J. Phys. Chem. 1992,96, 5715-5719
-
820
-
800
-
780
-
Q)
TS12
A+ H20
......
0
E 760 -
'=Y
d
740 -
/
TS23
/ /
720
r->
loniz. Onset
U 700 680 -
CH3CHzCHzOH+
Ob)
I ..... : I./ .... ...
'\
10.5 eV,lSrespectively. The photoelectron spectrum of propanol19 exhibits a moderately broad and structureless band in the region of the IP. It is, for instance, twice as broad as the first band in the thiopropanol PES. This suggests that there is a considerable geometry change accompanying ionization, thus supporting the existence of a lower energy Ib structure. On the other hand, the PES peak appears too narrow to be consistent with a shift of 0.9 eV between the adiabatic and vertical ionization energies. It may well be that the n-type structure is stable but that a small barrier separates it from the a-type structure. Higher level calculations may shed more light on this interesting ion.
A,...ergyAcknowledgment. We are grateful to the Department of Enfor financial support of this work, and to the North Carolina
I
'
CHzCHzCHzOHz
5715
H20
(Ill)
+
(11)
Potential energy diagram based on the 0 K heat of formation scale. The solid line is b a d on energy calculated with the 6-31GSSbasis set at the MP2 level. The dashed line rectangles are the experimental energies. Figure 2.
density thereby slowing down the dissociation rates into the microsecond domain. Third, the region of the potential energy surface of the distonic ion and ion/dipole complex is also fairly deep compared to the dissociation asymptote, and thus may also support a large density of states. An important feature in the potential energy surface in this region is the low barrier between the distonic ion and ion/dipole complex. Thus, it may be that the reacting configuration for H20loss is actually an equilibrium between these two structures. Another low energy ion is the allyl radical/hydronium ion complex. However, it is separated from the distonic ion via a substantial barrier and it does not lead to dissociation a t low energies. Thus, it does not appear to be implicated in the overall dynamics of the dissociation. Holmes et a1.2 have suggested that this ion is important in the dissociation of 2-methylpropane- 1,3-diol. The collision induced dissociation spectrum of this C3H80+isomer was very different from that of the propanol ion. Perhaps the most spectacular result of this a b initio study is the low MP2/6-31G** heat of formation of the a-type molecular ion (Ib) which suggests that the adiabatic ionization energy of propanol is 9.6 eV, which is 0.6 and 0.9 eV below the photoionization onset of 10.22 eV1 and the vertical ionization energy of
Supercomputer Center for a generous grant.
References and Notes (1) Rafaey, K. M. A.; Chupka, W. A. J . Chem. Phys. 1968, 48, 3205. (2) Holmes, J. L.; Mommers, A. A.; Szulejko, J. E.; Terlouw, J. K. J . Chem. SOC.,Chem. Commun. 1984, 165. (3) Cao, J. R.; George, M.; Holmes, J. L. Org. Mass Spectrom. 1991,26, 481. (4) McAdoo, D. J.; Ahmed,M. S.; Hudson, C. E.; Giam, C. S . In!. J. Mass Spectrom. Ion Processes 1990, 100, 579. (5) Yamamoto, M.; Takeuchi, T.; Nishimoto, K. In?.J.Mass Spectrom. Ion Processes 1983, 46, 239. (6) Takeuchi, T.; Ueno, S.; Yamamoto, M.; Matsushita, T.; Nishimoto, K. Int. J . Mass Specirom. Ion Processes 1985, 64, 33. (7) Derrick, P. J.; Gardiner, T. M.; Loudon, A. G. Adu. Mass Spectrom. 1978, 7A, 77. (8) Shao, J. D.; Baer, T.; Morrow, J. C.; Fraser-Monteiro, J. L. J . Chem. Phys. 1987,87, 5242. (9) Zha, Q.; Meisels, G. G.; Nishimura, T. Conference on Mass Spectrometry and Allied Topics, ASMS Meeting Abstracts, San Francisco, 1988. (10) Bellville, D. J.; Bauld, N. L. J . Am. Chem. SOC.1982, 104, 5700. (11) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley-Interscience: New York, 1986. (12) Lunell, S.; Yin, L.; Huang, M. B. Chem. Phys. 1989, 139, 293. (13) Nobes, R. H.; Radom, L. Org. Mass Spectrom. 1984, 19, 385. (14) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Re$ Data 1988, 17, Suppl. 1. (15) Crow, F. W.; Gross, M. L.; Bursey, M. M. Org. Mass Spectrom. 1981, 16, 309. (16) Yamdagni, R.; Kebarle, P. J . Am. Chem. SOC.1976, 98, 1320. (17) Tsang, W. J . Am. Chem. SOC.1985, 107, 2872.
