An Improved CP-FTMW Analysis of the Structures of Phenol Dimer and Trimer

Nathan A. Seifert, Cristobal Perez, Amanda L. Steber, Daniel P. Zaleski, Justin L. Neill, Brooks H. Pate University of Virginia

Alberto Lesarri Universidad de Valladolid

Previous Experimental Structural Studies:• CP-FTMW; previously by us: RC11, 2011 (Steber et al.)1

• Substitution structure assigned, issues with Kraitchman agreement

• Less assigned transitions poorer determination of constants, distortion poorer structure

• Rotational coherence spectroscopy2 and UV-Vis3;• Some issues with these results: O⋯O lengths not determined (RCS) or anomalously long (UV-Vis), ≥3 Å• Poor precision with respect to FTMW

• Phenol trimer detected using UV/IR doubleresonance, but no structural information4

• IR data consistent with symmetric “barrel”structure

1. A. L. Steber, et al. Faraday Discuss., 2011, 150, 227.2. M. Schmitt, et al., ChemPhysChem, 2006, 7, 1241.3. A. Weichert, et al., J. Phys. Chem. A, 2001, 105, 5679.4. T. Ebata, et al. J. Phys. Chem., 1995, 99, 5761.

• Inter-ring dispersion motif: π stacking-like, or C-H⋯π dominated?

Unsolved Mysteries of (phenol)2:

• (H2O)2: r[O ⋯O] is 2.98 Å; is hydrogen bonding a major player in (phenol)2?

• Necessitated reduced bandwidth measurementto assign isotopologues in natural abundance

On a side note:

Introduction - Experiment

UV/Vis r0 structure M06-2X/6-311++g(d,p)

Introduction - Theory

• (phenol)2: Important benchmark molecule for modeling the interplay of various non-covalent interactions

----H-bonding:Electrostatic dominated

-------------------

-------------

π-π:Dispersion(exchange)dominated

CH-π:dispersion/electrostaticcompetition

• Important to fine tune the computational balance betweenelectrostatics and exchange for biochemical or (in this case) spectroscopicpredictions

Experimental

• New horns enable use of 5 pulsed nozzles at 2-8 GHz(previous setup was 2 nozzles)

• New TWT with 600W peak output (previous was 200W)

• New oscilloscope (Tek DPO73304D) enables detection of 8 frames at 3.3 Hz! (previous 2-8GHz rate was ~0.5Hz, at 4 frames/acq!)

9.2 million averages, 2-8 GHzDynamic range: 30000:14000 lines over 4:1 SNR

See Cristobal’s talk, TH10, for more information on the instrument

At 3.3 Hz, this is about 4 days of averaging!

V(θ) = (0.232 cm-1)θ2 - (0.00447 cm-1)θ3

Δ<θ> = ±4° (1σ)

MO6-2X/6-311++g(d,p) scan of the hinge potential

Calculating ψ with this potential gives a QM hinge fluctuation of

Ab initio PES fits with R2 = 0.99 to:

Fundamental mode is 28 cm-1 by thesecalculations; experimentally1 it’s 9 cm-1

Smaller than the σ seen in the theoretical results (next slide)!

Phenol Dimer - Analysis

Question: Will this be a problem for our determined structure?

“Hinge potential” – anharmonic and large amplitude?

1. M. Schmitt, et al. J. Chem. Phys., 1995, 103, 9918.

Answer: Yes and no.Experimental determinationis quite acceptable, but thereare issues with theory.

Phenol Dimer - Analysis 

A (MHz) B (MHz) C (MHz)Hinge Angle

MP2/6-311g(d,p) 1165.2 410.3 363.3 49.0MP2/6-311++g(d,p) 1365.2 325.8 305.7 69.5MP2/cc-pVTZ-cp a 1286.0 356.1 317.6 64.4CCSD/6-311g(d,p) 1545.2 282.4 275.6 88.7CCSD/6-311++g(d,p) 1459.6 308.9 286.8 79.5M06-2X/6-311g(d,p) 1352.7 342.1 318.3 68.2M06-2X/6-311++g(d,p) 1382.5 336.6 306.4 59.6M06-2X/6-311++g(df,pd) 1382.9 338.4 307.6 59.6B3LYP/6-31G(d,p) 1800.0 250.8 243.7 111.9B3LYP/6-311++g(d,p) 1946.3 231.8 229.6 110.5RI-DFT-D/aQz' b 1399.7 318.5 292.5 61         

