Electronic Spectra of Gas-Phase Polycyclic Aromatic Nitrogen

Apr 12, 2012 - Department of Chemistry, La Trobe Institute for Molecular Sciences, La ... corresponding isoquinoline+ and quinoline+ molecular cations...
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Electronic Spectra of Gas-Phase Polycyclic Aromatic Nitrogen Heterocycle Cations: Isoquinoline+ and Quinoline+ Viktoras Dryza,† Julian A. Sanelli,† Evan G. Robertson,‡ and Evan J. Bieske*,† †

School of Chemistry, The University of Melbourne, Victoria, Australia 3010 Department of Chemistry, La Trobe Institute for Molecular Sciences, La Trobe University, Bundoora, Victoria, Australia 3086

‡

S Supporting Information *

ABSTRACT: Electronic spectra of the gas-phase isoquinoline+-Ar and quinoline+-Ar complexes are recorded using photodissociation spectroscopy by monitoring the Ar loss channel. The D3←D0 and D4←D0 band origins for isoquinoline+-Ar are observed at 15245 ± 15 cm−1 and 21960 ± 15 cm−1, respectively, whereas for quinoline+-Ar they appear at 16050 ± 15 cm−1 and 21955 ± 15 cm−1, respectively. Strong vibronic progressions for the D3←D0 band systems of both isoquinoline+-Ar and quinoline+-Ar are modeled and assigned in terms of ring deformation and carbon−carbon stretch vibrational modes using time-dependent density functional theory calculations in conjunction with Franck−Condon simulations. The properties of the isoquinoline+ and quinoline+ molecules are compared with those of the isoelectronic naphthalene+ molecule. The existence of strong progressions in the visible spectra of isoquinoline+-Ar and quinoline+-Ar suggests that the corresponding isoquinoline+ and quinoline+ molecular cations are unlikely to be responsible for diffuse interstellar bands.

1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are fundamental molecules that play a role in combustion processes and which have found applications as discotic liquid crystals, organic semiconductors, and fluorescence dyes.1,2 However, it is the proposed widespread existence of PAHs in the interstellar medium (ISM) that has largely driven recent investigations of their spectroscopic and photophysical attributes.3 The main foundation for this conjecture is the detection of mid-infrared (IR) emission bands from regions of space that are characteristic of PAH structural motifs, along with the fact that the composite elements are relatively abundant in space.3 Charged PAHs (cations or protonated species) have been linked with the diffuse interstellar bands (DIBs), a series of unassigned absorption features occurring across the visible spectrum.4−7 However, claims that charged PAHs are responsible for DIBs remain inconclusive and so far no convincing matches have been found between laboratory and astronomical spectra of PAHs in the optical region. Polycyclic aromatic nitrogen heterocycles (PANHs) are closely related to PAHs, with one or more CH groups being substituted by a nitrogen atom. Because nitrogen is the fourth most abundant element in space, it has been proposed that PANHs should also be present in the ISM.3 PANHs are also interesting from an astrobiological perspective, because of their possible involvement in prebiotic chemistry and eventual transformation into the nitrogen containing aromatic systems that are essential ingredients in biomolecules. This theme has motivated several studies of PANH spectroscopy8−13 and chemistry. 14−16 Although PAHs and PANHs such as naphthalene (Np), quinoline (Qn), and isoquinoline (i-Qn), shown in Figure 1, are generally considered too small to be © 2012 American Chemical Society

Figure 1. Naphthalene (Np), quinoline (Qn), and isoquinoline (iQn).

photostable in the ISM,14,17 extraterrestrial i-Qn and Qn molecules have been identified in meteorites,18 and there have been contentious reports for detection of Np+ in the ISM.19−21 In the current study, we have investigated the D3←D0 and D4←D0 electronic transitions of the isoquinoline and quinoline radical cations, the simplest PANH cations, using resonanceenhanced photodissociation spectroscopy of the argon tagged complexes. The D3←D0 and D4←D0 electronic transitions of iQn+ and Qn+ are found to occur in the visible region, whereas the S1←S0 transitions for the neutral i-Qn and Qn molecules occur in the 300 nm ultraviolet region.22 The D3←D0 bands of i-Qn+ and Qn+ consist of several strong, well-resolved progressions involving ring deformation and C−C stretch vibrational modes that are assigned using a combination of time-dependent density functional theory (TD-DFT) calculations and Franck−Condon (FC) simulations. Eventually, spectra of the argon tagged i-Qn+ and Qn+ cations reported in this paper should help facilitate measurements of cold, untagged PANH+ molecules, providing the requisite data for testing whether they exist in the ISM. Received: February 14, 2012 Revised: April 11, 2012 Published: April 12, 2012 4323

