From Norbornadiene to Norcamphor and Camphor: Reduced-Cost Semiexperimental Structural Refinement from Limited Isotopologue Data

We present an efficient semiexperimental protocol for determining spectroscopically accurate molecular structures from limited isotopologue data with a focus on medium-sized organic molecules. The availability of all monosubstituted isotopologues of norbornadiene enabled the determination of a complete semiexperimental (<i>r</i>_eq^SE) equilibrium structure, establishing a reference for validating reduced-dimensionality approaches that avoid deuterium substitution. The rotational spectrum of norcamphor is reported here for the first time, providing a critical benchmark for assessing the method's accuracy. By combining composite quantum-chemical calculations with a cost-effective vibrational correction scheme, the protocol achieves near-spectroscopic accuracy while substantially reducing the computational effort. This approach enables the structural characterization of large systems where extensive isotopic substitution is impractical, thereby broadening the applicability of semiexperimental methods in modern molecular spectroscopy.