Photoelectron spectra and electronic structures of substituted pyrimidines

Article abstract of DOI:10.1002/ejoc.200400534

The electronic structures of pyrimidine (1) and its substituted derivatives 2-15 have been investigated by ultraviolet photoelectron spectroscopy and quantum chemical methods. The ionisation potentials corresponding to the π MOs π₁-π₃

Full text of DOI:10.1002/ejoc.200400534

FULL PAPER
Photoelectron Spectra and Electronic Structures of Substituted Pyrimidines
[a]
[a]
[b]
[a]
Ursula Lottermoser, Paul Rademacher,* Monika Mazik, and Klaus Kowski
Keywords: Photoelectron spectroscopy / Density functional theory calculations / Electronic structure / Ionisation
potentials / Linear free energy correlation / Multiple linear regression analysis
The electronic structures of pyrimidine (1) and its substituted
p
substituent constants indicated that Hammett σ values
derivatives 2−15 have been investigated by ultraviolet photo- performed well for all these IPs, whereas the resonance para-
+
electron spectroscopy and quantum chemical methods. The
ionisation potentials corresponding to the π MOs π -π and
the two n orbitals of the pyrimidine unit could be deter-
N
meters R and R were satisfactory for π and poor for n ionis-
ations.
1
3
N
mined and assigned for 1−15. Multiple linear regression
analyses of the IPs related to these orbitals with different
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
Results and Discussion
PE Spectra and Electronic Structures of Pyrimidines 1؊15
Substituents can modify the electronic structure of a par-
ent molecule and knowledge of its electronic properties and
the nature of its interactions with substituents could help
in better understanding its chemical properties. Previously,
we have investigated the ultraviolet photoelectron (PE)
spectra and electronic structures of 1-substituted 1H-benzo-
The molecular structure of pyrimidine (1) has been deter-
[10]
mined by X-ray structure analysis.
The molecule is
planar and has C2v symmetry. In 1 there are 15 occupied
valence MOs, consisting of three π type MOs (π Ϫπ ) be-
1
3
longing to the symmetry groups b and a , and twelve σ
1
2
type MOs which belong to the groups a and b . The elec-
1
2
[
1]
triazoles. With only a few exceptions, the ionisation po-
tentials corresponding to all occupied π MOs (π -π ) and
tron lone-pairs of the two N atoms occupy the n type or-
Ϫ
ϩ
1
5
bitals nN and nN of b and a symmetry, respectively.
2 1
the two nN orbitals of the 1H-benzotriazole unit could be
determined and assigned for the parent 1H-benzotriazole
and 19 of its derivatives. Linear free energy (LFE) corre-
lations of the ionisations related to these orbitals with in-
These two n type and the three π type MOs can be assumed
to occur with more or less modified shapes and energies in
all substituted pyrimidines and we call them the ‘‘character-
istic pyrimidine MOs’’. These MOs, as calculated by the
B3LYP method, are shown in Figure 1.
ductive substituent parameters σ and F were reasonable
I
except for IP(π ). The energies of the π and n MOs, except
3
for π , are thus affected primarily by the electronegativity
3
of the substituents. The different behaviour of π is caused
3
by direct interactions with substituent orbitals. This work
presents the results for substituted pyrimidines 1Ϫ15. PE
spectra of pyrimidine and some of its derivatives have been
investigated previously[
2Ϫ9]
but no systematic research of
substituent effects on the electronic structures has been per-
formed until now.
[
[
a]
b]
Institut für Organische Chemie, Universität Duisburg-Essen,
Campus Essen
Universitätsstr. 5Ϫ7, 45117 Essen, Germany
Institut für Organische Chemie, Technische Universität
Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Figure 1. Characteristic MOs of pyrimidine (1) (B3LYP results)
522
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200400534
Eur. J. Org. Chem. 2005, 522Ϫ531

Photoelectron Spectra and Electronic Structures of Substituted Pyrimidines
FULL PAPER
Scheme 1
The PE spectra of compounds 1Ϫ15 (Scheme 1) are de- limit the amount of data, we have omitted some σ orbitals
picted in Figures 2 and 3. The measured ionisation poten- which are irrelevant for spectroscopic assignments.
tials are summarised in Tables 1Ϫ15 together with the rel-
evant results of quantum chemical calculations. In order to B3LYP
We performed density functional theory (DFT)
[
11,12]
calculations with the basis set 6-31ϩG*. As-
Figure 2. PE spectra of pyrimidines 1Ϫ8
Eur. J. Org. Chem. 2005, 522Ϫ531
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
523

U. Lottermoser, P. Rademacher, M. Mazik, K. Kowski
FULL PAPER
Figure 3. PE spectra of pyrimidines 9Ϫ15
signments of the IPs can be achieved using Koopmans’ the- tween Ϫε(HOMO) and the calculated IP1v in order to ob-
[
13]
[16]
orem,
IP ϭ Ϫε , by which vertical ionisation energies tain higher IP values.
i
i
v
and SCF MO energies are related. Although KohnϪSham
(
Whereas for the compounds studied here typical energy
KS) orbitals obtained by DFT methods[ are not SCF differences between IP and Ϫε
14]
B3LYP
values are about 3 eV,
i
i
MOs and their physical meaning is still debated, it has been experimental and calculated IPi values differ only by
B3LYP
shown that they can be used with high confidence for the 0.1Ϫ0.8 eV. Furthermore, both Ϫεi
and calculated
interpretation of PE spectra.[
15,16]
Much better agreement IP (calcd.) values are linearly correlated with the exper-
i
between experimental and theoretical values can be ex- imental IP (exp) values with correlation coefficients (both
i
pected for the first vertical IP (IP ) when the energies of r ϭ 0.992) close to 1.000.
