Spectroscopic Identification and Characterization of Three Rotamers of m-Ethoxyphenol: Combined REMPI, MATI, and Quantum Chemical Study

Abstract

Rotational isomers (rotamers) of substituted aromatic molecules exhibit distinct physicochemical properties that are fundamental to understanding their reactivity and biological functions. However, resolving individual rotamers spectroscopically remains challenging due to their similar transition energies and overlapping spectral features. Herein, we report the conformer-specific identification and characterization of three stable rotamers of m-ethoxyphenol using a combination of resonance-enhanced multiphoton ionization (REMPI), hole-burning (HB) spectroscopy, and mass-analyzed threshold ionization (MATI) techniques, complemented by high-level quantum chemical calculations at the B3PW91/aug-cc-pVTZ and G4 levels of theory. The S₁ ← S₀ electronic origins of rotamers I, IV, and III were determined to be 35,966 ± 2, 36,031 ± 2, and 36,198 ± 2 cm⁻¹, respectively, while their corresponding adiabatic ionization energies (IEs) were precisely measured as 64,574 ± 5, 64,122 ± 5, and 64,994 ± 5 cm⁻¹. The vibrational spectra of both the S₁ excited state and the D₀ cationic ground state were assigned, with most active modes corresponding to in-plane benzene ring vibrations. Structural analysis reveals that the benzene ring undergoes slight expansion upon S₁ ← S₀ excitation and contraction upon D₀ ← S₁ ionization, while the overall molecular geometry remains remarkably similar across all three electronic states, a key factor underlying the excellent agreement between experimental and simulated Franck–Condon spectra. Comparison with m-methoxyphenol demonstrates that the stronger electron-donating ability of the ethoxy group leads to systematically lower excitation and ionization energies. The distinct spectroscopic fingerprints established herein provide a definitive reference for identifying specific m-ethoxyphenol rotamers in future studies of this molecule and its complexes.

1. Introduction

Phenolic derivatives constitute a fundamental class of compounds with widespread importance in biological systems, pharmaceutical chemistry, and materials science. As the base chromophore of the essential amino acid tyrosine, phenol and its substituted derivatives have attracted sustained research interest due to their pivotal role in ultraviolet (UV) absorption processes that underlie photochemical and photobiological phenomena [1,2]. Among these derivatives, alkoxyphenols—bearing both hydroxyl and alkoxy substituents—are particularly intriguing because both functional groups can participate in chemical reactions through electron donation to the aromatic ring [3]. The rotational flexibility about the single bonds connecting these substituents to the benzene ring gives rise to multiple rotational isomers, or rotamers, which can coexist in chemical samples and exhibit distinct physicochemical properties [4,5].

The existence of multiple rotamers poses both opportunities and challenges. While rotamers may display markedly different chemical reactivity and biological activity [6], their similar molecular structures lead to electronic excitation energies (E₁) and ionization energies (IEs) that differ by only tens to hundreds of wavenumbers [7]. Conventional spectroscopic techniques such as infrared (IR), Raman, or fluorescence emission spectroscopy generally lack the resolution to disentangle the overlapping spectral features arising from different rotamers in a mixture. This limitation necessitates the application of conformer-specific spectroscopic methods combined with supersonic molecular beam cooling, which isolates individual rotamers in a low-temperature environment and suppresses conformational interconversion [8,9].

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy coupled with time-of-flight (TOF) mass spectrometry provides a powerful approach for recording vibronic spectra of mass-selected species [10,11,12,13,14,15]. However, even with mass selectivity, spectra of mixtures containing multiple rotamers can be congested due to overlapping vibronic transitions, as different rotamers share the same mass. Hole-burning (HB) spectroscopy addresses this limitation by optically selecting individual conformers, enabling unambiguous assignment of vibronic bands to specific rotamers [16,17,18,19,20,21]. For investigating cationic state properties, mass-analyzed threshold ionization (MATI) spectroscopy offers unique advantages [22,23,24,25,26]. This technique combines the high resolution of zero-kinetic energy (ZEKE) photoelectron spectroscopy with mass selectivity [27], allowing precise determination of adiabatic IEs and vibrational frequencies of selected rotamer cations. The MATI method has proven particularly valuable for studying isotopes [28,29], molecular clusters [30,31,32], and radicals [33] due to its mass-resolving capability.

