Search

Nuclear spin isomers of molecules play a pivotal role in our understanding of quantum mechanics and can have significant implications for various fields. In this work, we report the isolation and characterization of stable nuclear spin isomers as well as conformational isomers of a reactive compound, namely carbonyl diisothiocyanate. It can exist as three rotational conformers, two of which, the syn - syn and syn - anti , were observed in a pulsed supersonic jet by chirped pulse Fourier transform microwave spectroscopy in the 2-12 GHz frequency region. The rotational spectra of two distinct nuclear spin isomers of syn - syn -carbonyl diisothiocyanate, ortho and para , were recorded and analyzed. Experimental molecular rotational parameters for the identified rotational and nuclear spin isomers were determined, including rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants. The two nuclear spin isomers are distinguished by unique hyperfine splitting signatures in their rotational spectra as an outcome of their different nuclear spin states. The relative abundances of the two observed conformers in the gas phase were estimated from the intensity of their rotational transitions. Following detection of singly substituted rare isotopologues of the syn - syn conformer, a partial substitution ( r s ) structure was determined.

To assess the presence of oxygen-containing polycyclic aromatic hydrocarbons (OPAHs) in the interstellar medium and understand how water aggregates on an OPAH surface, we present a comprehensive gas-phase spectroscopy investigation of the OPAH xanthene (C 13 H 10 O) and its complexes with water using IR-UV ion dip spectroscopy and chirped-pulse Fourier transform microwave spectroscopy. The infrared spectrum of xanthene shows weak features at 3.42, 3.43, and 3.47 μm, which have been suggested to partly originate from vibrational modes of PAHs containing sp 3 hybridized carbon atoms, in agreement with the molecular structure of xanthene. The high resolution of rotational spectroscopy reveals a tunneling splitting of the rotational transitions, which can be explained with an out-of-plane bending motion of the two lateral benzene rings of xanthene. The nature of the tunnelling motion is elucidated by observing a similar splitting pattern in the rotational transitions of the singly-substituted 13 C isotopologues. The rotational spectroscopy investigation is extended to hydrates of xanthene with up to four water molecules. Different xanthene-water binding motifs are observed based on the degree of hydration, with O-H⋯π interactions becoming preferred over O-H⋯O xanthene interactions as the degree of hydration increases. A structural comparison with water complexes of related molecular systems highlights the impact of the substrate's shape and chemical composition on the arrangement of the surrounding water molecules.

The recent astronomical observations of the simplest aromatic nitrile benzonitrile, c -C 6 H 5 CN, followed by a five-membered and a bicyclic CN-functionalized ring in TMC-1 have provided a significant impetus to the field for searches of cyclic complex organic molecules in space. One such example is 2,4,6-cycloheptatriene-1-carbonitrile, a seven-membered ring with a -CN group attached to the sp 3 -hybridized carbon atom. With a permanent electric dipole moment of 4.3 D, this molecule is an excellent candidate for laboratory rotational spectroscopy. In this study, experiments were performed in the 2-8 GHz, 18-26 GHz, and 75-110 GHz frequency ranges in a supersonic expansion setup and a room temperature flow cell setup. The measurements across the broad frequency range of 2-110 GHz have enabled the identification and assignment of the vibronic ground state, singly substituted rare-atom isotopologues, and vibrationally excited states. Here, we report the precise determination of the rotational constants, quartic centrifugal distortion constants, nitrogen nuclear quadrupole coupling constants, as well as molecular structure in its vibronic ground state. The comprehensive rotational spectroscopy study of this molecule, covering a large frequency range, forms the basis for its future astronomical detection and thus for extending the pool of detected complex cyclic molecules.

We present an investigation of the ultrafast dynamics of the polycyclic aromatic hydrocarbon fluorene initiated by an intense femtosecond near-infrared laser pulse (810 nm) and probed by a weak visible pulse (405 nm). Using a multichannel detection scheme (mass spectra, electron and ion velocity-map imaging), we provide a full disentanglement of the complex dynamics of the vibronically excited parent molecule, its excited ionic states, and fragments. We observed various channels resulting from the strong-field ionization regime. In particular, we observed the formation of the unstable tetracation of fluorene, above-threshold ionization features in the photoelectron spectra, and evidence of ubiquitous secondary fragmentation. We produced a global fit of all observed time-dependent photoelectron and photoion channels. This global fit includes four parent ions extracted from the mass spectra, 15 kinetic-energy-resolved ionic fragments extracted from ion velocity map imaging, and five photoelectron channels obtained from electron velocity map imaging. The fit allowed for the extraction of 60 lifetimes of various metastable photoinduced intermediates.

