60
J. Phys. Chem. 1982, 86, 60-66
Multlphoton Ionization-Fragmentation Patterns of Alkyl Iodides D. H. Parker and R. B. Bernsteln' Department of chemistry, Columbia University, New York, New York 10027 (Received: September 1, 1981)
Multiphoton ionization (MP1)-fragmentation patterns are reported for six alkyl iodides (methyl,ethyl, n-propyl, isopropyl, n-butyl, and tert-butyl) in the 4W360-nm region. Three- or four-photon ionization is resonantly enhanced via two-photon excitations to various spin-orbit components of the 5pa 6s molecular Rydberg transition. For each of these molecules a minimum of seven photons (overall) must be absorbed to account '). For methyl iodide it appears that different for the observed ion fragments (which include C+ and H intermediate Rydberg states can affect the rate of ionization and thus the subsequent ion fragmentation process, possibly due to retention by the ion of the differing Rydberg-state geometries. Such distinct fragmentation effects are not observed for different resonant states in the larger alkyl iodides. Although the resonant-state spectra are essentially continuous, significant structure-sensitiveinformation can be obtained by means of the MPI mass spectral (MPIMS) technique. For the isomers n-propyl iodide and isopropyl iodide (whose electron impact mass spectra are essentially identical) the MPIMS patterns are significantly different; similarly for tert-butyl iodide and n-butyl iodide. This is attributed to differences in the spectroscopic and dynamics properties of the first-formed fragment ion in the MPI-fragmentation process. Specifically, the n-propyl and n-butyl cations do not isomerize appreciably within the 5-ns laser pulse duration but absorb the 368-nm radiation and photodissociate, whereas the isopropyl and tert-butyl cations are essentially transparent at this wavelength and thereby persist without fragmentation.
-
Introduction Much progress has been made over the past several years in understanding resonance-enhanced multiphoton ionization-dissociation (REMPID) processes in polyatomic molecules. REMPI is, in itself, a mature branch of twoand three-photon spectroscopy' in which the intensity of a (typically) three- or four-photon ionization process is strongly enhanced by the existence of an intermediate excited electronic state, often a molecular Rydberg state,' reached via two- or three-photon absorption. Analysis of the usually extensive ion photofragmentation which takes place during the laser pulse duration has been the subject of much recent research.2 One motivation for such study is in the development of REMPI as a tool for analytical mass ~pectrometry.~ Several techniques have been used to examine the successive steps in the REMPID process. Kinetic-energy analysis4 of the photoelectrons, cations, and anions5 has established for several molecules that fragmentation takes place up the "ionic ladder" (rather than via the neutral (1)(a) P. M. Johnson, Acc. Chem. Res., 13, 20 (1980); (b) P. M. Johneon, Annu. Reu. Phys. Chem., in press; (C) D. H. Parker, J. 0. Berg, and M. A. El-Sayed in "Advancea in Laser Chemistry", A. H. Zewail, Ed., Springer, West Berlin, 1978, p 320. (2)(a) U.Boesl, H. J. Neueser, and E. W. Schlag, Z . Naturjorsch. A, 33, 1546 (1978); (b) L. Zandee, R. B. Bernstein, and D. A. Lichtin, J. Chem. Phys., 69, 3427 (1978;L. Zandee and R. B. Bernstein, ibid., 70, 2574 (1979);71, 1359 (1979); (c) V. S. Antonov, I. N. Knyazev, V. S. Letokhov, V. M. Matiuk, V. G. Movshev, and V. K. Potapov, Opt. Lett., 3, 37 (1978); (d) S. D. Rockwood, J. P. Reiily, K. Hohla, and K. L. Kompa, Opt. Commun., 28,175(1979);J. P.Reilly and K. L. Kompa in 'Laser Spedroscopy", VoL IV,Springer-Verlag,West Berlin, 1979;p 631; (e) D. A. Lichtin, S. Datta-Ghoeh, K. R. Newton, and R. B. Bemstein, Chem. Phys. Lett., 75,214 (1980);K. R. Newton, D. A. Lichtin, and R. B. Bemstein, J. Phys. Chem., 85, 15 (1981); (f) T.G. Dietz, M. A. Duncan, M. G. Liverman, and R. E. Smalley, J. Chem. Phys., 73, 4816 (1980). (3)D. A. Lichtin, L. Zandee, and R. B. Bemstein, Chap. 6 in "Lasers in Chemical Andy&",G. Hieftje, J. Travis, and F. Lytle, Eds., Humana Press, Clifton, NJ, 1981,p 125. (4)(a) J. T.Meek, R. K. Jones, and J. P. Reilly, J.Chem. Phys., 73, 3503 (1980); (b) J. C. Miller and R. N. Compton, ibid., 75, 22 (1981). (5)M. S. DeVries, N. J. A. Van Veen, T. Baller, and A. E. DeVries, Chem. Phys., 56, 157 (1981). 0022-3654/82/2086-0060$0 1.2510