1
v
the molecule M and the radical cation M· are calculated
ϩ
In the following section, a brief interpretation and dis-
by the B3LYP method. Since a vertical IP corresponds to cussion of the PE spectra of compounds 1Ϫ15 are given.
the transition with the highest FranckϪCondon factor MOs are classified and labelled according to their symmetry
without any structural change, a single point calculation properties. For the labelling, only valence MOs were taken
can be performed for M· using the molecule’s geometry in into account. The core orbitals (i.e., 1s orbital except for
order to obtain IP . The corresponding energy values, hydrogen, 2s and 2p for third-row elements, etc.) were neg-
ϩ
1
v
which do not include any zero-point corrections, are sum- lected. In certain cases (compounds 5, 6 and 12), the exact
marised in Table S1 (see Supporting Information). We can molecular symmetry may be lower than that used for the
B3LYP
now correct the other ε
values by the difference be- assignments but deviations are very small. Because of their
524
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org Eur. J. Org. Chem. 2005, 522Ϫ531

Photoelectron Spectra and Electronic Structures of Substituted Pyrimidines
FULL PAPER
unsymmetrical substitution, compounds 7Ϫ9, 11 and is quite similar to that of the corresponding band in
[
18Ϫ20]
13Ϫ15 have no molecular symmetry and accordingly their ethynylbenzene
which is isoelectronic with 3. In con-
point group is C . Many of the observed ionisation bands, trast to the former compound, the second ionisation related
1
in particular those related to ionisations from π and n MOs, to the triple bond, π(CϵC) (2b ), is not characterised by a
1
exhibit vibrational splitting. The corresponding frequencies similar fine structure but is part of a composite band to
are given in the respective tables. No attempt was made to which three ionisations contribute. The ionisations related
Ϫ
assign the frequencies to certain vibrations of the respective to the two highest occupied MOs, nN (6b ) and π (3b ),
2
3
1
radical cations but, generally, it can be stated that they be- are quite close in energy and form a common band. Interac-
long to valence or deformation vibrations of the pyrimidine tion of the orbitals π and π(CϵC), which are of the same
3
ring or a substituent. Sharp peaks at 15.76 and 15.94 eV, symmetry, destabilises the former relative to its value in py-
present in some spectra, originate from argon that was used rimidine (1) by 0.66 eV. On the other hand, π in 3 is stabil-
2
as a calibration gas. IP values which could be determined ised relative to its value in 1 by about 0.3 eV. By the same
ϩ
Ϫ
only approximately from the spectra are given in parenth- value, nN of 3 is also shifted while nN remains nearly
eses. unchanged. 5-Ethynylpyrimidine (4) has C2v molecular
The spectrum of pyrimidine (1) (Figure 2, Table 1) has symmetry and its structure has been determined by X-ray
[
3]
[21]
been investigated previously by Gleiter and Heilbronner.
crystallography.
The spectrum of this compound (Fig-
Most assignments given by these authors are confirmed by ure 2, Table 4) is similar to that of its isomer 3 and that of
[
18Ϫ20]
the present study. Our data and assignment (14.0 eV, σ, 6a1) ethynylbenzene
deviate from those of the previous investigation (13.9 eV, π, and 4 with only minor energy differences.
b1) only for the fifth ionisation band. The spectrum of 4-
and the IP sequence is the same for 3
1
methylpyrimidine (2) (Figure 2, Table 2) is rather similar to
that of the parent compound 1. The IPs related to the
characteristic pyrimidine MOs (Figure 1) are lowered by the
inductive effect of the methyl group by about 0.3 eV from
Table 3. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[cm ] and orbital energies ε [eV] of 2-ethynylpyrimidine (3)
ϪεB3LYP
IPB3LYP
IP/ν
those of 1. C molecular symmetry can be assumed with a
mirror plane lying in the molecular plane.
