Rotational spectra and semi-experimental structures of furonitrile and its water cluster.

Rotational spectroscopy represents an invaluable tool for several applications: from the identification of new molecules in interstellar objects to the characterization of van der Waals complexes, but also for the determination of very accurate molecular structures and for conformational analyses. In this work, we used high-resolution rotational spectroscopic techniques in combination with high-level quantum-chemical calculations to address all these aspects for two isomers of cyanofuran, namely 2-furonitrile and 3-furonitrile. In particular, we have recorded and analyzed the rotational spectra of both of them from 6 to 320 GHz; rotational transitions belonging to several singly-substituted isotopologues have been identified as well. The rotational constants derived in this way have been used in conjunction with computed rotation-vibration interaction constants in order to derive a semi-experimental equilibrium structure for both isomers. Moreover, we observed the rotational spectra of four different intermolecular adducts formed by furonitrile and water, whose identification has been supported by a conformational analysis and a theoretical spectroscopic characterization. A semi-experimental determination of the intermolecular parameters has been achieved for all of them and the results have been compared with those obtained for the analogous system formed by benzonitrile and water.

Valorization Strategies in CO2 Capture: A New Life for Exhausted Silica-Polyethylenimine.

The search for alternative ways to give a second life to materials paved the way for detailed investigation into three silica-polyethylenimine (Si-PEI) materials for the purpose of CO2 adsorption in carbon capture and storage. A solvent extraction procedure was investigated to recover degraded PEIs and silica, and concomitantly, pyrolysis was evaluated to obtain valuable chemicals such as alkylated pyrazines. An array of thermal (TGA, Py-GC-MS), mechanical (rheology), and spectroscopical (ATR-FTIR, 1H-13C-NMR) methods were applied to PEIs extracted with methanol to determine the relevant physico-chemical features of these polymers when subjected to degradation after use in CO2 capture. Proxies of degradation associated with the plausible formation of urea/carbamate moieties were revealed by Py-GC-MS, NMR, and ATR-FTIR. The yield of alkylpyrazines estimated by Py-GC-MS highlighted the potential of exhausted PEIs as possibly valuable materials in other applications.

Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran-Formaldehyde Complex.

The 1:1 benzofuran-formaldehyde complex has been chosen as model system for analyzing π→π* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint "rotational spectroscopy-quantum chemistry" strategy unveiled the dominant role of π→π* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low-energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone-pair→π interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi-experimental equilibrium structure for a molecular complex of such a dimension.

A Journey from Thermally Tunable Synthesis to Spectroscopy of Phenylmethanimine in Gas Phase and Solution.

Phenylmethanimine is an aromatic imine with a twofold relevance in chemistry: organic synthesis and astrochemistry. To tackle both aspects, a multidisciplinary strategy has been exploited and a new, easily accessible synthetic approach to generate stable imine-intermediates in the gas phase and in solution has been introduced. The combination of this formation pathway, based on the thermal decomposition of hydrobenzamide, with a state-of-the-art computational characterization of phenylmethanimine laid the foundation for its first laboratory observation by means of rotational electric resonance spectroscopy. Both E and Z isomers have been accurately characterized, thus providing a reliable basis to guide future astronomical observations. A further characterization has been carried out by nuclear magnetic resonance spectroscopy, showing the feasibility of this synthetic approach in solution. The temperature dependence as well as possible mechanisms of the thermolysis process have been examined.

Theory meets experiment for elucidating the structure and stability of non-covalent complexes: water-amine interaction as a proof of concept.

Several gas-phase spectroscopic investigations have focused on a better understanding of the nature of weak, non-covalent interactions in model systems. However, their characterization and interpretation are still far from being satisfactory. A promising route to fill this gap is offered by strategies in which high-resolution rotational spectroscopy is deeply integrated with state-of-the-art quantum-chemical methodology to accurately determine intermolecular parameters and interaction energies, with the latter interpreted by means of powerful energy decomposition analyses (EDAs). As a proof of concept of this approach, we have selected the adducts formed by n-propylamine (PA) and iso-propylamine (IPA) with water. Among the stable structures computationally predicted, four (out of five) isomers of the PA-water complex and two isomers (trans and gauche) of the IPA-water adduct have been characterized with supersonic jet Fourier transform microwave spectroscopy. Starting from the experimental rotational constants for different isotopic species, computation of the corresponding vibrational corrections allowed a semi-experimental determination of the intermolecular parameters. Different EDAs point out that in all cases a strong O-HN hydrogen bond is the primary interaction. Accurate computations indicate that the length and ramification of the alkyl chain do not significantly affect the water-amine interactions, which - on the contrary - modify the stability order of PA conformers with respect to the isolated systems.

