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Conformational Landscape and Torsion-Rotation-Vibration Effects in the Two Conformers of Methyl Vinyl Ketone, a Major Oxidation Product of Isoprene Olena Zakaharenko, Roman A. Motiyenko, Juan Ramon Aviles Moreno, and Therese R. Huet J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06360 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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The Journal of Physical Chemistry

Conformational Landscape and Torsion-RotationVibration Effects in the Two Conformers of Methyl Vinyl Ketone, a Major Oxidation Product of Isoprene Olena Zakharenko, Roman A. Motiyenko, Juan Ramon Aviles Moreno, Thérèse R. Huet* Univ. Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers Atomes et Molécules, F-59000 Lille, France. Corresponding Author * Prof. Thérèse R. Huet

ABSTRACT. Methyl vinyl ketone is the second major oxidation product of isoprene, and as such an important volatile organic compound present in the troposphere. In the present study, quantum chemical calculations coupled to high resolution millimeter-wave spectroscopy have been performed to characterize the ground and first excited vibrational states of the two stable conformers. Equilibrium structures, internal rotation barriers, and relative energies have been calculated at the MP2 and M062X levels of the theory. Experimental molecular parameters have been obtained that model the rotational and torsional structures, including splitting patterns due to the internal rotation of methyl group. For the most stable antiperiplanar (s-trans) conformer,

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the set of parameters obtained for the ground state should be useful to further model IR spectra up to room temperature. By combining theoretical and experimental data we obtained a relative energy value of 164 ± 30 cm-1 in the gas phase between the more stable antiperiplanar and the less stable synperiplanar conformers. Moreover, we compared our system with related molecules the variation in the barriers of methyl rotors in different molecular environments. In addition, the inverse sequence of A and E tunneling sub-states for the rotational lines of the first excited skeletal torsional state and Coriolis type coupling with methyl torsion have been observed. For the less stable synperiplanar (cis) conformer, molecular parameters for the ground and first excited torsional states as well as of the first excited skeletal torsional state are presented.

Introduction The emissions of volatile organic compounds (VOCs) from anthropogenic and biogenic sources make contribution to the air pollution, and thereby attracted a lot of attention in recent years. Indeed the sources of atmospheric VOCs include nonmethane biogenic emissions from terrestrial ecosystems and the ocean (1150 TgC/yr), and anthropogenic emissions (142 TgC/yr).1 Methyl vinyl ketone (MVK, butenone CH3—C(O)CH=CH2), like methacrolein (MAC, H2C=C(CHO)—CH3), is a major oxidation product of the important biogenic volatile organic compound isoprene.2 MVK and MAC can also originate from the primary emissions produced from fuel evaporation and combustion in urban areas.3 These species remaining in the gas phase have short lifetimes and are highly active in atmospheric chemical reactions. Measurements of the ratio of isoprene and its oxidation products, such as MVK/MAC and [MVK+MAC]/isoprene, were used in many studies to investigate the magnitude and location of sources of isoprene and its photochemical transformations.4,5 Isoprene is removed from the atmosphere primarily by reaction with OH radicals during the day and with O3 and NO3 radicals during the night.5,6 In


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areas where isoprene is coming from biogenic emissions, the oxidation chemistry is destroying isoprene and is producing and destroying MAC and MVK during the day.5 Tropospheric lifetimes are estimated at 1.7 hours for isoprene, 8.6 h and 6.1 h for MVK and MAC respectively.7 The atmospheric oxidation of isoprene and its oxidation products MVK and MAC contribute significantly to the formation of tropospheric ozone O3 and secondary organic aerosols (SOAs).8,9 These air pollutants can have a profound effect on the radiation balance of the Earth and human health. However, SOA effects on climate have not yet been fully studied, and we need a better understanding of their sources, chemical formation and physical and chemical characteristics.10 From a physicochemical point of view, an important effort has been produced to report on the first generation product yields from the OH initiated oxidation of MVK under both low and high NO conditions, by combining laboratory experiments and quantum chemistry calculations.11 It is also important to study the spectroscopic properties of MVK because it provides an unambiguous laboratory tool, especially when more than one conformer is energetically relevant. For this purpose, we recently studied methacrolein in the gas phase.12 As second major oxidation product of isoprene, we believe that methyl vinyl ketone deserves the same interest. Methyl vinyl ketone (MVK, CH3COCH=CH2) exists as a mixture of two stable conformations with respect to the CC single bond, antiperiplanar (ap, the two double bonds are trans to each other) and synperiplanar (sp, the two double bonds are cis to each other). All atoms are in a plane except for two hydrogens of the methyl group. The gas phase spectrum of MVK was characterized for the first time in 1965 by Foster et al. in the microwave frequency range (7 – 33 GHz).13 In this work Curl and his colleagues identified the most stable ap conformer (previously


