Configuration Interaction Effects on He( I) Photoelectron Spectra of Nucleotide Bases: Evidence for Valence Electron Hole-Mixing

in 1,9-Dirnethylguanine

MIN YU, QING JIANG, AND PIERRE R. LEBRETON Department of Chemistry, The University of Illinois at Chicago, Chicago, Illinois 60680

Abstract

&miempirical HAM/^ molecular orbital configuration interaction (CI) calculations have been employed to interpret the gas-phase He(1) photoelectron (PE) spectrum of 1,9dimethylguanine ( 1,9-DMG). Previous interpretation of the PE spectrum of this molecule, which has a tautomeric structure similar to that of guanine in DNA and RNA, has relied on sm calculations. Test calculations were performed on pyrimidine. Because HAM/) calculations without CI were parameterized to describe PE data, ionization potentials (IPS) obtained at the SCF level agree better with experiment than IPS obtained with CI calculations. However, when pyrimidine results from the HAM/^ CI calculations are compared to previously reported results obtained using the outer-valence Green function ( OVGF) method or the two-particle-hole Tamm- Dabcoff Green function approximation (2ph-TDA) method, the agreement is satisfactory. The average difference between corresponding values for the first four IPS of pyrimidine obtained from HAM/) CI, OVGF, and the 2ph-TDA calculations is less than 0.5 eV. Furthermore, for the seven lowest-energy cation states of pyrimidine, the three computational methods yield normalized coefficients associated with the dominant Koopmans configurations that have squares that differ by less than 0.14. For I,9-DMG. the results Of HAM/3 CI calculations indicate that five of the seven lowestenergy ionization events, previously assigned to the first-to-third and to the sixth- and seventh-highest occupied orbitals, are well described within the framework of Koopmans theorem. On the other hand, the fourth and fifth ionization events in 1,9-DMG result in cation states in which there is significant hole-mixing of Koopmans configurations associated with removal of electrons from the second- and third-highest occupied K orbitals. 0 1992 John Wiley & Sons, Inc.

Introduction

The combined use of gas-phase uv photoelectron ( PE) spectroscopy and molecular orbital theory has provided detailed and experimentally verified descriptions of the valence electronic structures of many of the components occurring in DNA and RNA, including bases, nucleosides, phosphate esters, and, most recently, nucleotides [ 1-41. Much of the interpretation of ionization events in nucleotides and nucleotide components has been carried out within the framework of Koopmans theorem [ 5 ] and has relied on single-configuration descriptions of closed-shell initial states and open-shell final states. In a previous examination of one of the RNA bases, uracil, an ab initio configuration interaction ( CI) calculation of the PE spectrum was per- formed that employed a double-zeta quality basis set and singly and doubly excited

International Journal of Quantum Chemistry: Quantum Biology Symposium 19,27-41 (1992) 0 1992 John WiIey & Sons, Inc. CCC 0360-8832/92/010027-15

28 YU, JIANG, AND LEBRETON

configurations [ 61. The ionization potential ( IP) of uracil obtained from the CI calculation differed from the experimental value by less than 0.1 eV and was more accurate than values obtained from earlier semiempirical and ab initio SCF calcu- lations [ la, 1 m,7 1. However, the results from the CI calculations also interestingly pointed out that all the well-resolved bands in the uv PE spectrum of uracil (the five lowest-energy bands with vertical IPS less than 12.63 eV) are associated with radical cation states that are qualitatively well described by a single Koopmans configuration. For all five of these cation states, the coefficient associated with the major Koopmans configuration is greater than 0.87 [ 61.

Configuration interaction can influence PE spectra by causing weak satellite shake- up bands that arise from the mixing of a Koopmans configuration with a non- Koopmans configuration that has the same symmetry and that arises from a two- electron excitation [ 8,9]. Normally, access to the non-Koopmans configuration is forbidden via a PE transition because of orbital selection rules. In practice, the accessibility of shake-up states relies on CI matrix elements that are made up of two-electron Coulomb integrals between occupied orbitals in the Koopmans and non-Koopmans configurations. As the configuration interaction matrix elements increase, the coefficient of the Koopmans configuration in the CI wave function increases. The intensity of the satellite band increases as the square of the coefficient of the Koopmans configuration increases. When the symmetries match, the mixing of Koopmans and non-Koopmans configurations increases as the energy difference between the configurations decreases. Figure 1 shows Koopmans and non-Koop mans configurations for a radical cation.