Rich Collection of n-Propylamine and Isopropylamine Conformers: Rotational Fingerprints and State-of-the-Art Quantum Chemical Investigation.

The conformational isomerism of isopropylamine and n-propylamine has been investigated by means of an integrated strategy combining high-level quantum-chemical calculations and high-resolution rotational spectroscopy. The equilibrium structures (and thus equilibrium rotational constants) as well as relative energies of all conformers have been computed using the so-called "cheap" composite scheme, which combines the coupled-cluster methodology with second-order Møller-Plesset perturbation theory for extrapolation to the complete basis set. Methods rooted in the density functional theory have been instead employed for computing spectroscopic parameters and for accounting for vibrational effects. Guided by quantum-chemical predictions, the rotational spectra of isopropylamine and n-propylamine have been investigated between 2 and 400 GHz with Fourier transform microwave and frequency-modulation millimeter/submillimeter spectrometers. Spectral assignments confirmed the presence of several conformers with comparable stability and pointed out possible Coriolis resonance effects between some of them.

Rotational Spectroscopy Meets Quantum Chemistry for Analyzing Substituent Effects on Non-Covalent Interactions: The Case of the Trifluoroacetophenone-Water Complex.

The most stable isomer of the 1:1 complex formed by 2,2,2-trifluoroacetophenone and water has been characterized by combining rotational spectroscopy in supersonic expansion and state-of-the-art quantum-chemical computations. In the observed isomer, water plays the double role of proton donor and acceptor, thus forming a seven-membered ring with 2,2,2-trifluoroacetophenone. Accurate intermolecular parameters featuring one classical O-H···O hydrogen bond and one weak C-H···O hydrogen bond have been determined by means of a semi-experimental approach for equilibrium structure. Furthermore, insights on the nature of the established non-covalent interactions have been unveiled by means of different bond analyses. The comparison with the analogous complex formed by acetophenone with water points out the remarkable role played by fluorine atoms in tuning non-covalent interactions.

State-of-the-art computation of the rotational and IR spectra of the methyl-cyclopropyl cation: hints on its detection in space.

Recent measurements by the Cassini Ion Neutral Mass Spectrometer demonstrated the presence of numerous carbocations in Titan's upper atmosphere. In [Ali et al., Planet. Space Sci., 2013, 87, 96], an analysis of these measurements revealed the formation of the three-membered cyclopropenyl cation and its methyl derivatives. As a starting point of a future coordinated effort of laboratory experiments, quantum-chemical calculations, and astronomical observations, in the present work the molecular structure and spectroscopic properties of the methyl-cyclopropenyl cation have been investigated by means of state-of-the-art computational approaches in order to simulate its rotational and infrared spectra. Rotational parameters have been predicted with an expected accuracy better than 0.1% for rotational constants and on the order of 1-2% for centrifugal-distortion terms. As for the infrared spectrum, despite the challenge of a large amplitude motion, fundamental transitions have been computed to a good accuracy, i.e., the uncertainties are expected to be smaller than 5-10 wavenumbers.

Interactions between azines and alcohols: a rotational study of pyridine-tert-butyl alcohol.

We assigned the rotational spectra of the parent and the OD isotopologues of the intermolecular complex pyridine-tert-butyl alcohol. The rotational and 14N quadrupole coupling constants are in agreement with a σ-type shape and a Cs symmetry of the complex. The two subunits are held together by a "classical" O-HN intermolecular hydrogen bond. Structural features of these hydrogen bonds are given and compared to those of similar molecular adducts. The ON distance decreases by 4 mÅ upon deuteration of the hydroxyl group, denoting a marked reverse Ubbelohde effect of the O-HN hydrogen bond.

Internal dynamics of cyclohexanol and the cyclohexanol-water adduct.

Two conformers of cyclohexanol and the cyclohexanol-water adduct have been characterized in a jet expansion using rotational spectroscopy. In the gas phase, cyclohexanol adopts an equatorial position for the hydroxyl group, with the two conformers differing in the orientation of the hydroxylic hydrogen, either gauche or trans with respect to the aliphatic hydrogen at C(1). Axial cyclohexanol was not detected in the jet. The transitions of the gauche conformer are split into two component lines due to the tunneling effect of the O-H internal rotation, which connects two equivalent gauche minima. The tunneling splitting in the vibrational ground state has been determined to be ΔE0+0- = 52(2) GHz. From this splitting, the inversion barriers connecting the two equivalent gauche conformers have been determined using a flexible model to be B2 = 377 cm-1. A single isomer is detected for the cyclohexanol-water dimer, in which the water molecule acts as a proton donor to the equatorial gauche ring. The presence of torsional tunneling in the adduct suggests a concerted large-amplitude-motion in which the internal rotation in the ring is accompanied by a torsion of the water molecule, to produce an equivalent enantiomer. The torsional tunneling in the adduct is reduced to ΔE0+0- = 32.7(4) GHz and the potential barrier in the complex increases to B2 = 494 cm-1.

