Qian Gou

Conformational equilibria in carboxylic acid bimolecules: a rotational study of acrylic acid-formic acid.

The bimolecule acrylic acid-formic acid has been investigated by pulsed jet Fourier transform microwave spectroscopy. The complex adopts two different conformational shapes, according to the cis or trans forms of the acrylic acid moiety. The measurements have been extended to four deuterated and to four (13)C (natural abundance) species, and their combinations, for each conformer. These data allowed us to determine the carbon skeleton structures and to size quantitatively the structural effect caused by the OH → OD isotopic substitutions (the Ubbelohde effect).

Lone‐Pair⋅⋅⋅π Interaction: A Rotational Study of the Chlorotrifluoroethylene–Water Adduct

The rotational spectrum of the chlorotrifluoroethylene–water adduct shows that water links to a fully halogenated alkene through a lone-pair (lp)···p interaction. Water is ubiquitous in chemical, physical, and biological systems, and the knowledge of the ways it interacts with the various kind of molecules and biomolecules helps to understand solvation processes in aqueous environments and its effects on gas-phase reactions. The typology of the complexes that water forms with organic molecules has been described and classified in a recent article. Using alcohols, ethers, amines, amides or N-containing aromatics, water is linked to the partner molecule with relatively strong (15–25 kJmol ) O H···O, O H···N or N H···O hydrogen bonds. With ethers, aliphatic amines, diazines, alcohols, 6] water acts generally as a proton donor. However, when making an adduct with phenols and with NH groups inserted in an aromatic ring, water takes the role of a proton acceptor. With amides and amino acids, water forms a two-hydrogen-bond ring structure with the double role of proton donor and proton acceptor. With hydrogen-containing freons, water forms weak (4–6 kJmol ) OH···halogen hydrogen bonds, such as OH···F or OH···Cl. When both Cl and F atoms are present in a freon molecule, sometimes the OH···Cl linkage is favorite, but the OH···F one is preferred in other cases. However, when a aliphatic freon molecule is perhalogenated, and no hydrogen atoms are present in the molecule, then a halogen bond (6–10 kJmol ), rather than a hydrogen bond is formed. The geometries of the adducts of water with molecules containing a p-electron system, such as ethylene–water and benzene–water have been found to be stabilized by OH···p interactions, although with high dynamical effects. No rotational investigations of an adduct of water with a molecule fully halogenated and with a p-electron system has been reported. For this reason, we decided to investigate the microwave spectrum of the adduct of water with chlorotrifluoroethylene (C2ClF3, freon-1113). Before collecting the spectra, the full geometry optimization of the complex has been done with ab initio calculation at the MP2/6-311 ++ G(d,p) level using the Gaussian03 program package. Six plausible conformers were found. The relative energies and spectroscopic constants were obtained and collected in Table 1. To have a better estimate of the

Halogen bond and free internal rotation: the microwave spectrum of CF3Cl-dimethyl ether.

The rotational spectra of two isotopologues of the molecular complex CF3Cl-dimethyl ether have been measured with pulsed jet Fourier transform microwave spectroscopy. The rotational spectrum encodes information on the global minimum configuration, stabilized by a Cl···O halogen bond, and on the almost free rotation of the -CF3 group.

Frontiers in Rotational Spectroscopy: Shapes and Tunneling Dynamics of the Four Conformers of the Acrylic Acid—Difluoroacetic Acid Adduct

The rotational spectra of four conformers of the acrylic acid-difluoroacetic acid adduct (CH2 =CHCOOH-CHF2 COOH, AA-DFA) are reported and information on their internal dynamics is supplied. This represents an unprecedented result for the conformational analysis, with microwave spectroscopy, of such a heavy molecular adduct.

Ubbelohde Effect within Weak C–H···π Hydrogen Bonds: The Rotational Spectrum of Benzene–DCF3

The Fourier transform microwave investigation of C6H6-DCF3 outlines a shortening of the distance between the two constituent molecules of about 0.0044(2) Å upon H → D substitution of the hydrogen atom involved in the C-H···π hydrogen bond. This proves that the Ubbelohde effect takes place also within weak hydrogen bonding. The measure of the spectra of several (13)C isotopologues in natural abundance has been useful to obtain structural information.