s
Ϫ
9.6 sh
.76/700
0.76/1950
11.50/1200
11.7 sh
7.31
7.41
8.04
8.92
9.12
9.04
9.14
9.77
n
π
N
3
(6b
1
)
2
)
9
(3b
1
πЈ(CϭC) (5b
2
)
ϩ
Table 1. Vertical ionisation potentials IP [eV], fine structure ν
10.65
10.85
10.88
13.65
13.78
13.97
15.28
16.26
n
π
N
1
(9a )
Ϫ1
[
cm ] and orbital energies ε [eV] of pyrimidine (1)
2
(1a
2
)
1
1.8 sh
14.2
4.4
14.55
5.75
16.81
9.15
1
π(CϭC) (2b )
ϪεB3LYP
IPB3LYP
11.92
12.05
12.24
13.55
14.53
σ (4b
σ (8a
(1b
σ (7a
σ (6a
2
)
IP/ν
1
1
)
1
Ϫ
π
1
)
9
.70/1150
7.25
8.16
8.58
8.98
11.54
11.78
9.20
10.11
10.53
10.93
13.49
13.73
13.85
n
π
n
π
N
(5b
2
)
1
1
)
1
1
1
1
1
0.42/900
1.22/950
1.38/800
4.0
3
(2b
1
)
(7a
ϩ
1
)
N
2
1
)
(1a
σ (6a
σ (4b )
1 1
π (1b )
2
)
1
)
4.3
2
1
1.90
Table 4. Vertical ionisation potentials IP [eV], fine structure ν
cm ] and orbital energies ε [eV] of 5-ethynylpyrimidine (4)
Ϫ1
[
IP/ν
ϪεB3LYP
IPB3LYP
Table 2. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[cm ] and orbital energies ε [eV] of 4-methylpyrimidine (2)
Ϫ
9
9
.65
.75/800
7.34
7.40
8.53
8.88
9.22
9.27
9.33
n
π
N
3
(6b
1
)
2
)
(3b
IP/ν
ϪεB3LYP
IPB3LYP
11.14/2000
1.27
10.46
10.81
11.15
11.61
13.74
13.94
14.16
15.97
16.68
πЈ(CϭC) (5b
2
)
ϩ
1
n
N
1
(9a )
Ϫ
9
.52/900
7.14
7.91
8.41
9.40
10.17
10.67
10.88
13.15
13.42
14.58
n
π
n
π
N
3
(14aЈ)
(4aЈЈ)
11.52/900
12.19
13.95
14.08
14.44
16.25
16.79
π
2
2
(1a )
1
1
1
1
1
1
0.08/1100
0.95/900
1.09/1000
3.36
3.66
4.54
9.68
π(CϭC) (2b
σ (8a
σ (4b
(1b
σ (7a
σ (3b
1
)
ϩ
N
2
(13aЈ)
(3aЈЈ)
11.81
12.01
12.23
14.04
14.75
1
)
8.62
2
)
10.89
11.16
12.32
σ (12aЈ)
π(CH ) (2aЈЈ)
(1aЈЈ)
π
1
1
)
1
)
3
π
1
2
)
The molecular structure of 2-ethynylpyrimidine (3) has
In the spectrum of 2-aminopyrimidine (5) (Figure 2,
been determined by X-ray structure analysis[ and the mo- Table 5) there are two groups of bands in the energy region
17]
lecular symmetry is C . In the spectrum of 3 (Figure 2, below 12 eV which can be assigned to ionisations from the
2
v
Ϫ
ϩ
Table 3), the ionisation related to the orbital πЈ(CϵC) (5b2) MOs π , π , n , nN and n(NH ). The molecule has a
3
2
N
2
can be easily recognised by its distinct fine structure which pyramidal amino group and only one plane of symmetry.
Eur. J. Org. Chem. 2005, 522Ϫ531
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
525

U. Lottermoser, P. Rademacher, M. Mazik, K. Kowski
FULL PAPER
Accordingly, the molecular symmetry is C . Again, most IPs Table 7. Vertical ionisation potentials IP [eV], fine structure ν
s
Ϫ1
related to the characteristic MOs of the pyrimidine system [cm ] and orbital energies ε [eV] of 2-amino-4-chloro-6-methyl-
are lowered by about 0.3 eV relative to the values of 1. A
pyrimidine (7)
larger shift of 1.58 eV was found for IP(π ). This can be
3
IP/ν
ϪεB3LYP
IPB3LYP
explained by the fact that π has a rather large orbital coef-
3
ficient on the C atom which carries the substituent and,
because of this, substituents in this position will affect the
MO energy to a greater extent than those on ring atoms 10.46/550
with smaller coefficients. In the energy region below 12 eV
there are three bands in the spectrum of 2-amino-4,6-di-
methylpyrimidine (6) (Figure 2, Table 6). The band at about
8
9.63
.78/650
6.67
7.37
8.16
8.76
9.04
8.64
9.34
3
π (24a)
Ϫ
nN (23a)
(22a)
n(NH ) (21a)
(20a)
10.13
10.73
11.01
10.76
12.26
12.93
13.22
13.58
13.98
14.64
π
2
11.09
11.28
11.62
12.78
2
ϩ
n
n
n
N
8.79
π
Ј(Cl) (19a)
10.29
10.96
11.25
11.61
12.01
12.67
π
(Cl) (18a)
10.8 eV is a superposition of two ionisations as indicated 13.3
σ (17a)
σ (16a)
π(CH ) (15a)
3
1
3.5
13.93
4.61
14.9)
by its high intensity. The ionisations can be assigned to the
Ϫ
ϩ
orbitals π , n , π , n
and n(NH ). Again, C molecular
3
N
2
N
2 s
1
(
n
π
σ
(Cl) (14a)
(13a)
symmetry is assumed. Compared with compound 5, the ad-
ditional two methyl groups induce a substantial destabilis-
ation of π by about 0.9 eV, while π and the two n orbitals
1
2
3
N
are shifted upwards by 0.2Ϫ0.4 eV.
In the spectrum of 2-amino-4-hydroxy-6-methylpyrim-
idine (8) (Figure 2, Table 8), there are four groups of bands
in the energy region below 12 eV which can be assigned to
Table 5. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[cm ] and orbital energies ε [eV] of 2-aminopyrimidine (5)
IP/ν
ϪεB3LYP
IPB3LYP
ionisations from π
, π , both n orbitals, n(NH ) and
3 2 N 2
n (O). The last mentioned orbital can be found above n (O)
because the latter is stabilised considerably by its interaction
σ
π
8
.84/1000
.44
6.50
6.98
8.71
8.71
9.17
11.33
11.69
12.04
8.99
9.47
π
n
n
π
3
(11aЈ)
Ϫ
9
N
N
2
(7aЈЈ)
with π . Compared with the unsubstituted parent com-
ϩ
2
1
1
1
1
1
1.01/1050
1.14/950
1.6
4.0
4.43
11.20
11.20
11.66
13.82
14.18
14.53
(6aЈЈ)
(10aЈ)
pound 1, all characteristic pyrimidine MOs are destabilised
Ϫ
n(NH
2
N 3
) (9aЈ) by between 0.15 eV (n ) and 1.84 eV (π ), whereas relative
σ (8aЈ)
σ (5aЈЈ)
π (7aЈ)
1
to 2-aminopyrimidine (5), destabilisation of these MOs
Ϫ
amounts to between 0.11 eV (nN ) and 0.99 eV (π ). In the
2
(
15.0)
spectrum of 2-amino-4-methoxy-6-methylpyrimidine (9)
(Figure 3, Table 9), the same ionisation bands can be found
as for compound 8. However, all of them are shifted to
lower values by 0.2Ϫ0.3 eV because of the replacement of
the hydroxy by a methoxy group.
