Adiabatic ionization energy molecule

The adiabatic ionization energy of any molecule AB (mono-, di-, or polyatomic), represented by ) (AB), is the minimum energy required to remove an electron from the isolated molecule at 0 K ... 

Figure 4.2 Potential energy curves for the molecules AB and AB+ showing the vertical and the adiabatic ionization energies of AB. r is the A-B bond length, and v represents the vibrational quantum number.

Transition energies between cations and neutral species were calculated by two procedures. In the first one, the vertical ionization energy of the neutral molecule was determined with the P3 method. These values were compared with experimental adiabatic ionization energies of the neutral molecules, which were adjusted according to... 

In Fig. 14 the magnified central part of the spectrum shown in Fig. 13 is presented together with the positions of the Rydberg peaks of two series calculated using Eq. (6). The assignment of the series limits leads to accurate values of the lowest adiabatic ionization energy of the benzene (C6D6> molecule and the rotational constants of its cation [89]. 

One further consideration is that the measured IE may not be the adiabatic ionization energy. If the ion structure differs significantly from that of the neutral, the measured IE may exceed the adiabatic value. In this situation, substituting the measured IE in eqn. (20) will tend to make the calculated critical energy [eqn. (20)] too small. Examples of molecules with which the adiabatic IE has not been clearly observed are (C2HS)+from (C2H5) [16] and [(CH3)3CH]t from (CH3)3CH [794]. 

The adiabatic ionization energy and the adiabatic electron affinity of a molecule AB are the minimum energies required to remove or attach an electron from the isolated molecule at zero Kelvin (reactions (14) and (15)) respectively. That means that the transition is between the ground states of all molecules present, assumed to be perfect gases with no interaction among them. 

FIGURE 6. Schematic representation of the PE spectrum of a hypothetical molecule. Abscissa Ionization energy / in eV. Ordinate Intensity C in counts per second (cps). Pj = adiabatic ionization energy, / = vertical ionization energy, I f = ionization energy corresponding to the maximum of the band... 

The principles of ion themiochemistry are the same as those for neutral systems however, there are several important quantities pertinent only to ions. For positive ions, the most fiindamental quantity is the adiabatic ionization potential (IP), defined as the energy required at 0 K to remove an electron from a neutral molecule [JT7, JT8and 1191. 

Even the photoelectron spectroscopy of closed-shell molecules is valuable for the physical chemistry of radicals because a difference between the nth and the first adiabatic ionization potentials determines the excitation energy in a radical cation for a transition from the ground doublet state to the (n — 1) excited doublet state. 

The adiabatic ionization potential (1A) of a molecule, as shown in Figure 4.1, equals the energy difference between the lowest vibrational level of the ground electronic state of the positive ion and that of the molecule. In practice, few cases would correspond to adiabatic ionization except those determined spectroscopically or obtained in a threshold process. Near threshold, there is a real difference between the photoabsorption and photoionization cross sections, meaning that much of the photoabsorption does not lead to ionization, but instead results in dissociation into neutral fragments. 

Table 3.5. Adiabatic and vertical ionization energies of small molecules (cf. Table 3.4), compared with Koopmans -theorem (KT) estimates using MO or NBO orbital energies, and with experimental adiabatic values...

First of all, quantum calculations allow one to predict basicity scales in agreement with experiment provided that the calculations are performed on the preferred conformation of the isolated molecule. If this is not done, a given term within a consistent series may jump from one rank to another as a function of the conformation used for the calculations. The determinant role of preferred conformation on any property (barrier to internal rotation and inversion, dipole moment, first adiabatic ionization potential, acidity and basicity in the gas phase, energy of complexation to BF3, etc.) was clearly demonstrated. We further show the importance of the role of preferred conformation in explaining some of the anomalies in Drago s systematics. 

Adiabatic detachment energy [7]. Abbreviations used rcoy (Em) = covalent radius of element E in a trivalent compound BE(E-E) = bond enthalpy of a single E-E bond D°298(E2) = dissociation enthalpy of the E2 molecule at standard conditions IE = ionization enthalpy EA = electron affinity AHf°(E2 g) = standard enthalpy of formation of the gaseous E2 molecule. 

Another consequence of the stronger interactions upon ionization is that the equilibrium geometry of the ionized complex may differ signihcantly from that of the neutral states. Broadened ionization onsets are frequently attributed to the spectral superposition of ionization into several vibrational levels for which Franck-Condon factors are more favorable. As a result, the adiabatic ionization potential may be considerably lower than the vertical potential, and the observed ionization onsets may occur above the adiabatic potential. Another factor to be considered is the conformation-dependent efifect, due to the different conformations of the solvent molecules. The most populated form of a complex may involve a less stable form of the solvent. After photoionisation, the lowest-energy dissociation channel in the complex ion leads to the most stable form of isolated solvent, which has to be taken into account for the estimate of the binding energy. 

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