Structures of guaiacol and the guaiacol-argon van der waals complex from rotational spectroscopy of guaiacol isotopologues

•First assigned rotational spectra for guaiacol and eight isotopologues.•First assigned rotational spectrum for the guaiacol-argon van der Waals complex.•Detailed experimental guaiacol monomer structure from Kraitchman analysis.•Precise location of the argon in the guaiacol-argon complex. High-resolution rotational spectra were recorded for guaiacol and eight isotopologues as well as the guaiacol-argon van der Waals complex using a resonant-cavity Fourier transform microwave spectrometer. The monomer spectra were assigned to an anti-syn conformation with an internal hydrogen bond from the hydroxyl group to the methoxy oxygen. Atomic coordinates of the carbon and hydroxyl hydrogen atoms were determined from the experimental moments of inertia using the Kraitchman method, and these coordinates are in excellent agreement with the corresponding coordinates in the MP2/6-311++G(d,p) model structure. The recorded rotational transitions showed no evidence for methyl internal rotation tunneling, and a potential energy scan (ωB97XD/6-311++G(d,p)) identified a 1006.0 cm−1 barrier to methyl internal rotation. The rotational spectrum of guaiacol-argon was used determine the location of the argon with respect to the anti-syn monomer conformation: RCM = 3.533 Å, θ = 105.4 °, and χ = -100.0 °. The argon is positioned towards the hydroxyl and methoxy groups because of relatively strong van der Waals interactions between argon and the oxygen atoms. [Display omitted]