Vibrational Spectra of Lumazine in Water at pH 2−13: Ab Initio Calculation and FTIR/Raman Spectra

Abstract

Harmonic vibrational frequencies and transition intensities of lumazine (2,4-(1H,3H)pteridinedione in the neutral state) have been calculated in a self-consistent reaction field of high dielectric medium (ε = 78.54) using ab initio Hartree−Fock (HF) and hybrid HF/density functional theory (DFT) methods. For the DFT method, the 4-31G basis set and the three parameter exchange functional of Becke combined with the Lee, Yang, and Parr correlational functional are used. The 6-31+G* basis set is used in the HF calculations. Both the spherical cavity model (SCM) and the self-consistent isodensity polarizable continuum model (SCIPCM) are used to simulate an aqueous environment for the N-protonated lumazines. For the N-deuterated lumazines, only the SCM is used. Simple scaling of the vibrational frequencies resulting from the DFT calculations incorporating the reaction field models of neutral lumazine, lumazine A1/N3-H monoanion, and lumazine A1,A3-dianion compare closely with Raman and Fourier transform infrared spectra of lumazine taken in aqueous media (both H2O and D2O) over a wide pH/pD range. The mean deviation between calculated and experimental vibrational frequencies is 9.10 cm-1 for neutral lumazine (9.37 cm-1 in D2O), 9.18 cm-1 for the monoanion (10.77 cm-1 in D2O), and 16.06 cm-1 for the dianion (16.00 cm-1 when prepared in D2O). Calculated vibrational energy shifts with changes in ionization exhibit many trends that are evident in the experimental data although the absolute magnitudes of many of the individual shifts do not agree exactly. The calculated H/D isotopic shifts for the neutral and monoanionic species of lumazine agree well with those seen experimentally. Correlating shifts with ionization and the H/D isotopic shifts of the vibrational modes of each species of lumazine investigated have resulted in normal mode assignments of all in-plane vibrational modes observed in the 300−1750 cm-1 range in aqueous solution.