(18) Pedley, J. B.; Naylor, R. D.; Kirby, S . P. Thermochemical Data of Organic Compounds, 2nd ed.; Chapman and Hall: London, 1986. (19) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S . Handbook of He(l) Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Society Press: Tokyo, 198 1.
Dissociation Dynamics of Energy Selected Propanol Ions from a a-Type Ion Structure Jon A. Booze and Tomas Baer* Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599- 3290 (Received: December 16, 1991) The dissociation rates of energy selected propanol ions and partially deuterated propanol ions have been investigated by photoelectron photoion coincidence. Cold CH3CH2CH20Hand CD3CH2CH20Hsamples were introduced into the photoionization region of a time-of-flight mass spectrometer through a skimmed molecular beam. The ions dissociated via the loss of H and HzO, the latter dominating near threshold. These rates ranged from los to lo7 s-* near the dissociation onset. Slow dissociation from the ion-dipole complex C3H6+--H20is inconsistent with the simultaneous Occurrence of metastable H loss and the lack of hydrogen scrambling prior to dissociation. However, the dissociation rates for H 2 0 loss and H loss are described well by assuming statistical dissociation from the a-type electronic ground state of the molecular ion, which lies 0.5 eV below the reported ionization onset. This mechanism is consistent with the simultaneous Occurrence of metastable H loss and lack of hydrogen scrambling at low energies. Hydrogen tunneling does not play a role in either loss channel.
5716 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
parent ion rearranges to a more stable isomer prior to dissociation thereby increasing the activation energy and bringing the measured and calculated rates into accord. Among these are butyne ions which rearrange to the more stable butadiene ions,2 and 2,4hexadiyne ions which rearrange to the benzene ion ~ t r u c t u r e . ~ However, a number of small ions such as ethanol: ethyl fl~oride,~ and l-propanol,8 which appear to have no reasonable lower energy isomers, have remained on the list. Recently, the slow dissociation of ethyl chloride ions, which dissociate to C2H4+ + HCl with an activation energy of only 0.3 eV, has been explained by Booze et al.9 who have shown that tunneling through a substantial H atom transfer barrier causes the slow rates. Modeling the rates for normal and deuterated ethyl chloride ions with the statistical theory modified to include tunneling gave good agreement between theory and experiment. The 1-propanol ion dissociation to c-C3H6++ H 2 0is also on the list of anomalous ions. Its phenomenological activation energy is about 0.3 eV giving calculated dissociation rate constants of lo9 s-I, while the experimentally determined rate constants were about lo5 s-l. The dissociation rates as a function of the ion internal energy were studied recently by Shao et al.s using both photoelectron photoion coincidence (PEPICO) and PEPICOphotodissociation techniques. To account for the slow rates, Shao proposed that an ion-dipole complex consisting of a cyclopropane ion and water, which is located along the dissociation path, has an extremely anharmonic well so that the density of vibrational states becomes extremely large, thereby reducing the dissociation rates into the 105-s-' region. This mechanism was supported by the findings of Holmes et al.IOwho noted that the kinetic energies released upon dissociation of the 1-propanol ion and a distonic ion [CH2CH2CH20H2+]were identical. The distonic ion was prepared from a dissociative ionization of 1,Cbutanediol. In an accompanying paper we show that this distonic ion and the iondipole complex are of comparable energy and are connected via only a small barrier. Thus, these two ions are dynamically coupled and can be considered as a single structure located along the dissociation