Binary conformers of a flexible, long-chain fluoroalcohol: dispersion controlled selectivity and relative abundances in a jet.

The complex conformational panorama of binary 4,4,4-trifluoro-1-butanol (TFB) aggregates was investigated using chirped-pulse Fourier transform microwave spectroscopy, aided by conformational searches using CREST (Conformer-Rotamer Ensemble Sampling Tool) and quantum chemistry calculations. From nearly 1500 initial dimer geometries, 16 most stable binary candidates were obtained within a relative energy window of ∼4 kJ mol-1. Rotational spectra of five binary conformers were experimentally observed in supersonic expansion and assigned. Interestingly, three out of the five observed binary conformers are composed solely of monomer conformers, which were not observed in their isolated gas phase forms in jet expansion. In addition, an observed dimer that is made exclusively of the most stable TFB monomer subunits does not correspond to the global minimum. The intricate kinetically and thermodynamically controlled dimer formation mechanisms are discussed, and a modified kinetic-thermodynamic model was developed, providing conformational abundances that are in good agreement with the experiment. Subsequent non-covalent interaction analyses reveal that the observed conformers are held together by one primary O-H⋯O hydrogen bond and secondary intermolecular C-H⋯O, C-H⋯F, and/or O-H⋯F interactions, as well as C-H⋯H-C London dispersion interactions between the methylene groups. Further symmetry-adapted perturbation theory analyses of the TFB dimer conformers and related alcohol dimers reveal a considerable rise in dispersion contributions with increasing n-alkyl carbon chain length and highlight the role of dispersion interactions in preferentially stabilizing the global minimum of the TFB dimer.

Unlocking a new hydrogen-bonding marker: C-O bond shortening in vicinal diols revealed by rotational spectroscopy.

The conformational space of cis-1,2-cyclohexanediol, a model molecule for cyclic vicinal diols, was investigated using rotational spectroscopy and density functional theory calculations. Four low energy conformers within an energy window of 5 kJ mol-1 were identified computationally. A rotational spectrum of jet-cooled cis-1,2-cyclohexanediol was recorded with a chirped pulse Fourier transform microwave spectrometer. Two sets of rotational transitions were observed and could be assigned to conformers of cis-1,2-cyclohexanediol. The non-observation of other low energy conformers was explained by conformational conversion barrier height calculations and results from experimental spectra recorded with different carrier gases. Eight isotopologues, including those with 13C and 18O, of the lowest energy conformer were observed, allowing the determination of the semi-experimental equilibrium structure, reSE. Interestingly, the structural analysis revealed that the C-O bond length of the intramolecular hydrogen-bond donor is shorter than that of the acceptor. This appears to be a general characteristic of vicinal diols and can be used as a novel hydrogen-bond marker in such compounds.

Wetting vs Droplet Aggregation: A Broadband Rotational Spectroscopic Study of 3-Methylcatechol⋠⋠⋠Water Clusters.

Two competing solvation pathways of 3-methylcatechol (MC), an atmospherically relevant aromatic molecule, with up to five water molecules were explored in detail by using a combination of broadband rotational spectroscopy and computational chemistry. Theoretically, two different pathways of solvation emerge: the commonly observed droplet pathway which involves preferential binding among the water molecules while the solute serves as an anchor point for the formation of a water cluster, and an unexpected wetting pathway which involves interactions between the water molecules and the aromatic face of MC, i.e., a wetting of the π-surface. Conclusive identification of the MC hydrate structures, and therefore the wetting pathway, was facilitated by rotational spectra of the parent MC hydrates and several H2 18 O and 13 C isotopologues which exhibit splittings associated with methyl internal rotation and/or water tunneling motions. Theoretical modelling and analyses offer insights into the tunneling and conversion barriers associated with the observed hydrate conformers and the nature of the non-covalent interactions involved in choosing the unusual wetting pathway.

Femtosecond Rotational Dynamics of D_{2} Molecules in Superfluid Helium Nanodroplets.

Rotational dynamics of D_{2} molecules inside helium nanodroplets is induced by a moderately intense femtosecond pump pulse and measured as a function of time by recording the yield of HeD^{+} ions, created through strong-field dissociative ionization with a delayed femtosecond probe pulse. The yield oscillates with a period of 185 fs, reflecting field-free rotational wave packet dynamics, and the oscillation persists for more than 500 periods. Within the experimental uncertainty, the rotational constant B_{He} of the in-droplet D_{2} molecule, determined by Fourier analysis, is the same as B_{gas} for an isolated D_{2} molecule. Our observations show that the D_{2} molecules inside helium nanodroplets essentially rotate as free D_{2} molecules.

