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Millimeter Wave Spectrum of Iso-Propanol A. MAEDA, I. MEDVEDEV, E. HERBST and F. C. DE LUCIA Department of Physics, The Ohio State University Iso-Propanol • Iso-Propanol [(CH3)2CHOH] – One of the structural isomers of propanol [C3H7OH]: N-propanol [CH3CH2CH2OH] Iso-propanol [(CH3)2CHOH] – Three internal rotors: Two CH3 tops One OH top – Two different structural conformers: Gauche & Trans Gauche • Astrochemical Interest • Spectroscopic Interest Trans OH-Torsional Potential 600 tunneling coupling 500 Gauche Trans Gauche’ -1 Energy [cm ] 400 300 gauche (a) 200 trans gauche (s) 100 0 0 60 120 180 Torsional Angle ( 240 300 360 Calculated OH torsional potential barrier and energy levels of iso-propanol (F. Inagaki, I. Harada and T. Shimanouchi, JMS 46, 381, 1973) Iso-Propanol •Astrochemical Interest – Saturated organic molecule Important role in hot molecular cores & corinos – Interstellar Saturated Alcohols Methanol (CH3OH), Ethanol (C2H5OH) – Next largest alcohol is Propanol (C3H7OH) – detectable? N-propanol; submillimeter-wave observation Iso-propanol; only microwave data (< 30 GHz) available Predictions at higher frequency not enough • Spectroscopic Interest Iso-Propanol • Spectroscopic Interest – Previous studies Microwave1, Millimeter-wave2, Far-infrared (OH-torsional fundamental band)3 – Torsion-rotation interaction for a molecule with an internal rotor – Relative energy of the trans torsional substate 1. Kondo & Hirota (1970), Hirota (1979), Hirota & Kawashima (2001) 2. Ulenikov et al. (1991) 3. Inagaki, Harada & Shimanouchi (1973) Experiment --- FASSST WI04 (Fast Scan Submillimeter-wave Spectroscopic Technique) • Radiation source BWOs sweep very fast Wide range! Short time! • Frequency range 100-370 GHz region • Measurement * 200 scans accumulation * Up & down sweeps • Production condition Commercial iso-propanol 14 mTorr SO2 (calibration gas) 3 mTorr • Room temperature FASSST Spectrum of Iso-Propanol 110-370 GHz region : ~70,000 lines Assignment with the CAAARS program Assignment with CAAARS (Computer Aided Assignment of Asymmetric Rotor Spectra) Blended b-type R (ΔJ=+1) pure rotational transitions of (J,Ka,Kc) = (13,0,13) ← (12,1,12) & (13,1,13) (12,0,12) trans gauche (a) gauche (s) • Assigned lines — Spectrum Iso-Propanol in the Ground State Assignment with CAAARS ~ 7,600 lines b, c - type rotational transitions within g(s), g(a), trans a, x - type torsional transitions between g(s) & g(a) Through J = 68 Kc = 52 x-type: Perturbation allowed transition ↓ ΔKa = 0, ΔKc = 0 between different torsional states • Assigned lines — Spectrum OH-Torsional Potential 600 trans → perturbation free gauche (s) & gauche (a) → interact with each other 500 -1 Energy [cm ] 400 300 A estimation 8.7 cm-1 ? – Inagaki et al. 1.56 cm-1 200 g (a) trans g (s) 100 0 0 60 120 180 Torsional Angle ( 240 300 Calculated OH torsional potential barrier and energy levels of iso-propanol (F. Inagaki, I. Harada and T. Shimanouchi, JMS 46, 381, 1973) 360 Analysis with SPFIT Separate Fits for gauche & trans • gauche (s) & gauche (a) – Two-state torsional rotational Hamiltonian Heff = HR + HTR + HT up to sextic centrifugal distortion terms fifth order terms • trans – Rotational Hamiltonian for a semi-rigid rotor HTR (completed through 5th order) 1st 2nd 3rd 4th Explain gauche (s) & (a) substates very well ! 5th σ; torsional substate (σ ≠ σ’) Analysis with SPFIT Separate Fits for gauche & trans • gauche (s) & gauche (a) – Two-state torsional rotational Hamiltonian Heff = HR + HTR + HT • trans – HR for a semi-rigid rotor (Watson type A-reduced Hamiltonian) Heff = HR (up to sextic centrifugal distortion) Perturbation in the trans Substate • Centrifugal distortion • Interaction with an excited vibrational state These ~320 lines were excluded from the fit ? ~3 MHz • Coriolis interaction with gauche Molecular Constants of Iso-Propanol in the Ground State / MHz Prediction for astronomical observation A. Maeda, I. R. Medvedev, F. C. De Lucia, E. Herbst ApJ Supplement, accepted 53 parameters for gauche (s) & (a) (~6300 lines) RMS = 76 kHz 15 parameters for trans (~1500 lines) RMS = 63 kHz Distribution of Intensity Ratio & Relative Energy [Baskakov et al. (2006) HCOOH] Intensity ratio of identical rotational transitions in different torsional substates 2 -1 2 Mean ΔE(trans, g(s)) = 83 (42) cm ET ' , ' i , ' Rint ' , exp 2 2 kT i , • Infrared study σ’,σ = torsional substates • Microwave 8.7 cm-1 158 cm-1 Compared 559 lines • Theoretical calculation 55.96 cm-1 in each trans & gauche (s) Summary • c.a.7,600 spectra of iso-propanol in the ground state have been newly assigned and analyzed. • A prediction has been made accurate enough for astronomical observation. • Perturbation was found in trans at J > 50. • Relative energy of the trans conformer was estimated from distribution of relative intensity of lines. Acknowledgement NASA for its support program Brenda P. Winnewisser Manfred Winnewisser Torsion-Rotation Interaction for an asymmetric molecule with an internal rotor • Quade & Lin (1963) Deuterated Methanol; Effective Hamiltonian with FFAM (FrameworkFixed Axis Method) • Pearson, Sastry, Herbst, & De Lucia (1996) Ethanol (J up to 30); HTR expanded up to 5th order terms (no 4th order) • Duan, Zhang &Takagi (1996), Duan, Wang &Takagi (1999) Methanol; Higher order HTR terms for a molecule with an internal rotor derived with sequential contact transformation technique Present study HTR complete up to 5th order Distribution of Intensity Ratio & Relative Energy Mean ΔE(g(a), g(s)) = 3.6 (10) cm-1 Comparable to ΔE(g(a), g(s)) = 1.56 cm-1 556 lines in each gauche (s) & gauche (a) Baskakov et al. (2006) HCOOH Energy Difference / cm-1 Vibrational Excited State Unassigned ~62,000 lines 3~4 times weaker intensity — Spectrum — Unassigned lines