path. Holmes concluded that the rate limiting steps for both the propanol ion and the lower energy distonic ion are the same, namely the dissociation out of the lower energy isomer. This implies that the isomerization of the molecular ion to the ion-dipole complex is faster than the dissociation of the complex. The Shao model is consistent with these results and explains qualitatively why the rates are slow. However, one finding was incompatible with the Shao model. This is that H loss from the I-propanol ion is also slow and that the departing hydrogen comes exclusively from the a-carbon with no isotopic s ~ r a m b l i n g . ~The ~ , ' ~absence of scrambling means that the ion-dipole complex does not revert back to the 1-propanol ion structure because that would result in considerable scrambling of the H atoms. Yet, the H loss reaction is also slow, and in fact appears to be in competition with H 2 0loss. How can this slow H loss, which involves no dipolar neutral fragment, be explained? A similar slow H loss reaction has been reported by Rafaey and Chupka6 for the ethanol ion which has a phenomenological activation energy of about 0.2 eV. Clearly one would like to develop a general model that accounts for these observations. The iondipole complex mechanism, although interesting and intriguing, appears to be inconsistent with the whole body of results. What are some alternative explanations for slow rates? A rate limiting tunneling through an H-atom transfer barrier was recently demonstrated for the HCI loss from ethyl chloride ions.9 Our molecular orbital calculations of the 1-propanol ion and the various isomers and dissociation products showed that both H 2 0and H loss proceed via reverse activation barriers.I4 Therefore, tunneling through these barriers may be the explanation for the slow reactions. Finally, we note that some small alkane ions as well as ions containing oxygen, fluorine, and chlorine have an interesting feature in common. Ab initio calculations of a number of these molecular ions by Radom et al.I5 and Bellville and BauldI6 have shown that the lowest energy state of the ion does not correspond to the ejection of an electron from the HOMO of the neutral,
Booze and Baer which is typically a nonbonding orbital and is referred to as n-type ionization. Rather, in the lowest energy ionic configuration, the electron hole is located at the a-bond somewhere along the carbon chain, in a process referred to as u-type ionization. This means that the minimum-energy geometry of the ground electronic state of the ion has a one-electron u-bond, usually between the a and B carbons, which may be as long as 2 A. The ion geometry is thus significantly different from that of the neutral, which is consistent with the broadened and vibrationally unresolved first band of the PES for these ions. The resulting difference in the ionic minimum and the ionization onset (determined mainly by Franck-Condon factors) may be quite large, as for example in CH3CH2F+where IP, - IP, = 0.6 eV.7 Hence, these ions may actually lie in potential wells which are much deeper than indicated by the ionization onsets, and it is possible that these deep wells in some cases are the key to the slow dissociations. In an accompanying study, we have shown that the potential energy surface for H20 loss from 1-propanol has both a stable ion-dipole complex structure as well as a u-type ion ground state geometry, separated by a large hydrogen transfer barrier.14 We have undertaken this study of the rates of the propanol ion dissociation in order to test the various models more quantitatively than previously possible. An important difference between this and our previous studys is that we have employed a supersonic jet to cool the sample to a few kelvin, and that both normal (unlabeled) and deuterated 1-propanol have been investigated in order to provide additional contraints on the analysis, especially as a test of the tunneling model. Finally, the availability of the calculated potential energies for the various isomers and the calculated vibrational frequencies for both the molecular ion and the transition statal4provide further opportunities for quantitative modeling of the dissociation dynamics. 11. Experimental Method
