Furanosic forms of sugars: conformational equilibrium of methyl beta-D-ribofuranoside

The investigation of an isolated ribofuranose unit in the gas phase reveals the intrinsic conformational landscape of the biologically active sugar form.We report the rotational spectra of two conformers of methyl b-D-ribofuranoside in a supersonic jet expansion. Both conformers adopt a near twisted (3T2) ring conformation with the methoxy and hydroxymethyl substituents involved in various intramolecular hydrogen bonds.MINECO-FEDER CTQ2015-68148-C2-

SEMI-EXPERIMENTAL (rs/re)(r_s/r_e) STRUCTURES FOR THE HEAVY ATOM BACKBONES OF TWO MODERATELY LARGE MOLECULES OBTAINED FROM MICROWAVE SPECTROSCOPY AND QUANTUM CHEMICAL CALCULATIONS

Author Institution: Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074; Departamento de Quimica Fisica y Quimica Inorganica, Facultad de Ciencias, Universidad de Valladolid, E-47011 Valladolid, Spain; Departamento de Química Física y Química Inorganica, Facultad de Ciencia y; Tecnologia, Universidad del Pais Vasco, Ap. 644, E-48080 Bilbao, Spain; Institut fur Physikalische Chemie und Elektrochemie, Gottfried-Wilhelm-Leibniz-Universitat Hannover, Callinstrasse 3A, D30167 Hannover, Germany.From recent microwave investigations of 1-methyl-4-piperidone and tropinone ground state rotational constants are available for the equatorial conformers of the normal species and the isotopologues with single substitution of all the heavy atoms. Vibration-rotation constants (alphas) were computed with Gaussian 03 (G03) for the B3LYP/cc-pVTZ model and used to convert ground state rotational constants into equilibrium rotational constants. Using the Kraitchman equations ({\em r}$_{s}$ method), the equilibrium ({\em r}$_{e}$) Cartesian coordinates were determined for all the heavy atoms in the principal axis framework. Equilibrium bond lengths and bond angles are compared with those computed with the B3LYP/cc-pVTZ model. We have compared the ground state rotational constants computed with G03, after scaling by factors based on the normal species, with observed values. The agreement is within 0.1\% for the full set of constants (0.04\% for methyl-piperidone and 0.1\% for tropinone). This agreement between experiment and theory is so good that it seems possible to use calculated ground state rotational constants in place of experimental ones for determining {\em r}$_{s}$/{\em r}$_{e}$ structures for organic molecules of this size

Rotational spectroscopy of antipyretics: Conformation, structure, and internal dynamics of phenazone

The conformational and structural preferences of phenazone (antipyrine), the prototype of nonopioid pyrazolone antipyretics, have been probed in a supersonic jet expansion using rotational spectroscopy. The conformational landscape of the two-ring assembly was first explored computationally, but only a single conformer was predicted, with the N-phenyl and N-methyl groups on opposite sides of the pyrazolone ring. Consistently, the microwave spectrum evidenced a rotational signature arising from a single molecular structure. The spectrum exhibited very complicated fine and hyperfine patterns (not resolvable with any other spectroscopic technique) originated by the simultaneous coupling of the methyl group internal rotation and the spins of the two 14N nuclei with the overall rotation. The internal rotation tunnelling was ascribed to the C–CH3 group and the barrier height established experimentally (7.13(10) kJ mol−1). The internal rotation of the N–CH3 group has a lower limit of 9.4 kJ mol−1. The structure of the molecule was determined from the rotational parameters, with the phenyl group elevated ca. 25◦ with respect to the average plane of the pyrazolic moiety and a phenyl torsion of ca. 52◦. The origin of the conformational preferences is discussed in terms of the competition between intramolecular C–H· · ·N and C–H· · ·O weak hydrogen bonds

