Unravelling the structural features of monosaccharide glyceraldehyde upon mono-hydration by quantum chemistry and rotational spectroscopy.

Water is a fundamental molecule for life, and investigating its interaction with monosaccharides is of great interest in order to understand its influence on their conformational behavior. In this study, we report on the conformational landscape of monosaccharide glyceraldehyde, the simplest aldose sugar, in the presence of a single water molecule in the gas phase. This investigation was performed using a combination of Fourier transform microwave spectroscopy and theoretical calculations. Out of the nine calculated conformers, only the lowest energy conformer was experimentally observed and characterized. Interestingly, the presence of water was found to induce structural features in the lowest energy conformer of the glyceraldehyde monomer, with water positioned between the alcohol groups. To analyze this interaction further, non-covalent interaction plots were employed to map the intermolecular interactions in the observed species. Additionally, natural bond orbital analysis was conducted to study the effects of charge transfer in the monohydrate system. Furthermore, topological analysis based on Bader's Atoms in Molecules theory was performed to gain insights into the observed complex. The results of all three analyses consistently showed the formation of relatively strong hydrogen bonds between water and glyceraldehyde, leading to the formation of a seven-member ring network.

The gas-phase structure determination of α-pinene oxide: An endo-cyclic epoxide of atmospheric interest.

The gas-phase rotational spectra of α-pinene oxide have been recorded using a chirped-pulse Fourier transform microwave spectrometer in the 6-18 GHz frequency range. The parent species and all heavy atom isotopologues (13C and 18O) have been observed in their natural abundance. The experimental rotational constants of all isotopic species observed have been determined and used to obtain the substitution (rs) and the effective (r0) structures of the most stable conformer of α-pinene oxide. Calculations using the density functional theories B3LYP, M06-2X, and MN15-L and the ab initio method MP2 level of theory were carried out to check their performance against experimental results. The structure of the heavy atom's skeleton of α-pinene oxide has been compared to that of α-pinene and has shown that epoxidation does not overly affect the structure of the bicycle, validating its robustness. Furthermore, the structural features have been compared to those of other bicyclic molecules, such as nopinone and ß-pinene.

The conformational landscape of myrtenol: The structure of the hydroxymethyl group and its robustness upon hydration.

The conformational landscape of myrtenol (2-pinen-10-ol) and its robustness upon hydration were investigated theoretically and experimentally by employing a synergic combination of quantum chemical calculations and Fourier transform microwave spectroscopy coupled to a supersonic jet expansion. Relaxed potential energy surfaces have been carried out, and the lowest energy conformers of the monomer were found to be associated with different geometries of the hydroxymethyl group from those previously reported [Sedo et al., J. Mol. Spectrosc. 356, 32 (2019)]. Geometry optimizations and harmonic vibrational frequency calculations allowed characterization of the equilibrium structure of the possible conformers of myrtenol. Among the nine predicted structures, four have been observed, analyzed, and identified. The controversy on the geometry was solved with the deuteration of the hydroxyl group, which led to the determination of substitution (rs) geometry, in agreement with the present theoretical results. Interestingly, the four observed conformers exhibit the same orientation of OH as in the allyl alcohol molecule. Furthermore, hydrogen bonding linking myrtenol to water was studied. One monohydrate has been observed and identified. Non-covalent interactions and natural bond orbital analysis were performed to depict the interactions responsible for the stabilization of the observed structure. We conclude that the structure of the hydroxymethyl group is robust and does not change upon hydration.

The impact of water vapor on the OH reactivity toward CH3CHO at ultra-low temperatures (21.7-135.0 K): Experiments and theory.

The role of water vapor (H2O) and its hydrogen-bonded complexes in the gas-phase reactivity of organic compounds with hydroxyl (OH) radicals has been the subject of many recent studies. Contradictory effects have been reported at temperatures between 200 and 400 K. For the OH + acetaldehyde reaction, a slight catalytic effect of H2O was previously reported at temperatures between 60 and 118 K. In this work, we used Laval nozzle expansions to reinvestigate the impact of H2O on the OH-reactivity with acetaldehyde between 21.7 and 135.0 K. The results of this comprehensive study demonstrate that water, instead, slows down the reaction by factors of ∼3 (21.7 K) and ∼2 (36.2-89.5 K), and almost no effect of added H2O was observed at 135.0 K.

Gas-Phase Conformational Map of the Amino Acid Isovaline.

Four conformers of the non-proteinogenic α-amino acid isovaline, vaporized by laser ablation, are characterized by Fourier-transform microwave techniques in a supersonic expansion. The comparison between the experimental rotational and 14 N nuclear quadrupole coupling constants and the ab initio calculated ones provides conclusive evidence for the identification of the conformers. The most stable species is stabilized by an N-H⋅⋅⋅O =C intramolecular hydrogen bond and a cis-COOH interaction, whereas the higher-energy conformers exhibit an N⋅⋅⋅H-O intramolecular hydrogen bond and trans-COOH, as in other aliphatic amino acids. The spectroscopic data herein reported can be used for the astrophysical purpose in a possible detection of isovaline in space.