Carboxylic Acids, Reactivity with Alcohols and Clustering with Esters: A Rotational Study of Formic Acid-Isopropylformate.

The rotational spectrum of the 1:1 complex formic acid-isopropylformate (FA-IPF) has been first observed when trying to assign the pulsed jet Fourier transform microwave (FTMW) spectrum of the adduct formic acid-2-propanol, by expanding a binary mixture of HCOOH and 2-propanol in He. The strong FTMW spectrum of isopropylformate, formed by the esterification reaction, was observed instead. However, when HCOOH was in excess in the binary mixture, it was possible to observe and assign the rotational spectrum of FA-IPF. Later on a much intense spectrum of FA-IPF was obtained, when combining FA with IPF. Finally, the spectra of five isotopologues of the most stable isomer of formic acid-isopropylformate have been observed by means of rotational spectroscopy in supersonic expansion. Some of them, HCOOH-(CH3)2CHOOCD and HCOOH-(CH3)2CDOOCH have been synthesized in the MW cavity by using DCOOH or (CH3)2CDOH as precursors in the esterification process. In the observed isomer of the complex, the two subunits are linked to each other by a standard O-H···O and a weak C-H···O hydrogen bond. The dissociation energy has been estimated to be 34.1 kJ·mol-1.

The Unexplored World of Cycloalkene-Water Complexes: Primary and Assisting Interactions Unraveled by Experimental and Computational Spectroscopy.

The intermolecular interactions in cycloalkene-water adducts were computationally characterized, thus demonstrating that the primary O-H⋅⋅⋅πC=C hydrogen bond is dominated by the electrostatic interaction. A deeper investigation by means of a joint rotational spectroscopy/state-of-the-art quantum chemistry approach also led to the determination of an accurate semi-experimental equilibrium structure for the cyclopentene adduct.

Unveiling the Sulfur-Sulfur Bridge: Accurate Structural and Energetic Characterization of a Homochalcogen Intermolecular Bond.

By combining rotational spectroscopy in supersonic expansion with the capability of state-of-the-art quantum-chemical computations in accurately determining structural and energetic properties, the genuine nature of a sulfur-sulfur chalcogen bond between dimethyl sulfide and sulfur dioxide has been unveiled in a gas-jet environment free from collision, solvent and matrix perturbations. A SAPT analysis pointed out that electrostatic S⋅⋅⋅S interactions play the dominant role in determining the stability of the complex, largely overcoming dispersion and C-H⋅⋅⋅O hydrogen-bond contributions. Indeed, in agreement with the analysis of the quadrupole-coupling constants and of the methyl internal rotation barrier, the NBO and NOCV/CD approaches show a marked charge transfer between the sulfur atoms. Based on the assignment of the rotational spectra for 7 isotopologues, an accurate semi-experimental equilibrium structure for the heavy-atom backbone of the molecular complex has been determined, which is characterized by a S⋅⋅⋅S distance (2.947(3) Å) well below the sum of van der Waals radii.

Theory Meets Experiment for Noncovalent Complexes: The Puzzling Case of Pnicogen Interactions.

A gas-phase nitrogen-nitrogen noncovalent interaction has been unveiled in the nitroethane-trimethylamine complex in an environment free from solvent and matrix effects using rotational spectroscopy in supersonic expansion. Different quantum chemical models (NOCV/CD and NBO) agree in indicating that this interaction largely prevails over the C-H⋅⋅⋅O and C-H⋅⋅⋅N hydrogen bonds. Furthermore, a SAPT analysis shows that electrostatic and dispersion interactions play a comparable role in stabilizing the complex. The conformational landscape exploration and stationary points characterization have been performed using state-of-the-art quantum-chemical computations providing significant insights on structure determination.

Noncovalent Interactions and Internal Dynamics in Pyridine-Ammonia: A Combined Quantum-Chemical and Microwave Spectroscopy Study.

