Quantum mechanical and experimental infrared and Raman studies of 1-methyluracil and its hydrogen-bonded dimer†

Density functional theory at the B3LYP/6-31G(d,p) level has been used to calculate the structure, force field, and vibrational [infrared (IR) and Raman] spectra of the 1-methyluracil monomer and its dimer with the cyclic N3H … OC4 hydrogen bonds. The experimental IR spectrum of the 1-methyluracil monomer isolated in a low-temperature Ar matrix and the IR and Raman spectra of its polycrystalline solid have been reinvestigated. The cyclic hydrogen bond for the calculated dimer is the same as that in the 1-methyluracil crystal. It appears that calculation at this level of theory does provide IR and Raman spectra closely resembling the corresponding experimental spectra. We were amazed to find that the agreement between the calculated spectrum of the dimer and the experimental spectrum of the crystalline solid is not worse than the agreement between the calculated spectrum of the monomer and the experimental spectrum of 1-methyluracil isolated in an Ar matrix. The results of calculation appear to be useful in the interpretation of the experimental spectra of the monomer and crystal. The close agreement found between the calculated and experimental spectra suggests that force constants and geometries of the monomer and dimer are also predicted well by the calculation at this level. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002