Laser-ion Beam Photodissociation Studies of C4H4Radical Cations
R. E. Krailler and D. H. Russell? Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA
The laser-ion beam photodissociation for [C4H4]+' ions produced from a variety of precursors has been studied. Based on the data it is apparent that two structurally distinct forms of the [C4H4]+*ion are produced by fragmentation of larger systems. The relative population of the various structural forms is very dependent on the internal energy of the fragmenting ion, with 1-buten-3-yne [C4€L,]+'ions being favored at low internal energies. As the internal energy of the reactant ion is increased, the relative population of butatriene [C,H4]+' ions increases. The laser-ion beam photodissociation technique is able to selectively sample these two structural forms.
calculations are correct, i.e. AH, for 1-buten-3-yne is INTRODUCTION c. 1 eV greater than that for methylene cyclopropene, why is formation of the higher energy form favored; The [C,H,]+' radical cation (m/z52) is a system which and (ii) can information concerning structural illustrates the problems associated with structural equilibration of [C&6]+' radical cations be obtained assignments of small hydrocarbon ionic systems. indirectly by examining the [C4H4]+*ions. That is, if Rosenstock et a1.l proposed that [C,H,] +' ions formed the precursor [C6H6]+' ions isomerize to a common at threshold energies from benzene and pyridine structure prior to formation of [C4H4]+',then each radical cations have a cyclic structure. Weininger and [C6H6]+' ion should produce the same [C,H,]+' Thorton2 proposed a cyclobutadiene structure for structural products. Levsen et ~1.~have reported on [C,H,]+' ions formed by fragmentation of pyridazine the photodissociation of [C,H,]+' and raised similar and substituted pyridazines. Similarly, Schwartz et al .3 questions, but their data and interpretations are not proposed a cyclobutadiene structure for substituted conclusive. [C4H4J+'ions. Baer el a1., suggested that [C,H,]+' ions formed at threshold energies from 2,4-hexadiyne, 1,Shexadiyne and benzene radical cations have the methylene cyclopropene structure; however, they did EXPERIMENTAL not rule out the accessibility of linear structures at higher energies. Baer and coworkers further sug- The experimental apparatus used in this work has gested, based on ab initio calculations, that among the been described in detail elsewhere'" so it will only be possible structures for the [C4H4]+'ion, the methylene briefly reviewed here. The apparatus consists of a cyclopropene form should be c. 1eV lower in energy Kratos (AEI) MS-902 double focusing mass spectro- than the linear isomers. Lastly, Ono and Ng5 meter and a Coherent CR-18 argon ion laser. Ions are proposed a linear structure, e.g. l-buten-3-yne, for extracted from the ion source and accelerated into the [(C,H,),]+' (acetylene dimer). first field-free region. The laser beam intersects the Bowers et ~1.~have suggested that stable [C,H,]+' ion beam and the photoproducts are focused and ions exist as two structural forms, viz. I-buten-3-yne detected by conventional accelerating voltage scan- and methylene cyclopropene. This work is in ning techniques." The laser beam can intersect the agreement with metastable ion and collision induced ion beam in two ways, coaxially and perpendicularly. dissociation (CID) studies reported by Lifshitz et ~1.~The coaxial alignment gives better sensitivity but as well as with the ion-molecule reactivity data of critical data were remeasured in a perpendicular Ausloos.' Ausloos proposed that stable [C,H,] +. ions laser-ion beam configuration. No significant differ- have a 1-buten-3-yne structure with additional ences in the experimental data were observed between structures being accessible at both higher and lower the two configurations. energies. The butatriene structure is assigned to the Discrimination against product ions with a large higher energy form and a methylene cyclopropene velocity component perpendicular to the ion beam structure is proposed for the lower energy form. direction is a problem with this experiment because of Ausloos' data seems to provide the best quantitative the finite slit widths in the 2 direction. Of the data assessment for the structural population of [C,H,]+' reported in this paper the branching ratio measure- ions. ments will be most sensitive to 2 axial discrimination. Our interest in the problem is two-fold: (i) if the To minimize this error the branching ratios were calculated using peak areas. Also instead of reporting t Author to whom correspondence should be addressed kinetic energy release distributions we calculated the
