7721
J . Phys. Chem. 1991, 95, 7721-7726
20
1
O !
0.0
0.1
1.0 p1/2
1.I
2.0
Figure 5. (?=-I - 1) vs pl/z. Conditions: 50 pM R~(bpy),~+, 20 mM MV2+,0.1 M TEOA, pH 10.0, continuous photolysis (0):50 pM Ru2 mM MV2+,0.1 M TEOA, pH 10.0, flash photolysis (0): 50 (bp~),~+, pM Ru(bpy)32+,2 mM MV2+, pH 10.0 (A),flash photolysis.
Figure 5 as a function of ionic strength would be the same, and all the data would be superimposable. The fact that the slopes are different indicates that the presence of TEOA has an effect
on the nature of the species that are within, or that make up the solvent cage, and that the effect is manifested as p is increased. The net result is that k,, which decreases, in general, as p is increased for the escape of like-charged species into bulk solution, has an even lower value in the presence of 0.1 M TEOA than in its absence at those higher ionic strengths. It would appear that the combination of TEOA and an ionic medium is effective in retarding, albeit to a limit extent, the rate of cage escape of the redox pair. The results suggest that even neutral species that would normally be viewed as "innocent" solutes can cause subtle variations in the structure of the solvent cage and the value of qcc. From the data in Figure 4 for 0.1 M TEOA at p = 0.06 M, we see that 7, has a limiting upper value of -0.1 at [Ru(bpy):+]
1 50 pM,and a lower plateau value of -0.06 at I20 pM. Although the magnitude of the effect and the concentration range over which it occurs is somewhat different than was observed with 0.1 M EDTA at pH 11.0, which was attributed to the presence of R~(bpy)~+...EDTA~-...Ru(bpy)~~+ ion-paired aggregated specie^,^^*^' the behavior with TEOA follows the same general pattern. It is difficult to visualize how TEOA could mediate the formation of aggregates of the photosensitizer in the same way that EDTA has been proposed to do. We must conclude that it is CI-, the counterion of both R ~ ( b p y ) ~and ~ +MV2+ present at a concentration of 40 mM in these experiments a t ambient ionic strength, that causes the effect by mediating the ion-paired aggregation of the photosensitizer. As [ R ~ ( b p y ) ~ is~ +increased ] across the 20-50 pM regime, the formation of ion-paired aggregates could be enhanced, thereby increasing the electrostatic repulsion of the geminate pair, and the values of k, amd v,. Conchrsiorw TEOA can be used as an effective sacrificial donor in the Ru(bpy):+/MV2+ model photochemical system at 10.05 M; at lower concentrations, its efficiencies of scavenging Ru(bpy)?+ and its oxidized radical are less than unity. At 0.1 M, TEOA is effective in the pH 8.5-11.5 range; in less alkaline solution, its protonation diminishes its ability to act as a scavenger for the same species. In more alkaline solution, secondary processes between MV2+ and the products of the TEOA degradation reactions yield additional equivalents of M V . For [ R ~ ( b p y ) ~ ~ + ] = 50 pM in the absence of TEOA, the dependence of ,7 on p in the quenching of *Ru(bpy):+ by MV2+ is given by the following expression: ,7 = (3.8 4.8pl/2)-l. In the presence of 0.1 M TEOA at pH 10.0, = (3.8 + 9.0p'/2)-'.
+
Acknowledgment. This research was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy. We thank Dr. M. DAngelantonio (Istituto FRAE-CNR, Bologna, Italy) for carrying out the computer simulation.
Formation of Thermodynamically Stable Dications in the Gas Phase by Thermal Ion-Molecule Reactions: Ta2+ and Zr2+ with Small Alkanes Yasmin A. Ranasinghe, Timothy J. MacMahon,t and Ben S. Freiser* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: August 3, 1990; In Final Form: May 9, 1991)
The reactions of doubly charged tantalum and singly and doubly charged zirconium with small linear alkanes are reported. The rate constants for the reactions of singly and doubly charged Zr and Ta with methane are also reported. Ta" reacts with methane formally by carbene and hydride abstraction along with some charge transfer. Reactions of Ta2+with longer chain alkanes result in complete charge transfer. Zr2+ also reacts with methane formally by carbene and hydride abstraction and undergoes dehydrogenation and demethanation reactions and hydride and methide abstraction in addition to charge transfer with propane and butane. With alkanes larger than butane only charge transfer is observed. Zr+ reacts similarly to other early transition metals to multiply dehydrogenate alkanes and, to a lesser extent, yield C-C cleavage fragments.
