Data Bond Energies

These examples show that in a number of cases the bond energy is found to be an additive quantity, but the extension of these calculations to other series of molecules is unfortunately not possible owing to the lack of accurate data. Bond energies of other groups must therefore be obtained by employing approximate data and the values may have to undergo alteration as more accurate data become available. The values calculated... 

In the present set of calculations we have used the functional proposed by Becke (7), which adopts a non-local correction to the HFS exchange, and treats correlation between electrons of different spins at the local density functional level. All calculations presented here were based on the LCAO-HFS program system due to Baerends et al, (2) or its relativistic extension due to Snijders et al.(3), with minor modifications to allow for Becke s non-local exchange correction as well as the correlation between electrons of different spins in the formulation by Stoll et al, (4) based on Vosko s parametrization (5) from homogeneous electron gas data. Bond energies were evaluated by the Generalized Transition State method (6), or its relativistic extensions (7). 

These data also show that the bond energies decrease as the atomic number of the metal increases in the same Group of the Periodic Table. 

The resulting data for liquid metals indicate a systematic relationship witlr the bonding energy of the element, which is reflected in the heat of vaporization... 

Table 1.3 gives some bond-energy data. Part A includes bond energies for some simple diatomic molecules and generalized values for some of the types of bonds found... 

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. 

The C-F bonds in 1 fluoroalkenes and fluorobenzenes also are very strong (Table 17), but alkene k bond strengths vary with the level of fluonnabon (Table 18) Both CHF=CF2 and CF2=Cp2 have significantly weaker 7t bonds than CH2= CH2, CH7=CHF, and CH2=Cp2, consistent with other data indicatuig that tn- and tetrafluonnation thermodynamically destabilize double bonds [75] The low n bond energy in Cp2=CF2 underlies its propensity to undergo thermal biradical [2-1-2] cycloaddibons [103] (see p 767 )... 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

Table 16-111 collects some data. The existence of a correlation between the ionization energy difference, Ei(X) — Ei( Y), and the bond energy discrepancy, AHxr — i(AHx, + is obvious. 

From the following bond energy data and the ionization energies given in Table 15-111, calculate the entries in the last two columns of Table 16-111 for the compounds LiF and LiBr. The ionization energy, (, for bromine atom is 273 kcal/mole. 

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. 

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