autoionization route). Two-laser studies6 have confirmed this for several cases and have also indicated the importance of the parent-ion absorption spectrum in determining the extent and the laser-power dependence of fragmentation. Much more detailed information on this aspect is provided by a combined electron impact-laser technique' where (parent and daughter) ions are formed by electron impact and then selectively dissociated by a pulsed tunable dye laser. A number of other studies, experimental"" and theoretical,12J3have contributed to the understanding of the REMPID process. In many cases it appears that the total energy available to the parent ion after multiphoton ionization occurs is important,'OJ1 but the manner in which the ion was formed is not. This premise is tested for a number of simple alkyl iodides; evidence to the contrary will be presented for methyl iodide. The usefulness of REMPID to analytical mass spectroscopy will be governed by sensitivity and selectivity. Although the duty factor of present pulsed lasers is low (cf. CW electron impact sources),the ionization probability per pulse is typically greater for photons. Spectroscopic selectivity is the major advantage of MPI, however. Selective photoionization of isomeric and isotopic molecules has been demonstrated.'&"j For the isomers azulene and (6)(a) U.Boesl, H. J. N e w e r , and E. W. Schlag, J. Chem. Phys., 72, 4327 (1980); (b) K.R. Newton, D. A. Lichtin, and R. B. Bernstein, J. Phys. Chem., 85, 15 (1981). (7)K. R. Newton and R. B. Bematein, to be submitted for publication. (8)G. J. Fisanick, T. S.Eichelberger, B. A. Heath, and M. B. Robin, J. Chem. Phys., 72,5571 (1980). (9)R. Pandolfi, D. A. Gobell, and M. A. El-Sayed, J. Phys. Chem., 85, 1779 (1981). (10)T.Carney and T. Baer, J. Chem. Phys., 75,477 (1981). (11)D. H. Parker, D. A. Lichtin, and R. B. Bemstein, J. Chem. Phys., 75,2577 (1981). (12)J. Silberstein and R. D. Levine, Chem. Phys. Lett., 74,6 (1980). (13)(a) F. Rebentrost and A. Ben-Shaul, J. Chem. Phys., 74,3255 (1981); (b) F. Rebentrost, K. L. Kompa, and A. Ben-Shad, Chem. Phys. Lett., 77,394 (1981). (14)D. M. Lubman, R. Naaman, and R. N. Zare, J. Chem. Phys., 72, 3034 (1980).
0 1982 American Chemical Society
MPI-Fragmentation Patterns of Alkyl Iodides
The Journal of Physical Chemistry, Vol. 86, No. 1, 1982 61
naphthalene the laser can be wavelength-tuned to a resonance of one molecule, ionizing it without exciting the other isomer.14 Similar results have been reported for substituted anilines.’& In both studies the fragmentation patterns, both E1 and MPI, of the two isomers were readily distinguishable. The geometric isomers cis- and transdichloroethylene can be selectively ionized by appropriately tuning the laser.l6 Here the fragmentation patterns, both E1 and MPI, were found to be essentially identical. In the present study it will be shown that the isomers isopropyl iodide and n-propyl iodide, whose E1 fragmentation patterns are almost indistinguishable, cannot be wavelength-selected because of overlapping absorption spectra; yet they can be easily distinguished by their very different REMPID patterns. Similar results are also noted for the isomers tert-butyl iodide and n-butyl iodide. As in the previously reported MPI-fragmentation study of the tertiary amines,” considerable structure-sensitive information can be obtained from the REMPID results despite the absence of resolvable spectroscopic features in the REMPI bands. Thus, many molecules formerly believed to be less interesting for study by MPI spectroscopy now become important subjects of investigation via laser mass spectrometry. These include the alkyl iodide family, six members of which have been chosen for the present study.
