Intra-host π-π interactions in crown ether complexes revealed by cryogenic ion mobility-mass spectrometry.

Cryogenic ion mobility-mass spectrometry was performed to investigate the relative abundance of conformers of dinaphtho-24-crown-8 (DN24C8) complexes with alkali metal cations M+ (M = Li, Na, K, Rb, and Cs). The "closed" conformers of M+(DN24C8) with short distances between two naphthalene rings in the crown ethers were predominantly observed for all complexes at 86 K. The two noncovalent interactions, host-guest and intra-host interactions, were analyzed separately by density functional theory calculations to reveal the origin of the stability of the closed conformers. As a result, it was revealed that the intra-host π-π interactions have a more critical role in determining the stability of the conformers than the host-guest interactions. The closed conformers of M+(DN24C8) also have wider regions of the π-π interactions than those of the M+(dibenzo-24-crown-8) complexes.

Gas-Phase Characterization of Hypervalent Carbon Compounds Bearing 7-6-7-Ring Skeleton: Penta- versus Tetra-Coordinate Isomers.

In this study, we afford explicit characterizations of the electronic and geometrical structures of recently reported hypervalent penta-coordinate carbon compounds by using gas-phase characterization techniques: photodissociation spectroscopy (PDS) and ion mobility-mass spectrometry (IM-MS). In particular for a compound with moderately electron-donating ligands, bearing p-methylthiophenyl substituents, the coexistence of tetra- and penta-coordinate isomers is confirmed, consistent with solution characterizations. It is in sharp contrast to the exclusive tetra-coordinate form (with normal valence of the central carbon atom) in the single crystal. This suggests that a non-polar environment makes the penta-coordinate structure thermodynamically most stable. This delicate difference between the tetra- and penta-coordinate structures, which depends on the environment, is a close reflection of the lower activation barrier of the SN 2 reaction found in neutral solvent or gas-phase reactions.

Fragment imaging in the infrared photodissociation of the Ar-tagged protonated water clusters H3O+-Ar and H+(H2O)2-Ar.

Infrared photodissociation of protonated water clusters with an Ar atom, namely H3O+-Ar and H+(H2O)2-Ar, was investigated by an imaging technique for mass-selected ions, to reveal the intra- and intermolecular vibrational dynamics. The presented system has the advantage of achieving fragment ion images with the cluster size- and mode-selective photoexcitation of each OH stretching vibration. Translational energy distributions of photofragments were obtained from the images upon the excitation of the bound (νb) and free (νf) OH stretching vibrations. The energy fractions in the translational motion were compared between νbI and νfI in H3O+-Ar or between νbII and νfII in H+(H2O)2-Ar, where the labels "I" and "II" represent H3O+-Ar and H+(H2O)2-Ar, respectively. In H3O+-Ar, the νfI excitation exhibited a smaller translational energy than νbI. This result can be explained by the higher vibrational energy of νfI, which enabled it to produce bending (ν4) excited H3O+ fragments that should be favored according to the energy-gap model. In contrast to H3O+-Ar, the νbII excitation of an Ar-tagged H2O subunit and the νfII excitation of an untagged H2O subunit resulted in very similar translational energy distributions in H+(H2O)2-Ar. The similar energy fractions independent of the excited H2O subunits suggested that the νbII and νfII excited states relaxed into a common intermediate state, in which the vibrational energy was delocalized within the H2O-H+-H2O moiety. However, the translational energy distributions for H+(H2O)2-Ar did not agree with a statistical dissociation model, which implied another aspect of the process, that is, Ar dissociation via incomplete energy randomization in the whole H+(H2O)2-Ar cluster.

Size-Dependent Geometrical Structures of Platinum Oxide Cluster Cations Studied by Ion Mobility-Mass Spectrometry.

