Pure Rotational and Ultraviolet-Microwave Double Resonance Spectroscopy of

Two Water Complexes of para-methoxyphenylethylamine (pMPEA)

Justin L. Neill, Matt T. Muckle and Brooks H. Pate,Department of Chemistry, University of Virginia

Ryan G. Bird, David W. Pratt,Department of Chemistry, University of Pittsburgh

Spectroscopy of pMPEA and pMPEA-waterpMPEASeven conformers reported: Martinez et al, J. Mol. Spectrosc. 158 (1993) 82-92.Complete (correct) structural assignment: Robertson, Simons, and Mons, J. Phys. Chem. A 105 (2001) 9990. Yi, Robertson, and Pratt, Phys. Chem. Chem. Phys. 4 (2002) 5244-5248. (rotationally resolved LIF)Douglass et al., MF02, ISMS (2006) (CP-FTMW and UV-MW)Cortijo, Alonso, and López, Chem. Phys. Lett. 466 (2008) 214-218. (MW)

pMPEA-waterTwo clusters found, binding energies measured,assigned to structures: Unamuno et al, Chem. Phys. 271 (2001) 55-69.

Unamuno et al.

Unamuno et al.

Conformational Landscape of pMPEA

Conformer E (cm-1)

E-8 0

D-5 34

C-7 38

A-4 54

B-1 351

F-2 489

G-3 498

6 675

9 678

Yi, Robertson, and Pratt, Phys. Chem. Chem. Phys. 4 (2002) 5244-5248.

mp2/6-31g**

A

B

C

D

E

F GB F G

AD

EC

New CP-FTMW MeasurementsSample acquired from Aldrich (98%); placed in reservoir within nozzle, heatedto approximately 100°C, seeded in He/Nesupersonic expansion. (No water added)

CP-FTMW sample reduction techniques:3 nozzles10 FIDs per valve pulse

Collected 995,000 FIDs, using298,500 valve pulses, equivalentsensitivity to 8.955 million pulses witha single nozzle!

Measurement time: 48 hours (consecutive)

G.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.

New CP-FTMW Measurements

3 nozzles, 995,000 FIDs (298,500 pulses) versus 1 nozzle, 80,000 FIDs (80,000 pulses)Scaled to match signal heights on strongest transitions

New CP-FTMW Measurements

2008 spectrum much richer than 2006…(used same bottle)

3 nozzles, 995,000 FIDs (298,500 pulses) versus 1 nozzle, 80,000 FIDs (80,000 pulses)Scaled to match signal heights on strongest transitions

New Transitions

Several unassigned Q-branches observed. For near-prolate top, b/c-type Q-branches are locatedat approximately (A-(B+C)/2)*(2K-1), so the ratio between two Q-branches gives you their K assignmentsand A-(B+C)/2.

Pattern of the Q-branches gives (B-C), then (A+B+C) can be varied until the strong b/c-type R-branchesare fit. Two new spectra were assigned this way.

New Transitions

pMPEA-water Fit Parameters

Conformer D-water Experiment TheoryA/MHz 1740.6781(7) 1769.85B/MHz 430.5044(4) 426.01C/MHz 380.0915(4) 376.65χaa/MHz 0.20(6) 0.41

χbb-χcc/MHz -2.419(26) -2.19Nlines 224

rms error/kHz 24.2µa/D 0.64µb/D b ≈ c 2.84µc/D 2.60

Conformer E-water Experiment TheoryA/MHz 1533.8873(10) 1552.23B/MHz 457.8598(5) 452.47C/MHz 398.9191(9) 383.61χaa/MHz 0.785(47) 0.97

χbb-χcc/MHz -2.31(44) -2.04Nlines 137

rms error/kHz 12.7µa/D 0.47µb/D c only 0.49µc/D 2.32

Observed structures are analogous to those of other similar structures: tryptamine (Felker, J. Phys. Chem. 96 (1992) 7844);2-phenylethylamine (Melandri, et al, RC13)

Ab initio: b3pw91/6-311+g(df,pd), using effective Q and recommended basis set of W.C. Bailey (http://homepage.mac.com/wcbailey/nqcc/)

All fits performed using SPFIT (Pickett), with standard errors determined by PIFORM (Kisiel). (Quartic distortion parameters not listed)

