Thermodynamic Isotope Effects Research Articles

Hydrogen activation and surface exchange over La0.8Sr0.2Ga0.8Mg0.2O3-z

Hydrogen mass-transfer between molecular hydrogen and La0.8Sr0.2Ga0.8Mg0.2O3-z (LSGM) has been studied by means of hydrogen isotope exchange with gas phase equilibration. The measurements were carried out within a temperature range of 300–700 °C, with molecular hydrogen pressures of 100, 150 and 200 Pa. It was found that LSGM is capable of consuming hydrogen from the gas phase. The relative number of hydrogen atoms in the LSGM structure varied between 1 and 4 mol.%. The kinetics of hydrogen isotope redistribution revealed that the mass-transfer mechanism between molecular hydrogen in the gas phase and LSGM includes at least three steps. The first two relate to dissociative and associative/molecular adsorption of molecular hydrogen, while the third, identified as the rate-determining step in the exchange process, relates to hydrogen incorporation into LSGM structure. The kinetic and thermodynamic isotope effects of the exchange were estimated.

Temperature dependence of hydrogen isotope effect in LaNi5-H(D) system

To study inverse effect of hydrogen isotopes in LaNi5 for hydrogen isotope separation at ambient temperature, temperature dependence of hydrogen isotope effect in LaNi5－H(D) system was investigated in the temperature range -70 °C to 3 °C. Isotherms of hydrogen isotopes were tested in an automated Sieverts apparatus with a thermostat, and pressure differences were studied between H2 and D2. Isotope exchange process was carried out and H2－HD－D2 equilibrium was judged by online chromatography, separation factors were calculated to investigate the thermodynamic isotope effect. In the H2, D2 isotherms, average time interval (0.5h) between each equilibrium point was relatively short due to high precision temperature control. From the plateau pressures at a constant hydrogen (or deuterium) concentration in LaNi5 (2D or 2H atoms/mole of LaNi5) the partial molar enthalpies, ⊿H, and entropies, ⊿S of formation were calculated. ⊿H and ⊿S kept good accordance with the values from references, which indicated good performance of the Sieverts apparatus. The isotope exchange equilibrium was reached within 1h. The results showed that ratio between H2 pressure and D2 pressure, r(H－D) = P(D2)/P(H2), dropped as temperature decreased and the inverse effect was found (r(H－D)<1.0) below -5 °C according to the absorption branch of isotherms, whereas the effect was estimated to appear below 60 °C from the desorption branch. The separation factor of the H2－HD－D2 mixture, decreased as temperature increased and the inverse effect happened below -20 °C. The inverse effect was affected by the isotherm hysteresis and HD existence.

The preparation and characterization of chemically deuterium incorporated cotton fibers

Preparation of deuterium incorporated cellulose is a vital tool to investigate cellulose internal structure and to expand the application fields of cellulose materials. In this study, cellulosic cotton fibers with anti-rehydration (exchange-resistant) deuterium incorporated in cellulose were prepared by chemical hydrogen–deuterium exchange treatment. The chemical hydrogen–deuterium exchange process, along with exchange time, were characterized by nuclear magnetic resonance hydrogen spectroscopy (1H-NMR). The anti-rehydration deuterium incorporation was determined by Fourier Transform infrared spectroscopy (FTIR) and Stable Isotope Ratio Mass Spectrometer (IRSM). The effect of the deuterium hydroxyl substitution on cotton fiber’s spectral data, microstructure, crystalline information, degree of polymerization, as well as it’s thermogravimetric analysis (charcoalization and combustion) are explored. Analysis of the chemical exchange process indicated that the hydrogen–deuterium exchange occurred preferentially in the amorphous cellulose component over the first several minutes. Deuterium exchange in the anti-rehydration crystalline phase took several hours. Increasing the treatment time, enhanced exchange-resistant deuterium incorporation to as high as about 60% of the cotton fibers’ cellulose hydroxyl groups was achieved. The characterization of FTIR, Fourier transform Raman (FT-Raman), and near-infrared spectra (NIR) all exhibited the deuterium spectral isotope effect on cellulose hydroxl groups. While, apart from the effect of reaction temperature, deuterium incorporation isotope effect did not affect the cellulose microstructure, crystalline index and the degree of polymerization properties. Furthermore, the thermogravimetric analysis of deuterated cotton fibers under N2 and air atmosphere were both altered due to the thermodynamic isotope effect. These observations revealed the hydrogen–deuterium exchange treatment process and impacts on cellulose fiber properties, which helped us to better understand the cellulose internal structure and may facilitate the potential utilization of deuterated cellulosic materials.

