Shapes of Benzyl Benzoate: A Rotational Spectroscopic and Computational Study

To gain molecular-level insights into how weak intramolecular noncovalent interactions govern the conformational preference of large semivolatile organic compounds, benzyl benzoate (BnBz), an ester with terminal benzyl and phenyl groups, was investigated using chirped-pulse Fourier transform microwave spectroscopy and quantum chemical calculations. Systematic conformational searches followed by DFT calculations identified four BnBz conformers: two low-energy species with a planar benzoate (C₆H₅-COO) motif and two significantly higher-energy conformers with a nonplanar benzoate moiety. The rotational spectra of the two most stable conformers, BnBz-<i>g</i> and BnBz-<i>t</i>, along with 16 of their ¹³C isotopologues, were observed and assigned. The DFT-predicted stability ordering of these two conformers is reversed when based on zero-point-corrected energies and free energies. Experimental results confirm BnBz-<i>g</i> as the global minimum, resolving the theoretical ambiguity and underscoring the importance of benchmarking computational predictions with conformer-specific data. The conformational conversion barrier was investigated both experimentally using helium, neon, and argon as carrier gases, and computationally, revealing a low barrier height of less than 5 kJ mol^-1. Noncovalent interactions analyses indicate that multiple CH···O hydrogen bonds govern the structural preferences of the low-energy conformers, while additional model calculations show that the π-π stacking motif becomes more prominent with the addition of further bridging methylene groups.