Constitutional and conformational isomers of bromopropanol are vibrationally and rotationally characterised with parallels drawn to the structural chlorine analogues. A previous microwave spectroscopic study of the chloropropanols is re-examined and all systems are explored by Raman jet spectroscopy. For bromine, the entire nuclear quadrupole coupling tensors are accurately determined and compared to their chlorine counterparts. Tensor asymmetry parameters are determined and linked with the hydrogen bond strength as indicated by the downshift of the OH-stretching frequency. The spectroscopic constants derived from the observed transitions are used as benchmarks for a large variety of electronic structure methods followed by harmonic and anharmonic rovibrational treatments. The CCSD(T) electronic structure calculations provide the best performance, in particular once anharmonic and relativistic corrections are applied or implied. Standard DFT approaches vary substantially with respect to their systematic error cancellation across the investigated species, and cost-effective compromises for the different observables are proposed.

Gas phase homodimers of 3,3,3-trifluoro-1,2-epoxypropane (TFO), a molecule which has shown promise as an effective chiral tag for determining the absolute stereochemistry and the enantiomeric composition of chiral analytes, are explored using a variety of quantum chemistry models and rotational spectroscopy. The potential surface governing the interaction of the two molecules is rapidly explored using the artificial bee colony algorithm for homodimer candidates that are subsequently optimized by quantum chemistry methods. Although all model chemistries employed agree that the lowest energy form of the heterochiral homodimer of TFO ( RS or SR ) is lower in energy than that of the homochiral dimer ( RR or SS ), the energy spacings among the lower energy isomers of each and indeed the absolute energy ordering of the isomers of each are very model dependent. The experimental results suggest that the B3LYP-D3BJ/def2-TZVP model chemistry is the most reliable and provides excellent estimates of spectroscopic constants. In accord with theoretical predictions the non-polar lowest energy form of the heterochiral homodimer is not observed, while two isomers of the homochiral dimer are observed and spectroscopically characterized. Observation and assignment of the spectra for all three unique singly-substituted 13 C isotopologues, in addition to that of the most abundant isotopologue for the lowest energy isomer of the homochiral homodimer of TFO, provide structural information that compares very favorably with theoretical predictions, most notably that the presence of three fluorine atoms on the trifluoromethyl group removes their direct participation in the intermolecular interactions, which instead comprise two equivalent pairs of CH⋯O hydrogen bonds between the two epoxide rings augmented by favorable dispersion interactions between the rings themselves.

We present a rotational spectroscopy study of alpha-methoxy phenylacetic acid in the gas phase. This acid is a derivative of mandelic acid and is used in various organic reactions. The conformational landscape of alpha-methoxy phenylacetic acid was explored to gain insight into its intramolecular dynamics. A rich rotational spectrum was obtained using chirped-pulse Fourier transform microwave spectroscopy in the 2-8 GHz range. Five conformers out of six calculated low-energy forms were identified in the spectrum, and the assignment of the 13 C singly substituted isotopologues for the lowest-energy conformer led to its accurate structure determination. Splitting patterns were analyzed and attributed to the internal rotation of a methyl top. The analysis of the non-covalent interactions within the molecule highlights the subtle balance in the stabilization of the different conformers. We thus provide high-level structural and intramolecular dynamics information that is also used to benchmark the performance of quantum-chemical calculations.

We investigated the dissociation of dications and trications of three polycyclic aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and pyrene. PAHs are a family of molecules ubiquitous in space and involved in much of the chemistry of the interstellar medium. In our experiments, ions are formed by interaction with 30.3 nm extreme ultraviolet (XUV) photons, and their velocity map images are recorded using a PImMS2 multi-mass imaging sensor. Application of recoil-frame covariance analysis allows the total kinetic energy release (TKER) associated with multiple fragmentation channels to be determined to high precision, ranging 1.94-2.60 eV and 2.95-5.29 eV for the dications and trications, respectively. Experimental measurements are supported by Born-Oppenheimer molecular dynamics (BOMD) simulations.

Binary complexes between the chiral monoterpenoids camphor and α-fenchol were explored with vibrational and rotational jet spectroscopy as well as density functional theory in order to explore how chirality can influence the binding preferences in gas-phase complexes. The global minimum structures of the two diastereomers were assigned. It is found that chirality recognition leads to different compromises in the fine balance between intermolecular interactions. While one isomer features a stronger hydrogen bond, the other one is more tightly arranged and stabilized by larger London dispersion interactions. These new spectroscopic results help understand the influence of chirality in molecular aggregation and unveil the kind of interactions involved between a chiral alcohol and a chiral ketone with large dispersion contributions.

