Abstract Optical metrology and high-resolution spectroscopy, despite impressive progress across diverse regions of the electromagnetic spectrum from ultraviolet to terahertz frequencies, are still severely limited in the region of vibrational bending modes from 13 to 20 µm. This long-wavelength part of the mid-infrared range remains largely unexplored due to the lack of tunable single-mode lasers. Here, we demonstrate bending modes frequency metrology in this region by employing a continuous-wave nonlinear laser source with tunability from 12.1 to 14.8 µm, optical power up to 110 µW, MHz-level linewidth and comb calibration. We assess several CO2-based frequency benchmarks with uncertainties down to 30 kHz and we provide an extensive study of the v11 band of benzene, a significant testbed for the resolution of the spectrometer. These achievements pave the way for long-wavelength infrared metrology, rotationally-resolved studies and astronomic observations of large molecules such as aromatic hydrocarbons.