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Mixed complexes of acetylene-ethylene are studied using vacuum-ultraviolet (VUV) photoionization mass spectrometry and theoretical calculations. These complexes are produced and ionized at different distances from the exit of a continuous nozzle followed by reflectron time-of-flight mass spectrometry detection. Acetylene, with a higher ionization energy (11.4 eV) than ethylene (10.6 eV), allows for tuning the VUV energy and initializing reactions either from a C2H2+ or a C2H4+ cation. Pure acetylene and ethylene expansions are separately carried out to compare, contrast, and hence identify products from the mixed expansion: these are C3H3+ (m/z = 39), C4H5+ (m/z = 53), and C5H5+ (m/z = 65). Intensity distributions of C2H2, C2H4, their dimers and reactions products are plotted as a function of ionization distance. These distributions suggest that association mechanisms play a crucial role in product formation closer to the nozzle. Photoionization efficiency (PIE) curves of the mixed complexes demonstrate rising edges closer to both ethylene and acetylene ionization energies. We use density functional theory (ωB97X-V/aug-cc-pVTZ) to study the structures of the neutral and ionized dimers, calculate their adiabatic and vertical ionization energies, as well as the energetics of different isomers on the potential energy surface (PES). Upon ionization, vibrationally excited clusters can use the extra energy to access different isomers on the PES. At farther ionization distances from the nozzle, where the number densities are lower, unimolecular decay is expected to be the dominant mechanism. We discuss the possible decay pathways from the different isomers on the PES and examine the ones that are energetically accessible. [ABSTRACT FROM AUTHOR]

Hartree–Fock(HF) is known to suffer from drawbacks in the description of the relativestabilities between the hemi-bonded (HB) and proton-transferred (PT)isomers of the water dimer cation, (H2O)2+. The energy difference predicted by HF is too large, approximately27 kcal/mol, which is lowered to 7 kcal/mol when correlation effectsare added. The error in HF has been previously attributed to the largedynamic correlation effects in the HB structure as well to the largesymmetry breaking this structure exhibits. In this study we use therecently developed projected Hartree–Fock (PHF) methods tostudy the relative stability of the two isomers of (H2O)2+as well as its second and third row analogs,namely, (H2S)2+and (H2Se)2+. In PHF, symmetries are broken and restoredin a variation-after-projection approach and thus can deal easilywith systems for which HF itself spontaneously breaks symmetry. Weuse different flavors of PHF (SUHF, KSUHF, SGHF, and KSGHF) to exploretheir ability in capturing dynamic correlation effects and to comparetheir performance to different wave function based methods. We studythe role of the symmetry-breaking in the above systems, using wavefunction based methods with unrestricted and restricted wave functionsas well as performing a single-shot symmetry restoration (a projection-after-variationscheme). [ABSTRACT FROM AUTHOR]