- Email: [email protected]
Infrared spectra of polycyclic aromatic hydrocarbons: nitrogen substitution
Chemical Physics 234 Ž1998. 87–94
Infrared spectra of polycyclic aromatic hydrocarbons: nitrogen substitution Charles W. Bauschlicher, Jr. Mail Stop 230-3, NASA Ames Research Center, Moffett Field, CA 94035, USA Received 26 January 1998
Abstract The B3LYPr4-31G approach is used to compute the harmonic frequencies of substituted naphthalene, anthracene, and their cations. The substitutions include cyano ŽCN., aminio ŽNH 2 ., imino ŽNH., and replacement of a CH group by a nitrogen atom. All unique sites are considered, namely 1 and 2 for naphthalene and 1, 2, and 9 for anthracene, except for the imino, where only 2-iminonaphthalene is studied. The IR spectra of these substituted species are compared with those of the unsubstituted molecules. The addition of a CN group does not significantly affect the spectra except to add the C-N stretching frequency. Replacing a CH group by N has only a small effect on the IR spectra. The addition of the NH 2 group dramatically affects the neutral spectra, giving it much of the character of the cation spectra. However, the neutral 2-iminonaphthalene spectra looks more like that of naphthalene than like the 2-aminonaphthalene spectra. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction We have been interested in generating infrared spectra of polycyclic aromatic hydrocarbons ŽPAHs. to aid in the understanding of the different PAH contributions w1–6x to the unidentified infrared emission ŽUIR. bands. We have considered w7–10x the neutral and cation spectra of bare PAH molecules, as well as those with some substituents. Overall the agreement with the available experimental data has been very good, despite the rather modest level of theory used. The computational studies are very useful for understanding the cation spectra, where the experiments are difficult to perform and some of the cation bands are obscured by neutral molecules left in the matrix. In this paper we give a more complete picture of our studies of the nitrogen substituted PAHs. We should note that while there is no
direct spectroscopic evidence that nitrogen is present in the interstellar PAHs, there is strong evidence w11–14x for nitrogen substitution from studies of meteorites and interstellar dust particles. In addition, calculations show w15x that the PAH–CN and PAH– NH 2 bond energies are comparable to those for methyl side groups, which are believed w5x to exist in interstellar space under some conditions. 2. Computational methods The geometries are fully optimized and the frequencies and intensities computed using the double harmonic approximation. The 4-31G basis set w16x basis set is used throughout. The hybrid w17x B3LYP w18x approach is used for all calculations, except for the isoquinoline cation Ži.e. replacing the CH at position 2 in naphthalene with a nitrogen atom.,
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 3 9 - 6
88
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
Fig. 1. The parent molecules, naphthalene and anthracene, with the unique sites labeled.
Fig. 2. The theoretical IR spectra of naphthalene, 1cyanonaphthalene, and 2-cyanonaphthalene. 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.
where the Becke-Perdew 86 w19,20x ŽBP86. approach is used. As discussed previously w8x, the B3LYP approach breaks symmetry for this molecule using the 4-31G basis set, but not using the 6-31G) set. Recent work suggests w21,22x that while the BP86 frequencies are slightly less accurate than the B3LYP frequencies, in general, in those cases where B3LYP breaks symmetry, BP86 is the more accurate approach. We should note that in this case, the B3LYPr6-31G) spectra is very similar to the BP86r4-31G spectra. The calculations are performed using the Gaussian 94 computer codes w23x. Previous work has shown w7–10x that the B3LYPr4-31G approach has a very favorable cancellation of errors for these systems, such that the scaled Ž0.958. harmonic frequencies are in very good agreement with the experimental fundamentals. ŽThe scale factor for the BP86r4-31G approach is 0.986 w8x.. The basis set is small enough that large systems can be treated. The calibration calculations show that the computed B3LYPr4-31G intensities are accurate except for C-H stretches, which on the average are
Fig. 3. The theoretical IR spectra of anthracene, 1-cyanoanthracene, 2-cyanoanthracene, and 9-cyanoanthracene.