Coherence-Converted Population Transfer UV-FTMW Spectroscopy

T.J. Balle and W.H. Flygare, Rev. Sci. Instrum. 52, 33 (1981)M. Nakajima, Y. Sumiyoshi, and Y.Endo, Rev. Sci. Instrum. 73, 165 (2002)

MW Synthesizer

ν0

ν0

Free Induction Decay(30 MHz Carrier)1 Gs/s Oscilloscope

R.D. Suenram, J.U. Grabow, A. Zuban, and I. Leonov, Rev. Sci. Instrum. 70, 2127 (1999)Douglass, Johns, Nair, Brown, Rees, and Pate, J. Mol. Spectrosc. 239, 29 (2006)

2 GS/s AFG

v0 + 30 MHzSingle Sideband

Pulsed 1 watt amp

Nd:YAG

Continuum

10 Hz rep. rate

200 mJ/p 532 nm

5 mJ/p UV0.025 cm-1

bandwidth

Dye laser

Lambda PhysikAll spectra are ~0.1 cm-1 blue-shifted due to coaxial arrangement.

Rhodamine 6G dye,doubled with BBOSHG crystal

pMPEA-water UV-FTMW

Flowed He/Ne gas over cooled (0°C) water reservoir before entering chamber; increased signals by around a factor of 5 (as strong asmonomer)

With the water reservoir at roomtemperature, signal started todrop again (higher water clusters?)

Ab Initio Relative Energies (cm-1)Conformer A-4 B-1 C-7 D-5 E-8 F-2 G-3 6 9

Monomer 54 351 38 34 0 489 498 675 678

Water cluster 1023 1427 1005 44 0 918 946 1210 816

mp2/6-31g**

pMPEA(E)-waterpMPEA(C)-water pMPEA(9)-water

pMPEA-water UV-FTMW

The assignments of Unamuno et al are correct—35670 cm-1 feature is due towater with conformer 5; 35681 cm-1 feature is due to water withconformer 8.

Their assignments were based on structural stability—conformer 8+water goesto strongest peak, conformer 5+water to second-strongest—andlow-frequency vibrational mode calculations.

Residual Spectrum

No residual transitions with resolved quadrupole hyperfine splitting—not pMPEA, or simply a functionof cluster size? (large number of hyperfine-resolved transitions for assigned pMPEA-H2O clusters)

Possibilities:Other conformers with water; water molecule on the methoxy group? (Unlikely due to energetics)Two waters or more? (more likely—ab initio calculations needed)Remeasure CP-FTMW spectrum with water added! MW-MW double resonance spectroscopy needed

Strongest pMPEA transition intensity 120 µV

Acknowledgements

Funding:NSF CRIF:ID (CHE-0618755)Jefferson Scholars Foundation (J.Neill)

Tektronix

pMPEA-water Fit Parameters

Conformer D-water Experiment TheoryA/MHz 1740.6781(7) 1769.85B/MHz 430.5044(4) 426.01C/MHz 380.0915(4) 376.65DJ/kHz 0.0496(13)DJK/kHz -0.223(6)DK/kHz 2.586(13)dJ/kHz 0.0114(5)dK/kHz 0.32(5)

χaa/MHz 0.20(6) 0.41χbb-χcc/MHz -2.419(26) -2.19

Nlines 224rms error/kHz 24.2

µa/D 0.64µb/D b ≈ c 2.84µc/D 2.60

Conformer E-water Experiment TheoryA/MHz 1533.8873(10) 1552.23B/MHz 457.8598(5) 452.47C/MHz 398.9191(9) 383.61DJ/kHz 0.0573(35)DJK/kHz -0.063(12)DK/kHz 1.468(34)dJ/kHz 0.0113(20)dK/kHz 0.23(8)

χaa/MHz 0.785(47) 0.97χbb-χcc/MHz -2.31(44) -2.04

Nlines 137rms error/kHz 12.7

µa/D 0.47µb/D c only 0.49µc/D 2.32

Observed structures are analogous to those of other similar structures: tryptamine (Felker, J. Phys. Chem. 96 (1992) 7844);2-phenylethylamine (Melandri, et al, RC13)