Rotational study of cis- and trans-acrylic acid-trifluoroacetic acid.

The rotational spectra of the normal and deuterated species of the acrylic acid-trifluoroacetic acid complex have been assigned for both cis and trans conformations with the pulsed jet Fourier transform microwave technique. Relative intensity measurements suggest that the cis species is more stable than the trans one, in agreement with the ab initio results. The deuteration of the carboxylic groups generates an increase of the distance between the two moieties of the complex (Ubbelohde effect) and prevents the possibility of locating the OH hydrogen atoms. The changes of the Paa planar moment of inertia upon OH → OD substitution allowed for a quantitative estimate of the Ubbelohde effect.

Interaction between freons and amines: the C-H···N weak hydrogen bond in quinuclidine-trifluoromethane.

The 1:1 complex between quinuclidine and trifluoromethane has been investigated using pulsed jet Fourier transform microwave spectroscopy. The two constituting molecules are held together through a weak C-H···N hydrogen bond, with a H···N length of 2.070(1) Å. The C3 symmetric axes of the two moieties are collinear, making the overall molecular system a symmetric top. The HCF3 subunit is freely rotating with respect to the amine moiety. Transitions for the ground (m = 0) and for the first excited (|m| = 1) torsional states, with K up to 2, have been measured, all of them showing the (14)N nuclear quadrupole hyperfine structure. The dissociation energy of the complex has been estimated to be 10.2 kJ mol(-1).

Rotational Study of Dimethyl Ether–Chlorotrifluoroethylene: Lone Pair···π Interaction Links the Two Subunits

The rotational spectra of two isotopologues of chlorotrifluoroethylene-dimethyl ether show that the two constituent molecules are held together by a lone pair···π interaction. The ether oxygen is linked to the (CF2) carbon atom, with a C-O distance of 2.908 Å.

Probing the Lone Pair···π-Hole Interaction in Perfluorinated Heteroaromatic Rings: The Rotational Spectrum of Pentafluoropyridine·Water

The rotational spectrum of the weakly bound complex pentafluoropyridine·water has been investigated with pulsed jet Fourier transform microwave spectroscopy. From the analysis of the rotational parameters of the parent species and of three water isotopologues, the structural arrangement of the adduct has been unambiguously established. The results show that the full ring fluorination of pyridine has a dramatic effect on its binding properties: It inverts the electron density distribution above the ring, creating a π-hole, with respect to the typical π-cloud of benzene and pyridine. In the complex the water moiety lies above the aromatic ring with the oxygen lone pairs pointing toward its center. This lone pair···π-hole interaction stabilizes the adduct, and it is more stable than the in-plane O-H···N hydrogen bond normally found in the complexes involving nitrogen heterocyclic aromatic rings. Evidence of a large amplitude motion involving the weakly bound water molecule has also been observed and discussed.

How CO2 Interacts with Carboxylic Acids: A Rotational Study of Formic Acid–CO2

The rotational spectra of the 1:1 formic acid-carbon dioxide molecular complex and of its monodeuterated isotopologues are analysed in the 6.5-18.5 and 59.6-74.4 GHz frequency ranges using a pulsed jet Fourier transform microwave spectrometer and a free-jet absorption millimetre wave spectrometer, respectively. Precise values of the rotational and quartic centrifugal distortion constants are obtained from the measured frequencies, and quadrupole coupling constants are determined from the deuterium hyperfine splittings. Structural parameters are estimated from the moments of inertia and their differences among isotopologues: the complex has a planar structure with the two subunits held together by a HC(O)OH⋅⋅⋅O=C=O (2.075 Å) and a HC(OH)O⋅⋅⋅CO2 (2.877 Å) interactions. The ab initio intermolecular binding energy, obtained at the counterpoise corrected MP2/aug-cc-pVTZ level of calculation, is De =17 kJ mol(-1).