Triple bond dissociation

Figure 3. Calculations of the triple bond dissociation ofN2 in the STO-3G basis, using full Cl, conventional CCD, variational CCD, and quadratic CCD,...

It is estimated that each year approximately 150 million tonnes of nitrogen are fixed biologically compared to 120 million tonnes fixed industrially by the Haber process (p. 421). In both cases N2 is converted to NH3, requiring the rupture of the N=N triple bond which has the highest dissociation energy (945.41 kJmol )... 

FIGURE 2.16 The bond dissociation energies, in kilojoules per mole of nitrogen, oxygen, and fluorine molecules. Note how the bonds weaken in the change from a triple bond in N, to a single bond in F,... 

By contrast, addition-elimination mechanisms in their simplest form begin with formation of an addition complex resulting from a well on the PES, followed by dissociation of the complex, yielding products. Both the entrance to and exit from the well may be hindered by barriers on the PES. Addition mechanisms are uncommon in radical -b saturated closed-shell reactions due to the difficulty of bond formation with the saturated species (ion-molecule reactions are exceptions). By contrast, additions are more common in radical -b unsaturated closed-shell species, where the double or triple bond allows a low barrier or barrierless pathway for addition of the radical into the 7i-bond of the stable species, such as the reaction... 

The nature of the bonding in this molecule has been the cause of considerable discussion. Its short length (112.8 pm) and its great strength (bond dissociation enthalpy 1072 kJ mol- ) are consistent with the usual triple-bond Lewis structure... 

Typical derivatives of this type of compounds are obtained by addition to carbon-carbon double and triple bonds, usually accompanied by metal-metal bond scission. The highly crowded cyclotristannane 89 reacts readily with phenylacetylene to yield a distannacyclobutene 31 (see Table 4), as shown in reaction 14. This points to intermediate 69 (Table 6) as possible dissociation product of 89 before undergoing (2 + 2) cycloaddition271. 

Nitrogen. Molecular nitrogen N2 has a dissociation energy of 950 kJ/mol, and the N-N triple bond is one of the strongest known chemical bond. Shock-wave experiments disclosed the possibility of N-N dissociation in condensed phases [224, 319-322]. From this an interest arose in the possible obtainment of arrays of N-N single bonds that could form in potentially energetic materials. Ab initio calculations of various kinds [323-327] showed that actually at high pressure... 

A dissociative elimination-addition pathway has also been proposed to account for the kinetics of alkaline hydrolysis of 2,4-dinitrophenyl 4 -hydroxyphenylpropionitrile in 40% (v/v) dioxane-water, although participation of the associative Bac2 mechanism cannot be ruled out since it may be facilitated by the electronic effect of the triple bond. Formation of intermediate (15), having a conjugated and cumulated double-bond system, should favour the ElcB mechanism and thereby account for the contrasting entropies of activation found for hydrolysis of (14) and the corresponding 4 -methoxyphenylpropionate. 

The primary motivation behind the QMMCC approximations is the need to improve the CR-CCSD(TQ) description of more complicated types of multiple bond breaking, such as triple bond breaking in N2. The CRCCSD[T], CR-CCSD(T), and CR-CCSD(TQ) methods are capable of providing an excellent description of PESs involving single and double bond dissociation (9, 13, 15, 17-21, 111), but the CR-CCSD(T) and CR-CCSD(TQ) results for triple bond breaking are less accurate (14, 18, 21). This can be seen by analyzing the CR-... 

The present paper is devoted to display the performance of both procedures in situations of streched bonds. These areas of the molecular energy hypersurface are typically placed outside of the domain of good performance of the standard CCSD method [19-23]. In particular, we consider the dissociation of HF, H2O and N2 molecules (i.e., a single bond, two single bonds and a triple bond). 

We have studied the performance of the so-called (SC)2CAS-SDCI and ec-CCSD methods versus standard CAS-SDCI method on one hand, and CCSD on the other hand, in quasidegenerate situations that arise when exploring dissociation channels of simple molecules. In particular, we dissociate a single bond, two single bonds simultaneously and a triple bond. 

The condition wherein the concentration of a reactant, ligand, product, or effector is significantly greater than the dissociation constant of that same entity. 2. A solution becomes saturated with a particular substance when that substance has been dissolved to its maximum extent. 3. The process (or degree of) by which the number of CT-bonds in a molecular entity decrease (that is, the removal of double and triple bonds and increase of (T-bonds). 

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