Mass-analyzed-threshold-ionization (MATI) spectroscopy of 1,2,3-substituted halogenated benzenes via different intermediate vibrational states in the S₁ state

Abstract

For the first time, two color resonant mass analyzed threshold ionization (MATI) spectroscopy has been applied in order to investigate the ionic properties of 1,3-dichloro-2-fluoro-benzene (1,3,2-DCFB) and 1,3-difluoro-2-chloro-benzene (1,3,2-DFCB) radical cations in their electronic ground state. The ionic ground state of the different samples has been investigated via different S₁ intermediate states and compared to 1,2,3-trichlorobenzene measured in previous work. Additionally quantum chemical calculations at DFT (density functional theory) and TDDFT (time-dependent density functional theory) level of theory have been performed to support experimental findings. From the MATI spectra the adiabatic ionization energies of 1,3-dichloro-2-fluorobenzene and 1,3-fluoro-2-chlorobenzene could be determined to be 75.242 ± 6 cm⁻¹ and 75.627 ± 6 cm⁻¹, respectively. Several vibrational modes of both compounds have been assigned by comparison of the experimental and theoretical results.

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1. Introduction

Halogenated aromatic molecules became a topic of great interest due to their widespread presence in industrial processes and hence presence in the environment as potentially toxic and cancerogenic pollutants. Moreover halogenated benzenes are important model substances to validate theoretical concepts, such as vibronic coupling and others. As one of the spectroscopically best-investigated molecules of all, benzene can serve as an outstanding reference system to study the influence of tailored perturbations, such as careful choice of substitution pattern, to the (ro)vibronic structure. As an immediate effect a shift in transition energies, ionization energy, molecular geometry and vibrational frequencies can be observed. The characteristics of these effects strongly depend on the number, type and localization of the substituents. As the magnitude of these effects are rather small (from a few wavenumbers up to a hundred wavenumbers), high resolution techniques, such as mass analyzed threshold ionization (MATI) or zero kinetic energy (ZEKE) spectroscopy are ideally suited to investigate such phenomena.^1–4 Chloro- and fluoro-benzenes have been subject to various studies investigating the vibronic properties of the first electronic excited state (S₁) as well as cationic ground state (D₀).^1,5–13 In this paper we will focus particularly on out-of-plane b₁-symmetric modes. Modes of this symmetry are connected to interesting phenomena observed in a whole variety of halobenzenes. For example, difluorobenzenes exhibit large frequency lowering of b₁ (b_3u under D_2h in case of p-DFB) symmetric modes in going from S₀ to S₁ or from D₀ to S₁,¹⁴ respectively. The intensity gain of this symmetry forbidden mode was interpreted in terms of the pseudo-Jahn–Teller effect (PJTE). The PJTE is based on the idea that a geometrical distortion blurs the difference in symmetry between two or more electronic states of virtually equal energy. In 1,2,4,5-tetrafluorobenzene (D_2h) it was found that the b_2g-symmetric mode 11 shows an unusual strong activity in the form of a long progression (v²ⁿ₁₁). Recent results show that the planarity of this molecule is distorted along the eigenvector of the 11 mode during excitation to ¹B_2uS₁ state. It is an interesting question to elucidate if these phenomena can be extended to further congeners and identified as a general feature. In particular in the case of the molecular geometry change it is crucial to determine the involved vibrational modes and participating electronic states. For this purpose, the experimental data are compared to theoretical calculations, which enable the possibility of assigning the observed vibrational modes. In this paper we present mass-selected two color two photon Resonance Enhanced Multi Photon Ionization (2C2P-REMPI) spectra of the S₁ ← S₀ transition and MATI spectra via different S₁-intermediate states of 1,3-dichloro-2-fluorobenzene and 1,3-dichloro-2-fluorobenzene. Some assignments of 1,2,3-trichlorobenzene reported previously¹ were reconsidered and compared to the newly gained knowledge.

2. Experimental setup

The experimental setup consists of a homemade time-of-flight (TOF) mass spectrometer as described in detail previously.^1,15,16 Briefly, the spectrometer consists of a standard second order corrected reflectron time-of-flight mass spectrometer equipped with single stage ion source. The laser system used for excitation and ionization consists of two dye lasers (Laser Analytical Systems LDL 205, Lambda Physics FL 3002). Each dye laser is pumped by a dedicated Nd:YAG laser (Lumonics HY-1200, Continuum Surelite II). A Quantum Composers 9600+ delay pulse generator fed by an external clock operated at 10 Hz controls flash lamp and Q-switch delays. The output of each dye laser is frequency-doubled by a BBO-I crystal yielding tunable ranges from 245 to 285 nm. Wavelength calibration of both dye lasers is performed by recording an opto-galvanic spectrum with a neon hollow cathode lamp yielding accuracy better than 2 cm⁻¹. Initially a supersonic molecular beam of sample molecules and seed gas (argon) with a backing pressure of approximately 2 bar is expanded via a pulsed jet valve (General Valves Series 9) into the ion source. To obtain a sufficient vapor pressure the sample is heated to approximately 80 °C. Excitation or ionization is accomplished by multi photon absorption under field-free conditions. In the MATI modus, promptly generated ions are discriminated against Rydberg neutrals through the application of a weak electrical field of 1–2 V cm⁻¹ by a subsequent delay (∼100 ns) to multi-photon excitation. Finally, a high voltage pulse (890 V cm⁻¹) switched by a Behlke HS56-01 fast thyristor ionizes the Rydberg neutrals and accelerates them into the mass spectrometer. In REMPI modus, the generated ions are accelerated directly into the TOF without the retarding field. The ion signal is detected by a conventional dual micro-channel-plate detector and transferred to a LeCroy 534M digital oscilloscope. A computer, linked to the oscilloscope by a GPIB connection performs the data acquisition and processing. The 123-DCFB was purchased from Aldrich, 126-DFCB from ABCR and were used without further purification.