For many molecules, valence electron shake-up ( VESU) satellite bands for inner valence-shell electrons, in the energy region 20-50 eV, have been examined using both ab initio and semiempirical methods [ 8,9]. The many-body Green function method using the two-particle-hole Tamm-Dancoff approximation ( 2ph-TDA) predicts that the molecular orbital description of inner valence-shell ionization

T 1

k

m Y vv

i"

Figure 1 . Koopmans configurations, associated with ground and excited cation states (Type A) and non-Koopmans configurations (Types B and C) .

CI EFFECTS ON SPECTRA OF NUCIEOTIDE BASES 29

processes often breaks down [ 8e-n,9a-f]. The Green function calculations also predict that, in some molecules, outer-shell VESU satellites occur in He(1) spectra at energies less than 20 eV. Theoretical investigations of unsaturated hydrocarbons [ 9b,k,l,n 1, such as ethylene, butatriene, cycloproparenes, and pquinodimethanes, and of mines [ 9e], such as pyrimidine and pyrazine, suggest that these bands fre- quently contribute to the PE spectra at energies lower than 16.0 eV and, in some cases, lower than 10.0 eV. However, the assignment of low-energy VESU bands has sometimes resulted in controversy [ 9k,n, lo].

In addition to leading to predictions of satellite bands, CI can also result in pre- dictions of hole-mixing [ 1 11. This involves the mixing of Koopmans configurations of the same symmetry and of nearly equal energy. When only two Koopmans configurations, I p-l ) and I q-' ), contribute, a CI wave function, Q,, that describes mixing between these configurations is given by Eq. ( 1 ):

+ contributions from more highly excited configurations . Here, I p - ' ) and 1q-l) describe cation configurations in which an electron is re- moved from orbitals p and q, in a closed-shell, neutral molecule. The ratio c A , q /

CA,D is given by Eq. (2):

In Eq. (2), the summation is over non-Koopmans radical cation configurations, each of which is defined by the three indicesj-I, k - ' , and 1. For the ground-state neutral molecule from which the radical cation is formed, the j and k'th orbitals are doubly occupied and the l'th orbital is unoccupied. The non-Koopmans radical cation configuration, I j - ' , k-' , l), is formed by removing an electron from the j'th orbital and promoting an electron from the k'th orbital to the l'th orbital. In Eq. (2), ei is the one-electron energy associated with orbital i, and the coupling matrix elements, V ( p - ' , j - ' , k- ' , 1) and V ( q - ' , j - ' , k-I, l), are made up of Coulomb integrals between electrons in configuration I j- ' , k-' , 1) and electrons in configurations I p-l ) and I q-' ), respectively [ 1 11. The mixing of Koopmans configurations, which is mediated by non-Koopmans configurations, gives rise not to weak satellites, but to strong PE bands with energies different from those of the single Koopmans configurations from which they are formed. In Eq. (2) , the de- nominators ej + &k - el - ep represent the energy differences between the Koopmans configuration I p - ' ) and the non-Koopmans configuration I j- ' , k-' 1). The mag- nitudes of the coefficients, CA,q and CAs, become similar when cj + &k - el - ep is small and when there is one or several non-Koopmans configurations that simul- taneously have large coupling matrix elements to the two Koopmans configurations, I p - ' ) and Iq - ' ) .

In Figure 1, the configurations labeled Type A represent ground- and excited- state Koopmans configurations of a radical cation. The configurations labeled Type B represent non-Koopmans configurations of a radical cation formed from a ground-

state, closed-shell neutral molecule in which all the orbitals, occupied in the neutral, are either empty or doubly occupied. The configurations labeled Type C represent non-Koopmans configurations of the radical cation in which two of the orbitals, occupied in the neutral, are singly occupied.

The self-consistent-field HAM (hydrogenic atoms in molecules) method is a fre- quently successful semiempirical approach for interpreting the PE spectra of organic molecules [ 12- 16 1. In this approach [ 12,16 1, Slater-type shielding coefficients are employed to account for the electron correlation energy. A transition-state method has been used to account for reorganization. In several cases, IPS calculated using this approach agree with experimental IPS to within a few tenths of an electron- volt [ 14,161.

