Examined here are the structures of complexes of benzophenone microsolvated with up to three water molecules by using broadband rotational spectroscopy and the cold conditions of a molecular jet. The ...analysis shows that the water molecules dock sideways on benzophenone for the water monomer and dimer moieties, and they move above one of the aromatic rings when the water cluster grows to the trimer. The rotational spectra shows that the water trimer moiety in the complex adopts an open‐loop arrangement. Ab initio calculations face a dilemma of identifying the global minimum between the open loop and the closed loop, which is only solved when zero‐point vibrational energy correction is applied. An OH⋅⋅⋅π bond and a Bürgi‐Dunitz interaction between benzophenone and the water trimer are present in the cluster. This work shows the subtle balance between water–water and water–solute interactions when the solute molecule offers several different anchor sites for water molecules.
The subtle discrepancy between the experimental and theoretical structures of the Ph2CO‐(H2O)3 complex has been unveiled. An open‐looped structure, instead of the cyclic structure, of the water trimer moiety above one of the π clouds was determined by rotational spectroscopy.
Fluorinated derivatives of biological molecules have proven to be highly efficient at modifying the biological activity of a given protein through changes in the stability and the kind of docking ...interactions. These interactions can be hindered or facilitated based on the hydrophilic/hydrophobic character of a particular protein region. Diadamantyl ether (C20H30O) possesses both kinds of docking sites, serving as a good template to model these important contacts with aromatic fluorinated counterparts. In this work, an experimental study on the structures of several complexes between diadamantyl ether and benzene as well as a series of fluorinated benzenes is reported to analyze the effect of H→F substitution on the interaction and structure of the resulting molecular clusters using rotational spectroscopy. All experimentally observed complexes are largely dominated by London dispersion interactions with the hydrogen‐terminated surface areas of diadamantyl ether. Already single substitution of one hydrogen atom with fluorine changes the preferred docking site of the complexes. However, the overall contributions of the different intermolecular interactions are similar for the different complexes, contrary to previous studies focusing on the difference in interactions using fluorinated and non‐fluorinated molecules.
The effect of H→F substitution on the interactions and structure of complexes between diadamantyl ether and benzene and a series of fluorinated benzenes have been studied using rotational spectroscopy. London dispersion is the governing factor in directing spatial arrangement. The preferred binding position changes upon partial or full substitution, whereas the overall contributions of the intermolecular interactions involved are similar.
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
13
C 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.
A semi-quantitative analysis as well as determination of the structures and internal dynamics of components of two natural essential oils have been carried out using rotational spectroscopy.
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.
For complexes involving aromatic species, substitution effects can influence the preferred geometry.
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.
Five conformers of the flexible molecule alpha-methoxy phenylacetic acid were identified using rotational spectroscopy. The conformational landscape, internal dynamics, and intramolecular interactions were investigated.
The first examples of luminescence Ir(iii) complexes derived from a tridentate tetrazole ligand, 2-(tetrazole-5-yl)-1,10-phenanthroline (Hphenttz), are reported here. Two cationic complexes, ...heteroleptic Ir(tpy)(phenttz)2+ (1) (tpy = 2,2′:6′,2′′-terpyridine) and homoleptic Ir(phenttz)2+(2), have been synthesized and fully characterized from a chemical, structural and photophysical point of view. Both complexes exhibit green luminescence with a prevalent 3LC (Ligand-Centered) character, which is evidenced by their structured emission profiles and low kr values (of about 103 s−1), and supported by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. In air-equilibrated solutions, 1 and 2 show emission lifetimes (respective values of 1.6 μs and 1.2 μs) comparable to that of the reference complex Ir(tpy)23+ (1.0 μs) and quantum yields slightly lower (1.5% (1), 1.3% (2)) than that obtained for Ir(tpy)23+ (2.5%). Under an oxygen-free atmosphere, the emission lifetimes and quantum yields (τ/Φ) of the complexes increase significantly up to 5 μs/4.2% (1) and 3 μs/3.3% (2). 1 has been embedded within amorphous silica nanoparticles leading to the hybrid material 1@SiO2. This material shows enhanced photochemical stability and higher luminescence efficiency compared to the free complex, which demonstrates that silica hampers the diffusion of O2, restrains the mobility of the complex and stimulates the radiative decay of the excited state.
We report on a combined experimental and computational study of the chiral recognition of the amino acid serine in protonated form (l/d-SerH+), by the crown ether ...(all-S)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (S-18c6H4). Infrared and vibrational circular dichroism spectroscopies (IR-VCD) are employed to characterize the chiroptical response of the complexes formed by S-18c6H4 with the l-SerH+ and d-SerH+ enantiomers in dried thin films obtained from aqueous solutions. The study focuses on vibrational modes directly related to the intermolecular hydrogen bonds between the crown ether derivative and serine, responsible for crown–serine binding, namely, the CO and CO stretching modes, and on the COH bending mode, which yield intense IR and VCD signals in the range of wavenumbers 900–2000 cm–1. The experimental spectra are analyzed in combination with a computational structural survey and optimization at different levels of density functional theory. The conformational landscape of the complexes is found to be primarily governed by a bowl-like structure of the crown ether host and a tripodal coordination of the protonated R-NH3 + group of serine with the oxygen atoms of the central ether ring. Additionally, one or two of the carboxylic side groups of the crown ether interact with the −COH and −COOH groups of serine. Chiral selectivity is probed by recording the IR and VCD spectra of dried thin films obtained from aqueous solutions with equimolar concentrations of the two serine enantiomers and the macrocycle. The results demonstrate a marked chiral recognition of l-SerH+ relative to d-SerH+ by the S-18c6H4 substrate, which arises from the favorable host–guest coordination through H-bonds at optimum distances and collinear orientations, also involving a limited distortion of the crown ether backbone.
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.
Complexes between the chiral monoterpenoids camphor and α-fenchol were explored with vibrational and rotational jet spectroscopy as well as density functional theory to explore how chirality can influence the binding preferences in the gas phase.
Diadamantyl ether (DAE, C20H30O) represents a good model to study the interplay between London dispersion and hydrogen‐bond interactions. By using broadband rotational spectroscopy, an accurate ...experimental structure of the diadamantyl ether monomer is obtained and its aggregates with water and a variety of aliphatic alcohols of increasing size are analyzed. In the monomer, C−H⋅⋅⋅H−C London dispersion attractions between the two adamantyl subunits further stabilize its structure. Water and the alcohol partners bind to diadamantyl ether through hydrogen bonding and non‐covalent Owater/alcohol⋅⋅⋅H−CDAE and C−Halcohol⋅⋅⋅H−CDAE interactions. Electrostatic contributions drive the stabilization of all the complexes, whereas London dispersion interactions become more pronounced with increasing size of the alcohol. Complexes with dominant dispersion contributions are significantly higher in energy and were not observed in the experiment. The results presented herein shed light on the first steps of microsolvation and aggregation of molecular complexes with London dispersion energy donor (DED) groups and the kind of interactions that control them.
Alkyl groups matter: Complexes between diadamantyl ether (DAE) and water and alcohols with increasing side‐chain size have been studied by using rotational spectroscopy. DAE and the clustering molecules interact through an O−H⋅⋅⋅O hydrogen bond and London dispersion (LD) interactions. The H−C⋅⋅⋅H−C LD attractions take place between the alkyl groups of the alcohols and those of DAE. The LD interactions increase with increasing size of the alcohol.
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.
New unassigned spectral lines were found in a reinvestigation of the rotational spectrum of estradiol-17β. Several possibilities were considered leading to the assignment of the transitions to the steroid molecule estrone, a precursor of estradiol.
PDF
