Photoionization Spectroscopic and Theoretical Study on the Molecular Structures of cis- and trans-3-Chlorothioanisole

Abstract

Resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra are measured for the cis- and trans-3-chlorothioanisole (3ClTA). The first electronic excitation energy (E₁) and the adiabatic ionization energy (IE) of the cis-rotamer are determined to be 33 959±3 and 65 326±5 cm^–1, respectively, and those of the trans-rotamer are determined to be 34102±3 and 65 471±5 cm^–1, respectively. Density functional theory (DFT) calculations confirm that both the cis- and trans-rotamers of 3ClTA are stable and coexist in their respective S₀, S₁, and D₀ states. Both rotamers adopt planar structures with cis- being slightly more stable than trans- in the respective S₀, S₁, and D₀ states. The conformation, substitution, and isotope effects on the molecular structure, active vibrations, and electronic transition and ionization energies of 3ClTA are analyzed.

1. Introduction

There have been numerous experimental and theoretical studies for investigating the stable structures and properties of anisole and thioanisole (TA) derivatives in their respective electronic ground (S₀), first excited (S₁), and cationic ground (D₀) states.¹⁻⁹ Due to strong conjugation effect between the benzene ring and the substituent functional groups, most of those molecules are found to be planar. However, several exceptions have been observed. For example, a nonplanar stable structure was observed for trans-2-fluoroanisole (2-F anisole) in its S₀ state with Fourier transform infrared (FTIR) spectroscopy¹⁰ and a small structure tilt was observed for trans-2-fluorothioanisole (2FTA) in its S₁ state through resonance-enhanced two-photon ionization (R2PI) spectroscopy.¹¹ In 2FTA, the enhanced steric effect and inductive effect caused by the 2-substituted (ortho-substituted) F atom play important roles.

For TA derivatives, there exist a lot of controversies, from previous calculations and spectroscopic results,^6,9,12−14 about their stable structures, particularly in their S₁ states. Some theoretical calculations predict their molecular skeletons to be nonplanar in the S₁ state,^12,15 in which the thiomethyl (−SCH₃) group was nearly perpendicular to the benzene ring. The whole molecule has a vertical gauche-like structure. This contradicts the general rule that the benzene ring tends to keep a planar structure as favored by π-orbital delocalization.¹⁶ The nonplanar structures obtained by theoretical calculations do not exactly agree with the spectroscopic experiments.^6,9,15 Nevertheless, most TA derivatives have now been determined to be planar or quasi-planar,^{6,8,9,12−15} and the competition between the steric effect and the inductive effect caused by −SCH₃ was used to explain the difference between calculations and spectroscopic measurements.^6,9,12 However, the reason for the structure tilt seen in the theoretical calculation is still unclear, and it is worthwhile to investigate the stable structures of halogen-substituted anisole and TA derivatives in various electronic states.

The deformation changes of benzene derivatives upon photoexcitation and photoionization are usually similar to each other for molecules with similar structures.^1,6,16−20 For example, benzene ring is usually enlarged in the S₁ state and a quinoidal structure is often found in the D₀ state. It is reasonable to assume that the conjugation effects of −OCH₃ and −SCH₃ groups with the benzene ring are similar since O and S atoms belong to the same group in the periodic table. Compared with the O atom, the S atom is larger in size, which could make the −SCH₃ group more flexible than the −OCH₃ group.^2,14 Investigation on how the −SCH₃ group substitution affects the molecular properties is limited.

Besides the conjugation effect mentioned above, the steric effect and inductive effect caused by the halogen substitutions should also be considered. For example, the cis-rotamer is observed to be stable for 3-fluoroanisole (3-F anisole)²¹ but not for 2-F anisole.^10,22 That is due to the fact that the steric effect between the F atom and the −OCH₃ group is weaker in 3-substituted (meta-substituted) anisole than that in the 2-substituted one. The transition energies (E₁) of Cl-substituted anisoles were found to be red-shifted compared to those of anisole,²³ while those of F-substituted anisoles were found to be blue-shifted.^17,24−26 This is mainly caused by the different inductive effects of F and Cl atoms.

Based on the above discussion, the 3-chlorothioanisole (3ClTA) molecule can be a suitable system for simultaneously comparing the substitution effects between −SCH₃ and −OCH₃ groups and between Cl and F atoms. To the best of our knowledge, the vibrational spectra of 3ClTA in both S₁ and D₀ states have not been reported, and its structural properties have not been investigated. In this work, we reported a combined theoretical and spectroscopic study on the cis- and trans-rotamers of 3ClTA. The vibrational spectra of 3ClTA in S₁ and D₀ states were measured by R2PI and mass-analyzed threshold ionization (MATI) spectroscopy. The stable molecular structures and the rotational barriers between the cis- and trans-rotamers of 3ClTA are calculated by comparing the spectroscopic measurements with the theoretical calculations. The structural changes induced by photoexcitation and photoionization are discussed in detail.

2. Results

2.1. Calculated Results

The chemical structures and the atomic labelings of cis- and trans-3ClTA are shown in Figure 1. The optimized geometric parameters and atomic charges for the two rotamers in S₀, S₁, and D₀ states are listed in Table 1, and their Cartesian coordinates are provided in the Supporting Information (Table S1). The calculated one-dimensional potential energy curves (PECs) of 3ClTA in S₀, S₁, and D₀ states are presented in Figure 2. All PECs were scanned along the C2–C1–S–C8 dihedral angle from −10° to 190° with a 5° increment, and all other structural parameters were optimized without any constraints. The presented energies for the stationary points corresponding to minimum structures include zero-point energy (ZPE) corrections, but ZPE corrections are not included for other points on the PECs.

