Heats of Formation of Triplet Ethylene, Ethylidene, and Acetylene

Abstract

Heats of formation of the lowest triplet state of ethylene and the ground triplet state of ethylidene have been predicted by high level electronic structure calculations. Total atomization energies obtained from coupledcluster CCSD(T) energies extrapolated to the complete basis set limit using correlation consistent basis sets (CBS), plus additional corrections predict the following heats of formation in kcal/mol: ∆H f 0 (C 2 H 4 , 3 A 1 ) )80.1 at 0 K and 78.5 at 298 K, and ∆H f 0 (CH 3 CH, 3 A′′) ) 86.8 at 0 K and 85.1 at 298 K, with an error of less than (1.0 kcal/mol. The vertical and adiabatic singlet-triplet separation energies of ethylene were calculated as ∆E S-T,vert ) 104.1 and ∆E S-T,adia ) 65.8 kcal/mol. These results are in excellent agreement with recent quantum Monte Carlo (DMC) values of 103.5 ( 0.3 and 66.4 ( 0.3 kcal/mol. Both sets of computational values differ from the experimental estimate of 58 ( 3 kcal/mol for the adiabatic splitting. The computed singlet-triplet gap at 0 K for acetylene is ∆E S-T,adia (C 2 H 2 ) ) 90.5 kcal/mol, which is in notable disagreement with the experimental value of 82.6 kcal/mol. The heat of formation of the triplet is ∆H f 0 (C 2 H 2 , 3 B 2 ) ) 145.3 kcal/mol. There is a systematic underestimation of the singlet-triplet gaps in recent photodecomposition experiments by ∼7 to 8 kcal/mol. For vinylidene, we predict ∆H f 0 (H 2 CC, 1 A 1 ) ) 98.8 kcal/mol at 298 K (exptl. 100.3 ( 4.0), ∆H f 0 (H 2 CC, 3 B 2 ) ) 146.2 at 298 K, and an energy gap ∆E S-T-adia (H 2 CC) ) 47.7 kcal/ mol.

A systematic approach to vertically excited states of ethylene using configuration interaction and coupled cluster techniques

“…= 466.40 kcal/mol, H f (0K) = 80.1 kcal/mol and H f (298K) = 78.5 kcal/mol with their largest basis sets. Our best D 0 is somewhat smaller, but remains in good agreement with the Nguyen et al 61 value in part due to cancellation of errors in the latter. We find H f (0K) = 80.5 ± 0.2 kcal/mol and H f (298K) = 78.8 ± 0.2 kcal/mol.…”

“…A study by Nguyen et al 61 using a similar coupled cluster approach employed somewhat smaller basis sets, scaled UMP2/aVTZ frequencies, a different technique for computing the scalar relativistic correction and no estimate of the effect of higher order correlation. They found D 0 158 The uncertainty is based on the spread in CBS values among five formulas.…”

“…We find H f (0K) = 80.5 ± 0.2 kcal/mol and H f (298K) = 78.8 ± 0.2 kcal/mol. In addition to the heat of formation, Nguyen et al 61 reported both vertical (104.1 kcal/mol) and adiabatic (65.6 kcal/mol) excitation energies for the triplet state. Corresponding diffusion Monte Carlo values of 103.5 ± 0.3 kcal/mol (vertical) and 66.4 ± 0.3 kcal/mol (adiabatic) were obtained by El Akramine et al 54 Our best composite adiabatic excitation energy ( 1 A g → 3 A 1 ) is 65.8 ± 0.6 kcal/mol, where the estimated uncertainty assumes the worst case scenario of no cancellation of error in the energy estimates for each state.…”

Generalized valence bond wave functions in quantum Monte Carlo

“…We use the same N state QMC energies as before, but include them again in this table for completeness. Although there does not appear to be sufficient experimental data to make a good comparison, we do have the recent high quality CCSD͑T͒/CBS result 45 of 68.8 kcal/mol to compare against, and with which our recommended result of 69.20Ϯ 0.17 kcal/ mol only differs by 0.4 kcal/mol. El Akramine and co-workers 43 also recently studied this transition using QMC to obtain 69.6Ϯ 0.3 kcal/ mol, and our results are in agreement with theirs, even given the differences in our wave functions.…”

The quest for a triplet ground‐state alkene: Highly twisted C═C double bonds

“…Triplet ethylene ( D 2 d ) has a 90° twist angle, lies roughly 65 kcal/mol above the planar ( D 2 h ) singlet ground state, and may be considered as a 1,2‐biradical because twisting to the perpendicular form completely disrupts π‐bonding. Several groups, both experimental and theoretical, have reported the singlet‐triplet gap of ethylene with mostly minor discrepancy among the determined values . A rotational barrier for ethylene of 65 kcal mol −1 was measured for the cis‐trans isomerization of 1,2‐dideuteroethylene at high pressure .…”