+ Ç B = 6.0. The discrepancy is within experimental error. Similarly, Ç c + Çpn = 7.6 is within experimental error of Qn = 9.0, while Q = 12.4 may be compared with Ç + Ç> + Q = 14.7 and Q = 14.6 with Q + Çac + Can = 14.9. pA
P
P
iG
E
aA
aB
iH
F
Although this method appears to give reasonably satisfactory and self-consistent results, there are many complications. The variation of
In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
158
RADIATION CHEMISTRY
II
ionizing voltage and the change of pressure and hence ion current may affect space charge, contact potentials, collection efficiencies and other parameters with a consequent effect on reaction probability and ion energy. As noted previously, the success of this method and its reproducibility are its best justification (12, 26).
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14
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Figure 3. Evaluation of probabilities of hydrogen atom and hydrogen ion transfer from methyl and acyl groups of acetaldehyde to methanol Curve Curve
1: 2:
UCHiCDO+l/UiCH&H*). ip(CD CHO+)/i (CH OH+). 3
p
s
Other and probably more severe problems are the formation of fragment ions of low appearance potential and that of long-lived excited ions. Clearly such fragments as C H O which undergo proton transfer reactions with the reactant molecules could seriously alter the results. W e have evaluated appearance potentials with reference to A r and find AP ( C H C H O ) = 10.25 ± 0 . 1 e.v. in good agreement with the literature (10.21 e.v.) (31), and A F ( C H O ) = 13.0 ± 0.2 e.v., which may be compared with 12.5 e.v. obtained by the R P D technique (18) and ca. 13.0 by the second differential method (6). The participation of C H . . C O , whose appearance potential is approximately 10.5 e.v. (6) to 10.9 e.v. (19), in ion-molecule reactions is not likely to lead to protium or proton transfer but probably leads to hydride transfer. To minimize the contribution from such reactions the increase in ionizing voltage was terminated +
3
+
In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
9.
MEISELS AND LEGER
159
Dipole Effects
when fragment ion currents exceeded 10% of the parent ion current. The experimental points at the lowest C H O H V C H C H O ion current ratios must therefore be regarded as more reliable. Unfortunately, it is also in this range that ion currents are reduced significantly so that the signal-to-noise ratio becomes poorer. W e have not found any evidence for the charge transfer reaction 3
+
3
C H O H + C H C H O -> C H O H + C H C H O +
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3
3
3
3
(3)
+
which is energetically allowed. This reaction would have been observed under our conditions if it occurred with a cross section of the order of that for proton transfer reactions. Discussion If one assumes that every collision leads to a long-lived intermediate complex, one might regard the summary in Table II as a comparison of the fate of intermediates differing only in energy content. Since the ionization potential of methanol is higher, there is 0.64 e.v. additional energy available when the reactant ion is C H O H \ However, the colliding partners w i l l initiate their encounter on different potential energy surfaces. 3
Table I.
Relative Cross Sections of Specific Hydrogen Atom and Hydrogen Transfer Reactions Estimated Mixture
Cross Section
Proton Hydrogen Transfer Atom Transfer Q Q
Ion Current Ratio
CH3CDO
p
(A) + CH OH CH OHD /CH OH (B) C D C H O + C H O H C H O H D / C H O H (C) C D C D O + C D O H C D C D O H / C D C D O (D) C H C H O + C D O H C H C H O D / C H C H O 3
3
3
3
3
3
3
3
3
3
3
Table II.
+
3
+
+
3
+
+
3
+
3
a
0. ± 0.2 6. ± 1. 4.1 ± 0.6 3.5 ± 1.
+
+
0.5 ± 0.2 1.8 ± 1. 3.0 ± 0.1 0.8 ± 0.3
Cross Sections of Ion-Molecule Reactions in Undeuterated Components and Mixtures Cross Section
Mixture (E) (F) (G) (H)
CH CH CH CH
3 3 3 3
Intercept Q
Ion Current Ratio
OH CHO OH + CH CHO OH +
CH3CHO 3
CH CH CH CH
3 3 3 3
OH /CH OH CHOH /CH CHO OH /CH OH CHOH7CH CHO 2
+
+
3
+
2
+
3
+
+
3
3
Slope Q
{
+
12.4 14.6
s
12.4 11.1
In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
8.3 9.0
160
RADIATION CHEMISTRY
Table III.
Reactions of C H C H O and Reactions of C H O H +
3
3
Relative Cross Section (4) C H C H O + C H 3 O H - > CHaOrV + CH CO (5) C H C H O + + CH CHO (6) C H C H O + CH CHOH + CH3O (7) C H C H O + CH CHOH + CH OH
+
AH(kcal. mole' ) 1
+
3
6.
