Search

In this paper, we examine decay and fragmentation of core-excited and core-ionized water molecules combining quantum chemical calculations and electron-energy-resolved electron–ion coincidence spectroscopy. The experimental technique allows us to connect electronic decay from core-excited states, electronic transitions between ionic states, and dissociation of the molecular ion. To this end, we calculate the minimum energy dissociation path of the core-excited molecule and the potential energy surfaces of the molecular ion. Our measurements highlight the role of ultra-fast nuclear motion in the 1 a 1 − 1 4 a 1 core-excited molecule in the production of fragment ions. OH+ fragments dominate for spectator Auger decay. Complete atomization after sequential fragmentation is also evident through detection of slow H+ fragments. Additional measurements of the non-resonant Auger decay of the core-ionized molecule (1 a 1 − 1 ) to the lower-energy dication states show that the formation of the OH+ + H+ ion pair dominates, whereas sequential fragmentation OH+ + H+ → O + H+ + H+ is observed for transitions to higher dication states, supporting previous theoretical investigations. [ABSTRACT FROM AUTHOR]

The ASEP/MD method has been employed for studying the solvent effect on the conformational equilibrium of the alanine dipeptide in water solution. MP2 and density functional theory (DFT) levels of theory were used and results were compared. While in gas phase cyclic structures showing intramolecular hydrogen bonds were found to be the most stable, the stability order is reversed in water solution. Intermolecular interaction with the solvent causes the predominance of extended structures as the stabilizing contacts dipeptide-water are favoured. Free-energy differences in solution were calculated and PPII, αR, and C5 conformers were identified as the most stable at MP2 level. Experimental data from Raman and IR techniques show discrepancies about the relative abundance of αR y C5, our results support the Raman data. The DFT level of theory agrees with MP2 in the location and stability of PPII and αR forms but fails in the location of C5. MP2 results suggest the possibility of finding traces of C7eq conformer in water solution, in agreement with recent experiments. [ABSTRACT FROM AUTHOR]

The nudged elastic band (NEB) method is a successful optimization method for obtaining minimum energy reaction paths if only the initial and final structures are known. However, the original implementation of the method had some limitations, which has meant that there has been considerable interest in proposing alternative NEB formulations, which show improved convergence behavior. In this work, we present two modifications to the standard NEB procedure. The first involves the use of a second-order quasi-Newton optimization technique applied separately to each of the images that form the path. The second consists of the use of an interpolating spline to represent the path. This ensures that the images along the path are evenly spaced and means that the arbitrary spring forces employed in the standard NEB method are no longer necessary. We tested these modifications on a set of small, but relatively complex, chemical systems and found that the computation time was reduced by as much as 90% compared with the standard method. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [ABSTRACT FROM AUTHOR]

In this paper, we examine decay and fragmentation of core-excited and core-ionized water molecules combining quantum chemical calculations and electron-energy-resolved electron-ion coincidence spectroscopy. The experimental technique allows us to connect electronic decay from core-excited states, electronic transitions between ionic states, and dissociation of the molecular ion. To this end, we calculate the minimum energy dissociation path of the core-excited molecule and the potential energy surfaces of the molecular ion. Our measurements highlight the role of ultra-fast nuclear motion in the 1a 1 -1 4a 1 core-excited molecule in the production of fragment ions. OH + fragments dominate for spectator Auger decay. Complete atomization after sequential fragmentation is also evident through detection of slow H + fragments. Additional measurements of the non-resonant Auger decay of the core-ionized molecule (1a 1 -1 ) to the lower-energy dication states show that the formation of the OH + + H + ion pair dominates, whereas sequential fragmentation OH + + H + → O + H + + H + is observed for transitions to higher dication states, supporting previous theoretical investigations.