John D. Roberts, an organic chemist who pioneered the use of nuclear magnetic resonance spectroscopy and other techniques to reveal the structures of molecules and the dance of atoms as they rearrange in chemical reactions, died on Oct. 29 at his home in Pasadena, Calif. He was 98.

The cause was a stroke, his daughter, Anne, said.

In the 1950s, Dr. Roberts played a crucial role in the explosive growth of physical organic chemistry, a field that studies the reactivity of biological compounds. One notable contribution was in popularizing nuclear magnetic resonance spectroscopy, which was developed by physicists, to understand chemical structures and reaction mechanisms.

“This was a real revolution for organic chemists at the time,” said Peter B. Dervan, a professor of chemistry at Caltech, where Dr. Roberts worked for more than 60 years. “Jack saw the potential use of this spectroscopy and mastered it, and then, by publishing papers in this area, convinced other organic chemists that this was a powerful tool.”

Dr. Roberts was a leader in understanding how organic reactions occurred, and he shared his techniques with the scientific community.