C. F. Wilcox

The number of Kekulé structures and the thermodynamic stability of conjugated systems

Abstract The dependence of Huckel π-electron energies, E π , on the basic graph theoretical parameters N (the number of vertices), ν (the number of edges) and ASC (the algebraic structure count) is explored. The form with the ASC enters E π is established and an equation for E π is developed. It is shown how the early and apparent success of the (resonance) theory rested on the fortunate fact that all Kekule structures for benzenoid hydrocarbons and acyclic polyenes have the same parity. The significance of ASC in determining chemical stability and reactivity is dicussed briefly.

ON MALONALDEHYDE AND ACETYLACETONE : ARE THEORY AND EXPERIMENT COMPATIBLE?

Abstract For malonaldehyde (MA) and acetylacetone (AcAc) the available experimental structural and spectroscopic data are briefly summarized and contrasted with extensive ab initio calculations of their salient features. Attention is directed to a clear discrepancy between ab initio computed stability of the two degenerate structures of enol AcAc (as published, and confirmed in this report) and the observed species populations. MA and AcAc are prototypes of a unique class of compounds for which significant differences in structure are anticipated for small differences in thermochemical parameters. Temperature dependent FTIR spectra were recorded; the latter confirm that a discrepancy exists.

Synthesis and properties of unusual π-networks : Cycloocta[1,2,3,4]-def]benzo[3,4]cyclobuta[6,7[biphenylene☆

Abstract The title compound, an example of a hydrocarbon containing three linearly fused 4n-membered rings, has been synthesized and characterized as a thermally stable species possessing no unusual chemical reactivity. Despite the presence of these conventionally antiaromatic structural units this hydrocarbon is predicted to have a net diamagnetic ring current. The NMR results, which are discussed in some detail, are shown to be consistent with this expectation. The impact of this result on the NMR criterion of aromaticity is discussed briefly.

Estimation of homogeneous nucleation flux via a kinetic model

The proposed kinetic model for condensation under homogeneous conditions, and the onset of unidirectional cluster growth in supersaturated gases, does not suffer from the conceptual flaws that characterize classical nucleation theory. When a full set of simultaneous rate equations is solved, a characteristic time emerges, τn, for each cluster size, at which the production rate, and its rate of conversion to the next size (n+1) are equal. Procedures for estimating the essential parameters are proposed; condensation fluxes Jsskin are evaluated. Since there are practical limits to the cluster size that can be incorporated in the set of simultaneous first‐order differential equations, a code was developed for computing an approximate Jssth, based on estimates of a ‘‘constrained equilibrium’’ distribution, Nequ, and identification of its minimum, Neql.

The structure of 4-chloronortricyclene

Abstract The structure of 4-chloronortricyclene in the gas phase was determined by electron diffraction. Interatomic distances and mean square amplitudes of vibration were evaluated. The cyclopropyl ring bonds are 1.510 A, typical of C 3 rings. A possible correlation between increased cyclopentyl bond lengths and increased flap angle is noted, as is the apparent decrease in these bond lengths with substitution of electronegative groups. A summary of CCl bond lengths in a variety of molecular environments completes the discussion.

The thermochemistry of TNAZ (1,3,3-trinitroazetidine) and related species: G3(MP2)//B3LYP heats of formation

The heats of formation of TNAZ(1,3,3-trinitroazetidine) and related compounds have been calculated with the G3(MP2)// B3LYP model to give a mean absolute error of 0.85 kcal/mol for 16 singlet species and 0.61 kcal/mol for 12 free radical intermediates. These uncorrected errors are better than our previous B3LYP/6-31G(d,p)-based scheme that employed seven auxiliary parameters. The mean absolute deviation of the G3(MP2)//B3LYP energies of the combined set of 28 singlet and radical species, 0.75 kcal/mol, can be reduced to 0.42 kcal/mol when supplemented with a minimal four-parameter atom-based correction scheme. It was found that the previous B3LYP/6-31G(d,p) energies when fitted to a combination of 39 experimental and G3(MP2)//B3LYP singlet and radical species gave a mean absolute deviation of 3.7 kcal/mol, but that the error is reduced to 1.52 kcal/mol when the B3LYP/6-31G(d,p) energies are supplemented with a five parameter additive correction scheme. The results of these calculations are used to clarify the initial mechanistic steps in the decomposition of TNAZ. q 2001 Elsevier Science B.V. All rights reserved.

The crystal and molecular structure of an unusual conjugated hydrocarbon : Cycloocta[1,2,3,4-def]benzo[3,4]cyclobuta[6,7]biphenylene☆

Abstract The conjugated hydrocarbon, cyclo[1,2,3,4-def]benzo[3,4]cyclobuta[6,7]biphenylene, crystallizes in the space group P212121 with a = 15.136(4), b = 16.453(4), c = 5.584(1) A, Z = 4, ϱcal = 1.32, ϱobs = 1.31 g cm−3. The structure was solved by application of direct methods after reducing the dominance of hk0 reflections in starting set selection. Full-matrix, least-squares refinement of 1910 independent data collected with MoKα radiation on a four-circle diffractometer yielded a final conventional R = 0.068. Bond lengths in this unusual conjugated hydrocarbon range from 1.34 to 1.53 A. Calculated iterated-Huckel bond lengths that include effects of σ strain reasonably correspond to the observed bond distances, with the largest differences (0.03 A) occurring in the four membered rings. The molecule is essentially planar, but exhibits an overall bowing that is probably due to crystal packing forces.

Kinetics of Condensation in Supersonic Expansion (Ar)

The temperature and density in an overexpanded jet (Ar) were mapped by the method of characteristics. The kinetics of condensation in the supersaturated gas along the central streamline was analyzed on the basis of four models, subject to previously developed thermodynamic relations which must apply during the homogeneous condensation of any supersaturated vapor. Here we report on detailed dynamic analyses of the rates of cluster growth. Each association: An−1+A1⇄An was represented either by a single stabilization step, or by a sequence of two energy removing collisions, for which kinetic parameters based on Lennard‐Jones (LJ) potentials were estimated. Coupled differential rate equations (up to 192) were solved, both for cases when the heat of condensation was neglected, and when it was partially or fully distributed throughout the fluid. The distribution of cluster sizes declines exponentially, according to N(n)=A exp(−an)+B exp(−bn), in agreement with generally observed distributions for monatomic spec...

DFT calculations of the thermochemistry and structures of tetrazine derivatives

Abstract Enthalpies of formation and standard state entropies were calculated for tetrazine, amino- and nitrotetrazines, and four extended ditetrazines using DFT programs. The derived values were corrected with the previously derived supplementary set of four parameters. The basic geometric structural features of the minimum energy states of the tetrazines are summarized.

Kinetic mechanism for condensation from supersaturated vapors that incorporate high levels of dimers

The kinetic molecular mechanism (KMM) for the onset of avalanche condensation from supersaturated vapors, described in this report, is more general than the one we used previously. While the expressions for the rate constants needed to compute the steady-state populations are the same, additional steps were introduced to take into account cluster growth via accretions with dimers. In the present version, the magnitudes for the stepwise enthalpy decrements are assigned using all the “best” available experimental and computed values, so that the program requires less arbitrary adjustable parameters. The derived values for the entropy decrements upon successive accretions present a basis for estimating the total entropy of clusters (size n), and thus impose constraints on any model for dynamics within clusters. For high ambient pressures an option is provided for correcting the condensation rate constants due to restricted diffusion of accreting units (monomers or dimers) into the reaction shell of the growi...