meta-Ethoxyphenol (m-ethoxyphenol, C₆H₄(OH)(OC₂H₅)) presents an interesting case for conformational analysis. With both hydroxyl and ethoxy groups attached to the benzene ring in a meta relationship, multiple stable rotamers can arise from different orientations of these substituents. The ethoxy group itself possesses two rotationally flexible bonds (O–CH₂ and CH₂–CH₃), further enriching the conformational landscape. Previous studies on related molecules have established important precedents. Ullrich et al. thoroughly characterized three rotamers of m-methoxyphenol using ZEKE and hole-burning spectroscopy [34,35], while Wilke et al. employed rotationally resolved electronic spectroscopy to elucidate the conformational space of the same molecule [36]. For para-substituted analogues, Zheng et al. investigated p-ethoxyphenol rotamers using MATI spectroscopy [37], and Li et al. studied p-methoxyphenol cations [38]. These investigations demonstrate that alkoxy substitution systematically lowers excitation and ionization energies, with the magnitude of the shift correlating with the electron-donating strength of the substituent.

In this work, we present a comprehensive spectroscopic investigation of m-ethoxyphenol rotamers using complementary techniques: potential energy surface (PES) scanning to identify stable conformers, REMPI spectroscopy combined with HB to resolve vibronic spectra of individual rotamers, and MATI spectroscopy to precisely determine adiabatic IEs and characterize cationic vibrations. High-level quantum chemical calculations at the B3PW91/aug-cc-pVTZ and G4 levels support experimental assignments and provide insights into structural changes accompanying electronic excitation and ionization. By comparing our results with those for m-methoxyphenol [34,35,36], we elucidate the influence of ethoxy versus methoxy substitution on molecular properties. The conformer-specific spectroscopic fingerprints established herein serve as essential references for future studies of m-ethoxyphenol-containing systems, including molecular clusters and complexes relevant to atmospheric chemistry and materials science.

2. Results

2.1. Theoretical Conformational Landscape of m-Ethoxyphenol

The ground-state potential energy surface (PES) of m-ethoxyphenol was systematically explored by scanning two key torsional angles of the ethoxy group: α (∠C6–C1–O11–C14) and β (∠C1–O11–C14–C15), as defined in Figure 1. It is well known that the hydroxyl group exhibits only two stable in-plane orientations (0° and 180°). A 180° rotation around the C–O bond from one orientation to the other entails an energy barrier of over 1000 cm⁻¹ (see Figure S1), rendering this process negligible at room temperature. For the up (0°) and down (180°) orientations of the hydroxyl group, PES calculations were performed at the B3LYP/6-311++G(d,p) level. Figure 1a,d show the PES obtained by scanning the two torsional angles of the ethoxy group with the OH group in the up and down orientations, respectively. Eight local minima were identified on each PES. The two lowest-energy minima on PES (a) correspond to conformers I (b) and II (c). Their analogous structures with the methyl group oriented out of the aromatic plane are denoted as I’ and II’, respectively. Similarly, the two dominant low-lying conformers on PES (d) are labeled III (e) and IV (f), along with their out-of-plane methyl analogs III’ and IV’. Atomic labeling is shown for the four representative stable rotamers illustrated herein.

To facilitate comparison with the well-characterized m-methoxyphenol system, the naming convention for rotamers I–IV has been kept consistent with that in the referenced literature [34,35,36,39,40].

Table 1. Calculated relative zero-point energies (ZPEs) of the m-ethoxyphenol rotamers in the S₀ ground state (cm⁻¹) ^a.

