Finite-Temperature IR Spectroscopy of Polyatomic Molecules: A Theoretical Assessment of Scaling Factors

Structure Elucidation of Menthol-Based Deep Eutectic Solvent using Experimental and Computational Techniques

The structural properties and nonbonding interactions of a menthol-based deep eutectic solvent (DES) were investigated in detail employing experimental and computational methods. A mass spectrometry analysis confirmed the formation of 1:1 l-menthol/acetic acid. A molecular dynamics simulation was used to figure out energetically most favorable cluster conformers of the 1:1 l-menthol/acetic acid system. Density functional theory at the ωB97XD/6-311G (d,p) level of theory was employed to optimize the isolated structures and to calculate their thermochemical properties. Both experimental and computed IR spectra were analyzed for the samples. Additionally, vibrational circular dichroism (VCD) spectra of the samples were measured to prove the chirality transfer. Principal component analysis (PCA) was used to make the data interpretation more vivid. All the spectral data analyses and nanostructure elucidation proved the spontaneous formation of the DES through the formation of strong hydrogen bonding. Experimental solvatochromism and computed highest occupied molecular orbital-lowest unoccupied molecular orbital gaps validated the reasoning. Moreover, comparative VCD and IR spectral analyses clearly indicated a chirality transfer from the chiral menthol to achiral acetic acid. This study suggests that various techniques, such as mass spectrometry, IR, solvatochromism, and computed IR-VCD could be useful and important tools to elucidate nanostructure and nonbonding interactions of a DES. VCD could be used as an excellent complementary technique to IR spectroscopy for a chiral molecule-based DESs.

Far-IR Absorption of Neutral Polycyclic Aromatic Hydrocarbons (PAHs): Light on the Mechanism of IR-UV Ion Dip Spectroscopy

Gas-phase IR-UV double-resonance laser spectroscopy is an IR absorption technique that bridges the gap between experimental IR spectroscopy and theory. The IR experiments are used to directly evaluate predicted frequencies and potential energy surfaces as well as to probe the structure of isolated molecules. However, a detailed understanding of the underlying mechanisms is, especially in the far-IR regime, still far from complete, even though this is crucial for properly interpreting the recorded IR absorption spectra. Here, events occurring upon excitation to vibrational levels of polycyclic aromatic hydrocarbons by far-IR radiation from the FELIX free electron laser are followed using resonance-enhanced multiphoton ionization spectroscopy. These studies provide detailed insight into how ladder climbing and anharmonicity influence IR-UV spectroscopy and therefore the resulting IR signatures in the far-IR region. Moreover, the potential energy surfaces of these low-frequency delocalized modes are investigated and shown to have a strong harmonic character.

Experimental Approach to the Study of Anharmonicity in the Infrared Spectrum of Pyrene from 14 to 723 K

Quantifying the effect of anharmonicity on the infrared spectrum of large molecules such as polycyclic aromatic hydrocarbons (PAHs) at high temperatures is the focus of a number of theoretical and experimental studies, many of them motivated by astrophysical applications. We recorded the IR spectrum of pyrene C16H10 microcrystals embedded in KBr pellets over a wide range of temperatures (14-723 K) and studied the evolution of band positions, widths, and integrated intensities with temperature. We identified jumps for some of the spectral characteristics of some bands in the 423-473 K range. These were attributed to a change of phase from crystal to molten in condensed pyrene, which appears to affect more strongly bands involving large CH motions. Empirical anharmonicity factors that quantify the linear evolution of band positions and widths with temperature for values larger than ∼150-250 K, depending on the band, were retrieved from both phases and averaged to provide recommended values for these anharmonicity factors. The derived values were found to be consistent with available gas phase data. We conclude about the relevance of the methodology to produce data that can be compared with calculated anharmonic IR spectra and provide input for models that simulate the IR emission of astro-PAHs.

