Thermochemical data bond energies

Thermodynamic data in the area of transition metal chemistry is available, but additional studies would be desirable. One of the early indications that C—Ti bonds are not notoriously weak was obtained from the heats of combustion of Cp2Ti(CH3)2 and Cp2Ti(C6H5)2 with subsequent estimation of the a-bond dissociation energies (250 kJ/mol-1 and 350 kJ/mol 1, respectively)49. From heats of alcoholysis of a number of titanium, zirconium and hafnium compounds, and heats of solution of the products as well as subsidiary data, Lappert estimated heats of formation (AHf°) and thermochemical mean bond energy terms (EM X) of metal—X bondsso> (Table 2). 

The activation energy for ro tation about a typical carbon-carbon double bond IS very high—on the order of 250 kj/mol (about 60 kcal/ mol) This quantity may be taken as a measure of the ir bond contribution to the to tal C=C bond strength of 605 kJ/mol (144 5 kcal/mol) in ethylene and compares closely with the value esti mated by manipulation of thermochemical data on page 191... 

Interatomic distances calculated from the detailed analysis of rotational fine structure of the UV spectrum of pyrazine are in close agreement with those observed in (7) and (8), with the calculated bond lengths for C—C of 1.395, C—N 1.341 and C—H 1.085 A (60DIS(20)4291). Thermochemical data have provided a figure of 75 kJ moP for the delocalization energy of the pyrazine ring (B-67MI21400). 

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

Because these various quantities are characteristics of the reactants and products but are independent of the reaction path, they cannot provide insight into mechanisms. Information about AG, AH, and AS does, however, indicate the feasibility of any specific reaction. The enthalpy change of a given reaction can be estimated from tabulated thermochemical data or from bond-energy data such as those in Table 1.3 (p. 14) The exan le below illustrates the use of bond-energy data for estimating the enthalpy of a reaction. 

Whether AH for a projected reaction is based on bond-energy data, tabulated thermochemical data, or MO computations, there remain some fundamental problems which prevent reaching a final conclusion about a reaction s feasibility. In the first place, most reactions of interest occur in solution, and the enthalpy, entropy, and fiee energy associated with any reaction depend strongly on the solvent medium. There is only a limited amount of tabulated thermochemical data that are directly suitable for treatment of reactions in organic solvents. Thermodynamic data usually pertain to the pure compound. MO calculations usually refer to the isolated (gas phase) molecule. Estimates of solvation effects must be made in order to apply either experimental or computational data to reactions occurring in solution. 

In the following paper of this series6 a value of about 1.7 v.e. has been found from thermochemical data for the resonance energy of benzene. Equating the negative of this quantity to 1.1055a, we calculate the value of a to be about —1.5 v.e. This value may not be very reliable, however, since it is based on the assumption that values of bond energies obtained from aliphatic compounds can be applied directly to aromatic compounds. 

The Nature of the Chemical Bond. VI. The Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures1... 

In formulating a set of bond-energy values we first calculated the energies of formation of molecules from experimental values of the heats of combustion of the compounds6 and thermochemical data pertaining to the products of combustion—carbon dioxide, water, nitrogen, etc. The same values for the latter quantities were used as previously.4... 

This study of thermochemical data for a very large number of substances has shown that in every case for which the data are reliable the observed energy of the molecule is equal to that calculated for an assumed distribution of bonds or differs from it in the direction corresponding to greater stability, in accord with the quantum-... 

Simple amides of this type are the bis(trimethylsilyl)amides M[N(SiMe3)2]2 (M = Cd and Hg) the essential thermodynamic data of which have been determined in calorimetric measurements of the heats of hydrolysis in dilute H2S04.146 Evaluation of the measured data yielded the standard enthalpies of formation AH° = —854(21)kJmoU1 and —834(9)kJmol-1 for M =Cd and Hg, respectively. Using subsidiary data, the average thermochemical bond energies E—(Cd—N) 144 and E(Hg—N) 108 kJ mol-1 were also obtained, i.e., the Cd—N bonds are considerably stronger than the Hg—N bonds. 

This type of side-on bending, which has been established by X-ray crystallographic methods for the related acyl complexes (r 5-C5H5)2Zr(COMe)Me (38) and (T>5-C5H5)2Ti(COMe)Cl (39), could overcome the thermodynamic objection, previously discussed, against the formation of a normal, linearly bonded formyl by CO insertion into a metal-hydride bond. Thermochemical data obtained from alcoholysis of zirconium tetralkyl species show that the mean bond energy of Zr—O is 50 kcal/mole greater than that of Zr—C (40). 

The mean bond dissociation energies (E ) given in Table 12 are based on thermochemical data at 25 C19. Unless previously discussed, the heat of formation of the metal alkyl used is that given by Long60. The higher values of E and D2 for dimethyl mercury are obtained when Long s recommended value for the heat... 

RSE values can also be calculated from experimentally measured X - H bond dissociation energies or heats of formation (where available). In order to be directly comparable to the RSE values calculated at the ROMP2 or G3(MP2)-RAD level described above, this requires thermochemical data for the species in Eqs. 1-4 at 0 K. One straightforward approach is the back correction of experimentally measured heats of formation at 298.15 K to 0 K values using thermochemical corrections calculated using the rigid ro-tor/harmonic oscillator model in combination with scaled DFT or UMP2 frequencies [19,23]. 

V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon Free Radicals," J.Chem.Physics 1 (1933) 362374 Linus Pauling and J. Sherman, "The Nature of the Chemical Bond. VI. Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures," J.Chem.Physics 1 (1933) 606617 and Pauling and Sherman, "The Nature of the Chemical Bond. VH. The Calculation of Resonance Energy in Conjugated Systems," J.Chem.Physics 1 (1933) 679686. 

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