Rotationally-Resolved Spectra of 2-methylfuran from the cm-wave to the

far IRSteven T. Shipman, Ian A. Finneran, Susanna L. Widicus Weaver, and

Jennifer van Wijngaarden

2-methylfuran

W. G. Norris and L. C. Krisher, J. Chem. Phys. 51, 403-406 (1969).U. Andresen and H. Dreizler, Z. Naturforschung 25a, 570 (1970).

Small, volatile organic compoundMethyl rotor complicates spectrum

First assignments by Norris and Krisher (1969)Andresen and Dreizler measured dipole (1970)

ma = 0.31 D, mb = 0.57 D, mc = 0

D (planar)

V3 = 412.8 cm-1

Mid-to-high Ka: level mixing c-types borrow intensity from b-types

Forbidden c-types

Forbidden c-types are > 80% of E-state peaks from 8.7 – 18.3 GHz at 273 K

1) Chirped-pulse generated by AWG, mixed with PLDRO.

2) Sideband is amplified and sent into sample cell. (10 mTorr, 0 °C typical)

3) Molecular FID is amplified and mixed with PLDRO.

4) Downconverted FID is detected with oscilloscope.

Chirped-Pulse Spectrometer

New this yearAmps and multipliers for 18 – 26.5 GHz.

Working on DR across full range.

Spectra from 8 – 26.5 GHz

A-E splittingGS: 3.2 MHzv = 1: 251.8 MHz

725 - 716

3 M shots3091 peaks above 3:1

Tallest peaks are bQ Remainder are bR, bP, cR, cP

10 12 14 16 18

1.5 M shots740 peaks above 3:1

1 – 50 GHzFrequencySynthesizer

VDI Multiplier chain50 GHz – 1.2 THz Detector

Gas Flow Cell

To Computer

Sample Input To Vacuum Pump

Carroll, Drouin, & Widicus Weaver ApJ 723, 2010

Source modulated at 15 kHz

HEB signal detected with lock-in at 2f

100 kHz point spacing, 2.5 GHz / hour.

Direct Absorption Flow Cell

2-mf mm- and submm-wave datamainly aR, bRweak bQ

bR, cR

Conservative estimate:45,806 peaks from 75 – 960 GHz

11,509 assigned to GS2,580 assigned to vtors = 1

Assigned ~31% of the features(Qvib at 298 K is ~ 5)

Fitting Strategies / Challenges

Needed “unusual” tunneling parameters in ERHAM:• GS uses an S-reduction parameter• Both use parameters for sextic distortion constants fixed to 0.

• Initial fitting done with recompiled XIAM

• Fine for cm-wave data, but problems near Ka = 11 with high frequency data

• Switching to ERHAM resolved this

• Extensive use of combination loops in assignment of weak bQ transitions

GS v = 1

Fit RMS Parameters Fit RMS Parameters

XIAM 263 kHz 18 > 16 MHz 18

ERHAM 108 kHz 19 113 kHz 23

Far IR Spectroscopy at the CLS

Bruker IFS 125HR

0.000959 cm-1 resolution (28.75 MHz)2 m multipass cell – 36 passesData collected at 298 K

Located in Saskatoon, SK

Beam at 2.9 GeV

Most beamlines are X-ray, but two

for IR: • Far IR (5 – 1000 cm-1)• Mid IR (560 – 6000 cm-1)

2-mf Vibrational Modes

237.4 cm-1, 6.6out-of-plane bend

339.3 cm-1, 2.4in-plane bend

603.3 cm-1, 7.2ring puckering mode

628.7 cm-1, 1.3ring puckering mode

B3LYP / 6-311++G(d,p), with anharmonic frequency corrections

Si detector

Ge/Cu detector

Low pressure

High pressure

50 – 400 cm-1:130 mTorr, 88 IFGs960 mTorr, 314 IFGs

500 – 1100 cm-1:125 mTorr, 103 IFGs 525 mTorr, 309 IFGs

Roughly 5 IFGs / hour

CLS Data Overviewn29

c-type

n20

b-type

n28

c-type

n27

c-type

960 mTorr 960 mTorr

525 mTorr

CLS Data – Expanded ViewsHot bands!

Multiple Q-branches (oop bend)

R-branch (ip bend)

130 mTorr

960 mTorr

130 mTorr

P-branch (oop bend)

960 mTorr

Summary and Future Work

• Beam time from August 8 – 13. Use chilled cell to reduce number of

hot bands.

• Fit far IR data, use constants as starting point for U-lines in 8.7 – 960

GHz data.

• Use full data set as testbed for developing more automated fitting

routines.

Future work:

Comprehensive fit of 2-methylfuran ground and first excited states.

Fitting with ERHAM was more successful than with XIAM; some strangeness with tunneling parameters, though.

