J. Phys. Chem. 1993,97, 1 1122-1 1123
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Electron Attachment to Uracil. Theoretical ab Initio Study Nathan A. Oyler and Ludwik Adamowicz' Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received: May 4, 1993Q
Ab initio calculations indicate that the uracil molecule can bind an electron and form a stable dipole-bound anion. The adiabatic electron affinity of this process is estimated to be only 0.003 15 hartree (0.086 eV). Theoretical calculation of the electron affinities (EA) of larger biological molecules present a complicated task. Usually a significant electron correlation energy contribution mandates a treatment that goes beyond the HartreeFock model. Spatial expansion of the electron density for the anion requires additional diffused basis functions. The electron affinities of nucleic acid bases and related compounds in the gas phase are not well-known, despite a significant interest in the mechanism for excess electron attachment to DNA.' In a conventional ab initio approach to calculate the EA of a molecule, one usually takes a standard basis set with additional diffused functions (e.g., 6-31+G*) and performs an SCF MP2 calculation (self-consistent field calculation followed by the many-body perturbation theory calculation of the second-order electron correlation correction) on the anion and the neutral system. The difference in the total energies gives an estimate of the EA. This scheme was recently followed by Sevilla and co-workers.2 Unfortunately, such an approach leads to negative values of the EAs of the pyrimidine nucleic bases, indicating that the anions are unstable and that witha lossofenergy they shoulddissociateinto theneutralsystems and free electrons. The only reason the detachment of the electron is not seen in the theoretical calculation is the limited basis set usually confined to the areas around the atoms. If the energy of the anion is higher than the energy of the neutral molecule, the theoretically evaluated properties and structure of the anion may not correspond to a real anionic system existing in the gas phase but rather to a fictitious anion which exists by the virtue of artificially imposed limits on its spatial expansion. One may certainly question the relevance of such an artificial model to the physical reality of the electron attachment. The question which should be answered first by any theoretical calculation pertains to the stability of the nucleic bases anions. In the present work we study the EA of the uracil molecule. Our initial SCF calculations on the uracil ground state revealed that this molecule possesses a significant dipole moment of 5.07 D (the SCF/6-3 1+G* results). This result immediately suggests that even if there is no covalent-bound anion of uracil (i.e., with electron attached to a valence shell of the molecule), there must exist a stable dipole-boundanion, in which the electron is attached to the attractive field of the permanent molecular dipole moment. Such dipole-bound states, long predicted by theoertical calculations: were recently observed by means of ultrahigh-resolution photodetachment spectroscopy (Maed et al.4). Studies, in which one of us was involved: conducted with the use of the numerical HartreeFock and MCSCF methods allowed us to examine some properties of dipole-bound states for diatomic polar molecules including the basis set requirements. More recently, we presented a study on the dipole-bound anionic state of nitromethane6 in which a floating, diffused Gaussian basis set of three sp shells with exponents determined based on an even-tempered ansatz was included in the calculations to describe the orbital occupied by the dipole-bound electron.
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Abstract published in Advance ACS Absrracfs,October 1, 1993.
0022-3654/93/2097-11122$04.00/0
The present calculation for uracil was done with the geometry optimized at the SCF/3-21G level of theory with the use of the GAUSSIAN 92 program.' All other calculations presented in this work were also done with this program. The following summarizes our computational work which led to determination of the EA of uracil for the dipole-electron attachment: (1) A set of 12 orbitals (three sp sets with exponents 0.01, 0,001, and 0.0001) were localized in a point 1 A away from the nearest neighboring atomic center in the direction of the positive side of the molecular dipole. When the three sets of orbitals were included in the SCF/3-21G calculation on the neutral system, the LUMO (lowest unoccupied molecular orbital) energy became negative. The exponents of the sp orbitals were subsequently optimized by arbitrarily choosing a common multiplier of the exponents and varying it so the lowest LUMO energy wasreached. This optimization procedure resulted in exponent values of 0.07, 0.007, and 0.0007. The additional basis functions with the optimized exponent values will be referred to as X in basis designations (e.g., 3-2 1GX). (2) The position of the extra orbitals was optimized with the use of the UHF/3-21GX procedure to get the lowest energy of the dipole-bound anion, holding the geometry of the uracil structure constant. (3) SCF and MP2 calculations were performed with the basis set combining the standard 6-31+G* basis and the X set