exert significant influence on the appearance of the spectra. RESULTS

AND DISCUSSION

A large number of organic ions have been examined as to their tendency to undergo ion/surface reactions under standardized experimental condi-

134 TABLE 1 Daughter spectra of some hydrocarbon ions obtained at 25 eV collision energy qb Ion

Precursor

Mf’

(2-G

Hexane Hexane Hexane Benzene

27 28 29 38

C,H:’ GH: C,H;’

(100) (50) (25) (21)

(M+H)+

Six most abundant fragment ions d

39 (15)

26 27 27 50

(32), 15 (11) (XI), 26 (31) (100) (NO), 51 (25), 27(18),

CsH:

Benzene

39 (100)

C,H:’

Cyclopropane Cyclopropane Hexane

40 (76) 41 (32) 42 (21) 43 (6) 50 (100) 51(100) 52 (100)

GH:

Hexane Benzene Benzene Benzene Hexane

C,H,+’

Hexane

56 (4)

C,H,+

Hexane

57 (1)

15 41 55 29

C,H: GH:

Anisole Cyclopentadiene Cyclopentadiene Methylenenorbomane Cyclohexane

63 (100)

55 (1) 62 (15), 37 (8)

C,H: C,H;’ C,H: C,H;’ GH: C,H:’

C,H6+’ GH: C,H;

49 (4) 43 (“)

51(44) 53 (15)

55 (8)

66 (6)

39 (w,

66 (100)

67 (6)

65 (24), 40 (24), 41 (ll), 51 (6) 39 (6), 27 (1) 41 (77), 65 (8) 39 (6)

69 (6)

41(100), 77 (4)

78 (8)

79v)

79 (100)

GH:;

Methylenenorbomane Methylenenorbomane Cyclohexane

GH:,’

Hexane

C,H:

Toluene

GH: C,H,C’

27 (4) 40 (1)

67 (100)

76 (41) 77 (37)

C,H: C,H,+’

(3) (lOO), 29 (18), 28 (14), 39 (lo), (4) 27 (3) (NW), 41 (35), 39 (7), 27 (6)

65 (46)

1,5-Hexadiyne 1,5-Hexadiyne 1,5-Hexadiyne

C,H:’

62 (ll),

63 (8), 74 (8) 38 (6), 57 t4), 27 (3), 50 (2), 52 (2) 39 (lOO), 15 (34), 27 (7) 39 (lOO), 15 (85) 27 (7) 27 (NW), 41(41), 40 (24), 15 (15), 39 (14), 16 (4) 27 (loo), 41(21), 15 (13), 39 (3) 39 (20), 49 (75) 50 (51), 39 (3), 27 (3) 51(61), 26 (42), 50 (32), 27 (31) 29 (UN), 27 (62), 39 (21), 53 (7),

80 (70)

91 (86)

39 50 51 52 50

29 (20), 53 (3), 27 (3),

(1X 43 (1) (100) 51 (28) 27 (6) (100) 27 (23) 39 (4) 41 (1) (NO), 39 (55) 77 (32) 51 (28) (17), 63 (13)

77 (94) 53 (4) 51 (l), 39 (1X 81(4)

79 49 69 42 43 44 65 63

(lOO), 78 (ll), (4) 77 (1) (loo), 41 (96) (28), 43 (24) (100) 29 (39) (ll), 42 (6) (UN), 41 (62) (lo), 27 (1)

65 (7), 52 (4), 56 (68), 55 (34) 41 (28) 57 (21) 39 (34) 51 (18),

135 TABLE 1 (continued) Ion C,Hs+’

Precursor Toluene

M+’ 92 (61)

(M+H)+

Six most abundant

93 (3)

91 (lOO), 65 (18), 77 (14) 64 (lo), 52 (g), 41(g)

fragment ions d

a Numbers in the table refer to m/z values followed by relative abundances given in brackets (base peak = 100). b Values printed in italics are due to fragmentation of the charge-exchanged ion of twice the mass. ’ Trace. d When fewer than 6 ions are listed, no other ions above 1% relative abundance were observed.

