Laboratory Rotational Spectrum and Radio-Astronomical Search of Acetoin.

The rotational spectrum of acetoin (3-hydroxy-2-butanone) was measured by using Fourier transform microwave spectroscopy with the aid of quantum chemical calculations. Only one conformer of acetoin was detected in the pulsed jet, whose spectrum featured the splittings raised from the internal rotation of the methyl top linking to the C═O group. Based on the spectroscopic result, radio-astronomical searches for acetoin were carried out toward the massive star-forming region Sgr B2(N) using the Shanghai Tianma 65 m and IRAM 30 m radio telescopes. No lines belonging to acetoin were detected toward Sgr B2(N). Its upper limit of column density was calculated.

Quantum mechanical modeling of interstellar molecules on cosmic dusts: H2O, NH3, and CO2.

Since the first detection of CH molecule in interstellar medium (ISM), more than 270 molecules have been identified in various astronomical sources in ISM. These molecules include big complex ones, such as fullerene (C60) and polycyclic aromatic hydrocarbons (PAHs), which are the main components of carbonaceous dust. Dust surface chemistry plays an important role in explaining the formation of interstellar molecules. However, many of the dust surface chemical parameters, such as the adsorption energies, are still of uncertainty. Here we present a study of the adsorption of water (H2O), ammonia (NH3), and carbon dioxide (CO2) on graphene-like substrate within the framework of density functional theory (DFT). We used Gaussian 16 software and adopted the corrected generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functions. We determined the optimal accretion position of the studied molecules on the graphene-like surface and calculated the adsorption energies. Furthermore, according to the density of states and molecular orbitals of the adsorbed states, we analyzed the charge transfer between the molecules and the graphene-like surface. These results can provide more accurate parameters for calculating the chemical reaction rates on the dust surface, thus contributing to the understanding of dust-surface reactions in ISM.

Laboratory Measurements and Astronomical Search for Methoxyacetone and Methyl Methoxyacetate.

High-resolution pure rotational spectra of methoxyacetone and methyl methoxyacetate have been recorded and analyzed by using pulsed jet-expansion Fourier transform microwave spectroscopy with the aid of quantum calculations. The global minima for both target molecules have been detected in pulsed jet, whose spectra are featured with the splittings raised from the methyl internal rotations. On the basis of the spectroscopic results, a radio-astronomical search of methoxyacetone and methyl methoxyacetate was carried out toward the high-mass star-forming region Sgr B2(N) using the Shanghai Tianma 65 m radio telescope. No lines belonging to either of the target molecules were detected, and the upper limits to the column density were derived.

Three body photodissociation of the water molecule and its implications for prebiotic oxygen production.

The provenance of oxygen on the Earth and other planets in the Solar System is a fundamental issue. It has been widely accepted that the only prebiotic pathway to produce oxygen in the Earth's primitive atmosphere was via vacuum ultraviolet (VUV) photodissociation of CO2 and subsequent two O atom recombination. Here, we provide experimental evidence of three-body dissociation (TBD) of H2O to produce O atoms in both 1D and 3P states upon VUV excitation using a tunable VUV free electron laser. Experimental results show that the TBD is the dominant pathway in the VUV H2O photochemistry at wavelengths between 90 and 107.4 nm. The relative abundance of water in the interstellar space with its exposure to the intense VUV radiation suggests that the TBD of H2O and subsequent O atom recombination should be an important prebiotic O2-production, which may need to be incorporated into interstellar photochemical models.

The temperature-dependence of rapid low temperature reactions: experiment, understanding and prediction.

Despite the success of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method in measuring rate coefficients for neutral-neutral reactions of radicals down close to the very low temperatures prevalent in dense interstellar clouds (ISCs), there are still many reactions of potential importance in the chemistry of these objects for which there have been no measurements of low temperature rate coefficients. One important class of reactions is that between atomic and molecular free radicals and unsaturated hydrocarbons; that is, alkynes and alkenes. Based on semi-empirical arguments and correlations of 'room temperature' rate coefficients, k(298 K), for reactions of this type with the difference between the ionisation energy of the alkyne/alkene and the electron affinity of the radical, we suggest which reactions between the radicals, C(3P), O(3P), N(4S), CH, C2H and CN, and carbon chain molecules (Cn) and cyanopolyynes (HC2nCN and NCC2nCN) are likely to be fast at the temperature of dense ISCs. These reactions and rate coefficients have been incorporated into a purely gas-phase model (osu2005) of ISC chemistry. The results of these calculations are presented and discussed.

Femtosecond dynamics of flavin cofactor in DNA photolyase: radical reduction, local solvation, and charge recombination.

We report here our femtosecond studies of the photoreduction dynamics of the neutral radical flavin (FADH) cofactor in E. coli photolyase, a process converting the inactive form to the biologically active one, a fully reduced deprotonated flavin FADH(-). The observed temporal absorption evolution revealed two initial electron-transfer reactions, occurring in 11 and 42 ps with the neighboring aromatic residues of W382 and F366, respectively. The new transient absorption, observed at 550 nm previously in photolyase, was found from the excited-state neutral radical and is probably caused by strong interactions with the adenine moiety through the flavin U-shaped configuration and the highly polar/charged surrounding residues. The solvation dynamics from the locally ordered water molecules in the active site was observed to occur in approximately 2 ps. These ultrafast ordered-water motions are critical to stabilizing the photoreduction product FADH(-) instantaneously to prevent fast charge recombination. The back electron-transfer reaction was found to occur in approximately 3 ns. This slow process, consistent with ultrafast stabilization of the catalytic cofactor, favors photoreduction in photolyase.