Structures of platinum oxide cluster cations (PtnOm+) were studied by ion mobility-mass spectrometry in combination with theoretical calculations. Structures of oxygen-equivalent PtnOn+ (n = 3-7) clusters were discussed from the comparison between their collision cross sections (CCSs) obtained by mobility measurement and simulated CCSs of their structural candidates from structural optimization calculations. Assigned structures of PtnOn+ were found to be composed of Pt frameworks and bridging O atoms, which follows the previous theoretical prediction on the neutral clusters. The structures change from planar (n = 3 and 4) to three-dimensional (n = 5-7) with increasing cluster size by deforming platinum frameworks. Comparison with other group-10 metal oxide cluster cations (MnOn+; M = Ni and Pd) showed that the PtnOn+ structures have a similar tendency to PdnOn+ rather than NinOn+.

Structural assignments of yttrium oxide cluster cations studied by ion mobility mass spectrometry.

The geometric structures of yttrium oxide cluster ions, YnOm+ (n = 3-11), were experimentally assigned for stable compositions by ion mobility mass spectrometry combined with theoretical calculations. The stable compositions were firstly determined by collision induced dissociation experiments in mass spectrometry as YO(Y2O3)x+ and YO2(Y2O3)x+ for odd numbers of Y atoms (n = 2x + 1) and (Y2O3)x+ and O(Y2O3)x+ for even numbers of Y atoms (n = 2x). The structures of the ions with the above compositions were assigned by comparing the collision cross sections obtained in the ion mobility measurement with those obtained by theoretical calculations. The assigned structures have the following two characteristic features. Firstly, metal-metal or oxygen-oxygen bonds were rarely observed, and most of the oxygen atoms bridge two Y atoms, which is due to the ionic bonding nature between Y3+ and O2- ions. Secondly, common Y-atom frameworks were obtained for the ions with the same number of Y atoms n. For example, for the clusters with even numbers of Y atoms, one atomic oxygen radical anion (O-) in the most stable structures of (Y2O3)x+ was replaced with a superoxide ion (O2-) to form the most stable structures of O(Y2O3)x+ ions, keeping the Y-atom framework geometries.

Large Conformational Change in the Isomerization of Flexible Crown Ether Observed at Low Temperature.

The dynamic processes of conformational changes of supramolecules are important to understand the motion in synthetic supramolecules. Although a host-guest complex is the most basic supramolecule, a detailed mechanism of its conformational changes has rarely been studied. Here, we observed the large conformational change of a dibenzo-24-crown-8 complex with four guest ions (Ag+, Na+, K+, and NH4+) at low temperature in the gas phase. The isomerization between the two types of conformers, which have different distances between the two benzene rings, proceeds even at 86 K. Using variable-temperature ion mobility-mass spectrometry (IM-MS) at 100-210 K, the activation energy for the isomerization is determined to be rather small (4.8-9.0 kJ mol-1). Reaction pathway calculations revealed that the isomerization is caused by the sequential rotation of two single bonds in the crown ether ring. The present cryogenic IM-MS study of the host-guest complexes at the molecular level opens an approach to detailed understanding of the motion in supramolecules.

Geometrical Structures of Gas-Phase Cerium Oxide Cluster Cations after Reaction with Nitric Oxide Studied by Ion Mobility Mass Spectrometry.

Cerium oxide cluster cations were reacted with nitric oxide molecules and then measured by ion mobility mass spectrometry (IMMS). CenO2n+1N+ species appeared as products of the reaction CenO2n+ + NO → CenO2n+1N+, and their collision cross sections (CCSs) with helium were obtained by IMMS. The experimental CCSs of CenO2n+1N+ were 2-6 Å2 larger than those of CenO2n+ for n = 4-10. Geometrical structures of Ce4O9N+ and Ce5O11N+ were assigned by comparing experimental CCSs with theoretically calculated CCSs of candidate structures. The suggested structures showed that the adsorbed NO molecule is oxidized by the CenO2n+ cluster into a nitrite (NO2-) or nitrate (NO3-). The CenO2n+1N+ species are regarded as intermediates of the NO oxidation reaction CenO2n+ + NO → CenO2n-1+ + NO2, and therefore, the present results are helpful for understanding redox reactions involving gas-phase CenO2n+ cluster ions.

Conformer Separation of Dibenzo-Crown-Ether Complexes with Na+ and K+ Ions Studied by Cryogenic Ion Mobility-Mass Spectrometry.

