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 TechnologyDivision, NIST

A. J. Remijan, National Radio Astronomy Observatory

Centers for Chemical Innovation

Conformers of 1,2-propanediol: mp2/aug-cc-pVTZO

2-C

3-C

4-C

7 dihedral = 180º O

2-C

3-C

4-C

7 dihedral = 60º

O2 is

H-bondacceptor

O2 is

H-bonddonor

conf. 1E = 192 cm-1

conf. 2E = 83 cm-1

conf. 3E = 0 cm-1

conf. 5E = 87 cm-1

conf. 4E = 338 cm-1

conf. 6E = 213 cm-1

conf. 7E = 345 cm-1

conf. 8E = 441 cm-1

Detected by CaminatiaDetected by Caminatia

Detected by Lockley et alb

aW. Caminati, J. Mol. Spectrosc. 86 (1981) 193-201. bT.J.L. Lockley et al., J. Mol. Struct. 612 (2002) 199-206.

New MeasurementsTwo spectrometers employed:

1) Balle-Flygare-type FTMW spectrometer at NISTa

discovered conformer 4Stark 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

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.

Sample (mix of enantiomers) purchased from Aldrich (>99.5%, <0.2% H

2O); internal reservoir, heated to 60ºC

(CP-FTMW: strongest (H2O)2 line factor of 3,000 downfrom strongest 1,2-propanediol line)

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 collectedper 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)

CP-FTMW Modifications 2008-09FastFrame

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

All simulations from SPCAT, with ab initio dipoles, at 0.9 K.

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

Observations of Previously Assigned Conformers

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

Observations of New Conformers

x17.5

Parameter Conformer 2 Theory

A (MHz) 8393.4003(16) 8451.8

B (MHz) 3648.5661(7) 3678.9

C (MHz) 2778.2963(6) 2802.6

ΔJ (kHz) 0.797(15) 0.772

ΔJK (kHz) 4.485(70) 4.88

ΔK (kHz) 3.16(35) 3.44

δJ (kHz) 0.1827(60) 0.177

δK (kHz) 3.14(21) 2.96

Nlines 61

Wt. Std. 0.90

µa (D) 2.496(2) -2.64

µb (D) 0.309(20) 0.28

µc (D) 0.45(8) -0.57

Parameter Conformer 3 Theory

A (MHz) 8572.0553(8) 8643.1

B (MHz) 3640.1063(5) 3672.6

C (MHz) 2790.9666(4) 2818.1

ΔJ (kHz) 0.738(7) 0.719

ΔJK (kHz) 5.276(30) 5.56

ΔK (kHz) 2.53(10) 2.97

δJ (kHz) 0.1631(16) 0.155

δK (kHz) 3.180(31) 3.16

Nlines 57

Wt. Std. 0.88

µa (D) 1.201(3) 1.21

µb (D) 1.916(6) -2.10

µc (D) 0.365(36) 0.45Parameter Conformer 5 Theory

A (MHz) 8536.770(2) 8608.5

B (MHz) 3604.198(1) 3630.1

C (MHz) 2778.331(1) 2802.3

ΔJ (kHz) 0.751(14) 0.714

ΔJK (kHz) 5.29(7) 5.66

ΔK (kHz) 2.75(22) 2.99

δJ (kHz) 0.152(6) 0.143

δK (kHz) 3.34(14) 3.12

Nlines 44

Wt. Std. 1.1

µa / µb 0.28 0.22

µb / µb 1 1

µc / µb 0.88 0.81

Parameter Conformer 6 Theory

A (MHz) 8327.599(5) 8371.4

B (MHz) 3642.001(4) 3674.6

C (MHz) 2776.902(3) 2801.0

ΔJ (kHz) 0.76(12) 0.767

ΔJK (kHz) 5.1(6) 4.81

ΔK (kHz) 2.9(fixed) 2.89

δJ (kHz) 0.24(11) 0.166

δK (kHz) 2.8(fixed) 2.85

Nlines 18

Wt. Std. 1.9

µa / µa 1 1

µb / µa 0.28 0.31

µc / µa 0.49 0.53

Parameter Conformer 1 Theory

A (MHz) 6642.4488(9) 6672.3

B (MHz) 4163.5949(9) 4213.2

C (MHz) 3365.3627(7) 3407.2

ΔJ (kHz) 1.774(29) 1.80

ΔJK (kHz) 6.354(82) 5.55

ΔK (kHz) -4.51(12) -3.28

δJ (kHz) 0.267(13) 0.254

δK (kHz) 1.74(18) 0.89

Nlines 46

Wt. Std. 0.63

µa (D) 2.202(4) 2.35

µb (D) 0 (fixed) -0.03

µc (D) 0.616(10) 0.70

Parameter Conformer 4 Theory

A (MHz) 6634.7621(7) 6654.0

B (MHz) 4160.6347(9) 4217.7

C (MHz) 3377.9063(8) 3424.7

ΔJ (kHz) 1.751(31) 1.74

ΔJK (kHz) 8.21(11) 7.47

ΔK (kHz) -6.51(12) -4.86

δJ (kHz) 0.244(17) 0.244

δK (kHz) 2.72(23) 1.61

Nlines 32

Wt. Std. 0.57

µa / µa 1 1

µb / µa 0.56 0.62

µc / µa 0.56 0.49Parameter Conformer 7 Theory

A (MHz) 6627.612(8) 6659.2

B (MHz) 4146.287(5) 4192.7

C (MHz) 3363.345(6) 3407.8

ΔJ (kHz) 1.84(3) 1.83

ΔJK (kHz) 6.2(2) 5.85

ΔK (kHz) -5.0(3) -3.84

δJ (kHz) 0.23(3) 0.249

δK (kHz) 1.8(3) 1.19

Nlines 20

Wt. Std. 0.50

µa / µc 0.43 0.51

µb / µc 0.30 0.42

µc / µc 1 1

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.

blown up 140x from original spectrum

Astronomical Search

New model incorporates grain-surface radical reactions, predicting high abundances of avariety of complex astrochemical species.

CH2OH + CH2OH (CH2OH)2(abundance predicted accurately)

Not incorporated into this model, butpossible similar propanediol formation route existsin 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)

Astronomical Search

Since ethylene glycol has been found in Sgr B2(N), both1,2- and 1,3-propanediols were sought in the same source.

For 1,2-propanediol, a total of 12 transitions (six from conformer2, six from conformer 3) were sought. The lowest noise levelattained was ~4 mK. Assuming a temperature of 10 K, the upperlimit on the 1,2-propanediol conformer 3 column density is8 x 1014 cm-2.

For 1,3-propanediol, a total of 22 transitions of conformer 1 weresought; the lowest noise level attained was ~5 mK. The upperlimit on the 1,3-propanediol conformer 1 column density is2 x 1013 cm-2.

For comparison, ethylene glycol column density: 3.3 x 1014 cm-1

Acknowledgements

Funding:National Science Foundation Centers for Chemical Innovation

grant 0847919University of VirginiaJefferson Scholars Foundation (J. Neill)

Tektronix

http://www.virginia.edu/ccu

Conformer 8?

Ab initio (mp2/aug-cc-pvtz):A = 6647.6 MHzB = 4160.1 MHzC = 3369.6 MHzµa = -0.35 Dµb = -2.49 Dµc = 0.35 D

only ~5 transitions might bevisible at current sensitivity