On the Cl⋯C halogen bond: a rotational study of CF3Cl–CO

A Cl⋯C halogen bond links CF₃Cl to CO in their symmetric top 1 : 1 adduct.

Hydrated forms of fluoroacetic acid: a rotational study

Two conformers have been observed in the rotational spectrum of the 1 : 1 adduct of fluoroacetic acid and water.

The Cage Structure of Indan-CHF3 is Based on the Cooperative Effects of C-H⋅⋅⋅π and C-H⋅⋅⋅F Weak Hydrogen Bonds

The structural and energetic features of the C-H⋅⋅⋅π interaction and the internal dynamics of the CHF3 group change drastically in going from benzene-CHF3 to indan-CHF3 , according to the analysis of the rotational spectrum of the latter complex generated in a supersonic expansion.

Theory vs. experiment for molecular clusters: Spectra of OCS trimers and tetramers

All singly substituted (13)C, (18)O, and (34)S isotopomers of the previously known OCS trimer are observed in natural abundance in a broad-band spectrum measured with a chirped-pulse Fourier transform microwave spectrometer. The complete substitution structure thus obtained critically tests (and confirms) the common assumption that monomers tend to retain their free structure in a weakly bound cluster. A new OCS trimer isomer is also observed, and its structure is determined to be barrel-shaped but with the monomers all approximately aligned, in contrast to the original trimer which is barrel-shaped with two monomers aligned and one anti-aligned. An OCS tetramer spectrum is assigned for the first time, and the tetramer structure resembles an original trimer with an OCS monomer added at the end with two sulfur atoms. Infrared spectra observed in the region of the OCS ν1 fundamental (≈2060 cm(-1)) are assigned to the same OCS tetramer, and another infrared band is tentatively assigned to a different tetramer isomer. The experimental results are compared and contrasted with theoretical predictions from the literature and from new cluster calculations which use an accurate OCS pair potential and assume pairwise additivity.

Average orientation of water in CH2F2⋯H2O from the (17)O quadrupole effects in the rotational spectrum of CH2F2⋯H2(17)O

The rotational spectrum of the CH2F2⋯H2(17)O adduct has been measured in the 6.5-18.5 GHz range using a pulsed jet Fourier-transform microwave spectrometer. The experimental values of the (17)O quadrupole coupling constants have been determined to be χaa=-3.853(6), χbb=-3.259(1) and χcc=7.112(9) MHz, respectively. These data correspond to an average value of the angle between the water plane and the FCF plane of about 23°.

Molecular Structure and Chirality Detection by Fourier Transform Microwave Spectroscopy

We describe a three-wave mixing experiment using time-separated microwave pulses to detect the enantiomer-specific emission signal of the chiral molecule using Fourier transform microwave (FTMW) spectroscopy. A chirped-pulse FTMW spectrometer operating in the 2-8 GHz frequency range is used to determine the heavy-atom substitution structure of solketal (2,2-dimethyl-1,3-dioxolan-4-yl-methanol) through analysis of the singly substituted (13)C and (18)O isotopologue rotational spectra in natural abundance. A second set of microwave horn antennas is added to the instrument design to permit three-wave mixing experiments where an enantiomer-specific phase of the signal is observed. Using samples of R-, S-, and racemic solketal, the properties of the three-wave mixing experiment are presented, including the measurement of the corresponding nutation curves to demonstrate the optimal pulse sequence.

Oligomers based on weak hydrogen bond networks: a rotational study of the tetramer of difluoromethane

We report the rotational spectrum of one conformer of the tetramer of difluoromethane (CH2F2)4 (the first pure rotational spectrum of a tetramer of an asymmetric rotor), and describe the network of weak hydrogen bonds which connect the four subunits.

Conformational flexibility of mephenesin

The mephenesin molecule (3-(2-methylphenoxy)propane-1,2-diol) serves as a test bank to explore several structural and dynamical issues, such as conformational flexibility, the orientation of the carbon linear chain relative to the benzene plane, or the effect of substituent position on the rotational barrier of a methyl group. The molecule has been studied by rotational spectroscopy in the 4-18 GHz frequency range by Fourier-transform methods in a supersonic expansion. The experiment has been backed by a previous conformational search plus optimization of the lowest energy structures by ab initio and density functional quantum calculations. The three lowest-lying conformers that can interconvert to each other by simple bond rotations have been detected in the jet. Rotational parameters for all structures have been obtained, and methyl torsional barriers have been determined for the two lowest-lying rotamers. The lowest-lying structure of mephenesin is highly planar, with all carbon atoms lying nearly in the benzene ring plane, and is stabilized by the formation of cooperative intramolecular hydrogen bonding. An estimation of the relative abundance of the detected conformers indicates that the energetically most stable conformer will have an abundance near 80% at temperatures relevant for biological activity.

O-H⋅⋅⋅N and C-H⋅⋅⋅O hydrogen bonds control hydration of pivotal tropane alkaloids: tropinone⋅⋅⋅H(2)O complex

The effect of monohydration in equatorial/axial isomerism of the common motif of tropane alkaloids is investigated in a supersonic expansion by using Fourier-transform microwave spectroscopy. The rotational spectrum reveals the equatorial isomer as the dominant species in the tropinone⋅⋅⋅H2 O complex. The monohydrated complex is stabilized primarily by a moderate OH⋅⋅⋅N hydrogen bond. In addition, two CH⋅⋅⋅O weak hydrogen bonds also support this structure, blocking the water molecule and avoiding any molecular dynamics in the complex. The water molecule acts as proton donor and chooses the ternary amine group over the carbonyl group as a proton acceptor. The experimental work is supported by theoretical calculations; the accuracy of the B3LYP, M06-2X, and MP2 methods is also discussed.

How water interacts with halogenated anesthetics: the rotational spectrum of isoflurane-water

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.

Competition between weak hydrogen bonds: C–H⋯Cl is preferred to C–H⋯F in CH2ClF–H2CO, as revealed by rotational spectroscopy

CH₂O is linked to CH₂ClF through a C–H⋯Cl weak hydrogen bond, and it rotates along its symmetry axis with a barrier V₂ ∼ 125 cm⁻¹.