Hydroxyacetone (HA) is an important starting material in the organic synthesis of gem-diols, acetals, and ketals. 1 If such molecules are synthesized in interstellar medium, they may lead to the production of complex prebiotic chiral molecules, such as hydroxy aldehydes (sugars) and hydroxy acids. 2 HA is formed in the atmosphere during oxidation of the important biogenic hydrocarbon, isoprene. 3 Atmospheric HA reacts with an OH radical under sunlight irradiation 4 and also contributes to the formation of secondary organic aerosols. 5 The overall rate coefficient for the tropospherically relevant reaction, HA+OH, was proposed 6 to depend on the initial conformation of the HA molecule.Theoretical calculations on HA monomers in the gas phase 7,8 show that there are only two conformers (Cc and Tt) of this compound ( Figure 1). The lowest energy conformer of HA is Cc, whereas Tt was predicted to be higher in energy by 994 cm -1 . Hence, the Cc conformer should represent 99% of the conformational equilibrium at room temperature, 8 which is why only the Cc conformer has been observed so far. 2,8,9 In this work, we report the first experimental observation of the elusive high-energy Tt conformer of hydroxyacetone. We show that, for HA isolated in a low-temperature Ar matrix, the Tt conformer can be produced by narrow-band near-infrared (NIR) vibrational excitation of the conformational ground state Cc. When isolated in an argon matrix at 15 K, the photogenerated Tt form is stable. In the present study, the NIR-induced back transformation of the Tt conformer into the Cc form was also observed. This photoreversibility makes the HA molecule a system satisfying the criterion of a molecular switch. 10 In the experiments, tunable NIR radiation was provided by a Quanta-Ray MOPO-SL optical parametric oscillator (fwhm ∼0.2 cm -1 , repetition rate 10 Hz, pulse energy ∼3 mJ) pumped with a pulsed Nd:YAG laser. HA (Aldrich, >90%) was premixed with argon in a 1:1000 ratio and deposited onto a CsI window at 15 K. The FTIR spectra were recorded with 0.5 cm -1 resolution, in the 4000-400 cm -1 range. Other details of the experimental setup and of calculations are described elsewhere. 8 In a freshly deposited argon matrix the HA monomer only exists in Cc geometry, which is separated by a global barrier of 1417 cm -1 from the higher-energy Tt form. 8 Narrow-band NIR irradiation of the matrix at 6785 cm -1 led to a decrease of bands in the IR spectrum of the Cc form. Simultaneously, a set of new bands appeared due to a photogenerated species. The observed changes in the IR spectrum of HA are shown in Figures 1a and 2a.Comparison of the experimental difference spectrum with those calculated for the two conformers of HA (Figures 1b and 2b) reveals doubtlessly that the photogenerated species corresponds to the higher energy Tt conformer. The assignment of the nine most intense infrared Tt absorptions (along with Cc) is shown in Table 1.The observed shifts of the νOH and νCdO modes to higher frequencies (hypsochromic shifts) result from disru...

The structure of hydroxyacetone (HA) isolated in an argon matrix (at 12 K) and in a neat solid phase (at 12-175 K) was characterized by using infrared (IR) spectroscopy. The interpretation of the experimental results was supported by high-level quantum chemical calculations, undertaken by using both ab initio (MP2) and density functional theory methods. A potential-energy surface scan, carried out at the MP2/6-311++G(d,p) level of theory, predicted four nonequivalent minima, Cc, Tt, Tg, and Ct, all of them doubly degenerate by symmetry. The energy barriers for conversion between most of the symmetrically related structures and also between some of the nonequivalent minima (e.g., Tg --> Tt and Ct --> Tt) are very small and stay below the zero-point vibrational level associated with the isomerization coordinate in the higher-energy form in each pair. Therefore, only Cc and Tt conformers have physical significance, with populations of 99 and 1%, respectively, in gas phase at room temperature. For the matrix-isolated compound, only the most stable Cc conformer was observed. On the other hand, the polarizable continuum model calculations indicated that in water solution, the population of Tt and Ct conformers might be high enough (ca. 6 and 11%, respectively) to enable their experimental detection, thus supporting the conclusions of a previous IR spectroscopy study [ Spectrochim. Acta A 2005, 61, 477] in which the presence of more than one HA conformer in aqueous solution was postulated. The signatures of these minor conformers, however, do not appear in the spectra of the neat HA crystal, and the crystal structure was rationalized in terms of centrosymmetric hydrogen-bonded dimers consisting of two Cc-like units. Finally, we calculated (1)H, (13)C, and (17)O NMR chemical shifts at different levels of theory and found them to agree with available experimental data.

