Computational Spectroscopy of the Cr–Cr Bond in Coordination Complexes

We report the accurate computational vibrational analysis of the Cr–Cr bond in dichromium complexes using second-order multireference complete active space methods (CASPT2), allowing direct comparison with experimental spectroscopic data both to facilitate interpreting the low-energy region of the spectra and to provide insights into the nature of the bonds themselves. Recent technological development by the authors has realized such computation for the first time. Accurate simulation of the vibrational structure of these compounds has been hampered by their notorious multiconfigurational electronic structure that yields bond distances that do not correlate with bond order. Some measured Cr–Cr vibrational stretching modes, ν(Cr2), have suggested weaker bonding, even for so-called ultrashort Cr–Cr bonds, while others are in line with the bond distance. Here we optimize the geometries and compute ν(Cr2) with CASPT2 for three well-characterized complexes, Cr2(O2CCH3)4(H2O)2, Cr2(mhp)4, and Cr2(dmp)4. We obtain CASPT2 harmonic ν(Cr2) modes in good agreement with experiment at 282 cm−1 for Cr2(mhp)4 and 353 cm−1 for Cr2(dmp)4, compute 50Cr and 54Cr isotope shifts, and demonstrate that the use of the so-called IPEA shift leads to improved Cr–Cr distances. Additionally, normal mode sampling was used to estimate anharmonicity along ν(Cr2) leading to an anharmonic mode of 272 cm−1 for Cr2(mhp)4 and 333 cm−1 for Cr2(dmp)4.

Computational Spectroscopy of the Cr–Cr Bond in Coordination Complexes

We report the accurate computational vibrational analysis of the Cr–Cr bond in dichromium complexes using second-order multireference complete active space methods (CASPT2), allowing direct comparison with experimental spectroscopic data both to facilitate interpreting the low-energy region of the spectra and to provide insights into the nature of the bonds themselves. Recent technological development by the authors has realized such computation for the first time. Accurate simulation of the vibrational structure of these compounds has been hampered by their notorious multiconfigurational electronic structure that yields bond distances that do not correlate with bond order. Some measured Cr–Cr vibrational stretching modes, ν(Cr2), have suggested weaker bonding, even for so-called ultrashort Cr–Cr bonds, while others are in line with the bond distance. Here we optimize the geometries and compute ν(Cr2) with CASPT2 for three well-characterized complexes, Cr2(O2CCH3)4(H2O)2, Cr2(mhp)4, and Cr2(dmp)4. We obtain CASPT2 harmonic ν(Cr2) modes in good agreement with experiment at 282 cm−1 for Cr2(mhp)4 and 353 cm−1 for Cr2(dmp)4, compute 50Cr and 54Cr isotope shifts, and demonstrate that the use of the so-called IPEA shift leads to improved Cr–Cr distances. Additionally, normal mode sampling was used to estimate anharmonicity along ν(Cr2) leading to an anharmonic mode of 272 cm−1 for Cr2(mhp)4 and 333 cm−1 for Cr2(dmp)4.

Identification of DNA Bases and Their Cations in Astrochemical Environments: Computational Spectroscopy of Thymine as a Test Case

Increased importance of vibrational fingerprints in the identification of molecular systems, can be highlighted by the upcoming interstellar medium (ISM) observations by the James Webb Space Telescope, or in a context of other astrochemical environments as meteorites or exoplanets, Mars robotic missions, such as instruments on board of Perseverance rover. These observations can be supported by combination of laboratory experiments and theoretical calculations, essential to verify and predict the spectral assignments. Astrochemical laboratory simulations have shown that complex organic molecules (COMs) can be formed from simple species by vacuum ultraviolet or X-ray irradiation expanding interest in searching for organic biological and prebiotic compounds. In this work an example of nucleobase, thymine, is selected as a test case for highlighting the utility of computational spectroscopic methods in astrochemical studies. We consider mid-infrared (MIR) and near-infrared (NIR) vibrational spectra of neutral (T) and cationic (T+) thymine ground states, and vibrationally-resolved photoelectron (PE) spectra in the far UV range from 8.7 to 9.4 eV. The theoretical framework is based on anharmonic calculations including overtones and combination bands. The same anharmonic wavenumbers are applied into the simulations of vibrationally-resolved photoelectron spectra based on Franck-Condon computations. The infrared and vibrationally-resolved photoelectron spectra are compared with the available experimental counterparts to verify their accuracy and provide assignment of the observed transitions. Finally, reliable predictions of spectra, going beyond currently available experimental data, either dealing with energy ranges, resolution or temperature, which can support astrochemistry studies are provided.