Rotamer	B3LYP/6-311++G(d,p)	B3PW91/6-311++G(d,p)	B3LYP/aug-cc-pVTZ	B3PW91/aug-cc-pVTZ
I/down-up	58	61	47	48
II/up-up	226	226	200	198
III/down-down	90	90	66	65
IV/up-down	0	0	0	0
I’	644	620	639	628
II’	839	813	823	810
III’	678	652	663	649
IV’	593	569	596	585

^a The ground-state ZPE values of rotamer IV calculated by the four methods are as follows: B3PW91/6-311++G(d,p): −461.096815 Hartree; B3LYP/6-311++G(d,p): −461.280991 Hartree; B3PW91/aug-cc-pVTZ: −461.142351 Hartree; B3LYP/aug-cc-pVTZ: −461.327201 Hartree.

presents the relative zero-point energies (ZPEs) of all rotamers calculated using four different theoretical methods: B3LYP/6-311++G(d,p), B3PW91/6-311++G(d,p), B3LYP/aug-cc-pVTZ, and B3PW91/aug-cc-pVTZ. Rotamer IV consistently exhibits the lowest ZPE across all methods, in excellent agreement with the conformational preferences observed for m-methoxyphenol [34,36].

Rotamers I’, II’, III’, and IV’ lie significantly higher in energy (≥569 cm⁻¹ above rotamer IV). According to the Maxwell–Boltzmann distribution, the ground-state populations of these high-energy rotamers are negligible under supersonic jet expansion conditions [41]. Consequently, subsequent discussion focuses on the four low-energy rotamers I–IV. Based on the behavior of m-methoxyphenol, where rotamer II was not experimentally observable due to its higher energy and steric hindrance [34,36], we anticipated that only rotamers I, III, and IV would be detectable in our experiments.

2.2. Vibronic and Hole-Burning Spectra

Figure 2 presents the two-color resonance-enhanced two-photon ionization (2C-R2PI) spectrum of m-ethoxyphenol in the region near its S₁ ← S₀ electronic transition. The ionization laser was fixed at 33,393 cm⁻¹ to avoid fragmentation of larger clusters into the monomer mass channel. The 2C-R2PI technique offers the advantage of accurately reflecting S₁ ← S₀ transition intensities by increasing the ionization laser intensity to ionize most molecules in the resonantly excited S₁ state.

The HB spectra (Figure 2c,e,g) confirm the presence of three distinct rotamers. With the probe laser fixed at the origin of a selected rotamer, the burn laser wavelength is scanned. When the burn laser is resonant with any energy level of the probed rotamer, that rotamer is depleted, leading to a significant decrease in the probe signal. The resulting dips correspond to the vibrational energy levels of the targeted rotamer. All prominent vibronic features in the REMPI spectrum exhibit corresponding holes, unequivocally demonstrating that three stable rotamers of m-ethoxyphenol coexist in the supersonic molecular beam.

The assignment of these three rotamers to specific structures (I, IV, and III) was achieved through comprehensive consideration of multiple theoretical and experimental criteria:

• Ground-state energies: The relative ZPEs (Table 1) predict rotamer IV as the most stable, with rotamers I and III lying slightly higher (48–90 cm⁻¹). Using the Boltzmann distribution, the expected intensity ratio of the 0₀⁰ bands is IV:I:III ≈ 1:0.8:0.77, consistent with the experimental REMPI intensities.
• Franck–Condon simulations: Figure 2b,d,f display the simulated S₁ ← S₀ vibronic spectra of rotamers IV, I, and III, calculated at the TD-B3PW91/aug-cc-pVTZ level. The excellent agreement between experimental and simulated spectra for each rotamer provides strong support for the assignment.
• Ionization energies: As detailed in Section 2.3, the experimentally determined IEs and their relative ordering (IV < I < III) match theoretical predictions.
• Rotamer II absence: Simulated Franck–Condon factors for rotamer II were found to be extremely small, indicating negligible S₁ ← S₀ transition probability (See Figure S2). This parallels observations for resorcinol or its cluster with CO and water [42,43] and m-methoxyphenol [34,36], where the analogous high-energy rotamer (with both substituents oriented unfavorably) is not observed experimentally due to steric hindrance.

