Kinetic energy release of protonated methanol clusters using the low-temperature fast-atom bombardment: experiment and theory combined

Mass spectrometry evidence for self-rigidification of π-conjugated oligomers containing 3,4-ethylenedioxythiophene groups using RRKM theory and internal energy calibration

The self-rigidification of ionized π-conjugated systems based on two combinations of thiophene (T) and 3,4-Ethylenedioxythiophene (E) is investigated using mass-analyzed ion kinetic energy spectrometry (MIKES) of ions produced from electron impact ionization at 70 eV. The m/z 446 radical cations of the two isomers ETTE and TEET lead to detect m/z 418 and 390 daughter ions. The MIKE spectra differ only by the intensities of these fragment ions. As the m/z 418 daughter ion is produced through a same retro-Diels Alder reaction whatever the fragmenting isomer, the difference in daughter ion intensities is interpreted in term of unimolecular dissociation rate constants ( k( E_int)) ratios. Considering that the transition state (TS) of such reaction is attributed to a quinoid form, equivalent vibration modes are assumed for the TS of both dissociating ETTE and TEET radical cations. As a result, by using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory, the difference in daughter ion intensities is interpreted by considering that the fragmenting ion is more or less ordered in its ground state than at the transition state, resulting from the influence of the number of the S^…O interactions in the planarization of the TEET ion toward the ETTE charged species. The comparison of this behavior in MIKES experiments is supported by the modeling of ion behavior in mass spectrometer and the calibration in internal energy of the radical cations produced in an EI source.

SORI excitation: collisional and radiative processes

Theoretical modeling of sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments in Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is described in the present paper. Manipulation of various analytical expressions yield the result that the average laboratory frame collision energy is equal to 2/3 of the maximum kinetic energy in SORI. Survival yields (the fraction of nondecomposed molecular ions) as a function of excitation time, collision energy, and source temperature have been considered: results of MassKinetics-type reaction kinetics modeling were compared with experimental results obtained by Guo et al. (Int. J. Mass Spectrom.2003, 225, 71-82). The results show that radiative cooling has a major influence in SORI-CID. They also suggest that collisional cooling is significant only at very low (less than 0.02 eV) center of mass collision energy; therefore it has a very small influence on the SORI process. Survival yield curves showed excellent agreement between experiments and calculations optimizing two parameters only (collisional energy-transfer efficiency and radiative cooling rate). Using leucine enkephalin as a model compound, the results indicate 0.128 +/- 0.021 energy deposition in a single collision and 7.5 +/- 0.5 s(-1) infrared cooling rate. We also present that these two physical parameters cannot be properly deconvoluted. This effect shows the importance of the parallel consideration of different physical processes.