The ground-state rotational spectrum of the dimethyl ether dimer, (DME)(2), has been studied by molecular beam Fourier transform microwave and free jet millimeter wave absorption spectroscopies. The molecular beam Fourier transform microwave spectra of the (DME-d(6))(2), (DME-(13)C)(2), (DME-d(6))...(DME), (DME-(13)C)...(DME), and (DME)...(DME-(13)C) isotopomers have also been assigned. The rotational parameters have been interpreted in terms of a C(s) geometry with the two monomers bound by three weak C-H...O hydrogen bonds, each with an average interaction energy of about 1.9 kJ/mol. The experimental data combined with high-level ab initio calculations show this kind of interaction to be improper, blue-shifted hydrogen bonding, with an average shortening of the C-H bonds involved in the hydrogen bonding of 0.0014 A. The length of the C-H...O hydrogen bonds, r(O...H), is in the range 2.52-2.59 A.

The concentration dependence of the Raman noncoincidence effect (NCE) of the C-O and O-H stretching bands of methanol is investigated in methanol/CCl 4 mixtures in the range of 1.0 g x m g 0.1, where x m is the mole fraction of methanol, by performing Raman spectroscopic measurements and molecular dynamics (MD) simulations. Band asymmetry observed for both bands is carefully taken into account. The experimental and simulation results are in satisfactory agreement with each other. For the C-O stretching band, it is observed that the magnitude of the negative NCE gets larger upon dilution in CCl 4 down to x m ∼ 0.2, contrary to the expectation of becoming smaller from simple guess that the NCE arises from intermolecular vibrational resonant interactions between methanol molecules, which, on average, get separated from each other upon dilution. For the O-H stretching band, the magnitude of the positive NCE remains almost the same upon dilution down to x m ∼ 0.3. These apparently peculiar experimental results are reasonably explained by the MD simulations on the basis of the transition dipole coupling (TDC) mechanism of intermolecular resonant vibrational interactions and the simulated hydrogen-bonded liquid structures. In the case of the C-O stretching band, the negative NCE arises mainly from positive vibrational coupling between hydrogen-bonded pairs of molecules, which is partially canceled by negative vibrational coupling between molecules in different hydrogenbonded chains. In the case of the O-H stretching band, the positive NCE arises predominantly from negative vibrational coupling within hydrogen-bonded chains. As a result, a locally anisotropic change in the liquid structure that occurs upon dilution, in which, around each molecule, intermolecular distances do not change very much along hydrogen-bond directions but do change significantly in other directions, gives rise to the apparently peculiar behavior of the NCE described above.

Hydrogen bonding involves many scientific areas and is invoked to explain the energetic and structural features of inorganic, organic, and biological chemical systems. [1,2] Herein, we show that a water molecule in a solvent-free environment prefers to form a hydrogen bond with a Cl atom rather than with a F atom. For this purpose, several isotopologues of chlorofluoromethane/water were studied by molecular-beam Fourier transform rotational spectroscopy.

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The trimer of difluoromethane, (CH2F2)3, has been characterized by supersonic jet Fourier transform microwave spectroscopy. The rotational spectrum displays all types (mu(a), mu(b), and mu(c)) of transitions, showing that the adduct does not possess any element of molecular symmetry. The investigation of the three 13C species in natural abundance indicates that the three carbon atoms form a triangle where the C-C distances are 3.648(2), 3.825(8), and 3.942(6) A, respectively. The three subunits are held together by nine CH...F weak hydrogen bonds.

The molecular beam Fourier transform microwave spectra of two isotopomers of the 1:1 complex between indole and water have been measured. The water molecule has been reliably located in the complex from these experimental data. The complex has a Cs symmetry with an N–H⋯O hydrogen bond and the plane of the H2O molecule perpendicular to the indole plane. The two-dimensional potential energy surface of the internal rotation and inversion of water in the complex, evaluated with B3LYP/6-31G** or MP2/6-31G** quantum chemical calculations, suggests the tunneling motion of water to take place with the contribute of both motions. The experimental evidence combined with flexible model calculations, indicate, however, that the tunneling motion is mainly an internal rotation of water around its C2 symmetry axis.

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We report the free-jet rotational spectra of methylsalicylate, a molecule with a possible tautomeric and conformational equilibrium. In the ground electronic state, the molecule adopts a form stabilized by an intramolecular hydrogen bond between the phenolic hydrogen and the carbonylic oxygen, and this structure is characterized as the lowest-energy form by quantum chemical calculations. All rotational transitions are split because of the internal rotation of the methyl group, and the value of the barrier for this motion was determined to be V(3) = 5.38 kJ mol(-1).