The self-consistent-field HAM method has been extended to permit a more rigorous description of valence electron ionization processes through the introduction of CI [ 9k, 16,17 1. Results from HAM/^ CI calculations are found, in some cases, to predict CI effects, similar to those obtained from more computationally demanding Green functions methods [ 9k]. The HAM/3 CI approach employs the single-excitations CI method for calculating IPS. Because experimental IPS have been used to parameterize HAM/3, the values of IPS obtained from the HAM/3 CI calculations are not as accurate as those obtained from HAM/^ calculations. However, the HAM/^ CI approach has proven useful for predicting the breakdown of Koopmans theorem [ 9j 1.

In the present investigation, HAM/^ CI calculations have been employed to de- termine the importance of CI effects on the He(1) PE spectrum of 1,9-dimethyl- guanine ( 1 ,9-DMG) at energies less than 12.5 eV where previous assignments have been carried out [ 21. Guanine, which contains 78 electrons, is the most complicated of the common bases occurring in DNA and RNA. For this reason, there is a higher probability that CI effects contribute to the PE spectrum of guanine than to the spectra of the other bases. The present investigation has focused on the 1,9dimethyl derivative of guanine because methyl substitution at the 1- and 9-positions results in tautomeric structure that is identical to that of guanine in the guanine-cytosine, Watson-Crick base pair, and eliminates the possibility of rare tautomeric forms that occur in nonpolar environments [ 18 1, The PE spectrum of 1,9-DMG contains a broad region of high PE intensity between 9.0 and 10.5 eV that arises from several overlapping bands. In an earlier interpretation of the PE spectrum of 1,9-DMG, this energy region was assigned to four bands associated with the second- and third- highest occupied 7~ orbitals and with the first- and second-highest occupied lone- pair orbitals [ 21. This energy region, containing several cation states with nearly equal energy, is of special interest to the present investigation of CI effects.

Methods

Results from HAM/^ and HAM/^ CI calculations were compared to previously reported He( I) PE spectra of pyrimidine [ 191 and 1,9-DMG [2]. The geometries of pyrimidine [ 201 and 1,9-DMG [ 21 employed in the calculations were obtained from crystallographic data. For the HAM/^ CI calculations, the geometries of the

CI EFFECTS ON SPECTRA OF NUCIEOTIDE BASES 31

radical cations were assumed to be the same as those of the parent molecules. The doublet CI matrix elements were calculated by applying the INDO approximation [ 2 1,221 to the LCAO-zDO formalism [ 231. The relative intensities of the PE bands were estimated to be equal to the sum of the square of the coefficients of the Koop mans configurations (C;) that contribute to the CI wave function.

The HAM/^ CI method employs a reference orbital to obtain an energy difference between the neutral, closed-shell, ground-state molecule and a reference Koopmans state of the cation. The ionization potential associated with the cation state generated by removal of an electron from the reference orbital is obtained by employing Koopmans theorem. Ionization potentials of other cation states are calculated by comparing excitation energies obtained from the CI calculations to the energy of the reference state. The choice of the reference orbital is controlled by the adjustable parameter IHALF. For the HAM/^ CI calculations on pyrimidine and 1,9-DMG, the values of IHALF employed were 12 and 34, respectively. For pyrimidine, 200 con- figurations were used. These were made up of 12 Type A, 18 Type B, and 170 Type C configurations. For 1,9-DMG, 68 configurations were used. These were made up of 15 Type A, 7 Type B, and 46 Type C configurations.

Results

Pyrimidine

Figure 2 shows the He(1) PE spectrum of pyrimidine, labeled A, together with ionization potentials obtained from HAM/^ and HAM13 CI calculations. In the PE spectrum of pyrimidine, the first and second PE bands are well resolved and have maxima at 9.7 and 10.5 eV. The energy region from 11.0 to 18.0 eV contains five maxima. These occur at 11.5, 14.5, 15.8, 16.9, and 17.4 eV.

The HAM/^ ionization potentials, labeled B in Figure 2, are numbered according to the Koopmans state from which they arise. The HAM13 CI ionization potentials are labeled C in Figure 2. Ionization potentials in C that are associated with cation states arising primarily from a single Koopmans configuration are numbered without circles. Ionization potentials associated with shake-up states are labeled with circled numbers that denote the Koopmans state to which the shake-up state is predom- inantly coupled.

