Porous materials primarily have their structural properties determined using materials science tools such as powder X-ray diffraction, solid-state NMR, and gas adsorption. Crystalline porous materials including covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and zeolites can be seen to respond to external stimuli, like pressure, using diamond-anvil cells, but few experiments are capable of directly investigating the material’s response to temperatures as low as 30K. Here, we show how our new cryogenic FTIR instrument is capable of observing structural changes in “flexible” COFs as a function of temperature, down to as low as 30K. Using a functionalized series of COF-300 materials, we show that different temperature-induced structural changes are revealed in situ through changes in the IR spectrum. Further comparison of the materials shows different degrees of change, both through the quantitative degree of the peak shift and a qualitative assignment of which peaks shift. Finally, we discuss the utility of this experimental technique for future gas-uptake and adsorption applications.

Small conjugated organic molecules play an important role in many areas of chemistry, including organic synthesis, atmospheric chemistry, and industrial chemistry. These small conjugated species also have an interesting conformational energy landscape, with a barrier to conversion between trans and gauche conformers. We have measured the high-resolution infrared spectrum of the strongest vibrational band of 1,3-pentadiene (C₅H₈), one of the simplest conjugated organic molecules, under supersonic jet-cooled conditions using a quantum cascade laser-based spectrometer. Under the jet-cooled conditions, we have observed rotationally-resolved spectra with a rotational temperature of approximately 15 K. We will present our intial spectral analysis and fit, and discuss the presence of "extra" peaks that appear in the spectrum. We will also compare the jet-cooled spectrum to previous measurements of the high-resolution room temperature spectrum recorded in our research group.

Cationic, protonated, and hydrogenated polycyclic aromatic hydrocarbons (PAH) and their nitrogen-substituted derivatives (PANH) have been proposed as possible carriers of unidentified infrared (UIR) emission bands. These compounds might also be promising candidates for carriers of the diffuse interstellar bands (DIB), electronic absorption bands of interstellar origin observed in the visible to near-IR region. However, laboratory spectra of such species suitable for comparison with astronomical observations are rarely available. We present the fluorescence excitation and dispersed fluorescence spectra of N-protonated quinoline (C₉H₇NH⁺) and isomers of hydrogenated quinoline (HC₉H₇N)), derivatives of the smallest PANH, isolated in solid para-H₂ (p-H₂) at 3 K. These molecules were generated by electron bombardment during the deposition of a mixture of quinoline (C₉H₇N) and p-H₂. We located the 0₀⁰ band of the S₁–S₀ transition at 27778 cm⁻¹for C₉H₇NH⁺, indicating a red-shift induced by the p-H₂ environment. This shift was  ∼ 90 cm⁻¹relative to gas-phase experiments conducted at room temperature by Hansen et al.Hansen et al. Phys. Chem. Chem. Phys. 17, 25882-25890 (2015).nd  ∼ 122 cm⁻¹relative to gas-phase experiments at 40 K by Féraud et al.,Féraud et al. J. Phys. Chem. A, 121, 2580-2587 (2017).oth using photodissociation spectroscopy. Furthermore, we observed the spectral signature of at least one HC₉H₇N isomer, which exhibited a fluorescence lifetime of  ∼ 40 ns. We tentatively assign this band system with its 0₀⁰ band at 18756 cm⁻¹to 5-HC₉H₇N and provide the first vibronic assignments for the dispersed fluorescence and fluorescence excitation spectra of this molecule according to the Franck-Condon Herzberg-Teller simulations using the (TD-)B3LYP/6-311++G(2d,2p) method.

