Millimeter-wave spectroscopy of HDC=CH

Area of Science:

• Molecular Physics
• Spectroscopy
• Quantum Chemistry

Background:

• Understanding the structure and dynamics of small organic radicals is crucial for astrochemistry and reaction mechanisms.
• Isotopic substitution, like deuteration, provides sensitive probes for molecular structure and vibrational dynamics.

Purpose of the Study:

• To investigate the rotational transitions and molecular structure of the mono(β)-deuterated vinyl radical (HDC=CH).
• To determine the isomeric form and potential energy surface of HDC=CH using high-resolution spectroscopy and computational methods.

Main Methods:

• Production of HDC=CH via ArF excimer laser photolysis in a supersonic jet expansion.
• Observation of rotational transitions using millimeter-wave spectroscopy.
• Ab initio calculations at the CCSD(T)/cc-pVTZ level to determine relative energies and structures.

Main Results:

• Observed b-type and weak a-type rotational transitions for the lower tunneling doublet component.
• Determined molecular constants (A rotational constant, Fermi contact, magnetic dipolar interactions) indicating the trans-isomer.
• Calculations confirmed the trans-form is energetically favored over the cis-form by 30.04 cm^-1 due to zero-point energy differences.

Conclusions:

• The observed spectrum is assigned to the trans-isomer of HDC=CH.
• Tunneling components in the ground state effectively act as distinct isomers in a double minimum potential.
• Observed hyperfine constants are consistent with those of the vinyl radical (H₂C=CH).