Introduction Although the existence of multiply charged ions has been known since the early days of mass spectrometry, their low abundances relative to singly charged ions, as well as interferences from isobaric singly charged ions, have inhibited their thorough investigation. In the past couple of decades, however, there have been significant advances in the instrumentation and methodology for studying multiply charged ions, particularly involving sector instruments, together with a concomitant increase in theoretical treatments of these interesting species.' 'Current address: IBM, Hopewell Junction, NY.
0022-3654/9 1/2095-7721$02.50/0
One of the fascinating aspects of the work is the origins of the stability of multiply charged ions of very small molecules. For example, it is surprising that even triatomic trications such as CS23+have been observed to be stable on the microsecond time scale despite the expected Coulombic repulsion between positive (1) (a) Ast, T. Ado. Muss Spectrom. 1980, A8, 555. (b) Koch, W.; Maquin, F.: Stahl, D.; Schwarz, H. Chimiu 1985, 39, 376. (c) Koch,W.; Heinrich, N.; Schwarz. H.; Maquin, F.; Stahl, D. Int. J . Muss Spcfrom. Ion Processes 1985.67, 305. (d) Cooks, R. G.; Ast, T.; Beynon, J. H. Inr. J. Mass Spectrom. Ion Phys. 1973, 11,490. (e) Guilhaus, M.; Brenton, A. G.; Beynon, J . H.; Rabrenovic. M.; Schleyer, P. v. R. J . Chem. Soc., Chem. Commun. 1985, 210. (0 Drewello, T.; Schwarz, H. Int. J . Muss Specrrom. Ion Processes 1989, 87, 135.
0 1991 American Chemical Society
7722 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 charges. In many cases these species exhibit kinetic stability.2 Thermodynamic stability, however, is also possible, particularly in species containing transition metals3 Despite the growing number of studies on the generation and properties of multiply charged species, there have been relatively few studies of thermal or near-thermal ion-molecule reactions of doubly charged ions.4 Examples involving transition-metalcontaining ions are even f e ~ e r . ~In, view ~ of the higher stability and hence the greater prevalence of multiply charged metal ion species in solution, studies of analogous species in the gas phase are of increased significance. While simple charge transfer might be expected to be the predominant process observed between a bare doubly charged metal ion and an organic reagent, Tonkyn and Weisshaar demonstrated that the early transition metal Ti2+with its relatively low second ionization potential undergoes clustering reactions with methane and hydride abstraction reactions with ethaneeSaThis encouraged us to examine the behavior of other doubly charged early transition metals. We recently reported on the gas-phase reactions of Nb2+ with small alkanes at thermal e n e r g i e ~ . ~Dehydrogenation ,~~ was observed to be a prominent pathway for methane and ethane, while charge transfer was virtually the only reaction pathway observed for propane and butane. These results were found to be in qualitative agreement with a simple curve-crossing model. In addition, NbCH;+ and NbC2H?+ formed in the reactions of Nb2+ with methane and ethane, respectively, were determined to be thermodynamically stable, attributable in part to the chargestabilizing effect of the metal center. In this paper we investigate tantalum, which at 16.2 eV6 has one of the highest second ionization potentials of the early transition metals, and zirconium, whose second ionization potential of 13.13 eV6 is near that of titanium (13.58 eV).6 This range in ionization potentials permits a further test of the curve-crossing model and an interesting comparison of the chemistries. Finally, the reactivity of the doubly charged ions is briefly compared to that of the singly charged ions. The chemistry of Ta+ has been previously reported,14aand that of Zr+ is reported here. Experimental Section
