Applications of charge stripping/charge exchange spectra of organic ions

The application of a new technique which involves the combination of charge stripping and charge exchange processes has been investigated. Ions are charge-stripped in the second field-free region of a triple-sector mass spectrometer (BEE geometry), and then subjected to a charge exchange reaction in the third field-free region. The resulting charge stripping/charge exchange (CS/CE) spectrum is free from interference, which is otherwise common in charge stripping spectra. Comparisons between charge stripping and CS/CE spectra are made in cases where both kinds of spectra are obtainable. In order to assess the applicability of this new technique to studies of isomeric ion structures, species for which charge stripping spectra have previously been unobtainable have been chosen. CS/CE spectra of [C₆H₆]⁺˙, [C₆H₅]⁺, [C₈H₁₀]⁺˙ and [C₇H₇O]⁺ ions from a variety of precursors are recorded: in most cases, sufficient differences are observed to permit distinction between isomeric structures (or mixtures of structures). Previous studies which have shown that stable doubly charged molecular ions of ethane cannot be formed by the charge stripping technique are confirmed from its CS/CE spectrum.     

Differences between the internal energy depositions induced by collisional activation and by electron transfer of W(CO)6(2+) ions on collision with Ar and K targets

Doubly charged tungsten hexacarbonyl W(CO)(6) (2+) ions were made to collide with Ar and K targets to give singly and doubly charged positive ions by collision-induced dissociation (CID). The resulting ions were analyzed and detected by using a spherical electrostatic analyzer. Whereas the doubly charged fragment ions resulting from collisional activation (CA) were dominant with the Ar target, singly charged fragment ions resulting from electron transfer were dominant with the K target. The internal energy deposition in collisionally activated dissociation (CAD) evaluated with the Ar target was broad and decreased with increasing internal energy. The predominant peaks observed with the K target were associated with singly charged W(CO)(2) (+) and W(CO)(3) (+) ions: these ions were not the result of CA, but arose from dissociation induced by electron transfer (DIET). The internal energy deposition resulting from the electron transfer was very narrow and centered at a particular energy, 7.8 eV below the energy level of the W(CO)(6) (2+) ion. This narrow internal energy distribution was explained in terms of electron transfer by Landau-Zener potential crossing at a separation of 5.9 x 10(-8) cm between a W(CO)(6) (2+) ion and a K atom, and the coulombic repulsion between singly charged ions in the exit channel. A large cross section of 1.1 x 10(-14) cm(2) was estimated for electron capture of the doubly charged W(CO)(6) (2+) ion from the alkali metal target, whose ionization energy is very low. The term "collision-induced dissociation," taken literally, includes all dissociation processes induced by collision, and therefore encompasses both CAD and DIET processes in the present work. Although the terms CID and CAD have been defined similarly, we would like to propose that they should not be used interchangeably, on the basis that there are differences in the observed ions and in their intensities with Ar and K targets.     

Multiple ionization,charge separation and charge stripping reactions involving polycyclic aromatic compounds

The 70 eV electron ionization mass spectra of polycyclic aromatic compounds are characterized by the presence of relatively stable multiply charged molecular ions [M]ⁿ⁺ (n=2–4). When generated from the compounds benzene, napthalene, anthracene, phenanthrene, 2,3-benzanthracene, 1,2-benzanthracene, chrysene, 9,10-benzophenanthrene and pyrene, the relative abundances of the multiply charged ions increase dramatically with the number of rings. These compounds form multiply charged molecular ions (n=2, 3) which undergo unimolecular decompositions indicative of considerable ionic rearrangement. The main charge separation processes observed here [M]²⁺→m₁⁺+m₂⁺, [M]³⁺˙→m₃⁺+m→+m₄²⁺) involve, in almost every case, one or more of the products [CH₃]⁺, [C₂H₃]⁺˙ and [C₃H₃]⁺. This suggests the existence of preferred structures amongst the metastable parent ions. Information on the relative importance of the various fragmentation pathways is presented here along with translational energy release data. Some tentative structural information about the metastable ions has been inferred from the translational energy release on the assumption that the released energy is due primarily to coulombic repulsion within the transition state structure. For the triply charged ions these interpretations have necessitated the use of a coulombic repulsion model which takes account of an extra charge. Vertical ionization energies for the process [M]ⁿ⁺+G→[M](ⁿ⁺¹)+G+e^? (charge stripping) have also been determined where possible for n=1 and 2 and the results from these experiments allow the derivation of simple empirical equations which relate successive ionization energies for the formation of [M]²⁺ and [M]³⁺˙ to the appearance energy of [M]⁺˙.     

