Quantitative vibronic coupling calculations. The visible spectrum of propadienylidene

High-level ab initio calculations that employ the equation-of-motion coupled-cluster method in the singles, doubles and triples approximation (EOMEE-CCSDT) have been used in conjunction with a quasidiabatic ab initio approximation to construct a vibronic Hamiltonian for the strongly coupled A1A2 and B1B1 states of propadienylidene (CCCH2). Simulations based on the model Hamiltonian reproduce all of the major features of the visible absorption spectrum of this molecule, with regard to both intensities and band positions, especially in the region that is more than 1000 cm(-1) above the B1B1 origin. Despite the fact that a great number of individual vibronic levels contribute to each major band in this higher energy region, the spectrum is qualitatively similar to that from a Franck-Condon treatment, apart from a consequent broadening of the features. By contrast, the range from 15 800-16 500 cm(-1), which comprises the B1B1 <-- X1A1 origin, is profoundly affected in appearance by the vibronic interaction. The simulations do a credible job of accounting for the spectrum in this region, although a thorough analysis of all features cannot be made at this time. A combination of ab initio calculations, experimental observations and the spectral simulation allows an estimate for the position of the (electric dipole forbidden) A1A2 <-- X1A1 origin: 13 600 +/- 200 cm(-1).