Secondary kinetic deuterium isotope effects on unimolecular cleavage reactions: Zero-point vibrational energy and qualitative RRKM theory.

Secondary kinetic isotope effects arise as the result of transition-state zero-point vibrational energy differences. Unimolecular simple cleavage reactions of gas-phase ions in mass spectrometers allow detailed studies of isotope effects on competing reactions, particularly when examined in intramolecular competition experiments where interpretation requires very few simplifying assumptions. The zero-point energy differences reflect changes of isotope sensitive vibrational properties, and both α- and β-secondary deuterium isotope effects are related to the sp 3 → sp 2 hybridization changes that accompany bond cleavage. Deuterium substitution three bonds or more removed from the bond broken also gives rise to isotope effects, but their origin is less easily interpreted. The magnitude and variation of the observed effects depend not only on zero-point energy differences; a number of additional factors play a role. The influence of the critical energy, the excess energy, the size of the reactant, and the presence of competing reactions can be rationalized within a simple, qualitative RRKM framework. The distinction between kinetic and thermodynamic isotope effects is not always obvious.

Can nitrogen-based complex hydrides be a hydrogen isotope separation material?

A nitrogen-based complex hydride is investigated for hydrogen isotope separation for the first time. The experimental results show that a lithium amide-lithium hydride composite (Li-N-H) possesses a distinct positive isotope effect with a separation factor of 1.42. The H-D exchange process in this system occurs at 373 K and can be accelerated with the increase of temperature.

Unusual behavior of temperature-dependent solvent H/D isotope effects in the enthalpy and heat capacity of hexamethylenetetramine (urotropine) hydration

Based on the identified inconsistencies in the existing literature data on the enthalpies and heat capacities of dissolution (solvation) of hexamethylenetetramine (HMTA) in ordinary (H2O) and heavy (D2O) water, we carried out carefully the calorimetric study of these two binary systems at seven temperatures from (278.15 to 318.15) K. It is established that the considered D2O–H2O enthalpy-isotopic effect exhibits a pronounced exothermic maximum between two sign-inversion points near 293 K and 318 K. The same goes the negative (up to ~305 K) solvent isotope effect (IE) in the partial molar heat capacity of HMTA in water. Such an unusual temperature-dependent behavior of the thermodynamic IEs points out the dualistic character of the HMTA hydration process which is dependent on the initial structure state of the solvent.

CO2 hydrogenation to methanol over Cu catalysts supported on La-modified SBA-15: The crucial role of Cu–LaOx interfaces

The direct hydrogenation of CO2 to methanol has become a very active research field because CO2 can be prospectively recycled to mitigate greenhouse effect and store clean synthetic fuels. This reaction can be catalyzed by supported Cu catalysts and the catalysts display strong support or promoter effects. Sintering of Cu species accelerates the separation of Cu–oxide interfaces, reduces the active component, and diminishes the methanol selectivity. In this work, we report a Cu catalyst supported on La-modified SBA-15, where the Cu–LaOx interface is generated through the interaction of highly dispersed Cu nanoparticles with LaOx species bedded into the SBA-15 pore wall. The optimized Cu1La0.2/SBA-15 catalyst can achieve methanol selectivity up to 81.2% with no deterioration in activity over 100 h on stream compared with the La-free catalyst. A thorough study reveals that La species not only significantly improve the CO2 adsorption but also enhance Cu dispersion to produce well-dispersed active sites. The H/D exchange experiments show that the methanol synthesis displays a strong thermodynamic isotope effect and the Cu–LaOx interface plays a crucial role for the methanol synthesis rate in CO2/D2 feed. In situ DRIFTS studies reveal that *HCOO and *OCH3 species are the key intermediates formed during the activation of CO2 and methanol synthesis over the Cu1La0.2/SBA-15 catalyst.

