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.

Enhanced reactivity of Li+@C60 toward thermal [2 + 2] cycloaddition by encapsulated Li+ Lewis acid.

Lithium ion-endohedral fullerene (Li+@C60), a member of the burgeoning family of ion-endohedral fullerenes, holds substantial promise for diverse applications owing to its distinctive ionic properties. Despite the high demand for precise property tuning through chemical modification, there have been only a few reports detailing synthetic protocols for the derivatization of this novel material. In this study, we report the synthesis of Li+@C60 derivatives via the thermal [2 + 2] cycloaddition reaction of styrene derivatives, achieving significantly higher yields of monofunctionalized Li+@C60 compared to previously reported reactions. Furthermore, by combining experimental and theoretical approaches, we clarified the range of applicable substrates for the thermal [2 + 2] cycloaddition of Li+@C60, highlighting the expanded scope of this straightforward and selective functionalization method.

Synthesis and Characterization of Ionic Li+ @C70 Endohedral Fullerene.

Ion-endohedral-fullerene has attracted growing interest due to the unique electronic and structural characteristics arising from its distinctive ionic nature. Although there has been only one reported ion-encapsulated fullerene, Li+ @C60 , a significant number of fundamental and applied studies have been conducted, making a substantial impact not only in chemistry and physics but also across various interdisciplinary research fields. Nevertheless, studies on ion-endohedral fullerenes are still in their infancy due to the limitations in variety, and hence, it remains an open question how the size and symmetry of fullerene, as well as the motion and position of the encapsulated ion, affect their physical/chemical properties. Herein, we report the synthesis of lithium-ion-endohedral [70]fullerene (Li+ @C70 X- , X=PF6 - and TFSI- ), a novel ionic endohedral fullerene. X-ray crystallography confirmed the encapsulation of Li+ by C70 cage as well as its ion-pair structure stabilized by external TFSI- counter anion. The encapsulated Li+ drastically lowered the orbital energy of the C70 cage by Coulomb interactions but did not affect the orbital energy gap and degeneracy. DFT studies were also performed, which supported the experimentally observed electronic effects caused by the encapsulated Li+ .

Highly Efficient Intramolecular Proton Transfer in p-Aminobenzoic Acid by a Single Ammonia Molecule as a Vehicle.

Proton transfer is classified into two mechanisms: the Grotthuss (proton-relay) and vehicle mechanisms. It has been well studied on gas-phase proton transfer by a proton relay involving multiple molecules. However, a vehicle mechanism in which a single molecule transports a proton has rarely been reported. Here, we have obtained clear evidence that the proton transfers efficiently between the two protonation sites in protonated p-aminobenzoic acid (PABA·H+) by a single ammonia molecule as a vehicle. The gaseous PABA·H+ ions were reacted with NH3 or ND3 under single-collision conditions in a cold ion trap, and the proton-transferred ions were identified by cryogenic ion mobility-mass spectrometry. A reaction intermediate PABA·H+·NH3 was also detected for the first time. The reaction pathway search calculations and ab initio molecular dynamics simulations supported the present experimental finding that intramolecular proton transfer occurs very efficiently by the vehicle mechanism.

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+.

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.

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.

Ultraviolet photodissociation of Mg+-NO complex: Ion imaging of a reaction branching in the excited states.

Ultraviolet photodissociation processes of gas phase Mg+-NO complex were studied by photofragment ion imaging experiments and theoretical calculations for excited electronic states. At 355 nm excitation, both Mg+ and NO+ photofragment ions were observed with positive anisotropy parameters, and theoretical calculations revealed that the two dissociation channels originate from an electronic transition from a bonding orbital consisting of Mg+ 3s and NO π* orbitals to an antibonding counterpart. For the NO+ channel, the photofragment image exhibited a high anisotropy (ß = 1.53 ± 0.07), and a relatively large fraction (∼40%) of the available energy was partitioned into translational energy. These observations are rationalized by proposing a rapid dissociation process on a repulsive potential energy surface correlated to the Mg(1S) + NO+(1Σ) dissociation limit. In contrast, for the Mg+ channel, the angular distribution was more isotropic (ß = 0.48 ± 0.03) and only ∼25% of the available energy was released into translational energy. The differences in the recoil distribution for these competing channels imply a reaction branching on the excited state surface. On the theoretical potential surface of the excited state, we found a deep well facilitating an isomerization from bent geometry in the Franck-Condon region to linear and/or T-shaped isomer. As a result, the Mg+ fragment was formed via the structural change followed by further relaxation to lower electronic states correlated to the Mg+(2S) + NO(2Π) exit channel.

Synthesis of neutral Li-endohedral PCBM: an n-dopant for fullerene derivatives.

Li@PCBM, the first neutral Li@C60 derivative, was synthesized. The Li@PCBM exists in a monomer-dimer equilibrium in solution but as a monomer in the PCBM matrix. The fully dispersed Li@PCBM n-doped the surrounding empty PCBM, raising the Fermi level by 0.13 eV compared with the undoped PCBM film. The hybrid films were utilized as an ETL for PSCs, promoting the efficiency of the device.

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.

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.

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.

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.

A fast and robust trajectory surface hopping method: Application to the intermolecular photodissociation of a carbon dioxide dimer cation (CO2)2.

Our recently developed trajectory surface hopping method uses numerical time derivatives of adiabatic potential gradients to estimate the nonadiabatic transition probability and the hopping direction. To demonstrate the practicality of the novel method, we applied it to the intermolecular photodissociation of a carbon dioxide dimer cation (CO2)2 +. Our simulations reproduced the measured velocity distribution of CO2 + fragments consisting of two (fast and slow) components and revealed that nonadiabatic transitions occur promptly toward the electronic ground state regardless of the fragment velocity. The structure of (CO2)2 + at optical excitation governs the fate of subsequent nonadiabatic dynamics leading to a fast or slow dissociation. Our method gave similar results to the fewest switches algorithm at lower computational expense. Our fast and robust surface hopping method is promising for the investigation of nonadiabatic dynamics in large and complex systems.

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.

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.

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.

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.

Photodissociation processes of a water-oxygen complex cation studied by an ion imaging technique.

Photochemistry of molecular complex ions in the atmosphere affects the composition, density, and growth of chemical species. Photodissociation processes of a mass-selected O2+(H2O) complex ion in the visible and ultraviolet regions were studied by ion imaging experiments and theoretical calculations. At 473 nm excitation, O2+ was the predominant photofragment ion produced. In this O2+ channel, the kinetic energy release was comparable to that estimated using a statistical dissociation model, and the anisotropy parameter was determined to be ß = 1.0 ± 0.1. On the other hand, the H2O+ photofragment ion was mainly produced at 355 nm excitation. The kinetic energy release for the H2O+ channel was large and nonstatistical, and the anisotropy parameter was ß = 1.9 ± 0.2. Theoretically, the 473 and 355 nm excitations were assigned to the B[combining tilde]2A''← X[combining tilde]2A'' and D[combining tilde]2A''← X[combining tilde]2A'' transitions, respectively, both of which were characterized by positive charge transfer from O2 to H2O subunits. To further investigate the dissociation mechanisms, potential energy curves (PECs) and surfaces (PESs) for the O2+(H2O) ion were calculated for the ground and excited states. As a result, the H2O+ channel at 355 nm excitation was explained by rapid dissociation on the repulsive PES of the D[combining tilde] state, while rapid electronic relaxation from the B[combining tilde] to X[combining tilde] state followed by dissociation in the ground state was inferred in the O2+ channel at 473 nm excitation.

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.