THE MILLIMETER-WAVE SPECTRUM OF METHACROLEIN THE MILLIMETER-WAVE SPECTRUM OF METHACROLEIN. TORSION-ROTATION-VIBRATION EFFECTS IN THE EXCITED STATES OLENA ZAKHARENKO, JUAN-RAMON AVILES MORENO, ROMAN A. MOTIYENKO, THERESE R. HUET Laboratoire PhLAM, UMR8523 CNRS - Université Lille 1, Villeneuve d’Ascq, France.

Volatile organic compounds (VOCs) sources lifetime is short  oxidation and degradation products formation of SOAs  influence on the global climate and human health Antropogenic  100 TgC/yr Natural  1170 TgC/yr Others VOCs Monoterpenes Isoprene Biomass burning Energy use and transfer

Isoprene oxydation products Isoprene C5H8 Methacrolein (MAC) Trans Cis Methyl vinyl ketone (MVK) Reaction with OH /NOx/ O3 Objectives: Analysis of ground state and low lying excited states Study of large amplitude motion (internal rotation of methyl group) Providing required information for analysis of high resolution IR spectra can also originate from primary emissions such as fuel evaporation or combustion (vehicular emissions)

Quantum chemical calculations C3v symmetry group the internal rotation potential function: V α = V 3 2 1−cos3α + V 6 2 1−cos6α V3(CH3) Trans most stable Cis + 1203.7 cm-1 244,20-234,19 E A J.-R. Alives Moreno et al., 69th ISMS, 2014, TI01

Rho-Axis Method (RAM) B > A θRAM Ir (a, b, c→ z, x, y) a= z   Ir IIl PAM (MHz) RAM (MHz) A 8612.39 4510.68 8504.79 B 4403.08 C 2965.33 θRAM 80.8° -9.2° μx 2.67 D 2.77 D μz 0.84 D 0.40 D θRAM ρ Ir (a, b, c→ z, x, y) B > A b = x IIl (a, b, c→ x, z, y) OK PAM to RAM: 𝐴 𝑅𝐴𝑀 = 𝐴 𝑃𝐴𝑀 cos 2 𝜃 𝑅𝐴𝑀 + 𝐵 𝑃𝐴𝑀 sin 2 𝜃 𝑅𝐴𝑀 𝐵 𝑅𝐴𝑀 = 𝐴 𝑃𝐴𝑀 𝑠𝑖𝑛 2 𝜃 𝑅𝐴𝑀 + 𝐵 𝑃𝐴𝑀 𝑐𝑜𝑠 2 𝜃 𝑅𝐴𝑀 𝐶 𝑅𝐴𝑀 = 𝐶 𝑃𝐴𝑀 𝐷 𝑎𝑏 =− 𝐴 𝑃𝐴𝑀 − 𝐵 𝑃𝐴𝑀 cos 𝜃 𝑅𝐴𝑀 sin 𝜃 𝑅𝐴𝑀 𝜇 𝑎 𝜇 𝑏 𝑅𝐴𝑀 = 𝑐𝑜𝑠 𝜃 𝑅𝐴𝑀 𝑠𝑖𝑛 𝜃 𝑅𝐴𝑀 −𝑠𝑖𝑛 𝜃 𝑅𝐴𝑀 𝑐𝑜𝑠 𝜃 𝑅𝐴𝑀 𝜇 𝑎 𝜇 𝑏 𝑃𝐴𝑀 RAM Hamiltonian: 𝑯 𝑹𝑨𝑴 = 𝑯 𝑹 + 𝑯 𝒄𝒅 +𝑯 𝑻 + 𝑯 𝒊𝒏𝒕 , where HR is the rotational Hamiltonian, Hcd the centrifugal distortion Hamiltonian, HT the torsional Hamiltonian, Hint contains higher-order torsional- rotational interaction terms.

