1368
J. Phys. Chem. 1984,88, 1368-1373
modifications were made on the simplest puckering model in order to adjust the reduced mass ratio. The kinetic energy expansions were simply used as calculated. Figures 5 and 6 show this potential function along with the observed infrared transitions for 2-phospholene and 2phospholene-I-d, respectively. As can be seen, the function is asymmetric and single minimum, indicating that only one conformer of the molecule would be stable. This is similar to 3phospholene,6 which has a single minimum, corresponding to the endo conformation (both the P-H and ring inverted in the same direction), in its potential function. Without additional (e.g. microwave) data it cannot be ascertained which conformation is more stable for 2-phospholene. However, analogy with 3phospholene suggests the endo conformation would be at lower energy. The potential function of 2-phospholene has inflection points at -0.08 A, where V = 205 cm-', and at -0.19 A, where V = 524 cm-'. Most likely, then, the less stable planar structure and the less stable puckered conformation (exo) correspond approximately to these values. For the 3-phospholene molecule, for which microwave data were also available,22 the planar and exo conformations occurred at -0.1 1 and -0.23 A, respectively. The potential energies for these conformations are 331 and 785 cm-', respectively. In comparison, then, 2-phospholene appears to have a somewhat less puckered conformation and the energy of the planar form is only about 205 cm-I higher, as compared to 33 1 cm-' for the 3-ene. A lower energy for the planar ring conformation of 2-phospholene may well be an indication of some d r p n bonding between the phosphorous and the olefinic double bond. Moreover, the quartic constant of 6.05 X lo5 cm-'/A4 in eq 5 is considerably higher than for 3-phospholene (2.86 X lo5 cm-'/A4). Since the quartic constant is a measure of angle strain and ring rigid it^,^ this also suggests additional forces are present in 2-phospholene (22) J. R. Durig, B. J. Streusand, Y. S. Li, J. Laane, and L. W. Richardson, J. Chem. Phys., 73, 5564 (1980).
that are lowering the energy of the planar structure. Nonetheless, the molecule has only one stable conformation and this is puckered. The lower C=C stretching frequency of 1560 cm-' for 2phospholene vs. 1626 cm-' for 3-phospholene further suggests some d F p n bonding for the 2-ene. Each of these observations parallels the comparisons made for 1-silacyclopent-3-ene and l-silacyclopent-2-ene, where the latter molecule is believed to show substantial dn-pn bonding between silicon d orbitals and the carbon-carbon double bond. Both of these silicon compounds are planar, but the 2-ene is considerably more rigid. The quartic constant for the 2-ene is approximately 18.5 X lo5 vs. 1.99 X lo5 cm-'/A4 for the 3-ene, and the carbon-arbon stretching frequency for the former is 1560 vs. 1610 cm-' for the 3-ene. Comparison of the 2-phospholene potential energy function to those of other asymmetric ring molecules is more difficult because reliable kinetic energy expressions for the other molecules have not been published. However, preliminary calculations in our l a b ~ r a t o r y indicate '~ that the quartic constant of 6.05 ( X lo5 cm-'/A) is similar to that in the sulfur compound (6.40) but substantially less than for the silicon compound (18.5) with its substantial dn-pn bonding. This x4coefficient is generally higher I when M in CH=CHCH2CH,M involves a first-row element (with higher ring strain). Thus, values of about 7.1 and 10.0 have been found for M = CH, and 0, respectively. Except for M = SiH, and C=O, where n bonding for M to the C==C occurs, the quadratic constant, which reflects primarily torsional interaction between adjacent CH, groups, is negative. For 2-phospholene, however, the quadratic term is small and positive, reflecting the fact that it is dominated by the x3 coefficient. The latter gives rise to the asymmetry resulting from the phosphine hydrogen lying off of the molecular ring plane. Acknowledgment. We thank the National Science Foundation and the Robert A. Welch Foundation for financial support. Registry No. I, 69220-51-5; 11, 695-63-6; 111, 1769-52-4; IV, 8876664-7; PCI3, 7719-12-2; C6H,SiH3, 694-53-1; butadiene, 106-99-0.