Table 2. Experimental and theoretical vibrational frequencies (cm⁻¹) of the m-ethoxyphenol rotamers in the S₁ excited state, with tentative vibration assignments ^a.

I			IV			III			Assignment b
Energy	Shift	Calc.	Energy	Shift	Calc.	Energy	Shift	Calc.
35,966	0	0	36,031	0	0	36,198	0	0	00
			36,176	145	140				βOC2H5
36,177	211	204							10a1
			36,210	179					βOC2H5δOC2H5
			36,271	239	238				10b1
						36,455	257		τCH3
			36,320	289					βOC2H52
			36,346	315					βOC2H52δOC2H5
36,305	339	339				36,538	340	339	9a1
36,417	451	463	36,486	455	457	36,652	454	457	6b1
36,519	553	570	36,570	539	536	36,747	549	553	6a1
36,698	732	731	36,750	719	718	36,922	724	730	11
			36,953	922	935				18b1
36,927	961	960	36,995	964	964	37,158	960	962	121
			37,330	1299	1300				131

^a Experimental values are shifts from the origin bands at 35,966 ± 2, 36,031 ± 2, and 36,198 ± 2 cm⁻¹ for the respective rotamers. Theoretical vibrational frequencies at the TD-B3PW91/aug-cc-pVTZ level, scaled by 0.98. ^b Vibrations of the substituents: ν, stretching vibration; β, in-plane bending; γ, out-of-plane bending; τ, CH₃ rotation; δ, other bending.

lists the observed S₁ state vibrational bands, their frequency shifts relative to the origin, calculated frequencies, and tentative vibrational assignments for each rotamer. Vibrational modes are labeled according to Wilson’s notation for benzene derivatives as adapted by Varsanyi and Szoke [44,45]. Most active vibrations correspond to in-plane benzene ring deformation modes and substituent-sensitive bending motions.

Notably, the benzene ring breathing vibration (mode 1¹) appears at distinctly different frequencies for the three rotamers: 732, 719, and 724 cm⁻¹ for rotamers I, IV, and III, respectively. This sensitivity to the relative orientation of the substituents underscores the utility of HB spectroscopy for distinguishing closely related rotamers.

2.3. Cationic Spectra and Ionization Energies

Prior to the MATI experiments, adiabatic IEs of the four low-energy rotamers were calculated using the high-level composite methods G4 and CBS-QB3, which are known to provide reliable energetics for small to medium-sized molecules.

Table 3. Calculated relative energies of the S₀ and D₀ states and ionization energies of the m-ethoxyphenol rotamers from G4 and CBS-QB3 calculations (Unit: cm⁻¹) ^a,b.

Isomer	S0 Relative Energy		D0 Relative Energy		IE
	G4	CBS-QB3	G4	CBS-QB3	Exp.	G4	Error	CBS-QB3	Error
I	50	50	718	714	64,574	64,373	−201	65,116	542
II	207	203	0	0		63,497		64,249
III	70	65	1183	1112	64,994	64,817	−177	65,500	506
IV	0	0	198	187	64,122	63,902	−220	64,639	517

^a S₀ state energies of rotamer IV: G4(0 K) = −461.052005 Hartree, CBS-QB3(0 K) = −460.530971 Hartree. ^b D₀ state energies of rotamer II: G4(0 K) = −460.761746 Hartree, CBS-QB3(0 K) = −460.237304 Hartree.

presents the relative energies of the S₀ and D₀ states, the calculated IEs, and the experimental IEs, together with the theoretical errors.

The calculations consistently predict rotamer IV as the most stable in the S₀ state, while rotamer II becomes the most stable in the D₀ state—a reversal analogous to that observed for resorcinol [42,43,46] and m-methoxyphenol [34,36]. The calculated IEs follow the order II < IV < I < III, with rotamer II showing the lowest IE.

