The Lowest Vibrational States Of Urea From The Rotational Spectrum

Abstract

The urea molecule, (NH$_2$)$_2$CO, has a complex potential energy surface

resulting from a combination of the NH$_2$ torsion and NH$_2$ inversion motions.

This leads to a distribution of lowest vibrational states that is expected to be significantly

different from the more familiar picture from simple inversion or normal

mode models.\footnote{P.D.Godfrey, R.D.Brown, A.N.Hunter {\it J.~Mol.~Struct.},

{\bf 413-414}, 405-414 (1997).}$^,$\footnote{N.Inostroza, M.L.Senent,

{\it Chem.~Phys.~Lett.}, {\bf 524}, 25 (2012).}

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The broadband 207-500 GHz spectrum of urea recorded in Dayton has

signal to noise sufficient for assignment of rotational transitions in

excited vibrational states up to at least 500 cm$^{-1}$. In addition to

the previously reported analysis of the ground and the lowest excited

state we have been able to assign transitions in at least five other

excited vibrational states. Strongly perturbed

transitions in a close doublet of such states have been fitted to within experimental

accuracy with a coupled fit and a splitting in the region of 1 cm$^{-1}$.

These assignments combined with vibrational energy estimates from relative intensity measurements allow

for empirical discrimination between different models for the energy level manifestation of the large amplitude motions in urea.$^b$