Experimental Section The laser ionization time-of-flight mass spectrometer (TOFMS) has been described in detail previously.2BLaser wavelengths useful in this study were generated by mixing the fundamental of a Nd:YAG laser with the output of a tunable dye laser, pumped by the NdYAG second harmonic. A 250-mm fl lens was used to focus the final laser beam into the ionization region of the TOFMS. Laser pulse energies (mJ) were measured by a calorimeter. Electron impact mass spectra a t 70 eV were recorded for comparison with MPI-fragmentation patterns obtained in the same apparatus. Sample pressures were typically 1 x iO-5-2 x 10“ torr. The samples used were commercial grade, degassed before use. The larger alkyl iodides were found to contain small fractions of isomeric impurities as well as some methyl iodide. No effort was made to correct for these, except that laser wavelengths were chosen to avoid methyl iodide absorptions. The integrated ion signal vs. wavelength (i.e., the MPI spectrum) for each molecule was found to be in good agreement with known MPI cell spectral7 (except for isopropyl iodide and tert-butyl iodide, whose spectra have not been published). In the next section TOFMS results for all six alkyl iodides will be summarized and discussed. Results and Discussion REMPID patterns as a function of laser pulse energy will be presented for methyl, ethyl, n-propyl, isopropyl, t n-butyl, and tert-butyl iodides and compared with E1 spectra at 70 eV. Additional MPI mass spectra at different wavelengths will be displayed for methyl iodide. The excitation wavelengths were chosen to coincide with the vibrationless origins of the various spin-orbit components (15)(a) A. Herrmann, S. Leutwyler, E. Schumacher, and L. Wbete, Chem. Phys.Lett., 52,418 (1977);(b) E.W.Rothe, B. P. Mathur, and G. P. Reck, ibid., 53,74(1978);(c) C.T.Rettner and J. H. Brophy, Chem. Phys.,56,53 (1981). (16)J. W.Hudgens, M. Seaver, and J. J. DeCorpo,J. Phys.Chem., 86, 761 (1981). (17)D. H.Parker, R. Pandolfi, P. R. Stannnrd, and M. A. El-Sayed, Chem. Phys.,45, 27 (1980).
nethyl iodide
CH31
d P I 3 7 0 n m (A1 v.0)
3.0mj CH?
I
I+
1
2.4mj
I
0.4mj
20
0
120 MASS
140
Figwe 1. REWID mass spectra of methyl iodide, two-photon resonant with the %, A, origin, at several specified laser pulse energies (mJ pulse-‘). Bottom panel shows E1 mass spectrum at 70 eV for comparison. methyl iodide
CH3I
M P I 3 7 0 n m 1.6mj
I+
H+ P
CHf
C+
hh
‘
5
L
15
130
140
MASS
Flguro 2. Higher-resoiution REMPID mass spectrum of methyl iodide extended down to H+.
-
of the 5 p r 6s molecular Rydberg transition.l8 Of particular interest are the Al and E components (3?r0 and ‘r) lying near 370 and 366 nm for each m01ecule.l~.No significant differences in the total ion currents were observed between isopropyl iodide and n-propyl iodide or (18)(a) G.Herzberg, “Molecular Spectra and Molecular Structure”, Vol. 111, Van Nostrand, Princeton, NJ, 1967. (b) J. D. Scott, W. S. Felps, G. L. Findley, and S. P. McGlynn, J. Chem. Phys.,68, 4678 (1978).
