Supplement F2: The Chemistry of Amino, Nitroso, Nitro and Related Groups.
Edited by Saul Patai Copyright 1996 John Wiley & Sons, Ltd.
ISBN: 0-471-95171-4
CHAPTER 4
Photoelectron spectra of amines, nitroso and nitro compounds
PAUL RADEMACHER
Institut fur¨ Organische Chemie der Universitat¨ Essen, D-45117 Essen, Germany Fax: (0201)-183-3082; e-mail: RADEM@OC1.ORGCHEM.UNI-ESSEN.DE
I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
160 |
II. AMINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
161 |
A. Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
161 |
B. Aliphatic Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
163 |
C. Aromatic Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
171 |
1. Anilines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
171 |
2. Aminonaphthalenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
176 |
D. IP Values and Proton Affinities . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
178 |
E. IP Values and Biological Activity . . . . . . . . . . . . . . . . . . . . . . . . . . |
179 |
F. Transannular Interaction of Amino Groups with |
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Other Functional Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
181 |
G. Diamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
183 |
H. Polyamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
186 |
I. Miscellaneous Amino Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . |
188 |
III. NITROSO COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
188 |
A. Nitrosomethane and Other Nitrosoalkanes . . . . . . . . . . . . . . . . . . . . . |
188 |
B. Nitrosobenzene and Related Compounds . . . . . . . . . . . . . . . . . . . . . |
191 |
C. Miscellaneous Nitroso Compounds . . . . . . . . . . . . . . . . . . . . . . . . . |
192 |
IV. NITRO COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
192 |
A. Nitromethane and Other Nitroalkanes . . . . . . . . . . . . . . . . . . . . . . . . |
193 |
B. Nitrobenzene and Related Compounds . . . . . . . . . . . . . . . . . . . . . . . |
197 |
C. Miscellaneous Nitro Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . |
200 |
V. ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
201 |
VI. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
201 |
159
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Paul Rademacher |
I. INTRODUCTION
Molecular photoelectron spectroscopy (PES) is widely used to study the electronic structure of molecules, and compounds can be characterized by their PE spectra. In this chapter the results of ultraviolet PE spectroscopic (UPS) studies of molecules which incorporate amino, nitroso or nitro groups will be summarized.
The basic principles of PES, the experimental methods, the interpretation procedures and the applications have been described in several books1 10. There are also some more recent review articles11 13. Extensive data collections are available, e.g. by Robinson14. Also ‘the early days’ of PES have been highlighted15,16. Only a few words are hence necessary to provide a basis for the following statements.
The fundamental principle of PES is the photo-electric effect. A molecule M in the gas phase is irradiated with monochromatic UV light which is usually generated by a helium discharge source (HeI 21.22 eV, 58.43 nm; HeII 40.81 eV, 30.38 nm). Electrons can be ejected when their binding energy is lower than the photon energy leaving behind a radical cation MCž in a certain electronic and vibrational state.
h |
|
M ! MCž C e |
1 |
The kinetic energy of the ejected electrons Ekin(e) is measured and the ionization energy or ionization potential IP is obtained from the energy conservation condition.
IP D h Ekin e |
2 |
Measuring a PE spectrum, the number of photo-electrons per time unit (‘count rate’) is recorded as a function of the kinetic energy or IP. Since the radical cation may be excited to different vibrational states, the ionization band exhibits in general vibrational fine structure and the adiabatic IP, i.e. transition to the vibrational ground state of MCž , can be distinguished from the vertical IP, i.e. transition with the greatest Franck Condon factor. The latter property (IPv) is of higher relevance when studying the electronic structure of a molecule because it is linked with the energy ε of the molecular orbital (MO), from which it was ejected, by Koopmans’ theorem17
IPi D εiSCF |
3 |
stating that the vertical ionization energies are equal to the negative values of the SCF orbital energies which are obtained by quantum chemical calculations. Actually, this is an approximation, which can fail and lead to wrong interpretation of spectral data. Quantitative characteristics of a PE spectrum, such as position and intensity of the ionization bands, vibrational structure, Jahn-Teller and spin-orbital effects, are in general sufficient for reliable assignments of the IPs of simple molecules. The spectra of polyatomic molecules are usually analysed with the aid of quantum chemical calculations making use of Koopmans’ approximation.