The photoelectron photoion coincidence experiment with jetcooled samples has been described previou~ly.'~ Briefly, vacuum ultraviolet (vacuum-UV) radiation from a hydrogen discharge lamp is dispersed by a 1-m normal incidence monochromator. The radiation was refocused by a toroidal mirror to intersect with a skimmed molecular beam consisting of neat propanol vapor. The 300-pm nozzle was located about 15 mm from a 1 mm diameter skimmer. The total distance between the nozzle and the photoionization region was 55 mm. Electrons and ions were accelerated in opposite directions by a 20 V/cm electric field. The zero energy electrons were passed through a 10-cm lens system with 3 mm entrance and exit holes resulting in a resolution of about 35 meV. The ions were first accelerated through 5 cm of the 20 V/cm region, followed by a 3-mm acceleration region in which they gained 50 V, and finally drifted through a 30-cm long drift region at the end of which was located a multichannel plate assembly for ion detection. A neat beam of 1-propanol was employed in this study, which provided a backing pressure equal to the room-temperature vapor pressure of about 15 Torr. This pressure was ideal since it led to cooling of the sample without the formation of clusters. On the other hand, the introduction of Ar resulted in significant cluster formation. Cooling of the gas in the neat expansion was evidenced by the difference in the daughter to parent ion ratio in the vicinity of the dissociation onset when the effusive room-temperature source was replaced by the skimmed molecular beam. The shift in the experimental onset toward higher photon energies for the molecular beam is a direct result of cooling. The time-of-flight spectra were slightly contaminated by a thermal background signal. However, the cold and warm signals could be easily distinguished on the basis of their time of flight (TOF) peak widths. The 20% contribution from the warm ambient gas was removed by subtraction of spectra acquired from the effusive source at the same photon energy. The normal propanol was obtained from Aldrich, while 10 g of CD3CH2CH20Hwas purchased from Merck Sharp and Dohme.
Dissociation Dynamics of Energy Selected Propanol Ions
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5717 A
B
. hv = 10.67 eV
hv = 10.77 eV
13.5
14.5
15.5
16.5
17.5
Time of Flight, ksec. Figure 1. Time of flight distribution of propanol ion and its fragments at a photon energy of 10.77 eV. The asymmetric C3H6+signal is due to the slow dissociation of the parent propanol ion.
h v = 10.83 eV
h v = 10.83eV
i
. I
TABLE I: ExperimentaP and Calculated"Branching Ratios for l-ROpmOld
l-Rop.nol-d3
CH3CHZCHzOH+ photon energy, eV 10.67 10.77 10.83 11.13
m l z 42 0.38 (0.40) 0.56 (0.59) 0.53 (0.57) 0.40 (0.43)
m l z 59 m l z 60 159111421 0.00 (0.00) 0.62 (0.60) 0.00 (0.00)
0.10 (0.10) 0.33 (0.31) 0.18 (0.16) 0.17 (0.16) 0.30 (0.27) 0.32 (0.28) 0.39 (0.33) 0.21 (0.24) 0.98 (0.76)
L
CD3CH2CHZOH' photon energy. eV m l z 44 m l z 62 m l z 63 162111441 0.26 (0.29) 0.25 (0.24) 0.49 (0.47) 0.96 (0.82) 10.77 10.83 0.27 (0.31) 0.32 (0.35) 0.41 (0.34) 1.19 (1.11) 11.13 0.20 (0.25) 0.54 (0.48) 0.26 (0.27) 2.70 (1.97) "The experimental values are followed by the calculated ones in parentheses.
380
1480 1580 Time of Flight, p e c
1525 1625 Time of Flight. p s e c
1725
Figure 2. Product ion TOF distributions at various propanol ion internal energies. The points are the experimental data while the solid line is a fit of the data assuming the decay rates shown in Figure 4.
