Simple Bond Cleavage Research Articles

Comparative Studies of 193-nm Photodissociation and TOF-TOFMS Analysis of Bradykinin Analogues: The Effects of Charge Site(s) and Fragmentation Timescales

The dissociation reactions of [M + H]+, [M + Na]+, and [M + Cu]+ ions of bradykinin (amino acid sequence RPPGFSPFR) and three bradykinin analogues (RPPGF, RPPGFSPF, PPGFSPFR) are examined by using 193-nm photodissociation and post-source decay (PSD) TOF-TOF-MS techniques. The photodissociation apparatus is equipped with a biased activation cell, which allows us to detect fragment ions that are formed by dissociation of short-lived (<1 mus) photo-excited ions. In our previously reported photodissociation studies, the fragment ions were formed from ions dissociating with lifetimes that exceeded 10 mus; thus these earlier photofragment ion spectra and post-source decay (PSD) spectra [composite of both metastable ion (MI) and collision-induced dissociation (CID)] were quite similar. On the other hand, short-lived photo-excited ions dissociate by simple bond cleavage reactions and other high-energy dissociation channels. We also show that product ion types and abundances vary with the location of the charge on the peptide ion. For example, H+ and Na+ cations can bind to multiple polar functional groups (basic amino acid side chains) of the peptide, whereas Cu+ ions preferentially bind to the guanidino group of the arginine side-chain and the N-terminal amine group. Furthermore, when Cu+ is the charge carrier, the abundances of non-sequence informative ions, especially loss of small neutral molecules (H2O and NH3) is decreased for both photofragment ion and PSD spectra relative to that observed for [M + H]+ and [M + Na]+ peptide ions.

Reactions in trifluoroacetic acid (CF3COOH) induced by low energy electron attachment

Abstract Dissociative electron attachment to trifluoroacetic acid (CF 3 COOH) is characterized by an intense low energy shape resonance located near 1 eV and a comparatively weaker core excited resonance located near 7 eV. The shape resonance decomposes into the fragment ions CF 3 COO − , CF 2 COO − , and CF 2 - . The underlying reactions include simple bond cleavage but also more complex sequences involving multiple bond cleavages, rearrangement in the precursor ion and formation of new molecules (HF, CO 2 ). The core excited resonance additionally decomposes into F − , CF 3 - and probably metastable CO 2 - .

Thermochemistry of organic and heteroorganic species. Part XIII. Structural aspects and thermochemical approach to isomerization and fragmentation of negative ions. Simple bond cleavage and hydrogen rearrangement processes

Many hundreds of isomerization and fragmentation processes for negatively charged ions obtained in electron attachment dissociation, collision induced dissociation and negative chemical ionization have been analyzed to find out certain regularities in gas-phase monomolecular chemistry of negative ions like it was earlier performed for positive ions [Eur. J. Mass Spectrom., 8 (2002) 409]. This analysis combined with systematic application of the data on energies of the processes (appearance energies, isolation of stable resonance states on energetical scale in the range 0–15 eV, enthalpies of formation of even negative ions and electron affinities of corresponding free radicals) allowed to come to quite definite conclusion of validity of Eyring's quasi-equilibrium theory (QET) to mass spectrometry of negative ions. However, it is valid only in the limits of a stable resonance state rather than in a whole range of energies being applied. The validity of QET was illustrated by many diverse simple bond cleavage and rearrangement processes. Those hundreds of known processes for negative ions dissociation were reduced to a few isomerization and fragmentation types for odd- and even-charged negative ions—8 types for simple bond cleavage and 7—for H-migration, all being in certain aspect common with those suggested earlier for positive ions. The isomerization and fragmentation types were assigned to about 370 processes of simple bond cleavage and H-migration with presentation of possible structures for isomeric molecular and fragment ions. Possibility of such specification of the processes for negative ions, partial thermochemical approach for their description, general and specific application of QET make unnecessary the appeal to participation of high-energetical electron excitation, nonergodic fragmentation, to any specifity in charge-remote processes—all these used in literature to explain occurrence of some unusual processes. The latter could be described by the introduced reaction types or their combination, by formation of many (instead of a single one) neutrals in one-step process, H-wandering over ionized benzene ring, ring opening as a routine event (e.g. to explain the appearance of molecular ions in several domains of energies for aromatic compounds.).