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Footnotes:

Hansen et al. Phys. Chem. Chem. Phys. 17, 25882-25890 (2015).a Féraud et al. J. Phys. Chem. A, 121, 2580-2587 (2017).b

The mid-IR spectra observed from different parts of the interstellar medium (ISM) are dominated by a set of IR features referred to as the unidentified infrared bands (UIBs). Although it has been accepted that these emission features originate from polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives, the individual detection of the emitting species is challenging due to their similar IR fingerprint in the 5-15 μm spectral region. Cyano-substituted PAHs were recently detected in the ISM based on their rotational spectra. These species have dominant features (CH and CN stretch) in the less explored 1-5 μm spectral region that can be diagnostic in further detection. However, there is still a lack of laboratory data to compare with the observed mid-IR spectra. Here we present the low temperature gas-phase vibrational spectra of a set of ionised cyano-group containing PAHs in the mid-infrared region. Experimentally, the cyano-PAH ions are cooled below 10 K in a cryogenic ion trapping apparatus, tagged with He atoms and probed with tunable radiation. Quantum-chemical calculations are carried out at a density functional theory level. The spectra are dominated by the CN stretch around 4.5 μm, with weaker CH modes near 3.2 μm.

The gas-phase rotational spectrum from 85 to 750 GHz and the high-resolution infrared spectrum of cyclobutanone (C₄H₆O, Cs) have been analyzed for the ground and low-energy vibrationally excited states of the ring-puckering vibration (ν₂₀). The extension of the ground-state rotational spectrum provides the foundation for astronomical searches across most of the frequency range covered by modern radiotelescopes. Observation of the rotational spectrum in frequency range above 350 GHz, reveals transition frequencies perturbed by b-axis Coriolis-coupling interactions between the ground vibrational state and the ring-puckering vibration (ν₂₀). As a result, meaningful analysis of the rotational spectrum of the ground vibrational state of cyclobutanone cannot be accomplished via a single-state, distorted-rotor Hamiltonian. Thus, a combined global fit of the rotational and infrared data has been obtained using a sextic, centrifugally distorted rotor Hamiltonian with Coriolis coupling between appropriate states. Prior to the acquisition of the high-resolution infrared spectrum from the Canadian Light Source (CLS), initial analysis of the ground state and several quanta of the ν₂₀ series provided accurate and precise band origins of ν₂₀ at 35.730957 (40) cm⁻¹and 2ν₂₀ at 92.031027 (47) cm⁻¹, due to the observation of many nominally interstate transitions and resonant transitions. Experimental spectroscopic constants and vibrational energies are compared to their computed values.

We measured transition frequencies of the ν₃ fundamental band transitions of methane from P(1) to P(12), from Q(1) to Q(14), and R(0) to R(8) with an uncertainty from a few to seventeen kilohertz using a difference-frequency-generation source, an enhanced-cavity absorption cell, and an optical frequency comb.S. Okubo, H. Nakayama. K. Iwakuni, H. Inaba, and H. Sasada, Opt. Express, vol. 19, p. 23878 (2011).^, M. Abe, K. Iwakuni, S. Okubo, and H. Sasada, J. Opt. Soc. Am. B, vol. 30, p. 1027 (2013).T

PAHs are ubiquitous in astrochemical environments and have been proposed to contribute to the emission from diffuse infrared emission bands. Most spectroscopy of PAHs has been focusing on cationic and neutral species, since anions may not survive the UV radiation in circumstellar regions. However, anionic PAHs may be present in darker, carbon rich regions in space. We present infrared spectra of anionic pyrene and anthracene measured using Argon predissociation spectroscopy, together with density functional theory calculations. The spectra reveal congested CH stretching regions, which defy assignment by harmonic calculations, but can be interpreted using vibrational perturbation theory.We gratefully acknowledge funding from the Department of Energy (Award No. DE-SC0021387). html:<hr /><h3>Footnotes: We gratefully acknowledge funding from the Department of Energy (Award No. DE-SC0021387).

Typically resulting from the combustion of coal and wood, nitrophenols are found throughout the earths atmosphere. Being highly photochemically active, they may undergo photolysis in sunlight to produce nitrous acid, among other pollutants. Despite their clear atmospheric relevance, their electronic properties and photophysics remain poorly characterised. Here, we report an investigation of, cryogenically cooled 3-nitrophenol’s, electronic and vibrational structure using photoelectron and photodetachment spectroscopy as well as resonant photoelectron imaging (PEI). The electron affinity of 3-nitrophenol is measured to be 2.997 eV and an additional electronically excited state was observed approximately 3.900 eV above the ground state. Photofragmentation of the 3-nitrophenol obfuscates direct measurement of the dipole bound state meaning that the exact energy of the dipole bound state is impossible to determine from the photodetachment spectrum. The photodetachment spectrum shows around 10 clearly resolved peaks corresponding to vibrational Feshbach resonances associated with 13 of the total 36 vibrational modes present to be assigned. The electronic and vibrational data acquired as part of this work provides valuable information on the role of 3-nitrophenol compounds as atmospheric pollutants and their potential photochemistry.

