J. Phys. Chem. 1983, 87, 2213-2221
s-l found for pure 2-MCPD at 13221 cm-l. The higher value for k ( E ) derived from the data in Figure 6 may indicate that n-pentane is a less efficient deactivator of vibrationally excited 2-MCPD than 2-MCPD is itself. If so, k, for n-pentane would be less than the value calculated from collision theory and the resulting value of k ( E ) calculated from the slope of the line in Figure 6 would be smaller. The major point to be noted from Figure 6 is that, within the scatter of the data, there is no evidence for curvature over the pressure range studied, and thus no evidence for a fast (nonrandom) component to the reaction when the methylenic CH stretching overtone is excited. At 100 torr, the photochemical production of 1-MCPD is sufficiently slow that -25% of the 1-MCPD is formed by the dark reaction. This renders values of k for the photochemical reaction at higher pressures uncertain since the correction term for the dark reaction is comparable in magnitude to the value of k for the photochemical reaction. Therefore, experiments were not performed at pressures higher than 100 torr.
2213
Conclusion Overtone activation has been used to induce a unimolecular reaction which involves significant H atom motion along the reaction coordinate. There is no obvious enhancement of the unimolecular rate constant for the reaction when the CH overtone transition for this hydrogen atom is excited compared to excitation of other CH overtone transitions. The result suggests that the nuclear motions excited when the methylenic CH overtone transition of 2-MCPD is pumped with a continuous wave dye laser are not coupled more strongly to the reaction coordinate than they are to the other nuclear motions. The observed rate constants can best be explained by assuming that energy is randomized before any significant fraction (
0.5 -
0
c
5 0
E
n t-
0
Pn
L
I
I
I
I
I
920 930 940 950 960 970
I
900
I 990
FREQUENCY I C " '
Figure 5. Frequency dependence of the singie-photon photodissociation of CF31+ (a) at 300 K and (b) the major feature peaking at 946 cm-' measured with ion source temperature of 177 K (0)and 458 K (0). The lines drawn through the data are shown to illustrate the differences between the data sets.
as a function of ion source temperature, no significant change in intensity was found over the temperature range 170-470 K. The absorption around 946 cm-' shown in Figure 5b has been assigned by previous workers to the u1 C-F symmetric stretch of CF31+in the X2E1 &tea3 From simulation of the v(0-1) transition using d e rotational constants known for neutral CF31,15the rotational envelope is estimated to be 10 cm-' (fwhm) at 300 K and doubles over the temperature range 200-500 K. The measured photodissociation bandwidth is -30 cm-' and increases by less than 15% over the temperature range 177-458 K. Hence, the absorption shown in Figure 5 is not a simple u(0-1) transition. Ions sampled in these experiments are close to the dissociation threshold and contain up to 0.45 eV of vibrational energy. However, most of this energy will be contained in low-frequency modes such as the v3 C-I stretch at 286 cm-' in CF3I,l8and the V g C-I bend at 260 (18) P. R. McGee, F. F. Cleveland, A. G. Meister, C. E. Decker, and
S.I. Miller, J. Chem. Phys., 21, 242 (1953).
2218
The Journal of Physical Chemistry, Vol. 87, No. 12, 1983
cm-' in CF31.18 The high-frequency modes such as the vl C-F stretch will not be substantially populated. Assuming a statistical population in the vibrational levels, we estimate that only 15-20% of the population of v1 resides in the u1 = 1 level at internal energies within one photon of dissociation. Hence the absorption is probably best described as arising mainly from bands of the type nu, nu, + u1 where the ul(O-+l)transition occurs. A smaller contribution is expected from hot bands of the type nu, (n+l)ul. In addition, combination and overtone bands of the lower frequency modes may also be important. In neutral CF31,the vl spectral region contains several combination transitions which have become strong by intensity b o r r o ~ i n g 'from ~ the very strong v1 transition. In summary, the observed bandwidth in our experiments is probably primarily due to anharmonic distortion of the surface in the vl region by vibrational excitation of other modes and possibly to intensity enhanced combination and overtone bands. We predict that this composite absorption feature will broaden less with temperature than a single transition since the overall broadening will arise mainly from the weaker transitions in the wings. The photodissociation spectrum from our ion beam experiments is compared in Figure 6 to the spectrum obtained from multiphoton dissociation in an ICR spect r ~ m e t e r In . ~ the ICR experiments the ions are produced by electron impact and have a range of internal energies between zero and the dissociation threshold. Hence, multiple photon absorptions are required by the majority of ions in order for them to dissociate. The photodissociation spectra for these two experiments are strikingly dissimilar both in width and location of the band maxima. As we discuss in more detail below we believe that the most reasonable explanation for the differences between the two photodissociation spectra is that a red shift occurs in the ~ ~ ( 0 - 1 transition ) with increasing vibrational excitation in the other modes. If we assume that the u1(0-1) transition red shifts with increasing vibrational energy, then the ICR photodissociation spectrum represents a convolution of absorptions over a range of internal energies from 0 to -0.45 eV (the dissociation threshold). This leads to a much broader, blue-shifted spectrum than that which would be observed in the ion beam experiment where the energy range of the CFJ+ is higher and much narrower (0.33-0.45 eV). There are alternative explanations for the differences between these two photodissociation spectra. One possibility is that the ion beam spectrum results from a vl hot-band absorption which is red shifted by anharmonicity, whereas the MPD-ICR spectrum includes fundamental and hot-band absorptions in the v l mode. This possibility can be ruled out because, as discussed above, adding 0.45 eV of vibrational energy heavily populates the low frequency modes but only populates the u1 = 1 level by 15-2070 and does not significantly populate the u1 = 2 level. A second possibility is that the high-frequency portion of the MPD-ICR spectrum arises from combination bands and/or overtone bands of the lower frequency modes and at high vibrational energies these are red shifted by anharmonicity to coincide with the v l type absorptions. This is clearly a tenuous explanation since it requires all the combination and/or overtone bands to be located on the high-frequency side of the v1 absorption. In any case, this possibility can be ruled out since the higher order transitions are normally very weak and only become strong by "intensity borrowing" from the strong v1 absorption,
Jarrold et al. u)
.c _
L
0
z 0
-
(19)F. Kohler, H. Jones, and H. D. Rudolph, J . Mol. Spectrosc., 80,
/
a n
-
56 (1980).