The theory, instrumentation, and methodology of Fourier transform mass spectroscopy (FTMS) have been discussed elsewhere.' All experiments were performed on a prototype FTMS-1000 from Nicolet which is equipped with a Nd:YAG pulsed laser.* Doubly charged tantalum and zirconium, as well as singly charged zirconium, were generated by focusing the fundamental frequency ( I 064 nm) of the laser onto the appropriate high-purity metal target.g A constant trapping voltage of 2.5 (2) (a) Newton, A. S.J . Chem. Phys. 1964, 40, 607. (b) Morvay, L.; Cornides, I . f n t . J . Mass Spectrom. Ion Processes 1984,62,263. (c) Singh, S.; Boyd, R. K.; Harris, F. M.; Beynon, J. H. fnt. J . Mass Spectrom. Ion Processes 1985, 66, 167. (3) Gord, J. R.; Freiser, B. S.; Buckner, S. W. J . Chem. Phys. 1989, 91,
75311 (4) (a) Maier 11, W. B. J . Chem. Phys. 1974, 60, 3588. (b) Spears, K. G.;Fehsenfeld, F. C. J . Chem. Phys. 1972, 56, 5698. (c) Spears, K. G.; Fehsenfeld, F. C.; McFarland, M.; Ferguson, E. E. J . Chem. Phys. 1972,56, 2562. (d) Viggiano, A. A,; Howorka, F.;Futrell, J. H.; Davidson, J. A,; Dotan, I.; Albritton, D. L.;Fehsenfeld, F. C. J . Chem. Phys. 1979,71,2734. (e) Howorka, F. J . Chem. P M . 19% 68, 804. (f) Holzscheiter, H. M.; Church, D. A. J . Chem. Phys. 1981, 74, 2313. (5) (a) Tonkyn, R.; Weisshaar, J. C. J . Am. Chem. Soc. 1986,108,7128. (b) Buckner, S. W.; Frciser, B. S. J . Am. Chem. Soc. 1987. 109, 1247. (c) Huang, Y . ; Freiser, B. S. J . Am. Chem. Soc. 1988, 110, 4435. (6) Moore, C. Natl. Stand. ReJ Data Ser. (US., Narl. Bur. Stand.) 1970, No. 34. (7) (a) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282. (b) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974,26,489. (c) Marshall, A. G . Arc. Chem. Res. 1985, 18, 316. (d) Freiser, B. S. In
Techniquesfor the Study of Gas Phase Ion Molecule Reactions: Techniques ofchemisrry; Farrar, J. M., Saundcrs, Jr., W. H., Eds.; Wiley-lnterscience: New York, 1988; Vol. XX, Chapter 11. (8) (a) Freiser, B. S. Talanta 1985,32,697. (b) Freiscr, 9. S. Chemtracts: Anal. Phys. Chem. 1989, I , 65. (9) Cody, R. B.; Burnier, R. C.; Rents, Jr., W. D.; Carlin, T.J.: McCrery, D. A.; Lengel, R . K.; Freiser, B. S. Int. J . Mass Spectrom. Ion Phys. 1980, 33, 37.
Ranasinghe et al. V was maintained throughout the experiment. The laser power was adjusted to obtain the best intensities of these ions, and the singly charged ions were continuously ejected. The FTMS-1000 has a 5.2-cm cubic cell which is situated between the poles of a Varian 15-in. electromagnet maintained at around 0.85 T. The two transmitter plates of the cell have been replaced with 80% transmittance stainless steel screens. All samples were used as supplied except for multiple freeze-pumpthaw cycles to remove any noncondensable gases. The chemicals were admitted into the cell through Varian leak valves in order to maintain a constant background pressure. Pressures were measured with an uncalibrated Bayard-Alpert ionization gauge. Multiply charged metal ions produced by laser desorption have been observed to exhibit high kinetic and internal energies.I0 To try to ensure that all of the ions studied were thermal, the ions were allowed to collide with a background of Ar at -6 X 10" Torr for 1 s. This allowed the ions to undergo around 100 to 200 thermalizing collisions, depending on whether the ion was singly or doubly charged." At the end of this time, all ions were ejected from the cell by typical ejection pulses except for the ion to be studied. For most experiments with the singly/doubly charged zirconium, the 89.9/44.95 (52%) ion was isolated by ejecting the 90.9/45.45 (1 l%), 91.9/45.95 (17%), and 93.9/46.95 (17%) isotopes prior to the reactions to simplify the identification of the reaction products. For the charge-splitting reactions in which M+, MH+ and MCH3+ are generated, often the corresponding low-mass alkyl ions are observed to have greater variations in their intensities from trial to trial. Thus, unless otherwise noted, product ion distributions are obtained from the metal ion species. This procedure may overestimate the amount of simple charge transfer observed somewhat, Le., the intensity of M+,since charge transfer with background species is possible. The branching ratios are the average of at least three experiments performed under similar conditions, with a precision of *IO% absolute. The static background pressure of argon also served as collision gas for collision-induced dissociation (CID) experiments.