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Infrared spectra of polycyclic aromatic hydrocarbons: methyl substitution and loss of H
Chemical Physics 234 Ž1998. 79–86
Infrared spectra of polycyclic aromatic hydrocarbons: methyl substitution and loss of H Charles W. Bauschlicher, Jr., Stephen R. Langhoff Mail Stop 230-3, NASA Ames Research Center, Moffett Field, CA 94035, USA Received 12 January 1998
Abstract The B3LYP approach, in conjunction with the 4-31G basis set, is used to compute the harmonic frequencies of 1- and 2-methylnaphthalene, 1-, 2-, and 9-methylanthracene, and their cations. The IR spectra of the methyl substituted species are very similar to the parent spectra, except for the addition of the methyl C-H stretch at lower frequency than the aromatic C-H stretch. The loss of a single hydrogen from naphthalene, anthracene, and their cations is shown to have a very small effect on the IR spectra. Loss of a methyl hydrogen from 1- or 2-methylnaphthalene, or their cations, is shown to shift the side group C-H frequencies from below aromatic hydrogen stretching frequencies to above them. The loss of H from 2-methylenenaphthalene shows only a small shift in the side group C-H stretching frequency. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The unidentified infrared emission ŽUIR. bands are observed in many astronomical objects; the most intense bands fall at 3.29, 6.2, 7.7, 8.7, and 11.2 mm. There is strong evidence w1–3x that these bands are due to the polycyclic aromatic hydrocarbons ŽPAHs.. The evidence also suggests that the emission is due to the PAH cations or at least a mixture of neutrals and cations, where the ratio of cation to neutral depends on the conditions. The 3.29 mm Ž3040 cmy1 . band actually consists of a strong band at 3040 cmy1 , a broad, weak pedestal from 3125 to 2700 cmy1 and a series of weaker features on the pedestal. The 3040 cmy1 band is usually interpreted as the aromatic C-H stretch. The weaker features have been suggested as arising from; 1. hot bands of PAHs w4x, 2. C-H
stretches due to aliphatic side groups on the PAHs w5x and 3. C-H stretches from PAH molecules containing excess H atoms w6x. Other molecules may also contribute to this region of the spectra, for example relatively stable species created by the loss of a hydrogen atom, such as the loss of a methyl hydrogen to yield a CH 2 side group w7x. This radical is expected to be stabilized by the interaction of the CH 2 p orbital with the aromatic p system. The PAH growth process is expected to involve some loss of hydrogen, but due to the large excess of H, high levels of dehydrogenation seem unlikely. Using density functional theory to compute the harmonic frequencies, we study the changes in the spectra that arise from methyl substitution on naphthalene, anthracene, and their cations. We also investigate the changes in the IR spectra due to the loss of hydrogen from the naphthalene, anthracene, and their
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C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86
cations. The effect of hydrogen loss has been considered previously w8x for naphthalene and naphthalene cation, but using a lower level of theory. We also consider the loss of a methyl hydrogen, which is significantly less strongly bound than an aromatic hydrogen.
2. Computational methods
Fig. 1. The parent molecules, naphthalene and anthracene, with the unique sites labeled.
Fig. 2. The theoretical IR spectra of naphthalene, 1-methylnaphthalene and 2-methylnaphthalene. Note that for all plots in this paper one tick on the y axis represents a constant unit of intensity and the full width at half maximum ŽFWHM. is assumed to be 20 cmy1 . The B3LYP frequencies are scaled by 0.958.
We follow the approach used in our previous studies w9,10x of PAHs. All calculations are performed using the B3LYP w11x hybrid w12x functional in conjunction with the 4-31G basis set w13x. The geometries are fully optimized and the frequencies and intensities computed using the double harmonic approximation. The B3LYP calculations are performed using the Gaussian 94 computer codes w14x on the Computational Chemistry IBM RISC Systemr6000 computers. The geometries can be obtained at http:rrccf.arc.nasa.govr; cbauschlr astro.geometry.
Fig. 3. The theoretical IR spectra of anthracene, 1-methylanthracene, 2-methylanthracene, and 9-methylanthracene.
