Rotational Spectrum, Tunneling Motions and Intramolecular Potential

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Rotational Spectrum, Tunneling Motions and Intramolecular Potential Barriers in Benzyl Mercaptan Rizalina Tama Saragi, Marcos Juanes, Walther Caminati, Alberto Lesarri, Lourdes Enriquez, and Martin Jaraiz J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b06921 • Publication Date (Web): 05 Sep 2019 Downloaded from pubs.acs.org on September 5, 2019

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The Journal of Physical Chemistry

Rotational Spectrum, Tunneling Motions and Intramolecular Potential Barriers in Benzyl Mercaptan Rizalina Tama Saragi,† Marcos Juanes,† Walther Caminati,*,†,‡ Alberto Lesarri,*,† Lourdes Enríquez,§ Martín Jaraíz§

†Departamento de Química Física y Química Inorgánica - IU CINQUIMA, Facultad de Ciencias, Universidad de Valladolid, Paseo de Belén, 7, E-47011, Valladolid, Spain, §Departamento de Electrónica, ETSIT, Universidad de Valladolid, Paseo de Belén, 11, E-47011, Valladolid, Spain. ‡Permanent address: Via della Cavriola, 20, Bologna, Italy

1 Environment ACS Paragon Plus

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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ABSTRACT: The rotational spectrum of benzyl mercaptan (parent and four isotopologues) has been assigned in a supersonic jet expansion using chirped-pulse Fourier transform microwave spectroscopy. The spectrum is characterized by torsional tunneling doublings, strongly perturbed by Coriolis interactions. The experimental rotational constants reveal that the sulfur atom is located above the ring plane in a gauche conformation. The torsion dihedral 0 =  (SC-C1C2) is approximately 74°, according to a flexible molecular model calculation reproducing the energy separation (E01~2180.4 MHz) between the first two torsional sub-states. The global minimum configuration is fourfold degenerate, corresponding to potential minima with 0   74° and (180-74)°. The four equivalent minima are separated by potential barriers at  =  90°, 0° or 180°. The tunneling splittings are caused by the potential barrier at  =  90°, while the barriers at torsions of 0° or 180° are too large to generate detectable splittings. The tunnelling barrier has been determined as 248 cm-1, similar to the value obtained with high-level MP2 ab

initio calculations (259 cm-1), but smaller than in benzyl alcohol (280 cm-1).




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1. INTRODUCTION

Interesting conformational and spectroscopic features can be observed when short alkyl chains are attached to a benzene ring. For two-atom chains containing C, O, N or S atoms, the benzene derivatives have quite different configurations, as observed by highresolution microwave (MW) spectroscopy. The alkylated prototype of this series is ethyl benzene, which presents a side-chain perpendicularly oriented with respect to the aromatic ring (0 =  (CC-C1C2)  90°).1-3 Conversely, the molecules of anisole4,5 and thioanisole6 have their methoxy or methoxythiol side-chains in the ring plane. Benzyl amine exhibits two conformations, where the amino group may be either tilted ca. 40° (conformer I) or perpendicular (conformer II) to the aromatic plane.7 In conformer I the nitrogen lone pair points towards the hydrogen atom in the ortho position. In conformer II the amino hydrogens are pointing towards the  cloud. Conformation I exhibits a largeamplitude motion in which the -CH2NH2 methylene amino group tunnels between equivalent forms with 0 = ± 40° above and below the phenyl plane. The observation of tunneling splittings in the rotational spectrum allowed the calculation of a torsional barrier



