In this study, we present the structures of MEA-H2O and MEA-CO2 complexes identified in pulsed jet expansion using the broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy COMPACT that operated in the 2-18 GHz range[1]. We reveal the microhydration processes of MEA from these observed MEA-H2O clusters and show that the structures of MEA-(H2O)n are closely analogous to the pure water clusters (H2O)n+2. For example, the MEA monohydrate, dihydrate, and trihydrate adopt cyclic topologies similar to water trimer, tetramer, and pentamer, respectively. With four water molecules participating, we identified two isomers of MEA-(H2O)4, named Cage and Prism. In the observed MEA-H2O clusters, the intramolecular OH-NH2 H-bond of MEA is broken to form an energetically favorable intermolecular H-bond with water. In contrast, without the presence of water, CO2 is unable to break the intramolecular H-bond based on the observed MEA-CO2 complexes[2]. Interestingly, we discovered that the OH lone pair-CO2 interaction has higher energetic priority than the NH2 lone pair-CO2 interaction, which is in sharp contrast to the results from aqueous solutions. How would the CO2 bind to MEA with the presence of a few water molecules? A short discussion on the topic will be made based on what we learned from the observed structures of MEA-H2O and MEA-CO2. [1] D. Schmitz, V. A. Shubert, T. Betz, M. Schnell, J. Mol. Spectrosc. 2012, 280, 77–84. [2] F. Xie, W. Sun, P. Pinacho, M. Schnell, Angew. Chem. Int. Ed. 2023, e202218539; Angew. Chem. 2023, e202218539.

We have detected the \textit{tert}-butyl hydroperoxide dimer (\textit{t}-BuOOH)$_2$ in gas phase at room temperature using conventional FTIR techniques. The dimer is identified by an asymmetric absorbance band assigned to the hydrogen-bound OH$_\text{b}$-stretch. The gas-phase dimer assignment is supported by Ar matrix isolation FTIR experiments at 12 K. The dimer OH$_\text{b}$-stretching band is located at $\sim$3452 \wn{} redshifted by $\sim$145 \wn \ from the monomer OH-stretching band. Theoretically, we find the lowest energy structure of ($t$-BuOOH)$_2$ to be a doubly hydrogen bound 6-membered ring. The ring strain in this structure leads to non-optimal H-bond angles and a smaller than expected redshift. We calculate the transition frequency and oscillator strength of the OH$_\text{b}$-stretching transition with a 1D local mode model and DFT electronic structure methods. The constant of dimer formation \textit{K} is estimated from the experimental integrated absorbance and the theoretically calculated oscillator strength of the OH$_\text{b}$-stretching band. The unitless \textit{K} is estimated to be $\sim$0.41 for ($t$-BuOOH)$_2$.

Polyol water clusters can act as a proxy for alcohol water ices in the interstellar medium. By studying these clusters, conclusions can be drawn about ionization and solvation processes of atmospheric and interstellar chemical interest, as was done with the polyols ethylene glycol, 1,2 propylene glycol, and 1,3 propylene glycol. These polyol water clusters are generated in a continuous supersonic jet expansion, then photionized with synchrotron based tunable vacuum ultraviolet light, and detected by a reflectron time-of-flight mass spectrometer. Polyol water cluster fragments are observed and the appearance energies for these clusters as well as water clusters are determined. To explain the experimentally obtained mass spectra, theoretical calculations are performed on neutral and ionized polyol water clusters as well as the detected fragments and fragment clusters. From these calculations, interaction and ionization energies are determined and hydrogen bond networks may be visualized.

Copper oxides exhibit several useful electronic attributes such as electron correlation effects, spin dynamics, magnon behavior, and high temperature superconductivity. The atomic precision and tunability of gas phase neutral clusters provides insights into the charge carrier excitation dynamics of strongly correlated bulk materials by revealing how their excited state dynamics change with subtle changes in stoichiometry, local geometry, and electronic density. I will present our recent measurements of the ultrafast relaxation dynamics of sub-nanometer neutral copper oxide clusters investigated with femtosecond pump-probe spectroscopy coupled with time-of-flight mass spectrometry and supported by theoretical calculations. The carrier dynamics of neutral copper oxides clusters can be tuned upon the addition/subtraction of each atom and is attributed to the ligand-to-metal charge-transfer (LMCT) character of the photoexcitation. The sub-picosecond excited state lifetimes decrease almost linearly upon sequential oxidation, for Cu_nO_x (n ≤ 7), of  ∼ 30-100 fs per oxygen atom. Stoichiometric clusters show an increase in excited state lifetimes as cluster size increases and is supported by DFT calculations attributing the trend to the localization of electrons in the excited state. Understanding the electronic properties and dynamics of copper oxide clusters promises to shape a new era of semiconductor and superconductor physics through development of new materials containing tunable properties.

Here we report the study on the cis and trans isomers of decahydro-2-naphthol and various complexes with it. Decahydro-2-naphthol is structurally analogous to a section of the steroid hormone androsterone, which has been recently studied by rotational spectroscopy.Caliebe, S. V. M.; Pinacho, P., Schnell, M. J. Phys. Chem. Lett. 2022, 13, 11913-11917.or the large (from a rotational spectroscopy point of view) molecule androsterone, no experimental gas-phase structure was observed. In addition, intermolecular interactions in complexes with androsterone and smaller molecules cannot be experimentally studied due to the size of the steroid hormone. r0pt Figure Because of that, decahydro-2-naphthol is an excellent system to indicate intermolecular interactions with other molecules and gain knowledge of the experimental gas-phase structure and thus, the conformational flexibility and arrangement in order to start to understand reaction mechanisms and functions of steroid hormones. Further, the orientations of the OH-groups in the decahydro-2-naphthol molecules were determined and also compared with the results from androsterone. The molecules were studied in the gas phase in a cold and isolated environment generated by a supersonic expansion. The spectra were recorded using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2-8 GHz frequency region. The assignment was supported by quantum-chemical calculations.

