H2 adsorption on 3d transition metal clusters: a combined infrared spectroscopy and density functional study

The adsorption of H2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H2 is found to be exclusively dissociative on Sc(n)O+, V(n)+, Fe(n)+, and Co(n)+, and both atomic and molecularly chemisorbed hydrogen is present in Ni(n)H(m)+ complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2- or 3-fold coordinated to cationic metal clusters.     

Oxidation of methanol to formaldehyde on supported vanadium oxide catalysts compared to gas phase molecules

The oxidation of methanol to formaldehyde on silica supported vanadium oxide is studied by density functional theory. For isolated vanadium oxide species silsesquioxane-type models are adopted. The first step is dissociative adsorption of methanol yielding CH3O(O=)V(O-)2 surface complexes. This makes the O=V(OCH3)3 molecule a suited model system. The rate-limiting oxidation step involves hydrogen transfer from the methoxy group to the vanadyl oxygen atom. The transition state is biradicaloid and needs to be treated by the broken-symmetry approach. The activation energies for O=V(OCH3)3 and the silsesquioxane surface model are 147 and 154 kJ/mol. In addition, the (O=V(OCH3)3)(2) dimer (a model for polymeric vanadium oxide species) and the O=V(OCH3)3(*+) radical cation are studied. For the latter the barrier is only 80 kJ/mol, indicating a strong effect of the charge on the energy profile of the reaction and questioning the significance of gas-phase cluster studies for understanding the activity of supported oxide catalysts.     

Size-specific reactions of copper cluster ions with a methanol molecule

Chemisorption of a methanol molecule onto a size-selected copper cluster ion, Cu(n)+ (n = 2-10), and subsequent reactions were investigated in a gas-beam geometry at a collision energy less than 2 eV in an apparatus based on a tandem-type mass spectrometer. Mass spectra of the product ions show that the following two reactions occur after chemisorption: dominant formation of Cu(n-1)+(H)(OH) (H(OH) formation) in the size range of 4-5 and that of Cu(n)O+ (demethanation) in the size range of 6-8 in addition to only chemisorption in the size range larger than 9. Absolute cross sections for the chemisorption, the H(OH) formation, and the demethanation processes were measured as functions of cluster size and collision energy. Optimized structures of bare copper cluster ions, reaction intermediates, and products were calculated by use of a hybrid method (B3LYP) consisting of the molecular orbital and the density functional methods. The origin of the size-dependent reactivity was explained as the structural change of cluster, two-dimensional to three-dimensional structures.     

Gas phase infrared spectroscopy of mono- and divanadium oxide cluster cations

The vibrational spectroscopy of the mono- and divanadium oxide cluster cations VO(1-3)+ and V2O(2-6)+ is studied in the region from 600 to 1600 wave numbers by infrared photodissociation of the corresponding cluster cation-helium atom complexes. The comparison of the experimental depletion spectra with the results of density functional calculations on bare vanadium oxide cluster cations allows for an unambiguous identification of the cluster geometry in most cases and, for VO(1-3)+ and V2O(5,6)+, also of the electronic ground state. A common structural motif of all the studied divanadium cluster cations is a four-membered V-O-V-O ring, with three characteristic absorption bands in the 550-900 wave number region. For the V-O-V and V=O stretch modes the relationship between vibrational frequencies and V-O bond distances follows the Badger rule.     

Gas-phase oxidation of isomeric butenes and small alkanes by vanadium-oxide and -hydroxide cluster cations

Bare vanadium-oxide and -hydroxide cluster cations (V(m)O(n)H(o)+, m = 2-4, n = 1-10, o = 0, 1) were generated by electrospray ionization in order to examine their intrinsic reactivity toward isomeric butenes and small alkanes using mass spectrometric techniques. Two of the major reactions described here concern the activation of C-H bonds of the alkene/alkane substrates resulting in the transfer of two hydrogen atoms and/or attachment of the dehydrogenated hydrocarbon to the cluster cations; these processes are classified as oxidative dehydrogenation (ODH) and dehydrogenation, respectively. For the dehydrogenation of butene, it evolved as a general trend that high-valent clusters prefer ODH resulting in the addition of two hydrogen atoms to the cluster concomitant with elimination of neutral butadiene, whereas low-valent clusters tend to add the diene with parallel loss of molecular hydrogen. Deuterium labeling experiments suggest the operation of a different reaction mechanism for V2O2(+) and V4O10(+) compared to the other cluster cations investigated, and these two cluster cations also are the only ones of the vanadium-oxide ions examined here that are able to dehydrogenate small alkanes. The kinetic isotope effects observed experimentally imply an electron transfer mechanism for the ion-molecule reactions of the alkanes with V4O10(+).     

