Large yet bounded: Spin gap ranges in carbenes

TL;DR

This study systematically explores the electronic structure of 8,000 diverse carbene molecules, revealing a universal upper limit for the singlet-triplet gap and providing a comprehensive database of high-accuracy quantum calculations.

Contribution

It introduces a large, systematic computational analysis of carbene electronic structures, establishing a universal upper limit for the singlet-triplet gap and creating a valuable quantum chemistry database.

Findings

Universal upper limit of ~30 kcal/mol for singlet-triplet gap in carbenes

Large vertical and adiabatic spin gap ranges observed within carbene classes

Development of a comprehensive QMspin database with ~13,000 high-accuracy calculations

Abstract

Despite its relevance for chemistry, the electronic structure of free carbenes throughout chemical space has not yet been studied in a systematic manner. We explore a large and systematic carbene chemical space consisting of eight thousand diverse and common carbene scaffolds in their singlet and triplet state computed at controlled accuracy (higher order multireference level of theory) and with verified carbene character in the electronic structure. Originating in strong electron correlation, a hard upper limit for the singlet-triplet gap is found to emerge at around 30 kcal/mol for all the carbene classes in this chemical space. We also observe large vertical and adiabatic spin gap ranges within many carbene classes ($>$100 and $>$60 kcal/mol, respectively), and we report novel relationships between compositional, structural, and electronic degrees of freedom. Our QMspin data base includes numerical results for $\approx$13'000 MRCI calculations on randomly selected carbene scaffolds.