Table 6. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[cm ] and orbital energies ε [eV] of 2-amino-4,6-dimethylpyrim-
The spectrum of 2-amino-4,6-dichloropyrimidine (10)
idine (6)
(
Figure 3, Table 10) is dominated by three very strong and
IP/ν
ϪεB3LYP
IPB3LYP
sharp ionisation bands between 11.0 and 12.5 eV which
originate from the ejection of electrons from the chlorine
atoms. For the assignment of these bands, a comparison
8
9
.44
.07
6.30
6.81
8.05
8.41
8.65
10.96
10.96
12.30
8.30
8.81
π
n
π
n
3
(14aЈ)
Ϫ
N
2
(10aЈЈ)
(9aЈЈ)
[
22,23]
with the spectrum of 1,3-dichlorobenzene
Ionisations from the pyrimidine MOs π , n
was helpful.
1
1
1
1
0.25/1200
0.79
1.0
2.7
10.05
10.41
10.65
12.96
12.96
14.30
Ϫ
ϩ
and π can
N
(13aЈ)
) (12aЈ) be found in the lower energy part of the spectrum while
) (11aЈ)
σ (7a")
(8aЈ)
3
N
2
n(NH
π(CH
3
2
(
(
13.6)
15.0)
π
1
Table 8. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[cm ] and orbital energies ε [eV] of 2-amino-4-hydroxy-6-methyl-
pyrimidine (8)
Below 12 eV in the spectrum of 2-amino-4-chloro-6-
methyl-pyrimidine (7) (Figure 2, Table 7), four bands occur IP/ν
again from the same orbitals as in 6 and an additional band
ϪεB3LYP
IPB3LYP
8
9
.58
.55
6.38
7.19
7.79
8.53
8.82
10.57
10.74
11.54
12.56
8.40
9.21
9.81
10.55
10.84
12.59
12.76
13.56
14.58
π
n
π
3
(24a)
Ϫ
from the orbital n Ј(Cl) of the chloro group. The latter or-
π
N
2
(23a)
(22a)
bital is localised in the molecular plane, in contrast to
1
0.15/1200
n (Cl) which is perpendicular to it. Three orbitals contrib- 10.90/1350
n(NH ) ϩ n (O) (21a)
π
2
π
ϩ
ϩ
ute to the fourth band ionisations, namely n(NH ), n and 11.13
n
n
N
(20a)
2
N
12.8
13.1
14.1
σ
(O) (19a)
) Ϫ n
nπЈ(Cl). The ionisations of 7 differ by up to 0.7 eV from
those of 2 and by around 1 eV from those of the parent
compound 1. In the latter case, an even greater deviation
n(NH
2
π
(O) (18a)
n
π
π
(O) (16a)
(14a)
(
15.1)
1
(1.64 eV) can be found only for π3.
526
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2005, 522Ϫ531

Photoelectron Spectra and Electronic Structures of Substituted Pyrimidines
Table 9. Vertical ionisation potentials IP [eV], fine structure ν sity and sharp appearance. The three pyrimidine bands at
FULL PAPER
Ϫ1
[
cm ] and orbital energies ε [eV] of 2-amino-4-methoxy-6-methyl- lower energies can be assigned as in compound 10. The de-
pyrimidine (9)
stabilising effect of the methoxy group, which has replaced
a chlorine atom in the latter compound, is mainly mani-
IP/ν
ϪεB3LYP
IPB3LYP
fested in the shift of IP(π ) by 0.61 eV. In the region below
2
1
3.0 eV, in the spectrum of 2-amino-4,6-dimethoxypyrim-
8
9
9
.36
.33
.82
6.21
7.04
7.57
8.34
8.60
9.74
9.92
10.53
11.17
13.08
8.16
8.99
9.52
10.29
10.55
11.69
11.87
12.48
13.12
15.03
π
n
π
3
(27a)
Ϫ
N
(26a)
(25a)
) (24a)
(23a)
(O) (22a)
(O) (21a)
idine (12) (Figure 3, Table 12), four bands of different inten-
sities may be found. Some of them are composite bands to
which several ionisations contribute. In particular, this
holds for the very strong and broad band between 10.8 and
2
1
1
1
1
1
1
0.63/1350
0.83
1.1 sh
2.3
2.8
n(NH
2
ϩ
n
n
n
N
σ
13.0 eV to which four ionisations can be assigned. To the
π
σ (20a)
π(CH ) (19a)
1
π (17a)
second band, which lies at about 9.6 eV, ionisations from
nN and π contribute in such a way that individual IPs
cannot be determined. C symmetry is assumed for the mol-
3.50
3
Ϫ
2
(
15.0)
s
ecule, although this is probably a simplification since the
methoxy groups may adopt different conformations.
