J. Phys. Chem. 1992, 96, 7882-7886
7882
Theoretical I R Spectra of Ionized Naphthalene F. Pauzat,tJ D. Talbi,f**M. D. Miller,! D. J. DeFrees:*! and Y. Ellinger*.*,! Molecular Research Institute, 845 Page Mill Road, Palo Alto, California 94304, Equipe dAstrochimie Quantique, ENS and Observatoire de Paris, 24 rue Lhomond, 75005 Paris, France, and IBM Almaden Research Center, 650 Harry Road, San Jose, California 951 20-6099 (Received: March 24, 1992; In Final Form: June 16, 1992) We report the results of a theoretical study of the effect of ionization on the IR spectrum of naphthalene, using ab initio molecular orbital theory. For that purpose we determined the structures, band frequencies, and intensities of neutral and positively ionized naphthalene. The calculated frequencies and intensities allowed an assignment of the most important bands appearing in the newly reported experimental spectrum of the positive ion. Agreement with the experimental spectrum is satisfactory enough to take into consideration the unexpected and important result that ionization significantly affects the intensities of most vibrations. A possible consequence on the interpretation of the IR interstellar emission, generally suppased to originate from polycyclic aromatic hydrocarbons (PAHs), is briefly presented.
Introduction The emission lines observed in many interstellar infrared sources at 3050, 1610, 1300, 1150, and 885 cm-I (3.3, 6.2, 7.7, 8.6, and 11.3 gm) are hypothesized to originate from polycyclic aromatic hydrocarbon molecules (PAHs).'** These assignments are based on analysis of laboratory infrared spectra of neutral PAHs. But, as pointed out by Allamandola et al.,3"although the IR emission band spectrum resembles what one might expect from a mixture of PAHs, it does not match in details such as frequency, band profile, or relative intensities predicted from the absorption spectra of any known PAH molecule". Moreover, it has been proposed that PAHs, in the regions where they are observed in the interstellar medium, are mostly ionized, i.e., positively ~ h a r g e d . ~If. ~ that is the case, then the comparison of the interstellar emission with laboratory data of neutral PAHs is highly questionable; there is no reason for an ionized molecule to have the same IR spectrum as the corresponding neutral precursor. Information on the spectra of ionized PAHs is then urgently needed. The charged systems, which are generated from the parent PAHs, are highly unstable in laboratory conditions. Experiments specifically designed for their study are extremely difficult to realize for many reasons, the most obvious being the uncertainty about the nature and quantity of the species really formed. Only recently, Vala's group6 and Salama and Allamandola' have succeeded in producing a partial IR vibrational spectrum and UV spectrum of the naphthalene cation, respectively. Another source of information for IR spectra is ab initio molecular orbital theory. It can be used to compute the geometries, vibrational frequencies, and vibrational intensities for neutral and ionized PAHs. The main object of this paper is to present a comparison of the theoretical (ab initio) IR spectra of neutral and ionized naphthalene which is the simplest representative of polycyclic aromatic species with condensed rings. It is structured as follows: After a presentation of the computational methods used to perform this study, optimized geometries for naphthalene and its cation are presented and compared to existing data. The theoretical IR spectra of these species are first discussed and interpreted in terms of vibrational normal modes with the effect of the ionization pointed out. A comparison with the few available experimental data follows. The consequence of the effect of ionization on the hypothesis of PAHs being at the origin of the observed UIR bands is presented in the conclusion.
Theoretical Model We have used the Gaussian 888 computer codes to determine the infrared emission spectra. Such determinations are considered
* Send all correspondence to Y.Ellinger, Paris address
' Molecular Research Institute.
tENS and Observatoire de Paris. IBM Almaden Research Center.