Experimental and theoretical studies of reactions of neutral vanadium and tantalum oxide clusters with NO and NH3

Reactions of neutral vanadium and tantalum oxide clusters with NO, NH(3), and an NO/NH(3) mixture in a fast flow reactor are investigated by time of flight mass spectrometry and density functional theory (DFT) calculations. Single photon ionization through a 46.9 nm (26.5 eV) extreme ultraviolet (EUV) laser is employed to detect both neutral cluster distributions and reaction products. Association products VO(3)NO and V(2)O(5)NO are detected for V(m)O(n) clusters reacting with pure NO, and reaction products, TaO(3,4)(NO)(1,2), Ta(2)O(5)NO, Ta(2)O(6)(NO)(1-3), and Ta(3)O(8)(NO)(1,2) are generated for Ta(m)O(n) clusters reacting with NO. In both instances, oxygen-rich clusters are the active metal oxide species for the reaction M(m)O(n)+NO→M(m)O(n)(NO)(x). Both V(m)O(n) and Ta(m)O(n) cluster systems are very active with NH(3). The main products of the reactions with NH(3) result from the adsorption of one or two NH(3) molecules on the respective clusters. A gas mixture of NO:NH(3) (9:1) is also added into the fast flow reactor: the V(m)O(n) cluster system forms stable, observable clusters with only NH(3) and no V(m)O(n)(NO)(x)(NH(3))(y) species are detected; the Ta(m)O(n) cluster system forms stable, observable mixed clusters, Ta(m)O(n)(NO)(x)(NH(3))(y), as well as Ta(m)O(n)(NO)(x) and Ta(m)O(n)(NH(3))(y) individual clusters, under similar conditions. The mechanisms for the reactions of neutral V(m)O(n) and Ta(m)O(n) clusters with NO/NH(3) are explored via DFT calculations. Ta(m)O(n) clusters form stable complexes based on the coadsorption of NO and NH(3). V(m)O(n) clusters form weakly bound complexes following the reaction pathway toward end products N(2)+H(2)O without barrier. The calculations give an interpretation of the experimental data that is consistent with the condensed phase reactivity of V(m)O(n) catalyst and suggest the formation of intermediates in the catalytic chemistry.     

Polynuclear vanadium complexes from thermal decomposition of [V3O(O2CPh)6(H2O)3]Cl

Solid-state decomposition of [V3O(O2CPh)6(H2O)3]Cl at 300 degrees C followed by alcoholysis of the product gives the new vanadium complexes [V6O6(PhCO2)6(CH3O)6(CH3OH)3] (1), [V6O6(PhCO2)6(C2H5O)6(C2H5OH)3] (2), [V6O6(PhCO2)6(C3H7O)6(C3H7OH)3] (3), [V6O6(PhCO2)6(C4H9O)6(C4H9OH)3] (4) and [V4O4(OCH3)6(O2CPh)2(HOCH3)2] (5). Complexes 2, 3 and 5 have been crystallographically characterised. DC magnetic susceptibility studies on complex shows antiferromagnetic coupling leading to a S = 0 spin ground state.     

过渡金属氧化物(M2O5)+m=1,2(M=V, Nb, Ta)与C2H4气相反应机理的密度泛函研究

采用密度泛函理论研究了过渡金属钒族氧化物阳离子团簇(M2O5)+m=1,2(M=V, Nb, Ta)与C2H4气相反应机理. 反应为(M2O5)m++C2H4→(M2O5)m-1M2O4++C2H4O, 反应物先化合生成C—O键相连的化合物, 经过过渡态后M—O键断裂, 从而发生氧原子转移到碳氢化合物上的反应. 对于V2O5+与C2H4的反应, 存在经顺式和反式两种过渡态结构路径, 从能量上看, 经反式过渡态结构的路径更有利. 计算结果表明, 发生反应时C2H4与钒氧化物阳离子反应大量放热, 而与铌、钽氧化物阳离子反应却放热较少甚至不放热, 这与实验结果一致. 钒、铌、钽氧化物阳离子团簇发生氧转移反应活性不同的原因是金属-氧键的强弱不同所致.     

