Isotope Effects in the Spectra of Hydrogen-Bonded Complexes. Experimental and Theoretical Study of an IR Absorption Spectrum of the (¹²CH₃)${}_2^{13}$CO…HF Complex

IR absorption spectra of mixtures (¹²CH₃)${}_2^{13}$CO/HF and free (¹²CH₃)${}_2^{13}$CO molecules are recorded in the region of 4000−800 cm⁻¹ with a Bruker IFS-125 HR vacuum Fourier spectrometer at room temperature with a resolution of 0.05 cm⁻¹. Absorption bands of the (¹²CH₃)${}_2^{13}$CO…HF complex are obtained by subtracting the absorption bands of free HF and acetone molecules and absorption lines of atmospheric water from the experimental spectrum of mixtures. Spectral characteristics of the 2ν(¹³C=O) overtone band of free acetone were also recorded. Comparison of the spectral data obtained with the analogous data measured earlier for the (¹²CH₃)${}_2^{12}$CO…HF complex shows changes in the absorption spectra of complexes caused by ¹²C → ¹³C isotopic substitution. The frequencies and intensities for absorption bands of both complexes are calculated using the perturbation theory and methods MP2/6-311++G(2d,2p) and MP2/6-311++G(3df,3pd) with allowance for the error of superposition of basis functions of the monomers. The calculated results are in good agreement with the experimental data and are used to interpret the observed spectra. The influence of anharmonic effects on the frequencies and intensities of the strongest ν(H–F) and ν(¹³C=O) bands is examined with the help of variational calculations. It is shown that, unlike the (¹²CH₃)${}_2^{12}$CO…HF complex, the 2ν(¹³C=O)/ν(H–F) resonance is virtually absent in the (¹²CH₃)${}_2^{13}$CO…HF complex.