Dissociation of multiply ionized alkanes from methane to n-butane due to electron impact

The dissociation of up to triply ionized alkanes from methane to n-butane has been studied through electron impact ionization at an electron energy of 200 eV. The ionic fragments are recorded by a covariance mapping technique using a focusing time-of-flight mass spectrometer. The absolute cross-sections for the numerous dissociation channels of all the alkane monocations and dications, as well as propane and n-butane trications have been obtained. All the alkane dications are unstable and dissociate mostly into ion pairs, in which the production of proton pairs is found to be a major dissociation channel. The ionic products from the dissociation of ethane, propane and n-butane dications have a similar distribution, in which the first few abundant ion species are H⁺, CH₃⁺, C₂H₃⁺ and C₂H₂⁺. The total single ionization cross-sections agree well with the results of the binary-encounter-Bethe method. The distortion of the ion coincidence islands has been explained as due to the deviation from the space focusing condition of the mass spectrometer. Metastable decays, such as C₃H₅²⁺→CH₃⁺+C₂H₂⁺ in propane and C₄H₁₀⁺→C₃H₇⁺+CH₃ in n-butane have been observed and identified using the initial and final angles of the decay traces.     

多次碰撞条件下金阳离子诱导的甲烷碳―碳偶联

The reactivity of atomic metal cations toward CH₄ has been extensively investigated over the past decades. Closed-shell metal cations in electronically ground states are usually inert with CH₄ under thermal collision conditions because of the extremely high stability of methane. With the elevation of collision energies, closed-shell atomic gold cations (Au⁺) have been reported to react with CH₄ under single-collision conditions to produce AuCH₂⁺, AuH⁺, and AuCH₃⁺ species. Further investigations found that the ion-source-generated AuCH₂⁺ cations can react with CH₄ to synthesize C―C coupling products. These previous studies suggested that new products for the reaction of Au⁺ with CH₄ can be identified under multiple-collision conditions with sufficient collision energies. However, the reported ion-molecule reactions involving methane were usually performed under single- or multiple-collision conditions with thermal collision energies. In this study, a new reactor composed of a drift tube and ion funnel is constructed and coupled with a homemade reflectron time-of-flight mass spectrometer. Laser-ablation-generated Au⁺ ions are injected into the reactor and drift 120 mm to react with methane seeded in the helium drift gas. The reaction products and unreacted Au⁺ ions are focused through the ion funnel and accumulate through a linear ion trap and are then detected by a mass spectrometer. In the reactor, the pressure is approximately 100 Pa, and the electric field between the drift tube and ion funnel can regulate the collision energies between ions and molecules. The reaction of the closed-shell atomic Au⁺ cation with CH₄ is investigated, and the C―C coupling product AuC₂H₄⁺ is observed under multiple-collision conditions with elevated collision energies. Density functional theory calculations are performed to understand the mechanism of the coupling reaction (Au⁺+ 2CH₄ → AuC₂H₄⁺ + 2H₂). Two pathways involving Au―CH₂ and Au―CH₃ species can separately mediate the C―C coupling process. The activation of the second C―H bond in each process requires additional energy to overcome the relatively high barrier (2.07 and 2.29 eV). Ion-trajectory simulations under multiple-collision conditions are then conducted to determine the collisional energy distribution in the reactor. These simulations confirmed that the electric fields between the drift tube and ion funnel could supply sufficient center-of-mass kinetic energies to facilitate the C―C coupling process to form AuC₂H₄⁺. The following catalytic cycle could then be postulated: $\mathrm{AuC}_{2} \mathrm{H}_{4}^{+}+\mathrm{CH}_{4} \stackrel{\Delta}{\longrightarrow} \mathrm{AuCH}_{4}^{+}+\mathrm{C}_{2} \mathrm{H}_{4}, \mathrm{AuCH}_{4}^{+}+\mathrm{CH}_{4} \stackrel{\Delta}{\longrightarrow} \mathrm{AuC}_{2} \mathrm{H}_{4}^{+}+2 \mathrm{H}_{2}$, and $\mathrm{CH}_{4} \stackrel{\mathrm{Au}^{+}, \Delta}{\longrightarrow} \mathrm{C}_{2} \mathrm{H}_{4}+2 \mathrm{H}_{2}$. Thus, this study enriches the chemistry of both gold and methane.     

