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Vinyl alcohol (VyA) and cyanide (CN) radicals are relatively abundant in the interstellar medium (ISM). VyA is the enolic tautomer of acetaldehyde and has two low-lying conformers, characterized by the syn or anti placement of hydroxyl hydrogen with respect to the double bond. In this paper, we present a gas-phase model of the barrierless reactions of both VyA's conformers with CN employing accurate quantum chemical computations in the framework of a master equation approach based on the transition state theory. Our results indicate that both VyA conformers feature a similar reactivity with CN, starting with a barrierless addition to the double bond and followed by different isomerization, dissociation, and/or hydrogen elimination steps. The rate constants computed for temperatures up to 600 K show that several reaction channels are open even under the harsh conditions of the ISM, with the favoured one providing the first feasible formation route of a prebiotic molecule not yet detected in the ISM, namely cyanoacetaldehyde. This finding suggests looking for cyanoacetaldehyde in regions where both VyA and CN have already been detected, like, e.g. , Sagittarius B2N or G+0.693-0.027.

Stochastic modeling approaches can be used to rationalize complex molecular dynamical behaviours in solution, helping to interpret the coupling mechanisms among internal and external degrees of freedom, providing insight into reaction mechanisms and extracting structural and dynamical data from spectroscopic observables. However, the definition of comprehensive models is usually limited by (i) the difficulty in defining - without resorting to phenomenological assumptions - a representative reduced ensemble of molecular coordinates able to capture essential dynamical properties and (ii) the complexity of numerical or approximate treatments of the resulting equations. In this paper, we address the first of these two issues. Building on a previously defined systematic approach to construct rigorous stochastic models of flexible molecules in solutions from basic principles, we define a manageable diffusive framework leading to a Smoluchowski equation determined by one main tensorial parameter, namely the scaled roto-conformational diffusion tensor, which accounts for the influence of both conservative and dissipative forces and describes the molecular mobility via a precise definition of internal-external and internal-internal couplings. We then show the usefulness of the roto-conformational scaled diffusion tensor as an efficient gauge of molecular flexibility through the analysis of a set of molecular systems of increasing complexity ranging from dimethylformamide to a protein domain.

Chemical bonding in a set of six cuprous complexes with simple nitriles (CN-, HNC, HCN, CH3NC, and CH3CN) is investigated by means of a recently devised analysis scheme framed in density-functional theory and quantitatively singling out concurrent charge flows such as σ donation and π backdonation. The results of our analysis are comparatively assessed against qualitative models for charge redistribution based on the popular concepts of octet rule and resonance structures, and the relative importance of different charge-flow channels relating to σ donation, π back-donation, polarization, and hyperconjugation is discussed on a quantitative basis.

Circular dichroism spectra in the IR range (VCD = vibrational circular dichroism) and in the UV range (ECD = electronic circular dichroism) have been recorded for both enantiomers of simple mono-substituted ferrocenes containing chiral pendants: 1-acetoxyethylferrocene, 1, 1-methoxyethylferrocene, 2, and 1-hydroxyethylferrocene, 3; the related disubstituted 1,1'-bis(1-hydroxyethyl)ferrocene, 4, was also considered. These two types of spectra, with the support of DFT calculations, concur to unequivocally confirm the absolute configuration for 1-4. In particular, our computational results point out the clear advantage of using an anharmonic oscillator model for the interpretation of VCD spectra of chiral ferrocenes. Interesting conformational properties are either confirmed or established by the technique, like the eclipsed conformation of the two cyclopentadienyl rings and an intra-molecular interaction involving the OH for 3. For 4, NMR, VCD and IR spectra are compatible with dimer formation and in this case a distorted conformation is predicted. Of utmost importance for the absolute configuration assignment in mono-substituted ferrocenes, we were able to identify a diagnostic VCD band at 950 cm-1 and a (low intensity) ECD band that clearly indicate the absolute configuration of the whole series.

Through an analysis of eighty tetrahedral and square-planar metal carbonyls of general formula [M(CO)(L')(L) 2 ] including newly synthesized chlorocarbonyl rhodium complexes with chelating atropoisomeric diphosphanes, we show how coordination geometry can switch the carbonyl stretching frequency into a selective probe of the σ-donor and π-acceptor abilities of the ligands. We thus provide a framework whereby the σ-donation and π-backdonation constituents of the Dewar-Chatt-Duncanson model can be quantitatively predicted through spectroscopic data on coordinated CO moieties and vice versa.

The relation between spectroscopic observables and the detailed metal-ligand bonding features in chelation complexes is addressed using both experimental and state-of-the-art theoretical and computational methods. We synthesized and characterized a set of six nickel dicarbonyl complexes of general formula [Ni(CO) 2 (PP)], where PP is an atropoisomeric chelating diphosphine ligand. The analysis of the obtained experimental data and the basicity and oxidative potentials of the free ligands suggests a close relation between the donor ability of the chelating ligand and the carbonyl stretching frequencies observed in the complexes. We then use theory to unravel the detailed mechanisms of chelation-bond formation in terms of partial charge flows between the molecular orbitals of the fragments. By extending the promising, recently published natural orbitals for chemical valence/charge displacement (NOCV/CD) analysis scheme we provide a thorough, quantitative description of the several charge fluxes following the metal-ligand bond formation and demonstrate that the carbonyl stretching frequencies in the considered complexes selectively respond to the σ-donation charge flow from the phosphorus lone pairs of the ligands, with the frequency shift being in quantitative correlation with the extent of the ligand-to-metal charge transfer.

The CO stretching response upon coordination to a metal M to form [(L) n M(CO)] m complexes (L is an auxiliary ligand) is investigated in relation to the σ donation and π back-donation components of the M-CO bond and to the electrostatic effect exerted by the ligand-metal fragment. Our analysis encompasses over 30 carbonyls, in which the relative importance of donation, back-donation and electrostatics are varied either through the ligand in a series of [(L)Au(CO)] 0/+ gold(i) complexes, or through the metal in a series of anionic, neutral and cationic homoleptic carbonyls. Charge-displacement analysis is used to obtain well-defined, consistent measures of σ donation and π back-donation charges, as well as to quantify the σ and π components of CO polarization. It is found that all complexes feature a comparable charge flow of σ symmetry (both in the M-CO bonding region and in the CO fragment itself), which is therefore largely uncorrelated to CO response. By contrast, π back-donation is exceptionally variable and is found to correlate tightly with the change in CO bond distance, with the shift in CO stretching frequency, and with the extent and direction (C → O or C ← O) of the CO π polarization. As a result, we conclusively show that π back-donation can be an important bond component also in non-classical carbonyls and we provide the framework in which the spectroscopic data on coordinated CO can be used to extract quantitative information on the π donor properties of metal-ligand moieties.

State-to-state exact quantum probabilities of the N + N2 exchange reaction have been calculated on the recently proposed L4 potential energy surface fitted to high level ab initio points using full-dimensional time-independent quantum techniques. Thermal rate coefficient values calculated on L4 were found not to differ from those calculated on a previously proposed potential energy surface. On the contrary, state-specific reaction probabilities calculated on the two surfaces are shown to differ significantly. Arguments for attributing the difference to specific features of the considered potential energy surfaces are provided.