Modeling Gas-Phase Unimolecular Dissociation for Bond Dissociation Energies: Comparison of Statistical Rate Models within RRKM Theory

Abstract

The Rice-Ramsperger-Kassel-Marcus (RRKM) theory provides a simple yet powerful rate theory for calculating microcanonical rate constants. In particular, it has found widespread use in combination with gas-phase kinetic experiments of unimolecular dissociations to extract experimental bond dissociation energies (BDE). We have previously found several discrepancies between computed BDE values and the respective experimental ones, obtained with our empirical rate model, named L-CID. To investigate the reliability of our rate model, we conducted a theoretical analysis and comparison of the performance of conventional rate models and L-CID within the RRKM framework. Using previously published microcanonical rate data as well as reaction cross-section data, we show that the BDE values obtained with the L-CID model agree with the ones from the other rate models within the expected uncertainty bounds. Based on this agreement, we discuss the possible rationalization of the good performance of the L-CID model.

Model Compounds for Intermediates and Transition States in Sonogashira and Negishi Coupling:d8–d10Bonds in Large Heterobimetallic Complexes Are Weaker than Computational Chemistry Predicts

“…Treating the results in reverse order, threshold collision-induced dissociation (T-CID) of electrosprayed 4 , reported in our previous work, was clean in the mass spectrometer at the low energies included in the fit, producing only products of Pd–Cu bond cleavage. The fitting of the energy-resolved CID cross-section with L-CID accounts for, among other factors, the kinetic shift in the threshold by applying a version of statistical rate theory . Especially the treatment of the density-of-states function, ρ­( E ), for large molecules, large meaning up to some hundreds of atoms, requires approximations, which in the case of L-CID amounts to treating the ensemble of 3N-6 vibrational frequencies with a single “effective” frequency obtained as a fit parameter.…”

“…The fitting of the energy-resolved CID cross-section with L-CID 11 accounts for, among other factors, the kinetic shift 28 in the threshold by applying a version of statistical rate theory. 13 Especially the treatment of the densityof-states function, ρ(E), for large molecules, large meaning up to some hundreds of atoms, requires approximations, which in the case of L-CID amounts to treating the ensemble of 3N-6 vibrational frequencies with a single "effective" frequency obtained as a fit parameter. Testing and calibration of the approximation against a panel of reference molecules found that ρ(E) from a Beyer−Swinehart direct state count with explicit frequencies could be reliably reproduced by the effective frequency approximation over a range of over 100 kcal/mol in E, and 50 orders of magnitude in ρ(E).…”

“…11 We have further validated the approximation against more explicit kinetic treatments, starting with experimentally measured microcanonical dissociation rates, finding a good agreement at the level of the extracted E 0 . 13 We have also verified experimentally that our method behaves properly for a set of test molecules, starting with small molecules for which independent bond energies have been measured, and then extending to larger molecules approaching the size of interest in the present study. 12 Nevertheless, for molecules, such as 4, with 111 atoms, the kinetic shift reaches a significant fraction of the bond strength itself, making an accurate extraction of that bond strength from the experimental threshold sensitively dependent on a reliable modeling of the kinetic shift.…”

“…Electrospray ionization tandem mass spectrometry experiments were performed as described in our previous work . While the instrument, a ThermoFinnigan TSQ Quantum mass spectrometer, can be used for quantitative bond dissociation energy measurements by threshold collision-induced dissociation (T-CID) experiments, the present work refrains from measuring the bond energies with T-CID experiments on large molecules, − the validation of that latter experiment being one of the principal motivations for the present report. NMR spectra were recorded with Bruker Ascend 500 MHz or Bruker Ascend 400 MHz.…”

2‐Aminoalkylgold Complexes: The Putative Intermediate in Au‐Catalyzed Hydroamination of Alkenes Does Not Protodemetalate

“…Experimental errors are determined by the degree of reproducibility, as well as an error analysis of the L-CID model. [11] Unsaturated hydrocarbon E 0,exp. Upon treatment of [LAuCl] complexes containing different dialkylbiaryl phosphine-and NHC-ligands with the metalated (2-phenylethyl)dimethylamine nucleophile, obtained by the abovementioned method, we were pleased to observe that the transmetalation took place and that the desired 2-aminoalkyl gold complexes were formed.…”