Article pubs.acs.org/JPCA
Valence-Bound and Diffuse-Bound Anions of 5‑Azauracil H. H. Corzo, O. Dolgounitcheva, V. G. Zakrzewski, and J. V. Ortiz* Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849-5312, United States ABSTRACT: Structures, isomerization energies, and electron binding energies of 5-azauracil and its anions have been calculated ab initio with perturbative, coupledcluster, and electron-propagator methods. Tautomeric structures, including those produced by proton transfer to a CH group, have been considered. Dyson orbitals and pole strengths from electron-propagator calculations validated a simple, molecular-orbital picture of anion formation. In one case, an electron may enter a delocalized π orbital, yielding a valence-bound (VB) anion with a puckered ring structure. The corresponding electron affinity is 0.27 eV; the vertical electron detachment energy (VEDE) of this anion 1.05 eV. An electron also may enter a molecular orbital that lies outside the nuclear framework, resulting in a diffusebound (DB) anion. In the latter case, the electron affinity is 0.06 eV and the VEDE of the DB anion is 0.09 eV. Another VB isomer that is only 0.02 eV more stable than the neutral molecule has a VEDE of 2.0 eV.
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INTRODUCTION Since the discovery of their therapeutic, bacteriostatic, fungicidal properties as well as their uses as antineoplastic agents and enzyme inhibitors, modified purines and pyridines have been of great interest in the biological and medical fields.1−9 In particular, in the aza analogues of uracil, a minor structural modification in the pyrimidinic ring such as the replacement of a carbon atom at position 5 or 6 for a nitrogen leads to marked alterations in the physico-chemical and biological properties of the resultant bases.1,3,4,6,10−13 Substitution of a CH group by a nitrogen atom at position 6 in the uracil ring results in the 6-azauracil molecule, a pharmacologically active compound1,4 which inhibits growth of solid animal tumors and has been used in its nucleoside form for the treatment of leukemia.4 On the other hand, substitution of a CH group by a nitrogen atom at position 5 results in 5-azauracil (see Figure 1 for the canonical form), a molecule that has been shown to exhibit fungistatic and bacteriostatic properties.4,11
Both molecules have been studied extensively in their neutral form.1,4,6,10 Exposure of nucleobases to radiation, such as ultraviolet light, leads to formation of anions and cations that could have important pharmacological as well as biological properties.14−20 Uracil itself does not display the same biological activity as its derivatives, such as thiouracils, fluorouracils, or azauracils. Neither does it form long living, stable anions. On the other hand, 5-fluorouracil,19 4-thiouracil,21,22 2,4-dithiouracil,21,22 and 6-azauracil23 are known to be capable of capturing an electron with formation of stable, canonical anions. There are no data pertaining to the 5-azauracil anion or its possible forms. The study of the electronic structure of 5-azauracil with an excess electron could bring new perspectives about possible applications of this molecule. Two types of anions of the nucleic-acid bases are known: valence-bound (VB) anions in which an electron is captured by a valence, π-type virtual molecular orbital (MO) of a neutral molecule and diffuse-bound (DB) anions, where an extra electron resides in a very diffuse, σ-type MO. The latter structures also are known in the literature as dipole-bound anions. This concept was introduced by Fermi and Teller in their discussion of an electron interacting with a meson−proton pair.24 Later it was shown that at a certain, critical value of the dipole moment in a system of two separated charges, an electron may become bound.25−27 This simple model stimulated the observation of many anionic states of molecules with large dipole moments, especially by means of photoelectron spectroscopy.28 Interpretations of these spectra with ab initio theory are usually based on self-consistent field calculations that are followed by an approximate treatment of electron correlation. The former step accounts for the Received: May 29, 2014 Revised: August 4, 2014 Published: August 7, 2014
Figure 1. Enumeration of atoms in the ring of 5-azauracil. © 2014 American Chemical Society
