CASPT2 Study of Photodissociation Pathways of Ketene

Abstract

The mechanism of various photodissociation channels of ketene involving the three lowest singlet states (S0, S1, and S2) and the three lowest triplet states (T1, T2, and T3) was investigated by means of the (MS-)CAS(8e,8o)PT2/6-31+G* method. Stationary structures, i.e., global minima (GMs), local minima (LMs), transition states (TSs), minimum energy conical intersections (MECIs), and minima on seam of crossing (MSXs), were explored systematically by the global reaction route mapping (GRRM) strategy. On the basis of these structures, we discussed related dissociation channels starting from S2 that have been studied experimentally with 193-215 nm excitation wavelength. Five working pathways were found for relaxation to the low-lying S0, S1, and T1 potential energy surfaces (PESs) from the Franck-Condon region of S2, and the relaxation is expected to occur very quickly. On these low-lying states, five dissociation channels are open: three CH2 + CO channels for different CH2 electronic states, H + HCCO, and H2 + C2O. Pathways for all of these five channels were identified and discussed, including new minor paths leading to H2 + C2O.

Accurate Excited-State Geometries: A CASPT2 and Coupled-Cluster Reference Database for Small Molecules

“…79 For diazomethane, a 1969 work concluded that the excited state should really be designated A ″ in the point group C s . 80 On the theoretical side, one can find a very complete CASPT2/6-31+G(d) investigation of the Morokuma group devoted to the dissociation paths of ketene, 81 as well as an earlier investigation of Szalay and co-workers with CC methods for the same molecule. 82 The structure of the lowest ES structure of diazomethane, constrained in the C 2 v symmetry, was previously determined at both CASPT2/6-31G(d) 26 and CC2/TZVP 25 levels of theory, whereas a CASSCF/6-31G(d) investigation of the ES pathways relating diazirine to diazomethane has also been performed.…”

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…125 Ion imaging studies of the CO products formed upon photolysis of H2CCO at  = 208 and 213 nm, 126 earlier studies of the IR emission from CO products formed by photolysis at  = 193 nm, 127 and PTS studies of the H + HCCO products formed via photolysis in the range 193    215 nm, 128 all return product distributions that are broadly consistent with the decay of 'hot' S0 parent moleculesin accord with later theoretical studies. 129,130 PTS studies at yet shorter wavelengths ( = 157.6 nm), however, return a very different outcome. CH2 + CO products represent ~97% of the total dissociation yield.…”

Assessing the Performances of CASPT2 and NEVPT2 for Vertical Excitation Energies

“…As in the previous systems, CASPT2(NOIPEA) underestimates systematically all the TBEs. Ketene was previously investigated at the MS-CASPT2/6-31+G(d) level by the Morokuma group, 152 who reported VTEs of 3.72, 5.97, 3.62, 5.42, and 5.69 eV for the 1 𝐴 2 , 1 𝐵 1 , 3 𝐴 2 , 3 𝐴 1 , and 3 𝐵 1 excitations, respectively, all values being slightly below the TBEs. Despite the different basis sets used, one also notes a reasonable match between these previous MS-CASPT2 values and the present results that rely on rather comparable active spaces.…”