of 121° scattering angle and 25 eV collision energy. A representative selection of their daughter spectra is shown in Table 1 (hydrocarbon ions), Table 2 (nitrogen-containing ions), Table 3 (oxygen-containing ions), and Table 4 (sulfur-containing ions). As an example of an ion that efficiently undergoes reactive collisions, the daughter spectrum of pyrazine molecular ion (m/z SO), colliding at 25 eV on a platinum surface, is shown in Fig. 1. The reflected molecular ion appears in moderate abundance only; the base peak in the spectrum is due to the reaction. The (M+H) + ion (m/z Sl), the product of an ion/surface fragmentation of the molecular ion of pyrazine, as established by low-energy collision activated dissociation (CAD) using a triple quadrupole, proceeds via loss of HCN to give a peak at m/z 53 and further loss of HCN giving rise to m/z 26. Thus, the additional peaks in the SID spectrum in Fig. 1 are due to the fragmentation of the (M + H)+ ion: loss of HCN gives m/z 54 and further losses of C,H, and HCN yield peaks at m/z 28 and 27, respectively. This assumption was confirmed by CAD of the protonated molecular ion of pyrazine generated in a chemical ionization source. One must consider three possible sources of hydrogen for the hydrogen atom abstraction reaction: (1) residual gas in the analyzer could donate hydrogen in an ion/molecule reaction, (ii) pyrazine itself might act as the hydrogen source, in a process of self-hydrogenation (e.g. with pyrazine molecules neutralized and adsorbed at the surface), and (iii) various hydrogen-containing adsorbates on the surface (such as residual hydrocarbons from pump oil) could be the source. To test the first possibility, the voltage applied to the extraction lenses was raised slightly, diverting the ion beam so that it passed close to but did not collide with the surface. The current read at the picoammeter connected to the surface dropped to zero, indicating the absence of any ion/surface interactions. The spectrum obtained under these conditions showed only the molecular ion, thus ruling out the possibility of tions

146

corresponding hydrogen abstraction; a large positive value for aniline is in agreement with the very small (M + H)+ peak observed. The data for cyclic and acyclic oxygen-containing ions show similar agreement. Note that absolute values are uncertain because the enthalpies of the adsorbed species are not known; however, the trends reproduce the experimental behavior. Multiple hydrogen atom and methyl abstraction

Although simple hydrogen atom abstraction from the surface is the common ion/surface reaction for the majority of polyatomic ions examined, in a number of ions derived from simple nitriles, abstraction of up to four hydrogen atoms were observed. Figure 5 shows the daughter spectrum of the C,HN+‘ion (m/z 39) derived from acetonitriIe obtained by collision with a stainless steel surface at 25 eV. The hydrogen abstraction peak at m/z 40 is the base peak, but an (M + 2I-I)+’ ion appears with nearly the same abundance. In addition, smaller peaks are observed at m/z 42 and 43, representing processes in which three and four hydrogen atoms have been abstracted by the incident ion. These processes are further reflected in the fragmentation pattern. Thus, for example, the peak at m/z 29 is most probably due to the loss of 14 mass units from m/z 43. Similar behavior was observed for other nitrile ions derived from acetonitrile: C2H2N+ (m/z 40) picks up one, two, and three hydrogen atoms (although it is an even-electron species) and the molecular ion C,H,N+’ (m/z 41) abstracts both one and two hydrogen atoms. It is interesting to note that the maximum number of hydrogen atoms abstracted by all three ions derived from acetonitrile is that needed to form C,H,N+’ (m/z 43), which is an

40

IOO2’

39w+9

14

41

6 -a 26 iz B s

50-

.

15

9 26 ? ‘F 0 z

. .

26

I3

36

42 43

52

CL

IO

20

30

40

50

60

m/z Fig. 5. Surface-induced daughter spectrum of 25 eV C2HN+’ ions derived from acetonitrile (stainless steel surface).