Figure 3 displays the MATI spectra of rotamers IV (a), I (c) and III (e), recorded via their respective S₁ ← S₀ 0⁰ origins (36,031, 35,966 and 36,198 cm⁻¹). The simulated D₀ ← S₁ vibronic spectra at the B3PW91/aug-cc-pVTZ level (scaling factor 0.98) are shown in Figure 3b, Figure 3d and Figure 3f for rotamers IV, I and III, respectively.

From the sharp 0⁰ bands in the MATI spectra, the adiabatic IEs were precisely determined as: Rotamer IV: 64,122 ± 5 cm⁻¹; Rotamer I: 64,574 ± 5 cm⁻¹; and Rotamer III: 64,994 ± 5 cm⁻¹. The measured IE trend (IV < I < III) is fully consistent with G4 calculations. All deviations remain below 0.3%, demonstrating reliable computational performance for ionization energy evaluation. The distinct IE values serve as definitive fingerprints for identifying specific m-ethoxyphenol rotamer cations.

Table 4. Experimental and theoretical vibrational frequencies (cm⁻¹) of the m-ethoxyphenol rotamer cations, together with their tentative assignments ^a,b.

I		IV		III		Assignment b
Exp.	Calc.	Exp.	Calc.	Exp.	Calc.
		141	136	146	142	βOC2H5
355	347	311	310	330	320	9a1
395	396					16b1
476	476					γOH
		568	566			6a1
739	736	731	730	733	732	11
836	823	865	865	830	837	νO-C2H5
		998	998			νO-CH2-CH3
1098	1094	1098	1096			18a1
1176	1161					βOH
		1354	1347			131
		1532	1539			8a1

^a Calculated values are obtained from the B3PW91/aug-cc-PVTZ calculations scaled by 0.98. ^b Vibrations of the substituents: ν, stretching vibration; β, in-plane bending; γ, out-of-plane bending.

presents the measured vibration assignments for the D₀-state cations. The spectra exhibit rich vibrational structure, reflecting the influence of conformational differences on cationic vibrational properties.

3. Discussion

3.1. Structural Changes upon Electronic Excitation and Ionization

The excellent agreement between experimental and simulated vibronic spectra (Figure 2 and Figure 3) indicates that the theoretical methods employed (B3PW91/aug-cc-pVTZ for S₀ and D₀ states; TD-B3PW91/aug-cc-pVTZ for S₁ state) reliably describe the molecular and spectral properties of m-ethoxyphenol. Tables S1–S3 present the optimized geometric parameters for rotamers IV, I and III in the S₀, S₁, and D₀ states, respectively, along with changes upon electronic excitation and ionization.

Several important trends emerge from the structural analysis:

• Upon S₁ ← S₀ excitation: The benzene ring undergoes slight expansion, with C–C bond lengths increasing by 0.013–0.039 Å. This is consistent with the π* ← π electronic excitation, which weakens the bonding character of the ring carbon–carbon bonds. Concurrently, the C–O bonds (C1–O11 and C3–O12) shorten slightly (by 0.006–0.011 Å), indicating increased double-bond character due to electron redistribution.
• Upon D₀ ← S₁ ionization: The benzene ring contracts, with C–C bond lengths decreasing by 0.019–0.043 Å relative to the S₁ state. The removal of an electron restores and even strengthens the π-bonding character of the cationic ring. The C–O bonds shorten substantially (by 0.030–0.044 Å), reflecting the increased electron density withdrawal toward the positively charged ring. The O11–C14 bond elongates (by 0.033–0.038 Å), suggesting weakening of this bond in the cation.
• Substituent geometry: The C6–C1–O11 bond angle deviates by approximately 4° from the ideal sp² angle of 120°, suggesting a weak non-covalent interaction between the lone-pair electrons of the ethoxy oxygen and the adjacent hydrogen atoms on the benzene ring. This interaction induces a slight geometric distortion at the substitution site—a feature common to all three rotamers.
• Hydroxyl group orientation: The angular geometry of the hydroxyl group remains nearly unchanged upon S₁ ← S₀ excitation (Δ < 0.5°), but undergoes a small reorientation (≈3.5°) upon D₀ ← S₁ ionization, reflecting altered electron density distribution in the cationic state.