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equivalence of electrons and Fermi−Dirac statistics through the inclusion of the exchange operator. The latter step often discloses that a description of electron correlation is necessary to obtain the correct sign of an anion’s electron detachment energy. Molecules with zero dipole moments, but nonvanishing quadrupole or higher moments are capable of forming anions in diffuse molecular orbitals.29 We therefore employ the term diffuse-bound to describe this class of anions. VB and DB anions differ in their electron binding energies and structures and give rise to distinct patterns in experimental photoelectron spectra. A typical DB anion is identified by a sharp, narrow peak with an electron binding energy close to zero,30 whereas a typical VB anion is characterized by a broad, structured peak with a maximum usually at about 1−1.5 eV.23 In principle, a molecule could form anions of both types. The canonical form of uracil was found to produce a weakly bound DB anion30−32 with a vertical electron detachment energy (VEDE) measured at 0.086 eV. The VB canonical anion of uracil was observed in collision experiments with Ar atoms in Rydberg states, but neither the VEDE nor the adiabatic electron affinity (AEA) of such a species was measured.33 Density functional calculations provided an AEA of about 0.07 eV and a VEDE of about 0.7 eV.33 In a recent publication,34 a positive AEA of uracil corresponding to a canonical VB anion was reported. The calculations were performed with the Brueckner doubles and disconnected triples, or BD(T), coupled-cluster method and a number of basis sets. The extrapolation to the largest basis, augh-cc-pVQZ +2df, gave an unbound AEA value of −0.091 eV. A positive AEA value of 0.034 eV was obtained by including zero-point energy corrections calculated at the B3LYP/cc-pVTZ level. The VEDE of a VB anion of uracil was finally measured and ab initio calculations revealed that a noncanonical, very rare isomer was responsible for a broad peak in the experimental spectrum.35−37 This anionic isomer can be considered to be the result of a proton transfer from N3 to C5 in uracil to form an imino−oxo structure. The anion’s six-member ring was significantly puckered and gave rise to a broad peak with a maximum at 2.58 eV. VB anions of thiouracils were studied experimentally through their photoelectron spectra21 and with ab initio calculations.22 VB anions of 2-thiouracil, 4-thiouracil and 2,4-dithiouracil were found to be both vertically and adiabatically bound. Anions of 4-thiouracil and 2,4-thiouracil produced experimental bands that were typical for VB species. 2-thiouracil has not been studied experimentally. The VB anion of 6-azauracil has been observed experimentally and studied theoretically.23 This anion also was stable adiabatically (and therefore vertically) and gave rise to a broad band with a maximum at 1.2 eV. This peak was assigned to electron detachment from a canonical structure of 6-azauracil. In the current paper, we present results of ab initio studies on the tautomeric structures, relative energies and electron binding energies of the 5-azauracil molecule and its anions.
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Figure 2. Tautomeric valence-bound anions of 5-azauracil.
COMPUTATIONAL DETAILS All calculations were performed with the Gaussian-09 package of programs.38 Structures of canonical 5-azauracil, its nine tautomers, their VB and DB anions and three very rare isomeric anions resulting from proton tranfer from amino nitrogens to C5 or N6, were optimized with the MP2 or UMP2 methods and the 6-311++G(2df,2p) basis.39−41 Harmonic frequency analysis confirmed minima for all stationary points obtained in
this way. AEAs of tautomers were calculated with the coupledcluster singles and doubles (CCSD)42 and CCSD-with-perturbative-triples-corrections, or CCSD(T),43 methods. VEDEs of the anions were obtained with electron propagator methods44−46 in the Outer Valence Green’s Function (OVGF),47−49 Partial Third-Order (P3)50−52 and P3+53 approximations as well as with CCSD and 6909
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Table 1. Relative Energies of Molecules and VB Anions, eVa neutral
VB anion
tautomer
CCSD(T)
CCSD(T)+ZPE
CCSD(T)
CCSD(T)+ZPE
⟨S2⟩UHF
1 2 3 4 5 6 7 8 9 10 11b 12b 13
0 0.40 0.40 0.41 0.43 0.47 0.48 0.79 0.83 1.25
0 0.40 0.41 0.42 0.44 0.46 0.48 0.77 0.80 1.20
0.93
0.90
0 0.82 0.84 0.38 0.84 0.67 0.44 0.67 1.20 1.54 0.79 0.83 0.24
0 0.79 0.83 0.38 0.85 0.68 0.44 0.68 1.19 1.51 0.80 0.81 0.25
0.77 0.79 0.78 0.78 0.79 0.82 0.77 0.82 0.77 0.77 0.84 0.77 0.77
a
Single-point CCSD(T)/6-311++G(2df,2p) calculations at the MP2 (UMP2 for anions)/6-311++G(2df,2p) optimized geometries. bNeutral analogs of anions 11 and 12 do not exist, for optimization of the corresponding rings leads to ring opening.