Surface-induced dissociation and chemical reactions of C2D 4 + on stainless steel, carbon (HOPG), and two different diamond surfaces

Surface-induced interactions of the projectile ion C₂D₄⁺ with room-temperature (hydrocarbon covered) stainless steel, carbon highly oriented pyrolytic graphite (HOPG), and two different types of diamond surfaces (O-terminated and H-terminated) were investigated over the range of incident energies from a few eV up to 50 eV. The relative abundance of the product ions in dependence on the incident energy of the projectile ion [collision-energy resolved mass spectra, (CERMS) curves] was determined. The product ion mass spectra contained ions resulting from direct dissociation of the projectile ions, from chemical reactions with the hydrocarbons on the surface, and (to a small extent) from sputtering of the surface material. Sputtering of the surface layer by low-energy Ar⁺ ions (5–400 eV) indicated the presence of hydrocarbons on all studied surfaces. The CERMS curves of the product ions were analyzed to obtain both CERMS curves for the products of direct surface-induced dissociation of the projectile ion and CERMS curves of products of surface reactions. From the former, the fraction of energy converted in the surface collision into the internal excitation of the projectile ion was estimated as 10% of the incident energy. The internal energy of the surface-excited projectile ions was very similar for all studied surfaces. The H-terminated room-temperature diamond surface differed from the other surfaces only in the fraction of product ions formed in H-atom transfer surface reactions (45% of all product ions formed versus 70% on the other surfaces).     

A Gaussian-2 ab initio study of [C2H5S] ions: III. H2 and CH4 eliminations from CH3CHSH and CH3SCH2

Gaussian-2 ab initio calculations were performed to examine the six modes of unimolecular dissociation of cis-CH₃CHSH⁺ (1⁺), trans-CH₃CHSH⁺ (2⁺), and CH₃SCH₂⁺ (3⁺): 1⁺→CH₃⁺+trans-HCSH (1); 1⁺→CH₃+trans-HCSH⁺ (2); 1⁺→CH₄+HCS⁺ (3); 1⁺→H₂+c-CH₂CHS⁺ (4); 2⁺→H₂+CH₃CS⁺ (5); and 3⁺→H₂+c-CH₂CHS⁺ (6). Reactions (1) and (2) have endothermicities of 584 and 496 kJ mol⁻¹, respectively. Loss of CH₄ from 1⁺ (reaction (3)) proceeds through proton transfer from the S atom to the methyl group, followed by cleavage of the C–C bond. The reaction pathway has an energy barrier of 292 kJ mol⁻¹ and a transition state with a wide spectrum of nonclassical structures. Reaction (4) has a critical energy of 296 kJ mol⁻¹ and it also proceeds through the same proton transfer step as reaction (3), followed by elimination of H_2. Formation of CH₃CS⁺ from 2⁺ (reaction (5)) by loss of H₂ proceeds through protonation of the methine (CH) group, followed by dissociation of the H₂ moiety. Its energy barrier is 276 kJ mol⁻¹. On both the MP2/6-31G* and QCISD/6-31G* potential-energy surfaces, the H₂ 1,1-elimination from 3⁺ (reaction (6)) proceeds via a nonclassical intermediate resembling c-CH₃SCH₂⁺ and has a critical energy of 269 kJ mol⁻¹.     

Vibrational spectrum of [(CD3)3S]I and normal coordinate calculations for [(CH3)3S] and [(CD3)3S] cations

IR (4000-30 cm⁻¹) and Raman (4000-0 cm⁻) spectra of [(CD₃)₃S]I have been observed, together with those of [(CH₃)₃S]I. By assuming a C_3v molecular symmetry for the cations [(CH₃)₃S]⁺ and [(CD₃)₃S]⁺, all the active fundamentals of [(CD₃)₃s]⁺ have been assigned and normal coordinate calculations have been carried out by a symmetry force field for [(CH₃)₃S]⁺ and [(CD₃)₃S]⁺. The strength of the S---C and C---H bonds in the compound has been compared with that in dimethyl sulfide by using their valence stretching force constants.     

XeF+, IF+, and other unusual ions generated by reactions of hyperthermal ion beams at self-assembled monolayer surfaces

Collisions of atomic and molecular ions (I⁺, Xe^+·, CH₃I^+·, I^+·₂) with self-assembled fluoroalkyl-monolayer surfaces result in reactions involving the net transfer of fluorine atoms or fluorocarbon radicals from the surface to the projectile ions. The scattered products, which include unusual ionic species such as IF^+·, IF⁺₂, CFI^+·, CF₂I⁺, I₂F⁺, and XeF⁺, are generated in endothermic ion-surface reactions. These reactions are not observed when the collision partner is a gas-phase (rather than a surface-bound) perfluoroalkane. Evidence is presented which suggests that in some cases molecular projectiles undergo surface-induced dissociation to yield atomic species which subsequently react with the surface. Fluorine abstraction is favored for projectiles containing highly polarizable elements.     