Deprotonated 2-Hydroxytriphenylene anions are known to be crucial precursors for synthesizing polycyclic diaryl ethers, which are commonly applied in antibiotics. Here, we report our investigation on photodetachment via two-photon resonant absorption of cryogenically-cooled deprotonated 2-Hydroxytriphenylene anions utilizing photodetachment spectroscopy and high-resolution photoelectron imaging. The electron affinity was measured as 2.629 ± 0.001 eV (21200 ± 10 cm⁻¹) from the high-resolution photoelectron spectrum acquired near threshold. The existence of low-lying electronic states below threshold essentially allowed anion absorption spectroscopy to be performed through the lens of photodetachment spectroscopy. In brief, forty-six vibrational levels of the anion first excited singlet (S₁) state were resolved via photodetachment spectroscopy and six vibrational levels for the neutral ground electronic state (N₀) were resolved via above threshold photoelectron spectroscopy. Furthermore, direct experimental evidence for the S₀ to T_N transition was acquired from the photodetachment spectrum. Finally, characterization of the two-photon resonant detachment mechanism below threshold for this anion is also provided and discussed in great detail.

Porous materials containing under-coordinated Cu(I) centers can play an important role in dihydrogen adsorption as well as the efficient isotope separation of H₂/D₂. The vibrational spectra of the copper(I) cation–dihydrogen complexes of Cu⁺(H₂)₄ and Cu⁺(H₂O)(H₂)₂, as well as their isotoplogues, Cu⁺(D₂)₄, Cu⁺(D₂)₃(H₂)₁, Cu⁺(H₂O)(D₂)₂, Cu⁺(D₂O)(H₂)₂ and Cu⁺(D₂O)(D₂)₂ are therefore studied using cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations, for an in-depth understanding of the binding nature between dihyrogen and under-coordinated copper(I) cation complexes. The infrared photodissociation (IRPD) spectra measured in the HH (or DD) stretch regions (2500 cm⁻¹ to 4400 cm⁻¹) are assigned based on the calculated frequencies using vibrational second-order perturbation theory (VPT2). The observed vibrational features are attributed to the excitations of dihydrogen stretching fundamentals (ν_HH) and combination bands of these fundamentals with low-frequency mode excitations. The ν_HH frequencies in Cu⁺(H₂)₄ shows large red-shifts by 432 cm⁻¹ relative to the frequency of a free H₂ molecule (4161 cm⁻¹), indicating a stronger bonding of dihydrogen to the Cu(I) center.Jin et.al., Phys. Chem. Chem. Phys., 2023, 25, 5262.he ν_HH shift is even larger in Cu⁺(D₂O)(H₂)₂ cation, due to the presence of the water molecule. The studies of isotoplogues and the combination bands reveal that the anharmonic effect plays a profound role in the vibrational spectra and the thermodynamic properties. Further, the non-ergodic dissociation behavior in the Cu⁺(D₂)₃(H₂)₁ and a fluxional nature are discussed.

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Footnotes:

Jin et.al., Phys. Chem. Chem. Phys., 2023, 25, 5262.T

In contrast to the traditional analysis of molecules presenting local mode behavior, where the degree of locality is given through a function in terms of the Morse potential parameters, new criteria of local/normal (LN) degree suitable to be applied to any molecular system are proposed. The approach is based on the connection between the algebraic normal and the local mode representations. It is shown that both descriptions are equivalent as long as the polyad in the local representation is not conserved. The imposition of a local polyad conservation naturally provides a criterion to assign a LN degree in quantitative form. This approach assumes bosonic operators, where only the fundamental energies are involved. The correlation between the different parameters are studied as well as their connection with spectroscopic properties. The importance of this connection is exemplified by the Raman spectra of isotopologues of carbon dioxide.