Compounds with a certain functional group have a certain number of atoms in common and thus their PE spectra should resemble each other. In particular, this holds for the members of a homologous series of compounds. And for large molecules seldom will a total assignment of all IPs be intended but the IPs related to the characteristic MOs of the functional groups. These are, e.g., the orbitals of double or triple bonds and the n (lonepair) orbitals on heteroatoms like nitrogen, phosphorus, oxygen, sulphur and the halogens. Of particular value are PE spectroscopic studies of structural effects on functional groups. These can be electronic perturbations by substituents, steric strain, conjugation etc.
Many of the investigations on compounds to be included in this chapter were performed in the 1970s. Because instrumentation has made little progress in UPS in more recent
4. Photoelectron spectra of amines, nitroso and nitro compounds |
161 |
years, there can be no reservation to including these spectra. However, some of the computational methods used at that time are no longer adequate today and therefore are generally given lower priority. In a few cases, semi-empirical AM118 or outer valence Green’s function (OVGF)19 calculations were added. Experience has shown that for organic compounds with heteroatoms of the second period of the PSE, in particular nitrogen and oxygen, these methods can give even better results than high-level ab initio calculations.
Some unpublished PE spectra or spectral data from the author’s laboratory are included. These have been measured using a Leybold Heraeus UPG 200 spectrometer with a HeI radiation source. Orbitals were plotted with the program PERGRA20.
In the following sections ionization energies are given as vertical IP values unless stated otherwise.
II. AMINES
The structure, reactivity and biological properties of amines are largely determined by the electron lone-pair at the nitrogen atom. Most amines have a pyramidal structure similar to that of ammonia. Typical bond angles at the nitrogen atom are little different from 109.5° and the nN orbital can be described as an sp3 hybride. However, the classification of the electron lone-pair as ‘non-bonding’ should not lead to the false conception that removal of an electron has no effect on the structure. The ionization bands of electrons from nN orbitals are usually rather broad indicating substantial structural differences between molecule and radical cation. This can be explained by delocalization of the electron lonepair or interaction of the nN orbital with orbitals of the substituents. On the other hand, removal of an electron is accompanied by a flattening of the N pyramid as the most important structural change.
A. Ammonia
The parent compound of all amines, ammonia, has a pyramidal structure with N H bond lengths of 101.5 pm and H N H bond angles of 106.6°. The electronic structure of the ground state may be represented as
1a1 2 2a1 2 1e 4 3a1 2.
The 3a1 D nN orbital (HOMO) is largely made up by the 2pz orbital of the nitrogen atom and houses the electron lone-pair. The doubly degenerate orbital 1e is strongly N H bonding and can be termed NH. The orbital 2a1 can be described as the symmetric combination of the nitrogen 2s and the hydrogen 1s atomic orbitals. It is strongly N H bonding. The 1a1 orbital is essentially the nitrogen 1s AO. Only electrons from the two highest occupied MOs of ammonia are accessible by HeI radiation usually employed in UPS.
The PE spectrum of NH3 (Figure 1) has been analysed frequently and is described even in textbooks1,4,21. The first band (IPa D 10.073, IPv D 10.90 eV4,22) consists of a long single series in the out-of-plane bending vibration mode ( 2) having at least 18 member peaks. The progression shows negative anharmonicity with spacings ranging from about 111 to 140 meV (895 1130 cm 1). The structure of this band, which at first sight would not have been expected for the removal of a non-bonding electron, is a consequence of the large geometrical change associated with the ionization, since the ground state of NH3Cž is planar.
The second band (IPa D 14.725, IPv ³ 15.8 and 16.5 or 16.8 eV4,21) has a completely different shape. As a result of Jahn-Teller splitting there are two maxima. Vibrational structure is found in the regions 14.7 15.9 eV and 16.3 17.8 eV. The mean separation is ca 165 meV D 1330 cm 1. In the middle region (15.9 16.3 eV) there are only weak
162 |
Paul Rademacher |
FIGURE 1. PE spectrum of ammonia: (a) full spectrum, (b) first band expanded
4. Photoelectron spectra of amines, nitroso and nitro compounds |
163 |
indications of vibrational bands, which is caused by predissociation of the radical cation. The third band has its maximum at 21.22 eV1.
It is well known that a change in the H N H valence angle of NH3 is important for the energy of the nN 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).
