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New spectroscopic experiments and state‐of‐the‐art quantum‐chemical computations of creatinine in different aggregation states unequivocally unveiled a significant tuning of tautomeric equilibrium by the environment: from the exclusive presence of the amine tautomer in the solid state and aqueous solution to a mixture of amine and imine tautomers in the gas phase. Quantum‐chemical calculations predict the amine species as the most stable tautomer by about 30 kJ mol−1 in condensed phases. On the contrary, moving to the isolated forms, both Z and E imine isomers become more stable by about 7 kJ mol−1. Since the imine isomers and one amine tautomer are separated by significant energy barriers, all of them should be present in the gas phase. This prediction has indeed been confirmed by high‐resolution rotational spectroscopy, which provides the first experimental characterization of the elusive imine tautomer. The interpretation of the complicated hyperfine structure of the rotational spectrum, originated by three 14N nuclei, makes it possible to use the spectral signatures as a sort of fingerprint for each individual tautomer in the complex sample., A journey among the aggregation states of a key biomolecule, creatinine, is reported, guided by state‐of‐the‐art quantum‐chemical computations and spectroscopic techniques. The fine tuning of the creatinine tautomeric equilibrium by different environments is unveiled and fully understood by vis‐à‐vis comparison between simulated and experimental signatures.

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.

Der 1:1-Benzofuran-Formaldehyd-Komplex wurdeals Modellsystem fürp!p*-Wechselwirkungen in supramo-lekularen Umgebungen mit heteroaromatischen Ringen undCarbonylgruppen gewählt. Die Strategie einer gemeinsamen„Rotationsspektroskopie-Quantenchemie“-Analyse enthülltdie Rolle dominanter intermolekularerp!p*-Wechselwir-kungen in solchen Addukten. Die Untersuchung der intermo-lekularen Potentialfläche identifiziert 14 tiefliegende Minima,wobei 4 gestapelte Isomere stabiler sind als die durch Wasser-stoffbrücken oder „freies Elektronenpaar“!p-Wechselwir-kungen verbundene. Alle lokalen Minima sind nur lose durchÜbergangszustände getrennt, was unter den experimentellenBedingungen eine effektive Relaxation zum globalen Mini-mum erwarten lässt. Der Nachweis nur einer Spezies, die dankBerechnung genauer spektroskopischer Parameter und Cha-rakterisierung von 11 Isotopologen eindeutig zugeordnetwerden konnte, bestätigt dies. Die große Isotopologenanzahlermçglichte die Bestimmung der ersten semi-experimentellenGleichgewichtsstruktur für einen Molekülkomplex dieserGrçßenordnung.

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.

The intermolecular interactions in cycloalkene-water adducts were computationally characterized, thus demonstrating that the primary O-H...pi(C=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.

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.

