ION-MOLECULE REACTIOK OF PEXTYL AKD CYCLOPEKTYL HALIDES
August, 1963
1709
THE ENERGY DEPENDENCE FOR REACTION CROSS SECTIONS OF ION-MOLECULE REACTIONS OF SOME PENTYL AND CYCLOPENTYL HALIDES’ B Y ANDR6E J. LORQUET AXD
’\IVILLIAM H.
HAMILL
Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana Received March 27, 1963 Ion-molecule reactions of cyclopentyl chloride, bromide, and iodide, of 3-broniopentane, and of 1-iodopentane have been observed in a mass spectrometer. Primary and secondary ion appearance potentials have been measured. Several of the secondary ion abundance curves exhibit maxima or concealed maxima at electron energies approximating excited state levels of the corresponding primary methyl halide ions. The dependence of reaction cross sections for secondary ions upon repeller field has been correlated with earlier studies.
Introduction Earlier studies of bimolecular processes in a mass spectrometer have provided evidence in support of the following descriptions.2-4 The secondary ion current is is proportional to the primary ion current i,, the concentration N of molecules in the ion source, the distance ,Z from the electron beam to the exit slit, and the energy-dependent reaction cross section u(E)
E’
i, = ipArZe
u(E)dZ
(14
Since the translational energy E of the ion in the electric field is proportional to I, then
. .
ts,‘tPArle=
Q
=
E,-l
u(E)dE
(Ib)
tail; they were not examined more carefully because no secondary ions have similar behavior and high appearance potentials as well. The proposed ion-molecule reactions appear in Table 11. In the case of C5HsC1,two additional secondary ions appeared a t m/e 173+ and 102+. The ion current of the first is quite small, but by analogy with other reactions we postulate CSHsCl+
+ (36H&l---+
(CEHs)zCl+
+ C1
which is the most important reaction type for the other halides. We could not study the second reaction because a t mass 102 there is also a primary peak due to C&H7C1+. A possible mechanism for the formation of secondary CsH7Clf is
The cross section is assumed to be resolvable into contributions from ion-induced dipole forces uLE-l12, and knock-on collisions, C ~ K . u ( ~ == )
-
uK)
+
(IC) The P’s are the corresponding chemical reaction probabilities. Equations I b and IClead to the relat’ion PL(~~LE-’”
PKCK
+
~ P L c ~ L E ~ -U~K’ (~P K- PL) (Id) Above a transitional ion energy .Et for which UK eclipses q E - ‘ l 2we have Q
=
(~PzuLE~ ”PLLTKE~)E~-’ ~ - PRUK (2) and above a further :limit E,, which always leads to decomposition of the secondary ion
Q
=:
Q
=:
+
(PLuIE~’” P K U K E J E ~ - ~( 3 ) Experimental
All substances were purified by gas-liquid chromatography and the purity was established by mass analysis. Operation of the mass spectrometer (CEC 21-103A, modified) has been described . 3 Appearance potentials were determined both by the vanishing current method and also by linear extrapolation since the secondary ion current is frequently very small. The secondary ions were identified by pressure dependence, effect of repeller field, and appea.rance potential. The electron energ>’ scale was calibrated using xenon. In the high mass range it was necessary to use several mass standards, including C3H5Br3, CHBr3, CHI3.
Results Appearance potentials for both secondary and various possible primary ions appear in Table I. The values for ions at masses 40+, 39+, 28+, 27+, 26+, and 15+of C6Hd are not reliable because of a long exponential (1) Supported in part, under AEC Contract 4t(11-1)-38. (2) N. Boelrijk a n d W H. HarniH, J . Am. Chem. Sac., 84, 730 (1962). (3) R. P. Pottie, 4 . J. Lorquet, and W. H. Hamill, ibid., 84, 529 (1962). (4) D. A. Kubose and W. H. Hamill, ibid., 88, 125 (1963).
There is no similar reaction in the case of C5HgBr and C5H91where the peak for C5Hx+ is small. The most important secondary ions from cyclopentyl halides which contain only C and H are C10H17’-, CBH18+, C7Hll+, and CCH9+. All originate from CEH9+ and are of one type, vix., C,H2n-3. Several secondary ion current us. electron energy curves showed maxima, or concealed maxima, a t energies slightly above onset. The more prominent of these appear in Table 111. The displacement of a maximum from the foot of the curve is expressed as AE e.v. Excited slates of the methyl halide ions are included for c ~ m p a r i s o n . ~It should be observed that primary CjHllI+ also exhibits a small maximum at the foot of the ion abundance curve. These results suggest primary ions in two or more states which produce secondary ions with unequal cross sections. Such effects have been reported previously.6 Results for pentyl chloride were well behaved in terms of eq. 1-3, for which the results appear in Tablo IV. In the other cases anomalous results suggested complications arising from space charge. This possibility was tested by measuring Q for CD5+ from CDfL with added pentyl iodides or xenon a t high pressures Effects were marked at very low repeller field but values of Q for CD,+ were normal4 from E, = 1.2 to 7 e.v. Consequently, eq. 1 cannot be applied to these reactions, since its energy range coincides with that of space charge effects. Equations 2 and 3 do appear to describe the results for the secondary ions for which results are summarized in Table V. (5) D C Frost and C A McDoiiell, Proc. Roy Soc (London), A241, 194 (1957) ( 6 ) R. F. Pottie a n d Vi’. H. Hamlll, J . Phya. Chew., 6S, 877 (1959).