,\ -\
E
c
5
0
0 v)
2
a 0 c
T = 458 K
P a
J
!L' 0
9bO
950
1000
FREQUENCY / c m - l
Figure 6. Comparison between frequency dependence of CF$ infrared photodissociationfrom singlephoton photodissociationion beam experiments (this work) and multiphoton dissociation ICR experiments (data from ref 2). The normalization is arbitrary.
which is not expected provide such a disproportionately large intensity enhancement. In summary, the differences between the MPD-ICR and ion-beam photodissociation spectra cannot be explained by the different contributions of overtone and/or combination bands of the lower modes or vl-type hot bands (nu, (n+l)ul) in the two experiments. This leads us to suggest that the differences originate primarily from a lowering of the ul(O-1) transition energy in highly vibrationally excited CFJ'. The origin of the lowering of the ul(O-+l) transition energy with vibrational energy can be understood quite simply: at high vibrational energy the average positions of the atoms are shifted away from their equilibrium positions and the overall bonding is weakened. As a result of this, the molecule has lower force constants and the ~ ~ ( 0 - 1 )transition is lower in energy. Further experiments on other ions would be useful to confirm these conclusions. It should be noted that this phenomena is relevant to all multiphoton dissociation experiments and may play a significant role in red shifting (or blue shifting in the case of strained molecules) the absorption profile for photodissociation with respect to the ground-state adsorption known from the infrared spectra. The above interpretation of the data suggests that the v1 fundamental frequency of CF31+in the ground vibrational state will be at around 970 cm-', the maximum in the MPD-ICR spectrum, rather than at 946 cm-' which is the maximum in the single-photon photodissociation spectrum. Quenching Experiments. The IR photodissociation spectrum of CF31+ shows two features: an intense absorption at 950 cm-', and a weaker absorption at 1080 cm-'. Evidence from the photoelectron spectrum and Rydberg spectra of CF313suggests that the v1 mode is expected to occur at around 950 cm-' for the 2E1,2state and around 1090 cm-' for the 2E3/2state. This observation led Thorne adn Beauchamp3 to assign the absorption at 950 cm-' to the v, mode in the 2E1/2state. However, the assignment of the feature at 1080 cm-' is not unambiguous because it could also arise from the v4 asymmetric C-F stretch in the 2E1 state. In other experiments, Thorne and Beauchampi measured the rate at which Xe collisionally quenches the multiphoton dissociation of CF31+. They found a rate of 2.5 X mol-' cm3 s-'. This rate is 4-5 times the calculated ion-neutral capture rate (5.2 X mol-' cm3 s-'1. They suggested that this large rate could be due to a long-range interaction which induces an electronic transition from the 'Eli2 state to the 2E3,2state
-
Photodissociation of Vibrationaily Excited CF,I+ and CF,Br+
which absorbs at different frequencies. The initial motivation behind the experiments discussed in this section was to measure the rate at which Xe collisionally quenched the absorption at 1081 cm-'. If this absorption arises from state, rapid electronic quenching the v., mode in the 2E112 is expected (from the work of Thorne and Beauchamp) whereas if it arises from the v1 mode of the 2E,12state, slower vibrational quenching is predicted and the intensity could be enhanced by the rapid 2El12 2E312transition. Thus the plan was to obtain data useful in assigning the weaker absorption. Figure 7 shows a plot of the relative photoinduced signal measured by using the 946-cm-l laser lines as a function of Xe pressure in the ion source. When the measured ion source residence time distributions are used, it is possible to obtain a rate constant, k,from the data shown in Figure 7. This was done by fitting the data shown in Figure 7, averaging over the measured ion source residence time distributions at a number of different pressures, and adjusting the value of k to give the best match with the experimental data. A value of 2.5 X 10-lomol-l cm3s-l for k yields the line shown in Figure 7 which is essentially identical with a linear least-squares fit to the data. Thus from this data we derive a rate constant of (2.5 f 0.5) X mol-l cm3 s-l. This value is an order of magnitude small than that reported by Thorne and Beauchamp2 for the quenching of CF31+multiphoton dissociation at 953 cm-l by Xe. The two experiments are not the same and sample different ion populations. However, we can think of no reasonable explanation for the order of magnitude difference between the derived rates. In fact, we might predict the quenching rate for single-photon dissociation would be larger than the rate measured for multiphoton dissociation since we expect the quenching rates to increase with increasing internal excitation. The discrepancy between this work and the results of Thorne and Beauchamp led us to critically evaluate the experimental method employed here. One potential problem in our experiment is the possibility of forming CF31+ by ion-molecule reactions as well as by electron impact. In Xe/CF31 mixtures in our ion source, Xe+ ions are produced. Charge transfer from Xe+ to CFJ is very exothermic and could be a source of highly excited CF31+. So that this possibility could be investigated, measurements at the product distribution from Xe+/CF31charge transfer were performed on the tandem ICR at UCSBe20 The data are shown in Figure 8. The product distribution obtained by extrapolating to zero CF31pressure to remove the effects of secondary reactions was CF3+ (0.93), I+ (0.072), CF21+(