Figure 1.

Chemical structures and atomic labelings of (left) cis- and (right) trans-3ClTA.

Table 1. Calculated Geometric Parameters and Atomic Charges of cis- and trans-Rotamers of 3ClTA.

	cis-			trans-
	S0	S1	D0	S0	S1	D0
Bond Length (Å)
S–CH3	1.815	1.804	1.809	1.815	1.806	1.811
S–C1	1.774	1.754	1.717	1.775	1.754	1.716
C1–C2	1.394	1.395	1.406	1.399	1.412	1.417
C2–C3	1.389	1.434	1.377	1.383	1.431	1.375
C3–C4	1.385	1.401	1.415	1.390	1.389	1.409
C4–C5	1.392	1.390	1.393	1.387	1.402	1.400
C5–C6	1.384	1.440	1.378	1.391	1.433	1.375
C6–C1	1.400	1.403	1.427	1.395	1.389	1.419
C3–Cl	1.755	1.746	1.719	1.754	1.761	1.722
Bond Angle (o)
C8-S-C1	103.9	106.9	106.7	103.8	106.7	107.2
S-C1-C2	124.1	122.2	125.1	116.0	112.2	114.8
S-C1-C6	116.5	112.9	114.2	124.7	122.9	124.5
C2-C1-C6	119.4	124.9	120.7	119.4	124.8	120.7
C1-C2-C3	119.2	116.0	118.6	119.5	115.6	119.1
C2-C3-C4	122.1	120.7	121.0	121.8	120.9	120.3
C3-C4-C5	118.2	122.0	120.2	118.2	121.9	120.4
C4-C5-C6	121.0	119.0	119.9	121.2	118.7	120.5
C5-C6-C1	120.2	117.5	119.6	119.9	117.8	110.1
C2-C3-Cl	118.6	118.8	120.3	119.0	118.6	120.5
Atomic Charges (e)
H of −CH3 group	0.208	0.238	0.248	0.207	0.238	0.248
	0.208	0.238	0.248	0.207	0.235	0.248
	0.226	0.233	0.261	0.226	0.233	0.261
C8	–0.705	–0.736	–0.729	–0.704	–0.738	–0.729
S	0.270	0.638	0.668	0.270	0.646	0.681
C1	–0.137	–0.195	–0.170	–0.136	–0.200	–0.169
C2 (Ha)	–0.279	–0.348	–0.241	–0.257	–0.372	–0.199
	(0.218)	(0.208)	(0.240)	(0.222)	(0.212)	(0.248)
C3	0.017	–0.104	0.010	0.014	–0.074	–0.014
C4 (Ha)	–0.251	–0.153	–0.079	–0.250	–0.153	–0.072
	(0.218)	(0.208)	(0.243)	(0.218)	(0.207)	(0.243)
C5 (Ha)	–0.165	–0.281	–0.181	–0.162	–0.301	–0.162
	(0.209)	(0.204)	(0.246)	(0.207)	(0.202)	(0.244)
C6 (Ha)	–0.230	–0.320	–0.117	–0.253	–0.282	–0.161
	(0.210)	(0.203)	(0.239)	(0.205)	(0.192)	(0.229)
Cl	–0.018	–0.031	0.116	–0.015	–0.044	0.104

^aThe H atom in parentheses is directly connected to the C atom before the parentheses.

Figure 2.

Calculated one-dimensional potential energy curves (PECs) along the dihedral angle between the −SCH₃ group and the benzene ring in S₀, S₁, and D₀ states of 3ClTA. The presented energies for the stationary points corresponding to minimum structures include zero-point energy (ZPE) corrections, but ZPE corrections are not included for intermediate points on the PECs.

The calculations indicate that different chlorine isotopes have negligible effects on the investigated properties of 3ClTA, such as molecular geometry, excitation (E₁) and ionization (IE) energies, and vibrational frequencies. Such finding is similar to previous studies on several aromatic molecules containing the chlorine substituent.^{1,3,20,23,27,28} Hence, we will focus on the results of the ³⁵Cl-substituted 3ClTA in the following sections for clarity.

The PECs presented in Figure 2 indicate that the cis-rotamer of 3ClTA is slightly more stable than its trans-rotamer in S₀ and D₀ states, while the calculated energies including ZPE corrections for the stationary points in the S₁ state show that the trans-rotamer is slightly more stable than the cis-rotamer. Nonetheless, these energy differences are relatively small, and a more confident assignment needs experimental support. In the S₀ state, the calculated energy difference between the cis- and trans-rotamers is 68 cm^–1, including ZPE corrections, and the isomerization energy barrier is about 500 cm^–1 with respect to the cis-rotamer. A previous study showed that flexible organic molecules with interconversion barriers larger than 400 cm^–1 would not relax significantly during the supersonic expansion.²⁵ Hence, both the cis- and trans-rotamers of 3ClTA could exist in the supersonic molecular beam with an estimated population ratio N_cis/N_trans of about 1.4 according to the Maxwell–Boltzmann distribution under the assumption of thermal equilibrium.

