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The guanine functionalization reaction uses singlet oxygen to covalently link single-wall carbon nanotubes to guanine bases in their ssDNA coatings. This creates shallow but densely spaced exciton traps that modulate nanotube band gaps with energetic and spatial control, giving red-shifted electronic transitions. To better understand guanine functionalization, we used quantum chemical computations to compare the stabilities of several candidate addends in multiple orientations on the nanotube surface. Structures of three possible isomers of guanine peroxide (GPO), the reactive intermediate formed through reaction of 9-methyl guanine with singlet O2, were optimized using the semi-empirical PM3 method. To examine effects of nanotube diameter on adduct stability, we then computed the enthalpy changes for bonding of each GPO isomer to a 6 nm segment of (5,4), (6,5), (7,6) and (8,7) SWCNT. Six orientations of the addend on the SWCNT surface were considered for each (n,m) species, giving a total of 72 adduct structures. The results showed that for all four SWCNTs, the most energetically stable adduct is the 4,5-GPO isomer bonded in the ortho L-30 orientation. This adduct can be considered a derivative of 1,4-dioxane. Subsequent ab initio DFT and TDDFT computations comparing bonding orientations of one guanine addend on a 12 nm long SWCNT segment found that ortho L -30 gives a slightly reduced HOMO-LUMO gap, a mildly localized exciton structure, and a slightly red-shifted E11 optical transition as compared to the pristine SWCNT, in agreement with experiment. We conclude that guanine functionalization of near-armchair SWCNTs leads mainly to 4,5-GPO addends bonded in the ortho L -30 orientation.