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A constrained geometry optimisation scheme in combination with density functional theory is applied to probe the effect of an external force on a molecule. When the external force overcomes a critical value, one bond will break. In a polyatomic molecule, it is of special interest to see which bond breaks and why. In the present work, a series of organic molecules have been studied and it is found that it is not necessarily the thermodynamically weakest bond that breaks. It is found that the dissociation energy, the harmonic force constant and the angle between the given bond and the external force are all important factors in this process. [ABSTRACT FROM AUTHOR]

Experimental mass abundance spectra are used to extract evaporative activation energies (dissociation energies) for protonated water clusters, (H2O)NH+, and deprotonated water clusters, (H2O)NOH-, in the size range up to hundred molecules. The inversion is achieved by application of the shell correction method adapted from nuclear physics to the abundance spectra. The well known abundance anomaly for protonated clusters which occurs for N=20–22 is found to have the characteristic behavior of a shell closing, whereas other apparent magic numbers are only prominent peaks in the abundance spectra because of the instability of the evaporative precursor. For the deprotonated clusters, we find a similar shell closing for N=55–56. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Determines the stabilities of heavy water clusters. • Slightly smaller magicity of the N = 21 cluster compared to normal water. • No difference observed for N = 28: The difference ascribed to the special geometric structure of the N = 21 cluster. Abstract The abundance spectrum of deuterated heavy water clusters has been measured, and relative dissociation energies were extracted from the abundances. The dissociation energies are similar to those of protonated light water clusters, except for a slightly reduced excess stability of the N = 21 cluster relative to the N = 22 cluster. The difference is interpreted in terms of the special geometry of the N = 21 cluster. The absence of a similar effect for N = 28/29 suggests that the stability of the N = 28 cluster is not due to a closed shell structure. [ABSTRACT FROM AUTHOR]

We have studied the collision induced dissociation reactions of proton-bound diastereomeric adducts of S-1-phenylethanol and enantiomers of three different amino acids (tryptophan, phenylalanine, methionine). In all cases, the loss of S-1-phenylethanol from the adduct ion is the only observed process, and the relative abundance is found to be independent of the chirality of the amino acid. This is in contrast to earlier experiments on the dissociation of protonated tryptophan–2-butanol adducts, where chirality affected the results. Results obtained from quantum chemical computations support and provide a rationale for the experimental observations and highlight temperature as a possible factor of importance for the chiral effect in these types of systems. [ABSTRACT FROM AUTHOR]

Dimethyl disulfide (DMDS) and N-methylacetamide are two first choice model systems that represent the disulfide bridge bonding and the peptide bonding in proteins. These molecules are therefore suitable for investigation of the mechanisms involved when proteins fragment under electron capture dissociation (ECD). The dissociative recombination cross sections for both protonated DMDS and protonated N-methylacetamide were determined at electron energies ranging from 0.001 to 0.3 eV. Also, the branching ratios at 0 eV center-of-mass collision energy were determined. The present results give support for the indirect mechanism of ECD, where free hydrogen atoms produced in the initial fragmentation step induce further decomposition. We suggest that both indirect and direct dissociations play a role in ECD. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Here we report on the gas-phase interactions between protonated enantiopure amino acids ( l- and d-enantiomers of Met, Phe, and Trp) and chiral target gases [( R)- and ( S)-2-butanol, and ( S)-1-phenylethanol] in 0.1-10.0 eV low-energy collisions. Two major processes are seen to occur over this collision energy regime, collision-induced dissociation and ion-molecule complex formation. Both processes were found to be independent of the stereo-chemical composition of the interacting ions and targets. These data shed light on the currently debated mechanisms of gas-phase chiral selectivity by demonstrating the inapplicability of the three-point model to these interactions, at least under single collision conditions. [ABSTRACT FROM AUTHOR]