It is noted that in the calculated PEC for the S₁ state (Figure 2), the benzene ring is distorted and the Cl atom is optimized to be out-of-plane for most of the data points, except at the points where the C2–C1–S–C8 dihedral angles are 0 and 180°. Similar behavior has been found previously for the S₁ state of TA.^9,12 Therefore, caution should be taken for quantitative evaluation of the calculated PEC and rotational barrier of 3ClTA in the S₁ state. Nonetheless, at the C2–C1–S–C8 dihedral angles of 0 and 180°, the benzene ring was all optimized to be planar with the Cl atom in the same plane for S₀, S₁, and D₀ states.

A discontinuity at 90° was noted in the PEC of the D₀ state. It is mainly caused by the rotation of the −CH₃ group, and such discontinuity does not affect the optimized results for both cis- and trans-rotamers at 0 and 180° in the D₀ state. Similar discontinuities were observed previously in the PECs of 2-N-methylaminopyridine.²⁶ In this regard, the calculated IEs and relative stabilities of the two rotamers in the D₀ state are assumed to be not affected by the discontinuity at 90°.

2.2. 1C-R2PI Spectra

A typical time-of-flight (TOF) mass spectrum of 3ClTA recorded at the UV wavelength of ∼285 nm is presented in Figure 3. The mass peaks at 159 and 161 amu are assigned, respectively, to the ion signals of ³⁵Cl-3ClTA and ³⁷Cl-3ClTA isotopomers, showing that the mass resolution of the setup is enough for collecting the individual spectra of the two isotopomers of 3ClTA. The 1C-R2PI spectra of the ³⁵Cl and ³⁷Cl isotopomers of 3ClTA in the range of 0–1200 cm^–1 are shown in Figure 4. Tentative assignments of the observed vibrational features are listed in Table 2, and the Varsányi’s labeling system is adopted to approximately describe the observed benzene-like vibrational modes of 3ClTA, which should be classified as an meta-di-heavy substituted benzene derivative.²⁹

Figure 3.

Typical time-of-flight (TOF) mass spectrum of 3ClTA.

Figure 4.

1C-R2PI spectra of ³⁵Cl-3ClTA (top) and ³⁷Cl-3ClTA (bottom). Letters “c” and “t” in parentheses represent the cis-rotamer and trans-rotamer, respectively.

Table 2. Observed Vibrational Frequencies (in cm^–1) in the 1C-R2PI Spectra of cis- and trans-3ClTA and Their Tentative Assignments.

cis-3ClTA				trans-3ClTA
exptl.	vib.	theo.a	assignmentb	exptl.	vib.	theo.a	assignmentb
33 959	0		band origin	34 102	0		band origin
34 005	46	48	τ01,torsion	34 126	24	33	τ01,torsion
34 056	97	96c	τ02,torsion	34 143	41	66c	τ02,torsion
34 168	209	213	1501	35 065	963	954	101, breathing
34 308	349	360	6a01, β(C–C–C)
34 368	409	408c	6a01τ0
34 576	617	621	νs1, stretching
34 774	815	790	1201, β(C–C–C)
34 912	953	960	101, breathing

^aThe theoretical values have been scaled by a scaling factor of 0.967.

^bThe torsion vibration and stretching vibration are denoted by τ and ν, respectively. The bending mode is designated by β.

^cThe calculated frequency for these overtone and combination modes are taken from the sum of the calculated individual frequency of each mode.

As shown in Figure 4, the first electronic excitation energies of cis- and trans-³⁵Cl-3ClTA are identical to those of ³⁷Cl-3ClTA, confirming that the Cl isotope substitution has negligible effects on the first electronic excitation energies of 3ClTA. Most of the vibrational frequencies are close to each other for the two 3ClTA isotopomers within 3 cm^–1. For example, the 6a₀¹, ν_s, and 1₀¹ bands locate at 349, 617, and 953 cm^–1 for ³⁵Cl-3ClTA, and 351, 615, and 955 cm^–1 for ³⁷Cl-3ClTA, respectively. The relative intensities of several vibrational bands are different between the two isotopomers, which is presumably attributed to the edge eliminating effect in choosing the mass range to obtain the reduced spectra during experiments. Those experimentally observed similarities between the two 3ClTA isotopomers verified our predictions derived from theoretical calculations, allowing us to discuss only the spectrum of ³⁵Cl-3ClTA in the following sections.

3ClTA has 42 normal modes, and the intensity of the vibronic bands is proportional to the Franck–Condon factors (FCFs), and thus not all vibrational modes, for instance, in the cases of very small FCFs, could be observed in the 1C-R2PI process. The assignments of the observed bands to a specific rotamer have been confirmed by measuring the respective IEs using the MATI spectroscopic method.

According to the calculations, the cis-rotamer is slightly more stable than the trans-rotamer in the S₀ state and should have a higher population in the supersonic molecular beam and thus corresponds to a stronger band origin peak in the 1C-R2PI spectrum. Hence, the observed vibrational band at 33959 cm^–1 is tentatively assigned to the cis-rotamer’s band origin (S₁ ← S₀; 0₀⁰ band). This band origin corresponds to E₁. The band at 34102 cm^–1 is tentatively assigned to the trans-rotamer’s 0₀ band. The experimentally measured 0₀⁰ band of the trans-rotamer is blue-shifted by 143 cm^–1 with respect to that of the cis-rotamer. These assignments are also consistent with the calculated results that the cis-rotamer is more stable than the trans-rotamer in the D₀ state. However, these results seem not to be consistent with the calculations in the S₁ state. Such discrepancy could be attributed to the expected difficulty of TDDFT in accurately predicting the electronically excited states.