B. Aliphatic Amines
In Tables 1 8 the first IP values of various amines are summarized. These values refer to the removal of an electron from the nN orbital of the amines. The other ionizations of simple saturated aliphatic amines are of no particular interest. They are associated with the orbitals of the framework of the molecules. More interesting are the effects of substituents and other structural factors on the energy of the nN ionization or in the
limits of Koopmans’ theorem17, IPi D εiSCF |
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on the energy of the nN orbital. |
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TABLE 1. nN |
ionization potentials (eV) of primary |
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amines RNH2 |
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R |
IP |
References |
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Me |
9.64 |
22 |
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Et |
9.46 |
22 |
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Pr |
9.34 |
22 |
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i-Pr |
9.36 |
22 |
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c-Pr |
9.41 |
21 |
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All |
9.43 |
25 |
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Bu |
9.29 |
22 |
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i-Bu |
9.28 |
22 |
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s-Bu |
9.27 |
22 |
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t-Bu |
9.25 |
22 |
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Pen |
9.30 |
22 |
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Hex |
9.31 |
26 |
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c-Hex |
9.15 |
26 |
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Oct |
9.25 |
26 |
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c-Oct |
9.13 |
26 |
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Me3SiCH2 |
9.07 |
27 |
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PhCH2CH2 |
8.99 |
28 |
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Ph |
10.80 |
21 |
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TABLE 2. nN ionization potentials (eV) of acyclic sec- |
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ondary amines R2NH |
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R |
IP |
References |
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Me |
8.94 |
22 |
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Et |
8.74 |
26 |
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Pr |
8.55 |
22 |
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i-Pr |
8.42 |
22 |
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All |
8.79 |
25 |
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Bu |
8.49 |
22 |
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i-Bu |
8.47 |
22 |
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Pen |
8.45 |
22 |
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c-Hex |
8.14 |
26 |
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Me3SiCH2 |
8.36 |
27 |
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Ph |
10.64 |
29 |
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164 |
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Paul Rademacher |
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TABLE 3. nN ionization potentials (eV) of acyclic secondary |
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amines R1R2NH |
|
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|
R1 |
R2 |
IP |
References |
|
Me |
PhCH2CH2 |
8.66 |
28 |
|
Me |
Me3SiCH2 |
8.55 |
27 |
|
Me |
Cyclodecyl |
8.46 |
26 |
|
Me |
PhCH2 |
8.78 |
30 |
|
Et |
Me3SiCH2 |
8.46 |
27 |
|
Ph |
C6F5 |
11.16 |
31 |
TABLE 4. nN ionization potentials (eV) of acyclic tertiary amines R3N
R |
IP |
References |
|
|
|
H |
10.92 |
32 |
Me |
8.53 |
33, 34 |
Et |
8.08 |
33, 34 |
Pr |
7.92 |
33, 34 |
i-Pr |
7.18 |
35 |
c-Pr |
8.44 |
36 |
All |
8.30 |
25 |
Bu |
7.90 |
33, 34 |
Pen |
7.85 |
22 |
Me3SiCH2 |
7.66 |
27 |
Ph |
7.00 |
37 |
CHF2 |
11.65 |
38 |
F3C |
12.52 |
38 |
H3Si |
9.7 |
39 |
Me3Si |
8.58 |
40 |
H3Ge |
9.2 |
39 |
TABLE 5. nN ionization potentials (eV) of tertiary amines R1R2R3N
R1 |
R2 |
R3 |
IP |
References |
Me |
Me |
Et |
8.22 |
34 |
Me |
Me |
i-Pr |
8.20 |
41 |
Me |
Me |
Bu |
8.35 |
41 |
Me |
Me |
i-Bu |
8.31 |
41 |
Me |
Me |
t-Bu |
8.08 |
41 |
Me |
Me |