Identification of the maze in the conformational landscape of fenchol.

The rotational spectrum of the bicyclic molecule fenchol (C10H18O, 1,3,3-trimethylbicyclo[2.2.1]heptan-2-ol) - a biogenic volatile organic compound - was recorded in the gas phase using an impulse Fourier transform microwave spectrometer coupled to a supersonic jet expansion over the 2-20 GHz range. Quantum chemical calculations were performed to characterize the conformational landscape of the two diastereoisomers, endo-fenchol and exo-fenchol, with respect to the orientation of the hydroxyl group. The three most stable structures for each diastereoisomer were optimized at the MP2/6-311++G(2df,p) level of theory. Two of them were found to be very close in energy. Molecular parameters obtained from the analysis of observed signals led to the observation of one conformer per diastereoisomer. For the endo-fenchol molecule the rs geometry associated with the hydroxyl group was obtained, from the observation and analysis of the rotational spectra associated with the deuterated hydroxyl group. The nuclear quadrupolar hyperfine signature was identified. The hydroxyl group was found to be oriented into the direction of the methyl groups attached to C3, for the more stable conformer of endo-fenchol. For exo-fenchol, it is oriented into the methyl group attached to C1.

The effect of N-methylation on the conformational landscape of alanine: the case of N-methyl-l-alanine.

The non-proteinogenic amino acid N-methyl-l-alanine has been brought into the gas phase using laser ablation techniques and studied by high resolution chirped pulse and molecular-beam Fourier transform microwave spectroscopies coupled to supersonic expansion. Four conformers showing the three types of hydrogen bond interactions I (NHO[double bond, length as m-dash]C), II (OHN) and III (N-HO-H) have been unambiguously identified, based on the comparison of the experimental rotational and 14N nuclear quadrupole constants with the calculated ab initio values. The observation of a type III conformer evidences the role of methyl groups in both sides to impose the steric hindrance, precluding the relaxation from type III to type I conformers and explains the responsibility for the unique conformational landscape observed in the case of NMA.

The quasi-unchanged gas-phase molecular structures of the atmospheric aerosol precursor ß-pinene and its oxidation product nopinone.

The rotational spectra of the two bicyclic molecules ß-pinene and its oxidation product nopinone were investigated in the gas phase, using Fourier transform microwave spectroscopy coupled to a supersonic jet, in the 2-20 GHz range. The parent species and all heavy atom isotopologues have been observed in their natural abundance. The spectroscopic parameters of the ground states were determined from a Watson's Hamiltonian in the A reduction. The rotational constants were used together with geometrical parameters obtained from ab initio calculations to determine the r0 and r structures of the skeletons, without any structural assumption in the fit concerning the heavy atoms. Comparison with solid phase and other bicyclic monoterpenes unveiled an unprecedented complete set of geometrical parameters for the rigid cages. The structures of ß-pinene and nopinone are very close, except for the substituents at C2. In the gas phase C2 is a centre of planarity in both molecules.

Comprehensive rotational study and astronomical search for cyclopropanecarboxaldehyde.

CONTEXT: At least a dozen molecules with a formyl group (HCO) have been observed to date in the interstellar medium (ISM), suggesting that other such species exist and remain to be discovered. However, there is still a lack of high-resolution spectroscopic data for simple molecular species of this type that could provide a basis for their detection. AIMS: Cyclopropanecarboxaldehyde, c-C3H5CHO, is a small molecule containing a formyl group and is therefore an interesting candidate for astrophysical detection. The rotational spectrum of cyclopropanecarboxaldehyde has been observed before, but its experimental rotational parameters are not precise enough to allow its detection in the millimetre-wave domain. METHODS: We measured the rotational spectrum of cyclopropanecarboxaldehyde in the frequency ranges 31.5-50 GHz and 72-116.5 GHz using the GACELA (GAS CEll for Laboratory Astrophysics) broadband high-resolution rotational spectrometer constructed at the Yebes Observatory. The spectroscopic study was supported by high-level theoretical calculations which were used in the identification of the vibrational excited states of cyclopropanecarboxaldehyde. RESULTS: Our analysis of the rotational spectrum of cyclopropanecarboxaldehyde allowed us to obtain accurate rotational parameters for the ground state of both cis and trans isomers, which were used to derive sufficiently reliable predictions up to 300 GHz. In addition to the ground states, we identified 12 and 6 vibrationally excited states for the trans and cis isomers, respectively, including fundamental modes, multiple excitation quanta, and combination states. We find that the gas phase concentration of the trans isomer is almost 1.2 times larger than that of the cis one. These new experimental rotational parameters were employed to search for cyclopropanecarboxaldehyde in the warm molecular clouds Orion KL and Sgr B2(N) using the spectral surveys captured by ALMA (Orion) and IRAM 30 m (Sgr) at 1 mm and 3 mm, respectively.