1632
J. Phys. Chem. A 1998, 102, 1632-1646
Infrared Spectra of Substituted Polycyclic Aromatic Hydrocarbons Stephen R. Langhoff,† Charles W. Bauschlicher, Jr.,*,‡ Douglas M. Hudgins,§ Scott A. Sandford,§ and Louis J. Allamandola§ National Aeronautics and Space Administration Ames Research Center, Moffett Field, California 94035 ReceiVed: September 29, 1997; In Final Form: NoVember 26, 1997
Calculations are carried out using density functional theory (DFT) to determine the harmonic frequencies and intensities of 1-methylanthracene, 9-methylanthracene, 9-cyanoanthracene, 2-aminoanthracene, acridine, and their positive ions. The theoretical data are compared with matrix-isolation spectra for these species also reported in this work. The theoretical and experimental frequencies and relative intensities for the neutral species are in generally good agreement, whereas the positive ion spectra are only in qualitative agreement. Relative to anthracene, we find that substitution of a methyl or CN for a hydrogen does not significantly affect the spectrum other than to add the characteristic methyl C-H and CtN stretches near 2900 and 2200 cm-1, respectively. However, addition of NH2 dramatically affects the spectrum of the neutral. Not only are the NH2 modes themselves strong, but this electron-withdrawing group induces sufficient partial charge on the ring to give the neutral molecule spectra characteristics of the anthracene cation. The sum of the absolute intensities is about four times larger for 2-aminoanthracene than those for 9-cyanoanthracene. Substituting nitrogen in the ring at the nine position (acridine) does not greatly alter the spectrum compared with anthracene.
1. Introduction Because of the tremendous stabilization afforded by π-electron delocalization, polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in our everyday surroundings. For example, they are the dominant component of coals and coal-derived liquids and are important byproducts of petroleum manufacture.1 They also constitute a major part of the soot formed in combustion processes2 and, as such, given their well-documented biological activity,3 represent important environmental pollutants. In addition to their terrestrial importance, PAHs are also one of the major classes of molecular species found in the interstellar medium (ISM).4,5 Because of their relative importance for both terrestrial and astrophysical concerns, the spectral behavior of PAHs and PAHrelated species has received extensive theoretical and experimental attention in recent years. Ab initio calculations are now available for the vibrational frequencies and intensities of a substantial number of neutral and ionized PAHs,6-8 and extensive laboratory work has been carried out on both PAHs in the gas phase9-11 and in matrix isolation.12,13 The majority of this work has concentrated on simple, unfunctionalized PAHs (i.e., PAHs that contain only carbon and hydrogen arranged in fused benzenoid rings). Only recently has some theoretical and laboratory work begun to appear on the spectral properties of PAHs containing peripheral chemical side groups.10,11,14 Nonetheless, PAHs with side groups are of great interest for both terrestrial and astrophysical issues. It has been shown that PAHs in the terrestrial atmosphere can gain chemical side groups through a variety of processes.15 The presence of side groups can substantially effect the biological activity of these molecules. For example, the presence of methyl side groups can either increase or decrease
the tumorigenicity of a PAH depending on its location.16 Similar effects are seen for side groups containing oxygen1 and nitrogen.18 Studies have also indicated that PAHs with side groups are of astrophysical importance as well. A major fraction of organic material in most meteorites is in the form of aromatic materials,19 at least some of which have an interstellar heritage as evidenced by their large deuterium excesses.20 PAHs have also been detected in interplanetary dust particles (IDPs) thought to come from asteroids and comets.21,22 PAHs from both sources frequently carry functional groups. Methyl side groups are the most common, but ethyl, biphenyl, isobutyl, isopropyl, hydroxyl, and N-bearing functionalities are also seen.22,23 PAHs with side groups may also be the carriers responsible for certain enigmatic emission bands seen in the infrared spectra of nebulae in the interstellar medium.5,24 Recent ab initio calculations have shown25 that some of these side groups, such as NH2 and CN, are more strongly bound to anthracene and anthracene cation than the methyl side groups. Given the potential importance of PAHs with side groups, and the relative paucity of relevant theoretical and laboratory data, we have undertaken a study of the mid-infrared spectra of a number of these species. In this paper we compare the results of ab initio calculations with laboratory matrix-isolation spectra for the PAHs 9-cyanoanthracene (9-anthracenecarbonitrile), 2-aminoanthracene, acridine, 1-methylanthracene, and 9-methylanthracene. These molecules are shown in Figure 1. This paper is organized as follows. The computational details are given in section 2, and the experimental details of our studies are briefly discussed in section 3. The laboratory spectra and computational results are compared in section 4. Our results are summarized in section 5. 2. Computational Methods
†
Computational Chemistry Branch, Mail Stop 230-3. ‡ Space Technology Division, Mail Stop 230-3. § Astrophysics Branch, Mail Stop 245-6.