Pen |
8.33 |
26 |
Me |
Me |
Me3SiCH2 |
8.20 |
27 |
Me |
Me |
PhCH2CH2 |
8.35 |
28 |
Me |
Me |
Ph |
9.85 |
42 |
Me |
Me |
H2B |
9.51 |
43 |
Me |
Me |
Me2B |
8.92 |
43 |
Me |
Me |
F2B |
9.71 |
43 |
Me |
Me |
Cl2B |
9.56 |
43 |
Me |
Me |
Br2B |
9.60 |
43 |
Me |
Me |
H3Si |
8.5 |
39 |
Me |
Me |
Me3Si |
8.03 |
40 |
Me |
Et |
Et |
8.44 |
34 |
Me |
H3Si |
H3Si |
9.2 |
39 |
(continued)
4. Photoelectron spectra of amines, nitroso and nitro compounds |
165 |
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TABLE 5. |
(continued) |
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R1 |
R2 |
R3 |
IP |
References |
|
Me |
Me3Si |
Me3Si |
8.21 |
40 |
|
Et |
Et |
Me3SiCH2 |
7.93 |
27 |
|
Et |
Et |
Ph |
9.70 |
42 |
|
Et |
i-Pr |
i-Pr |
7.66 |
26 |
|
Et |
Me3SiCH2 |
Me3SiCH2 |
7.82 |
27 |
|
Pr |
Pr |
Ph |
9.63 |
26 |
|
Pr |
Pr |
Me3Si |
7.83 |
40 |
|
Pr |
Me3Si |
Me3Si |
8.18 |
40 |
|
i-Pr |
i-Pr |
c-Pr |
7.79 |
36 |
|
i-Pr |
i-Pr |
Ph |
8.93 |
26 |
|
i-Pr |
c-Pr |
c-Pr |
8.14 |
36 |
|
c-Pr |
t-Bu |
t-Bu |
7.76 |
36 |
|
Bu |
Bu |
Ph |
9.52 |
26 |
|
Ph |
Ph |
Me3Si |
8.05 |
31 |
|
Ph |
Ph |
SiMe2C6F5 |
7.69 |
31 |
|
Ph |
Ph |
Ph3Si |
7.32 |
31 |
|
Ph |
Ph |
(C6F5)3Si |
8.07 |
31 |
|
Ph |
Me3Si |
Me3Si |
10.43 |
26 |
|
CF3 |
CHF2 |
CHF2 |
12.08 |
38 |
|
TABLE 6. nN ionization potentials (eV) of cyclic secondary amines
|
IP |
References |
|
|
|
(CH2)2NH |
9.84 |
44, 45 |
(CH2)3NH |
9.04 |
23 |
(CH2)4NH |
8.77 |
23, 46 |
2,5-Dihydropyrrole |
8.51 |
23 |
(CH2)5NH |
8.64 |
23, 46 |
1,2,5,6-Tetrahydropyridine |
8.64 |
23 |
(CH2)6NH |
8.41 |
23 |
O(CH2CH2)2NH |
8.91 |
46 |
(CH2)7NH |
8.41 |
26 |
7-Azanorbornane |
9.00 |
47 |
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TABLE 7. nN ionization potentials (eV) of cyclic tertiary amines
|
IP |
References |
|
|
|
(CH2)2NMe |
9.23 |
24, 45 |
(CH2)3NMe |
8.93 |
24 |
(CH2)4NMe |
8.41 |
23, 24 |
(CH2)4NHex-c |
7.96 |
46 |
1-Methyl-2,5-dihydropyrrole |
8.61 |
23 |
(CH2)5NMe |
8.35 |
23, 24, 48 |
(CH2)5NHex-c |
7.93 |
46 |
(CH2)5NCH2SiMe3 |
8.18 |
27 |
1-Methyl-1,2,5,6-tetrahydropyridine |
8.29 |
23 |
(CH2)6NMe |
8.29 |
23, 24 |
(CH2)7NMe |
8.02 |
24, 48, 49 |
(CH2)7NEt |
7.93 |
49 |
(continued overleaf )
166 |
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Paul Rademacher |
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TABLE 7. (continued) |
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IP |
References |
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(CH2)7NPr-i |
|
7.69 |
49 |
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(CH2)7NBu-t |
|
7.64 |
49 |
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(CH2)8NMe |
|
7.93 |
24 |
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(CH2)9NMe |
|
7.99 |
24, 48 |
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(CH2)10NMe |
|
8.00 |
24 |
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(CH2)11NMe |
|
8.12 |
26 |
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(CH2)12NMe |
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8.11 |
24 |
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(CH2)15NMe |
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8.16 |
24 |
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O(CH2CH2)2NMe |
|
8.64 |
26 |
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O(CH2CH2)2NHex-c |
|
8.18 |
46 |
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CH2(SiMe2CH2)2NMe |
|
7.90 |
27 |
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1-Azabicyclo[1.1.0]butane |
³9.75 |
50 |
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1-Azabicyclo[2.2.2]octane (quinuclidine) |
8.06 |
51 |
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1-Aza-5-borabicyclo[3.3.0]octane |
8.06 |
43 |
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1-Azabicyclo[3.3.3]undecane (manxine) |
7.13 |
52 |
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1-Azabicyclo[4.4.4]tetradecane |
7.84 |
52 |
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9-Methyl-9-azabicyclo[3.3.1]nonane |
7.84 |
53 |
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1-Azadamantane |
|
7.94 |
54 |
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1-Azatwistane |
|
7.98 |
51 |
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TABLE 8. nN |
ionization potentials |
(eV) of |
simple |
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halogenoamines |
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IP |