Remarkably, the overall molecular geometry remains very similar across all three electronic states for each rotamer. This structural similarity underlies the excellent Franck–Condon overlap observed in both S₁ ← S₀ and D₀ ← S₁ transitions, explaining the dominance of the origin bands and the good agreement between experimental and simulated spectra.

Although the optimized torsional angles of the ethoxy group do not show significant variations across the S₀, S₁, and D₀ electronic states (as summarized in Tables S1–S3), the torsional potential energy surface (see Figure 1) exhibits multiple local minima separated by low energy barriers, indicating that the ethoxy group is highly flexible. Consequently, the low-frequency torsional motion of the ethoxy group cannot be adequately described by the harmonic approximation, and a global scaling factor derived from high-frequency vibrational modes is expected to have limited accuracy for these torsional modes. This anharmonicity is a direct consequence of the shallow and multi-minimum nature of the potential energy surface governing the ethoxy rotation. The present torsional scan provides explicit evidence of this anharmonic behavior, and we therefore caution that the vibrational assignments involving the ethoxy group in Table 2 and Table 4 should be considered as tentative, pending more rigorous theoretical treatments (e.g., full anharmonic calculations) in future studies.

3.2. Comparison with m-Methoxyphenol: Substituent Effects

Table 5. Comparison of S₁ ← S₀ excitation energies (E₁) and adiabatic ionization energies (IE) for m-ethoxyphenol and m-methoxyphenol rotamers (cm⁻¹).

Rotamer	m-Ethoxyphenol a		m-Methoxyphenol b		ΔE1	ΔIE
	E1	IE	E1	IE
I	35,966	64,574	35,974	65,228	−8	−654
IV	36,031	64,122	36,034	64,741	−3	−619
III	36,198	64,994	36,202	65,648	−4	−654

^a This work. ^b Refs. [34,36].

compares the E₁ and IE values of m-ethoxyphenol (this work) with those of the corresponding rotamers of m-methoxyphenol [34,36].

Both E₁ and IE of m-ethoxyphenol are consistently lower than those of the corresponding m-methoxyphenol rotamers. The E₁ values differ by only 3–8 cm⁻¹, while the IE differences are substantial (619–654 cm⁻¹). This trend directly reflects the slightly stronger electron-donating ability of the ethoxy group (OC₂H₅) relative to methoxy (OCH₃): the longer alkyl chain enhances the inductive effect, increasing electron density on the benzene ring and the entire molecular framework. Consequently, less energy is required to promote a π electron to the π* orbital (S₁ ← S₀ excitation) and to remove a valence electron (ionization) in m-ethoxyphenol.

The much larger effect on IE compared to E₁ can be rationalized by considering the nature of the two processes. S₁ ← S₀ excitation involves promotion of an electron within the π system, while ionization completely removes an electron. The cation experiences the full inductive effect of the ethoxy group, whereas the neutral excited state benefits only partially from the enhanced electron donation.

3.3. Absence of Rotamer II

The calculations predict rotamer II to lie 198–226 cm⁻¹ above rotamer IV in the S₀ state (Table 1), which would suggest a non-negligible population under supersonic jet conditions. Yet, no spectroscopic features attributable to rotamer II were observed in either REMPI or HB experiments. This absence parallels observations for m-methoxyphenol [34,36], resorcinol [42,43,46], m-dimethoxybenzene [47] and m-diethoxybenzene [40].