., f
0022-365419212096-7882$03.00/0
difficult, especially concerning absolute intensities. It is well-known from previous studies on various small mOleculesgJOrealized with HF/3-21G and HF/6-31G* calculationsthat, after simple scaling, ab initio frequencies are accurate within 50-100 cm-I; most of the time, intensities are considered correct within a factor of 2 to 3. Calculations performed on benzene show that these conclusions are also valid for an unsaturated hydrocarbon ring." Extensive studies on simple moleculesI2indicate that improvements in basis sets (from 3-21G to 6-31++G**) and level of theory (from independent particle approximation, HF, to correlated wavefunctions, MP2) lead to a better accuracy of the results, especially of the intensities. However, our extensive study on the smallest cyclic aromatic molecule, i.e., pyridineI3in both neutral and ionized states, shows that a HF treatment (RHF and ROHF, respectively) in a split valence basis set (3-21G), eventually extended with polarized functions (6-31G**), is a reasonable compromise between accuracy and computational effort since it gives coherent enough orders of magnitude for the intensities to account for the effects of ionization on the IR spectra. A similar choice has already proved to be wise for investigating IR spectra of polymer~.~~ Concerning the vibrational frequencies themselves, we used a more sophisticated correcting procedure than the usual multiplying factor applied uniformely to all frequencies (0.89 for HF/3-21G or HF/6-3 1G); considering each vibration type separately (stretching, bending, out-of-plane torsions, ...), we optimized a scaling factor for each type of force constant for neutral naphthalene, following in that a philosophy expressed by Pulay et al.;" thus, we improved the theoretical results to values accurate within a few tens of cm-l when compared to experiment. The same set of scaling factors was applied to the force constant matrix of the daughter ionized molecule.
Optimized Geometries for Naphthalene Neutral and Cation
'$
The ground state of neutral naphthalene is closed shell. In accordance with the results of the studies quoted previously, we chose the following standard calculations: HF/3-21G and HF/6-31G*. The positive ion being *Au open shell system, only restricted open-shell Hartree-Fock (ROHF) calculations have been performed to avoid any possible artifact due to uncontrolled spin-contamination. Table I gives computed optimized geometries for CloH8and C]d&+r together with experimental geometries available actually (see Figure 1 for definitions). As already shown by X-ray data, we find cl&8to be planar with Du geometry. The same structure is found for the corresponding cation. This table shows the following: (1) Geometries, as suspected, are almost unchanged when calculated with the two different basis sets. The most affected parameters are those describing the CH bonds for which variations of less than 0.003 A and 0.2O are observed. The same conclusion is valid for the cation. 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7883
Theoretical IR Spectra of Ionized Naphthalene
TABLE I: Comparison of Calculated and Observed Structures for Neutral rad I d z e d Naphthalene neutral molecule experiment 3-21G 6-31G* b c CICI’ 1.409 1.409 1.412 1.421 ClC2 1.419 1.421 1.422 1.424 C2C3, 1.357 1.358 1.381 1.377 CJC3 1.414 1.417 1.417 1.411 C2H2 1.073 1.076 1.092 1.095 CIH, 1.072 1.075 1.092 1.098 C2ClCl’ 118.975 118.969 119.5 119.0 C3C2CI 120.738 120.778 120.2 C$&’ 120.286 120.253 120.5 H2C2CI 118.685 118.838 117.0 H3C3C2 120.346 120.253 119.9
cation 6-31G’ 1.422 1.402 1.400 1.378 1.075 1.073 119.117 120.668 120.214 119,709 119.548
3-21G 1.421 1.401 1.398 1.376 1.072 1.070 119.119 120.628 120.252 119.647 119.521
“Bond lengths in angstrams, angles in degrees, energies in au. Experimental geometries obtained in gas phase from photoelectronic spectra (ref 20). CExperimental geometries obtained in solid phase from X-ray diffraction (ref 21). H,
E
Figure 1. Structure of naphthalene and definition of internal coordinate types used in the vibrational analysis: R,CC bonds; r, CH bonds; a, CCC angles; b, (b1-b2)in-plane CH bending; c, bending of CH out of CCC plane; 7 , dihedral angle between planes C,C& and C/CkC,.
(2) The calculated geometries for neutral naphthalene are in excellent agreement with the experimental one. Bonds are reproduced within 0.02 A and angles within 0.5’ for CCC angles; the positions of the hydrogens differ at most by 2’ from the values obtained by deconvolution of X-rays diffraction patterns. This agreement shows that a RHF/3-21G level of computation gives a description of the naphthalene structure good enough to be generalized to larger systems. (3) The geometry of naphthalene is little affected by ionization. The modifications are dictated by the bonding and antibonding properties of the au orbital from which the electron is removed: the CICI’ and C2C3bonds increase by -0.01 and 0.04 A, while the ClC2 and C3C{ bonds decrease by -0.02 and 0.04 A, respectively. The other bonds and angles are almost unchanged (see Figure 1 for definitions). The net result of ionization is only to slightly extend the molecule along the principal axis of linear PAHs.
