Argon Complex Research Articles

Tuning Intermolecular Interactions for Chiral Analysis: The Microwave Spectra and Molecular Structures of the Chiral Tag Candidates cis- and trans-2-Fluoro-3-(trifluoromethyl)oxirane and Their Gas-Phase Heterodimers with the Argon Atom.

The cis and trans isomers of the chiral tagging candidate molecule, 2-fluoro-3-(trifluoromethyl)oxirane, as well as the lowest energy gas-phase heterodimer of each with the argon atom, are characterized via quantum chemistry calculations and microwave rotational spectroscopy from 5 to 18 GHz and their ground state, vibrationally averaged structures, are determined. Apart from the cis/trans nature of the ring substitution and small differences in the dihedral angle specifying the rotation of the trifluoromethyl group, the two oxirane molecules and their respective argon complexes each have remarkable structural similarity. In contrast, the binding mode of argon to the oxirane, while similar for the two complexes here, is distinct from those modes observed in previous argon-fluorooxirane species. The ability to tune the preferred mode of binding with differing levels of fluorine substitution may prove advantageous in applications of chiral tagging to a wide variety of analytes.

The microwave spectra and molecular structures of the chiral and achiral rotamers of 2,3,3-trifluoropropene and their gas-phase heterodimers with the argon atom

The microwave spectra of two rotamers, connected by rotation of the –CHF2 group, of 2,3,3-trifluoropropene and all three of the singly substituted 13C isotopologues of each are obtained in the 5.6 – 18.1 GHz region using broadband chirped pulse Fourier transform microwave spectroscopy. The lower energy rotamer, stabilized by a weak intermolecular hydrogen bond, is chiral, existing as a pair of enantiomers, while the higher energy isomer, lacking this interaction but having a plane of symmetry, is achiral. Heterodimers of both rotamers with the argon carrier gas are also observed in the broadband spectrum and further analyzed using spectra obtained from 5.0 to 20.6 GHz with a Balle-Flygare cavity Fourier transform microwave spectrometer. Although all singly substituted 13C isotopologues are observed, in addition to the normal species, for the argon complex with the lower energy rotamer, only a single isotopologue of the argon complex, the parent, is observed with the higher energy rotamer. Despite differing slightly in detail, the argon atom occupies a similar position in the heterodimer with each rotamer, one that is also similar to the location of the argon atom in argon-2,3,3,3-tetrafluoropropene.

Benchmarking density functional theory methods for modelling cationic metal–argon complexes

Noble gas chemistry is fascinating because noble gases can make formal chemical bonds with metal ions, despite their closed electronic configuration. Argon–metal ion complexes are particularly interesting since their bonding is halfway between dispersion and covalent interactions. Although many metal ion–noble gas complexes have been synthesized, there are still disagreeing theoretical descriptions about their bonding, which is not yet fully understood. Accurate experimental data are important as solid reference for theoretical methodologies, but such data are currently scarce for complexes of a metal ion with noble gas atoms. We measured infrared spectra of MArn+ (n = 3–5; M = Au, Ag, Pd) complexes and used these spectra as benchmark data for different theory levels within the density functional theory formalism. Several basis sets, exchange–correlation functionals, and the inclusion of dispersion corrections were considered. The agreement between the measured spectra and the calculations strongly depends on the applied level of theory. Functionals of a higher level of complexity do not consistently provide a better agreement with the experiment; this is particularly the case for the B3LYP hybrid functional that performs worse than the PBE GGA functional. On the other hand, the inclusion of dispersion corrections and the use of a large basis sets are crucial for a good description of the interaction between M+ and argon atoms.

Infrared spectroscopy of RG-Co+(H2O) complexes (RG = Ar, Ne, He): The role of rare gas "tag" atoms.

