Conformational Cooling Dynamics in Matrix-Isolated 1,3-Butanediol

Abstract

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.

Conformational changes in matrix-isolated 6-methoxyindole: Effects of the thermal and infrared light excitations

“…37 Based on these predictions, the two conformers should be present in the gaseous sample of 6MOI before the matrix deposition. Since they are separated by a medium-high energy barrier (∼9 kJ mol −1 in the anti → syn direction, see Figure 1), no conformational cooling 5,6,38,39 is expected to occur during the deposition, and therefore both of them should be preserved in both matrices immediately after the deposition, as it has already been confirmed in our previous experiments. Figure 2 shows the mid-infrared spectra of 6MOI isolated in an argon ( Fig.…”

“…[1][2][3][4] Two methodologies are commonly employed to alter the relative abundance of the trapped conformers: annealing of the sample 5,6 or subjecting it to narrowband or broadband radiation of the IR or UV spectral regions. 2,[7][8][9][10][11][12][13][14] In the first case, the population of less stable forms is reduced in favour of the most stable ones, providing that the energy barriers for the isomerizations can be surmounted.…”

The Interplay Between Conformation and Absolute Configuration in Chiral Electron Dynamics of Small Diols

“…Our calculated properties for the quasi-degenerate conformers I and II and for the next most stable conformer, III, which is >2 kJ mol -1 higher in energy, are summarized in Table S2. FTIR measurements [10] on an annealed, cold 10K matrix isolated 13BD found that the population collapsed to a 1:1 mix of the two thermodynamically most stable conformers.…”

“…In Figure 1 we compare the two most stable conformers calculated for 13PD [8,9] and those calculated for 13BD. [10,11] The four letter designations used in this literature -g′GG′t and tGG′g -describe the four dihedral angles defining the structures; for brevity we also show and will use an alternative Roman numeral numbering.…”

“…[12] This is fully corroborated by the observation of uncoupled asymmetric top spectra for the I and II pair of 13BD, indicating their existence as distinct conformers. [10,11] But such addition of a methyl also creates an asymmetric substitution at the C(3) site, so generating permanent configurational enantiomerism in 13BD, with separable R,S enantiomers. Nevertheless, as can be seen in Figure 2, the calculated HOMO ionization cross section and anisotropy parameter, , (both enantiomer independent properties) retain many similarities to those for 13PD.…”