We performed cryogenic ion mobility-mass spectrometry (IM-MS) to study conformations of dibenzo-crown-ether complexes with Na+ and K+ ions at 86 K in the gas phase. Four dibenzo-crown-ethers (dibenzo-18-crown-6, dibenzo-21-crown-7, dibenzo-24-crown-8, and dibenzo-30-crown-10) with different cavity ring sizes were investigated. For dibenzo-18-crown-6 complexes with Na+ and K+, only one type of conformer was assigned by comparing the experimental collision cross sections with those predicted theoretically for candidate structures. In this conformer, the distance between two benzene rings in the complexes was long due to the open form of the dibenzo-18-crown-6. This open conformer was consistent with the previous laser spectroscopic studies of the cold complex ions in the gas phase. For dibenzo-21-crown-7 and dibenzo-24-crown-8 complexes with Na+ and K+, two types of conformers were clearly separated by IM-MS. These two conformer types were assigned to "open" and "closed" forms in which benzene-benzene distances were long and short, respectively. Observed relative abundances of the open and closed conformers qualitatively agreed with the Boltzmann distribution using Gibbs energies of the conformers calculated by quantum chemical calculations. For the Na+(dibenzo-30-crown-10) complex, open and closed conformers were also observed in IM-MS. On the other hand, only the closed conformer was observed for the K+(dibenzo-30-crown-10) complex. This closed conformer was similar to the "wraparound" structure, which was proposed in the previous studies in the solution. In conclusion, the closed conformers were formed by the deformation of flexible crown ethers with large cavity ring sizes. In addition, the diameter of the K+ ion was suitable to form the closed conformer by deformation of the molecular structure of dibenzo-30-crown-10.

Photofragment ion imaging in vibrational predissociation of the H2O+Ar complex ion.

Vibrational predissociation processes of the H2O+Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H2O+ unit were studied by photofragment ion imaging. The anisotropy parameters, ß, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, ß > 0 for the P-branch excitations, while ß < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H2O+ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H2O+ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H2O+ fragment ions was discussed based on the steric configuration of the H2O+ and Ar units at the moment of dissociation.

Long-distance proton transfer induced by a single ammonia molecule: ion mobility mass spectrometry of protonated benzocaine reacted with NH3.

Long-distance proton transfer is a ubiquitous phenomenon in chemical and biological systems. Two mechanisms of proton transfer in solids are well established; the Grotthuss mechanism (proton-relay) and the vehicle mechanism. Previously, intramolecular proton transfer has been extensively studied in the gas phase to understand the proton transfer mechanism microscopically. However, only the Grotthuss mechanism was proposed so far for intramolecular proton transfer. Here we show the first evidence for long-distance proton transfer (ca. 0.7 nm) via the vehicle mechanism in a gas-phase protonated molecule. Using ion mobility mass spectrometry, we observed that intramolecular proton transfer between two structural isomers with different protonation sites of protonated benzocaine (BC; p-NH2C6H4COOC2H5) is induced by a single NH3 molecule. In combination with theoretical calculations of the reaction pathway for the bimolecular reaction of BC·H+ + NH3, it was concluded that intramolecular proton transfer to produce the O-protomer (protonated BC at the C[double bond, length as m-dash]O group) proceeds in the N-protomer (protonated BC at the NH2 group) by NH3 coordination. In the calculated pathway, the NH4+ ion formed by proton transfer from the NH2 group of the N-protomer to NH3 donates a proton to the C[double bond, length as m-dash]O group after hopping on the benzene ring of BC. Our results demonstrate that we can investigate microscopically not only the Grotthuss mechanism but also the vehicle mechanism using gas-phase spectroscopic methods.

Sequential growth of iridium cluster anions based on simple cubic packing.

Geometric structures of free iridium cluster anions, Irn-, were examined by means of ion mobility mass spectrometry and density functional theory calculation for n = 3-15 with the additional help of photoelectron spectroscopy for n = 4-10. It has been revealed that Irn- clusters with n ≥ 5 favor a square facet and take a cubic motif in contrast to the face-centered cubic structures in the corresponding nanoparticles and bulk. A growth sequence of Irn- for n = 5-15 is proposed: single Ir atoms are sequentially attached to one side of the square plane of Ir4- to form a cubic Ir8-, and are then continuously attached on one of the square facets of Ir8- for n = 9-12 and Ir12- for n = 13-15.