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Figure 2. Lowest energy structures of the Np+-(Ar)n, i-Qn+-(Ar)n, and Qn+-(Ar)n (n = 1, 2) complexes. For each complex the argon binding energy (D0) and the distance from the argon atom to the nearest carbon atom (Re) are also shown.

The i-Qn+ and Qn+ cations are isoelectronic with the Np+ cation, whose electronic absorptions have been thoroughly characterized through cavity ringdown spectroscopy,23,24 photodissociation spectroscopy of the Np+-Ar complex,25 and direct absorption in a rare gas matrix.26,27 The electronic spectra of the C9H7N+ isomers reported in the current study, together with the previous Np+ data, allow one to follow the effects of nitrogen substitution on the aromatic π electronic structure. The new spectra also complement recent near-IR electronic spectra of larger PANH cations obtained in an argon matrix.11

shift associated with irradiation of the counter-propagating beam of Np+-Ar, i-Qn+-Ar, Qn+-Ar ions with 10 eV translation energy. For example, the correction for the D2←D0 origin of Np+-Ar at 14873 cm−1 is +0.17 cm−1, whereas for the D3←D0 origin of Np+-Ar at 21959 cm−1 it is +0.25 cm−1. The contribution of Doppler broadening to the band shapes is also minimal. Assuming at worst, an energy spread of ±5 eV in the ion beam, the Doppler broadening is 0.1 cm−1, several orders of magnitude less than the observed band widths (80 cm−1). The uncertainty in the peak positions is estimated to be ±15 cm−1. There are several possible photodissociation mechanisms for the charged complexes. For Np+-Ar, Pino et al. suggested that single photon excitation of the D2 state precedes rapid internal conversion into a lower Dn state, followed by internal vibrational redistribution, and transfer of sufficient energy into the Np+ ···Ar bond to cause its rupture.25 This sequence of nonradiative events, which are also likely to occur for i-Qn+-Ar and Qn+-Ar, should give unity quantum yield for photodissociation. Alternatively, radiative relaxation to vibrational levels in the ground electronic state with energies exceeding the Ar binding energy will inevitably lead to dissociation. 2.2. Computational Methods. The C10H8+ and C9H7N+ ions and their Ar tagged counterparts were computationally examined using DFT within Gaussian 09.30 Ground state geometry optimizations and harmonic vibrational frequency calculations were carried out with the ωB97X-D functional31 and aug-cc-pVDZ basis set on a large grid of size (99,590) using the “tight” optimization criteria. The Ar binding energies were corrected for zero-point vibrational energy and basis set superposition effects. The ωB97X-D functional was designed to account for dispersion forces, which play an important role in the interaction between the Ar atom and the molecular cation. The suitability of the ωB97X-D/aug-cc-pVDZ approach for the Np+-Ar, i-Qn+-Ar, and Qn+-Ar systems was gauged by calculating the dissociation energies of the well characterized Ar2 and C6H6+-Ar systems. For the Ar2 dimer, which is bound solely by dispersion forces, the current DFT functional gives a well depth of 48 cm−1 and dissociation energy of 36 cm−1, underestimating the experimental values (99 and 84 cm−1, respectively; ref 32). For C6H6+-Ar, the predicted D0 of 301 cm−1 also underestimates the experimental value of 532 cm−1,33 although the discrepancy is partly a consequence of correcting for zero-point energy using harmonic vibrational frequencies, which for the intermolecular modes, will be slightly too high.