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called s-trans). The analysis of A-E splittings from the ground state allowed to determine a value of the barrier to internal rotation of the methyl group to 437(7) cm-1. In 1987, the microwave spectrum was measured in the frequency range 26.4 – 40 GHz by Fantoni et al.,14 evidencing the two first excited states, i.e. the skeletal torsion ( ) and the methyl torsion ( ) normal modes.

For the treatment of ro-vibrational interaction between the two torsions, in ref. 14, a Hamiltonian based on flexible models was developed. It was shown that satisfactorily description of the interaction may be achieved using the Hamiltonian that contains both kinetic and potential energy terms.14 In ref. 14, the value of the barrier to internal rotation of the methyl group was obtained slightly smaller, 424.6(2) cm-1. This difference in barrier heights can be explained by different methods used in Ref. 13 and Ref. 14. More recently the rotational spectrum (7.5 - 18.5 GHz) of MVK was re-investigated, using a Fourier transform microwave spectrometer coupled to a molecular beam, by Wilcox et al.15 The high-resolution spectrum of the sp conformer was identified for the first time, in the ground state. The values of the barrier to internal rotation of the methyl group were determined to 433.8(1) cm-1 and 376.6(2) cm-1 for the ap and sp conformers respectively. The components of the permanent dipole moment were also calculated (µa = -3.13 D , µb = -2.35 D for the ap conformer, and µa = 0.66 D, µb = 3.19 D for the sp conformer). However, the presence of a second stable MVK sp conformer (previously called scis) was first observed by infrared and NMR spectra analysis of vapor, liquid and solid states.1618

According to the measurements of enthalpies the ap conformer is more stable. The enthalpy

difference between the two conformers for the vapor phase, was measured by Bowles et al (198±18 cm-1) and by Durig and Little (280 cm-1).18,16 It should be noted that the last value was obtained from an effective model, by adjusting to experimental data a torsional potential as a Fourier cosine series in the internal rotation angle.


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The geometries and energies of the two MVK conformers were also determined by quantum chemical calculations.15,19-21 The energy difference between both conformers varies a lot depending on the chosen method. The calculations by different quantum chemistry methods (B3LYP, MP2, QCISD, CCSD(T), CASSCF and others) carried out by Bokareva et al. illustrates this point.21 The geometrical parameters, the relative energy of the two conformers (-110 to +183 cm-1) and the barriers to internal rotation (387 - 461 cm-1 and 300 - 356 cm-1 for the ap and sp conformers respectively) were obtained. The goal of the present study is to provide a set of accurate spectroscopic parameters for the ground state of both conformers, in order to permit the future analysis of high resolution infrared spectra in the atmospheric window, namely by using the combination difference technique which allows unambiguous assignments. To this end, it is needed to obtain an overview of the energy levels populated at room temperature. Therefore, we have recorded the millimeter-wave and THz spectra of MVK and performed their analysis with a model taking into account the torsionrotation effects. All the microwave data from the literature were included in our global analysis. The study was prepared and completed by quantum chemical calculations. The structure of the two conformers, their relative energy, and the large amplitude motion associated with the methyl top were characterized, using ab initio method and density functional theory (DFT). At the end we do present an accurate and coherent set of molecular parameters characterizing the gas phase energy structure of this molecule in its ground and first excited states, for the two stable conformers. Quantum chemical calculations results All the calculations were performed using the Gaussian 09 (G09), Rev. D.01, software package.22 The most stable geometries, relative energies, and rotation barriers of the methyl


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group were evaluated. We retained the DFT (M06-2X)23 and the Møller-Plesset second order theory (MP2)24 with different Pople and Dunning’s basis sets.25 At first a relaxed potential energy surface (PES) scan (the geometry is optimized in each point of the scan) of methyl vinyl ketone was performed in order to characterize the two possible conformers. It was carried out at the MP2/6-311++G(2d,2p) level of theory along the O7C6C4C1 dihedral angle (see Figure 1), from 0° to 360° with a step of 5°.

