J. Phys. Chem. 1992, 96, 4845-4851 1956, 10, 483. (b) Middleton, W. J. J. Org. Chem. 1983.48, 3845. For the generation of HNOH+, the trifluoro derivative was preferred as it gave the highest yield of this ion upon electron impact. The collisional activation mass spectra of HNOH+ derived from CHINHOH and CFINHOH are almost identical. (4) For a review on the methods for the generation of HNO, see: Seel, F.; Bliefert, C. 2.Anorg. Allg. Chem. 1974, 277, 406. (5) HNO+ arises from the protonation of NO, one of the thermolysis products of SBH, in the ion source; CH20H+stems from the recrystallization of SBH from ethanol. The latter interference can be reduced by recrystallization of SBH from CD,CD,OH. When CFICONHOH is em loyed as a precursor, the [H2,N,0]+ion has to be resolved from isobaric CF+. (6) (a) Cooks,R. G., Ed. Collision Spectroscopy; Plenum Press: New York, 1978. (b) Levsen, K.; Schwarz, H. Mass Specrrom. Rev. 1983,2,77. (c) Bordas-Nagy, J.; Jennings, K. R. Inr. J . Mass Spectrom. Ion Proc. 1990, 100, 105. (7) Holmes, J. L. Org. Mass Specrrom. 1985, 20, 169. (8) Cooks,R. G.; Beynon, J. H.; Caprioli, R. M.; Lester, R. G. Metastable Ions; Elsevier: Amsterdam, 1973. (9) In the present study the last sector E(2) of our modified tandem mass spectrometer was used to perform energy-release measurements. Unfortunately, a software problem does not yet permit us to measure KER with To,5 < 50 meV. (10) All computational details are fully described in ref 1 and will not be repeated here. ( I I ) Pople, J. A.; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtis, L. A. J. Chem. Phys. 1989, 90, 5622. (12) A similar, high-energy transition structure was found for the [1,2]-H2 elimination from the hydroxymethylcation: Hvistendal, G.; Uggerud, E. Org. Mass Spectrom. 1991, 26, 67. (13) The trans (2) and cis (3) isomers of HNOH' cannot be distinguished yet by means of mass spectrometry. For the sake of simplicity, in the discussion of the experimental results the two conformers will be referred to indiscriminately as 2. (14) Lifshitz, C.; Ruttink, P. J. A.; Schaftenaar, G.; Terlouw, J. K. Mass
P
4845
Spectrom. Rapid Commun. 1987, 1, 6 1. (1 5) The thermochemical data of the fragments will be given for reasons of comparison with the most stable isomer 1 (M,"(exp) = 225 kcal mol-[): H N O " + H' (308 kcal mol-'), NOH" + H'(326 kcal mol-'), NO+ + H2 (235 kcal/mol), N+ + H 2 0 (390 kcal mol-'), NH" + OH' (410 kcal mol-'), NH2++ 0 (362 kcal mol-'), N + H20'+ (346 kcal mol-]); NH + OH+ (399 kcal mol-'), and NH2. + 0" (419 kcal mol-'), respectively. All values are taken from ref 16. (16) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref.Data 1988, 17, Suppl. I . (17) (a) Ma, N. L.; Smith, B. J.; Pople, J. A.; Radom, L. J. Am. Chem. SOC.1991,113,7903. (b) Wlodek, S.; Bohme, D. K.; Herbst, E. Mon. Not. R . Astron. SOC.1990, 242, 674. (18) Uggerud, E.; Koch, W.; Schwarz, H. Int. J. Mass Spectrom. Ion Proc. 1986, 73, 187. (19) HruSBk, J.; Bohme, D. K.; Wlodek, S.; Schwarz, H. J. Phys. Chem.,
in press. (20) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984, 13, 771. (21) Minkwitz, W.; Meckstroth, M., unpublished results. (22) Mason, R.; Milton, D.; Harris, F. J. Chem. SOC.,Chem. Commun. 1987, 1453. (23) Wong, H. W.; Radom, L. J. Phys. Chem. 1989, 93, 6303. (24) For the charge transfer reaction of N+ with water see: (a) Bolden, R. C.; Twiddy, N. D. Faraday Discuss. Chem. Soc. 1972,53,192. (b) Clary, D. C.; Dateo, C. E.; Smith, D. Chem. Phys. Lett. 1990, 167, 1. (25) The state-selected charge-transfer reactions in the systems NO+/H2
and NO/H;+ have been described, and no evidence for the formation of an observable encounter complex [H,,N,O]+ or reaction products thereof was reported: Koyano, I.; Tanaka, K.; Kato, T.; Suzuki, S. Faraday Discuss. Chem. SOC.1987, 84, 265. (26) (a) For a theoretical study of the [H2,N,0]-potential energy surface, see: Sheldon, J. C.; Bowie, J. H. J. Am. Chem. SOC.,in press. (b) For a PE spectroscopic investigation of [H2,N,0]', see: Baker, J.; Butcher, V.;Dyke, J. M.; Morris, A. J. Chem. SOC.,Faraday Trans. 1990, 86, 3843.