Room-temperature data collected from 50 – 400 cm-1 and 500 – 1100 cm-1. Analysis greatly complicated by hot bands.

Acknowledgments

Funding

Dr. Brant Billinghurst

Canadian Light Source

van Wijngaarden groupUniversity of Manitoba

Widicus Weaver group

Emory University

Shipman groupNew College of

Florida

Noah Anderson (2012)Brittany Gordon (2013)

Erin Kent (2013)Sophie Lang (2014)

Morgan McCabe (2014)

Sam McCamant (2013)Christian Metzger

(2013)Maria Phillips (2013)

Ben Rooks (2013)Suzanne Setti (2014)

κ ~ -0.67 for 2-methylfuran Adapted from Gordy and Cook, fig. 12.7 and 12.8

Example: R-Branch, vt=1

2-mf features (simulated)

aR and bR

dominant

Sparse bR and cR

Weaker bQ (offset)

bR and cR

dominant

Very weak bQ

(offset)

Ground State Fit Table (Partial) GS Calculated ERHAM XIAM

A (MHz) 8756.7754 8791.54486(12) 8792.22489(33)

B (MHz) 3537.1200 3543.321804(46) 3542.64071(20)

C (MHz) 2559.8639 2565.603243(36) 2565.560151(38)

DJ (kHz) 0.25545138 0.2645388(58) 1.539273(68)

DJK (kHz) 1.3677974 1.408544(66) -2.41323(31)

DK (kHz) 1.0616181 1.000997(83) 0.99269(31)

dJ (kHz) 0.070719708 0.0724207(24) 0.564919(32)

dK (kHz) 0.60351449 0.687123(86) -0.46139(17)

V3 (cm-1) 368.8 412.873(74)

< (i,a) 4.48 4.725(32) 3.30(13)

< (i,b) 85.52 85.275(32) 86.70(13)

< (i,c) 90.00  [90.00] 90.009(18)

J max 120 95

Ka max 54 53

N 11793 8006

sfit (MHz) 0.108 0.263

sw 0.718 1.754

• ERHAM distortion constants much closer to ab initio (MP2/6-311++G(d,p)) than XIAM.

• N is distinct frequencies (blends only counted once); 19,143 GS transitions in fit.

Rest of the fit tables

  v = 1 (ERHAM)

A (MHz) 8790.0922(18)

B (MHz) 3540.94203(22)

C (MHz) 2564.946450(65)

DJ (kHz) 0.268855(50)

DJK (kHz) 1.8300(31)

DK (kHz) -1.947(21)

dJ (kHz) 0.074930(24)

dK (kHz) 0.6824(14)

FJK (mHz) -2.852(63)

fK (mHz) -7.01(64)

v = 1 (cont’d)

e1 (MHz) 3781.55(72)

B0201 (MHz) -0.3910(35)

B2001 (MHz) 0.017418(49)

B2201 (kHz) -0.1102(27)

B0221 (kHz) -0.0523(14)

B2021 (Hz) -5.335(12)

B0401 (kHz) 0.806(10)

B4001 (kHz) -0.010640(23)

B2401 (Hz) 0.0785(35)

B0421 (Hz) 0.0523(25)

B4201 (mHz) 1.354(61)

 GS XIAM   ERHAM

FJK (mHz) -0.669(45) -0.6653(83)

FKJ (mHz) 0.568(50) [0.0]

FK (mHz) [0.0] 1.882(20)

fJK (mHz) -0.502(29) [0.0]r 0.0549511(44) 0.0550406(14)

DJm (MHz) -0.0950(30) B0201e (kHz) 1.051(29)

DKm (MHz) 0.2333(73) B2201 (Hz) -0.1781(88)

dm (MHz) 0.0406(27) B2021 (Hz) 0.0133(11)

    B0041 (Hz) -0.03557(84)

    B6001 (mHz) 0.00603(23)

e1 (MHz) -119.459(11)

v = 1 (cont’d)r 0.0554463(90)

b (degrees) 2.1785(31)

J max 114

Ka max 15

N 2580b

sfit (MHz) 0.113

swa 0.753

Other Excited States?

W. G. Norris and L. C. Krisher, J. Chem. Phys. 51, 403-406 (1969).

Assigned just over 30% of the peaks to GS and v=1 of methyl torsion.

Qvib at 298 K is ~ 5.

Norris and Krisher: 14 peaks from 20 – 30 GHz assigned to v=1 of ring-puckering mode.Probably out-of-plane / in-plane bend instead (237 / 339 vs 603 cm-1)

We observe 9 of these, but only 6 have correct intensities. Can’t extend fits using N&K constants.

Plan: Get constants from CLS for excited states and go back to 8.7 – 960 GHz data.