We report the reinvestigation of the high-resolution rotational spectrum of estradiol. After removing the known spectral lines corresponding to three conformers of estradiol identified in the gas phase before, a large number of spectral lines remained unassigned in the spectrum. The observation of remaining lines is a common feature in spectra obtained by broadband rotational spectroscopy. In our reinvestigation, the detection of certain patterns resulted in two new sets of experimental rotational constants. Here we describe a systematic analysis, which together with quantum-chemical computations culminated in the assignment of two estrone conformers, namely exhibiting the trans - and the cis -arrangement of the hydroxy group attached to the rigid steroid backbone. Estrone and estradiol only differ in two atomic mass units, and they show a dynamic interconversion equilibrium under certain conditions, which might also have been the case in our experiments due to the heating temperature of 195 °C. The results illustrate the potential of high-resolution rotational spectroscopy to discern between structurally related molecules and to provide their gas-phase structures without information beforehand exploiting the benefit of having remaining unassigned rotational transitions in the spectrum.

For complexes involving aromatic species, substitution effects can influence the preferred geometry. Using broadband rotational spectroscopy, we report the structures of three naphthol-aromatic ring complexes with different heteroatoms (furan and thiophene) and alkyl groups (2,5-dimethylfuran). The aim was to analyze the influence of the presence of heteroatoms or alkyl groups on the structure of the complex and the kind of intermolecular forces that control it. Face or edge arrangements can take place in these complexes via π-π or O-H⋯O/O-H⋯π interactions, respectively. All the experimentally observed complexes present O-H⋯O/O-H⋯π interactions with the hydroxyl group, with different structures and intermolecular interactions depending on the heteroatom present in the five-membered aromatic rings, yielding different symmetries in the experimental structure. Structures are experimentally identified through the use of planar moments of inertia. Further results from SAPT calculations show that dispersion and electrostatic interactions contribute similarly to the stabilization of all the studied complexes. These new spectroscopic results shed light on the influence of dispersion and hydrogen bonding in molecular aggregation of systems with substituted aromatic residues.

Molecular aggregation is of paramount importance in many chemical processes, including those in living beings. Thus, characterization of the intermolecular interactions is an important step in its understanding. We describe here the aggregation of benzyl alcohol at the molecular level, a process governed by a delicate equilibrium between OH⋯O and OH⋯π hydrogen bonds and dispersive interactions. Using microwave, FTIR, Raman and mass-resolved double-resonance IR/UV spectroscopic techniques, we explored the cluster growth up to the tetramer and found a complex landscape, partly due to the appearance of multiple stereoisomers of very similar stability. Interestingly, a consistently homochiral synchronization of transiently chiral monomer conformers was observed during cluster growth to converge in the tetramer, where the fully homochiral species dominates the potential energy surface. The data on the aggregation of benzyl alcohol also constitute an excellent playground to fine-tune the parameters of the most advanced functionals.

We report on the noncovalent intermolecular interactions established between the polycyclic aromatic hydrocarbons phenanthrene and phenanthridine with water. Such noncovalent interactions involving extended aromatic systems and water molecules are ubiquitous in a variety of chemical and biological systems. Our study provides spectroscopic results on simple model systems to understand the impact that an extended aromatic surface and the presence of a heteroatom have on the nature of the noncovalent interactions established with the solvent. Microhydrated phenanthrene and phenanthridine clusters with up to three water molecules have been observed and unambiguously characterised by means of broadband rotational spectroscopy and quantum chemical calculations. The presence of a nitrogen atom in the backbone of phenanthridine remarkably affects the geometries of the water clusters and the interaction networks at play, with O-HN and C-HO interactions becoming preferred in the phenanthridine-water clusters over the O-Hπ interactions seen in the phenanthrene-water clusters. The presence of this heteroatom induces nuclear quadrupole coupling, which was used to understand the cooperativity effects found with increasing cluster size. Our results provide important insight to draw a more complete picture of the noncovalent interactions involving solvent molecules and aromatic systems larger than benzene, and they can be significant to enhance our understanding of the aromatic-polar interactions at play in a myriad of chemical and biological contexts.

We report on a detailed multi-spectroscopic analysis of the structures and internal dynamics of diphenylether and its aggregates with up to three water molecules by employing molecular beam experiments. The application of stimulated Raman/UV and IR/UV double resonance methods as well as chirped-pulse Fourier transform microwave spectroscopy in combination with quantum-chemical computations yield the energetically preferred monomer and cluster geometries. Furthermore, the complex internal dynamics of the diphenylether monomer and the one-water clusters are analysed. In the cluster with three water molecules, water forms a cyclic structure similar to the isolated water trimer. The interactions ruling the structures of the higher-order water clusters are a combination of the ones identified for the two monohydrate isomers, with dispersion being a decisive contribution for systems that have a delicate energetic balance between different hydrogen-bonded arrangements of similar energy.

