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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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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

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



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



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.

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.

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




RH08—diethylsilane (A.Steber)‏

trans-methyl formate (M.Muckle)

CP-FTMW Modifications 2008-09


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

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.

Observations of Previously Assigned Conformers

Noise level ~500 nV (20,000:1 S/N on strongest line)

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.

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:


(likely more stable) (1,2-propanediol)



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



National Science Foundation Centers for Chemical Innovation

grant 0847919

University of Virginia

Jefferson Scholars Foundation (J. Neill)


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