M. Eugenia Sanz

Seven conformers of L-threonine in the gas phase: a LA-MB-FTMW study

Rotational spectroscopy in combination with molecular beams and laser ablation (laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectroscopy) has proved to be successful in characterizing the conformers of natural amino acids. The procedure usually followed to assign and identify the different conformers of an amino acid from the rotational spectrum is described through the study of the natural amino acid L-threonine. The solid sample of L-threonine was vaporized by laser pulses, diluted in Ne and supersonically expanded between the mirrors of a Fabry-Pérot resonator where it was spectroscopically probed by microwave radiation. The rotational and nuclear quadrupole coupling constants extracted from the analysis of the rotational spectrum are directly compared with those predicted by ab initio methods to achieve the conclusive identification of seven different conformers. A complex hydrogen bonding network arises as a consequence of the polar side chain of threonine.

Probing thymine with laser ablation molecular beam Fourier transform microwave spectroscopy

Laser ablation in combination with molecular beam Fourier transform microwave spectroscopy has been used to establish unambiguously the presence of the diketo form of thymine in the gas phase and to obtain its structure.

Rotational Probes of Six Conformers of Neutral Cysteine

has made possible the gas-phase study of solidbiomolecules with high melting points. In this approach,solidsarevaporizedbyahigh-energylaserpulse,supersoni-callyexpandedintoavacuumchamber,andcharacterizedbytheirrotationalspectrum.Ofthebiomoleculesthathavebeenstudiedbythistechnique,aliphaticaminoacidshavereceivedspecial attention because of the lack of experimental infor-mationandtheirbiologicalrelevance.Aminoacidsarewellknowntoexistaszwitterions(NH

Rotational transitions of SO, SiO, and SiS excited by a discharge in a supersonic molecular beam: Vibrational temperatures, Dunham coefficients, Born–Oppenheimer breakdown, and hyperfine structure

Fourier transform microwave spectroscopy has been used to investigate vibrational excitation and relaxation of diatomic molecules produced by an electric discharge in the throat of a supersonic nozzle. Rotational transitions of SO, SiO, and SiS, in vibrational states up to v=33 for 32S16O, v=45 for 28Si16O, and v=51 for 28Si32S in their ground electronic states have been detected. The isotopic species 33S16O, 34S16O, 29Si16O, 28Si18O, 29Si32S, and 28Si34S have also been observed in highly excited vibrational states. Microwave transitions include up to v=22 for the second lowest excited electronic state b 1Σ+ of SO (∼10 510 cm−1 above ground) have also been detected. Effective vibrational temperatures have been derived for each species, and a general model is proposed to qualitatively explain the observations. Vibrational excitation is caused by inelastic collisions with the hot electrons produced in the discharge. The subsequent vibrational populations are largely determined by vibration–vibration energy ...

Six conformers of neutral aspartic acid identified in the gas phase

The multiconformational landscape of the proteogenic amino acid aspartic acid, the simplest amino acid with two carboxylic groups, has been explored for the first time in the gas phase. Solid aspartic acid (m.p. > 300 degrees C) was vaporized by laser ablation and expanded in a supersonic jet where rotational spectra of various conformers were obtained by Fourier transform microwave spectroscopy. Six different conformers have been conclusively identified by their distinct rotational and (14)N nuclear quadrupole coupling constants. The relative stability of the conformers rests on a delicate balance of the different intramolecular hydrogen bonds established between the two carboxylic groups and the amino group.

Vibrational excitation and relaxation of five polyatomic molecules in an electrical discharge.