CCC-0030-493x/85/0020-0606$04.00 606 ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 10, 1985 0Wiley Heyden Ltd, 1985 LASER-ION BEAM PHOTODISSOCIATION STUDIES OF C4H4 RADICAL CATIONS kinetic energy release values from the peak width at Table 1. Precursor-dependent photodissociation branching half-height. ratios for [C,H,]+' ions The elemental composition of the mlz 52 ion from ionized pyridine and pyridazine was checked by 514.5nm 488.0 nm H' loss H, loss H' loss H, loss high-resolution mass measurements. No detectable abundances of nitrogen-containing fragment ions at Benzene 0.44 0.56 0.55 0.45 2,4-Hexadiyne 0.69 0.31 0.76 0.23 mlz52 were observed, e.g. [C3H2N]+or [C,N,]+. A 1,5-Hexadiyne 0.64 0.36 0.75 0.25 referee has suggested that the presence of such ions Pyri d ine 0.65 0.35 0.68 0.32 could be the reason for the low photodissociation 2,4,6-Cycloheptatrienone 0.48 0.52 0.50 0.50 cross-sections for these two [C4H4]+'ions. Benzene-& 0.52 0.48 0.57 0.43 The experiments were performed under low 1-Buten-3-yne 0.63 0.37 0.67 0.33 pressure (3-5 x Torr) electron impact conditions. All reagents were obtained from commercial sources: benzene, pyridine (MCB), pyridazine (Aldrich), broader than the spectra for [C4H,]+' ions formed by 2,4-hexadiyne, 1,5-hexadiyne, 1-buten-3-yne (Pfaltz fragmentation of larger systems. and Bauer), benzene-d, (MSD Canada) and 2,4,6- The photofragment ion branching ratios were cycloheptatrienone (Alpha). These samples were obtained from the peak areas for each reaction purified by a freeze-pump-thaw cycle to remove any channel. The data for 514.5 and 488nm (the two non-condensable gases. maxima) are given in Table 1. The branching ratios for [C,H,]+' from 2,4-hexadiyne, 1,5-hexadiyne, pyridine and 1-buten-3-yne are similar with loss of H' accounting for approximately 65% (514.5 nm) and RESULTS 68% (488 nm) of the total photodissociation signal. Loss of H, accounts for approximately 35% As part of this work we investigated the visible (514.5 nm) and 32% (488 nm) of the photofragmenta- (514.5-454 nm) wavelength photodissociation of a tion. For [C4H4]+' from benzene, tropone and relatively large number of [C,H,]+' ions, i.e. from benzene-d, loss of H' (D') accounts for approximately secondary fragmentation of 2,4,6-~ycloheptatrienone, 48% and 52% of the signal at 514.5 and 488nm, from primary fragmentation of benzene, pyridine, respectively. Loss of H, (D2) accounts for ap- pyridazine, 2,4-hexadiyne and 1,5-hexadiyne and from proximately 52% of the 514.5nm signal and direct ionization of 1-buten-3-yne. All the systems approximately 48% of the 488 nm signal. studied, except pyridazine, give rise to [C,H,]+' ions The relative photofragment ion yield was measured which photodissociate in the blue-green region. for [C,H,]+' from benzene as a function of ionizing The photodissociation reactions observed for energy and the data are reported in Table 2. The data [C4H4]+'are loss of H', H2 and C2H2: were obtained in the following manner. Peak areas for loss of H' and H, were measured at 70, 50, 30 and [C,H,]++H' (1) 20 eV of ionizing energy and corrected for the change in the reactant ion intensity by dividing the peak area [C,H,]+' + hv [C&zI+'+ Hz (2) by the reactant ion intensity. The photofragment [C,HJ'+ CPz (3) intensities for the two reaction channels were summed -E at each electron energy and divided by the photon Studies of the laser power dependence for the flux. The relative photofragment ion yield increased photofragment ion abundances are consistent with a from 0.72 to 1.20 at 514.5 nm and decreased from 1.00 single photon photodissociation process. Owing to the to 0.62 at 488 nm as the ionizing energy was decreased weak signal for C2H2loss (4%H' loss) this reaction from 70 to 20 eV. Similar data were taken for [C,H,]+' channel was not studied in great detail. from 1-buten-3-yne by using ionizing energies of 70, The photofragment spectra were derived in the 50, 30, 20 and 10eV. The same general trend was following manner. The photofragment ion yields for observed in the data (see Table 2). each reaction channel were obtained from the peak The relative photofragment ion yield was measured areas. The signal intensities for loss of H' and H2 were for each [C,H,]+' ion precursor. These values were summed together and divided by the photon flux. The calculated from the photofragment peak areas for loss result is a relative photofragment ion yield which is of