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94 Table 1 Total B3LYP IR intensities, in kmrmol Molecule
Neutral
Cation
naphthalene 1-cyanonaphthalene 2-cyanonaphthalene 1-aminonaphthalene 2-aminonaphthalene 2-iminonaphthalene substitute N at site 1 substitute N at site 2
336.30 319.39 323.81 1273.36 1421.86 546.78 352.27 360.46
586.29 793.96 918.48 1374.88 1716.00 1309.69 608.32 579.67
anthracene 1-cyanoanthracene 2-cyanoanthracene 9-cyanoanthracene 1-aminoanthracene 2-aminoanthracene 9-aminoanthracene substitute N at site 1 substitute N at site 2 substitute N at site 9
431.71 433.90 448.45 438.87 1425.25 1748.23 1452.76 461.84 483.01 458.68
1086.42 1153.09 1271.85 1170.15 2184.78 2953.56 1597.52 991.79 1178.43 1363.47
89
about a factor of two larger than those determined in the matrix studies w24x. While the gas-phase data are very limited, it appears that the gas-phase intensities tend to lie between the matrix 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 nearly constant. a
a
The BP86 approach is used; the B3LYP value is 1507.20 kmrmol.
Fig. 4. The theoretical IR spectra of the cations of naphthalene, 1-cyanonaphthalene, and 2-cyanonaphthalene.
3. Results and discussion The number of frequencies determined in this work is very large and therefore they are not tabulated here, but are available on the web at http:rr ccf.arc.nasa.govr; cbauschlrastro.data2. ŽThe geometries and energies are available at http:rr ccf.arc.nasa.govr; cbauschlrastro.geometry.. 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 spectra of
Fig. 5. The theoretical IR spectra of the cations of anthracene, 1-cyanoanthracene, 2-cyanoanthracene, and 9-cyanoanthracene.
90
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
w9x for 9-cyanoanthracene suggests that any underestimation is relatively small. The total intensity of the CN substituted species and the unsubstituted PAHs are very similar, see Table 1. The effect of ionization on the spectra of the cyano-PAHs is shown in Figs. 4 and 5. A comparison of these figures with those for the neutral species shows that the C-H stretching intensity at about 3050 cmy1 is dramatically reduced and the intensity of C-H bending and C-C stretching modes is increased greatly. The addition of CN does not significantly affect this trend. It is interesting to note that with ionization, the C-N stretch decreases in intensity for cyanoanthracene, but increases for cyanonaphthalene. We suspect that this is due to spreading the positive charge over more C atoms in the larger anthracene. Therefore, the anthracene is probably more representative of larger PAH cations, however, this remains to be confirmed. As found for the neutrals, the C-H bending and C-C stretching region looks similar for the PAH cations and the cyano-PAH cations. For both the neutral and cation, the C-N Fig. 6. The theoretical IR spectra of naphthalene, 1-aminonaphthalene, 2-aminonaphthalene, and 2-iminonaphthalene.
naphthalene, anthracene, 9-cyanoanthracene, 2aminoanthracene, and acridine Žsubstituting N for the CH group at the 9 position in anthracene., and their cations are taken from the previous studies w7,9x, where the computed results are shown to be in good agreement with the available experimental results. The parent molecules are shown in Fig. 1, with the unique sites labeled. The substitution of a CN group for one of the aromatic H atoms makes relatively small changes in the spectra, other than the addition of the C-N stretch at about 2220 cmy1 — see Figs. 2 and 3. The lower symmetry spreads out the intensity in the 500–1600 cmy1 ŽC-H bending and C-C stretching. region somewhat, but the changes in this region are smaller than the variation in the spectra obtained for different PAH molecules, see for example the results w7x for the unsubstituted PAHs. We should note that if we have underestimated the electron withdrawing power of the CN group, we have also underestimated the intensity of this region of the spectra. However, a comparison of this level of theory with experiment
Fig. 7. The theoretical IR spectra of anthracene, 1-aminoanthracene, 2-aminoanthracene, and 9-aminoanthracene.
