Journal of Molecular Structure - HHU...Rotationally resolved electronic spectroscopy study of the conformational space of 3-methoxyphenol Martin Wilke a, Michael Schneider a, Joseﬁn

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Journal of Molecular Structure 1140 (2017) 59e66

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Journal of Molecular Structure

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Rotationally resolved electronic spectroscopy study of theconformational space of 3-methoxyphenol

Martin Wilke a, Michael Schneider a, Josefin Wilke a, Jos�e Arturo Ruiz-Santoyo b,Jorge J. Campos-Amador b, M. Elena Gonz�alez-Medina b, Leonardo �Alvarez-Valtierra b,Michael Schmitt a, *

a Heinrich-Heine-Universit€at, Institut für Physikalische Chemie I, D-40225 Düsseldorf, Germanyb Divisi�on de Ciencias e Ingenierías, Universidad de Guanajuato-Campus Le�on, Le�on, Guanajuato 37150, Mexico

a r t i c l e i n f o

Article history:Received 1 September 2016Received in revised form27 October 2016Accepted 31 October 2016Available online 3 November 2016

Keywords:3-MethoxyphenolElectronic spectroscopyStructural analysisab initio calculationsConformational landscape

* Corresponding author.E-mail address: [email protected] (M.

http://dx.doi.org/10.1016/j.molstruc.2016.10.0960022-2860/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

Conformational preferences are determined by (de-)stabilization effects like intramolecular hydrogenbonds or steric hindrance of adjacent substituents and thus, influence the stability and reactivity of theconformers. In the present contribution, we investigate the conformational landscape of 3-methoxyphenol using a combination of high resolution electronic spectroscopy and ab initio calcula-tions. Three of the four possible conformational isomers were characterized in their electronic groundand lowest excited singlet states on the basis of their rotational constants and other molecular param-eters. The absence of one conformer in molecular beam studies can be explained by its non-planarstructure in the excited state, which leads to a vanishingly small Franck-Condon factor of the respec-tive origin excitation.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

The investigation of conformational isomers of a given molec-ular structure is vital for interpretation of chemical and biologicalrecognition processes [1,2]. In contrast to intuitive expectations, thelowest-energy conformer of flexible molecules does not alwaysbind to specific receptor molecules [2]. Thus, a thorough study ofthe conformational space including structural and energetic as-pects helps in the interpretation of molecular recognition.

Methoxyphenol has three constitutional isomers called guaiacol(2-methoxyphenol), mequinol (4-methoxyphenol) and 3-methoxyphenol. Each of the molecules has the same connectivitiyof the methoxy and hydroxyl group to the benzene ring. However,both substituents can exist in various configurations resulting fromdifferent orientations of the hydrogen atom of the hydroxyl groupand the methyl group of the methoxy group. Since different con-formers are always present in chemical samples, but differ in theirstabilities; it can be quite difficult to distinguish the conformers andcharacterize their properties, especially in the electronically excited

Schmitt).

state. Rotationally resolved electronic spectroscopy offers apowerful tool to tackle this problem. Since the rotational constantsare characteristic of each conformer, our technique is sensitiveenough to identify the structures in the ground and lowest elec-tronically excited states.

Recently, the molecular structures of guaiacol and mequinolhave been studied and identified using high-resolution electronicspectroscopy [3]. While for guaiacol only the most stable cis-conformer was observed experimentally, both conformers (cis andtrans) of mequinol are identified in supersonic jet experiments[3e7]. Experimental studies of the conformational space of 3-methoxyphenol were performed by Fujimaki et al. who identifiedtwo different rotational isomers by means of IR-UV double reso-nance spectroscopy [7] and by Caminati et al. who found one of fourpossible rotational isomers using microwave spectroscopy [8]. Theconformational space of the bare molecule and of its water clustersof three rotational isomers was investigated by two-color reso-nance-enhanced multiphoton ionization (REMPI), UV-UV holeburning spectroscopy, and zero kinetic energy (ZEKE) photoelec-tron spectroscopy in the group of Müller-Dethlefs [9,10].