Two factors likely contribute to the non-observability of rotamer II:

• Franck–Condon factors: The simulated Franck–Condon factors for the S₁ ← S₀ transition of rotamer II were found to be extremely small (see Supporting Information Figure S2). This is in sharp contrast to rotamers I, III, and IV, where the origin band dominates the spectrum. Such a low Franck–Condon factor for the origin transition would make rotamer II undetectable under typical experimental conditions.
• Steric hindrance: In rotamer II, both the hydroxyl hydrogen and the ethoxy group are oriented “upward” (pointing toward the same side of the ring), creating steric repulsion between the ethoxy and hydroxyl groups. This steric strain elevates the ground-state energy and may also distort the excited-state geometry, further reducing Franck–Condon factors.

The absence of rotamer II underscores an important caveat in conformational analysis: the relative intensities of origin bands in electronic spectra do not necessarily reflect ground-state populations, as differing Franck–Condon factors can dramatically affect transition probabilities.

4. Materials and Methods

4.1. Experimental Methods

meta-Ethoxyphenol (≥98% purity, Shanghai TCL Chemical Co., Ltd., Shanghai, China) was used without further purification. The sample was heated to approximately 150 °C to achieve sufficient vapor pressure and seeded in 3 bar of krypton carrier gas. The gas mixture was expanded into a vacuum through a pulsed nozzle (General Valve, Series 9, 0.5 mm orifice, Parker Hannifin Corporation, Cleveland, OH, USA) operating at 10 Hz. The resulting supersonic molecular beam passed through a skimmer (1 mm diameter) located 15 mm downstream from the nozzle, entering the ionization chamber where it intersected perpendicularly with the laser beams. Typical background pressures were ~10⁻⁴ Pa in the source chamber and ~10⁻⁶ Pa in the ionization chamber.

Two tunable dye lasers (Sirah, CBR-D-24 and Precision Scan-D, Sirah GmbH, Göttingen, Germany) pumped by a Q-switched Nd:YAG laser (Quantel, Qsmart 850, Les Ulis, Essonne, France) provided the UV radiation for two-color resonant two-photon excitation. The appropriate laser dyes (Rhodamine 6G, Coumarin 153, Exciton Inc., Dayton, OH, USA) were selected based on the expected S₁ ← S₀ excitation and ionization energy ranges. The output of both dye lasers was frequency-doubled using BBO crystals. A delay/pulse generator (Stanford Research Systems, DG535, Sunnyvale, CA, USA) controlled the timing between the two laser pulses, typically set to 0–10 ns for an optimal two-color signal.

For REMPI experiments, the ion extraction plates were configured as follows: the first plate was grounded, while the second and third plates (separated by 0.7 cm) were biased with pulsed electric fields of +500 V and +400 V, respectively, to accelerate photoionized ions toward the microchannel plate (MCP) detector.

For MATI experiments, a two-pulse field ionization scheme was employed [16,17]. Approximately 18 ns after the laser pulses, a pulsed electric field of −0.55 V cm⁻¹ was applied to reject prompt ions produced by direct photoionization. After a delay of about 11.8 μs, a second pulsed electric field of +143 V cm⁻¹ was applied to field-ionize molecules in high Rydberg states. The resulting threshold ions were then accelerated and passed through a 48 cm field-free drift tube before detection by the MCP detector. An electrostatic lens assembly was used to focus ions from different spatial positions, enhancing signal intensity.

Ion signals were processed by a multichannel scaler (Stanford Research Systems, SR430, Sunnyvale, CA, USA), accumulating data over 300 laser shots per data point. Laser wavelengths were calibrated using a wavemeter (HighFinesse WS-7, HighFinesse GmbH, Offenburg, Germany).

For hole-burning experiments, the burn laser was fired ~200 ns before the probe laser [40,48]. The probe wavelength was fixed at a selected REMPI peak (e.g., the electronic origin), while the pump wavelength was scanned. When the burn laser wavelength is resonant with any energy level of the probed rotamer, it leads to a significant decrease in the probe signal. Ions generated by the burn laser were rejected by appropriate timing and gating of the detector. The resulting dips correspond to the vibrational energy levels of the targeted rotamer. More experimental details are available in our previous publications [40,41,48,49,50].