4-Fluoro-threonine, the only fluoro amino acid of natural origin discovered so far, is an interesting target for both synthetic and theoretical investigations. In this work, we lay the foundation for spectroscopic characterization of 4-fluoro-threonine. First, we report a diastereoselective synthetic route, which is suitable to produce synthetic material for experimental characterization. The addition of the commercially available ethyl isocyanoacetate to benzyloxyacetaldehyde led to the corresponding benzyloxy-oxazoline, which was hydrolyzed and transformed into ethyl (4S*,5S*)-5-hydroxymethyl-2-oxo-4-oxazolidinecarboxylate in a few steps. Fluorination with diethylamino sulfur trifluoride (DAST) afforded ethyl (4S*,5S*)-5-fluoromethyl-2-oxo-4-oxazolidinecarboxylate, which was deprotected to give the desired diastereomerically pure 4-fluoro-threonine, in 8-10% overall yield. With the synthetic material in our hands, acid-base titrations have been carried out to determine acid dissociation constants and the isoelectric point, which is the testing ground for the theoretical analysis. We have used machine learning coupled with quantum chemistry at the state-of-the-art to analyze the conformational space of 4-fluoro-threonine, with the aim of gaining insights from the comparison of computational and experimental results. Indeed, we have demonstrated that our approach, which couples a last-generation double-hybrid density functional including empirical dispersion contributions with a model combining explicit first-shell molecules and a polarizable continuum for describing solvent effects, provides results and trends in remarkable agreement with experiments. Finally, the conformational analysis applied to fluoro amino acids represents an interesting study for the effect of fluorine on the stability and population of conformers. 4-Fluoro-threonine, the only fluoro amino acid of natural origin discovered so far, is an interesting target for both synthetic and theoretical investigations. In this work, we lay the foundation for spectroscopic characterization of 4-fluoro-threonine. First, we report a diastereoselective synthetic route, which is suitable to produce synthetic material for experimental characterization. The addition of the commercially available ethyl isocyanoacetate to benzyloxyacetaldehyde led to the corresponding benzyloxy-oxazoline, which was hydrolyzed and transformed into ethyl (4S*,5S*)-5-hydroxymethyl-2-oxo-4-oxazolidinecarboxylate in a few steps. Fluorination with diethylamino sulfur trifluoride (DAST) afforded ethyl (4S*,5S*)-5-fluoromethyl-2-oxo-4-oxazolidinecarboxylate, which was deprotected to give the desired diastereomerically pure 4-fluoro-threonine, in 8-10% overall yield. With the synthetic material in our hands, acid-base titrations have been carried out to determine acid dissociation constants and the isoelectric point, which is the testing ground for the theoretical analysis. We have used machine learning coupled with quantum chemistry at the state-of-the-art to analyze the conformational space of 4-fluoro-threonine, with the aim of gaining insights from the comparison of computational and experimental results. Indeed, we have demonstrated that our approach, which couples a last-generation double-hybrid density functional including empirical dispersion contributions with a model combining explicit first-shell molecules and a polarizable continuum for describing solvent effects, provides results and trends in remarkable agreement with experiments. Finally, the conformational analysis applied to fluoro amino acids represents an interesting study for the effect of fluorine on the stability and population of conformers.

The rotational spectra of the parent and eight isotopologues of the 1:1 complex formic acid – tert-butyl alcohol (FA-TBA) have been measured by pulsed jet Fourier transform microwave spectroscopy. The spectra have been observed in the supersonic expansion of a mixture of FA and TBA in Helium, differently with respect to the mixtures of FA with primary and secondary alcohols, which undergo the esterification reaction upon supersonic expansion. In the complex, the two subunits are linked to each other by two different O-H···O hydrogen bonds (HB) in which FA and TBA are alternate their roles of bond acceptor and donor. Upon H → D substitution of the corresponding O-H···O HB, a small Ubbelohde effect is observed.

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&middot, &middot, O hydrogen bond and one weak C-H&middot, 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.

In the past decade, many gas-phase spectroscopic investigations have focused on the understanding of the nature of weak interactions in model systems. Despite the fact that non-covalent interactions play a key role in several biological and technological processes, their characterization and interpretation are still far from being satisfactory. In this connection, integrated experimental and computational investigations can play an invaluable role. Indeed, a number of different issues relevant to unraveling the properties of bulk or solvated systems can be addressed from experimental investigations on molecular complexes. Focusing on the interaction of biological model systems with solvent molecules (e.g., water), since the hydration of the biomolecules controls their structure and mechanism of action, the study of the molecular properties of hydrated systems containing a limited number of water molecules (microsolvation) is the basis for understanding the solvation process and how structure and reactivity vary from gas phase to solution. Although hydrogen bonding is probably the most widespread interaction in nature, other emerging classes, such as halogen, chalcogen and pnicogen interactions, have attracted much attention because of the role they play in different fields. Their understanding requires, first of all, the characterization of the directionality, strength, and nature of such interactions as well as a comprehensive analysis of their competition with other non-covalent bonds. In this review, it is shown how state-of-the-art quantum-chemical computations combined with rotational spectroscopy allow for fully characterizing intermolecular interactions taking place in molecular complexes from both structural and energetic points of view. The transition from bi-molecular complex to microsolvation and then to condensed phase is shortly addressed.

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.

