Experimental Observation and Quantum Chemical Characterization of

Article pubs.acs.org/JPCA

Experimental Observation and Quantum Chemical Characterization of the S1 ← S0 Transition of Protonated Naphthalene−Argon Clusters Alexander Patzer,† Markus Schütz,† Christophe Jouvet,‡ and Otto Dopfer*,† †

Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany Laboratoire de Physique des Interactions Ioniques et Moléculaires (PIIM/UMR CNRS 7345), Aix Marseille Université, Avenue Escadrille Normandie-Niémen, 13397 Marseille cedex 20, France

‡

S Supporting Information *

ABSTRACT: We report on the photodissociation spectrum of protonated naphthalene+−argon complexes (NpH+−Ar) recorded by excitation into the first excited singlet electronic state. Unlike previous electronic spectra of the free molecule (NpH+), both the α and the β isomer could be observed for the Ar adducts. Detailed information on the S0 and S1 state of both isomers is provided by quantum chemical calculations. An assignment of observed vibrational bands is proposed based on Franck−Condon simulations.

1. INTRODUCTION Protonated aromatic hydrocarbon molecules (H+PAH) are of high spectroscopic interest. They occur as intermediates in electrohilic aromatic substitution reactions,1 which can be considered as one of the most fundamental reaction mechanisms of aromatic molecules.2 H+PAH ions are also considered to be present in the interstellar medium and invoked as possible carriers of the unidentified infrared emission (UIR) bands and the diffuse interstellar bands (DIB).3−9 Protonation of PAH in terrestrial and extraterrestrial plasmas occurs efficiently by proton transfer with H+3 , which was spectroscopically identified first in both the laboratory and in space by pioneering work of Oka and co-workers.10,11 The excited states of H+PAH have long only been known from the condensed phase.12 It is only recently that vibrationally resolved neon matrix absorption and fluorescence spectra of (H+PAH) have been published.13−16 Similarly, high-resolution infrared spectra of protonated benzene in an H2 matrix were reported recently.17 Early pioneering gas phase studies have been limited to very low resolution spectra for which the properties of the excited state deduced from the vibrational analysis cannot be obtained.18,19 We have recently shown that the optical spectrum of the free protonated molecule is strongly perturbed by the presence of solvent molecules leading to spectral shifts as large as 0.5 eV.7,20,21 Spectroscopic information to directly determine the most stable protonation sites in isolated H+PAH ions have been lacking until recently because of the difficulties in producing sufficient ion concentrations. Recent progress in the development of sensitive infrared (multiphoton) photodissociation (IR(M)PD) schemes allowed for the first time the spectroscopic characterization of isolated and microsolvated arenium ions in the gas phase through their IR vibrational signature.5,6,22−33 Especially in the case of microsolvated H+PAH, high-level ab initio calculations, preferably taking © 2013 American Chemical Society

anharmonicity into account, are required to interpret spectroscopic data. These have become available only very recently.34 Since distinct conformers of H+PAH are expected to have different electronic spectra, electronic spectroscopy is another option for the characterization of the protonation site. Due to the availability of accurate ab initio methods for excited state calculations, identification of an observed isomer is also possible on the basis of the predicted S1 ← S0 band origins and excited state vibrational structure. Due to recent advances in experimental techniques, vibrationally resolved electronic gas phase spectra are now feasible and have been reported for a number of protonated aromatic molecules and their clusters.8,35−40 In particular, the photodissociation spectrum of protonated naphthalene (NpH+) by Alata et al. represents the first observation of vibrationally resolved electronic absorption spectra of NpH+ in the gas phase.7 The experiment utilized a discharge source for ion production and detection of the neutral photofragments. The NpH+ spectrum shows the band origin of an electronic transition at 503.36 nm (19866 cm−1), which was assigned to the S1 ← S0 transition of α-NpH+ (Figure 1) based on quantum chemical calculations. However, no indication of the slightly less stable β-NpH+ isomer was found. Previously reported IR(M)PD spectra of the free molecular ion and its Ar clusters also did not show clear spectroscopic evidence for the presence of β-NpH+, although minor contributions could not be ruled out.28,33 Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 20, 2012 Revised: February 19, 2013 Published: March 8, 2013 9785