-
HI. Experimental Results A typical photoion time of flight (TOF) distribution of normal 1-propanol ions, corrected for the ambient room temperature signal, is shown in Figure 1. The asymmetric C3H6+TOF distribution is a result of the slow dissociation of the propanol ions in the first acceleration region. The narrow portion of the parent peak is signal of ions which survived for the entire flight time as parents and is indicative of the low translational temperature of the sample. The broad base of the parent peak is caused by ions which dissociate in the drift region and thus have the same nominal TOF as the parent but are broadened due to kinetic energy release. The TOF distribution in Figure 1 shows the presence of the H loss channel which becomes important at higher ion internal energies. Because of the small TOF difference between the parent ion and the H loss fragment ion, it was not possible to determine if the C3H70+peak was asymmetric. It might have been possible to determine this by the use of a fully deuterated sample, which would have lost the heavier D atom. However, this sample was not available to us. The T O F distributions for the daughters of both CH3CH2CH20H+and CD3CH2CH20H+dissociation at several photon energies are shown in Figure 2. The branching ratios for the two products and parent ions at several photon (ion internal) energies are given in Table I. One of the important questions is the relationship between the H and H 2 0 loss. Is H loss slow and in competition with slow H 2 0 loss, or is it rapid and not in competition with the slow channel? Cao et and McAdoo et al.12 both report H loss product from metastable propanol ions produced by electron impact in a mass spectrometer. Because our ratio of H to H 2 0 loss is similar to that found in the metastable signals in the sector-type mass
1425
1680
kisom
----P
kH
820 800
-
I
a
700
F
1
ki.Om,reY
0-
C3H6 +-
H20
CH~CH~CHZOH' CHzCHzCHzOHz
+
660
640
Figure 3. Heats of formation at 0 K of the propanol ion, two of its
isomers, and the two dissociation products obtained from the ab initio molecular orbital calculations (solid lines). The rectangular boxes indicate experimentally determined heats of formation, while the dashed line below the molecular ion is the propanol ion heat of formation assumed for fitting the rate data in Figure 2. spectrometers, we conclude that all of our H loss product must come from a slow dissociation. Several studies of isotopically labeled 1-propanol ions have reported no hydrogen scrambling prior to dissociation. Our results are consistent with these studies since we observe only HDO loss and H loss from CD3CH2CH20H+.
IV. The Statistical Theory (RRKM) Rate Analysis A. Tunneling througb tbe Barrier. The ab initio calculations of the potential energy surface indicate that there is a substantial
5718 The Journal of Physical Chemistry, Vol. 96. No. 14, 199'2
reverse activation barrier for both the H loss and isomerization transition states (Figure 3). In fact, these barriers are above the observed appearance energy for both H and H 2 0 loss. Furthermore, the normal coordinate associated with the imaginary frequency at each barrier corresponded almost entirely to motion of a hydrogen atom. The surface in Figure 3 would thus appear to be ideal for tunneling. We attempted to simulate the experimental rates with tunneling using an Eckart potential with a curvature given by the imaginary frequency of the transition state. This analysis resulted in a predicted isotope effect on the isomerization rate constant, kisomrwhich was orders of magnitude too great except in the extreme case where the isomerization barrier was broadened (imaginary frequency lowered) to such extent that tunneling was effectively eliminated. We conclude that tunneling is not important in this reaction and that the calculated barriers €or isomerization and H atom loss must be too high. It is instructive to compare this conclusion with our recent study9 of the HCl loss from CH3CH2Cl+ions, which showed that hydrogen tunneling dominates the dynamics for this 1,2 elimination of HCl. In that case, the dissociation rates can be understood only when tunneling through the four-centered transition state is included into the RRKM rate expression. An extremely steep k(E) function as well as a large shift in k(E) (isotope effect) of several orders of magnitude for CD3CH2Cl+are the direct consequences of tunneling in the ethyl chloride system. In contrast to the ethyl chloride data, the present data for 1-propanol are characterized by rates which are not unusually steep nor do they show such a dramatic isotope effect (Figure 2). B. Dissociation from the u-Type Complex. Since tunneling cannot explain the low dissociation rates, we turn next to the low energy CH3CH2CH20H+a-type complex. Is its energy sufficiently low to account for the rates? In this model, the H loss from the CY carbon and the isomerization to the ion-dipole complex are in direct competition because they both proceed from the same molecular ion (seeFigure 3). At a given ion internal energy Eion, the total rate constant for the production of daughters is given by