IR Spectra of Naphthalene Neutral and Cation Theoretical Spectra a d Effect of Ionization. Neutral naphthalene as well as its cation, are characterized by 48 normal modes classiicd a m r d i i to the following symmetry types: 9A1, 8B3, 8BIu 8Bh 3B1, 4B28 4AIu 4Bs. Our computed IR spectra of naphthalene and its cation are reported in Tables I1 and 111. Comparing theoretical spectra calculated with two different basis sets, we notice that extending the basis set from 3-21G to 6-31G’ did not affect the IR spectra of both Cl& and CloHs+. Band positions (raw values from the H F calculations) are comparable within 50 cm-’. Intensities are of the same order of magnitude and differ no more than by a factor of 2. Thus we verify that differences observed between the two types of calculations, for band positions and intensities, are well within the error bar for this type of calculation. Comparing the theoretical IR spectrum of the neutral naphthalene to that of its corresponding cation calculated in the same
+
+
+
+
+
+
+
basis set, we notice that band positions (frequencies) are only slightly changed. Differences stay, most of the time, within the error bar. By contrast, intensities are significantly affected. In order to get a more refined description of the calculated vibrations, a normal coordinate analysis has been carried out, allowing an assignment of each vibration according to the preponderant internal modes. It is presented in Tables I1 and 111, using the following notation: r(CH) and R(CC) for CH and CC stretching vibrations; /3(CH) and a(CCC) for respectively CH and CCC in-plane bending vibrations; e(CH) and T(CCC) for CH and CCC out-of-plane bending vibrations (see Figure 1). A number of comments can be made from these tables: (1) Let us first consider neutral naphthalene. Apart from the r(CH) and the a(CCC) modes, all in-plane modes are delocalized and cannot be assigned to the vibration of any individual bond or angle. This is a characteristic feature of cyclic compounds in general, and cyclic aromatics in particular. For a better understanding of these modes, which describe ring vibrations, we signal the dominant displacement in bold character. The present assignments are in complete agreement with a previous theoretical study of neutral naphthaleneI5 using semiempirical methods. However, in this study, each mode was assigned to a unique vibration type of motion while, in our work, it corresponds to the dominant components of a delocalized vibration, which is probably a more realistic way to present the vibrational movements. The localized C H and CCC out-of-plane bending vibrations also confirm the assignments proposed on semiempirical grounds. (2) If we now consider the assignments obtained for the &tion, for which-to the best of our knowledge-there is no other theoretical work, it is interesting to notice that, the in-plane modes, which were represented by a blend of R(CC), B(CH), and a(CCC) internal coordinates, acquire an increasing contribution of the modes relevant to the ring motion (Le. R(CC) and a(CCC). Vibrations such as 1212 and 1075 cm-l of b3 symmetry, 1280 and 1097 cm-I of bl, symmetry, and 1140 cm- B of Bzusymmetry, which can be related to CH in-plane bending vibrations, have a much stronger R(CC) component in the cation than in the neutral molecule. This can be explained by the fact that removing an electron from the II system of the aromatic molecule softens the bondings of its skeleton, which then can vibrate more intensively. This results in a signifcant effect of ionization on the IR intensities. Thus, parallel to a tentative vibration to vibration comparison of the intensities reported in Tables I1 and 111, we present, in Tables IV and V, a type to type comparison inside each symmetry. In the same tables we report the ratio of each type of vibration to the CH out-of-plane bending, the only strong feature unaffected by ionization. Besides, considering that the final purpose of our work is astrophysical, i.e. to provide evidence for a better understanding of the unidentified IR bands observed in space, we must stress the fact that summed intensities, which are very seldom used in laboratory spectroscopy, are the only quantities useful for a comparison with observations. A close look at the details shows that ionization has a number of important effects. The CH stretching vibrations of blu and bzu symmetry have their positions slightly shifted toward larger frequencies by about 30 cm-I in the positive ion, which may be not significant though systematic, because within the error bar, their intensities decrease by 1 order of magnitude. The CC stretching vibrations of bl, and of symmetry around 1600 and 1500 cm-I in the neutral, are shifted downward by about 50 cm-I in the cation and have their intensities increased by at least 1 order of magnitude. The CH in-plane bending vibrations of bluand bzusymmetry between 1400 and 1100 cm-l which contain also a strong R(CC) contribution are so slightly shifted apart that they can be considered stable, and their intensities increase by less than 1 order of magnitude. The CC stretching vibration of bzusymmetry, blended with a strong /3(CH) contribution and located around 1100 cm-’, increases by more than 2 orders of magnitude, which, even if somehow overestimated, is remarkable.