Closely related systems are dimethoxybenzene and dihydrox-ybenzene, where the number and geometry of the stable confor-mations is different from the systems discussed above. Only one

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M. Wilke et al. / Journal of Molecular Structure 1140 (2017) 59e6660

conformer is observed for 1,2-dimethoxybenzene and 1,2-dihydroxybenzene. The most stable geometry for 1,2-dimethoxybenzene is identified as the trans-conformer (up/down)[11e13]. In the case of the para-substituted benzenes (1,4-dimethoxy- and 1,4-dihydroxybenzene) always two rotamers arepresent [14,15]. If two equivalent substituents are in 1,3-positionthree conformers are possible. One with both substituents point-ing upwards the benzene ring (up/up), the other where both arepointing downwards the chromophore (down/down) and the lastone with one substituent oriented upwards and one downwardsthe benzene ring (up/down). However, for 1,3-dihydroxybenzeneonly two origin bands were found and assigned to the most sta-ble conformers down/down and up/down [16]. Although the up/upconformation is still highest in energy for 1,3-dimethoxybenzene,three bands are identified as the origin bands of all possibleconformer in resonant two-photon ionization (R2PI) and time-of-flight mass spectroscopy (TOFMS) experiments [12,17].

If both substituents are different, the up/down and down/upconformations are no longer the same. This leads to anotherpossible conformer, see Fig. 1 for all possible conformers of 3-methoxyphenol. In this work, we investigate the conformationalspace of 3-methoxyphenol by a combination of rotationallyresolved electronic spectroscopy and high-level ab initio calcula-tions. The geometries of the three most stable conformers wereassigned from their molecular parameters in the electronic groundand lowest excited singlet states. The variation of the transitiondipole moment orientations for the different dihedral angles of themethoxy and the hydroxy groups is analyzed and used, along withthe rotational constants, their changes upon electronic excitationand the shifts of origin frequencies for assignment of the observedbands to the conformers. Several reasons for the absence of the lastmost unstable conformer are discussed.

2. Experimental section

2.1. Experimental procedures

3-Methoxyphenol (�96%) was purchased from Sigma-Aldrichand used without further purification. The experimental set-upfor the rotationally resolved laser induced fluorescence spectros-copy is described in detail elsewhere [18]. In brief, the laser systemconsists of a single frequency ring dye laser (Sirah Matisse DS)operatedwith Rhodamine 110, pumpedwith 7Wof the 514 nm lineof an Arþ-ion laser (Coherent, Sabre 15 DBW). The dye laser outputwas coupled into an external folded ring cavity (Spectra PhysicsWavetrain) for second harmonic generation. The resulting outputpower was constant at about 5 mW during the experiment. Themolecular beam was formed by co-expanding 3-methoxyphenol,heated to 160 �C, and 750 mbar of argon through a 200 mm nozzleinto the vacuum chamber. The molecular beammachine consists ofthree differentially pumped vacuum chambers that are linearlyconnected by skimmers (1 mm and 3 mm, respectively) in order to

Fig. 1. Structures of the four possible conformers of 3-methoxyphenol wi

reduce the Doppler width. In the third chamber, 360 mm down-stream of the nozzle, the molecular beam crosses the laser beam ata right angle. The resulting resolution is 18 MHz (FWHM) in thisset-up. The imaging optics set-up consists of a concave mirror andtwo plano-convex lenses to focus the resulting fluorescence onto aphotomultiplier tube, which is mounted perpendicularly to theplane defined by the laser and molecular beam. The signal outputwas then discriminated and digitized by a photon counter andtransmitted to a PC for data recording and processing. The relativefrequency was determined with a quasi confocal Fabry-Perotinterferometer. The absolute frequency was obtained bycomparing the recorded spectrum to the tabulated lines in theiodine absorption spectrum [19].

2.2. Quantum chemical calculations

Structure optimizations were performed employing Dunning'scorrelation consistent polarized valence triple zeta (cc-pVTZ) basisset from the TURBOMOLE library [20,21]. The equilibrium geometriesof the electronic ground and the lowest excited singlet states wereoptimized using the approximate coupled cluster singles anddoubles model (CC2) employing the resolution-of-the-identityapproximation (RI) [22e24]. Vibrational frequencies and zero-point corrections to the adiabatic excitation energies have beenobtained from numerical second derivatives using the NumForcescript [25] implemented in the TURBOMOLE program suite [26]. Thepotential energy surface (PES) was performed using the Scankeyword in GAUSSIAN 09 package at DFT/B3LYP/cc-pVTZ level oftheory [27]. It was built by scanning the two dihedral torsionalangles in steps of 5� from 0� to 360�.