Intramolecular Processes Revealed Using UV-Laser-Induced IR-Fluorescence: A New Perspective on the "Channel Three" of Benzene

Radiative relaxation in the infrared (IR) is common following nonradiative electronic relaxation processes, but it is rarely measured. We present ultraviolet laser-induced infrared fluorescence (UV-LIIRF) excitation spectroscopy and dispersed UV-LIIRF spectroscopy of gas phase benzene vibronically excited around the onset of channel 3, using a homemade spectrometer. We found that the vibrational IR fluorescence yield is clearly higher when benzene is excited above the onset than when it is excited below. Significant changes in dispersed IR emission profiles resulting from excitations below and above the onset of channel 3 were also observed. These results suggest that isomerization of benzene toward fulvene occurs efficiently below the opening of channel 3 and confirm that channel 3 involves a photophysical relaxation pathway that efficiently competes with isomerization.

Probing the spin multiplicity of gas-phase polycyclic aromatic hydrocarbons through their infrared emission spectrum: a theoretical study

The anharmonic infrared emission spectrum following an optical excitation has been calculated for a variety of polycyclic aromatic hydrocarbon molecules in their ground singlet electronic state or in their triplet state. The computational protocol relies on second-order perturbation theory and involves a quartic vibrational Hamiltonian, the vibrational quantum numbers being sampled according to a Monte Carlo procedure. In the case of neutral naphthalene, the IR spectrum obtained in the (ground) singlet state differs significantly from the spectrum in the triplet state, especially for out-of-plane CH bending modes. Although not as prominent, spectral differences in larger molecules are still observable.

Temperature effects on the rovibrational spectra of pyrene-based PAHs

Absorption infrared spectra have been computed for a variety of polycyclic aromatic hydrocarbon molecules of the pyrene family, taking into account anharmonicity and temperature effects, rovibrational quantization, and couplings. The energy levels are described by a second-order perturbative expansion of the rovibrational Hamiltonian in the vibrational and rotational quantum numbers, as relevant for a symmetric-top molecule, with ingredients obtained from quantum chemistry calculations. Multicanonical Monte Carlo simulations are carried out to compute bidimensional IR intensity histograms as a function of total energy and vibrational frequency, which then provide the absorption spectrum at arbitrary temperatures via a Laplace transformation. The main spectral features analyzed for neutral, anionic, and cationic pyrene indicate a strong dependence on temperature, in agreement with existing laboratory experiments, and a significant contribution of rotational degrees of freedom to the overall broadenings. The spectral shifts and broadenings reveal some sensitivity of anharmonicities to the charge and protonation states and, in the case of protonated pyrene and pyrenyl cation, on possible isomers and between aromatic and aliphatic C-H bands. Implications of the present work to the general issue of interstellar emission features are discussed.

Molecular dynamics simulations on [FePAH]+ π-complexes of astrophysical interest: anharmonic infrared spectroscopy

In this article, classical Born-Oppenheimer molecular dynamics (MD) simulations in the microcanonical ensemble are performed on neutral and cationic polycyclic aromatic hydrocarbon (PAH) species, focusing on [FePAH](+)π-complexes. Their anharmonic mid-infrared (mid-IR) spectra in the classical approximation are derived. This approach allows us to describe the influence of the energy of a system on its IR spectrum in terms of band-shifts and broadenings. The MD simulations are performed on a potential energy surface (PES) described at the self-consistent-charge density functional tight-binding level of theory. The PES is benchmarked on DFT calculations, showing the validity of the approach for complexes of Fe(+) with PAHs larger than coronene (C(24)H(12)) that are of astrophysical interest. MD simulations at high temperature show the occurrence of the diffusion of the Fe cation on the surface of the PAH. It proceeds through the edge of the carbon skeleton which is the lowest energy pathway presenting barriers smaller than 1 eV. Although only qualitative information on the band broadenings can be obtained, we show that the dependence of the computed positions of the main bands of [C(24)H(12)](0/+)and [FeC(24)H(12)](+)π-complexes on temperature can be fit by linear laws. The spectral trends determined for [FeC(24)H(12)](+) are compared to those of N-substituted [C(23)NH(12)](+)and [SiC(24)H(12)](+)π-complexes of astrophysical interest.