References |
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H2NF |
11.62 |
32 |
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(F3C)2NF |
12.45 |
38 |
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HNF2 |
12.38 |
32 |
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F2CHNF2 |
12.33 |
38 |
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F3CNF2 |
12.62 |
38 |
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H2NCl |
10.60 |
55 |
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HNCl2 |
10.52 |
55 |
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NF3 |
13.83 |
32 |
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H2NBr |
10.18 |
56, 57 |
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HNBr2 |
10.1 |
58 |
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NBr3 |
10.10 |
56 |
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MeNHCl |
9.70 |
59 |
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MeNCl2 |
10.06 |
59 |
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Me2NCl |
9.25 |
60 |
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MeNHBr |
9.60 |
56 |
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MeNBr2 |
9.62 |
56 |
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Me2NBr |
9.15 |
56, 60 |
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Morishima and coworkers23 observed that the first IPs of cyclic secondary amines and their N-methyl derivatives (Tables 6 and 7) fall in the order of increasing ring size, i.e. aziridine > azetidine > pyrrolidine > piperidine > hexahydroazepine (hexamethyleneimine). The authors suggested that this is because of changes in the overall hybridization of the nitrogen atom as the ring size is increased, i.e. change of the lonepair orbital nN from an sp2 hybride (in aziridine) towards an sp3 orbital. Similar as in cycloalkanes the s character of the carbon hydrogen bond increases with decreasing ring size, in cyclic amines the s character of the lone-pair electrons increases. This is confirmed by a linear correlation of the first IPs of cyclic amines and the 13C 1H nuclear spin coupling constants of the corresponding cycloalkanes.
4. Photoelectron spectra of amines, nitroso and nitro compounds |
167 |
IPv (eV)
9.2
9.0
8.8
8.6
8.4
8.2
8.0
7.8
3 |
5 |
7 |
9 |
11 |
13 |
15 |
17 |
19 |
Ring size, n
FIGURE 2. Plot of experimental nN IPs for cyclic N-methyl amines ( ). Experimental ( ) and calculated (ž) IPs for open-chain methyldialkylamines having the same number of carbon atoms are also shown. Reproduced with permission from Reference 24
The higher members in the series of N-methyl derivatives (ring size 8) exhibit rather constant IP(nN) values with minor variations probably owing to conformational effects24 (Figure 2). The cyclic N-methylamines with an even number of ring atoms seem to exhibit a somewhat larger IP than those with an odd ring size.
The energy of the nN orbital of amines varies appreciably with the substitution of the nitrogen atom. Accordingly, the lone-pair ionization is a sensitive indicator of the electron donating or withdrawing power of substituents. The main effect of alkyl groups can be attributed to inductive destabilization of the nitrogen valence orbitals. But also steric and conformational effects are of importance. The contribution of hyperconjugative effects or in other words interaction of the nN orbital with or pseudo orbitals of the substituents can best be studied in compounds with a rigid or at least minor flexible conformation.
From the IP values of primary, secondary and tertiary amines, R NH2, R2NH, R2NH and R3N with R being simple alkyl groups like Me, Et, Pr. . . (Tables 1 5), it is obvious that the effects of the substituents on lowering the ionization energy of the lone-pair electrons are not additive but that groups already present ‘dilute’ the action of the next one. The second and the third substituent have roughly only half the effect of the first one. However, a striking exception is triisopropylamine which has an abnormally low IP(nN) of only 7.18 eV35. As has been shown by Bock and coworkers35, this tertiary amine has a planar configuration of the nitrogen atom. The ‘extra’ destabilization of the nN orbital
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Paul Rademacher |
of this compound caused by the planarization can be estimated by comparison with other amines to about 0.5 eV.