Mass-Spectrometric Investigation of the Klnetlcs and the Deactivation of C,H,+ and (C,H,),+ Ions Formed by Photoionization of Isobutene in Nonpolar Gas Mixtures Kenzo Nagaset and Jan A. Herman* Department of Chemistry and Centre de Recherches sur les Atomes et les Molicules, Universiti Laval, Quibec, QuZbec, GIK 7P4 Canada (Received: June I I, 1982; In Final Form: April 6, 1983)
Ion/molecule reactions of isobutene leading to C4H9+(C4H8)n and C8Hl4+(c4H8),species formation were investigated in a high-pressure photoionization (10.03 eV) mass spectrometer. The kinetics and the deactivation by nonpolar gases of the precursor ions, C4H8'* and (C4H8)2*,were evaluated. Some relationships between the deactivation efficiency and some molecular parameters were examined.
Introduction The ion/molecule reactions in gaseous isobutene have been investigated on several occasions by different mass-spectrometric techniques.lM8 At low pressure torr) the mass-spectrometric results from different laboratories agree fairly well and most discrepancies can be rationalized by taking into account the influence of the kinetic energy of the parent and/or their fragmentary ions on reactions with isobutene neutral molecules.' High-pressure mass spectrometry using a photoionization source (krypton resonqnce line at 10.03 eV or Lyman (Y hydrogen line at 10.2 eV) has the advantage over electron impact sources in that Visiting Professor, 1981-82. Permanent address: College of General Education, Tohoku University, Kawauchi, Sendai, Japan.
0022-3654/84/2088-1368$01.50/0
it leads to the appearance of the parent ion, i-C4H8+,only; therefore, one can follow without ambiguities its subsequent reaction s e q ~ e n c e . ~The , ~ *second ~ krypton resonance line at 10.63 (1) I. Koyano, J . Chem. Phys., 45, 706 (1966). (2) V. Aquilanti, A. Galli, A. Giardini-Guidoni, and G. G. Volpi, Trans. Faraday SOC.,63, 926 (1967). (3) F. P. Abramson and J. H. Futrell, J. Phys. Chem., 72, 1994 (1968). (4) M. hie, K. Aoyagi, K. Hayashi, J. Sohma, and T. Miiyamae, Bull. Chem. SOC.Jpn., 44,2261 (1971). (5) L. W. Sieck, S. G. Lias, L. Hellner, and P. Ausloos, J . Res. Natl. Bur. Stand., Sect. A 76, 115 (1972). (6) L. W. Sieck and P. Ausloos, J. Chem. Phys., 56, 1010 (1972). (7) M. S. Lin and A. G. Harrison, Int. J. Mass Spectrom. Ion Phys., 17, 97 (1975). (8) Z . Luczynski and J. A. Herman, Int. J. Mass Spectrum. Ion Phys., 31, 237 (1979).
0 1984 American Chemical Society
Kinetics and Deactivation of C4H8+and (C4H&+
.
0.71
e
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1369
-
v
6 0.5 + v)
“;1! t 0
-
P(i C,H,)/Torr
Figure 1. Pressure dependence of the fractional intensities for the two major ion families in pure isobutene.
