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Effects of Intramolecular Hydrogen Bonding on the Ionization Energies of Proline.

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Angewandte
Chemie
Computational Chemistry
DOI: 10.1002/ange.200504039
Effects of Intramolecular Hydrogen Bonding on
the Ionization Energies of Proline**
Shan Xi Tian* and Jinlong Yang
Understanding the nature of inter- and intramolecular Hbonding is the focus of extensive research in the fields of
chemistry and biochemistry because the thermodynamics and
kinetics of many chemical and biochemical systems are
determined to a large degree by H-bonding.[1] Structural
information regarding the intermolecular H-bonding in complexes can be obtained from various spectroscopic experiments,[1] and the energetics can be predicted within a supermolecular, high-level ab initio approach. Although the nature
of the intramolecular H-bonding is the same as for the
intermolecular H-bonding, the evaluation of the intramolecular H-bonding energy has been challenging both experimentally and theoretically because intramolecular H-bonding is
an intrinsic feature of the molecule involved.[1, 2] Ionization
energies (IEs) are important data for understanding the
charge transfer, electrophilicity, and for modification of the
diverse reactivity patterns of biological molecules.[1] Experimental studies indicate that the IE of a lone-pair orbital (n)
of the H-bonding molecule is usually higher than that of the
molecule without (or with very weak) intramolecular Hbonds,[3] and the excitation transition energy of the lone-pair
orbital for the solvated molecule frequently differs from that
for the free molecule in the gas-phase.[4] The effects of
intramolecular H-bonding on the IEs are difficult to determine by ultraviolet photoelectron spectroscopy (UPS)
because of the coexistence of several conformers.[5, 6] For
amino acids, both intramolecular H-bonding and intermolecular interactions play a role in protein folding and peptide
assembly.[1] The IE values of isolated amino acids and their
derivatives have been obtained by gas-phase UPS experiments.[5] Herein, we provide the first assignment of the UP
spectrum of proline[5] based on ab initio electron-propagator
theory[7] combined with natural bond orbital (NBO)[8] analyses and show that intramolecular H-bonding has a strong
[*] Prof. Dr. S. X. Tian, Prof. Dr. J. Yang
Hefei National Laboratory for Physical Sciences at Microscale
Department of Chemical Physics
University of Science and Technology of China
Hefei, Anhui 230026 (China)
Fax: (+ 86) 551-360-2969
E-mail: sxtian@ustc.edu.cn
[**] This work was partially supported by the NSFC (20503026,
20533030, and 50121202), the Virtual Laboratory for Computational
Chemistry of CNIC, and the Supercomputing Center of CNIC,
Chinese Academy of Sciences. We thank Prof. J. V. Ortiz at Kansas
State University for helpful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. 2006, 118, 2123 ?2126
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2123
Zuschriften
influence on the electronic structure, and therefore the IEs, of
proline.
Proline usually forms a kink as the b- or g-turn in the
peptides,[9] and it is a favorite organocatalyst for asymmetric
synthesis.[10] These features depend strongly on the properties
of the isolated species. Theoretical studies showed that four
proline conformers (due to the carboxy cis and trans
orientations and pyrrolidine ring puckering, labeled Ia, Ib,
IIa, and IIb in Figure 1) have notable populations at low and
H
H
C
C
H
N
C
C
1.869
H
HH
C
O
1.885
O
Ia
2.243
Ib
2.365
IIa
IIb
Figure 1. Proline conformers with their intramolecular H-bond
lengths [B].
moderate temperatures;[11] this result was supported by rovibrational spectroscopy experiments.[12] Conformers of types
I and II have populations around 63 and 37 %, respectively,
according to the high-level calculations[11, 12b] and supposing
Boltzmann distributions at 430 K (see Supporting Information). The low-resolution He II UP spectrum of proline[5] has
not been theoretically analyzed until now. In particular, it is
unclear whether the two peaks in the first band[5] correspond
to two cationic states of the most stable conformer or arise
from pyrrolidine ring puckering or the variance in the
intramolecular H-bonding strengths of cis and trans conformers.
While the density functional method does not provide
reliable values for the relative energies of proline conformers,
it can be used to predict structures and infrared vibrational
frequencies that are in good agreement with the experimental
observations.[12b] We optimized the geometrical parameters of
Ia, Ib, IIa, and IIb by the hybrid density functional B3LYP
method[13] using Dunning?s aug-cc-pVDZ basis set.[14] The
vertical IE values were calculated according to electronpropagator theory in the partial third-order (P3) approximation,[7] where a Dyson orbital (an integral of the wave
functions between the neutral and cationic states) is proportional to a canonical Hartree?Fock orbital.[7] This method
gives an average absolute error of less than 0.2 eV for the IEs
below 20 eV.[7, 15]
Table 1 shows the excellent agreement between the
calculated IE values and the energies of the peak positions
observed in the UP spectrum.[5] The small calculated IE
2124
Ia
Ib
9.41
10.51
11.15
12.25
9.52
10.45
11.17
12.34
nN
nO
pCO
sCC
IIa
IIb
8.83
8.71
10.67
11.86
12.04
10.62
11.95
11.97
Exptl[a]
9.0
9.5
10.6
11.6
12.0
[a] Taken from the He II UP spectrum in reference [5].