dx.doi.org/10.1021/jp312581v | J. Phys. Chem. A 2013, 117, 9785−9793

The Journal of Physical Chemistry A

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the ion source. The molecular beam passes through a skimmer to select the colder central part of the plasma expansion. First, a quadrupole mass spectrometer selects the ionic NpH+-Ar clusters. The mass-selected cluster ion beam is then directed into an octopole ion guide, where it is irradiated by a tunable visible laser pulse. Resonant excitation of the S1 ← S0 transition in NpH+−Ar (m = 169 u) leads to fragmentation into NpH+. Fragment ions with mass m = 129 u are then selected by a second quadrupole and sensed by a Daly detector. Thus, the S1 ← S0 excitation spectrum of NpH+−Ar is recorded by monitoring the NpH+ fragment ion signal. Tunable laser radiation is generated by an optical parametric oscillator (OPO) or a dye laser. For fast wavelength scans, the OPO laser with a bandwidth of 4 cm−1 is used. After identification of the required spectral range, higher resolved spectra are recorded using a dye laser with a bandwidth of 0.04 cm−1. An average laser energy of 200 μJ/pulse is used to record the spectrum. Both tunable lasers are pumped by a Q-switched nanosecond Nd:YAG laser operating at 20 Hz. Wavelength calibration of the dye laser is accomplished by simultaneous acquisition of an iodine absorption spectrum and comparison to a reference spectrum.44

Figure 1. Structures of the three stable NpH+ isomers with employed atom numbering.

In this paper we present the electronic photodissociation spectrum of the S1 ← S0 transition of the NpH+−Ar complex. Good agreement between the experimental spectra and ab initio calculations allow for a firm assignment of the electronic spectra to the two most stable NpH+ isomers. These are denoted as α and β isomers, depending on whether the protonation occurs on the carbon atom C1 or C2, respectively (Figure 1). In the case of the NpH+−Ar cluster, the Ar atom can be located at different positions with respect to the molecule. Although these ligand binding sites are energetically not equivalent, differences in binding energy are small, and the exact Ar binding site is less certain than the determination of the protonation site.

3. COMPUTATIONAL TECHNIQUES Detailed analysis of the electronic ground state potential energy surface of the NpH+ monomer has been published previously.33 The reported energetic sequence at the B3LYP/6-311G (2df,2pd) level is α < β ≪ γ with energy differences of 12 and 80 kJ/mol relative to the global minimum structure (α isomer). We therefore restrict our considerations to the α and β isomers. Quantum chemical density functional calculations including dispersion corrections (DFT-D) and ab initio calculations are used to identify the most stable cluster structure in the electronic ground state. There are eight possible local minima in the case of a NpH+−Ar cluster with Cs symmetry for both αand β-NpH+−Ar (i.e., the Ar is located in the aromatic plane). The binding position of the Ar atom is denoted as (i,j) in this case, with i and j being the two closest carbon atoms. To quickly identify the most stable structures with respect to the Ar binding site, geometry optimizations are carried out at the B3LYP-D/def2-QZVP level of theory for the clusters with Cs symmetry as well as for the π bound structures with no symmetry, where the Ar is located above the ring containing the excess proton. The resolution-of-the-identity approximation (RI) is used to reduce computational costs for the DFT-D as well as the CC2 calculation. The charge distribution is analyzed using the natural bond orbital (NBO) analysis. To characterize the S1 ← S0 transition, full geometry optimization and calculation of harmonic vibrational frequencies in both electronic states is performed at the RI-CC2/ccpVTZ level for the two most stable structures found by the DFT-D calculation. To account for its high polarizability, the aug-cc-pVTZ basis set is used for the Ar atom. Although not of spectroscopic quality, this computational method has proven to be a reliable and very efficient tool to characterize the S1 ← S0 transition in small protonated aromatic molecules with nonpolar ligands.8,20,36,45 All quantum chemical calculations are performed using version 6.1 of the TURBOMOLE program package.46 Franck−Condon simulations of the S1 ← S0 transition are carried out using PGOPHER47 on the basis of the unscaled harmonic vibrational frequencies obtained from the CC2 calculation.