1884 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
Pauzat et al.
TABLE II: Comwted and Expcrimcatal Infrared Spectra for Napbthlene'
experiment RHF/3-21G vc I 3383 0 0 3354 0 1749 0 1635 0 1464 0 1321 0 1106 0 831 0 567 0 3365 0 3345 0 181 1 1626 0 0 1399 0 1293 0 1057 0 578 54 3368 2 3347 12 1785 4 1567 8 1410 4 1266 1 884 2 40 1 46 3381 1 3349 15 1664 3 1481 0 1333 3 1208 3 1070 4 706 0 1130 0 831 0 442 0 1185 0 1041 0 898 0 537 0 1176 0 976 0 718 0 210 6 1154 168 915 33 548 3 192
RHF/6-3 lG*
V
vc
P
I
3387 3361 1783 1626 1487 1287 1117 831 55 1 3371 3350 1845 1623 1373 1273 1023 555 3374 3353 1806 1543 1390 1245 859 389 3385 3355 1683 1476 1311 1193 1075 674 1080 81 1 432 1124 993 854 520 1117 944 685 206 1098 887 535 188
3085 3056 1645 1453 1355 1134 1043 780 517 3069 3046 1673 1461 1219 1149 917 49 1 307 1 3047 1654 1364 1261 1130 78 1 364 3083 3052 1539 1321 1165 1091 988 596 968 708 397 985 880 785 478 970 824 606 184 977 79 1 485 172
0 0
0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 88 3 8 4 7 3 2 2 73 2 12 2 0.1 3 2 5 0 0 0
0 0 0 0 0
0 0 0 4 125 19 2
(ref 16) 3060 3031 1577 1460 1376 1145 1025 758 512 3092 3060 1624 1438 1239 1158 935 506 3065 3058 1595 1389 1265 1125 753 359 3090 3027 1506 1361 1209 1138 1008 618 943 717 386 980 876 846/770 46 1 970 841 581 195 958 782 476 176
Z (ref 18)
4 4 0 2 2 2 0 0.5 1 W
2 0 1 0.5 3 0.5
3 10 2
'Frequencies are expressed in cm-I; computed IR intensities are in (km/mol). bSee Figure 1 for definition of notations; the displacement in bold character is the dominant one. CFrequencies,V, are calculated unscaled. dFrequencies,V , are scaled (the scaling factors applied to the force constants matrix are 0.89 for r, 0.829 for R , 0.765 for & 0.830 for a,0.82 for z, and 0.955 for 7). The nearby CC stretching vibration of blu symmetry has a similar behavior, though somewhat weaker. The intensity of the CCC bending vibration of bzusymmetry at about 600 cm-I in the neutral (40 cm-' lower in the cation) increases also by 2 orders of magnitude. The intensities of the CH and CCC out-of-plane bending vibrations of b3, symmetry remain constant under ionization. This study of the theoretical spectra of neutral and ionized naphthalene has revealed an unexpected effect of the ionization which leads to the decrease of the intensities of the CH stretching vibrations (by about 1 order of magnitude), with a concomitant increase of the ring CC stretching and CH bending vibrations (by about 2 orders of magnitude). There is no perceptible transfer of intensity between out-of-plane CH and CC vibrations which remain constant after ionization. The tendency of this unexpected behavior is confirmed by the experimental work of Vala and *workers6 and briefly discussed in the following paragraphs. IR Spectrum of Neutral NapComparison with Experimeatal Data. The experimental IR spectrum of naphthalene can be found in Table I1 where it is compared to the theoretical
one. The shifts between scaled quantum mechanical values and experimental band positions are less than 50 cm-1 in the worst cases and less than 20 cm-I in most cases. The shifts are well within the error bar usually accepted for this kind of calculation and confirm the validity of the scaling procedure by type of vibration. Our vibrational assignments support those reported by KrainoP and Mitral7 from force field calculations. It should be noticed that in both approaches, ring vibrations do appear as delocalized on the same modes i.e. R(CC), B(CH), and a(CCC). Comparison of the computed intensities with the experimental ones is difficult, since the only experimental numbers available are rough relative estimations and no indication is given on the procedure used for their determination.'* Moreover, when vibrations of the same type in the Same symmetry are close enough, they overlap to such an extent that it is hardly possible to separate the contributions of individual vibrations, which is the case, for example, of the two vibrations r(CH) of b,, symmetry. It is another reason that led us to consider the sum of the intensities of closely related bands and their ratios to the C H out-of-plane bending vibration as reported in Table IV.