*J2 The CID energy is variable, and the spread in kinetic energies is dependent on the average kinetic energy (35% at 1 eV, 10% at 10 eV, and 5% a t 30 eV).I3 The rates of the reactions of the singly and doubly charged Zr and Ta with methane were studied by calibrating the pressure using the reaction of Nb2+ with methane (rate constant of 9.5 X cm3/(molecule s ) ) . The ~ ~ formation of oxides from background water and oxygen is unavoidable in all these reactions. However, by selectively ejecting the respective ion, we could minimize its interference, except in the determination of the rate constants of Zr+ and Zr2+. Here, because of very slow rates, reaction times up to a few seconds had to be used in order to obtain high product ratios. The best data were acquired by monitoring the reaction to about 60% completion as shown in Figure 4, from which the rate constant was calculated. The faster Ta2+reaction was observed to 90% completion, under similar conditions (Figure 51
" I *
Results
Tantalum. Methane is the only alkane studied which reacts with Ta2+ by other channels h i d e s charge transfer. The primary 2 C + H and ~ the secondary reactions 4-1 3 reactions 1-3 of ~ ~ with are strikingly similar to those observed for Nb2+.' As in the Nb2+
Ta2*
+
CH4
-f
TaCH$*
Hz
+ CH3*
TaH* Ta'
+
+
CH4*
(52%)
(23%) (25%)
(1) (2) (3)
(IO) Kang, H.; Beauchamp, J. L. J . Phys. Chem. 1985,89, 3364. ( I I ) Based on the Langevin collision rate calculated using uA,= I .64 A' from: Miller, T.M., Bederson, B. Adu. At. Mol. Phys. 1977, 13, I . (12) Cody, R. B.; Freiser, B. S. Int. J . Mass Spectrom. fon Phys. 1982, 41, 199. (13) Huntress, W. T.;Mosesman, M. M.; Elleman, D. D. J . Chem. Phys. 1971, 54, 843.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7723
Formation of Thermodynamically Stable Ta2+ and Zr2+
+
TaCH;'
CHI
TaC2H22+ + 2H2 (90%)
1
TaC2H12+
+
2.0
-
(10%)
H2
TaC2HZ2+
TaC3H12+ (90%)
TaC2H12+
TaC3H:+
I"-
(10%)
4
TaC3Ht+
TaC3H:+
s 1 1
(10%)
d1.0
%
0.5 -
i
1 2
case, complete isolation of the higher order products could not be accomplished, and thus, the higher order reactions were studied by isolating ions in groups of given n, TaC,Hm2+. In addition, multiple dehydrogenation once again was quite sensitive to experimental parameters, and the products described by reactions 4-13 represent the most reproducible and abundant ion signals obtained. It seems reasonable to assume that the ligands are coupling in reactions 4-1 3 to avoid electronic saturation of the metal center in analogy to that observed for Nb+ and Nb2+.3J4 For example, TaC,H?+ generated in reaction 4 undergoes collision-induced dissociation (CID) to give Ta+ and C2H2+almost exclusively (reaction 14), with some neutral loss of C2H2observed (reaction 15). A CID breakdown for TaC2H;+ is shown in Figure 1.
4
8
6
10
12
(W
Figure 1. Collision-induced dissociation breakdown plot of TaC2Hz2+
generated from reaction 4. 2.0
%"3 41 0
2 '
Unlike its analogue NbCH22+,TaCH22+showed no substantial CID processes under the conditions used in our instrument, presumably indicating a greater stability. Zirconium. The second ionization potential of Zr at 13.13 eV is considerably lower than that of Ta at 16.2 eV, and accordingly Zr2+ exhibits far less of a tendency to undergo charge-transfer reactions. Unlike Ta2+,where charge transfer is virtually the only process observed with alkanes larger than methane, for Zr2+ resonable intensities of doubly charged product ions are observed with alkanes up to butane. The predominant product of the reaction of Zr2+with methane is ZrCH2H with the hydride abstraction product, ZrH+, observed in minor amounts, reactions 16 and 17, respectively. Although Zr+ is occasionally observed, the absence of any CH,+ precludes its presence as arriving from charge transfer with methane. Z?'