Table 10. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[cm ] and orbital energies ε [eV] of 2-amino-4,6-dichloropyrim-
idine (10)
Table 12. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[cm ] and orbital energies ε [eV] of 2-amino-4,6-dimethoxypyrim-
IP/ν
ϪεB3LYP
IPB3LYP
idine (12)
9
.11/650
7.00
7.91
8.41
8.66
8.99
9.54
9.18
10.61
10.79
12.09
12.48
13.04
8.79
9.70
π
n
π
n
n
n
n
3
(14aЈ)
ϪεB3LYP
IPB3LYP
IP/ν
Ϫ
1
1
1
1
1
1
1
1
1
1
1
0.13
N
2
(10aЈЈ)
(9aЈЈ)
0.71/1100
1.05/700
1.56
1.7
2.12/450
2.88
3.38
4.3
4.47
5.02
10.20
10.45
10.78
11.33
10.97
12.40
12.58
13.88
14.27
14.83
8
9
.27/1700
.62
6.32
7.13
8.27
9.08
9.18
π
n
π
n
n
n
3
(17aЈ)
Ϫ
ϩ
π
π
(Cl) (13aЈ)
N
2
(13aЈЈ)
(12aЈЈ)
Ϫ
Ј(Cl) (8aЈЈ)
7.23
ϩ
N
(12aЈ)
ϩ
1
1
1
1
1
1
1
1
0.05
0.97
2.2
2.29
2.5
2.7
3.84
4.60
7.80
8.55
9.46
9.75
π
(O) (16aЈ)
ϩ
π
Ј(Cl) (11aЈ)
) (10aЈ)
(Cl) (7aЈЈ)
(Cl) (6aЈЈ)
(Cl) (9aЈ)
ϩ
10.50
11.41
11.82
12.03
12.16
13.79
14.01
15.35
N
(15aЈ)
Ϫ
n(NH
2
Ϫ
σ
(O) (11aЈЈ)
n
n
n
π
π
9.87
n(NH
2
) (14aЈ)
Ϫ
σ
σ
1
ϩ
10.08
10.21
11.84
12.06
13.40
n
n
σ
(O) (13aЈ)
ϩ
Ϫ
π
(O) (10aЈЈ)
(8aЈ)
π(CH
σ (11aЈ)
(8aЈ)
3
) (12aЈ)
(
15.2)
π
1
ϩ
that of nN occurs between the second and third nCl ionis-
ations. The molecular point group is Cs.
The amino group of 2-amino-4-chloro-6-methoxypyrim-
idine (11) is pyramidal and there is only one mirror plane
vertical to the molecular plane if the methoxy groups are
In addition to the IPs related to the characteristic pyrim-
idine MOs, in the low-energy part of the spectrum of 2,4-
bis(dimethylamino)-6-methylpyrimidine (13) (Figure 3,
Table 13) two ionisations from orbitals that are mainly
localised in the two dimethylamino groups can be expected.
arranged accordingly. We have assumed C symmetry for
s
this molecule. As in the previous compound, the ionisation
bands related to the chlorine atoms in the spectrum of 11
ϩ
Ϫ
These are labelled as n(NMe2) and n(NMe ) , although
2
(Figure 3, Table 11) can be recognised by their high inten-
the molecule has no symmetry and linear combinations ac-
cording to different symmetry properties can only refer to
Table 11. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[
cm ] and orbital energies ε [eV] of 2-amino-4-chloro-6-methoxy-
pyrimidine (11)
Table 13. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[
cm ] and orbital energies ε [eV] of 2,4-bis(dimethylamino)-6-
methylpyrimidine (13)
ϪεB3LYP
IPB3LYP
IP/ν
8
.71/1250
.78 sh
6.57
7.59
7.89
8.22
9.10
8.73
10.05
8.49
9.51
9.81
π
n
π
n
n
n
n
3
(27a)
Ϫ
IP
7.33
8.00
8.56
ϪεB3LYP
IPB3LYP
9
0.10
0.58/750
1.31
1.57/850
2.80
N
2
(26a)
(25a)
1
1
1
1
1
5.45
6.08
6.45
7.69
8.01
7.12
7.75
8.12
9.36
9.68
10.40
12.81
13.38
14.59
3
π (36a)
ϩ
10.14
11.02
10.65
11.97
12.17
12.25
13.43
14.20
π
(Cl) (24a)
n(NMe
2
)
(35a)
(31a)
ϩ
Ϫ
N
(23a)
Ј(Cl) (22a)
(O) (21a)
) (20a)
(O) (19a)
n
π
n
N
(34a)
(33a)
π
9.71
2
ϩ
σ
10.08
10.69
13.2
13.9
(15.0)
N
(32a)
Ϫ
1
1
0.25
0.33
n(NH
2
8.73
n(NMe
σ (23a)
σ (20a)
2
)
n
n
π
π
11.14
11.71
12.92
1
4.00
5.08
11.51
12.28
σ
(Cl) (18a)
(17a)
1
1
1
π (16a)
Eur. J. Org. Chem. 2005, 522Ϫ531
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
527

U. Lottermoser, P. Rademacher, M. Mazik, K. Kowski
FULL PAPER
the approximate local symmetry of the two groups in meta Table 15. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
positions of the pyrimidine ring. Below 11.5 eV, there are [cm ] and orbital energies ε [eV] of 2,4-bis(dimethylamino)-5-iso-
propyl-6-methylpyrimidine (15)
six ionisation bands, the second and the sixth being as-
signed to the two n(NMe ) ionisations and the remaining
2
IP
7.32
ϪεB3LYP
IPB3LYP
four can be related to the characteristic pyrimidine MOs.
