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-BAR2(1)M CRYSTAL PHASE

In this study the geometrical structure and vibrational spectrum are s tudied 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 u rea 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 calcu lations at the RHF/6-31++G* level, the equilibrium geometry and harmo nic force field for both the gas phase and the crystal phase are deter mined. The crystal phase is modeled using a 15 molecule cluster surrou nded by 5312 point charges. Calculated structure and frequencies are i n excellent agreement with experiment. Shifts in frequencies for the a bove-mentioned isotopomers are reproduced with a root mean square devi ation and largest difference of 15 and 43 cm(-1), respectively. Shifts in frequencies in going from the gas phase to the crystal phase for t he parent and deuterated isotopomer are reproduced with a root mean sq uare deviation and largest difference of 14 and 31 cm(-1), respectivel y.