+
CHI
-21 5 Do(Ta2'-C2H4) > 48 D0(Tat-C2H4') > -83
D0(Zr2'-CH2) > 11 1 D0(Zr2'-C2H4) > 48 Do(Zr2'-CH2) > 95 Do(Zr2'-C2H4) > 33 Do(Zr2'-C2H2) > 75 Do(Zr2'-C3H4) > 71 Do(Zr2'-C2H2) > 62 D0(Zr2'-C4H4) > 113 D0(Zr2'-C3H4) > 58 Do(Zr2'-C4H6) > 65
16 18 26 27 28 32b 33 37( 38 3gd
D0(Zrt-CH2') > 48 D0(Zrt-CzH4') > -12 D0(Zrt-CH2') > 32 D0(Zrt-C2H4+) > -27 D0(Zrt-C2H2') > 35 D0(Zrt-C3H4') > -8 D0(Zr'-C2H2') > 22 Do(Zrt - C4H4') > 31 D0(Zrt-C3H4+) > -21 D0(Zrt-C&') > -17
a Derived by assuming that the observed reaction is exothermic and by using the standard thermochemical data in Table Ill. bAssumes C3H4is allene. CAssumesC4H4is 1-buten-3-yne. "Assumes C4H6is
1,3-butadiene. TABLE 111: Standard Thermochemical Data" reaction AHrxn,kcal/mol
-
CH4 CH2 + H2 C2H6 C2H4 + Hz C2H2 + 2H2 CHI + CH4 C3Hs C3H4 + 2H2 C2H2 + CH4 + H2 C4HIo C4H4 + 3H2 C3H4 + CH4 + H2 C4H6 + 2H2 CH2 + CH4 C2H4 + H2 -.+ C2H2 + 2Hz species IE, eV 10.40 CH2 11.40 C2H2 CZH4 10.51 C3H4 9.69
111
+
33
75 95
+
-+
The reactions of Zr2+ with propane and butane are given in (32)-(36) and (37)-(41), respectively. Note that in reactions 34 and 40 the parent organic cations were observed. The larger alkanes, pentane through nonane, were all observed to react exclusively via charge exchange. In addition, dissociative charge transfer was observed, indicating that the organic parent ion formed initially contains sufficient internal energy to fragment. Zr2'
+
C3HB
ZrC3H42*
+
2H2
(8%)
ZrC2H;+
+
CHI
+
Zr' ZrH'
+ C3H2 + C3H;
ZrCH3' Zr2'
+
+
H2
(22%)
C2H;
(9%)
F
ZrC3H42'
+
CH4
+
ZC4H:'
+
2H2
(1%)
+
C4Htl
+
C3H;
57
-63 -2 1
+
species C4H4 C4H6 Tat Zr+
IE, eV
9.58 9.07
16.20 13.13
yne. 1,3-Butadiene.
3H2
Zr'
58
-+
(38%)
+
L ZCH;
+
-+
"Supplementary data from: Lias, S.H.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. ReJ Dam 1988, 17; Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. Ref. Data Suppl. 1977,6. bAllene. Ol-Buten-3-
(24%)
C 4 H I o 7 ZrC4H42'
-t
(8%)
71 62 1 I3
-+
H2
(9%)
(79%) (2%)
Comparison of the Chemistries of M2+and M'.
The primary reactions of Ta+ and Zr+ are given in Table I. As is the case for other early transition metal ions such as Ti+, V+, Sc+, Nb+, Y+,and La+, both Ta+ and Zr+ eliminate H2 and multiple H2 units from small alkanes.I4 While Tat is more effective at eliminating multiple H, units than Zr+, Zr+ exhibits a significantly greater tendency to undergo C-C cleavage processes than does Ta+. As a rough generalization, the elimination reactions of the doubly charged ions are surprisingly similar to those of the singly charged ions with the predominant processes observed being H, and multiple H2 loss. In addition, there is a greater degree of multiple dehydrogenation observed for M2+ than M+ which may be attributed to several factors including the enhanced complexing and bond energies of the doubly charged species due to its increased electrostatic attraction, as well as electronic factors.I5 (14) (a) Buckner, S. W.; MacMahon, T. J.; Byrd, G. D.; Freiser, B. S . Inorg. Chem. 1989. 28,351 I . (b) Huang, Y.;Wise, M. B.; Jacobson, D.B.; Freiser, 8. S . OrganomeraNics 1987.6, 346. (c) Byrd, G. D.; Burnier, R. C.; Freiser, B. S . J. Am. Chem. Soc. 1982, 104, 3565. (d) Jackson, T. C.; Carlin, T. J.; Freiser, B. S . J. Am. Chem. SOC.1986, 108, 1120. (e) Tolbert, M.; Beauchamp, J. L. J. Am. Chem. SOC.1984, 106, 6117.
