Document Type : Original Article

Authors

• Sima Esmailpour ¹
• Hossein Nikoofard ²
• Mohsen Sargolzaei ²

¹ Department of Physical Chemistry, Faculty of Chemistry, Shahrood University of Technology, Shahrood, Iran

² Faculty of Chemistry, Shahrood University of Technology, Shahrood, Iran

Abstract

This study investigates the tautomerization process of hypoxanthine using the B3LYP/6-31G(d) theoretical approach. The energy profiles derived from calculations are supplemented by kinetic rate coefficient analyses, employing transition state theory (TST). The results demonstrate that both tautomeric forms of hypoxanthine are planar, with HX1 as the more stable isomer under the studied conditions, exhibiting spontaneous and exothermic tautomerization. Kinetic analyses reveal an increased rate constant for the lactam form's formation, enhanced by tunneling effects. Transition state calculations indicate a lower energy barrier in the gas phase compared to aqueous conditions, leading to predominantly slow tautomerization rates in the absence of catalysts. Notably, both tautomers exhibit promising nonlinear optical (NLO) properties, characterized by higher hyperpolarizability relative to para-nitroaniline (pNA), even though they display lower polarizability values. Their greater dipole moments suggest potential effectiveness in NLO applications. Natural bond orbital (NBO) analysis reveals significant differences in second-order energy perturbation between HX1 and HX2, with HX1 showing reduced electron transfer from σ and π* orbitals. Additionally, the proximity of the hydroxyl group in HX2 increases perturbation energy and electron transfer, potentially contributing to the observed elongation of the C6-O10 bond length in HX1. Collectively, these tautomers represent promising candidates for further exploration in nonlinear optics.

Keywords

• Hypoxanthine
• Hydrogen Shift
• Kinetics
• Tautomerism
• Tunneling Effect
• Wigner