2. EXPERIMENTAL TECHNIQUES Electronic photodissociation spectra are recorded in a tandem quadrupole mass spectrometer.21,36,41−43 The molecular beam is produced by expanding the carrier gas seeded with naphthalene into a vacuum chamber. Stagnation and residual gas pressures in the source chamber are 10 bar and 10−5 mbar, respectively. The carrier gas consists of an Ar, He, and H2 mixture in a 100:95:5 ratio. The low H2 concentration of 2.5% is sufficient to produce protonated naphthalene in a concentration comparable to that of the nonprotonated species. The naphthalene sample from Sigma Aldrich with purity ≥99% is used without further purification. The adiabatic expansion is followed by electron ionization, producing Np+ as well as NpH+ cations. NpH+−Ar clusters are generated by subsequent threebody aggregation. Figure 2 shows a typical mass spectrum of

Figure 2. Mass spectrum of the ion source. The inset shows the spectrum in the mass range of the clusters of interest with increased detector sensitivity (x15). 9786



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Figure 3. Experimental overview photodissociation spectrum of NpH+−Ar recorded with the dye (a) and OPO laser (b).

4. RESULTS AND DISCUSSION Figure 3 shows the vibrationally resolved photodissociation spectrum of the NpH+−Ar dimer. Spectra for larger NpH+−Ln

the formation process of the protonated ions in the Orsay setup,7 where the ions are produced by a pulsed discharge occurring 2−3 mm down stream of a pulsed valve. Under these conditions, it has been possible to record vibrationally resolved spectra of the free molecule, but the complexes with Ar could not be obtained. The Franck−Condon analysis indicates that this discharge source produces ions at around 100−200 K,8 which may be too hot to stabilize the Ar complex. The calculations at the B3LYP-D/def2-QZVP level reveal nine local minima for both the α and β isomers of NpH+−Ar. In the case that the Ar is π bonded, the calculations converge to a structure in which the Ar is located above the ring containing the excess proton. This structure is denoted as NpH+−Ar(π). Attempts to find a stable structure where the Ar is located above the nonprotonated ring converge to the NpH+−Ar(π) structure. The latter can be rationalized by a NBO charge analysis, which is carried out on basis of the structures obtained at the CC2 level (vide infra). The DFT-D calculation shows a clear preference for π binding of the Ar atom to the molecule with a binding energy De on the order of 600 cm−1. The clusters in which the Ar atom binds in the aromatic plane, denoted as NpH+−Ar(i,j), are significantly less stable (De about 370−500 cm−1). A similar conclusion was recently derived from highlevel ab initio calculations for Ar complexes of protonated benzene.34 Table 1 lists the binding energies (De) of different NpH+−Ar clusters for the α and β isomer. The relative energy difference of the α and β isomers of the NpH+ monomer in the electronic ground state calculated at the RI-CC2/cc-pVTZ level including zero point correction amounts to 12.5 kJ/mol and agrees well with the reported value.33 The corresponding minima in the S1 state of the α and β isomer exhibit the same energetic sequence as in the S0 state, but the difference reduces to 1.1 kJ/mol. This change in the energetic difference equals a predicted 952 cm−1 redshift of the β-NpH+ band origin compared to α-NpH+. The calculated adiabatic excitation energy for the α monomer of 19929 cm−1 agrees well with the observed transition at 19866 cm−1.7 For βNpH+, an adiabatic excitation energy of 18977 cm−1 is calculated. This value agrees well with the observed band

Table 1. Binding Energies of Different NpH+−Ar Cluster Isomers Evaluated at the B3LYP-D/def2-QZVP Level isomer

binding position

α

π 1,2 3,4 4a 8a 2,3 7,8 5,6 6,7 π 1,2 8a 2,3 4a 3,4 5,6 7,8 6,7

β

De/cm−1 (kJ/mol) −623 −480 −454 −451 −445 −433 −392 −387 −374 −595 −497 −462 −449 −406 −404 −403 −399 −390

