bi-radical-ion clusters: interrupted sigma bonds in the gas phase?

JOURNAL OF MASS SPECTROMETRYJ. Mass Spectrom. 34, 198È205 (1999)

Bi-radical–Ion Clusters : Interrupted Sigma Bondsin the Gas Phase?

G. Chen, J. W. Denault, N. Kasthurikrishnan and R. G. Cooks*Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393, USA

Novel cation-bound bi-radicals are generated in a chemical ionization source from either the stable free radical,2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), or the phenoxyl radical and cations including carbonyl isocyanate,( [OCNCO‘ ] ), and, in one case, the proton. The dimeric cluster ions, mildly activated under collision-S‘~, CS‘~induced dissociation (CID) conditions, dissociate to yield the cationized monomer (e.g. the [OCNCO ]‘-boundTEMPO radical, [OCNCO ]‘-bound phenoxyl radical, TEMPO radical or TEMPOS‘~-bound CS‘~-boundradical). Such facile dissociation suggests a loosely bound structure comparable to that of a proton-bound dimer.Binding of this sort, radical–cation–radical, suggests the possible formation of an “interrupted sigma bondÏ(electron–electron interactions mediated by the cation). The cluster ion comprised of Cl‘ and two TEMPO rad-icals behaves di†erently and gives a complex set of lower abundance products under similar activation conditions.This indicates that in this case the dissociating cluster has a conventional covalently bound structure. Analogousbehavior is observed in the cases of proton-bound bis-TEMPO clusters and the corresponding mixed phenoxyl–TEMPO species, the Cl‘-bound bis-phenoxyl cluster and the Cl‘-bound cluster with mixed TEMPO–phenoxylcluster, as well as the bis-phenoxyl and bis-phenoxyl complexes. The CID of the putativeS‘~-bound CS‘~-boundproton-bound bis-phenoxyl cluster also suggests a conventional structure and its dissociation shows a strong depen-dence on collision energy, possibly because of facile rearrangement from the desired weakly bound bi-radicalcluster ion. Copyright 1999 John Wiley & Sons, Ltd.(

KEYWORDS: ion structure ; radicals ; kinetic method ; cluster ions

INTRODUCTION

Free radicals have been the subject of extensive researchfor several decades, in part owing to their importance asintermediates in chemical reactions.1,2 Mass spectro-metric studies of free radicals in the gas phase provideunique information regarding the thermochemistry,reactivity and structures of these species.3h17 Forexample, Squires and co-workers reported a general,regioselective method for producing diradical negativeions in the gas phase and characterized their heats offormation.18h20 Subsequent negative ion photoelectronspectroscopic measurements have yielded the electronaffinities and singletÈtriplet energy gaps in the corre-sponding neutral diradicals. Kentta� maa and co-workers,5,6 Eberlin7 and others have used distonic ionsto study free radical chemistry with the inert ionic“labelÏ allowing mass spectrometry to be employed asthe experimental tool. In this lab we demon-

* Correspondence to : G. Department of Chemistry,R. Cooks,Purdue University, West Lafayette, Indiana 47907-1393, USA

Contract/grant sponsor : National Science Foundation ; Contract/grant number : CHE 97-32670.

Contract/grant sponsor : Office of Naval Research.

strated that it is possible to generate cluster ions com-prised of a cation, a free radical and a closed-shellneutral molecule. Proton-bound dimers of this typewere generated and the kinetic method, an approximatemethod for making thermochemical demonstrationsbased on the relative rates of dissociation of clusterions,21,22 was used to measure the proton affinity of thephenoxyl radical and of some substituted phenoxyl rad-icals.23 More recently, the same method was extendedto measure the proton affinity of the stable free radical2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) by gen-erating proton-bound dimers between TEMPO andvarious reference compounds in a chemical ionizationsource.24 The proton affinity of the TEMPO radicalwas estimated to be 209.5^ 1.0 kcal mol~1 (1kcal\ 4.184 kJ). Comparisons with radicals of knownproton affinity suggest that these results are consistentwith an oxy-centered radical.24

In the present study, we attempted to extend theseresults by generating gas-phase cluster ions consisting ofa cation and two radicals. The work was undertaken inview of the observation that solutions of the stabletris(2,6-dimethoxyphenyl)methyl radical and lithiumsalts form solids with unusual magnetic behavior.25 Thetwo radicals appears to “sandwichÏ a cation (e.g. Li`) ofappropriate size between them and the spatial relation-ships dictate that intermolecular electron coupling

CCC 1076È5174/99/030198È08 $17.50 Received 24 June 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 3 December 1998