Calculation of Transition Moments on Isolated Adenine and Guanine and Their Methyl-Substituted Derivatives Andrey Volosov* and Robert W. Woody Department of Biochemistry, Colorado State University, Fort Collins, Colorado 80523 (Received: January 16, 1992)
The effects of substituents upon the transition energies and transition moment directions of adenine and guanine and their methyl-substituted derivatives are discussed. The CNDO/OPTIC-2 method with a large number of interacting configurations has been used for calculating spectroscopic properties of these compounds. The calculated results are compared with the experimentally obtained average transition moment directions of the adenine and guanine chromophores. It is shown that despite a significant difference in the CI contributions the pattern of transition monopoles is very similar for every transition within the group with the same chromophore. However, in the case of substitution of the hydrogen atom by the methyl group, especially in position 9 of the purine ring, some transitions, particularly those with higher energies, may change their structure in CI so much that there is a change in the order of transitions, their energies, and/or transition moments.
Introduction The biological importance of nucleic acid bases as well as of nucleosides and nucleotides has led to a large number of experimental and theoretical investigations of their electronic structure and especially of excited states and spectroscopic characteristics of these compounds. Two excellent reviews of this field have been published by Callis.1.2 However, there are still many unanswered questions even about such basic spectroscopic properties as the number and location of transitions and their symmetries and polarizations. Recent investigations, both experimental3" and the~retical,~.'-l~ have significantly improved our knowledge about the compounds under discussion. At the same time the latter publications show the complexity of the problem. Experimentally, there is the problem of a large number of overlapping transitions so that not only are many of the weaker transitions obscured, but the transitions also interact with each other in such a complex way that resolving and characterizing them is often very difficult.
In the theoretical calculations, there is the problem of reliability. The traditionally used semiempirical methods, such as PPP or CNDO/S, do not describe transition moments satisfactorily even at the qualitative leve1.2,8 The description of the excited states using these methods is still less reliable. The a b initio methods require such extensive basis sets and configuration interaction (CI) that they are near the limit of available computer resources for molecules as large as the methylated purines. Even stateof-theart ab initio calculations on such systems overestimate the transition energies by 1-2 eV. Scaling of the transition energies is commonly applied, but this assumes that the order of the excited-state energies is faithfully reproduced in the calculations, and this is not necessarily true. Nevertheless, recent publications show certain progress in the calculation of the spectrwxpic properties of the nucleic acid bases. Including doubly excited states in the CI, as in INDO/Sl2 and CNDO/OPTIC-2,8 seems to be an appropriate tool for investi-
0022-3654/92/2096-4845%03.Q0/0 0 1992 American Chemical Society
4846 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Volosov and Woody
TABLE I: Calculated Chrr~cteristics~ of the mr* Electronic spectrum of Adenine CNDO/OPTIC-2 wavelength (nm) 296 287 243
transition 1 2 3
204 20 1
4 5
oscillator strength 0.015 0.129 0.021
wavelength 06
(nm)
strength
e
-2
275 259 248 223 209 204 204 199 197 193 193 190 187 182
0.00 1
-33 +41 -54 -48 -68 -58 -82 -51 -22 -23 +62 +9 -60 +38
+62 -58
0.196 0.054
-45 -13
6
196
0.270
+67
7 8
187 179
0.042
-43 -72
0.006
ab initio MRCI" oscillator
0.318 0.003
0.071 0.372 0.119 0.110
0.075 0.056
0.054 0.08 1
0.227 0.101
0.310
"The transition correlation of the two types of calculation is arbitrary (see text). *Angle (degrees) between the C 4 C 5line and the transition moment measured toward the C6 atom (counterclockwise). gations on the semiempirical leve1.2q8 A special approach has been suggested to incorporate the crystal field environment into semiempirical calculations.' The latest applications of the RPA?J3 MRCI,I4-l6 and CAS SCFi7-I9 methods to the calculation of transition moments and excited states in nucleic acid bases at the a b initio level have given valuable information about cytosine, adenine, and g ~ a n i n e . ~ - ~ ~ One of the important questions about nucleic acid bases concerns the effects of substituents upon the transition energies and transition moment directions. At present, it is often assumed that these differences are small enough to be neglected when analyzing and interpreting the experimental data, and one discusses transition moments in different chromophores, such as the cytosine chromophore, the adenine chromophore, etc., without taking into account specific features of various derivative^.