(7.5) (5.7) (5.4) (5.4) (5.3) (5.2) (4.7) (4.6) (4.5) (7.1) (5.9) (5.5) (5.4) (4.9) (4.8) (4.8) (4.8) (4.7)

clusters with L = Ar (n ≤ 4) and L = N2 (n ≤ 3) have also been recorded. These have, however, not been analyzed in detail due to their substantially lower spectral resolution (Figure S3). The NpH+−Ar spectrum shows a first peak at 18745 cm−1, which is attributed to the band origin of the S1 ← S0 transition of βNpH+−Ar. The corresponding origin of α-NpH+−Ar is observed at 19860 cm−1. This means that the Ar tagging introduces a red shift of only 6 cm−1 from the origin of the free ion (19866 cm−1),7 confirming that the monomer spectrum is only slightly influenced by the weakly bound Ar ligand. The spectra obtained for α-NpH+−Ar are quite different from the ones obtained previously for the free molecular ion,7 which were recorded in a different setup. This may be due to 9787



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Table 2. Structural Parameters of α-NpH+ and β-NpH+ in the S0 and S1 Electronic State α S0 Ci,j C−C bond length

1,2 2,3 3,4 4,4a 4a,5 5,6 6,7 7,8 8,8a 4a,8a 8a,1

circumference 8a,1,2 1,2,3 2,3,4 3,4,4a 4,4a,8a 4a,8a,1 8a,4a,5 4a,5,6 5,6,7 6,7,8 7,8,8a 8,8a,4a

S0 Δri,j/mÅ

ri,j/Å 1.469 1.371 1.409 1.405 1.419 1.379 1.409 1.394 1.393 1.428 1.486 14.132 117.0 121.9 119.3 123.4 119.0 119.3 120.6 119.5 119.7 121.5 119.9 118.8

S1 Δri,j/mÅ

ri,j/Å

1.489 1.388 1.385 1.468 1.399 1.442 1.379 1.402 1.450 1.392 1.488 14.290

20 17 −24 63 −20 63 −30 8 57 −36 3 157 Δ∠i,j,k/°

1.461 1.473 1.359 1.431 1.402 1.389 1.416 1.374 1.428 1.447 1.381 14.114

114.5 122.2 122.1 119.0 120.8 121.4 117.1 122.7 120.4 117.3 122.5 119.9

−2.6 0.3 2.8 −4.4 1.8 2.1 −3.5 3.3 0.7 −4.2 2.6 1.1

122.1 116.8 121.0 121.0 120.3 118.8 118.0 120.3 121.9 119.7 119.6 120.5

∠i,j,k/°

Ci,j,k C−C−C angle

β

S1

1.485 1.491 1.374 1.430 1.415 1.424 1.374 1.439 1.387 1.414 1.438 14.258

24 18 15 −1 13 35 −42 65 −41 −33 57 144 Δ∠i,j,k/°

121.9 113.2 123.6 121.4 119.3 120.6 119.8 121.7 118.1 120.3 121.9 118.1

0 −4 3 0 −1 2 2 1 −4 1 2 −2

∠i,j,k/°

The circumference is calculated as the sum of all C−C bonds excluding the 4a,8a bond. Δri,j and Δ∠i,j,k refer to the difference of the bond length and bond angle in the S1 and S0 state, respectively.

Table 3. NBO Charge Distribution of α-NpH+ and β-NpH+ in the S0 State ring (element)

α-NpH+

β-NpH+

1 (C) 1 (H) 2 (C) 2 (H) total 1 2

−0.51 1.20 −0.56 0.87

−0.56 1.20 −0.52 0.88

0.69 0.31

0.64 0.36

Given is the sum of individual charges of all H and all C atoms in the ring containing the excess proton (ring 1) and ring 2. The charges of the C atoms 4a and 8a are equally distributed over the two rings.

intramolecular modes. Interestingly, the structural changes depend largely on the protonation site. Therefore, the vibrational structure of the S1 ← S0 transition is very different for both NpH+ isomers. In general, electronic excitation leads to an increase of the circumference of both isomers. Only the NpH+−Ar(π) and NpH+−Ar(1,2) cluster isomers are considered in the CC2 calculations, as they are predicted to be the most stable ones by the DFT-D calculations. Stable structures as shown in Figure 4 for NpH+−Ar(π) and NpH+− Ar(1,2) are found in the electronic ground and excited states. In the π bound clusters, the Ar ligand is located above the ring with the excess proton. The NBO charge analysis carried out on the basis of the monomer structures (Table 3) shows that most of the charge (0.69 e for α-NpH+ and 0.64 e for β-NpH+) is located on the ring containing the excess proton, which