H
H
H
Table 1: Calculated vertical IEs [eV] in comparison with experimental
data.
www.angewandte.de
difference (ca. 0.1 eV) for the HOMOs of Ia and Ib (or IIa
and IIb) demonstrates that pyrrolidine ring puckering cannot
lead to the peak split (0.5 eV) of the first band in the UP
spectrum.[5] It is somewhat surprising that for conformers of
types I and II distinctly different IEHOMO values are calculated. According to the Dyson orbital plots with contour
values of 0.03 shown in Figure 2 a, two HOMOs are
characterized as the lone-pair orbitals of N atoms (nN).
Thus, two peaks with a splitting of 0.5 eV[5] can be ascribed
to the different energy levels of HOMOs (nN) of the
respective conformers, which means that the peak observed
at IE = 9.0 eV is mainly due to the HOMOs of conformers IIa
and IIb, while the next peak at IE = 9.5 eV is due to those of
conformers Ia and Ib. The calculated IEHOMO for Ia (or Ib) is
0.58 eV (or 0.81 eV) higher than that for IIa (or IIb), in
agreement with the experimental peak split (0.5 eV).
The vertical IEHOMO can be evaluated as the difference
between the total energies of the neutral and cationic species
at the neutral equilibrium structure. Intramolecular H-bonding will enhance the stabilities of both the neutral and cationic
species, perhaps leading to more negative total energies. To
elucidate the distinctly different IEHOMO values for these two
types of conformers, the intramolecular H-bonding strengths
were compared in detail. The NBO analysis transforms the
delocalized MOs into localized ones that are closely tied to
chemical-bond concepts.[8] The H-bonding interaction can be
treated by the second-perturbation energy, E(2) = nsF 2ij/
de,[8, 16] where Fij is the Fock matrix between the unperturbed
occupied lone pair (n) and unoccupied antibonding OH or
NH natural orbitals (s*) of proline, ns is the lone-pair orbital
population, and de is the energy difference between the
unperturbed n and s* orbitals.
The NBO analyses for the neutral conformers (Ia, Ib, IIa,
and IIb) and their vertical ionized cations (Ia+, Ib+, IIa+, and
IIb+) are given in Table 2. In NBO theory, the energy level of
the n orbital involved in the H-bonding is lowered and its
electron distribution becomes delocalized due to the perturbation of the n!s* interaction. For example, as shown in
Figure 2 a, the energy level of the unperturbed nO orbital is
slightly lowered by 0.08 eV to form the next HOMO
(HOMO1, the perturbed nO orbital) of IIa because of the
weak NHиииO bond; the energy-level of the unperturbed nN
orbital is significantly lowered by 0.78 eV to form the HOMO
(perturbed nN orbital) of Ia because of the strong OHиииN
bond. The H-bond distances of OHиииN are 1.869 and
1.885 G (Figure 1), while the NHиииO distances are much
longer. It is known for neutral proline that the OHиииN bonds
are much stronger than the NHиииO bonds, as derived from
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 2123 ?2126
Angewandte
Chemie
a)
C
C
C
?*OH
C
N
O
C
nN
H
E(2) = 0.78 eV
O
Ia
C
C
O
C
C
N
?*NH
C
H
nO
O
E(2) = 0.08 eV
IIa
E(2) = 0.19
E(2) = 0.17
b)
1.2
1.0
0.8
-399.81
-399.82
0.4
0.2
0.0
Ia
Ib
E(2) = 0.03
IE' = 8.70
IE = 8.71
IIa
-399.83
-399.84
~
Etot /
-400.13 Hartree
-400.14
E(2) = 0.04
0.6
E(2) = 0.71
E(2) = 0.78
0.8
IE' = 8.82
1.0
E(2) = 0.08
1.2
IE = 8.83
Erel. / eV ~
IE' = 8.98
0.0
IE = 9.41
0.2
IE = 9.52
IE' = 8.82
0.4
E(2) = 0.07
0.6
-400.15
-400.16
IIb
Figure 2. a) Dyson orbital plots of the HOMOs of Ia and IIa, and E(2)
values for their intramolecular H-bonding interactions. The unperturbed energy levels are shown in blue and red, the perturbed (real)
levels are shown in black. b) The relationship among the IEHOMO,
IE?HOMO, and E(2) values (top row: cations, bottom row: neutral
species). The perturbed energy levels are shown in black, the unperturbed levels in red; IEHOMO = IE?HOMO + DE(2); DE(2) = E(2)(neutral)E(2)(cation).