^" Different methyl-substituted derivatives are used to obtain different crystal forms of compounds with the same chromophore to resolve ambiguities in assignments of transition polarizations. The substitution of one or several hydrogen atoms by methyl groups perturbs the electronic structure of a molecule, of course much less than protonation, which is used as an alternative way to create different crystal structures with the same chromophore. However, currently there is no experimental information which can provide an independent test of such an approach. Theoretical calculations, on the contrary, show that there can be significant differences between transition moments in different pyrimidine bases containing the same chromophore.8 In this paper we investigate the differences in transition moments in the purine bases, adenine and guanine, and their methyl-substituted derivatives, using the CNDO/OPTIC-2 method. We have two reasons for the choice of this semiempirical method for our investigation: (i) it gives results for transition moments in pyrimidines which are fairly close to the results of advanced ab initio methods and to the experimental data for these compound$ (ii) thii method does not have such severe restrictions on the size of molecules as the ab initio methods. Metbod of Calculation
For the calculation of the electric dipole transition moments Moa
= e(*olrl*a)
(1)
we used the semiempirical CNDO/OPTIC-2 method recently published in detail.8 The method is based on the spectroscopic version of the CNDO approximation of Del Bene and JaffeZ0and includes a large-scale CI of singly- and doubly-excited states. The wave function for the ath state in the CNDO/OPTIC-2 method is
Here H is the Hamiltonian operator, and the wave function of the dth singly- or doubly-excited singlet configuration, &, is an appropriate combination of Slater determinants. A,,, is an expansion coefficient of the low-dimensional CI of the 100 configurations with the lowest energy. E , is the energy of the ath state obtained in this CI. 4, and E/ are the wave function and the energy of thefth configuration, not included in the low-dimensional CI. In our calculation F was taken equal to 15 000. The electric dipole transition moments were calculated in the strictly analytical way without any approximations, using the CNDO wave functions transformed by the method described in ref 33. The transition monopolesz1and bond orders3' were calculated, including all the contributions corresponding to eq 2.
Adenine and Its Derivatives We considered four molecules containing the adenine chromophore (Figure la): adenine (A), 9-methyladenine (9MA), W-methyladenine (N6MA), and N6,9-dimethyladenine (N69DMA). For A, N6MA, and N69DMA we applied the idealized structure with a planar ring corresponding to refs 22 and 23. For 9MA we used the structure from the crystal.z4 Control calculations showed that the deviations from the planar ring structure do not significantly change the results for the strong transitions. The results of our calculation for these molecules are given in Tables I-IV. The calculated transition monopoles and bond orders of the first two transitions of adenine and its methyl derivatives are consistent with the classification of Callis3*for these transitions: the first transition has Lb character, the second one has La character. In all cases, the lowest energy transition is a ?TA* transition with the main contribution of the transition from the highest occupied molecular orbital (HOMO) to the next lowest unoccupied molecular orbital (LUMO + 1). The second transition is a m* transition dominated by the transition from HOMO to LUMO. Analysis of the remaining TT* transitions and their contributions to the CI expansion shows that transitions 3 and 6 are of the same nature in all the molecules. Transition 4 in A and N6MA corresponds to transition 5 in 9MA and N69DMA. Transition 5 in A and N6MA corresponds to transition 7 in 9MA. Transition 7 in A and N6MA corresponds to transition 4 in 9MA. Other mr* transitions in different adenines are composed of different contributions in the C I and cannot be clearly correlated. Thus we obtain three different kinds of m* transitions in adenine and its derivatives: (a) transitions which correspond to each other, with the same order number and wavelengths varying over a range of several nanometers (numbers 1, 2, 3, and 6); (2) transitions which correspond to each other, but have different order numbers, some of which differ significantly in wavelength (compare transitions 5 and 7 in A with corresponding transitions in its derivatives); (3) transitions which cannot be connected with certainty
