Effective fragment potential structures

In general, for each acid HA, the HA-(H20) -Wm model reaction system (MRS) comprises a HA (H20) core reaction system (CRS), described quantum chemically, embedded in a cluster of Wm classical, polarizable waters of fixed internal structure (effective fragment potentials, EFPs) [27]. The CRS is treated at the Hartree-Fock (HF) level of theory, with the SBK [28] effective core potential basis set complemented by appropriate polarization and diffused functions. The W-waters not only provide solvation at a low computational cost they also prevent the unwanted collapse of the CRS towards structures typical of small gas phase clusters by enforcing natural constraints representative of the H-bonded network of a surface environment. In particular, the structure of the Wm cluster equilibrates to the CRS structure along the whole reaction path, without any constraints on its shape other than those resulting from the fixed internal structure of the W-waters. 

Methanol-water mixtures were studied by Adamovic and Gordon who used the so called effective-fragment-potential method, an approach of the type discussed in section IID, for finite (Me0H/H20) clusters with n = 2,3,. .., 8. Some of their findings were tested by using accurate, parameter-free, electronic-structure methods. The main outcome is the organization of the H2O and MeOH molecules relative to each other, which then is expected to be representative for macroscopic mixtures of water and methanol. 

Nemukhin, A. V., Grigorenko, B. L., Bochenkova, A. V, Topol, 1. A., 8c Burt, S. K. (2002). A QM/MM approach with effective fragment potentials applied to the dipeptideiiL water structures. Journal of Molecular Structure (THEOCHEM), 581,167. 

Traditionally, conventional electron impact techniques have been used to effect sample ionization. The often complex fragmentation pattern which results can provide invaluable information for elucidating the structure of unknown compounds. Vast computer libraries of electron impact spectra are available to assist the analyst with identifications. However, electron impact is an ionization technique that is somewhat limited in scope. By contrast, sample ionization by chemical ionization techniques offers the potential for selectivity, increased sensitivity, complimentary fragmentation, and confirmation or determination of molecular weight. (7). In this paper we will describe several examples in which the versatility of the chemical ionization technique has simplified or improved an analysis. 

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