2.3. Fits of the rovibronic spectra using evolutionary algorithms

The search algorithm employed for the fit of the rotationallyresolved electronic spectra is an evolutionary strategy (ES) adaptingnormal mutations via a covariance matrix adaptation (CMA)mechanism. This (CMA-ES) algorithmwas developed by Ostermeierand Hansen [28,29] and is designed especially for optimization onrugged search landscapes that are additionally complicated due tonoise, local minima and/or sharp bends. It belongs to a group ofglobal optimizers that were inspired by natural evolution. For adetailed description of these evolutionary strategies refer to refs.[30e33].

3. Results and discussion

3.1. Computational results

For the assignment of the experimental spectra and identifica-tion of the respective structures CC2/cc-pVTZ calculations of thefour possible conformers have been performed. Table 1 summarizesthe calculated molecular parameters, which are the rotational

th their main inertial axes according to the nomenclature of ref. [7].

Table 1Molecular properties of the four possible conformers of 3-methoxyphenol at their respective CC2/cc-pVTZ optimized geometries. This includes the ground and excited staterotational constants. Double-primed constants belong to the ground state and single-primed to the excited state. The angle of the transition dipole moment with the maininertial axis a is given by q and the adiabatic excitation energy by n0. A positive sign of the angle corresponds to a clockwise rotation of the TDM vector onto the a-axis (cf. Fig. 1).The uncertainties of the parameters are given in parentheses and are obtained as standard deviations by performing a quantum number assigned fit.

Calculation Experiment

Conformer I Conformer II Conformer III Conformer IV A band B band C band

A00/MHz 3641.40 2839.75 3634.53 2830.76 3628.47(74) 2841.06(11) 3624.13(62)

B00/MHz 1132.48 1307.00 1135.29 1313.51 1129.75(3) 1303.46(2) 1131.59(2)

C00/MHz 868.58 900.16 869.84 902.32 866.68(3) 899.02(2) 867.48(2)

DI00/amu Å2 �3.20 �3.21 �3.20 �3.20 �3.50 �3.46 �3.47

A'/MHz 3496.46 2778.00 3500.64 2774.16 3492.35(75) 2786.29(11) 3496.75(63)B0/MHz 1126.77 1297.22 1123.37 1292.23 1122.42(4) 1282.35(3) 1120.44(3)C0/MHz 857.38 890.98 855.07 886.82 854.87(3) 883.94(3) 853.89(2)DI0/amu Å2 �3.62 �4.29 �3.21 3.39 �3.80 �3.75 �3.73q/� þ7 þ19 þ3 þ16 ±9.9(3) ±19.9(1) ±8.8(2)t/ns e e e e 5.7(1) 3.6(1) 7.6(1)n0/cm�1 37 086 36 632 37 164 36 957 35 974.66 36 034.43 36 201.24s/MHz e e e e 0.47 0.61 0.38

Fig. 2. Rovibronic spectrum of the electronic origin of the A band of 3-methoxyphenolalong with a simulation using the best parameters obtained from a CMA-ES fit, given inTable 1.

M. Wilke et al. / Journal of Molecular Structure 1140 (2017) 59e66 61

constants in the ground (A00, B, C) and excited state (A0 , B0, C0), and

the inertial defects of the respective states (DI). Additionally, theangles q of the transition dipole moment (TDM) with the maininertial a-axis, and the center frequencies n0 are given.

Since the rotational constants of 3-methoxyphenol are mainlydetermined by the orientation of the methoxy group, we can dividethe four conformers into two groups (up and down). The first onecontains conformer I and conformer III with the methyl grouppointing downwards the benzene ring and the second one containsconformer II and IV with the methyl group pointing upwards thechromophore (cf. Fig. 1). Inside a family a differentiation of theconformers is quite difficult due to the small influence of the hy-droxyl group orientation on the rotational constants.