Table I lists the HAM/3 predictions of symmetries of the orbitals from which electrons are removed to form the Koopmans configurations given in Figure 2. The table also lists lowenergy experimental IPS and IPS obtained from the HAM/^ and HAM/^ CI calculations. Along with the Ips obtained from the HAM13 CI calculations, values of the square of the coefficient (C;) for the most important Koopmans configuration are given. Table I1 lists CI matrix elements, obtained from the HAM/ 3 CI calculations, for the four lowest-lying cation states in which CI effects are large.

The results in Table I indicate that for all, except the second band, the values of IPS obtained from the HAMIS calculations agree better with experiment than do values obtained from the HAM/ 3 CI calculations.

In addition to HAM/^ and HAM/3 CI results for pyrimidine, Table I also gives results from previous ab initio CI calculations [ 9e3 using the outer-valence Green

32 YU, JIANG, AND LEBRETON

h JJ .rl

a, JJ C H

C 0 k +, u a, rl a, 0 JJ 0 c pc

z

5 I 6 7

B 2 3

C

1

I l l 2,3 4

I 1 Hi

8.9 10

ll I

~~

0 11.0 13.0 15.0 17.0

Ionization Potential (eV) Figure 2. IPS of pyrimidine: (A) He(1) PE spectrum taken from [19]. Numbers above the spectrum refer to assignments based on results from HAM/J calculations. (B) Ionization potentials obtained from results of H A M / 3 calculations. Orbital symmetries are given in Table 1. ( C ) Ionization potentials obtained from results of HAM/3 CI calculations. Circled numbers refer to states associated with shake-up bands. The number describing each shake- up state refers to the most important Koopmans configuration contributing to the state.

The heights of lines indicating cation-state energies denote relative band intensities.

CI EFFECTS ON SPECTRA OF NUCIEOTIDE BASES 33

TABLE I. Theoretical and experimental IPS of pyrimidine.'

Koopmans configuration HAMJJ HAM13 Clb OVGFb'C'd. 2ph-TDAb3'' Experiment'

9 (3b;')

10 14~;')

9.88 10.77 11.14 11.45 13.77 14.34 14.38 15.88

15.99

17.29

9.22 (0.93) 10.40 (0.96) 10.41 (0.91) 10.87 (0.94) 12.75 (0.82) 13.68 (0.90) 13.70 (0.88) 15.13 (0.60) 15.23 (0.22)

15.18 (0.14) 15.27 (0.48) 15.45 (0.16) 16.49 (0.78)

9.50 (0.90) 10.54 (0.91) 10.87 (0.90) 1 1.23 (0.89) 14.46 (0.84) 14.40 (0.84) 14.25 (0.89) 16.02 (0.89)

9.86 (0.92) 10.43 (0.94) 11.15 (0.90) 11.16 (0.93) 13.95 (0.75) 14.73 (0.88) 14.46 (0.87) 16.31 (0.83)

17.23 (0.45) 17.52 (0.31) 18.22 (0.78)

9.7 10.4 11.2 11.5 13.9 14.5 14.5 15.8

16.9

17.4

a All IPS in eV. Values of the square of the coefficients (Ci) for the most important Koopmans configuration are

given in parentheses. Only states for which C: is greater than 0.1 are listed. ' Obtained from outer-valence Greens function (OVGF) calculations.

Taken from [9e]. Obtained from two-particle-hole Tamm-Dabcoff approximation (2ph-TDA) Green function calcu-

Taken from [ 191. lations.

function ( OVGF) and the 2ph-TDA methods. For the cation states associated with experimental IPS below 15.0 eV, the ab initio CI results support the qualitative description provided by the HAM13 CI calculations. Both the semiempirical and the ab initio CI results predict that the first seven bands in the He(1) PE spectrum of pyrimidine are associated with cation states with large contributions from a single Koopmans configuration. For these states, the results from the HAM/3 CI calculations indicate that the squares of the coefficients of the most important Koopmans con- figuration (Cfi) lie in the range 0.82-0.96. The results from the OVGF method indicate that the seven lowest-energy cation states have Cfi values between 0.84 and 0.9 1. The 2ph-TDA results indicate that the Cfi values are in the range 0.75- 0.94 [ 9e].