H' and H, (70eV) and summed together for each plotted as a function of wavelength for each precursor precursor ion. Because the abundance of the reactant in Fig. 1. The photofragment spectra shown in Fig. 1 appear Table 2. Relative photodissociation cross-section measure- to be bimodal with maxima at 514.5 and 488nm. ments for benzene and 1-buten-3-yne Although all spectra are similar in this respect, there are notable differences for the various precursors. For Ionizing 514.5 nm 488 nm example, the photofragment ion yields for benzene energy (eV) Benzene 1-Buten-3-yne Benzene 1-Buten-3-yne [C4H4]+.are roughly equivalent at 514.5 and 488 nm. 70 0.72 0.68 1.oo 1.oo 50 0.80 0.80 0.97 0.95 On the other hand, the photofragment ion yields at 30 1.01 1.oo 0.72 0.80 514.5 and 488nm differ somewhat for the other 20 1.20 1.10 0.62 0.72 [C4H4]+' ions. Finally, the photofragment spectrum 10 - 1.23 - 0.60 for the 1-buten-3-yne [C,H,]+' ion is significantly
ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 10, 1985 607 R. E. KRAILLER AND D. H. RUSSELL
2,4,6 - Cycloheptatrienone
h cIn 3 P P -; c>\ c E - Pe m
I I I I I I I I I I I I 1 I I I I 520 510 500 490 480 470 460 450 520 510 500 490 480 470 460 450
Wavelength (nrn) Wavelength (nm)
(a) ( b) 1,5- Hexadiyne A Pyridine
L
I I I I 1 I 1 I 520 510 500 490 480 470 460 450 520 510 500 490 480 470 460 450 Wavelength (nrn) Wavelength (nrn)
(C 1 (d) Figure 1. Relative photofragment ion yields as a function of excitation wavelength for [C,H,]+' ions produced from (a) 2,4,6-~ycloheptatrienone, (b) 2,4-hexadiyne, (c) 1,5-hexadiyne, (d) pyridine, (e) benzene, (f) benzene-d, and (9)I-buten-3-yne. The line joining the points is a guide to give an indication of the shape of the absorption profile. The experimental conditions were: ionizing electron energy, 70 eV and ion source temperature, 450 K.
608 ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 10, 1985 LASER-ION BEAM PHOTODISSOCIATION STUDIES OF C,H, RADICAL CATIONS
~~ Benzene - d6
T
L
I I I I I I I I 520 510 500 490 480 470 460 450 520 510 500 490 480 470 460 450
Wavelength (nm) Wavelength (nm)
(e) (f)
n 1- Buten -3 - yne '\
1 I I I I I I I 520 510 500 490 480 470 460 450
Wavelength (nm) (g)
Figure 1-( continued)
ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 10, 1985 609 R. E. KRAILLER AND D. H. RUSSELL
Table 3. Precursor-dependent photodis- DISCUSSION sociation cross-sections for [c4&1+* The bimodal photofragment spectra for [C,H,]+' can Precursor 514.5 nm 488.0 nm be rationalized in two ways: (i) there are two Benzene 1.00 1.00 structurally distinct [C,H,]+' ions, one which photo- 2.4-Hexadiyne 0.40 0.60 dissociates at a wavelength maximum of 514.5 nm and 1,5-Hexadiyne 0.36 0.52 the other which photodissociates at a wavelength Pyridine 0.21 0.28 2,4,6-Cycloheptatrienone 0.78 1.01 maximum of 488nm or (ii) the bimodality is due to 1-Buten-3-yne 0.52 0.91 absorption fine structure of a single photodissociating [C,H,]+' ion structure. This point will be discussed further as it relates to the ionizing energy dependence [C,H,]+' ion changes as a function of the precursor, for the photodissociation cross-section. If there are the data were corrected for this change by dividing the indeed two structures, the second question is: what summed peak areas by the [C4H,]+' reactant ion are the structures of the photodissociating [C,H,]+' intensities. The data were taken at 514.5 and 488nm ions? and normalized with respect to the photofragment ion A critical aspect of photodissociation measurements yield for benzene. The result is a measurement of the is the correlation between the photodissociation precursor-dependent photofragment ion yield at each spectrum and the electronic structure of a molecule as wavelength. The data are tabulated in Table 3. determined by photoelectron spectroscopy (PES). The photofragment relative kinetic energies were The early work of Beauchamp et a1.l' and Dunbar et measured for [C,H,]+ and [C,H,]+' products at 514.5 al.l3 shows clearly that the photodissociation maxi- and 488nm. Data for other wavelengths are not mum often corresponds to the observed photoelectron reported due to low sensitivity at these wavelengths. bands. Thus, the PES data can be utilized to predict The photofragment ion relative kinetic energies were photodissociation maxima provided such data are obtained from the full width at half maximum available. The PES data for two [C,H,]+' isomers (FWHM) of the photofragment peaks (corrected for have been reported, viz. butatriene and l-buten-3- the main beam contribution to the