Beside the rotational constants, the transition dipole moment(TDM) orientation, given by the angle q between the TDM vectorand the inertial a-axis, is a valuable tool to distinguish between theconformers. A rotation of the hydroxyl group with a fixed methoxygroup orientation (going from one family to the other) leads to adecrease of the angle by around 3�, while a rotation of the methoxygroup with a fixed hydroxyl group orientation (going fromconformer I to II and from conformer III to IV) causes an increase ofthe q angle of around 13�. All calculated angles are positive, whichmeans that the TDM vector must be rotated clockwise onto the a-axis.

3.2. Experimental results

Fig. 2 shows the rotationally resolved spectrum of the electronicorigin of the lowest energy band of the resonance enhancedmultiphoton ionization (REMPI) spectrum recorded by Fujimakiet al. [7], denoted as the A band. It is accompanied by a simulationusing the best parameters from a CMA-ES fit, given in Table 1. Thespectra and simulations of the other conformers (B and C) are givenin the online supporting material. For all conformers the deviationsof the origin wavenumbers from to the low-resolution values [7,9]is less than one wavenumber.

The band type of all conformers is ab hybrid as can be inferredfrom the value q in Table 1. The fit of the line shapes to Voigt profilesusing a Gaussian (Doppler) contribution of 18 MHz yielded a Lor-entzian contribution of 28.1 ± 0.3 MHz for the A band,44.1 ± 0.6 MHz for the B band and 20.8 ± 0.3 MHz for the C band.These line widths are equivalent to excited state lifetimes of5.7 ± 0.1 ns for the A band, 3.6 ± 0.1 ns for the B band and 7.6 ± 0.1ns for the C band. All experimental parameters of the three con-formers, including the rotational constants, their changes upon

excitation, the inertial defects, the TDM orientation, the excitedstate lifetimes and the origin frequencies, are given in Table 1. Theagreement between the experimental and simulated spectrum canbe inferred from the standard deviation s of the respective fits,based upon 1238 assigned transitions of for the A band,1458 for theB band and 1266 for the C band (cf. Table 1).

3.3. Conformational assignment

The experimental bands can be divided into two groups on thebasis of their TDM orientations. The A and C band show similar qangles with the TDM vector rotated by around 9� away from theinertial a-axis, while the respective value of the B band is about 19�.A similar behavior is observed from the calculated q angles inTable 1, so that the A and C bandmust belong to a down (conformer Iand III) and the B band to a up conformation (conformer II and IV).From the comparison of the experimental and calculated constantsin Table 1, the aforementioned division gets confirmed.

In the next step we make the final assignment of the structuresof the A and the C band by taking a closer look at the differences ofthe rotational constants in both states between two conformers of

Fig. 3. Relative stabilities of the four conformers of 3-methoxyphenol according toCC2/cc-pVTZ calculations. All energies are zero-point corrected and given in cm�1.


one family. A direct comparison of the ab initio and the experi-mental rotational constants is difficult, since the experimentallydetermined rotational constants are vibrationally averaged, whilethe ab initio constants are equilibrium constants. In first approxi-mation vibrational averaging between conformers of the samemolecule is similar. Thus, the vibrational averaging effect cancelsout in the difference of the rotational constants of differentconformers.

Between the A and C band an experimental differenceof�4.34MHz (þ4.40MHz) in the A00 (A0),þ1.84MHz (�1.98MHz) inthe B

00(B0) and þ0.80 MHz (�0.98 MHz) in the C00 (C0) rotational

constant is observed. For conformer I and III a change of�6.87MHz(þ4.18 MHz) in the A00 (A0), þ2.81 MHz (�3.40 MHz) in the B00 (B0)and þ1.26 MHz (�2.31 MHz) in the C00 (C0) constants are calculatedfrom the optimized ab initio geometries. Due to the good agree-ment in the absolute values as well as in the sign of the changes, weassign the A band to conformer I (down/up) and the C band toconformer III (down/down). The assignment of the A band getsconfirmed by the results of a microwave study, where oneconformer was identified as conformer I with the ground staterotational constants in excellent agreement with those of the Aband [8].

Next, we need to decide whether the B band belongs toconformer II or conformer IV. Since the rotational constants withinthis family are too close, another characteristic is needed to assignthe structure of the B band.