Besides the 0₀⁰ bands, the bands at 46 and 97 cm^–1 are assigned as the methyl torsion vibrational mode τ₀ and its overtone τ₀² of the cis-rotamer, in good agreement with the calculated methyl torsion vibration of 48 cm^–1 and its overtone of 96 cm^–1 (Table 2; also see Figure S1 in the Supporting Information for the vibrational vectors); the band at 209 cm^–1 is assigned as the cis-15₀ mode, and the corresponding calculated value is 213 cm^–1; the bands at 349, 617, 815, and 953 cm^–1 are assigned to the in-plane ring deformation modes 6a₀¹, ν_s, 12₀¹, and 1₀ of the cis-rotamer, also agreeing with the calculated values of 360, 621, 790, and 960 cm^–1; and a combination band of 6a₀¹ and τ₀ is observed at 409 cm^–1 (Figure 4; the calculated 6a₀¹ + τ₀ sum frequency is 408 cm^–1). As for the trans-rotamer, the observed bands at 24 and 41 cm^–1 are assigned to the methyl torsion mode τ₀¹ and its overtone τ₀, and the band at 963 cm^–1 is assigned to the in-plane ring deformation mode 1₀¹. Those measured values are well reproduced by the calculations, i.e., 33 and 954 cm^–1 for τ₀ and 1₀¹, respectively. The above mode assignments are summarized in Table 2, and in Figure S1 in the Supporting Information, their vibrational vectors are provided.

2.3. MATI Spectra

The MATI spectra and the assignments of the vibrational bands observed for the cis- and trans-rotamers of ³⁵Cl- and ³⁷Cl-3ClTA are presented in this section. For the cis-rotamer, MATI spectra were recorded via the vibrational peaks at 0, 46, 349, and 409 cm^–1, which show relatively high intensities in the 1C-R2PI spectra; for the trans-rotamer, MATI spectra via the band origin 0₀⁰ at 0 cm^–1 and the τ₀ mode at 24 cm^–1 were collected. Except for several vibrational peaks that were not observed due to the low intensity, the MATI spectra of ³⁷Cl-3ClTA are nearly identical to those of ³⁵Cl-3ClTA for both rotamers (Figure 5). All vibrational frequencies of the two isotopomers were observed within 7 cm^–1, close to the experimental uncertainty (±5 cm^–1). Thus, only the MATI spectra of ³⁵Cl-3ClTA are discussed here.

Figure 5.

MATI spectra of cis-³⁵Cl-3ClTA (top) and ³⁷Cl-3ClTA (bottom) recorded via the 0₀⁰ vibrational mode in the S₁ state.

As shown in Figures 5 and 6, a strong prominent band origin in the D₀ state was observed for both cis- and trans-rotamers via the 0₀⁰ vibrational mode in the S₁ state, suggesting that the molecular shapes of both rotamers do not change significantly upon photoionization from the S₁ to D₀ state. The structural change has been quantitatively evaluated by calculating the root-mean-square deviation (RMSD) for distances, and it can be seen that the RMSD values between S₁ and D₀ are 0.029 and 0.123 Å for cis- and trans-rotamers, respectively (Figure S2). The band origins were measured to be 65 323 and 65 468 cm^–1 for the cis- and trans-rotamers, respectively. After correcting the direct-current Stark shift, the IEs of cis- and trans-rotamers were determined, respectively, to be 65 326±5 and 65 471±5 cm^–1, giving a measured difference of IE values to be 145 cm^–1. These values agree reasonably with the calculated respective values of 63 205, 63 456, and 251 cm^–1 including ZPE corrections (Figure 2).

Figure 6.

MATI spectra of trans-³⁵Cl-3ClTA (top) and ³⁷Cl-3ClTA (bottom) recorded via the 0₀⁰ vibrational mode in the S₁ state.

The assignments of all of the vibrational modes observed in the MATI spectra are listed in Table 3. Most of the observed active vibrations are found to be related to the ring deformation and substituent-sensitive modes. For the cis-³⁵Cl-3ClTA, the bands at 88 and 177 cm^–1 are assigned as the methyl torsion vibrational mode τ¹ and its overtone τ², respectively. Note that the calculated methyl torsion vibrational frequency is 83 cm^–1. The bands at 223, 652, and 980 cm^–1 are assigned as the vibrational modes of 15¹, ν_s¹, and 1¹, respectively, agreeing with the calculated respective values of 270, 629, and 978 cm^–1; the bands at 289 and 446 cm^–1 are assigned as combination modes. The assignment of the prominent band at 398 cm^–1 in the MATI spectra via the 0₀ mode (Figures 5 and 6) is uncertain. It could be due to either the 6a¹ or 16b¹ mode. The 6a¹ mode is observed to be prominent in the 1C-R2PI spectra (Figure 4) with a vibrational frequency of 349 cm^–1. This observation seems to support the assignment of the calculated 381 cm^–1 band to the 6a¹ mode. However, the vibrational frequency of the 6a¹ mode is measured to be ∼366 cm^–1 instead of 398 cm^–1 in the MATI spectra when the τ₀¹ and 6a₀ modes in the S₁ state were used as the intermediate levels (Figure 7a,b). Similar phenomena have been observed previously for 2FTA.¹¹ For example, the 6a¹ band in the D₀ state of 2FTA was observed at 411 cm^–1via the 0₀⁰ mode in the S₁ state, while it was measured to be 400 cm^–1 when the τ₀6a₀¹ mode was used as the intermediate level. For the trans-³⁵Cl-3ClTA, the frequencies of most benzene-ring-related vibrational modes are observed to be close to those of the cis-³⁵Cl-3ClTA. The most prominent peak in the MATI spectrum of trans-³⁵Cl-3ClTA via the methyl torsion mode τ₀ at 24 cm^–1 (Figure 7e) is tentatively assigned as β(C-SCH₃) + β(C–Cl), in reasonable agreement with the calculated value of 154 cm^–1 (Table 3; see also Figure S1 in the Supporting Information for the vibrational vectors). The vibrational frequency for overtone τ³ should be also close to ∼120 cm^–1, while activation of τ³ is usually highly unfavored.