The HAM13 CI calculations predict that shake-up bands occur in the PE spectrum of pyrimidine at smaller energies than those predicted by the ab initio CI calculations. According to the HAM/3 CI results, CI leads to five shake-up bands with calculated IPS below 17.0 eV. In two of the lowest-energy shake-up states, the major contributing Koopmans configuration is I5a;' ); in the other three, the major Koopmans con- figuration is I3b;'). For the 15~;') shake-up bands, the values of C i predicted by HAM/^ CI calculations are 0.60 (for the major band) and 0.22 (for the satellite

34 YU, JIANG, AND LEBRETON

TABLE 11. HAM/^ CI CI matrix elements for the four lowest-energy radical cation states of

pyrimidine for which CI effects are large.

IP (eV) CI matrix elements"

15.13 (5a;' ( H I5&', la?', 36') (0.04) 15.18 15.23 15.27

(36:' ( H I562', 2b;', 361) (0.37) ( 5 ~ ; ' JH I 562', la;', 361) (0.04)

(362' I H 126;', 7aT', 2 ~ 2 ) (-0.38) (36:' IH156T', IU;', 2 ~ 2 ) (-0.07) (362' ( H I2bT', 7ai', 2 ~ 2 ) (-0.07) 15.45

Koopmans configurations are given on the left. Non-Koopmans configurations are given on the right. The notation describing the Koopmans and non-Koopmans configurations is given in the text. Values of matrix elements in eV are given in pa- rentheses.

band). In these states, the most important non-Koopmans configuration is I5bT', la;', 3bl). As Table I1 indicates, this configuration, like the non-Koopmans configurations mixing with the 13b:') configuration, is Type C.

According to the 2ph-TDA calculations, which have been used to describe all states with IPS lower than 22.84 eV, no shake-up bands occur below 17.0 eV [ 9e]. Results from the 2ph-TDA calculations indicate that the I 5a;') configuration does not give rise to cation states with significant configuration mixing and that the lowest-energy shake-up states are associated with the I3b;') configuration. Results from the 2ph-TDA calculations indicate that the 13b:') configuration gives rise to two shake-up bands.

The HAM/^ CI calculations predict that the splitting between the major bands and the satellite bands of the 15~;') and I3b;' ) configurations is less than 0.4 eV. The 2ph-TDA calculations predict that the splitting of the bands of the 13b;') configuration is less than 0.3 eV. If they exist, the satellites associated with the I 52;') and I 3b:') shake-up bands are predicted to lie in an energy region near the bands that occur at 15.8 and 16.9 eV in the He( I) PE spectrum of pyrimidine. The poor resolution of the spectrum in this region makes experimental verification of the satellite structure impossible at this time.

The HAM/^ CI results predict that the next band appearing in the PE spectrum of pyrimidine, at an energy higher than those associated with states arising from the 15~;') and the I3b;') configurations, has an IP of 16.49 eV and arises from the 1 4a ; ) Koopmans state. The HAM/ 3 CI calculations also predict that this state is not significantly mixed with non-Koopmans states. The 2ph-TDA calculations predict the occurrence of a similar state with an IP of 18.22 eV. It is likely that the

CI EFFECTS ON SPECTRA OF NUCIEOTIDE BASES 35

Koopmans state predicted by the HAM/^ CI and 2ph-TDA calculations gives rise to the band at 17.4 eV in the spectrum of pyrimidine.

It is interesting to note that while the HAM/3 CI calculations indicate that shake- up states do contribute to the He(1) PE spectrum of pyrimidine, these states are predicted to give rise to bands experimentally observed at IPS above 15.7 eV. For these bands, the assignments based on HAM/^ CI results are in agreement* with earlier assignments of the PE spectrum of pyrimidine [ 19 1, with the HAM/^ assign- ments, with results from experimentally scaled ab initio SCF calculations [ 241, and with results from the more rigorous ab initio CI calculations [ 9e].

1,9-DMG

The PE spectrum and assignments for 1,9-DMG are shown in Figure 3. The energy region from 7.5 to 1 1.8 eV contains four maxima. These occur at 8.09, 9.5, 10.0, and 11.0 eV.

Table I11 lists IPS of 1,9-DMG obtained from HAM/^ calculations, as well as results from previous [ 21 ab initio SCF calculations using 3-21G [ 251 and 4-31G [ 261 basis sets. The table also gives experimental IPS. Figure 3 compares the PE spectrum of 1,9-DMG with results from HAM/3 and HAM/^ CI calculations, labeled B and C, respectively.