From the origin frequencies of the four conformers in Table 1, itbecomes obvious that the rotation of the methoxy group by a fixedOH-group orientation is accompanied with a red shift of 454 cm�1

in the adiabatic excitation energies by going from down/up to up/up.A smaller value of 207 cm�1 is observed between the down/downand up/down conformers with the OH-group pointing upwards thebenzene ring. With the knowledge that the A band belongs to thedown/up and the C band to the down/down conformation, we cancalculate the experimental differences in the origin frequencies andcompare it to the aforementioned values. Between the A and the Bband the difference is approximately �60 cm�1 and between the Cand the B band around 167 cm�1, which matches with the valuebetween conformer III and IV (cf. Table 2). Consequently, the B bandis assigned as the up/down conformer, denoted as conformer IV.

3.4. Searching the missing conformer

Fig. 3 compares the relative energies of the four possible con-formers of 3-methoxyphenol, derived from CC2/cc-pVTZ calcula-tions. In the ground state conformer IV is the most stable andconformer II the highest energy conformer, which is in goodagreement with the calculations from Ullrich et al. made atdifferent levels of theory [9]. In the lowest electronically excitedstate the energetic order changes and conformer II becomes themost stable conformer and conformer III is highest in energy: n0(conf II) < n0 (conf IV) < n0 (conf I) < n0 (conf III). For the experi-mental excitation energies, a similar trend becomes apparent.

Table 2Calculated and experimental differences of the origin frequencies of two conformersof 3-methoxyphenol. The calculated values belong to two conformers with the sameOH-group orientation arising from the methyl group rotation. The experimentaldifferences are made to assign the structure of the B band as conformer II or IV.

Calculation Experiment

Conformers Dn0 Dn0 bands

IeII 454 cm�1 �59.77 cm�1 AeBIIIeIV 207 cm�1 166.81 cm�1 CeB

Conformer III is the conformer with the highest excitation energy,while the ordering of conformer IV and I changes with respect tothe calculations. In this context, one has to keep in mind that thedifference in the origin frequencies of conformer IV and I is under100 cm�1.

Since only three of the four possible conformers are observedexperimentally, the question arises why conformer II is missing. Onthe basis of the experimental origin frequencies of the other threeconformers and the incremental energies arising from the rotationof the hydroxyl or methyl group we made an estimation of theorigin frequency of conformer II (cf. Fig. 4). Going from the A to theC band defines the effect of the hydroxyl group rotated down,which causes a blue shift in the origin frequency of around227 cm�1. Rotating the methyl group down leads to blue shift ofaround 167 cm�1. To obtain conformer II, which corresponds to theup/up conformation either the methyl or hydroxyl group must berotated up (starting from the A or B band) or both substituentsrotate simultaneously (starting from the C band). Independent ofthe starting point, the estimated origin frequency for the missingconformer is expected at 35 807 cm�1; the lowest value of allconformers in agreement with the theoretical results.

While this region is not shown in the low-resolution spectrumof Ullrich et al. [9], there is no signal in the respective spectrumrecorded by Fujimaki et al. at this position [7]. So why thisconformer does not appear in molecular beam studies? In order toanswer this question, we calculated the barriers separating theminimum structures of all conformers in the ground state. Inspiredby the theoretical study on the conformational preferences of 2-methoxyphenol [34], we assume that the transition states belongto structures with the substituent being perpendicular to thechromophore. Fig. 3 shows the different ground state barriers,where the maxima belong to a transition state with one of thesubstituents rotated by 90� out of the molecular plane. In all fourcases, the transition states are destabilized by around 1500 cm�1 in

Fig. 4. Estimation of the adiabatic excitation energy of conformer II from the addition of incremental energies arising from the rotation of the substituents.


comparison to the most stable ground state conformer IV (seeFig. 3). Due to the height of these barriers, a conversion ofconformer II into conformer I or IV via collisions with the buffer gasduring the expansion seems not plausible. Depopulation ofconformer II by conversion to conformer III, with both substituentsrotating out of the molecular plane, is improbable for its barrier ofaround 2800 cm�1 according to CC2/cc-pVTZ calculations and cantherefore also be excluded. Since the excited state barriers do notprovide any additional information about the absence of conformerII and the computational effort is too high, Fig. 3 only shows therelative energies of the respective minima.