H/D Isotope Effect in the Conductivity of CaZr1 – xScxO3 – α in Reducing Atmospheres

The ionic (proton and deuteron) conductivity of the system CaZr1 – xScxO3 – α (x = 0.03–0.20) is studied experimentally in H2 + H2O + N2 and D2 + D2O + N2 reducing atmospheres at pH2O = pD2O = 3.2 kPa and in the temperature range of 600–900°C. This system exhibits a considerable H/D isotopic effect in conductivity, and its magnitude is estimated. Using theoretical elaborations, we also estimate the magnitude of the thermodynamic isotope effect in the solubility of H2O/D2O in the considered system under reducing conditions.

Isotopic exchange between hydrogen from the gas phase and proton-conducting oxides: Theory and experiment

The interaction of hydrogen isotopes from the gas phase with proton-conducting oxides with the perovskite structure La1−x SrxScO3−α (x = 0; 0.04) was studied by means of the hydrogen isotope exchange with gas phase equilibration in the temperature range T = 300–800 °C and in hydrogen pressure range pH2 = 2–20 mbar.A novel kinetic model for the hydrogen isotope exchange experimental data treatment taking into account the isotopic effects was developed. This model was implemented for the obtained experimental results. The heterogeneous exchange rates of hydrogen isotopes with investigated oxides La1−xSrxScO3−α (x = 0; 0.04) were calculated. The mole fractions of hydrogen isotopes were determined for the investigated materials. It was found that deuterium saturation level is higher in comparison with protium, whereas the deuterium surface exchange coefficient for the proton-conducting oxide La0.96Sr0.04ScO3–α is smaller in comparison with the protium surface exchange coefficient. The thermodynamic isotope effect can be caused by the difference of energy of zero-point oscillations between OH- and OD-defects and molecular H2 and D2. The kinetic isotope effect can be explained by the different strength of OH and OD bonds. It is shown that the rate determining stage of hydrogen surface exchange is the process of the exchange between the forms of hydrogen in the gas phase and in the adsorption layer of the proton-conducting oxides (the stage of dissociative adsorption of hydrogen). A new statistical criterion is proposed for the first time allowing dividing the observed surface inhomogeneities caused by not only the natural surface roughness but also the presence of different isotopes of hydrogen (protium and deuterium) with different binding energies on a solid surface.The activity of the investigated proton-conducting oxides with respect to the hydrogen heterogeneous exchange is comparable to the hydrogen heterogeneous exchange activity for the oxides based on cerates and zirconates of alkaline earth metals. High catalytic activity with respect to the process of hydrogen exchange from the gas phase in reducing atmospheres allows us to consider proton-conducting oxides based on the lanthanum scandates as very promising electrolytes for numerous electrochemical applications.

Изотопный эффект H/D в проводимости CaZr-=SUB=-1-x-=/SUB=-Sc-=SUB=-x-=/SUB=-O-=SUB=-3-alpha-=/SUB=- в восстановительных атмосферах

AbstractThe ionic (proton and deuteron) conductivity of the system CaZr_1 – _ x Sc_ x O_3 – α ( x = 0.03–0.20) is studied experimentally in H_2 + H_2O + N_2 and D_2 + D_2O + N_2 reducing atmospheres at p H_2O = p D_2O = 3.2 kPa and in the temperature range of 600–900°C. This system exhibits a considerable H/D isotopic effect in conductivity, and its magnitude is estimated. Using theoretical elaborations, we also estimate the magnitude of the thermodynamic isotope effect in the solubility of H_2O/D_2O in the considered system under reducing conditions.

Quantum-chemical analysis of the thermodynamic isotope effect in quasi-one-dimensional H-bonded Pb(H/D)PO4 ferroelectrics

The thermodynamics of the structural phase transition of H-bonded ferroelectric materials, Pb(H/D)PO4, were considered in terms of the pseudo-spin Ising model with inclusion of tunneling and longrange effects. The pseudo-spin Hamiltonian parameters needed for analysis of the transition were determined by a procedure based on an independent quantum chemical method. A simplified scheme for the selection of model clusters was proposed, which allows the application of various quantum chemical methods, including high-level methods (CCSD/6-311+G** and so on), in the calculations of double-well potential profiles and Slater parameters. The calculation results were discussed in terms of two statistic models: molecular field approximation (MFA) and Bethe cluster method (BCM). The theoretical estimates of critical transition temperature for both systems are discussed and it is shown that the (semi)quantitative reproduction of experimental data is possible only in terms of BCM taking into account the tunneling effects. The explanation is given for the observed isotope effect caused by very pronounced increase in the critical transition temperature upon deuteration (ΔTс ≈ 140 K). The crucial role belongs to the difference between tunneling effects in the ferroelectric crystals in question. It is emphasized that the observed differences between the crystal lattice and H/D bond geometries, including the mutual orientation of the bonds, must be accurately included in the calculations.