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
91
Thus, amino substitution leads to a different ratio of the intensity in the C-C stretching and C-H bending modes compared with ionization of the parent PAH molecules. Thus without observing the N-H stretches, it might be difficult to separate amino substitution from a mixture of unsubstituted PAH neutrals and cations. The Mulliken populations show some charge on the carbon atom near the NH 2 group, thereby giving the amino species some similarities with the parent cations. The total intensities of the amino compounds are the largest of any of the neutral systems — see Table 1. Calculations w15x have shown that the amino hydrogen is slightly easier to remove than the entire NH 2 group or an aromatic hydrogen. Therefore we have also included the spectra of 2-iminonaphthalene in Fig. 6. The very intense bands below 600 cmy1 and near 1650 cmy1 in the aminonaphthalene spectra are weak in the 2-iminonaphthalene spectra. The 600–1600 cmy1 region for the imino is broken into many more bands that in naphthalene, but overall the spectra of 2-iminonaphthalene looks more like naphFig. 8. The theoretical IR spectra of the cations of naphthalene, 1-aminonaphthalene, 2-aminonaphthalene, and 2-iminonaphthalene.
stretch seems like the best approach for detecting CN substitution. The amino spectra are shown in Figs. 6 and 7. Also shown in Fig. 6 is the 2-iminonaphthalene spectra. The addition of the amino group results in the N-H stretches that are clearly visible at frequencies higher than the C-H stretches. The NH 2 bending mode is responsible for the strong band at about 1625 cmy1 . However, the far more dramatic change with the addition of the amino group is the large increase in the intensities of the C-C stretching and C-H bending modes. The increase in these modes is similar to that found with ionization of the parent naphthalene or anthracene: compare Figs. 6 and 7 with the parent cations in Figs. 4 and 5. For the naphthalene and anthracene cations, the C-H stretching modes become very weak while the C-C stretching and C-H bending modes become intense, while for the amino substituted species, the C-C stretching and C-H bending modes become intense without the dramatic reduction in the C-H stretching modes.
Fig. 9. The theoretical IR spectra of the cations of anthracene, 1-aminoanthracene, 2-aminoanthracene, and 9-aminoanthracene.
92
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
intensity is hardly affected by ionization, making it different from either the aromatic C-H stretches, which are reduced in intensity for all systems studied, or the N-H stretches in aminonaphthalene, which increase in intensity with ionization. Figs. 10 and 11 show the spectra that result from replacing a CH group with a nitrogen atom, which lowers the symmetry and hence splits some of the bands. Overall the changes in the spectra associated with replacing a CH group with a nitrogen atom are very small. The cation spectra are shown in Figs. 12 and 13. As noted in the methods section and discussed previously w8x, the B3LYP approach breaks symmetry for this molecule using the 4-31G basis set, but not using the 6-31G) set, and is therefore not considered as reliable as the BP86 spectra. However, we have included the B3LYP spectra in Fig. 12, to show the magnitude of the problem associated with the B3LYP symmetry breaking. As for the neutrals, the spectra of the nitrogen substituted cations is very similar to the bare PAH cation specFig. 10. The theoretical IR spectra of naphthalene and with the CH group at site 1 or site 2 replaced by a nitrogen atom.
thalene than like aminonaphthalene. We also note that N-H stretch in 2-iminonaphthalene is much closer to the C-H stretch than to the N-H stretches in the amino systems. The amino cation spectra are shown in Figs. 8 and 9. Unlike the C-H stretches which drop in intensity with ionization, the N-H stretches increase in intensity with ionization. The bands near 1650 cmy1 remain strong in the amino cations, while there are no strong bands in the naphthalene or anthracene cation spectra near this frequency. The 600–1600 cmy1 region of the amino cation spectra clearly has more strong bands than the parent cations, which is consistent with the lower symmetry. Overall, the amino and parent cation spectra are more similar than the amino and parent neutral spectra. Fig. 8 contains the 2-iminonaphthalene cation spectra. The strongest band in the 2-iminonaphthalene cation spectra is at 1615 cmy1 , which more similar to the aminonaphthalene cation spectra than to naphthalene cation spectra. The N-H stretching
Fig. 11. The theoretical IR spectra of anthracene and with the CH group at site 1, site 2, or site 9 replaced by a nitrogen atom.