Table 3. Observed Vibrational Frequencies (in cm^–1) in the MATI Spectra of the cis- and trans-Rotamers of 3ClTA and Their Tentative Assignments.

cis-							trans-
exptl.							exptl.
via 000	via 46 cm–1	via 349 cm–1	via 409 cm–1	via 617 cm–1	theo.a	assignmentb	via 000	via 24 cm–1	theo.a	assignmentb
0	0	0	0	0			0	0
88					83	τ1	34	33	70	τ1
177	178				166c	τ2		81		τ2
								127	154	β(C-SCH3) + β(C–Cl) or τ3
223	225				270	151	177	186	182	151
289	310				353c	151τ1	223	210	252c	151τ1
398	367	366			381	6a1 or 16b1	399		400	6a1 or 16b1
446	446		441		464c	6a1τ1
652				653	629	νs1	655		627	νs1
980					978	11	981		968	11

^aThe theoretical values have been scaled by a scaling factor of 0.967.

^bThe denotations are according to the 1C-R2PI spectra in Table 2, and the torsion vibration and stretching vibration are denoted by τ and ν, respectively. The bending mode is designated by β.

^cThe calculated frequency for these overtone and combination modes are taken from the sum of the calculated individual frequency of each mode.

Figure 7.

MATI spectra of cis- and trans-³⁵Cl-3ClTA recorded via different vibrational modes in the S₁ state.

In Figure 7, MATI spectra via several other vibrational bands are presented. It can be seen that when the MATI spectra were recorded via the in-plane ring deformation modes 6a₀¹, ν_s, and the combination mode 6a₀¹τ₀ as the intermediate levels, the most intensive vibrational peaks observed in the MATI spectra correspond to the same modes of the intermediate levels (Figure 7b,d). This indicates that the molecular geometry and the corresponding vibrational coordinates of the 6a₀¹, ν_s, and 6a₀¹τ₀ modes do not change much upon photoionization from the S₁ state to the D₀ state, as verified by the RMSD analysis (Figure S2). Similar phenomena via the benzene-ring-related vibrational levels were previously observed for anisole and its derivatives.^{1,17,21,23,30} However, when the methyl torsion mode τ₀¹ was used as the intermediate level, the MATI spectra showed dramatically different characteristics as those via the benzene-ring-related vibrational levels. For the cis-rotamer, the most prominent peak was due to the 0⁺ band, and many other vibrational modes were also significantly excited (Figure 7a). The MATI spectrum of the trans-rotamer via the methyl torsion mode τ₀ at 24 cm^–1 is completely different from that of the cis-rotamer, the 0⁺ band was much weaker, and several low-frequency vibrational modes were strongly excited (Figure 7e). These observations indicate that the methyl torsion mode vibrations are strongly coupled to many other vibrational modes for both the cis- and trans-rotamers during the photoionization process, but their detailed coupling mechanisms should be quite different due to the different orientations of the −SCH₃ groups in the two rotamers (see the torsion mode vibration vectors in Figure S1).

3. Discussion

3.1. Molecular Structures of cis- and trans-3ClTA in S₀, S₁, and D₀ States

3.1.1. Molecular Structures in S₀ State

Both cis- and trans-3ClTA are stable and adopt planar structures possessing a C_s point group in the S₀ state. The cis-rotamer is slightly more stable than the trans-rotamer, suggesting that the inductive effect of the Cl atom dominates over the steric effect between the two substituent groups in determining the relative stability of the two rotamers. As seen in Table 1, the S–C8 bond in the −SCH₃ group is 1.815 Å in the cis-rotamer and trans-rotamer, and ∠SC1C2 is 124.1° in the cis-rotamer and 116.0° in the trans-rotamer. The slightly larger ∠SC1C2 in the cis-rotamer may arise from the larger steric repelling effect between the Cl atom and the −SCH₃ group in the cis-rotamer than that in the trans-rotamer. This is similar to the larger ∠NC1C2 in cis-3-Cl-N-methylaniline than that in the trans-rotamer.¹⁶

As for the benzene ring, all of the calculated bond length differences between the two rotamers are less than 0.007 Å and all the bond angle differences are less than 0.4°, indicating that the influences on the structure of the benzene ring caused by the different orientations of the substituent groups in the two rotamers are small.

From the perspective of intramolecular charge distributions, the two rotamers are close to each other. Table 1 shows that their calculated atomic charges in the S₀ state are almost identical, except for the C2 and C6 atoms. The C2 atom is more negative in the cis-rotamer (−0.279 e for the cis-rotamer and −0.257 e for the trans-rotamer), while the C6 atom is more negative in the trans-rotamer (−0.230 e for the cis-rotamer and −0.253 e for the trans-rotamer). This could be caused by the electron-donating effect of the −CH₃ group, and the C atom closer to the −CH₃ group gets more negative charge.

