Negative Ion Photoelectron Spectroscopy Studies of Organic Reactive Intermediates

Nearly 100 years ago, Moses Gomberg introduced the scientific community to the study of organic radicals,1 concluding this classic manuscript with the comment that, “This work will be continued and I wish to reserve the field for myself.” Gomberg’s studies of triphenylmethyl radical were only the beginning of intense research into the properties of the newly discovered trivalent carbon, and despite Gomberg’s proclamation, work was carried out by scientists all over the world. By the middle of the century, new experimental techniques such as matrix isolation2 and flash photolysis3 allowed for the direct spectroscopic study of these transient species. Armed with this array of new tools, in 1964 Porter and Ward politely requested,4 in reference to Gomberg, that “after the respectable period of more than half of a century, we may perhaps be permitted to look further into the problem”. With this comment, they ushered in the modern era of organic radical spectroscopy. Since that time, spectroscopic studies using techniques, including infrared, visible and ultraviolet absorption spectroscopy, Raman spectroscopy, and electron spin resonance spectroscopy, have lead to a much better understanding of the physical properties of many reactive intermediates, including carbenes and radicals. Over the past decade, we have applied the technique of negative ion photoelectron spectroscopy (NIPES) to such systems. In this gas-phase experiment, a beam of mass-selected negative ions is intersected with an intense ultraviolet laser beam, resulting in photodetachment of an electron (Figure 1). A portion of the photodetached electrons are collected and energy-analyzed, giving the photoelectron spectrum, which is a plot of the number of photoelectrons detected as a function of electron kinetic energy (eKE). It is frequently useful to plot the number of electrons versus electron binding energy (eBE), the difference between the laser energy and eKE. The advantage of such a plot is that the positions of features observed in the spectrum are not dependent on the energy of the laser used in the experiment. The information that can be obtained from negative ion photoelectron spectroscopy studies is multifold. Accurate values of vibrational frequencies for the isolated, gas-phase molecule can be determined from vibrational progressions observed in the spectra. Moreover, vibrational frequencies for negative ions can often be obtained from the positions of “hot bands”, i.e., transitions from excited vibrational states of the ion. However, the most important information obtained from photoelectron spectra often turns out to be the electron affinity and electronic state term energies for the neutral intermediate. Electron affinities are of particular value in calculating bond dissociation energies, as illustrated in the thermochemical cycle shown in eq 1. If the gas-phase acidity of a molecule