RGn-Co+(H2O) cation complexes (RG = Ar, Ne, He) are generated in a supersonic expansion by pulsed laser vaporization. Complexes are mass-selected using a time-of-flight spectrometer and studied with infrared laser photodissociation spectroscopy, measuring the respective mass channels corresponding to the elimination of the rare gas "tag" atom. Spectral patterns and theory indicate that the structures of the ions with a single rare gas atom have this bound to the cobalt cation opposite the water moiety in a near-C2v arrangement. The O-H stretch vibrations of the complex are shifted compared to those of water because of the metal cation charge-transfer interaction; these frequencies also vary systematically with the rare gas atom attached. The efficiencies of photodissociation also vary with the rare gas atoms because of their widely different binding energies to the cobalt cation. The spectrum of the argon complex could only be measured when at least three argon atoms were attached. In the case of the helium complex, the low binding energy allows the spectra to be measured for the low-frequency H-O-H scissors bending mode and for the O-D stretches of the deuterated analog. The partially resolved rotational structure for the antisymmetric O-H and O-D stretches reveals the temperature of these complexes (6 K) and establishes the electronic ground state. The helium complex has the same 3B1 ground state as the tag-free complex studied previously by Metz and co-workers ["Dissociation energy and electronic and vibrational spectroscopy of Co+(H2O) and its isotopomers," J. Phys. Chem. A 117, 1254 (2013)], but the A rotational constant is contaminated by vibrational averaging from the bending motion of the helium.

The microwave spectrum and molecular structure of the gas-phase heterodimer formed between argon and glycidol

The microwave rotational spectrum of the gas-phase heterodimer formed between glycidol and the argon atom is measured from 5.6 to 18.1 GHz. Despite the existence of two low energy conformers for the glycidol molecule, the spectrum of only one argon-glycidol complex is observed, corresponding to the global minimum energy structure. Higher energy isomers, and indeed less abundant isotopologues of the observed species, are presumably not produced with sufficient number density to be detected under the conditions of the experiment. The spectroscopic constants obtained from the analysis of the spectrum are combined with ab initio quantum chemistry predictions for the dipole moment components to determine the structure of the lowest energy arrangement for argon-glycidol. This structure is compared with those previously observed for the argon complexes of analogous fluoromethyloxirane molecules.

Competition between the hydrogen bond and the halogen bond in a [CH3OH-CCl4] complex: a matrix isolation IR spectroscopy and computational study.

Methanol (CH3OH) is the simplest alcohol and carbon tetrachloride (CCl4) is widely used as a solvent in the chemical industry. CH3OH and CCl4 are both important volatile substances in the atmosphere and CCl4 is an important precursor for atmospheric ozone depletion. Moreover, mixtures of CH3OH and CCl4 are an important class of non-aqueous mixtures as they exhibit a large deviation from Raoult's law. The specific interaction between CH3OH and CCl4 is not yet investigated experimentally. The interaction between CH3OH and CCl4 at the molecular level can be twofold: hydrogen bond (O-HCl) and halogen bond (C-ClO) interaction. One halogen bonded minimum and two hydrogen bonded minima are identified in the dimer potential energy surface. Herein, the 1 : 1 complex of [CH3OH-CCl4] has been characterised using matrix-isolation infrared spectroscopy and electronic structure calculations to investigate the competition between hydrogen bonded and halogen bonded complexes. Vibrational spectra have been monitored in the C-Cl, C-O, and O-H stretching regions. The exclusive formation of halogen bonded 1 : 1 complexes in argon and nitrogen matrices is confirmed by a combination of experimental and simulated vibrational frequency, stabilisation energy, energy decomposition analysis, and natural bond orbital and atoms-in-molecules analyses. This investigation helps to understand the specific interactions in the [CH3OH-CCl4] mixture and also the possibilities of formation of halogen bonded atmospheric complexes that may influence the atmospheric chemical activities, and enhance aerosol formation and deposition of CCl4.