C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86 Table 1 Total B3LYP IR intensities, in kmrmol.
81
Calibration calculations w15x, which have been carried out for selected unsubstituted PAH molecules and cations, show that a single scale factor of 0.958
brings the B3LYP harmonic frequencies computed using the 4-31G basis set into excellent agreement with the experimental fundamentals; for example, in naphthalene the average absolute error is 4.4 cmy1 and the maximum error is 12.4 cmy1 . While the error can be reduced by improving the basis set, provided the C-H stretches are scaled separately, the B3LYPr4-31G results are of sufficient accuracy to allow a critical evaluation of experiment. While fewer calibration calculations have been carried out for the substituted PAHs, the agreement between the B3LYPr4-31G approach and experiment was very good for anthracene with methyl, NH 2 , and CN substitution w10x. While scaling the 4-31G harmonics yields very reliable frequencies, the calibration calculations also show that the computed B3LYPr4-31G intensities are accurate except for C-H stretches, which on the average are about a factor of two larger than those determined in the matrix studies w16x. While the gas-phase data are very limited, it appears that the gas-phase intensities tend to lie between the matrix
Fig. 4. The theoretical IR spectra of the C-H stretching region for the neutral molecules.
Fig. 5. The theoretical IR spectra of the cations of naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene.
Molecule
Neutral
Cation
naphthalene 1-methylnaphthalene 2-methylnaphthalene 1-CH 2 -naphthalene 2-CH 2 -naphthalene 2-CH-naphthalene naphthalene minus HŽ1. naphthalene minus HŽ2.
336.30 415.60 424.74 373.03 372.17 345.48 285.91 289.29
586.29 714.68 845.87 1103.50 1143.25 1114.91 569.15 567.76
anthracene 1-methylanthracene 2-methylanthracene 9-methylanthracene anthracene minus HŽ1. anthracene minus HŽ2. anthracene minus HŽ9.
431.71 516.44 530.53 513.54 382.54 384.39 383.50
1086.42 1276.08 1365.14 1070.97 1079.18 1045.50 1041.86
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and B3LYP values. We also observe that when two bands of the same symmetry are close in energy, their relative intensities are sensitive to the level of theory, but that the sum of their intensities is very reliable. As discussed previously w15x, many of the methyl PAHs are found to have lower symmetry than expected, due to a slight rotation of the methyl about the C-C bond. Relative to the higher symmetry results, this methyl rotation eliminates the imaginary frequency, but has negligible effect on the other vibrational bands. Since we believe that this effect is caused by a numerical problem, we avoid it by performing all of the calculations involving the methyl-substituted PAHs in no symmetry, even though the true symmetry may be C s .
3. Results and discussion The number of frequencies determined in this work is very large and therefore they are not tabuFig. 7. The theoretical IR spectra of the C-H stretching region for the cation species.
Fig. 6. The theoretical IR spectra of the cations of anthracene, 1-methylanthracene, 2-methylanthracene, and 9-methylanthracene.
lated here, but are available on the web at http:rrccf.arc.nasa.govr; cbauschlrastro.data1. To illustrate the results we present synthetic spectra. In all spectra, one unit Žone tick. on the y axis is an equal unit of intensity. The full width at half maximum ŽFWHM. is assumed to be 20 cmy1 . The naphthalene and anthracene results are taken from the previous study of unsubstituted PAHs w9x and the 1- and 2-methylanthracene results are taken from a recent comparison of theory and experiment w10x. The parent molecules are shown in Fig. 1, with the unique sites labeled. The computed spectra of naphthalene, 1- and 2-methylnaphthalene are given in Fig. 2 and the spectra of anthracene, 1-, 2-, and 9-methylanthracene are given in Fig. 3. Excluding the methyl C-H stretches in the region 2900–3000 cmy1 , the spectra of the methyl substituted species look similar to the unsubstituted species. The naphthalene and anthracene analogs look similar. The total intensities of the methyl substituted species are larger than the parents — see Table 1. The intensity in the C-C
C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86
83
stretching and C-H bending regions Ž400–1600 cmy1 . is spread out somewhat more than in the unsubstituted species, which is consistent with the lower symmetries and additional modes. However, a comparison of naphthalene and anthracene with other unsubstituted PAH molecules w9x shows considerable variation in this spectral region with PAH molecule. Therefore it would be appear impossible to definitively identify methyl substitution using this region of the spectra. Unlike the C-C stretching and C-H bending region, the C-H stretching region Ž2900– 3100 cmy1 . shows a clear difference upon methyl substitution; the methyl C-H stretches are lower in frequency than the aromatic C-H stretches. Fig. 4 shows the C-H stretching region in more detail. Excluding 9-methylanthracene, the three methyl C-H stretches are clearly seen; for 9-methylanthracene, the highest frequency methyl C-H stretch is the shoulder on the low energy side of the aromatic C-H stretches. At higher temperatures, we expect rotation
Fig. 9. A comparison of the spectra of naphthalene cation with 1-methylenenaphthalene and 2-methylenenaphthalene cations Ži.e. 1- and 2-methylnaphthalene cation after the loss of a methyl hydrogen.. Also shown is 2-CH-naphthalene cation.