Rotational Spectroscopy of the 2,2,3,3,3-Pentafluoropropanol⋠⋠⋠Water Complex: Conformations and Large Amplitude Motions.

The 2,2,3,3,3-pentafluoropropanol (PFP) monomer can exist in five conformations defined by the CCCO and CCOH dihedral angles: four mirror-imaged pairs (G+g+/G-g-, G+g-/G-g+, G+t/G-t, Tg+/Tg-) and an achiral Tt form. We examined the conformational landscape of the PFP⋅⋅⋅water complex using chirped pulsed Fourier transform microwave spectroscopy and theoretical calculations. Rotational spectra of two PFP⋅⋅⋅water conformers, PFPG+g+⋅⋅⋅WH and PFPTg+⋅⋅⋅WH , and seven deuterated isotopologues of each, were assigned. Tunneling splittings were observed for both conformers and are attributed to the exchange of the bonded and non-bonded hydrogen atoms of water. On the other hand, the tunneling splitting associated with the OH flipping motion in PFPTg+/Tg- appears to be quenched upon hydrogen bonding with water. The large amplitude motions associated with the water subunits were examined in detail to explain the very different magnitudes of the experimental and theoretical permanent electric dipole moment components. The study highlights the challenge in correctly identifying the conformers observed when large amplitude motions are involved. Quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses, as well as electrostatic potential (ESP) calculations were carried out to explore the nature of the non-covalent interactions and to appreciate the effects of fluorination.

Structural study of 1- and 2-naphthol: new insights into the non-covalent H-H interaction in cis-1-naphthol.

Previous microwave studies of naphthol monomers were supplemented by measuring spectra of all 13C mono-substituted isotopologues of the cis- and trans-conformers of 1-naphthol and 2-naphthol in their natural abundances. The resulting data were utilized to determine substitution structures and so-called semi-experimental effective structures. Results from electronic structure calculations show that the OH group of cis-1-naphthol points ≈6° out of plane, which is consistent with the inertial defect data of cis- and trans-1-naphthol. The non-planarity of cis-1-naphthol is a result of a close-contact H-atom-H-atom interaction. This type of H-H interaction has been the subject of much controversy in the past and we provide here an in-depth theoretical analysis of it. The naphthol system is particularly well-suited for such analysis as it provides internal standards with its four different isomers. The methods used include quantum theory of atoms in molecules, non-covalent interactions, independent gradient model, local vibrational mode, charge model 5, and natural bond orbital analyses. We demonstrate that the close-contact H-H interaction is neither a purely attractive nor repulsive interaction, but rather a mixture of the two.

2,2,3,3,3-Pentafluoro-1-propanol and its dimer: structural diversity, conformational conversion, and tunnelling motion.

Rotational spectra of 2,2,3,3,3-pentafluoro-1-propanol (PFP) were measured using cavity and chirped pulse Fourier transform microwave spectrometers. Of the nine possible PFP configurations which include four mirror-imaged pairs and an achiral conformer, the two most stable monomeric PFP imaged pairs, i.e., PFPG+g+/G-g- and PFPTg+/Tg- were observed and assigned, along with the 13C, 18O and deuterated isotopologues of PFPG+g+/G-g-. The rotational transitions of PFPTg+/Tg- exhibit large tunnelling splittings and were analyzed in detail. CREST, a recently developed conformational search tool that was used for systematic conformational searches of possible binary PFP conformers and the subsequent DFT calculations at the B3LYP-D3(BJ)/def2-QZVP level produced nearly 80 stable, binary PFP geometries, where ten of them are within a narrow energy window of ∼1 kJ mol-1, highlighting the structural diversity of the system. Rotational spectra of five (PFP)2 conformers were assigned and were identified as the five most stable binary conformers predicted. A closer examination reveals that the assigned binary conformers are made exclusively of the two most stable PFP monomeric subunits observed experimentally. A combined kinetic and thermodynamic model was proposed to explain the observation or non-observation of low energy conformers, and the analysis was further verified by the 'argon test'. The non-covalent intermolecular interactions of PFP and its binary conformers are also discussed with the aid of quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses, as well as the effects of fluorination by comparing with 1-propanol and its dimers.