Field investigation of major and minor ions along Summit (Central Greenland) ice cores by ion chromatography

As a part of the European EUROCORE and GRIP (Greenland Ice Core Project) operations aimed at recovering deep ice cores at Summit (Central Greenland), we have for the first time successfully performed ion chromatography measurements in the field and investigated in detail the soluble impurities, including Na⁺, NH⁺₄, K⁺, Mg²⁺, Ca²⁺, F⁻, CH₃COO⁻, CH₂ OHCOO⁻, HCOO⁻, CH₃SO₃⁻, Cl⁻, NO⁻₂, SO₄²⁻ and C₂O₄²⁻, trapped in ice deposited over some 200 000 years in Greenland.     

Pathway studies in the fragmentation of Ge(CH3)4 involving Ge(3d,3p,3s) and C(1s) inner-shell photoexcitation in the range 49.5–450 eV

The dissociative multiple photoionization of tetramethylgermane (Ge(CH₃)₄) in the valence, and in the Ge(3d,3p,3s) and C(1s) inner-shell regions has been studied by using time-of-flight mass spectrometry coupled to synchrotron radiation in the range 49.5–450 eV. Total and individual photoion yields have been recorded as a function of the incident photon energy. Several discrete resonances over a structureless giant resonance are observed below the Ge(3p), Ge(3s) and C(1s) threshold regions. The structureless giant resonance corresponding to the Ge(3d) presumably arises from the continuum enhancement caused by the 3d→εf transition. Various monocations of H⁺, H₂⁺, CH_n⁺ (n=0–4), C₂H_n⁺ (n=0–5), GeH_n⁺, GeCH_n⁺, GeC₂H_n⁺, and GeC₃H_n⁺ are detected in the whole energy range. Dissociation processes have also been investigated by photoelectron–photoion and photoion–photoion coincidence methods. The dominant dissociation channel is found to be CH_n⁺–GeCH_n⁺ in the whole energy examined. Specific energy dependence of dissociation processes is observed in the Ge(3p) and Ge(3s) regions. With the help of ab initio HF/6-311++G(2df,p) calculation, we roughly estimated the photoabsorption positions and symmetries for the discrete core hole states.     

NH3R++NHR2(R=H,CH3)体系质子传递反应的从头算研究

用从头计算法在HF/6-31G^*基组水平上研究了NH₄⁺+NH₃→NH₃+NH₄⁺,NH₄⁺+NH₂CH₃→NH₃+NH₃CH₃⁺,NH₄⁺+NH(CH₃)₂→NH₃+NH₂(CH₃)₂⁺以及NH₃CH₃⁺+NH₂CH₃→NH₂CH₃+NH₃CH₃⁺等4个体系的质子传递反应的机理.结果表明:(1)上述质子传递反应均具有双阱型的势能面,质子沿N(1)、N(2)连线直接传递;(2)质子受体分子中的甲基对质子传递起促进作用,而质子给体离子中的甲基则阻碍质子的传递。     

Translational to vibrational energy conversion during surface-induced dissociation of n-butylbenzene molecular ions colliding at self-assembled monolayer surfaces

Translational to vibrational (T-->V) energy conversion in the course of inelastic collisions of n-butylbenzene molecular ions with thiolate self-assembled monolayer (SAM) gold surfaces is studied to better understand internal energy uptake by the hyperthermal projectile ions. The projectile ion is selected by a mass spectrometer of BE configuration and product ions are analyzed using a quadrupole mass analyzer after kinetic energy selection with an electric sector. The branching ratio for formation of the fragment ions m/z 91 and m/z 92, measured over a range of collision energies, is used to estimate the average internal energy with the aid of calculations based on unimolecular dissociation kinetics [Rice-Ramsperger-Kassel-Marcus (RRKM) theory]. The measured T-->V conversion efficiencies (the fraction of the laboratory kinetic energy converted into internal energy) are 11 approximately 12% for dodecanethiolate SAM (H-SAM) and 19 approximately 20% for 2-perfluorooctylethanethiolate SAM (F-SAM), respectively, over ranges of a few 10s of eV. The values are similar to those reported earlier for other thermometer molecules undergoing surface collisions. Chemical sputtering leading to ionization of the surface is a prominent feature of the surface-induced dissociation (SID) spectra of n-butylbenzene acquired using the H-SAM surface but not the F-SAM surface because of the lower ionization energy of the former.     

