Theoretical Study of Nascent Solvation in Ni⁺(Benzene)_<i>m</i>, <i>m</i> = 3 and 4, Clusters

The ligand versus solvent behavior of Ni⁺(C₆H₆)_3,4 complexes was studied using density functional theory all-electron calculations. Dispersion corrections were included with the BPW91-D2 method using the 6-311++G­(2d,2p) basis set. The ground state (GS) for Ni⁺(C₆H₆)₃ has three benzene rings 3d−π bonded to the metal. A two-layer isomer with two moieties coordinated η³–η² with Ni⁺, and the other one adsorbed by van der Waals interactions to the Ni⁺(C₆H₆)₂ subcluster, i.e., a 2 + 1 structure, is within about 8.4 kJ/mol of the GS. Structures with 3 + 1 and 2 + 2 ligand coordination were found for Ni⁺(C₆H₆)₄. The binding energies (<i>D</i>₀) of 28.9 and 26.0 kJ/mol for the external moieties of Ni⁺(C₆H₆)_3,4 are much smaller than that for Ni⁺(C₆H₆)₂, 193.0 kJ/mol, obtained also with BPW91-D2. This last <i>D</i>₀ overestimates somehow the experimental value, of 146.7 ± 11.6 kJ/mol, for Ni⁺(C₆H₆)₂. The abrupt fall for <i>D</i>₀(Ni⁺(C₆H₆)_3,4) shows that such molecules are bound externally as solvent species. These results agree with the <i>D</i>₀(Ni⁺(C₆H₆)₃) < 37.1 kJ/mol limit found experimentally for this kind of two-layer clusters. The ionization energies also decrease for <i>m</i> = 2, 3, and 4 (580.8, 573.1, and 558.6 kJ/mol). For Ni⁺(C₆H₆)_3,4, each solvent moiety bridges the benzenes of Ni⁺(C₆H₆)₂; their position and that of one internal ring mimics the tilted T-shape geometry of the benzene dimer (Bz₂). The distances from the center of the external to the center of the internal rings for <i>m</i> = 3 (4.686 Å) and <i>m</i> = 4 (4.523 Å) are shorter than that for Bz₂ (4.850 Å). This and charge transfer effects promote the (C^δ−–H^δ+)_int dipole−π_ext interactions in Ni⁺(C₆H₆)_3,4; π–π interactions also occur. The predicted IR spectra, having multiplet structure in the C–H region, provide insight into the experimental spectra of these ions

Uranium Oxo and Superoxo Cations Revealed Using Infrared Spectroscopy in the Gas Phase

The UO₄⁺ and UO₆⁺ cations are produced in a supersonic molecular beam by laser vaporization and studied with infrared laser photodissociation spectroscopy using rare gas atom predissociation. The argon complexes UO₄⁺Ar₂ and UO₆⁺Ar₂ are mass-selected in a reflectron time-of-flight spectrometer and excited with an IR-OPO laser system in the range of the O–U–O and O–O stretching vibrations. These same systems are studied with computational quantum chemistry. UO₄⁺ is found to have a central UO₂ core, with an additional η² coordinated oxygen molecule. Charge transfer/oxidation gives the system the character of a UO₂²⁺, O₂^– ion pair. UO₆⁺ has this same core structure, with an additional weakly bound oxygen molecule in an η¹ coordination configuration. The O–U–O stretch is sensitive to the local environment and approximates the vibration of the isolated uranyl cation in these systems

Infrared Spectroscopy of Solvation in Small Zn⁺(H₂O)_<i>n</i> Complexes

Singly charged zinc-water cations are produced in a pulsed supersonic expansion source using laser vaporization. Zn⁺(H₂O)_<i>n</i> (<i>n</i> = 1–4) complexes are mass selected and studied with infrared laser photodissociation spectroscopy, employing the method of argon tagging. Density functional theory (DFT) computations are used to obtain the structures and vibrational frequencies of these complexes and their isomers. Spectra in the O–H stretching region show sharp bands corresponding to the symmetric and asymmetric stretches, whose frequencies are lower than those in the isolated water molecule. Zn⁺(H₂O)_<i>n</i>Ar complexes with <i>n</i> = 1–3 have O–H stretches only in the higher frequency region, indicating direct coordination to the metal. The Zn⁺(H₂O)_2–4Ar complexes have multiple bands here, indicating the presence of multiple low energy isomers differing in the attachment position of argon. The Zn⁺(H₂O)₄Ar cluster uniquely exhibits a broad band in the hydrogen bonded stretch region, indicating the presence of a second sphere water molecule. The coordination of the Zn⁺(H₂O)_<i>n</i> complexes is therefore completed with three water molecules

