Cation Vibrations of 1-Methylnaphthalene and 2-Methylnaphthalene

Article Cite This: J. Phys. Chem. A 2019, 123, 5969−5979

pubs.acs.org/JPCA

Cation Vibrations of 1‑Methylnaphthalene and 2‑Methylnaphthalene through Mass-Analyzed Threshold Ionization Spectroscopy Published as part of The Journal of Physical Chemistry virtual special issue “Hai-Lung Dai Festschrift”. Sheng Yuan Tzeng, Vidya S. Shivatare, and Wen Bih Tzeng*

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Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, 1 Section 4, Roosevelt Road, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: Vibrationally resolved cation spectra of 1-methylnaphthalene (1MN) and 2methylnaphthalene (2MN) were obtained using the two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy. MATI spectra were obtained through ionization via several intermediate vibronic levels. Due to hindrance effect, no spectral features related to methyl torsion were observed in the MATI spectra of 1MN. By contrast, most of the in-plane ring deformation vibrations of the 2MN cation were found to couple with methyl torsion because of its small internal rotational barrier. These experimental findings were well supported by our theoretical calculations.

Kawamura10 employed rotationally resolved LIF and theoretical calculations to investigate the torsional dynamics of the methyl group. Chalyavi et al.11 used resonant two-color (2C) two-photon excitation and dispersed fluorescence spectroscopy techniques to obtain the excitation and emission spectra of jetcooled 1-naphthylmethyl radicals produced using a pulsed discharge nozzle. A crucial step to investigating the ionic properties of a molecule is to determine its ionization threshold. As determined using electron impact ionization, charge transfer, photoelectron, and photoionization methods, the ionization energies (IEs) of 1MN and 2MN range from 7.8 to 8.5 eV.12−18 Friha et al.19 used argon tagging to obtain the photodissociation spectra of the 1MN+-Ar, 1MN+-N2, 2MN+Ar, and 2MN+-N2 cations and obtained information on several vibrational modes involved in the D2 ← D0 electronic transition of these two cationic species. In addition, the authors concluded that, because of steric inhibition, the conformation of the 1MN cation does not change on electronic excitation. By contrast, the 2MN cation undergoes a conformational change, leading to a specific vibrational progression. More detailed cation spectra of 1MN and 2MN are required. Resonance-enhanced multiphoton ionization (REMPI) spectroscopy can be employed to obtain vibronic spectra and information similar to that obtained using the excitation LIF

1. INTRODUCTION Polycyclic aromatic hydrocarbons and their derivatives are atmospheric pollutants that may have adverse effects on human health.1 These compounds are emitted into the atmosphere from various incomplete combustion sources. Numerous spectroscopic experiments have been conducted to determine the photochemical properties of naphthalene and its derivatives.2−6 In methylnaphthalene, electrons in the σ orbitals of the methyl group interact with the π orbitals of the aromatic ring through hyperconjugation. Consequently, the molecular geometry and normal vibrations of methylnaphthalene differ from those of naphthalene. Some molecular properties of 1-methylnaphthalene (1MN) and 2-methylnaphthalene (2MN) in the ground state S0 and electronically excited state S1 have been extensively investigated using various spectroscopic techniques and theoretical calculations.7−11 Warren et al.7 reported the supersonic jet excitation laser-induced fluorescence (LIF) spectra of naphthalene, 1MN, and 2MN. However, they did not attempt to assign the observed vibrational bands. On the basis of their single vibronic level (SVL) dispersed fluorescence data, they suggested that vibrationally induced mode mixing in S1 was responsible for the observed intramolecular energy redistribution in 2MN. Jacobson et al.8 also performed excitation LIF and SVL dispersed fluorescence experiments to investigate jetcooled fluoronaphthalene, chloronaphthalene, and methylnaphthalene. The authors considered the substituent effect and tentatively assigned the detected vibronic bands of these substituted naphthalenes. Tan et al.9 and Nakai and © 2019 American Chemical Society