BI-RADICALÈION CLUSTERS 199

Figure 1. (A) Product ion tandem mass spectrum of the mass-selected ÍOCNCOË½-bound cluster ion with two TEMPO radicals. Activationwas achieved with 2 eV collisions using argon at a pressure of 0.4 mTorr. (B) Product ion tandem mass spectrum of the mass-selectedproton-bound dimer of 2,4,6-trimethylpyridine. Activation was achieved with 2 eV collisions using an argon target at a pressure of 0.4mTorr.

occurs. Kahr and co-workers25 introduced the term“interrupted sigma bondÏ to describe a radical pair inwhich electron interactions are mediated by a cation.Ab initio calculations on linear suggestH3C~Li`~CH3that the triplet state is energetically favored.25 Theinterrupted sigma bond phenomenon may be an impor-tant consideration in the synthesis of organic materialswith colligative magnetic properties.26,27 Improvedunderstanding of systems with interrupted sigma bondsmay be useful and is of fundamental chemical interest.Here, we used chemical ionization to investigate clusterions consisting of TEMPO or phenoxyl radicals boundby the cations H`, Cl` and [OCNCO]`, and also theradical cations and Structural character-S`~ CS`~.ization of these novel cluster ions was achieved bycollision-induced dissociation (CID).

EXPERIMENTAL

Experiments were performed using a Finnigan TSQ 700triple-quadrupole mass spectrometer (Finnigan MAT,San Jose, CA, USA). Acetyl chloride, ethoxycarbonylisocyanate and carbon disulÐde (Aldrich Chemical, Mil-waukee, WI, USA) were introduced into the ion sourcethrough the GC inlet via a Granville Phillips (Boulder,CO, USA) variable leak valve at sufficient pressure tocreate chemical ionization conditions in the source(indicated pressure, 0.5 Torr (1 Torr \ 133.3 Pa)). Thesource temperature was maintained at 100 ¡C and themanifold at 70 ¡C for all experiments. For experimentsinvolving TEMPO, a 1 ll aliquot of a methanol solu-

Copyright ( 1999 John Wiley & Sons, Ltd. J. Mass Spectrom. 34, 198È205 (1999)

200 G. CHEN ET AL .

Figure 2. (A) Product ion tandem mass spectrum of the mass-selected ÍOCNCOË½-bound cluster ion with two phenoxyl radicals. Activa-tion was achieved with 2 eV collisions using argon at a pressure of 0.4 mTorr. (B) Product ion tandem mass spectrum of the mass-selectedproton-bound dimer of phenol. Activation was achieved with 2 eV collisions using an argon target at a pressure of 0.4 mTorr.

tion (1 M) of TEMPO (Aldrich Chemical) was depositedon the rhenium wire Ðlament of a direct evaporationprobe. Similarly, for experiments involving phenoxylradicals, a 1 ll aliquot of a methanol solution (1 M) ofphenol (Aldrich Chemical) was deposited on therhenium wire Ðlament of a direct evaporation probe.The direct evaporation probe was not heated ; thesample was simply allowed to evaporate into the ionsource.

The cluster ions of interest, generated in the ionsource, were mass selected using the Ðrst quadrupolemass analyzer operated at unit mass resolution. CID ofthe mass-selected cluster ions was achieved in thesecond quadrupole under very mild conditions, viz.nominal 2 eV collision energy and an argon target at anominal pressure of 0.40 mTorr. A typical main beamattenuation was 20È30%. The fragment ion abundances

were measured from the product ion mass spectrumgenerated by scanning the third quadrupole at a scanspeed of 500 Thomson s~1, (1 Thomson (Th)\ 1 Daper unit charge28). Each spectrum is the average of15È30 scans.

RESULTS AND DISCUSSION

Ionization of the TEMPO radical in the presence ofethoxycarbonyl isocyanate yielded the carbonyl iso-cyanate cation [OCNCO]`,29 its adduct with TEMPOand the [OCNCO]`-bound TEMPO dimer. The latterion, the cation-bound bi-radical, was mass selected anddissociated under mild CID conditions. As shown in theproduct ion mass spectrum [Fig. 1(A)], this cluster ion



Figure 3. (A) Product ion tandem mass spectrum of the mass-selected cluster ion with two TEMPO radicals. Activation wasS½~-boundachieved with 2 eV collisions using an argon target at a pressure of 0.4 mTorr. (B) Product ion tandem mass spectrum of the mass-selected

dimer with two TEMPO radicals. Activation was achieved with 2 eV collisions using an argon target at a pressure of 0.4 mTorr.CS½~-bound

fragments to yield just one product ion at 226 Th. Thisfragment ion corresponds to the monomeric[OCNCO]`-bound TEMPO radical. The ease of disso-ciation under mild activation conditions suggests thatthe precursor ion is a loosely bound cluster ion com-posed of [OCNCO]` and two TEMPO radicals.30 Evi-dence for this interpretation is found in the fact that aproton-bound dimer (loosely bound) composed of 2,4,6-trimethylpyridine shows similar fragmentation behavior[Fig. 1(B)]. Note that ab initio calculations on the[OCNCO]` cation show that it has a linear structurewith the positive charge mainly localized on the twocarbon atoms.29 The high positive charge density on thecarbon atoms suggests that one or both of the these isthe binding site of the TEMPO radicals ; binding to theoxygen of TEMPO is assumed but binding via nitrogen

cannot be ruled out. If both TEMPO radicals bind tonitrogen, any interaction of the two TEMPO radi-cals with each other will be mediated by the cation[OCNCO]`. This type of interaction would resemblethe “interrupted sigma bondingÏ proposed to occur insolution.25