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
93
sity of the C-C stretching and the C-H bending modes between 600–1700 cmy1 . Especially pronounced is a band at 1650 cmy1 . This gives the amino spectra the look of an unsubstituted PAH cation. Imino ŽNH. substitution does not have the same effect on the neutral systems. After ionization, the amino, imino, and unsubstituted PAH systems look much more similar, excluding the N-H stretches. Replacing a C-H group by a nitrogen atom makes very small changes to the spectra. The astronomical implications of this work are as follows. Because there does not appear to be any strong indication of bands at 2200 cmy1 or between 3450–3650 cmy1 , we conclude that CN and NH 2 side groups are not an important component of the PAH molecules in the emission regions. Recent studies w25x of the photochemistry of ices has shown the addition of nitrogen and oxygen side groups to PAH molecules, thus suggesting that the molecules containing side groups that have been observed in meteorites and interstellar dust particles are produced in Fig. 12. The theoretical IR spectra of the cations of naphthalene and with the CH group at site 1 or site 2 replaced by a nitrogen atom.
tra, provided the more reliable BP86 approach is used for the isoquinoline cation Ži.e. N at site 2..
4. Conclusions The IR spectra of several substituted naphthalenes and anthracenes have been reported. In general, the change in the spectra caused by the addition of side groups is very similar for naphthalene and anthracene. The side groups lower the symmetry and hence spread out the existing PAH bands and add new bands associated with the side group. The cation spectra are also reported, and again the general spectral features arising from the side groups are similar for the naphthalene and anthracene species. The addition of a cyano group adds the distinctive C-N stretch at about 2200 cmy1 . The addition of an amino group adds the N-H stretches between 3450– 3650 cmy1 , but also dramatically increases the inten-
Fig. 13. The theoretical IR spectra of the cations of anthracene and with the CH group at site 1, site 2, or site 9 replaced by a nitrogen atom.
94
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
ice grains, not in the gas phase in the emission regions. The computational results show that PAH molecules with nitrogen in the rings cannot be ruled out in the emission regions. 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 S.R. Langhoff, J. Phys. Chem. 100 Ž1996. 2819. w8x C.W. Bauschlicher, S.R. Langhoff, Spectrochim. Acta A 53 Ž1997. 1225. w9x S.R. Langhoff, C.W. Bauschlicher, D.M. Hudgins, S.A. Sandford, L.J. Allamandola, J. Phys. Chem. 102 Ž1998. 1632. w10x C.W. Bauschlicher, S.R. Langhoff, S.A. Sandford, D.M. Hudgins, J. Phys. Chem. 101 Ž1997. 2414. w11x S. Clemett, C. Maechling, R.N. Zare, P. Swan, R. Walker, Science 262 Ž1993. 721. w12x M.H. Studier, R. Hayatsu, E. Anders, Geochim. Cosmochim. Acta 36 Ž1972. 189.
w13x T.N. Tingle, C.H. Becker, R. Malhotra, Meteoritics 26 Ž1991. 117. w14x L.J. Kovalenko, C.R. Maechling, S.J. Clemett, J.M. Philippoz, R.N. Zare, C.M.O’D. Alexander, Anal. Chem. 64 Ž1992. 682. w15x C.W. Bauschlicher, S.R. Langhoff, Mol. Phys., in press. w16x M.J. Frisch, J.A. Pople, J.S. Binkley, J. Chem. Phys. 80 Ž1984. 3265, and references therein. w17x A.D. Becke, J. Chem. Phys. 98 Ž1993. 5648. w18x P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 Ž1994. 11623. w19x A.D. Becke, Phys. Rev. A 38 Ž1988. 3098. w20x J.P. Perdew, Phys. Rev. B 33 Ž1986. 8822 and 34 Ž1986. 7406 ŽE.. w21x C.W. Bauschlicher, D.M. Hudgins, L.J. Allamandola, unpublished. w22x M. Rosi, C.W. Bauschlicher, G.V. Chertihin, L. Andrews, Theor. Chem. Acc. 99 Ž1998. 106. w23x 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. w24x D.M. Hudgins, S.A. Sandford, J. Phys. Chem. 102 Ž1998. 329, 344, 353. w25x M.P. Bernstein, personal communication.