The microwave spectrum and molecular structure of 3,3-difluoro-1,2-epoxypropane and its complex with the argon atom

As a small, chiral molecule with a strong and simple microwave rotational spectrum, 3,3-difluoro-1,2-epoxypropane is a promising candidate as a chiral tag for conversion of enantiomeric analytes into spectroscopically distinct diastereomers via formation of a non-covalently bound complex. The Fourier transform microwave spectra of this species and its complex with the argon atom are obtained from 5.1 to 21.0 GHz, and their structures determined. Not surprisingly, these structures for both the monomer and the argon complex show strong similarities with those previously determined for 3,3,3-trifluoro-1,2-epoxypropane and 3,3,3-trifluoro-1,2-epoxypropane–argon, but are different from those of the analogous propylene oxide species. This suggests that fluoromethyl and methyl substitution affect the bonding in the epoxy moiety and intermolecular interactions in distinctly different ways.

Microwave Spectrum and Molecular Structure of the Chiral Tagging Candidate, 3,3,3-Trifluoro-1,2-epoxypropane and Its Complex with the Argon Atom.

The rotational spectrum of the chiral tagging candidate molecule, 3,3,3-trifluoro-1,2-epoxypropane (TFO), and of its heterodimer with the argon atom, is obtained using Fourier transform microwave spectroscopy from 5.6 to 18.1 GHz. With a strong, simple rotational spectrum, TFO shows promise for applications in chiral analysis through the conversion of enantiomers into spectroscopically distinct diastereomeric species through noncovalent attachment. The structure of the argon complex of TFO, determined from analysis of the microwave spectrum, is extremely similar to that previously found for ethylene oxide-argon but quite different from that suggested for propylene oxide-argon.

FTIR study of acetylacetone, D2-acetylacetone and hexafluoroacetylacetone–water complexes in argon and nitrogen matrices

Association of acetylacetone molecules with water was studied by means of infrared absorption spectroscopy aided by matrix isolation technique. The spectra of acetylacetone–water mixtures isolated in low temperature argon and nitrogen matrices revealed additional spectral bands, not observed in the spectra of pure substances, thus confirming the formation of hydrogen bonded complexes. The precise assignment of the spectral bands was performed by varying the sample concentration, performing annealing experiments and DFT B3LYP 6-311++G(3df, 3pd) calculations. Positions of the associated water bands indicate a medium strength hydrogen bond comparable to the one observed in the water trimers. The effect of hydrogen bond formation is rather minimal for the acetylacetone molecule and our experiments confirm no significant influence on an internal hydrogen bond structure or dynamics in the acetylacetone molecule. Similar conclusions are valid in the case of the D2O D2-acetylacetone complex. Different situation is observed when CH3 groups in acetylacetone are replaced with CF3 groups. The calculated energy of the complex is twice as small. This is also confirmed by a very small bounded OH stretch shift. This observation confirms that the electronic structure of the molecular groups even relatively far away from the hydrogen bond accepting atom has a large influence on its possibility to form a hydrogen bond.

Microwave spectra of 2-phenylethyl methyl ether and 2-phenylethyl methyl ether-argon: Conformation-dependent tunneling and complexation

High-resolution rotational spectra were recorded for 2-phenylethyl methyl ether and the 2-phenylethyl methyl ether – argon complex using a cavity-based Fourier transform microwave spectrometer. The spectra were assigned to the aao and g−ao monomer conformations and the g−ao – argon complex. Small tunneling splittings arising from more than one internal motion were resolved for eight transitions of the aao conformer; no tunneling splittings were observed in the spectra of the g−ao conformer or the argon complex. The calculated barriers (ωB97XD/6-311++G(d,p)) for methyl and phenyl internal rotations were found to be very similar for both conformers. The observation of tunneling splittings is limited to the aao conformer by relatively large barriers for the internal motions and sensitivity to the rotor axis angles within the two conformers.

Structural assignment of small cationic silver clusters by far-infrared spectroscopy and DFT calculations.

The structures of small cationic silver clusters Agn+ (n = 3-13) are investigated by comparing measured far-infrared multiple photon dissociation spectra of cluster-argon complexes with the calculated harmonic vibrational spectra of different low-energy structural isomers. A global structure search was carried out using the CALYPSO structure prediction method, after which isomers were locally optimized with the meta GGA functional TPSS. The obtained structures of the cationic silver clusters are mostly consistent with earlier ion mobility measurements and photodissociation spectroscopy studies for Agn+ (n = 3-11) and allowed excluding several structural isomers that were considered in those earlier studies, which illustrates the strength of combining multiple experimental techniques for conclusive structural identification. The growth pattern of the cationic silver clusters is discussed and differences with other cationic coinage metal clusters are highlighted.