Similar observations were made with [OCNCO]`-bound phenoxyl radicals. Figure 2(A) shows the production mass spectrum of [OCNCO]` bound to two phe-noxyl radicals. This cluster ion displays analogous frag-mentation behavior, viz. one fragment ion occurs inrelatively high abundance. Similar dissociation behaviorof an authentic loosely bound proton-bound dimer, thatof phenol, is illustrated in Fig. 2(B) and conÐrms aloosely bound structure for [OCNCO]`-bound bis-phenoxyl complex. The observation of [OCNCO]`-


202 G. CHEN ET AL .

Figure 4. Product ion tandem mass spectrum of the mass-selected complex Activation was achieved with 2 eVÍ(TEMPO)2½35Cl½Ë.

collisions using an argon target at a pressure of 0.4 mTorr.

bound bis-TEMPO and bis-phenoxyl complexes sug-gests that the possibility that species with interruptedsigma bonds can be prepared in the gas phase beexplored further.

More notably, gas-phase cluster ions with three for-mally unpaired electrons can also be generated. Theproduct ion mass spectra of andS`~-bound CS`~-

bis-TEMPO radicals are shown in Fig. 3(A) andbound(B), respectively. In the former case, both radicals mustbe bound to the same cationic site. Both cluster speciesdisplay fragmentation in which just one fragment ionoccurs and in relatively high abundance. This suggests aloosely bound structure for the cluster ion, and stronglyimplies an “interrupted sigma bondÏ phenomenon.

The generation of the proton-bound dimer ofTEMPO was also attempted. The mass spectrum ofTEMPO in methanol shows an ion at mass/chargeratio 313 Th. However, dissociation of this ion results infragment ions at 156 and 157 Th (data not shown).These ions correspond to the ionized TEMPO cationand the protonated TEMPO radical cation, respec-tively. The low abundance of these fragment ions andthe fact that two fragment ions were observed indicatethat the ion has a covalently bound structure. Furtherattempts were made to generate a Cl`-bound bis-TEMPO cluster ion. Ionization of acetyl chloride30 inthe presence of TEMPO results in the formation of anion at 347 Th. This ion corresponds to [(TEMPO)2and fragments under mild activation condi-]35Cl`],tions to produce fragment ions at 191, 156, 157 and 142Th (Fig. 4). These fragment ions correspond to[TEMPO] 35Cl`], [TEMPO]`, [TEMPO] H]`and respectively. To conÐrm[TEMPO ] H [ CH3]`,these results, the cluster ion composed of the isotopicform of chlorine, was mass selec-[(TEMPO)2]37Cl`],ted and dissociated under similar conditions (data notshown). The fragments of the 37Cl-labeled cluster was

shifted by two mass/charge units and the relative abun-dances were similar to that of the 35Cl cluster ion.These results suggest that the cluster ion [(TEMPO)2is covalently bound.] Cl`]

Covalently bound structures have been reported inprevious work involving the 3-methylphenoxylÈTEMPO proton-bound cluster ion.24 Further CIDstudies of the proton-bound bis-phenoxyl andphenoxylÈTEMPO complexes, the Cl`-bound bis-phenoxyl and phenoxylÈTEMPO complexes and the

bis-phenoxyl and bis-phenoxylS`~-bound CS`~-boundcomplexes indicate the formation of covalently boundstructures, viz. each mass-selected ion yields severalfragment ions with low abundance.