We follow the approach used in our earlier study8 of unsubstituted PAHs in that we compute the harmonic frequen-
S1089-5639(97)03156-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/07/1998
Polycyclic Aromatic Hydrocarbons
Figure 1. The molecules considered in this work: (A) 9-cyanoanthracene, (B) 2-aminoanthracene, (C) acridine, (D) 1-methylanthracene, and (E) 9-methylanthracene.
cies using the density functional theory (DFT) approach. We use the B3LYP26 hybrid27 functional which includes some rigorous Hartree-Fock exchange as well as including both a gradient correction for exchange and correlation. The B3LYP calculations were performed using the Gaussian 94 computer codes28 on the Computational Chemistry IBM RISC System/ 6000 computers. The geometries can be obtained at http:// ccf.arc.nasa.gov/∼ cbauschl/astro.geometry. We have used the 4-31G basis set29 throughout. Calibration calculations,30 which have been carried out for selected unsubstituted systems, 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 cm-1 and the maximum error is 12.4 cm-1. While the error can be reduced by increasing the basis set, provided the C-H stretches are scaled separately, the B3LYP/4-31G results are of sufficient accuracy to allow a critical evaluation of experiment. In addition, this basis set facilitates a comparison with our earlier work for the hydrogenated species and can be used to study larger systems. While scaling the 4-31G harmonics yields very reliable frequencies, the calibration calculations also show that the computed B3LYP/4-31G intensities are accurate except for C-H stretches, which on the average are about a factor of 2 larger than those determined in the matrix studies.31 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 very reliable. As discussed previously,30 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 Cs. 3. Experimental Methods The experimental apparatus and methodology used for these matrix-isolation studies have been described in detail previously.7,13,32 Briefly, the PAHs were codeposited onto a cold
J. Phys. Chem. A, Vol. 102, No. 9, 1998 1633 (10 K) infrared transparent window (CsI) along with a large overabundance of argon to a thickness appropriate for measurement of their infrared spectra. The window was suspended inside an ultrahigh vacuum chamber (P ≈ 10-8 mTorr), cooled by a closed-cycle helium refrigerator, and mounted so that the infrared window can be rotated to face a number of different ports without breaking vacuum. The PAHs used in this investigation were 9-cyanoanthracene (9-anthracenecarbonitrile; Aldrich, 97% purity), 2-aminoanthracene (Aldrich, 96% purity), acridine (Aldrich, 97% purity), 1-methylanthracene (Aldrich, 99% purity), and 9-methylanthracene (Aldrich, 98% purity). All samples were used without further purification. Matheson prepurified argon (99.998% min) was used in these studies. The argon deposition line was liquid nitrogen trapped to further minimize contamination. These PAHs have insufficient vapor pressure at room temperature to permit their preparation as a gaseous mixture with argon and thus have to be thermally vaporized before codeposition with larger amounts of argon. The PAHs were thermally vaporized by placing them in resistively heated Pyrex tubes mounted on the sample chamber. Sample temperatures were monitored using a chromel/alumel type K thermocouple mounted on the exterior of the tube. During PAH volatilization, argon was admitted through an adjacent inlet port in such a way that the two “streams” coalesced and froze together on the surface of the cold window. Sample quality was found to be optimal for PAH deposition vapor pressures in the range from 10 to 30 mTorr. Optimum tube temperatures for the PAHs discussed here were: 9-cyanoanthracene, heated to 65 °C, 2-aminoanthracene, heated to 105 °C, acridine, maintained at room temperature, 1-methylanthracene, heated to 35 °C, and 9-methylanthracene, heated to 35 °C. The optimal argon flow rate was estimated to be between 0.5 and 1.0 mmol/h. On the basis of our argon flow rates and the PAH deposition rates estimated from measured band areas using the theoretical intrinsic band strengths presented later in this paper, we estimate the Ar/PAH ratios for our samples were all in excess of 2000/1. Thus, all of our PAH samples were well-isolated in argon. After sample deposition, the cold head was rotated to face the beam of an infrared spectrometer and a spectrum was recorded and ratioed against a “blank” spectrum taken prior to sample deposition. The sample chamber was equipped with CsI vacuum windows and suspended in the sample compartment of an FTIR spectrometer (Nicolet Analytical Instruments, Model 740). Mid-infrared spectra (7000-500 cm-1) were collected using an MCT-B detector/KBr beam splitter combination. All spectra reported here were measured at 0.5 cm-1 resolution. Spectra were typically generated through co-addition of 5 blocks of 200 scans, a number which optimized both the signal-tonoise ratio and time requirements of each experiment. 4. Results and Discussion A. Overview. Since the theoretical data spans the full spectral range, we use simulated spectra based on the computed results to provide an overview and to demonstrate the overall good agreement between theory and experiment. In Figure 2, we compare simulated spectra for neutral anthracene and methylsubstituted anthracene. Adding a methyl side group in either the 1 or 9 position has little impact on the spectra apart from introducing the characteristic methyl C-H stretching bands in the 3000-2850 cm-1 range. The C-C stretch and C-H bending regions (400-1600 cm-1) become a little more complex for the substituted system, because the lower symmetry mixes the bands that were allowed with those that were forbidden in
1634 J. Phys. Chem. A, Vol. 102, No. 9, 1998
Figure 2. The theoretical IR spectra of anthracene, 1-methylanthracene, and 9-methylanthracene. The B3LYP frequencies are scaled by 0.958. Note that for all plots in this manuscript one tick on the y axis represents a constant unit of intensity and the full width at half-maximum (fwhm) is assumed to be 20 cm-1.