Figure 4. Structures of the NpH+−Ar(π) and NpH+−Ar(1,2) isomers with α-NpH+ and β-NpH+ in the S0 state obtained at the RI-CC2/ccpVTZ level. Distances are given in Å.

origin at 18745 cm−1. Hence, the calculations fully support the assignments of the S1 band origins given in Figure 3. Table 2 lists important structural parameters (bond lengths and bond angles) for both NpH+ isomers in both electronic states. Several bond lengths and bond angles change drastically, giving rise to large Franck−Condon activity of several 9788



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Table 4. Vibrational Frequencies (cm−1) in the S0 and S1 State Obtained at the RI-CC2/cc-pVTZ Level α +

NpH N

S0

in-plane 1 3244 2 3241 3 3223 4 3213 5 3210 6 3208 7 3190 8 3006 9 1648 10 1594 11 1579 12 1548 13 1507 14 1466 15 1453 16 1438 17 1371 18 1324 19 1292 20 1262 21 1216 22 1189 23 1181 24 1148 25 1108 26 1041 27 974 28 916 29 789 30 745 31 593 32 497 33 491 34 345 a b out-of-plane 35 3029 36 1159 37 1022 38 1007 39 1002 40 978 41 908 42 851 43 782 44 727 45 657 46 473 47 424 48 391 49 229 50 170 51 103 c d e

β

NpH −Ar(π)

+

NpH −Ar(1,2) +

0

NpH

+

NpH −Ar(π)

NpH+−Ar(1,2)

+

S1

S0

S1

S

S1

S0

S1

S0

S1

S0

S1

3245 3223 3223 3208 3202 3198 3194 2980 1629 1590 1537 1497 1471 1436 1408 1387 1381 1349 1265 1216 1201 1177 1166 1103 1081 1027 948 897 772 732 586 489 465 336

3243 3240 3223 3212 3209 3207 3189 3001 1648 1593 1579 1548 1507 1466 1453 1438 1371 1324 1291 1262 1216 1188 1181 1148 1108 1041 973 916 789 745 593 497 491 345

3245 3223 3221 3207 3201 3197 3193 2978 1628 1592 1537 1496 1472 1436 1408 1387 1381 1349 1266 1217 1201 1177 1166 1103 1080 1027 948 897 772 732 586 490 465 336

3244 3240 3223 3213 3213 3207 3190 3006 1647 1593 1579 1547 1507 1465 1452 1438 1371 1318 1292 1262 1216 1189 1179 1149 1108 1041 973 916 789 746 593 497 492 347 49 26

3245 3223 3223 3210 3203 3197 3193 2980 1628 1590 1537 1496 1471 1436 1408 1383 1379 1348 1264 1216 1201 1177 1166 1103 1080 1027 948 897 772 733 586 489 465 337 47 26

3241 3230 3225 3214 3213 3209 3191 2992 1649 1614 1566 1530 1517 1466 1457 1424 1373 1307 1287 1263 1243 1194 1169 1150 1049 1035 938 924 775 748 601 502 491 351

3245 3228 3215 3203 3200 3197 3183 2983 1708 1604 1523 1490 1460 1428 1409 1395 1368 1332 1284 1243 1213 1184 1173 1071 1041 1021 908 903 770 755 572 497 479 340

3240 3229 3225 3213 3212 3209 3190 2991 1649 1613 1566 1530 1517 1466 1457 1424 1373 1306 1286 1262 1243 1194 1168 1150 1049 1035 939 924 775 748 601 502 491 350

3244 3227 3215 3203 3200 3197 3182 2983 1708 1604 1522 1491 1459 1428 1408 1395 1368 1333 1284 1242 1213 1184 1172 1071 1040 1021 908 898 770 755 572 497 479 340