Comparison of IP(nN) values of cyclic secondary amines (Table 6) with those of the respective 1-methyl derivatives (Table 7) reveals that the effect of the additional methyl group decreases from 0.6 eV in aziridine to 0.1 eV in hexahydroazepine (hexamethyleneimine).
An interesting interplay of steric and electronic effects is presented by tertiary cyclopropylisopropylamines c-Prni-Pr3 nN n D 1 3 36 (Tables 4 and 5):
|
c-Pr3N |
c-Pr2(i-Pr)N |
c-Pr(i-Pr2)N |
i-Pr3N35 |
IP(nN): |
8.44 |
8.14 |
7.79 |
7.18 eV |
The surprisingly large difference of the IP(nN) values [ IP nN D 1.26 eV] of the first and the last member of this series indicates that tricyclopropylamine behaves more like a simple tertiary amine with linear alkyl groups [IP nN D 8.5 7.9 eV, Table 4] than like its acyclic analogue i-Pr3N. This is confirmed by an X-ray structure analysis of c-Pr3N which revealed C N C angles of 110.1° with approximate C3v symmetry indicating normal pyramidality of the nitrogen atom61. In the series c-Pr3N ! c-Pr2 i-Pr N ! c-Pr i-Pr2 N ! i-Pr3N, the shift IP(nN) for replacing a cyclopropyl by an isopropyl group increases from 0.30 eV for the first pair to 0.61 eV for the last pair, which is in accord with increasing planarization of the nitrogen atom.
Another very interesting system is the series of fluoroamines NH3 nFn (n D 0 3) (Table 8) which has been investigated by Bock and coworkers32. As expected from the difference in electronegativity between H and F, all ionizations energies increase with F substitution. For NH3 ! H2NF and for H2NF2 ! NHF2 both increments IP(nN) are about 0.7 eV. For NHF2 ! NF3 a considerably larger shift of 1.45 eV is observed. This is rationalized by MNDO results indicating that for NF3, nN is bonding, while for the other members of this series it is anti-bonding32. In addition, also an increase of the s character in the hybride orbital of the lone-pair may contribute to this effect. The F N F bond angle of NF3 is about 106°. NF3 has the highest nitrogen lone-pair ionization energy known so far, even exceeding the value 12.52 eV for the nearly planar N(CF3)338 (Table 4). Relative to the planar amines i-Pr3N35 and 1-azabicyclo[3.3.3]undecane52 (see Section II.D) with their rather low lone-pair ionization of only ca 7.1 eV, the span of nN ionizationsIP nN D 13.8 7.1 D 6.7 eV results, indicating that the NH3Cž radical cation ground state is extremely substituent-sensitive. Compared to ammonia [IP nN D 10.9 eV], the
fluorine ligands cause a stabilization of 2.9 eV, while |
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on the other side |
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destabilization |
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up to 3.8 eV has been found. An exceptionally |
low IP n |
N |
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D |
7.66 eV was also found |
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for tris(trimethylsilylmethyl)amine, N(CH2SiMe3)3 |
, indicating that trimethylsilylmethyl |
substituents are powerful electron donors.
The reduced energy difference between the ground state of the neutral molecules and the resulting radical cation states are best rationalized in terms of stabilizing delocalization of the positive charge. Applying Koopmans’ theorem17, IPi D εSCFi , and using perturbation arguments, the replacement of substituents, e.g. Me by CH2SiMe3, destabilizes the nN orbital both inductively and due to increased hyperconjugation (nN/ CSi).
By comparison with IP data from silylamines, the vertical IP corresponding to a nitrogen 2p orbital in planar trimethylamine was estimated to be 7.7 7.9 eV40.
Larger alkyl groups are well-known to decrease the IPs of heteroatomic compounds relative to smaller ones. Danby and coworkers62 have shown that vertical ionization potentials can be described empirically by using substituent parameters R ( Me D 0, R
is negative for larger alkyl groups) with equation 4. |
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IP(RX) D IP(MeX) C RX Ð R |
4 |