John D. Vaughan

Computer-based molecular mechanics techniques for accurate prediction of thermodynamic properties of chemically reactive systems

Abstract Computer-based molecular mechanics (or force field) techniques are proposed as logical current methods for accurate prediction of the thermodynamic properties of chemically reactive systems involving nontrivial molecules. Although chemists have increasingly employed molecular mechanics models in their predictive and interpretive work, chemical engineers have largely ignored these developments. We illustrate the potential power of these molecular mechanics models by applying them to the prediction of thermodynamic equilibrium constants, K eq ( T ), for the two Diels-Alder reactions: 1,3-cyclopentadiene + C 2 H 2 ⇌norbornadiene (A) 1,3-cyclopentadiene + C 2 H 4 ⇌norbornene. (B) Our results agree quite well with literature data for reactions (A) and (B). In addition, our computational approach requires substantially less time than laboratory study (with obvious economic benefits), and further critique of important results is often quickly possible by carrying out additional computations for related systems.

Thermodynamic measurements for the Diels-Alder adduct of anthracene and maleic anhydride

In the present experimental work, the energy of combustion of the crystalline Diels-Alder adduct of anthracene and maleic anhydride: C18H12O3, was measured with a model 1241 Parr automatic calorimeter and a Parr model 1710 calorimeter controller. The standard molar enthalpy of combustion of the (anthracene + maleic anhydride) adduct at po = 0.1 MPa was determined to be ΔcHmo(C18H12O3, cr, 298.15 K) = −(8380.0±5.9) kJ·mol−1. The molar enthalpy of fusion of this adduct at its melting temperature (534.07 K), as measured by a 910 DuPont d.s.c. and a 9900 DuPont thermal (analyzer + digital computer), was found to be (36.3±4.2) kJ·mol−1. The other thermodynamic properties of the (anthracene + maleic anhydride) adduct derived from those measured properties are: ΔfHmo(C18H12O3, cr, 298.15 K) = −(418.2±6.4) kJ·mol−1 and ΔfHmo(C18H12O3, 1, 298.15 K) = −(389.7 ± 7.7) kJ·mol−1.

Thermodynamic molecular mechanics force field: Modified QCFF program

The QCFF program originated by Warshel and Karplus4a was modified to compute accurate thermodynamic properties So, C  po , (H  To – H  0o )/T, and ΔH  fo for various acyclic and cyclic alkenes and alkadienes. Modifications consisted of adjusted bond angle, dihedral angle, bond stretch, and bond energy parameters that improved calculated vibrational frequencies, zero point energies, and thermodynamic functions. Supplemental torsional potential energy functions that were added to existing torsional functions led to greatly improved relative conformer energies and ΔH  f0 values. It was shown that inclusion of hindered internal rotation leads to significantly better agreement of calculated thermodynamic functions with observed values for acyclic alkenes at high temperatures. The calculated thermodynamic properties of the alkenes and alkadienes were deemed sufficiently accurate for calculation of standard enthalpies and Gibbs free energies of gas phase chemical reactions at various temperatures.

14N(n,p)14C recoil atom chemistry in sodium azide

Abstract The distribution of 14 C-labeled products resulting from various post-irradiation treatments of n -irradiated NaN 3 was investigated. Water dissolution of target crystals yielded 14 CN − , 14 CN 2 2− , and 14 CH 3 NH 2 . Hydrazine dissolution yielded 14 CN − and 14 CN 2 2− , but no 14 CH 3 NH 2 . Target samples thermally decomposed in vacuo at 400° yielded essentially only 14 CN 2 2− , but at 500° both 14 CN − and 14 CN 2 2− were found. Decomposition at 500° in the presence of hydrogen gas gave the same result as decomposition in vacuo , but decomposition in the presence of oxygen gas at 500° yielded only Na 2 CO 3 , while decomposition in the presence of gaseous iodomethane at 500° produced CH 3 14 CN, labeled tarry residue, 14 CN 2 2− and 14 CN − .