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 Centers for Chemical Innovation

Conformers of 1,2-propanediol: mp2/aug-cc-pVTZ O 2 -C 3 -C 4 -C 7 dihedral = 180ºO 2 -C 3 -C 4 -C 7 dihedral = 60º O 2 is H-bond acceptor O 2 is H-bond donor conf. 1 E = 192 cm -1 conf. 2 E = 83 cm -1 conf. 3 E = 0 cm -1 conf. 5 E = 87 cm -1 conf. 4 E = 338 cm -1 conf. 6 E = 213 cm -1 conf. 7 E = 345 cm -1 conf. 8 E = 441 cm -1 Detected by Caminati a Detected by Lockley et al b a W. Caminati, J. Mol. Spectrosc. 86 (1981) b T.J.L. Lockley et al., J. Mol. Struct. 612 (2002)

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

CP-FTMW Modifications 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)

CP-FTMW Modifications 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 1Frame 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

ParameterConformer 2Theory A (MHz) (16) B (MHz) (7) C (MHz) (6) Δ J (kHz)0.797(15)0.772 Δ JK (kHz)4.485(70)4.88 Δ K (kHz)3.16(35)3.44 δ J (kHz)0.1827(60)0.177 δ K (kHz)3.14(21)2.96 Nlines61 Wt. Std.0.90 µ a (D)2.496(2)-2.64 µ b (D)0.309(20)0.28 µ c (D)0.45(8)-0.57 ParameterConformer 3Theory A (MHz) (8) B (MHz) (5) C (MHz) (4) Δ J (kHz)0.738(7)0.719 Δ JK (kHz)5.276(30)5.56 Δ K (kHz)2.53(10)2.97 δ J (kHz)0.1631(16)0.155 δ K (kHz)3.180(31)3.16 Nlines57 Wt. Std.0.88 µ a (D)1.201(3)1.21 µ b (D)1.916(6)-2.10 µ c (D)0.365(36)0.45 ParameterConformer 5Theory A (MHz) (2) B (MHz) (1) C (MHz) (1) Δ J (kHz)0.751(14)0.714 Δ JK (kHz)5.29(7)5.66 Δ K (kHz)2.75(22)2.99 δ J (kHz)0.152(6)0.143 δ K (kHz)3.34(14)3.12 Nlines44 Wt. Std.1.1 µ a / µ b µ b / µ b 11 µ c / µ b ParameterConformer 6Theory A (MHz) (5) B (MHz) (4) C (MHz) (3) Δ J (kHz)0.76(12)0.767 Δ JK (kHz)5.1(6)4.81 Δ K (kHz)2.9(fixed)2.89 δ J (kHz)0.24(11)0.166 δ K (kHz)2.8(fixed)2.85 Nlines18 Wt. Std.1.9 µ a / µ a 11 µ b / µ a µ c / µ a

ParameterConformer 1Theory A (MHz) (9) B (MHz) (9) C (MHz) (7) Δ J (kHz)1.774(29)1.80 Δ JK (kHz)6.354(82)5.55 Δ K (kHz)-4.51(12)-3.28 δ J (kHz)0.267(13)0.254 δ K (kHz)1.74(18)0.89 Nlines46 Wt. Std.0.63 µ a (D)2.202(4)2.35 µ b (D)0 (fixed)-0.03 µ c (D)0.616(10)0.70 ParameterConformer 4Theory A (MHz) (7) B (MHz) (9) C (MHz) (8) Δ J (kHz)1.751(31)1.74 Δ JK (kHz)8.21(11)7.47 Δ K (kHz)-6.51(12)-4.86 δ J (kHz)0.244(17)0.244 δ K (kHz)2.72(23)1.61 Nlines32 Wt. Std.0.57 µ a / µ a 11 µ b / µ a µ c / µ a ParameterConformer 7Theory A (MHz) (8) B (MHz) (5) C (MHz) (6) Δ J (kHz)1.84(3)1.83 Δ JK (kHz)6.2(2)5.85 Δ K (kHz)-5.0(3)-3.84 δ J (kHz)0.23(3)0.249 δ K (kHz)1.8(3)1.19 Nlines20 Wt. Std.0.50 µ a / µ c µ b / µ c µ c / µ c 11

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 a variety of complex astrochemical species. CH 2 OH + CH 2 OH  (CH 2 OH) 2 (abundance predicted accurately) Not incorporated into this model, but possible similar propanediol formation route exists in this type of chemistry: CH 3 CHOH + CH 2 OH  CH 2 (OH)CH(OH)CH 3 (likely more stable)(1,2-propanediol) CH 2 CH 2 OH + CH 2 OH  CH 2 (OH)CH 2 CH 2 (OH) (1,3-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 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 cm -2. For comparison, ethylene glycol column density: 3.3 x cm -1

Acknowledgements Funding: National Science Foundation Centers for Chemical Innovation grant University of Virginia Jefferson Scholars Foundation (J. Neill) Tektronix

Conformer 8? Ab initio (mp2/aug-cc-pvtz): A = MHz B = MHz C = MHz µ a = D µ b = D µ c = 0.35 D only ~5 transitions might be visible at current sensitivity