Intramolecular Dispersion Attraction in Tetraalkylammonium Cations Revealed by Cryogenic Ion Mobility Mass Spectrometry.

We performed cryogenic ion mobility mass spectrometry and quantum chemical calculations of tetraalkylammonium (TAA) cations, (CnH2n+1)4N+ (n = 4-8), to study geometrical structures of TAA cations. We measured collision cross sections (CCSs) of TAA cations with He buffer gas atoms at 86 K. In addition, CCSs were calculated for optimized structures of TAA ions to compare with experimental CCSs. For n = 4 and 5, calculated CCSs of nearly planar conformers with all-trans four alkyl chains agree well with experimental CCSs. On the other hand, for n = 8, calculated CCSs of a conformer with two gauche alkyl chains reproduce experimental CCSs. The structural transition from all-trans to gauche conformers occurs around n = 6-8. The dispersion attraction between alkyl chains is a major interaction to stabilize the gauche conformer of n = 8.

Structures of Magnesium Oxide Cluster Cations Studied Using Ion Mobility Mass Spectrometry.

Structural assignments of gas-phase magnesium oxide cluster cations, MgnOn+ (n ≤ 24), have been achieved from a comparison of experimental collision cross sections (CCSs) measured using ion mobility mass spectrometry and theoretical CCSs calculated for equilibrium structures optimized by quantum chemical calculations. Various structures based on rock-salt and hexagonal-tube structures were assigned for n = 5-13 and 15. On the other hand, only rock-salt type structures were assigned for n = 4, 14, 16-21, and 24. The CCS values and the total energies were close for the hexagonal-tube and rock-salt structures of a given size in the small size range (n ≤ 15 except for 4 and 14). The hexagonal-tube structures of the cluster ions with n ≥ 16 were less stable and had larger CCSs than the rock-salt structures. These results indicate that the structures of the MgnOn+ clusters were changed from mixtures of rock-salt and hexagonal-tube structures to pure rock-salt structures with the growth of the cluster size. All atoms in the hexagonal-tube structures are located on the surface of the clusters, even if the cluster size increases. In contrast, the assigned rock-salt structures with n = 13, 14, and n ≥ 17 had atoms inside the clusters, which means that the average coordination numbers are substantially higher for the rock-salt structures than for the hexagonal tube structures. The structural change from the mixed structures to pure rock-salt with an increase in size n can be attributed to this difference in the coordination number.

Conformation of K+(Crown Ether) Complexes Revealed by Ion Mobility-Mass Spectrometry and Ultraviolet Spectroscopy.

The conformation and electronic structure of dibenzo-24-crown-8 (DB24C8) complexes with K+ ion were examined by ion mobility-mass spectrometry (IM-MS), ultraviolet (UV) photodissociation (UVPD) spectroscopy in the gas phase, and fluorescence spectroscopy in solution. Three structural isomers of DB24C8 (SymDB24C8, Asym1DB24C8, and Asym2DB24C8) in which the relative positions of the two benzene rings were different from each other were investigated. The IM-MS results at 86 K revealed a clear separation of two sets of conformers for the K+(SymDB24C8) and K+(Asym1DB24C8) complexes whereas the K+(Asym2DB24C8) complex revealed only one set. The two sets of conformers were attributed to the open and closed forms in which the benzene-benzene distances in the complexes were long (>6 Å) and short (<6 Å), respectively. IM-MS at 300 K could not separate the two conformer sets of the K+(SymDB24C8) complex because the interconversion between the open and closed conformations occurred at 300 K and not at 86 K. The crown cavity of DB24C8 was wrapped around the K+ ion in the complex, although the IM-MS results availed direct evidence of rapid cavity deformation and the reconstruction of stable conformers at 300 K. The UVPD spectra of the K+(SymDB24C8) and K+(Asym1DB24C8) complexes at ∼10 K displayed broad features that were accompanied by a few sharp vibronic bands, which were attributable to the coexistence of multiple conformers. The fluorescence spectra obtained in a methanol solution suggested that the intramolecular excimer was formed only in K+(SymDB24C8) among the three complexes because only SymDB24C8 could possibly assume a parallel configuration between the two benzene rings upon K+ encapsulation. The encapsulation methods for K+ ion (the "wraparound" arrangement) are similar in the three structural isomers of DB24C8, although the difference in the relative positions of the two benzene rings affected the overall cross-section. This study demonstrated that temperature-controlled IM-MS coupled with the introduction of appropriate bulky groups, such as aromatic rings to host molecules, could reveal the dynamic aspects of encapsulation in host-guest systems.

