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 P(4)over-bar21m crystal phase

In this study the geometrical structure and vibrational spectrum are studied for the urea molecule (CO(NH2)(2)) in the gas phase as well as in its P(4)over-bar2(1)m crystal phase. Experimental frequencies for urea in the crystal phase are determined for the CO(NH2)(2), (CO)-O-18(NH2)(2), CO((NH2)-N-15)(2),(CO)-C-13(NH2)(2) CO(ND2)(2) (CO)-O-18(ND2)(2), CO((ND2)-N-15)(2), and (CO)-C-13(ND2)(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.