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Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH3)2CHCH(NH2)COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the π* orbital of the carboxyl group. The dominant negative ion (M–H)- (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying π* and through higher lying resonances at about 5.5 and 8.0–9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies. [ABSTRACT FROM AUTHOR]

Using electron attachment time-of-flight mass spectrometry, we show how high-precision structural constants of van der Waals aggregates may be obtained for two kinds of homogeneous clusters, (SF[sub 6])[sub N] and (CO[sub 2])[sub N]. Furthermore, we obtain size-specific structural information over a wide range of aggregate sizes. Mass spectrometric data are presented regarding the size needed to facilitate the transition from "cluster packing," dominated by nearest-neighbor interactions, to bulk-like packing. For both examples, it appears that the cluster-to-bulk packing transition may occur even for aggregates where the majority of the molecules resides at the surface. The critical size for the cluster-to-bulk transition may be related to the size at which molecules packed as bulk crystals can begin forming nearly spherical shapes. A discussion of the mechanism by which geometrical shell closings are visualized in electron attachment time-of-flight mass spectrometry is also presented. We postulate that these observations reflect the dynamics of electron localization in ordered crystallites with and without defects. [ABSTRACT FROM AUTHOR]