ANDREEJ. LORQUET AND WILLIAM H. HAMILL
1710
Vol. 67
TABLE I APPEARANCE POTENTIALS OF PRIMARY AND SECONDARY Ioxs Compound
Cyclopentyl chloride
Primary ion
CsHg(I.1' C5H9 CsHe+ +
d e
Secondary ion
+
+
CaHd3r + CrHllBr + C5Hll'
CbHQBr+ CsHsBr + CsH@+
+
c;"iI' CaHJ + CzHJ + CHzI' HI +
I+ C6Hll'
CzHJ C6H9' C6Hsf C6H7' C4H5 C3H6+ C&+ C3H4 C3Ha C&+ CZH3 +
+
+
'
CzHz
+
CHa +
396+ 324 + 269' 241 +
9.3 11.4 15.1 20.6 10.4 15.3 10.1 9.5 13.4 9.3 11.o
9.1 11 .o 12.6 16.1 10.3 13.7 9.9 9.1 12.4
392+ 322+ 265+ 137+ 109+ 95 + 93 + 91 + 81+
9.2 15.1 10.2 10.2(10.7; 11.65)" 12.5(12.8; 13.85)" 13.5(14.2; 16.3)" 13.3 12.8 18 .O( 19 .O)" 16.2(17.6; 19 .O)" 17.0 20.4 23 .O 21.3 9.3 9.1 9.1 10.4 12.3 10.3 12.4( 15.15)" 12.5( 13.4)" 10.1
196+ 154 69+ 68+ 67+ 53 42+ 41+ 40+ 39 28 + 27+ 26+ 15+ +
+
+
CioHdz CsH8Ie" C15H18Tf CioHi?+ Ci"3 Ci"i C7Hs+ C7H7 Ce" These values refer to higher breaks in the ionization efficiency curve.
+
+
+
+
+
a
10 1 10.1 10.9 10.1 10.1
'
+
CKH~TT
217" 219+ 137+
10.4 10.4 11.2 10.4 10.55 11.2
198+ 184+ 155+ 141+ 128 127+ 71+
CioHzzIz+ C&oIz+ CioHzzI CsHiJ+ Cyclopentyl iodide
10.2 10.2 10.5 10.25 10.25
148+ 150+ 69+
CdL8Br C1oHlPBr.i. ClOHl?+ 1-Iodopentane
221 + 223+
10.5 10.5 10.9 10.8 10.5
+
+
Cyclopentyl bromide
95 + 137' 109+
10.5 11.6 11.o 11.6
150+ 152+ 71 CloHzzBr C10H22Br+
Curves for Q us. Ee-l for the following secondary ions were abnormal: 109+, 265+, 392+ from C6HJ.; 269+, 324+, 396ffrom C6HI1I; 217+from C5H9Br;and 221 from C5HI1Br. The abnormality was shown not to arise from space charge or focus effects. Thus, ion focus was excellent for 265+ at E, 7 7 v. and for 396+ at E, 7 10 v. The phenomenon is much more pronounced at low repeller field. Since Q at constant & increased with increasing pressure it appears that a tertiary process is responsible. An example of this behavior is illustrated by the two upper curves of Fig. l .
A.P. (linear extrap.)
10.9 12 .o 11.4 12.15 12.15 12.50
104+ 69+ 68+ CTHu CioHi, CsHia+
3-Bromopentane
m/e
A.P. (vanishing current)
11.6
..
..
10.1 9.1
.. 9.8 10.2 11.5 13.3
..
12.3
..
.. .. .. 8.9 9.1 9 .o 9.5 11.4
9.7
.. 9.5
Discussion Let us consider the formation of a tertiary ion T + from secondary ion 5' and also from primary ion P+ 1
+ R I +S+ + RI S+ + &I +T + + Rz P+
2
The rate of formation of T + at distance 1 from the electron beam in the direction of the exit slit is
ION-MOLECULE REACTIONS OF PENTYL AND CYCLOPEXTYL HALIDES
August, 1963
1711
dnT = Nnsu(l)ndZ (4) where ns is the nuinber of S+, u is the cross section for reaction, and N is the concentration of molecules. TABLE I1 IO~V-R~OLECULE I~EACTIONS Q
x
1016
nt 12 v./cin. (crn.2)
’ 6 100
(RI)z+
2” 90
+ RI +RzI+ + I + R I
This niechanism is consistent with the appearance of a small metastable peak a t apparent mass 182.7 which can only be understood as evidence for
5 x 2 4 6
Vol. 67
70
(CsHiiI)z+
60
-
+
(CBHII)Z~+1
Fig. 2.-The termolecular contribution to ( CSHB)J+ from CaHJ for the same runs described in Fig. 1 in terms of us. Ee-l according to eq. 9.
