© A.Yu. Ivanov, Yu.V. Rubin, S.A. Egupov, L.F. Belous, and V.A. Karachevtsev, 2013 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 6, pp. 704–711 Fermi resonance in Ne, Ar and Kr-matrix infrared spectra of 5-bromouracil A.Yu. Ivanov, Yu.V. Rubin, S.A. Egupov, L.F. Belous, and V.A. Karachevtsev B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine 47 Lenin Ave., Kharkov 61103, Ukraine E-mail: ivanov@ilt.kharkov.ua Received December 21, 2012, revised February 13, 2013 Low-temperature matrix isolation Fourier-transform infrared spectroscopy and quantum-chemical calcula- tions with DFT/B3LYP and MP2 methods were used for investigation of isolated 5-bromouracil (BrU) mole- cules. Only one tautomeric form of BrU was dominated in the low-temperature Ne, Ar, and Kr matrices. It was revealed that population of minor hydroxy-tautomers did not exceed 0.2%. Appearance of additional absorption bands in the region of stretching vibrations CO (about 1710 cm –1 ) as well as of deformation ones (1297, 1093, 901 cm –1 ) was explained by Fermi resonance. In Ne matrices the peak intensities of absorption bands assigned to the out-of-plane vibrations of the ring and exocyclic atoms were decreased sharply. For the first time, least square method with the using of polynomial was proposed for the corrective scaling of calculated frequencies of vibrations. It is shown that the correction of calculated frequencies with the polynomial of degree two permits to decrease the root-mean-square discrepancy between the calculated and experimental ones to 4–5 cm –1 in the re- gion of 1500–500 cm –1 . The same polynomial may be applied for the correction of spectra of molecules with a similar structure. PACS: 33.15.–e Properties of molecules; 33.20.–t Molecular spectra; 33.20.Ea Fourier transform spectra; 82.30.Qt Isomerization and rearrangement. Keywords: FTIR spectroscopy, matrix isolation, DFT method, Fermi resonance. 1. Introduction Modern science shows constant interest in studies of biological molecules under conditions free of strong inter- molecular interactions [1,2]. The low-temperature spectral methods are the basis of this scientific direction [1–5]. Low-temperatures permit to freeze separate isomers of biological molecules and their complexes in the inert sur- rounding and to model biological processes in the space [6]. Besides, absent of molecule rotation in the inert medi- um permits essential improve of the resolution of vibra- tional spectra. Owing to this, the low-temperature matrix isolation infrared spectroscopy revealed self-descriptive- ness technique during studies on tautomerism of deoxyri- bonucleic acid (DNA) bases and of their biologically ac- tive derivatives [7]. Such biologically active molecules as halogen-substituted DNA bases and nucleosides have a vi- tal part in the various biological processes [8]. For example, 5-haloderivatives of uracil are able to replace thymine in DNA [8,9], have increased mutagenic activity [8,10] and increase DNA sensitivity to ionizing radiation [11]. Some works were devoted to studies of vibrational spectra of 5-bro- mouracil (BrU ) molecules isolated in Ar matrices [12–14]. On the basis of these spectra, conclusions have been made on BrU flat structure [12] and on Fermi resonance in the region of CO stretching vibrations [13,14]. As well, the conclusion on the predominance of the tautomeric structure of BrU_0 was drawn (Fig. 1). However, in spite of the predominance BrU_0 tauto- mer, the number of absorption bands in the experimental vibrational spectra of BrU exceeds noticeably theoretically possible 3N-6 fundamental vibrations. Also, quantitative data on BrU tautomeric equilibrium and on low-intensity spectral bands in the deformation range of 1500–500 cm –1 were absent. Meanwhile, analysis of the uracil spectra reveals Fermi resonance in this spectral region (1550–500 cm –1 ) too [15]. Besides, spectra of BrU were obtained only for Ar matrices, and this is able to complicate their analysis