3. Quantum-chemical calculations

Quantum chemical calculations were performed in order to assign the observed vibrational bands and to support the experimental findings. The software package Turbomole^17–20 was used for all quantum chemical calculations. Geometry optimizations and subsequent frequency analyses for molecules in the electronic ground state (S₀) and the cationic ground state (D₀) were conducted at the density functional theory (DFT) level of theory with both the gradient corrected functional BP86 ^21a and the hybrid functional B3LYP.^21b The triple basis set TZVPP^21c has been applied to all DFT calculations. Calculations for the S₁ were done analogously using the time dependent density functional theory (TDDFT). For comparison reasons, geometry calculations and frequency analyses were also treated at the coupled cluster (CC2) level of theory for the S₀- and S₁-state using the Ahlrichs basis set cc-pVTZ.^21d The nomenclature used for assignment of vibrational bands is according to Varsanyi and Szoke,²² which is derived from Wilson's notation of the Benzene modes.²³

We refrained from the use of scaling factors to fit the calculated frequencies.

4. Experimental results

4.1. 1,3-Dichloro-2-fluorobenzene (1,3,2-DCFB)

4.1.1. REMPI spectrum. According to electric dipole selection rules the transition to first excited singlet state ¹B₂S₁(π* ← π) in 1,3,2-DCFB is allowed (y-polarized). For benzene, the corresponding transition A¹B_2u ← X¹A_1g is dipole forbidden under D_6h symmetry. The reduced symmetry in 1,3,2-DCFB (C_2v) leads to an electronically as well as vibronically allowed transition. Starting from the premise that the molecules are sufficiently cooled with regards to their degrees of freedom, only total symmetric (a₁) vibrations are allowed according to the Franck–Condon (FC) principle. Due to vibronic coupling to intensive, nearby states, vibrations of a₂ and b₂ symmetry can gain intensity as well. The REMPI spectrum shown in Fig. 1 in units of internal energy could be recorded in the range up to 1300 cm⁻¹. The excitation energy of the first excited state S₁ could be determined for the first time to be 36 460 ± 2 cm⁻¹.

Fig. 1 (1 + 1′) REMPI spectrum of 1,3-dichloro-2-fluoro-benzene (1,3,2-DCFB).

Normal coordinate analysis for 1,3,2-DCFB yields following distribution of fundamental modes under C_2v symmetry: Γ_vib = 11 × a₁, 10 × b₂, 3 × a₂, 6 × b₁. Clearly, the spectrum is dominated by the total symmetric modes 1¹ (565 cm⁻¹) and 18a (977 cm⁻¹). Moreover the a₁-symmetric modes 9a¹, 6a¹, 7a¹, 12¹ (186 cm⁻¹, 349 cm⁻¹, 792 cm⁻¹, 1104 cm⁻¹) could be identified with lower intensity. With 16a¹ (360 cm⁻¹) also an a₂-symmetric mode could be assigned. As expected, no modes of b₁-symmetry could be assigned to the spectrum in accordance with selection rules. Particular attention should be paid to the low frequency band at 120 cm⁻¹. A band with such a low frequency can with certainty be assigned to an out-of-plain mode. Based on correlation with MATI spectra we assigned the first even overtone of the b₁-symmetric mode 17b to this band. The frequency of 52 cm⁻¹ calculated with the coupled cluster method is in reasonable agreement with the experimental value for the half of the overtone for 17b². The bands at 247 cm⁻¹ (15¹), 337 cm⁻¹ (6b¹), 500 cm⁻¹ (3¹) and 1236 cm⁻¹ (18b¹) were assigned to b₂-symmetric modes. The latter two experimental values are in excellent agreement with both calculated values (TDDFT, CC2) for the first excited singlet state of 1,2,3-DCFB. Comparing the calculated values for the 15¹ and 6b¹ mode a major deviation becomes apparent, just like in the case of 17b. All performed calculations show good agreement with the experiment for the modes 9a¹, 6a¹, 1¹, 7a¹, 18a¹, 12¹, 16a¹, whereas the best results were obtained with the CC2 level of theory (see Table 1).