ktot(Eion) = kH(Eion) + kisom(Eion) where kHand khm are the rate constants for loss of hydrogen and isomerization to the ion-dipole complex, respectively, as depicted in Figure 3. Although the TPEPICO experiment selects the ion energies, a small dispersion of ion energies remains. That is, the sample at a particular photon energy consists of a narrow distribution of ion internal energies P(Eion)which results from the bandpass of the monochromator, the instrument function of the electron energy analyzer, and the internal energy distribution of the neutral. All of these are well-known in the present experiment, and so the internal energy distribution function P(Eion)can be readily calculated and used to improve the analysis. In the simulations, the total daughter signal appearing in channel i of the multichannel analyzer (MCA) is expressed as
where kD is the rate-limiting rate constant for formation of daughter D, and ti and ti+]are the ion lifetimes which result in time of flights appearing at the leading edge of channel i and i 1, respectively, of the MCA. The rate constants as a function of energy, k(E), in eq 1 can be either estimated or obtained directly from the statistical theory [Rice-Ramsperger-Kassel-Marcus (RRKM)I*]. We chose the latter, which can be calculated by eq 2, where Eo is the transition state barrier height, G# is the sum
+
of states at the transition state, and N is the density of states of the molecular ion, and u is the reaction path degeneracy, which we have assumed here to be equal to 2 for H loss from the CY
Booze and Baer position and equal to 3 for the isomerization step which involves the transfer of one of the three terminal H atoms. The calculations of N or G# are carried out by direct count using the BeyerSwinehartI9 method which uses the harmonic oscillator approximation. The vibrational frequencies were taken directly from the ab initio molecular orbital calculations of the molecular ion (Ib) and the two transition states, TSH and 7312, which are listed in the preceding paper. The calculated rate constants are a strong function of the assumed molecular ion energy as well as the dissociation limits for H loss, eo,^ and HzO loss, EOisom. The important features of the MP2/6-31G** potential energy surface are illustrated in Figure 3. It was already pointed out that the ab initio barrier energies are somewhat higher than the experimentally measured appearance energies for H and H 2 0loss. Because the tunneling calculations showed that the calculated barriers are too high, we have chosen them here to be equal to the experimentally observed onsets for H and H 2 0 loss. [A kinetic shift20q21could be ruled out on the basis of the measured minimum rates.] On the 0 K heat of formation scale, these barriers lie at a AHom(H-loss)of 802 kJ/mol and at a AHom(isom)of 795 kJ/mol for the normal 1-propanol. The measured onset is, of course, not a precise quantity but is good enough for the present purposes. Of greater importance for assessing the isotope effect is the difference in the onsets between normal and deuterated propanol ions. In order to be internally consistent, we calculated these energies for the deuterated sample using the zero point energy differences. The barriers for the deuterated propanol ions were then assumed to be 810 (H-loss), and 809 (isom) kJ/mol. The internal energy of the ion is given by the expression Eion= hv - AHOm(ion)
+ AHOm(neutra1) + Encutral - KE,
where KE, is the kinetic energy of the electron detected in coincidence with the ion and Encutral is the average internal energy of the neutral gas sample. As mentioned previously, the daughter ion signal at a given photon energy in the supersonic beam was significantly smaller than in the room temperature effusive beam, indicating a high degree of cooling. Efficient vibrational cooling of neat beams operated under similar conditions as those used in this study has been reported.17~22 Although the temperature of the molecular beam could not be deduced directly for 1-propanol, these previous studies indicate that the temperature is considerably closer to 0 K than to room temperature. Thus, it is assumed here that the Encutla, = 0 for all molecules. The uncertainty in Eionis then due to the uncertainties in hv and KE, (quantities which are known) and the uncertainty in the a b initio value PHom(ion), which is not known and may be reasonably large, i.e. as much as 20 kJ/mol. Therefore, we have treated the W m ( i o n ) as an adjustable parameter in order to obtain the best agreement between the simulated and measured time of flight distributions. This single parameter model gave the results illustrated in Figure 2. The derived AHom(ion) of 703.3 kJ/mol is within 8 kJ/mol of the MP2/6-31G** value, and is 53 kJ/mol or 0.55 eV below the experimental 1-propanol ionization onset. The calculated branching ratios are summarized and compared to the experimental values in Table I. The corresponding RRKM rate constants are shown in Figure 4. V. Discussion
