β‐Carboline (norharman)

The structure of β‐carboline, also called norharman (systematic name: 9H‐pyrido[3,4‐b]indole), C₁₁H₈N₂, has been determined at 110 K. Norharman is prevalent in the environment and the human body and is of wide biological interest. The structure exhibits intermolecular N—H...N hydrogen bonding, which results in a one‐dimensional herringbone motif. The three rings of the norharman molecule collectively result in a C‐shaped curvature of 3.19 (13)° parallel to the long axis. The diffraction data show shorter pyridyl C—C bonds than those reported at the STO‐3G level of theory.     

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

This perspective describes research, carried out in the authors' labs over the past forty years, aimed at understanding, predicting, and measuring the singlet-triplet energy differences (ΔE(ST)) in diradicals. A theory for qualitatively predicting the ground states of diradicals and the use of Negative Ion Photoelectron Spectroscopy (NIPES) for measuring ΔE(ST) are described. The application of this theory, ab initio calculations, and NIPES to the prediction and measurement of ΔE(ST) in a wide variety of organic diradicals is detailed. Among the diradicals that are discussed in this perspective are HN, CH(3)N, PhN, CH(2), trimethylenemethane (TMM), oxyallyl (OXA), meta-benzoquinodimethane (MBQDM), meta-benzoquinone (MBQ), tetramethyleneethane (TME), 1,2,4,5-tetramethylenebenzene (TMB), and D(8 h) cyclooctatetraene (COT). All of these diradicals have been studied in one and, in most cases, in both of the authors' laboratories. The studies of OXA and D(8h) COT were, in fact, collaborations between the research groups of the authors. These two projects both took advantage of the ability of NIPES to provide information about transition states. Transition-state spectroscopy was used to measure the carbonyl stretching frequency in the singlet state of OXA and to establish that D(8h) COT violates the strictest version of Hund's rule.