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of ca. 375 cm-1. Both propargyl benzene (C6H5CH2CCH)8 and benzyl cyanide (C6H5CH2CN)9 have planar (0 = 0°) structures. Concerning benzyl alcohol, the IR, IRUV and NMR spectra were inconclusive and the assignment of its rotational spectrum resisted up to 2010, because of the complications associated to a tunneling motion connecting four equivalent gauche conformations.10 Finally a gauche conformation characterized by a torsional angle of ca. 55° and a barrier of 280 cm-1 was determined using MW spectroscopy. Later investigations of the rotational spectra of several monoand di- ring-fluorinated derivatives of benzyl alcohol have shown that fluorination in ortho and para favors the tunneling trough two equivalent gauche conformations on the same side of the ring, while the opposite is true for meta-fluorination.11-15

For the thiol analog of benzyl alcohol, i.e. benzyl mercaptan (BM), there are only lowresolution spectroscopic studies16 and the rotational spectrum has not yet been reported. It is plausible that, similarly to the alcohol, a low barrier hindering the (SC-C1C2) internal

rotation

generates

severe

Coriolis

coupling


between

the

equivalent

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conformations in Figure 1, making he spectral assignment and the calculation of the torsional barrier difficult.

We succeeded to unravel this problem, providing comparative information with benzyl alcohol and related compounds, and offering a first step in the conformational investigations of larger side-chains in benzyl derivatives.

Figure 1. The four equivalent configurations of gauche benzyl mercaptan.



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2. EXPERIMENTAL AND COMPUTATIONAL METHODS

Benzyl mercaptan was purchased commercially (99%) and used without further purification. The rotational spectrum was recorded with a broadband direct-digital chirpedpulse Fourier-transform MW spectrometer covering the frequency range 2-8 GHz, which follows Pate’s design.17 In this spectrometer a 4 s chirped pulse created by an arbitrary waveform generator is amplified to 20 W and radiated perpendicular to the propagation of the jet expansion through a horn antenna. A molecular transient emission spanning 40 s is then detected through a second horn, recorded with a digital oscilloscope and Fourier-transformed to the frequency domain. Optimal conditions of the jet required backing pressures of ca. 0.4 MPa and neon as carrier gas. The accuracy of the frequency measurements is better than 5 kHz. The spectra of the

34S

observed in natural abundance (4% and 1%, respectively).


and some

13C

species were

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The experiment was supported by molecular orbital calculations using density functional theory (B3LYP18,19) and ab initio methods (MP220). The B3LYP calculations were conducted with Alrichs’ balanced basis set functions def2TZVP,21 while the MP2 calculations used triple- basis sets from Pople (6-311++G(d,p) and 6-311++G(3df,3pd)22) and Dunning (aug-cc-pVTZ23). Vibrational frequency calculations were done at the same level of the optimizations, using the harmonic approximation. All calculations were carried out with Gaussian16.24 The calculations had different objectives, including the initial conformational search, optimization of the most stable structures and determination of the harmonic force field used for the calculation of the centrifugal distortion constants. In a later stage, MP2 calculations were used to model the potential function representing the interconversion between equivalent conformations in the molecule. The calculated structural and electric parameters included the equilibrium rotational constants, electric dipole moment components and Watson’s S-reduced quartic centrifugal distortion constants.25 The theoretical predictions are shown in Tables 1 and S1 (Supporting Information). The most stable gauche conformation of BM is depicted in Figure 2.



Figure 2. The gauche global minimum conformation of benzyl mercaptan on the ab principal-inertial-axes plane.


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Table 1. Predicted spectroscopic parameters for the gauche conformer of benzyl mercaptan.

Ae / MHza Be / MHz Ce / MHz DJ / kHz DJK / kHz DK / kHz d1 / kHz d2 / kHz Pbb / u Å2 μa, / D μb / D μc / D    (SCα-C1C2) / degb  (HS-CαC1) / deg