Because of the strong destabilising effect of the two NMe2
groups, IPs related to the latter MOs are lowered by up to
5.44
6.18
7.01
7.75
π3 (45a)
ϩ
8.30
n(NMe ) (44a)
2
Ϫ
3
.09 eV relative to their positions in the parent compound
8.6 sh
6.47
7.63
8.07
8.31
8.04
9.20
9.64
9.88
10.96
12.68
13.23
14.41
n
π
n
N
(43a)
(42a)
9.59
2
(1). The appearance of the spectrum of 2,4-bis(dimethylam-
ϩ
1
1
1
0.0
N
(41a)
ino)-5,6-dimethylpyrimidine (14) (Figure 3, Table 14) is
similar to that of compound 13 although the bands are less
Ϫ
0.30
1.1
2
n(NMe ) (40a)
9.39
π(iPr) (39a)
σ (28a)
σ (26a)
well separated. The calculation shows no symmetry for the 13.6
11.11
11.66
12.84
1
(
4.8
molecule (point group C ) with relatively large deviations
from C symmetry. The sequence of the IPs is the same as
1
15.1)
1
π (19a)
s
in 13. The spectrum of 2,4-bis(dimethylamino)-5-isopropyl-
6
-methylpyrimidine (15) (Figure 3, Table 15) is little differ-
Linear Free Energy Correlation Analysis of Ionisation
Energies
ent from that of compound 14 but the assignments are the
same for both compounds. As a new feature in the spectrum
of 15, a band at 11.1 eV appears which can be assigned to
ionisation from a pseudo π type orbital mainly localised on induced shifts on an orbital is dependent upon the elec-
the isopropyl group.
In substituted compounds, the magnitude of substituent-
tronegativity of the substituent (through inductive effects)
and on the ability of orbitals located on the substituent to
interact with orbitals of the parent molecular core (reson-
[
24]
ance effect). The orbitals of the parent moiety and those
of the substituent generally become mixed in the composite
system so that localisation on one subunit no longer exists.
However, even though the orbitals of the substituted system
are delocalised, it is helpful to refer to them in terms of
their origin.
Table 14. Vertical ionisation potentials IP [eV], fine structure ν
Ϫ1
[
cm ] and orbital energies ε [eV] of 2,4-bis(dimethylamino)-5,6-
dimethylpyrimidine (14)
IP
ϪεB3LYP
IPB3LYP
7
8
8
9
.30
.30
.6
5.40
6.19
6.47
7.66
8.16
7.03
7.82
8.10
9.29
9.79
10.04
12.41
12.62
14.47
3
π (39a)
ϩ
In Figure 4, a correlation diagram for the ionisation po-
tentials IP associated with the characteristic pyrimidine
MOs (Figure 1) is shown for compounds 1Ϫ15. As is clear
from Figure 4, for all pyrimidines studied here, there is a
comparable influence of the substituents on the energies of
these MOs. In particular, the MO sequence remains un-
changed for compounds 6Ϫ12 and when the remaining
compounds are included, only two MO crossings occur.
n(NMe
2
)
(38a)
(34a)
Ϫ
n
π
n
N
(37a)
(36a)
.58
2
ϩ
1
1
1
1
0.2 sh
0.43
3.2
N
(35a)
Ϫ
8.41
n(NMe
σ (26a)
σ (24a)
2
)
10.78
10.99
12.84
3.9
(15.0)
1
π (17a)
Figure 4. Correlation diagram of the ionisation potentials IP of pyrimidines 1Ϫ15
28 www.eurjoc.org
5
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2005, 522Ϫ531

Photoelectron Spectra and Electronic Structures of Substituted Pyrimidines
FULL PAPER
Thus, π is always the highest occupied MO (HOMO) ex- single-parameter LFE analysis would only be of limited rel-
3
Ϫ
cept in compounds 1Ϫ4 in which it is replaced by nN
evance. In order to analyse all relevant IPs of compounds
which is itself the second highest occupied MO (HOMO-1) 1Ϫ15, we have performed multiple linear regression analy-
[
28]
1
4
in all other compounds. Furthermore, for most compounds, ses
including all four substituents (R ϪR ) simul-
ϩ
except for 1Ϫ5, π is found above nN . However, a closer taneously. The regression equation with substituent param-
inspection of the data reveals that the spread of IP values eters X(R ) as independent variables has the form [Equa-
2
k
differs for the individual MOs: π3 (7.3Ϫ10.4 eV), π2 tion (1)]:
Ϫ
(
9.6Ϫ11.7 eV), π (14.3Ϫ15.1 eV), n
(8.6Ϫ10.1 eV) and
1
N
ϩ
nN (10.0Ϫ11.7 eV). From these observations it can be con-
cluded that the substituents affect both types of MOs (n
and π) in a comparable way but to different extents. The
1
2
3
X(R4)
IP
i
ϭ a ϩ c
1
X(R ) ϩ c
2
X(R ) ϩ c
3
X(R ) ϩ c
4
(1)
energy variation of IP(π ) is considerably larger then that
Thus, regression coefficients c can be determined. The
k
3
of IP(π ) and nearly four times as large as that of IP(π ). intercept term a should ideally be equal to the respective IP
2
1
ϩ
Ϫ
On the other hand, the variations of IP(nN ) and IP(nN
)
value of the parent compound (1) and c Ϫc indicate how
1 4
k
are quite similar. For a quantitative investigation of sub- important the substituent R is for IPs related to the re-
stituent effects, we have carried out linear free energy (LFE) spective MO. The statistical significance of the regression
correlation analyses of the IP values with the aid of sub- coefficients is given by their standard errors.
stituent parameters in order to understand the nature of the
electronic effects on the various energy levels.