Partial ionisation cross-sections of 2-propanol and ethanal

Electron impact ionisation of 2-propanol and ethanal is studied using mass spectrometry. Cross-sections of the formation of molecular ions and ionic fragments are measured between 14 and 86 eV. Free energy changes are evaluated using ab initio calculations. For 2-propanol, two ions, identified as CH 3CHOH + (45 amu) and CH 3 CHCH 3 + (43 amu), contribute more than 75% to the total cross-section over the whole range of electron energies and are produced by simple bond cleavage in the molecular ion. Both processes occur spontaneously, leaving the molecular ion as a minority species. For ethanal, two ions, identified as HCO + (29 amu) and CH 3CO + (43 amu), and the molecular ion (44 amu) contribute more than 80% to the total cross-section. The ions of 29 and 43 amu result from a simple bond cleavage in the molecular ion. These sprocesses are not spontaneous and the contribution of the molecular ion becomes predominant at 15 eV and is therefore significant over the whole range of ionisation energies.

Thermochemistry of organic and heteroorganic species. Part XIX. Structural aspects and thermochemical approach to isomerization and fragmentation of negative ions: Skeletal rearrangements, molecular ion and artifacts

Many hundreds of isomerization and fragmentation processes for negatively charged ions obtained in electron attachment dissociation, collision induced dissociation and negative chemical ionization have been analyzed to find out certain regularities in gas-phase monomolecular chemistry of negative ions like it was earlier performed for positive ions (Eur.J.Mass Spectrom., 8 (2002) 409). This analysis combined with systematic application of the data on energies of the processes (appearance energies, isolation of stable resonance states on energetical scale in the range 0–15 eV, enthalpies of formation of even negative ions and electron affinities of corresponding free radicals) allowed to come to quite definite conclusion of validity of Eyring's quasi-equilibrium theory (QET) to mass spectrometry of negative ions. However, it is valid only in the limits of a stable resonance state rather than in a whole range of energies being applied. The validity of QET was illustrated by many diverse simple bond cleavage and rearrangement processes. Those hundreds of known processes for negative ions dissociation were reduced to a few isomerization and fragmentation types for odd-and even-charged negative ions—eight types for simple bond cleavage and seven—for H-migration, all being in certain aspect common with those suggested earlier for positive ions. The isomerization and fragmentation types were assigned to about 350 processes of hydrogen and skeletal rearrangements with presentation of possible structures for isomeric molecular and fragment ions. Possibility of such specification of the processes for negative ions, partial thermochemical approach for their description, general and specific application of QET make unnecessary the appeal to participation of high-energetical electron excitation, nonergodic fragmentation, to any specifity in charge-remote processes—all these used in literature to explain occurrence of some unusual processes. The latter could be described by the introduced reaction types or their combination, by formation of many (instead of a single one) neutrals in one-step process, H-wandering over ionized benzene ring, ring opening as a routine event (e.g. to explain the appearance of molecular ions in several domains of energies for aromatic compounds.).

Ground and Excited State Dissociation Dynamics of Ionized 1,1-Difluoroethene

The kinetic energy release distributions (KERDs) for the fluorine atom loss from the 1,1-difluoroethene cation have been recorded with two spectrometers in two different energy ranges. A first experiment uses dissociative photoionization with the He(I) and Ne(I) resonance lines, providing the ions with a broad internal energy range, up to 7 eV above the dissociation threshold. The second experiment samples the metastable range, and the average ion internal energy is limited to about 0.2 eV above the threshold. In both energy domains, KERDs are found to be bimodal. Each component has been analyzed by the maximum entropy method. The narrow, low kinetic energy components display for both experiments the characteristics of a statistical, simple bond cleavage reaction: constraint equal to the square root of the fragment kinetic energy and ergodicity index higher than 90%. Furthermore, this component is satisfactorily accounted for in the metastable time scale by the orbiting transition state theory. Potential energy surfaces corresponding to the five lowest electronic states of the dissociating 1,1-C2H2F2+ ion have been investigated by ab initio calculations at various levels. The equilibrium geometry of these states, their dissociation energies, and their vibrational wavenumbers have been calculated, and a few conical intersections between these surfaces have been identified. It comes out that the ionic ground state X2B1 is adiabatically correlated with the lowest dissociation asymptote. Its potential energy curve increases in a monotonic way along the reaction coordinate, giving rise to the narrow KERD component. Two states embedded in the third photoelectron band (B2A1 at 15.95 eV and C2B2 at 16.17 eV) also correlate with the lowest asymptote at 14.24 eV. We suggest that their repulsive behavior along the reaction coordinate be responsible for the KERD high kinetic energy contribution.

