Observation of conformation-specific pathways in the photodissociation of 1-iodopropane ions

Abstract

Many molecules can rotate freely around single bonds and thereby interconvert between different conformations, such as gauche and anti 1,2-disubstituted ethane, a classic example of conformational isomerism. Even though rotation occurs rapidly at room temperature, the product selectivity seen in some reactions has been explained by conformation-dependent reaction mechanisms: if reactant molecules differing only in their conformation are located at different positions on the reaction path, they may undergo different reactions. But a direct verification of this effect is difficult, because the energy barrier separating conformational isomers is so low that under ambient conditions reactants with more than one conformation will be present. But by using temperatures low enough to suppress the interconversion between different conformations, gauche-1-iodopropane ions and anti-1-iodopropane ions have been selectively generated. Here we show that the kinetic energy released during the photodissociation of 1-iodopropane ions depends strongly on the conformation of the ions. Thermodynamic arguments and ab initio calculations indicate that this difference in kinetic energy release results from differences in the reaction mechanism, with gauche-1-iodopropane ions forming 2-propyl ions and anti-1-iodopropane ions forming protonated cyclopropane ions. These findings suggest that the well-known concept of conformation selection forms the basis of a simple scheme for reaction control, thus providing in some cases an attractive alternative for more involved schemes that utilize the phase and pulse shape of laser beams to control chemical reactions.

Spectroscopic Investigation of cis-2,4-Difluorophenol Cation by Mass-analyzed Threshold Ionization Spectroscopy

“…16 To the best of our knowledge, detailed spectroscopic data about the 24DFP in electronically excited S 1 and cationic ground D 0 state are not yet available in the literature. Both zero-kinetic energy (ZEKE) photoelectron and mass-analyzed threshold ionization (MATI) spectroscopic [17][18][19][20][21][22][23] techniques can provide information about the precise IE and active cation vibrations in the D 0 state. The latter method provide mass information and is suitable for studies of radicals, complexes, clusters, or molecular systems in which impurities may be present.…”

Tandem time-of-flight mass spectrometer for photodissociation of biopolymer ions generated by matrix-assisted laser desorption ionization (MALDI-TOF-PD-TOF) using a linear-plus-quadratic potential reflectron

“…Excitation via multiple collisions is thought to be important in the CID tandem mass spectrometry of high mass biopolymers with many degrees of freedom. Recently, dissociation of multiply protonated molecules induced by electron capture, or electron capture dissociation (ECD) [10,11], is attracting a lot of attention as a method to obtain site-specific information.Photodissociation (PD) has been utilized in the field of tandem mass spectrometry mostly as a method to study the structure and dissociation dynamics of small polyatomic ions [12]. Infrared multiphoton dissociation (IRMPD) [13,14] has been used to induce dissociation of biopolymer ions also, even though a large amount of internal energy needed for such a dissociation necessitates absorption of a large number of infrared photons.…”

“…Photodissociation (PD) has been utilized in the field of tandem mass spectrometry mostly as a method to study the structure and dissociation dynamics of small polyatomic ions [12]. Infrared multiphoton dissociation (IRMPD) [13,14] has been used to induce dissociation of biopolymer ions also, even though a large amount of internal energy needed for such a dissociation necessitates absorption of a large number of infrared photons.…”

Sensitivity of Photoelectron Angular Distributions to Molecular Conformations of Anions

“…Gas-phase ion spectroscopy in particular has provided a wealth of insight into the intrinsic interactions leading to specific conformations in biological, chemical and catalytic contexts. [1][2][3][4][5][6][7] Electronic structure is of course closely linked to molecular structure and, generally, specific geometric structures can have very different electronic structures that in turn lead to differing physical or chemical properties. Anion photoelectron (PE) spectroscopy is a powerful and well-established method that probes electronic structure directly and, by comparison with calculations and vibrational analysis of the Franck-Condon profiles of the PE spectra, can also probe geometric structure.…”