147

odd-electron ion and thus an unexpected common product. The tendency of nitrile ions readily to accept hydrogen atoms might be rationalized on the basis of their highly unsaturated character. Indeed, it is a well-known fact that the electron impact mass spectra of aliphatic nitriles exhibit an (M + 1)’ peak due to ion/molecule reactions even at low ion source pressures [21-23). Martin and Melton 1241 have found that HCN+’ and CH,CN+’ readily abstract hydrogen or deuterium from H,, D,, or CH,. The spectrum in Fig. 5 also shows a peak of moderate abundance at m/z 52. The only plausible explanation of its origin is an ion/surface reaction in which the incident m/z 39 ion picks up a methyl group from the surface followed by loss of two hydrogen atoms. The molecular ion of acetonitrile (m/z 41) exhibits the same reaction: a peak at m/z 54 is observed in this case, although of much smaller abundance. In the spectrum of CH3N+’ (m/z 29), a weak signal was observed at m/z 42, again suggesting similar behavior. However, the even-electron m/z 40 ion derived from acetonitrile behaves somewhat differently, showing two small peaks at m/z 52 and 54. While C, group abstraction still seems to be involved, the actual route by which these ions are formed is not clear. The molecular ion of benzene (m/z 78) also undergoes a C, abstraction reaction: a peak at m/z 91 is observed in this case, apparently due to methyl abstraction followed by loss of two hydrogen atoms (Fig. 5). In this case, the assignment is substantiated by the presence of m/z 55 peak in the daughter spectrum of C,Hz’. This peak is absent in the mass spectrum of benzene, but is known to represent the favored fragmentation of the C,Hq

SURRCE 0J -

76tdj

w

1+

+ 91

/

39

27

IO

20

R 30

40

I 50

60

70

60

90

m/z

Fig. 6. Surface-induced daughter spectrum of 35 eV benzene molecular ions (stainless steel surface).

148

ion (loss of C,H,). Another case of a methyl group abstraction followed by H, loss has been encountered in the surface-induced daughter spectrum of C,H,F+’ (m/z 46), which exhibits a peak at m/z 59 attributed to this process [3]. The spectra of pyrazine (Fig. 1) and d,-pyrazine (Fig. 2) molecular ions show peaks at m/z values of 95 and 99, respectively, due to methyl radical abstraction. In these cases, the (M + CH,)+ ion did not undergo subsequent hydrogen loss. Doubly charged ions

Hydrogen abstraction reactions have also been observed in studies of charge exchange of doubly charged ions at metal surfaces [4]. For example, when doubly charged C,Hi’ ions (m/z 45) derived from toluene are collided with the surface, a daughter spectrum is obtained that contains only singly charged ions. While no peak occurs at m/z 90, a peak does appear at m/z 91, pointing to the fact that C,Hz’ is formed by charge exchange and then undergoes a hydrogen abstraction reaction. (The alternative, hydrogen abstraction followed by charge exchange is unlikely given the rate of electron transfer.) The products of fragmentation are also observed in the spectrum. This is a remarkable result since charge exchange is a highly exothermic process, the recombination energies of doubly charged ions being significantly greater than typical hydrocarbon adsorbate ionization energies. In spite of the fact that the charge-exchanged ion is generated with considerable internal energy, it still undergoes hydrogen atom abstraction in what must be a rapid process compared with dissociation. As expected, this is followed by extensive fragmentation. CONCLUSION

Reactive polyatomic ion/surface collisions are shown to represent a general phenomenon. The most common reaction is hydrogen atom abstraction from the metal surface covered with hydrocarbon adsorbate. The reaction occurs for n-hexane but not for water adsorbate. (M + H)+ ions thus generated exhibit varying stabilities; products of their fragmentation are usually also observed in the daughter spectra. With a few exceptions, only odd-electron ions undergo hydrogen abstraction, in line with their tendency to acquire a more stable even-electron configuration. The recent report [15] that hydrogen abstraction is also observed during surface collisions in a modified triple quadrupole instrument points to the fact that it does, indeed, represent a general type of ion/surface interaction. Occurrence of ion/surface reactions provides an extra dimension to SID spectra. Depending on the particular application, this may constitute either