CP-FTMW (r0, this study) 1415.3275(11) 313.36802(32) 287.96128(30) 62.3(14)

UV-VIS (r0, Schmitt et al.) 1416.99(39) 313.51(1) 288.11(1) 63.0

Observations:• MP2/CCSD overestimates the dispersive contributions (a known problem) • B3LYP fails completely• Dispersion-corrected hybrid functionals such as M06-2X(Truhlar) and RI-DFT-D (Hobza) perform the best

a. P. Jurečka, J. Šponer, J. Černy and P. Hobza, Phys. Chem. Chem. Phys. 2006, 8, 1985.b. M. Koláŕ and P. Hobza, J. Phys. Chem. A, 2007, 111, 5851.

A bold claim: On the basis of efficientspectroscopic discovery, new hybrid functionalsmight be the future.

Phenol Dimer - Structure

• Six parameters required to fit intermolecular structure:1)R[1d⋯1a] (hydrogen bond length)2)θ[1d⋯1a-2a] & θ[1a⋯1d-2d] (planarity of H-bond)3)φ[1a⋯1d-2d-3d] & φ[1d⋯1a-2a-3a] (“ring tilt”)4)φ[2d-1d⋯1a-2a] (hinge angle)

• Fix phenol monomer geometry to ab initio geometry (excellent agreement with previous expt. results)

Schema (also used in UV/Vis study):

r0 intermolecular fit structure:

Our “toolkit”:• All 13C and both 18O isotopologuesin natural abundance• 103 better certainty on parent speciesconstants (plus distortion!)

UV/Vis “toolkit”:• C6H5-OD measurements• 13C at the 1- position• Just A/B/C on parent species with ~50-100 kHz precision

M06-2x/6-311++g(d,p) [frame]r0 [spheres]

Phenol Dimer - Structure

M06-2x/6-311++g(d,p) [frame]rs [spheres]

Parameter Value (r0 fit, this

study)

Value (r0 fit, UV/VIS1)

M06-2X/6-311++g(d,p)

r(Hd ⋯ Oa) 1.873(22)

2.354(49) 1.890

r(Od ⋯ Oa) 2.833(21)

3.211(25) 2.844

θ(Od-Hd ⋯ Oa) 170.5(21)

150.6(18) 168.1

θ(Ca-Oa ⋯ Hd) 122.5(10)

138.6(15) 118.8

φ(Oa-Ha ⋯ Od-Cd) 75.5(59)

109.6(45) 76.8

φ(Cd-Od-Hd ⋯ Oa) -27.7(47)

-26.5(46) -23.4

φ(C2a-Ca-Oa⋯ Hd) 10.6(17) -1.0(19) 14.2

φ(Cd-Od ⋯ Oa-Ca) [hinge]

64.0(13)

63.0 59.6

Cd Od

Oa

Ca

Hd

C2a

Take home points:

• Water dimer-like hydrogen bonding:• (H2O)2 : r(O ⋯ O) = 2.98(1) Å

• π stacking dominant, not

the “twisted” C-H ⋯π-like motif seen in UV-VIS structure

Phenol Dimer - Structure

Brown: UV-Vis r0

Purple: CP-FTMW r0

1. M. Schmitt, M. Böhm, C. Ratzer, D. Krügler, K. Kleinermanns, I. Kalkman, G. Berden and W. L. Meerts, ChemPhysChem, 2006, 7, 1241.

(donor rings overlapping)

CP-FTMW r0 σfit = 0.059 amu Å2

Phenol Trimer

• Oblate symmetric top

Parent 1-13C 6-13C 2-13C 5-13C 3-13C 4-13C A(MHz

) --282.19749(1

4)282.21553(1

7)282.26789(2

3)282.23405(5

2)281.96181(3

1)281.89683(6

6)B

(MHz)282.280790(

56)281.30995(1

4)280.33503(1

7)281.44815(2

3)279.47608(5

3)280.56222(3

0)279.57290(6

5)C

(MHz) -- 187.33(14) 187.069(50) 187.36(21) 186.897(44) 187.34(14) 186.824(66)   

DJ

(kHz) 0.10300(19) [0.0315] [0.0315] [0.0315] 0.0297(10) [0.0315] 0.0334(12)

DJK

-0.13600(28) [0.0752] [0.0752] [0.0752] 0.0779(26) [0.0752] 0.0725(34)

