The ν1+ν3 combination mode of C3 in Ar and Kr matrices: Evidence for a bent structure

The ν1+ν3 combination mode of laser-ablated C3 has been observed in Ar and Kr matrices for all six possible 12C/13C isotopomers. The ν1 totally symmetric mode frequencies have been determined by difference with the known ν3 asymmetric stretching frequencies, corrected for anharmonicity effects. A comparison of certain isotopomer pairs reveals that C3 is bent in these matrices. A normal coordinate analysis using four force constants resulted in an excellent fit of the 12 observed frequencies (ν1 and ν3 for six isotopomers). From the fit, a value for the ν2 bending mode frequency of 82 cm−1 was obtained. This is shown to be in good agreement with the value obtained from the C3 phosphorescence spectrum. Finally, the molecular bending angle which was determined to be 160.0° (in Ar and Kr) is shown to match very well with the predicted value from ab initio theory (161.6°) and from a fitting of a Morse oscillator–rigid bender potential to gas phase vibrational frequencies (162.5°). The influence of the matrix environment on the vibrational force constants and frequencies of C3 is discussed.