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Caliebe, S. V. M.; Pinacho, P., Schnell, M. J. Phys. Chem. Lett. 2022, 13, 11913-11917.F

Hydrogen bonding (H-bonding) of aromatic clusters in dilute aqueous solutions has been a subject of great interest in recent years. In the present work, we investigated the hydrogen-bonded structures of jet-cooled benzoic acid-water clusters, (BA)_m-(H₂O)_n (m, n = 1 or 2), with IR spectroscopy and quantum-chemical calculations. We measured the IR spectra of mass selected (BA)_m-(H₂O)_n in the C-H and O-H stretching regions by IR excitation -VUV ionization with a reflectron time-of-flight mass spectrometer (RF-TOF-MS). The IR spectrum of BA monomer was obtained by 1+1 IR-VUV photoionization technique, and those of the clusters were obtained by the depletion of the VUV-induced ion signal upon IR predissociation. The observed IR spectra were analyzed by comparison with spectra of possible structures predicted with anharmonic calculations at the B3LYP/6-311++G(d,p) level of theory. We discuss the structures of (BA)_m-(H₂O)_n and how the H-bonding network developed in these clusters.

Aluminum oxides are an affordable, abundant, and commonly utilized material for many applications such as magnetic devices, gas sensing, and catalysis. To maximize catalytic activity, a modern frontier of material science is aimed at creating ever-decreasing sizes of materials which drives new demands to understand the electronic properties and geometric structures down to the molecular (cluster) scale. I will present recent measurements investigating the excited state dynamics of neutral aluminum clusters collected with femtosecond pump-probe spectroscopy coupled with time-of-flight mass spectrometry. Neutral clusters are excellent mimics of bulk material properties and show tunable ultrafast dynamics and excited state lifetimes with the addition and subtraction of each atom in their composition. Our preliminary results on the ultrafast dynamics of aluminum oxide clusters reveals a trend where sequential addition of oxygen atoms decreases the measured excited state lifetimes. For example, in the Al₄O_y series (y = 0-2), the measured excited state lifetimes decrease by 37% with the addition of 2 oxygen atoms, which is consistent across our measurements for other aluminum oxide series. By understanding the electron dynamics and charge carrier separation in aluminum oxide clusters, bulk material defects and vacancies can be capitalized upon to increase the efficiency of aluminum containing species.

Sugars are one of the most important families of biomolecules. They act as mediators of molecular recognition processes. Among the possible biomedical applications, one example is the addition of mannosides to photosensitizers used in PhotoDynamic Therapy, targeting Mannose Receptor (MR) proteins on pathogenic cells. A precise description of the structural basis of Mannose-MR local and direct interplay, which can be provided by gas phase spectroscopy, critically lacks to design PSs with improved selectivity. In the gas phase, combining experimental mass resolved and conformer selective double resonance vibrational spectroscopy and theoretical chemistry studies,Cocinero E.J.; Çarçabal P., Top. Curr. Chem., 2015, 364, pp. 299-333.e have already been able to observe several complexes between sugars and peptide models.Cocinero E.J.; Çarçabal P.; Vaden T.D.; Simons J.P.; Davis B.G., Nature, 2011, 469, 76-79.Cocinero E.J.; Çarçabal P.; Vaden T.D.; Davis B.G.; Simons J.P., J. Amer. Chem. Soc., 2011, 133, 4548-4557. We can now also observe such complexes with a controlled number of water molecules. Our most recent results on complexes of mannose with different peptide models, either hydrated or not, allow resolving the nature of the interactions between the molecules, for each donor and acceptor molecular group involved in the non-covalent bonds governing the complexes. These results evidence the adaptability of the sugar moiety to its peptide receptor. In particular, the study of the complexes formed with few water molecules may highlight the role of water in molecular recognition processes in an unprecedented manner.

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Cocinero E.J.; Çarçabal P., Top. Curr. Chem., 2015, 364, pp. 299-333.w Cocinero E.J.; Çarçabal P.; Vaden T.D.; Simons J.P.; Davis B.G., Nature, 2011, 469, 76-79.

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2-(4-Hydroxyphenyl)ethanol, commonly known as Tyrosol, is a powerful antioxidant in the human diet, mainly found in olive oil. It also presents other biological properties such as cardiovascular drug or geroprotector agent, among others. As it widely known, these biological properties are related with its molecular structure and its conformational landscape. Here we presented the study of the molecular structure and conformational panorama of tyrosol and its mono- and dihydrated complexes, by means of gas-phase high-resolution rotational spectroscopy. From a vast number of predicted conformers, three of them have been experimentally identified for the monomer. The large amplitude motions interconverting the predicted and observed forms have been studied by means of computational methods. Five different conformers were observed for the monohydrated complex, all of them preserving the monomer structures with the water being able to interact as a proton acceptor with both hydroxy groups. In this competition between these two groups the interaction with the hydroxy group attached to the phenyl ring seems to present a higher stability. This is corroborated with the observation of two conformers of the dihydrated complex, which also preserves the structures of the two most stable monomer forms with water dimer forming a cycle only with the hydroxy group attached to the phenyl ring.