Revealing the Structure and Noncovalent Interactions of Isolated Molecules by Laser-Desorption/Ionization-Loss Stimulated Raman Spectroscopy and Quantum Calculations

Harmonic and anharmonic vibrational computations for biomolecular building blocks: Benchmarking DFT and basis sets by theoretical and experimental IR spectrum of glycine conformers

Advanced vibrational spectroscopic experiments have reached a level of sophistication that can only be matched by numerical simulations in order to provide an unequivocal analysis, a crucial step to understand the structure-function relationship of biomolecules. While density functional theory (DFT) has become the standard method when targeting medium-size or larger systems, the problem of its reliability and accuracy are well-known and have been abundantly documented. To establish a reliable computational protocol, especially when accuracy is critical, a tailored benchmark is usually required. This is generally done over a short list of known candidates, with the basis set often fixed a priori. In this work, we present a systematic study of the performance of DFT-based hybrid and double-hybrid functionals in the prediction of vibrational energies and infrared intensities at the harmonic level and beyond, considering anharmonic effects through vibrational perturbation theory at the second order. The study is performed for the six-lowest energy glycine conformers, utilizing available "state-of-the-art" accurate theoretical and experimental data as reference. Focusing on the most intense fundamental vibrations in the mid-infrared range of glycine conformers, the role of the basis sets is also investigated considering the balance between computational cost and accuracy. Targeting larger systems, a broad range of hybrid schemes with different computational costs is also tested.

Conformational Mapping, Interactions, and Fluorine Impact by Combined Spectroscopic Approaches and Quantum Chemical Calculations

Noncovalent interactions and their careful variation can be crucial in understanding molecular structures, conformational topographies, and properties. Here, we examine the fluorination impact on the structure and conformational behavior of 2-(2-fluorophenyl)ethyl alcohol (2-FPEAL) by monitoring the first individual ionization-loss-stimulated Raman spectra of the jet-cooled molecule. The comparison of two different broad-range spectra and predicted equivalents discloses two distinct structures. One possesses a folded side chain (gauche) and the other an extended chain (anti) with the terminal hydrogen atom pointing opposite or toward the fluorine side, indicating the improper previous tentative assignment of the latter. These conformers resemble and differ from the nonfluorinated analogue structures. Theoretical analyses reveal interconversion pathways of 2-FPEAL conformers during expansion and the delicate balance between attractive (C-H···F and O-H···π) and repulsive interactions. These findings show the achievements of our integrated approach, suggesting its potential for overcoming future structural challenges.

Synergistic Spectroscopic and Computational Characterization Evidencing the Preservation or Flipping of the Hydroxyl Group of 2-Phenylethyl Alcohol upon Single and Double Hydration

Even apparently simple, obtaining and analyzing observations on molecules and clusters and unambiguously assigning their structures is challenging. We report here the first ionization-loss Raman spectra compared to quantum chemical predictions for establishing the structural preferences of hydrates of the neurotransmitters hydroxy analogue, 2-phenylethyl alcohol (PEAL). The spectra encode two monohydrates and two previously unnoticed dihydrates, consequences of water insertion and sidewise attachment to the O-H group of gauche PEALs, in PEAL-H2O and PEAL-(H2O)2, or the higher-energy gauche-trans PEAL in the latter. The electronic structures retain the stable PEAL or flip its O-H to convert the gauche-trans PEAL conformer to the global minimum-energy dihydrate. We disclose conventional and bifurcated hydrogen bonds and electron steric repulsions by noncovalent interaction analysis and correlations between the experimental O-H stretching vibrational frequencies and the O-H and H···X bond lengths and electron densities, pointing to implications on hydrate forms and our approach virtue.

Experimental/Computational Study on the Impact of Fluorine on the Structure and Noncovalent Interactions in the Monohydrated Cluster of ortho-Fluorinated 2-Phenylethylamine

Fluorine-containing medicinal compounds frequently allow modulation of physical-chemical properties. Here, we address the effect of fluorine, near the ethylamino side chain, on conformational flexibility and noncovalent interactions (NCIs) of the selected jet-cooled monohydrated cluster of 2-(2-fluoro-phenyl)-ethylamine (2-FPEA) by mass-selected resonance-enhanced two-photon ionization and ionization-loss stimulated Raman spectroscopies. Our results show that Raman spectral signatures of the 2-FPEA-H₂O cluster match the scaled harmonic vibrational Raman frequencies, resulting from density functional theory calculations of the most stable 2-FPEA gauche conformer hydrogen-bonded (HB) to water, confirming the three-dimensional cluster structure. This predicted electronic structure, together with NCI analysis, allows visualization and assessment of the attractive and repulsive interactions. The comparison of the NCIs and revealed red (O-H and N-H stretches) and blue shifts (C-H stretches and CH₂ out-of-plane bends) of the cluster to other class members confirm O-H···N, N-H···π, C-H···O, and C-H···F HB formation and their contribution to structure stabilization, uncovering the potential of the approach.

Attaching Onto or Inserting Into an Intramolecular Hydrogen Bond: Exploring and Controlling a Chirality-Dependent Dilemma for Alcohols

Prereactive complexes in noncovalent organocatalysis are sensitive to the relative chirality of the binding partners and to hydrogen bond isomerism. Both effects are present when a transiently chiral alcohol docks on a chiral α-hydroxy ester, turning such 1:1 complexes into elementary, non-reactive model systems for chirality induction in the gas phase. With the help of linear infrared and Raman spectroscopy in supersonic jet expansions, conformational preferences are investigated for benzyl alcohol in combination with methyl lactate, also exploring p-chlorination of the alcohol and the achiral homolog methyl glycolate to identify potential London dispersion and chirality effects on the energy sequence. Three of the four combinations prefer barrierless complexation via the hydroxy group of the ester (association). In contrast, the lightest complex predominantly shows insertion into the intramolecular hydrogen bond, such as the analogous lactate and glycolate complexes of methanol. The experimental findings are rationalized with computations, and a uniform helicality induction in the alcohol by the lactate is predicted, independent of insertion into or association with the internal lactate hydrogen bond. p-chlorination of benzyl alcohol has a stabilizing effect on association because the insertion motif prevents a close contact between the chlorine and the hydroxy ester. After simple anharmonicity and substitution corrections, the B3LYP-D3 approach offers a fairly systematic description of the known spectroscopic data on alcohol complexes with α-hydroxy esters.