2. EXPERIMENTAL AND COMPUTATIONAL APPROACH 2.1. Experimental Methods. The Np+, i-Qn+, and Qn+ molecular cations were studied using photodissociation spectroscopy. Because the bare cations do not dissociate following absorption of a single visible photon, we have obtained spectra of argon tagged C10H8+-Ar and C9H7N+-Ar complexes by monitoring the photoinduced argon atom loss channel. For similar molecular cations, a weakly attached Ar atom has a minor effect on the chromophore’s electronic transitions.5,25,28 The tandem mass spectrometer used for the photodissociation experiments has been described previously.29 Briefly, Np+-Ar, iQn+-Ar, and Qn+-Ar complexes were synthesized by directing an electron beam through a supersonic expansion of Ar gas (6 bar) seeded with the relevant neutral precursor (naphthalene, isoquinoline, or quinoline). The target ions (Np+-Ar, i-Qn+-Ar, Qn+-Ar) were mass-selected by a quadrupole mass filter, deflected through 90° by a quadrupole bender, and passed into an octopole ion guide. There, the ions encountered a counterpropagating light beam that served to detach the Ar atom when the light was resonant with a vibronic transition of the complex. Photofragmentation was monitored by directing the ions through a second quadrupole mass filter tuned to pass fragment ions to the ion detector. An absorption spectrum was obtained by monitoring the Ar loss channel as a function of wavelength. The light source for the photodissociation experiments was an optical parametric oscillator (Opotek Vibrant) with a bandwidth of ∼5 cm−1. The photofragment signal intensities were normalized with respect to laser power. Wavelength calibration was achieved with a spectrometer (Ocean Optics HR4000) in conjunction with a mercury−argon lamp. The transition wave numbers were corrected for the minor Doppler 4324

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Table 1. Experimental and Theoretical Parameters for Optical Absorptions of Np+, i-Qn+, and Qn+, Including Experimental Transition Energies, Calculated Adiabatic/Vertical Transition Energies, and Oscillator Strengths ( f)a transition Np

+

i-Qn+ Qn+

2

energy (exp) eV 2

D2( B2g)←D0( Au) D3(2B3g)←D0(2Au) D3(2A″)←D0(2A″) D4(2A″)←D0(2A″) D3(2A″)←D0(2A″) D4(2A″)←D0(2A″)

b,c,d

1.848 2.725b,c 1.896b 2.728b 1.995b 2.727b

energy (calc) eV

f (calc)

f (exp)c

2.013/2.148 2.973/3.018 2.023/2.171 3.036/3.274 2.088/2.258 2.938/3.010

0.052 0.005 0.053 0.005 0.041 0.010

0.052 0.010

a

Transition energies obtained from Ar tagged photodissociation experiments have been corrected for the Ar induced energy shift. bCurrent work (Ar tagged). cRef 25. (Ar tagged). dRef 24. (bare ion).

interact with different rings (C2h symmetry), with an Ar binding energy of 472 cm−1 (with respect to the Np+-Ar +Ar limit). The Np+-(Ar)2 geometry in which the Ar atoms sit above and below the same ring was not found to be a minimum. An isomer in which both Ar atoms are attached to the same side of Np+, is predicted to have a dissociation energy of 323 cm−1 (with respect to the Np+-Ar + Ar limit). For this structure, one Ar atom sits nearby the shared C atoms, as for Np+-Ar, while the second Ar atom sits above a C−H bond on the other ring such that the Ar···Ar separation is 4.01 Å. The most stable i-Qn+(Ar)2 and Qn+-(Ar)2 isomers have structures in which the Ar atoms are attached above and below the homocycle ring, and have Ar binding energies of 525 and 529 cm−1, respectively. Because the ωB97X-D/aug-cc-pVDZ approach apparently underestimates dispersion interactions, the calculated geometries and binding energies should be viewed as rough guides to the actual structures.

TD-DFT calculations were performed to determine the vertical and adiabatic electronic transition energies of the bare molecular cations, using the B3LYP functional and aug-ccpVTZ basis set. The spin expectation values were within 15% of the theoretical value (0.75). To help interpret the vibronic structure in the experimental spectra, spectral simulations were performed using the PGOPHER program, which calculates the Franck−Condon factors from the ground and excited state geometries and vibrational frequencies.34 For the simulations, optimized geometries and harmonic vibrational frequencies for the ground and excited electronic states were obtained using the B3LYP functional and the cc-pVDZ basis set. Vibrational frequencies in each electronic state were scaled by 0.9717, as recommended by Merrick et al.35 Vibrational frequencies for the ground state were calculated analytically, whereas those for the excited state were calculated numerically. The vibrational modes of the molecular cations in their ground states were numbered using the Mulliken convention on the basis of their calculated frequencies.

