Conformational landscape and tunneling dynamics in 3-fluorobenzylamine: a rotational spectroscopy and computational study

The conformational landscape and intramolecular dynamics of 3-fluorobenzylamine were investigated by high-resolution rotational spectroscopy combined with quantum-chemical calculations. Broadband, cavity-based Fourier transform and free-jet microwave measurements allowed the identification of three conformers in the supersonic jet and the accurate determination of rotational constants, 14 N quadrupole coupling parameters, and tunneling splittings. Isotopic substitution in natural abundance and selective deuteration supported the structural assignment and enabled the derivation of substitution and effective geometries. The tunneling splittings were analyzed using a two-dimensional flexible model treating the coupled CCN(lp) and CCCN torsions within a discrete variable representation framework. The model successfully reproduces the experimental energy splittings and provides a detailed description of the underlying potential energy surface. Comparison with benzylamine and 2-fluorobenzylamine highlights the role of substitution position in modulating conformational stability and tunneling dynamics, demonstrating the sensitivity of rotational spectroscopy to subtle electronic effects in flexible fluoroaromatic systems. • Three conformers of 3FBA identified by rotational spectroscopy. • Meta-fluorination subtly modulates conformational stability. • 2D model reproduces tunneling splittings and PES features. • Fluorine position controls dynamics in benzylamine derivatives.