eV accounts for less than 10% of the total intensity of the light emission in the vacuum-UV and does not seem to have any particular effect on the fragmentation of the parent ion. At 10.03-eV ionization energy the parent ion retains most of its original structure, the excess of internal energy being insufficient to rearrange it into another i ~ o m e r . ~ Two main channels for disappearance of the parent ions have been proposed: (i) proton or hydrogen atom transfer (reaction 1). followed by a “polymerization” reaction (reaction la), and (ii) i-C,&+
+ i-C4Hg
C4H9++ C4H7
-+
(1)
the C8H14+ ion formation (reaction 2), where IB stands for neutral i-C4H8+ + i-CdH8
-
(C4Hg)z’
IB
C ~ H I+ ~ i-C4Hlo +
(2)
isobutene. Most of the CgH14+ do not cluster further; only less than 10% cluster to form C8H14+(C4H8)and C8H14+(C4H8)2 species. If these two reaction channels, proton or hydrogen atom transfer (reaction 1) and the condensation reaction (reaction 2), were mutually independent, then the ratio of the fractional intensities, I(C4H9+)/I(C8Hl6+),should be independent of the isobutene pressure. However, the C4H9+and (c4H8)2+ ions, owing to their high reactivity, are not observed under our experimental conditions; therefore, this relation cannot be verified. Nevertheless, the shape of the sums of fractional intensities of the ions originating from C4H9+and (C4Hg)z’ species, namely, zZ(C4H9+(C4H&) and ~Z(c8H14+(c&&,,), as a function of the pressure of isobutene suggests strongly the existence of an interrelation between channels 1 and 2 (Figure 1). One of the purposes of the present study is a reexamination of the kinetics of this double-channel mechanism at high pressure. Under the present experimental conditions the parent ions are formed with a maximum excess internal energy [photon energy - IP(i-C4H8) = 10.03 - 9.23 = 0.8 eV], which, although insufficient to promote intramolecular r e a r r a n g e m e ~ ~can t , ~ play an important role in the proton- or atom-transfer process 1. On the other hand, the formation of the dimer ion, (C4H&+, in reaction 2 is also an exothermic process by about 1 eV and this excess of internal energy may play a role in subsequent processes, particularly in a reactive dissociation of the ion dimer. Therefore, one can conceive that the energy-transfer processes between excited (c4&+)* and (C4H8+)2*species and the neutral molecules can orient the subsequent reaction path depending on the residual energy content in the reactants. Besides this practical aspects of deactivation studies in isobutene, the energy transfer occurring during collisions between excited ion and chemically “inert” neutral molecules is theoretically and experimentally interesting to investigate. There are several the~~~
~
(9) S.G.Lias and P. Ausloos in “Ion-Molecule Reactions, Their Role in Radiation Chemistry”, American Chemical Society, Washington, DC, 1975, p 22.
oretical and semiempirical approaches to the calculation of the efficiency of deactivation processes: (i) the energy-transfer efficiency as a function of the depth of the interaction potential, Le., the dependence on -(dU/8r),lo (ii) the dependence on p L / 2 (where is the reduced mass of the reactive system), which is a measure of efficiency in relation to the duration of the collision,” (iii) the transfer efficiency as a function of the number of transitional modes available in a long-lived c o m p l e ~ , and ~ ~ -(iv) ~ ~a modified statistical approach assuming that the energy of the A+M complex is randomized among some degrees of freedom whose number is determined by matching the A+M lifetime with the collision d ~ r a t i 0 n . l ~Experimental data on the energy transfer between excited ions and neutral molecules have been reported for several systems, but experiments carried out systematically for many neutral bath gases seem to be limited. Ethylene and C5H9+,14J6benzene,” and 1,l-difluoroethylene” systems were investigated more thoroughly. The results on 1,l-difluoroethylene were interpreted by model ii, but those of ethylene did not fit this model and were explained by model iii. Clearly there is a need for experimental data on other systems. Consequently, another purpose of the present work is to determine the energy-transfer efficiency for (i-C4H8+)* and (C4H8+)2*excited ions for a number of nonpolar chemically “inert” gases. In order to experimentally determine such data we ionized selectively isobutene (IP = 9.23 eV) by 10.03-eV photons in mixtures in which the bath or “inert” gas has an