Conformational landscape and free-jet millimeter-wave spectrum of eugenol: The role of dispersion and relaxation dynamics

The rotational spectrum of eugenol (2-methoxy-4-(prop-2-en-1-yl)phenol) has been investigated using a free-jet absorption Stark-modulated spectrometer in the 59.6-74.4 GHz frequency range. Conformational analysis at the B3LYP-D3(BJ)/def2-TZVP level identified twelve non-equivalent minima, with three low-energy conformers stabilized by an internal hydrogen bond. These species have been unambiguously assigned in the experimental spectrum: the global minimum HB-E1 ( A =1427.0280(1), B =628.66978(9), C =450.45805(8) MHz); the folded form HB-F ( A =1427.988(3), B =644.49(2), C =503.69(3) MHz); and the second extended form HB-E2 ( A =1775.238(6), B =526.33(7), C =417.32(7) MHz). A global fit for HB-E1, incorporating previously reported microwave data (3–18 GHz), yields a highly precise set of rotational and quartic centrifugal distortion constants, resolving prior ambiguities in structural assignment. The detection of the HB-E2 species was found to be strictly carrier-gas dependent. Its absence in argon expansions, contrasted with its presence in helium, evidences a conformational relaxation pathway to the global minimum facilitated by a low interconversion barrier ( ≃ 6 kJ mol −1 ) that is overcome during the efficient collisional cooling with argon. • Millimeter-wave spectrum (59.6–74.4 GHz) reveals three hydrogen-bonded conformers. • Conformational landscape mapped at the B3LYP-D3(BJ)/def2-TZVP level. • Dispersion forces stabilize the folded form via CH ⋯ π interactions. • Fit of global minimum combines new MMW data with prior microwave measurements. • Absence of third conformer in Ar jet provides evidence of collisional relaxation.