Fig. 8. A comparison of the spectra of naphthalene with 1-methylenenaphthalene and 2-methylenenaphthalene Ži.e. 1- and 2-methylnaphthalene after the loss of a methyl hydrogen.. Also shown is 2-CH-naphthalene.
of the methyl group to cause the methyl C-H stretches to coalesce into one band. The analogous cation data is presented in Figs. 5–7. Comparing these figures with those of the neutral systems shows a dramatic increase the intensity of the C-C stretches and C-H bends relative to the C-H stretches with ionization Žalso see Table 1.. A similar effect has been noted previously w9,17x for the unsubstituted PAHs, but the increase in intensity with ionization is larger for the substituted species. As for the neutrals, the cation C-C stretching and C-H bending region looks very similar with and without the methyl groups. The C-H stretching region ŽFig. 7. shows that both the methyl and aromatic C-H stretches shift to slightly higher frequency and decrease in intensity, however, there is some variation in the reduction of the relative intensities. For 1- and 2-methylnaphthalene the intensity of the aromatic C-H stretches decrease relative to methyl C-H stretches, while for 9-methylanthracene, the
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methyl C-H stretches decrease in intensity relative to the aromatic C-H stretches. Thus we are unable to make any general conclusion of how ionization changes the relative intensities of the methyl and aromatic C-H stretches, but we can conclude that the relative frequencies of the methyl and aromatic C-H stretches are very similar for the neutrals and cations. This observation is important because it is difficult to observe the C-H stretching region for the cations in matrix experiments due to the strong C-H bands arising from neutrals that remain in the matrix. We next consider the loss of a methyl hydrogen to form 1-methylenenaphthalene and 2-methylenenaphthalene, which are expected to be reasonably stable and therefore of potential importance in understanding the interstellar PAH molecules w7x. The computed spectra, Fig. 8, shows that the C-C stretch and C-H bending region looks very similar to naphthalene. A comparison of Figs. 2 and 8 shows that the C-C stretch and C-H bending region looks similar for the methyl and methylene substituted systems. As found for the methyl systems, the C-H stretching Fig. 11. The theoretical IR spectra of the cations of anthracene and anthracene with one H atom missing.
Fig. 10. The theoretical IR spectra of naphthalene and naphthalene with one H atom missing.