Theoretical Study of Nascent Hydration in the Fe⁺(H₂O)_<i>n</i> System

The interactions of the iron monocation with water molecules and argon atoms in the gas phase were studied computationally to elucidate recent infrared vibrational spectroscopy on this system. These calculations employ first-principles all-electron methods performed with B3LYP/DZVP density functional theory. The ground state of Fe⁺(H₂O) is found to be a quartet (<i>M</i> = 2<i>S</i> + 1 = 4, <i>S</i> is the total spin). Different binding sites for the addition of one or two argon atoms produce several low-lying states of different geometry and multiplicity in a relatively small energy range for Fe⁺(H₂O)–Ar₂ and Fe⁺(H₂O)₂–Ar. In both species, quartet states are lowest in energy, and sextets and doublets lie at higher energies from the respective ground states. These results are consistent with the conclusion that the experimentally determined infrared photodissociation spectra (IRPD) of Fe⁺(H₂O)–Ar₂ and Fe⁺(H₂O)₂–Ar are complicated because of the presence of multiple isomeric structures. The estimated IR bands for the symmetric and asymmetric O–H stretches from different isomers provide new insight into the observed IRPD spectra

Imaging Charge Transfer in a Cation−π System: Velocity-Map Imaging of Ag⁺(benzene) Photodissociation

Ag⁺(benzene) complexes are generated in the gas phase by laser vaporization and mass selected in a time-of-flight spectrometer. UV laser excitation at either 355 or 266 nm results in dissociative charge transfer (DCT), leading to neutral silver atom and benzene cation products. Kinetic energy release in translationally hot benzene cations is detected using a new instrument designed for photofragment imaging of mass-selected ions. Velocity-map imaging and slice imaging techniques are employed. In addition to the expected translational energy release, DCT of Ag⁺(benzene) produces a distribution of internally hot benzene cations. Compared with experiments at 355 nm, 266 nm excitation produces only slightly higher translational excitation and a much greater fraction of internally hot benzene ions. The maximum kinetic energy release in the photodissociation sets an upper limit on the Ag⁺(benzene) dissociation energy of 32.8 (+1.4/–1.5) kcal/mol

IR Spectroscopy of Gas Phase V(CO₂)_<i>n</i>⁺ Clusters: Solvation-Induced Electron Transfer and Activation of CO₂

Ion–molecule complexes of vanadium and CO₂, i.e., V­(CO₂)_<i>n</i>⁺, produced by laser vaporization are mass selected and studied with infrared laser photodissociation spectroscopy. Vibrational bands for the smaller clusters (<i>n</i> < 7) are consistent with CO₂ ligands bound to the metal cation via electrostatic interactions and/or attaching as inert species in the second coordination sphere. All IR bands for these complexes are consistent with intact CO₂ molecules weakly perturbed by cation binding. However, multiple new IR bands occur only in larger complexes (<i>n</i> ≥ 7), indicating the formation of an intracluster reaction product whose nominal mass is the same as that of V­(CO₂)_<i>n</i>⁺ complexes. Computational studies and the comparison of predicted spectra for different possible reaction products allow identification of an oxalate-type C₂O₄ anion species in the cluster. The activation of CO₂ producing this product occurs via a solvation-induced metal→ligand electron transfer reaction

Mid-Infrared Spectroscopy of C₇H₇⁺ Isomers in the Gas Phase: Benzylium and Tropylium