anionic tautomers. All neutral tautomers are higher in energy than the canonical structure. The closest tautomer, structure 2, is about 0.40 eV above structure 1, i.e., canonical 5-azauracil. In addition to ten classic tautomers (the canonical and nine imino−oxy structures resulting from proton transfers from amino nitrogens to carbonyl oxygens), three additional structures (11, 12, and 13) were included in the VB group. These are the very rare isomers35 that result from proton transfer from either NH group to atoms in the fifth or sixth positions in the ring. The canonical, VB anion of 5-azauracil is the lowest-energy structure among the 12 isomers. One of the very rare isomers, structure 13, is the nearest in energy and differs from the canonical structure by ∼0.25 eV. All other anions, except structure 4, are much higher in energy. Relative Stabilities: DB Anions. Table 2 contains relative CCSD(T) energies obtained at the optimized geometries of
CCSD(T). Electron propagator methods also yielded Dyson orbitals, ϕDyson VEDE, where Dyson ϕVEDE (x1) =
N
∫ Ψ*molecule(x2 ,x3 ,x4 ,...,xN )
Ψanion(x1 ,x 2 ,x3 ,...,xN ) dx 2 dx3 dx4 ... dxN
(1)
and where N is the number of electrons in the anion. From a Dyson orbital, one may obtain a corresponding pole strength, p, where p=
∫ |ϕDyson(x1)|2 dx1
(2)
When a pole strength is close to unity, the MO description of an electron binding energy is qualitatively valid. For VB anions, the 6-311++G(2df,2p) basis was used, whereas for the DB anions this basis was augmented with extra diffuse functions centered on all atoms.32 The MOLDEN graphing package54 was used to generate the figures. Contours of ±0.03 are depicted in the orbital plots.
Table 2. Relative Energies of Molecules and DB Anions, eVa
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RESULTS Enumeration of C, N, and O atoms in the 5-azauracil ring is given in Figure 1. Imino−oxy tautomers result from proton transfer from either nitrogen atom to either oxygen atom or to both. When a proton is transferred from either nitogen atom to C6 or to N5, imino−oxo tautomers are formed. In the case of 5-azauracil, nine imino−oxy and three imino−oxo isomers are possible. All tautomeric structures are depicted in Figure 2. Relative Stabilities: Molecules and VB Anions. Table 1 presents relative energies of isomers obtained with CCSD(T) at the MP2 or UMP2 optimized geometries. Zero-point corrections also are included. VB and DB anions may be distinguished by the Dyson orbitals that correspond to their VEDEs. In the case of VB anions, all Dyson orbitals retained quasi-π character that is typical for such species.22,23,32 Of all the neutral structures, only the canonical 5-azauracil and tautomeric structures 3, 5, and 8 possessed a plane of symmetry. Other neutral tautomers were nonplanar. All VB anions have C1 symmetry, with a significantly puckered pyrimidinic ring. Inclusion of zero-point corrections to the energies does not significantly change relative stabilities of either neutral or VB
tautomer
neutral
DB anion
⟨S2⟩UHF
1 2 3 4 5 6 7 8 9 10
0 0.40 0.40 0.41 0.44 0.47 0.48 0.79 0.83 1.25
0 0.62 0.60 0.46 0.58 0.54 0.47 0.59 0.76 0.80
0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75
a
Single-point CCSD(T)/6-311++(2df,2p) calculations where the basis was augmented with diffuse s and p functions on all atoms at the MP2(UMP2 for anions)/6-311++G(2df,2p) optimized geometries.