A structural investigation of [C6H6]2+ and [C6H6]+˙ ions involved in charge exchange reactions

Mass-analysed ion kinetic energy spectra for collisional activation (CA) of [C₆H₆]⁺˙ formed via electron capture by [C₆H₆]²⁺ ions in collision with neutral benzene molecules have been compared for the C₆H₆ isomers benzene, 1,5-hexadiyne and 2,4-hexadiyne. Comparisons of fragment abundance and total CA fragment yields were also made for [C₆H₆]⁺˙ ions generated by electron ionization (EI). CA conditions of ion velocity and collision gas pressure were identical in these comparisons. In general the fragment abundance patterns for the ions formed by charge exchange were very similar to those for singly charged benzene ions generated by EI. However, significant variations in CA fragment yield (the ratio of the total CA fragment ion abundance to the abundance of the incident unfragmented ions) were observed. It is not clear from the results whether these variations reflect structurally different ions or ions of different internal energies. The CA spectra of [C₆H₆]⁺˙ ions derived from charge exchange reactions between the benzene dication and the target gases He, Ne, Ar, Kr and Xe have also been recorded and, once again, very similar fragment abundance patterns were observed along with large variations in total CA fragment yields. Charge exchange efficiency measurements are reported for reactions between the benzene dication and the targets He, Ne, Ar, Kr, Xe and C₆H₆ (benzene) and also for the doubly charged ions derived from the linear C₆H₆ isomers. In the latter case Xe and benzene targets were used. The energetics and efficiency measurements for the former reactions suggest that for targets such as He and Ne the processes probably involve excited states of the doubly charged ions. The efficiencies measured for the latter reactions were distinctly different for the three C₆H₆ isomers and may indicate a strong dependence of charge exchange cross-section on doubly charged ion structure.     

A study of the electron capture induced decompositions and charge separation reactions of the doubly charged isomeric ions of the methylpyridines and aniline

The doubly charged isomeric ions [C₆H₇N]²⁺ formed from 2-, 3- and 4-methylpyridine and aniline were investigated via their unimolecular charge separation reactions and by electron capture induced decompositions (ECID). The ECID spectra were compared with the collision induced decomposition (CID) spectra of the singly charged ions in an attempt to investigate the structure of the doubly charged ions. The four isomers could be unambiguously identified by their unimolecular charge separations. These differences were greater than in the mass spectra, ECID spectra or CID spectra of singly charged ions.     

Double-ionization energies to ground and excited states of the tungsten hexacarbonyl dication

Tungsten hexacarbonyl was investigated by double-charge-transfer (DCT) spectroscoPy, and the double-ionization energies to ground and electronically excited states of W(CO) ₆ ²⁺ determined. The double-ionization energies corresponding to the first two distinct peaks in the spectra are 22.8 ± 0.3 eV and 28.5 ± 0.3 eV, but numerous overlapping peaks at higher energies are evident. It is shown that the DeI spectra can explain the main features of a previously determined (J. Am. Soc. Mass Spectrom. 1990, 1, 16–27) internal energy distribution curve for W(CO) ₆ ²⁺ ions formed by 70-eV electron ionization of W(CO)₆ molecules.     

Energy dependence of the fragmentation of some isomeric [C6H12]+˙ ions

The charge exchange mass spectra of 14 C₆H₁₂ isomers have been determined using [CS₂]^+˙, [COS]^+˙, [Xe]^+˙, [CO]^+˙, [N₂]^+˙ and [Ar]^+˙ as the major reactant ions covering the recombination energy range from ∼10.2 eV to ∼15.8 eV. From the charge exchange data breakdown graphs have been constructed expressing the energy dependence of the fragmentation of the isomeric [C₆H₁₂]^+˙ molecular ions. The electron impact mass spectra are discussed in relation to these breakdown graphs and approximate internal energy distribution functions derived from photoelectron spectra.     