Two examples of the trends described above are the secondary reactions of Ta+ and TaZt with methane and the reactions of Zr+ and Zr2+with ethane. TaCH2+reacts sequentially with CHI three times by reaction 42. In comparison TaCHz2+reacts sequentially TaCnH2,'
+ CH4
-
TaCn+,H2,+2+ + Hz ( n = 1-3)
(42)
in an analogous fashion with CHI five times, reactions 4-1 3, with a greater extent of multiple dehydrogenation observed. Similarly, Zr' eliminates one and two molecules of H2 from ethane in roughly a 3:1 ratio, while the ratio for Zr2+ is reversed a t about 1:4. It is also interesting to compare the rate constants of the singly charged verses the doubly charged ions. As shown in Table VI, the doubly charged ions react more than twice as fast as the singly charged ions, in contrast to the collision rates predicted by Langevin theory. This may be due in part to the greater exothermicity of the doubly charged ion reaction pathways. The rate constant for the fastest reaction, involving Ta2', agrees nicely with Langevin theory. Finally, the charge-splitting reactions observed for the doubly charged ions are clearly not possible for the singly charged ions, but the processes involved such as electron, hydride, and alkyl (15) Electronic factors can be more important than electrostatic factors. Recently, for example, Lat with its two valence electrons has been found both experimentally and theoretically to bond more strongly to selected ligands than La2' with its one valence electron. (MacMahon, T.; Ranasinghe, Y.;Freiser, B. S. Unpublished results. Rosi, M.; Bauschlicher, Jr., C. W. Chem. Phys.
Lett. 1990, 166, 189.)
The Journal of Physical Chemistry, Vol. 95, NO. 20, 1991 1125
Formation of Thermodynamically Stable Ta2+ and Zr2+
\
4
r = 6 . 9 d
6
Internuclear Distance
1 E
(i)
TIME
Figure 3. Potential energy surfaces calculated for the reaction of Zr2+
with propane. The crossing of the attractive ion-induced dipole (-aq2/2r') and the ion-ion repulsive potential ( 9 2 / r+ AH,,") occurs at r = 6.9 A for charge transfer and at 5.1 8, for hydride transfer.
(SECONDS)
Figure 5. Temporal variation of ion abundance for the reaction of Ta2+ with methane at 3 X IO-*Torr. The Ta+ and TaH+ ions are left out for
clarity. TABLE IV: Calculated Exothermicities and Curve-Crossing Distances for Charge-Transfer Reactions
Ta2++ N AHr,,: eV -3.7 -4.7 -5.2 -5.7 -5.8
+
N
CH4 C2H6
C'H8 C4H10
CSHl2
TaL2+
0.5
1.0 TIME (SECONDS)
fl:
A3
2.60 4.44 6.29 8.14 9.98
r, A
4.2 3.6 3.6 3.5 3.6
Ta2++ L % Ta+ + L+
L
AH,,, eV
CZH2
-4.8
a:
A3
3.33
r, A
3.6
OData from ref a in Table 111. bPolarizabilitieswere obtained from the calculated values in: Miller, K. J.; Savchik, J. A. J . Am. Chem. SOC.1979, 101, 7206.
1.5
Figure 4. Temporal variation of ion abundance for the reaction of Zr2+ with methane at 4 X IO-' Torr. The ZrH+ was left out for clarity.