The tracking of symmetry-breaking events in space is a long-lasting goal of astrochemists, aiming at an understanding of homochiral Earth chemistry. One current effort at this frontier aims at the detection of small chiral molecules in the interstellar medium. For that, high-resolution laboratory spectroscopy data is required, providing blueprints for the search and assignment of these molecules using radioastronomy. Here, we used chirped-pulse Fourier transform microwave and millimeter-wave spectroscopy and frequency modulation absorption spectroscopy to record and assign the rotational spectrum of the chiral aromatic molecule styrene oxide, C6H5C2H3O, a relevant candidate for future radioastronomy searches. Using experimental data from the 2-12, 75-110, 170-220, and 260-330 GHz regions, we performed a global spectral analysis, which was complemented by quantum chemistry calculations. A global fit of the ground state rotational spectrum was obtained, including rotational transitions from all four frequency regions. Primary rotational constants as well as quartic and sextic centrifugal distortion constants were determined. We also investigated vibrationally excited states of styrene oxide, and for the three lowest energy vibrational states, we determined rotational constants including centrifugal distortion corrections up to the sextic order. In addition, spectroscopic parameters for the singly-substituted 13C and 18O isotopologues were retrieved from the spectrum in natural abundance and used to determine the effective ground state structure of styrene oxide in the gas phase. The spectroscopic parameters and line lists of rotational transitions obtained here will assist future astrochemical studies of this class of chiral organic molecules.

Isoleucinol, a potential precursor to the essential α-amino acid isoleucine, has been studied using microwave spectroscopy from 2-26 GHz. The measurements between 18-26 GHz were performed with a newly developed segmented chirped-pulse Fourier transform microwave spectrometer, which has reduced the cost of the instrument by half compared to a single pulse excitation and direct detection chirped-pulse microwave spectrometer in the same frequency range. The performance of the instrument has been demonstrated and found to be comparable to the previous design. For isoleucinol, the flexibility of the sec-butyl side chain (R = -CH(CH3)CH2CH3) can result in more than 200 different conformers from its five dihedral angles, and experimentally, seven conformers have been assigned. A fit including the hyperfine splitting due to nitrogen nuclear quadrupole coupling for the rotational transitions is reported for all conformers, along with the experimental structures of the three lowest energy conformers. The observed conformers have intramolecular NH-O hydrogen bond interactions, similar to the second energetically favorable conformer of the analogous amino acid, isoleucine. A complete linelist has been provided to facilitate a search for isoleucinol in the interstellar medium.

A semi-quantitative analysis of the components of two natural essential oils has been carried out using broadband rotational spectroscopy, which is inherently molecule specific. The samples under study were two thyme essential oils from Spain with different compositions: (a) with thymol as the most abundant species (thyme I) and (b) with linalool and 4-carvomenthenol being the most abundant ones (thyme II). Relative intensity measurements of selected rotational transitions were carried out to estimate the abundances of the different species present in these complex mixtures, taking into account the square of the respective dipole moment components. One strength of rotational spectroscopy is its structure sensitivity. Here, we also re-investigated the microwave spectrum of linalool and determined the accurate experimental gas-phase structures of thymol and linalool through the assignment of all 13C isotopologues of their lowest energy conformers. A characteristic splitting pattern of the rotational transitions due to internal rotation of two non-equivalent methyl groups of linalool was observed in the thyme II spectrum. Their internal rotation barriers were experimentally determined to 4.7703(96) kJ mol-1 and 9.2581(74) kJ mol-1, respectively.

The structural preferences within a series of dibenzofuran-solvent complexes have been investigated by electronic, vibrational, and rotational spectroscopic methods probing supersonic jet expansions. The experimental study is accompanied by a detailed theoretical analysis including dispersion-corrected density functional theory, symmetry adapted perturbation theory, as well as coupled cluster approaches. The complementary, multi-spectroscopic results reveal a preferred OHO structure for the smallest complex of dibenzofuran-water, whereas for the methanol complex an OHπ isomer is simultaneously observed. For the largest complex, dibenzofuran-tert-butyl alcohol, only a π-bound structure is found. These comprehensive investigations show that a completely inverse trend regarding the docking preference is observed by comparing the present results with the ones for analogous diphenyl ether complexes. This can be rationalized on the basis of the planarity/non-planarity and rigidity/flexibility of the different systems, providing valuable insight into the interplay between different non-covalent interactions. This analysis is a further step towards a quantitative description of very delicate energetic balances with the overall goal of yielding reliable structural predictions for non-covalently bound systems.