Infrared spectroscopy of the Ti(H2O)Ar+ ion–molecule complex: Electronic state switching induced by argon

Titanium–water complexes are produced by laser vaporization in a pulsed supersonic expansion, mass selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy using argon complex predissociation. Density functional theory calculations provide structures and vibrational spectra for these ions. Spectra in the O–H stretching region indicate that the water is intact in these complexes, even though inserted structures are computed to be more stable. Although the ground state of Ti(H2O)+ is established to be a quartet, that predicted and observed for Ti(H2O)Ar+ is a doublet. Attachment of argon changes the electronic ground state of this system.

Structural determination of niobium-doped silicon clusters by far-infrared spectroscopy and theory.

In this work, the structures of cationic SinNb(+) (n = 4-12) clusters are determined using the combination of infrared multiple photon dissociation (IR-MPD) and density functional theory (DFT) calculations. The experimental IR-MPD spectra of the argon complexes of SinNb(+) are assigned by comparison to the calculated IR spectra of low-energy structures of SinNb(+) that are identified using the stochastic 'random kick' algorithm in conjunction with the BP86 GGA functional. It is found that the Nb dopant tends to bind in an apex position of the Sin framework for n = 4-9 and in surface positions with high coordination numbers for n = 10-12. For the larger doped clusters, it is suggested that multiple isomers coexist and contribute to the experimental spectra. The structural evolution of SinNb(+) clusters is similar to V-doped silicon clusters (J. Am. Chem. Soc., 2010, 132, 15589-15602), except for the largest size investigated (n = 12), since V takes an endohedral position in Si12V(+). The interaction with a Nb atom, with its partially unfilled 4d orbitals leads to a significant stability enhancement of the Sin framework as reflected, e.g. by high binding energies and large HOMO-LUMO gaps.

Electron-impact-induced dissociation of small argon clusters

We study electron-impact-induced dissociation of small van der Waals--bound argon complexes at a projectile energy of 120 eV. Kinetic-energy-release (KER) spectra of the ${\text{Ar}}_{2}$ and ${\text{Ar}}_{3}$ parent species for the final charge states $2{\text{Ar}}^{+}, {\text{Ar}}^{+}+{\text{Ar}}^{2+}$, and ${\text{Ar}}_{2}^{+}+{\text{Ar}}^{+}$ and electron energies have been measured together with angular distributions of fragment ions. They are used to identify dissociation mechanisms such as interatomic Coulombic decay (ICD).

The microwave spectra and molecular structures of 2-chloro-1,1-difluoroethylene and its complex with the argon atom

The microwave spectra of six isotopologues each of 2-chloro-1,1-difluoroethylene and its argon complex are obtained in the 5.6–20.5GHz region using a combination of broadband chirped pulse and Balle–Flygare cavity Fourier transform microwave techniques. The structure of the halogen substituted ethylene has been determined using a combination of theory and experiment. Argon is found to form a nonplanar complex with this haloethylene, locating in the ClCCF cavity, which maximizes its interactions with the heavy atoms in the ethylene subunit. A combined analysis of the chlorine quadrupole coupling constants of the monomer and of the complex allows the sign of the only nonzero off-diagonal component of the quadrupole coupling tensor for the monomer to be determined; thus, the complete tensor for the monomer has been derived.