Reactive landing of peptide ions on self-assembled monolayer surfaces: an alternative approach for covalent immobilization of peptides on surfaces

Soft landing of mass-selected peptide ions onto reactive self-assembled monolayer surfaces (SAMs) was performed using a newly constructed ion deposition apparatus. SAM surfaces before and after soft landing were characterized ex situ using time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and infrared reflection-absorption spectroscopy (IRRAS). We demonstrate that reactive landing (RL) results in efficient covalent linking of lysine-containing peptides onto the SAM of N-hydroxysuccinimidyl ester-terminated alkylthiol on gold (NHS-SAM). Systematic studies of the factors that affect the efficiency of RL revealed that the reaction takes place upon collision and is promoted by the kinetic energy of the ion. The efficiency of RL is maximized at ca. 40 eV collision energy. At high collision energies the RL efficiency decreases because of the competition with scattering of ions off the surface. The reaction yield is independent of the charge state of the projectile ions, suggesting that peptide ions undergo efficient neutralization upon collision. Chemical and physical properties of the SAM surface are also important factors that affect the outcome of RL. The presence of chemically reactive functional groups on the SAM surface significantly improves the reaction efficiency. RL of mass- and energy-selected peptide ions on surfaces provides a highly specific approach for covalent immobilization of biological molecules onto SAM surfaces.     

Electronic and vibronic dipole moments of CH2D

Isotopic substitution of a hydrogen atom by deuterium turns a non-polar ion such as CH₃⁻ into a non-centrosymmetric system, CH₂D⁻, suitable for rotational spectra detection in space. The vibrational contribution to the dipole moment of the ion is investigated in this paper. The electronic potential surface has been obtained in a first-order CI, starting from a full valence active space built by using projected localized orbitals, to which core valence correlation effects have been added to account for rehybridization. The vibrational functions are obtained in a basis of harmonic oscillators. Three levels of vibrational treatment are presented, i.e. harmonic, non-coupled and coupled modes. The final result for the transition moment in the lowest vibrational level, μ_v = 0.327 Debye, shows that CH₂D⁻ should be observed and may be a useful tracer of CH₃⁺ in interstellar chemistry.     

The decomposition of C4H8 complexes at controlled internal energies

A coincidence technique is used to study the influence of the internal energy of the reactant ion on the cross section of the ion-molecule reactions in the C₂H₄⁺ + C₂H₄ system. The experiment is performed at thermal collision energies. In the ion-molecule reactions of C₂H₄⁺ + C₂H₄ our measurements indicate a barrier between the initially formed collision complex (C₂H₄)₂⁺* and a tight complex (C₄H₈⁺)*. Using an extension of our earlier developed statistical model, now including a potential barrier between the initially formed loose complex (C₂H₄)₂⁺* and the tight complex (C₄H₈⁺)*, our experimental data can be reproduced. For comparison also the internal energy dependence of the unimolecular decomposition of photoionised 1-C₄H₈⁺ is measured. Assuming that the photoionised 1-C₄H₈⁺ is identical with the tight (C₄H₈⁺)* complex, the model applied to the ion-molecule reactions describes also the unimolecular decay of 1-C₄H₈⁺ correctly, using the same set of parameters.     

Collisions of slow polyatomic ions with surfaces: dissociation and chemical reactions of C2H2+*, C2H3+, C2H4+*, C2H5+, and their deuterated variants C2D2+* and C2D4+* on room-temperature and heated carbon surfaces