Table 1 Comparison experimental and calculated normal modes observed in the S₀, S₁ and D₀ state of 1,3,2 DCFB

S0(C2v)						S1	(C2v)	(C2v)	(CS)	D0	(C2v)
Wilson	Sym.	exp.	B3LYP	BP86	CC2	exp.	B3LYP	BP86	CC2	exp.	B3LYP	BP86
a IR-band.^24 b From combination band. c From overtone.
9a	a1	256^a	192	186	189	186	241	232	183	190	195	189
6a	a1	376^a	379	369	380	349	355	361	343	361	381	372
1	a1	597^a	601	584	596	565	538	522	558	578	603	586
7a	a1	803^a	839	813	829	792	831	815	791	798	820	797
18a	a1	1058^a	1092	1061	1080	977	1013	998	991		1047	1027
12	a1	1112^a	1116	1083	1124	1104	1077	1046	1079		1143	1104
13	a1	1205^a	1277	1240	1277		1314	1320	1242		1339	1295
19a	a1	1462^a	1490	1444	1485		1213	1229	1383		1459	1412
8a	a1	1572^a	1613	1564	1608		1509	1476	1512		1618	1567
20a	a1	3084^a	3189	3119	3220		3159	3080	3210		3197	3126
2	a1		3212	3142	3243		3220	3147	3246		3218	3147
10a	a2	215^a^,^b	208	201	207		120	123	71	174	180	173
16a	a2	530^a^,^b	553	534	518	360	458	495	313	512	498	476
17a	a2	894^a	921	876	890		1045	1060	616		936	897
17b	b1	115^a^,^b	107	103	107	60^c	31	38	52	95	88	85
10b	b1	260^a^,^b	282	271	278	270^c	263	259	178	264	274	265
16b	b1	513^a	533	512	542		502	454	364	452	455	441
4	b1	705^a	732	699	680		766	755	460	732	743	715
11	b1	770^a	794	758	771		847	827	601	678	826	792
5	b1	965^a	983	937	949		1004	966	782		1009	966
15	b2	279^a	258	248	252	247	62	61	227	242	233	253
6b	b2	401^a	404	393	401	337	202	209	319	331	300	331
3	b2	539^a	545	528	538	500	500	498	519	471	513	504
7b	b2	829^a	798	779	810		698	683	759	827	812	796
9b	b2	1154^a	1180	1145	1170		1177	1144	1105		1083	1104
18b	b2	1205^a^,^b	1233	1193	1223	1236	1250	1185	1170		1330	1192
14	b2	1255^a	1313	1322	1421		1413	1360	1497		1226	1296
19b	b2	1446^a	1481	1432	1465		1446	1407	1389		1514	1466
8b	b2	1583^a	1615	1566	1608		1574	1545	1683		1371	1384
20b	b2	3084^a	3207	3137	3235		3188	3177	3141		3215	3144

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4.1.2. MATI spectra. The MATI spectra via the S₁ intermediate states 0⁰, 17b¹, 9a¹, 1¹ are shown in Fig. 2. The origin of the D₀(²B₁) state and with that the adiabatic ionization energy was found to be 75.242 ± 6 cm⁻¹ (9.3288 ± 0.0007 eV). This value is in good accordance with the previously by conventional photoelectron spectroscopy determined value of 9.32 ± 0.02 eV.²⁵

Fig. 2 MATI spectra of 1,3,2-DCFB of different vibrational intermediate states of the first excited electronic state: (a) via 1¹, (b) via 9a¹, (c) via 17b², (d) via the electronic origin (0⁰).

Besides the 0⁰ transition, the MATI spectrum obtained via the electronic origin exhibits several more additional resonances. The most prominent peak in the spectrum is the band assigned to the 17b mode, which is constituting a violation of the Δv = 0 propensity rule. Additionally, the mode 17b¹ at 95 cm⁻¹ appears as a combination band (17b¹9a¹) at 288 cm⁻¹. Moreover the spectrum is conspicuously rich in b₁-symmetric modes. In addition to 17b¹ we assigned the 10b¹ (264 cm⁻¹), 16b¹ (452 cm⁻¹), 11¹ (678 cm⁻¹) and 4¹ (732 cm⁻¹). Another out-of-plane vibration (10a¹) with a₂-symmetry and low intensity has been assigned to the band at 174 cm⁻¹. The bands at 361 cm⁻¹, 578 cm⁻¹ have been assigned to the total symmetric modes 6a¹ and 1¹, respectively. b₂ symmetric modes were identified at 242 cm⁻¹ (15¹), 331 cm⁻¹ (6b¹), 471 cm⁻¹ (3¹) and 827 cm⁻¹ (7b¹). The band at 657 cm⁻¹ fits the overtone 6b¹.

The MATI spectrum obtained via the S₁17b² shows a short, three-membered regular progression with transitions corresponding to the excitation of one, two and three quanta. In contradiction to the Δv = 0 propensity rule, the band labeled 17b¹ is the most prominent peak in the spectrum. It is notable that the 17b mode also appears in the combination bands 17b¹9a¹ (288 cm⁻¹) and 16b¹6b¹ (423 cm⁻¹). Also apparent is the richness in b₁-symmetric modes, first and foremost the intense 16b¹ (448 cm⁻¹), but also the 10b¹ (264 cm⁻¹), 11¹ (680 cm⁻¹), 4¹ (728 cm⁻¹). The MATI spectrum obtained via the S₁9a¹ mode also shows a breakdown of the Δv = 0 propensity rule. Not the vertical transition into the D₀9a¹ state is the dominating one, but the band at 242 cm⁻¹ (15¹). In addition the 17b mode appears again, also as a combination in 17b¹9a¹. A further band at 331 cm⁻¹ could be identified with 6b¹. The measured values are in good accordance with the calculated values (195 and 189 cm⁻¹ for the 9a¹, 233 and 253 cm⁻¹ for the15¹ and also 300 and 331 cm⁻¹ for the 6b¹).