149

an advantage or a disadvantage. In problems of isomer differentiation, the extra information, provided by the presence of these additional peaks in the SID spectra, can be viewed as a distinct advantage. Indeed, in a number of such studies [7-91, peaks due to the (M + H)+ ion and its fragments were crucial distinguishing features. On the other hand, if solving a particular ion structure is the goal of investigation, the mixture of peaks due to both M+‘and (M + H)+ ions may complicate the analysis. However, as the mechanisms of ion/surface reactions become better understood, and it becomes possible to control the nature of the adsorbate, these reactions may take on increased importance in studies aimed at characterizing adsorbates or ion structures. Clean metal surfaces, available through ultrahigh vacuum techniques, might be found to eliminate reactive ion/surface collisions. ACKNOWLEDGEMENTS

This work was supported by NSF grant CHE 8408258 and by the U.S.-Yugoslav Joint Fund for Scientific and Technological Cooperation under grant NST 514 and by the Science Research Fund of Serbia. REFERENCES 1 Md.A. Mabud, M.J. DeKrey and R.G. Cooks, Int. J. Mass S.-r worn. Ion Processes, 67 (1985) 285. 2 M.J. DeKrey, Md.A. Mabud, R.G. Cooks and J.E.P. Syka, Int. J. Mass Spectrom. Ion Processes, 67 (1985) 295. 3 M. Vincenti and R.G. Cooks, Org. Mass Spectrom., submitted for publication. 4 Md.A. Mabud, M.J. DeKrey, R.G. Cooks and T. Ast, Int. J. Mass Spectrom. Ion Processes, 69 (1986) 277. 5 M.E. Bier, J.W. Amy, R.G. Cooks, J.E.P. Syka, P. Ceja and G. Stafford, Int. J. Mass Spectrom. Ion Processes, 77 (1987) 31. 6 M.E. Bier, M. Vincenti and R.G. Cooks, submitted for publication. 7 Md.A. Mabud, T. Ast and R.G. Cooks, Org. Mass Spectrom., 22 (1987) 418. 8 M. Hayward, Md.A. Mabud and R.G. Cooks, J. Am. Chem. Sot., in press. 9 Md.A. Mabud, T. Ast, S. Verma, X.-Y. Jiang and R.G. Cooks, J. Am. Chem. Sot., in press. 10 P. Vainiotalo, H.I. Kenttamaa, Md.A. Mabud, J.R. O’Lear and R.G. Cooks, J. Am. Chem. Sot., 109 (1987) 3187. 11 R.G. Cooks, Md.A. Mabud, Y.-X. Jiang, C. Paradisi and P. Traldi, submitted for publication. 12 M. Vincenti, S.R. Horning and R.G. Cooks, Org. Mass Spectrom., in press. 13 M.J. DeKrey, H.I. Kenttlimaa, V.H. Wysocki and R.G. Cooks, Org. Mass Spectrom., 21 (1986) 193. 14 V.H. Wysocki, H.I. Kenttarnaa and R.G. Cooks, Int. J. Mass Spectrom. Ion Processes, 75 (1987) 181.

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15 D.P. Stevenson, J. Phys. Chem., 61 (1957) 1453. 16 G.L. Glish, S.A. McLuckey, T.Y. Ridley and R.G. Cooks, Int. J. Mass Spectrom. Ion Phys., 41 (1982) 157. 17 A.E. Schoen, Ph.D. Thesis, Purdue University, 1981. 18 M.P. D’Evelyn and R.J. Madix, Surf. Sci. Rep., 3 (1984) 413. 19 H. Ehrhardt and 0. Osberghaus, Z. Naturforsch. Teil A, 15 (1960) 575. 20 H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6 (1977). 21 J.H. Beynon, Mass Spectrometry and Its Applications to Organic Chemistry, Elsevier, Amsterdam, 1960, p. 404. 22 F.W. McLafferty, Anal. Chem., 34 (1962) 26. 23 R. Bengelmans, D.H. WiIliams, H. Budzikiewicz and C. Djerassi, J. Am. Chem. Sot., 86 (1964) 1386. 24 T.W. Martin and C.E. Melton, J. Chem. Phys., 32 (1960) 700.