3.1.2. Structural Changes during the S₁←S₀ Transition

The intense 0₀⁰ bands observed in the 1C-R2PI spectra in Figure 4 suggest that there are only minor geometric changes upon excitation from the S₀ state to the S₁ state, as verified by the RMSD analysis (Figure S2). According to the calculated results shown in Table 1, the C1–C2, C2–C3, C3–C4, C5–C6, and C6–C1 bonds in the benzene ring become a little longer upon excitation while the C4–C5 bond stays about the same for the cis-rotamer; for the trans-rotamer, the C1–C2, C2–C3, C4–C5, and C5–C6 bonds become longer during the S₁ ← S₀ transition, while the C3–C4 and C6–C1 bonds are contracted by 0.001 and 0.006 Å, respectively. The above observations are consistent with the previous studies, which showed that the S₁ ← S₀ transitions of benzene derivatives are mainly subject to the π* ← π electronic excitation and a benzene ring distortion usually occurs, making it deviate from a perfect hexagon.^31,32 Such transition scheme is confirmed by analyzing the molecular orbitals involved in the first electronic transition (Figure S3).

Besides the benzene ring distortion, the C3–Cl bond connecting the benzene ring and the Cl atom is shortened by 0.009 Å for the cis-rotamer during the S₁ ← S₀ transition, while it is elongated slightly by 0.007 Å for the trans-rotamer. This may indicate that the steric effect between the Cl atom and the −SCH₃ group becomes weaker and/or the interaction between the Cl atom and the benzene ring becomes stronger in the S₁ state for the cis-rotamer. The −SCH₃ group is also distorted during the S₁ ← S₀ transition. The S–C1 bond connecting the −SCH₃ group with the benzene ring is shortened by 0.020 Å for the cis-rotamer and 0.021 Å for the trans-rotamer. The S–C8 bond connecting the S atom with the −CH₃ group is shortened by 0.011 Å for the cis-rotamer and 0.009 Å for the trans-rotamer. This is consistent with the change of the intramolecular charge distribution upon photoexcitation. As shown in Table 1, all H atoms in the −CH₃ group and the S atom become more positive upon the excitation, while all H atoms directly connected to the benzene ring, and all of the C atoms of the benzene ring become more negative in the S₁ state, except for C4, which becomes less negative and is furthest from the −SCH₃ group. The shortened bond lengths and the intramolecular charge distribution changes in the S₁ state enhance the p−π conjugation interaction between the −SCH₃ group and the benzene ring (Figure S3). Some anisole derivatives, such as 3-F anisole²¹ and 3-Cl anisole,²³ undergo similar ring deformation upon excitation from the S₀ state to the S₁ state.

3.1.3. Structural Changes during Photoionization

Both the measured spectra and theoretical calculations suggest that both of the 3ClTA rotamers adopt a stable planar structure in the D₀ state. The calculated results show that for both rotamers, the benzene ring of 3ClTA has longer C1–C2, C3–C4, C4–C5, and C6–C1 bonds and shorter C2–C3 and C5–C6 bonds in the D₀ state than those in the S₀ state, showing a quinoidal structure. The chlorine atom and all the C atoms of the benzene ring except for C1 that connects with the −SCH₃ group become more positive (or less negative) in the D₀ state than those in the S₁ state upon photoionization; while the changes of the charges on C1 and the −SCH₃ group are comparably smaller. This implies that the ejected photoelectron is mainly contributed by the nonbonding p orbitals of the S and Cl atoms and the π orbital of the benzene ring (see the plotted highest molecular orbitals in Figure S3).

For the −SCH₃ group, the S–C1 bond shows a very strong double bond character in the D₀ state, and it is significantly shorter (1.718 Å for the cis-rotamer and 1.716 Å for the trans-rotamer) than that in S₀ (1.774 Å for the cis-rotamer and 1.775 Å for the trans-rotamer) and S₁ states (1.754 Å for the cis-rotamer and 1.754 Å for the trans-rotamer). This is consistent with the spectroscopic measurements that the frequencies of the methyl torsion mode τ¹ for both rotamers become significantly larger in the D₀ state (88 cm^–1 for the cis-rotamer and 34 cm^–1 for the trans-rotamer) than those in the S₁ state (46 cm^–1 for the cis-rotamer and 24 cm^–1 for the trans-rotamer). The above observations indicate that the conjugation interaction between the p orbital occupied by the remaining nonbonding electron on the S atom and the π orbital on the benzene ring is enhanced in the photoionization process.

For the Cl atom, the C3–Cl bond is also significantly shorter in the D₀ state (1.719 Å for the cis-rotamer and 1.722 Å for the trans-rotamer) than that in S₀ (1.755 Å for the cis-rotamer and 1.754 Å for the trans-rotamer) and S₁ states (1.746 Å for the cis-rotamer and 1.761 Å for the trans-rotamer). The Cl atom can interact with the benzene ring through the p-π conjugative effect, which arises from the donation of a lone pair electron from the Cl atom to the benzene ring, thus the conjugative effect between the Cl atom and the benzene ring may also be enhanced due to the shortened C3–Cl bond in the D₀ state and the rigidity of the benzene ring could be increased. This is in accordance with the spectroscopic observations. For example, the frequencies of the in-plane ring deformation mode 1¹ in the D₀ state (980 cm^–1 for the cis-rotamer and 981 cm^–1 for the trans-rotamer) are slightly larger than those in the S₁ state (953 cm^–1 for the cis-rotamer and 963 cm^–1 for the trans-rotamer), indicating a more rigid benzene ring in the D₀ state.