Depending upon the internal energy of the dimeric ion and its velocity, collision with a molecule may either stabilize or decompose. Values of E, for various reactions have been collected in Table VI.
The resulting definite integrals may be indicated by the constants K1 and KZ3. Replacing i/Nnpl, by Q gives
VALUESOF E,
0.0
0.2
0.4 Ee-l,
0.6 e.v.-l.
0.8
1.0
+
Q = KiE,-l K23Lv12e-2 (9) Consequently QE,us. E , - I should be linear and show an intercept independent of pressure and a slope proportional to pressure. Equation 9 evidently can be used to “correct” the observed Q for tertiary ions, leaving the secondary component alone, and then applying eq. 2 and 3. We illustrate for the case of 396’ from 198+ using C5HJ. At E, = 1.5 e.v. and reservoir pressures 281 and 516 p, Q (396’) was 9.6 and 11.9 X Plots of Q us. E,-] were nonlinear from E, of 1 to 10 e.v. Plots of &Eeus. Ee-l in the same interval were linear; intercepts were 21 and 20 X 10-l6 e.v. and slopes e.v.2 at low and high were 0.64 and 1.1 X 10-l6 pressures. respectively. The calculated Q (396 +) for secondary ions alone as a function of Ee-l was then well behaved, with an intercept zero as required by eq. 3 for the high energy region and a well defined E , = 2.5 e.v. a t both pressures. The ternary contribution may be collision stabilization. The behavior of (CsHg)J+ from C5HgI is entirely similar. At niolecular concentrations N = 1.82 and 8.99 X 10l2moles/cc., Q (265+) was 45 and 67 X cm.2, respectively, at E, = 1.5 e.v. The nonlinearity of Q us. E,-], and its pressure dependence are shown in Fig. 1. When Qoor = Qobsd - K23NZ,E,-2from eq. 9 is employed instead, both sets of measurements exhibit a Q vs. E,-] linear dependence with zero intercept (see Fig. 1) and conform to eq. 3. The procedure for evaluating the contribution of the termolecular process is demonstrated in Fig. 2 for the same runs in plots of QE, us. E,-l according to eq. 9. In four runs, of which only two are shown, pressure ratios were 4.9, 2.7, and 1.4 relative to the lowest while the ratios of the corresponding slopes of QE, us. Ee-l were 3.5, 2.5, and 1.5. In the preceding instance and also in the correspond-
FOR
TABLE VI I o s s FORMED BY SECOKDARY AND TERTIARY PROCESSES
Molecule
Primary and secondary ions
C6HJ + + CioHisI + CsHJ * CioHisIz C5Hg+ * CeHiS’ GHs+ CsHia+ GHiiI + CioHzzIz CjHJ + ---c CioHzzI + CzHJ + + C6HioI2 + CbHgBr+ + C1oHIIBr-k C;HI1Br+ + CIOHZ2Br
C6Hd
+
+
+
C6HiJ
+
C5H8Br 3-C~H11Br
+
E o , e.\’.
1.4 2.5 2.3
2.5
... 2 3.2 ...
Uniinolecular decomposition of (CjH111)2+ within the source can be detected as an energy-shifted peak for favorable values of accelerating and repeller voltages (V, and V,, respectively). It is indicated by a peak situated between 269+ and 268+ (the latter attributed to CHJZ impurity). At Va = 760 v. the peak is shifted toward smaller m/e as Vr increases from 5 to 6 v. At 8, = 350 v. the peak is detectably shifted by increasing V , from 1.6 to 1.8 v. and the apparent mass approximates 268 a t V, = 9 v. At V, = 200 v. and V, = 3 v. the apparent mass approximates 268. We interpret these results in terms of the preceding unimolecular decomposition. Since this shifted peak appears even a t values of E, well below the observed E, = 2.5 e.v. (Table VI), it may arise entirely from metastable collision complexes. Since the collision complex can form at any primary ion energy between zero and E, or E,, the energy loss AE to the neutral fragment will be proportioiial to V,. The apparent mass m* is given by m” since E, E5
= m(1
- AE)/e(V,
+ l/2Vr)
eV,, and we have approximately (m
- m*)/m = V J V ,
which accounts qualitatively for the observations.
