Investigation of activation energies for dissociation of host‐guest complexes in the gas phase using low‐energy collision induced dissociation

Abstract

A low‐energy collision induced dissociation (CID) (low‐energy CID) approach that can determine both activation energy and activation entropy has been used to evaluate gas‐phase binding energies of host‐guest (H‐G) complexes of a heteroditopic hemicryptophane cage host (Zn (II)@1) with a series of biologically relevant guests. In order to use this approach, preliminary calibration of the effective temperature of ions undergoing resonance excitation is required. This was accomplished by employing blackbody infrared radiative dissociation (BIRD) which allows direct measurement of activation parameters. Activation energies and pre‐exponential factors were evaluated for more than 10 H‐G complexes via the use of low‐energy CID. The relatively long residence time of the ions inside the linear ion trap (maximum of 60 s) allowed the study of dissociations with rates below 1 s−1. This possibility, along with the large size of the investigated ions, ensures the fulfilment of rapid energy exchange (REX) conditions and, as a consequence, accurate application of the Arrhenius equation. Compared with the BIRD technique, low‐energy CID allows access to higher effective temperatures, thereby permitting one to probe more endothermic decomposition pathways. Based on the measured activation parameters, guests bearing a phosphate (―OPO32−) functional group were found to bind more strongly with the encapsulating cage than those having a sulfonate (―SO3−) group; however, the latter ones make stronger bonds than those with a carboxylate (―CO2−) group. In addition, it was observed that the presence of trimethylammonium (―N(CH3)3+) or phenyl groups in the guest's structure improves the strength of H‐G interactions. The use of this technique is very straightforward, and it does not require any instrumental modifications. Thus, it can be applied to other H‐G chemistry studies where comparison of bond dissociation energies is of paramount importance.

Benchmarking higher energy collision dissociation (HCD) by investigation of binding energies of gas‐phase host–guest complexes of hemicryptophane cages

“…. 11 An advantage of the BIRD technique is that the real temperatures (T real ) of the ions are taken into account and therefore, in contrast to the two other approaches, there is no need for temperature calibration. However, only a relatively low temperature range is accessible (Figure 7), which limits BIRD's application to only systems possessing relatively low dissociation energies.…”

“…The second technique included in Figure 7 is low-energy CID, which implicates the effective temperature (T eff ) of ions, that is, the temperature corresponding to the Maxwell-Boltzmann distribution of the internal energy of the ions undergoing low-energy CID. 11,36 As is evident from Figure 7, the main advantage of this resonant activation technique is that it enables access to higher temperatures compared with BIRD because the collisional heating inherent to low-energy CID is more efficient than the IR heating used in BIRD to reach an equilibrium temperature under REX limit conditions. Finally, with the third technique, that is, HCD, that encompasses the characteristic temperature of the ions (T char ), one can gain access to even higher energy regimes than those of low-energy CID in a shorter activation time.…”

“…This HCD investigation into dissociations of hemicryptophane cage H-G complexes complements our previous studies of these same H-G complexes by high-pressure CID 12 and low-energy CID. 11…”

Development of a photoinduced fragmentation ion trap for infrared multiple photon dissociation spectroscopy

“…Examples are UV-visible photodissociation, [15][16][17] infrared and UV-visible [18][19][20] and infrared double resonance experiments 21,22 and blackbody infrared dissociation. 23,24 The main advantages of IRMPD spectroscopy over methods based on PID in other spectral regions [25][26][27][28][29][30][31] is that the IR spectral signatures allow the discrimination of functional groups, the analysis of conformational isomers and the identification of protonation/ coordination sites, especially when supported by theoretical calculations. 32,33 Consequently, IRMPD experiments have been used in an extensive number of systems and analytical applications, [34][35][36][37][38][39] from biomolecules, 40-42 drugs [43][44][45] and metabolites [46][47][48][49] to the evaluation of catalysis and reaction mechanisms, 19,[50][51][52][53][54][55][56] nanocalorimetry determinations 57,58 and in studies evaluating the electrospray ionization process 59 and aerosol composition.…”

Extended kinetic method and RRKM modeling to reinvestigate proline’s proton affinity and approach the meaning of effective temperature

“…3,4 These REX limit conditions are also achieved in the case of very soft collisional activation leading to a Maxwell-Boltzmann internal energy distribution at equilibrium. In this case, the term ''effective temperature'' [5][6][7] is used but the ''thermal collisional temperature'' term would be more adequate to avoid any ambiguity with the T eff term used for the kinetic method (discussed later). 8 For smaller systems, the highest energy part of the Maxwell-Boltzmann distribution is truncated and one speaks accordingly about truncated or depleted thermal distribution at equilibrium.…”