the higher symmetry anthracene. In Figure 3, we compare the spectrum of neutral anthracene with 2-amino- and 9-cyanosubstituted anthracene and with acridine (nitrogen substitution in the ring at the 9 position). As observed with methyl substitution, the spectra of anthracene and acridine are very similar, except that the lower symmetry in acridine splits bands in the C-C stretching and C-H bending region of the spectrum. Replacing a ring H with CN also has little effect on the spectrum, other than introducing the CtN stretch and splitting the C-C stretching and C-H bending regions. On-the-otherhand, replacing a hydrogen with NH2 dramatically affects the spectrum. In addition to splitting the bands in the C-C stretching and C-H bending regions, NH2 substitution increases the intensity of these bands greatly, in analogy with that observed upon ionization.8 However, the addition of NH2 does not reduce the C-H stretch intensity as dramatically as ionization. In Figures 4 and 5 we show the analogous simulated spectra for the positive ions, as was shown for the neutrals in Figures 2 and 3, respectively. Relative to the neutrals there is a dramatic decrease in the intensities of the C-H and CtN stretches and a dramatic increase in the intensity of the C-C stretches and C-H in-plane bends. Note that the intensity scale is significantly larger for the ions than the neutrals. Due to the larger intensity scale, the small differences between anthracene and the substituted systems, excluding 2-aminoanthracene, in the C-C stretching and C-H bending regions are less apparent for the ions than the neutrals. The spectrum for 2-aminoanthracene cation is still quite distinct. For the C-C stretching and C-H bending regions, there are more bands with sizable intensity and they cover a larger spectral range. Also in the
Langhoff et al.
Figure 3. The theoretical IR spectra of anthracene, 9-cyanoanthracene, 2-aminoanthracene, and acridine. The B3LYP/4-31G are frequencies scaled by 0.958.
Figure 4. The theoretical IR spectra of the cations of anthracene, 1-methylanthracene, and 9-methylanthracene. The B3LYP frequencies are scaled by 0.958.
higher frequency region, the N-H stretch is not decreased in intensity upon ionization as is the C-H and CtN stretches. Before comparing the theoretical and experimental data and making band assignments for several substituted anthracenes,
Polycyclic Aromatic Hydrocarbons
Figure 5. The theoretical IR spectra of the cations of anthracene, 9-cyanoanthracene, 2-aminoanthracene, and acridine. The B3LYP/431G frequencies are scaled by 0.958.
