Predissociation Measurements of Bond Dissociation Energies

Abstract

CONSPECTUS: A fundamental need in chemistry is understanding the chemical bond, for which the most quantitative measure is the bond dissociation energy (BDE). While BDEs of chemical bonds formed from the lighter main group elements are generally well-known and readily calculated by modern computational chemistry, chemical bonds involving the transition metals, lanthanides, and actinides remain computationally extremely challenging. This is due to the simultaneous importance of electron correlation, spin−orbit interaction, and other relativistic effects, coupled with the large numbers of low-lying states that are accessible in systems with open d or f subshells. The development of efficient and accurate computational methods for these species is currently a major focus of the field. An obstacle to this effort has been the scarcity of highly precise benchmarks for the BDEs of M−X bonds. For most of the transition metal, lanthanide, or actinide systems, tabulated BDEs of M−X bonds have been determined by Knudsen effusion mass spectrometric measurements of high-temperature equilibria. The measured ion signals are converted to pressures and activities of the species involved in the equilibrium, and the equilibrium constants are then analyzed using a van't Hoff plot or the third-law method to extract the reaction enthalpy, which is extrapolated to 0 K to obtain the BDE. This procedure introduces errors at every step and ultimately leads to BDEs that are typically uncertain by 2−20 kcal mol −1 (0.1−1 eV). A second method in common use employs a thermochemical cycle in which the ionization energies of the MX molecule and M atom are combined with the BDE of the M + −X bond, obtained via guided ion beam mass spectrometry, to yield the BDE of the neutral, M−X. When accurate values of all three components of the cycle are available, this method yields good resultsbut only rarely are all three values available. We have recently implemented a new method for the precise measurement of BDEs in molecules with large densities of electronic states that is based on the rapid predissociation of these species as soon as the ground separated atom limit is exceeded. When a sharp predissociation threshold is observed, its value directly provides the BDE of the system. With this method, we are able in favorable cases to determine M−X BDEs to an accuracy of ∼0.1 kcal mol −1 (0.004 eV). The method is generally applicable to species that have a high density of states at the ground separated atom limit and has been used to measure the BDEs of more than 50 transition metal−main group MX molecules thus far. In addition, a number of metal−metal BDEs have also been measured with this method. There are good prospects for the extension of the method to polyatomic systems and to lanthanide and actinide-containing molecules. These precise BDE measurements provide chemical trends for the BDEs across the transition metal series, as well as crucial benchmarks for the development of efficient and accurate computational methods for the d-and f-block elem...

On Achieving High Accuracy in Quantum Chemical Calculations of 3d Transition Metal-Containing Systems: A Comparison of Auxiliary-Field Quantum Monte Carlo with Coupled Cluster, Density Functional Theory, and Experiment for Diatomic Molecules

“…Morse also shows that the measurements are also amenable to testing via a thermodynamic cycle with other precise measurements. 49 In our study we convert D 0 to D e using the ZPE data in Ref. 7.…”

“…7 When possible, we substitute the high quality experimental results obtained via the predissociation technique of Ref. 49. We then consider the independently selected best experimental values in the work of Dixon and co-workers, 23 67 Papakondylis, who used various theoretical methods to calculate the D e of ZnS, 68 pointed out that the aforementioned experimental papers used outdated values for the equilibrium bond lengths and frequencies as compared to more recent measurements, 69,70 bringing the older experimental measurements into doubt.…”

“…Fourthly, we include new experimental values published in Ref. 49, and we identify one case (ZnS) where we believe that the experimental result is problematic, i.e. outside the error bars reported in the experimental papers.…”

“…23 as well, and plot their choice only when it is not within error bars of the corresponding value from Ref. 7.We would like to highlight that the measurements in Ref 49. were published after the ph-AFQMC calculations in this work were completed.…”

Performance of Density Functional Theory for Transition Metal Oxygen Bonds

“…In particular, the use of the BDEs of metalligand bonds are receiving significant attention as best-practice benchmark properties for chemical reactivity across the dblock. [30,[33][34][35][36][37][38][39][40] The success largely relates to the fact that the diatomic MÀ O BDEs correlate better than any other known single descriptor to the chemisorption energies of pure metal surfaces, with R 2 approaching 0.89 when computed with the same methods, [30] across a range of more than 500 kJ/mol in both O 2 chemisorption energies and MÀ O BDEs within the dblock. Coordinative saturation, metalÀ metal modulations and solvent effects contribute smaller errors to the DFT-modelled process than the local MÀ O bond, because the relative MÀ O bond energies vary by an order of magnitude more than the metalÀ metal modulating influences, or correspondingly, the trend in cohesive energy changes upon ligand dissociation from different metal surfaces.…”