Examining the gas-phase homodimers of 3,3,3-trifluoro-1,2-epoxypropane using quantum chemistry and microwave spectroscopy.

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 13C 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.

Conformational Landscape of the Hydrogen-Bonded 1-Phenyl-2,2,2-Trilfuoroethanol···1,4-Dioxane Complex: Dispersion Interactions and Conformational Conversion.

A rotational spectrum of the hydrogen-bonded complex between 1-phenyl-2,2,2-trilfuoroethanol (PhTFE), a chiral fluoroalcohol, and 1,4-dioxane, a common solvent for organic reactions, was measured using a chirped pulse Fourier transform microwave spectrometer. Initial theoretical conformational searches were carried out using CREST, a recently developed conformational searching tool. Subsequent geometry optimization and harmonic frequency calculations at the B3LYP-D3(BJ)/def2-TZVP level of theory yielded nearly 30 binary conformers of which 13 are within an energy window of ∼5 kJ mol-1. Interestingly, while the O-H···O hydrogen bond dominates the attractive binary interactions, the complex conformational landscape is mainly controlled by subtle dispersion interactions between the phenyl and 1,4-dioxane rings. Two sets of rotational transitions were assigned in the experimental spectrum and attributed to the two most stable conformers of PhTFE···1,4-dioxane. The quantum theory of atoms in molecules (QTAIM), noncovalent interactions (NCI), and symmetry-adapted perturbation theory (SAPT) analyses were employed in order to appreciate how the phenyl ring and O-H functional groups influence the intermolecular interaction and conformational distribution of the binary complex. The main PhTFE conformation within the complex, identified experimentally, is different from that of the isolated PhTFE monomer reported previously.

Conformers of Vanillic Acid and Its Monohydrate: A Rotational Spectroscopic and Theoretical Study.

Organic acids are released during wildfire combustion and can influence aerosol formation and growth. Conformational flexibility is thought to be advantageous in stabilizing the precritical nucleus in the process of aerosol particle formation and allowing for further complexation with other atmospheric constituents. We describe here a study of the conformational flexibility of vanillic acid and its monohydrate using electronic structure calculations and Fourier transform microwave spectroscopy. Computationally, 12 and 28 conformers were found for the monomer and monohydrate, respectively. The two lowest energy conformers of both the vanillic acid monomer and the vanillic acid-water complex could be experimentally identified. The deviation between experimental and theoretical rotational constants determined at the MP2/aug-cc-pVTZ and DFT B3LYP-D3(BJ)/def2-TZVP levels of theory is less than 1%. No tunneling splittings were observed, which suggests a relatively high barrier to methyl internal rotation, in agreement with other, previously studied vanillin derivatives. Furthermore, no c-type transitions could be observed for the vanillic acid monomer, in agreement with the computed zero c-dipole moment component of the two lowest energy structures. For the monohydrate the absence of c-type transitions is rationalized by averaging over a large amplitude motion involving the free H atom of the water unit. From the theoretical structures, it is apparent that intramolecular hydrogen bonds play a significant role in stabilizing the lowest energy conformers. To further characterize the intramolecular interactions in the monomer and intra- and intermolecular interactions in the monohydrate, quantum theory of atoms-in-molecules (QTAIM), noncovalent interactions (NCI), and intrinsic bond strength index (IBSI) analyses were performed. The atmospheric abundance of the vanillic acid monohydrate relative to the monomer was evaluated to assess its atmospheric significance.

Conformational Landscape, Chirality Recognition and Chiral Analyses: Rotational Spectroscopy of Tetrahydro-2-Furoic Acid⋠⋠⋠Propylene Oxide Conformers.