CH3OH — > CH3OIV CH3OH —» CH3OH —» 3
-17.0
± 1.
+
3
2
0.
-4.0
3.0 ± .1
- 4 . 0 ( + 13.0)
0.8 ± .3
-5.0 (+ 12.0)
+
3
+
3
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II
+
3
+
8
2
2Q
= 9 . 8 ± 1.5
M
(8) C H O H + C H 3 C H O - > CH CHOH + CH3O (9) C H O H + C H 3 C H O - > CH CHOH + CH OH (10) C H O H + C H 3 C H O - > CH OH + CH3CO (11) C H O H + CH3CHO —» CH OH2 + CH CHO +
3
4.1
+
3
-18.0 (-1.0)
.6
+
3
+
3
2
3.5 ± 1.
-19.0 (-2.0)
1.8
-32.0
0.5
-19.0
+
3
3
2
+
+
3
3
+
2
SÇ = 9.9±2 A
There exists more serious objection to any comparison based merely on the energetics: the impossibility of assigning a critical collision radius within which orbiting or capture occurs, and thus the probable absence of any long lived intermediate complex. Therefore, it is preferable to compare the fates of encounters when reaction is initiated only by acetaldehyde ions or solely by methanol ions. Such a regrouping of reactions is shown i n Table III, and the energetics of each process (4, 19) are included for comparison. The ion C H 0 can exist i n two configurations (24), C H C H O H having the lower energy content. The thermodynamic values i n parentheses are those calculated for C H C H 0 . It is apparent that the formation of this ion can be excluded from several reactions on energetic grounds. If product formation proceeded via an intermediate long-lived ion whose dissociation were governed by competitive channeling of energy into the possible dissociation coordinates, one would expect the complex to fragment at the weakest bond—i.e., i n the most thermodynamically favorable direction. This is clearly not the case. The most exothermic reaction (17) does not dominate the fate of the complex formed by collision of methanol ion with acetaldehyde. Similarly, one would expect Reaction 4 to be favored and Reactions 5, 6, and 7 to occur with about equal probability. A n alternate approach to analysis of these results may be based on the separation of hydrogen atom and proton transfer reactions. This 2
3
5
+
+
3
2
+
In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
9.
MEisELS AND LÉGER
Dipole Effects
161
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would indicate that the position from which proton transfer occurs is determined by the energetics of the dissociation since the more exothermic Reaction 4 proceeds at the exclusion of 5, and the thermodynamically nearly equal Reactions 8 and 9 are equally probable. O n the other hand, the hydrogen atom transfer, where one might expect participation of the permanent dipole in selecting the group from which transfer occurs, shows a clear preference by a factor of over 3:1 for transfer from the electronegative end of the neutral species. Such a separation of hydrogen atom and proton transfer reactions implies that the two reaction paths are predestined before the collisions. Although it would be tempting to associate one of these with ion-induced dipole and the other with ion-permanent dipole long-range forces in the manner in which maximum cross sections (9, 23) and averaged cross sections (29) are calculated, this does not represent the physical reality of the collision process (7) which clearly demonstrates the inseparability of these contributions. This is further supported by simple considerations of maximum cross sections based on known polarizabilities and dipole moments (20, 22). The ion-induced dipole interaction (23) is independent of the nature of the reactant ion in this instance since the disparity of the polarizabilities is exactly balanced by the difference in masses. The ion-dipole interaction should be 16% larger for reactions initiated by methanol ions assuming dipole "lock-in." Our relative cross section measurements are too uncertain to assess the veracity of a slightly higher cross section for C H O H but suggest (Table III) that the specificity of the atom transfer from the negative group is independent of dipole moment and that over-all contribution of the neutral transfer is actually smaller when the dipole interaction cross section should be larger. The simple theory of maximum cross sections based on lock-in of the dipole (9, 23) cannot readily account for these observations. 3
+
Another way in which the reaction rates could be determined a pnori would be association of one of the two processes with an intimate collision (complex formation) and the other with a transfer at longer ranges, perhaps by a stripping mechanism (14, 15). This differs only slightly from the assignment of critical reaction separations completely devoid of the assumption of any contribution of long-lived complexes. In view of the inability to assign a critical capturing radius on theoretical grounds (7) the last alternate appears to be the most attractive one. Acknowledgments W e are indebted to the U.S. Atomic Energy Commission for their support of this research, and to the Manned Spacecraft Center of the National Aeronautics and Space Administration for permission to use
In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
162
RADIATION CHEMISTRY II
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their facilities. We thank James Kluetz for providing the analyses by microwave spectrometry.
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In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