The pulsed supersonic jet expansion microwave spectra of the parent and of the CD=O monodeuterated species of cyclohexylformate have been measured in the 6.5–18 GHz range. The samples have been obtained by direct esterification of HCOOH (or DCOOH) with cyclohexyl alcohol during a supersonic expansion in a microwave cavity. Most of the observed transitions are split into two component lines, due to the tunnelling motion connecting two equivalent gauche forms. The tunnelling barrier has been determined to be B2 = 267 (10) cm−1. Structural information has been obtained from the 6 available rotational constants.

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., A multidisciplinary approach involving organic synthesis and high‐resolution molecular spectroscopy has led to the definition of a thermally tunable formation route for imines. Its application to phenylmethanimine combined with an integrated rotational spectroscopy–quantum chemistry protocol has enabled the full characterization of the latter, thus providing the means for future astronomical observations.

Molecules containing sulfur play an important role in atmospheric chemistry, but their reactivity in such environment is far from being well characterised. Therefore, important transient species might still have to be disclosed. In this regard, this paper aims at contributing to this field by means of the computational spectroscopic characterisation of potentially relevant atmospheric species, such as the (Formula presented.) isomers. Based on our computational investigation, the most stable species are the carbonothionic O,S- and O,O-acids. In addition to them, the sulfene molecule has also been considered. For all these species, the equilibrium structure and the corresponding rotational constants have been accurately evaluated using a coupled-cluster-based composite approach. All the other parameters required for the simulation of the rotational and ro-vibrational spectra have been obtained by exploiting density functional theory.

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.

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 has been observed. However, when formic acid was mixed with tertiary alcohols, adducts have been formed and their rotational spectra have been easily measured. Quantum mechanical calculations have been performed to interpret the experimental evidence.

The rotational spectra of several isotopologues of the 1:1 complex between the inhaled anesthetic isoflurane and water have been recorded and analyzed by using Fourier transform microwave spectroscopy. The rotational spectrum showed a single rotamer, corresponding to the configuration in which the most stable conformer of isolated isoflurane is linked to the water molecule through an almost linear C-H⋅⋅⋅O weak hydrogen bond. All transitions display a hyperfine structure due to the (35)Cl (or (37)Cl) nuclear quadrupole effects.

The pulsed jet Fourier transform microwave spectrum of the molecular adduct pyridine–difluoromethane shows that the two subunits are linked to each other through a bifurcated CH 2 ···N and a CH···F weak hydrogen bond. Energies and structural information of these links are given.

Formic acid and dimethyl ether are combined in a supersonic expansion to form a molecular adduct with the two subunits held together by a "classical" OH···O hydrogen bond and a bifurcated weak CH2···O hydrogen bond. The rotational spectra of the parent and of two (13)C isotopologues in natural abundance show that the complex has Cs symmetry, with the heavy atom symmetry planes of HCOOH and (CH3)2O perpendicular to each other.

The rotational spectra of the parent species and of four water isotopologues (-DOH, -HOD, -DOD, and -H(18)OH) of the adduct 3-fluoropyridine-water have been investigated using pulsed supersonic-jet Fourier-transform microwave spectroscopy. From the rotational constants, the structure of the adduct was deduced where the water is linked to the aromatic ring through an intermolecular O-H···N hydrogen bond with a bond distance of 1.9961(5) Å and an O-H···N angle of 156.8(1)°. The shape of the complex is such that the water oxygen is in the plane of the aromatic ring, on the opposite side of fluorine, and forming a C-H···N weak hydrogen bond with the adjacent aromatic hydrogen.

The rotational spectra of two isotopologues of chlorotrifluoroethylene-dimethyl ether show that the two constituent molecules are held together by a lone pair center dot center dot center dot pi interaction. The ether oxygen is linked to the (CF2) carbon atom, with a C-O distance of 2.908 angstrom.

The investigation of an isolated ribofuranose unit in the gas phase reveals the intrinsic conformational landscape of the biologically active sugar form.We report the rotational spectra of two conformers of methyl b-D-ribofuranoside in a supersonic jet expansion. Both conformers adopt a near twisted (3T2) ring conformation with the methoxy and hydroxymethyl substituents involved in various intramolecular hydrogen bonds., MINECO-FEDER CTQ2015-68148-C2-P

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.