The MATI spectrum via S₁1¹ continues the series of MATI spectra characterized by a breakdown of the Δv = 0 propensity rule in favor for the vibronic transition into the D₀17b¹ state. Further active modes that could be assigned in the recorded range up to 750 cm⁻¹ are the 15¹ (241 cm⁻¹), 6b¹ (331 cm⁻¹) and 1¹ (578 cm⁻¹). The experimentally determined frequency for the 1¹ mode is in good accordance with the calculated values of 578 cm⁻¹ or 603 cm⁻¹ respectively. It should be noticed that, owing to the heavy substituents, the displacement pattern of the ‘ring-breathing-mode’ shows a striking deviation from the original benzene pattern. It resembles clearly the pattern of mode 6a found for benzene. The assignment of the band at 578 cm⁻¹ to the mode 6a is excluded since it was already doubtlessly assigned to the band at 361 cm⁻¹.

4.2. 1,3-Difluoro-2-chloro-benzene (1,3,2-DFCB)

4.2.1. REMPI spectrum. 1,3,2-DFCB belongs to C_2v point group and has a dipole allowed transition to the ¹A₁S₁(π* ← π) first excited state. According to FC-principle only transitions in total symmetric (a₁) modes are allowed, b₁ and b₂ can gain intensity due to vibronic coupling mechanism, a₂ modes are symmetry forbidden. The REMPI spectrum shown in Fig. 3 in units of internal energy could be recorded in the range up to 800 cm⁻¹. The excitation energy of the first excited state S₁ could be determined for the first time to be 37 449 ± 2 cm⁻¹.

Fig. 3 (1 + 1′) REMPI spectrum of 1,3-difluoro-2-chloro-benzene (1,3,2-DFCB).

The electronic origin and the total symmetric mode 9a¹ at 228 cm⁻¹ dominates clearly the measured REMPI spectrum. It should be noted that the remaining bands at 381 cm⁻¹ and 607 cm⁻¹ that were both assigned to the total symmetric modes 6¹ and 1¹, respectively. Quite unusual for total symmetric modes we found poor consistency between calculated and observed bands: calculation substantially underestimate the 6a¹ and 1¹ with 188 cm⁻¹ to 202 cm⁻¹ and 488 cm⁻¹ to 517 cm⁻¹, respectively. The 9a¹ has been overestimated with 364 cm⁻¹ to 374 cm⁻¹. Nevertheless the correlation with MATI spectra backups the assignment of the total symmetric modes. Moreover, the calculations suggest the mode 7a¹, that cannot be seen in the spectrum, to appear in the spectrum between 710 cm⁻¹ and 750 cm⁻¹. As expected, no modes of a₂ symmetry could be assigned to the spectrum in accordance with selection rules. In contrast to 1,2,3-TCB and 1,3,2-DCFB, transitions to b₁-symmetric modes are allowed in 1,3,2-DFCB. The bands at 25, 98 and 404 cm⁻¹ could be identified with the b₁-symmetric modes 17b¹, 10b¹ and 16b¹. In this case, the calculated frequencies are in good agreement with the observed ones. The 17b¹ has been predicted to appear between 31 cm⁻¹ and 51 cm⁻¹, the 10b¹ between 176 cm⁻¹ and 215 cm⁻¹ and the 16b¹ between 351 cm⁻¹ and 428 cm⁻¹. With 15¹ and 6b¹ two a₂-symmetric modes could be assigned. Noteworthy is the appearance of the 15 mode as a short, three-membered progression of overtones (15¹, 15², 15³). The measured frequencies of 443 cm⁻¹ for 6b¹ and 63 cm⁻¹ for 15¹ are well reproduced by the B3LYP calculation with 439 and 64 cm⁻¹. The BP86 calculation predicts the 15¹ correctly with 67 cm⁻¹, while it slightly overestimates the 6b¹ with 463 cm⁻¹. The CC2 calculation gives good results in predicting the 6b¹ with 439 cm⁻¹, while it overestimates the 15¹ with 154 cm⁻¹.

4.2.2. MATI spectra. The MATI spectra via the S₁ intermediate states 0⁰, 17b¹, 9a¹ are shown in Fig. 4. The origin of the D₀(²B₁) state and with that the adiabatic ionization energy was found to be 75.627 ± 6 cm⁻¹ (9.3765 ± 0.0007 eV). This value is in good accordance the previously by photoelectron spectroscopy determined value of 9.37 ± 0.02 eV.²⁵

Fig. 4 MATI spectra of 1,2,6 DCFB of different vibrational intermediate states of the first excited electronic state: (a) via 9a¹, (b) via 17b², (c) via the electronic origin (0⁰).

In accordance with the Δv = 0 propensity rule, the MATI spectrum obtained via the electronic origin is dominated by the 0⁰-band. Furthermore the spectrum is characterized by strong activity of the two total symmetric modes 9a¹ and 6a¹. Within the recorded range of 1000 cm⁻¹, the spectrum exhibits a progression in 9a composed of the first three overtones 9a¹ (293 cm⁻¹), 9a² (588 cm⁻¹) and 9a³ (882 cm⁻¹). The 6a appears as ground vibration 6a¹ (418 cm⁻¹), first overtone 6a² (839 cm⁻¹) as well as combination vibration 9a¹6a¹ (706 cm⁻¹).