4. CALCULATED ELECTRONIC TRANSITIONS We turn now to a discussion of the excited electronic states and electronic transitions of Np+, i-Qn+, and Qn+. The TD-DFT predictions for the electronic transitions of Np+, i-Qn+, and Qn+ are summarized in Table 1. First, it is useful to examine the performance of the B3LYP/aug-cc-pVTZ approach for the well characterized, isoelectronic Np+ molecule, for which the D2(2B2g)←D0(2Au) and D3(2B3g)←D0(2Au) electronic bands occur in the visible region with origins at 1.848 and 2.725 eV, respectively.23−27,36−41 Encouragingly, the energies for the D2←D0 and D3←D0 origin transitions of Np+ are only slightly overestimated (by 0.165 and 0.248 eV, respectively), and the predicted oscillator strengths closely match the experimental values,25 suggesting that the method should also reliably estimate energies and oscillator strengths for the optical transitions of i-Qn+ and Qn+. The symmetry forbidden D1(2B1u)←D0(2Au) transition of Np+ is predicted from the naphthalene photoelectron spectroscopy (PES) spectrum to occur in the near-IR.41 Photoelectron spectra show that the frontier aromatic π molecular orbitals (MOs) of i-Qn and Qn are only slightly modified from those of Np, the key difference being that the C9H7N isomers possess a MO that is essentially the N atom’s sp2 lone pair orbital, and which has been assigned as the HOMO-2, although it is energetically close to the HOMO-1.42 Our DFT calculations for the neutral C9H7N isomers display this ordering, yet predict that upon ionization the lone pair MO becomes the HOMO-1. For i-Qn+ and Qn+, the vertical D1←D0 transitions are of n-π character and are predicted to occur in the near-IR (0.842 and 0.718 eV, respectively), although with very low oscillator strength. The D2←D0, D3←D0, and D4←D0 transitions of

3. DFT STRUCTURES DFT optimized geometries were calculated for Np+ (D2h symmetry), i-Qn+ (Cs symmetry), and Qn+ (Cs symmetry). The i-Qn+ cation was found to be slightly more stable than Qn+ (by 0.04 eV). Several structural isomers were identified for the Ar tagged complexes at the ωB97X-D/aug-cc-pVDZ level of theory. Isomers in which the Ar atom sits above a π-aromatic ring were found to be slightly more stable than planar isomers in which the Ar atom is attached to H atoms, presumably because the attractive dispersion forces are maximized when the Ar atom interacts with the π-orbitals. The lowest energy πbound isomers for the Np+-(Ar)n, i-Qn+-(Ar)n, and Qn+-(Ar)n (n = 1, 2) complexes are shown in Figure 2. For Np+-Ar, the Ar atom is predicted to sit atop one of the rings, close to the shared C atoms. The energy barrier for the Ar atom crossing from one ring to the other is predicted to be only ∼6 cm−1, suggesting that the Ar atom is probably delocalized over both rings. The predicted binding energy is 508 cm−1, somewhat greater than calculated for C6H6+-Ar (301 cm−1). For i-Qn+-Ar and Qn+-Ar, the Ar atom prefers to sit above the homocycle adjacent to the two central C atoms (as for Np+-Ar). Dissociation energies for i-Qn+-Ar and Qn+-Ar are predicted to be 519 and 514 cm−1, respectively, slightly larger than for Np+-Ar. For the Np+-(Ar)2, i-Qn+-(Ar)2, and Qn+-(Ar)2 complexes, structures containing π-bound Ar atoms disposed on opposite sides of the planar molecular cation (Figure 2) were predicted to be more stable than structures in which the Ar atoms are on the same side. For example, as shown in Figure 2, the most stable configuration of Np+-(Ar)2 is such that the Ar atoms 4325

