2016. 57, 1 UDC 548.737:543.422 VIBRATIONAL ANALYSIS, CONFORMATIONAL STABILITY, FORCE CONSTANTS, INTERNAL ROTATION BARRIERS, MP2=FULL AND DFT CALCULATIONS OF 1,3-DIMETHYLURACIL TAUTOMERS U.A. Soliman Department of Chemistry, Faculty of Science (boys), Al Azhar University, Nasr City, Cairo, Egypt E-mail: usama_a_soliman@hotmail.com Received May, 30, 2014 Revised September, 30, 2014 The molecular structure of 1,3-dimethyluracil (C6H8N2O2; 1,3-DMU) is studied theoretically and experimentally using Gaussian 98 calculations and different spectroscopic techniques. <...> The vibrational spectrum for 1,3-DMU in the solid phase is recorded in the IR range 4000— 400 cm–1. <...> Initially, in order to get the most stable structure, twelve structures were proposed for the titled compound as a result of the internal rotation of CH3 around C—N bonds and keto-enol tautomerism. <...> The single point energy and frequency calculations are obtained by MP2 (Full) and DFT/B3LYP methods with the 6-31G(d) basis set using the Gaussian 98 computation package. <...> After the complete relaxation of twelve isolated isomers, the (diketo) tautomer was the only favored structure owing to its low energy relative to the other isomers and the prediction of real frequencies. <...> This interpretation is supported by the recorded infrared spectrum that shows the presence of only the diketo tautomer. <...> Aided by the normal coordinate analysis and potential energy distributions, a confident vibrational assignment of the fundamental frequencies is calculated. <...> DOI: 10.15372/JSC20160109 Keywords: 1,3-dimethyluracil, normal coordinate analysis, vibrational frequencies, ab initio calculation, internal rotation barriers. <...> INTRODUCTION Uracil is one of the consequential derivatives of pyrimidine, which are known for their biological and pharmaceutical importance [ 1 ]. <...> They belong to the family of pyrimidine bases that are of great interest since they control the protein formation and cell functions in living organisms [ 2, 3 ] and also due to the important role of N-hetero cyclic molecules (i.e., cytosine, uracil, thymine, etc.) in the structural problems of nucleic acid. <...> Only two purines (adenine, guanine) and three pyrimidines (cytosine, thymine, and uracil) occur widely in nucleic acids [ 7 ]. <...> In RNA, uracil base pairs with adenine replace thymine during DNA transcription. <...> In recent times, pyrimidine derivatives have <...>