The experimental vertical IP associated with the 1 ?r : ' ) configuration, which arises from removal of the highest-occupied ?r orbital in 1,9-DMG, is 8.09 eV and is labeled 1 in Figure 3. According to the HAM/3 results, the energy region between 8.84 and 9.71 eV contains four bands arising from the In:'), In?'), IT;'), and I T:') configurations. These are associated with removal of electrons from the first- and second-highest occupied lone-pair orbitals, and from the second- and third- highest occupied ?r orbitals. In the previous investigation of 1,9-DMG [ 21, these four bands, labeled 2, 3, 4, and 5 in Figure 3, were assigned to the energy region between 9.5 and 10.0 eV. The HAM/3 calculations predict that next-highest-energy states are associated with the I n i l ) and I ?r;' ) configurations and have energies of 10.46 and 10.53 eV, respectively. The bands arising from these states were assigned to the energy region of the spectrum containing a maximum at 11.0 eV and are labeled 6 and 7 in Figure 3. According to the HAM/^ calculations, the next-highest cation state occurs at 12.25 eV and arises from the I T?') configuration. This state was assigned to the shoulder at 12.31 eV in the spectrum of 1,9-DMG, which is labeled 8 in Figure 3.

The results in Table I11 demonstrate that the absolute values for the IT:' ) ion- ization potentials obtained from the ab initio 3 - 2 1 ~ and 4 - 3 1 ~ calculations are less accurate than the value obtained from the HAM/^ calculations. The 3-21G and 4- 3 1 ~ calculations yield I T;') ionization potentials that are 0.35 and 0.17 eV smaller

* In [ 9e], the results from the OVGF and 2ph-TDA calculations indicate that the energetic ordering of bands associated with the 1 lb;'), 14b;') and 16~;') configurations is different than that found here and in [ 191 and [24]. However, the spectrum of pyrimidine shown in Figure 2 indicates that the energy region containing the 1 b, , 4&, and 6ul bands is poorly resolved and that these configurations have similar energies.

36 YU, JIANG, AND LEBRETON

h c, 4

i! c, fi H

c 0 k c, U al 4 al 0 c,

9, 2

Figure 3.

A

9.5-10.0

f i \ 11.0 67

B

C

'C

V

i 'i I'

9 h 13.0 '

8 12.3

i i

1 9.0 11.0 13.0 15.0

Ionization Potential (ev) IPS of 1,9-DMG: (A) He( I) PE spectrum [ 21. The upper set of numbers above

the spectrum refers to assignments based on results from HAM/3 and ab initio SCF calcu- lations. The lower set of numbers above the spectrum gives IPS. (B) IPS obtained from results O f HAM/3 calculations. Orbital symmetries are given in Table 111. (C) IPS obtained from results of HAM/^ CI calculations. Circled numbers refer to bands that are associated with cation states in which there is significant CI. The number describing each CI state refers to the most important Koopmans configuration contributing to the state. Relative

band intensities are indicated by line heights.

CI EFFECTS ON SPECTRA OF NUCIEOTlDE BASES 37

TABLE III. Theoretical and experimental ionization potentials for I,9DMG."

Koopmans configuration HAMI) 3-2 icib 4-3 icb Experiment

8.05 8.84 9.5 I 9.56 9.7 1

10.46 10.53 12.25 12.93

7.74 10.93 11.43 10.27 11.07 12.62 12.02 14.48 15.37

7.92 11.16 1 1.62 10.37 11.21 12.82 12.16 14.60 15.46

8.09 9.5-10.0 9.5-10.0 9.5-10.0 9.5-10.0 10.9-1 I. 1 10.9-1 1 . 1

12.31 13.00

All IPS in eV. Obtained from [2].

than the experimental value. The HAM/^ value agrees with experiment to within 0.04 eV. For the cation states associated with the first-to-seventh excited Koopmans configurations, which according to the previous investigation [ 21 give rise to bands labeled 2-8 in the spectrum of 1 ,9-DMG, the ab initio 3-2 IG and 4-3 1G calculations predict IPS that are 0.3-2.3 eV larger than the experimental values. The HAM13 calculations predict IPS that agree with experiment to within 1.2 eV.