anion transfer (e& reactions 34-36) are possible for the singly charged ions. For Ta+ and Zr+, however, these reactions are endothermic and, therefore, not observed. Thermochemical Implications. A variety of doubly charged metal-ligand species, ML2+, have been observed in this study. Assuming that they result from exothermic reactions involving thermalized M2+ions, lower limits on the bond dissociation energy Do(M2+-L) can be obtained. This data in turn yields a lower limit for Do(M+-L+) via 43 where IE is the ionization energy. Table I1 lists the bond energy limits derived from the observed reactions Do(M+-L+) = Do( M2+-L ) - IE(M+) + IE(L)
Ta+ + N+
(43)
and the standard thermochemical data in Table Ill. Of particular note are the ions ZrCH*+ and ZrC2Ht+ for which Do(M+-L+1 > 0 indicates thermodynamic stability. Because the structure of is unknown, the result that Do(Zr+-C4H4+)> 0 is inconclusive. Furthermore, for the majority of the ions in which Do(M+-L+) is shown as greater than a negative value, no information on the stability of the ion can be inferred. As was the case for NbCH*+ and NbC2H22+,3the stability of the analogous zirconium ions is most likely due to the charge-stabilizing effect of the metal center. Comparison of the Results to the Simple Curve-Crossing Model. As has been previously observed in the reactions of other doubly charged ions in the gas phase, the reactions of Ta2+and ZrZ+with small alkanes show an obvious trend toward more charge transfer with increasing size of the alkane. Also expected is the observation that Ta2+, with its higher second ionization energy, exhibits a greater propensity to undergo charge transfer than Zr2+. A simple curve-crossing model has been found to provide some insight into explaining these types of r e a ~ t i o n s ,and ~ * ~in~particular, this model
TABLE V: Calculated Exothermicities and Curve-Crossing Distances for Charge-TransferReactions N
CH4 C2H6
C3H8 C4HI0
CSH12
Zr2++ N AH,..," eV 4.6 -1.6 -2.2 -2.6 -2.1
+
Zr+ + N+ fl? A3
2.60 4.44 6.29 8.14 9.98 ZrL2+CID Zr2+ + L % Zr+ + L+
L
AH...." eV
CH2
-2.7 -1.7
fl,b
A'
1.30 3.33
r. A
23.2 9.1 6.9 6.0 5.7 r.
A
5.4 8.4
CzH2 "Data from ref a in Table 111. bPolarizabilitieswere obtained from the calculated values in: Miller, K. J.; Savchik, J. A. J . Am. Chem. SOC.1979, 101, 7206. was found to be in qualitative agreement with our earlier results on Nb2+.3 Figure 3 depicts the model for the reaction of Zr2+with propane via charge- and hydride-transfer pathways. The reactants enter on an attractive ion-induced dipole potential, -(uq2/2#, while the charge- and hydride-transfer products experience a repulsive Coulombic potential, q2/r. The separation of the curves at infinite distance is given by the exothermicity of the reaction channel in question and, in this case, is presumed to correspond to the ground-state products. The calculated exothermicities and curve-crossing distances for the charge-transfer pathways for the reactions of Ta2+and ZrZ+ with CH2, C2H2,and CnH2n+2 (n = 1-5) are compiled in Tables 1V and V, respectively. Lindinger and co-workers have proposed a "reaction window" ranging from internuclear separations of 2 6 A in which thermal transfer reactions proceeding via curve
J. Phys. Chem. 1991, 95,7726-7732
7726
TABLE VI: Rate Constants for the Reactions of Doubly and Singly Charged Zr and Ta with Methane
M"++ CH,
L,O Zrt ZrZt Tat TaZt
-
products ( n = 1, 2 ) kL
(7.1 f 4) X (5.0 f 4) X (4.4 f 2) X (1.95 & 1 ) X 10"
1.03 X 10" 2.05 X IO4 9.84 X 1.97 X IO"
k,X,lkL 0.01 0.02 0.45 1 .oo
"Values are the average and precision of three trials each. crossings are most favored.16 One consequence of this reaction window is that charge transfer will not be observed unless the reaction is sufficiently exothermic (21 eV). Otherwise, the curve crossing distance is too long to permit electron transfer. For example, the reaction Zr2+ CHI Zr+ + CH4+ is 0.6 eV exothermic but is not observed due to a curve crossing distance of about 23 A (Table V). Similarly, Ar+ is not produced during the cooling period for Ta2+even though the reaction is about 0.44 eV exothermic. In addition, the product branching ratios for Zr2+, and in our earlier study for Nb2+, can also be explained in terms of this reaction window. Weisshaar et aL5*observed mainly the adduct ion in the reaction of Ti2+with CH4 and hydride transfer almost exclusively with C2H6. Bond insertion and hydride transfer occur in the reactions of Zr2+with both methane and ethane. For both Ti2+and Zr2+ hydride abstraction is a major pathway with ethane. Bond insertion reactions and hydride transfer compete effectively with charge transfer in the reactions of Zr2+with CsHs, while for Ti2+only hydride and charge transfer were observed. The percentage of charge transfer rises sharply for C4Hloand the higher alkanes in accord with the relevant curve crossings lying within the reaction window. The general tendency for production
+
-
(16) Smith, D.;Adams, N. G.;Alge, E.; Villinger, H.; Lindinger, W. J . Phys. 1980, 813, 2187.