Characterizing the electrospray ionization mass spectral fragmentation pattern of indole derivatives synthesized from 2‐keto glycosides

Four indole derivatives synthesized from 2-keto glycosides were analyzed by electrospray ionization tandem mass spectrometry (ESI-MSn) with low-energy collision-induced dissociation to establish a general structural elucidation of indole derivatives. Their fragmentation pathways are proposed on the basis of the MSn studies and deuterium-labeled experiments. Indole derivatives undergo complicated gas-phase rearrangements in addition to simple bond cleavages. A rearrangement, which involves a contraction of the six-membered ring, was observed and a mechanism was proposed. The observations may have some potential applications in the interpretation of the mass spectra of indole derivatives.

Fragmentation of tryptophan by low-energy electrons

Decomposition of gas phase tryptophan (trp) following low energy (<10 eV) electron attachment is investigated by means of a crossed electron/molecular beam experiment. Various negatively charged species are observed, attributed to (trp − H) − (203 amu), (gly-yl) − (74 amu), HCO 2 - (45 amu), CN − (26 amu), OH − (17 amu), and O - / NH 2 - (16 amu). These fragments arise from either simple bond cleavages (e.g., (trp − H) −) or more complex reaction involving molecular rearrangements within the transitory negative ion (e.g., (HCO 2) −). Surprisingly, by far the most predominant dissociation processes occur at sub-excitation energies (below the threshold of the electronic excitation). The present Letter may contribute to a better understanding of the initial molecular processes taking place during the interaction of radiation with living systems.

The role of hydride migration in the mechanism of alcohol elimination from protonated ethers upon chemical ionization: Experiment and theory

An enhanced elimination of methanol under isobutane-chemical ionization (CI) conditions, resulting in highly abundant [MHCH 3OH] + ions, has been observed in several primary and secondary methyl ethers having a tertiary β-position (methine), as compared with those with β-methylene. This elimination is stereospecific in stereoisomeric 2-methyl-1-methoxycyclohexanes and in other ethers affording significantly more abundant [MHCH 3OH] + ions in the cis-isomers than in their trans-counterparts. These findings suggest involvement of a 1,2-hydride migration from the β- to α-position in the course of the alcohol elimination from the MH + ions of the above cis-ethers, resulting in stabilized tertiary carbocation structures. The possible pathways of methanol elimination from protonated cis-2-methyl-1-methoxycyclohexane were explored by density functional calculations at the B3LYP/6-31+G(d,p) level of theory. The transition states for MeOH elimination involving 1,2-hydride migration were located and the activation energy of the process was evaluated. The activation barrier of the alcohol elimination assisted by 1,2-hydride migration is lower by ∼10 kcal/mol than the simple bond cleavage (9.6 kcal/mol vs. 20.5 kcal/mol). These computational results support the mechanistic pathway involving the 1,2-hydride transfer. A step-wise mechanistic pathway is proposed for the less efficient elimination of methanol from protonated trans-2-methyl-1-methoxycyclohexane.

Electrospray ionization and liquid secondary ion mass spectrometric study of N-heterocyclic carbenes and their 1,2,4-triazolium salt precursors

N-Heterocyclic carbenes and their 1,2,4-triazolium salt precursors were analyzed by electrospray ionization (ESI) and liquid secondary ion mass spectrometry (LSIMS). It was found that under applied conditions the C5 carbene center is formed from [M+ClO 4] + ion, were M corresponds to the 1,2,4-triazolium dicationic system, by loss of HClO 4 molecule. Further decomposition of formed carbene ions, namely [M−H] + ion, consist in the breaking of triazolium ring and loss bulky substituents from N1 and N4 atoms through the hydrogen transfer to the carbene center. This is in contrast to the [M] 2+ carbene precursor ions for which only the loss of N1 substituent occurred as a simple heterolytic bond cleavage.