Fragmentation of 1- and 3-methoxypropene ions another part of the C4H8O+• potential energy surface

The fragmentation mechanisms of metastable ionized 1? and 3?methoxypropene have been examined in detail by using ionization and appearance energy measurements, metastable ion and collisional activation mass spectra, and a variety of isotopically labeled molecules. These metastable C₄H₈O^+? ions fragment by loss of H; CH₃, and H₂CO, and the experimental observations allowed the construction of the potential energy diagram which describes their interconversion and the participation of four other distonic and carbene C₄H₈O^+? ions. It was found that these two methyl alkenyl ether ions had no common reaction channel with either the 2?methoxy isomer or with any of the alcohol, keto, or enol C₄H₈O^n+? isomers which previously have been extensively studied.     

Internal energy distributions of tungsten hexacarbonyl ions after neutralization—Reionization

The internal energy distributions P(ε) transferred to W(CO)^+√₆ during the kiloelectronvolt collisions that occur upon neutralization-reionization (NR) have been estimated based on the relative abundances of the W(CO)^+√₀₋₆ products present in NR spectra (thermochemical method). The average internal energy of the incipient W(CO)^+√₆ ^* ions arising after near thermoneutral neutralization with trimethylamine followed by reionization with O₂ is −9 eV for 8-keV precursor ions and is mainly deposited during reionization. For comparison, the mean internal energy of W(CO)^+√₆* after electron ionization (EI) or collisionally activated dissociation (CAD) is −6 eV. Making the neutralization step endothermic slightly increases the overall excitation gained; however, a large increase in endothermicity ( > 16 eV) causes only a modest rise of the average internal energy (< 2 eV). The P(ε) curve for NR increases exponentially up to 6 eV and levels off at higher energies, showing that the probability of imparting large internal energies (6–17 eV) is high. In sharp contrast, the most probable excitation on CAD is ≤ 6 eV, and the probability of deposition of larger energies declines exponentially. The mean internal energies after CAD and NR decrease steadily when the kinetic energy is lowered. The structure (minima-maxima) observed in the P(ε) distribution for EI, which most likely originates from Franck-Condon factors, is not reproduced in the distributions for NR or high energy CAD, despite the fact that all three methods involve electronic excitation. Because of the large internal energies transferred upon NR, NR mass spectrometry could be particularly useful in the differentiation of ionic isomers with high dissociation but low isomerization thresholds. (J Am Soc Mass Spectrom 1994, 5, 1093-1101)     

Charge Transfer Dissociation (CTD) Mass Spectrometry of Peptide Cations Using Kiloelectronvolt Helium Cations

A kiloelectronvolt beam of helium ions is used to ionize and fragment precursor peptide ions starting in the 1+ charge state. The electron affinity of helium cations (24.6 eV) exceeds the ionization potential of protonated peptides and can therefore be used to abstract an electron from—or charge exchange with—the isolated precursor ions. Kiloelectronvolt energies are used, (1) to overcome the Coulombic repulsion barrier between the cationic reactants, (2) to overcome ion-defocussing effects in the ion trap, and (3) to provide additional activation energy. Charge transfer dissociation (CTD) of the [M+H]⁺ precursor of Substance P gives product ions such as [M+H]^2+? and a dominant series of a ions in both the 1+ and 2+ charge states. These observations, along with the less-abundant a + 1 ions, are consistent with ultraviolet photodissociation (UVPD) results of others and indicate that C–C_α cleavages are possible through charge exchange with helium ions. Although the efficiencies and timescale of CTD are not yet suitable for on-line chromatography, this new approach to ion activation provides an additional potential tool for the interrogation of gas phase ions. Graphical Abstract

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Fragmentation of the tryptophan cluster [Trp9-2H]2- induced by different activation methods

Electrospray ionization (ESI) of tryptophan gives rise to multiply charged, non‐covalent tryptophan cluster anions, [Trp_n–xH]^x?, in a linear ion trap mass spectrometer, as confirmed by high‐resolution experiments performed on a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The smallest multiply charged clusters that can be formed in the linear ion trap as a function of charge state are: x = 2, n = 7; x = 3, n = 16; x = 4, n = 31. The fragmentation of the dianionic cluster [Trp₉–2H]^2? was examined via low‐energy collision‐induced dissociation (CID), ultraviolet photodissociation (UVPD) at 266 nm and electron‐induced dissociation (EID) at electron energies ranging from >0 to 30 eV. CID proceeds mostly via charge separation and evaporation of neutral tryptophan. The smallest doubly charged cluster that can be formed via evaporation of neutral tryptophans is [Trp₇–2H]^2?, consistent with the observation of this cluster in the ESI mass spectrum. UVPD gives singly charged tryptophan clusters ranging from n = 2 to n = 9. The latter ion arises from ejection of an electron to give the radical anion cluster, [Trp₉–2H]^?.. The types of gas‐phase EID reactions observed are dependent on the energy of the electrons. Loss of neutral tryptophan is an important channel at lower energies, with the smallest doubly charged ion, [Trp₇–2H]^2?, being observed at 19.8 eV. Coulomb explosion starts to occur at 19.8 eV to form the singly charged cluster ions [Trp_x–H]^? (x = 1–8) via highly asymmetric fission. At 21.8 eV a small amount of [Trp₂–H–NH₃]^? is observed. Thus CID, UVPD and EID are complementary techniques for the study of the fragmentation reactions of cluster ions. Copyright © 2010 John Wiley & Sons, Ltd.     