Visible photodissociation of the CO2 dimer cation: fast and slow dissociation dynamics in the excited state.

Velocity and angular distributions of photofragment CO2+ ions produced from mass-selected (CO2)2+ at 532 nm excitation were observed in an ion imaging experiment. The velocity distribution was assigned to two components, fast and slow velocity components, which was consistent with the previous study by Bowers et al. The anisotropy parameters of the angular distributions for the fast and slow velocity components were experimentally determined to be ßfast = 1.52 ± 0.14 and ßslow = 0.46 ± 0.10, respectively. In the theoretical approach, potential energy surfaces (PESs) of (CO2)2+ were calculated along two coordinates, the intermolecular distance and mutual orientations of the CO2 monomers. In addition, molecular dynamics simulations were performed. The visible transition of the most stable staggered structure of (CO2)2+ was attributed to C[combining tilde]2Ag ← X[combining tilde]2Bu by an excited state calculation. On the PES of the C[combining tilde] state, a potential well was found in which the two CO2 monomers lay side by side to each other, in addition to a repulsive slope along the intermolecular distance. The results of the simulations confirmed that the fragment CO2+ ions with fast velocity and large anisotropy originated from the rapid dissociation of (CO2)2+ on the repulsive slope. Meanwhile, the fragment CO2+ ions with slow velocity and small anisotropy were expected to emerge from statistical dissociation after large amplitude libration of CO2 molecules which was caused by the potential well in the excited state PES.

Ion Imaging of MgI+ Photofragment in Ultraviolet Photodissociation of Mass-Selected Mg+ICH3 Complex.

We have observed images of MgI+ fragment ions produced in ultraviolet laser photodissociation of mass-selected Mg+ICH3 ions at 266 nm. Split distribution almost perpendicular to the polarization direction of the photolysis laser was observed in the photofragment image. Potential energy curves of Mg+ICH3 were obtained by theoretical calculations. Among these curves, the excited complex ion dissociated along almost repulsive potentials with several avoided crossings, which was connected to MgI+ + CH3. In the ground state of Mg+ICH3, the CH3I was bonded with Mg from the iodine side, and the Mg-I-C bond angle was calculated to be 101.1°. The theoretical results also indicated that the dissociation occurred after the 52A' ← 12A' photoexcitation, where the transition dipole moment was almost parallel to the Mg-I bond axis. The MgI+ and CH3 fragments dissociated each other parallel to the direction connecting those center-of-masses, which was 67° with respect to the transition dipole moment of 52A' ← 12A' photoexcitation. Therefore, the fragment recoil direction was assumed to approach perpendicular tendency against the polarization direction under the fast dissociation process. However, calculated potential energy curves showed a complicated reaction pathway for MgI+ production, including nonadiabatic processes, although the experimental results indicated the fast dissociation reaction.

Correlation between Electronic Shell Structure and Inertness of Cu n+ toward O2 Adsorption at n = 15, 21, 41, and 49.

The inertness of metal clusters in air is important for their application to novel materials and catalysts. The adsorption reactivity of copper clusters with O2 has been discussed in connection with the electronic structure of clusters because of its importance in electron transfer from the cluster to O2. Mass spectrometry was used to observe the reaction of Cu n+ + O2 ( n = 13-60) in the gas phase. For O2 adsorption on Cu n+, the relative rate constants of the n = 15, 21, 41, and 49 clusters were clearly lower than those with other n. Theoretical calculations indicated that the inertness of Cu15+ with 14 valence electrons was related to the large HOMO-LUMO gap predicted for the oblate Cu15+ structure. The Clemenger-Nilsson model was used to predict that the electronic subshell of oblate Cu49+ with 48 electrons was closed. This electronic shell closing of Cu49+ corresponds to the inertness for O2 adsorption.

