The equilibrium structure of 36 small molecules containing OH bonds, including molecules with an internal hydrogen bond, is reviewed from the literature. The structures are redetermined or determined, if necessary, using high level ab initio calculations at the CCSD(T) level of theory with basis sets of quintuple zeta quality and with a correction for the core correlation. The possibility of calculating accurate ab initio OH bond lengths at different levels of theory (MP2/VTZ, MP2/VQZ, CCSD(T)/VTZ, and CCSD(T)/AVTZ) is examined. All methods agree provided a small offset correction is taken into account. In most cases, the addition of diffuse functions does not improve the results. The MP2 method appears to be satisfactory whenever the non-dynamical electron correlation is small, which is the most typical situation. The various results are merged to demonstrate the following quantitative correlation between r e(OH) and ‘isolated’ OH stretching frequencies, relevant for predictive use: r e(OH) [A] = 1.2261(...

The equilibrium OH bond length

Terpenes in the gas phase: The structural conformation of S-(–)-perillaldehyde investigated by microwave spectroscopy and quantum chemical calculations

The structural properties of the three open chain C4H8O4 sugars, i.e. two aldoses (erythrose and threose) and one ketose (erythrulose), have been investigated by DFT and ab initio calculations to get accurate structures and relative energies. The structure of all the conformers predicted within 10 kJ/mol has been optimized at the B3LYP/6-311++G(d,p) level of the theory. Two types of intramolecular hydrogen bonds have been clearly identified. They are related to the hydroxyl and to the carbonyl oxygen atoms and are of weak and middle strength, respectively. The most stable structures have been optimized at the B3LYP/6-311++G(2df,p) and at the MP2/6-311++G(2df,p) levels of the theory in order to calculate accurate rotational parameters and dipole moment for their future detection in the microwave range in the gas phase. Their corresponding harmonic IR spectra have also been calculated and their fingerprint signature is discussed in the region of the OH stretching vibrations, of the torsion of the C–O bonds and of the deformation of the C–C skeleton.

Sugars in the gas phase: The conformational properties of erythrose, threose, and erythrulose characterized by quantum chemistry calculations

Despite many experimental and theoretical investigations, a quantitative explanation of the factors governing the stability of sugars is rather scarcely attempted in the literature. A quantitative understanding of such factors is important for correlating the stability of these molecules with their function. Recent experimental–theoretical studies report the global minimum structure of fructose and provide qualitative information about the predominant factors that determine the overall stability. In the present work, we quantitatively show that the relative stability of different conformers of fructose in the gas phase, albeit somewhat approximately, can be obtained in terms of the collective effect of (i) the sum of the energies of all the hydrogen bonds in a given conformer, (ii) the strain energy of a bare fructose ring, and (iii) the sum of anomeric stabilization (endo + exo) energies. The combined effect of these three factors is indeed useful for explaining the conformational landscape of fructose. The calculated relative stability of fructose is in good agreement with the one obtained from the relative energies of these sugar molecules. The large energetic gap between pyranose and furanose conformers is also well-explained. It is concluded that the small ring strain, the sufficiently large sum of the energies of the intramolecular hydrogen bonds and the higher stabilization due to anomeric interactions in β-fructo-pyranose make it a conformationally locked predominant structure in the gas phase.

Stability of conformationally locked free fructose: theoretical and computational insights

Gas phase, isolated fructose anionic species, fructose-, (fructose-H)-, (fructose-OH)-, and (fructose-H2O)-, are investigated employing anionic photoelectron spectroscopy (PES) combined with density functional theory (DFT) calculations. The PES vertical detachment energies (VDEs) for these anions are determined and, based on these experimental values, their calculated anionic structures are assigned. Generation of these four species through the matrix assisted laser desorption ionization (MALDI) process is sample desorption substrate dependent. The parent anion fructose- exists as a single, dominant open chain structure in the gas phase, with substrate dependent specific conformational isomers. (Fructose-H)- and (fructose-OH)- are mainly produced from the laser ablation process rather than from fragmentation reaction pathways associated with the parent anion species. Both conformational and positional isomers are identified in the gas phase for these latter anions. (Fructose-H2O)- has two types of positional isomers, both of which contribute to two different components of the observed PES feature. The fixed positions for losing an OH group and an H atom, in addition to thermodynamic calculations, provide reaction pathways for generating a dehydration product (open chain structures) from the parent anion (open chain and furanose structures), further demonstrating the active nature of fructose upon capturing an extra electron.

Anionic fructose-related conformational and positional isomers assigned through PES experiments and DFT calculations.

The hyperfine structure of sugars investigated by microwave spectroscopy and quantum chemical calculations

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The hyperfine structure of sugars investigated by microwave spectroscopy and quantum chemical calculations