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C9H7N+, all of which are of π−π character, are analogous to the D 1 ←D 0 , D 2 ←D 0 , and D 3 ←D 0 transitions of C 10 H 8 + , respectively. Accordingly, i-Qn+ and Qn+ are predicted to have two electronic transitions in the visible region. First, the D3(2A″)←D0(2A″) transitions for i-Qn+ and Qn+ have vertical/ adiabatic excitation energies at 2.171/2.023 and 2.258/2.088 eV, respectively, whereas the D4(2A″)←D0(2A″) transitions have vertical/adiabatic excitation energies at 3.274/3.036 and 3.010/ 2.938 eV, respectively. The D2←D0 transitions for i-Qn+ and Qn+, with predicted vertical energies of 1.379 and 0.916 eV, respectively, have close to zero oscillator strength. The nature of the optical absorptions, the relevant MOs, and the operative electron promotions are comprehensively illustrated in the Supporting Information. For i-Qn+ and Qn+, the D3←D0 transitions are well described by the excitation of a β electron from the HOMO-3 to the singly occupied MO (SOMO). On the other hand, there is substantial double excitation character for the D4←D0 transition, with promotion of the HOMO-4 β electron to the singly occupied MO (SOMO), in conjunction with promotion of the SOMO α electron to the lowest unoccupied MO (LUMO). Similar electron rearrangements occur for the analogous transitions of Np+.

Table 2. Wavelengths (Air), Wave Numbers (Vacuum), and Assignments for Vibronic Transitions of Np+-Ar, i-Qn+-Ar, and Qn+-Ara ν̃/cm−1

λair/nm

Δν̃/cm−1

Δν̃/cm−1 ref 25

assignment

+

Naphthalene -Ar D2(2B2g)←D0(2Au) 672.2 650.1 639.8 635.7 629.4 613.5 595.3 583.9 577.6 564.3 549.0 539.3 534.4 D3(2B2g)←D0(2Au) 455.3 D3(2A″)←D0(2A″) 655.8 634.5 623.3 614.6 603.4 600.2 582.2 572.6 554.3 D4(2A″)←D0(2A″) 455.2

5. OPTICAL PHOTODISSOCIATION SPECTRA Resonance-enhanced photodissociation spectra for massselected Np+-Ar, i-Qn+-Ar, and Qn+-Ar complexes over the 14000−22500 cm−1 range are shown in Figure 3. Spectra of the

D3(2A″)←D0(2A″) 622.9 604.0 595.6 586.2 578.8 572.6 562.8 556.3 D4(2A″)←D0(2A″) 455.3

14873 15377 15625 15727 15883 16296 16793 17121 17307 17716 18208 18538 18708

0 504 752 854 1010 1423 1920 2248 2434 2843 3335 3665 3835

21959

0 Isoquinoline+-Ar

15245 15756 16040 16265 16568 16657 17170 17460 18034

0 511 795 1020 1323 1412 1925 2215 2789

21960

0 Quinoline+-Ar

16050 16552 16785 17055 17272 17459 17763 17972

0 503 735 1005 1222 1410 1714 1922

21955

0

000 910 810 1220 920 410 410910 4101220 410920 420 420910 4201220 420920

0 500 750 845 1001 1417 1941 2258 2427 2841 3337 3692 3819

000

000 3010 2710 3020 27103010 1310 13103010 13102710 1320 000

000 2910 2710 2920 27102910 1410 27102920 14102910 000 −1

Uncertainties in the measured band positions are ±15 cm . Spectra are shown in Figure 3 and Figure 4. a

Figure 3. Optical spectra of Np+-Ar (top), i-Qn+-Ar (middle), and Qn+-Ar (bottom) complexes, recorded by monitoring the Ar loss channel.