peak width) and yne.', Based on this data photodissociation maxima converted to kinetic energy release (KER) values for these isomers should occur at 513 nm (l-buten-3- using Eqn (4).11 yne) and 487 nm (butatriene). These predicted values agree quite well with the strong photodissociation bands at 514.5 and 488 nm. In the following sections it will be argued that two In Eqn (4) V is the main beam accelerating voltage, structural forms of [C,H,]+' ion are sampled by the AVmwis the FWHM of the product ion peak, AVmb is photodissociation process. However, there must also the FWHM of the main beam peak and m,, m, and m, be at least one additional [C4H4J+'ion which does not are the masses of the reactant ion, product ion and photodissociate in the wavelength range of the argon product neutral, respectively. For a Gaussian peak ion laser, e.g. the [C,H,]+' ion produced by shape the KER values obtained from Eqn (4) fragmentation of pyridazine did not photodissociate! correspond to the most probable kinetic energy. l1 The It is difficult to make absolute photodissociation data for the various [C4H4]+'ions are contained in cross-section measurements but, comparing the Table 4. The unimolecular KER values are all photodissociation signal intensity for the [C,H,]+' ions between 15 and 20meV for H' loss and, with the with other systems we have studied, it is probable that exception of tropone, are all between 200 and a substantial fraction of the [C,H,]+' ions produced by 260meV for H, For each [C,H,]+' ion, H' loss fragmentation of benzene do not photodissociate in KER values for photodissociation are 20-40 meV the blue-green spectral range. Thus, a non- greater than the unimolecular KER value. Photoin- photodissociating [C,H,J+' ion structure is highly duced loss of H, does not show a great deal of probable in all the systems studied. It is difficult to consistency, with the photofragment KER value being make a structural assignment for this third ion and it is anywhere from 0 to 115meV greater than the quite possible, based on thermochemical data, that corresponding unimolecular KER value. there are two additional structures formed (see Fig. 2). It is generally that methylene ~ ~~ ~~~~ cyclopropene [C,H,]+' is the lowest energy structure; Table 4. Kinetic energy release values for [C,H,]+' ions however, it is difficult to say whether the non- Kinetic energy release values (me!!) photodissociating structure(s) lie above, below or Unimolecular 514 5 nm 488 0 nm bracket the photodissociating structures in terms of Compound H loss H, loss H loss H, loss H loss H, loss energy. Benzene 15 225 40 270 30 270 Based on available thermochemicai data for 1.5-Hexadiyne 20 260 40 260 40 270 [C,H4]+' ions it may be argued that as many as four 2,4-Hexadiyne 15 230 35 275 55 310 [C,H,]+' ions are produced by fragmentation of larger Pyridine 15 215 50 245 35 330 2,4,6-Cyclohepta- systems. In Fig. 2 we have placed the four neutral t r ie no ne 20 340 40 355 40 355 structures (l-buten-3-yne, butatriene, methylene 1-Buten-3-yne 10 200 50 245 45 270 cyclopropene and cyclobutadiene) on a common energy scale and added to the neutral heats of
610 ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 10, 1985 LASER-ION BEAM PHOTODISSOCIATION STUDIES OF C,H, RADICAL CATIONS
'*.OI16.0 - -
PES
/P
.I.-.=. .=.=.=. A III Figure 2. Total energy diagram for four possible structures of [C,H,I" ion; l-buten-3-yne, butatriene, methylene cyclopropene and cyclobutadiene. The neutral heats of formation, ionization potentials and available PES data are taken from the literat~re.8.'"~~
formation the best available ionization energy. From would be required to reach the excited state. If the the available literature, it is clear that the heats of photodissociation cross-section is dependent upon formation for these four [C,H,]+' isomers may differ internal energy, i.e. photon absorption cross-section by as little as 0.3 eV. The data for 1-buten-3-yne and changes or the photodissociation is thermochemically butatriene are collected from both the~retical'~and limited, one would expect to see the photofragment e~perimental~~~~,~~,'~results. The heat of formation for ion yields decrease at lower photon energies and neutral methylene cyclopropene neutral comes from increase at higher photon energies. Our observations an ab initio study15 and the experimental work of for the [C,H,]+' ion produced by fragmentation of Rosenstock et ~1.'