Additionally we assigned the band at 925 cm⁻¹ to a combination band of modes 9a¹1¹. The relative intensive band at 630 cm⁻¹ has been identified with the mode 1¹. The shoulder at 649 cm⁻¹ in the peak labeled with 1¹ has been assigned to the mode 4¹. Exhibiting a similar weak intensity as the 4¹, another b₁-symmetrical mode (7a¹) could be identified at 778 cm⁻¹. In the view of previous work,¹ the assignment of the bands at 95 cm⁻¹ and 196 cm⁻¹ to the modes 17b¹ and 17b² respectively seems appropriate. Assignments that are more tentative are the 16b¹ and 11¹ to the bands at 439 cm⁻¹ and 839 cm⁻¹. The a₂-symmetrical modes 10a¹ and 16a¹ are assigned to the weak bands at 201 cm⁻¹ and 546 cm⁻¹. With exception of 4¹ all calculated frequencies are in good accordance with the experimentally determined ones.

Provided the validity of the Δv = 0 propensity rule, the MATI spectrum obtained via the S₁17b¹ mode ought to be characterized by a towering 17b¹ band. Contrary to expectations, the band 17b¹ could not be observed in the spectrum shown in Fig. 4. Instead, the spectrum shows the 9a¹ at 291 cm⁻¹ as the most intensive resonance absorption. As can be seen from the correlation between the MATI spectra in Fig. 4, the first overtone 9a² appears at 586 cm⁻¹ as well. The signals at 419 cm⁻¹ and 629 cm⁻¹ are identified as the 6a¹ and 1¹ total symmetric modes. Obviously, the signals for the 11-vibration is broadened. Based on the MATI spectrum of the 0⁰ transition, we assigned this signal both to the 1¹ and 4¹ vibrations, the latter of which gives rise to the shoulder. The MATI spectrum obtained via the S₁9a¹ shows three prominent peaks: the 6a¹, the 9a¹6a¹ and the 18a¹. Without folding the spectrum with the laser power, no absolute statement can be made in regards to the peak intensities. The three peaks are of nearly equal intensity (approx. 5% difference), so it is hard to tell which one is the highest in intensity. Nevertheless it can be stated that in contrary to the predictions of the propensity rule, transitions to 6a¹ and 9a¹6a¹ are at least as same as intensive while the vertical transition into the D₀9a¹ state is scarcely visible in the spectrum. Nevertheless the mode 9a appears in a series of overtones 9a¹6a¹ (710 cm⁻¹), 9a¹1¹ (924 cm⁻¹) and 9a²6a¹ (1003 cm⁻¹). Remarkably, the spectrum exhibits a strong activity in 6a modes: the bands assigned to the modes 6a¹ and 9a¹6a¹ are among the three most intense bands in the spectrum. The 6a¹ vibration shows progression activity (6a¹ and 6a² in the recorded range). In addition, the 6a¹ mode contributes to overtone bands as already discussed above. The weak features at 97, 193 and 213 cm⁻¹ have been assigned on the basis of our previous studies^7,8 to the out-of-plane modes 17b¹, 17b² and in-plane mode 15¹.

5. Discussion

5.1. 1,3,2-DCFB

Compared to electronic and cationic ground states the 17b exhibits a drastically decreased frequency in the first electronic excited state (see Table 1). Such a frequency lowering suggests a geometrical distortion along the eigenvector of 17b in going from S₀ to S₁ or from D₀ to S₁, respectively. As considered earlier by Tsuchiya et al.¹⁴ for difluorobenzene it is highly suggested that this phenomenon is the result of strong vibronic coupling between the S₁ and nearby states. But the most important indication for a such a distortion shows up in the MATI spectra via the electronic origin and, first and foremost, via the 17b². The latter exhibits a three-membered progression of the mode 17b. Not just that this progression shows a shift of the Franck–Condon maximum between S₁ and D₀, the fact that the transition from D₀17b¹ ← S₁17b² is favored could be interpreted as a replanarisation of the molecular geometry in going from S₁ to D₀ along that mode.

Also during ionization via the vibrationless level of the first excited state, the transition D₀17b¹ ← S0⁰ is clearly favored over D₀0⁰ ← S₁0⁰ (see MATI via 0⁰Fig. 2). A major role of the 17b mode during ionization is aggravated by the fact the 17b also appears as combination bands (17b¹9a¹ in each of the recorded MATI spectra, 17b¹6b¹ in the MATI via 17b²). The strong activity of the mode 16b¹ in the MATI via 17b² and the general, unusual appearance of multiple other out-of-plane modes throughout the MATI spectra give rise to the assumption that the geometry in the S₁ is a product of a distortion along several modes. The CC2 computed geometry optimization seem to support the statements derived from the experimental data: the geometry shown in Fig. 5 is obviously not just the product of a distortion along a single (out-of-plane) normal mode (Table 2).

Fig. 5 Excited state geometry of 1,3,2-DCFB obtained by CC2-optimization.

