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Microwave Spectroscopy of Seven Conformers of 1,2-Propanediol

Centers for Chemical Innovation. Microwave Spectroscopy of Seven Conformers of 1,2-Propanediol. Justin L. Neill , Matt T. Muckle, and Brooks H. Pate , Department of Chemistry, University of Virginia F. J. Lovas, D. F. Plusquellic,  Optical Technology Division, NIST

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Microwave Spectroscopy of Seven Conformers of 1,2-Propanediol

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  1. Centers for Chemical Innovation Microwave Spectroscopy of Seven Conformers of 1,2-Propanediol Justin L. Neill, Matt T. Muckle, and Brooks H. Pate, Department of Chemistry, University of Virginia F. J. Lovas, D. F. Plusquellic, Optical Technology Division, NIST A. J. Remijan, National Radio Astronomy Observatory

  2. Conformers of 1,2-propanediol: mp2/aug-cc-pVTZ O2-C3-C4-C7 dihedral = 180º O2-C3-C4-C7 dihedral = 60º conf. 4 E = 338 cm-1 conf. 3 E = 0 cm-1 O2 is H-bond acceptor Detected by Lockley et alb conf. 8 E = 441 cm-1 conf. 5 E = 87 cm-1 conf. 1 E = 192 cm-1 conf. 2 E = 83 cm-1 O2 is H-bond donor Detected by Caminatia Detected by Caminatia conf. 7 E = 345 cm-1 conf. 6 E = 213 cm-1 aW. Caminati, J. Mol. Spectrosc. 86 (1981) 193-201. bT.J.L. Lockley et al., J. Mol. Struct. 612 (2002) 199-206.

  3. New Measurements Two spectrometers employed: 1) Balle-Flygare-type FTMW spectrometer at NISTa discovered conformer 4 Stark effect measurements (conformers 1-3) high-resolution measurements (all conformers) for final fits 2) Chirped-pulse FTMW spectrometer at UVab operating between 6.5-18.5 GHz—288,000 averaged FIDs Sample (mix of enantiomers) purchased from Aldrich (>99.5%, <0.2% H2O); internal reservoir, heated to 60ºC (CP-FTMW: strongest (H2O)2 line factor of 3,000 down from strongest 1,2-propanediol line)‏ aF.J. Lovas and R.D. Suenram, J. Chem. Phys. 87 (1987) 2010-2020. bG.G. Brown, B.C. Dian, K.O. Douglass, S.M. Geyer, S.T. Shipman, and B.H. Pate, Rev. Sci. Instrum. 79 (2008) 53103-1-13.

  4. CP-FTMW Modifications 2008-09 -24 GS/s AWG (Tektronix AWG7122B); more accurate intensities over full spectral range -50 GS/s oscilloscope (Tektronix DPO71022); all signals directly digitized (no peaks due to mixing bleedthroughs)‏ -Sample conservation techniques: 2 nozzles, 10 FIDs per gas pulse -nozzle slowed to 0.6 Hz (limited by oscilloscope's data processing speed) at a 20 μs FID length (10,000,000 points collected per valve pulse)‏ 21,000 FIDs collected per hour of averaging—14 hours to collect 288k average spectrum -limited by oscilloscope processing speed—potential factor of 16 enhancement Other talks using UVa CP-FTMW: MH08—propofol (A.Lesarri)‏ TA05—strawberry aldehyde (S.Shipman)‏ TA09—chloropentafluorobenzene (A.Osthoff)‏ WI06—isomers of HSCN in electric discharge (M.McCarthy)‏ RC11—p-methoxyphenethylamine—water (J.Neill)‏ RH08—diethylsilane (A.Steber)‏ trans-methyl formate (M.Muckle)

  5. CP-FTMW Modifications 2008-09 FastFrame Arbitrary waveform generator puts out 10- MW pulse chain (with 25 µs buffer between pulses) Oscilloscope saves spectrum every ~2.5 h (in case of power outages, phase shifts) Puts greater stress on passive diode limiter (Advanced Control Components) cannot reliably run with 1 kW TWT, used 300 W TWT instead Oscilloscope collects 10 acquisitions before moving data into computer memory Also keeps “average” frame as frame 11 Not efficiently processed: averages frames 1-10 over time as well as frame 11—could delete frames 1-10 after averaging together Need to use longer valve pulse (~700 µs) Frame 1 Frame 7

  6. Observations of Previously Assigned Conformers Noise level ~500 nV (20,000:1 S/N on strongest line) All simulations from SPCAT, with ab initio dipoles, at 0.9 K.

  7. Observations of Previously Assigned Conformers Noise level ~500 nV (20,000:1 S/N on strongest line)

  8. Observations of New Conformers x17.5

  9. blown up 140x from original spectrum 1441 lines present in spectrum at 3:1 S/N or better; 1141 remain unassigned MW-MW double resonance techniques are necessary to assign these spectra.

  10. Astronomical Search New model incorporates grain-surface radical reactions, predicting high abundances of a variety of complex astrochemical species. CH2OH + CH2OH  (CH2OH)2 (abundance predicted accurately) Not incorporated into this model, but possible similar propanediol formation route exists in this type of chemistry: CH3CHOH + CH2OH  CH2(OH)CH(OH)CH3 (likely more stable) (1,2-propanediol) CH2CH2OH + CH2OH  CH2(OH)CH2CH2(OH) (1,3-propanediol)

  11. Astronomical Search Since ethylene glycol has been found in Sgr B2(N), both 1,2- and 1,3-propanediols were sought in the same source. For 1,2-propanediol, a total of 12 transitions (six from conformer 2, six from conformer 3) were sought. The lowest noise level attained was ~4 mK. Assuming a temperature of 10 K, the upper limit on the 1,2-propanediol conformer 3 column density is 8 x 1014 cm-2. For 1,3-propanediol, a total of 22 transitions of conformer 1 were sought; the lowest noise level attained was ~5 mK. The upper limit on the 1,3-propanediol conformer 1 column density is 2 x 1013 cm-2. For comparison,ethylene glycol column density: 3.3 x 1014 cm-1

  12. Acknowledgements Funding: National Science Foundation Centers for Chemical Innovation grant 0847919 University of Virginia Jefferson Scholars Foundation (J. Neill) Tektronix http://www.virginia.edu/ccu

  13. Conformer 8? Ab initio (mp2/aug-cc-pvtz): A = 6647.6 MHz B = 4160.1 MHz C = 3369.6 MHz µa = -0.35 D µb = -2.49 D µc = 0.35 D only ~5 transitions might be visible at current sensitivity

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