Theoretical and Experimental Dipole Moments of Purines

As a follow-up on our previous study of a series of purines (purine, 6-chloropurine, purine-6-thiol, hypoxanthine, theobromine, theophylline, caffeine, and uric acid), we have investigated six additional biologically important purines (adenine, guanine, isoguanine, thioguanine, xanthine, and kinetin). Their ground-state dipole moments were measured in dioxane at 293 K. The first excited singlet-state dipole moments were obtained using the solvatochromic shift equations (McRae, Suppan, Bakhshiev, and Kawski-Chamma-Viallet). The theoretical dipole moments were calculated as a combination of the π-moment (PPP method) and the σ-moment (vector sum of the σ-bond and σ-group moments). The same approach was used to obtain their first excited singlet-state dipole moments (excited state π-moment; σ-moment assumed to be the same as in the ground state). Ab initio HF 6-31G** calculations were also used to obtain ground-state dipole moments for all the fourteen purines under study. In addition, a DFT/B3PW91/6311++(2df,2p) calculation has been carried out for purine for comparison. The different sets of theoretical dipole moments were compared with the respective experimental values. There is an approximately equally good agreement among the experimental dipole moments and the PPP + σ dipole moments (±6.9%) and the ab initio dipole moments (±7.4%). The effect of structure on the dipole moments is discussed.