Table 2: NBO analyses of the intramolecular H-bonds in the neutral and
cationic species.[a]
Ia: nN !s*OH
Ib: nN !s*OH
IIa: nO !s*NH
IIb: nO !s*NH
+
Ia : nN !s*OH
Ib+: nN !s*OH
IIa+: nO !s*NH
IIb+: nO !s*NH
IIb+. Moreover, the OHиииN bond strength in the neutral
species (E(2) = 0.78 eV for Ia and 0.71 eV for Ib) decreases
dramatically after ionization (E(2) = 0.19 eV for Ia+ and
0.17 eV for Ib+), while the NHиииO bond strengths are
similar for the neutral and cationic type-II conformers. The
natural atomic charge analyses in the Supporting Information
are in line with the above scenario.
If the E(2) values are approximated to be the contributions of H-bonding stabilization to the total energies of the
species, the relationship between the calculated IEHOMO,
IE?HOMO (the ionization energies excluding the H-bonding
stabilizations), and E(2) values (both of the neutral and
cationic species) can be determined (Figure 2 b). The IE?HOMO
values are estimated to be 8.82, 8.98, 8.82, and 8.70 eV for Ia,
Ib, IIa, and IIb, respectively. Without the effects of the
OHиииN or NHиииO interactions, the IE?HOMO values are
appealingly close to the IEHOMO(nN) (ca. 8.8 eV) of pyrrolidine.[17] The E(2) changes (DE(2)) from the neutral species to
the cations are 0.59, 0.54, 0.01, and 0.01 eV for Ia, Ib, IIa, and
IIb, respectively. The DE(2) differences between Ia and IIa,
and between Ib and IIb, are 0.58 and 0.53 eV, respectively.
The calculated IEHOMO differences?0.58 eV between Ia and
IIa and 0.81 eV between Ib and IIb?can be explained from
the above DE(2) differences. If the small E(2) values of the
cations are omitted in the above calculations, the calculated
IEHOMO differences are approximately the E(2) differences for
the neutral species, that is, 0.70 eV between Ia and IIa and
0.67 eV between Ib and IIb.
The effect of intramolecular H-bonding on the IEHOMO of
Ia was further examined with P3 calculations by scanning the
value for the H-O-C-N dihedral angle of Ia while keeping the
residual geometrical parameters fixed. It can be seen from
Figure 3 that IE(nN) decreases while IE(nO) remains almost
constant when the H atom in the carboxy group leaves the HO-C-N plane. The strong OHиииN bond disappears completely at a H-O-C-N dihedral angle around 958, which gives
an energy of about 0.75 eV for this intramolecular H-bond.
In general, strong intramolecular H-bonds lead to higher
IEs for related MOs (e.g., the lone-pair orbitals), as was
explained by the NBO analyses. The present theoretical
approach could elucidate the effects of both inter- and
intramolecular H-bonding on the electronic structures and
E(2) [kcal mol1]
de [au]
Fij [au]
10.6
18.01
16.31
1.88
1.01
1.14
1.16
1.16
1.16
0.130
0.124
0.043
0.041
10.4
4.37
3.92
1.64
0.72
1.39
1.41
1.30
1.05
0.099
0.094
0.037
0.035
HOMO?1(nO)
10.2
N
10.0
9.8
IE / eV
dH
Ia
9.6
C
O
9.4
9.2
9.0
~ 0.75 eV
HOMO(nN)
8.8
[a] With the neutral equilibrium structures.
8.6
-100
analysis of the electron densities, 1, at the H-bond critical
points.[11b] In Table 2, the intramolecular OHиииN bonds in
Ia+ and Ib+ are found to be stronger than those in IIa+ and
Angew. Chem. 2006, 118, 2123 ?2126
-80
-60
-40
-20
0
Dihedral angle HOCN / deg
Figure 3. Variation of IE values when scanning the H-O-C-N dihedral
angle in Ia.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
2125
Zuschriften
related properties of other species. Furthermore, the different
IEHOMO values tuned by the strong H-bonding interactions
profoundly affect charge transfer in biological systems since
the IEHOMO is closely related to the molecular chemical
potential and the electrophilicity index.[18]
Received: November 14, 2005
Published online: February 22, 2006
.
Keywords: ab initio calculations и amino acids и hydrogen
bonds и ionization potentials и photoelectron spectroscopy
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2126
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