region clearly shows the largest difference; the methylene C-H stretch is at higher frequency than the aromatic C-H stretch Žalso see Fig. 4.. This is the reverse of methyl substitution, where the methyl C-H stretch is lower in frequency than the aromatic C-H stretch. Ionization increases the total intensity Žsee Table 1., decreases the C-H stretching intensity, and increases C-C stretching and C-H bending intensities Žsee Fig. 9., as expected. For the methylene substituted cations, the C-C stretching and C-H bending region extends to slightly higher frequency than for the unsubstituted naphthalene cation. The C-H stretches shift to slightly higher frequency with ionization, but the methylene C-H stretching intensities become very small and are hardly visible in Fig. 7. However, as noted for the methyl cations, the intensities of the C-H stretches depend somewhat on the ring, so the methylene C-H stretches in other cations might be more intense than for naphthalene. It is clear that if CH 2 side groups exist they will contribute to the spectra at frequencies higher than the
C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86
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aromatic C-H stretches at 3040 cmy1 Žthe 3.29 mm band.. The loss of a methylene hydrogen is shown in Figs. 8 and 9 for one site. There are no dramatic changes with the loss of an additional hydrogen atom. The C-H stretch is weaker and at slightly lower frequency for 2-CH-naphthalene than for 2methylenenaphthalene. However, the side group C-H stretching frequency is still at higher frequency than the aromatic C-H stretch. For the cation the C-H stretch is somewhat stronger than for the parent 2-methylenenaphthalene. With one fewer hydrogen atom, it is not surprising that the total intensity decreases with the loss of one of the methylene hydrogens. Figs. 10 and 11 show the loss of H from naphthalene and anthracene, respectively. In both cases there are only small changes in the spectra. As expected the C-H stretching intensity decreases because there is one less H. Fig. 4 shows that the naphthalene frequencies are almost unaffected by the loss of H. The loss of H from anthracene also makes very small Fig. 13. The theoretical IR spectra of the cations of anthracene and anthracene with one H atom missing.
changes, and therefore the results are not included in Fig. 4. Since the C-H stretch is a very strong mode, it is not surprising that the total intensity decreases with the loss of a hydrogen atom — see Table 1. The spectra arising from the loss of H from naphthalene and anthracene cations are shown in Figs. 12 and 13. As for the neutrals there are only small changes in the spectra. Thus our B3LYP results are similar to the SCF results of Pauzat et al. w8x. From the small changes in the spectra with the loss of one H atom, it is clear that it would be very difficult to detect the presence of slightly dehydrogenated PAH’s from the spectra of celestial objects.
4. Conclusions
Fig. 12. The theoretical IR spectra of anthracene and anthracene with one H atom missing.
The addition of a methyl side group to naphthalene, anthracene, and their cations adds the characteristic methyl C-H stretch to the IR spectra. Both the methyl and aromatic C-H stretching intensities decrease dramatically with ionization; both frequencies
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C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86
also shift to slightly higher energy. Removing an aromatic hydrogen makes a very small change in the IR spectra, with only the C-H stretching intensity being slightly decreased since there is one less hydrogen. Loss of a methyl hydrogen produces a stable methylene side group. The methylene C-H stretch is at higher frequency than the aromatic C-H stretch. The astronomical implications are as follows. The calculations show that ionization causes a similar shift in the aromatic and methyl C-H stretches, which is required if any of the weak features near the 3.29 mm band are to be assigned to aliphatic side groups. The calculations predict that CH 2 side groups will contribute to the spectra at frequencies higher than 3040 cmy1 . Since there is little evidence for such bands, we conclude that there must be a low population of PAH molecules with CH 2 side groups. The calculations also show that it will very difficult to detect the presence of slightly dehydrogenated PAH molecules or ions.
References w1x L.J. Allamandola, A.G.G.M. Tielens, J.R. Barker, Astrophys. J. Supp. 71 Ž1989. 733. w2x J. Szczepanski, M. Vala, Nature 363 Ž1993. 699. w3x A. Leger, J.L. Puget, Astron. Astrophys. 137 Ž1984. L5.