B3LYP/ MP2/ def2TZVP 6-311++G(d,p) 4185.0 [0.4%]c 4126.3 [-1.0%] 1000.6 [-0.1%] 1000.2 [-0.1%] 891.0 [-0.1%] 892.0 [0.0%] 0.127 0.138 1.809 0.760 1.726 2.328 0.0038 -0.0013 0.0048 0.0046 91.444 91.886 1.2 1.3 0.6 0.6 0.4 0.5 75.3 80.8 53.7 51.5

aEquilibrium

rotational contansts (Ae, Be, Ce), Watson’s S-reduction centrifugal distortion constants (DJ, DJK, DK, d1, d2), Planar moment of inertia 𝑃𝑏𝑏 = (𝐼𝑎 + 𝐼𝑐 ― 𝐼𝑏)/2 = ∑𝑖𝑚𝑖𝑏2𝑖 and electric dipole moment components (, = a, b, c). bThiol group dihedrals. cRelative deviations (theory-experiment)/theory.

3. RESULTS

a. ROTATIONAL SPECTRUM

Following the results of the ab initio calculations, the first spectral surveys targeted low J, a Rtype transitions, expected to be more intense. The a–type R-branch bands J= 21 and 32 were identified around 3700 and 5700 MHz, respectively. These transitions were split into frequency doublets separated 2-30 MHz, as illustrated in Figure 3. The spectral doublets were attributable to the CH2SH methylene thiol group internal rotation connecting four equivalent minima and splitting the ground vibrational state in two torsional sublevels denoted v = 0 and v = 1. It was quite difficult to fit these initial transitions to deviations within the experimental measurement error, as they



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required Coriolis coupling terms. After the first successful fitting we could then measure more a and many c-type transitions, improving the quality of the fit. Unlike the a and c intra-state (v = 0 ↔ 0 or

1 ↔ 1) selection rules, the b-type lines were expected to be inter-state v = 0 ↔ 1

transitions, because the sign of b is inverting when going from form Ia to form Ib (or from Ic to Id, see Figures 1-2). The b transitions presented large splittings exceeding 4000 MHz, so in these cases only one of the two torsional components could be measured. All measured transitions have been fitted with a coupled two-state (𝑣 = 0, 1) torsion-rotation Hamiltonian based on Watson’s semirigid rotor Hamiltonian25 and coupling terms expressed in the Pickett’s reduced axis system.26 The fit used Pickett’s CALPGM programs,27 using the following expressions

𝑯=

∑

(𝑯𝑅𝑖 + 𝑯𝐶𝐷 𝑖 )+

𝑯𝑖𝑛𝑡

(1)

𝑣 = 0,1

and 𝑯𝑖𝑛𝑡 = ∆𝐸01 + 𝐹𝑎𝑏 × (𝑷𝑎𝑷𝑏 + 𝑷𝑏𝑷𝑎) + 𝐹𝑏𝑐 × (𝑷𝑏𝑷𝑐 + 𝑷𝑐𝑷𝑏)

(2)

where 𝑯𝑅𝑖 represents the rotational Hamiltonian for the state i. 𝑯𝐶𝐷 𝑖 accounts for the centrifugal distortion corrections, corresponding to the S reduction in the Ir-representation,25 assumed to be the same for both states. The energy difference between the v = 0 and v = 1 states is denoted ∆𝐸01 and 𝐹𝑎𝑏 and 𝐹𝑏𝑐 are the rotation-vibration coupling parameters between the two states. The derived spectroscopic constants are reported in Table 2. After the assignment of the parent species, it was possible to assign the rotational spectrum of the monosubstituted


34S

isotopologue in natural

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abundance (~ 4%) by scaling the model calculated rotational constants. The results of the fitting are shown also in Table 2. Figure 3. A section of the microwave spectrum of benzyl mercaptan (3-8 GHz) showing tunneling splittings in the a transitions.



Table 2. Spectroscopic parameters for the parent and monosubstitued isotopologues of benzyl mercaptan.