Infrared spectra of polycyclic aromatic hydrocarbons: nitrogen substitution Charles W. Bauschlicher, Jr. Mail Stop 230-3, NASA Ames Research Center, Moffett Field, CA 94035, USA Received 26 January 1998
Abstract The B3LYPr4-31G approach is used to compute the harmonic frequencies of substituted naphthalene, anthracene, and their cations. The substitutions include cyano ŽCN., aminio ŽNH 2 ., imino ŽNH., and replacement of a CH group by a nitrogen atom. All unique sites are considered, namely 1 and 2 for naphthalene and 1, 2, and 9 for anthracene, except for the imino, where only 2-iminonaphthalene is studied. The IR spectra of these substituted species are compared with those of the unsubstituted molecules. The addition of a CN group does not significantly affect the spectra except to add the C-N stretching frequency. Replacing a CH group by N has only a small effect on the IR spectra. The addition of the NH 2 group dramatically affects the neutral spectra, giving it much of the character of the cation spectra. However, the neutral 2-iminonaphthalene spectra looks more like that of naphthalene than like the 2-aminonaphthalene spectra. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction We have been interested in generating infrared spectra of polycyclic aromatic hydrocarbons ŽPAHs. to aid in the understanding of the different PAH contributions w1–6x to the unidentified infrared emission ŽUIR. bands. We have considered w7–10x the neutral and cation spectra of bare PAH molecules, as well as those with some substituents. Overall the agreement with the available experimental data has been very good, despite the rather modest level of theory used. The computational studies are very useful for understanding the cation spectra, where the experiments are difficult to perform and some of the cation bands are obscured by neutral molecules left in the matrix. In this paper we give a more complete picture of our studies of the nitrogen substituted PAHs. We should note that while there is no
direct spectroscopic evidence that nitrogen is present in the interstellar PAHs, there is strong evidence w11–14x for nitrogen substitution from studies of meteorites and interstellar dust particles. In addition, calculations show w15x that the PAH–CN and PAH– NH 2 bond energies are comparable to those for methyl side groups, which are believed w5x to exist in interstellar space under some conditions. 2. Computational methods The geometries are fully optimized and the frequencies and intensities computed using the double harmonic approximation. The 4-31G basis set w16x basis set is used throughout. The hybrid w17x B3LYP w18x approach is used for all calculations, except for the isoquinoline cation Ži.e. replacing the CH at position 2 in naphthalene with a nitrogen atom.,
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 3 9 - 6
88
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
Fig. 1. The parent molecules, naphthalene and anthracene, with the unique sites labeled.
Fig. 2. The theoretical IR spectra of naphthalene, 1cyanonaphthalene, and 2-cyanonaphthalene. 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.
where the Becke-Perdew 86 w19,20x ŽBP86. approach is used. As discussed previously w8x, the B3LYP approach breaks symmetry for this molecule using the 4-31G basis set, but not using the 6-31G) set. Recent work suggests w21,22x that while the BP86 frequencies are slightly less accurate than the B3LYP frequencies, in general, in those cases where B3LYP breaks symmetry, BP86 is the more accurate approach. We should note that in this case, the B3LYPr6-31G) spectra is very similar to the BP86r4-31G spectra. The calculations are performed using the Gaussian 94 computer codes w23x. Previous work has shown w7–10x that the B3LYPr4-31G approach has a very favorable cancellation of errors for these systems, such that the scaled Ž0.958. harmonic frequencies are in very good agreement with the experimental fundamentals. ŽThe scale factor for the BP86r4-31G approach is 0.986 w8x.. The basis set is small enough that large systems can be treated. The calibration calculations show that the computed B3LYPr4-31G intensities are accurate except for C-H stretches, which on the average are
Fig. 3. The theoretical IR spectra of anthracene, 1-cyanoanthracene, 2-cyanoanthracene, and 9-cyanoanthracene.