A chiral adduct formed between a chiral carboxylic acid, tetrahydro-2-furoic acid (THFA), and a chiral ester, propylene oxide (PO), was investigated using rotational spectroscopy and DFT calculations. Isolated THFA exists dominantly as three different conformers: I, II, and III in a jet, with I and II taking on the trans-COOH configuration and III having the cis-COOH configuration. We utilized CREST, a conformational ensemble space exploration tool, to identify the possible conformations of the binary adduct, THFA⋅⋅⋅PO. Subsequent DFT geometry optimizations predicted about two hundred homochiral and heterochiral binary structures with 28 low energy structures within an energy window of 15 kJ mol-1 . A rich broadband rotational spectrum was obtained with a mixture of trace amounts of THFA+PO in neon in a supersonic jet expansion. Six THFA⋅⋅⋅PO conformers were identified experimentally. Kinetically favored binary products which contain trans-COOH I dominate among the observed conformers, while thermodynamically more stable adducts were also detected. Detailed analyses of the structures of the observed conformers show interesting chirality-controlled structural preferences. Such non-covalently bound chiral contact pairs are the foundation of chiral-tag rotational spectroscopy, an exciting new analytical application of rotational spectroscopy for determination of enantiomeric excess. Enantiomeric excess analyses were performed and the results are discussed.

Global change of surgical and oncological clinical practice in urology during early COVID-19 pandemic.

OBJECTIVES: While the coronavirus disease 2019 (COVID-19) pandemic captures healthcare resources worldwide, data on the impact of prioritization strategies in urology during pandemic are absent. We aimed to quantitatively assess the global change in surgical and oncological clinical practice in the early COVID-19 pandemic. METHODS: In this cross-sectional observational study, we designed a 12-item online survey on the global effects of the COVID-19 pandemic on clinical practice in urology. Demographic survey data, change of clinical practice, current performance of procedures, and current commencement of treatment for 5 conditions in medical urological oncology were evaluated. RESULTS: 235 urologists from 44 countries responded. Out of them, 93% indicated a change of clinical practice due to COVID-19. In a 4-tiered surgery down-escalation scheme, 44% reported to make first cancellations, 23% secondary cancellations, 20% last cancellations and 13% emergency cases only. Oncological surgeries had low cancellation rates (%): transurethral resection of bladder tumor (27%), radical cystectomy (21-24%), nephroureterectomy (21%), radical nephrectomy (18%), and radical orchiectomy (8%). (Neo)adjuvant/palliative treatment is currently not started by more than half of the urologists. COVID-19 high-risk-countries had higher total cancellation rates for non-oncological procedures (78% vs. 68%, p = 0.01) and were performing oncological treatment for metastatic diseases at a lower rate (35% vs. 48%, p = 0.02). CONCLUSION: The COVID-19 pandemic has affected clinical practice of 93% of urologists worldwide. The impact of implementing surgical prioritization protocols with moderate cancellation rates for oncological surgeries and delay or reduction in (neo)adjuvant/palliative treatment will have to be evaluated after the pandemic.

A rotational spectroscopic and ab initio study of cis- and trans-(-)-carveol: further insights into conformational dynamics in monoterpenes and monoterpenoids.

Broadband rotational spectra of cis- and trans-(-)-carveol were recorded using a chirped pulse Fourier transform microwave spectrometer in the 2-6 GHz region. To aid in spectroscopic assignments a theoretical conformational search was carried out using a combination of a two dimensional potential energy scan, scanning over the isopropenyl and hydroxyl groups torsional angles, and the Conformer-Rotamer Ensemble Sampling Tool. The theoretical results yielded a total of 23 conformers for the trans- and 19 for the cis-conformer. Utilizing these results, a total of five conformers could be assigned in the spectra, two for trans- and three for cis-(-)-carveol. In both conformers of trans-carveol, the isopropenyl group is in an equatorial position and adopts the gauche- conformation in one and the the antiperiplanar conformation in the other, with the hydroxyl group in the axial position and adopting the antiperiplanar conformation in both. For cis-carveol the analogous conformers were found but with the hydroxyl in a equatorial position, in addition to an axial isopropenyl conformer. To interpret the experimental intensity patterns and examine conformational cooling effects, transition states were identified using the Synchronous Transit Quasi-Newton method. We found that most of the higher energy conformers cool out to the five experimentally observed ones and the others are too high in energy to be sufficiently populated in the molecular expansion for an experimental observation. To investigate the interesting preference for the axial position of the isopropenyl group in cis-(-)-carveol, which has not been seen before in monoterpenoids, non-covalent interactions and quantum theory of atoms-in-molecules analyses were carried out. These analyses reveal a hydrogen bonding interaction between the hydroxyl group and the isopropenyl π-system. A natural bond orbital analysis of the hydrogen bond allowed us to decompose the interaction into its constituent natural bond orbitals, and to quantify its strength. Although relatively weak, the hydrogen bond tips the balance towards the axial position of the isopropenyl group.