w4x J.R. Barker, L.J. Allamandola, A.G.G.M. Tielens, Astrophys. J. 315 Ž1987. L61. w5x M. Jourdain de Muizon, L.B. d’Hendecourt, T.R. Geballe, Astron. Astrophys. 235 Ž1990. 367. w6x M.P. Bernstein, S.A. Sandford, L.J. Allamandola, Astrophys. J. 472 Ž1996. L127, and references therein. w7x T.R. Geballe, A.G.G.M. Tielens, L.J. Allamandola, A. Moorhouse, P.W.J.L. Brand, Astrophys. J. 341 Ž1989. 278. w8x F. Pauzat, D. Talbi, Y. Ellinger, Astron. Astrophys. 293 Ž1995. 263. w9x S.R. Langhoff, J. Phys. Chem. 100 Ž1996. 2819. w10x S.R. Langhoff, C.W. Bauschlicher, D.M. Hudgins, S.A. Sandford, L.J. Allamandola, J. Phys. Chem. 102 Ž1998. 1632. w11x P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 Ž1994. 11623. w12x A.D. Becke, J. Chem. Phys. 98 Ž1993. 5648. w13x M.J. Frisch, J.A. Pople, J.S. Binkley, J. Chem. Phys. 80 Ž1984. 3265, and references therein. w14x GAUSSIAN 94, Revision D. 1, M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, GAUSSIAN, Inc., Pittsburgh PA, 1995. w15x C.W. Bauschlicher, S.R. Langhoff, Spectrochim. Acta A 53 Ž1997. 1225. w16x D.M. Hudgins, S.A. Sandford, J. Phys. Chem. 102 Ž1998. 329, 344, 353. w17x D.J. De Frees, D.M. Miller, D. Talbi, F. Pauzat, Y. Ellinger, Astrophys. J. 408 Ž1993. 530.
Infrared spectra of polycyclic aromatic hydrocarbons: methyl substitution and loss of H Charles W. Bauschlicher, Jr., Stephen R. Langhoff Mail Stop 230-3, NASA Ames Research Center, Moffett Field, CA 94035, USA Received 12 January 1998
Abstract The B3LYP approach, in conjunction with the 4-31G basis set, is used to compute the harmonic frequencies of 1- and 2-methylnaphthalene, 1-, 2-, and 9-methylanthracene, and their cations. The IR spectra of the methyl substituted species are very similar to the parent spectra, except for the addition of the methyl C-H stretch at lower frequency than the aromatic C-H stretch. The loss of a single hydrogen from naphthalene, anthracene, and their cations is shown to have a very small effect on the IR spectra. Loss of a methyl hydrogen from 1- or 2-methylnaphthalene, or their cations, is shown to shift the side group C-H frequencies from below aromatic hydrogen stretching frequencies to above them. The loss of H from 2-methylenenaphthalene shows only a small shift in the side group C-H stretching frequency. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The unidentified infrared emission ŽUIR. bands are observed in many astronomical objects; the most intense bands fall at 3.29, 6.2, 7.7, 8.7, and 11.2 mm. There is strong evidence w1–3x that these bands are due to the polycyclic aromatic hydrocarbons ŽPAHs.. The evidence also suggests that the emission is due to the PAH cations or at least a mixture of neutrals and cations, where the ratio of cation to neutral depends on the conditions. The 3.29 mm Ž3040 cmy1 . band actually consists of a strong band at 3040 cmy1 , a broad, weak pedestal from 3125 to 2700 cmy1 and a series of weaker features on the pedestal. The 3040 cmy1 band is usually interpreted as the aromatic C-H stretch. The weaker features have been suggested as arising from; 1. hot bands of PAHs w4x, 2. C-H
stretches due to aliphatic side groups on the PAHs w5x and 3. C-H stretches from PAH molecules containing excess H atoms w6x. Other molecules may also contribute to this region of the spectra, for example relatively stable species created by the loss of a hydrogen atom, such as the loss of a methyl hydrogen to yield a CH 2 side group w7x. This radical is expected to be stabilized by the interaction of the CH 2 p orbital with the aromatic p system. The PAH growth process is expected to involve some loss of hydrogen, but due to the large excess of H, high levels of dehydrogenation seem unlikely. Using density functional theory to compute the harmonic frequencies, we study the changes in the spectra that arise from methyl substitution on naphthalene, anthracene, and their cations. We also investigate the changes in the IR spectra due to the loss of hydrogen from the naphthalene, anthracene, and their
0301-0104r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 1 4 0 - 2
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C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86
cations. The effect of hydrogen loss has been considered previously w8x for naphthalene and naphthalene cation, but using a lower level of theory. We also consider the loss of a methyl hydrogen, which is significantly less strongly bound than an aromatic hydrogen.
2. Computational methods
Fig. 1. The parent molecules, naphthalene and anthracene, with the unique sites labeled.