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7085
Theoretical IR Spectra of Ionized Naphthalene
TABLE IIk Computed rad E x p e h W Intrued Spectra for Naphthalene Cationa ROHF/3-2 1G
vc
typeb
3407 3379 1745 1647 1467 1338 1143 829 567 3393 3373 161 1 1587 1361 1188 1040 517 3394 3376 1658 1583 1431 1207 889 398 3406 3375 1659 1466 1299 121 1 1058 635 1134 861 42 1 1205 1088 851 503 1200 1015 646 207 1165 898 483 175
I(4) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 147 (0.13) 19 (0.02) 5 22 (0.02) 0 0 1 11 180 (0.16) 3 31 1123 (1) 662 (0.6) 324 0 0 0 0 0 0 0 0 0 0 0 1 162 48 5
ROHF/6-3 1G' J
P
3421 3400 1780 1638 1503 1302 1147 829 549 3409 3393 1598 1589 1345 1181 1005 48 1 3410 3395 1678 1563 1415 1195 859 382 3420 3396 1656 1466 1261 1188 1058 571 1098 837 415 1153 1047 795 483 1149 987 621 205 1124 870 465 169
3115 3093 1637 1471 1370 1151 1071 780 516 3103 3086 1432 1395 1212 1075 899 42 1 3104 3087 1519 1387 1280 1097 780 358 3114 3090 1490 1289 1140 1101 1004 484 982 739 390 990 928 762 438 984 877 564 190 1002 777 426 156
experiment Wr)
V(W
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.3 0 133 (0.12) 33 (0.03) 9 19 (0.02) 0 0.1 7 13 323 (0.28) 10 14 1136 (1) 835 (0.7) 563
1519 (0.08) 1401 (0.04) 1023 (0.06)
1525 (0.16) 1218 (1.00) 1016 (0.2)
0 0 0 0 0 0 0 0 0 0 0 1 116 27 3
Frequencies are expressed in cm-I; computed IR intensities in (km/mol). b S Figure ~ 1 for definition of notations; the displacement in bold character is the dominant one. Frequencies, v, are calculated unscaled. Frequencies, v, are scaled (the scaling factors applied to the force constants matrix are 0.89 for r, 0.829 for R, 0.765 for 6, 0.830 for a,0.82 for t, and 0.955 for 7). 'The experimental intensities are reported relative to the experimentally strongest one at 1218 cm-I; the relative computed ones are taken relative to the strongest corresponding one at 1101 cm-I.
TABLE I V Relative Intensities of IR Bands of Cl&18
symmetry blu bzu blu bzu
vibration type r(CH) r(CH) cycle vib. + @(CH) cycle vib. + @(CH)
b,U
a(CCC)
bzu b3u b3U
a(CCC) 4CH) T(CCC)
band frequency expt, cm-' 3070-3050 3090-3030 1600-1 100 1500-1000 753-359 618 958-782 476-176
calculated 3-21G/6-3 1G* ratio to the CH out-of-plane intensity, bending vibration km/mol at 791 cm-I 56/91 0.32/0.70 47/75 0.27J0.58 28/22 0.16/0.17 24/19 0.14/0.15 0.02/0.03 314 415 0.02/0.04 1741129 1.0011.00 36/21 0.21 10.16
experiment' ratio to the C H out-of-plane bending vibration at 782 cm-' 0.62 0.10 0.46 0.50 0.04 0.04 1.oo 0.15
'These numbers are obtained by summing the experimental intensities given in ref 18. W h e n comparing these intensity ratios, we see that the calculated numbers c a n be considered to be in good agreement with the experimental ones; indeed, t h e less satisfactory agreement occurs for the delocalized vibrations mixing R(CC) and a(CCC)
ring deformations with CH bendings B(CH), where we found a ratio of about 3 between computed a n d experimental relative values. This result is particularly encouraging if one takes into account the usual error bar known for computed intensities at this
7886 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
TABLE V Rehtlve Intensitlw of IR Bands of C,&Ia+
symmetry
vibration 4CH) r(CH) cycle vib.
