Acetylene molecules motions

The metallacyclobutadiene formation from acetylene and molybdenum carbyne complex CI3M0CH was studied [45] along a postulated least-motion pathway as shown in Fig. 12. The CI3M0CH was brought together with the acetylene molecule such that the three carbon atoms and the metal center remained coplanar throughout the course of the reaction. It has been found... 

A linear molecule, such as any diatomic molecule, carbon dioxide, and ethyne (acetylene, HC=CH), can rotate about two axes perpendicular to the line of atoms, and so it has two rotational modes of motion. Its average rotational energy is therefore 2 X jkT = kT, and the contribution to the molar internal energy is NA times this value ... 

In some cases, steric interactions can prevent unimolecular reactions. Tetrahe-drane (18) has been the subject of a number of studies, and the conclusion is that, if formed, it would rapidly decompose to form two molecules of acetylene. However, tetra-tert-butyltetrahedrane (19) is a quite stable substance, and on heating rearranges to tetra-tert-butylcyclobutadiene. An orbital symmetry " analysis of the cleavage of tetrahedrane to acetylene indicates that it involves a torsional motion that in the case of the tert-butyl substituted derivative would bring the tert-butyl groups very close to each other. As a result, this mode of reaction is not possible, and the compound is relatively stable. 

The polyad model for acetylene is an example of a hybrid scheme, combining ball-and-spring motion in a two-dimensional configuration space [the two Franck-Condon active modes, the C-C stretch (Q2) and the tram-bend (Q4)] with abstract motion in a state space defined by the three approximate constants of motion (the polyad quantum numbers). This state space is four dimensional the three polyad quantum numbers reduce the accessible dimensionality of state space from the seven internal vibrational degrees of freedom of a linear four-atom molecule to 7 - 3 = 4. 

Hydrocarbon sorbate vibrations. IINS spectra have been recorded for a number of simple sorbate molecules within aluminosilicate zeolites, including hydrogen in A (40, 41). acetylene in X (4, ethylene in A (42) and X (44-46). and p-xylene (42) in X type materials. In addition to intramolecular modes, where interaction between the sorbate and the non-framework cations is strong (for example in the ethylene - silver zeolite A system (42)), vibrational transitions associated with sorbate motion with respect to the zeolite s internal surface can be observed. The latter modes, and the dependence of their frequencies on loading, structure and composition are of particular interest as they convey detailed information about the character of the zeolite - sorbate... 

Early attempts to synthesize the intriguing cyclonona-l,4,7-triyne molecule, 1 [1], grew from a recognition that the in-plane p-orbitals of the three acetylenes around its perimeter should encroach upon each other s space while the out-of-plane p-orbitals should consitute an essentially ideal trishomobenzene. The tantalizing prospect that a [2 + 2 + 2] cycloaddition requiring very little atomic motion might transform this monocylic triyne into tricyclopropabenzene (2, Fig. 9-1) added further incentive to prepare 1. 

One-bond X-Y couplings have been computed by Del Bene et for 18 HmX-YHn molecules, with X = C, N and P, using the equation-of-motion coupled-cluster singles and doubles method. The ealculated values cover the range from ca. 200 Hz in acetylene to ca. -450 Hz in -diphosphene. However, in the latter case agreement between the experimental and calculated values is rather poor. The same applies to some other molecules, such as, for example, methyl phosphine for which the calculated and experimental J values are 0.8 and 9.3 Hz, respectively. 

For the tetraatomic system HXXH, representing both the linear acetylene and the non-linear hydrogen peroxide, we expect to be able to construct twelve symmetry coordinates. Three of them are translational, whereas two of the remaining nine in the linear conformation and three in the non-linear one are reserved for rotations. Linear tetraatomics thus have seven vibrational coordinates, motion along which changes the potential energy, whereas their nonlinear counterparts have six. Those of the linear HXXH molecule are shown in Fig. 4.4 with the subgroup into which each is taken, if only momentarily, by the displacement. 

Other linear molecules (acetylene, CjHj, for example) have similarly described vibrational spectra either stretching vibrations or bending vibrations. It is only when a molecule becomes nonlinear that additional complexities arise. Unfortunately, most molecules are nonlinear. Fortunately, similar rough descriptions of the vibrations can be applied. Also fortunately, symmetry considerations combine with the change-in-dipole-moment selection rule to limit the number of IR-active vibrational motions of large, symmetric molecules. The next few sections will illustrate some of the procedures used to simplify our understanding of molecular vibrations. 

For example, let us consider the vibrational spectrum of methylacetylene, CH3C=CH. The 15 normal vibrations reduce to five Aj-labeled motions and five -labeled motions. Table 14.6 lists the 10 unique vibrational frequencies of methyl-acetylene. Also listed are other absorptions that are attributed to various overtones and combination bands. The nonideality of the molecule permits some of these combinations to appear with detectable intensity. 

From the results of classical trajectory calculations intrinsic non-RRKM behavior has been predicted for ethane dissociation, ethyl radical dissociation,and methyl isocyanide isomerization. These predictions are supported by classical trajectory calculations for model H-C-C -> H + C=C dissociation. To generalize, classical trajectory calculations have predicted intrinsic non-RRKM behavior for molecules with isolated high frequency modes [e.g, CH3NC, clusters like Li (H20)j, and van der Waals molecules], molecules like acetylene with linear geometries for which bending and stretching motions are nearly separable, and molecules with tight activated complexes. 

For larger polyatomic molecules the vibrational motions can become very complicated, with dozens of normal modes contributing to the observed spectra. However, the basic principles outlined here apply, at least as a first order approximation, to the motion of molecules as large as methyl acetylene (C3H4), shown in Fig. 3.4.4, and propane (C3H8), shown in Fig. 3.4.5. For some large molecules with simple structures, such as ring-shaped benzene (CeHe), the observed spectra are often quite simple (see, for example, Sadtler, 1972). 

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