In this work we report on the experimental and theoretical investigations of the progressional complexation of the polycyclic aromatic hydrocarbon (PAH) acenaphthene with itself and with water. In the interstellar medium, PAH complexes are an important link between molecular gas and solid state configurations of carbon, and in the form of grains they are postulated to serve as chemical catalysts. However, no direct detection of PAHs or their (microhydrated) complexes in interstellar space has been achieved as of yet. Therefore, we provide UV and far-infrared ion dip spectra of homogeneous PAH multimers and their hydrated clusters. The far-IR region of the IR spectrum is especially interesting since it contains the most spectral features that arise due to complexation or microhydration. We present microhydrated PAH complexes up to the third order, where we show that the water clusters are locked with little perturbation on the different PAH platforms. Density functional theory (DFT) calculations involving hydrogen bond interactions still seem challenging for predicting the far-IR frequency range, although applying anharmonic corrections leads to slight improvements.

15-Crown-5 ether (15C5) and its complexes with water have been studied using broadband Fourier transform microwave spectroscopy in a supersonic jet. A new conformer of 15C5 has been observed and established as the new global minimum out of a total of nine isolated structures. In addition, two 15C5-H2O and two 15C5-(H2O)2 clusters have been observed. The cluster structures have been unambiguously identified through the observation of water 18O isotopologue spectra. In all the clusters, at least one water molecule, located close to the axis of the 15C5 ring, interacts through two simultaneous hydrogen bonds to the endocyclic oxygen atoms. This interaction reshapes the 15C5 ring to reduce its rich conformational landscape to only two open structures, related to those found in complexes with Li+ or Na+ ions. In the most abundant 15C5-(H2O)2 form, the two water molecules repeat the same interaction scheme while binding to opposite sides of the ring. In the second most abundant dihydrated form the two water molecules lie on the same side of the ring. This finding is exceptionally rare because water-water interactions typically prevail over the formation of additional solute-water contacts, and it showcases the particular binding features of crown ethers.

A high-resolution molecular spectroscopy study was carried out on the cyclic ether tetrahydropyran (THP), one of the smallest molecules composed of a pyranose ring. As this ring structure is closely related to carbohydrates, THP can offer relevant insight into structural variations of this unit. Thus, an extensive probe of THP using three broadband instruments ranging from the microwave to the far-infrared (2-8 GHz, 75-110 GHz and 100-650 cm-1 frequency ranges) was performed to accrue both accurate sets of rotational constants and structural information. This array of experimental setups provided an accurate set of data to improve the description of the ground state of THP and revise the principal parameters of its backbone structure. The structural information was deduced from the assignment of the 13C and 18O isotopologues present in natural abundance. In addition, the complementary dataset obtained from these experiments led to a better characterization of the vibrational motions involving the skeletal ring of the molecule. In particular, the vibrational frequencies of four of these modes (ν23 (∼250 cm-1), ν22 (∼403 cm-1), ν21 (∼430 cm-1), and ν20 (∼562 cm-1)) have been determined from the analysis of the first rotationally resolved vibrational spectrum reported for THP. Quantum-chemical calculations aided in the analysis of the experimental results.

The rotational spectra of 2- and 3-nitrobenzonitrile were recorded via chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 2-8 GHz. These molecules each display large dipole moments, making them viable candidates for deceleration and trapping experiments with AC-electric fields. For both molecules, the main isotopologues and all isotopologues of the respective 13C-, 15N-, 18O-monosubstituted species in their natural abundance were assigned. These assignments allowed for the structural determination of 2- and 3-nitrobenzonitrile via Kraitchman's equations as well as a mass-dependent least-squares fitting approach. The experimentally determined structural parameters are then compared to those obtained from quantum-chemical calculations for these two molecules and 4-nitrobenzonitrile. Structural changes caused by steric interaction and competition for the electron density of the phenyl ring highlight how these strong electron-withdrawing substituents affect one another according to their respective positions on the phenyl ring.

Hydrogen-bonded complexes between ibuprofen and water generated in a supersonic expansion were characterized using chirped-pulse Fourier transform microwave spectroscopy in the 2-8 GHz frequency range. Four spectra were observed allowing the determination of their rotational parameters. Comparison with quantum-chemical calculations led to their identification as the lowest energy 1 : 1 ibuprofen-water complexes. These correspond to the complexes between water and the four different conformers of ibuprofen previously detected in the gas phase, owing to their similar stabilization energies and abundances. Water seems to not change the conformational distribution of ibuprofen.