Figure 1. Relaxed potential energy surface scan of methyl vinyl ketone at the MP2/6311++G(2d,2p) level of theory along the O7C6C4C1 dihedral angle. The two stable antiperiplanar (ap) and synperiplanar (sp) conformers are displayed with the a and b principal axes.


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Table 1. Experimental and calculated internal rotation barriers (V3 and V6 terms), rotational constants and dipole moment components in the principal axis system, for the ap and sp conformers of methyl vinyl ketone at different levels of theory (DFT and ab initio). ap-MVK

sp-MVK

Parameter

B3LYP1

M062X1

MP22

MP23

Exp.

B3LYP1

M062X1

MP22

MP23

Exp.

A(MHz)

8968

9015

8990

9013

8924.9

10267

10333

10200

10245

10220.5

B(MHz)

4276

4327

4301

4319

4276.8

3994

4030

4023

4037

3996.8

C(MHz)

2948

2977

2963

2973

2942.9

2927

2952

2937

2948

2926.2

|µa|(D)

2.6

2.5

2.9

2.9

0.4

0.3

0.5

0.5

|µb|(D)

2.2

2.2

2.5

2.5

2.9

2.8

3.2

3.1

|µc|(D)

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

V3(cm-1)

276.5

469.0

389.1

-

307.7

384.7

371.9

-

V3(cm-1)*

278.5(1.5)

464.0(2.6)

387.7(2.1)

-

309.4(2.2)

380.7(2.6)

371.2(2.4)

-

V6(cm-1)*

-6.5(1.5)

-9.2(2.6)

-9.2(2.1)

-

-6.5(2.2)

-8.1(2.6)

-10.6(2.4)

-

443.236(78)

385.28(30)

*

All the relative energies are zero point corrected (ZPE, ∆EZPE), except those calculated at the MP2/aug-cc-pVQZ level, which are equilibrium relative energies (∆Ee); * * Fitted valued according to the following equation = 1 − cos3 + 1 − cos6 . 1: 6-311++G(2df,p) basis set; 2: aug-cc-pVTZ basis set; 3: aug-cc-pVQZ basis set.


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According to the scan results, the sp conformation is less stable than the ap conformation, with an equilibrium relative energy of 229 cm-1. After vibrational zero-point energy (ZPE) corrections (harmonic level), the ground states of the two conformers are found to be separated by 170 cm-1. In addition, the Gaussian-326 (G3) and Gaussian-427 (G4) methodologies were applied in order to obtain more accurate energies. Indeed, the calculated equilibrium energy is corrected in terms of zero point, electronic correlation and basis size distribution, making these methodologies a method of choice to obtain accurate relative energies. As a result, we obtained a relative energy between the higher in energy sp conformer and the lower in energy ap conformer of 167.2 cm-1 (G3), 133.7 cm-1 (G4) and 183.9 cm-1 (MP2/aug-cc-pVTZ). The value calculated at the M062X/6-311++G(2df,p) level of theory is 125 cm-1. Our calculated values clearly indicate that the ap conformer is the most stable. Moreover, a new experimental relative energy value of 164 ± 30 cm-1 between the ground states was obtained in the present work (see next section) to assess the present calculated relative energy values. It should be noted that both experimental and theoretical values are in good agreement, except for the value calculated with the B3LYP method which is only 50.2 cm-1. Moreover, we retained the MP2/6-311++G(2d,2p) and the G3 and G4 results as being compatible with the experimental value, taking into account the experimental uncertainty. Second, equilibrium geometry calculations were performed at the MP2/aug-cc-pVQZ level of theory. The main structural parameters are shown in Figure 2. The equilibrium structure (Cartesian coordinates) is presented in Table 1S of the Supporting Information. Both conformers present a planar skeleton, with all atoms in a plane except two hydrogen atoms of the methyl group (H9 and H10). The equilibrium structure of the vinyl and methyl groups, and of the ketone bond are similar. In addition to the value of the O7-C6-C4-C1 dihedral angle, the main


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significant differences in the ap and sp conformers are the C6-C4 bond length (1.480 Å versus 1.490 Å) and the methylene angle (117.33° versus 118.95°). The structural optimization resulted in sets of rotational constants for both conformations. In addition, we used the results of the structural optimizations to calculate some other parameters of the molecular Hamiltonian applied for the analysis of the rotational spectrum of MVK. In particular, we calculated the internal rotation constant F associated with the methyl top torsion, as well as , the angle between the principal and the rho axis systems, and consequently the non-diagonal constant in the rho axis system (see the Analysis section for further details).

