Unraveling the spectroscopy of coupled intramolecular tunneling modes: a study of double proton transfer in the formic-acetic acid complex.

The rotational spectrum of the hetero dimer comprising doubly hydrogen-bonded formic acid and acetic acid has been recorded between 4 and 18 GHz using a pulsed-nozzle Fourier transform microwave spectrometer. Each rigid-molecule rotational transition is split into four as a result of two concurrently ongoing tunneling motions, one being proton transfer between the two acid molecules, and the other the torsion/rotation of the methyl group within the acetyl part. We present a full assignment of the spectrum J = 1 to J = 6 for the ground vibronic states. The transitions are fitted to within a few kilohertz of the observed frequencies using a molecule-fixed effective rotational Hamiltonian for the separate A and E vibrational species of the G(12) permutation-inversion symmetry group. Interpretation of the motion problem uses an internal-vibration and overall-rotation angular momentum coupling scheme and full sets of rotational and centrifugal distortion constants are determined. The tunneling frequencies of the proton-transfer motion are measured for the ground A and E methyl rotation states as 250.4442(12) and -136.1673(30) MHz, respectively. The slight deviation of the latter tunneling frequency from being one half of the former, as simple theory otherwise predicts, is due to different degrees of mixing in wavefunctions between the ground and excited states.

From transient to induced permanent chirality in 2-propanol upon dimerization: a rotational study.

Five different hydrogen-bonded conformers of the dimer of 2-propanol have been characterized by Fourier transform microwave spectroscopy. In all observed species, the proton donor moiety adopts a gauche conformation. While in the gauche monomer a transient chirality takes place, all dimers are classical chiral systems. The Ubbelohde effect has been observed upon OH → OD isotopic substitution and its structural effects--a 5 mŠreduction of the O---O distance--have been quantitatively defined.

Hydrates of trans- and gauche-difluoroacetic acids: a high-resolution microwave spectroscopic study.

Although identical in formula, trans- and gauche-difluoroacetic acids behave as different molecules in rotational spectra due to their distinct sets of rotational constants. In this study, high-resolution microwave spectra of monohydrates, dihydrates, and trihydrates of both trans- and gauche-difluoroacetic acids were recorded using a Fourier transform microwave spectrometer. Rotational and centrifugal distortion constants of these hydrates were obtained with high accuracy via fitting their microwave spectra. It was found that the subtle structural difference between the trans and gauche forms of the acid gives different tunneling motions in their monohydrates. An unusual mixing of wave functions of energetically nondegenerate conformations was observed in the monohydrate of gauche-difluoroacetic acid. Ab initio calculations using the second-order Møller-Plesset perturbation method were also performed to optimize structures and to predict magnitudes of dipole moments of these hydrates. Close agreement between experimental and theoretical rotational constants confirms the feature of hydrogen-bonded ring structures in which all incoming H(2)O molecules bind to the carboxylic group for all hydrates.

The monohydrate and dihydrate of acetic acid: a high-resolution microwave spectroscopic study.

High-resolution microwave spectra of the monohydrate and dihydrate of acetic acid were recorded using a pulsed nozzle Fourier transform microwave spectrometer. The rotational and centrifugal distortion constants of these species were determined, which confirms the structures predicted by ab initio calculations that the H(2)O molecules bind to the carboxylic group to form hydrogen-bonded ring complexes. The dependence of the intensity of the rotational transitions on the power of the microwave pulses suggests that both hydrates have small a-and b-dipole moments of less than 0.3 Debye. All rotational transitions were split into two by internal rotation of the methyl group. Analysis of the splitting using both the PAM and the CAM methods allows the orientation and the height of the three-fold barrier to internal rotation (V(3)) of the methyl group to be determined accurately. A consistently declining trend of V(3) from the acid monomer [168.16 cm(-1), B. P. van Eijck, J. van Opheusden, M. M. M. van Schaik and E. van Zoeren, J. Mol. Spectrosc., 1981, 86, 465] through the monohydrate (138.396 cm(-1)) and the dihydrate (118.482 cm(-1)) was observed, which suggests that the amount of decrease of V(3) may be correlated with the strength of hydrogen bonding in these complexes.