Rho-Axis Method (RAM) B > A θRAM Ir (a, b, c→ z, x, y)   Ir IIl PAM (MHz) RAM (MHz) A 8612.39 4510.68 8504.79 B 4403.08 C 2965.33 θRAM 80.8° 9.2° μx 2.67 D 2.77 D μz 0.84 D 0.40 D ρ θRAM Ir (a, b, c→ z, x, y) B > A b = z IIl (a, b, c→ x, z, y) OK PAM to RAM: 𝐴 𝑅𝐴𝑀 = 𝐴 𝑃𝐴𝑀 cos 2 𝜃 𝑅𝐴𝑀 + 𝐵 𝑃𝐴𝑀 sin 2 𝜃 𝑅𝐴𝑀 𝐵 𝑅𝐴𝑀 = 𝐴 𝑃𝐴𝑀 𝑠𝑖𝑛 2 𝜃 𝑅𝐴𝑀 + 𝐵 𝑃𝐴𝑀 𝑐𝑜𝑠 2 𝜃 𝑅𝐴𝑀 𝐶 𝑅𝐴𝑀 = 𝐶 𝑃𝐴𝑀 𝐷 𝑎𝑏 =− 𝐴 𝑃𝐴𝑀 − 𝐵 𝑃𝐴𝑀 cos 𝜃 𝑅𝐴𝑀 sin 𝜃 𝑅𝐴𝑀 𝜇 𝑎 𝜇 𝑏 𝑅𝐴𝑀 = 𝑐𝑜𝑠 𝜃 𝑅𝐴𝑀 𝑠𝑖𝑛 𝜃 𝑅𝐴𝑀 −𝑠𝑖𝑛 𝜃 𝑅𝐴𝑀 𝑐𝑜𝑠 𝜃 𝑅𝐴𝑀 𝜇 𝑎 𝜇 𝑏 𝑃𝐴𝑀 RAM Hamiltonian: 𝑯 𝑹𝑨𝑴 = 𝑯 𝑹 + 𝑯 𝒄𝒅 +𝑯 𝑻 + 𝑯 𝒊𝒏𝒕 , where HR is the rotational Hamiltonian, Hcd the centrifugal distortion Hamiltonian, HT the torsional Hamiltonian, Hint contains higher-order torsional- rotational interaction terms.

THz Spectrometer in Lille Previous study : 7.6 - 25 GHz up to J = 10 only ground state Present study : 150 – 465 GHz up to J = 76 and Ka = 17 ground state + low lying excited energy states J.R. Durig et al. Spectrochimica Acta A 42, 89-103 (1986). M. Suzuki, K. Kozima, J. Mol. Spectrosc. 38, 314-321 (1971). Spectrometer performance: 150 – 990 GHz (was recently extended up to 1.5 THz) frequency multiplication chain output power: 5 µW - 5 mW sensitivity: 10-6 - 10-7 cm-1 Doppler limited resolution measurement accuracy: 30 kHz, 50 kHz, 100 kHz depending on the observed S/N ratio and the frequency range Spectrometer based on solid state sources Pathway - 2.2 m Frequency modulation technique

Spectrum of MAC Examples of the spectrum in frequency range from 150 to 210 GHz. H2O 150 – 465 GHz up to J = 76 and Ka = 17 ground state + low lying excited energy states Dipole moments PAM: μa = 2.67 D and μb = 0.84 D

CH3 torsion Analysis of g.s. Prediction of v27=1 Assignment of v27=1 Global fit of g.s.+ v27=1 Prediction of v27=2 Parameter (v=0 and v27=1 ) cm-1 F 5.51076 fixeda Ρ 0.0279648(41) V3 493.848(24) V6 -33.514(22) A 0.2844277(23) B 0.1501602(25) C 0.9893682(45) ∙10-1 2Dab -0.42266(17) ∙10-1 θRAM -8.737° n 3411 rms(MHz) 0.0319 wrms 0.776 + 19 parameters CH3 barrier (cm-1) Method Trans Cis B3LYP/6-311++G(2df,p) 468 451 M062X/6-311++G(2df,p) 493 516 MP2/AVTZ  501 509 Experimental This work 494  - [1] v=0 [2] IR 444 441 Global fit of the ground state and the first excited torsional state: Up to J, Ka = 76, 17 > 3000 transitions rms = 0.0319 MHz wrms = 0.776 [1] M. Suzuki, K. Kozima, J. Mol. Spectrosc. 38, 314-321 (1971). [2] J.R. Durig et al. Spectrochimica Acta A 42, 89-103 (1986). a the MP2 aug-cc-pVTZ calculations v27 = 2: assignments only for rotational transitions with Ka<5 E: 275,23 – 265,22 E: 274,23 – 264,22