Unusual binary aggregates of perylene bisimide revealed by their electronic transitions in helium nanodroplets and DFT calculations.

The S1 ← S0 electronic transition of perylene bisimide (PBI) and its binary aggregates were investigated using a combination of helium nanodroplet isolation spectroscopy and computational methods. First, well-resolved vibronic bands of the PBI monomer obtained under the superfluid helium nanodroplet environment were compared to simulated vibronic spectra with anharmonic corrections of the band positions. Second, about ten sets of weaker vibronic bands were observed, which show similar vibronic patterns as that of the PBI monomer and have their band origins red-shifted by about 8 to 218 cm-1. Experimental Poisson curve analyses, performed at the origins of these new sets of bands and the PBI monomer, indicate that the carriers of these weaker red-shifted vibronic bands are binary adducts of PBI. Three types of PBI dimer structures where the electronic transition dipole moments of the two subunits are perpendicular to each other were proposed as possible carriers of these red-shifted vibronic patterns. Extensive vibronic simulations were carried out in a multi-step procedure with TD-DFT, vertical Hessian, and finally adiabatic Hessian approaches. Small red-shifted band origins and very similar vibronic patterns to that of the monomer were predicted for unusual, T-shaped, type I dimer structures and are in close agreement with the experimental data. The combined experimental and theoretical results indicate that the helium nanodroplet environment enables the formation of these unusual T-shaped dimers and stabilizes them.

Rotational Spectrum and Molecular Structures of the Binary Aggregates of 1,1,1,3,3,3-Hexafluoro-2-propanol with Ne and Ar.

The structures and binding topologies of two binary van der Waals complexes 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)···Ne and ···Ar were investigated. The rotational spectra of these two complexes including several isotopic species containing 20Ne, 22Ne, 40Ar, 13C, and hydroxyl D were measured using a chirped pulse Fourier transform microwave spectrometer and a cavity-based Fourier transform microwave spectrometer. While HFIP was shown to exist in both the gauche and trans configurations based on previous reports, the rare gas atom is predicted to attach to HFIP in several different binding topologies, leading to a total of nine possible structural isomers for each complex. Only one isomer was detected for each species, and it corresponds to the most stable one predicted, based on the comparison of the experimental rotational constants and electric dipole moment components with the theoretical predictions and on the isotopic data. We applied quantum theory of atoms in molecules (QTAIM) and electrostatic potential calculations to examine the different rare gas binding sites and to explore the nature of the interactions in these two complexes and several previously reported alcohol···Ar complexes. The effects of fluorination are also discussed by comparison with the binary complexes of isopropanol···Ne and ···Ar.

Higher-Energy Hexafluoroisopropanol···Water Isomer and Its Large Amplitude Motions: Rotational Spectra and DFT Calculations.

Rotational spectra of the 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)···water complex were measured using a chirped pulse Fourier-transform microwave spectrometer. The spectral analyses, aided by density functional theory calculations, reveal two HFIP···water isomers: one previously reported, trans HFIP (HFIPt)···water (Phys. Chem. Chem. Phys. 2015, 119, 5650-5657), and a new isomer, gauche HFIP (HFIPg)···water. To confirm the identity of the new isomer, rotational spectra of seven of its deuterated species were also measured and analyzed. Both the experimental and theoretical pieces of evidence indicate that the intermolecular interaction with water preferentially stabilizes the HFIPg monomer configuration over the global minimum configuration, HFIPt. The relative energy difference between these monomeric forms is 4.1 kJ mol-1 and decreases to 2.5 kJ mol-1 in the respective monohydrates at the B3LYP-D3(BJ)/def2-QZVP level of theory. Both rigid and relaxed potential energy surface scans were carried out to gain insights into the large-amplitude water motions in HFIPg···water. The nonobservation of a water tunneling splitting in HFIPt···water has been explained to be a result of a barrier-less (after zero-point-energy correction) pathway for the water motion, whereas in HFIPg···water, a relatively large water tunneling barrier was identified as the cause of barely resolved water tunneling splittings. Noncovalent interaction and quantum theory of atoms and molecule analyses were used to evaluate the changes in HFIPg···water when going from the minimum to the transition state in terms of attractive interactions such as the OH···H and OH···F contacts. The effect of fluorination is discussed by comparing the vastly different binding topologies of isopropanol···water and HFIP···water.

