A reaction surface Hamiltonian study of malonaldehyde

Area of Science:

• * Theoretical Chemistry
• * Molecular Dynamics
• * Spectroscopy

Background:

• * Accurate modeling of molecular vibrations and large amplitude motions is crucial for understanding chemical reactions.
• * Tunneling dynamics, particularly proton transfer, present significant challenges in computational chemistry.
• * Previous studies on malonaldehyde have provided benchmarks for theoretical calculations.

Purpose of the Study:

• * To develop and apply a reaction surface Hamiltonian for calculating vibrational energy levels and tunneling splittings.
• * To investigate the internal hydrogen transfer dynamics in malonaldehyde.
• * To re-evaluate the assignment of the OH stretching fundamental in malonaldehyde's infrared spectrum.

Main Methods:

• * Derivation of the kinetic energy operator in a coordinate system including normal coordinates (Q) and large amplitude motions (s(1), s(2)).
• * Representation of the potential energy surface using quadratic and anharmonic terms.
• * Variational solution of secular equations using a basis of Hermite polynomials and functions of s(1), s(2).

Main Results:

• * Calculations of zero-point tunneling splitting for malonaldehyde show good agreement with previous theoretical results.
• * The model successfully captures the tunneling dynamics inherent in malonaldehyde's internal hydrogen transfer.
• * The computed OH stretching fundamental appears at a significantly lower frequency than experimentally assigned, suggesting a potential misassignment.

Conclusions:

• * The reaction surface Hamiltonian provides a robust framework for studying tunneling dynamics in molecules like malonaldehyde.
• * The study challenges the current interpretation of malonaldehyde's infrared spectrum, specifically the OH stretching fundamental.
• * The calculated low frequency for the OH stretch indicates substantial transition state character, necessitating a re-evaluation of spectral assignments.