82
The Journal of Physical Chemistty, Vol. 86,No. 1, 1982
24
20
Parker and Bernstein
It
-
! ~
CH,I
I
0 '
I
Figure 9. Energy-level dlagram for REMPID of methyl lcdkle showlng two-photon resonant three-photon ionization followed by successive photofragmentation of CH31+ (level 3). Dashed lines are estlmated; solid Hnes are known appearance potentials of (possiMe) ion fragments.
between tert-butyl iodide and n-butyl iodide. Methyl Iodide. Figure 1shows the REMPID patterns for methyl iodide at several laser pulse energies, for a laser wavelength of 370 nm, resonant with the Al origin by two-photon absorption. To be noted is the typically observed continuous shiftii of fragmentation toward smaller masses with increasing laser pulse energy. The 70-eV E1 spectrum of methyl iodide is shown in the bottom panel of Figure 1. The total ion current was found to increase as the square of the laser pulse energy, consistent with a two-photon resonant-state "bottleneck". Higher-resolution scans of the methyl iodide fragmentation pattern are shown in Figure 2 for low- and high-maas regions. These spectra were taken in separate scans in order to include H+, which has not been monitored in the other REMPID spectra in this paper. (The apparent relative intensity of H+ was found to vary from time to time much more than that for the larger ions.) Although the similarity of the CH,+ and CH,I+ patterns (n = 0, 1, 2,3) is striking in this figure, such similarity was not always observed at other laser wavelengths and pulse energies. The low relative abundance of CH+ and CHI+ is general for REMPID of methyl iodide. However, this is not found in the E1 spectrum. The qualitiative features of REMPID of methyl iodide can be interpreted with the aid of Figure 3. Shown are the appearance potentials of the possible ionic fragments of methyl iodide vs. the minimum number of near-UV photons. Level 1 is a virtual state in the two-photon resonant three- or four-photon ionization process. Two ionization potentials are shown for methyl iodide (level 3) corresponding to the spin-orbit components 2E3/2 and ?E+!. The REMPID spectra shown in Figures 1and 2 are resonance enhanced via the AI (3?ro) Rydberg state which correlates with the upper (2El/2)ionization potential. Solid lines in Figure 3 indicate measuredlg appearance potentials; those with dashed lines are estimated. Disso(19) H.M. Roaenstock, K.Draxl, D. W. Steiner, and J. T. Herron, J. Phys. Chem. Ref. Data, 6, Suppl. No. 1 (1977).
INTENSITY
IO PHOTON ENERGY (cm-') Flgure 4. Detalled energy-level diagram for REMPI of methyl iodide via several two-photon resonant states. Threaphoton ionization is posdble for photon energies above 27 250 cm-'. The lower 3 ~ E1state correlates with the *Ejl2 CH3I' otentlal; the upper 37r0A, and 'iTl E states correlate with E,,, CH31 . Also shown are the photoelectron spectrum and the UV absorption spectrum of CHJ including the A, ( v = 0, 1) bands, marked by asterisks, which appear only in the two-photon spectrum.
P
ciation to CH3+ can produce two atomic iodine components, U2P32) and I*(2P1/2).The lower appearance pois based upon an empirically adjusted heat tential for of formation.20 From this diagram it is evident that at least seven photons are required to produce C', H+, and CH+ from CH31. It is possible that even more photons are actually absorbed since the intermediate dissociation species are known4to possess kinetic and internal excess energy. More information on the detailed photophysics of methyl iodide REMPID can be obtained by varying the laser wavelength. Three separate electronic transitions are accessible by two-photon resonant MPI in this region, corresponding to the 3 ~ E1 state and the 3 ~ Al 0 and lr1E excited states from the 5p7r 6s molecular Rydberg transition. The energy positioning of these states and other relevant information for methyl iodide REMPID is shown in Figure 4. The Al state, which is not observed by conventional one-photon absorption spectroscopy, is included in the UV absorption spectrum. Asterisks mark its vibrationless origin and the first quantum of the main vibrational progression-the Yg totally symmetric methyl umbrella stretch.l* The lowest-energy excited state, the broad absorption from 4 to 6 eV, is responsible for the well-studied methyl iodide photochemistry.21 The photoelectron spectrum22of methyl iodide also included in Figure 4 shows the two spin-orbit components
d+
-
(20) J. Silberstein and R. D. Levine, J.Chem. Phys., 75,5735 (1981). (21)(a) M.Dzvonik, S.Yang, and R. Bersohn, J. Chem. Phys., 61,4408 (1974); (b) M.Shapiro and R. Bersohn, ibid., 73, 3810 (1980). (22) J . H. D. Eland, R. Frey, A. Kueetler, H. Schulte, and B. Brehm, Int. J. Mass Spectrom. Ion Phys., 22, 155 (1976).