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94 Table 1 Total B3LYP IR intensities, in kmrmol Molecule
Neutral
Cation
naphthalene 1-cyanonaphthalene 2-cyanonaphthalene 1-aminonaphthalene 2-aminonaphthalene 2-iminonaphthalene substitute N at site 1 substitute N at site 2
336.30 319.39 323.81 1273.36 1421.86 546.78 352.27 360.46
586.29 793.96 918.48 1374.88 1716.00 1309.69 608.32 579.67
anthracene 1-cyanoanthracene 2-cyanoanthracene 9-cyanoanthracene 1-aminoanthracene 2-aminoanthracene 9-aminoanthracene substitute N at site 1 substitute N at site 2 substitute N at site 9
431.71 433.90 448.45 438.87 1425.25 1748.23 1452.76 461.84 483.01 458.68
1086.42 1153.09 1271.85 1170.15 2184.78 2953.56 1597.52 991.79 1178.43 1363.47
89
about a factor of two larger than those determined in the matrix studies w24x. While the gas-phase data are very limited, it appears that the gas-phase intensities tend to lie between the matrix 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 nearly constant. a
a
The BP86 approach is used; the B3LYP value is 1507.20 kmrmol.
Fig. 4. The theoretical IR spectra of the cations of naphthalene, 1-cyanonaphthalene, and 2-cyanonaphthalene.
3. Results and discussion The number of frequencies determined in this work is very large and therefore they are not tabulated here, but are available on the web at http:rr ccf.arc.nasa.govr; cbauschlrastro.data2. ŽThe geometries and energies are available at http:rr ccf.arc.nasa.govr; cbauschlrastro.geometry.. 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 spectra of
Fig. 5. The theoretical IR spectra of the cations of anthracene, 1-cyanoanthracene, 2-cyanoanthracene, and 9-cyanoanthracene.
90
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
w9x for 9-cyanoanthracene suggests that any underestimation is relatively small. The total intensity of the CN substituted species and the unsubstituted PAHs are very similar, see Table 1. The effect of ionization on the spectra of the cyano-PAHs is shown in Figs. 4 and 5. A comparison of these figures with those for the neutral species shows that the C-H stretching intensity at about 3050 cmy1 is dramatically reduced and the intensity of C-H bending and C-C stretching modes is increased greatly. The addition of CN does not significantly affect this trend. It is interesting to note that with ionization, the C-N stretch decreases in intensity for cyanoanthracene, but increases for cyanonaphthalene. We suspect that this is due to spreading the positive charge over more C atoms in the larger anthracene. Therefore, the anthracene is probably more representative of larger PAH cations, however, this remains to be confirmed. As found for the neutrals, the C-H bending and C-C stretching region looks similar for the PAH cations and the cyano-PAH cations. For both the neutral and cation, the C-N Fig. 6. The theoretical IR spectra of naphthalene, 1-aminonaphthalene, 2-aminonaphthalene, and 2-iminonaphthalene.
naphthalene, anthracene, 9-cyanoanthracene, 2aminoanthracene, and acridine Žsubstituting N for the CH group at the 9 position in anthracene., and their cations are taken from the previous studies w7,9x, where the computed results are shown to be in good agreement with the available experimental results. The parent molecules are shown in Fig. 1, with the unique sites labeled. The substitution of a CN group for one of the aromatic H atoms makes relatively small changes in the spectra, other than the addition of the C-N stretch at about 2220 cmy1 — see Figs. 2 and 3. The lower symmetry spreads out the intensity in the 500–1600 cmy1 ŽC-H bending and C-C stretching. region somewhat, but the changes in this region are smaller than the variation in the spectra obtained for different PAH molecules, see for example the results w7x for the unsubstituted PAHs. We should note that if we have underestimated the electron withdrawing power of the CN group, we have also underestimated the intensity of this region of the spectra. However, a comparison of this level of theory with experiment
Fig. 7. The theoretical IR spectra of anthracene, 1-aminoanthracene, 2-aminoanthracene, and 9-aminoanthracene.