Experimental and theoretical study of the reactions between neutral vanadium oxide clusters and ethane, ethylene, and acetylene

Reactions of neutral vanadium oxide clusters with small hydrocarbons, namely C2H6, C2H4, and C2H2, are investigated by experiment and density functional theory (DFT) calculations. Single photon ionization through extreme ultraviolet (EUV, 46.9 nm, 26.5 eV) and vacuum ultraviolet (VUV, 118 nm, 10.5 eV) lasers is used to detect neutral cluster distributions and reaction products. The most stable vanadium oxide clusters VO2, V2O5, V3O7, V4O10, etc. tend to associate with C2H4 generating products V(m)O(n)C2H4. Oxygen-rich clusters VO3(V2O5)(n=0,1,2...), (e.g., VO3, V3O8, and V5O13) react with C2H4 molecules to cause a cleavage of the C=C bond of C2H4 to produce (V2O5)(n)VO2CH2 clusters. For the reactions of vanadium oxide clusters (V(m)O(n)) with C2H2 molecules, V(m)O(n)C2H2 are assigned as the major products of the association reactions. Additionally, a dehydration reaction for VO3 + C2H2 to produce VO2C2 is also identified. C2H6 molecules are quite stable toward reaction with neutral vanadium oxide clusters. Density functional theory calculations are employed to investigate association reactions for V2O5 + C2H(x). The observed relative reactivity of C2 hydrocarbons toward neutral vanadium oxide clusters is well interpreted by using the DFT calculated binding energies. DFT calculations of the pathways for VO3+C2H4 and VO3+C2H2 reaction systems indicate that the reactions VO3+C2H4 --> VO2CH2 + H2CO and VO3+C2H2 --> VO2C2 + H2O are thermodynamically favorable and overall barrierless at room temperature, in good agreement with the experimental observations.     

Photodissociation of chromium oxide cluster cations

Chromium oxide cluster cations, Cr(n)O(m)+, are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. The mass spectrum exhibits a limited number of stoichiometries for each value of n, where m > n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic output of a Nd:YAG laser. At either wavelength, multiphoton absorption is required to dissociate these clusters, which is consistent with their expected strong bonding. Cluster dissociation occurs via elimination of molecular oxygen, or by fission processes producing stable cation species and/or eliminating stable neutrals such as CrO3, Cr(2)O(5), or Cr(4)O(10). Specific cation clusters identified to be stable because they are produced repeatedly in the decomposition of larger clusters include Cr(2)O(4)+, Cr(3)O(6)+, Cr(3)O(7)+, Cr(4)O(9)+, and Cr(4)O(10)+.     

Reaction of Cationic Vanadium Oxide Clusters with Ethylene in a Flow Tube Reactor

A time of °ight mass spectrometer coupled with a cluster formation and reaction source is employed to study the reactivity of cationic vanadium oxide clusters (VmOn+) toward ethylene (C2H4) in the gas phase. The cationic vanadium oxide clusters with m=1-10 and n=1-26 (depending on m) are generated by reaction of laser ablation created vanadium plasma with O2 in a supersonic expansion and then reacted with the ethylene in a °ow tube reactor. Hydrogen atoms are attached in most of the oxygen saturated clusters(2n?5m) in our experimental condition. The reactivity of VmOn+ toward C2H4 is usually higher than that of hydrogen containing clusters, VmOnH2x+. Larger clusters show less reactivity than smaller ones. Most of the observed products are in the forms of VmOnC2H4+ and VmOnH2xC2H4+ due to direct association. C2H4 clustering products ((C2H4)n+, n=2-6) are also observed.     

