Circumstellar carbon chain molecules: A density function theory study of C<i>n</i>O, <i>n</i>=3–9

Abstract

The infrared vibrational frequencies and intensities of the CnO linear chains, n=3–9, in their electronic ground state is predicted at the Becke–Lee–Yang–Parr (BLYP) level of theory. The computational model is assessed in three steps: (i) comparison of calculated and experimental rotational parameters for the whole series; (ii) comparison of experimental and calculated infrared frequencies, intensities and isotopic shifts for C3O (this molecule can be considered the prototype of the chains whose ground electronic state is 1Σ+); (iii) comparison of calculated and experimental infrared frequencies and intensities for C4O (this molecule can be considered the prototype of the chains whose ground electronic state is 3Σ−). The excellent agreement between experimental and computational results allows the prediction of the infrared pattern to 20 cm−1 for the frequencies and a few percent for the relative intensities. Analysis of the infrared intensities in terms of local atomic oscillators within the chains shows that while for short chains the intensity arises from the motion of the two carbon atoms nearest to the oxygen, for C7O and C9O the intensity arises in conjunction with the motion of carbon atoms close to, but not at, the other end of the molecule. For these two molecules, the infrared intensity is therefore similar in nature to that of pure carbon chains.