strong progressions. In contrast, the D3 ←D0 band is comparatively weak, and only the origin peak at 21959 ± 15 cm−1 is discernible. Energies for the origin transitions of the Np+-Ar complex are close to those reported by Pino et al. (D2←D0 origin at 14863 cm−1, D3←D0 origin at 21959 cm−1) and Biennier et al. (D2←D0 origin at 14865 cm−1).24,25 The transitions of the Np+-Ar complex are shifted to lower energy with respect to the transitions of the bare molecular ion (D2← D0 origin at 14906 cm−1 ; ref 24), representing an effective increase for the dissociation energy in the excited electronic state (red shift = D0′ − D0″). The shifts of the Np+-Ar and Np+Ar2 D2←D0 origin transitions with respect to the transition of the bare Np+ cation (14906 cm−1; ref 24, 25) are 33 ± 15 cm−1

three molecular cations are similar, and consist of a band system with an intense origin transition at around 650 nm, accompanied by several progressions extending to roughly 520 nm, and a second weaker system with a lone origin band transition near 455 nm. Peak positions for the three species are provided in Table 2. The Np+-Ar photodissociation spectrum is similar to previous gas-phase spectra24,25 and matrix isolation spectra,26,27 and is dominated by the D2←D0 band system, with an intense origin transition at 14873 ± 15 cm−1, accompanied by several 4326

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and 66 ± 15 cm−1, respectively, indicating a linear dependence of the band shift on the number of attached Ar atoms. There are two key differences between our Np+-Ar spectrum and the one reported by Pino et al.25 First, the vibronic bands in our spectrum have widths of ∼80 cm−1, whereas those in the spectrum of Pino et al. have widths of ∼20 cm−1. Second, the D2←D0 vibronic peaks in our spectrum are shifted by ∼10 cm−1 to higher energy with respect to those of Pino et al. The disparities presumably arise from variations in the internal energy of the complexes stemming from different methods of ion preparation. Pino et al. formed internally cold Np+-Ar complexes by two-color resonance-enhanced multiphoton ionization of cold, neutral naphthalene-Ar, so that the peak widths are probably governed primarily by lifetime broadening.25 Our Np+-Ar complexes, on the other hand, were generated through electron impact of an argon supersonic expansion seeded with naphthalene, and therefore probably possess more internal energy, particularly in the intermolecular vibrational modes. Presumably, overlapping hot band transitions involving the intermolecular stretch and bend vibrational modes (s, bx and by), contribute to the broad band profiles and also lead to shifts in the apparent band maxima. Because the frequencies of the intermolecular modes increase in the excited state because of a stronger excited state intermolecular interaction, hot bands of the type s11, s22, bx11, by11, and so forth will generally be shifted to higher energy from the 000 transition. Therefore, as the complexes become hotter, the observed peak, which represents an unresolved collection of hot band transitions, shifts back toward the 000 transition of the bare Np+ cation. A similar dependence of the band shift on ion temperature (diminishing red shift with increasing temperature) has been observed for the B←X transition of the N2+-Ne complex.43 Note however, that the transition energy of the bare Np+ cation (14906 cm−1 ; ref 23) can be recovered from the measured peak positions for the n = 1 and 2 complexes by presuming that the red shift scales linearly with the number of attached Ar atoms. This is important when we come to estimate the band positions for the bare PANH+ cations. The effect of hot bands involving the intermolecular vibrational modes on the apparent center of the electronic transitions of Ar solvated molecules will be largest for transitions that feature a large Ar induced band shift, for which the frequencies of the intermolecular modes undergo a large change upon electronic excitation. Therefore, one can anticipate that the D3←D0 origin transition of Np+ (shift = 20 cm−1) will be less affected than the D2←D0 transition (shift = 43 cm−1) by excitation of the intermolecular modes. It might be argued that the broad band contours found in the current study represent rotational contours. We believe that this is unlikely. Generally, the ion source, which creates ions through electron impact on a supersonic expansion, tends to produce complexes and clusters that are rotationally cold, but which may contain vibrational energy.44 Rotational temperatures of charged complexes usually lie in the 30−50 K range. To assess the likely rotational contour width, we simulated the D2←D0 origin transition band, using the DFT rotational constants for the structure shown in Figure 2, and with the simplifying assumptions of no change of rotational constants (A′ = A″ = 0.034 cm−1, B′ = B″ = 0.027 cm−1, C′ = C″ = 0.021 cm−1) and a transition moment directed along the long axis of the Np molecule (hybrid a-b type transition). At temperatures of 30, 100, and 300 K, the contours had widths of 6, 10, and 15 cm−1, far less than the observed bandwidth (80 cm−1). Tests