DB, tautomeric 5-azauracil anions and corresponding neutrals. Electron correlation was estimated with the CCSD(T) method and the 6-311++G(2df,2p) basis augmented with extra s and p diffuse functions on each atom.32 As was the case with the VB anions, the canonical isomer is the lowest energy structure among all DB anions. Adiabatic Electron Affinities: VB Anion Tautomers. CCSD and CCSD(T) predictions of AEAs do not differ much. (See Table 3.) Inclusion of zero-point corrections to the total 6910
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Table 3. Adiabatic Electron Affinities, eVa valence-bound tautomer 1 2 3 4 5 6 7 8 9 10 13
CCSD
CCSD(T)
0.18 −0.25 −0.27 0.23 −0.23 −0.03 0.23 0.30 −0.22 −0.13 0.93
Table 4. Vertical Electron Detachment Energies of VB Anions, eVa
b
CCSD(T)+ZPE
tautomer
CCSD
CCSD(T)
OVGF
P3
P3+
0.27 −0.11 −0.15 0.31 −0.13 0.06 0.31 0.36 −0.16 −0.08 0.93
1 2 3 4 5 6b 7 9 10 11 12 13
1.15 0.45 0.43 1.18 0.48 0.76 1.18 0.79 0.87 4.15 4.61 2.20
1.05 0.38 0.36 1.07 0.40 0.69 1.07 0.72 0.80 3.65 3.81 2.02
1.20 0.49 0.46 1.23 0.52 0.80 1.22 0.85 0.92 4.03 4.25 2.21
1.48 0.72 0.69 1.53 0.75 1.08 1.53 1.05 1.13 4.26 4.44 2.56
1.39 0.63 0.60 1.44 0.66 0.99 1.43 0.95 1.03 4.09 4.27 2.46
0.19 −0.24 −0.25 0.22 −0.22 −0.01 0.23 0.31 −0.19 −0.10 0.88 diffuse-boundc
tautomer
CCSD
CCSD(T)
CCSD(T)+ZPE
μ, D
1 2 3 4 5 6 7 8 9 10
0.02 −0.21 −0.17 −0.03 −0.12 −0.06 0.03 0.20 0.09 0.46
0.04 −0.18 −0.17 −0.01 −0.10 −0.03 0.05 0.24 0.12 0.49
0.06
5.02 1.52 2.82 4.04 2.47 5.69 6.96 5.05 8.26 10.73
0.01 0.04 0.05
a
6-311++(2df,2p) basis. bAnions 6 and 8 are identical.
a
Calculated as total energy differences between equilibrium structures of an anion and its parent neutral. bSingle-point calculations with CCSD and CCSD(T) and the 6-311G++(2df,2p) basis. Zero-point corrections were calculated with MP2/UMP2 and the same basis. c Single-point calculations with CCSD and CCSD(T) and the 6-311G+ +(2df,2p) augmented with extra diffuse s and p functions.32
Figure 3. Molecular orbital plot for a valence-bound, canonical 5azauracil anion.
Table 5. Vertical Electron Detachment Energies of DB Anions, eVa
energies increases the AEAs at the CCSD(T) level and in one case (structure 6) converts the anion from a slightly unbound to a slightly bound species. Anions 1, 4, 6, 7, 8, and 13 are adiabatically bound with respect to their corresponding neutral structures and could be observed experimentally. However, only the first of these four anions is more stable the canonical form (i.e., structure 1) of 5-azauracil. Adiabatic Electron Affinities: DB Anion Tautomers. Table 3 also shows AEAs of 5-azauracil toward formation of DB anions. At the CCSD and CCSD(T) levels, only five tautomeric anions are adiabatically bound. However, when zero-point corrections are included, two more anions become marginally bound. Binding energy values for anions 1, 4, 6, and 8 are typical for DB anions. The AEA of structure 9, which is unusually large, has the largest dipole moment of all tautomeric neutrals. Vertical Electron Detachment Energies: VB Anions. VEDEs of VB anions obtained with several electron correlation methods are shown in Table 4. All anions are vertically bound. All pole-strength values exceed 0.85. Electron propagator VEDEs are systematically larger than those obtained with CCSD or CCSD(T). The VEDE of the most stable, canonical VB anion of 5-azauracil is predicted to exceed 1 eV and is comparable to its counterpart obtained experimentally and theoretically for 6-azauracil.23 The closest anionic structure, 13, a very rare tautomer, has a VEDE above 2 eV. Thus, these two structures might be represented in photoelectron spectra of anionic 5-azauracil. A typical Dyson orbital of a VB 5-azauracil anion is depicted in Figure 3.