MPI-Fragmentatlon Patterns of Alkyl Iodides
The Journal of phvsical Chemistty, Vol. 86, No. I, 1982 63
methyl iodide C H S I M P I 2.4mj
0
20
120
140
MASS
Flgure 5. REMPID mass spectra for methyl Iodide at constant laser pulse energy via separate (Indicated) Intermediate states.
of ground-state CH81+and the first excited state lying near 12 eV. The photodissociation spectrum of CH31+,which has also been reportedB for the lower region of the first excited state, is similar (while resolving some vibrational fine structure). The lowest observed state, E ( 3 ~ 1 )with vibrationless origin at 6.12 eV, correlates in the limit of strong spin-orbit coupling with the E3/2 component of CH31+while the other two components, Al and E and ~ T J correlate , with the ElIz component. For this upper level four laser photons (A 1 367 nm) are required for ionization. The vibrationless origin of the second E state and the 6; band of the Al state lie in the three-photon ionization region. MPI cell spectral7 show only a slow rise in ionization signal when crossing this limit. Figure 5 shows the REMPID patterns of methyl iodide a t constant laser power for the four resonances shown in Figure 4. The longest-wavelength excitation (402 nm) results in the most extensive fragmentation observed-as indicated by the almost complete loss of parent ion. The total ion yield is low because of the four-photon requirement, however, and is consistent with the relative intensities of the REMPI cell spectra.17 Such fragmentation behavior can be easily understood from Figure 4, where the fourth photon is seen to lie in the region of the first excited states of CHSI+. The “doorway” to further absorption and resulting fragmentation is thus “open” compared to the shorter wavelengths where the fourth photon terminates in a region of weak ion absorption. This (23)D.C. McGilvery and J. D. Morrison, J. Chem. Phys., 67, 368 (1977).
mechanism has recently been shown to control the extent of fragmentation in other molecules also, notably hexadiyne’O and the tertiary amines.’, Further information can be obtained from the three higher-energy resonances. The results shown in Figure 5 indicate a much larger degree of fragmentation for the (u = 0 and 1) Al bands than for the interposed E(v = 0) band. (To obtain the same degree of fragmentation, it is necessary to double the laser pulse energy when resonant with the E state?, The total ion current is again consistent with the REMPI cell spectra where the relative ion yield is roughly 2:1:0.5 for the A,(u = 0), E(u = 0), and A,(u = 1) bands, respectively. Since the ion absorption spectrum is essentially continous in this it is difficult to rationalize this fragmentation trend through a chance doorway variation. A correlation with the type of intermediate resonant state selected (A, vs. E) seems indicated by the marked similarity of the two A, band fragmentation patterns. A qualitative explanation for this possible dependence on the resonant intermediate state arises from consideration of the excited-state geometry. Rotational line-shape analysis17of the REMPI spectra suggested that the HCH bond angle is larger for the A, state than for the ground state, while the E-state and ground-state geometries are very similar. An approach toward planarity of the methyl group would elongate the C-I bond, making the molecule less stable. As noted, REMPID through the A, levels results in significantly more fragmentation (at a given laser pulse energy) that the E level. If the different geometries of the two Rydberg states are retained in the ion, different fragmentation could be expected. Successive photoabsorption must take place very quickly compared to collisional relaxation effects-with a long-time limit of the -5-ns laser pulse length. Similar effects were not detected in the REMPID patterns of the larger alkyl iodides. Becuase of the almost diffuse spectra of these systems, caused in part by sequence congestion, it is much more difficult to excite a single resonant state component. REMPZD Patterns for Larger Alkyl Zodides. Total ion currents were found to increase quadratically with laser pulse energies, as expected, for the four larger RI molecules. REMPID patterns for ethyl iodide at several laser pulse energies are compared with the 70-eV E1 mass spectrum in Figure 6. The laser wavelength at 372 nm is two-photon resonant with the Al absorption region. Similar fragmentation patterns were observed with other wavelengths at similar laser powers. Almost no parent ion or parent minus hydrogen(s) is observed with MPI, even for the lowest laser pulse energies, indicating strong parent-ion absorption at these wavelengths. The appearance of C+ and Cz+at higher pulse energy is also noted in Figure 6. The observed changes in the fragmentation patterns for a fourfold increase in laser power are rather small and will be discussed later. However, since the “expensive” C+ ions are formed, a large number of photons must be absorbed at high laser powers. Another interesting difference between the REMPID and E1 mass spectra concerns mass 28 (CZH4’). As shown in more detail in Figure 7, almost no CzH4+is observed with MPI, in contrast to EI. REMPID of molecules with similar alkyl groups (e.g., triethylamine) at similar laser wavelengths has been shown to produce an abundance of C2H4+.I1 This ion is simply not formed in the REMPID of ethyl iodide. The REMPID and E1 patterns for the isomers n-propyl iodide and isopropyl iodide are compared in Figure 8. The