Received: April 22, 2019 Revised: June 25, 2019 Published: June 25, 2019 5969

DOI: 10.1021/acs.jpca.9b03756 J. Phys. Chem. A 2019, 123, 5969−5979

Article

The Journal of Physical Chemistry A method.20−22 Most substituted benzenes and naphthalenes have an IE in the range of 6.5−9.0 eV.12 One-color resonant two-photon ionization (1C-R2PI) can be employed to obtain a vibronic spectrum if the energy of the S1 ← S0 electronic transition (E1) is higher than one-half of the IE. However, the 2C-R2PI scheme must be used to obtain the vibronic spectrum if E1 is lower than one-half of the IE, as is the case for naphthalene.3 Both the zero kinetic energy (ZEKE) photoelectron 23−25 and mass-analyzed threshold ionization (MATI)26−28 spectroscopic techniques are invaluable for obtaining the ionic spectra of molecules with a spectral resolution of a few wavenumbers. These two methods involve excitation of the long-lived high n Rydberg neutrals and their subsequent ionization by a delayed pulsed electric field, leading to formation of threshold ions in well-defined energy states.29,30 The LIF and ZEKE methods detect photons or electrons with high sensitivity and spectral resolution. By contrast, the REMPI and MATI approaches detect ions and provide unambiguous mass information. In this study, we used the 2C-R2PI and MATI techniques to obtain vibronic and cation spectra and prevent spectral congestion caused by impurities, complexes, and clusters. In particular, 2C resonant two-photon MATI spectra of 1MN and 2MN were obtained through ionization via several intermediate levels. Detailed analysis of these spectra yielded information about the active vibrations of the molecules in the electronically excited state S1 and cationic ground state D0 in addition to E1 and the adiabatic IE. Comparing the collected data of 1MN and 2MN with those of naphthalene and naphthalene derivatives2,3,8,31−33 provided insight into the methyl substitution effect on the transition energy and molecular vibration.

states for the neutral species as well as the electronic ground state for the cation. To explore the excited state, we used the time-dependent (TD) variant42,43 of the aforementioned functional, TD-B3PW91, whereas for cation calculations, we used the unrestricted variant, UB3PW91. All theoretical calculations were performed using the 6-311++G(d,p) basis set44−46 and by using the Gaussian 16 package.47 The configuration interaction singles (CIS) and TD B3PW91 calculations were performed to predict the vibrational frequencies of S1. The scaling factors were applied to correct combined errors caused by basis set incompleteness, the neglect of electron correlation, and vibrational anharmonicity. The calculated vibrational frequencies were in favorable agreement with the values measured in the vibronic spectroscopic experiments. The vibrational frequencies of D0 were calculated at the unrestricted Hartree−Fock (UHF) and B3PW91 levels. The adiabatic IE was deduced from the difference in the zero-point energy (ZPE) levels of the cation in D0 and the corresponding neutral species in S0. The spin multiplicity was 2, and the expectation value of the operator ⟨S2⟩ was 0.75 for the cation. Simulated MATI spectra were obtained by calculating Franck−Condon factors from the predicted harmonic vibrational frequencies and geometries of the molecule in S1 and D0.

3. RESULTS 3.1. Measured and Simulated Vibronic Spectra of 1MN. Considering the reported E1 and IE,7−9,12−18 we performed the 2C-R2PI experiment by scanning the excitation laser while fixing the ionization laser at 307.84 nm (32 484 cm−1). As illustrated in Figure 1a, the band origin of the S1 ← S0 electronic transition (E1) of 1MN appears at 31 771 ± 2 cm−1, which is in excellent agreement with that reported

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Experimental Method. All experiments reported herein were performed using a photoionization time-of-flight (TOF) mass spectrometer equipped with two tunable ultraviolet (UV) lasers, as described elsewhere.34,35 Detailed description is included in Supporting Information. Briefly, 1MN and 2MN (both 99% purity) were purchased from Sigma-Aldrich Corporation and used without further purification. The 2C-R2PI, photoionization efficiency (PIE), and MATI experiments were performed using two independent tunable UV lasers. The wavelengths of the obtained spectra were calibrated by conducting high-resolution electronic spectroscopic experiments for aniline-NH2, aniline-NHD, and aniline-ND2.36,37 The spatial overlap of the molecular beam with the excitation and laser beams was crucial in these 2C resonant two-photon spectroscopic experiments. The laser beam waist was measured using retarding potential analysis.38 The spatial widths of the counterpropagating excitation and ionization UV lasers were controlled at ∼0.5 and 1.0 mm, respectively.34 In the MATI experiment, the spoiling field strength was 1.0 V·cm−1. This caused the measured IE to be lower than the true adiabatic IE by 4 cm−1 because of the Stark effect.29,30,39 The signal from the ion detector was accumulated and analyzed using a multichannel scaler (MCS, Stanford Research Systems, SR430), which was interfaced to a personal computer (PC). 2.2. Computational Method. To analyze the vibronic and cation spectra, we performed density functional theory (DFT) calculations using the B3PW91 functional.40,41 We optimized the geometries for both the electronic ground and excited

Figure 1. Vibronic spectra of 1MN (a) obtained in the 2C-R2PI experiment and (b) simulated using B3PW91/6-311++G(d,p) calculation. 5970


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The Journal of Physical Chemistry A previously.7−9 Under this condition, the excess energy was estimated to be no more than 1250 cm−1. No ion signals corresponding to fragments are present in the TOF spectrum. The CIS and TD-B3PW91 calculations performed using the 6311++G(d,p) basis set predicted the E1 of 1MN to be 37 828 and 31 364 cm−1, respectively, which deviate from the measured value by 19.1% and 1.3%, respectively. Table 1 lists the vibronic bands observed in Figure 1a and their possible assignments. Spectral assignment was performed

Mode 38 corresponds to in-plane ring−CH3 bending, whereas modes 48 and 25 mainly represent out-of-plane and in-plane ring−H bending vibrations, respectively. The band at 1075 cm−1 is tentatively assigned to combination vibronic transition 31103810. Some of the observed fundamental vibrations are illustrated in Figure 2.