Gas-phase reactivity of metavanadate [VO3]- towards methanol and ethanol: experiment and theory

The gas-phase reactivity of the metavanadate anion [VO3]- towards methanol and ethanol was examined by a combination of ion-molecule reaction and isotope labelling experiments in a quadrupole ion-trap mass spectrometer. The experimental data were interpreted with the aid of density functional theory calculations. [VO3]- dehydrated methanol to eliminate water and form [VO2(eta2-OCH2)]-, which features an [eta2-C,O-OCH2]2- ligand formed by formal removal of two protons from methanol and which is isoelectronic with peroxide. [VO3]- reacted with ethanol in an analogous manner to form [VO2(eta2-OCHCH3)]-, as well as by loss of ethene to form [VO2(OH)2]-. The calculations predicted that important intermediates in these reactions were the hydroxo alkoxo anions [VO2(OH)(OCH2R)]- (R: H, CH3). These were predicted to undergo intramolecular hydrogen-atom transfer to form [VO(OH)2(eta1-OCHR)]- followed by eta1-O-->eta2-C,O rearrangements to form [VO(OH)2(eta2-OCHR)]-. The latter reacted further to eliminate water and generate the product [VO2(eta2-OCHR)]-. This major product observed for [VO3]- is markedly different from that observed previously for [NbO3]- containing the heavier Group 5 congener niobium. In that case, the major product of the reaction was an ion of stoichiometry [Nb, O3, H2]- arising from the formal dehydrogenation of methanol to formaldehyde. The origin of this difference was examined theoretically and attributed to the intermediate alkoxo anion [NbO2(OH)(OCH3)]- preferring hydride transfer to form [HNbO2(OH)]- with loss of formaldehyde. This contrasts with the hydrogen-atom-transfer pathway observed for [VO2(OH)(OCH3)]-.     

Guided ion-beam studies of the reactions of Co(n)+ (n=2-20) with O2: cobalt cluster-oxide and -dioxide bond energies

The kinetic-energy dependence for the reactions of Co(n)+ (n=2-20) with O2 is measured as a function of kinetic energy over a range of 0 to 10 eV in a guided ion-beam tandem mass spectrometer. A variety of Co(m)+, Co(m)O+, and Co(m)O2+ (m < or = n) product ions is observed, with the dioxide cluster ions dominating the products for all larger clusters. Reaction efficiencies of Co(n)+ cations with O2 are near unity for all but the dimer. Bond dissociation energies for both cobalt cluster oxides and dioxides are derived from threshold analysis of the energy dependence of the endothermic reactions using several different methods. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry and the stabilities of oxygenated cobalt clusters are discussed. The bond energies of Co(n)+-O for larger clusters are found to be very close to the value for desorption of atomic oxygen from bulk-phase cobalt. Rate constants for O2 chemisorption on the cationic clusters are compared with results from previous work on cationic, anionic, and neutral cobalt clusters.     

Metastable decompositions of gem-dialkoxyalkanes upon electron impact. III. Diethoxymethane (CH(2)(OCH(2)CH(3))(2))

Unimolecular metastable decomposition of diethoxymethane (CH(2)(OCH(2)CH(3))(2), 1) upon electron impact has been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and theD-labeling technique in conjunction with thermochemistry. The m/z 103 ion ([M - H](+) : CH(OCH(2)CH(3)) = O(+)CH(2)CH(3)) decomposes into the m/z 47 ion (protonated formic acid, CH(OH) = O(+)H) by consecutive losses of two C(2)H(4) molecules via an m/z 75 ion. The resulting product ion at m/z 47 further decomposes into the m/z 29 and 19 ions by losses of H(2)O and CO, respectively, via an 1,3-hydroxyl hydrogen transfer, accompanied by small kinetic energy release (KER) values of 1.3 and 18.8 meV, respectively. When these two elimination reactions are suppressed by a large isotope effect, however, another 1,1-H(2)O elimination with a large KER value (518 meV) is revealed. The m/z 89 ion ([M - CH(3)](+) : CH(2)(OCH(2)CH(3))O(+) = CH(2)) decomposes into the m/z 59 ion (CH(3)CH(2)O(+) = CH(2)) by losing CH(2)O in the metastable time window. The source-generated m/z 59 ion ([M - OCH(2)CH(3)](+) : CH(2) = O(+)CH(2)CH(3)) decomposes into the m/z 41 (CH(2) = CH(+)CH(2)) and m/z 31 (CH(2) = O(+)H) ions by losses of H(2)O and C(2)H(4), respectively, with considerable hydrogen scrambling prior to decomposition. Copyright 2000 John Wiley & Sons, Ltd.     