Interestingly, the CID of proton-bound bis-phenoxylcomplexes exhibits fragmentation that shows strongdependence on collision energy. The product ion massspectrum of the protonated bis-phenoxyl complex isshown in Fig. 5 (nominal zero collision energy). Thefragment ion at 94 Th corresponds to a protonated phe-noxyl radical cation, which could be due to simple bondcleavage in the precursor cluster ion. However, the ionsat 159 and 169 Th correspond to the loss of CO and

from the precursor ion. These fragments mustH2Ooccur via rearrangement processes. At a higher collisionenergy (2 eV), the same fragment ions are observed [Fig.5(B)], but their abundances relative to the ion at 94 Thhave decreased. This may indicate competition betweensimple bond cleavage and rearrangement processes.31At higher collision energy, simple bond cleavage in theproposed cluster ion is favored, resulting in a moreabundant fragment ion at 94 Th. At lower collisionenergy, rearrangement to a covalent complex is domi-nant, leading to more abundant fragment ions at 159and 169 Th. Dissociation of the isotopically labeledproton-bound cluster (data not shown)bis-d5-phenoxylconÐrms the observations drawn from Fig. 5 : (i) an ion



Figure 5. Product ion tandem mass spectra of the putative proton-bound dimer of the phenoxyl radical. Activation was achieved using anargon target at a pressure of 0.4 mTorr. Collision energy (A) nominally 0 eV and (B) 2 eV.

corresponding to the protonated radicald5-phenoxylcation is observed at 99 Th, (ii) ions corresponding tothe loss of CO and HDO from the precursor ion areobserved at 169 and 178 Th and (iii) an ion correspond-ing to the loss of from the precursor ion isD2Oobserved at 177 Th. The proposed mechanisms for themain fragmentation pathways, assuming that the orig-inal cluster is in fact the loosely bound structure, aregiven in Scheme 1. Isomerization of a weakly boundcluster, even in low-energy activation experiments, isunexpected but may be facilitated by the high reactivityof the free radicals. The alternative explanation, that thecluster ion is covalently bound, cannot be excluded.Additional evidence suggesting the occurrence of aloosely bound, interrupted sigma bond structure in

which the separating cation is the proton comes fromthe dissociation of the protonated mixed cluster ioncomposed of TEMPO and the phenoxyl radical. Thetandem mass spectrum of this ion is illustrated in Fig. 6.The fragmentation observed is characteristic of proton-bound dimers, as demonstrated in Figs 1(B) and 2(B).Further, the dissociation is consistent with that reportedearlier for proton-bound dimers of TEMPO with even-electron partners.24 Note further that the two fragmentions observed in Fig. 6 occur in an abundance ratio ofD8, and under the mild activation conditions used, aratio of D8 in fragment ion relative abundances forproton-bound dimers is associated with a di†erence ofD2 kcal mol~1 in proton affinity of the neutral constit-uents.22 This is in reasonable agreement with indepen-


204 G. CHEN ET AL .

Figure 6. Product ion tandem mass spectrum of the putative proton-bound mixed dimer consisting of the phenoxyl and the TEMPOradical. Activation was achieved with 2 eV collisions using an argon target at a pressure of 0.4 mTorr.

Scheme 1. Fragmentation pathways of proton-bound bis-phenoxyl cluster ion.

dent estimates22,24 of the proton affinity of the phe-noxyl (206 kcal mol~1) and TEMPO (210 kcal mol~1)radicals and supports the possibility that a proton-interrupted sigma bond is generated in this case.

CONCLUSIONS

The free radical TEMPO, and also the phenoxylradical, can form loosely bound cluster ions with thecations [OCNCO]`, or The last two systemsS`~ CS`~.formally contain three unpaired electrons. The CIDbehavior suggests a loosely bound cluster structure. Thetwo radicals are assumed to interact with each other

through the binding cation (at least in the case of S`~)but their spin states are not known. These observationsare analogous to those made regarding interruptedsigma bond structures in solution. This is the Ðrst casereported of interrupted sigma bonding in the gas phase.The spin states of the gas-phase bi-radicals remain to beinvestigated, although computations on the linear

model show the high-spin state to beH3C~Li`~CH3favored.25Given the high reactivity of free radicals, it is not sur-

prising that attempts to generate cluster ions composedof two radicals and a cation failed in other systems. Thisis presumably a result of the formation of a closed-shellcovalently bound structure of lower energy. This wasobserved for Cl`-bound bis-TEMPO, bis-phenoxyl andmixed TEMPOÈphenoxyl clusters, proton-bound bis-TEMPO and TEMPOÈphenoxyl clusters and

bis-phenoxyl radicals. The proton-S`~/CS`~-boundbound bis-phenoxyl radicals display behavior suggest-ing fragmentation from a covalent structure formed byrearrangement of the precursor dimer. The proton-bound homodimers of TEMPO and the phenoxylradical fragment simply and appear to have a looselybound, interrupted sigma bond structure.

Acknowledgements

This work was supported by the National Science Foundation, CHE97-32670, and the Office of Naval Research. We thank Professor BartKahr and the late Professor Robert R. Squires for valuable dis-cussions and Dr S. H. Hoke, II, and Dr J. S. Patrick for contributionsto the study. G.C. acknowledges Ðnancial support from an AmericanChemical Society, Division of Analytical Chemistry Fellowship, spon-sored by Dow Chemical.



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bi-radical-ion clusters: interrupted sigma bonds in the gas phase?

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