+
@(CHI
cycle vib. B(CW cycle vib.
calculated 3-216/6-3 1G* ratio to the band CH out-of-plane frequency, intensity, bending vibration cm-I km/mol at 777 cm-I 3100-3080 310.3 0.02/0 3 110-3090 12/20 0.07/0.17 1500-1300 1711175 1.051 1.5 1
+
1500-1200 2141347
1.32/3.0
+
1100
221 19
0.14/0.16
1100
112311136
6.9319.80
1000 780-358 484 777 426-156
6621835 010.1 3241563 1621116 53/30
4.0917.20 010 2.014.85 111 0.3210.26
@(CHI
cycle vib. + @(CW cycle vib. a(CCC) a(CCC)
4CW
T(CCC)
level of calculation and the possible influence of the experimental conditions. IR Specbum of Naphthalene Cation: Comparison witb Experimental Data. The experimental IR bands assigned to naphthalene cation by Vala and co-workers are reported in Table I11 together with the results of the quantum mechanical calculations. As a guideline to the discussion, observed intensities are quoted relative to the strongest observed feature at 1218 cm-l and compared to theoretical values, themselves normalized to the corresponding intensity at 1101 cm-I. As already said above, the intensities are delocalized on several vibrations of same type. It is not surprising that the only experimental absorptions observed are related to ring vibrations. The vibrations are indeed the most affected by ionization and thus they can be distinguished from the bands of the neutral molecule, even for a low concentration of the ion in the matrix. The parallel between experiment and theory is fairly good if one considers that the larger difference is found for the band at 1016 cm-l, where we obtain 3 times more intensity than for the experimental values.
Concluding Remarks The main result of this study concerns the unexpected and striking behavior of IR intensities when comparing neutral and positively charged naphthalene. HartretFock calculations show that IR intensities are quite sensitive to ionization and that their variations are strongly related to the type of vibration implied. The CH stretching intensities decreases by 1 order of magnitude while ring deformations, which in fact imply both CC stretching and CH in-plane-bending vibrations, increase by 2 orders of magnitude. Though the same tendency seems to be observed experimentally, it is not yet fully clear if transfer of scale factors between a neutral system and the corresponding positive ion can yield force fields with similar accuracy for the two entities. The difference in electronic correlation between a molecule and its positive ion is an effect that is well-known in the theoretical treatment of ionization potentials where it gives the correction to Koopmans theorem. However, nothing systematic is known about a possible differential effect of electron correlation on quantities such as anharmonicity, vibrational coupling between vibrations, or a modification of IR band intensities. A correlated study of a smaller aromatic molecule,I3pyridine, has shown the same trends between the related CSHSNand CSH5N+as for naphthalene and its cation, which led us to consider that the effect is general, at least for small aromatic species. From an astrophysical point of view, the most interesting conclusions have to be drawn from Tables IV and V which give the intensities summed up for each type of vibration in relation
Pauzat et al. to the unindentified bands at 3.3, 6.2, 1.1, 8.6, and 11.3 pm observed in the interstellar medium and commonly assigned to CH stretching, ring deformations, and CH in-plane and out-ofplane bending, respectively. The ring deformation to CH stretching intensity ratios calculated in this ab initio study of the simplest polycyclic aromatic molecule project a new light on the PAH hypothesis and its integration into the interstellar medium patchwork. At our best level of theory, these ratios are 1(6.2+7.7)/1().3) = 0.25 for neutral naphthalene 1(6,2+7,7)/1(),3)
= 20 for the corresponding positive ion
The values of this ratio, deduced from astrophysical observations, range from 10 to 100 depending on the type of source ~0nsidered.I~ It is then obvious that, if the interstellar ratio cannot correlate to that of a neutral PAH (0.25 from our calculated C&8 spectrum) it compares favorably to that of the ionized specia (=20 from our calculated IR spectrum of cloH8+).These theoretical results show that, contrary to the usual interpretation of the observations, it is no longer necessary to postulate that PAHs are stripped of most of their hydrogens to explain the observed ratio of carbon skeleton band intensities to CH stretching ones. This study of the effect of ionization on an IR spectra of the first member of the PAH family provides a strong argument for the existence of ionized PAHs in the interstellar medium.