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4841
Transition Moments in Purine Bases
TABLE 11: Calculated ChPrrcteristics of the m* Electronic Spectrum of N6-Methyladenine
transition 1 2 3 4 5 6 7 8 9
CNDO/OPTIC-2 wavelength oscillator (nm) strength 293 286 245 208 202 198 186 181 171
0.017 0.157 0.019 0.186 0.044 0.291 0.032 0.022 0.168
experiment from single crystal6 wavelength oscillator (nm) strength e
8" -65 +55 -46 -27 -2 1 t72 -35 -55 +44
278 267
0.09 0.18
+83 +25
212 208 182
0.25 0.11 0.30
-45 +15 +72
175 160
0.10 0.23
-45 +6
INDO/S calculationZ5 wavelength oscillator (nm) strength 287 274 234 213 20 1 193 186 179 172
0.182 0.300 0.073 0.612 0.310 0.173 0.037 0.001 0.188
e +30 +71 -15 -30 +75 +52 -4 +56 t62
"See footnote b in Table I. TABLE 111. Calculated Characteristics of the m* Electronic Spectrum of 9-Methyladenine
wavelength transition
(nm)
1 2 3 4 5 6 7 8 9
29 1 288 243 207 207 198 192 187 174
CNDO/OPTIC-2 oscillator strength
e"
0.005
wavelength (nm)
+4 +57 -42 -88 -4 3 +62 -7 +67 t42
0.115 0.023 0.025 0.248 0.3 13 0.007 0.008 0.084
280 262 227 213 205 199 186 178 172
INDO/S calc~lation~~ oscillator strength 0.099 0.305 0.213 0.564 0.364 0.21 1 0.0 17 0.020 0.089
e +30 +62 -33 -52 +4 1 +44 +69 -50 +6 1
"See footnote b in Table I. TABLE I V Calculated Characteristics of the mr* Electronic Swctrum of N6.9-Dimethvhdenine ~~~~~
~~
transition
wavelength (nm)
oscillator strength
e"
1 2 3 4 5 6 7 8 9
292 286 244 209 205 199 189 186 175
0.0 19 0.142 0.018 0.071 0.159 0.356 0.042 0.001 0.113
-86 +52 -36 -27 -26 +7 1 -8 t4 +55
"See footnote b in Table I to any transitions of other molecules within the wavelength region considered. Of course, there are no pronounced boundaries between these three groups. However, such an assignment makes it easier to follow the differences between the electronic transitions under discussion if we regard their transition monopoles (Figure 2). A survey of the transition monopoles immediately reveals that despite the formally different nature of identically numbered transitions in groups 2 and 3 according to the CI analysis, transitions with the same numbers have a similar pattern of transition monopoles within each molecule. There is a strong mechanism of compensation redistributing the MOs of every molecule in such a way that the common transition charge pattern is conserved under CI. This gives the chromophore approach a certain degree of legitimacy. At the same time, on some atoms of different methylated adenines we obtain significant quantitative differences between transition monopoles with the same transition numbers. As seen from Tables I-IV, these differences correspond to significant differences between transition moment directions. The main feature of the transitions of the first group is that there are relatively small transition monopoles centered on atoms N9 and NIO,immediately connected to the methyl groups in the adenine derivatives under study. The differences between these monopoles do not contribute significantly to the differences in transition moments between the molecules under discussion, even in the case of the first transition where transition moment directions are
ti
(CHI)
Figure 1. Structure of compounds for which calculations are reported: (A) adenine and its derivatives; (B) guanine and its derivatives.
clearly different in different molecules. As an illustration, the difference between the first transition moment directions in A and N6MA is 63'. The contribution of the monopoles centered on N9 and N,o to this difference is only a few degrees. At the same time we would like to point out that in the case of 9MA, where the first transition is the weakest of any transition in any of the derivatives considered here, its moment direction is much more sensitive to any change of the computational scheme. The calculation of the transition moments using the simple summation of all products of transition monopoles and atomic p i t i o n vectors, i.e. excluding all the two-center integrals and one-center integrals of the type (slrlp,), leads to a noticeable deviation of the transition moment obtained from the transition moment which we obtain by applying the usual CNDO/OPTIC-2 equation. Nevertheless, this discrepancy is much less than the differences between tran-
4848
The Journal of Physical Chemistry, Vol, 96, No. 12, 1992 ADENINE
N1-METHYLADENINE
9-METHYLADENINE
0
0
0
N6,9-DIMETHYLADENINE
oe
O@
0.050
0.100
0
0.200
Figure 2. Transition monopoles in adenines. The numbers designate the transitions in order of increasing energy. The radii of the circles are proportional to the magnitude of the transition monopoles, and shaded vs unshaded circles indicate the relative signs of the monopoles.