Internal energy deposition in doubly charged tungsten hexacarbonyl on low-energy Charge exchange with organic targets

Estimates are reported for the internal energy distribution and the average internal energy deposited in the molecular ion of tungsten hexacarbonyl, W(CO)˙, when it is generated by charge exchange of the corresponding doubly charged ion. Charge exchange was performed in the collision energy range from nominal zero to 40 eV using a triple quadrupole mass spectrometer with a variety of polyatomic targets including alkanes, aromatics and tungsten hexacarbonyl itself. At all energies, dissociative charge exchange was accompanied by collision-induced dissociation and this meant that the energetics of charge exchange had to be measured by extrapolation to zero collision energy and zero pressure. Under these conditions, the average internal energy deposited into the charge-exchanged product W(CO)˙ was 1.0, 0.9, 1.3 and 2.3 eV for the targets isobutane, n-pentane, n-heptane and toluene, respectively. Data for the symmetrical charge exchange reaction demonstrated that partitioning of internal energy between the two products occurred equally. Hence, assuming equipartitioning over all degrees of freedom (at threshold) in the other cases also, the total internal energies deposited in the target molecules were determined to be 1.1, 1.1, 2.3 and 2.7 eV, respectively. Comparison of these values with the reaction exothermicities revealed that ～2 eV (1.2–2.6 eV) of additional energy is liberated on charge exchange in each case. This is suggested to appear as translational energy of separation of the two charged products. Charge exchange in this system is suggested to occur by a Landau-Zener curve-crossing mechanism and the estimated values of the coulombic energies correspond to electron transfer at internuclear distances of the order of 7 ± 2 Å.     

Organic neutralization agents for neutralization-reionization mass spectrometry

Porter has shown that excited neutrals of specified internal energies can be prepared by neutralization of an ion beam with metal vapors of low ionization potential (IP). For specific problems in neutralization-reionization mass spectrometry, a metal with the desired IP value may not be available, or it may present experimental problems such as a high vaporization temperature, instrument contamination, or detector instability. The use of organic neutralization agents such as tetra-p-anisylethylene (IP = 6.0 eV) can minimize these problems (although cross sections for neutralization with these are a factor of 5 lower than those with metals), and can provide a much wider range of IP values. Their utility is demonstrated in the neutralization of C₄H₄ ^+? and CH₈ ^+? ions to produce C₄H₄ and C₄H₈ of selected internal energies. However, for CH₄ ^+? neutralization, the CH₄ neutrals formed have a much lower internal energy than predicted, indicating that electron transfer from the neutralization agent predominantly produces its ions in excited states.     

Selenoketene (H2CCSe)+• and selenoketyl cumulene (HCCSe)+ ions and their neutral counterparts: a tandem mass spectrometric and computational study

Dissociative electron ionization (70eV) of selenophene (C₄H₄Se) generates m/z 106 ions of composition [H₂, C₂, ⁸⁰Se]^+? and m/z 105 ions of [H, C₂, ⁸⁰Se]⁺. From tandem mass spectrometric experiments, Density Functional Theory (DFT) and ab initio calculations, it is concluded that these ions have the structure of selenoketene H₂C?C?Se^+? (1a^+? )and selenoketyl HC?C?Se⁺ (2a⁺) ions respectively. The calculations predict that selenoketene ion 1a^+? is separated by high energy barriers from its isomers selenirene (H e)^+? 1b^+?, ethyne selenol (HCCSeH)^+? 1c^+?, (CCHSeH)^+? 1d^+? and (CCSeH₂)^+? 1e^+?. The selenoketyl ion 2a⁺ is separated by high barriers from its isomers (CCHSe)⁺ 2b⁺, and (CCSeH)⁺ 2c⁺. Neutralization‐reionization mass spectra (NRMS) of these structurally characterized ions confirmed that the corresponding neutral analogues, selenoketene H₂CCSe 1a and selenoketyl radical HCCSe 2a^? are stable in the rarefied gas phase. The relative, dissociation, and isomerization energies for selenoketene and selenoketyl ions and neutrals studied at B3LYP/6–31G(d,p) and G2/G2(MP2) levels are used to support and interpret the experimental results. Copyright © 2005 John Wiley & Sons, Ltd.     