Table 1. Vibronic Bands (in cm−1) of 1MN Observed in Figure 1aa calcd

energy

shift

31 771 32 032 32 190 32 257 32 430 32 449 32 536 32 589 32 706 32 738 32 846 32 859

0 261 419 486 659 678 765 818 935 967 1075 1088

ref 8

419 484 661

815 970

CIS

TDB3PW91

268 418 474 653 693 771 807 909 962

266 415 470 660 669 758 815 915 991

1121

1056

assignment and approximate descriptionb,c 000, origin 3810, β(ring−CH3) 3710, β(CCC) [8̅10] 3510, β(CCC) [910] 3310, breathing [810] 4810, γ(ring−H) 3210, β(CCC) 3110, β(CCC) [7̅10] 3010, β(CCC) 2910, β(CCC) [710] 31103810 2510, β(ring−H)

Figure 2. Some observed vibrations of 1MN in S1 and D0. (○) The original locations of the atoms. (●) The displacements. The measured (calculated) frequencies are included for each mode.

The experimental values are shifts from the 000 band at 31 771 cm−1, whereas the calculated values are obtained from CIS (scaled by 0.90) and B3PW91 (scaled by 0.96) calculations using the 6-311++G(d,p) basis set. bThe numbering of the normal vibrations follows Mulliken’s convention48 for 1MN, whereas symbols in square brackets are Stockburger’s notation49 for naphthalene. cβ, in-plane bending; γ, outof-plane bending. a

The vibronic spectrum can be simulated on the basis of the Franck−Condon principle. In 2008, Bloino et al.51 and Barone et al.52 successfully simulated vibrationally resolved electronic spectra of benzene derivatives by using the Gaussian package. Recently, Helle et al.53 used the quantum chemical approach to obtain reliable results in multidimensional Franck−Condon simulations for detailed analysis of the vibronic and cation spectra of phenetole. We employed the procedure provided by the Gaussian 16 package47 for spectral simulation, which involves calculations of electronic spectra and frequency calculations for both initial and final states. Figure 1b presents the simulated vibronic spectrum of 1MN, which was obtained using the results of the RB3PW91 and TD-B3PW91 calculations for S0 and S1, respectively. Although the computed spectrum does not match the measured spectrum, this comparison may be used by theoreticians to improve the computational details. The vibronic spectrum of trans-2fluorostyrene was successfully simulated using B3PW91 calculations with the 6-311++G(d,p) basis set previously.54 3.2. Measured and Simulated Cation Spectra of 1MN. Before obtaining a vibrationally resolved cation spectrum, information about the ionization threshold must be determined. We performed the PIE experiment by using the 2CR2PI technique, in which the ionization (probe) laser was scanned, while the excitation (pump) laser was fixed at the S100 level (31 771 cm−1). Analysis of the abruptly rising step of the PIE curve (Figure S1) yielded the IE of 1MN as 64 198 ± 15 cm−1. MATI provided results as a sharp peak at the ionization limit (Figure 3a). The adiabatic IE was determined to be 64 198 ± 5 cm−1 (7.9595 ± 0.0006 eV) when considering the uncertainty in laser photon energy, spectral width, and the Stark effect.29,30 The IE of 1MN has been reported to be 7.90 ± 0.02, 7.96 ± 0.01, 7.85 ± 0.01, and 7.80 ± 0.03 eV through