Compositions and structures of niobium oxide cluster ions, NbmOn±, (m = 2-12), revealed by ion mobility mass spectrometry.

Herein, the compositions and geometrical structures of niobium oxide cluster ions were studied and compared with those of the lighter Group 5 counterpart vanadium oxide cluster ions by ion-mobility mass spectrometry (IM-MS). As a result of collision-induced dissociation in IM-MS, the compositions were found to be dependent on an odd and even number of niobium atoms, whereby the ions with (NbO2)(Nb2O5)(m-1)/2+ and (NbO3)(Nb2O5)(m-1)/2- were identified as stable compositions for an odd number of Nb atoms, whereas (Nb2O5)m/2± and (Nb2O6)(Nb2O5)(m-2)/2- were identified as stable compositions for an even number of Nb atom clusters. Furthermore, structural transitions were observed between m = 8 and 9 for cluster cations and m = 7 and 8 for cluster anions for experimental collision cross-sections (CCSs), which were determined from the arrival times in the ion-mobility measurements. Quantum chemical calculations were conducted on several structural candidates of these compositions for m = 2-12. For cluster cations with the sizes between m = 2 and 8 and cluster anions with m = 2-7, the structures were found to be similar to those of vanadium oxide cluster ions upon comparing the experimental CCSs with the theoretical CCSs of optimized structures. As compared to the vanadium oxide cluster ions, niobium oxide cluster cations with m ≥ 9 and anions with m ≥ 8 consisted of structures where some niobium atoms had more than five oxygen-atom coordination; thus, compact structures could be achieved in the case of niobium oxide cluster ions.

Geometrical Structures of Gas Phase Chromium Oxide Cluster Anions Studied by Ion Mobility Mass Spectrometry.

Structural assignments of gas phase chromium oxide cluster anions, CrmOn- (m = 1-7), have been achieved by comparison between experimental collision cross sections measured by ion mobility mass spectrometry and theoretical collision cross sections of optimized structures by quantum chemical calculations. In the mass spectrum, significant magic behavior between the numbers m and n was not observed for CrmOn-, while wide ranges of compositions were observed around CrmO2m+2- to (CrO3)m- as reported previously. The (CrO3)m- (m = 3-7) ions were assigned to have monocyclic-ring structures for m = 3-5 and bicyclic rings for m = 6 and 7. In addition, gradual structural change from these cyclic structures of (CrO3)m- to three-dimensional structures of CrmO2m+2- was found for m = 4-7. The energy levels of molecular orbitals of a calculated monocyclic structure of Cr5O15- were also found to be consistent with previous results of photoelectron spectroscopy, although those of the bicyclic isomer exhibited a different behavior. Moreover, the observation of abundant ions generated by collision induced dissociations at the inlet of the ion drift cell indicates that the larger sized (CrO3)m- (m > 5) series were unstable and easily dissociated to smaller ions.

Photofragment ion imaging from mass-selected Mg+BrCH3 complex: Dissociation mechanism following photoinduced charge transfer.

We have observed fragment ion images produced by ultraviolet photodissociation of Mg+BrCH3 complex ions using a reflectron time-of-flight mass spectrometer combined with an imaging detector. The BrCH3+ fragment ion was produced after the 266-nm excitation of Mg+BrCH3. In the image of the BrCH3+ ions, a split distribution was observed parallel to the polarization direction of the photolysis laser. In calculated potential energy curves, we found a repulsive potential correlated with a dissociation limit of Mg + BrCH3+: The calculation results indicate that the dissociation and the charge transfer occurred via non-adiabatic process after the 52A' ← 12A' photoexcitation. The obtained energy and angular distributions of BrCH3+ photofragments were consistent with the fast BrCH3+ formation process on the repulsive potential energy curve.