Free Radicals in Mass Spectrometry. Part I. General Fragmentation Rules for Organic and Organometallic Compounds under Electron Impact

General fragmentation rules for isomerization and fragmentation processes based on firm thermochemical grounds are presented for positive odd- and even-electron ions. The thermochemical approach by ions' and free radicals' heats of formation, appearance and critical energies allowed the unification of many known fragmentation and isomerization processes. It is shown that to describe more than 180 classes of organic and hetero-organic compounds it is sufficient to apply only 21 types of isomerization and fragmentation processes. Most of these types are characterized with a specific mechanism and the stabilization morde for ions and/or neutral species. Many original fragmentation schemes are given. In addition to simple bond cleavages, it was demonstrated that the quasi-equilibrium theory of Eyring et al. Is applicable to each step of the multi-stage rearrangement rather than to the summary rearrangement process.

Why are smaller fragments preferentially lost from radical cations at low energies and larger ones at high energies?: An experimental and theoretical study

A long-standing mystery in gas phase ion chemistry is: why are smaller fragments preferentially lost at low energies and larger fragments at higher energies in competing α-cleavages? This is addressed here by studies of dissociations of ethanol, 2-propanol, 2-butanol, 2-methylpropane and 2-butanone radical cations. The onsets and energy dependencies of the reactions were obtained by photoionization mass spectrometry. Stationary point geometries, critical energies and vibrational frequencies were generated by B3LYP/6-31G(d) and B3LYP/6-311G(d,p) theory (DFT). Dependencies of reaction rates on internal energy were calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Results obtained establish the generality of a previous finding that loss of H is slower than competing losses of polyatomic fragments. This is attributable to substantial lowering of the frequencies of the vibrations that are converted to rotations and translations in the transition states for the latter reactions, but not in transition states for H losses. It is concluded that lower dissociation thresholds produce preferred losses of smaller alkyl fragments at low energies and that looser transition states favor losses of larger fragments at higher energies. DFT results and abundances of parallel alkane eliminations (two step processes also initiated by simple CC bond cleavage) indicate that cleavage of the CC bond to the larger alkyl fragment is most frequent even below the threshold for complete dissociation (energies at which ion-induced dipole alkyl complexes are formed), i.e., at all energies of bond cleavage, even if not reflected in simple cleavage abundances near threshold. Obtaining realistic rates by RRKM theory required increasing the lowest transition state vibrational frequencies (for modes that become rotations and translations) to well above those obtained by DFT for alkyl losses, further evidence that the rates of those reactions are controlled by minimum entropy transition states occurring earlier than the single imaginary frequency transition states found by DFT. In addition, comparison of DFT critical energies, photoionization critical energies and critical energies that give the best RRKM rates support relaxation of the ground states to lower energy species with long CC bonds following ionization. Preferred losses of larger alkanes at low energies, dominant losses of larger alkyl groups at high energies and formation of long CC bonds to the larger alkyl in the ground state ion all appear to have a common origin.

Photodissociation Studies of M(Furan)+ (M = Cu, Ag, and Au) and Au(C3H4)+ Complexes

The group 11 metal/furan cationic complexes were generated using a laser vaporization technique combined with a supersonic beam expansion in a time-of-flight mass spectrometer. From the viewpoint of the ionization energies, these complexes were treated as Cu+−furan, Ag+−furan, and Au−furan+. The photodissociative ligand-to-metal charge transfer with an exclusive furan cation was observed for Cu and Ag, whereas a simple bond cleavage with furan+ formation was inspected for Au. The photofragment spectra were recorded as a function of the laser wavelength. The continuous and structureless bands were measured in each complex. The thresholds of the fragment appearance determined the upper limits of the ground-state binding energy with 37 kcal/mol for Cu+−furan, 28 kcal/mol for Ag+−furan, and 62 kcal/mol for Au−furan+. An ab initio approach at the MP2 level was employed to optimize the geometries of the furan complexes and the binding energies were obtained using CCSD(T) single point calculations. The measured binding energies in both Cu and Ag complexes approximate to the theoretical predictions. Both the experimental and theoretical measurements yielded the enhanced bond strength for Au complex. In addition, a furan ring opening process leading to Au+−C3H4 production was observed in the reactions of a gold atom with a furan molecule. The binding energy was taken as a reference to discern three possible isomers, i.e., allene, cyclopropene, and propyne, as C3H4 species by means of experimental and theoretical approaches.