by comparing the present experimental data with available naphthalene derivative data elsewhere2,3,8,31−33 and the results predicted from the CIS and TD-B3PW91 calculations by using the 6-311++G(d,p) basis set. The numbering system of the normal vibrations of 1MN follows Mulliken’s convention.48 The notation used by Stockburger et al.49 for naphthalene (symbols in square brackets) is also included, where totally symmetric (ag) modes are denoted by their numbers alone, and b1g modes are indicated by numbers with bars over them (e.g., 8̅). As indicated by Figure 1a, the distinct bands at 419, 486, 659, 818, and 967 cm−1 result from the 3710 [8̅10], 3510 [910], 3310 [810], 3110 [7̅10], and 2910 [710] transitions, which are related to in-plane ring deformation vibrations. These findings are in good agreement with those of LIF experiments.8 The characteristic band 3310 [810] of 1MN corresponds to in-plane ring deformation vibration similar to the breathing motion of benzene derivatives.50 The vibronic transitions [8̅10], [910], and [810] were, respectively, observed at 436, 501, and 700 cm−1 for naphthalene; 410, 498, and 699 cm−1 for 1-fluoronaphthalene; 448, 507, and 659 cm−1 for 1-cyanonaphthalene; and 448, 507, and 659 cm−1 for trans-1-methoxynaphthalene.8,31−33 This indicates that the nature of a substituent can affect the frequency of each normal vibration. The newly observed bands at 261, 678, 765, 935, and 1088 cm−1 correspond to vibronic transitions 3810, 4810, 3210, 3010, and 2510, respectively. Vibrations 32 and 30 correspond to in-plane ring deformation. 5971


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Figure 4. MATI spectra of 1MN obtained through ionization via (a) S100, (b) S1371, and (c) S1351.

Figure 3. Cation spectra of 1MN (a) obtained in the MATI experiment with ionization through S100 at 31 771 cm−1 and (b) simulated using B3PW91/6311++G(d,p) calculation.

PIE, photoionization, photoelectron, and electron impact ionization methods, respectively. 12−18 The UHF and B3PW91 using the 6-311++G(d,p) basis set predicted the IE to be 52 938 and 62 143 cm−1, respectively, which were smaller than our measured value by 17.5% and 3.2%, respectively. When S100 was used as the intermediate level, the resulting 0+ band was the most intense feature in the MATI spectrum (Figure 3a). However, the MATI bands corresponding to cation vibrations are weak. We applied the Franck−Condon principle to the B3PW91/6-311++G(d,p) calculations to simulate the MATI spectrum associated with the D0 ← S1 transition from S100, with the calculations performed using the Gaussian 16 package.47 The procedure involved using the results of the TD-B3PW91 and UB3PW91 calculations for S1 and D0, respectively. As illustrated in Figure 3b, the simulated spectral features corresponding to cation vibrations 371, 351, 341, 331, 451, and 251 are in partial agreement with the MATI bands displayed in Figure 3a. However, the relative intensity in the simulated spectrum is slightly different from that in the measured spectrum. The MATI spectra of o-, m-, and pfluorophenylacetylene were successfully simulated using B3PW91/6-311++G(d,p) calculations in a previous study.55 We obtained MATI spectra through ionization via several vibronic levels to detect additional active cation vibrations and examine whether a considerable change occurred in the molecular geometry on the D0 ← S1 transition. Figures 4 and 5 present the MATI spectra of 1MN, obtained through ionization via the 00 (31 771 cm−1), 371 (00 + 419 cm−1), 351 (00 + 486 cm−1), 331 (00 + 659 cm−1), 311 (00 + 818 cm−1), and 301 (00 + 935 cm−1) vibrational levels in S1. When the vibronic states S1371, S1351, S1331, S1311, and S1301 were used as the intermediate levels, the most intense MATI bands corresponded to the same vibrations of the cation, as observed in Figures 4 and 5. The finding of this ν = 0


propensity rule indicates a favorable Franck−Condon correspondence associated with the D 0 ← S 1 transition. Consequently, the molecular geometry and vibrational coordinates of 1MN in the cationic ground state D0 must resemble those of the neutral species in the electronically excited state S1. Similar findings have been reported for naphthalene,3 1-fluoronaphthalene,31 2-fluoronaphthalene,31 1cyanonaphthalene,32 1-methoxynaphthalene,33 and tetracene.56 As listed in Table 2, the MATI bands shifted from the 0+ band by 434, 495, 549, 690, 768, 849, 969, and 1024 cm−1 result 5972


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The Journal of Physical Chemistry A Table 2. Bands (in cm−1) Observed in the MATI Spectrum of 1MNa intermediate level in the S1 state 00

371

351

331

435 495 549 691

434 497

495

433 497 690 768

calcd 311

301

691 849

957 969 1024 1190

UHF

UB3PW91

429 492 562 681 789 854 951 992 1022 1184

429 491 547 689 785 848 926 963 1032 1170

assignment approximate descriptionb,c 371, 351, 341, 331, 321, 311, 451, 301, 291, 251,

β(CCC) [8̅1] β(CCC) [91] β(CCC) breathing [81] β(CCC) β(CCC) [7̅1] γ(ring−H) β(CCC) β(CCC) [71] β(ring−H)

