Rotationally resolved infrared spectroscopy of adamantane

Role of the solvent polarity on the optical and electronic characteristics of 1-iodoadamantane

The natural absorbance caused by the chromophore and chemical behavior of 1-iodoadamantane is highly influenced by the polarity of different solvent environments. This gives rise to the solvatochromatic shifts in the optical absorption and electronic structure and the experimentally measured UV-vis absorption spectra show significant solvatochromic shifts with respect to the solvent polarity. The absorption shift for both σ to σ*and n to σ* electronic transitions are more dominant in polar solvents than in nonpolar solvents. To obtain a better understanding of the impact of solvent polarity on the 1-iodoadamantane at the molecular level, computational calculations were carried out through implicit solvation. According to this, changes in the HOMO and LUMO energies and electron density distributions of various solvent continuums demonstrate the influence of solvent polarity on the HOMO and LUMO energy levels of the chemical system. This also shows an increment in the HOMO-LUMO gap with respect to the polarity of the solvent.

Supramolecular metal-organic frameworks as host-guest nanoplatforms for versatile and customizable biomedical applications

Molecular imaging of disease with multifunctional nanoparticles has improved specificity and sensitivity but also raises the complexity, potential toxicity, and cost. Here, we show a facile and degradable self-assembly β-cyclodextrin metal-organic framework (β-CD-MOF) nanoplatform for customizable multifunctional imaging. These β-CD-MOF nanoparticles were obtained with favorable morphology and size by controlling the degradation time. The β-CD-MOF were used as nanoplatforms for facile functionalization with adamantane (Ad)-modified probes through host-guest interactions between the surface β-CD units and Ad molecules. We demonstrated the method's feasibility and capability by developing various contrast agents for multiple biomedical imaging, including fluorescence imaging, magnetic resonance imaging (MRI), and computed tomography (CT) imaging. The nanoprobes showed superior performance compared to the corresponding small molecular probes, including better physio-chemical properties (e.g., about 5 times of T1 relaxivity for MRI, 1.2 times of Hounsfield units for CT), improved pharmacokinetics, effective tissue imaging capability, and low safety concerns. These β-CD-MOF-based nanoparticles are promising host-guest nanoplatforms for developing multifunctional and safe imaging probes. STATEMENT OF SIGNIFICANCE: Molecular imaging of disease with multifunctional nanoparticles has improved specificity and sensitivity but also raises the complexity, potential toxicity, and cost. Here, we introduce facile and degradable self-assembly β-cyclodextrin metal-organic framework (β-CD-MOF) nanoplatforms for customizable multifunctional imaging. The significance of this work includes: 1) This work reports the tailoring of MOFs nanoparticles with suitable sizes and shapes for biomedical applications through controllable morphological transition and degradation; 2) The β-CD-MOF-based host-guest nanoplatforms are facile and feasible for developing multifunctional nanoparticular contrast agents for effective tissue imaging; 3) The nanoparticular contrast agents show low safety concerns with a long-term tissue deposition similar to the small molecular probes.

Microhydration of the adamantane cation: intracluster proton transfer to solvent in [Ad(H2O)n=1-5]+ for n ≥ 3

Radical cations of diamondoids are important intermediates in their functionalization reactions in polar solvents. To explore the role of the solvent at the molecular level, we characterize herein microhydrated radical cation clusters of the parent molecule of the diamondoid family, adamantane (C₁₀H₁₆, Ad), by infrared photodissociation (IRPD) spectroscopy of mass-selected [Ad(H₂O)_n=1-5]⁺ clusters. IRPD spectra of the cation ground electronic state recorded in the CH/OH stretch and fingerprint ranges reveal the first steps of this fundamental H-substitution reaction at the molecular level. Analysis of size-dependent frequency shifts with dispersion-corrected density functional theory calculations (B3LYP-D3/cc-pVTZ) provides detailed information about the acidity of the proton of Ad⁺ as a function of the degree of hydration, the structure of the hydration shell, and the strengths of the CH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. For n = 1, H₂O strongly activates the acidic C-H bond of Ad⁺ by acting as a proton acceptor in a strong CH⋯O ionic H-bond with cation-dipole configuration. For n = 2, the proton is almost equally shared between the adamantyl radical (C₁₀H₁₅, Ady) and the (H₂O)₂ dimer in a strong C⋯H⋯O ionic H-bond. For n ≥ 3, the proton is completely transferred to the H-bonded hydration network. The threshold for this size-dependent intracluster proton transfer to solvent is consistent with the proton affinities of Ady and (H₂O)_n and confirmed by collision-induced dissociation experiments. Comparison with other related microhydrated cations reveals that the acidity of the CH proton of Ad⁺ is in the range of strongly acidic phenol⁺ but lower than for cationic linear alkanes such as pentane⁺. Significantly, the presented IRPD spectra of microhydrated Ad⁺ provide the first spectroscopic molecular-level insight of the chemical reactivity and reaction mechanism of the important class of transient diamondoid radical cations in aqueous solution.