84
The Journal of Physical Chemistry, Vol. 86, No. 1, 7982
Parker and Bernstein
observed in the tertiary amines." A major difference, m2&Y---l however, is that here a large number of fragments are C2H i+
M P I 372nm
0.5mj
20
120 MASS
140
160
Flgure 8. REMPID mass spectra for ethyl Iodide at different lndlcated laser pulse energies, compared wth E1 at 70 eV. e l h y l iodide
QH5I
MPI 365 nm 0 . 4 mi
b
€ 1 70cV
-
7-
25
30 MASS
Figure 7. Higher-resoMionREMPID pattems for ethyl kdide compared with E1 mass spectrum. Note the near absence of mass 28 (C2H,+) via REMPID.
strong similarity of the E1 spectra, taken under identical conditions, precludes distinguishing these isomers by their E1 spectra. However, their REMPID patterns are very different at the same wavelengths and pulse energy. For both isomers the parent ions are *burned out" by the laser photons of 368 nm. But consider the REMPID patterns for isopropyl iodide. No significant change in fragmentation is observed over a threefold range of laser pulse energy. The total ion yield is found to increase strongly with laser power, as expected. Such behavior, although atypical of most REMPID patterns, has also been
observed and found independent of laser power in contrast to only two species (the parent and a large daughter ion) for the tertiary amines. It is to be expected that at least some of the fragments would further absorb photons from the laser pulse, leading to increasing fragmentation with increasing laser power. Similar effects are observed for n-propyl iodide, though less pronounced. For a fourfold increase in pulse energy, only a small fraction of fragments (C+,C2+,and CH+ ions) are generated. This is again atypical REMPID behavior. Only the I+ yield shows a dependence on the laser power, for both propyl iodide and butyl iodide. n-Butyl iodide and tert-butyl iodide are compared in Figure 9 for REMPID and 70-eV EI. The two molecules can be distinguished by EI, with the tert-butyl isomer fragmenting to a larger extent. The appearance of CzH4+ should also be noted in both E1 spectra. The REMPID patterns are most surprising for tert-butyl iodide. Essentially only one ion is observed (a fragmentation) over a threefold energy range. In contrast, the REMPID patterns for n-butyl iodide show extensive fragmentation (no parent ion present) which, once more, has only a slight dependence on the laser power. tert-Butyl iodide fragments mainly to the (CHd C+ ion, with almost no parent ion formation. (Asfor ethy! iodide and the propyl iodides, again no C2H4+is observed in the REMPID mass spectra.) For tert-butyl iodide, the lack of fragmentation with increasing laser pulse energy implies that the tert-butyl cation (or its precursor) does not dissociatively absorb at this laaer wavelength. Either this cation is formed directly via the absorption of a fourth photon or a precursor such as a highly excited parent ion produces the tert-butyl ion in a short time (ca.1 ps) after absorbing the fourth photon. The latter suggestion is attractive for interpretation of REMPID of the other alkyl iodides. If the precursor of the observed fragments has a lifetime 7 2 T ~the , laser pulselength, most of these fragments will not be excited (and further dissociated) by the laser. The complicated fragmentation pattern will be unchanged with increasing pulse energies (although the overall ion yield increases). Less-stable precursors will yield fragments earlier (7< 71) and thus show stronger fragmentation-pulse energy dependence. Metastable ions, commonly found in E1 studies, have been recently observed also in MPI mass spectra.24" Only when the precursor lifetime is of the order of the ion acceleration time (Le., microseconds) can the metastable processes be detected (via peak broadening). Metastable lifetimes 7 = T~ (-5 ns) are consistent with this mechanism, which accounts for most of the general features of the REMPID spectra. For experiments at increasing laser power, multiphoton absorption by the precursor should eventually dominate, yielding more complex fragmentation behavior. Further experiments are obviously necessary to gain a better understanding of the REMPID process.