sition moment directions in 9MA and N6MA or N69DMA. With regard to transition energies, the transitions of the first group can be considered as practically independent of the methyl substitution in positions N6 and 9. The transition moments of this group of transitions may differ in various degrees as reflected by differences between transition monopoles. The role of the transition monopoles on atoms N9 and Nlo increases in the second and third groups. The magnitudes of the transition monopoles on these atoms are comparable with the largest transition monopoles obtained for any of the molecules under consideration, and the substituents clearly change all the spectroscopic properties of A. All transitions in the adenines involve transition charges on several atoms. This agrees with the results of the INDO/S calculations of Callis2 but is in poorer agreement with the CNDO/S results for the lowest energy transition of Hug and Tinoco21which is predominantly localized in the N7=C8 bond. The disagreement seems to have its origin in the parameterization
Volosov and Woody used by Hug and Tinoco and in the much more extensive CI in our calculation. Another observation which may be made from the comparison of transition monopoles in different molecules is that in wellconjugated systems such as adenines the differences in transition moments are due to a global change of the electronic structure and do not necessarily involve a drastic change of the transition density at the site of substitution. The above example of the first weak transition having different transition directions in different methylated adenines can be complemented by an example of the strong transition 4 in A and N6MA and 5 in 9MA and N69DMA. In this case the magnitude of the transition monopoles on atoms N9 and Nlodo not define the directions of the transition moments even qualitatively. Generally, it may be stated that in the investigated adenines there are some corresponding transitions having similar directions of their moments, along with transitions having different moment directions. The largest variations are predicted for the lowest energy transition and for the highest energy transitions. The differences in transition moment directions do not depend on thustrength of the transitions under consideration. This implies that the usual argument that it is difficult to obtain a physically correct value for a weak transition in a quantum chemical calculation because of its mathematical instability is not enough to explain our result. Besides, in our calculation the result for the weakest transition was stable with changes in the number of interacting excited states by some thousand and more. Comparison of our results with experiment is difficult for several reasons. The most reliable transition moment directions are those derived from single-crystal polarized absorption or reflection spectroscopy. However, in highly polar crystals such as those formed by the molecules we are dealing with here, the crystal field can mix the excited states within each molecule, changing transition energies, intensities, and polarization directions.’~~~In addition, exciton coupling can alter transition moment directions. Furthermore, there are sometimes ambiguities due to overlapping transitions. For example, in the initial s t ~ d i e s of ~ ~9MA, l ~ ~ the 260 nm band was interpreted as a dominant band with 97% of the intensity, polarized at either -3O or + 4 5 O , with a weak band on the short-wavelength edge, polarized along the long axis of the molecule. Analysis of the polarized absorption spectrum of the 1:l complex between 9MA and 1-methylthymine led Stewart and D a v i d ~ o nto~ ~prefer the -3’ direction for the 9MA crystal. Recently, Clark4 has analyzed polarized reflectance data for N6MA. He found that he could account for the long-wavelength band of N6MA as consisting of a dominant band polarized at + 3 3 O , not far from the choice which Stewart and Davidson rejected for 9MA. However, the band could also be interpreted as containing two bands in a ca. 2:l intensity ratio: a weaker lowenergy component polarized at + 8 3 O , and a stronger component at slightly higher energy polarized at +25O. Differences in band shape for the N6MA spectra measured along the three crystal axes, together with linear dichroism data26from stretched films for 9MA and N69DMA, led Clark to prefer the assignment with two bands of comparable intensity for N6MA and 9MA, assuming no major alterations with changes in methyl substitution pattern. A comparison between our results for N6MA and the experimental crystal data6 is shown in Table 11. Our results agree better with the interpretation in which one component dominates the long-wavelength band, the interpretation disfavored by Clark.6 The third, fifth, and sixth transitions in the crystal are reproduced fairly well in our calculations, but the fourth and seventh are not. It is not certain whether those discrepancies are due to neglect of crystal interaction^,^ other defects in the theoretical method, or experimental error. In principle, it might be better to compare the theoretical results for an isolated molecule with measurements of transition moments of molecules oriented in stretched films. In these experiments the influence of the surrounding molecules may be significantly smaller. However, experiments with stretched films are beset by uncertainties about the direction of the orientation axes and the extent of orientation. The assignment of polarizations in A is also rather unclear because of the superposition of transitions, and the
