Experimental and theoretical study of the vibrational spectra of free 12-crown-4

The Raman and IR spectra of free 12-crown-4 (12c4) were measured in the solid, liquid, and solution phases. In the three phases, IR active modes were Raman inactive and IR inactive modes were Raman active. According to the exclusion rule, this is consistent with a conformation with a center of inversion. This indicates that 12c4 in the above-mentioned three phases exists in the C(i) conformation. Harmonic force fields were calculated for five of the lowest energy conformations of 12c4 of C(i), S(4), C(4), C(2), and C(s) symmetries at the corresponding optimized geometries at the B3LYP/6-31+G level. The five force fields were scaled using a six-scale-factor scaling scheme. The scale factors were varied to minimize the difference between the calculated and experimental fundamental frequencies, except that corresponding to the C-H stretching mode that was held fixed. The root-mean-square (rms) deviation of the experimental to the calculated vibrational frequencies was 6.2, 12.0, 10.8, 13.2, and 13.5 cm(-1), for C(i)(), S(4), C(4), C(2), and C(s) conformations, respectively. This supports the above conclusion that 12c4 in the solid, liquid, and considered solution phases exists in the C(i) conformation.