The 1:1 complex of ammonia with pyridine is characterized by using state-of-the-art quantum-chemical computations combined with pulsed-jet Fourier-transform microwave spectroscopy. The computed potential energy landscape indicates the formation of a stable σ-type complex, which is confirmed experimentally: analysis of the rotational spectrum shows the presence of only one 1:1 pyridine-ammonia adduct. Each rotational transition is split into several components owing to the internal rotation of NH3 around its C3 axis and to the hyperfine structure of both 14 N quadrupolar nuclei, thus providing unequivocal proof that the two molecules form a σ-type complex involving both a N-H⋅⋅⋅N and a C-H⋅⋅⋅N hydrogen bond. The dissociation energy (BSSE- and ZPE-corrected) is estimated to be 11.5 kJ mol-1 . This work represents the first application of an accurate yet efficient computational scheme, designed for the investigation of small biomolecules, to a molecular cluster.

The Borderline between Reactivity and Pre-reactivity of Binary Mixtures of Gaseous Carboxylic Acids and Alcohols.

By mixing primary and secondary alcohols with carboxylic acids just before the supersonic expansion within pulsed Fourier transform microwave experiments, only the rotational spectrum of the ester was observed. However, when formic acid was mixed with tertiary alcohols, adducts were formed and their rotational spectra could be easily measured. Quantum mechanical calculations were performed to interpret the experimental evidence.

Hydrated forms of fluoroacetic acid: a rotational study.

The rotational spectra of two conformers of the 1 : 1 adduct of fluoroacetic acid with water have been assigned by pulsed jet Fourier transform microwave spectroscopy. Their shapes differ according to the trans and cis forms of the fluoroacetic acid moiety. This is in contrast to the rotational spectrum of the monomer, for which the cis form has not been observed. Details of the hydrogen bond, structure, dynamics and energetic features of the two species are given.

A butterfly motion of formic acid and cyclobutanone in the 1 : 1 hydrogen bonded molecular cluster.

Upon supersonic expansion, formic acid and cyclobutanone (CBU) form a molecular cluster in which the two constituent molecules, linked by OHO and CHO hydrogen bonds, undergo a rapid interconversion between two equivalent forms. The tunneling motion takes place through the rupture and reformation of the C-HO hydrogen bond between the carbonyl oxygen of HCOOH and one of the two hydrogen atoms of the methylenic group adjacent to the cyclobutanone keto group. From the microwave spectra, tunneling energy splittings (ΔE01) have been determined for the parent (1122.756(3) MHz), DCOOHCBU (1084.538(1) MHz) and HCOODCBU (1180.282(4) MHz) isotopic species. From these splittings, the potential barrier to interconversion has been calculated to be B2 = 39.7(5) cm-1. The tunneling pathway is an asymmetric butterfly-like motion between the two moieties of the adduct, with a barrier at a configuration in which the ring plane of cyclobutanone is coplanar with formic acid.

Potential energy surface of fluoroxene: experiment and theory.

The potential energy surface (PES) of the general anesthetic fluoroxene (2,2,2-trifluoroethyl vinyl ether) was probed in a supersonic jet expansion using broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy and theoretical calculations. The PES is dominated by a single conformation, as other stable minima are shown to kinetically relax in the expansion to the global minimum. Consistently, the rotational spectrum reveals a single conformation. Fluoroxene adopts a CS heavy-atom planar skeleton structure in the gas phase, with a cis-trans conformation (cis for the CH2=CH-O-CH2- and trans for the =CH-O-CH2-CF3 part). The sensitivity of a recently-built CP-FTMW spectrometer at the UPV/EHU is demonstrated by the detection of five isotopologues of fluoroxene in natural abundance, corresponding to the (13)C and (18)O monosubstituted species. The rS and r0 structures were determined and are in good agreement with theoretical predictions using the MP2, B3LYP and M06-2X methods.

Conformational Equilibrium and Potential Energy Functions of the O-H Internal Rotation in the Axial and Equatorial Species of 1-Methylcyclohexanol.

The rotational spectra of four conformers (At, Ag, Et, Eg) of the tertiary alcohol 1-methylcyclohexanol were assigned by pulsed jet Fourier transform microwave spectroscopy. The transitions of two gauche conformers were split in two separated component lines, but it was not possible-from the available measured transitions-to accurately determine their vibrational ΔE0+0- ground-state splittings, respectively. In addition, the rotational spectra of the four OD deuterated isotopologues were measured and assigned. For the gauche species of this isotopologue we were able to determine the tunneling splittings, ΔE0+0-(Ag, OD) = 15.581(5) GHz and ΔE0+0-(Eg, OD) = 18.17(3) GHz, respectively. From these splittings the inversion barriers for Ag and Eg were determined, by using a flexible model, to be B2(Ag) = 356(10) and B2(Eg) = 320(10) cm(-1), respectively.