3241 3230 3225 3214 3213 3209 3195 2992 1649 1614 1566 1529 1517 1465 1457 1424 1372 1305 1282 1262 1243 1194 1169 1150 1047 1035 939 924 775 749 601 502 491 352 49 27

3245 3227 3215 3203 3199 3197 3188 2982 1708 1604 1523 1490 1460 1428 1407 1395 1367 1333 1284 1243 1213 1184 1173 1071 1040 1021 909 903 770 755 572 497 479 342 48 28

2994 1157 979 975 953 889 878 742 727 627 524 452 375 325 223 131 94

3033 1158 1019 1004 1000 976 905 849 780 725 654 470 424 390 229 172 115 61 42 23

2997 1158 975 973 952 885 877 740 724 626 521 450 373 324 227 134 106 63 41 23

3028 1156 1021 1006 1001 977 907 850 780 726 657 473 424 391 231 170 106 16

2993 1155 977 974 953 888 878 741 726 627 525 452 375 325 225 133 96 20

3007 1142 1008 1000 987 941 896 800 784 766 658 474 436 367 231 167 130

2992 1165 975 968 945 901 880 750 741 647 608 443 380 330 217 145 43

3007 1141 1007 996 985 938 894 797 782 762 655 472 434 365 226 168 134 64 41 21

2992 1165 973 964 944 902 877 745 740 643 604 443 379 328 219 145 70 60 40 25

3005 1139 1008 999 986 941 895 798 783 765 657 474 437 367 235 165 133 19

2991 1164 975 967 945 900 879 749 740 646 608 443 380 330 217 144 53 15

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Table 4. continued The usual vibrational mode numbering convention is applied to the monomers. The intermolecular modes arising from the Ar ligand are treated separately and labeled a−e.

Table 5. Excitation Energies (S1), Band Origin Shifts Relative to the Monomer (ΔS1) of the S1 ← S0 Transition, and Dissociation Energies of the Ar Clusters in the S0 and S1 State (D0) DS00 α-NpH+ β-NpH+ α-NpH+−Ar(π) α-NpH+−Ar(1,2) β-NpH+−Ar(π) β-NpH+−Ar(1,2)

902 467 879 484

DS01

Scalc 1

1001 434 951 457

19929 18977 19830 19962 18904 19004

ΔScalc 1

Sexp 1

ΔSexp 1

19866 −99 33 −72 27

19860

−6

18745

Calculated values are obtained at the RI-CC2/cc-pVTZ level. All values are given in cm−1. The experimental value for α-NpH+ is taken from ref 8. Figure 7. Experimental spectrum of NpH+−Ar (a) in the vicinity of the band origin of the α-NpH+−Ar isomer. Spectra b and c show simulations of β-NpH+−Ar(π) and β-NpH+−Ar(1,2) corresponding to peak 6. Spectra d and e are simulations of the band origin for the αNpH+−Ar(π) and α-NpH+−Ar(1,2) isomers, respectively.

Figure 5. Comparison of experimental photodissociation spectrum of NpH+−Ar and Franck−Condon simulation of the α-NpH+ (red) and β-NpH+ (blue) monomers. Only intramolecular modes are included. The assignment of labeled peaks is listed in Table 6.

Figure 8. Experimental spectrum of NpH+−Ar (a) in the vicinity of the intramolecular 3210 (peak 23) and 3110 (peak 26) transitions of the α-NpH+−Ar isomer. Spectra b and c are simulations of the α-NpH+− Ar(π) and α-NpH+−Ar(1,2) isomers, respectively.

explains the preference for Ar binding above this ring. A table containing the NBO charges for all individual atoms of α-NpH+ and β-NpH+ is provided in the Supporting Information (Table S1). An electrostatic potential map of α-NpH+ and β-NpH+ is also available in the Supporting Information (Figures S1 and S2). All structures in Figure 4 are confirmed to be true minima by calculating harmonic vibrational frequencies in the S0 and the S1 state. A full list of calculated vibrational frequencies including the notation used for the assignment of the transitions is available in Table 4. Calculated electronic excitation energies including zero point energy corrections are compared in Table 5 with the experimentally observed transition energies. In general, there is almost quantitative agreement between the measured and calculated S1 origins, which allows for a firm assignment of the protonation sites.