it is useful to compare the theoretical data for neutral anthracene and 9-cyanoanthracene. We choose these systems because of the high symmetry of the molecules and the perturbation of the CN group is sufficiently small that it is relatively unambiguous to make a one-to-one correspondence between the harmonic frequencies. In Table 1 we compare the frequencies and intensities of the harmonics of these two systems. The anthracene molecule has D2h symmetry, whereas the 9-cyanoanthracene has C2V symmetry. We do not include the b1g and au modes of anthracene, which correspond to the a2 modes of 9-cyanoanthracene, because they have zero intensity by symmetry. We can make the following approximate designation of the harmonics: 0-400 cm-1 (ring deformation and -C-N bend), 400-1000 cm-1 (C-H out-of-plane bend), 1000-1300 cm-1 (C-H in-plane bend), 1300-1620 cm-1 (C-C stretch), 2200 cm-1 (CtN stretch), and 3030-3100 cm-1 (C-H stretch). The sums of the integrated intensities for anthracene and CNsubstituted anthracene are very comparable. In other words, the small increase in intensity for the ring deformation, C-H in plane bend, and C-C stretch modes plus the addition of the relatively intense CtN stretch mode in 9-cyanoanthracene nearly cancels the reduction in C-H stretch intensity compared with anthracene. The approximately 37% reduction in C-H stretch intensity is clearly not entirely due to having one fewer C-H bond. Instead the CN group withdraws some charge from the anthracene giving it a small positive charge. Recall that the C-H stretch intensities for the anthracene cation have much reduced C-H intensities and much enhanced C-H in-plane and C-C stretch intensities. It is generally possible to make an unambiguous comparison of the modes of anthracene and 9-cyanoanthracene. There are, of course, some additional modes due to the CN group (e.g., the b1 mode at 110.3 cm-1 and the b2 mode at 128.9 cm-1 are due to CN out-of-plane and in-plane bends, respectively. Also
J. Phys. Chem. A, Vol. 102, No. 9, 1998 1635
Figure 6. A plot of the theoretical and experimental data for neutral 9-cyanoanthracene and 2-aminoanthracene, using the data from the tables and assuming bandwidths of 20 cm-1 for both. Note that no experimental data is available below 500 cm-1.
the mode at 2203.7 cm-1 is clearly due to the CtN stretch. Since one of the C-H bonds is replaced by a C-CN bond, there is one less symmetric C-H stretch mode in the substituted system. Overall the presence of the CN group in the nine position lowers the symmetry and spreads out the intensity into more bands. This is particularly noticeable for the “solo” outof-plane bend modes, which in the unsubstituted case is concentrated in the b3u mode at 885.3 cm-1, while in the CN substituted anthracene, the one solo hydrogen band mixes with another band, causing the intensity to be distributed into two nearly equally intense bands at 860.6 and 912.4 cm-1. The overall intensity of this band is reduced as there is now only one solo hydrogen and the band that it mixes with is forbidden in anthracene. While there are significant differences, the spectra are qualitatively similar in appearance and overall intensityssee Figure 3. B. Comparison of Neutral Spectra. We should first note that there is good overall agreement between the DFT and experimental band positions and relative intensities, with the one exception that the theoretical C-H stretching intensities are about a factor of 2 too large. The agreement between theory is illustrated in Figure 6 where we plot the theoretical and the experimental data using the same full width at half-maximum for two representative systems. The good agreement in both the position and intensities gives us confidence that we can assign most of the experimental bands, although there are cases noted below where the assignment is ambiguous. We should also note that the experimental bands corresponding to C-H stretching modes are sufficiently overlapping and the theoretical bands too close in energy to make symmetry assignments of the bands. Thus, we compare only the total experimental and theoretical relative intensities for these bands.
1636 J. Phys. Chem. A, Vol. 102, No. 9, 1998
Langhoff et al.
TABLE 1: Comparison of the Theoretical Infrared Frequencies and Intensities for Neutral Anthracene and 9-Cyanoanthracene anthracene irreducible representation
c
9-cyanoanthracene
frequencya
b3u
91.0
b1u b2g b3u ag b3g b3u
232.3 267.1 379.8 390.2 390.9 471.3
b3g b2g b2u
530.5 584.4 612.7
ag b1u b3u ag b2g b2u b1u b2g b3u b2g b3g b3u b2g ag b2u
637.9 651.6 730.2 742.9 767.1 795.9 908.0 836.3 885.3 909.9 918.3 962.0 990.6 1005.4 1000.7
b3g b2u b1u ag b2u b3g ag b1u b3g b1u b2u b2u ag b3g b2u b1u ag b2u ag b3g b3g b1u
1100.4 1157.7 1156.2 1181.2 1169.3 1202.0 1265.9 1274.6 1282.1 1311.2 1342.6 1383.2 1385.2 1397.1 1455.3 1456.1 1481.9 1533.7 1541.0 1578.0 1617.2 1620.0
b1u ag b3g b1u b2u ag b3g b1u b2u ag
3039.0 3041.1 3043.4 3044.1 3047.5 3049.0 3062.9 3063.2 3077.8 3078.4
intensityb
irreducible representation
frequencya
intensityb
0.62(0.01) 76.42(1.00)
b1 b1 b2 a1 b1 b1 a1 b2 b1 a1 b2 b1 b2 b2 b1 a1 a1 b1
79.4 110.3 128.9 227.8 270.6 364.5 386.4 391.3 446.5 459.7 469.5 562.1 594.0 631.4 642.0 648.1 683.9 743.8
2.21(0.03) 0.37(0.01) 4.62(0.06) 3.03(0.04) 0.28(