Insights into the molecular structure and infrared spectrum of the prebiotic species aminoacetonitrile

Aminoacetonitrile is an interstellar molecule with a prominent prebiotic role, already detected in the chemically-rich molecular cloud Sagittarius B2(N) and postulated to be present in the atmosphere of the largest Saturn's moon, Titan. To further support its observation in such remote environments and laboratory experiments aimed at improving our understanding of interstellar chemistry, we report a thorough spectroscopic and structural characterization of aminoacetonitrile. Equilibrium geometry, fundamental bands as well as spectroscopic and molecular parameters have been accurately computed by exploiting a composite scheme rooted in the coupled-cluster theory that accounts for the extrapolation to the complete basis set limit and core-correlation effects. In addition, a semi-experimental approach that combines ground-state rotational constants for different isotopic species and calculated vibrational corrections has been employed for the structure determination. From the experimental side, we report the analysis of the three strongest fundamental bands of aminoacetonitrile observed between 500 and 1000 cm^-1 in high-resolution infrared spectra. More generally, all computed band positions are in excellent agreement with the present and previous experiments. The only exception is the ν₁₅ band, for which we provide a revision of the experimental assignment, now in good agreement with theory.

Infrared Spectrum of the Adamantane+ -Water Cation: Hydration-Induced C-H Bond Activation and Free Internal Water Rotation

Diamondoid cations are reactive intermediates in their functionalization reactions in polar solution. Hydration is predicted to strongly activate their C-H bonds in initial proton abstraction reactions. To study the effects of microhydration on the properties of diamondoid cations, we characterize herein the prototypical monohydrated adamantane cation (C₁₀ H₁₆ ⁺ -H₂ O, Ad⁺ -W) in its ground electronic state by infrared photodissociation spectroscopy in the CH and OH stretch ranges and dispersion-corrected density functional theory (DFT) calculations. The water (W) ligand binds to the acidic CH group of Jahn-Teller distorted Ad⁺ via a strong CH⋅⋅⋅O ionic H-bond supported by charge-dipole forces. Although W further enhances the acidity of this CH group along with a proton shift toward the solvent, the proton remains with Ad⁺ in the monohydrate. We infer essentially free internal W rotation from rotational fine structure of the ν₃ band of W, resulting from weak angular anisotropy of the Ad⁺ -W potential.

Spectroscopic Characterization of the Product Ions Formed by Electron Ionization of Adamantane

A structural characterization of the products formed in the dissociative electron ionization of adamantane (C10 H16 ) is presented. Molecular structures of product ions are suggested based on multiple-photon dissociation spectroscopy using the Free Electron Laser for Infrared eXperiments (FELIX) in combination with quantum-chemical calculations. Product ions are individually isolated in an ion trap tandem mass spectrometer and their action IR spectra are recorded. Atomic hydrogen loss from adamantane yields the 1-adamantyl isomer. The IR spectrum of the C8 H11 + product ion is best reproduced by computed spectra of 2- and 4-protonated meta-xylene and ortho- and para-protonated ethylbenzenes. The spectrum of the product ion at m/z 93 suggests that it is composed of a mixture of ortho-protonated toluene, para-protonated toluene and 1,2-dihydrotropylium, while the spectrum of the m/z 79 ion is consistent with the benzenium ion. This study thus suggests that adamantane is efficiently converted into aromatic species and astrophysical implications for the interstellar medium are highlighted.

Characterising molecules for fundamental physics: an accurate spectroscopic model of methyltrioxorhenium derived from new infrared and millimetre-wave measurements

Precise spectroscopic analysis of polyatomic molecules enables many striking advances in physical chemistry and fundamental physics. We use several new high-resolution spectroscopic devices to improve our understanding of the rotational and rovibrational structure of methyltrioxorhenium (MTO), the achiral parent of a family of large oxorhenium compounds that are ideal candidate species for a planned measurement of parity violation in chiral molecules. Using millimetre-wave and infrared spectroscopy in a pulsed supersonic jet, a cryogenic buffer gas cell, and room temperature absorption cells, we probe the ground state and the Re[double bond, length as m-dash]O antisymmetric and symmetric stretching excited states of both CH₃¹⁸⁷ReO₃ and CH₃¹⁸⁵ReO₃ isotopologues in the gas phase with unprecedented precision. By extending the rotational spectra to the 150-300 GHz range, we characterize the ground state rotational and hyperfine structure up to J = 43 and K = 41, resulting in refinements to the rotational, quartic and hyperfine parameters, and the determination of sextic parameters and a centrifugal distortion correction to the quadrupolar hyperfine constant. We obtain rovibrational data for temperatures between 6 and 300 K in the 970-1015 cm^-1 range, at resolutions down to 8 MHz and accuracies of 30 MHz. We use these data to determine more precise excited-state rotational, Coriolis and quartic parameters, as well as the ground-state centrifugal distortion parameter D_K of the ¹⁸⁷Re isotopologue. We also account for hyperfine structure in the rovibrational transitions and hence determine the upper state rhenium atom quadrupole coupling constant eQq'.

