Conformational cooling dynamics in matrix-isolated 1,3-butanediol.

The complete conformational space of monomeric 1,3-butanediol has been characterized theoretically, and 73 unique stable conformers were found at the MP2/6-311++G(d,p) level. These were classified into nine families whose members share the same heavy atom backbone configurations and differ in the hydrogen atom orientations. The first and third most populated backbone families are governed by the formation of an intramolecular hydrogen bond; however, the second precludes this type of interaction and was frequently overlooked in previous studies. Its stability is determined by the relatively high entropy of its main conformers. The hydrogen bonding of four of the most important conformers was characterized by means of atoms in molecules (AIM, also known as QTAIM) and natural bond orbital (NBO) analyses. Using appropriate isodesmic reactions, hydrogen bonding energy stabilizations of 12-14 kJ mol(-1) have been found. Experimentally, monomeric molecules of 1,3-butanediol were isolated in low-temperature inert matrixes, and their infrared spectra were analyzed from the viewpoint of the conformational distribution. All the relevant transition states for the conformational interconversion reaction paths were characterized at the same level of theory to interpret the conformational cooling dynamics observed in the low-temperature matrixes. The energy barriers for rotation of the OH groups were calculated to be very low (<3 kJ mol(-1)). These barriers were overcome in the experiments at 10 K (Ar matrix), in the process of matrix deposition, and population within each family was reduced to the most stable conformers. Further increase in the substrate temperature (up to 40 K, Xe matrix) resulted in conformational cooling where the medium-height barriers (approximately 13 kJ mol(-1)) could be surmounted and all conformational population converted to the ground conformational state. Remarkably, this state turned to consist of two forms of the most stable hydrogen bonded family, which were predicted by calculations to be accidentally degenerated and were found in the annealed matrix in equal amounts. All of these experimentally observed conformational cooling processes were analyzed and supported by full agreement with the theoretical calculations.

Stepwise conformational cooling towards a single isomeric state in the four internal rotors system 1,2-butanediol.

The present work explores the possibilities of the matrix isolation technique in the structural characterisation of highly flexible molecules. To date, most studies of this type were carried out on molecules with three or less internal degrees of freedom and a few (less than 10) possible conformations. The molecule of 1,2-butanediol has four conformationally relevant three-fold rotational axes, which can result in 81 possible conformations. A detailed theoretical study, at the MP2 and DFT(B3LYP) levels of theory with the 6-311 + + G(d,p) basis set, revealed that more than 20 conformers of 1,2-butanediol have relative energies in a 0-10 kJ mol(-1) range and contribute appreciably to the gas phase equilibrium at room temperature. This fact renders conformational studies of the system extremely difficult under normal conditions. However, the method of matrix isolation permits the reduction of the number of populated conformational states in the experiment at low temperature due to the effect known as conformational cooling: low energy barriers promote the relaxation of the higher energy local minima into more stable structures. As a result of massive conformational cooling occurring upon matrix deposition, only five conformers of 1,2-butanediol were retained in the samples at 10 K. These conformers were identified using a combination of FTIR spectroscopy and extensive theoretical calculations of vibrational spectra. Annealing of the matrices up to 50 K resulted in the extreme case of conformational cooling related with the depopulation of all conformers into the most stable unique structure. The observed transformations were rationalized in terms of barriers to intramolecular rotation.

Conformational study of erythritol and threitol in the gas state by density functional theory calculations.

Density functional theory calculations using the B3LYP functional and the 6-311++G(d,p) basis set were carried out on the isolated molecules of erythritol and L-threitol. For the meso isomer, a relatively large number of conformers have to be considered to describe the gas state structure. The lowest energy conformer is characterized by the establishment of a strong intramolecular H-bond between the two terminal hydroxyl groups, giving rise to a seven-membered ring and two additional weaker H-bonds between vicinal OH groups. In the case of L-threitol, two conformers are predominant in the gas state, and both are stabilized by the formation of a cyclic system of four intramolecular hydrogen bonds involving all OH groups. The conformational stability in both diastereomers is discussed in terms of the electronic energy and of the Gibbs energy. The weighted mean enthalpy of both diastereomers in the gas state at 298.15 K was obtained from the thermodynamic data and Boltzmann populations of the low-energy conformers.