Aggregation in hydroxyacetone (HA) is studied using low-temperature FTIR, supersonic jet expansion, and X-ray crystallographic (in situ cryocrystallization) techniques. Along with quantum chemical methods (MP2 and DFT), the experiments unravel the conformational preferences of HA upon aggregation to dimers and oligomers. The O-H···O═C intramolecular hydrogen bond present in the gas-phase monomer partially opens upon aggregation in supersonic expansions, giving rise to intermolecular cooperatively enhanced O-H···O-H hydrogen bonds in competition with isolated O-H···O═C hydrogen bonds. On the other hand, low-temperature IR studies on the neat solid and X-ray crystallographic data reveal that HA undergoes profound conformational changes upon crystallization, with the HOCC dihedral angle changing from ~0° in the gas phase to ~180° in the crystalline phase, hence giving rise to a completely new conformation. These conclusions are supported by theoretical calculations performed on the geometry derived from the crystalline phase.

Two almost isoenergetic conformers of 4-methoxybenzaldehyde (p-anisaldehyde), O-trans and O-cis, are nearly equally populated in gas phase at room temperature. The existence of these two conformers of similar energy makes p-anisaldehyde an attractive molecule for conformational investigations, in which the relative populations of the two forms might be subjected to optical control. In the present study, monomers of the compound were trapped from the room-temperature gas phase into cryogenic argon and xenon matrices. The initial relative amount of the two conformers present in the freshly deposited matrices is shifted slightly in favor of the O-trans conformer. The ratio of the two forms could be reversibly varied by irradiating the sample with UV light in different wavelength ranges or by using the temperature variation. Increasing the temperature of the xenon matrix up to ca. 57 K led to conversion of the less stable O-cis form into the O-trans conformer, shifting the O-cis/O-trans ratio to ca. 1:7. A series of UV irradiations with different long-pass cutoff filters was carried out. UV excitation induced transformation of O-cis and O-trans conformers into each other. These transformations were leading to the UV-wavelength-specific photostationary equilibria characterized by the O-cis/O-trans ratios of about 1:2.2, 1:1.4, 1:1.1, and 1:0.89 for λ > 328, 295, 288, and 234 nm cutoff filters, respectively. The isomerization processes were probed by infrared spectroscopy and supported by quantum chemical calculations. The absorption bands observed in the infrared spectra of p-anisaldehyde isolated in argon and xenon matrices were assigned to the theoretically predicted normal modes.

β-aminoisobutyric acid (BAIBA) has been studied in isolation conditions: in the gas phase and trapped into a cryogenic N2 matrix. A solid sample of the compound was vaporized by laser ablation and investigated through their rotational spectra in a supersonic expansion using two different spectroscopic techniques: broadband chirped pulse Fourier transform microwave spectroscopy and conventional molecular beam Fourier transform microwave spectroscopy. Four conformers with structures of two types could be successfully identified by comparison of the experimental rotational and (14)N nuclear quadruple coupling constants with those predicted theoretically: type A, bearing an OH⋯N intramolecular hydrogen bond and its carboxylic group in the trans geometry (H-O-C=O dihedral ∼180°), and type B, having an NH⋯O bond and the cis arrangement of the carboxylic group. These two types of conformers could also be trapped from the gas phase into a cryogenic N2 matrix and probed by Fourier transform infrared (IR) spectroscopy. In situ irradiation of BAIBA isolated in N2 matrix of type B conformers using near-IR radiation tuned at the frequency of the O-H stretching 1st overtone (∼6930 cm(-1)) of these forms allowed to selectively convert them into type A conformers and into a new type of conformers of higher energy (type D) bearing an NH⋯O=C bond and a O-H "free" trans carboxylic group.

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