Synthesis, High-Resolution Infrared Spectroscopy, and Vibrational Structure of Cubane, C8H8

Carbon-cage molecules have generated a considerable interest from both experimental and theoretical points of view. We recently performed a high-resolution study of adamantane (C10H16), the smallest hydrocarbon cage belonging to the diamandoid family ( Pirali , O. ; et al. J. Chem. Phys. 2012 , 136 , 024310 ). There exist another family of hydrocarbon cages with additional interesting chemical properties: the so-called platonic hydrocarbons that comprise dodecahedrane (C20H20) and cubane (C8H8). Both possess C-C bond angles that deviate from the tetrahedral angle (109.8°) of the sp(3) hybridized form of carbon. This generates a considerable strain in the molecule. We report a new wide-range high-resolution study of the infrared spectrum of cubane. The sample was synthesized in Bari upon decarboxylation of 1,4-cubanedicarboxylic acid thanks to the improved synthesis of literature. Several spectra have been recorded at the AILES beamline of the SOLEIL synchrotron facility. They cover the 600-3200 cm(-1) region. Besides the three infrared-active fundamentals (ν10, ν11, and ν12), we could record many combination bands, all of them displaying a well-resolved octahedral rotational structure. We present here a preliminary analysis of some of the recorded bands, performed thanks the SPVIEW and XTDS software, based on the tensorial formalism developed in the Dijon group. A comparison with ab initio calculations, allowing to identify some combination bands, is also presented.

High-Resolution Far-Infrared Spectroscopy of N-Substituted Two-Ring Polycyclic Aromatic Hydrocarbons: An Extended Study

Polycyclic aromatic hydrocarbons (PAHs) and their N-substituted derivatives are among the largest species for which gas-phase high-resolution spectroscopy can be performed nowadays. In this paper we report the observation and analysis of spectra from several N-substituted two-ring PAHs, all taken in the "fingerprint" far-infrared region (<850 cm(-1)). Together with accurate measurements of their pure rotational transitions in the millimeter and submillimeter ranges, these synchrotron-based Fourier transform infrared (FTIR) measurements provide an accurate description of the rotational energy levels in the ground and low-energy excited vibrational states of these species. To complement the experimental data, anharmonic DFT calculations were performed to obtain relatively accurate rotational and vibrational parameters. The calculated results strongly support the rotational analysis and provide a good estimate of the equilibrium structures for each species. Extended measurements, analysis, and calculations are presented here for the far-IR bands of quinoline (C9H7N), isoquinoline (C9H7N), quinoxaline (C8H6N2), quinazoline (C8H6N2), [1,5]-naphthyridine (C8H6N2), [1,6]-naphthyridine (C8H6N2), and indole (C8H7N) molecules.

The far-infrared spectrum of azulene and isoquinoline and supporting anharmonic density functional theory calculations to high resolution spectroscopy of polycyclic aromatic hydrocarbons and derivatives

In the laboratory, the acquisition and analysis of the rotationally resolved spectra of large molecular systems remain challenging. We report in this paper the rotational analysis of the ν30-GS band of azulene and the ν41-GS band of isoquinoline recorded with synchrotron-based Fourier transform absorption spectroscopy in the far-IR. As a support to rotational analyses, we employed a method based on standard density functional theory calculations performed at the anharmonic level which accurately reproduced the rotational constants of 28 vibrational states of 16 Polycyclic Aromatic Hydrocarbons (PAHs) and aza-derivatives. This method appears as an invaluable support for the spectral assignment of the very congested rotational structures of the infrared bands of PAH species and should be very helpful in the active search of these molecules in space through their pure rotational or rovibrational spectra.

Synchrotron based FT-FIR pure rotational spectroscopy of the NH2 radical in its two lowest vibrational states

Six Fourier-transform FIR spectra of the NH2 radical have been recorded at high resolution (0.001 cm(-1)) using synchrotron radiation on the AILES beamline at SOLEIL Synchrotron. Three different experimental discharge setups have been used to observe, in absorption, 1009 pure rotational transitions of NH2 in the vibrational ground state (000) and 170 pure rotational transitions within the first excited vibrational state (010). These results constitute a significant extension of the observed quantum numbers for these two states. The spectra permitted several couplings to be resolved (asymmetric coupling, spin-rotation coupling, hyperfine structure) for relatively highly excited energy levels. An effective fit has been realized using both standard Watson-S and -A reductions despite an abnormal centrifugal distortion effect for this light hydride.

Gyroscopic destabilisation in polyatomic molecules: rotational structure of the low-frequency bending vibrational states ν(23) and ν(11) of dimethylsulfoxide

We give details of the spectroscopic observation of the gyroscopic destabilisation in the ν23 vibrational state of dimethylsulfoxide (DMSO) announced by Cuisset, Pirali, and Sadovskií [Phys. Rev. Lett. 109, 094101 (2012)]. Following the first successful high-resolution spectroscopic study of the rotational structure of the "perpendicular" band of DMSO at 324 cm(-1) associated with the ν23 bending vibrational mode, the rare subsystem of ν23 rotational levels consisting of a series of fourfold quasidegenerate levels (4-clusters) was identified. Our complete analysis of the underlying rotational dynamics uncovered a bifurcation leading to the gyroscopic destabilisation of one of the two stable principal axes of inertia, a phenomenon known previously only in a few triatomic molecules.