Concluding Remarks For methyl iodide several resonant states have been compared and found to affect the subsequent ion fragmentation process. For all of the alkyl iodide molecules, the importance of the parent-ion absorption spectrum has been noted. The isomer pairs n-propyl iodide-isopropyl iodide and n-butyl iodide-tert-butyl iodide have been (24) D. Proch, D. M. Rider, and R. N. Zare, Chem. Phys. Lett., 81,430 (1981). (25) Unpublished results on tertiary amines from this laboratory.
MPI-Fragmentation Patterns of Alkyl
Iodides
The Journal of Physlcal Chemlsrry, Vol. 86, No. 1, 1982 65
--IF n-propyl iodide
I
n-C3H7I M P I 368 n m
n,-C3H7+
CzHf
I
isDpropyliodide;,CzH I
'
d 1.8mj
1.0 mi
0.5 m j
P
1
20
120
40
140
160
180
-20
120
40
140
160
180
MASS
MASS
Figure 8. REMPID and E1 mass spectra of n-propyl iodide and isopropyl iodide. Presentation similar to Figure 6. n - b u t y l iodide
n-CqHgI
1-butyl iodide (CH+,CI
MPI
CZH2
CH?
1)
I 1,
368nm 1.5mj
M P I 368 nm 1.5mj
(CHs),C'
n- C4H9*
1
I+ I+ '1
p
, I
'
I
1.0 mj
1.0 m i
L
- , ,
0.5 mi
0.5 mi
I
20
40
60
120 MASS
140
160
180
20
.I
40
-
,
60
I
I
I
120
140
I60
180
MASS
Figure 0. REMPID and E1 mass spectra of n-butyl iodide and tert-butyl iodide. Presentation similar to Figures 6 and 8.
examined by REMPID and EI. Even in the absence of wavelength selectivity, REMPID mass spectra contain
significant structure-sensitive information on these molecules. A simple mechanism involving metastable precur-
J. Phys. Chem. 1082, 86,66-70
66
sors has been proposed to account for the atypical behavior of the REMPID fragmentation patterns of the alkyl iodide molecules with increasing laser pulse energy. A key finding of the present study is that the n-propyl and n-butyl cations do not isomerize appreciably (to isopropyl or tert-butyl cations, respectively) within the 5-ns laser puke duration but absorb the 368-nm radiation and photodis-
sociate, whereas the isopropyl and tert-butyl cations are essentially transparent at this wavelength and thereby persist without fragmentation. Acknowledgment. This work has been supported by NSF Grants CHE 78-25187 and CHE 77-11384, hereby acknowledged with thanks.
Klnetics of the Reactions of Methoxy and Ethoxy Radlcals wlth Oxygen D. Outman,'' N. Sanders,? and J. E. Butler Chemistry Divhbn, Nevel Research Laboratory. Washington, D.C. 20375 (Received: September 4, 1981)
Rate constants have been obtained as a function of temperature for the gas-phase reactions of methoxy and ethoxy radicals with oxygen. The alkoxy radicals were produced by the 266-nm photolysis of the corresponding nitrites, and their concentrationswere monitored by using laser-induced fluorescence. The CHBO+ O2reaction was studied from 140 to 355 "C, and the rate constants obtained were used to derive an Arrhenius expression, 6.3 X lo7exp(4.6 kcal/RT) M-' s-l. The C2HS0+ O2reaction was studied at two temperatures, and the rate constants obtained were 4.8 x lo6 M-' s-l (23 "C) and 5.9 x lo6 M-l s-l (80 "C). Upper limits for the rate of the isomerization reaction, CH30 + M CHzOH + M,are calculated, and the possible use of the results of this study in atmospheric smog modeling is discussed.