As can be seen from the simulated spectra in Figure 2, as well as the branching ratios in Table I, a model assuming slow dissociation from the u-type ground state of 1-propanol gives an excellent description of the reaction dynamics of both the normal and y-deuterated 1-propanol ion at several different energies. The good agreement between the derived and the MP2/6-31G** calculated heats of formation of the molecular ion is strong evidence in support of the model. It should be emphasized that only a single parameter was adjusted in fitting all four k(E) curves in Figure 4. An interesting feature of the experimental and calculated rates is the isotope effect on the two reactions. The H loss channel has rates that hardly vary when the molecule is
Dissociation Dynamics of Energy Selected Propanol Ions 10'
i
E
a). C H ~ C H ~ C H ~ O H + ,
o
4
1
2000 4000 6000 800010000
O9
b). CDsCH2CH20H'
, ', . , . , 2000 4000 6000 800010000
1041
I
Energy above loniz. Onset, cm-1 Figure 4. Statistical theory rate constants that were used to fit the data in Figure 2. Note the shift in the &- relative to the kH. As a result, H loss is considerably more important in the deuterated sample. deuterated at the terminal carbon. This is reasonable since the H atom is lost from the u carbon. On the other hand, the isomerization reaction, which involves the transfer of an H (or D) atom from the terminal carbon to the oxygen atom, has a rate that is significantly reduced upon deuteration. This is fully explained by the change in the zero point energy and the difference in the density of states upon loss of an H or D stretching vibration in the transition state. The agreements between the experimental results, the ab initio molecular orbital results, and the RRKM theory are very gratifying. However, we were surprised that tunneling is evidently unimportant, particularly so since the ab initio imaginary frequencies for motion across each transition state are relative large, indicating a large curvature and thus a narrow barrier. However, the ab initio normal modes give only very local information about the motion across each transition state. It is true that the reaction coordinate motion at the very top of the barriers is nearly all hydrogen motion, but at the same time the length of the C,-C, bond at each transition state is about 1.5 A, and the C,-O bond is 1.3 and 1.5 A at the H loss and isomerization transition states, respectively. In the u-type molecular ion the C,-C bond length is around 1.8 A and the C,-O bond is about 1.3 Therefore the reaction coordinate motion leading up to either the H loss or isomerization transition state must involve a considerable amount of motion which is not simply H atom movement. We conclude therefore that a simple &kart potential in which the barrier width is determined by only a single parameter, the curvature at the saddle point, does not accurately describe this barrier.
A.
VI. Conclusion The metastable dissociation of energy-selected 1-propanol ions is explained well by slow, statistical dissociation from the deep potential well of the u-type molecular ion of 1-propanol. The heat of formation of this structure, which contains a oneelectron bond between the a and j3 carbon, is predicted to be 703 kJ/mol, which is 53 kJ/mol, or 0.5 eV, below the reported ionization energy of I-propanol (10.22 eV). This model explains both the rates for H and HzO loss as well as the rates for H and HDO loss from the partially deuterated propanol ions.
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5719 Ionization from a u-bond rather than a nonbonding orbital has been proposed to explain the weak ionization onset as well as the lack of vibrational structure for the photoelectron spectra for a number of molecules of the form CH3(CH2)J (X = CH3, OH, F, CI; n = 1,2, 3).15J6*23 Many, if not all, of these molecules also have slow dissociations despite very small differences between the ionization onsets and the appearance energies of the dissociation products. It seems possible that the potential wells of the molecular ions may be deeper than is indicated by the ionization onset and that these deep wells may explain the puzzling metastable behavior of many of these ions. The adiabatic ionization potentials of these molecules can be measured experimentally either from charge exchange equilibrium measurements or from measurements of the appearance energies of dissociations which generate the ion of interest as a daughter. The latter method has been used to determine that the adiabatic ionization potential of CH3CH2F+ is 0.6 eV below the vertical ionization potential.' Such studies of these ions are extremely important for enhancing our understanding of their geometry, their electronic structure, as well as their dissociation dynamics.
Acknowledgment. We thank the Department of Energy for support of this work and the Department of Education for the partial support of J.A.B. RWtry NO. PIOH, 71-23-8; CD$H2CH20H, 61844-01-7; PrOH', 34538-82-4; CD3CH2CH20H+,131 178-66-0; cyclopropane cation, 34496-93-0.
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