ISOTOPIC EXCHANGE BETWEEN HYDROGEN FROM THE GAS PHASE AND PROTON-CONDUCTING OXIDES: THEORY AND EXPERIMENT

The interaction of gaseous hydrogen isotopes from the gas phase with proton-conducting oxides with the perovskite structure La 1−x Sr x ScO 3− α (x = 0; 0.04) was studied by means of the hydrogen isotope exchange with gas phase equilibration in the temperature range T = 300−800 ºC and in hydrogen pressure range pH 2 = 2−20 mbar. A novel kinetic model for the hydrogen isotope exchange experimental data treatment taking into account the isotopic effects was developed. This model was implemented for the obtained experimental results. The heterogeneous exchange rates of hydrogen isotopes with investigated oxides La 1−x Sr x ScO 3− α (x = 0; 0.04) were calculated. The mole fractions of hydrogen isotopes were determined for the investigated materials. It was found that deuterium uptake is higher in comparison with protium, whereas the deuterium surface exchange coefficient for the proton-conducting oxide La 0.96 Sr 0.04 ScO 3– α is smaller in comparison with the protium surface exchange coefficient. The thermodynamic isotope effect can be caused by the difference of energy of zero-point oscillations between OH- and OD-defects and molecular H 2 and D 2 . The kinetic isotope effect can be explained by the different strength of OH and OD bonds. The rate determining stage of hydrogen exchange is shown to be the process of exchange between the forms of hydrogen in the gas phase and in the adsorption layer of the proton-conducting oxide (the stage of dissociative hydrogen adsorption). For the first time, a new statistical criterion is proposed that allows dividing the observed surface inhomogeneities caused by not only the natural surface roughness but also the presence of different isotopes of hydrogen (protium and deuterium) with different binding energies on a solid surface. The activity of the investigated proton-conducting oxides with respect to the hydrogen heterogeneous exchange is comparable to the heterogeneous hydrogen exchange activity for oxides based on cerates and zirconates of alkaline earth metals. High catalytic activity with respect to the process of hydrogen exchange from the gas phase in reducing atmospheres allows us to consider the proton-conducting oxides based on the lanthanum scandates as the very promising electrolytes for numerous electrochemical applications.

Hydrogenation of CO2 to methanol and CO on Cu/ZnO/Al2O3: Is there a common intermediate or not?

H/D exchange experiments on a Cu/ZnO/Al2O3 catalyst have shown that methanol synthesis and RWGS display a strong thermodynamic isotope effect, which is attributed to differences in the zero-point energy of hydrogenated vs. deuterated species. The effect is larger for methanol synthesis and substantially increases the equilibrium yield in deuterated syngas. In the kinetic regime of CO2 hydrogenation, an inverse kinetic isotope effect of H/D substitution was observed, which is stronger for methanol synthesis than for CO formation suggesting that the two reactions do not share a common intermediate. Similar observations were also made on other catalysts such as Cu/MgO, Cu/SiO2, and Pd/SiO2. In contrast to CO2 hydrogenation, the CO hydrogenation on Cu/ZnO/Al2O3 did not show such a strong kinetic isotope effect indicating that methanol formation from CO2 does not proceed via consecutive reverse water gas shift and CO hydrogenation steps. The inverse KIE is consistent with formate hydrogenation being the rate-determining step of methanol synthesis from CO2. Differences in the extent of product inhibition by water, observed for methanol synthesis and reverse water gas shift indicate that the two reactions proceed on different surface sites in a parallel manner. The consequences for catalyst design for effective methanol synthesis from CO2 are discussed.