Doubly charged benzene and isomeric dications. Fragmentation energetics and charge distributions calculated by MINDO/3 molecular orbital theory

MINDO/3 calculations for singlet and triplet doubly charged benzene [C₆H₆]²⁺ are in satisfactory agreement with the experimentally determined values of the vertical double ionization energy of benzene; calculations for straight chain isomeric structures are consistent with the observed kinetic energy release on fragmentation to [C₅H₃]⁺ and [CH₃]⁺. Symmetrical doubly charged benzene ions relax to a less symmetrical cyclic structure having sufficient internal energy to fragment by ring opening and hydrogen transfer towards the ends of the carbon chain. Fragmentation of [CH₃C₄CH₃]²⁺ to [CH₃C₄]⁺ and [CH₃]⁺ is a relatively high energy process (A), whereas both (B): [CH₃CHC₃CH₂]²⁺ to [CHC₃CH₂]⁺ and [CH₃]⁺ and (C): [CH₃CHCCHCCH]²⁺ to [CHCCHCCH]⁺ and [CH₃]⁺ may be exothermic processes from doubly charged benzene. Furthermore, the calculated energy for the reverse of process (A) is less than the experimentally observed kinetic energy released, whereas larger energies for the reverse of processes B and C are predicted. Heats of formation of homologous series [HC_n]⁺, [CH₃C_n]⁺, [CH₂C_n?2CH]⁺, [CH₃C_n?2CH₂]⁺ and [CH₂?CHC_n?3CH₂]⁺ with 1 < n < 6 are calculated to aid prediction of the most stable products of fragmentation of doubly charged cations. The homologous series [CH₂C_n?2CH]⁺ is relatively stable and may account for ready fragmentation of doubly charged ions to [C_nH₃]⁺; alternatively the symmetrical [C₅H₃]⁺ ion [CHCCHCCH]⁺ may be formed. Dicoordinate carbon chains appear to be important stabilizing features for both cations and dications.     

Multiple ionization and Coulomb explosion of mercury clusters in femtosecond laser fields

We report on studies of multiple ionization and fragmentation of free Hg_n (n ≤ 80) clusters in the femtosecond time domain at wavelengths ranging from 255 nm to 800 nm. After excitation by single laser pulses of an intensity of 5 * 10¹¹ W/cm² we observe prompt formation of multiply charged Hg_n clusters. The Hg_n cluster size distribution observed up to n ≈ 80 shows in additon to singly charged also doubly and triply charged clusters with a surprisingly high amount of doubly charged clusters. The measured cluster size distribution is nearly independent of laser wavelengths. For higher laser intensities (2 * 10¹² W/cm²) we observe multiply charged mercury atoms up to Hg⁵⁺. At 10¹³ W/cm² molecules and clusters eventually disappear due to Coulomb explosion and complete Fragmentation. Only atomic ions, singly and multiply charged, with high kinetic energies are then observed.     

Multiphoton ionization and Coulomb explosion of C2H5Br clusters: a mass spectrometric and charge density study

Using time‐of‐flight mass spectrometry (TOFMS), laser‐induced photochemistry of ethyl bromide clusters has been investigated at three different wavelengths (viz. 266, 355 and 532 nm) utilizing nanosecond laser pulses of ~5 × 10⁹ W/cm². An interesting finding of the present work is the observation of multiply charged atomic ions of carbon and bromine at 355 and 532 nm, arising from the Coulomb explosion of (C₂H₅Br)_n clusters. At 266 nm, however, the (C₂H₅Br)_n clusters were found to exhibit the usual multiphoton dissociation/ionization behaviour. The TOFMS studies are complemented by measuring the total charge density of the ionized volume at 266, 355 and 532 nm, using the parallel plate method, and the charge densities were found to be ~2 × 10⁹, 6 × 10⁹ and 2 × 10¹¹ charges/cm³, respectively. The significantly higher charge density and the presence of energetic, multiply charged atomic ions at 532 nm are explained by the higher ponderomotive energy of the 532 nm photon, coupled with the Coulomb stability of the residual multiply charged ethyl bromide clusters generated upon laser irradiation, due to their larger effective cluster size at 532 nm than at 355 and 266 nm. Copyright © 2011 John Wiley & Sons, Ltd.     