Interaction of C2Hn+ (n = 2-5) hydrocarbon ions and some of their isotopic variants with room-temperature and heated (600 degrees C) highly oriented pyrolytic graphite (HOPG) surfaces was investigated over the range of incident energies 11-46 eV and an incident angle of 60 degrees with respect to the surface normal. The work is an extension of our earlier research on surface interactions of CHn+ (n = 3-5) ions. Mass spectra, translational energy distributions, and angular distributions of product ions were measured. Collisions with the HOPG surface heated to 600 degrees C showed only partial or substantial dissociation of the projectile ions; translational energy distributions of the product ions peaked at about 50% of the incident energy. Interactions with the HOPG surface at room temperature showed both surface-induced dissociation of the projectiles and, in the case of radical cation projectiles C2H2+* and C2H4+*, chemical reactions with the hydrocarbons on the surface. These reactions were (i) H-atom transfer to the projectile, formation of protonated projectiles, and their subsequent fragmentation and (ii) formation of a carbon chain build-up product in reactions of the projectile ion with a terminal CH3-group of the surface hydrocarbons and subsequent fragmentation of the product ion to C3H3+. The product ions were formed in inelastic collisions in which the translational energy of the surface-excited projectile peaked at about 32% of the incident energy. Angular distributions of reaction products showed peaking at subspecular angles close to 68 degrees (heated surfaces) and 72 degrees (room-temperature surfaces). The absolute survival probability at the incident angle of 60 degrees was about 0.1% for C2H2+*, close to 1% for C2H4+* and C2H5+, and about 3-6% for C2H3+.     

New temperature effect on tunneling reaction in hydrogen, alkane, and ethanol in the solid phase

Rate constants for the tunneling reaction (HD + D → h + D₂) in solid HD increase steeply with increasing temperature above 5 K, while they are almost constant below 4.2 K. The apparent activation energy for the tunneling reaction above 5 K is 95 K, which is consistent with the energy (91–112 K) for vacancy formation in solid hydrogen. The results above 5 K were explained by the model that the tunneling reaction was accelerated by a local motion of hydrogen molecules and hydrogen atoms. The model of the tunneling reaction assisted by the local motion of the reactans and products was applied to the temperature dependence of the proton-transfer tunneling reaction (C₆H₆⁻ + C₂H₅OH → C₆H₇ + C₂H₅O⁻) in solid ethanol, the tunneling elimination of H₂ molecule of H₂ molecule ((CH₃)₂ CHCH(CH₃)₂⁺ → (CH₃)₂ C = C(CH₃)₂⁺ + H₂) in solid 2,3-dimethylbutane, and the selective tunneling reaction of H atoms in solid neo-C₅H₁₂-alkane mixtures.     

Quadruple-stranded Eu-helicate Assembled from Bis-β-diketonate: Its Stability Towards Metal Ions

Herein, we reported the synthesis and investigation of highly luminescent quadruple-stranded helicate (C₆H₁₆N)₄[Eu₂(MBDA)₄]₂^.3C₄H₁₀O·4C₂H₃N(1-Eu)[H₂MBDA=N-methyl-4,4'-bis(4,4,4-trifluoro-1,3-dioxobutyl)di- phenylamine] for its stability toward metal ions in the solution. The material was characterized via X-ray crystallographic technique, Fourier transform infrared(FTIR) spectroscopy and electrospray ionization quadrupole time-of- flight(ESI-TOF) mass spectrometry. The results on the luminescence quantum yields clearly demonstrate that the ligand can effectively sensitize the luminescence of the Eu³⁺ ions(Φ_overall=15%). Upon the addition of different metal ions(i. e., Ag⁺, Cd²⁺, Zn²⁺, Fe³⁺, Al³⁺ and Ni²⁺) to the CH₃CN solution of compound 1-Eu, the emission intensities of Eu³⁺ ions at 612 nm were affected to some extent, which could be attributed to the presence of ion exchanges between Eu³⁺ ions and the metals ions, and the result was confirmed by ESI-TOF mass spectrometry.     

Thermal dissociation of the protein homodimer ecotin in the gas phase

The influence of charge on the thermal dissociation of gaseous, protonated, homodimeric, protein ecotin ions produced by nanoflow electrospray ionization (nanoES) was investigated using the blackbody infrared radiative dissociation technique. Dissociation of the protonated dimer, (E₂ + nH)ⁿ⁺ E₂ⁿ⁺ where n = 14–17, into pairs of monomer ions is the dominant reaction at temperatures from 126 to 175 °C. The monomer pair corresponding to the most symmetric charge distribution is preferred, although 50–60% of the monomer product ions correspond to an asymmetric partitioning of charge. The relative abundance of the different monomer ion pairs produced from E₂¹⁴⁺, E₂¹⁵⁺, and E₂¹⁶⁺ depends on reaction time, with the more symmetric charge distribution pair dominating at longer times. The relative yield of monomer ions observed late in the reaction is independent of temperature indicating that proton transfer between the monomers does not occur during dissociation and that the different monomer ion pairs are formed from dimer ions which differ in the distribution of charge between the monomers. For E₂¹⁷⁺, the yield of monomer ions is independent of reaction time but does exhibit slight temperature dependence, with higher temperatures favoring the monomers corresponding to most symmetric charge distribution. The charge distribution in the E₂¹⁵⁺ and E₂¹⁶⁺ dimer ions influences the dissociation kinetics, with the more asymmetric distribution resulting in greater reactivity. In contrast, the charge distribution has no measurable effect on the dissociation kinetics and energetics of the E₂¹⁷⁺ dimer.     

