Correlation-consistent Quadruple-ζ Basis Set Research Articles

Twisted Triplet Ethylene: Anharmonic Frequencies and Spectroscopic Parameters for C2H4, C2D4, and 13C2H4

Ethylene is an exceptional example of a stable closed-shell singlet molecule with a low-lying triplet state of very different symmetry. Triplet C2H4 (the ã (3)A1 state), which has a twisted D2d geometry, is studied herein with high-level theoretical methods, namely, CCSD(T) (coupled cluster theory with single, double, and perturbative triple excitations) and Dunning's correlation-consistent quadruple-ζ basis set (cc-pVQZ). Geometric parameters, including equilibrium (re) and vibrationally corrected (rg) values, are reported for C2H4, C2D4, and (13)C2H4. Harmonic and anharmonic vibrational frequencies are also predicted using second-order vibrational perturbation theory (VPT2). Challenges encountered for the wagging vibrational features are discussed.

Combustion Chemistry: Important Features of the C3H5 Potential Energy Surface, Including Allyl Radical, Propargyl + H2, Allene + H, and Eight Transition States

The C(3)H(5) potential energy surface (PES) encompasses molecules of great significance to hydrocarbon combustion, including the resonantly stabilized free radicals propargyl (plus H(2)) and allyl. In this work, we investigate the interconversions that take place on this PES using high level coupled cluster methodology. Accurate geometries are obtained using coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)] combined with Dunning's correlation consistent quadruple-ζ basis set cc-pVQZ. The energies for these stationary points are then refined by a systematic series of computations, within the focal point scheme, using the cc-pVXZ (X = D, T, Q, 5, 6) basis sets and correlation treatments as extensive as coupled cluster with full single, double, and triple excitation and perturbative quadruple excitations [CCSDT(Q)]. Our benchmarks provide a zero-point vibrational energy (ZPVE) corrected barrier of 10.0 kcal mol(-1) for conversion of allene + H to propargyl + H(2). We also find that the barrier for H addition to a terminal carbon atom in allene leading to propenyl is 1.8 kcal mol(-1) lower than that for the addition to a central atom to form the allyl radical.

A Systematic Study of the X̃ 2B1, Ã 2A1, and B̃ 2B2 States of the Neutral Radical PH2

The three lowest-lying electronic states of PH2, X̃ 2B1, Ã2 A1, and B̃ 2B2, have been investigated systematically using ab initio electronic structure theory. The SCF, CASSCF, CISD, CASSCF-SOCI, CCSD, and CCSD(T) levels of theory have been employed to determine total energies, equilibrium structures, and physical properties, including dipole moments, harmonic vibrational frequencies, and infrared intensities. The predicted geometries and physical properties of the two lowest states of PH2 are in good agreement with available experimental results. At the CCSD(T) level of theory with the correlation-consistent quadruple-ζ basis set (cc-pVQZ), the à 2A1 state of PH2 has a large bond angle of 121.9° and is predicted to lie 52.2 kcal/mol (2.26 eV, 18 300 cm-1) above the ground state. This is in excellent agreement with the experimental T0 values of 52.26 kcal/mol (2.266 eV, 18 276.59 cm-1) and 52.08 kcal/mol (2.258 eV, 18 215 cm-1). The second excited electronic state (B̃ 2B2), not studied previously, was predicted to possess an unusual acute HPH angle of 29.1° and a theoretical T0 value of 71.3 kcal/mol (3.09 eV, 24 900 cm-1) relative to the ground state.