Conformational Analysis. 5. Molecular Structure, Composition, Trans-Gauche Energy and Entropy Differences, and Potential Hindering Internai Rotation of Gaseous Formaldazine as Determined by Electron Diffraction

Kolbjørn Hagen, Vladimir Bondybey, Kenneth Hedberg

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Abstract

Formaldazine (2, 3-diaza-l, 3-butadiene, H2CNNCH2) has been studied by electron diffraction from the gas at nozzle-tip temperatures of -30, 60, and 225 °C. As in the cases of oxalyl chloride and bromide reported earlier, the molecules exist as a mixture of s-trans and gauche conformers with the trans the more stable. The composition (mol fraction) of the vapor with uncertainties estimated at 2a was found to be 0.911 (0.077), 0.785 (0.095), and 0.749 (0.108) at -30, 60, and 225 °C, respectively. These values correspond to an energy difference with estimated standard deviations ΔE° = Eg° - Et° = 1.2 ± 0.5 kcal mol-1 and an entropy difference ΔS° = Sg° — St° = -0.9 ± 1.5 cal/mol-1 K-1. Certain of the diffraction results permit the evaluation of an approximate torsional potential function of the form 2V = V1(1 — cosø) + V2(1 — cos 2ø) + K3(l - cos 3ø); the results are V1 = 1.10 ± 0.19, V2 = 0.50 ± 0.31, and V3 = 0.73 ± 0.79, all in kilocalories per mole. The barriers separating the conformers trans from gauche and gauche from gauche are respectively 1.50 ± 0.74 and 0.53 ± 0.83 kcal mol-1. The results for the distance (ra), angle, and rms amplitude (l) parameters obtained at the three temperatures are entirely consistent. At -30 °C the more important ones are, with estimated uncertainties of 2σ, r(N-N) = 1.418 (0.003) Å, r(C=N) = 1.277 (0.002) Å, r(C-H) = 1.094 (0.005) Å, ACNN = 111.4 (0.2°), ZHCN = 120.7 (0.8°), δ (rms amplitude of torsional vibration of the trans conformer) = 15.8 (3.7°), l(N-N) = 0.0558 (0.0026) Å, l(C=N) = 0.0448 (0.0022) Å, l(C⋯N) = 0.0596 (0.0029) Å, l(C⋯C) = 0.0600 Å (assumed).

Original languageEnglish
Pages (from-to)1365-1368
Number of pages4
JournalJournal of the American Chemical Society
Volume99
Issue number5
DOIs
StatePublished - 1977
Externally publishedYes

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