Table 2 Comparison experimental and calculated normal modes observed in the S₀, S₁ and D₀ state of 1,3,2 DFCB

S0(C2v)						S1	(C2v)	(C2v)	(CS)	D0	(C2v)
Wilson	Sym.	exp.	B3LYP	BP86	CC2	exp.	B3LYP	BP86	CC2	exp.	B3LYP	BP86
9a	a1		301	290	295	228	374	364	367	293	304	294
6a	a1		437	425	436	381	198	202	188	418	435	422
1	a1		617	602	618	607	517	500	488	630	636	620
7a	a1		783	760	776		748	729	713	778	784	761
19a	a1		1072	1142	1124		1506	1462	1360		1360	1318
18a	a1		1121	1042	1065		1113	1083	1059		1045	1021
12	a1		1314	1275	1317		1005	984	949		1150	1114
13	a1		1494	1448	1491		1294	1252	1268		1413	1372
8a	a1		1637	1587	1635		1577	1542	1507		1665	1613
20a	a1		3191	3121	3224		3201	3133	3212		3199	3128
2	a1		3212	3143	3248		3223	3155	3266		3217	3148
10a	a2		249	239	247		194	189	183	201	218	209
16a	a2		597	573	588		762	766	519	546	565	541
17a	a2		893	850	873		1043	1107	788		920	880
17b	b1		124	119	125	25	31	41	51	97	98	94
10b	b1		275	264	273	198	215	214	176	280	289	277
16b	b1		530	510	521	404	428	407	351	439	434	415
4	b1		717	684	675		805	779	649	649	729	700
11	b1		792	754	772		640	632	548	839	837	802
5	b1		968	922	944		979	946	781		1000	954
15	b2		212	204	208	63	64	67	154	213	221	212
6b	b2		509	494	501	443	451	463	439	367	386	369
3	b2		555	534	543		511	501	492	489	539	522
7b	b2		1020	990	1014		979	958	948	982	1018	986
9b	b2		1179	1088	1167		1137	1128	1133		1110	1087
18b	b2		1266	1226	1262		1208	1171	1194		1303	1259
14	b2		1322	1331	1424		1380	1393	1456		1354	1316
19b	b2		1503	1456	1495		1441	1382	1381		1548	1500
8b	b2		1626	1577	1623		1477	1451	1680		1381	1370
20b	b2		3206	3137	3242		3207	3139	3215		3214	3145

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The quantum chemical calculations predict a S₁(π* ← π) transition into the first excited state for 1,3,2-DCFB. However, due to σ*-orbitals localized on the Halogen–Carbon bond, it is possible for modes of appropriate symmetry to induce coupling to (πσ*)-states. In accordance with the assumption of a vibronic coupling effect we prevalently observed a substantial decrease in frequency comparing the S₀ state and the S₁ state of the neutral for formal forbidden modes with strong influence of halogen atoms on their displacement pattern. For the modes 15, 6b, 3 (b₂ symmetry) and 16a (a₂ symmetry) we observed such a decrease.

The decrease in frequency for the modes 15, 6b, 3 is explainable in accordance with the Herzberg–Teller (HT) effect. However, the frequency lowering of the symmetry allowed mode 17b² could not be described in terms of the HT effect in a sufficient way. It seems that the distortion along 17b blurs the symmetry differences between the (πσ*)- and (ππ*)-states which would be an indication for a pseudo Jahn–Teller effect.

5.2. 1,3,2-DFCB

In 1,3,2-DFCB the vibration 15 appears in the REMPI spectrum as a progression-forming and largely frequency lowered mode. Remarkably, the progression reveals a negative anharmonicity. The occurrence of the 15 as a b₂ symmetric mode is explained in terms of the Herzberg–Teller effect. Besides the mode 15, also the modes 6a and 9a exhibit a largely lowered frequency in the first excited state S₁. Both modes are likely to contribute in pseudo Jahn–Teller distortion in going from the electronic ground to excited state or from excited state to ionic ground state, respectively. This is seen from the different MATI spectra as well as the different calculated structures given in Tables 3–8. The MATI spectrum via the electronic origin exhibits a harmonic, three-membered progression of mode 9a (9a¹, 9a², 9a³), whereby the band 9a¹ is almost as intense as the 0–0 transition. Such a shift of the Franck–Condon maximum is a clear indication for a distortion along the eigenvector of this mode during ionization.

Table 3 B3LYP/TZVPP calculated geometries of 1,3,2-DCFB

B3LYP/TZVPP	C 2v	C 2	C 2v
Bond length [Å]	S0	S1	D0
C1–C2	1.391	1.364	1.438
C2–C3	1.391	1.445	1.438
C3–C4	1.389	1.403	1.373
C4–C5	1.389	1.370	1.407
C5–C6	1.389	1.370	1.407
C6–C1	1.389	1.380	1.373
C1–Cl7	1.739	2.358	1.698
C2–F8	1.334	1.325	1.290
C3–Cl9	1.739	1.700	1.698
C4–H10	1.080	1.080	1.081
C5–H11	1.081	1.082	1.082
C6–H12	1.080	1.085	1.081
Bond angles [°]
C1–C2–C3	120.125	123.523	122.001
C2–C3–C4	119.972	119.825	118.077
C3–C4–C5	119.648	117.525	119.488
C4–C5–C6	120.635	120.436	122.869
C5–C6–C1	119.648	123.643	119.488
C6–C1–C2	119.972	115.049	118.077
Cl7–C1–C2	119.336	131.346	118.378
C1–C2–F8	119.938	121.023	118.996
C4–C3–Cl9	120.692	120.051	123.541
Dihedral angle [°]
Cl7–C1–C6–H12	0	−0.018	0
Cl7–C1–C2–F8	0	0.016	0
Cl9–C3–C4–H10	0	−0.005	0

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Table 4 BP86/TZVPP calculated geometries of 1,3,2-DCFB