Microwave Spectra and Structure of the Argon–Cyclopentanone and Neon–Cyclopentanone van der Waals Complexes

The rotational spectra of cyclopentanone and its van der Waals complexes with argon and neon have been observed with a Balle-Flygare type pulsed jet Fourier transform microwave spectrometer in the 6 to 20 GHz region. This work improves the rotational constants and quartic centrifugal distortion constants for cyclopentanone and its five (13)C and the (18)O isotopologues. The argon-(12)C5H8(16)O van der Waals complex has rotational constants of A = 2611.6688, B = 1112.30298, and C = 971.31969 MHz. The (20)Ne-(12)C5H8(16)O complex has rotational constants of A = 2728.8120, B = 1736.5882, and C = 1440.4681 MHz. In addition, the five unique, singly substituted (13)C and (18)O isotopologues of the argon complex are reported. The five single-substituted (13)C of the (20)Ne complex and the (22)Ne-(12)C5H8(16)O complex are reported. The rare gases are in van der Waals contact with the carbonyl α carbon and nearly in contact with the hydrogen on β and γ carbons toward the back of the ring.

Rotational Spectrum and Structure of Cyclohexene Oxide and the Argon–Cyclohexene Oxide van der Waals Complex

The rotational spectra of cyclohexene oxide and its van der Waals complex with argon have been observed with a Balle-Flygare type pulsed jet Fourier transform microwave spectrometer in the 6 to 20 GHz region. This work improves the existing rotational and quartic centrifugal distortion constants of cyclohexene oxide, its six singly substituted (13)C, and the (18)O isotopologue. In addition, the (17)O isotopologue was observed in natural abundance. The quadrupole coupling constants for the (17)O isotopologue are χ(aa) = 8.855(5), χ(bb) = -4.560(4), and χ(cc) = -4.296(4) MHz. The argon-(12)C6H10(16)O complex has rotational constants of A = 2146.4825(2), B = 908.64292(8), and C = 859.00320(8) MHz. Additionally, the six unique singly substituted (13)C isotopologues of the argon complex are reported here. The position of the argon that is consistent with the parent and six (13)C complex rotational constants is above the ring on the side opposite the epoxide.

Experimental Observation and Quantum Chemical Characterization of theS1←S0Transition of Protonated Naphthalene–Argon Clusters

We report on the photodissociation spectrum of protonated naphthalene(+)-argon complexes (NpH(+)-Ar) recorded by excitation into the first excited singlet electronic state. Unlike previous electronic spectra of the free molecule (NpH(+)), both the α and the β isomer could be observed for the Ar adducts. Detailed information on the S0 and S1 state of both isomers is provided by quantum chemical calculations. An assignment of observed vibrational bands is proposed based on Franck-Condon simulations.

Hydrogen bonding to xenon: A comparison with neon, argon and krypton complexes

H-bonding-like interactions between AH acids and Ne, Ar, Kr and Xe are examined by analysis of predicted A–H⋯Rg geometry, energy, A–H stretching vibration (CCSD(T) level) as well as assumed proton affinitiy, pKa, polarizability and chemical hardness of the complex components. The Kr and Xe systems are described as mostly the acid–base contacts and reveal properties of typical H-bonded complexes. These contacts are also influenced by structural factors. Properties of the Ne contacts are qualitatively different. The A–H⋯Xe complexes can be divided into stronger and weaker ones using the 129Xe chemical shift as the criterion.

Anomalous fine structures of the 00 0 band of tetracene in large He droplets and their dependence on droplet size

The spectrum of the band of the S1 ← S0 transition of tetracene molecules embedded in superfluid helium droplets is studied via laser induced fluorescence spectroscopy. Measurements are reported for droplets of different average sizes from about 103 to 107 helium atoms. In agreement with previous observations in small droplets the band shows two components designated as α and β peaks split by about 1 cm−1. In addition the higher energy β-peak shows some sharp superimposed peaks separated by about 0.05 cm−1. A very similar fine structure of the band is found in the tetracene–argon complex and in pentacene which challenges a simple explanation in terms of a free molecule Hamiltonian. Presumably the rotational-translational coupling of very anisotropic molecules, such as tetracene in superfluid helium has a noticeable affect on the spectra. In droplets larger than about = 106, obtained in supercritical expansions, the β peak has a broad feature shifted by 0.1 cm−1 towards higher energies from the maximum found at smaller sizes which dominates the spectrum at = 107. Several mechanisms including the interaction with the phonons in the interior of the droplets and trapping by quantum vortices are discussed as possible explanations for the unexpected spectral behaviour in the large droplets.