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LASER-ION BEAM PHOTODISSOCIATION STUDIES of C4H4 RADICAL CATIONS Kinetic Energy Release Values from the Peak Width at Table 1

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LASER-ION BEAM PHOTODISSOCIATION STUDIES of C4H4 RADICAL CATIONS Kinetic Energy Release Values from the Peak Width at Table 1 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.
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