19 research outputs found
Theoretical Study of Nascent Solvation in Ni<sup>+</sup>(Benzene)<sub><i>m</i></sub>, <i>m</i> = 3 and 4, Clusters
The ligand versus solvent behavior
of Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>3,4</sub> 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<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>3</sub> has three benzene rings 3d−π bonded to
the metal. A two-layer isomer with two moieties coordinated η<sup>3</sup>–η<sup>2</sup> with Ni<sup>+</sup>, and the other
one adsorbed by van der Waals interactions to the Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>2</sub> 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<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>4</sub>. The binding energies (<i>D</i><sub>0</sub>) of 28.9 and 26.0 kJ/mol for the external moieties of
Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>3,4</sub> are much
smaller than that for Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>, 193.0 kJ/mol, obtained also with BPW91-D2. This last <i>D</i><sub>0</sub> overestimates somehow the experimental value,
of 146.7 ± 11.6 kJ/mol, for Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>. The abrupt fall for <i>D</i><sub>0</sub>(Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>3,4</sub>)
shows that such molecules are bound externally as solvent species.
These results agree with the <i>D</i><sub>0</sub>(Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>3</sub>) < 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<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>3,4</sub>, each solvent moiety bridges the benzenes
of Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>; their position
and that of one internal ring mimics the tilted T-shape geometry of
the benzene dimer (Bz<sub>2</sub>). 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 Bz<sub>2</sub> (4.850 Å). This and charge
transfer effects promote the (C<sup>δ−</sup>–H<sup>δ+</sup>)<sub>int</sub> dipole−π<sub>ext</sub> interactions in Ni<sup>+</sup>(C<sub>6</sub>H<sub>6</sub>)<sub>3,4</sub>; 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 UO<sub>4</sub><sup>+</sup> and UO<sub>6</sub><sup>+</sup> 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 UO<sub>4</sub><sup>+</sup>Ar<sub>2</sub> and UO<sub>6</sub><sup>+</sup>Ar<sub>2</sub> 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. UO<sub>4</sub><sup>+</sup> is found to have a central UO<sub>2</sub> core, with an additional η<sup>2</sup> coordinated oxygen molecule. Charge transfer/oxidation gives the system the character of a UO<sub>2</sub><sup>2+</sup>, O<sub>2</sub><sup>–</sup> ion pair. UO<sub>6</sub><sup>+</sup> has this same core structure, with an additional weakly bound oxygen molecule in an η<sup>1</sup> 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<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> Complexes
Singly charged zinc-water cations
are produced in a pulsed supersonic
expansion source using laser vaporization. Zn<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> (<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<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>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<sup>+</sup>(H<sub>2</sub>O)<sub>2–4</sub>Ar complexes have multiple bands here, indicating
the presence of multiple low energy isomers differing in the attachment
position of argon. The Zn<sup>+</sup>(H<sub>2</sub>O)<sub>4</sub>Ar
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<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> complexes is therefore completed with three water
molecules
Theoretical Study of Nascent Hydration in the Fe<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> 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<sup>+</sup>(H<sub>2</sub>O) 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<sup>+</sup>(H<sub>2</sub>O)–Ar<sub>2</sub> and Fe<sup>+</sup>(H<sub>2</sub>O)<sub>2</sub>–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<sup>+</sup>(H<sub>2</sub>O)–Ar<sub>2</sub> and Fe<sup>+</sup>(H<sub>2</sub>O)<sub>2</sub>–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
IR Spectroscopy of Gas Phase V(CO<sub>2</sub>)<sub><i>n</i></sub><sup>+</sup> Clusters: Solvation-Induced Electron Transfer and Activation of CO<sub>2</sub>
Ion–molecule complexes of vanadium and CO<sub>2</sub>, i.e.,
V(CO<sub>2</sub>)<sub><i>n</i></sub><sup>+</sup>, 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 CO<sub>2</sub> 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 CO<sub>2</sub> 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(CO<sub>2</sub>)<sub><i>n</i></sub><sup>+</sup> complexes. Computational studies and
the comparison of predicted spectra for different possible reaction
products allow identification of an oxalate-type C<sub>2</sub>O<sub>4</sub> anion species in the cluster. The activation of CO<sub>2</sub> producing this product occurs via a solvation-induced metal→ligand
electron transfer reaction
Imaging Charge Transfer in a Cation−π System: Velocity-Map Imaging of Ag<sup>+</sup>(benzene) Photodissociation
Ag<sup>+</sup>(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<sup>+</sup>(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<sup>+</sup>(benzene) dissociation energy of
32.8 (+1.4/–1.5) kcal/mol
Coordination and Spin States in Vanadium Carbonyl Complexes (V(CO)<sub><i>n</i></sub><sup>+</sup>, <i>n</i> = 1–7) Revealed with IR Spectroscopy
The vibrational spectra of vanadium carbonyl cations
of the form V(CO)<sub><i>n</i></sub><sup>+</sup>, 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)<sub>6</sub><sup>+</sup> complex is the fully coordinated experimental
species. A single band at 2097 cm<sup>–1</sup> was observed
for this complex red-shifted from the free CO vibration at 2143 cm<sup>–1</sup>
Mid-Infrared Spectroscopy of C<sub>7</sub>H<sub>7</sub><sup>+</sup> Isomers in the Gas Phase: Benzylium and Tropylium
Both
prominent C<sub>7</sub>H<sub>7</sub><sup>+</sup> 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<sup>–1</sup> 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<sup>–1</sup>, corresponding
to C–H stretching and C–C–H deformation/CC
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 (C<sub>2</sub>H<sub>8</sub>N<sub>2</sub>, <b>en</b>) vapor. Infrared spectra of these ions were
measured in the region from 1000 to 4000 cm<sup>–1</sup> 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<sup>–1</sup>, 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<sup>–1</sup> 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 NH<sub>3</sub><sup>+</sup> group, thereby localizing the excess proton and increasing the frequency
of the intramolecular shared proton vibration to 3157 cm<sup>–1</sup>. Other highly fluxional dimer structures with facile intermolecular
proton transfer and concomitant structural reorganization were computed
to lie within 2 kcal mol<sup>–1</sup> 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<sup>+</sup> and Y<sup>+</sup>
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<sup>+</sup> does not form the eight-coordinate 18-electron complex
but instead produces a 16-electron seven-coordinate species. However,
Y<sup>+</sup> forms the anticipated eight-coordinate structure. Density
functional theory computations provide structures and corresponding
vibrational spectra for these complexes. Sc(CO)<sub>7</sub><sup>+</sup> has a C<sub>3v</sub> capped octahedral structure, while Y(CO)<sub>8</sub><sup>+</sup> forms a D<sub>4d</sub> square antiprism. The
C–O stretches at 2086 and 2087 cm<sup>–1</sup> for Sc(CO)<sub>7</sub><sup>+</sup> and Y(CO)<sub>8</sub><sup>+</sup>, respectively,
are among the most red-shifted frequencies measured for any transition
metal carbonyl cation