Solids Modeled by Ab-Initio Crystal Field Methods. Part 17. Study of the Structure and Vibrational Spectrum of Urea in the Gas Phase and in Its P421m Crystal Phase

In this study the geometrical structure and vibrational spectrum are studied for the urea molecule (CO(NH 2 ) 2 ) in the gas phase as well as in its P 4̄2 1 m crystal phase. Experimental frequencies for urea in the crystal phase are determined for the CO(NH 2 ) 2, C 18 O(NH 2 ) 2, CO( 15 NH 2 ) 2, 13 CO(NH 2 ) 2, CO(ND 2 ) 2, C 18 O(ND 2 ) 2, CO( 15 ND 2 ) 2, and 13 CO(ND 2 ) 2 isotopomers. Using calculations at the RHF/6-31++G** level, the equilibrium geometry and harmonic force field for both the gas phase and the crystal phase are determined. The crystal phase is modeled using a 15 molecule cluster surrounded by 5312 point charges. Calculated structure and frequencies are in excellent agreement with experiment. Shifts in frequencies for the above-mentioned isotopomers are reproduced with a root mean square deviation and largest difference of 15 and 43 cm -1, respectively. Shifts in frequencies in going from the gas phase to the crystal phase for the parent and deuterated isotopomer are reproduced with a root mean square deviation and largest difference of 14 and 31 cm -1, respectively.