Multi-factorial in vivo stable isotope fractionation: causes, correlations, consequences and applications

Many physical and chemical processes in living systems are accompanied by isotope fractionation on H, C, N, O and S. Although kinetic or thermodynamic isotope effects are always the basis, their in vivo manifestation is often modulated by secondary influences. These include metabolic branching events or metabolite channeling, metabolite pool sizes, reaction mechanisms, anatomical properties and compartmentation of plants and animals, and climatological or environmental conditions. In the present contribution, the fundamentals of isotope effects and their manifestation under in vivo conditions are outlined. The knowledge about and the understanding of these interferences provide a potent tool for the reconstruction of physiological events in plants and animals, their geographical origin, the history of bulk biomass and the biosynthesis of defined representatives. It allows the use of isotope characteristics of biomass for the elucidation of biochemical pathways and reaction mechanisms and for the reconstruction of climatic, physiological, ecological and environmental conditions during biosynthesis. Thus, it can be used for the origin and authenticity control of food, the study of ecosystems and animal physiology, the reconstruction of present and prehistoric nutrition chains and paleaoclimatological conditions. This is demonstrated by the outline of fundamental and application-orientated examples for all bio-elements. The aim of the review is to inform (advanced) students from various disciplines about the whole potential and the scope of stable isotope characteristics and fractionations and to provide them with a comprehensive introduction to the literature on fundamental aspects and applications.

Hydrogen Isotope Effects of Ti, Zr Metals

The p-c-T curves of D2 and T2 absorption by Ti and Zr were measured. there are one plateau at temperature below 300 °C and two plateaus at temperature range of 500∼600 °C for Ti but one plateau below 525°C and two plateaus above 525°C for Zr. The thermodynamic parameters on different phases were determined and there are obvious thermodynamic isotope effects. The lag effect was not observed for Ti but its existent for Zr. The kinetic p-t curves of absorption were investigated at different temperature ranges and then the rate constants are calculated. The results show that the rate constants increase with raising temperature for Ti but decrease for Zr. The activation energy values are (110.2±3.0), (155.7±3.2) kJ.mol-1 respectively for Ti and (-25.9±0.7), (-6.8±0.8) kJ.mol-1 for Zr. The kinetic p-t curves of desorption were investigated too and the activation energy of desorption are (42.3±1.9), (62.1±1.6) kJ.mol-1 respectively for Ti and (40.1±0.8), (57.7±1.6) kJ.mol-1 for Zr. So there are remarkable kinetic isotope effects for Ti, Zr.

Depletion of the heaviest stable N isotope is associated with NH4+/NH3 toxicity in NH4+-fed plants

BackgroundIn plants, nitrate (NO3-) nutrition gives rise to a natural N isotopic signature (δ15N), which correlates with the δ15N of the N source. However, little is known about the relationship between the δ15N of the N source and the 14N/15N fractionation in plants under ammonium (NH4+) nutrition. When NH4+ is the major N source, the two forms, NH4+ and NH3, are present in the nutrient solution. There is a 1.025 thermodynamic isotope effect between NH3 (g) and NH4+ (aq) which drives to a different δ15N. Nine plant species with different NH4+-sensitivities were cultured hydroponically with NO3- or NH4+ as the sole N sources, and plant growth and δ15N were determined. Short-term NH4+/NH3 uptake experiments at pH 6.0 and 9.0 (which favours NH3 form) were carried out in order to support and substantiate our hypothesis. N source fractionation throughout the whole plant was interpreted on the basis of the relative transport of NH4+ and NH3.ResultsSeveral NO3--fed plants were consistently enriched in 15N, whereas plants under NH4+ nutrition were depleted of 15N. It was shown that more sensitive plants to NH4+ toxicity were the most depleted in 15N. In parallel, N-deficient pea and spinach plants fed with 15NH4+ showed an increased level of NH3 uptake at alkaline pH that was related to the 15N depletion of the plant. Tolerant to NH4+ pea plants or sensitive spinach plants showed similar trend on 15N depletion while slight differences in the time kinetics were observed during the initial stages. The use of RbNO3 as control discarded that the differences observed arise from pH detrimental effects.ConclusionsThis article proposes that the negative values of δ15N in NH4+-fed plants are originated from NH3 uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH4+/NH3 toxicity in plants species. Therefore, we hypothesise that the low affinity transport system for NH4+ may have two components: one that transports N in the molecular form and is associated with fractionation and another that transports N in the ionic form and is not associated with fractionation.