Prebiotic hydantoin and its complexes with one and two water molecules are investigated using high-resolution broadband rotational spectroscopy in the 2-8 GHz frequency range. The hyperfine structure due to the nuclear quadrupole coupling of the two 14 N atoms is analysed for the monomer and the complexes. This characteristic hyperfine structure will support a definitive assignment from low frequency radioastronomy data. Experiments with H 2 18 O provide accurate experimental information on the preferred binding sites of water, which are compared with quantum-chemically calculated coordinates. In the 2-water complexes, the water molecules bind to hydantoin as a dimer instead of individually, indicating the strong water-water interactions. This information provides first insight on how hydantoin interacts with water on the molecular level.

Aromatic ethers such as diphenyl ether (DPE) represent molecules with different docking sites for alcohols leading to competing OH-O and OH-π interactions. In a multi-spectroscopic approach in combination with quantum chemical calculations the complex of DPE with tert-butyl alcohol (t-BuOH) is investigated in the electronic ground state (S 0 ) and the electronically excited state (S 1 ). FTIR, microwave as well as mass- and isomer-selective IR/R2PI spectra are recorded, revealing co-existing OH-O and OH-π isomers in the S 0 state. Surprisingly, they are predicted to be of almost equal stability in contrast to the previously investigated DPE-MeOH complex, where the OH-π structure is preferred by both theory and experiment. The tert-butyl group in t-BuOH allows for a simultaneous optimization of hydrogen-bonding and dispersion interactions, which provides a sensitive meeting point between theory and experiment. In the electronically excited state of DPE-t-BuOH, vibrational spectra could be recorded separately for both isomers using UV/IR/UV spectroscopy. In the S 1 state the same structural binding motifs are obtained as in the S 0 state with the OH-O bond being weakened for the OH-O arrangement and the OH-π interaction being strengthened in the case of the OH-π isomer compared to the S 0 state.

We report the results of a broadband rotational spectroscopic study of corannulene, C 20 H 10 , all of its singly substituted 13 C isotopologues, and a complex of corannulene with one molecule of water. Corannulene is a polycyclic aromatic hydrocarbon (PAH) with a curved structure that results in a large dipole moment. Observation of 13 C isotopic species in natural abundance allowed us to precisely determine the molecular structure of corannulene. The differences between the experimental C-C bond lengths correlate to the double-bond character predicted using Kekule's resonance structures. In the case of C 20 H 10 -H 2 O, the water molecule is found to reside inside the bowl-like structure of corannulene. Our experimental and theoretical results indicate that the water molecule rotates freely around its C 2 axis and that dispersion interactions are the dominant contribution to the binding.

We report here further spectroscopic investigation of the astrochemically relevant molecule i-propyl cyanide. We observed and analysed the rotational spectra of the ground state of the molecule and ten vibrationally excited states with energies between 180-500 cm -1 . For this, we used a segmented W-band spectrometer (75-110 GHz) and performed the experiments under room temperature conditions. This approach thus provides access to high-resolution, pure rotational data of vibrational modes that occur in the far-infrared fingerprint region, and that can be difficult to access with other techniques. The obtained, extensive data set will support further astronomical searches and identifications, such as in warmer regions of the interstellar space where contributions from vibrationally excited states become increasingly relevant.

Dispersion interactions are omnipresent in intermolecular interactions, but their respective contributions are difficult to predict. Aromatic ethers offer competing docking sites for alcohols: the ether oxygen as a well known hydrogen bond acceptor, but also the aromatic π system. The interaction with two aromatic moieties in diphenyl ether can tip the balance towards π binding. We use a multi-spectroscopic approach to study the molecular recognition, the structure and internal dynamics of the diphenyl ether-methanol complex, employing infrared, infrared-ultraviolet and microwave spectroscopy. We find that the conformer with the hydroxy group of the alcohol binding to one aromatic π cloud and being coordinated by an aromatic C-H bond of the other phenyl group is preferred. Depending on the expansion conditions in the supersonic jet, we observe a second conformer, which exhibits a hydrogen bond to the ether oxygen and is higher in energy.