Vibrational excitation and relaxation of five linear polyatomic molecules, OCS, OC3S, HC3N, HC5N, and SiC2S, have been studied by Fourier transform microwave spectroscopy in a supersonic expansion after the application of a low-current dc electric discharge. For each chain, the populations in bending and stretching modes have been characterized as a function of the applied discharge current; for stable OCS and HC3N, vibrational populations were studied as well in the absence of a discharge. With no discharge present the derived vibrational temperatures are slightly below T, the temperature of the gas before the supersonic expansion (i.e., 300 K). In the presence of the discharge, vibrational excitation occurs via inelastic collisions with the electrons and the vibrational temperatures rise as the applied current increases. Global vibrational relaxation is governed by rapid vibration-vibration (VV) energy transfer and slow vibration-translation (VT) energy transfer. The latter process is rate-determining and depends primarily on the wave number of the vibration. Vibrational modes with wave numbers near and below kT/hc (where T = 300 K and kT/hc-210 cm(-1)) are efficiently cooled by VT transfer because a sufficient number of collisions occur in the initial stages of the supersonic expansion. Vibrational modes with wave numbers around 450 cm(-l) appear to be inefficiently cooled in the molecular beam; at these energies VV and VT rates are probably comparable. For high-frequency vibrations, VV energy transfer dominates. For the longer chains OC3S and HC5N, higher-lying modes are generally not detectable and vibrational temperatures of most lower-lying modes were found to be lower than those of OCS and HC3N, suggesting that as the size of the molecules increases, intermode VV transfer becomes more efficient, plausibly due to the higher density of vibrational levels. New high resolution spectroscopic data have been obtained for several vibrationally excited states of OC3S, HC3N, and HC5N. Rotational lines of the 13C and 15N isotopic species of HC5N have been measured, yielding improved rotational and centrifugal distortion constants; 14N nitrogen quadrupole coupling constants for the isotopic species of HC5N with 13C have been determined for the first time.

Tautomerism and microsolvation in 2-hydroxypyridine/2-pyridone.

The Fourier transform microwave spectra of the hydrated forms of the tautomeric pair 2-pyridinone/2-hydroxypyridine (2PO/2HP) have been investigated in a supersonic expansion. Three hydrated species, 2PO-H₂O, 2HP-H₂O, and 2PO-(H₂O)₂, have been observed in the rotational spectrum. Each molecular complex was confidently identified by the features of the ¹⁴N quadrupole hyperfine structure of the rotational transitions. The presence of water affects the tautomeric equilibrium -N═C(OH)- ↔ -NH-C(═O)-, which is shifted to the enol form for the bare molecules 2PO/2HP but to the keto tautomer for the hydrated forms.

Conformational behavior of norephedrine, ephedrine, and pseudoephedrine.

A conclusive identification of the different conformers of the neurotransmitters ephedrine, norephedrine, and pseudoephedrine has been carried out in the gas-phase by molecular beam Fourier transform microwave (MB-FTMW) spectroscopy. Three conformers of norephedrine, three conformers of ephedrine, and four conformers of pseudoephedrine have been unequivocally assigned through the analysis of their rotational spectra and the comparison of the experimental rotational and (14)N quadrupole coupling constants with those calculated ab initio. The conformational preferences have been rationalized in terms of the various intramolecular forces at play. The main stabilizing interaction is an O-H...N hydrogen bond established in the side chain of the neurotransmitters which adopt an extended disposition in their most stable form.

Hydrogen Bond in Molecules with Large-Amplitude Motions: A Rotational Study of Trimethylene Sulfide⋅⋅⋅HCl

Two different conformers (axial (in the left picture) and equatorial (right)) are formed in a supersonic jet stream when trimethylene sulfide, a floppy molecule with a large-amplitude ring-puckering motion, is complexed to hydrogen chloride. The axial conformer is the most stable. The relaxation from the equatorial to the axial conformational has been observed using different carrier gases.

Ab initio theory and rotational spectra of linear carbon chains SiCnS

On the basis of extensive coupled cluster calculations, the rotational spectra of the linear silicon- and sulfur-containing carbon chains SiC2nS (n=1–3) in their singlet electronic ground state and SiC3S in its triplet electronic ground state have been detected and characterized by means of molecular beam Fourier transform microwave spectroscopy. Rotational and centrifugal distortion constants have been determined to high accuracy as well as the spin-spin coupling constant for triplet SiC3S. In addition, the 29Si, 34S, and both 13C isotopic species of SiC2S have been detected, allowing the determination of both an effective r0 structure, as well as a mixed experimental–theoretical structure resulting from the combination of the measured rotational constants with the vibration–rotation coupling constants calculated ab initio. Several rotational satellite lines have also been observed for SiC2S and, on the basis of their predicted vibration–rotation and l-type doubling constants, were assigned to the two hi...