Figure 6. Experimental spectrum of NpH+−Ar (a) and simulated spectra of β-NpH+−Ar(π) (b) and β-NpH+−Ar(1,2) (c) in the vicinity of the band origin of the β-NpH+−Ar isomer. The assignment of labeled peaks is listed in Table 6.

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Table 6. Assignment of Spectral Features of the Measured Photodissociation Spectrum of NpH+−Ar Based on the Franck−Condon Simulations for the π Isomers ν/cm−1

isomer

assignment

Δνexp

Δνcalc

1 2 3 4 5 6 7 8

18745 19146 19234 19325 19582 19716 19730 19784

β β β β β β β β

000 33105101* 3310 1 2 330510; 3110 33103410 3320 32103310 1 250; 33105120; 31103310

0 401 489 580 837 971 985 1039

0 436 479 565; 572 819 958 976 1041; 1044; 1051

9 10 11 12 13 14 15 16 17 18

19806 19823 19844 19860 19867 19877 19896 19917 20063 20201

α α α

5111* m−1 n+1 dm e n 000

−36 −15 0

−38 −18 0

α α α β α or β

20209

α or β

20

20265

β

18 37 58 1318 342 (α); 1456 (β) 350 (α); 1464 (β) 1520

19

19

n−1 50105101*; dm+1 m en * cn+1 n 33203410 3410 (α) or 3330 (β) 1 340 (α) or 3330 (β) 25103310; 33305120; 31103320

21 22 23 24 25 26 27 28 29

20315 20335 20351 20359 20368 20446 20454 20459 20464

30

20583

α and β

31 32

20624 20677

α α and β

33

20693

α

63 1298 336 (α); 1437 (β) 336 (α); 1437 (β) 1520; 1523; 1530

α

3210

492

489

α

3110

587

586

α

311050105110*; m+1 3110en−1 n dm 1 300 (α); 17103310 (β) 2910 311034105111* (α); 3340 (β) 32103410

605

605

724(α); 1838(β) 765 1932

732(α); 1847(β) 772 1916

834

825

Figure 9. The lower traces show the measured photodissociation spectrum of NpH+−Ar. The upper traces are the same experimental spectrum shifted by 342, 491, 587, and 834 cm−1, illustrating the repetitive pattern of the α-NpH+−Ar band origin. The latter is marked with a filled circle.

and the most intense intramolecular modes. These simulations are carried out for the NpH+−Ar(π) and NpH+−Ar(1,2) structures, respectively, and are depicted in Figures 6−8. In these simulations, a temperature of 150 K was used and the stick spectra were convoluted with a Gaussian line profile of 2 cm−1 line width to account for unresolved rotational structure. The temperature is obtained from Franck−Condon simulations carried out for temperatures of 50−300 K in steps of 50 K. These simulations are available in the Supporting Information (Figures S4−S7). The intensity ratio of peaks 12 and 14 is determined to be 3.9 from experiment and is in good agreement with the value obtained from the simulation of αNpH+ with a temperature of 150 K (3.5). The intensity ratios for other temperatures are listed in Table S2 in the Supporting Information. The simulations are used to propose an assignment of the observed vibrational transitions in the S1 state. A full list of peak positions and a possible assignment of the observed spectral features is available in Table 6. In general, the Franck−Condon simulations fully support the assignment of the S1 origins and intramolecular transitions of the α-NpH+− Ar and β-NpH+−Ar isomers. By contrast, the simulation of the intermolecular structure is less clear and does not allow for an unambiguous assignment to a single Ar binding site. For example, in the case of α-NpH+, the strongest intramolecular modes in the experimental spectrum exhibit a repetitive pattern with a spacing of 8 and 18 cm−1, which is also found on the band origin (Figure 9). Unfortunately, the Franck−Condon simulation does not reproduce this pattern in all details. Nonetheless, the simulations for the most stable intermolecular α-NpH+−Ar(π) isomer show the largest similarity to the

Bands marked with an asterisk include a larger number of hot band transitions between the low-lying intermolecular modes. Only the corresponding intramolecular mode is listed in this case. Δν refers to the wavenumber difference of the observed peak and the band origin to which the peak is assigned.