We have tested different substituent constants such as σ ,
I
ϩ
Ϫ
[29]
σ , σ , σ , R, R , R and F in LFE analyses according
p
m
F
Linear correlations of IP values with substituent param- to Equation (1) using the IPs related to the pyrimidine MOs
[
25]
Ϫ
ϩ
eters have been demonstrated repeatedly
and numerous π Ϫπ , n
and nN (Figure 1). Most of these substituent
1
3
N
[
9,26]
benzene derivatives have been investigated. Hammett σ parameters performed rather poorly and only with Ham-
values are usually used in such analyses for aromatic com- mett σ and resonance constants R and R were satisfac-
pounds. For other compounds, e.g., quinuclidines,
ϩ
p
[
27]
IPs tory to fair results obtained. As representative examples,
were found to correlate well with Taft’s σ values and, as we the regression coefficients for the constants σ and R are
p
[
1]
have recently shown for 1-substituted 1H-benzotriazoles,
IPs related to all occupied π MOs (π -π ) and the two n
summarised in Table 16. For σ , the multiple correlation co-
p
efficients r have values greater than 0.90 indicating quite
1
5
N
orbitals of the parent 1H-benzotriazole unit correlate well satisfactory (r Ͼ 0.95) to fair (r ϭ 0.90 to 0.95) corre-
[
28,30]
ϩ
with inductive substituent parameters σ and F. For some lations
for all IPs. For R (or R ) the correlation is
I
substituents, for which no σ or F parameters are known, even a little better than for σ if only the π ionisations are
I
p
these could be determined from their IPs by correlation considered. For the nN IPs, the correlation is less satisfac-
analysis.
tory than with σ and r values of 0.81 and 0.83 can be
p
In the above-mentioned LFE analyses of IP data, series considered as poor.[28,30] This may be taken as an indication
of compounds have been studied in which only one sub- that the two types of orbitals are affected in a somewhat
stituent was varied. Similar investigations could also be per- different way by the substituents and the superior perform-
formed for subgroups of pyrimidines 1Ϫ15 which differ ance of σ can be explained by the fact that these constants
p
1
4
only in one of the four substituents R ϪR (Scheme 1). represent the sum of electronic influences (inductive and
Thus, compounds 6Ϫ9 differ only in R , compounds 1, 3 mesomeric effects).
2
1
4
and 5 in R , and compounds 7, 10 and 11 in R , etc. Since
As is clear from the data in Table 16, the regression coef-
none of these subgroups contains more than four members, ficients c cannot be considered as significant since in most
3
Table 16. Regression parameters a and c and correlation coefficients r of multiple linear regression analyses of IPs related to the character-
istic MOs of pyrimidines 1Ϫ15 with substituent parameters σ
p
and R; values in parentheses are standard deviations
Ϫ
ϩ
π
3
π
2
π
1
n
N
n
N
σ
p
r
0.965
0.956
0.918
0.914
0.942
a
9.92 (0.15)
1.76 (0.29)
1.09 (0.27)
Ϫ0.05 (1.21)
0.83 (0.67)
11.34 (0.13)
1.18 (0.25)
0.42 (0.23)
0.73 (1.05)
1.87 (0.58)
14.53 (0.07)
Ϫ0.68 (0.12)
0.10 (0.12)
Ϫ0.18 (0.53)
Ϫ0.24 (0.29)
9.65 (0.11)
0.04 (0.21)
1.04 (0.20)
0.27 (0.91)
0.36 (0.51)
11.29 (0.10)
0.15 (0.19)
1.12 (0.18)
0.13 (0.80)
0.43 (0.45)
c
c
c
c
1
2
3
4
R
r
a
c
c
c
c
0.965
0.975
0.947
0.810
0.832
10.06 (0.15)
1.54 (0.34)
0.90 (0.38)
1.85 (1.68)
0.07 (0.75)
11.46 (0.10)
0.61 (0.22)
1.00 (0.25)
0.01 (1.11)
1.61 (0.49)
14.47 (0.05)
Ϫ0.62 (0.12)
Ϫ0.01 (0.12)
0.43 (0.61)
Ϫ0.29 (0.17)
9.68 (0.16)
0.19 (0.36)
0.74 (0.41)
1.99 (1.81)
Ϫ1.13 (0.81)
11.30 (0.16)
0.09 (0.37)
0.89 (0.42)
2.01 (1.84)
Ϫ0.77 (0.82)
1
2
3
4
Eur. J. Org. Chem. 2005, 522Ϫ531
www.eurjoc.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
529

U. Lottermoser, P. Rademacher, M. Mazik, K. Kowski
FULL PAPER
cases the standard deviation is larger than its absolute respectively. NMR signals were referenced to TMS (δ ϭ 0 ppm) or
solvent signals recalculated relative to TMS. IR spectra were meas-
value. This can be simply explained since there are only
three compounds with a substituent R ϶ H, namely 4, 14
and 15.
