Gas-Phase Infrared and NMR Investigation of the Conformers of Diacetone Diperoxide (DADP).

Gas-phase infrared measurements of diacetone diperoxide (DADP) indicate a chair conformation with less than 5% of the predicted twist conformer. Vibrational frequencies are very similar to those previously measured in the solid state. Solution NMR measurements using 2D exchange spectroscopy (EXSY) also set a very low maximum limit on the equilibrium population of the twist conformer, with a room-temperature free-energy difference in excess of 14.5 kJ/mol. These experimental results are in accord with high-level quantum calculations incorporating full thermochemistry and solvation effects, which indicate a free-energy difference in the range of 14.7-17.5 kJ/mol in polar solvents.

Density functional theory and atoms-in-molecules investigation of intramolecular hydrogen bonding in derivatives of malonaldehyde and implications for resonance-assisted hydrogen bonding.

A density functional theory (DFT) and atoms-in-molecules (AIM) analysis has been applied to the intramolecular hydrogen bonding in the enol conformers of malonaldehyde and its fluoro-, chloro-, cyano-, and nitro-substituted derivatives. With the B3LYP/6-311++G(2d,p) method, good agreement between the DFT geometries and published experimental structures has been found. The donor-acceptor distance was also varied in a series of constrained optimizations in order to determine if energetic, structural, and topological trends associated with intermolecular hydrogen bonding remain valid in the intramolecular case. At very short donor-acceptor distances (<2.24 A), the hydrogen is symmetrically located between donor and acceptor; at distances longer than this, the hydrogen bonding is no longer symmetric. The AIM methodology has been applied to explore the topology of the electron density in the intramolecular hydrogen bonds of the chosen model systems. Most AIM properties for intramolecular hydrogen bond distances longer than 2.24 A show smooth trends, consistent with intermolecular hydrogen bonds. Integrated AIM properties have also been used to explore the phenomenon of resonance-assisted hydrogen bonding (RAHB). It is shown that as the donor-acceptor distance is varied, pi-electron density is redistributed among the carbon atoms in the intramolecular hydrogen bond ring; however, contrary to prior studies, the integrated atomic charges on the donor-acceptor atoms were found to be insensitive to variation of hydrogen-bonding distance.

Effects of Zero-Point and Thermal Vibrational Averaging on Computed NMR Properties of a Model Compound for Purine Nucleosides.

A method for the incorporation of thermal averaging into the calculation of nuclear magnetic resonance properties is given. These properties are computed using density functional theory, anharmonic first-order perturbation corrections to a normal-mode analysis, and standard statistical mechanical averaging. The method is applied to the calculation of chemical shieldings and spin-spin coupling constants (J couplings) of 1'-imidazolyl-2'-deoxy-ß-ribofuranose (IDR), a model compound for purine nucleosides, at the B3LYP/6-311++G(2d,p) level of theory. Thermal averaging causes substantial changes in the values of computed parameters. The calculated harmonic normal modes of IDR are also investigated; we find reasonable agreement with published results from vibrational spectroscopy on DNA fragments. Finally, the calculated magnetic and structural data regarding the reported hydrogen bond between H8 on the imidazole ring and O5' on the sugar ring are investigated; we find that such data do not strongly support the formation of a hydrogen bond between these two atoms.