-
Introduction Alkoxy radicals are important reaction intermediates formed during the gas-phase oxidation of most hydrocarbons. In the early, low-temperature stages of combustion these labile intermediates form largely through the decomposition of peroxidic species: ROOR' -* RO. + R'O.
(1)
Subsequent reactions of these radicals ultimately produce such products as aldehydes, ketones, and alcohol^.^,^ In photochemical smog cycles, alkoxy radicals are produced from the oxidation of nitric oxide by alkylperoxy radicals:
ROz
+ NO
--+
RO. + NOz.
(2)
Under atmospheric conditions RO- can react with oxygen
RCH20.
+02
---*
RCHO
+ H02
(3)
isomerize or d e c ~ m p o s e . ~ Although the reactions of alkoxy radicals have been the subject of numerous studies, our present knowledge of the reactivity of these intermediates is still largely based on indirect kinetic measurements. The primary obstacle to performing direct studies has been the lack of a suitably sensitive method for dynamically monitoring these free radicals under well-defined reaction conditions. Recently (1) On sabbatical from the Department of Chemistry, Illinois Institute of Technology, Chicago, IL 60616. (2) NRL/NRC Postdoctoral Research Associate (1979-1981). Present address: E u o n Research and Engineering Co, Linden, NJ 07036. (3) Pollard, R. T. In "Comprehensive Chemical Kinetics";Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: New York, 1977; Vol. 17, Chapter 2. (4) Bradley, J. N. "Flame and Combustion Phenomena"; Methuen: London, 1969. (5) Pitts, J. N., Jr.; Finlayson, B. J. Angew. Chem., Int. Ed. Engl. 1975, 14. 1 and references therein.
Inoue, Akimoto, and Okuda have observed and analyzed the laser-induced fluorescence (LIF) spectra of three simple alkoxy These studies have now provided the spectroscopic information needed to use LIF as a diagnostic method in future chemical kinetic studies of the reactions of the three radicals, CH30, C2Hs0,and C2H30. The first direct study of an alkoxy radical reaction was reported by Sanders, Butler, Pasternack, and M~Donald.~ The investigation involved the production of CH30 using pulsed UV laser photolysis of CH30N0 and the subsequent monitoring of the decay of CH30 by LIF in the presence of different reactant gases. A rate constant for the reaction with NO was obtained,1° as were upper limits on the rate constants of nine other reactions, all at room temperature. We wish to report that this same experimental procedure has now been used with a heated reaction vessel to study the reactions of CH30and C2H50with oxygen at several temperatures. Prior studies of RO + O2reactions have all been of the simplest of the series, namely CH30 + O2 CHzO + HOz (4) Three recent publications have focused on reaction 4, reporting rate constants at various temperatures."-13 All
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(6) Inoue, G.; Akimoto, H.; Okuda, M. J. Chem. Phys. 1980,72,1769. (7) Inoue, G.; Akimoto, H. J . Chem. Phys. 1980, 74,425. (8)Inoue, G.; Okudo, M.; Akimoto, H. J . Chem. Phys. 1981,75,2060. (9) Sanders, N.; Butler, J. E.; Paaternack, L. R.; McDonald Chem. Phys. 1980,48, 203. (IO) The room-temperaturevalue for the CH90 + NO rate constant reported in ref 9 is valid only for the 15 i 5 torr of SF, buffer gaa pressure used in the experiments on this reaction. Subsequent work at other pressures confirms that this reaction is in the falloff region at 15 torr. N.D.S. thanks Dr. L. S. Batt of Aberdeen University for pointing out the likelihood that the mechanism of the CHsO + NO reaction under the conditions of the earlier study waa primarily the pressure-dependent recombiantion process. (11) Barker, J. R.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1977, 9, 31. (12) Batt, L.; Robinson, G . N. Int. J . Chem. Kinet. 1979, 9, 1045.
This article not subject to U.S. Copyright. Published 1982 by the American Chemical
Society