Mechanistic Aspects and Reaction Pathways for Oxidative Coupling of Methane on Mn/Na2WO4/SiO2 Catalysts

Kinetic and isotopic methods were used to determine the identity, rate constants, and reversibility of elementary steps for primary and secondary reactions involved in the oxidative coupling of methane (OCM) on Mn/Na2WO4/SiO2. We provide evidence in this study for parallel C−H bond activation pathways, in which H-abstraction is mediated by either oxygen species on surfaces or by OH radicals formed via H2O/O2 equilibration on catalyst surfaces. OCM rates and C2+ yields are higher when H2O is present and OH-mediated pathways prevail, because of the high reactivity of OH radicals and of their lesser sensitivity to the energy of the C−H bond containing the hydrogen abstracted. These coupled homogeneous-catalytic sequences account for all observed kinetic effects of O2, CH4, and H2O on rates and selectivities for both CH4 conversion and for subsequent reactions of C2H6, C2H4 and C3 products; they are also consistent with measured kinetic and thermodynamic isotope effects for C−H bond activation mediated by sur...

Thermodynamic and Magnetic Properties of Pd&lt;SUB&gt;0.93&lt;/SUB&gt;Ag&lt;SUB&gt;0.07&lt;/SUB&gt; Hydride

Thermodynamic and magnetic properties of the Pd 0.93 Ag 0.07 hydrides were measured. The isotherms of Pd 0.93 Ag 0.07 -H and -D systems in the temperature range 373-523 K showed the plateau regions to be within 0.05 < [Q (Q = H or D)]/[Pd 0.93 Ag 0.07 ] < 0.4. From an investigation of the thermodynamic isotope effect for the formation and decomposition of hydride, the enthalpy changes, ΔH° β→α , for desorption of hydrogen and deuterium were evaluated to be 43.0 kJ/mol H2 and 36.4 kJ/mol D2 , respectively. It was observed that the magnetization of Pd 0.93 Ag 0.07 Hx at ambient temperature increased with increasing magnetic field. Since the magnetization was not saturated up to 7 T, it can be concluded that Pd 0.93 Ag 0.07 H x is paramagnetic. The magnetic susceptibility of Pd 0.93 Ag 0.07 H x progressively decreased with increasing hydrogen content and vanished at [H]/[Pd 0.93 Ag 0.07 ] = 0.4. The disappearance of the susceptibility coincided with the termination of the plateau region of the isotherms. This fact can be explained by the rigid band model, according to which the vacant part of d-band of Pd 0.93 Ag 0.07 can accept electrons from absorbed hydrogen atoms.

Estimation of solubility and thermodynamic characteristics of solvation of radon in deuterated water at 0.1 MPa and 278–318 K

The concentration of Rn in saturated solution in D2O at 101 325 Pa and 278.15–318.15 K (at 5 K interval) was estimated by the Karapet’yants method of comparative calculation from original and published data on the solubility of noble gases He-Xe in H/D water isotopomers and of Rn in H2O. Thermodynamic functions of Rn solvation (ΔsolG0, ΔsolH∞, ΔsolS0, and ΔsolCp∞) in H2O and D2O and the corresponding H/D isotope effects were calculated by the original method. The characteristics obtained for the D2O-Rn system are consistent with the previously found trends in variation of thermodynamic isotope effects of solvation of noble gases (He-Xe) in aqueous solution with increasing temperature.

Isomerization of the protonated acetone dimer in the gas phase

Mass spectrometry-based methods have been employed in order to study the reactions of non- (h(6)/h(6)), half (d(6)/h(6)), and fully (d(6)/d(6)) deuterium labeled protonated dimers of acetone in the gas phase. Neither kinetic nor thermodynamic isotope effects were found. From MIKES experiments (both spontaneous and collision-induced dissociations), it was found that the relative ion yield (m/z 65 vs m/z 59) from the dissociation reaction of half deuterium labeled (d(6)/h(6)) protonated dimer of acetone is dependent on the internal energy. A relative ion yield (m/z 65 vs m/z 59) close to unity is observed for cold, nonactivated, metastable ions, whereas the ion yield is observed to increase (favoring m/z 65) when the pressure of the collision gas is increased. This is in striking contrast to what would be expected if a kinetic isotope effect were present. A combined study of the kinetics and the thermodynamics of the association reaction between acetone and protonated acetone implicates the presence of at least two isomeric adducts. We have employed G3(MP2) theory to map the potential energy surface leading from the reactants, acetone and protonated acetone, to the various isomeric adducts. The proton-bound dimer of acetone was found to be the lowest-energy isomer, and protonated diacetone alcohol the next lowest-energy isomer. Protonated diacetone alcohol, even though it is an isomer hidden behind many barriers, can possibly account for the observed relative ion yield and its dependence on the mode of activation.