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Parent

34S

13C1

13C4

13C

A0 / MHza

4167.4584(15)d

4153.9682(61)

4158.3(15)

4164.7(18)

4142.6(42)

B0 / MHz

1001.61056(41)

974.8537(14)

1001.2317(59)

987.2886(69)

996.014(16)

C0 / MHz

891.69084(39)

870.9822(10)

891.7571(39)

880.5689(46)

888.583(11)

A1 / MHz

4167.7017(15)

4154.2161(63)

[4158.3(15)]e

[4164.7(18)]

[4142.6(42)]

B1 / MHz

1001.59720(36)

974.8423(12)

1001.2254(63)

987.2769(72)

996.007(17)

C1 / MHz

891.65660(37) 91.73518(24) 2180.4879(35) 96.3332(14) 17.88461(23) 11.5

870.9484(17) 91.74572(89) 2083.252(14) 95.8710(96) 17.2474(37) 11.9

891.7240(26) 91.755(24) 2175.7(14) 96.054(90) 17.935(11) 6.4

880.5412(29) 91.696(29) 2178.1(17) 94.44(10) 17.506(13) 7.7

888.5658(71) 91.672(69) 2136.9(37) 94.29(26) 18.156(32) 18.5

16

18

18

Pbb / u Å2 b E01 / MHzc Fab / MHz Fbc / kHz

 / kHz N

102

40

aRotational

constants for the first two torsional sub-states (A0, B0, C0 and A1, B1, C1). The Watson’s S-reduction centrifugal distortion constants for the parent species were kept fixed for the isotopologues (DJ=0.1550(41) kHz, DJK,= 2.506(22) kHz, DK=0.288(77) kHz, d1=d2=0). bPlanar moment of inertia 𝑃𝑏𝑏. cEnergy difference between the first torsional substates (E01), coupling parameters (Fab, Fbc), standard error of the fit () and number of transitions (N). cStandard errors in parentheses in units of the last digit. eThe rotational constants A0 and A1 were fixed to the same value for the three 13C isotopologues.

Then, we proceeded to the assignment of the spectra of the monosubstituted 13C species in natural abundance (~ 1%). There are two kinds of carbon atoms in the BM molecule. Carbons C1, C4 and C lie in the symmetry plane of the ground state vibrational wavefunction, while C2, C3, C5 and C6 (with C2 equivalent to C6 and C3 to C5 within the ground state vibrational wavefunction) do not. When C1, or C4 or C are substituted by a 13C atom, the generated isototopologues preserve the symmetry of the parent species. Viceversa, such a single isotopic substitution at the C2/C6 and C3/C5 pairs generate two different isotopologues for each pairs and quenches the tunneling splitting. The

13C1, 13C4

and

13C

species maintain the spectroscopic properties of the parent

species and can be fitted with the same procedure described above. For them, the model rotational constants can be scaled as measured for the parent species. Conversely, for the 13C2, 13C3, 13C5



and 13C6 species it is not possible to apply the empirical corrections mentioned above to model rotational constants, because the fittings do not include the Fab and Fbc Coriolis coupling constant. In consequence, the tunnelling vibrational effects are accounted for in the effective rotational constants. For this reason, we succeeded to measure and fit the rotational spectra of the 13C1, 13C4 and 13C, species, but failed with the remaining ones. The spectroscopic parameters of the 13C1, 13C4

and 13C isotopologues are listed in Table 2. The near-invariance of the planar moment of

inertia Pbb in Table 2, which gives the mass extension along the b inertial axis, confirms the molecular symmetry. The measured transitions are collected in Tables S2-S6 (Supporting Information).

b. EFFECTIVE STRUCTURE

The key molecular structural parameters of BM in the vibrational ground-state, in particular the position and orientation of the thiol group, can be investigated from the experimental rotational constants of the 5 observed isotopologues (averaged over the two torsional states) assuming a rigid structure. We determined a partial effective structure28 by a least-squares fit incorporating the MP2 ab initio structure of Table 1 and adjustable parameters. We present in Table S7 (Supporting information) the results of a fit floating two bond distances in the side-chain (r(SC and r(CC1), the valence angle at C (SCC1)), the orientation of the terminal thiol hydrogen atom ( (HSCC1)) and the sulfur elevation angle ( = (SC-C1C2)). The resulting structure reproduces the ground-state rotational constants below 0.1 MHz (except for the A rotational constants of the three 13C species, which deviate < 2.4 MHz). However, the results of this fit should be taken with caution