Fig. 2. The theoretical IR spectra of naphthalene, 1-methylnaphthalene and 2-methylnaphthalene. Note that for all plots in this paper one tick on the y axis represents a constant unit of intensity and the full width at half maximum ŽFWHM. is assumed to be 20 cmy1 . The B3LYP frequencies are scaled by 0.958.
We follow the approach used in our previous studies w9,10x of PAHs. All calculations are performed using the B3LYP w11x hybrid w12x functional in conjunction with the 4-31G basis set w13x. The geometries are fully optimized and the frequencies and intensities computed using the double harmonic approximation. The B3LYP calculations are performed using the Gaussian 94 computer codes w14x on the Computational Chemistry IBM RISC Systemr6000 computers. The geometries can be obtained at http:rrccf.arc.nasa.govr; cbauschlr astro.geometry.
Fig. 3. The theoretical IR spectra of anthracene, 1-methylanthracene, 2-methylanthracene, and 9-methylanthracene.
C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86 Table 1 Total B3LYP IR intensities, in kmrmol.
81
Calibration calculations w15x, which have been carried out for selected unsubstituted PAH molecules and cations, show that a single scale factor of 0.958
brings the B3LYP harmonic frequencies computed using the 4-31G basis set into excellent agreement with the experimental fundamentals; for example, in naphthalene the average absolute error is 4.4 cmy1 and the maximum error is 12.4 cmy1 . While the error can be reduced by improving the basis set, provided the C-H stretches are scaled separately, the B3LYPr4-31G results are of sufficient accuracy to allow a critical evaluation of experiment. While fewer calibration calculations have been carried out for the substituted PAHs, the agreement between the B3LYPr4-31G approach and experiment was very good for anthracene with methyl, NH 2 , and CN substitution w10x. While scaling the 4-31G harmonics yields very reliable frequencies, the calibration calculations also show that the computed B3LYPr4-31G intensities are accurate except for C-H stretches, which on the average are about a factor of two larger than those determined in the matrix studies w16x. While the gas-phase data are very limited, it appears that the gas-phase intensities tend to lie between the matrix
Fig. 4. The theoretical IR spectra of the C-H stretching region for the neutral molecules.
Fig. 5. The theoretical IR spectra of the cations of naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene.
Molecule
Neutral
Cation
naphthalene 1-methylnaphthalene 2-methylnaphthalene 1-CH 2 -naphthalene 2-CH 2 -naphthalene 2-CH-naphthalene naphthalene minus HŽ1. naphthalene minus HŽ2.
336.30 415.60 424.74 373.03 372.17 345.48 285.91 289.29
586.29 714.68 845.87 1103.50 1143.25 1114.91 569.15 567.76
anthracene 1-methylanthracene 2-methylanthracene 9-methylanthracene anthracene minus HŽ1. anthracene minus HŽ2. anthracene minus HŽ9.
431.71 516.44 530.53 513.54 382.54 384.39 383.50
1086.42 1276.08 1365.14 1070.97 1079.18 1045.50 1041.86
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C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86
and B3LYP values. We also observe that when two bands of the same symmetry are close in energy, their relative intensities are sensitive to the level of theory, but that the sum of their intensities is very reliable. As discussed previously w15x, many of the methyl PAHs are found to have lower symmetry than expected, due to a slight rotation of the methyl about the C-C bond. Relative to the higher symmetry results, this methyl rotation eliminates the imaginary frequency, but has negligible effect on the other vibrational bands. Since we believe that this effect is caused by a numerical problem, we avoid it by performing all of the calculations involving the methyl-substituted PAHs in no symmetry, even though the true symmetry may be C s .
3. Results and discussion The number of frequencies determined in this work is very large and therefore they are not tabuFig. 7. The theoretical IR spectra of the C-H stretching region for the cation species.