The experimental values are shifts from 64 198 cm−1, whereas the calculated values are obtained from the UHF (scaled by 0.95) and UB3PW91 (scaled by 0.98) calculations using the 6-311++G(d,p) basis set. bThe numbering of the normal vibrations is the same as that in Table 1. cβ, inplane bending; γ, out-of-plane bending. a

from the in-plane ring deformation vibrations 371, 351, 341, 331, 321, 311, 301, and 291 of the 1MN cation in D0. 3.3. Measured and Simulated Vibronic Spectra of 2MN. Figure 6 displays the vibronic spectrum of 2MN,

vibrational frequencies are listed in Table 3. The pronounced band at 717 cm−1 could have resulted from vibronic transition Table 3. Vibronic Bands (in cm−1) of 2MN Observed in Figure 6aa calcd energy

shift

31 704 31 801 32 089 32 132 32 181 32 386 32 421 32 474 32 516 32 560 32 607 32 622 32 676 32 706 32 804 32 862

0 97 385 428 477 682 717 770 812 856 903 918 972 1002 1100 1158

ref 8

385 427 478 720

967

CIS

TDB3PW91

99 373 424 480 671 721

97 368 422 482 675 736

901 980 1020 1114

907 1000 1032 1127

assignment approximate descriptionb,c 000, origin τ, CH3 torsion 3710, β(CCC) [8̅10] 3610, β(CCC) 3510, β(CCC) [910] 3310, β(CCC) 3210, breathing [810] 3720 37103610 3620 36103510 2910, β(CCC) 2810, β(CCC) [710] 2710, β(ring−H) 2510, β(ring−H) 33103510

The experimental values are shifts from the 000 band at 31 704 cm−1, whereas the calculated values are obtained from CIS (scaled by 0.90) and B3PW91 (scaled by 0.96) calculations using the 6-311++G(d,p) basis set. bThe numbering of the normal vibrations is the same as that in Table 1. cβ, in-plane bending; γ, out-of-plane bending; τ, CH3 torsion. a

Figure 6. Vibronic spectra of 2MN (a) obtained in the 2C-R2PI experiment and (b) simulated using B3PW91/6-311++G(d,p) calculation.

3210 [810], which is similar to the breathing motion of benzene derivatives.50 The vibronic transitions 3710 [8̅10], 3610, 3510 [910], 3310, and 2810 [710] are observed at 385, 428, 477, 682, and 972 cm−1, respectively, and are related to in-plane ring deformation vibrations in S1. The vibronic transitions [8̅10], [910], and [810] were, respectively, observed previously at 436, 501, and 700 cm−1 for naphthalene and 447, 480, and 711 cm−1 for 2-fluoronaphthalene.8,31 In this study, newly observed bands at 918, 1002, and 1100 cm−1 were assigned to vibronic transitions 2910, 2710, and 2510, respectively. A weak band corresponding to the transition related to CH3 torsion (designated τ) is present at 97 cm−1. A few transitions are related to overtone or combination vibrations, as detailed in

obtained by scanning the excitation laser while fixing the ionization laser at 305.77 nm (32 704 cm−1). The band origin appears at 31 704 ± 2 cm−1, which is in excellent agreement with that previously reported.7−9 Under this condition, the excess energy for the present 2C-R2PI spectrum is no more than 1250 cm−1. The CIS and TD-B3PW91 calculations using the 6-311++G(d,p) basis set predicted 38 070 and 31 617 cm−1, respectively, which deviate from the measured value by 20.1% and 0.3%. The vibronic bands of 2MN in Figure 6a along with the previously reported experimental values8 and calculated 5973


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the Gaussian 16 package.47 The simulated spectrum is displayed in Figure 3b. We obtained the MATI spectra of 2MN through ionization via the 00 (31 704 cm−1), S1 τ (00 + 97 cm−1), 371 (00 + 385 cm−1), 361 (00 + 428 cm−1), 351 (00 + 477 cm−1), 331 (00 + 682 cm−1), and 321 (00 + 717 cm−1) intermediate levels in S1. The obtained spectra are presented in Figures 9 and 10. The

Table 3. Some of these normal vibrations of 2MN are illustrated in Figure 7.

Figure 7. Some observed active vibrations of 2MN in S1 and D0. (○) The original locations of the atoms. (●) The displacements. The measured (calculated) frequencies are included for each mode.

