EVV 2DIR spectrum of a non-planar G-quartet. Four combinations of guanine carbonyl stretch modes at ~1700 cm -1 exist in a G-quartet. The guanine carbonyl stretch mode giving rise to the highest intensity couplings along the row of cross-peaks is the in-phase guanine carbonyl stretch mode at 1694 cm -1 as depicted in (f). Two degenerate out-of-phase guanine carbonyl stretch modes at 1704 cm -1 , as depicted in (g) and (h), give rise to the highest intensity couplings along the column of cross-peaks. b, Computational EVV 2DIR spectrum of a G-quadruplex. There are 12 combinations of guanine carbonyl stretches in a G4. The guanine carbonyl stretch mode giving rise to the highest intensity couplings along the row of cross-peaks at 1692 cm -1 involves the same motions as the 1694 cm -1 G-quartet mode, as seen in (f), across all three quartets of the G4. Similarly, two degenerate guanine carbonyl stretch modes give rise to the highest intensity couplings along the column of cross-peaks at 1679 cm -1 . They involve the same motions as the 1704 cm -1 G-quartet modes, as shown in (g) and (h), but across all three quartets in the G4. Animations of the G4 vibrational modes at 1692 cm -1 and 1679 cm -1 can be found in the Supplementary Information.