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
91
Thus, amino substitution leads to a different ratio of the intensity in the C-C stretching and C-H bending modes compared with ionization of the parent PAH molecules. Thus without observing the N-H stretches, it might be difficult to separate amino substitution from a mixture of unsubstituted PAH neutrals and cations. The Mulliken populations show some charge on the carbon atom near the NH 2 group, thereby giving the amino species some similarities with the parent cations. The total intensities of the amino compounds are the largest of any of the neutral systems — see Table 1. Calculations w15x have shown that the amino hydrogen is slightly easier to remove than the entire NH 2 group or an aromatic hydrogen. Therefore we have also included the spectra of 2-iminonaphthalene in Fig. 6. The very intense bands below 600 cmy1 and near 1650 cmy1 in the aminonaphthalene spectra are weak in the 2-iminonaphthalene spectra. The 600–1600 cmy1 region for the imino is broken into many more bands that in naphthalene, but overall the spectra of 2-iminonaphthalene looks more like naphFig. 8. The theoretical IR spectra of the cations of naphthalene, 1-aminonaphthalene, 2-aminonaphthalene, and 2-iminonaphthalene.
stretch seems like the best approach for detecting CN substitution. The amino spectra are shown in Figs. 6 and 7. Also shown in Fig. 6 is the 2-iminonaphthalene spectra. The addition of the amino group results in the N-H stretches that are clearly visible at frequencies higher than the C-H stretches. The NH 2 bending mode is responsible for the strong band at about 1625 cmy1 . However, the far more dramatic change with the addition of the amino group is the large increase in the intensities of the C-C stretching and C-H bending modes. The increase in these modes is similar to that found with ionization of the parent naphthalene or anthracene: compare Figs. 6 and 7 with the parent cations in Figs. 4 and 5. For the naphthalene and anthracene cations, the C-H stretching modes become very weak while the C-C stretching and C-H bending modes become intense, while for the amino substituted species, the C-C stretching and C-H bending modes become intense without the dramatic reduction in the C-H stretching modes.
Fig. 9. The theoretical IR spectra of the cations of anthracene, 1-aminoanthracene, 2-aminoanthracene, and 9-aminoanthracene.
92
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
intensity is hardly affected by ionization, making it different from either the aromatic C-H stretches, which are reduced in intensity for all systems studied, or the N-H stretches in aminonaphthalene, which increase in intensity with ionization. Figs. 10 and 11 show the spectra that result from replacing a CH group with a nitrogen atom, which lowers the symmetry and hence splits some of the bands. Overall the changes in the spectra associated with replacing a CH group with a nitrogen atom are very small. The cation spectra are shown in Figs. 12 and 13. As noted in the methods section and discussed previously w8x, the B3LYP approach breaks symmetry for this molecule using the 4-31G basis set, but not using the 6-31G) set, and is therefore not considered as reliable as the BP86 spectra. However, we have included the B3LYP spectra in Fig. 12, to show the magnitude of the problem associated with the B3LYP symmetry breaking. As for the neutrals, the spectra of the nitrogen substituted cations is very similar to the bare PAH cation specFig. 10. The theoretical IR spectra of naphthalene and with the CH group at site 1 or site 2 replaced by a nitrogen atom.
thalene than like aminonaphthalene. We also note that N-H stretch in 2-iminonaphthalene is much closer to the C-H stretch than to the N-H stretches in the amino systems. The amino cation spectra are shown in Figs. 8 and 9. Unlike the C-H stretches which drop in intensity with ionization, the N-H stretches increase in intensity with ionization. The bands near 1650 cmy1 remain strong in the amino cations, while there are no strong bands in the naphthalene or anthracene cation spectra near this frequency. The 600–1600 cmy1 region of the amino cation spectra clearly has more strong bands than the parent cations, which is consistent with the lower symmetry. Overall, the amino and parent cation spectra are more similar than the amino and parent neutral spectra. Fig. 8 contains the 2-iminonaphthalene cation spectra. The strongest band in the 2-iminonaphthalene cation spectra is at 1615 cmy1 , which more similar to the aminonaphthalene cation spectra than to naphthalene cation spectra. The N-H stretching
Fig. 11. The theoretical IR spectra of anthracene and with the CH group at site 1, site 2, or site 9 replaced by a nitrogen atom.