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4849
Transition Moments in Purine Bases
TABLE V: Calculated Characteristics‘ of the ?IT* Electronic Spectrum of Guanine CNDO/OPTIC-2 wavelength oscillator transition (nm) strength e* 1 336 0.173 -68 2 270 0.074 +49 3 258 0.021 +11 4 208 0.069 +48 5 195 0.157 +52 6 7 8
192 180 177
0.144 0.070 0.1 17
+lo3 (-77) +64 -12
wavelength (nm) 279 27 1 253 246 194 191 189 182
ab initio MRCI” oscillator strength 0.239 0.088 0.216 0.136 0.228 0.048 0.297 0.09 1
e -52 +3 +80 +5 1 +I3 +64 +73 +77
“IbSeeTable I. TABLE VI: Calculated Characteristics of the m* Electronic Swctrum of N*-Methrlrmanine wavelength oscillator transition (nm) strength ea 1 336 0.195 -69 2 268 0.068 +53 3 254 0.031 +23 4 206 0.086 +53 5 198 0.179 -78 6 193 0.105 +43 7 181 0.120 +53 8 175 0.079 -18 ‘See footnote 6 in Table I.
spectra have been interpreted on the basis of the results of single-ciystal measurements. The ambiguities in published data26 for adenine (+go or -7OO for the second AX* transition moment and +16O or - 7 5 O for the third AT* transition moment) are significant, and this information about the spectroscopic properties of adenine in stretched films may require revisi~n.~’Our result (+62O) for the second transition in A does not agree with these data. The results for the third AX* transition, however, are in an acceptable agreement (-58O and -75O, respectively). Recently two relevant advanced quantum chemical calculations on adenines have been published: an ab initio multireference configuration interaction (MRCI) calculation” on A, with an additional random-phase approximation calculation, and INDO/S semiempirical calculations on N6MA and 9MA.25 The comparison of the scaled MRCI result for AT* transitions in A with our results is given in Table I. Because of the more extensive basis set in the ab initio calculation, one obtains about twice as many transitions within the same wavelength region as we obtain in our CNDO/OPTIC-2 calculation. The correlation of transitions from the two types of calculation is rather arbitrary and based on the similarity of transition energies, oscillator strengths, and transition moment directions. The transition moment directions of the first few transitions are in fair agreement, except for the very first one. The correlation of the following transitions is so arbitrary that there is little basis for any serious comparison. However, the three strong transitions predicted by the ab initio calculation at 259, 209, and 190 nm may be compared with the strongest transitions predicted in this work, at 287,204, and 196 nm. The two lowest
energy transitions differ by 20° or less in transition moment direction, but the highest energy band shows a large discrepancy of nearly 60°. The results of the INDO/S calculation of N6MA and 9MA are shown in Tables I1 and 111. Despite the similarity of the INDO and CNDO models, there are disagreements between results for some transitions, primarily because of the different parameterization of the methods. The disagreement of the results for the first transition should be mentioned especially. The INDO/S method describes this transition as a relatively strong one while the CNDO/OPTIC-2 method gives a rather small oscillator strength for it. The results for transitions 5 and 8 in N6MA and transitions 7 and 8 in 9MA are also noticeably different. Qualitatively, other transitions agree fairly well. The most important result of the INDO/S calculation for our discussion is that in this calculation differences between some corresponding transition moments are also obtained, although they are not as pronounced as in the CNDO/OFTIC-2 calculation. The INDO/S model seems to be less sensitive to the change in the electronic structure due to a substituent. The sequence of transitions is much more stable in N6MA and 9MA. Nevertheless, transition energies and especially transition moment directions are different for some transitions. It should also be pointed out that in contrast to the ab initio methods discussed above the CNDO/OPTIC-2 method gives transition energies which are in fair agreement with experiment without applying any kind of scaling.
