Parahydrogen (pH2) quantum solids are excellent matrix isolation hosts for studying the rovibrational dynamics and nuclear spin conversion (NSC) kinetics of molecules containing indistinguishable nuclei with nonzero spin. The relatively slow NSC kinetics of propyne (CH3CCH) isolated in solid pH2 is employed as a tool to assign the rovibrational spectrum of propyne in the 600-7000 cm-1 region. Detailed analyses of a variety of parallel (ΔK = 0) and perpendicular (ΔK=±1) bands of propyne indicate that the end-over-end rotation of propyne is quenched, but K rotation of the methyl group around the C3 symmetry axis still persists. However, this single-axis K rotation is significantly hindered for propyne trapped in solid pH2 such that the energies of the K rotational states do not obey simple energy-level expressions. The NSC kinetics of propyne follows first-order reversible kinetics with a 287(7) min effective time constant at 1.7 K. Intensity-intensity correlation plots are used to determine the relative line strengths of individual ortho- and para-propyne rovibrational transitions, enabling an independent estimation of the ground vibrational state effective A″ constant of propyne.
Enol forms of trifluoroacetylacetone (TFacac) isolated in molecular and rare gas matrices were studied using infrared (IR) and Raman spectroscopy. Additionally, calculations using DFT B3LYP and M06-2X as well as MP2 methods were performed in order to investigate the possibility of coexistence of more than one stable enol form isomer of TFacac. Calculations predict that both stable enol isomers of TFacac, 1,1,1-trifluoro-4-hydroxy-3-penten-2-one (1) and 5,5,5-trifluoro-4-hydroxy-3-penten-2-one (2), could coexist, especially in matrices where the room temperature population is frozen, 1 being the most stable one. Raman and IR spectra of TFacac isolated in nitrogen (N2) and carbon monoxide (CO) matrices exhibit clear absorption bands, which cannot be attributed to this single isomer. Their relative band positions and intensity profiles match well with the theoretical calculations of 2. This allows us to confirm that in N2 and CO matrices both isomers exist in similar amounts. Careful examination of the spectra of TFacac in argon, xenon, neon, normal, and para-hydrogen (Ar, Xe, Ne, nH2, and pH2 respectively) matrices revealed that both isomers coexist in all the explored matrices, whereas 2 was not considered in the previous spectroscopic works. The amount of the second isomer (2) in the as-deposited samples depends on the host. The analysis of TFacac spectra in the different hosts and under various experimental conditions allows the vibrational characterization of both chelated isomers. The comparison with theoretical predictions is also investigated.
A HElium Nanodroplet Isolation (HENDI) experiment was performed to explore the absorption spectrum of the propyne-water complex (CH3CCH⋯H2O). Two spectral regions were investigated, near the CH stretch v1 of the propyne moiety and near the asymmetric stretch v3 of the water moiety. Ab-initio calculations were performed at the MP2/aug-cc-pVTZ level to estimate the spectroscopic constants of the free complex. This provided the necessary parameters to simulate the absorption spectrum of the complex and thus facilitate the interpretation of the experiment. The observed spectrum is consistent with a structure of the complex where two H-bonds between water and propyne form a five member ring. The later was predicted by Lopes et al. [J. Mol. Struct. 834, 258 (2007)].
A deposition model to simulate the growth of doped rare gas crystals is used. The study involves organic molecules with a single intramolecular hydrogen bond such as malonaldehyde, 2chloromalonaldehyde and acetylacetone as impurities. Different trapping sites were obtained depending on the rare gas properties for a given impurity, and depending on the molecular size and shape for a given crystal. Simulations were carried out by using classical molecular dynamics methods including an anharmonic thermal correction, to take into account the zero point movement of the crystal. The results are correlated to spectroscopic data previously achieved for these systems by steady state IR spectroscopy.
ABSTRACT Interstellar complex organic molecules (iCOMs) have been identified in different interstellar environments including star forming regions as well as cold dense molecular clouds. Laboratory studies show that iCOMs can be formed either in gas phase or in the solid state, on icy grains, from ‘non-energetic’ (atom-addition/abstraction) or energetic (UV-photon, particle bombardments) processes. In this contribution, using a new experimental approach mixing matrix isolation technique, mass spectrometry, and infrared and EPR spectroscopies, we want to investigate the COM formation at 35 K from a complex mixture of ground state radicals trying to draw a general reaction scheme. We photolyse (121 nm) CH3OH diluted in Ar at low temperature (below 15 K) to generate $\mathrm{H^.CO}$, $\mathrm{HO^.CO}$, $\mathrm{^.CH_2OH}$, $\mathrm{CH_3O^.}$, $\mathrm{^.OH}$, and $\mathrm{^.CH_3}$ radicals and ‘free’ H-atoms within the matrix. Radicals have been identified using infrared and EPR spectroscopies. With the disappearance of the Ar matrix (at 35 K), these unstable species are then free to react, forming new species in a solid film. Some recombination products have been detected using infrared spectroscopy and mass spectrometry in the solid film after Ar removal, namely methyl formate (CH3OCHO), glycolaldehyde (HOCH2CHO), ethylene glycol (HOCH2CH2OH), glyoxal (CHOCHO), ethanol (CH3CH2OH), formic acid (HCOOH), dimethyl ether (CH3OCH3), methoxymethanol (CH3OCH2OH), and CH4O2 isomers (methanediol and/or methyl hydroperoxide). The detected molecules are fully consistent with the radicals detected and strongly support the solid state scenario of iCOM formation in interstellar ices based on radical–radical recombination. We then discuss astrophysical implications of the radical pathways on the observed gas phase iCOMs.
Large amplitude motions involving hydrogen tunnelling can be preserved in molecules trapped in parahydrogen matrices, and observed through band splitting or under certain conditions by a temporal evolution of the spectra.
Samples of propyne trapped in solid parahydrogen show multiple peak structures in their infrared spectra. These structures are attributed to molecules in two distinct kinds of matrix sites. The most intense lines are assigned to propyne molecules executing a slightly hindered methyl rotation, as was extensively studied in our earlier publication from our two groups, and the other set of peaks to propyne trapped in a secondary site where the methyl rotation is quenched and replaced by methyl torsion within the matrix site. The assignment of the various rovibrational transitions is made possible by the observation of nuclear spin conversion (NSC) within the methyl group at long timescales. The NSC rate depends on the site and is much slower in the sites where the methyl rotation is quenched.
The energetic demands of modern society for clean energy vectors, such as H2, have caused a surge in research associated with homogeneous and immobilized electrocatalysts that may replace Pt. In particular, clathrochelates have shown excellent electrocatalytic properties for the hydrogen evolution reaction (HER). However, the actual mechanism for the HER catalyzed by these d-metal complexes remains an open debate, which may be addressed via Operando spectroelectrochemistry. The prediction of electrochemical properties via density functional theory (DFT) needs access to thermodynamic functions, which are only available after Hessian calculations. Unfortunately, there is a notable lack in the current literature regarding the precise evaluation of vibrational spectra of such complexes, given their structural complexity and the associated tangled IR spectra. In this work, we have performed a detailed theoretical and experimental analysis in a family of Co(II) clathrochelates, in order to establish univocally their IR pattern, and also the calculation methodology that is adequate for such predictions. In summary, we have observed the presence of multiple common bands shared by this clathrochelate family, using the B3LYP functional, the LANL2DZ basis, and effective core potentials (ECP) for heavy atoms. The most important issue addressed in this article was therefore related to the detailed assignment of the fingerprint associated with cobalt(II) clathrochelates, which is a challenging endeavor due to the crowded nature of their spectra.