3
ured on a Bio-Rad FTS 135 instrument. For mass spectra, a VG
Prospec 3000 instrument from Fisons was used. PE spectra were
recorded on a Leybold-Heraeus UPG200 spectrometer equipped
with a He(I) radiation source (21.21 eV). The temperature of the
inlet system was varied between 25 and 225 °C according to the
different volatilities of the compounds in order to reach sufficient
vapour pressure. Compound 9 was measured at 450 °C. The energy
If a multiple correlation analysis is performed simul-
taneously with two constants for each substituent such as
ϩ
F and R (or R ), which represent only the inductive and the
mesomeric electronic influences, respectively, a considerable
improvement is obtained. The correlation coefficients for all scale was calibrated with an argon/xenon mixture. The accuracy of
IPs become excellent (r Ͼ 0.99) to satisfactory (r Ͼ 0.95). the ionisation potentials is Ϯ0.03 eV for sharp peaks but only
However, since eight regression coefficients instead of four Ϯ0.1 eV for broad and overlapping signals.
Becke3LYP[
11,12]
calculations were performed with the program
have now been determined, the significance level is re-
duced accordingly.
If the characteristic pyrimidine MOs of the investigated
compounds are only slightly perturbed by substituents, the
effects of substituents may be expected to be more or less
GAUSSIAN 98.[ The latter DFT method was employed with the
basis set 6-31ϩG*. All calculations were carried out with full ge-
ometry optimisation. MOs were plotted by using the program
PERVAL.[
31]
32]
additive in polysubstituted pyrimidine derivatives. In order Materials: Compounds 1, 2 and 5Ϫ12 were purchased from
to test this assumption, we performed a final linear re- SigmaϪAldrich Chemie GmbH, Steinheim, Germany. Syntheses of
compounds 4[ and 13Ϫ15 have been described previously.
33]
[34]
gression of the IPs with the sum of the σ constants for
p
each compound. Only for π (r ϭ 0.951) and π (r ϭ 0.917)
3
2
2
-Ethynylpyrimidine (3): To a mixture of 2-bromopyrimidine (2.8 g,
Ϫ
were fair correlations found, while for π (r ϭ 0.669), n
1
N
17.6 mmol) and 2-methyl-3-butyne-2-ol (1.8 g, 21.5 mmol) in di-
ethylamine (25 mL) were added bis(triphenylphosphane)palladium
dichloride (126 mg, 0.2 mmol) and copper(i) iodide (19.8 mg,
ϩ
(r ϭ 0.809) and nN (r ϭ 0.860) only poor relations were
obtained.
0.1 mmol). The reaction mixture was stirred under argon at room
temperature for 15 h and the solvent was then removed under re-
duced pressure. Water (3 mL) was added and the mixture extracted
with diethyl ether (2 ϫ 40 mL). Toluene (67 mL) and sodium hy-
Conclusion
In this study, a relatively large number of compounds has droxide (560 mg, 140 mmol) were added and the mixture heated to
been investigated. The PE spectra were analysed according reflux for 1 h. After removal of the solvent in vacuo, the product
was recrystallised from chloroform. Yield 66 mg (4%), m.p. 95Ϫ96
to standard procedures including quantum chemical com-
1
°C. H NMR (500 MHz, CDCl
3
, 25 °C, TMS): δ ϭ 3.12 [s, 1 H,
putations. The set of IPs for ionisations related to all
characteristic pyrimidine MOs presented a good basis for
investigating the relationship between the electronic struc-
ture of the parent molecule and the substituents. This was
achieved by multiple regression analysis using different sub-
stituent constants. To the best of our knowledge, this is the
C(8)ϪH], 7.27 [t, J ϭ 4.9 Hz, 1 H, C(5)ϪH], 8.71 [d, J ϭ 4.9 Hz,
1
3
2
3
H, C(4)ϪH] ppm. C NMR (125.8 MHz, CDCl , 25 °C, TMS):
δ ϭ 75.9 [C(8)], 81.8 [C(7)], 120.5 [C(5)], 152.2 [C(2)], 157.3 [C(4)]
Ϫ1
ppm. IR: ν˜ ϭ 3257 cm [s, CϵCϪH], 3077 [s, Aryl-H], 2122 [s,
CϵC], 1632, 1567, 1504 [s, CϭC, CϭN], 803 [s]. MS (70 eV, EI):
ϩ
ϩ
ϩ
m/z (%) ϭ 104 (100) [M ], 105 (7) [M ϩ H], 77 (11) [M
Ϫ
ϩ
ϩ
first analysis of this type for ionisation potentials obtained HCN], 52 (11) [C H N ], 26 (7) [C H ]. MS (high-resolution):
3
2
2
2
by PE spectroscopy.
6 4 2
m/z ϭ 104.0359 [calcd. for C H N : 104.0374].
It was found that all IPs of compounds 1Ϫ15 related to
the characteristic pyrimidine MOs can be correlated with
Acknowledgments
substituent constants such as σ indicating that these par-
p
[
28]
ameters can be used as ‘‘explanatory variables’’
or We are grateful to Prof. Dr. Junes Ipaktschi and Dr. Thomas
‘
‘descriptors’’ for the MO energies and thus the electronic Eckert, Institute of Organic Chemistry, Justus Liebig University,
structures of pyrimidines. It is essential to include both in- 35392 Giessen, Germany, for providing a sample of 5-ethylnyl-
pyrimidine.
ductive and mesomeric effects in correlation analyses of the
IPs with substituent constants, in order to obtain satisfac-
[
[
1]
2]
tory results for π and n levels. A model that systematically
describes the nature and extent of substituent effects on π
and nN ionisations of pyrimidines is certainly of high inter-
est. Additivity of substituent effects in polysubstituted py-
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[
[
3]
4]
5]
rimidines could only be proved for Hammett σ constants
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530
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2005, 522Ϫ531

Photoelectron Spectra and Electronic Structures of Substituted Pyrimidines
FULL PAPER
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