Infrared levels of monomeric uracil in cryogenic matrices

Ar-Matrix IR Spectra of 5-Halouracils Interpreted by Means of DFT Calculations

“…The decrease of the band frequency predicted to take place with the increase in the halogen mass is valid for the (N 1 -H 7 ) band only. Our assignment of the (N 1 -H 7 ) band in the low-temperature IR spectra of uracil and halouracils is in agreement with most of the literature data; [29][30][31][32][33][34][35][36][37][38] some investigators have assigned the (N 1 -H 7 ) mode to the band positioned at ca. 100 cm -1 wavenumbers lower.…”

“…In conclusion, from among the three above hypotheses about the origin of the FR in the CdO stretching vibrations region in 5-halouracil IR spectra, the most reasonable is the one that assigns the (N 3 -H 9 ) + as (CdO) combination tone as resonating with a ν(CdO) mode because only this hypothesis can explain the earlier experimental findings. [30][31][32]37 The above hypotheses can be verified partially by calculations of the anharmonic frequencies accompanied by the calculated FRs. 57 Table 6 lists all of the B3PW91/6-311G**-calculated FRs that occur in the ν(CdO) stretching vibrations region.…”

“…It was shown for uracil that deuteration at the two uracil N atoms clarifies the pattern in the CdO stretching vibrations region so that two main bands appear. 30,32,37 Moreover, from among the IR matrix spectra of N 1 -, N 3 -, and C 5 -methylated uracils and those of N 1 -and N 3 -dimethylated uracils, only those that are methylated at the N 3 position exhibit two main bands in the ν(CdO) stretching a Calculated frequencies are scaled 62 by the straight line νscal ) 0.954νcalc + 27.6; IIR stands for IR intensities (km mol -1 ), RA denotes Raman activities (Å amu -1 ), and F stands for the depolarization ratio. For local mode definitions, see Table 1S in the Supporting Information.…”

“…This is why, for this region of spectra, some differences may occur in the interpretations. [29][30][31][32][33][34][35][36][37][38][39] (C 6 -H 12 ) Bending Vibrations. DFT Calculations.…”

“…However, just for BrU, the interpretation was supported by MINDO/3 calculations. On the other hand, the ample literature data devoted to the vibrational spectra of uracil [29][30][31][32][33][34][35][36][37][38] are helpful for the assignment of the 5-halouracils' vibrational spectra only partially: two C-H oscillators present in the uracil molecule result in CH and NH bending-mode couplings and make the spectral picture knotty.…”