Guanine and Its Derivatives Four molecules with the guanine chromophore (Figure 1b) were considered: guanine (G), N2-methylguanine (N2MG), 9methylguanine (9MG). and MY9-dimethylguanine(N29DMG). For all of them we started from the planar ring structure adopted from ref 22. The calculated spectroscopic characteristics of guanines are given in Tables V-VIII. In all the molecules, the first transition has L, character3*and is strongly dominated in CI by the TT* transition from HOMO to LUMO. The second transition has Lb character32and is mainly composed of the HOMO to LUMO + 1 and HOMO to LUMO + 2 transitions, with the contribution of the HOMO to LUMO + 1 transition being larger. The third transitions in G, N2MG, 9MG, and N29DMG have the same
TABLE VII: Calculated Characteristics of the m* Electronic Spectrum of 9-Methylgu~ine CNDO/OPTIC-2 INDO/S c a l c ~ l a t i o n ~ ~ wavelength oscillator wavelength oscillator transition (nm) strength 8’ (nm) strength 1 334 0.169 -70 315 0.28 0.064 +47 0.38 275 2 270 0.015 +16 3 254 242 0.01 +45 210 0.41 0.137 4 209 0.248 +67 5 197 20 1 0.19 194 0.055 6 192 -53 0.10 0.108 -1 0.10 190 I 181 0.066 -3 1 0.15 182 8 180 “See footnote b in Table I.
e -44 +57 -44 +56 -6 3 -22 +30 -4 3
Volosov and Woody
4850 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 GUANINE
TABLE VIII: Calculated Characteristics of the r r * Electronic Spectrum of N2,9-Dimethylguanine wavelength oscillator transition (nm) strength 80 -70 335 0.186 +53 270 0.062 +24 254 0.023 209 0.158 +50 -87 199 0.224 +27 0.076 193 +34 0.067 185 -20 179 0.097 +17 172 0.007
N2-METHVLGUANINE
aSee footnote b in Table I.
main contributors in CI as the second transition, but the HOMO to LUMO + 2 transition is dominant. The fourth transitions of G, N2MG, 9MG, and N29DMG also correspond to each other, however, with larger differences among the CI coefficients. The higher energy transitions show poor correlation among the four derivatives. Substitution in the 9-position significantly changes the electronic structure of other transitions with higher energies, and for guanines there is no clear correspondence between these transitions in 9MG and N29DMG or between them and higher energy transitions in G and N2MG. Therefore the attempt to unite these transitions in corresponding groups according to their energies or transition moments may lead to wrong conclusions. We obtained a similar effect of substituents in position 9 to that in the adenines, where some transitions were remarkably shifted and others significantly changed their nature. This result is not surprising because substitution in position 9 disturbs the electronic structure of purines much more than substitution in position N2 in G or N6 in A. The comparison of transition monopoles (Figure 3) shows that, except for the 5th and 6th transitions, the identically numbered transitions have a similar pattern of transition monopoles within each molecule, as was obtained for adenine derivatives. At the same time, we observe changes in the electronic structure of guanines caused by the substituents at positions 9 and N2, especially in the case of the higher energy transitions where we obtain dramatic changes of the magnitudes of transition charges on atoms N9 and Nlo. Qualitatively the degree of these changes increases consistently with the loss of the correspondence between transitions in the CI expansion. The observation on adenines that any transition involves transition charges on several atoms may also be made for guanines. In contrast to adenines, practically all the transition moments of corresponding transitions in G have similar directions and oscillator strengths. The transition parameters seem to be much less sensitive to the substituents at positions 9 and N2. This consistency between transition moments and oscillator strengths can be observed even for transitions with a relatively weak correspondence as transition 4 in G, N2MG, 9MG, and N29DMG, or transition 6 in G and 9MG and transition 5 in N2MG and N29DMG. Thus in guanines there is a more distinct difference, though still an arbitrary one, between the transitions which are conservative and do not change their properties much under the substitution of the H atom in position 9 by the methyl group and the transitions which are apparently influenced by this substitution. The conservative transitions may show some degree of response to the fact of the substitution and their energies may be slightly shifted. For example, transitions 5 and 6 in G and N2MG are reversed in these molecules, and the energies of the corresponding transitions are different by several nm. Taking into account the extent to which the spectroscopic properties of 9-ethylguanine are changed in the crystal compared with the gas phase,' a comparison of our results with the crystal data29130is not likely to be reliable. Measurement of transition moments of G oriented in a stretched film26gives a choice between +4" and -61" for the first transition and a choice between +31° and -88" for the second transition. Our calculation for G suggests
Q
1
Q
9
9-METHVLGUANINE
N2,B-DIMETHVLGUANINE
Q
1
I
? 08
O@
0.050
0.100
Figure 3. Transition monopoles in guanines. The numbers designate the transitions in order of increasing energy. The radii of the circles are proportional to the magnitude of the transition monopoles, and shaded vs unshaded circles indicate the relative signs of the monopoles.