The rotational spectrum of the 1:1 complex pyridine–chlorotrifluoromethane shows that the two moieties are held together through a C Cl···N halogen bond (RCl···N = 2.909(5) A). The quadrupolar effects of the 35Cl (or 37Cl) and 14N nuclei combined with the high density of the rotational transitions give rise to a very congested spectrum. The multidimensional large amplitude internal dynamics is reflected in a two times larger rotational constant A, on an unexpected positive values of the inertial defect of the ground state, and on a negative value of the DJ centrifugal distortion constant. This comes from a combination of a free internal rotation and of a very low energy in plane oscillation of CF3Cl with respect to pyridine. However, a coupled Hamiltonian including Coriolis interactions with an hypothetical (unobserved) v = 1 state solves the initial result of an unphysical negative value for the centrifugal distortion constant.

open 12 si MIUR “PRIN 2015” funds (Grant Number 2015F59J3R) 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. embargoed_20191010 Obenchain, Daniel A.; Spada, Lorenzo; Alessandrini, Silvia; Rampino, Sergio; Herbers, Sven; Tasinato, Nicola; Mendolicchio, Marco; Kraus, Peter; Gauss, Jürgen; Puzzarini, Cristina; Grabow, Jens-Uwe*; Barone, Vincenzo Obenchain, Daniel A.; Spada, Lorenzo; Alessandrini, Silvia; Rampino, Sergio; Herbers, Sven; Tasinato, Nicola; Mendolicchio, Marco; Kraus, Peter; Gauss, Jürgen; Puzzarini, Cristina; Grabow, Jens-Uwe*; Barone, Vincenzo

The pulsed supersonic jet expansion microwave spectra of the parent and all three 13C mono-substituted isotopologues of methyl trifluoroacetate have been measured in the 6.5–18 GHz range. All observed transitions are split into two component lines, due to the internal rotation of the methyl group. The corresponding barrier has been determined to be V 3 = 4.379(3) kJ/mol. Structural information has been obtained from the 12 available rotational constants.

C-cyanomethanimine (HNCHCN), existing in the two $Z$ and $E$ isomeric forms, is a key prebiotic molecule, but, so far, only the $E$ isomer has been detected toward the massive star-forming region. Sagittarius B2(N) using transitions in the radio wavelength domain. With the aim of detecting HNCHCN in Sun-like-star forming regions, the laboratory investigation of its rotational spectrum has been extended to the millimeter-/submillimeter-wave (mm-/submm-) spectral window in which several unbiased spectral surveys have been already carried out. High-resolution laboratory measurements of the rotational spectrum of C-cyanomethanimine were carried out in the 100-420 GHz range using a frequency-modulation absorption spectrometer. We then searched for the C-cyanomethanimine spectral features in the mm-wave range using the high-sensitivity and unbiased spectral surveys obtained with the IRAM 30-m antenna in the ASAI context, the earliest stages of star formation from starless to evolved Class I objects being sampled. For both the $Z$ and $E$ isomers, the spectroscopic work has led to an improved and extended knowledge of the spectroscopic parameters, thus providing accurate predictions of the rotational signatures up to $\sim$700 GHz. So far, no C-cyanomethanimine emission has been detected toward the ASAI targets, and upper limits of the column density of $\sim$ 10$^{11}$--10$^{12}$ cm$^{-2}$ could only be derived. Consequently, the C-cyanomethanimine abundances have to be less than a few 10$^{-10}$ for starless and hot-corinos. A less stringent constraint, $\leq$ 10$^{-9}$, is obtained for shocks sites. The combination of the upper limits of the abundances of C-cyanomethanimine together with accurate laboratory frequencies up to $\sim$ 700 GHz poses the basis for future higher sensitivity searches around Sun-like-star forming regions., 9 pages, 4 figures, 2 tables. A&A accepted

The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, O-H⋅⋅⋅Cl (R(H⋅⋅⋅Cl) =2.638(2) Å) and C-H⋅⋅⋅O (R(H⋅⋅⋅O) =2.501(2) Å), forming a five-membered ring. All transitions display the hyperfine structure due to the (35)Cl (or (37)Cl) nuclear quadrupole effects. Dynamical features in the spectrum are caused by two large-amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V(2) =320(10) cm(-1). Finally, an unexpected small value of the inertial defect (-0.96 uÅ(2) rather than -3.22 uÅ(2)) allowed the estimation of the barrier to the internal rotation of the CH(3) group, V(3) ≈8 cm(-1).