Note Added in Proof: We would like to add that our results on neutral naphthalene are directly comparable to those obtained by Sellers et a1.22 Acknowledgment. This work was supported by CNRS GR yPhysicochimiedes MolWea Interstellaires” and by NASA Grant NAG 2-16.
References and Notes (1) Leger, A.; Puget, J. L. Astron. Astrophys. 1984, 137, L5. (2) Allamandola, L. J.; Tielens, A. G.; Baker, J. R. Astrophys. J . 1985, 290, L25. (3) Allamendola, L. J.; Tielens, A. G.; Baker, J. R. Infrared emission from interstellar PAHs. In Physical Processes in Interstellar Clouds; Morfill, G. E. Scholer, M., Eds.; NATO AS1 Series; 1987; p 305. (4) Omont, A. Astron. Astrophys. 1986, 164, 159. (5) Puget, J. L.; Leger, A. Annu. Rev. Astron. Astrophys. 1989,27, 161. (6) Szczepanski, J.; Roser, D.; Personnette, W.; Eyring, M.; Pellow, R.; Vala, M. J . Phys. Chem., preceding paper in this issue. (7) Salama, F.; Allamendola, L. J. J . Chem. Phys. 1991, 94, 6964. (8) (a) Frisch, M. J.; Head-Gordon, M.; Schlegel, H.B.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Kahn, L. R.;Stewart, J. J. P.; Fluder, E. M.; Topiol, S.; Pople, J. A. Gaussian 88; Gaussian, Inc.: Pittsburgh, PA, 1988. (b) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab initio Molecule Orbital Theory; Wiley-Interscience: New York, 1986. (9) DeFrees, D. J.; McLean, A. D. J . Chem. Phys. 1985.82, 333. (10) Yamaguchi, Y.; Frisch, M.; Gaw, J.; Schaefer, H. F., 111; Binkley, J. S . J . Chem. Phys. 1986,84, 2262. (1 1) Pulay, P.; Fogarasi, G.; Boggs, J. E. J . Chem. Phys. 1981,74, 3999. (12) Frisch, M. J.; Pople, J. A.; Head-Gordon, M. Chem. Phys. Lett. 1988, 153,503. Miller, M. D.; Jensen, F.; Chapman, 0. L.; Houk, K.N. J. Phys. Chem. 1989, 93, 4495. Wiberg J. Mol. Struct. 1990, 244,61. (13) Ellinger, Y.;Talbi, D.; Pauzat, F.; DeFrees, D. To be submitted for publication in Theor. Acta. (14) Pacansky, J.; Miller, M.; Hatton, W.; Liu, B.; Scheiner, A. J . Phys. Chem. 1990. 111. 7132. (15) Rougeau, N.; Flament, J. P.; Youkharibache, P. J . Mol. Struct. THEOCHEM 1992, 254,405. (16) Krainov, E. P. Opt. Spektrosk. 1964, 16, 415 and 763. (17) Mitra, S. S.; Bernstein, H.J. Can. J . Phys. 1959, 37, 553. (18) Sverdlov, L. M.; Kovner, M. A.; Krainov, E. P. Vibrational spectra of polyaromic molecules; John Wiley & Sons: New York, 1968; p 365 and followinn. . ....
(19flourdain de Muizon, M.; d’Hendecourt, L. B.; Geballe, T. R. Astron. Astrophys. 1990, 235, 367. (20) Ketkar, N.; Fink, M. J. Mol. Struct. 1981, 77, 139. (21) Ponomarev, V. I.; Filipenko, 0. S.;Atovmyan, L. 0. Kristollografiva 1976, 21, 392. (22) Sellers, H.; Pulay, P.; Boggs, J. E. J . Am. Chem. SOC.1985, 107. 6487.
