Profiles in Chemistry: A Historical Perspective on the National Organic Symposium

This perspective delineates the history of the National Organic Chemistry Symposium (NOS) and, in doing so, traces the development of organic chemistry over the past 88 years. The NOS is the premier event sponsored by the ACS Division of Organic Chemistry (ORGN) and has been held in odd-numbered years since 1925, with the exceptions of 1943 and 1945. During the 42 symposia, 332 chemists have given 549 plenary lectures. The role the NOS played in the launch of <i>The</i> <i>Journal of Organic Chemistry</i> and <i>Organic Reactions</i> and the initiation of the Roger Adams Award are discussed. Representative examples highlighting the chemistry presented in each era are described, and the evolution of the field is examined by assigning each NOS talk to one of seven subdisciplines and analyzing how the number of talks in each subdiscipline has changed over time. Comparisons of the demographics of speakers, attendees, and ORGN members are made, and superlatives are noted. Personal interest stories of the speakers are discussed, along with the relationships among them, especially their academic lineage. Logistical aspects of the NOS and their historical trends are reviewed. Finally, the human side of science is examined, where over the past century, the NOS has been intertwined with some of the most heated debates in organic chemistry. Conflicts and controversies involving free radicals, reaction mechanisms, and nonclassical carbocations are discussed

Modulating Accidental Fermi Resonance: What a Difference a Neutron Makes

Vibrational reporters have shown significant promise as sensitive probes of local environments in proteins and nucleic acids. The utility of two potential vibrational probes, the cyanate and azide groups in phenyl cyanate and 3-azidopyridine, respectively, has been hindered by accidental Fermi resonance. Anharmonic coupling, between the fundamental −OCN or −N₃ asymmetric stretch vibration and a near resonant combination band, results in an extremely broad and complex absorption profile for each of these probes. A total of eight phenyl cyanate and six 3-azidopyridine isotopomers were synthesized and studied. Isotopic editing effectively modulated the accidental Fermi resonance; the absorption profiles of several isotopomers were greatly simplified, whereas others remained complex. The origins of the observed profiles are discussed. The addition of a single neutron to the middle atom of the oscillator converted the absorption profile to essentially a single band, resulting from either the cyanate or azide asymmetric stretch vibration

Synthesis and evaluation of the sensitivity and vibrational lifetimes of thiocyanate and selenocyanate infrared reporters

Two novel 2'-deoxyadenosine (dA) analogues, Si-2-dA-SCN and Si-2-dA-SeCN, and two novel phenylalanine (Phe) analogues, Boc-Me-PheCH(2)SCN and Boc-Me-PheCH(2)SeCN, have been synthesized and the thiocyanate (SCN) and selenocyanate (SeCN) functional groups evaluated as vibrational reporters. The syntheses of Si-2-dA-SCN and Si-2-dA-SeCN were accomplished in three steps in 16% and 32% overall yields, respectively, and the syntheses of Boc-Me-PheCH(2)SCN and Boc-MePheCH(2)SeCN were completed in four steps in 8.9% and 2.3% overall yields, respectively. The SCN and SeCN stretch vibrational modes were shown to be sensitive to the local environment by frequency shifts and full-width half-maximum (fwhm) changes in response to tetrahydrofuran (THF) and THF/water solvent mixtures. The vibrational lifetimes of the Si-2-dA-SeCN (237 +/- 12 ps) and Boc-Me-PheCH(2)SeCN (295 +/- 31 ps) in THF solution were determined by ultrafast infrared pump-probe spectroscopy to be 1.5 to 3 times longer than those for Si-2-dA-SCN (140 +/- 6 ps) and Boc-Me-PheCH(2)SCN (102 +/- 4 ps). The longer lifetimes for the SeCN analogues were attributed to the better insulating effects of the heavier selenium atom compared to the sulfur atom. The solvent sensitivity and longer vibrational lifetimes compared to other vibrational reporters suggest that SCN and SeCN vibrational reporters are well suited to studying several dynamic processes including protein and nucleic acid hydration and conformational changes, however stability issues may require post-synthetic modification methods to incorporate these reporters into biomacromolecules

Two-Dimensional Infrared Study of Vibrational Coupling between Azide and Nitrile Reporters in a RNA Nucleoside

The vibrations in the azide, N₃, asymmetric stretching region and nitrile, CN, symmetric stretching region of 2′-azido-5-cyano-2′-deoxyuridine (N₃CNdU) are examined by two-dimensional infrared (2D IR) spectroscopy. At earlier waiting times, the 2D IR spectrum shows the presence of both vibrational transitions along the diagonal and off-diagonal cross peaks indicating vibrational coupling. The coupling strength is determined from the off-diagonal anharmonicity to be 66 cm^–1 for the intramolecular distance of ∼7.9 Å, based on a structural map generated for this model system. In addition, the frequency–frequency correlation decay is detected, monitoring the solvent dynamics around each individual probe position. Overall, these vibrational reporters can be utilized in tandem to simultaneously track global structural information and fast structural fluctuations