Rotational and vibrational spectroscopy of a weakly bound hexafluoroisopropanol⋯dinitrogen complex: 14N hyperfine splittings, molecular geometry, and experimental benchmarks

Abstract

The rotational spectrum of a weakly bound binary complex of hexafluoroisopropanol (HFIP) with molecular nitrogen was measured using chirped-pulse and cavity-based Fourier transform microwave spectrometers. In addition, its infrared spectrum was measured in the OH stretching region. An extensive conformational search identified multiple binding sites on HFIP, with the global minimum structure featuring a trans-HFIP conformation and nitrogen weakly bound at the acidic proton (H_tN_H). Good agreement between the experimentally determined rotational constants and the relative intensity patterns of a-, b-, and c-type transitions with theoretical predictions conclusively identified the H_tN_H conformer. This assignment is further corroborated by an analysis of the ¹⁴N nuclear quadrupole hyperfine structure. The non-equivalence of the two ¹⁴N nuclei in H_tN_H is confirmed through a detailed molecular symmetry group analysis, as well as the ¹⁴N nuclear quadrupole hyperfine analysis. Examination of the experimental nuclear quadrupole coupling constants offers additional insights into the orientation and large-amplitude vibrational motions of the N₂ subunit. Furthermore, the experimentally derived rotational constants and the OH stretching band position of the complex, compared with previously known values for the isolated monomer, serve as complementary benchmarks for evaluating the systematic quality of predictions from electronic structure calculations across several levels of theory. This combined examination of vibrational energy levels and structural parameters aids in distinguishing fortuitously accurate predictions of individual properties.