Camilla Calabrese

Keto-enol tautomerism and conformational landscape of 1,3-cyclohexanedione from its free jet millimeter-wave absorption spectrum.

The free jet millimeter-wave absorption spectrum of 1,3-cyclohexanedione has been investigated in the 59.6-74.4 GHz frequency range, and the rotational spectra of two conformational species, the chair-diketo and boat-diketo, and probably one excited vibrational state belonging to the chair-diketo form have been assigned. Quantum chemical calculations, performed at the B3LYP/6-311++G** and MP2/6-311++G** levels, were used to characterize the potential energy surface minima. The potential energy surface related to the interconversion of the observed diketonic species was modeled at the DFT level.

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).

Conformational Equilibrium and Internal Dynamics of E-Anethole: A Rotational Study

The rotational spectra of the two conformers of E-anethole have been investigated using the free jet broadband millimeter-wave spectroscopic technique combined with theoretical calculations. Anti and syn conformers differ for the relative orientation of the propenyl and methoxy chains, with all heavy atoms coplanar to the benzene ring. Relative intensity measurements prove that the anti form is the global minimum, about 2.0(5) kJ mol(-1) lower in energy with respect to the syn conformer, solving the contrasting results supplied by different theoretical methods. For both conformers, the barriers to internal rotation of the propenyl -CH3 group are low enough to generate fully resolved A-E splittings of the rotational transitions. The corresponding V3 barriers have been determined to be 7.080(5) and 6.978(4) kJ mol(-1), respectively.

Millimeter Wave Spectrum of the Weakly Bound Complex CH2═CHCN·H2O: Structure, Dynamics, and Implications for Astronomical Search

The weakly bound 1:1 complex between acrylonitrile (CH2═CHCN) and water has been characterized spectroscopically in the millimeter wave range (59.6-74.4 GHz) using a Free Jet Absorption Millimeter Wave spectrometer. Precise values of the rotational and quartic centrifugal distortion constants have been obtained from the measured frequencies of the normal and isotopically substituted water moiety (DOH, DOD, H(18)OH). Structural parameters have been estimated from the rotational constants and their differences among isotopologues: the complex has a planar structure with the two subunits held together by a O-H···N (2.331(3) Å) and a C-H···O (2.508(4) Å) interaction. The ab initio intermolecular binding energy, obtained at the counterpoise corrected MP2/aug-cc-pVTZ level of calculation, is De = 24.4 kJ mol(-1).

Accurate spectroscopy of polycyclic aromatic compounds: From the rotational spectrum of fluoren-9-one in the millimeter wave region to its infrared spectrum

The rotational spectrum of fluoren-9-one, a small oxygenated polycyclic aromatic hydrocarbon, has been recorded and assigned in the 52-74.4 GHz region. The determined small negative value of the inertia defect (-0.3 u Å(2)) has been explained in terms of vibrational-rotational coupling constants calculated at the B3LYP/cc-pVTZ level of theory. Vibrational anharmonic analysis together with second-order vibrational perturbation theory approximation was applied both to fluorenone and its reduced form, fluorene, to predict the mid- and near-infrared spectra. The data presented here give precise indication on the fluorenone ground state structure, allow for an accurate spectral characterization in the millimeter wave and infrared regions, and hopefully will facilitate extensive radio astronomical searches with large radio telescopes.

Solving the Tautomeric Equilibrium of Purine through Analysis of the Complex Hyperfine Structure of the Four 14N Nuclei

The rotational spectra of two tautomers of purine have been measured by pulsed jet Fourier transform microwave spectroscopy coupled to a UV ultrafast vaporization system. The population ratio of the two main tautomers [N(7)H]/[N(9)H] is about 1/40 in the gas phase. It contrasts with the solid state where only the N(7)H species is present, or in solution where a mixture of both tautomers is observed. For both species, a full quadrupolar hyperfine analysis has been performed. This has led to the determination of the full sets of diagonal quadrupole coupling constants of the four (14)N atoms, which have provided crucial information for the unambiguous identification of both species. This work shows the great potential of microwave spectroscopy to study isolated biomolecules in the gas phase. All the work was supported by theoretical calculations.

Effects of Fluorine Substitution on the Microsolvation of Aromatic Azines: The Microwave Spectrum of 3-Fluoropyridine-Water

The rotational spectra of the parent species and of four water isotopologues (-DOH, -HOD, -DOD, and -H(18)OH) of the adduct 3-fluoropyridine-water have been investigated using pulsed supersonic-jet Fourier-transform microwave spectroscopy. From the rotational constants, the structure of the adduct was deduced where the water is linked to the aromatic ring through an intermolecular O-H···N hydrogen bond with a bond distance of 1.9961(5) Å and an O-H···N angle of 156.8(1)°. The shape of the complex is such that the water oxygen is in the plane of the aromatic ring, on the opposite side of fluorine, and forming a C-H···N weak hydrogen bond with the adjacent aromatic hydrogen.

Fluorine Substitution Effects on Flexibility and Tunneling Pathways: The Rotational Spectrum of 2-Fluorobenzylamine

The effect of ring fluorination on the structural and dynamical properties of the flexible model molecule 2-fluorobenzylamine has been studied by rotational spectroscopy in free-jet expansion and quantum chemical methods. The complete potential energy surface originating from the flexibility of the aminic side chain has been calculated at the B3LYP/6-311++G** level of theory and the stable geometries were also characterized with MP2/6-311++G**. The rotational spectra show the presence of two of the predicted four stable conformers: the global minimum (I), in which the side chains dihedral angle with the phenyl plane is almost perpendicular, is stabilized by an intramolecular hydrogen bond between the fluorine atom and one hydrogen of the aminic group; and a second conformer II (EII -EI ≈5 kJ mol(-1)) in which the dihedral angle is smaller and the amino group points towards the aromatic ortho hydrogen atom. This conformation is characterized by a tunneling motion between two equivalent positions of the amino group with respect to the phenyl plane, which splits the rotational transition. The ortho fluorination increases, with respect to benzylamine, the tunneling splitting of this motion by four orders of magnitude. The motion is analyzed with a one-dimensional flexible model, which allows estimation of the energy barrier for the transition state as approximately 8.0 kJ mol(-1).

Acrylic acid (CH2CHCOOH): the rotational spectrum in the millimetre range up to 397 GHz

In order to facilitate its detection in astronomical observations, the measurement of the rotational spectrum of acrylic acid has been extended to 397 GHz using a free space cell at room temperature. 295 new lines were assigned to the s-cis conformer and 286 new lines to the s-trans conformer of acrylic acid. Using the determined experimental parameters, the predictions of the rotational transition frequencies up to 900 GHz and their intensities were obtained at temperatures of 100 and 200 K and at room temperature. Based on these predictions, a search of the most intense transitions of acrylic acid in star-forming regions was performed using published data from the HERSCHEL Science Archive. No lines were found but the possibility of observing rotational transitions of acrylic acid in astronomical objects is discussed.