3.4. Measured and Simulated Cation Spectra of 2MN. When S100 was used as the intermediate level, numerous intense MATI bands appeared near the ionization threshold (Figure 8a). These bands resulted from CH3 torsion of the 2MN cation. Some weak bands related to in-plane ring deformation vibration with CH3 torsion are also present in the spectrum. We performed B3PW91/6-311++G(d,p) calculations to simulate the MATI spectrum associated with the D0 ← S1 transition from S100, with the calculations performed using Figure 9. MATI spectra of 2MN obtained through ionization via (a) S100, (b) S1τ1, (c) S1371, and (d) S1361.

observed MATI bands of the 2MN cation are listed in Table 4, along with their possible assignments. Analysis of these 0+ bands determined that the adiabatic IE of 2MN was 64 324 ± 5 cm−1 (7.9752 ± 0.0006 eV). In the literature, the IE of 2MN was reported to be 7.91 ± 0.02, 7.955 ± 0.01, 7.8 ± 0.01, and 8.10 ± 0.03 eV as determined through PIE, photoionization, photoelectron, and electron impact ionization methods.12−18 The UHF and UB3PW91 calculations using the 6-311+ +G(d,p) basis set predicted the IE of 2MN to be 52 792 and 62 264 cm−1, respectively, which were smaller than our measured value by 17.9% and 3.2%. As detailed in Table 4, when S100 was used as the intermediate level, pronounced bands at 42, 72, and 100 cm−1 corresponded to CH3 torsion of the 2MN cation, as observed in Figures 8a and 9a. A weak band at 157 cm−1 is tentatively assigned to out-of-plane ring deformation vibration 551. The weak bands at 501 and 775 cm−1 are concluded to result from fundamental vibrations 351 [91] and 321 [81], respectively. When S1τ was used as the intermediate level, a few bands related to CH3 torsion appeared near the ionization threshold. Notably, when the vibronic states S1371, S1361, S1351, S1 331, and S1321 were used as intermediate levels, the observed intense bands were related to the same vibration, as noted in Figures 9 and 10. In addition, bands resulting from coupled motion of the CH3 torsion with these fundamental

Figure 8. Cation spectra of 2MN (a) obtained in the MATI experiment through ionization via S100 at 31 704 cm−1 and (b) simulated using B3PW91/6311++G(d,p) calculation. 5974


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4. DISCUSSION 4.1. Effect of Methyl Substitution on Electronic Excitation and IEs. A substituent can interact with an aromatic ring when subjected to an inductive effect through σ bonds or a resonance (or mesomeric or conjugation) effect through π orbitals. Electrons in the σ orbital of the methyl group interacting with the π orbital of the adjacent aromatic ring is termed hyperconjugation. This causes a change in the electron density near the aromatic ring, resulting in a decrease in the ZPE level of the electronic state. The magnitude of the decrease is dependent on the strength of the interaction between the substituent and ring. If the decrease in the ZPE level of the upper electronic state (e.g., S1 or D0) is greater than that in the lower state (e.g., S0), the associated transition energy is smaller. This is often referred to as a red shift in the transition energy (e.g., E1 or IE). If the decrease in the ZPE level of the upper electronic state is smaller than that of the lower electronic state, a blue shift in the transition energy is observed. Table 5 details the calculated electronic energies of naphthalene, 1MN, and 2MN at the B3PW91/6-311++G(d,p) level. The ZPEs of these molecules are predicted to be −385.685 493, −424.970 417, and −424.972 162 hartree in S0 and −385.394 710, −424.687 273, and −424.688 467 hartree in D0. This indicates that the methyl substitution on naphthalene leads to a decrease in the ZPE level. Using these ZPE values, the IEs of naphthalene, 1MN, and 2MN are deduced to be 63 820, 62 143, and 62 263 cm−1, respectively, lower than the measured values of 65 687, 64 198, and 64 324 cm−1 by 1867, 2055, and 2061 cm−1. These theoretical calculations predict that the adiabatic IE follows the order 1MN < 2MN < naphthalene, as found in our MATI


vibrations are present in the MATI spectra. This indicates that the barrier to the CH3 torsion is low. Further discussion is provided in Section 4.3.

Table 4. Bands (in cm−1) Observed in the MATI Spectrum of 2MNa intermediate level in the S1 state 0

τ

42 72 100 157

72 100 157

0

1

1

37

36

1

1

35

calcd 33

32

UHF

UB3PW91

assignment approximate descriptionb,c

42 72

42 72 100

59

53

108 185 405

104 173 389

451

446

523

507

708

712

767

783

τ, CH3 torsion τ, CH3 torsion τ, CH3 torsion 551, γ(CCC) 371, β(CCC)[8̅1] 371+τ 361, β(CCC) 371+τ 361+τ 371+τ 351, β(CCC)[91] 361+τ 351+τ 351+τ 331,, β(CCC) 331+τ 331+τ 321, breathing[81] 331+τ 321+τ 321+τ 321+τ