Fig. 5 shows the three-dimensional potential energy surface(PES) of 3-methoxyphenol in the ground state as a function of the

Fig. 5. Left) Three-dimensional potential energy surface of 3-methoxyphenol in the ground sin steps of 5� at DFT/B3LYP/cc-pVTZ level of theory. Right) Overview of the possible structu

dihedral angles of both substituents with the aromatic plane,calculated at DFT/B3LYP/cc-pVTZ level of theory. For a betteridentification of the respective minima and maxima Fig. 5 containsa graphical overview of the possible structures along both co-ordinates. Also at this level of theory, the relative energies are ingood agreement to the results of the CC2 calculations and thosefrom Ullrich et al. [9].

From Fig. 5, it becomes obvious that no low barrier pathwayexists by varying the dihedral angles of both substituents, alongwhich conformer II can be depopulated. Since the calculatedoscillator strengths are almost similar for all four possible con-formers, there must be another explanation for the missingconformer.

tate created by varying the dihedral angles of both substituents with the aromatic planeres which belong to the minima and maxima of the PES shown on the left.


For 1,3-dimethoxybenzene, Tzeng and coworkers [17] foundthree origin bands assigned to all possible conformers with relativestabilities in the ground state comparable to those of the 3-methoxyphenol conformers. The up/up conformer is calculated tobe 250 cm�1 above the most stable up/down or down/up conformer

Fig. 6. Franck-Condon simulation of the excitation spectrum of conformer IV and II. Fordetails see text.

Fig. 7. Experimental and calculated TDM orientations of the four conformers of 3-methoxyphsigns of the angles are adapted from the theoretical results, whereby a positive sign belongsgiven in red and the CC2/cc-pVTZ calculated ones belong to the blue lines. (For interpretatversion of this article.)

[17]. However, the origin band assigned to the up/up conformer hasthe smallest intensity. In this context, different geometry changeswere mentioned to explain the large intensity changes. An exper-imental inertial defect of �3.50 amu Å2 in the electronic groundstate of conformer A, B, and C and of �3.80 amu Å2 for the elec-tronically excited state is in agreement with a planar symmetricchromophore that has only two hydrogen atoms of the methylgroup out of the aromatic plane. The slight increase upon excitationmight result from a decreased planarity of the heavy atoms or froma decrease of the twofold CeOCH3 torsional barrier. The calculatedinertial defects in Table 1 show that the value of conformer II issignificantly different from those of the other conformer in theelectronically excited state. Taking a deeper look at the optimizedexcited state geometry of conformer II reveals that the planarity ofthe benzene ring is lifted and the hydrogen atoms at C2 and O9 aretilted against each other. A similar, but less pronounced, behavior isobserved for conformer I; while conformer III and IV have a planargeometry in the excited state. The optimized geometries of allconformers in the ground and lowest electronically excited statesare given in the online supporting material. This suggests that theFranck-Condon (FC) factor for the origin excitation of conformer II isdifferent from that of the other conformers, what could explain theabsence of this conformer. The Franck-Condon simulations of theexcitation spectra for conformer II and conformer IV (the moststable one) are given in Fig. 6.

They have been obtained from the ab initio optimized groundand excited state structures of each conformer and the respective

enol in their principle axis system along with the respective q angles. The experimentalto a clockwise rotation of the PAS onto the inertial a-axis. The experimental vectors areion of the references to colour in this figure legend, the reader is referred to the web


Hessian using the program FCFit [35,36], which computes theexcitation spectrum in the FC approximation in the basis ofmultidimensional harmonic oscillator wavefunctions. Obviously,for conformer II, most of the total oscillator strength is distributedover higher vibronic levels, while for conformer IV, the FC factor ofthe 0,0 transition is largest. The ratio of the band area of the 0,0 andall other vibronic bands for both conformers shows, that the originof conformer IV is expected to be 10 times stronger (under theassumption of equal oscillator strengths for both band systems). Toconclude, the fact that the origin of conformer II is missing in theR2PI and LIF spectra in a molecular beam, is a combined effect ofsmall Boltzmann population of the most unstable conformer and asmall FC factor for this conformer. The possibility of depopulation ofan unstable conformer by collisions with the buffer gas, if thebarrier separating the two conformers, has often been discussed[37e42]. However, the effect of differing FC factors for differentconformers has to be kept in mind, when trying to deduce con-formers stabilities from intensity data in absorption spectra.