For 1,PDMG, both the 3 - ~ I G and the 4-31G ab initio calculations predict the same energetic ordering of the upper occupied orbitals. In some cases, where orbital energy differences are small, this ordering is different from that predicted by the HAM/^ calculations. Nevertheless, the patterns of Ips obtained from the HAM13 and the ab initio calculations are similar. In this sense, the assignment of states to different energy regions in the spectrum of 1,9-DMG is the same when based on results from either the HAM/3 or the ab initio calculations [ 21.

In the results from the HAM/^ CI calculations that are shown in Figure 3, cation states arising primarily from a single Koopmans configuration are numbered without circles. States in which there is significant CI are circled.

Table IV lists IPS and values of the sums of the squares of the coefficients of the Koopmans configurations associated with the PE accessible cation states having calculated IPS below 13.0 eV. Table IV also gives non-Koopmans configurations and values of major CI matrix elements contributing to cation states associated with shake-up bands. In Table IV, the notation for the non-Koopmans configu- rations is the same as that used for pyrimidine in Table 11. However, unlike the most important non-Koopmans configurations associated with low-energy cation states of pyrimidine, the important non-Koopmans configurations associated with low-energy cations states of 1,9-DMG are Type B configurations, rather than Type C configurations.

Except for cation states associated with I T;' ) and the I T ? ' ) configurations, the

38 YU, JIANG, AND LEBRETON

TABLE IV. IPS of lowest PE accessible, radical cation states of 1,9-DMG obtained from results of HAM/3 CI calculations.

IFb Major contributing Shake-up band CI configurations' matrix elementsa

7.73 (0.95) 1.7') 7.78 (0.87) In;') 9.1 1 (0.92) In;') 9.13 (0.9 1) I r;'), 1 r;') 9.41 (0.95) I T ; ' ) , I T ; ' )

10.00 (0.91) In;') 10.11 (0.89) 1rZ') 1 1.23 (0.50) I r;'), r;' , r;' , rf) -0.47 11.82 (0.19) In;'), r;', my', n:) -0.06 12.32 (0.30) I T ; ' ) , r;', ry', r:) -0.06 12.40 (0.17) In;'), r;', r;', rf) -0.47 12.91 (0.53) Iri'), a;', n;', r5) -0.37

a In eV. Relative intensities obtained from the sum of the squares

of the coefficients (Cfi) for all of the contributing Koopmans configurations are given in parentheses. IPS are listed for states for which the sum of the C: values are greater than 0.10. ' Only configurations for which the value of Cfi is greater

than 0.10 are listed. In the non-Koopmans configurations, r: , rf , and rf represent the first-, second- and third-lowest unoccupied r orbitals in ground-state neutral 1,9-DMG.

description obtained from the HAM/^ CI calculations of the cation states of 1,9- DMG with experimental IPS less than 1 I .O eV is similar to the description obtained from the HAM/3 and ab initio SCF calculations summarized in Table 111.

Shake-up States: According to the HAM/^ CI calculations, the IT?') and 1.1~6') Koopmans configurations, arising from removal of an electron from the eighth- and ninth-highest occupied orbitals in 1,9-DMG, mix with shake-up con- figurations to give rise to main bands and satellites that occur in the PE spectrum above 11.0 eV. The calculated IPS of CI states arising from the IT;') and 1 ~ 6 ' ) Koopmans configurations lie in the range 1 1.23- 12.9 1 eV. They are listed in Table IV along with values for the sum of C2 and with CI matrix elements. In the earlier investigation of the PE spectrum of 1,9-DMG [ 21, the PE emission occumng in the energy region around 12.31 eV was assigned to the I T ? ' ) Koopmans state. The present results provide evidence that, instead, this emission is due to shake-up bands coupled to the I T;' ) and, possibly, the IT;') configurations.

In addition to shake-up states arising from the I T ; ' ) and I T ; ' ) Koopmans configurations, the HAMI 3 CI calculations predict that numerous higher-energy, shake-up states also occur. States with calculated IPS below 15.0 eV are shown in

C1 EFFECTS ON SPECTRA OF NUCIEOTIDE BASES 39

Figure 3. Those with the highest IPS are mixed with Koopmans configurations formed from the removal of electrons from the 10th- to 15th-highest occupied orbitals.