1

1

389 431 437 461 479 489 501

501 535

540

542 571 700 741 770

775

775 798

812 844

816 844 875

The experimental values are shifts from 64 324 cm−1, whereas the calculated values are obtained from the UHF (scaled by 0.97) and UB3PW91 (unscaled) calculations using the 6-311++G(d,p) basis set. bThe numbering of the normal vibrations is the same as that in Table 1. cβ, in-plane bending; γ, out-of-plane bending; τ, CH3 torsion. a

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2MN. These data indicate that the methyl group substitution causes a frequency change. The frequency difference for each active vibrational mode is dependent on the vibrational pattern, location of the CH3 group, electronic state, and degree of the CH3 group involved in the overall vibration. 4.3. Barrier to Internal Rotation of the CH3 Group. Tan et al.9 reported the rotationally resolved fluorescence excitation spectra of 1MN and 2MN. The barrier to internal rotation of the group (called CH3 torsion) was, respectively, determined to be 809 and 563 cm−1 for 1MN and 234 and 128 cm−1 for 2MN in S0 and S1. The authors concluded that the magnitude of the torsional barrier height is dependent on the position of the methyl group and the electronic state of the naphthalene chromophore. Furthermore, no change in conformation occurs on the S1 ← S0 electronic transition for both 1MN and 2MN. Nakai and Kawamura10 performed ab initio calculations using the 6-31G** basis set to estimate the barrier to CH3 torsion for these two positional isomers and found that the conformation of the CH3 group of 1MN does not change on the compound’s electronic excitation and ionization. The barriers were, respectively, calculated to be 894, 557, and 382 cm−1 for this molecule in S0 and S1 and the cationic state D0. By contrast, the relative conformations of the CH3 group of 2MN in S1 and D0 are different from that in S0. The CH3 torsional barriers of 2MN are 294, −433, and −108 cm−1 in S0, S1 and D0, respectively. The negative signs indicate a different conformation of the CH3 group. We performed DFT (B3PW91/6-311++G(d,p)) calculations to estimate the barrier to CH3 torsion of 1MN and 2MN in different electronic states. As illustrated in Figure 11, the barrier heights are 663, 406, and 153 cm−1 for 1MN in S0, S1, and D0, respectively. Our calculated results are also consistent with those reported by Tan et al.9 and Nakai and Kawamura.10 However, under our experimental conditions the signal corresponding to CH3 torsion was absent in the 2C-R2PI and MATI spectra of 1MN, as shown in Figures 1 and 3−5. A weak band is present at 97 cm−1 in the 2C-R2PI (vibronic) spectrum of 2MN, displayed in Figure 6. This band resulted from vibronic transition related to CH3 torsion. In addition, when S100 was used as the intermediate level for obtaining the MATI spectrum, numerous strong spectral features resulting from CH3 torsion appeared near the 0+ band, as displayed in Figure 8a. To investigate the CH3 torsion of the 2MN cation, we obtained MATI spectra via the vibronic states S1τ1, S1371, S1361, S1351, S1331, and S1321. As shown in Figures 9 and 10, numerous MATI bands resulted from CH3 torsion and its coupled motion with the fundamental in-plane deformation vibrations. Hyperconjugation of the methyl group with the aromatic ring can cause different π electron densities in positions 1 and 2. This gives rise to different torsional barriers of the methyl group in 1MN and 2MN. Therefore, we also performed onedimensional potential energy surface calculations to estimate the barrier to CH3 torsion in 2MN. As illustrated in Figure 12, the conformations of the CH3 group of 2MN in S1 and D0 are different from that in S0. This calculated result is in good agreement with those reported by Tan et al.9 and Nakai and Kawamura.10 Because the torsional barrier of the CH3 group is relatively small, the MATI spectral features resulting from CH3 torsion are rich, as observed in Figures 9 and 10. Therefore, the results from theoretical calculations support our experimental findings. Pino et al.19 also found that the conformation of the 1MN cation does not change on the D2 ← D0 electronic

Table 5. Calculated Electronic Energies (in hartree) and Measured E1 and Adiabatic IE (in cm−1) of Naphthalene (Naph) and Its Methyl Derivativesa S0 state ER ZPC E″ S1 state ETD ZPC E′ D0 state EU ZPC E+ (E″ − E′)/cm−1 E1/cm−1(Exp.) (E+ − E″)/cm−1 IE/cm−1 (Exp.)