tautomer
CCSD
CCSD(T)
OVGF
P3
P3+
1 2b 3b 4 5b 6 7 8 9 10
0.078
0.087
0.061
0.122
0.122
0.034
0.025
0.008
0.052
0.052
0.033 0.079 0.300 0.151 0.539
0.051 0.087 0.326 0.164 0.560
0.019 0.061 0.284 0.135 0.526
0.039 0.112 0.303 0.193 0.608
0.039 0.112 0.303 0.193 0.608
a
6-311++G(2df,2p) basis augmented with extra diffuse s and p functions on each atom.32 bTautomers 2, 3, and 5 are not vertically bound.
Vertical Electron Detachment Energies: DB Anions. Table 5 contains VEDEs of ten tautomeric, DB anions of 5-azauracil. Seven anions out of ten are predicted to be vertically bound. The OVGF values are consistently smaller than the P3 and the P3+ energies. All pole strengths are greater than 0.85. MO plots of anions 1, 8, and 10 are given in Figure 4. These plots are typical for DB anions of pyrimidine bases, with the largest amplitudes occurring outside the nuclear framework and nearest to the positive end of the molecule’s dipole moment. Bearing in mind that the canonical, DB 5-azauracil anion is much lower in energy than any other DB tautomer, it is likely that only 6911
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corresponding AEA of the neutral (0.06 eV), it will be associated with a sharp peak in a photoelectron spectrum.
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AUTHOR INFORMATION
Corresponding Author
*J. V. Ortiz.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation through a grant (CHE-0809199) to Auburn University. REFERENCES
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Figure 4. Molecular orbital plots for diffuse-bound, tautomeric 5azauracil anions.
this structure will be observed in an experimental photoelectron spectrum.
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DISCUSSION The most reliable correlation method employed in this study, CCSD(T) with zero-point energy corrections, indicates that the most stable VB and DB anions have structures that resemble that of the canonical, neutral tautomer. Among the VB anions, structure 13 is the next most stable isomer, with an energy that is approximately 0.25 eV higher. The VB anion structure 13 is lower than the most stable isomer of the neutral molecule (structure 1) by 0.02 eV. None of the DB species except for the first is more stable than the neutral molecule’s lowest structure. AEAs corresponding to the VB and DB anions are 0.27 and 0.06 eV, respectively. VEDEs for the VB and DB anions are 1.05 and 0.09 eV, respectively. Dyson orbitals and pole strengths from the electron propagator calculations indicate that the MO picture of the vertical transition energies is qualitatively valid. The small discrepancy between the AEA and the VEDE corresponding to the DB anion and the close resemblance between the DB anion’s structure and that of the neutral molecule suggest that a sharp peak near 0.1 eV will be seen in a photoelectron spectrum when the DB anion is present. However, the VB anion is more stable than the DB anion. The large difference between the AEA and the VEDE corresponding to the VB anion indicate that a broad peak with an onset near 0.3 eV and a maximum near 1.1 eV will be observed in a photoelectron spectrum where the VB anion is present. Because VB anion structure 13 is marginally more stable than the neutral molecule, it is possible that a VEDE of 2.0 eV could be observed for this very rare tautomer.
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CONCLUSIONS Tautomeric 5-azauracil anions, both valence-bound and diffusebound, were studied with correlated ab initio methods. The canonical structures of both VB and DB anions were found to be the most stable and to have positive adiabatic electron detachment energies of 0.27 and 0.06 eV, respectively. The valence-bound, canonical anion of 5-azauracil is expected to be observed under experimental conditions comparable with those reported for 6-azauracil or thiouracils.23 A wide band is expected in a photoelectron spectrum of the VB anion, for its VEDE is 1.1 eV. A very rare, imino−oxo, VB tautomer also may be observed in such experiments and will have a peak near 2.0 eV. Because the DB anion has a VEDE, 0.09 eV, very close to the 6912
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