Do Thermally-Labile 1,4-Benzodiazepines Rearrange under Electrospray and Particle Bombardment?

Nordiazepam (N), diazepam (D), lorazepam (L), oxazepam (O)and temazepam (T)are 1,4-benzodiazepines. L, O and T are substituted in position 3 of the seven-membered ring by a hydroxyl group and are known to be thermally unstable. N and D are unsubstituted in position 3 and are expected to be thermally stable. We have studied the stability of all these molecules under electrospray conditions and under particle bombardment (MeV ions and UV photons). The fragmentations induced by low energy collision-induced dissociation and high energy collision-activated dissociation of molecules protonated by electrospray were compared with the spontaneous fragmentations of these molecules ionized by particle bombardment. The fragmentation mechanisms were determined using labeled compounds and by means of ab initio calculations using 1,4-diazepine and 3-hydroxy-1,4-diazepine as models. The fragmentation is dramatically dependent upon the substitution in position 3 and upon the internal energy of protonated molecules. At low collision energies, the non-hydroxylated benzodiazepines eliminate CO by opening of the diazepine ring whereas 3-hydroxy-1,4-benzodiazepines eliminate water after ring contraction. At high collision energies, all protonated benzodiazepines eliminate a hydrogen atom by simple bond cleavage. Molecular orbital calculations give arguments in favor of an isomerization in the gas phase of the protonated 3-hydroxybenzodiazepines and of a partial thermal decomposition of 1,4-benzodiazepines occurring before protonation under particle bombardment.

Experimental and Theoretical Studies of the VUV Photoionization of Chloropropylene Oxide

The photoionization and dissociative photoionization of chloropropylene oxide have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions C3H5O+, CHO+, C2H4+, C2H3+, C2H2+, CH3+, and CH2OH+ were obtained. Theoretically, the energetics of the dissociative photoionizations were examined by ab initio Gaussian-3 calculations. The computational results are useful in our attempt to establish the dissociative photoionization channels of chloropropylene oxide. These proposed channels include six simple bond cleavage reactions as well as a reaction involving intermediates and transition structures. The agreement between experimental and computational dissociation energies or barriers ranges from fair to excellent.

Unimolecular Reactions of Proton-Bound Cluster Ions: Competition between Dissociation and Isomerization in the Ethanol−Acetonitrile Dimer

The proton-bound dimer of acetonitrile and ethanol, (CH3CN)(CH3CH2OH)H+, exhibits three unimolecular reactions on the microsecond time scale: two simple bond cleavage reactions to form CH3CNH+ + CH3CH2OH and CH3CH2OH2+ + CH3CN, and the loss of water to form CH3CNCH2CH3+. The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH3CNCH2CH3)(H2O)+. The competition between the simple dissociation reactions and the isomerization reaction was modeled with ab initio calculations and RRKM theory to obtain relative energies for the reaction surface. The 0 K binding energy of the (CH3CN)(CH3CH2OH)H+ complex was calculated to be 152 kJ mol-1 at the G2(MP2,SVP) level of theory (relative to the dissociation products CH3CNH+ and CH3CH2OH). The isomerization barrier for the proton-bound dimer was estimated to be 22 kJ mol-1 lower than CH3CNH+ + CH3CH2OH. The greater polarizability of the ethyl group is believed to stabilize this transition structure over that found for the (CH3CN)(CH3OH)H+ ion (which was previously estimated to lie 6 kJ mol-1 below CH3CNH+ and CH3OH).

A Vacuum Ultraviolet Photoionization Mass Spectrometric Study of Propylene Oxide in the Photon Energy Region of 10−40 eV

The photoionization and dissociative photoionizations of propylene oxide have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions C3H6O+, C2H3O+, CH3O+, CH2O+, CHO+, C2H4+, C2H3+, C2H2+, CH3+, and CH2+ were obtained. Theoretically, the energetics of the dissociative photoionizations were examined by ab initio Gaussian-2 calculations. The computational results are useful in our attempt to establish the dissociation channels near the ionization thresholds. The dissociation channels we propose include simple bond cleavage reactions as well as reactions involving intermediates and transition structures.