BP86/TZVPP	C 2v	C 2	C 2v
Bond length [Å]	S0	S1	D0
C1–C2	1.400	1.377	1.444
C2–C3	1.400	1.445	1.444
C3–C4	1.396	1.411	1.382
C4–C5	1.395	1.379	1.411
C5–C6	1.395	1.434	1.411
C6–C1	1.396	1.395	1.382
C1–Cl7	1.740	2.325	1.701
C2–F8	1.341	1.335	1.300
C3–Cl9	1.740	1.708	1.701
C4–H10	1.088	1.088	1.089
C5–H11	1.089	1.090	1.090
C6–H12	1.088	1.094	1.089
Bond angles [°]
C1–C2–C3	120.007	123.861	121.791
C2–C3–C4	120.019	120.100	118.195
C3–C4–C5	119.613	117.257	119.447
C4–C5–C6	120.728	120.361	122.865
C5–C6–C1	119.613	124.461	119.477
C6–C1–C2	120.019	113.959	118.195
Cl7–C1–C2	119.198	130.690	118.269
C1–C2–F8	119.938	121.023	118.996
C4–C3–Cl9	120.783	119.621	123.535
Dihedral angle [°]
Cl7–C1–C6–H12	0	−0.030	0
Cl7–C1–C2–F8	0	0.038	0
Cl9–C3–C4–H10	0	−0.006	0

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Table 5 CC2/cc-pVTZ calculated geometries of 1,3,2-DCFB

BP86/TZVPP	C 2v	C 2
Bond length [Å]	S0	S1
C1–C2	1.395	1.432
C2–C3	1.395	1.432
C3–C4	1.393	1.426
C4–C5	1.393	1.421
C5–C6	1.393	1.421
C6–C1	1.393	1.426
C1–Cl7	1.725	1.719
C2–F8	1.333	1.326
C3–Cl9	1.725	1.719
C4–H10	1.080	1.079
C5–H11	1.081	1.082
C6–H12	1.080	1.079
Bond angles [°]
C1–C2–C3	119.992	121.970
C2–C3–C4	120.078	118.706
C3–C4–C5	119.618	118.910
C4–C5–C6	120.617	122.532
C5–C6–C1	119.618	118.910
C6–C1–C2	120.078	118.706
Cl7–C1–C2	119.105	118.817
C1–C2–F8	120.004	119.015
C4–C3–Cl9	110.817	120.760
Dihedral angle [°]
Cl7–C1–C6–H12	0	−17.842
Cl7–C1–C2–F8	0	17.222
Cl9–C3–C4–H10	0	−17.842

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Table 6 B3LYP/TZVPP calculated geometries of 1,3,2-DFCB

B3LYP/TZVPP	C 2v	C 2v	C 2v
Bond length [Å]	S0	S1	D0
C1–C2	1.393	1.365	1.440
C2–C3	1.393	1.365	1.440
C3–C4	1.384	1.444	1.365
C4–C5	1.389	1.384	1.409
C5–C6	1.389	1.384	1.409
C6–C1	1.384	1.444	1.365
C1–F7	1.340	1.326	1.309
C2–Cl8	1.729	2.361	1.665
C3–F9	1.340	1.326	1.309
C4–H10	1.081	1.081	1.081
C5–H11	1.081	1.080	1.082
C6–H12	1.081	1.081	1.081
Bond angles [°]
C1–C2–C3	117.528	111.693	118.393
C2–C3–C4	121.919	126.328	120.838
C3–C4–C5	118.997	118.962	118.528
C4–C5–C6	120.640	117.726	122.876
C5–C6–C1	118.997	118.962	118.528
C6–C1–C2	117.528	126.328	120.838
F7–C1–C2	118.843	120.164	117.338
C1–C2–Cl8	121.236	124.155	120.803
C4–C3–F9	119.237	113.508	121.823
Dihedral angle [°]
F7–C1–C6–H12	0	0.027	0
F7–C1–C2–Cl8	0	−0.204	0
F9–C3–C4–H10	0	−0.032	0

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Table 7 BP86/TZVPP calculated geometries of 1,2,3-DFCB

BP86/TZVPP	C 2v	C 2v	C 2v
Bond length [Å]	S0	S1	D0
C1–C2	1.401	1.378	1.446
C2–C3	1.401	1.378	1.446
C3–C4	1.391	1.446	1.374
C4–C5	1.396	1.391	1.414
C5–C6	1.396	1.391	1.414
C6–C1	1.391	1.446	1.374
C1–F7	1.348	1.337	1.318
C2–Cl8	1.730	2.332	1.671
C3–F9	1.348	1.337	1.318
C4–H10	1.089	1.089	1.089
C5–H11	1.089	1.087	1.090
C6–H12	1.089	1.089	1.089
Bond angles [°]
C1–C2–C3	117.514	110.326	118.508
C2–C3–C4	121.885	127.177	120.727
C3–C4–C5	118.999	118.870	118.543
C4–C5–C6	120.718	117.579	122.956
C5–C6–C1	118.999	118.870	118.543
C6–C1–C2	117.514	127.177	120.727
F7–C1–C2	118.763	119.276	117.392
C1–C2–Cl8	121.242	124.836	120.740
C4–C3–F9	119.349	113.548	121.870
Dihedral angle [°]
F7–C1–C6–H12	0	0.041	0
F7–C1–C2–Cl8	0	−0.191	0
F9–C3–C4–H10	0	−0.032	0

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Table 8 CC2/cc-pVTZ calculated geometries of 1,2,3-DFCB