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considering its large correlations, low convergence and possible ill-conditioning. For comparison, the MP2 atomic coordinates are given in Table S8 (Supporting information). A determination of the substitution structure29 was not attempted since the substituted atoms fall close to the a principal inertial axis, hampering the determination of the b coordinates.

c. POTENTIAL FUNCTION OF THE INTERNAL ROTATION

In presence of a large-amplitude motion in the molecule, the most accurate structural description is given by the potential function governing the intramolecular displacements. The potential function for the internal rotation of the -CH2SH methylene thiol group around the C-C1 bond was determined from the experimental energy difference E01 between the first two sub-torsional states v=0 and v=1, accurately determined for the parent species and the 34S and 13C isotopologues, together with a flexible molecular model describing this motion. The potential barrier height can later be used for a pertinent comparison with the alcohol group in benzyl alcohol. As previously discussed, we observed a direct tunneling doubling only for the b-type transitions, while a- and

c-type transitions are split only indirectly. Forms Ia and Ib (or Ic and Id) are connected through a motion which inverts b, while forms Ia and Ic (or Ib and Id) would be connected through a motion which inverts c (see Fig. 1). Because we only observe inter-state splittings in the b-type transitions, the effective torsional barrier at   (SC-C1C2) = 90 must be lower than that at  = 0 (or 180). In addition, the  (HS-CC1) torsion must be coupled with the main torsion to produce mirror equivalent configurations. We modelled these internal motions using MP2/6311++G(d,p) ab initio calculations in order to find the energies and the values of the (SC-C1C2)



and (HS-CC1) angles at the critical points of the main torsion coordinate. The results are summarized in Table 3. There, for the sake of simplicity in describing the potential energy function, which is symmetric with respect to (SC-C1C2) = 90, we use the parameters  (= 90 – (SC-C1C2), = 90– ) and  = (HS-CC1). Note that there are two possible transition states for tunneling through the phenyl plane (IaIc), with  = 0 (low barrier) or  = 180 (high barrier). There is no experimental evidence for tunneling through either of those barriers.

Table 3. Ab initio (MP2/6-311++G(d,p)) values of the energy and of the dihedral angles at the critical points of the potential energy surface for the internal rotation of the methylene thiol group.

Equilibrium IaIb barrier

IaIc barrier (low)

 / deg a 16 0 90 0 180 -52.2  / deg b E / cm-1 c 0 248 734 a  = 90 – (SC-C1C2). b= (HS-C-C1) c Absolute energy -668.4515159 E . h

For tunneling via the IaIb path, as  changes from 0 to 90,  changes from 0 to 180, i.e.,  must change, on average, at twice the rate of . This represents a considerable structural relaxation which is larger than the contributions from other small structural changes taking place during the internal rotation of . We included only the  structural relaxation. It is difficult, however, to model such a relaxation over a full 2 range. Because


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we only observe splittings associated with the  = 90 ( = 0) IaIb barrier, we considered the motion only on one side of the ring, and determined the two parameters required in the following double minimum potential

𝑉(𝜏) = 𝑉0 + 𝑉2 × (1 ― cos 2𝜏) + 𝑉90 × (1 + cos 2𝜏) × 𝑒 ―𝑎𝜏

2

(3)

where 𝑉0 is a constant to set to zero the minimum, 𝑉2 and 𝑉90 are, in a first approximation, the barrier at  = 0° and 90°, respectively, and the parameter a in the gaussian exponential determines the width of the exponential. In our case, it is useful to set up the value of  at the energy minima (0). We described the structural relaxation associated to  according to: (4)

𝛼(𝜏) = 𝑅𝜏

where R is the ratio between the amplitude of the  and  motions in proximity of the equilibrium conformation. We then applied Meyer’s one-dimensional Flexible Model,30 and obtained a reasonable reproduction of the five experimental splittings with the values of the five parameters of equations (3) and (4) shown at the bottom of Table 4.

