Theoretical study on the static and dynamic first-order hyperpolarisabilities of adenine tautomers

Static and dynamic electronic and vibrational first-order hyperpolarisabilities (β) of the lowest energy neutral adenine tautomers (amine forms A7 and A9) were obtained in gaseous and aqueous phases by using Hartree–Fock, Møller–Plesset second-order and fourth-order perturbation theory (MP2 and MP4-SDQ) and conventional and long-range corrected density functional theory methods with the Dunning's correlation-consistent cc-pVDZ, aug-cc-pVDZ, aug-cc-pVTZ and d-aug-cc-pVDZ basis sets. Frequency-dependent properties were calculated at the characteristic wavelength of the Nd:YAG laser (1064 nm) for the second harmonic generation and electro-optical Pockels effect nonlinear optical processes. Solvent effects were introduced under the polarised continuum model approximation. The electronic β^e values of the investigated isomers are noticeably affected by the theoretical level, basis set and solvation. In vacuum, the static and dynamic β^e values of A9 are greater than the corresponding data of A7, whereas the contribution of the solvent significantly enhances the hyperpolarisabilities of the A7 tautomer, resulting in β^e(A9)/β^e(A7) ratios between 0.5 and 0.6. The vibrational hyperpolarisabilities of the adenine tautomers are quite close to each other.