Hole-Mixing: In the energy region of the PE spectrum of 19-DMG below 11.0 eV, the only major CI effects involve hole-mixing of the 1 7r:' ) and I ?r 3' ) Koopmans configurations. These mixed states are labeled 4 and 5 in Figure 3 ( C ) . For state 4, the coefficients Ca,, and CA,r3 in Eq. (1) are 0.73 and -0.48, respectively. For state 5, the coefficients are -0.60 and -0.75. The most important non-Koopmans configurations that contribute to state 4 are IT;', T;', 7r:) and IT;', r;', 7r:). The coefficients are 0.13 and 0.12, respectively. The most important non-Koopmans configuration contributing to state 5 is IT;' , T;', x : ) , which has a coefficient of -0.13. The results from the HAM/3 CI calculations indicate that, compared to a pure Koopmans state with a relative intensity of 1.0, the intensities of the I 7r;' ) and I 7r;' ) hole-mixing states are 0.9 1 and 0.95, respectively. The hole-mixing that is predicted by the HAM/^ CI calculations occurs because of the small calculated energy separation (0.15 eV) between the I 7rT1) and I r?') configurations. However, the description of the hole-mixing was not strongly dependent upon the parame- terization of the calculation or on the number of configurations employed. While the energies shifted, the qualitative description of the mixing of the IT?') and

1 "3') configurations persisted when IHALF was changed from 34 to 3 1 and when the number of configurations employed was changed from 68 to 109.

Conclusions

For pyrimidine, the results from HAM/^ CI calculations agree with previously reported results from ab initio CI calculations employing the 2ph-TDA and OVGF methods. Both the semiempirical CI and the ab initio CI calculations indicate that the first seven PE bands, with experimental IPS below 15.0 eV, are associated with cation states described by Koopmans configurations and that CI effects are small. The HAM/^ CI results further indicate that the energetic ordering of the first seven PE bands is similar to the orderings obtained from the ab initio CI calculations. For pyrimidine, results from HAM/^ CI calculations, like results from the ab initio CI calculations, predict that the CI effects give rise to shake-up bands appearing in the He(1) PE spectrum, but that these shake-up bands all occur in the energy range above 15.0 eV.

For 1,9-DMG, the HAM/^ CI results describing shake-up states are similar to those for pyrimidine. Here, as in pyrimidine, shake-up states are not predicted to contribute to any of the first seven ionization potentials. According to the HAM/3 CI calculations, no shake-up states of 1,9-DMG have IPS below 1 1 .O eV. However, the calculations do predict that weak emission, occumng at 12.3 1 eV in the PE spectrum of 1Q-DMG and previously assigned [2] to the Koopmans state, I r;'), lies in an energy region where shake-up bands occur. The most important results obtained from the HAM/^ CI calculations on 1,9-DMG is that the cation states associated with the fourth and fifth PE bands, which have experimental IPS

40 YU, JIANG, AND LEBRETON

in the range 9.5-10.0 eV, do not arise from a single Koopmans configuration, but, instead, arise from hole-mixing of the 1 T;' ) and I ~ 7 ' ) configurations. This pre- diction of the HAM/^ CI calculation is supported by the observation that, in the PE spectrum of 1,9-DMG, the bands associated with cation states arising from the 1 T ; ' ) and I T ; ' ) configurations occur in an energy range that is poorly resolved and that contains several overlapping bands. Finally, the HAM/3 CI results and the experimental PE data for 1,9-DMG indicate that, although a change in the inter- pretation of the cation states arising from the I T:' ) and I "5') Koopmans config- urations is needed, the cation states that arise from hole-mixing lie in the same unresolved energy region of the spectrum as that earlier assigned to the 1 T : ' ) and I T 5' ) Koopmans states.

Acknowledgments

Support of this work by The American Cancer Society (Grant #CN-37), the Petroleum Research Fund (Grant #213 14-AC), The National Cancer Institute of the National Institutes of Health (Grant #CA41432), and Cray Research, Inc., is gratefully acknowledged. Computer access time has been provided by the Computer Center of the University of Illinois at Chicago, the Cornell National Supercomputer Facility, and the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign. The authors would like to thank Professors Thomas Koenig (University of Oregon) and Delano Chong (University of British Columbia) for helpful discussions.

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Received March 16, 1990 Accepted for publication April 13, 1992