Figure 2. Equilibrium geometries calculated at the MP2/aug-cc-pVQZ level of theory for the ap and sp conformers of methyl vinyl ketone. Some structural parameters are shown (distances in Å and angles in degrees). We also calculated the rotation barriers of the methyl group for the two conformers. As for methacrolein,12 our methodology aimed at calculating the methyl torsion barrier with a precision better than 5%. The quadratic synchronous transit QST3 methodology implemented in G09,28,29 starting from the geometry of two minima and the optimization of a transition state geometry


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between these minima, was used. At the MP2/aug-cc-pVQZ level of theory, we obtained 389 cm1

(ap-MVK) and 372 cm-1 (sp-MVK). Meanwhile a better result was obtained for both

conformers at the M062X/6-311++ G(2df, p) level, with a barrier height equal to 469 cm-1 (apMVK) and 385 cm-1 (sp-MVK). The difference between the last calculations and the present experimental values (see Table 1) is −6% (ap-MVK) and +0.1% (sp-MVK). The calculated rotational constants, dipole moments, and barriers to internal rotation are summarized in Table 1 and compared to experimental values.

Experimental results The sample of MVK (Sigma Aldrich, 90% purity) was used without purification. The millimeter-wave spectra were recorded in the frequency range between 50 and 650 GHz using the Lille spectrometer. Its detailed description can be found in Zakharenko et al.30 Briefly, the measurements were carried out at room temperature and at a pressure of about 0.015 mbar. The frequency ranges, 50 – 105 GHz, 150 –330 GHz, 400 – 650 GHz, were covered with solid state multiplied sources. The frequency of a reference source of radiation, the Agilent synthesizer (12.5–17.5 GHz), was first multiplied by six and amplified by a AMC-10 active sextupler from Virginia Diodes, Inc. (VDI), providing the output power of +15 dBm (∼32 mW) in the W-band range (75–110 GHz). This power is high enough to use VDI passive Schottky diode-based multipliers (×2, ×3, ×5, ×6) in the next stage of the frequency multiplication chain. The estimated uncertainties of the measured line frequencies are 30 kHz, 50 kHz and 100 kHz depending on the S/N ratio, the observed line shape and the frequency range. It should be noted that we observed on the spectrum CH3CN transitions, due to impurities. This molecule is well studied and the rotational transitions frequencies are already precisely measured and known.31 Thus, the


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presence of CH3CN does not influence our spectral analysis. As shown in Figure 3, the spectrum exhibits consecutive series of lines associated with the J rotational quantum number values.

a)

b)

Figure 3. Part of the spectrum showing a) the series of consecutive group of lines of MVK according to the J rotational quantum number values, and b) rotational transitions of the sp conformer of MVK in the ground and two first excited states. and are the methyl group


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torsion mode and the asymmetric out-of-plane skeletal torsion mode of the synperiplanar (sp) conformer, respectively. The rotational lines (label JKaKc) are split due to the internal rotation of the methyl top, and are assigned with the A and E labels.

Figure 4. Portion of the millimeter-wave spectrum displaying the JKaKc = 461,45 – 451,44, 461,45 – 452,44, 462,45 – 451,44, 462,45 – 452,44 lines associated with the ground state of both sp and ap conformers of MVK. The line intensities are proportional to the number of molecules of each conformer, which are related to the relative energy.

Due to the tunneling effect, the energy levels are split into one sub-level of A symmetry and two degenerated sub-levels of E symmetry of symmetry point group describing the methyl top. Consequently, transitions are split into two components of same intensity, and torsionrotational lines are labelled A and E. Compared to our previous study of methacrolein12 the presence of two populated conformers of MVK leads to higher spectral density.


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As done for HONO, the population ratio and the relative energy can be determined from intensity measurements.32 At the Beer-Lambert approximation, the integrated absorption coefficient !" (cm-2) at temperature T (K) of a line is given by: !" =

#$

' , + /0 1%&23 /5. %& ($)* !" -.

− 0 1%&26 /5. 78!" ,

where 8!" is the line strength (calculated using the RAM36 code, see next section), 9: is the ro-vibrational partition function estimated from quantum chemical calculations, ; is the number of molecules, +!" is the line wavenumber (cm-1), and

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