Vibrational Analysis of Urea

TLDR

Despite extensive work, contradictions remain in urea vibrational analysis due to interpreting crystalline spectra with isolated‑urea force fields rather than crystal‑phase models. The authors measured solid‑state IR and Raman spectra of eight urea isotopomers at various temperatures and a urea–water solution, and performed Hartree–Fock 6‑31++G** force‑field calculations on isolated and crystalline urea. They demonstrated that using isolated‑urea force fields is invalid for urea and that high‑pressure diamond‑anvil‑cell measurements provide no useful vibrational information.

Abstract

Although a lot of work has been done on the vibrational analysis of urea, there still remain some contradictions and uncertainties, mainly due to interpretation of the vibrational spectrum of crystalline urea based on force field calculations on isolated urea instead of on urea in its crystal phase. We have shown that this approach is not allowed in the case of urea. The vibrational spectrum of urea was interpreted by measuring the solid state infrared spectra of eight isotopomers of urea at room temperature and at −196 °C, and Raman spectra at room temperature and at −120 °C, and of a urea−water solution. Force field calculations from our recently published article on isolated urea and on urea in its crystal structure, at the Hartree−Fock level with a 6-31++G** basis set, were also used. We have also shown that high-pressure measurements using a "diamond anvil cell" are not useful by performing a vibrational analysis.