A novel cationic heteropolyoxovanadium(IV) cluster functionalized with organic ligands: synthesis and characterization of the fully reduced species [Mn(II)V(IV)6O6[(OCH2CH2)2N(CH2CH2OH)]6]Cl2

The solvothermal reaction of (HOCH2CH2)3N with [(n-C4H9)4N]3[H3V10O28] and MnCl2 x 4H2O in CH3CN and CH3OH yields a novel cationic heteropolyoxovanadium(IV) cluster, [Mn(II)V(IV)6O6[(OCH2CH2)2N(CH2CH2OH)]6]2+, containing a fully reduced new cyclic [MnV6N6O18] core with the Anderson structure.     

Comparison of hydrated hydroperoxide anion (HOO-)(H2O)n clusters with alkaline hydrogen peroxide (HOOH)(OH-)(H2O)(n-1) clusters, n = 1-8, 20: an ab initio study

Hydroperoxide anion (HOO(-)), the conjugate base of hydrogen peroxide (HOOH), has been relatively little studied despite the importance of HOOH in commercial processes, atmospheric science, and biology. The anion has been shown to exist as a stable species in alkaline water. This project explored the structure of gas phase (HOO(-))(H(2)O)(n) clusters and identified the lowest energy configurations for n ≤ 8 at the B3LYP/6-311++G** level of theory and for n ≤ 6 at the MP2/aug-cc-pVTZ level of theory. As a start toward understanding equilibration between HOO(-) and HOOH in an alkaline environment, (HOOH)(OH(-))(H(2)O)(n-1) clusters were likewise examined, and the lowest energy configurations were determined for n ≤ 8 (B3LYP/6-311++G**) and n ≤ 6 (MP2/aug-cc-pVTZ). Some studies were also done for n = 20. The two species have very different solvation behaviors. In low energy (HOOH)(OH(-))(H(2)O)(n-1) clusters, HOOH sits on the surface of the cluster, is 4-coordinated (each O is donor once and acceptor once), and donates to the hydroxide ion. In contrast, in low energy (HOO(-))(H(2)O)(n) clusters, (HOO(-)) takes a position in the cluster center surrounded on all sides by water molecules, and its optimum coordination number appears to be 7 (one O is donor-acceptor-acceptor while the other is a 4-fold acceptor). For n ≤ 6 the lowest (HOOH)(OH(-))(H(2)O)(n-1) cluster lies 1.0-2.1 kcal/mol below the lowest (HOO(-))(H(2)O)(n) cluster, but the lowest clusters found for n = 20 favor (HOO(-))(H(2)O)(20). The results suggest that ambient water could act as a substantial kinetic brake that slows equilibration between (HOOH)(OH(-)) and (HOO(-))(H(2)O) because extensive rearrangement of solvation shells is necessary to restabilize either species after proton transfer.     

Size-restricted proton transfer within toluene-methanol cluster ions

To understand the interaction between toluene and methanol, the chemical reactivity of [(C6H5CH3)(CH3OH) n=1-7](+) cluster ions has been investigated via tandem quadrupole mass spectrometry and through calculations. Collision Induced Dissociation (CID) experiments show that the dissociated intracluster proton transfer reaction from the toluene cation to methanol clusters, forming protonated methanol clusters, only occurs for n = 2-4. For n = 5-7, CID spectra reveal that these larger clusters have to sequentially lose methanol monomers until they reach n = 4 to initiate the deprotonation of the toluene cation. Metastable decay data indicate that for n = 3 and n = 4 (CH3OH)3H(+) is the preferred fragment ion. The calculational results reveal that both the gross proton affinity of the methanol subcluster and the structure of the cluster itself play an important role in driving this proton transfer reaction. When n = 3, the cooperative effect of the methanols in the subcluster provides the most important contribution to allow the intracluster proton transfer reaction to occur with little or no energy barrier. As n >or= 4, the methanol subcluster is able to form ring structures to stabilize the cluster structures so that direct proton transfer is not a favored process. The preferred reaction product, the (CH3OH)3H(+) cluster ion, indicates that this size-restricted reaction is driven by both the proton affinity and the enhanced stability of the resulting product.     