~The ionization energy for benzene indicates the exactly opposite trend! Recall methylene cyclopropene comes from both an ab initio that the photodissociation cross-section for [C,H,]+' study18 and the experimental work of Ausloos.8 The from benzene increases at 514.5 nm while it decreases heat of formation for neutral cyclobutadiene comes at 488 nm as the ionizing energy is lowered from 70 to from the calculations of Kollmar,15 Schweig and 20eV, and that a similar change is observed for the Theil19 and Dewar and Komornicki.'' The ionization [C,H,]+' ion from 1-buten-3-yne over the energy energy for cyclobutadiene was obtained from electron range 70-10eV. The changes in the photofragment impact measurementsZ1 and from the 'corrected' PES ion yield can be understood by examining the of some cyclobutadiene-iron cornpo~nds.~~~~~Much of thermochemical data for [C4H4]+'. That is, AHf the work on methylene cyclopropene and cyclobuta- (butatriene [C,H,]+') > AHf (1-buten-3-yne [C,H,]+') diene involves some form of estimation; nevertheless, and as the ionizing energy is lowered a larger fraction the values appear reasonable and indicate that the of the precursor ions dissociate to give the lowest difference in energy between methylene cyciopropene energy [C,H,]+' ion. Since the lowest energy [C4H4]+' and the other [C,H,]+' ion structures may not be as ion (1-buten-3-yne) photodissociates at 514.5 nm we much as generally believed. would predict the trend observed in Table 2. Since an Measurement of the relative photofragment ion internal energy effect would cause the exact opposite yields as a function of ionizing energy can be used to trend, we believe this evidence supports the argument address the question of two structures v. absorption that two distinct forms of photodissociating [C,H,] +. fine structure. In general, photofragment ion yields ions are produced and sampled selectively by the laser may depend on the initial ionizing energy if the photodissociation process. Note also that the observed absorption cross-section changes significantly with trend is consistent with the results of the ion-molecule internal energy.', As the ionizing energy is lowered, reaction chemistry reported by Ausloos.8 the ions will be formed with lower average internal A second piece of information gained from the energies, and a greater amount of photon energy photofragment spectra comes from the broadness of
ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 10, 1985 611 R. E. KRAILLER AND D. H. RUSSELL the 1-buten-3-yne spectrum as compared to the the reaction channel leading to [C4H4]+'may involve a photodissociation spectra of the other [C,H,]+' ions. different transition state, i.e. a tight versus orbiting [C,H,]+' ions formed by direct ionization of transition state.25 On the other hand, the precursor- 1-buten-3-yne may have a broader distribution of dependent differences in the [C,H,]+' ion photodis- internal energies than those formed by fragmentation sociation cross-section may arise from differences in of larger systems. This interpretation of the data the structure of the reactant ions. If this is the case, it indicates that the photodissociation process samples a would not be at all surprising for the various broad range of internal energies as opposed to precursors to give rise to very different structural selectively sampling specific rovibronic transitions. populations of the [C4H4]+' ion. In a recent study Honovich and Dunbar have reported that photodis- from our laboratory we present strong evidence that sociation cross-sections can increase significantly as 2,bhexadiyne [C,H,]+' ions do not isomerize to a the vibrational energy is increased, leading to selective cyclic structure prior to fragmentation to [C4H4]+'. sampling of particular vibrational transitions. This The similarity in the [C,H,]+' data for 2,4-hexadiyne effect, however, has been reported only for and 1,5-hexadiyne may indicate partial isomerization halogenated systems and has not been seen in between these two linear structures or a common aromatic hydrocarbons (i .e. styrene, l-methylnaph- transition state leading to [C4H4]+'. thalene and 2-methylnaphthalene) .24 The kinetic energy release (KER) data for the H' The photodissociation product ion branching ratios and H, loss reaction channels show dependence on show differences which reflect a consequence of wavelength and precursor. In each case the photodis- internal energy on the photodissociation process. For sociation KER values are greater than the unimolecu- example, photodissociation of the [C,H,]+' ions is lar (metastable) value. This is undoubtedly due to accomplished by photons in the range of 2.41 eV small