The experimentally determined red-shifts in the guanine carbonyl stretch frequencies caused by the addition of potassium ions are shown in Figures . The red-shift in the GCO I and GCO II frequencies are approximately reproduced in the calculated difference spectra between a G-quartet (Figure ) and a G4 (Figure ). Experimentally, the average frequency shifts in GCO I and GCO II are -8 cm -1 and -10 cm -1 , whilst the calculated frequency shifts are -2 cm -1 and -25 cm -1 , respectively. Therefore, our calculations, keeping in mind that they are based on harmonic vibrational wavefunctions, qualitatively agree with experimental observations, and suggest that potassium ion complexation causes a red shift overall. The mechanism of the red-shift of the guanine carbonyl stretch frequencies can be attributed to the ion coordination to the carbonyl oxygen and the stacking of G-quartets. The ion coordination results in the lengthening of the carbonyl bonds from 1.23 Å to 1.25 Å. Moreover, the resultant stacking of G-quartets, results in the delocalisation of the guanine carbonyl modes found in the G-quartet at 1704 cm -1 (Figure ) across all three quartets in the equivalent mode in the G4 found at 1679 cm -1 .