Formation of M+. ions under matrix fast atom bombardment conditions: Does charge exchange reaction occur?

The formation of molecular ions, M^+., under fast atom bombardment (FAB) conditions using a liquid matrix was examined by using a new type of synthesized compounds in which preferential M^+. peaks appear in their FAB spectra. The FAB spectra were compared with the corresponding mass spectra obtained by the electron impact (EI) ionization, chemical ionization (CI) and charge-exchange ionization (CEI) methods. All of the spectra showed preferential peaks of M^+. ion and a characteristic intense fragment ion peak originating from a β-fission. The FAB spectra were similar in the fragment ions appearing in the EI spectra and were very similar in the fragmentation pattern to the CEI spectra using Ar^+. and Xe^+. as the reagent ions. Further, the FAB spectra did not show any doubly charged ion peaks, while the 70 eV EI spectra showed the peaks of doubly charged molecular and/or fragment ions. The isobutane CI spectra of the synthesized compounds suggested that the formation of M^+. ions occurred through the CE reaction with isobutane ion, C₄H₁₀^+., and the CI spectra showed a marked intense fragment ion peak originating from the β-fission which seemed to occur characteristically in CEI processes. The results obtained suggested that the formation of M^+. ions under matrix FAB conditions occurred mainly by CE reactions between the analytes M and matrix molecular ions B^+. and/or fragment ions b^+..     

Doubly charged ion mass spectra: III—N-alkanes

Doubly charged ion mass spectra have been obtained for 15 n-alkane hydrocarbons. Spectra were measured using a Nier-Johnson geometry Hitachi RMU-7L mass spectrometer operated at 1.6kV accelerating voltage. Fragment ions, which resulted from C? C bond rupture and extensive H loss, dominated the spectra. Molecular ions have not been observed. The most intense ions in the doubly charged ion mass spectra of n-alkanes were [C₂H₄]²⁺, [C₃H₂]²⁺, [C₄H₃]²⁺, [C₅H₂]²⁺, [C₆H₆]²⁺, [C₆H₈]²⁺, [C₇H₆]²⁺, [C₇H₈]²⁺, [C₈H₆]²⁺ and [C₈H₈]²⁺. Appearance energies for forming the prominent doubly charged fragment ions have been measured and range from 27.5 eV to energies greater than 60eV. A geometry optimized SCF approach has been used to compute the energies and structures of prominent ions in the doubly charged mass spectra.     

Energy and substituent effects on gaseous ionic decompositions: Electron and charge exchange ionization of benzophenones and benzils

The correlation with substituent constants reported previously for [YØCO]⁺/[ØCO]⁺ ratios in the electron ionization mass spectra of substituted benzophenones and acetophenones has also been observed in the electron ionization spectra of substituted benzils. The [YØCO]⁺/[ØCO]⁺ ratio for the substituted benzils varied with energy of the ionizing electrons according to predictions from a simple kinetic and thermochemical analysis. [YØCO]⁺/[ØCO]⁺ ratios in the charge exchange spectra of benzophenones obtained with Xe, Kr, CO, N₂ and Ar gave good correlations with sub-stituent constants in agreement with the same analysis. Good correlations were also noted for [YØCO]⁺/[ØCO]⁺ ratios with substituent constants for [M]⁺ ions of the benzophenones of the same excess energy (5.5 eV). [YØCO]⁺/[ØCO]⁺ ratios for benzils obtained by charge exchange with [CO]⁺ also showed good correlations with substituent constant. It is suggested that [Ø]⁺ and [YØ]⁺ ions from the benzophenones may be formed primarily by one step decompositions of the molecular ions, but that the [Ø]⁺ and [YØ]⁺ ions from the benzils are formed primarily by decomposition of [ØCO]⁺ and [YØCO]⁺ ions.