showed that the contour width was relatively insensitive to small variations in the upper state constants, although the contour shape did vary. We turn now to a discussion of the PANH+ spectra. Comparisons with the Np+-Ar spectrum and with the TD-DFT calculations allow straightforward assignment of the low and high energy band systems as the D 3 ←D 0 and D 4 ←D 0 transitions, respectively. For i-Qn+-Ar, the D3←D0 and D4← D0 band origins occur at 15245 and 21960 cm−1, whereas for Qn+-Ar they occur at 16050 and 21955 cm−1, respectively. The visible spectrum of Qn+-Ar is similar to the unanalysed spectrum for Qn+ isolated in an argon matrix.27 The D3←D0 origin transition energies also compare well with those derived from PES studies of i-Qn and Qn (∼15150 and 15800 cm−1, respectively).42 The D3←D0 band origin energies for the bare PANH+ species were estimated by recording spectra for the i-Qn+(Ar)2 and Qn+-(Ar)2 complexes. The separation of the D3←D0 band origins for i-Qn+-Ar2 and i-Qn+-Ar is 51 ± 20 cm−1, whereas for Qn+-Ar2 and Qn+-Ar it is 42 ± 20 cm−1, which, assuming equal solvatochromic shifts arising from successive attachment of one and two Ar atoms, allows us to estimate the D3←D0 origin transitions for i-Qn+ and Qn+ as 15296 ± 25 cm−1 and 16092 ± 25 cm−1, respectively. This assumption of solvatochromic additivity holds for the D2←D0 transitions of Np+, Np+-Ar, and Np+-Ar2;24,25 one might expect that a similar additivity for the argon solvated i-Qn+ and Qn+ complexes given their predicted structural similarity to the argon solvated Np+ complexes (Figure 2). Corresponding estimates for the D4←D0 band origins of iQn+ and Qn+, again assuming equal solvatochromic shifts for the first two Ar atoms, are 22004 ± 30 cm−1 and 21992 ± 30 cm−1, respectively. The relatively large error is connected with the poor signal-to-noise ratio of the i-Qn+-Ar2 and Qn+-Ar2 spectra in the D4←D0 region. The TD-DFT adiabatic transition energies reproduce the experimental band origins reasonably well (Table 1) engendering confidence in the spectral assignments and in the quality of the calculations. Furthermore, the TD-DFT oscillator strengths correctly predict that the D4← D0 origin band is more intense for Qn+ than for i-Qn+.

6. VIBRONIC STRUCTURE We now consider the resolved vibrational progressions for the D2←D0 band system of Np+-Ar and the D3←D0 band systems of i-Qn+-Ar and Qn+-Ar . Measured peak positions and proposed assignments for the three species are summarized in Table 2. To help assign the electronic spectra and identify the active molecular vibrations, we generated theoretical spectra using TD-DFT and FC simulations. In Figure 4, the simulated and experimental spectra, for energies up to 2500 cm−1 above the origin, are compared and the main vibrational progressions are indicated. Vibronic features at higher energy are due to multiple overtone/combination excitations whose energies are influenced by anharmonicity and vibrational cross coupling, which are not accounted for in the simulated FC spectra. The vibronic structure in the D2←D0 band system of Np+-Ar has previously been assigned by Pino et al.,25 with reference to the FC simulations of Negri et al.36 and matrix studies of Salama et al.26 and Andrews et al.27 Three FC active ag modes contribute to the spectrum, with the main progressions being formed by the ring deformation mode ν9 at 500 cm−1 and the C−C stretch ν4 mode at 1417 cm−1. Also active is the ring deformation ν8 mode at 750 cm−1. Our experimental and 4327

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The main FC active modes, with estimated experimental and scaled calculated frequencies given in brackets, are ν30 (511/ 496 cm−1), ν27 (795/783 cm−1), and ν13 (1412/1462 cm−1) for i-Qn+ and ν29 (503/508 cm−1), ν27 (735/764 cm−1), and ν14 (1410/1434 cm−1) for Qn+ . The other FC active vibrational modes have similar vibrational motions and frequencies to the three dominant modes. No C−H bending or stretching modes are found to be FC active. It is apparent from Figure 4 that there is a good match between the simulated and experimental spectra for Np+-Ar, i-Qn+-Ar, and Qn+-Ar, particularly in terms of the peak frequencies, although intensities of the weaker transitions are exaggerated in the experimental spectra, possibly a consequence of the stronger transitions being saturated.