-68" for the first transition and +49" for the second one, which is in acceptable agreement with the experimental values. However, this agreement may be accidental because of the uncertainties in the stretched film experiments mentioned above. The calculated wavelengths for these two transitions are somewhat higher than the experimental ones (respectively, 336 vs 280 nm for the first transition, and 270 vs 248 nm for the second transition). Our results are compared with the ab initio MRCI result" for G in Table V. Both results are in acceptable agreement except for transition 6. It also seems that one of the methods reverses the order of transitions 2 and 3. The agreement between the INDO/S25 and our results for 9MG (Table VII) is satisfactory for all transitions but 3, 5, and 7. This disagreement, as we mentioned above, is a possible consequence of different parameterization. Unfortunately, in the case of guanines we do not have quantitative information about the INDO/S results for different guanine derivatives to see if the substitution or the change of
J. Phys. Chem. 1992, 96,4851-4859
substituent position in guanine leads to differences in the spectroscopic characteristics.
Conclusion The results of the CNDO/OPTIC-2 calculation of spectroscopic properties of adenine and guanine and their methyl-substituted derivatives show that despite significant differences in the CI contributions, the pattern of transition monopoles is very similar for every transition within the group with the same chromophore. However, in the case of substitution of a hydrogen atom by a methyl group, especially in position 9 of the purine ring, some transitions, particularly those with higher energies, may change their structure in CI so much that there is a change in the order of transitions, their energies, and/or their transition moments. There is not enough experimental or theoretical information to judge now how pronounced the effect of a substituent may be. We do not know whether the CNDO model overestimates or underestimates it. However, the fact that even such a small and "electronically nonagressive" group as the methyl group can change a calculated spectroscopic result, depending on its presence and position, may be a signal to the spectroscopists to be more observant to the differences between molecules with the same chromophore. Acknowledgment. This research was supported by NIH Grant GM22994. One of us (A.V.) expresses his gratitude to Dr. Sven Larsson for his encouragement to investigate theoretically nucleic acid bases. Registry No. Adenine, 73-24-5; N6-methyladenine, 443-72- 1; 9methyladenine, 700-00-5; N6,9-dimethyladenine, 2009-52-1; guanine, 73-40-5; N2-methylguanine, 10030-78-1; 9-methylguanine, 5502-78-3; N2,9-dimethylguanine, 67349-31-9.
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Stablllty and Structure of Lewls Adducts of Aluminum Hydrides and Halidest Marty Wilson,' Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
Michael B. Coolidge, F. J . Seiler Research Laboratory, US.Air Force Academy, Colorado Springs, Colorado 80840
and Gilbert J. Mains**$ Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: September 16, 1991) Structures for the adducts formed by AlH, AlF, AlC1, AIH2, AIFz, A1C12,AlH3, AlF,, AlC13,AIHF2, AIHC12, AIH2F, and AIH2Cl with HC1 and HF have been explored at the HF/6-31G* level. In all but two cases, AlF(311) and AIC1(311) with HCl, stable adducts were found. Vibrational analysis at the Hartree-Fock level was used to assure that the stationary points were minima and for determination of zero point energies. Correlation was taken into account by fourth-order perturbation theory, MP4(SDTQ)/6-3 lG*//HF/6-3 1G*. In a few instances the starting geometry rearranged to form hydrogen-bonded structures. Comparison of dissociation energies for the HF and HC1 adducts suggests that the F atom in HF is a better Lewis base than the CI atom in HCI. The adduct dissociation energies increased in the following order: A1F2H > AlF3> AICl, > A1FH2 > AlHC12 > AIH2CI >> AIH, whereas the order of increasing Lewis acidity was AIF, > AlF2H > AlC13 > AlFH2 > AlHC12 > A1ClH2 > AlH3.
Introduction The chemistry of aluminum hydrides and halides has the subject of numerous experimental and theoretical studies. These 'Contribution from the F. J. Seiler Research Laboratory. *Visiting Research Associates, 1991 Summer Research Program. 0022-3654/92/2096-485 1$03.00/0
compounds play important roles in 'a wide range of chemical processes. Aluminum chloride, for example, iS used eXtenSiVdy in Friedel-Crafts reactions.' Aluminum trifluoride is found in many glasses and is an important component of some optical fibersU2Both of these compounds find extensive use as catalysts and have favorable electrochemicalproperties. Aluminum hydride, 0 1992 American Chemical Society
