Ab initio study of the molecular structure, polarizability and first hyperpolarizability of 6-hydroxy-1-formylfulvene

Abstract

Ab initio calculations have been carried out on 6-hydroxy-1-formylfulvene(HFF) to study (a) the ground-state molecular structure and relative stability of conformers, (b) the strength and main features of the intramolecular hydrogen bond, including harmonic vibrational frequencies and proton tunnelling, (c) hydrogen-bond effects on linear and quadratic polarizabilities, (d) The role of electron correlation (at the MP2 level) on the computed quantites. The results show that the intramolecularly hydrogen-bonded C_s structure is the most stable form of HFF, the Barrier height for hydrogen transfer and the hydrogen-bond energy being 16.8 (MP3/6-31G**//MP2/6-31G**) and 66–78 Kj mol^–1, respectively. The electron correlation and H bonding greatly affect the vibrational frequencies and intensities, ν_OH being lowered by 932 and 1074 cm^–1, respectively, while on H-bond formation, δ_OH and γ_OH are raised by 267 and 665 mc^–1, respectively. A one-dimensional (1D) model for H-transfer tunnelling leads to a tunnelling splitting of 142 cm^–1, and a proton tunnelling time of 0.12 × 10^–12 s, in good agreement with the experimental microwave data. The overall magnitude of the calculated hyperpolarizability, It, of HFF is comparable to that of nitrobenzene. It is Strongly dependent on electron correlation, increasing by a factor of even more that three with respect to the SCF results. The intramolecular H bond favours an optical response within the chelate Structure, but its effect is overwhelmed by enhancement of the mono-dimensionality of the system, following OH group rotation. Cl-Singles calculations show that in the lowest five excited states, dipole moments are essentially in the same direction as in the ground state.