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 [(CH 3 ) 2 CHOH] –One of the structural isomers of propanol [C 3 H 7 OH]: N-propanol [CH 3 CH 2 CH 2 OH] Iso-propanol [(CH 3 ) 2 CHOH] –Three internal rotors: Two CH 3 tops One OH top –Two different structural conformers: Gauche & Trans Astrochemical Interest Spectroscopic Interest Gauche Trans

OH-Torsional Potential Calculated OH torsional potential barrier and energy levels of iso-propanol (F. Inagaki, I. Harada and T. Shimanouchi, JMS 46, 381, 1973) tunneling coupling Gauche Trans Gauche’ gauche (a) trans gauche (s)

Iso-Propanol Astrochemical Interest –Saturated organic molecule Important role in hot molecular cores & corinos –Interstellar Saturated Alcohols Methanol (CH 3 OH), Ethanol (C 2 H 5 OH) –Next largest alcohol is Propanol (C 3 H 7 OH) – detectable? Spectroscopic Interest N-propanol; submillimeter-wave observation Iso-propanol; only microwave data (< 30 GHz) available Predictions at higher frequency not enough

Iso-Propanol Spectroscopic Interest –Previous studies Microwave 1, Millimeter-wave 2, 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 (Fast Scan Submillimeter-wave Spectroscopic Technique) Radiation source BWOs sweep very fast Frequency range GHz region Measurement * 200 scans accumulation * Up & down sweeps Production condition Commercial iso-propanol 14 mTorr SO 2 (calibration gas) 3 mTorr Wide range! Short time! Room temperature WI04

GHz region : ~70,000 lines FASSST Spectrum of Iso-Propanol Assignment with the CAAARS program

Assigned lines — Spectrum Blended b-type R (ΔJ=+1) pure rotational transitions of (J,K a,K c ) = (13,0,13) ← (12,1,12) & (13,1,13) (12,0,12) trans gauche (a) gauche (s) Assignment with CAAARS (Computer Aided Assignment of Asymmetric Rotor Spectra)

Assignment with CAAARS ~ 7,600 lines Iso-Propanol in the Ground State Assigned lines — Spectrum b, c - type rotational transitions within g(s), g(a), trans a, x - type torsional transitions between g(s) & g(a) Through J = 68 K c = 52 x-type: Perturbation allowed transition ↓ ΔK a = 0, ΔK c = 0 between different torsional states

OH-Torsional Potential Calculated OH torsional potential barrier and energy levels of iso-propanol (F. Inagaki, I. Harada and T. Shimanouchi, JMS 46, 381, 1973) 1.56 cm -1 trans → perturbation free gauche (s) & gauche (a) → interact with each other g (a) g (s) trans A estimation 8.7 cm -1 ? – Inagaki et al.

Analysis with SPFIT Separate Fits for gauche & trans gauche (s) & gauche (a) – Two-state torsional rotational Hamiltonian H eff = H R + H TR + H T trans – Rotational Hamiltonian for a semi-rigid rotor up to sextic centrifugalfifth order terms distortion terms

H TR (completed through 5 th order) σ; torsional substate (σ ≠ σ’) 1 st 2 nd 3 rd 4 th 5 th Explain gauche (s) & (a) substates very well !

Analysis with SPFIT Separate Fits for gauche & trans gauche (s) & gauche (a) – Two-state torsional rotational Hamiltonian H eff = H R + H TR + H T trans – H R for a semi-rigid rotor (Watson type A-reduced Hamiltonian) H eff = H R (up to sextic centrifugal distortion)

Perturbation in the trans Substate Centrifugal distortion Coriolis interaction with gauche Interaction with an excited vibrational state These ~320 lines were excluded from the fit ? ~3 MHz

Molecular Constants of Iso-Propanol in the Ground State / MHz 53 parameters for gauche (s) & (a) (~6300 lines) RMS = 76 kHz 15 parameters for trans (~1500 lines)RMS = 63 kHz Prediction for astronomical observation A. Maeda, I. R. Medvedev, F. C. De Lucia, E. Herbst ApJ Supplement, accepted

Distribution of Intensity Ratio & Relative Energy [Baskakov et al. (2006) HCOOH] Intensity ratio of identical rotational transitions in different torsional substates σ’,σ = torsional substates Compared 559 lines in each trans & gauche (s) Mean ΔE(trans, g(s)) = 83 (42) cm -1 Infrared study8.7 cm -1 Microwave158 cm -1 Theoretical calculation55.96 cm -1

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 (Framework- Fixed Axis Method) Pearson, Sastry, Herbst, & De Lucia (1996) Ethanol (J up to 30); H TR expanded up to 5 th order terms (no 4 th order) Duan, Zhang &Takagi (1996), Duan, Wang &Takagi (1999) Methanol; Higher order H TR terms for a molecule with an internal rotor derived with sequential contact transformation technique Present study H TR complete up to 5 th 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 Baskakov et al. (2006) HCOOH 556 lines in each gauche (s) & gauche (a)

Energy Difference / cm -1

Unassigned ~62,000 lines 3~4 times weaker intensity Vibrational Excited State — Spectrum — Unassigned lines