Table 4. Results of Meyer’s Flexible Model calculations on benzyl mercaptan. Tunnelling splittings Obs. E01 (parent)/ MHz E01 (34S)/ MHz E01 (13C1)/ MHz E01 (13C4)/ MHz E01 (13C)/ MHz

2180.4879(35) 2083.252(14) 2175.7(14) 2178.1(17) 2136.9(37)


Calc. 2181.7 2058.8 2158.2 2175.0 2106.1


V0=-84 cm-1

Parameters used (see text) V2=409 cm-1 V90=162.1 cm-1 a=33

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R=-3.3

In this fit 𝑉2 has been fixed to the ab initio value, because we do not have experimental data useful for its determination. The parameter a has been adapted to reproduce the experimental value of 0, which needs to be 16.1°, in order to reproduce the experimental Pbb value of 91.735 uÅ2 (parent species). Finally, 𝑉90 has been adjusted in order to reproduce the experimental splitting. In the flexible model calculations the  coordinate has been considered in the ±50º range and solved into 61 mesh points. The shape of the potential energy function is shown in Figure 4, where it is compared to the ab initio potential. The barrier connecting the two equivalent minima (0= ± 16.1°) at  = 0 is derived to be B2 = 248 cm-1. The potential minima corresponds to a sulfur elevation angle of   (SCC1C2) = 73.9°, smaller than the MP2 structure of Tables 1 and S8 (Supporting information).


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Figure 4. Results of the flexible model calculations for the torsional motion along the coordinate  in benzyl mercaptan (continuous trace) compared to the ab initio potential (dashed).

4. CONCLUSIONS

We analyzed the intramolecular large-amplitude motion of the methylene thiol group in benzyl mercaptan, using a combination of microwave spectroscopy and ab initio calculations. In particular, the observation and identification of severe rotation-vibration perturbations between the two sets of rotational levels arising from the internal rotation of the methylene thiol group led to the determination of the rotational constants of five isotopic species and the energy difference between the two torsional sub-states (2083-2180 MHz depending on the isotopologue). The torsional energy splittings were fitted in a flexible molecular model to determine the internal rotation barrier, which, with a value of 248 cm-1, is ca. 10% smaller than in the corresponding alcohol. This work will further permit the future investigation of the clusters and microsolvation products of benzyl mercaptan, which is presently under way, complementing previous information on benzyl alcohol and thiol derivatives.

ASSOCIATED CONTENT

Supporting Information. Rotational transition frequencies and ab initio molecular structure.



AUTHOR INFORMATION

Corresponding Author *Email: [email protected]; [email protected]. Web: www.uva.es/lesarri. Phone: +34-983-185805.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Ministerio de Ciencia, Innovación y Universidades (MICINN-FEDER PGC2018-098561-B-C22) and the Junta de Castilla y León (VA056G18) for funds. The authors are thankful to Prof. Walther Caminati for his research visit leading to this investigation


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Figure 1. The four equivalent configurations of gauche benzyl mercaptan. 84x66mm (300 x 300 DPI)

ACS Paragon Plus Environment


Figure 2. The gauche global minimum conformation of benzyl mercaptan on the ab principal-inertial-axes plane. 68x72mm (300 x 300 DPI)


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Figure 3. A section of the microwave spectrum of benzyl mercaptan (3-8 GHz) showing tunneling splittings in the μa transitions. 170x238mm (300 x 300 DPI)


Rotational Spectrum, Tunneling Motions and Intramolecular Potential

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