However, as the predicted Ar complexation shifts are small, the comparison of the S1 origins in Table 5 does not allow for a reliable assignment of the Ar binding site. Figure 5 shows the comparison between the measured photodissociation spectrum of NpH+−Ar and the Franck− Condon simulation of intramolecular vibrational modes of the α-NpH+ and β-NpH+ monomers based on the harmonic frequency calculation at the RI-CC2/cc-pVTZ level. This simulation does not include the population of vibrationally excited states in the electronic ground state, i.e., the vibrational temperature is 0 K. Moreover, the calculated S1 origins are shifted to the observed ones. Additional simulations of the intermolecular modes involving the Ar atom are performed in the vicinity of the band origins 9791



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experimentally observed pattern, suggesting that the π binding site for Ar is probably the dominant intermolecular binding motif, in line with the calculated binding energies. However, the simulations also indicate substantial contributions of the slightly less stable isomers to the experimental spectrum. Figure S8 in the Supporting Information compares the NpH+−Ar spectrum with DIBs absorption spectra reported in the literature.48−50 It was already known that there is no coincidence between the α-NpH+ spectrum and the reported DIBs.7 The same is true for β-NpH+. This conclusion is consistent with the previous report that the linear protonated PAHs up to tetracene also do not show overlap with DIBs.7,8,51 On the other hand, for H+PAH to be photostable in the interstellar medium, they should be in the size range larger than 20 carbon atoms.

and Np+−Ar (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org/.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Technische Universität Berlin, Deutsche Forschungsgemeinschaft (DO 729/3), and the DAAD PROCOPE program (D0707510). C.J. acknowledges financial support from the PROCOPE 17832NK program.

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5. CONCLUSIONS The present work reports the first electronic spectrum of NpH+−Ar clusters. It is obtained in the visible spectral range by resonant photodissociation spectroscopy of mass-selected ions. The analysis is complemented by DFT-D, ab initio, and Franck−Condon calculations. A firm assignment of the majority of the observed intramolecular vibrational modes was proposed on the basis of harmonic frequency calculations. In contrast to previous spectroscopic studies in the gas phase, there is unambiguous evidence for the presence of the β-NpH+ isomer, which has escaped from spectroscopic clear-cut observation so far. Since there is very good agreement between the spectrum of α-NpH+7 and α-NpH+−Ar, it can be assumed that spectral shifts due to the Ar tagging are negligible in the case of β-NpH+ as well, and the presented Ar tagged spectrum closely resembles the one of the β-NpH + monomer. Furthermore, an assignment of intermolecular vibrational modes is proposed. Although the binding position of the Ar atom cannot be settled with the same certainty as the protonation sites, there is spectroscopic indication that π bonding of the Ar atom to the NpH+ ion is the preferred intermolecular binding motif. The latter is in accordance with the prediction of the energetic sequence from quantum chemical calculations, favoring a π bond to the aromatic ring containing the excess proton over H-bonding sites within the aromatic plane. In line with the hypothesis that small H+PAHs are not considered to be potential carriers of DIBs, both αNpH+ and β-NpH+ do not show coincidences between their photodissociation spectrum and the DIBs. After completion of this work, two publications appeared which report the detection of the α and β isomers of NpH+ by IR and electronic spectra recorded in Ne and H2 matrices.52,53 These reports are consistent with the interpretation of the current work.

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AUTHOR INFORMATION

REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Full NBO charge analysis for α-NpH+ and β-NpH+ (Table S1), electrostatic potential maps of α-NpH+ (Figure S1) and βNpH+ (Figure S2), photodissociation spectra of NpH+−Arn and NpH+−(N)2n (Figure S3), Franck−Condon simulations of α-NpH+−Ar and β-NpH+−Ar for different temperatures (Figure S4−S7), intensity ratios of peak 12 and 14 obtained from the Franck−Condon simulations (Table S2), a plot of the spectrum of NpH+−Ar compared to DIBs (Figure S8), and a comparison of the photodissociation spectrum of NpH+−Ar 9792



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