Structure, Anharmonic Vibrational Frequencies, and Intensities of NNHNN(+)

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• The NNHNN(+) cation is a molecule of interest for its unique proton transfer dynamics.
• Accurate theoretical models are crucial for understanding its spectroscopic properties.

Purpose of the Study:

• To develop a semiglobal potential energy surface (PES) and quartic force field (QFF) for NNHNN(+).
• To accurately calculate the vibrational and spectroscopic properties of NNHNN(+).

Main Methods:

• High-level electronic structure calculations were used to fit the PES and QFF.
• Vibrational second-order perturbation theory (VPT2) and vibrational self-consistent field/virtual state configuration interaction (VSCF/VCI) methods were employed.
• MULTIMODE calculations were performed to analyze vibrational modes and intensities.

Main Results:

• The equilibrium structure of NNHNN(+) is linear with D(∞h) symmetry.
• VSCF/VCI calculations successfully described the proton "rattle" motion, predicting a fundamental frequency of 759 cm⁻¹.
• This calculated frequency shows excellent agreement with experimental and diffusion Monte Carlo results (743-746 cm⁻¹).

Conclusions:

• The developed PES and QFF provide an accurate description of NNHNN(+) structures and spectroscopy.
• Advanced computational methods like VSCF/VCI are essential for capturing complex vibrational motions.
• The strong intensity of the proton-transfer fundamental suggests its significance in experimental spectra.