4.2. Theoretical Methods

All quantum chemical calculations were performed using the GAUSSIAN 16 program package [51]. Potential energy surface scanning of the ethoxy group torsional angles α (∠C6–C1–O11–C14) and β (∠C1–O11–C14–C15) was conducted at the B3LYP/6-311++G(d,p) level, scanning both angles in 10° steps. This level provides an excellent balance of computational efficiency and accuracy for conformational exploration. For identified stable rotamers, geometry optimizations and harmonic vibrational frequency calculations were performed at higher levels. The neutral ground state S₀ and the cationic ground state D₀ were calculated using the B3PW91 hybrid functional [52,53] with the aug-cc-pVTZ basis set [54]. The first electronically excited state S₁ was calculated using time-dependent DFT (TD-DFT) at the same B3PW91/aug-cc-pVTZ level [55]. All calculated harmonic vibrational frequencies were scaled by a factor of 0.98 to correct for systematic errors arising from basis set incompleteness, neglect of electron correlation, and vibrational anharmonicity [56].

Adiabatic ionization energies were also calculated using the composite methods G4 [57] and CBS-QB3 [58], which are known to provide reliable thermochemical data for small to medium-sized molecules. The IE was obtained as the difference in zero-point corrected total energies between the optimized cation D₀ and the corresponding neutral S₀. Vibronic spectra for S₁ ← S₀ and D₀ ← S₁ transitions were simulated within the Franck–Condon approximation, including Duschinsky rotation and Herzberg–Teller effects where appropriate [59,60]. Simulated spectra were convoluted with Gaussian line shapes (FWHM = 3.6 cm⁻¹ for S₁ spectra, 4.8 cm⁻¹ for D₀ spectra) to facilitate comparison with experimental data.

5. Conclusions

In this work, we have systematically characterized the rotamers of m-ethoxyphenol using a combination of high-resolution spectroscopic techniques and quantum chemical calculations. The main findings are summarized as follows:

Conformational identification: Three stable rotamers of m-ethoxyphenol were unambiguously identified in a supersonic molecular beam using REMPI and hole-burning spectroscopy. Based on ground-state energies, Franck–Condon simulations, and ionization energy measurements, these were assigned as rotamers I, IV, and III following the nomenclature established for m-methoxyphenol. The fourth low-energy rotamer (II) was not experimentally observed, attributed to unfavorable Franck–Condon factors for its S₁ ← S₀ transition and a lower ground-state population.

The electronic excitation energies for the S₁ ← S₀ origin transitions were precisely determined as 35,966 ± 2 cm⁻¹ for rotamer I, 36,031 ± 2 cm⁻¹ for rotamer IV, and 36,198 ± 2 cm⁻¹ for rotamer III. Accurate adiabatic ionization energies (IEs) were obtained via MATI spectroscopy, yielding values of 64,574 ± 5 cm⁻¹ for rotamer I, 64,122 ± 5 cm⁻¹ for rotamer IV, and 64,994 ± 5 cm⁻¹ for rotamer III. The distinct IE values serve as definitive fingerprints for identifying specific m-ethoxyphenol rotamer cations.

Complete vibrational assignments were established for both the S₁ excited state and the D₀ cationic ground state for all three rotamers. Most active modes correspond to in-plane ring vibrations and substituent-sensitive bending motions. The benzene ring breathing mode (1¹) exhibits characteristic frequencies that distinguish both rotamers and electronic states.

Quantum chemical calculations reveal that the benzene ring undergoes slight expansion upon S₁ ← S₀ excitation and contraction upon D₀ ← S₁ ionization, while the overall molecular geometry remains remarkably similar across all three electronic states. This structural similarity underlies the excellent Franck–Condon overlap observed experimentally.

The conformer-specific spectroscopic data presented herein provide a comprehensive foundation for understanding the photophysical and photochemical properties of m-ethoxyphenol. These results will facilitate future studies of this molecule and its derivatives, including hydrogen-bonded clusters and complexes relevant to atmospheric chemistry and materials science.