Alberto Lesarri

Construction of a molecular beam Fourier transform microwave spectrometer used to study the 2,5-dihydrofuran-argon van der Waals complex

Abstract The rotational spectrum of the van der Waals complex consisting of argon and 2,5-dihydrofuran has been observed in a molecular beam Fourier transform microwave spectrometer in the frequency range 5–18.5 GHz. Analysis of the derived spectroscopic constants shows that the dimer has a structure with a plane of symmetry in which the argon atom is located 3.48 A above the ring plane. The experimental setup of the molecular beam Fourier transform microwave spectrometer is also presented.

A laser-ablation molecular-beam Fourier-transform microwave spectrometer: The rotational spectrum of organic solids

A spectrometer that combines laser ablation of a solid sample with molecular-beam Fourier-transform microwave spectroscopy (MB-FTMW) has been constructed to obtain the rotational spectrum of solid organic molecules. Laser ablation is produced by visible radiation, using the second harmonic of a Nd:YAG laser. The laser hits a solid rod placed on a specially designed pulsed nozzle, coaxially oriented with the axis of the microwave spectrometer Fabry–Perot cavity. The vaporized molecules are seeded in the supersonic jet formed by the expansion of a carrier gas (Ar,Ne) and probed by FTMW spectroscopy. The high sensitivity of the spectrometer is exemplified with the observation of minor isotopomers in natural abundance of different organic molecules and natural amino acids. This instrument opens perspectives in the investigation of the gas phase structure of solid organic compounds.

Free Fructose Is Conformationally Locked

Fructose has been examined under isolation conditions using a combination of UV ultrafast laser vaporization and Fourier-transform microwave (FT-MW) spectroscopy. The rotational spectra for the parent, all (six) monosubstituted (13)C species, and two single D species reveal unambiguously that the free hexoketose is conformationally locked in a single dominant β-pyranose structure. This six-membered-chair skeleton adopts a (2)C(5) configuration (equivalent to (1)C(4) in aldoses). The free-molecule structure sharply contrasts with the furanose form observed in biochemically relevant polysaccharides, like sucrose. The structure of free fructose has been determined experimentally using substitution and effective structures. The enhanced stability of the observed conformation is primarily attributed to a cooperative network of five intramolecular O-H···O hydrogen bonds and stabilization of both endo and exo anomeric effects. Breaking a single intramolecular hydrogen bond destabilizes the free molecule by more than 10 kJ mol(-1). The structural results are compared to ribose, recently examined with rotational resolution, where six different conformations coexist with similar conformational energies. In addition, several DFT and ab initio methods and basis sets are benchmarked with the experimental data.

Proton Tunneling in Heterodimers of Carboxylic Acids: A Rotational Study of the Benzoic Acid−Formic Acid Bimolecule

Tunneling effects have been measured in the pulsed jet Fourier transform microwave spectra of two isotopologues of the benzoic acid-formic acid bimolecule. The tunneling splittings are originated by the concerted proton transfer of the two carboxylic hydrogens. From the values of these splittings for the OH-OH and OD-OD species, it has been possible to model/size the barrier to the concerted double proton transfer.

Rotational Spectrum of Jet-Cooled HfO2 and HfO

The rotational spectrum of jet-cooled hafnium dioxide obtained by laser ablation of a solid ceramic rod has been investigated by Fourier-transform microwave spectroscopy in the 8 to 28 GHz frequency range. Rotational transitions within the ground and several excited vibrational states of the lowest vibrational mode of the molecule have been assigned. The resulting spectra have been fit, yielding rotational parameters for the five most abundant isotopomers of HfO2. Centrifugal distortion effects are noticeable even for the lowest-J transitions. Very large quadrupole coupling effects for the isotopomers with nuclear quadrupole moments (179Hf (I=9/2) and 177Hf (I=7/2)) have been accounted for using the diagonal elements of the nuclear quadrupole coupling tensor. A ground-state effective C2v geometry has been obtained for HfO2, yielding ro(Hf–O)=1.7764(4) A and ∠(O–Hf–O)=107.51(1)°. The electric dipole moment has been determined for 180HfO2 from Stark-effect measurements, giving μ=26.42(3)×10-30C⋅m [7.92(1) D...

Six pyranoside forms of free 2-deoxy-D-ribose.