The rotational spectra of two isotopologues of the 1:1 complex between chlorotrifluoromethane and formaldehyde have been recorded and analyzed by using Fourier-transform microwave spectroscopy. Only one rotamer was detected, with the two constituent molecules held together through a Cl⋅⋅⋅O halogen bond (R(Cl⋅⋅⋅O) = 3.048 Å). The dimer displays two simultaneous large-amplitude intramolecular motions. The internal rotation of formaldehyde around its symmetry axis (V2 = 28(5) cm(-1)) splits all the rotational transitions into two component lines with a relative intensity ratio of 1:3. On the other hand, the almost free internal rotation (V3 ≈ 2.5 cm(-1)) of the CF3 symmetric top increases the "rigid" value of the rotational constant A by almost one order of magnitude. In addition, all the transitions display a hyperfine structure due to the (35)Cl (or (37)Cl) nucleus quadrupole effects.

The rotational spectra of 2 isotopologues of the molecular adduct CH2F2–CH2ClF show that the two subunits are held together through two weak C–H F–C and one C–H Cl–C linkages, rather than through three weak C–H F–C connections. This suggests the weak Cl H interaction to be observed preferentially to a weak F H one. Structural information on these weak hydrogen bonds has been obtained. The dissociation energy has been estimated from centrifugal distortion to be 5.3 kJ mol1.

The rotational spectra of trifluoroanisole (trifluoromethoxybenzene, C6H5OCF3) and of its 13C and 18O isotopologues in natural abundance have been measured in a supersonic expansion with pulsed-jet Fourier transform microwave spectroscopy. The spectrum is consistent with a perpendicular conformation of the CF3 group with respect to the phenyl ring.

The Virtual Multifrequency Spectrometer (VMS) is a tool that aims at integrating a wide range of computational and experimental spectroscopic techniques with the final goal of disclosing the static and dynamic physical-chemical properties "hidden" in molecular spectra. VMS is composed of two parts, namely, VMS-Comp, which provides access to the latest developments in the field of computational spectroscopy, and VMS-Draw, which provides a powerful graphical user interface (GUI) for an intuitive interpretation of theoretical outcomes and a direct comparison to experiment. In the present work, we introduce VMS-ROT, a new module of VMS that has been specifically designed to deal with rotational spectroscopy. This module offers an integrated environment for the analysis of rotational spectra: from the assignment of spectral transitions to the refinement of spectroscopic parameters and the simulation of the spectrum. While bridging theoretical and experimental rotational spectroscopy, VMS-ROT is strongly integrated with quantum-chemical calculations, and it is composed of four independent, yet interacting units: (1) the computational engine for the calculation of the spectroscopic parameters that are employed as a starting point for guiding experiments and for the spectral interpretation, (2) the fitting-prediction engine for the refinement of the molecular parameters on the basis of the assigned transitions and the prediction of the rotational spectrum of the target molecule, (3) the GUI module that offers a powerful set of tools for a vis-Ã -vis comparison between experimental and simulated spectra, and (4) the new assignment tool for the assignment of experimental transitions in terms of quantum numbers upon comparison with the simulated ones. The implementation and the main features of VMS-ROT are presented, and the software is validated by means of selected test cases ranging from isolated molecules of different sizes to molecular complexes. VMS-ROT therefore offers an integrated environment for the analysis of the rotational spectra, with the innovative perspective of an intimate connection to quantum-chemical calculations that can be exploited at different levels of refinement, as an invaluable support and complement for experimental studies.