Conformational flexibility is intrinsically related to the functionality of biomolecules. Elucidation of the potential energy surface is thus a necessary step towards understanding the mechanisms for molecular recognition such as docking of small organic molecules to larger macromolecular systems. In this work, we use broadband rotational spectroscopy in a molecular jet experiment to unravel the complex conformational space of citronellal. We observe fifteen conformations in the experimental conditions of the molecular jet, the highest number of conformers reported to date for a chiral molecule of this size using microwave spectroscopy. Studies of relative stability using different carrier gases in the supersonic expansion reveal conformational relaxation pathways that strongly favour ground-state structures with globular conformations. This study provides a blueprint of the complex conformational space of an important biosynthetic precursor and gives insights on the relation between its structure and biological functionality.

The rotational spectrum of trans-cinnamaldehyde ((E)-3-phenyl-2-propenal, C9H8O) was recorded by chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 2-8.5 GHz. The odourant molecule is the essential component of cinnamon oil and causes the characteristic smell. The rotational signatures of two conformers were observed: s-trans-trans- and s-cis-trans-cinnamaldehyde. The rotational spectra of s-trans-trans-cinnamaldehyde and all of its (13)C-monosubstituted species in natural abundance were assigned and the corresponding carbon backbone structure was determined. The second conformer s-cis-trans-cinnamaldehyde is about 9 kJ mol(-1) higher in energy and could also be identified in the spectrum.

Ibuprofen's pain-relieving properties arise from its ability to physically block the active site of an enzyme, thus making its structural and conformational properties highly interesting. We here present a conformer-selective high-resolution broadband rotational spectroscopy study of gas-phase ibuprofen. The interpretation of the spectroscopic results is supported by quantum-chemical calculations. We identify four low-energy conformers that differ in the structural arrangement of the isobutyl moiety with respect to the remainder of the molecule. While the isobutyl group shows high structural flexibility - resulting in distinct low-energy conformers - the propanoic acid group favors a stable arrangement.

Chirality plays a fundamental role in the activity of biological molecules and broad classes of chemical reactions. The chemistry of life is built almost exclusively on left-handed amino acids and right-handed sugars, a phenomenon known as "homochirality of life". Furthermore, most drugs developed in the last decade are of specified chirality. Thus, fast and reliable methods that can differentiate molecules of different handedness, determine the enantiomeric excess of even molecular mixtures, and allow for an unambiguous determination of molecular handedness are of great interest, in particular with respect to complex mixtures. In this perspective article, we discuss the recent developments, with an emphasis on modern spectroscopic methods using gas-phase samples, such as photoelectron circular dichroism, Coulomb explosion imaging, and microwave three-wave mixing.

We report a combined theoretical and microwave spectroscopy study of the internal dynamics of the benzene dimer, a benchmark system for dispersion forces. Although the extensive ab initio calculations and experimental work on the equilibrium geometry of this dimer have converged to a tilted T-shaped structure, the rich internal dynamics due to low barriers for internal rotation have remained largely unexplored. We present new microwave spectroscopy data for both the normal (C6H6)2 and partially deuterated (C6D6)(C6H6) dimers. The splitting patterns obtained for both species are unraveled and understood using a reduced-dimensionality theoretical approach. The hindered sixfold rotation of the stem can explain the observed characteristic 1 : 2 : 1 tunneling splitting pattern, but only the concerted stem rotation and tilt tunneling motion, accompanied by overall rotation of the dimer, yield the correct magnitude of the splittings and their strong dependence on the dimer angular momentum J that is essential to explain the experimental data. Also the surprising observation that the splittings are reduced by 30% for the mixed (C6D6)(C)(C6H6)(S) dimer in which only the cap (C) in the T-shaped structure is deuterated, while the rotating stem (S) monomer is the same as in the homodimer, is understood using this approach. Stark shift measurements allowed us to determine the dipole moment of the benzene dimer, μ = 0.58 ± 0.051 D. The assumption that this dipole moment is the vector sum of the dipole moments induced in the monomers by the electric field of the quadrupole on the other monomer yields a calculated value of μ = 0.63 D. Furthermore, the observed Stark behavior is typical for a symmetric top, another confirmation of our analysis.