Carbohydrates are one of the most versatile biochemical building blocks, widely acting in energetic, structural, or recognition processes. The interpretation of the biological activity of saccharides is based on the structure and relative stability of their conformers. One of the obstacles to resolving the basic structure issues arises from their ability to form strong intermolecular hydrogen bonds with polar solvents, which in turn can result in conformational changes. A clear picture of the conformational panorama of isolated 2-deoxyd-ribose has been revealed using Fourier-transform microwave spectroscopy in conjunction with a UV ultrafast laser ablation source. Additionally, the availability of rotational data has been the main bottle-neck for examining the presence of these building blocks in interstellar space, so these studies could also be useful to the astrochemistry community. 2-Deoxy-d-ribose (2DR, C5H10O4; Figure 1a) is an important naturally occurring monosaccharide, present in nucleotides, which are the building blocks for DNA. In DNA, 2DR is present in the furanose (five-membered) ring form, whereas free in aqueous solution it cyclizes into fiveor six-membered rings, with the latter—the pyranoid form—being dominant. By closing the chain into a six-membered ring, the C1 carbon atom is converted into an asymmetric center, yielding two possible stereochemical a and b anomeric species (Figure 1b). In aqueous solution, 2DR primarily exists as a mixture of nearly equal amounts of a and b pyranose forms, present in their low-energy chair conformations, C1 and C4 (Figure 1c). [4] Such configurations are connected through ring inversion, thus establishing the axial or equatorial position of the OH group for each conformer. In addition, the monossacharides exhibit an unusual preferential stabilization of pyranose rings containing an axial OH group at the C1 carbon over the equatorial orientation, widely known as the anomeric effect, although its physical origin remains controversial. Nevertheless, structural analysis of 2DRmust take into consideration the intramolecular hydrogen bonding between adjacent OH groups. The formation of hydrogenbond networks reinforces their stability owing to hydrogenbond cooperativity effects. Such networks are fundamental to the molecular recognition of carbohydrates. By dissecting all these factors we can determine the most stable conformers of 2DR and the relative arrangement of the different hydroxy groups under isolated conditions, such as in the gas phase. In vacuo theoretical calculations, carried out on a-/bpyranoses, a-/b-furanoses, and open-chain conformations, predict 15 furanose and pyranose forms (Figure 1d, Table 1) in an energy window of 12 kJmol 1 above the predicted cc-apyr C1 global minimum. The notation used to label the different conformers include the symbols a and b to denote the anomer type, C1 and C4 to denote the pyranose chair form, C2-endo or C3-endo to denote the furanose envelope forms, and “c” or “cc” to indicate a clockwise or counterclockwise configuration of the adjacent OH bonds, respectively. A number is added to provide theMP2 energy ordering within the same family. To validate the predicted conformational behavior, comparison with precise experimental data of 2DR is needed. Previous experiments to determine the conformation of monosaccharides were based on X-ray and NMR measurements. However, these data are influenced by environmental effects associated with the solvent or crystal lattice. Recently, an IR spectrum of 2DR in an inert matrix in

A rotational study of laser ablated thiourea

A laser ablation device in combination with a molecular beam Fourier-transform microwave spectrometer has allowed the observation of the rotational spectrum of solid thiourea for the first time. The sensitivity reached in the experiment allowed the observation of the isotopomers (34)S, (13)C, and (15)N in their natural abundance. The spectrum of D(4)-thiourea was also analyzed from an enriched sample. The complicated hyperfine structure arising from the presence of two (14)N quadrupolar nuclei has been fully resolved and analyzed. The substitution r(s) structure has been derived from the experimental moments of inertia. Thiourea in gas phase presents a planar heavy atom skeleton. Experimental inertial defect values and high-level ab initio calculations reveal that the amino groups hydrogen atoms lie out-of-plane with a C(2) symmetry configuration and are involved in large amplitude inversion motions.

Jet-cooled rotational spectrum of laser-ablated 1,3,5-trithiane ☆

Abstract The jet-cooled rotational spectra of the parent and three isotopic species (13CC2H6S3, C3H634SS2 and C3H633SS2) in natural abundance of 1,3,5-trithiane have been observed by laser ablation combined with molecular-beam Fourier transform microwave spectroscopy. The r0 and rs structures of the molecule have been determined from the rotational data. The sensitivity of the experimental set-up proved to be high enough to observe the 33S isotopomer in natural abundance. An analysis of the nuclear quadrupole hyperfine structure due to the 33S nucleus allowed the determination of the quadrupole coupling tensor, giving some insight about the electronic distribution around this atom.

The centimeter and millimeter microwave spectra of butadiene sulfone and α,α′-D4 butadiene sulfone

Abstract Microwave measurements on the ground and first eight excited states of the ring-puckering vibration of butadiene sulfone have been extended to millimeter wavelengths. The microwave spectra of the same vibrational states of α,α′-D 4 butadiene sulfone have been observed. For both isotopomers the Coriolis interaction between the v = 0 and v = 1 states has been analyzed to give the energy separation between these two states. These data and the variation of the rotational constants have been used to derive reduced potential functions for the ring-puckering vibration. The barrier to ring inversion is 49(2) cm −1 for butadiene sulfone and 44(2) cm −1 for the α,α′-D 4 isotopomer. The ring-puckering vibrational dependence of the quartic centrifugal distortion constants, including a small dependence of Δ J and δ J , has been accounted for.

N-Methyl stereochemistry in tropinone: the conformational flexibility of the tropane motif

The intrinsic conformational and structural properties of the tropinone azabicycle have been investigated in a supersonic jet expansion using rotational spectroscopy. The spectrum revealed the presence of equatorial and axial conformers originated by the inversion of the N-methyl group, with the tropane motif adopting a distorted chair configuration. The determination of substitution and effective structures for the two conformers reveals the flexibility and structural changes associated with the N-methyl inversion, mostly a flattening at the nitrogen atom and a simultaneous rising of the carbonyl group in the axial form. Relative intensity measurements indicate that the conformational equilibrium is displaced towards the equatorial form, with a relative population in the jet of N(eq)/N(ax) approximately 2/1, which would correspond to a relative energy of ca. 2 kJ mol(-1) in pre-expansion conditions. Supporting ab initio calculations of the molecular properties and inversion barrier complemented the experimental work.