7. COMPARISONS WITH ASTRONOMICAL DATA The fact that the PANH cations have absorptions in the visible part of the spectrum that could conceivably correspond to DIBs leads us to consider briefly the astrophysical relevance of the iQn+ and Qn+ ions. Obviously, it is difficult to make definite correspondences between the i-Qn+-Ar and Qn+-Ar transitions and astronomical absorptions because of shifts arising from the attached Ar atoms, and because the observed bands are relatively broad. Nevertheless, the presence of long, strong progressions in the D3←D0 visible band systems of i-Qn+ and Qn+ would seem to discount the possibility of these simple PANH cations being DIB carriers. This follows because there is a poor correlation between the strengths of any pair of DIBs collected from different ISM regions, leading to the expectation that any molecular carriers should have electronic spectra dominated by origin transitions. Nevertheless, completely eliminating i-Qn+ and Qn+ as DIB carriers would require electronic spectra of cold, bare PANH ions rather than the warm, Ar tagged complexes characterized in the current study, as the strong origin and vibronic peaks for both species encompass numerous DIB lines.

Figure 4. D2←D0 band system of Np+ (top), and D3←D0 band systems of i-Qn+ (middle), and Qn+ (bottom). Simulated spectra are for the bare Np+, i-Qn+, and Qn+ ions (red traces) whereas the experimental spectra are for Np+-Ar, i-Qn+-Ar, and Qn+-Ar (blue traces). As described in Section 2.2, peaks in the simulated spectra were assumed to be Gaussians with a full width half-maximum of 80 cm−1. Depictions of the main Franck−Condon active vibrational modes and simulated stick spectra for the three species are presented as Supporting Information.

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theoretical peak positions match well for these modes, although in our spectrum the 810 absorption appears as a shoulder on a peak at +854 cm−1. The vibronic feature at +854 cm−1, which is also apparent in previously reported spectra of Np+ (+845 cm−1 according to ref 25), has eluded convincing assignment, with Salama et al.26 suggesting that it is due to the fundamental of an out-of-plane C−H bending mode. Our FC simulations indicate that the peak is due to the 1220 transition, where the ν12 mode (b2g symmetry) is an out-of-plane C−H bending vibration, and the appreciable FC factor for the transition derives from a frequency reduction from 763 cm−1 in the D0 state to 329 cm−1 in the D2 state. This assignment implies that the scaled calculated ν12 frequency underestimates the experimental value, presumably because the scaling factor uniformly applied to the vibrational frequencies to account for anharmonicity is unsuitable for the low frequency out-of-plane C−H bending mode, which is associated with a flat-bottom potential energy curve and substantial positive anharmonicity. The D3←D0 band systems of i-Qn+-Ar and Qn+-Ar are more congested than the D2←D0 system of Np+-Ar, a consequence of lower symmetry and more FC active modes for the PANHs. For i-Qn+ and Qn+, progressions in the ring deformation and C−C stretching vibrational modes dominate the spectra, with 8 and 9 different vibrational modes of a′ symmetry contributing appreciably to the simulated spectra, respectively.

ASSOCIATED CONTENT

* Supporting Information S

Included are figures illustrating the nature of the optical absorptions, the relevant MOs, and the operative electron promotions of Np+, i-Qn+, and Qn+. Also included are simulated stick spectra for the D2←D0 transition of Np+ and the D3←D0 transitions of i-Qn+ and Qn+, and depictions of the main Franck−Condon active vibrational modes. The full citations for references 3 and 30 are also supplied. This material is available free of charge via the Internet at http:// pubs.acs.org..

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +613 83447082. Fax: +613 9347 5180. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under the Australian Research Council’s Discovery Project funding scheme (Project Number DP0986980). The computational portion of this research was undertaken with the assistance of resources provided at the NCI National Facility through the National Computational 4328

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Merit Allocation Scheme supported by the Australian Government.

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