Vanadogermanate cluster anions

Three novel vanadogermanate cluster anions have been synthesized by hydrothermal reactions. The cluster anions are derived from the (V(18)O(42)) Keggin cluster shell by substitution of V=O(2+) "caps" by Ge(2)O(OH)(2)(4+) species. In Cs(8)[Ge(4)V(16)O(42)(OH)(4)].4.7H(2)O, 1, (monoclinic, space group C2/c (No. 15), Z = 8, a = 44.513(2) A, b = 12.7632(7) A, c = 22.923(1) A, beta = 101.376(1) degrees ) and (pipH(2))(4)(pipH)(4)[Ge(8)V(14)O(50).(H(2)O)] (pip = C(4)N(2)H(10)), 2 (tetragonal, space group P4(2)/nnm (No. 134), Z = 2, a = 14.9950(7) A, c = 18.408(1) A), two and four VO(2+) caps are replaced, respectively, and each cluster anion encapsulates a water molecule. In K(5)H(8)Ge(8)V(12)SO(52).10H(2)O, 3, (tetragonal, space group I4/m (No. 87), Z = 2, a = 15.573(1) A, c = 10.963(1) A), four VO(2+) caps are replaced by Ge(2)O(OH)(2)(4+) species, and an additional two are omitted. The cluster ion in 3 contains a sulfate anion disordered over two positions. The cluster anions are analogous to the vanadoarsenate anions [V(18)(-)(n)()As(2)(n)()O(42)(X)](m)(-) (X = SO(3), SO(4), Cl; n = 3, 4) previously reported.     

含水杨醛缩对硝基苯甲酰腙的钒酰配合物和钴配合物的合成和晶体结构

本文合成了含水杨醛缩对硝基苯甲酰腙(简写为H2L)的钒酰配合物VOL(CH3OH)(CH3O)(1,C16H16N3O7V,Mr=413.26)和钴配合物[CoL(C5H5N)3]NO3C5H5N(2,C34H29N8O7Co,Mr=720.58)。配合物1属单斜晶系,空间群为P21/c,a=7.3253(3),b=18.8237(9),c=12.9014(5)?b=91.672(1),V=1778.2(1)3,Z=4,F(000)=848,m(MoKa)=0.603mm1,R=0.0470,wR=0.1312。配合物2属单斜晶系,空间群为P21/c,a=11.4196(8),b=17.157(1),c=17.081(1)?b=96.8233(9),V=3323.0(4)3,Z=4,F(000)=1488,m(MoKa)=0.578mm1,R=0.0455,wR=0.1311。在配合物1中,钒(V)原子由周围的酰氧基原子、配体L2的3个配位原子,去质子化甲醇的甲氧基原子和配位甲醇的氧原子配位,形成畸变的VO(ONO)(O)(O)八面体配位构型。晶体内每2个分子间通过氢键作用缔合成中心对称的分子对,OH…N氢键键长为2.861(4)?键角163.20。晶体中存在着弱p-p共轭作用。在配合物2中,钴(Ⅲ)原子由1个L2的3个配位原子和3个配位吡啶分子的3个氮原子配位,呈N4O2八面体配位构型。     

Ion/molecule reactions in the orifice-skimmer region of an atmospheric pressure Penning ionization mass spectrometer

Atmospheric pressure Penning ionization mass spectra of methanol were measured as functions of Ar or He gas pressure in the first vacuum chamber, the position of the skimmer, and the voltage applied between the orifice and the skimmer. When the orifice and the skimmer were coaxial with a distance of 4 mm, the distribution of CH3OH2+(CH3OH)n clusters was only weakly dependent on both Ar pressure (in the range of 19-220 Pa) and orifice-skimmer voltage (in the range of 1-45 V). The ion/molecule reaction CH3OH2+ + CH3OH --> CH3+(CH3OH) + H2O was observed in the free jet expansion, especially at high orifice-skimmer voltage values. When the orifice and the skimmer were off-centered and the distance between them was increased to 18 mm, the formation of large CH3OH2+(CH3OH)n clusters, as well as their dissociation, were seen. The endothermic proton transfer reaction, CH3+(CH3OH) + CH3OH --> CH3OH2+ + CH3OCH3, occurred at high orifice-skimmer voltage. The collision-induced dissociation of cluster ions by He gas in the first vacuum chamber was much more efficient than by Ar. These results demonstrated that the mass spectra are highly dependent on skimmer position and on orifice-skimmer voltage and that ions observed by mass spectrometry do not necessarily reflect the abundance of ions produced in the atmospheric pressure ion source.