differences in the total energies of the (514.5 nm) to 2.71 eV (4.58 nm). The appearance photoexcited ion. In addition, for loss of H2 the KER energies for [C4H3]+and [C4H2]+'from [C,H,]+' are values at 488nm are generally greater than those at c. 2.7-2.9 eV. Therefore, the photodissociation 514.5nm. This trend may arise from two sources: (i) process occurs at or near the thermochemical from structural changes as the [C4H4]+'ion structures threshold. The best available energetic data on sampled by the laser at 514.5nm and 488nm are [C4H3]+ and [C,H,]+' are the photoionization different or (ii) if the KER values for the two breakdown curves reported by Rosenstock. l6 These structures are similar, from small differences in excess data show that these two ions have approximately the internal energy of the photoexcited ions. A similar same appearance energy (AE = 13.05 eV); however, trend is not detectable in the H' loss reaction channel; at energies of 0.5-0.7eV above threshold, loss of H' however, for the H' loss reaction channel the is favored over loss of H2. That is, the rate of difference may not be detectable at the energy formation (k(E))for [C,H,]+ rises much more steeply resolution employed for these studies. The KER than the k(E) for [C,H,]+'. This effect could explain values are also precursor-dependent, which is the predominant loss of H' from 2,4-hexadiyne, probably due to changes in the internal energy of the 1,5-hexadiyne, pyridine and 1-buten-3-yne [C,H,]+' [C4H4]+'ions as a function of precursors. ions as compared to benzene and tropone [C,H,+' In an earlier study Ausloos' demonstrated, using ions. That is, the photodissociation results suggest that ion-molecule reaction chemistry, that dissociative the [C,H,]+' ions from 2,4-hexadiyne, 1,s-hexadiyne, ionization of benzene, 1,5-hexadiyne, 2,4-hexadiyne pyridine and 1-buten-3-yne have higher average and pyridine give rise to two ion structures for internal energies than do the [C,H,]+' ions from [C4H4]+'.In addition, he showed that the population benzene and tropone. This effect would also explain of the two structural forms have a strong dependence the observed increase in loss of H' at 488nm as on the internal energy of the fragmenting precursor. compared to 514.Snm. That is, the energy of the For example, dissociative charge exchange ionization photoexcited ion formed by absorption at 488 nm is of benzene at a total energy of 14.8-16.6eV yields a greater than that for 514.5nm because it has been [C,H,]+' ion population ranging from
612 ORGANIC MASS SPECTROMETRY, VOL. 20, NO. 10, 1985 LASER-ION BEAM PHOTODISSOCIATION STUDIES OF C4H4 RADICAL CATIONS
CONCLUSIONS cyclobutadiene and it may be that pyridazine radical cations do indeed produce a cyclobutadiene [C,H,1+"- ion in agreement with Weininger and Thorton.2 Based on the results from this study we conclude that Changes in internal energy can have quite a at least three structurally distinct [C4H4]+' ions are dramatic effect on photodissociation measurements, produced by fragmentation of larger systems as well as including the appearance of the photofragment by direct ionization of 1-buten-3-yne. The distribution spectra. It is therefore important to account for, or of the [C,H,]+' ions among the accessible structural rule out, this effect when making structural assign- forms is highly dependent upon the internal energy of ments by the photodissociation process. With this the fragmenting precursors. In agreement with the caution in mind, we suggest that the dramatic results of Baer and Ausloos, at threshold energies differences in the precursor-dependent relative photo- methylene cyclopropene is most probably the most fragment ion yield may be due to the lack of complete abundant structural form produced, but as structural isomerization of the reactant [C,H,] +* ions. the internal energ!y of the precursor ion is increased, the linear isomer~-(l-bute~-3-yneand butatriene) are produced in greater abundance. Initially, the domin- Acknowledgements ant linear [C4H4]" ion structure produced (at energies above threshold) is l-buten-3-yne; however, as the This work was supported by the US Department of Energy, Office of Basic Energy Sciences and the Robert A. Welch Foundation. energy Of the fragmenting precursor is increased, a Some of the equipment used for this work was purchased from substantial fraction of butatriene is formed. In funds provided by the TAMU Center for Energy and Mineral addition, we cannot rule out the formation of Resources.
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