3.5. Transition dipole moment orientations

The analysis of the rovibronic spectra only yields the projectionof the TDM vector onto the principle axis system (PAS) given by theangle q. Consequently, the absolute sign of the angle cannot bedetermined from this information alone and two orientations arepossible. Since the absolute values of the q angles derived fromtheory and experiment are in good agreement, the signs can beadapted from theory. This leads to a positive sign for all angleswhich corresponds to a clockwise rotation of the TDM vector ontothe a-axis. Fig. 7 summarizes the experimental and calculated TDMorientations of all conformers of 3-methoxyphenol in theirrespective principle axis system (PAS). Here, it can be seen that therotation of the hydroxyl group with a fixed methyl group orienta-tion only slightly affects the TDM orientation. Due to the low massof the hydrogen atom with respect to the rest of the molecule theaxis systems are practically coincident, which leads to almostsimilar q angles with deviations of under 3�. The rotation of themethyl group by a fixed hydroxyl group orientation changes the qangle by around 10�. This is a consequence of a simultaneousrotation of the TDM and PAS. The main influence comes from therotation of the inertial a- and b-axes with approximately 25�, whilethe TDM vectors rotate by around 15� by going from a down to an upconformation. This shows that the influence of the methoxy groupposition on the TDM orientation is much larger than those of thehydroxyl group.

4. Conclusion

The rotationally resolved electronic spectra of three conformersof 3-methoxyphenol were analyzed and assigned to a down/up (Aband), up/down (B band) and down/down conformation (C band) onthe basis of the rotational constants, excitation energies and TDMorientations. Our assignment is in good agreement with thosemade by Ullrich et al. based on vibrationally resolved spectroscopy[9]. Furthermore, a planar structure in the ground and lowestelectronically excited state for all three conformers can be deducedfrom the inertial defects, which was similiary observed for theparent molecules phenol [43,44] and anisole [45]. For the TDM acharacteristic dependence between the orientation and the posi-tion of the substituents can be observed. Changing the methoxygroup position leads to a much stronger change of the TDM vectorthan a hydroxyl group rotation. Similar to the experiments on 1,3-dihydroxybenzene the up/up conformer is experimentally notobservable, while for 1,3-dimethoxybenzene three origin bands areobserved and assigned to the possible conformers [12,16,17]. Since

the PES of 3-methoxyphenol along the two dihedral angles of themethoxy and hydroxyl group with the benzene ring is highlysymmetric, a collision induced depopulation along this coordinatescan be excluded. However, the geometry optimization of the up/upconformer shows a non-planar structure in the electronicallyexcited state. Thus, the absence of this conformer can be explainedwith a vanishingly small intensity due to its FC factor. In a similarway also for the constitutional isomer guaiacol (2-methoxyphenol)not all possible conformers are observed experimentally [3,4]. Here,the presence of only one conformer in cis-conformation can beexplained with the huge stability of this conformer and smallerbarriers between the other minimal structures leading to aconsiderably different three-dimensional PES [34].

Acknowledgements

Financial support of the Deutsche Forschungsgemeinschaft viagrant SCHM1043/12-3 is gratefully acknowledged. Computationalsupport and infrastructure was provided by the ”Center for Infor-mation and Media Technology” (ZIM) at the Heinrich-Heine-University Düsseldorf (Germany). Financial support provided byDirecci�on de Apoyo a la Investigaci�on y al Posgrado (DAIP)-Uni-versidad de Guanajuato, under the grant CIFOREA 76/2016 isgreatly appreciated. Computational access to the Kukulk�an in Cin-vestav, Unidad M�erida was provided by Dr. Gabriel Merino, towhom we are deeply grateful (M�exico).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.molstruc.2016.10.096.

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Rotationally resolved electronic spectroscopy study of the conformational space of 3-methoxyphenol1. Introduction2. Experimental section2.1. Experimental procedures2.2. Quantum chemical calculations2.3. Fits of the rovibronic spectra using evolutionary algorithms

3. Results and discussion3.1. Computational results3.2. Experimental results3.3. Conformational assignment3.4. Searching the missing conformer3.5. Transition dipole moment orientations

4. ConclusionAcknowledgementsAppendix A. Supplementary dataReferences

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Journal of Molecular Structure - HHU...Rotationally resolved electronic spectroscopy study of the conformational space of 3-methoxyphenol Martin Wilke a, Michael Schneider a, Joseﬁn