for the neutral uracil and uracil anion. The numerical results are summarized in Table I. (4) The geometry of the dipole-bound uracil anion was reoptimized with the use of UHF/3-21GX procedure. This includes the reoptimization of all the atomic centers as well as the position of the extra orbitals. In a subsequent calculation, the SCF and MP2 energies were computed for the reoptimized anion with the 6-3 1+G*X basis set. The results are included in Table I. Upon examining the results, one notices that the calculated electron affinity, of uracil is positive. This is mainly due to including diffused orbitals in the basis set which allowed an electron to attach to the molecular dipole. EA equal to 0.000 8 1 hartree at the Koopmans' level (the LUMO energy) raises to 0.000 96 hartree when the relaxation of the anion is accounted for in a separate UHF calculation. With inclusion of the electron correlation at the MP2 level, EA changes to 0.001 29 hartree. This change can be attributed to a change of the dipole moment of neutral uracil-an effect that frequently occurs when electron correlation is accounted for. The value 0.001 29 hartree (0.035 eV) should be considered our best estimate of the vertical EA of the electrondipole attachment to uracil. A more significant change of EA occurs when one allows the geometry of the anion to relax. The MP2/6-3 1+G*X result for adiabatic EA obtained as a difference between the total energies of neutral uracil with the geometry optimized at the SCF/3-21G level and the total energy of the anion with the geometry optimized at the UHF/ 3-21GX level is equal to 0.003 15 hartree (0.086 eV). Note the 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 11123
Electron Attachment to Uracil
TABLE I: Calculations of the Electron Affinity (EA) of Uracil (U)(All Enerdes in hartrees) U v EA SCF/3-21GX‘ SCF/6-31+G*Xa MP2/6-31+G*Xa SCF/3-21GXb SCF/6-3 1+G*Xb MP2/6-31+G*Xb
-410.230 041 1 -412.480 220 3 -413.642 209 7
-410.231 -412.481 -413.643 -410.232 -412.479 -413.645
403 7 178 3 503 0 254 5 081 4 361 5
0.001 36 0.000 96 0.001 29 0.002 21 4 . 0 0 1 14 0.003 15
a Molecular geometry optimized for neutral uracil at the SCF/3-21G level. Molecular geometryoptimizedfor theanion at theUHF/3-21GX level.
SCF EA now changes sign but the HOMO (highest occupied molecular orbital) energy still remains negative (-0.001 45 hartree). The electron attachment to the field arising from the molecular dipole moment leads to an orbital for the extra electron which is considerably diffused and positioned away from the area occupied by the molecular frame. In Figure 1 we present a plot for LUMO for uracilobtainedin the SCF/d31+G*Xcalculation. The position of the X basis set is indicated on the picture with a dot. To a good approximation, the orbital appears to be a diffused sp hybrid oriented along the direction of the molecular dipole. However, there exists a small component of the orbital which is localized around the atomic centers of the molecule, implicating that the electron attachment may have some “covalent” character. In conclusion, we determined that an electron attachment to the uracil molecule leads to formation of a bound dipole state. The EA for such an attachment is positive but very small. It has not been determined whether the dipole-bound state will deexcite to a more stable “covalent” anionic state* in a similar manner as occurs for nitromethane. However this may not be the case for uracil because as our SCF+MP2/6-31+G* calculations show, the neutral molecule is more stable than its “covalent” anion by 0.019 73 hartree. Ofcourse, higher ordercalculationsarerequired to confirm this finding. The present study shows that the crucial factor which allows one to determine the electron efficiency of a polar molecule such as uracil in an ab initio calculation is a proper selection of the basis set. Without very diffused orbitals located on the positive side of the molecular dipole, the stable electronic states of the anion can be completely missed.
Figure 1. Orbital occupied by the extra electron in the dipole-bound anion of uracil.
Acknowledgment. This work has been sponsored by a grant (No. DEFG 0393ER61605) from the Office of Health and Environmental Research, Department of Energy. References and Notes (1) Sevilla, M. D.; Baker, D.; Yan, M.; Summerfield, S . R. J. Chem. Phys. 1991, 95, 3409. (2) Colson, A.-0.; Baler, B.; Close, D. M.; Sevilla, M.D. J.Phys. Chem. 1992, 96, 661. (3) Fermi, E.; Teller, E. Phys. Rev. 1947, 72, 406. (4) Lykke, K.; Mead, R. D.; Lineberger, W. C. Phys. Rev. Lett. 1984, 52,2221. Mead, R. D.; Lykke, K. R.; Lineberger,W. C.; Marks, J.; Brauman, J. I. J. Chem. Phys. 1984,81,4883. ( 5 ) Adamowicz, L.; McCullough, E.A. Int. J. Quantum Chem. 1983, 24, 19; J . Phys. Chem. 1984,88, 2045; Chem. Phys. Lctt. 1984,107,12. (6) Adamowicz, L. J. Chem. Phys. 1989,91,1187. (7) Gaussian 92, Revision C.; Frisch, M. J.; Trucks, G. W.; Had-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Fore”, J. B.;Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.;Gomperts, R.; Andrea, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian, Inc.: Pittsburgh, PA, 1992. (8) Stockdale, J. A.; Davis, F. J.; Compton, R. N.; Klots,C. E.J . Chem. Phys. 1974,60,4279. Compton, R. N.; Weinhardt, P. W.; Cooper, C. D. J. Chem. Phys. 1978,68.4360.