Fig. 6. The theoretical IR spectra of the cations of anthracene, 1-methylanthracene, 2-methylanthracene, and 9-methylanthracene.
lated here, but are available on the web at http:rrccf.arc.nasa.govr; cbauschlrastro.data1. To illustrate the results we present synthetic spectra. In all spectra, one unit Žone tick. on the y axis is an equal unit of intensity. The full width at half maximum ŽFWHM. is assumed to be 20 cmy1 . The naphthalene and anthracene results are taken from the previous study of unsubstituted PAHs w9x and the 1- and 2-methylanthracene results are taken from a recent comparison of theory and experiment w10x. The parent molecules are shown in Fig. 1, with the unique sites labeled. The computed spectra of naphthalene, 1- and 2-methylnaphthalene are given in Fig. 2 and the spectra of anthracene, 1-, 2-, and 9-methylanthracene are given in Fig. 3. Excluding the methyl C-H stretches in the region 2900–3000 cmy1 , the spectra of the methyl substituted species look similar to the unsubstituted species. The naphthalene and anthracene analogs look similar. The total intensities of the methyl substituted species are larger than the parents — see Table 1. The intensity in the C-C
C.W. Bauschlicher, Jr., S.R. Langhoffr Chemical Physics 234 (1998) 79–86
83
stretching and C-H bending regions Ž400–1600 cmy1 . is spread out somewhat more than in the unsubstituted species, which is consistent with the lower symmetries and additional modes. However, a comparison of naphthalene and anthracene with other unsubstituted PAH molecules w9x shows considerable variation in this spectral region with PAH molecule. Therefore it would be appear impossible to definitively identify methyl substitution using this region of the spectra. Unlike the C-C stretching and C-H bending region, the C-H stretching region Ž2900– 3100 cmy1 . shows a clear difference upon methyl substitution; the methyl C-H stretches are lower in frequency than the aromatic C-H stretches. Fig. 4 shows the C-H stretching region in more detail. Excluding 9-methylanthracene, the three methyl C-H stretches are clearly seen; for 9-methylanthracene, the highest frequency methyl C-H stretch is the shoulder on the low energy side of the aromatic C-H stretches. At higher temperatures, we expect rotation
Fig. 9. A comparison of the spectra of naphthalene cation with 1-methylenenaphthalene and 2-methylenenaphthalene cations Ži.e. 1- and 2-methylnaphthalene cation after the loss of a methyl hydrogen.. Also shown is 2-CH-naphthalene cation.
Fig. 8. A comparison of the spectra of naphthalene with 1-methylenenaphthalene and 2-methylenenaphthalene Ži.e. 1- and 2-methylnaphthalene after the loss of a methyl hydrogen.. Also shown is 2-CH-naphthalene.
of the methyl group to cause the methyl C-H stretches to coalesce into one band. The analogous cation data is presented in Figs. 5–7. Comparing these figures with those of the neutral systems shows a dramatic increase the intensity of the C-C stretches and C-H bends relative to the C-H stretches with ionization Žalso see Table 1.. A similar effect has been noted previously w9,17x for the unsubstituted PAHs, but the increase in intensity with ionization is larger for the substituted species. As for the neutrals, the cation C-C stretching and C-H bending region looks very similar with and without the methyl groups. The C-H stretching region ŽFig. 7. shows that both the methyl and aromatic C-H stretches shift to slightly higher frequency and decrease in intensity, however, there is some variation in the reduction of the relative intensities. For 1- and 2-methylnaphthalene the intensity of the aromatic C-H stretches decrease relative to methyl C-H stretches, while for 9-methylanthracene, the
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methyl C-H stretches decrease in intensity relative to the aromatic C-H stretches. Thus we are unable to make any general conclusion of how ionization changes the relative intensities of the methyl and aromatic C-H stretches, but we can conclude that the relative frequencies of the methyl and aromatic C-H stretches are very similar for the neutrals and cations. This observation is important because it is difficult to observe the C-H stretching region for the cations in matrix experiments due to the strong C-H bands arising from neutrals that remain in the matrix. We next consider the loss of a methyl hydrogen to form 1-methylenenaphthalene and 2-methylenenaphthalene, which are expected to be reasonably stable and therefore of potential importance in understanding the interstellar PAH molecules w7x. The computed spectra, Fig. 8, shows that the C-C stretch and C-H bending region looks very similar to naphthalene. A comparison of Figs. 2 and 8 shows that the C-C stretch and C-H bending region looks similar for the methyl and methylene substituted systems. As found for the methyl systems, the C-H stretching Fig. 11. The theoretical IR spectra of the cations of anthracene and anthracene with one H atom missing.