ROTATIONAL SPECTRA AND STRUCTURE OF PROLINE: A JET-COOLED LASER ABLATION STUDY

Author Institution: Grupo de Espectroscop\'{i}a Molecular, Departamento de Quimica Fisica, Facultad de Ciencias, Universidad de ValladolidThe conformational behaviour of aminoacids is of critical interest to understand the dynamical role of these molecules in protein or polypeptide formation. Consequently, extensive structural research has been conducted in their natural condensed phases. This point is worth noting since solid or diluted aminoacids present a zwitterionic structure i.e., a bipolar ionized form of the type $^{+}H_{3}N-CH(R)-COO^{-}$, which does not occur in the polypetide chain. In order to obtain the structure of the neutral form of aminoacids research should be conducted in an environment essentially free of intermolecular interactions with other partners, either the solvent or other atoms of the molecule itself. In particular, the collisionless environment of a supersonic jet seems particularly well adapted for such studies. The obvious difficulties for the experimental studies in gas phase are due to the high melting points and associated low vapour pressures of aminoacids. For these species laser ablation can be an effective way of vaporizing neutral sample molecules, preventing the thermal decomposition of the heating methods. A laser vaporization source combined with Fourier transform microwave spectroscopy in a supersonic jet has been applied to the structural investigation of the essential aminoacid proline in gas phase. Analysis of the rotational spectrum has revealed the presence of two conformers of neutral proline in gas-phase, both exhibiting an intramolecular hydrogen bond between the hydrogen atom of the carboxylic group and the nitrogen atom of the ring $(OH{\ldots}N)$, the carboxyl moiety being in trans position with respect to the imino group. The five membered-ring of the molecule adopts in both conformers a bent-like structure, with near symmetrical endo or exo conformations respect to the carboxyl group. The endo form is the most stable species, as evidenced from a conformational relaxation from the exo to the endo form. The rotational spectra of the parent, $^{15}N, {^{13}}C$ (in natural abundance), N-D and O-D isotopic species have been anlyzed and $r_{0}$-like structures have been derived for both conformers

ALPHA-AMINO ACIDS IN GAS-PHASE: JET-COOLED ROTATIONAL SPECTRA OF LEUCINE, ISOLEUCINE AND 2-AMINOBUTIRIC ACID

{A. Lesarri, S. Mata, E. J. Cocinero, S. Blanco, J. C. Lopez and J. L. Alonso, \emph{Angew. Chem. Int. Ed. {A. Lesarri, E. J. Cocinero, J. C. Lopez and J. L. Alonso, \emph{Angew. Chem. Int. Ed. {S. Blanco, A. Lesarri, J. C. Lopez and J. L. Alonso, \emph{J. Am. Chem. Soc. {A. Lesarri, E. J. Cocinero, J. C. Lopez and J. L. Alonso, \emph{J. Am. Chem. Soc. {A. Lesarri, S. Mata, J. C. Lopez and J. L. Alonso, \emph{Rev. Sci. Instrum.Author Institution: Grupo de Espectroscop\ia Molecular (GEM), Departamento de Qu\imica F\isica y Qu\imica Inorganica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain; Lehrgebiet Physikalische Chemie A, Institut fur Physikalische; Chemie und Elektrochemie, Universtat Hannover, Callinstr. 3-3a, D-30167 Hannover, GermanyThe conformational behaviour of the aliphatic $\alpha$-amino acids leucine, isoleucine and 2-aminobutyric has been studied for the first time in gas-phase, continuing our recent studies on coded amino acids}, 41 (2002) 4673.}$^{,}$ }, 43 (2004) 605.}$^{,}$ }, 126 (2004) 11675.}$^{,}$ }, 127 (2005) 2572.}. The solid amino acids were vaporized using laser ablation (LA) from a Q-switched Nd:YAG laser, and probed spectroscopically in a supersonic jet using molecular-beam Fourier transform microwave spectroscopy (MB-FTMW)}, 74 (2003) 4799.}. The investigation of the jet-cooled rotational spectrum of the neutral forms of these amino acids revealed the presence of several conformers in the gas phase. The side chain is not affecting the conformational preferences of the amino acids, still dominated by the hydrogen bond interactions within the amino acid skeleton