Structures of gas-phase C2F 7 + ions

Structures of Gas-Phase C 7 F / Ions Robert W. Holman Department of Chemistry. Western Kentucky University, Bowling Green. Kentucky, USA Jeffrey P. Kiplinger Pfizer Inc., Groton, Connecticut, USA Albert A. Tuinman, John E. Bloor, and John E. Bartmess Department of Chemistry, University of Tennessee, Knoxville, Tennessee, USA In an ion cyclotron resonance spectrometer, less than 96% of the C 7Fi cation formed on electron ionization of perfluorotoluene reacts with hexamethyldisilazane. In contrast, the C 7Fi from perfluoronorbornadiene or perfluorobicydo[3.2.0]hepta-2,6-diene is nonreactive with hexarnethyldisilazane. Collision-induced dissociation results support this dichotomy, although the evidence is not as clear-cut. The reactive ion is assigned the benzyl structure and the nonreactive ion the tropyl structure, on the basis of analogy with the protio cases. By AMI calculations, the perfluorobenzyl ion is 25 kcaljmol more stable than the perfluorotropyl ion, the opposite of the situation for the protio analogs (-12 kcaly'mol). Ab initio calculations at the 3-21G level agree with the semiempirical energy difference to within 0.4 kcaljmol; at the more appropriate 6-31G* /MP2 level, the perfluorobenzyl cation is 9.7 kcaljmol more stable than the perfluorotropyl cation. (j Am Soc Mass Spectrom 1993, 4, 813-818) O ne of the classic problems in the chemistry of gas-phase ions is the question of the structure of the C7H~ ions obtained from electron ionization (El) of neutral precursors such as toluene [1]. It appears that two isomers of C7H~, the benzyl cation (1) and the tropyl cation (2), are formed in this process. The tropyl form is more stable than the benzyl form by 12 kcaljmol [2] and is produced in appreciable amounts even from toluene as a precursor [3-17]. The ratio of the two forms is a function of the structure of the neutral precursor, the mass spectral time frame in which the ion population is analyzed, and the energy of the ionizing electrons [18]. In some cases, the possibility of other C7H~ structures, such as tolyl ions, has been invoked [19]. At lower internal energies, the structural isomerization is thought to occur on the C 7Ht· radical cation surface, followed by loss of a hydrogen to produce C7H~. At higher internal energies, the C7H~ isomers can directly interconvert. 1 2 Address reprint requests to John E. Bartmess, Department of Chem- istry, University of Tennessee, Knoxville, TN 37996-1600. © 1993 American Society for Mass Spectrometry 1044-0305/93/$6.00 Although much research has centered on the structure of C7H~ ions emanating from C 7H s and C 7H 7X neutral precursors, there appears to be little information in the literature with regard to the structures of the analogous cations that arise from the perfluorinated analogs of the neutral precursors. The [M - F]+ ion is the base peak in the spectra of C 6FsCH 2X species, but the structure was assumed to be tropyl, with no evidence presented [20]. McLafferty and Amster [21] have examined the C 7H 6F+ ions from monofluorinated toluenes and cyclohepatatrienes via collision-induced dissociation (CID) and shown that there is less extensive isomerization than for the C 7Hi case, although some still occurs. In solution, a-fluorocarbenium and perfluorocarbenium ions, including benzyl, are stable species at temperatures where the protio analogs react [22]. The perfluorotropyl cation is also stable in solution [23]. The perfluorinated C 7Fs neutrals would be expected to yield C 7F7 cations on EI. The question of whether such ions would be of benzyl or tropyl structure is intriguing because our semiempirical molecular orbital calculations indicate that the perfluorobenzyl cation is 25 kcaljmol more stable than the perfluorotropyl cation, the opposite of the relative stabilities of the protio analogs. We investigated the benzyljtropyl ratio of [M - F] f ions arising from EI of three C 7Fs neutral precursors, using as structural probes both their chemical reactivity in an ion cyclotron resonance (ICR) spectrometer Received January 28,1993 Revised June 2,1993 Accepted June 3,1993 814 J Ant Soc Mass Spectrom 1993, 4,813-818 HOLMAN ET AL. and the low-energy (quadrupole mass spectrometer) CID mass spectrum. The perfluorinated precursors investigated are perfluorotoluene (PFT) (3), perfluorobicyclo[2.2.1]heptadiene [perfluoronorbornadiene (PFN)] (4), and perfluorobicyclo[3.2.0]hepta-2,6-diene (PFH) [5]. The intermediacy of perfluorocycloheptatriene (6) is also discussed. a 3 7'1 "'" 5 3 4 5 6 experimentally, benzyl is 12 kcaljmol greater than tropyl [2]. Thus, AMI calculations appear to be accurate for carbenium ion energetics of this type. Ab initio molecular orbital calculations were carried out with the Gaussian 90 program (Gaussian, Inc., Pittsburgh, PA) on an IBM 3094 computer. The perfluorobenzyl cation was constrained to C2 v symmetry and the perfluorotropyl cation to D 7h symmetry. PFT and hexamethyldisilazane were obtained from Aldrich Chemical Co. (Milwaukee, WI) and were pure by ICR and EI mass spectrometry. The two perfluorobicyclic isomers were obtained from Lemal and Dailey [23] and were used as received but were protected from light. The ICR and EI mass spectra of these showed no species other than C 7Fg • Experimental Results The ICR spectrometer is a homebuilt instrument that has been described elsewhere [24]. The C 7 F! ions were produced by EI at 30-eV electron energy. Reduction of this energy to 13 eV did not affect the reactivities reported below. The CID spectra were obtained on a VG Instruments (UK) ZAB-EQ mass spectrometer of BEqQ geometry [25] at 70-eV ionizing electron energy for primary ion formation. These experiments were also done by producing the C 7F! ions from all three precursors at 12-eV ionizing electron energy, with essentially no change in the CID spectra. For the CID experiment, the parent C 7F! ions were at 50-eV laboratory frame kinetic energy; the argon collision gas pressure was nominally 6 X 10- 6 torr. The actual argon pressure in the collision quadrupole is approximately 200 times greater than this value, which was measured downstream from the cell on the pumping line. Conditions for all three precursors were set identically rather than optimizing the conditions for each precursor. CID of "cold" C 7F! was recorded by setting the magnetic and electrostatic sectors to transmit the product of the first field-free region (FFR) metastable transition, C 7 Ft --> C 7 F; , then collision with CID gas in the first quadrupole, followed by mass analysis of CID products in the second quadrupole. The AMI semiempirical molecular orbital calculations [26] were carried out with the MOPAC 5.0 program (Program No. 455, Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN) on the UTK Vax 6000 system. All radical cations were calculated at the unrestricted Hartree-Fock level, and all geometric variables were optimized without restraint. Becau (...truncated)