Rotation- vibration interactions Scheme of the lowest vibrational states in s-trans methacrolein.[3] Fermi and c-type Coriolis interactions between v27=2 and v25 = 1 shift of approx. 9.5 cm-1 between observed and predicted v27 = 2-1 cis 1←0 (A,E) trans 1←0 (A,E) ~9.5 cm-1 2←1(E) 2←1(A) 1←0 (A,E) Spectral resolution: 0.1 cm-1 [3] J.R. Durig et al. Spectrochimica Acta A 42, 89-103 (1986).

Scheme of the lowest vibrational states in s-trans methacrolein. Vibrational modes v27 v26 Scheme of the lowest vibrational states in s-trans methacrolein. v27 – methyl torsion v26 – antisymmetric CCC out-of-plane bending vibration (skeletal torsion) v25 – symmetric CCC in-plane bending vibration

Skeletal torsion v26 The anomalous A-E splittings: the splittings of v26 mode are much bigger than these ones of ground state A and E tunneling sublevels are inversed compared to the typical order of transitions in the ground state The anomalous A-E splittings and unusual sequence of A-E transitions in the v26=1 state were explained by kinetic coupling between methyl and skeletal torsional modes

Scheme of the lowest vibrational states in s-trans methacrolein. Kinetic coupling The relative displacements of the atoms for the ν27 and ν26 modes obtained with the MP2 aug-cc-pVTZ calculations   Methyl torsion v27=1 Skeletal torsion v26=1 Atom X Y Z C1 0.00 0.04 0.13 H2 H3 0.15 0.32 C4 -0.05 -0.08 C5 -0.03 H6 0.50 -0.45 H7 0.46 -0.31 -0.38 0.06 0.22 H8 -0.46 0.03 0.38 -0.06 C9 -0.09 -0.17 H10 -0.29 -0.48 O11 0.11 0.12 Scheme of the lowest vibrational states in s-trans methacrolein. Hydrogen atoms of CH3 v27 v26 both normal modes may be represented as a mixture of pure methyl top torsion, and out-of-plane motion of others atoms

Skeletal torsion v26 By reason of inverted sequence of AE splittings ⟹The v26=1 state is assigned as a virtually first excited state of the methyl torsional mode ⟹ Separated fit of the v26 state using RAM Hamiltonian Parameter (v=0 and v27=1 ) cm-1 (v26=1) cm-1 F 5.51076 fixed ρ 0.0279648(41) 0.28253(45) ∙10-1 V3 493.848(24) 626.46(28) A 0.2844277(23) 0.281603(14) B 0.1501602(25) 0.153144(43) C 0.9893682(45) ∙10-1 0.989778(17) ∙10-1 2Dab -0.42266(17) ∙10-1 -0.56178(55) ∙10-1 θRAM -8.737° -11.81° n 3411 1054 rms(MHz) 0.0319 0.0376 wrms 0.776 0.844   + 19 parameters + 8 parameters Fit of the skeletal torsion state: up to J, Ka = 44, 17 1054 transitions rms = 0.0376 MHz wrms = 0.844 fit is purely effective v26

Conclusions analysis of the rotational spectrum of s-trans MAC significant advance in the knowledge on the molecular structure and dynamics, and on the low-lying excited vibrational states determined value of the barrier to internal rotation analyzed the first excited vibrational state of the skeletal torsional mode ν26 Provided information for analysis of high resolution infrared spectrum

THANK YOU FOR YOUR ATTENTION Support from the CaPPA project (Chemical and Physical Properties of the Atmosphere) is acknowledged. CaPPA is funded by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract "ANR-11-LABX-0005-01" and by the Regional Council “ Nord-Pas de Calais » and the  "European Funds for Regional Economic Development (FEDER)