ionization potential greater than 10.0 eV. Rare and usual atmospheric gases and Cl-C5 alkanesL7are not ionized at 10.03 eV and, therefore, were used for this study. We avoided the use of polar gases (HzO, NH3, etc.) as mixture components because the increased complexity of the mass spectrum would make a sensible interpretation more difficult. Experimental Section The experimental procedure and the high-pressure photoionization mass spectrometer have been already described.8 The measurements were made under field-free conditions inside the collision chamber with an electric field of low intensity ( - 5 V cm-I) applied in the ionic optics. The distance in the collision chamber between the plane of formation of the parent ions by photon impact and the exit slit was 4.2 mm. The experiments were done at room temperature, 25 f 2 OC, as indicated by a thermocouple connected to the collision chamber. The isobutene from Matheson of 99.5% purity was used without further purification. All other gases, also from Matheson, were of instrumental-grade purity and were used as such. The mixtures of known concentrations were prepared in a conventional highvacuum manifold. Kinetic Considerations For the purpose of the kinetic treatment, the mass spectra of pure isobutene were reproduced under the present experimental conditions in the range of 0.1-1.5 torr. The spectra obtained were, on the whole, similar to those already published.8 In spite of a number of m / z peaks in the mass spectrum, the sum of only two families, C4H9+(C4H8),and C8H14+(C4Hg)m, the former of which is the major product of reaction 1 and the latter of reaction 2, were taken into consideration. The fractional intensities of these (10) H. K. Shin, J . Chem. Phys., 42, 1739 (1965). (11) V. G. Anicich and M. T. Bowers, J . Am. Chem. SOC.,95, 1279 (1974). (12) E. E. Ferguson in “Ion-Molecule Reactions’’, J. L. Franklin, Ed., Plenum Press, New York, 1972. (13) P. Kebarle in ’Ion-Molecule Reactions”, J. L. Franklin, Ed., Plenum Press, New York, 1972. (14) P. G.Miasek and A. G. Harrison, J . Am. Chem. SOC.,97,714 (1975). (15) W.Forst and R. C. Bhattacharjee, Chem. Phys., 37, 343 (1979). (16) R. Houriet and J. H. Futrell, Ada Mass Spectrom., 7A, 335 (1977). (17) The other resonance line of Kr (10.63 eV) whose intensity is much weaker can ionize C4 and Csalkanes, but it seems that the ionization of these alkanes does not disturb the mass spectra to such an extent as to render the measurements of the deactivation efficiency meaningless. \ -
Nagase and Herman
1370 The Journal of Physical Chemistry, Vol, 88, No. 7, 1984 30 -
20 -
-
c
\
P
+.- 10-
1.o
0.5
1.5
P(r-C4He) Torr
Figure 2. Pressure dependence of the fractional intensity ratio for pure isobutene (0),and isobutene nitrogen mixtures: (0) 95%, (A)90%, and (0)80% NZ.
+
two families of ions reach -0.7 at 1 torr (Figure 1). The plot and ZS7 stand for of 1110/157 as a function of pressure, where ZIlo the fractional intensities of E [CsH14+(C4Hs)m]and E[C4H9+(C4Hs),,]ions, respectively, gave a straight line, which nearly passes through the origin of the axes (Figure 2), indicating the following relation: 1110/157
= (constant)Pi.C,Hs
Figure 3. Fractional intensity ratios for hydrogen and rare gases as a function of the total pressure. The concentrations of isobutene in the mixtures are tabulated in Table I.
(3)
When an “inert” gas is added to the system, the ratio 1110/157 tends to increase, but the plot of this ratio vs. the partial pressure of isobutene again gave a straight line passing through the origin of the axes. As an example, the plot of 1110/157 for the mixture N, i-C4Hs is shown in Figure 2. The C4H9+ion is found to be formed by a proton transfer or a hydrogen atom abstraction from the methyl group preferenti all^.^,^ It has been shown in several cases that the rate of proton-transfer reaction increases and approaches the collisional rate as the internal energy of the cation is decreased.l8 In the present case the parent ion has a maximum of 0.8-eV excess energy and, therefore, one should expect the rate of C4H9+ion formation by proton or hydrogen atom transfer to be lower than the collision rate. At low kinetic energy and low temperature most of the proton-transfer processes are taking place through a physically bonded ion-molecule complex (called association ion) whose lifetime extends between lo-” and 10” s, and in some cases such complex ions can survive the time of flight in the mass spectrometer and be detected. In fact, at low pressure (