An improved intermolecular potential surface for the benzene dimer is constructed from interaction energies computed by symmetry-adapted perturbation theory, SAPT(DFT), with the inclusion of third-order contributions. Twelve characteristic points on the surface have been investigated also using the coupled-cluster method with single, double, and perturbative triple excitations, CCSD(T), and triple-zeta quality basis sets with midbond functions. The SAPT and CCSD(T) results are in close agreement and provide the best representation of these points to date. The potential was used in calculations of vibration-rotation-tunneling (VRT) levels of the dimer by a method appropriate for large amplitude intermolecular motions and tunneling between multiple equivalent minima in the potential. The resulting VRT levels were analyzed with the use of the permutation-inversion full cluster tunneling (FCT) group G(576) and a chain of subgroups that starts from the molecular symmetry group C(s)(M) of the rigid dimer at its equilibrium C(s) geometry and leads to G(576) if all possible intermolecular tunneling mechanisms are feasible. Further information was extracted from the calculated wave functions. It was found, in agreement with the experimental data, that for all of the 54 G(576) symmetry species (with different nuclear spin statistical weights) the lower VRT states have a tilted T-shape (TT) structure; states with the parallel-displaced structure are higher in energy than the ground state of A symmetry by at least 30 cm(-1). The dissociation energy D(0) equals 870 cm(-1), while the depth D(e) of the TT minimum in the potential is 975 cm(-1). Hindered rotation of the cap in the TT structure and tilt tunneling lead to level splittings on the order of 1 cm(-1). Also intermolecular vibrations with excitation energies starting at a few cm(-1) were identified. A further small, but probably significant, level splitting was assigned to cap turnover, although in scans of the potential surface we could not find a plausible 'reaction path' for this process. Rotational constants were extracted from energy levels calculated for total angular momentum J = 0 and 1, and from expectation values of the inertia tensor. Although the end-over-end rotational constant B + C agrees well with the measured microwave spectra, there is disagreement with the measurements concerning the (a)symmetric rotor character of the benzene dimer. It is concluded from calculations for the 54 nuclear spin species that the microwave spectrum should show overlapping contributions from many different species. Another interesting conclusion regards the role of the quantum number K, for a prolate near-symmetric rotor the projection of the total angular momentum on the prolate axis. For the benzene dimer, K has a substantial effect on the energy levels associated with the intermolecular motions of the complex.

The rotational constants and the nitrogen nuclear quadrupole coupling constants of cis-3-aminophenol and trans-3-aminophenol are determined using Fourier-transform microwave spectroscopy. We examine several J = 2 <-- 1 and 1 <-- 0 hyperfine-resolved rotational transitions for both conformers. The transitions are fit to a rigid rotor Hamiltonian including nuclear quadrupole coupling to account for the 14N nuclear spin. For cis-3-aminophenol we obtain rotational constants of A = 3734.930 MHz, B = 1823.2095 MHz, and C = 1226.493 MHz, for trans-3-aminophenol of A = 3730.1676 MHz, B = 1828.25774 MHz, and C = 1228.1948 MHz. The dipole moments are precisely determined using Stark effect measurements for several hyperfine transitions to micro(a) = 1.7718 D, micro(b) = 1.517 D for cis-3-aminophenol and micro(a) = 0.5563 D, micro(h) = 0.5375 D for trans-3-aminophenol. Whereas the rotational constants and quadrupole coupling constants do not allow to determinate the absolute configuration of the two conformers, this assignment is straightforward based on the dipole moments. High-level ab initio calculations (B3LYP/6-31G* to MP2/aug-cc-pVTZ) are performed providing error estimates of rotational constants and dipole moments obtained for large molecules by these theoretical methods.

The microwave spectra of (CH(3))(3)(74)Ge(35)Cl and its isotopologues (CH(3))(3)(72)Ge(35)Cl and (CH(3))(3)(74)Ge(37)Cl have been studied in the frequency range from 3-24 GHz revealing the complex internal dynamics of this organometallic molecule with three internal rotors. The assignment of the complex spectrum has been facilitated by permutation-inversion theory and ab initio calculations. The V(3) barrier to internal rotation is determined to be 372.359(47) cm(-1). Furthermore, an analysis of the chlorine quadrupole coupling yields the description of the Ge-Cl bonding character which is estimated to be dominated by covalent contributions (46.5%) together with 37.6% ionic and 15.9% pi-bonding character. From isotopic substitution the Ge-Cl bond distance could be determined as 2.15198(97) Angstroms.

Equilibrium geometries have been optimized and harmonic vibrational frequencies calculated for HOOOCl using quadratic configuration interaction and coupled-cluster methods with Dunning's correlation-consistent basis sets. There are two conformers of HOOOCl (a cis and a trans structure) that are very close in energy and separated by a barrier of 3.28 kcal mol(-1). In experiments, these two forms may co-exist. Rotational constants that can be used to experimentally characterize and identify HOOOCl are presented. The electronic absorption spectrum for trans-HOOOCl is calculated using multi-reference configuration interaction methods in order to support the spectroscopic search. At 5.84 eV a remarkably strong transition is computed corresponding to a sigma(Cl-O') --> sigma*(Cl-O')-excitation. In addition, possible photodissociation pathways along the Cl-O' and O'-O bonds for trans-HOOOCl have been examined. Potential photochemistry of HOOOCl is suggested for the first time.