B3LYP/TZVPP	C 2v	C 2v
Bond length [Å]	S0	S1
C1–C2	1.396	1.416
C2–C3	1.396	1.416
C3–C4	1.387	1.431
C4–C5	1.394	1.407
C5–C6	1.394	1.407
C6–C1	1.387	1.431
C1–F7	1.339	1.337
C2–Cl8	1.716	1.825
C3–F9	1.339	1.337
C4–H10	1.080	1.825
C5–H11	1.081	1.337
C6–H12	1.080	1.083
Bond angles [°]
C1–C2–C3	117.753	110.391
C2–C3–C4	121.786	125.833
C3–C4–C5	118.972	120.624
C4–C5–C6	120.732	116.095
C5–C6–C1	118.972	120.624
C6–C1–C2	121.786	125.833
F7–C1–C2	118.680	118.508
C1–C2–Cl8	121.124	118.556
C4–C3–F9	119.534	115.651
Dihedral angle [°]
F7–C1–C6–H12	0	0.912
F7–C1–C2–Cl8	0	46.945
F9–C3–C4–H10	0	−0.912

---

A similar situation is found in the MATI spectrum via 9a¹. Here, in contradiction to the Δv = 0 propensity rule, the transition into the S₁6a¹ state is the most favorable while the vertical transition into the 9a¹ is scarcely observable. Nevertheless the mode 9a is strongly present in overtones (9a¹6a¹, 9a¹1¹, 9a²6a¹). The geometry found by the TDDFT calculations (Fig. 6a; Tables 6 and 7) support the experimental findings in predicting a C_2v-symmetric structure distorted along the 6a. The CC2 calculation suggests a 17b-like distorted structure (Fig. 6b; Table 8).

Fig. 6 Excited state geometry of 1,3,2-DFCB obtained by different quantum mechanical methods. (a) TDDFT, (b) CC2.

The CC2 calculation suggests a 17b-like distorted structure (Fig. 6b). Considering the vanishing low activity of 17b in the MATI spectra, the suggestion seems incorrect. Nevertheless the substantial decrease in frequency for 6a¹ and 9a¹ comparing the D₀ state and the S₁ state is accurately reproduced by all quantum chemical methods employed. Comparing experimental and calculated results for 1,3,2-DFCB, the unusual, striking deviation (up to 49%) for a₁-symmetrical S₁-frequencies was particularly noticeable. A poor reproduction of the molecular equilibrium structure in the S₁ (wrong minimum on the PES) could be one possible explanation.

A contrary indication is the fact that we find very similar frequencies for widely differing structures predicted by TDDFT (planar structure) and CC2 method (out-of-plane-structure). The planar structure resembles the experimental observations well, but since all frequency analyses were performed in harmonic approximation, a strong anharmonicity for this particular vibrations could be another explanation for this findings.

6. Conclusion

Comparing the results from two compounds presented in this paper with results obtained from different isomeric trichloro-benzenes,¹ the increasing number of fluorine atoms lead to a progressive decrease in some of the observed frequencies in going from S₀ to S₁ and from D₀ to S₁, respectively. This is especially true for modes with a strong fluorine participation in their vibrational pattern like 17b (cf.Table 1). We interpreted this as a strong indication for an out-of-plane distortion during excitation or a replanarisation during ionization along the eigenvector of those modes. This phenomenon could be attributed to an above, bounding S(πσ*) state which is stabilized by an increasing number of fluorine atoms.¹² It can be concluded that the fluorine atoms contribute a significant share in form of σ* ← π character to the transition.

For 1,3,2-DCFB the quantum chemical calculations gave excitation energies (EE) of 5.06 eV (B3LYP/TZVPP), 4.71 eV (BP86/TZVPP) and 4.44 eV (CC2/cc-pVTZ). The CC2 value reproduces the experimental value of 4.52 eV with a deviation of 0.08 eV best.

The experimental value for the ionization energy (IE) of 9.32 eV is underestimated by both DFT methods with 9.00 eV (B3LYP/TZVPP) and 8.97 eV (BP86/TZVPP). (The coupled cluster method CC2 is not suited for the ionic species!). In this case the frequency analysis performed by the CC2 method showed to be most appropriate to reproduce the experimental frequencies.

For 1,3,2-DFCB the quantum chemical calculations gave excitation energies (EE) of 5.23 eV (B3LYP/TZVPP), 4.90 eV (BP86/TZVPP) and 3.88 eV (CC2/cc-pVTZ). The TDDFT value reproduces the experimental value of 4.64 eV best, whereas BP86 underestimates and B3LYP overestimates the experimental value. Both DFT methods with 9.03 eV (B3LYP/TZVPP) and 9.02 eV (BP86/TZVPP) underestimate the experimental value for the ionization energy (IE) of 9.38 eV. Contrary to the 1,3,2 DCFB the frequency analysis performed by the DFT methods results in a better reproduction of the experimental frequencies for the 1,3,2 DFCB.

Furthermore the S₁ ← S₀ electronic excitation energies (EE) and D₀ ← S₀ adiabatic ionization energies (IE) could be determined very exactly:

EE(1,3,2-DFCB) = 36 460 ± 2 cm⁻¹, IE(1,3,2-DFCB) = 75.242 ± 6 cm⁻¹;

EE(1,3,2-DCFB) = 37 449 ± 2 cm⁻¹, IE(1,3,2-DCFB) = 75.627 ± 6 cm⁻¹.

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