We report the generation, spectroscopic characterization, and computational analysis of the first free (non-stabilized) organometallic bismuthinidene, BiMe. The title compound was generated in situ from BiMe3 by controlled homolytic Bi−C...

Mass-selective threshold photoelectron spectroscopy in the gas phase was employed to characterize the dialkynyl triplet carbenes pentadiynylidene (HC5H), methylpentadiynylidene (MeC5H) and dimethylpentadiynylidene (MeC5Me). Diazo compounds were employed as precursors to generate the carbenes by flash pyrolysis. The R1-C5-R2 carbon chains were photoionized by vacuum ultraviolet (VUV) synchrotron radiation in photoelectron photoion coincidence (PEPICO) experiments. High-level ab initio computations were carried out to support the interpretation of the experiments. For the unsubstituted pentadiynylidene (R1= R2= H) the recorded spectrum yields an adiabatic ionization energy (IEad) of 8.36 ± 0.03 eV. In addition, a second carbene isomer, 3-(didehydrovinylidene)cyclopropane, with a singlet electronic ground state, was identified in the spectrum based on the IEad of 8.60 ± 0.03 eV and Franck Condon simulations. We found that multireference computations are required to reliably calculate the IEad for this molecule. CASPT2 computations predicted an IEad =8.55 eV, while coupled-cluster computations significantly overestimate the IE. The cyclic isomer is most likely formed from another isomer of the precursor present in the sample. Stepwise methyl-substitution of the carbene leads to a reduction of the IE to 7.77 ± 0.04 eV for methylpentadiynylidene and 7.27 ± 0.06 eV for dimethylpentadiynylidene. The photoionization and dissociative photoionization of the precursors is investigated as well.

A recent review on the photoionisation of the CH isomer ortho-benzyne suggests that bands reported in earlier photoelectron spectra might be due to side products or contaminations, while computations raise doubts, whether the cation has a planar geometry. We therefore reinvestigate the photoionisation of ortho-benzyne, generated by pyrolysis from benzocyclobutenedione, by photoion mass-selected threshold photoelectron (ms-TPE) spectroscopy using synchrotron radiation. The experiments are accompanied by a theoretical study that investigates the structure of the ortho-benzyne cation systematically as a function of the computational method, up to CASPT2(11,14) ab initio computations. Our study leads to a re-evaluation of the ionisation energy of ortho-benzyne. It reveals that the ortho-benzyne cation has indeed a twisted C geometry rather than a C structure. A vertical ionisation energy IE of 9.77 eV and an adiabatic ionisation energy of IE = 9.56 eV are computed for ortho-benzyne. A Franck-Condon simulation of the photoelectron spectrum based on the CASPT2 results and including three electronic states of the cation is in agreement with the experiment and yields IE = 9.51 eV (+50 meV/-100 meV). Since this value is in contrast with previous work, the ionisation energy has to be revised based on our study. Computational methods based on density functional theory give a reasonable description of the cationic ground state, but fail for the corresponding excited electronic states that are indispensible for a proper assignment of the photoelectron spectrum.

We present ChemPager, a freely available tool for systematically evaluating chemical syntheses. By processing and visualizing chemical data, the impact of past changes is uncovered and future work guided. The tool calculates commonly used metrics such as process mass intensity (PMI), Volume−Time Output, and production costs. Also, a set of scores is introduced aiming to measure crucial but elusive characteristics such as process robustness, design, and safety. Our tool employs a hierarchical data layout built on common software for data entry (Excel, Google Sheets, etc.) and visualization (Spotfire). With all project data being stored in one place, cross-project comparison and data aggregation becomes possible as well as cross-linking with other data sources or visualizations.

The bioconjugation at tyrosine residues using cyclic diazodicarboxamides, especially 4-substituted 3 H-1,2,4-triazole-3,5(4 H)-dione (PTAD), is a highly enabling synthetic reaction because it can be employed for orthogonal and site-selective (multi)functionalizations of native peptides and proteins. Despite its importance, the underlying mechanisms have not been thoroughly investigated. The reaction can proceed along four distinctive pathways: (i) the SAr path, (ii) along a pericyclic group transfer pathway (a classical ene reaction), (iii) along a stepwise reaction path, or (iv) along an unusual higher order concerted pericyclic mechanism. The product mixtures obtained from reactions of PTAD with 2,4-unsubstituted phenolate support the SAr mechanism, but it remains unclear if other mechanisms also take place. In the present work, the various mechanisms are compared using high-level quantum chemistry approaches for the model reaction of 4 H,3 H-1,2,4-triazole-3,5(4 H)-dione (HTAD) with p-cresol and p-cresolate. In a protic solvent (water), the barriers of the SAr mechanism and the ene reaction are similar but still too high to explain the available experimental observations. This is only possible if the SAr reaction of cresolate with HTAD is taken into account for which nearly vanishing barriers are computed. This satisfactorily explains measured conversion rates in buffered aqueous solutions and the strong activation effects observed upon addition of bases.