Both prominent C₇H₇⁺ isomers, the benzylium and the tropylium cations, were generated in an electrical discharge/supersonic expansion from toluene and cycloheptatriene precursors. Their infrared spectra were measured in the region of 1000–3500 cm^–1 using photodissociation of the respective argon- and nitrogen-tagged complexes with a broadly tunable OPO/OPA laser system. Spectral signatures of both isomers were observed independent of the precursor, albeit in different relative intensities. The spectra were assigned based on scaled harmonic B3LYP-D3/cc-pVTZ frequency computations and comparisons to previous experimental studies. Consistent with its high symmetry, only two bands were observed for the (nitrogen-tagged) tropylium ion at 3036 and 1477 cm^–1, corresponding to C–H stretching and C–C–H deformation/CC stretching vibrations, respectively. Furthermore, the C–H stretching region of the benzylium ion is reported for the first time

Spectroscopy of Proton Coordination with Ethylenediamine

Protonated ethylenediamine monomer, dimer, and trimer were produced in the gas phase by an electrical discharge/supersonic expansion of argon seeded with ethylenediamine (C₂H₈N₂, <b>en</b>) vapor. Infrared spectra of these ions were measured in the region from 1000 to 4000 cm^–1 using laser photodissociation and argon tagging. Computations at the CBS-QB3 level were performed to explore possible isomers and understand the infrared spectra. The protonated monomer exhibits a <i>gauche</i> conformation and an intramolecular hydrogen bond. Its parallel shared proton vibration occurs as a broad band around 2785 cm^–1, despite the formally equivalent proton affinities of the two amino groups involved, which usually leads to low frequency bands. The barrier to intramolecular proton transfer is 2.2 kcal mol^–1 and does not vanish upon addition of the zero-point energy, unlike the related protonated ammonia dimer. The structure of the dimer is formed by chelation of the monomer’s NH₃⁺ group, thereby localizing the excess proton and increasing the frequency of the intramolecular shared proton vibration to 3157 cm^–1. Other highly fluxional dimer structures with facile intermolecular proton transfer and concomitant structural reorganization were computed to lie within 2 kcal mol^–1 of the experimentally observed structure. The spectrum of the trimer is rather diffuse, and a clear assignment is not possible. However, an isomer with an intramolecular proton transfer like that of the monomer is most consistent with the experimental spectrum

Testing the Limits of the 18-Electron Rule: The Gas-Phase Carbonyls of Sc⁺ and Y⁺

Scandium and yttrium carbonyl cations produced in the gas phase via laser vaporization are mass selected and studied with infrared laser spectroscopy in the C–O stretching region. Mass spectra, ion fragmentation behavior, and infrared spectra, complemented by computational chemistry, establish the coordination numbers and structures of these complexes. Sc⁺ does not form the eight-coordinate 18-electron complex but instead produces a 16-electron seven-coordinate species. However, Y⁺ forms the anticipated eight-coordinate structure. Density functional theory computations provide structures and corresponding vibrational spectra for these complexes. Sc­(CO)₇⁺ has a C_3v capped octahedral structure, while Y­(CO)₈⁺ forms a D_4d square antiprism. The C–O stretches at 2086 and 2087 cm^–1 for Sc­(CO)₇⁺ and Y­(CO)₈⁺, respectively, are among the most red-shifted frequencies measured for any transition metal carbonyl cation

Coordination and Spin States in Vanadium Carbonyl Complexes (V(CO)_<i>n</i>⁺, <i>n</i> = 1–7) Revealed with IR Spectroscopy

The vibrational spectra of vanadium carbonyl cations of the form V­(CO)_<i>n</i>⁺, where <i>n</i> = 1–7, were obtained via mass-selected infrared laser photodissociation spectroscopy in the carbonyl stretching region. The cations and their argon and neon “tagged” analogues were produced in a molecular beam via laser vaporization in a pulsed nozzle source. The relative intensities and frequency positions of the infrared bands observed provide distinctive patterns from which information on the coordination and spin states of these complexes can be obtained. Density functional theory is carried out in support of the experimental spectra. Infrared spectra obtained by experiment and predicted by theory provide evidence for a reduction in spin state as the ligand coordination number increases. The octahedral V­(CO)₆⁺ complex is the fully coordinated experimental species. A single band at 2097 cm^–1 was observed for this complex red-shifted from the free CO vibration at 2143 cm^–1