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
93
sity of the C-C stretching and the C-H bending modes between 600–1700 cmy1 . Especially pronounced is a band at 1650 cmy1 . This gives the amino spectra the look of an unsubstituted PAH cation. Imino ŽNH. substitution does not have the same effect on the neutral systems. After ionization, the amino, imino, and unsubstituted PAH systems look much more similar, excluding the N-H stretches. Replacing a C-H group by a nitrogen atom makes very small changes to the spectra. The astronomical implications of this work are as follows. Because there does not appear to be any strong indication of bands at 2200 cmy1 or between 3450–3650 cmy1 , we conclude that CN and NH 2 side groups are not an important component of the PAH molecules in the emission regions. Recent studies w25x of the photochemistry of ices has shown the addition of nitrogen and oxygen side groups to PAH molecules, thus suggesting that the molecules containing side groups that have been observed in meteorites and interstellar dust particles are produced in Fig. 12. The theoretical IR spectra of the cations of naphthalene and with the CH group at site 1 or site 2 replaced by a nitrogen atom.
tra, provided the more reliable BP86 approach is used for the isoquinoline cation Ži.e. N at site 2..
4. Conclusions The IR spectra of several substituted naphthalenes and anthracenes have been reported. In general, the change in the spectra caused by the addition of side groups is very similar for naphthalene and anthracene. The side groups lower the symmetry and hence spread out the existing PAH bands and add new bands associated with the side group. The cation spectra are also reported, and again the general spectral features arising from the side groups are similar for the naphthalene and anthracene species. The addition of a cyano group adds the distinctive C-N stretch at about 2200 cmy1 . The addition of an amino group adds the N-H stretches between 3450– 3650 cmy1 , but also dramatically increases the inten-
Fig. 13. The theoretical IR spectra of the cations of anthracene and with the CH group at site 1, site 2, or site 9 replaced by a nitrogen atom.
94
C.W. Bauschlicher, Jr.r Chemical Physics 234 (1998) 87–94
ice grains, not in the gas phase in the emission regions. The computational results show that PAH molecules with nitrogen in the rings cannot be ruled out in the emission regions. 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 S.R. Langhoff, J. Phys. Chem. 100 Ž1996. 2819. w8x C.W. Bauschlicher, S.R. Langhoff, Spectrochim. Acta A 53 Ž1997. 1225. w9x S.R. Langhoff, C.W. Bauschlicher, D.M. Hudgins, S.A. Sandford, L.J. Allamandola, J. Phys. Chem. 102 Ž1998. 1632. w10x C.W. Bauschlicher, S.R. Langhoff, S.A. Sandford, D.M. Hudgins, J. Phys. Chem. 101 Ž1997. 2414. w11x S. Clemett, C. Maechling, R.N. Zare, P. Swan, R. Walker, Science 262 Ž1993. 721. w12x M.H. Studier, R. Hayatsu, E. Anders, Geochim. Cosmochim. Acta 36 Ž1972. 189.
w13x T.N. Tingle, C.H. Becker, R. Malhotra, Meteoritics 26 Ž1991. 117. w14x L.J. Kovalenko, C.R. Maechling, S.J. Clemett, J.M. Philippoz, R.N. Zare, C.M.O’D. Alexander, Anal. Chem. 64 Ž1992. 682. w15x C.W. Bauschlicher, S.R. Langhoff, Mol. Phys., in press. w16x M.J. Frisch, J.A. Pople, J.S. Binkley, J. Chem. Phys. 80 Ž1984. 3265, and references therein. w17x A.D. Becke, J. Chem. Phys. 98 Ž1993. 5648. w18x P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 Ž1994. 11623. w19x A.D. Becke, Phys. Rev. A 38 Ž1988. 3098. w20x J.P. Perdew, Phys. Rev. B 33 Ž1986. 8822 and 34 Ž1986. 7406 ŽE.. w21x C.W. Bauschlicher, D.M. Hudgins, L.J. Allamandola, unpublished. w22x M. Rosi, C.W. Bauschlicher, G.V. Chertihin, L. Andrews, Theor. Chem. Acc. 99 Ž1998. 106. w23x 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. w24x D.M. Hudgins, S.A. Sandford, J. Phys. Chem. 102 Ž1998. 329, 344, 353. w25x M.P. Bernstein, personal communication.