The 1:1 complex of tert-butyl alcohol with difluoromethane has been characterized by means of a joint experimental-computational investigation. Its rotational spectrum has been recorded by using a pulsed-jet Fourier-Transform microwave spectrometer. The experimental work has been guided and supported by accurate quantum-chemical calculations. In particular, the computed potential energy landscape pointed out the formation of three stable isomers. However, the very low interconversion barriers explain why only one isomer, showing one O–H⋯F and two C–H⋯O weak hydrogen bonds, has been experimentally characterized. The effect of the H → tert-butyl-group substitution has been analyzed from the comparison to the difluoromethane-water adduct.

none 6 no 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 14N 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. mixed Spada, Lorenzo; Tasinato, Nicola; Vazart, Fanny; Barone, Vincenzo; Caminati, Walther; Puzzarini, Cristina Spada, Lorenzo; Tasinato, Nicola; Vazart, Fanny; Barone, Vincenzo; Caminati, Walther; Puzzarini, Cristina

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.

The rotational spectra of five isotopologues of the 1:1 complex trifluoroanisole-water have been investigated with pulsed jet Fourier transform microwave spectroscopy. The triple fluorination of the methyl group greatly affects the features of the rotational spectrum of the complex with water, with respect to those of the related complex anisole-water. The shape, the internal dynamics, and the isotopic effects turned out to be quite different from those of the anisole-water adduct ( Giuliano , B. M. ; Caminati , W. Angew. Chem., Int. Ed. 2005 , 44 , 603 - 606 ). However, as in anisole-water, water has the double role as a proton donor and proton acceptor, and it is linked to trifluoroanisole through two, O-H···O and C-H···O hydrogen bonds.

The rotational spectra of two isotopologues of a 1:1 difluoromethane-dichloromethane complex have been investigated by pulsed-jet Fourier-transform microwave spectroscopy. The assigned (most stable) isomer has C(s) symmetry and it displays a network of two C-H⋅⋅⋅Cl-C and one C-H⋅⋅⋅F-C weak hydrogen bonds, thus suggesting that the former interactions are stronger. The hyperfine structures owing to (35)Cl (or (37)Cl) quadrupolar effects have been fully resolved, thus leading to an accurate determination of the three diagonal (χ(gg); g=a, b, c) and the three mixed quadrupole coupling constants (χ(gg'); g, g'=a, b, c; g≠g'). Information on the structural parameters of the hydrogen bonds has been obtained. The dissociation energy of the complex has been estimated to be 7.6 kJ mol(-1).

The rotational spectra of four isotopologues of the 1:1 complex between formic acid and pyridine show that the two units are linked together through a “classical” (OH···N) and a weak (CH···O) hydrogen bond. The molecular system appears quite rigid and no effects of the internal motions have been observed in the spectrum. The dissociation energy obtained from the centrifugal distortion by applying an approximate model, 39.8 kJ/mol, is quite similar to the ab initio value, 41.7 kJ/mol. Its relatively high value suggests a small charge transfer to take place.

The hydration of 2-fluoropyridine with one water molecule has been studied with a pulsed jet Fourier transform microwave technique. The rotational spectra of five isotopologues of the 1:1 complex have revealed the formation of a single most stable isomer. The observed cluster is characterized by an O−H⋅⋅⋅N hydrogen bond and a C−H⋅⋅⋅O weak hydrogen bond, where water is coplanar to the aromatic ring and acts in a double role of proton donor and proton acceptor.

none 6 si 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. Feng, Gang; Gou, Qian; Evangelisti, Luca; Spada, Lorenzo; Blanco, Susana; Caminati, Walther Feng, Gang; Gou, Qian; Evangelisti, Luca; Spada, Lorenzo; Blanco, Susana; Caminati, Walther

The pulsed-jet Fourier transform rotational spectra of 4 isotopologues have been recorded for the most stable conformation of the molecular cluster pyridine-CH3F. Two weak C-H···N and C-H···F hydrogen bonds link the two subunits of the complex. Structural information on the hydrogen bridges has been obtained. The internal rotation of the CH3F subunit around its symmetry axis splits all rotational transitions into two (A and E) well resolved component lines, leading to a V3 barrier height of 1.55(1) kJ mol(-1).