Quantum chemical predictions of the electron affinities of carbon-hydrogen clusters C_2nH^·, the CH binding energies and the gas phase acidities of polyacetylenes C_2nH₂forn= 1–3

Abstract

Accurate data for the adiabatic electron affinities of the radicals C2.H', the CH bond dissociation energies and the gas phase acidities of the polyacetylenes C2nH 2 for n = 1-3 have been obtained using the complete active space SCF approach to optimize the geometries and coupled cluster, and for some cases multi-reference configuration interaction and averaged coupled pair functional methods to refine the energies in connection with the polarized basis sets due to Sadlej and the generally contracted atomic natural orbitals basis sets, respectively. Harmonic frequencies have been computed for all species using a density functional theory/Hartree-Fock hybrid approach employing the Becke3LYP functional and the 6-311G** basis set. Our final theoretical predictions for the electron affinities are 2.96 + 0.04, 3.46 + 0.07 and 3.69 + 0-07 eV for C2H', C4H" and (~6 H', respectively. For the CH binding energies and gas phase acidities we predict D O = 131.7 + 1"2, 130-2 + 2-0, and 127-5 + 2"0 kcalmo1-1 and AGacid, 298 = 369-5 + 2"1, 356"3 + 3"6 and 348"7 + 3"6 kcal mol-1 for C2H2, C4H2 and C6H2, respectively. Where available, these data compare very well with experimental results. In addition, the question of the ground state of the Call" radical has been settled. It is of 21-I symmetry, with the 2E § state lying only some 3 kcal mol 1 higher in energy. Hence there is no 'alternation rule' for the ground state of the species C,H" as all of them except C2 H" assume a 217 ground state.

Photoelectron spectra of the C2nH− (n=1–4) and C2nD− (n=1–3) anions

“…The electron affinity of C 4 H is in reasonable agreement with the value of 3.46Ϯ0.07 eV calculated by Natterer and Koch. 84 The signal at higher electron energies than peak A1 is most likely due to anion hot bands. We tentatively assign peak a to the X 2 ⌺ ϩ ( 7 Јϭ0)←X 1 ⌺ ϩ ( 7 Љϭ1) hot band affording a 7 frequency in the anion of 210 cm Ϫ1 and 169 cm Ϫ1 in C 4 H͑D͒, respectively.…”

“…This suggests that the energy ordering of the two neutral electronic states contributing to this spectrum is reversed, and that the ground state of C 6 H is a 2 ⌸ state, consistent with the earlier microwave spectra 39 and ab initio calculations. [83][84][85]87,[91][92][93] We take peak A to be the vibrational origin of the transition to the X 2 ⌸ state, yielding an electron affinity of 3.809Ϯ0.015 and 3.805Ϯ0.015 eV for C 6 H͑D͒, respectively. This is in reasonable agreement with values calculated by Feher 93 ͑3.6 eV͒ and Natterer 92 (3.69Ϯ0.05 eV).…”

“…Several ab initio studies have been carried out on C 4 H in which geometries, vibrational frequencies, and electronic term values were calculated. 30,81,[83][84][85][86][87] The experiments indicate that C 4 H has a 2 ⌺ ϩ ground state, but the ab initio calculations indicate that the lowest 2 ⌺ ϩ and 2 ⌸ states are nearly degenerate. Most of the calculations 81,85,86 agree with the experimental assignment of the 2 ⌺ state as the ground state but place the 2 ⌸ state within 100 cm Ϫ1 , although two calculations 84,87 predict the 2 ⌸ state to be the ground state.…”

“…Several ab initio studies have also been carried out in which geometries, vibrational frequencies, term values, and various thermodynamic quantities were calculated. 30,[83][84][85]87,[90][91][92][93] The calculations predict a low-lying 2 ⌺ ϩ state which has not been observed experimentally.…”

Nonisothermal crystallization kinetics and morphology of self‐seeded syndiotactic 1,2‐polybutadiene

“…The other electronic state, 2 Σ + , of C 4 H lies slightly higher in energy (0.76 kcal/mol) than 2 Π state at M06‐2X/6‐311++G(d,p) level of theory (moreover, the 2 Σ + state of C 4 H is not a local minimum, namely, it has two imaginary frequencies). As the different theory level applied in calculations for C 4 H gives a different ground state, the controversy about true electronic ground state of C 4 H still remains28–31. Recent calculations for C 4 H using CCSD(T)/aug‐cc‐pVQZ method, quite expensive calculation, has given that 2 Σ + state of C 4 H is the ground state, but the energy difference between 2 Σ + and 2 Π state is 0.000041 hartree, 0.026 kcal/mol30.…”