Fig. 10. The theoretical IR spectra of naphthalene and naphthalene with one H atom missing.
region clearly shows the largest difference; the methylene C-H stretch is at higher frequency than the aromatic C-H stretch Žalso see Fig. 4.. This is the reverse of methyl substitution, where the methyl C-H stretch is lower in frequency than the aromatic C-H stretch. Ionization increases the total intensity Žsee Table 1., decreases the C-H stretching intensity, and increases C-C stretching and C-H bending intensities Žsee Fig. 9., as expected. For the methylene substituted cations, the C-C stretching and C-H bending region extends to slightly higher frequency than for the unsubstituted naphthalene cation. The C-H stretches shift to slightly higher frequency with ionization, but the methylene C-H stretching intensities become very small and are hardly visible in Fig. 7. However, as noted for the methyl cations, the intensities of the C-H stretches depend somewhat on the ring, so the methylene C-H stretches in other cations might be more intense than for naphthalene. It is clear that if CH 2 side groups exist they will contribute to the spectra at frequencies higher than the
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aromatic C-H stretches at 3040 cmy1 Žthe 3.29 mm band.. The loss of a methylene hydrogen is shown in Figs. 8 and 9 for one site. There are no dramatic changes with the loss of an additional hydrogen atom. The C-H stretch is weaker and at slightly lower frequency for 2-CH-naphthalene than for 2methylenenaphthalene. However, the side group C-H stretching frequency is still at higher frequency than the aromatic C-H stretch. For the cation the C-H stretch is somewhat stronger than for the parent 2-methylenenaphthalene. With one fewer hydrogen atom, it is not surprising that the total intensity decreases with the loss of one of the methylene hydrogens. Figs. 10 and 11 show the loss of H from naphthalene and anthracene, respectively. In both cases there are only small changes in the spectra. As expected the C-H stretching intensity decreases because there is one less H. Fig. 4 shows that the naphthalene frequencies are almost unaffected by the loss of H. The loss of H from anthracene also makes very small Fig. 13. The theoretical IR spectra of the cations of anthracene and anthracene with one H atom missing.
changes, and therefore the results are not included in Fig. 4. Since the C-H stretch is a very strong mode, it is not surprising that the total intensity decreases with the loss of a hydrogen atom — see Table 1. The spectra arising from the loss of H from naphthalene and anthracene cations are shown in Figs. 12 and 13. As for the neutrals there are only small changes in the spectra. Thus our B3LYP results are similar to the SCF results of Pauzat et al. w8x. From the small changes in the spectra with the loss of one H atom, it is clear that it would be very difficult to detect the presence of slightly dehydrogenated PAH’s from the spectra of celestial objects.
4. Conclusions
Fig. 12. The theoretical IR spectra of anthracene and anthracene with one H atom missing.
The addition of a methyl side group to naphthalene, anthracene, and their cations adds the characteristic methyl C-H stretch to the IR spectra. Both the methyl and aromatic C-H stretching intensities decrease dramatically with ionization; both frequencies
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also shift to slightly higher energy. Removing an aromatic hydrogen makes a very small change in the IR spectra, with only the C-H stretching intensity being slightly decreased since there is one less hydrogen. Loss of a methyl hydrogen produces a stable methylene side group. The methylene C-H stretch is at higher frequency than the aromatic C-H stretch. The astronomical implications are as follows. The calculations show that ionization causes a similar shift in the aromatic and methyl C-H stretches, which is required if any of the weak features near the 3.29 mm band are to be assigned to aliphatic side groups. The calculations predict that CH 2 side groups will contribute to the spectra at frequencies higher than 3040 cmy1 . Since there is little evidence for such bands, we conclude that there must be a low population of PAH molecules with CH 2 side groups. The calculations also show that it will very difficult to detect the presence of slightly dehydrogenated PAH molecules or ions.
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