Naph

1MN

2MN

−385.832 753 0.147 260 −385.685 493

−425.145 229 0.174 812 −424.970 417

−425.146 607 0.174 450 −424.972 162

−385.822 543 0.142 375 −385.538 968

−425.135 195 0.169 871 −424.827 514

−424.997 856 0.169 751 −424.828 105

−385.541 524 0.146 814 −385.394 710 32 159 32 019 63 820 65 6873

−424.861 312 0.174 039 −424.687 273 31 364 31 771 62 143 64 198b

−424.862 214 0.173 747 −424.688 467 31 617 31 704 62 264 64 324b

a

Calculations are at the restricted B3PW91/6-311++G(d,p) level for S0, TD-B3PW91/6-311++G(d,p) level for S1, and unrestricted B3PW91/6-311++G(d,p) level for D0. bThis study.

experiments. As also listed in Table 5, the E1 of both 1MN and 2MN is slightly less than that of naphthalene. This indicates that the interaction between the CH3 and ring of naphthalene is slightly stronger in S1 than in S0. The D0 ← S1 transitions of naphthalene derivatives involve the removal of one electron from the aromatic ring.31−33 A methyl group of 1MN and 2MN has an electron-donating nature, which affects the electron density around the conjugative aromatic ring of naphthalene. Consequently, the IEs of 1MN and 2MN were lower than that of naphthalene by 1489 and 1364 cm−1, respectively. Thus, the methyl substitution on naphthalene leads to a red shift in both E1 and IE. 4.2. Methyl Substitution Effect on Molecular Vibrations. 1MN and 2MN have 57 normal vibrations including 48 naphthalene-like and 9 methyl modes. However, not all of these vibrations could be observed in the present photoexcitation and ionization experiments. Table 6 lists the Table 6. Measured Frequencies (in cm−1) of Active Vibrations of Naphthalene (Naph), 1MN, and 2MN in S1 and D0 1MNa

Naph3

vibration mode

2MNa

Naph/1MN/2MN

S1

D0

S1

D0

S1

D0

8̅/37/37 9/35/35 8/33/32

436 501 700

455 507 762

419 482 659

438 549 690

385 477 717

389 501 775

a

This study.

measured frequencies of some observed in-plane ring deformation vibrations of naphthalene, 1MN, and 2MN in S1 and D0. Vibrations 8̅, 9, and 8 are, respectively, found to have frequencies of 436, 501, and 700 cm−1 for naphthalene; 419, 482, and 659 cm−1 for 1MN; and 385, 477, and 717 cm−1 for 2MN in S1. Concerning the active vibrations of the cations in D0, the frequencies of modes 8̅, 9, and 8 are, respectively, found to be 455, 507, and 762 cm−1 for naphthalene;3 438, 549, and 690 cm−1 for 1MN; and 384, 501, and 775 cm−1 for 5976


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Figure 11. One-dimensional potential energy surfaces of CH3 torsion in 1MN in (a) D0, (b) S1, and (c) S0, as obtained from B3PW91/6311++G(d,p) calculations.

Figure 12. One-dimensional potential energy surfaces of CH3 torsion in 2MN in (a) D0, (b) S1, and (c) S0, as obtained from B3PW91/6311++G(d,p) calculations.

transition. The 2MN cation, however, does undergo a conformation change.

calculations to estimate the barriers to CH3 torsion in 1MN and 2MN in S0, S1, and D0. Our results revealed that the conformations of the CH3 group in 1MN were the same in these three electronic states. By contrast, the conformations of the CH3 group in 2MN in S1 and D0 were different from that in S0. Furthermore, the B3PW91/6-311++G(d,p) calculation predicted that the torsional barrier of the CH3 group of the 2MN cation in D0 was as small as 79 cm−1. This supported the experimental finding that numerous MATI bands in the cation spectra of 2MN result from CH3 torsion.

5. CONCLUSION We employed the 2C-R2PI and MATI methods to obtain vibrationally resolved spectra of 1MN and 2MN in the neutral electronically excited state S1 and cationic ground state D0. In particular, cation spectra were obtained through ionization via several intermediate vibronic levels. The results demonstrated that most of the active vibrations resulted from in-plane ring deformation. In addition, the molecular geometries and these vibrational coordinates of 1MN and 2MN in D0 resembled those of the neutral species in S1. Because of hindrance effect, CH3 torsion was not detected in the 1MN spectra. However, numerous MATI bands related to CH3 torsion were present in the 2MN cation spectra. Most of the observed in-plane ring deformation vibrations of the 2MN cation were found to be coupled to CH3 torsion because of its smaller internal rotational barrier. We conducted DFT calculations to obtain vibronic and cation spectra of 1MN and 2MN by using the Gaussian 16 package. Although the computed spectra did not satisfactorily match the measured spectra, the comparison might be useful to theoreticians wishing to improve their computational details. We also performed one-dimensional potential energy

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b03756. The detailed experimental method and photoionization efficiency curves of 1MN and 2MN (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wen Bih Tzeng: 0000-0002-3727-0886 5977


Article

The Journal of Physical Chemistry A Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for financial support of this work under Grant No. MOST-1072113-M-001-014.

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Cation Vibrations of 1-Methylnaphthalene and 2-Methylnaphthalene

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