High-resolution microwave spectroscopic and ab initio studies of propanoic acid and its hydrates.

High-resolution microwave spectra of the propanoic acid monomer (PPA) and two of its hydrates, the PPA-(H2O) and the PPA-(H2O)2, were recorded using a pulsed nozzle Fourier transform microwave spectrometer. The rotational and centrifugal distortion constants of these species were determined. Agreements between the experimental and ab initio results of these constants, and of the planar moment of inertia, the dipole moments, and the orientation of the PPA relative to the H2O confirm the geometry of the hydrates, i.e. H2O binds to the carboxylic group of PPA and forms hydrogen-bonded ring complexes. The equilibrium constant and the change of entropy and enthalpy for the formation of PPA-(H2O) were also derived, based on the calculation of partition functions, to evaluate the abundance of this monohydrate in the troposphere.

High-resolution microwave studies of ring-structured complexes between trifluoroacetic acid and water.

The rotational spectra of the complexes between one trifluoroacetic acid molecule and up to three water molecules have been recorded using a pulsed nozzle Fourier transform microwave spectrometer. The unambiguous assignments of them are made on the basis of the agreement between the experimentally determined rotational constants and the theoretical predictions from ab initio calculations using MP2/6-311++G(2df,2pd). All the complexes exhibit hydrogen-bonded ring structures. The fine splittings observed in some of the a-type transitions of the trifluoroacetic acid-H2O dimer were analyzed in terms of the likely tunneling motions of the hydrogens in the H2O molecule. Further calculations of the equilibrium constants for these three hydrated complexes of trifluoroacetic acid were also made to evaluate their fractions against the trifluoroacetic acid monomer in the atmosphere.

High-resolution spectroscopy of induced chiral dimers: a study of the dimers of ethanol by Fourier transform microwave spectroscopy.

We present the first recording of the high-resolution spectrum of an induced chiral dimer. Three conformers of the induced chiral dimers of ethanol have been observed using a pulsed molecular-beam Fourier transform microwave spectrometer. The rotational constants of the normal isotopomers of the three species have been determined to be (a) A=5113.826(5), B=1329.7214(4), and C=1257.5151(3) MHz, (b) A=5086.459(5), B=1316.6508(4), and C=1243.6329(4) MHz, and (c) A=4851.608(5), B=1369.7558(6), and C=1243.4184(4) MHz. The observed species have been assigned to calculated structures via Kraitchman double substitution analyses and ab initio calculations. The Kraitchman analyses and the fitted centrifugal distortion parameters suggest that the deuterium bond is significantly stronger than the hydrogen bond in the dimers of ethanol.

Prospective comparison of contrast-enhanced computed tomography versus magnetic resonance venography in the detection of occult deep pelvic vein thrombosis in patients with pelvic and acetabular fractures.

OBJECTIVE: To determine the rate of pelvic vein thrombosis following acetabular or pelvic fracture identified by enhanced computed tomography venography or magnetic resonance venography. DESIGN: Prospective evaluation of computed tomography venography and magnetic resonance venography in patients with pelvic and acetabular trauma as a screening tool for deep vein thrombosis. SETTING: Level I trauma center. RESULTS: Thirty patients with pelvic or acetabular fractures and who met the study criteria were prospectively screened with magnetic resonance venography and computed tomography venography to determine preoperative presence of pelvic venous thrombosis. Pelvic deep vein thrombosis was detected by computed tomography venography in two patients (7%) and by magnetic resonance venography in four patients (13%). Invasive selective pelvic venographies were performed on the five subjects who tested positive on either one or both screening tests. Only one computed tomography venography case was validated by invasive pelvic venography. The false-positive rate for computed tomography venography was 50%, and the false-positive rate for magnetic resonance venography was 100%. CONCLUSIONS: We cannot recommend the sole use of either computed tomography venography or magnetic resonance venography to screen and direct the treatment of asymptomatic thrombi in patients with fracture of the pelvic ring because of the high false positive rates. If these studies are used as screening tools, confirmation of the presence of thrombosis with selective venography should be performed prior to initiating invasive treatment with a vena cava filter. Clinical decisions based solely on one of these imaging techniques may result in inappropriate aggressive treatment due to the high false-positive rate.