Simulation Meets Experiment: Unraveling the Properties of Water in Metal-Organic Frameworks Through Vibrational Spectroscopy

<div> <div> <div> <p>In nanoporous materials, guest–host interactions affect the properties and function of both adsorbent and adsorbate molecules. Due to their structural and chemical diversity, metal-organic frameworks (MOFs), a common class of nanoporous materials, have been shown to be able to efficiently and, often, selectively adsorb various types of guest molecules. In this study, we characterize the structure and dynamics of water confined in ZIF-90. Through the integration of experimental and computational infrared (IR) spectroscopy, we probe the structure of heavy water (D₂O) adsorbed in the pores, disentangling the fundamental framework–water and water–water interactions. The experimental IR spectrum of D₂O in ZIF-90 displays a blue-shifted OD-stretch band compared to liquid D₂O. The analysis of the IR spectra simulated at both classical and quantum levels indicates that the D₂O molecules preferentially interact with the carbonyl groups of the framework and highlights the importance of including nuclear quantum effects and taking into account Fermi resonances for a correct interpretation of the OD-stretch band in terms of the underlying hydrogen-bonding motifs. Through a systematic comparison with the experimental spectra, we demonstrate that computational spectroscopy can be used to gain quantitative, molecular-level insights into framework–water interactions that determine the water adsorption capacity of MOFs as well as the spatial arrangements of the water molecules inside the MOF pores which, in turn, are key to the design of MOF-based materials for water harvesting.</p> </div> </div> </div>

Simulation Meets Experiment: Unraveling the Properties of Water in Metal-Organic Frameworks Through Vibrational Spectroscopy

<div> <div> <div> <p>In nanoporous materials, guest–host interactions affect the properties and function of both adsorbent and adsorbate molecules. Due to their structural and chemical diversity, metal-organic frameworks (MOFs), a common class of nanoporous materials, have been shown to be able to efficiently and, often, selectively adsorb various types of guest molecules. In this study, we characterize the structure and dynamics of water confined in ZIF-90. Through the integration of experimental and computational infrared (IR) spectroscopy, we probe the structure of heavy water (D₂O) adsorbed in the pores, disentangling the fundamental framework–water and water–water interactions. The experimental IR spectrum of D₂O in ZIF-90 displays a blue-shifted OD-stretch band compared to liquid D₂O. The analysis of the IR spectra simulated at both classical and quantum levels indicates that the D₂O molecules preferentially interact with the carbonyl groups of the framework and highlights the importance of including nuclear quantum effects and taking into account Fermi resonances for a correct interpretation of the OD-stretch band in terms of the underlying hydrogen-bonding motifs. Through a systematic comparison with the experimental spectra, we demonstrate that computational spectroscopy can be used to gain quantitative, molecular-level insights into framework–water interactions that determine the water adsorption capacity of MOFs as well as the spatial arrangements of the water molecules inside the MOF pores which, in turn, are key to the design of MOF-based materials for water harvesting.</p> </div> </div> </div>

Vibrational and Nuclear Magnetic Resonance Properties of 2,2′-Biquinolines: Experimental and Computational Spectroscopy Study

Experimental (IR, Raman and NMR) techniques and quantum chemical (DFT) methods have been applied to investigate the vibrational and NMR properties of a new ligand based on 2,2′-biquinoline (bq) functionalized with polar hydrophilic tetraethylene glycol monomethylether (TEG) chains (bq_TEG). Vibrational and NMR spectra of the ligand have been explained based on DFT computational data obtained at B3LYP/6-311+G(d,p) level of theory. For the spectroscopic analysis we started from the parent molecule 2,2′-biquinoline and explained the changes in the spectra of bq_TEG in close relation to the corresponding spectra of bq. Our data point to a trans conformation of bq_TEG in solid state, as wells as in liquid phase. The excellent agreement between the experimental and computed data allowed for a reliable assignment of the vibrational and NMR spectra, both for bq and bq_TEG.