of the charge-separated product in the CID of TaC2H?+, reaction 14, and the enhanced production of Zr2+ in the CID of the analogous zirconium species, reaction 30, are also in accord with the curve-crossing model. For Nb2+, 199% charge transfer was observed with C3Hs having an estimated curve crossing at 4.8 A, while for Zr2+ this occurred for n-C5HI2with a curve crossing at 5.7 A. Considering that the curve crossing for Ta2+ and CH4 is 4.2 A, it is somewhat surprising, therefore, that charge transfer is not the predominant reaction observed. This "discrepancy" underlines the fact that this model is after all an oversimplification. The limitations of this model and the associated Landau-Zener1' approach to calculating curve-crossing probabilities and charge-transfer cross sections have been discussed by a number of researchers,&J8 In the present study, several factors can limit the applicability of this approach, including the multiplicity of curve crossings and the possibility of extensive state coupling given the high density of states of the Mz+species. Furthermore, the surfaces do not include details of the impact parameter and collision geometry. A variety of other limitations have been previously discussed. Nevertheless, while clearly not rigorous, this and previous studies suggest that the model does provide a useful qualitative picture.
Acknowledgment is made to the Division of Chemical Sciences in the Office of Basic Energy Sciences in the United States Department of Energy (DE-FG02-87ER13766) for supporting this research and to the National Science Foundation (CHE-8920085) for continued support of FTMS methodology. Registry No. Ta2+,35831-23-3; ZrZt, 14995-75-6; Zr', 14701-19-0; CH4, 74-82-8; EtH, 74-84-0; PrH, 74-98-6; BuH, 106-97-8; H,C(CH,),CH,, 109-66-0. (17) (a) Landau, L. Z . Phys. Sowjer 1932,2,46. ( b ) Zener, C. Proc. R. Soc. London, Ser. A 1932, 137, 696. (18) (a) Bates, D. R.; Moiseiwitsch, B. L. Proc. Phys. Soc., London 1954, A67, 805. (b) Hasted, J. B.; Chong, A. Y . Proc. Phys. Soc., London 1962, A80, 44 1.
Kinetics of the Reactions of CS20H with 02,NO, and NOz Eric Wei-Cuang Diau and Yuan-Pern Lee*.+ Department of Chemistry, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu, Taiwan 30043. Republic of China (Received: January 29, 1991)
The laser-photolysis/laser-induced-fluorescencetechnique has been employed to study the reactions of CS2OH with 0 2 , NO, and NO2 in the presence of He at 26-202 Torr and 298 K. With excess CS2and without 02,NO,and NO2, a rapid approach of [OH] to equilibrium was observed, indicating reversible formation of a CS20H adduct. When 02,NO,or NOz was present at a small concentration, a rapid decrease of [OH] after reaching equilibrium was observed. Analysis of the [OH] temporal profiles yielded the rate coefficients for each reaction. The coefficients are k(CS20H+Oz) = (3.1 f 0.6) X IO-", k(CS20H+NO) = (7.3 f 1.8) X IO-", and k(CS20H+N02) = (4.2 f 1.0) X cm3molecule-I s-I: the uncertainties represent 95% confidence limits.
Introduction The oxidation of CS2 is important in the atmosphere and has been the subject of many investigations. The reaction of CS2 with O H has been generally accepted to be the major pathway for the degradation of CS2 to form OCS and SO2.' Previous laboratory studies of this reaction have reported rate coefficients that varied by 2 orders of magnitude?+ A rate enhancement in the presence The reaction mechanism of O2 has also been reported.l*I5
has been employed to explain the observed experimental results. With excess CS2 and without Oz, a rapid approach of [OH] to
Also affiliated with the Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan, Republic of China.
( I ) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry; Wiley: New York, 1986; p 663.
OH
+ CS2
CSZOH
-
CSzOH
(10
+ CS2
(Ir)
products
(2)
OH
C S 2 0 H + O2
0022-365419 112095-1726%02.5010 0 1991 American Chemical Society