How do dimethyl oxalate ions CH 3O–C(O)–C(O)–OCH 3·+ break in half? Loss of CH 3· + CO 2 Versus CH 3O–CO ·

The interesting unimolecular dissociation chemistry of dimethyl oxalate (DMO) ions, CH 3O–C(O)–C(O)–OCH 3 ·+, has been studied by vacuum ultraviolet photoionization and tandem mass spectrometry based experiments. The measured appearance energy (AE) for the generation of CH 3O–CO + (10.5 eV) is not compatible with a simple bond cleavage involving the cogeneration of the radical CH 3O–CO · whose calculated AE is 11 kcal/mol higher. However, because the CH 3O–CO · radical is thermodynamically less stable than its dissociation products CH 3 · and CO 2, by 19 kcal/mol, a two-step dissociation of ionized DMO into CH 3O–CO + + CH 3 · + CO 2 is energetically feasible. Collision induced dissociative ionization experiments clearly show that low energy DMO ions dissociate into CH 3 · + CO 2 without the intermediacy of CH 3O–CO ·. Experiments using a charged collision chamber further indicate that CO 2 is released first, followed by loss of CH 3 · and not vice versa and a mechanism is proposed. The measured AE, which we assign to the two-step process, is 8 kcal/mol higher than the calculated value. This could be due to a competitive shift caused by a prominent low energy decarbonylation reaction yielding the hydrogen bridged radical cation CH 2O · · · H · · · OC–OCH 3 ·+. However, from metastable ion observations and AE measurements on deuterium labeled DMO ions, it follows that there is no competitive shift and that the elevated AE for the two-step process corresponds to the barrier for the first step, loss of CO 2. Finally, neutralization–reionization experiments on ionized DMO and CH 3O–CO + provide evidence for the existence of CH 3O–CO · as a kinetically stable radical.

Theoretical Study of Dithioformic Acid, Dithiohydroxy Carbene and Their Radical Cations: Unimolecular and Assisted Rearrangements

The potential energy surfaces of dithioformic acid and its functional isomer, dithiohydroxy carbene, in both neutral and ionized states have been investigated using ab initio molecular orbital calculations at both (U)MP2 and (U)QCISD(T) levels with the 6-31G(d,p) and 6-311++G(d,p) basis sets. The unimolecular decomposition of HC(S)SH giving H2 + CS2 and H2S + CS, its rearrangement to HS−C−SH, and different simple bond cleavages have been examined. In both electronic states, the carbene is a stable isomer lying in a relatively deep potential well. Formation of H2S via a one-step decomposition of acid is found to be favored over that of CS2 whose reaction path involves a carbene intermediate. Carbene radical cations also participate as crucial intermediates in unimolecular transformation of ionized acids. Calculations on different hydrogen-bonded dimers of acid and carbene suggest that, in the dimer form, while the carbene is not stable, elimination of both H2 and H2S is accelerated. The role of H2S and H2 as possible catalysts in the acid decomposition has also been investigated. H2S induces an efficient catalytic effect in different hydrogen transfer pathways. The standard heats of formation are estimated to be ΔH°f,298[HC(S)SH] = 113 ± 8 kJ/mol and ΔH°f,298[HS−C−SH] = 267 ± 12 kJ/mol based on CCSD(T)/6-311++G(3df,2p) calculations. Adiabatic ionization energies are predicted to be IEa(HS−C−SH) = 8.2 ± 0.3 eV and IEa(HC(S)SH)] = 9.0 ± 0.3 eV, and proton affinities are PA[HC(S)SH] = 802 ± 12 kJ/mol and PA[HS−C−SH] = 956 ± 12 kJ/mol.

A Vacuum Ultraviolet Photoionization Mass Spectrometric Study of Ethylene Oxide in the Photon Energy Region of 10−40 eV

The photoionization and dissociative photoionizations of ethylene oxide have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions C2H4O+, C2H3O+, C2H2O+, C2HO+, CH3O+, CH2O+, CHO+, C2H4+, C2H3+, C2H2+, C2H+, CH4+, CH3+, CH2+, CH+, and C+ have been obtained. In addition, the energetics of the dissociative photoionizations have been examined by ab initio Gaussian-2 calculations. The computational results are useful in establishing the dissociation channels near the ionization thresholds. The dissociation channels established may be broadly divided into two types: simple bond cleavage reactions and those involving transition structures and activation energies.