Ionization potentials of amines

FIGURE 5. Relationship between log rate of quenching and ionization potentials of amines for quenching of singlet oxygen (methanol solution) [reprinted, with permission, from R. H. Young, R. L. Martin, D. Feriozi, D. Brewer, and R. Kayser, Photochem. Photobiol., JT, 233 (1973)1. 

Fig. 7. Ionization potentials of amines compared with ionization potentials of...

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

Background alkali metal chemistry. The alkali metals have the lowest ionization potentials of any group in the periodic table and hence their chemistry is dominated by the M+ oxidation state. However, it has been known for some time that a solution of an alkali metal (except lithium) in an amine or ether forms not only M+ ions and solvated electrons but also alkali anions of type M (Matalon, Golden Ottolenghi, 1969 Lok, Tehan Dye, 1972). That is, although an alkali metal atom very readily loses its single s-shell electron ... 

It is well known that a change in the H—N—H valence angle of NH3 is important for the energy of the mn orbital. In the transition from the pyramidal to planar conformation, this orbital destabilizes appreciably with decreasing contribution of the nitrogen 2s orbital. This is also reflected in the very low ionization potentials of planar amines (see below). 

Higher ionization potentials in amines The large difference in ionization potential between the nitrogen lone pair and the electrons of alkyl groupings has made it comparatively easy... 

The second ionization potentials (e.v.) of aliphatic amines compared with first ionization potentials of the isoelectronic hydrocarbons... 

Leland and Powell also studied ECL obtained from reaction of [(bpy)3Ru]3+ with trialkylamines [47], Since the mechanism involves an electron transfer from the amine to Ru3+, there exists an inverse relationship between the first ionization potential of the amine and ECL intensity. The relative intensity of [(bpy)3Ru]2+ ECL was found to be ordered tertiary > secondary > primary. Quaternary ammonium ions and aromatic amines do not produce ECL with Ru(II) diimine complexes. Brune and Bobbitt subsequently reported the detection of amino acids by [(bpy)3Ru]2+ ECL [28,29], Employing capillary electrophoresis for separation, the presence of various amino acids can be detected directly by reaction with [(bpy)3Ru]3+ generated in situ with up to femtomo-lar sensitivity and with a selectivity for proline and leucine over other amino acids. The formation of an amine radical cation intermediate is characteristic of proposed mechanisms of both aliphatic amines and amino acids. 

The PID allows for the detection of aromatics, ketones, aidehydes, esters, amines, organosulfur compounds, and inorganics such as ammonia, hydrogen sulfide, HI, HC1, chlorine, iodine, and phosphine. The detector will respond to all compounds with ionization potentials within the range of the UV light source, or any compound with ionization potentials of less than 12 eV will respond. 

Talamoni and coworkers17 found that in methanol, allylamine has no influence on the yield of 0-Ps. Other amines, such as w-propylamine and triethylamine, increase the yield of the o-Ps. The authors ascribe this difference to the higher ionization potential of allylamine, making positive charge transfer from the solvent ions (the holes) ineffective. The ionization potential of allylamine (9.6 V) is considerably higher than that of other amines (w-propylamine 8.8 eV, triethylamine 7.8 eV, aniline 7.7 eV). They found a correlation between the ionization potentials and the enhancement factors. In water, allylamine also enhances the formation of 0-Ps, due to the much higher ionization potential of water, 12.6 eV (while the value for methanol is only 10.8 eV). 

The ionization potential of the lone electron pair at the nitrogen site of both amines and enamines is unambiguously assigned. It may therefore be of much use in order to determine whether the enamine is protonated at the same site as its corresponding saturated amine (i.e. whether or not it is protonated at the nitrogen atom) on the basis of HA values. 

Some data for the rate constants of the triplet-state quenching by amines are given in Table 2. As expected, a correlation between the 3k H values and the ionization potential of the amine exists. However, the influence of the spectroscopic nature of the excited state (e.g., nre or nn ) is not adequately understood, so far. 

The efficacy of various amines in this electron-transfer-initiated process is governed by several factors [51]. While the ionization potential of the amine is obviously important, substituents present on the carbons beta to the nascent radical center also play a role. Thus, it has been shown that dimethylaniline is... 

There are a number of studies which indicate that certain TICT molecules undergo specific interactions with solvent or quencher molecules. One of the early proposals to explain the dual fluorescence of DM ABN was in fact that of a solute-solvent exciplex (excited complex) [26-28]. The first specific interaction documented as such, however, was that of DMABN (and model compounds) with saturated amines, where a mechanism necessitating a close approach was postulated. This was concluded from the lack of correlation of the quenching reactivity with the ionization potential of the amine quencher. However, a correlation was found between the reactivity pattern and the sterical demands of the saturated amine [140,141]. The complexes formed were non-emissive and were proposed to involve a two-center-three-electron bond between the aromatic and the saturated amino group. 

Leland and Powell [53] also studied ECL obtained from reaction of [(bpy)3Ru] + with tripropylamine. There is an inverse relationship between the first ionization potential of the amine and ECL intensity and therefore [(bpy)3Ru] + ECL intensity can be ordered as follows tertiary > secondary > primary. 

Because the delta chi indexes, A x> emphasize the role of the nonsigma electrons, they have become important in relating to properties which are more dependent on lone pair and 7t electrons. The delta chi indexes introduced above have also been found useful in an analysis of the ionization potentials of a set of 24 alkyl amines, alcohols, and ethers. The delta chi for the first three orders are defined as above. 

Although iodine has been most extensively studied, chlorine and bromine show similar behavior. For a given group of donors, the frequency of the intense charge-transfer absorption band in the ultraviolet is dependent upon the ionization potential of the donor solvent molecule, and electronic charge can be transferred either from a -electron system as in benzene or from lone-pairs as in ethers or amines. Charge-transfer spectra and complexes are of importance elsewhere in chemistry. 

Aliphatic amines have been found to quench the fluorescence of acridine efficiently and a linear relationship has been demonstrated between the logarithms of the Stern-Volmer quenching constants and the ionization potentials of the amines. The photoreduction of the acridine occurs in 20% MejCOH-MeCN via an exciplex formed between the ground state of the amine and the acridine (ti, n ) state and in 20% McaCOH-CgHg via the (n,n ) state of the acridine. A quantitative study has been reported of the photoreactivity of 2-nitrophenazine with tertiary amines. Excitation to the state leads via a non-emitting... 

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