3.2. Active Vibrational Modes and Their Frequencies

The present 1C-R2PI and MATI spectroscopic measurements show that the in-plane ring deformation modes 6a₀¹, ν_s, and 1₀¹ of the cis-rotamer have frequencies of 349, 617, and 953 cm^–1 in the S₁ state, and, respectively, become 366 (or 398), 652, and 980 cm^–1 in the D₀ state; the in-plane ring deformation mode 1₀ of the trans-rotamer has a frequency of 963 cm^–1 in the S₁ state, and becomes 981 cm^–1 in the D₀ state. Thus, almost all of the benzene ring-related vibrational modes have higher frequencies for both the cis- and trans-rotamers during the photoionization process, indicating that the conjugation effects between the benzene ring and the substituent groups are enhanced in the D₀ state. This increases the rigidity of the benzene ring. Similar vibrational frequency enhancement by photoionization was observed for the substituent-sensitive modes, for example, the methyl torsion mode τ₀¹, as presented above.

The current study along with several previous studies on other anisole derivatives^21,23,27,30 and TA derivatives^6,11 show that the frequencies of the benzene-ring-related modes are not very sensitive to the orientations of the substituents. In this study, the frequencies of the substituent-sensitive modes, e.g., the methyl torsion mode τ₀¹, are measured for both the cis- and trans-3ClTA. Its frequency for the cis-rotamer is 46 cm^–1 in the S₁ state, and 88 cm^–1 in the D₀ state, while it is only 24 and 34 cm^–1 in the S₁ and D₀ states of the trans-rotamer. The MATI spectra via the methyl torsion mode τ₀ for the two rotamers as shown in Figure 7a,e show that the coupling scheme of the methyl torsion mode τ₀¹ to other vibrational modes is strongly rotamer-dependent. This may be caused by the different through-space interactions between the two substituents Cl and −SCH₃ in the two rotamers. The two substituents are closer to each other, and thus should have stronger through-space interaction in the cis-rotamer than that in the trans-rotamer. That makes the methyl group in the cis-rotamer harder to move, resulting in a larger vibrational frequency.

It should also be noted that the methyl torsion mode τ₀¹ has been observed to be active in TA,^6,9,12trans-2FTA,¹¹ and cis- and trans-3ClTA, while it was not observed for stable anisole derivatives, such as 2-F anisole,²² 3-Cl anisole,²³ and 3-F anisole.²¹ This indicates that the methyl group on the more flexible −SCH₃ group is easier to be activated.

3.3. Substitution Effect on the Electronic Excitation and Ionization Energies

The measured electronic excitation transition energies (E₁) and ionization energies (IE) of several anisole and TA derivatives are listed in Table 4. It can be seen that the E₁ and IE values depend on both the nature and location of the substituent halogen atom.

Table 4. Transition and Ionization Energies (in cm^–1) of Anisole and TA Derivatives.

	E1	ΔE1	IE	ΔIE		E1	ΔE1	IE	ΔIE
thioanisole (TA)a	34 506		63 906		anisole	36 383d	0	66 399e
trans-o-fluorothioanisoleb	34 974	468	65 114	1208	trans-o-fluoroanisolef	36 611	228	67 354	955
cis-m-fluorothioanisolec	34 820	314	65 468	1562	cis-m-fluoroanisoleg	36 662	279	67 867	1468
trans-m-fluorothioanisolec	35 047	541	64 644	1738	trans-m-fluoroanisoleg	36 819	436	68 304	1905
cis-m-chlorothioanisole	33 959	–547	65 326	1420	cis-m-chloroanisolei	35 822	–561	67 645	1246
	(33 580)h		(63 205)h
trans-m-chlorothioanisole	34 102	–404	65 471	1565	trans-m-chloroanisolei	35 868	–515	68 008	1609
	(33 434)h		(63 456)h

^aFrom ref (6).

^bFrom ref (11).

^cFrom our unpublished data.

^dFrom ref (2)

^eFrom ref (44).

^fFrom ref (17).

^gFrom ref (21).

^hValues in parentheses represent theoretical values calculated in this work.

ⁱFrom ref (23).²³

For E₁ values shown in Table 4, the E₁ values of the Cl-substituted TA derivatives, cis- and trans-3ClTA, are red-shifted by 547 and 404 cm^–1, respectively, relative to that of TA,^6,9,12 while the E₁ values of the F-substituted TA derivatives, the trans-2FTA,¹¹ and cis- and trans-3FTA, are all blue-shifted compared to that of TA.^6,9,12 Similar phenomena have been observed between anisole and its F- and Cl-substituted derivatives.^2,17,21,23 A previous study showed that the substitution of a functional group on the benzene ring can lower its zero-point energy (ZPE) level.²¹ The red-shifted E₁ values of 3ClTA imply that the interaction of Cl with the benzene ring is stronger in the S₁ state than that in the S₀ state, resulting in a larger ZPE lowering of the S₁ state than that of the S₀ state.