Structural and dynamical features of the 2,2,2-trifluoroethanolammonia complex.

The 1 : 1 adduct of 2,2,2-trifluoroethanol (TFE) with ammonia was investigated using a combination of chirped pulse and cavity-based Fourier transform microwave spectroscopy and computational methods. Rotational spectra of the most stable TFENH3 conformer and seven deuterium and 15N isotopologues were identified, and this led to a determination of partial rs and ro structures. The observed complex exhibits a gauche conformation of TFE with ammonia inserted into the existing intramolecular hydrogen-bonded ring of TFE. The adduct is stabilised by a delicate interplay between the primary O-HN hydrogen-bond and secondary N-HF interactions between TFE and ammonia. Evidence for several internal-dynamics effects was found in the rotational spectra. The ammonia subunit shows an almost free internal rotation. Tunneling between the gauche forms, g+ and g-, of TFE is quenched by the hydrogen-bond interactions with ammonia.

Conformational Panorama and Chirality Controlled Structure-Energy Relationship in a Chiral Carboxylic Acid Dimer.

Chirality recognition in dimers of tetrahydro-2-furoic acid (THFA) was studied in a conformer-specific manner using rotational spectroscopy and theoretical approaches. THFA shows a strong preference for the trans- over the cis-COOH configuration. Two drastically different scenarios are possible for the detectable (THFA)2 : a kinetically preferred dimer bound by feeble interactions between two trans THFAs or a thermodynamically favored dimer with a double hydrogen-bonded ring structure between two cis subunits. To identify the conformers responsible for the extremely dense rotational spectra observed, it was essential not only to locate several hundred homo/heterochiral (THFA)2 minima in ab initio calculations but also to evaluate the energetic connectivities among the minima. The study further reveals an interesting chirality dependent structure-energy ordering relationship. A method for enantiomeric excess (ee) determination of THFA is presented using a recently proposed chiral self-tag approach.

Discovering the Elusive Global Minimum in a Ternary Chiral Cluster: Rotational Spectra of Propylene Oxide Trimer.

The chirality controlled conformational landscape of the trimer of propylene oxide (PO), a prototypical chiral molecule, was investigated using rotational spectroscopy and a range of theoretical tools for conformational searches and for evaluating vibrational contributions to effective structures. Two sets of homochiral (PO)3 rotational transitions were assigned and the associated conformers identified with theoretical support. One set of heterochiral (PO)3 transitions was assigned, but no structures generated by one of the latest, advanced conformational search codes could account for them. With the aid of a Python program, the carbon atom backbone and then the heterochiral (PO)3 structure were generated using 13 C isotopic data. Excellent agreement between theoretical and experimental rotational constants and relative dipole moment components of all three conformers was achieved, especially after applying vibrational corrections to the rotational constants.

A microwave spectroscopic and ab initio study of keto-enol tautomerism and isomerism in the cyclohexanone-water complex.

Cyclohexanone and water are two important components of secondary organic aerosol (SOA), and understanding the intermolecular interactions between these two species can provide insight to the initial formation mechanism of SOA particles. In this work, we have investigated the keto-enol tautomeric and conformational changes of the cyclohexanone monomer and its monohydrate by Fourier-transform microwave spectroscopy and ab initio calculations. Chirped-pulse and cavity-based FTMW spectrometers in the region of 7-14 GHz were used to measure rotational spectra of the most stable species prepared in a cold molecular beam. The experimental and theoretical results suggest that the chair conformer of the keto tautomer is the most stable structure. We have measured and assigned rotational spectra of ten isotopologues, including all six single 13C substitutions observed in natural abundance and four different isotopic species of water (H2O, D2O, DOH and HOD). The experimental structure of cyclohexanone-water was determined directly using this isotopic information. The analysis reveals the existence of both canonical and secondary hydrogen bonding, which we confirm using QTAIM analyses. To further elucidate the hydrogen bonding characteristics in ketone-water clusters, cyclohexanone-water is compared to a variety of other hydrated ketones, namely formaldehyde, acetone, cyclobutanone, and cyclopentanone, through utilization of the symmetry adapted perturbation theory (SAPT) energy decomposition method. The results of this study shed light on the effects of water on keto-enol tautomerization, and the role of hydrogen bonding in ketone monohydrates and the formation of related SOA particles.