Density functional and multiphoton ionization studies of N,N-dimethylformamide–(methanol)n clusters

The multiphoton ionization of the hydrogen-bonded clusters N,N-dimethylformamide–(methanol)_n (DMF–(CH₃OH)_n) was studied using a time-of-flight mass spectrometer at the wavelengths of 355 and 532 nm. At both wavelengths, a series of protonated DMF–(CH₃OH)_nH⁺ ions was obtained. The clusters were also investigated by density functional theory B3LYP method in conjunction with basis sets 6-31+G(d,p) and 6-311+G(2d,p). Equilibrium geometries of both neutral and ionic DMF–CH₃OH clusters, and dissociation channels and dissociation energies of the ionic clusters are presented. The results show that when DMF–CH₃OH is vertically ionized and dissociated, DMFH⁺ and CH₃O are the dominant products via proton transfer reaction. A high energy barrier makes another channel corresponding to the production of DMFH⁺ and CH₂OH disfavored. In the DMF–(CH₃OH)H⁺ ion, the proton prefers to link with the O atom of DMF molecule. Variation of atomic charges during proton transfer in hydrogen bond of the protonated cluster DMF–(CH₃OH)H⁺ ion is also discussed.     

Superoxometal-catalyzed co-oxidation of alcohols and nitrous acid with molecular oxygen

A superoxochromium(III) ion, Cr_aqOO²⁺, acts as a catalyst for the co-oxidation of alcohols and nitrous acid with molecular oxygen according to the stoichiometry: CH₃OH+HNO₂+O₂→CH₂O + NO₃⁻ + H₂O+H⁺. The kinetics are second order in [HNO₂] and independent of the concentrations of the superoxochromium catalyst, substrate, and O₂. The proposed mechanism features the disproportionation of HNO₂ to NO and NO₂, both of which react rapidly with Cr_aqOO²⁺. The Cr_aqOO²⁺/NO reaction generates another equivalent of NO₂ and a mole of Cr_aqO²⁺, the active oxidant. The two-electron oxidation of the alcohol by Cr_aqO²⁺ produces Cr_aq²⁺, which reacts rapidly with O₂ to regenerate the catalyst, Cr_aqOO²⁺. The NO₂/Cr_aqOO²⁺ reaction yields the peroxynitrato complex, Cr_aqOONO₂²⁺, in a dead-end equilibrium process that has no effect on the catalytic reaction. The disproportionation of NO₂ yields the final nitrogen-containing product, NO₃⁻, and regenerates an equivalent of HNO₂. Under a fixed set of conditions, the relative catalytic efficiency (CE) of Cr_aqOO²⁺ decreases as its concentration increases owing to the competition between O₂ and Cr_aqOO²⁺ for the intermediate Cr_aq²⁺.     

Differentiation of isomeric C8-substituted alkylaniline adducts of guanine by electrospray ionization and tandem quadrupole ion trap mass spectrometry

Product ion spectra from thirteen C8-substituted alkylaniline adducts of guanine and deoxyguanosine were generated using electrospray ionization and quadrupole ion trap mass spectrometry and studied to investigate the possibility of differentiating isomeric adduct structures based upon the relative abundances of fragment ions derived from the alkylaniline-modified guanine bases (BH₂⁺ ions). The structural discrimination of the BH₂⁺ ions formed by attachment of isomeric alkylanilines to the C8 position of guanine is a challenging problem because the ions tend to yield product ion spectra that are qualitatively identical upon collisional activation. In this study, a statistical method, referred to as a similarity index, was used to compare the product ion spectra of isomeric BH₂⁺ ions and differentiate their structures. All the adducts investigated could be distinguished from SIs calculated using 5–6 product ions. These results suggest that a searchable database of product ion spectra may be created and used to characterize DNA adducts from aromatic amines whenever they are detected at levels amenable to mass spectral analysis.