4. Conclusions

The 1C-R2PI and MATI spectra of cis- and trans-3ClTA are obtained, implying that both the cis- and trans-rotamers coexist in the experiment and have planar structures in their respective S₀, S₁, and D₀ states. The cis-rotamer is slightly more stable than the trans-rotamer with an isomerization barrier of ∼500 cm^–1. The first electronic excitation and ionization energies of cis-3ClTA are determined to be 33 959±3 and 65 326±5 cm^–1, and those of trans-3ClTA to be 34 102±3 and 65 471±5 cm^–1, respectively. The two Cl isotopes are found to have negligible effects on the molecular properties of 3ClTA. The E₁ and IE values are red-shifted by the Cl substitution for both rotamers, similar to the anisole derivatives.

Most of the active vibrations are found to be the in-plane ring deformation and substituent-sensitive modes. Their vibrational frequencies are generally measured to be higher in the D₀ state than those in the S₁ state, indicating that the conjugation interaction between the benzene ring and the substituted functional groups and the through-space interaction between the two substituents are enhanced during the photoionization process. This is confirmed by the molecular structural changes due to photoionization as revealed by theoretical calculations. The frequencies of the in-plane ring deformation modes are found to be not sensitive to the relative orientation between the two substituents, while the frequencies of the substituent-sensitive modes and their coupling schemes with other vibrational modes are found to be strongly rotamer-dependent.

5. Experimental and Computational Methods

5.1. Experimental Methods

The photoionization time-of-flight (TOF) mass spectrometer used in the current study has been described previously,^5,20 and only a brief description is given here. The 3ClTA sample with a purity of 99% was purchased from Alfa Aesar and used without further purification. It was seeded in the Ar carrier gas (∼2 atm) and then expanded into a vacuum chamber through a pulsed General valve with a nozzle diameter of 0.25 mm. The molecular beam passed through a skimmer with a diameter of 1 mm located at 20 mm downstream from the nozzle and then entered the photoionization region of the mass spectrometer, which is 70 mm from the nozzle. In the photoionization region, the molecular beam was crossed by one or two ultraviolet (UV) laser beams perpendicularly and photoionized for detection.

In the one color resonant two-photon ionization (1C-R2PI) spectroscopic measurement, only one UV laser beam was used. The 3ClTA molecule absorbed a single UV photon to be excited to the S₁ state first, and then the 3ClTA molecule in the S₁ state absorbed a second UV photon to go above the photoionization threshold. The ions thus produced by the UV laser were immediately accelerated by two direct-current electric fields of 200 and 2100 V/cm, respectively. In the MATI experiment, two counterpropagating UV laser beams of different wavelengths were used. The first UV laser beam was the same as that used in the 1C-R2PI experiment and was used to excite the 3ClTA molecule to its S₁ state; the second UV beam excited the molecules from the S₁ state to the high-n-Rydberg states, which are slightly below the ionization thresholds. The photoionization region was field-free during the photoexcitation process, and after about 200 ns, a pulsed electric field of −0.5 V/cm was switched on to reject the prompt ions, which were usually generated together with the high-n-Rydberg neutrals. After a delay of 11 μs, two pulsed electric fields of +200 and +2000 V/cm were switched on synchronously to field ionize the high-n-Rydberg neutrals and accelerate the produced ions to the detector. The ions produced in the above two processes were focused by an Einzel lens and flew through a 1.0 m long field-free tube toward a dual-stacked microchannel plate (MCP) detector. After passing through a preamplifier (SR445, Stanford Research System), the TOF signal was amplified by 25 times and collected by a multichannel scaler (SR430, Stanford Research System).

The UV lasers used in this experiment were generated by doubling the outputs of two tunable dye lasers (Sirah-CSTR), which were pumped by the second harmonic output of an Nd:YAG laser (Quanta-Ray, Pro-230-10E) at a repetition rate of 10 Hz. Three laser dyes, Pyrromethene 597, Pyrromethene 580, and DCM, were used in this work. Two pulse delay generators (DG535, Stanford Research System) were used to synchronize the whole system. Typical pressures of ∼3.0 × 10^–3 and ∼3.0 × 10^–5 Pa were maintained in the source and ionization chambers during operation.

5.2. Computational Methods

All calculations in this study were carried out with the Gaussian09 program package.³³ The stable structures and harmonic vibrational frequencies of 3ClTA in S₀, S₁, and D₀ states were calculated with density functional theory (DFT), in which time-dependent DFT (TDDFT)^34,35 was used for the excited state S₁. The B3LYP functional^36,37 and cc-pVTZ basis set³⁸⁻⁴⁰ were employed for all the calculations, and no imaginary frequencies were found for the optimized stationary points. The relaxed potential energy curve (PEC) in the S₀ state was scanned to illustrate the isomerization barrier between the cis- and trans-3ClTA. The calculated vibrational frequencies of 3ClTA in S₀, S₁, and D₀ states were scaled by 0.967⁴¹ to approximately account for the vibrational anharmonicity effect. The charge distributions based on natural population analysis (NPA)⁴² were conducted by Natural Bond Orbital (NBO) version 3.1 program implemented in the Gaussian09 program package.⁴³

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c06003.

• Vibrational vectors of selected modes (Figure S1), overlaid structures and RMSD values for distances (Figure S2), molecular orbitals (Figure S3), Cartesian coordinates (Table S1), and numerical data of Figures 4–7 (Tables S2–S5) (PDF)

The authors declare no competing financial interest.