DK -- [0] [0] [0] [0] [0] [0]dJ -- [0.03249] [0.03249] [0.03249] 0.03227(56) [0.03249] 0.03270(65)dK -- [0] [0] [0] [0] [0] [0]

   N 20 63 87 53 108 53 100σ

(kHz) 1.37 5.96 7.11 9.10 9.33 9.49 8.62

• 13C isotopologues are off-symmetry axis,so they are standard oblate asymmetric tops

• To ease the fitting process, isotopologueswere fit as pure c-type prolate asymmetric tops

1. T. Ebata, T. Watanabe, N. Mikami. J. Phys. Chem. 1995, 99, 5761.

Nature of Kraitchman substitution for this system requires some assumptions regarding C-C bond lengths in the phenol monomers (ask me about it after if you’re curious)

• Use of dummy coordinatesand forced C3v symmetry:

Only three fit parameters: r, θ, φ(w/ three-fold degeneracy)

Schema:

Phenol Trimer - Structure

• Use rm(1) model to fit intermolecular structure

• Assume phenol monomer ab initio structure• Similar assumptions to rs determination

(thanks to Z. Kisiel for the helpful tips!)

ParameterM06-2X/6-311+

+g(d,p) rm(1)

r(H ⋯ O) / Å 1.940 1.895(86)

r(O ⋯ O) 2.811 2.760(70)

r(C1 - C1') 4.010 3.967(83)<(O - O' -

O'') / °60.0 60.03(73)

<(O - H ⋯ O') 147.3 147.1(16)<(C1 - O'' ⋯

O')-23.4 -27.9(14)

<(C1'' - O'' ⋯ O)

117.9 114.0(16)

<(C1 ⋯ C1' ⋯ C1'')

60.0 60.00(75)

t(C1-O-O'-C1')

-6.42 -5.9(28)

t(O-O'-C1'-C6')

85.2 85.2(33)

caa = cbb / amu Å2

-- 1.140(85)

• r(O ⋯ O) shorter for phenol trimer than dimer• 2.833(21) Å 2.760(70) Å• Similar trend to (H2O)2 (H2O)3

2.98 Å 2.85 Å

Stabilizing multi-body effects are apparent:

• “hinge” is closed: C-H/π interactions much more important• 2- hydrogen with C5-C6 bond: ~2.6 Å (very typical for C-H/π)• Ring planes are nearly perpendicular 85.2(33)°

Phenol Trimer - Structure

Theory is much more well behaved!

  A (MHz) B (MHz) C (MHz)

M06-2X/6-311g(d,p)

199.494 292.600 --

M06-2X/6-311++g(d,p)

198.084 290.540 --

MP2/6-311g(d,p) 205.635 296.244 --MP2/6-311+

+g(d,p)211.380 300.117 --

B3LYP/6-311++g(d,p)

240.6 222.786 130.919

   

Experiment [188]282.280790(

56) 

Acknowledgements

Thanks to the NSF for funding:MRI-R2, Award CHE-0960074

Pate Group

Brooks Pate

Cristobal PerezSimon LobsigerLuca Evangelisti

Brent HarrisAmanda SteberNathan Seifert

Daniel Zaleski

Newcastle University

Brightspec

Justin Neill

Universidad de Valladolid

Alberto Lesarri

Thanks for your time!

Phenol Trimer - Structure

Off-axis substitution has easyform of Kraitchman’s equations:

|a| = 0|b| = |c| =

Problem: In the above frame, |a| = 0. In PA frame, |a| ≠ 0. How do we determine rotation matrix to get PA coordinates?

Solution: Assume ab-initio phenol monomer C-C bond lengths to solve Law of Cosines for required θ to convert KRA->PA

(oblate basis)

Phenol Dimer

• Detected all 12 13C and both 18O isotopologues

 M06-2X/6-311+

+g(d,p)CP-FTMW (2-8

GHz, new)CP-FTMW (7-9

GHz, old)

A (MHz) 1382.5 1415.32747(14) 1415.32633(48)

B 336.59 313.368020(41) 313.367093(60)C 306.41 287.961282(38) 287.960317(66)   

ΔJ (kHz) 0.908 0.372930(76) 0.372342(65)ΔJK -5.58 -3.92349(48) -3.91817(96)ΔK 15.6 13.0664(27) 13.079(16)δJ 0.0182 0.057377(17) 0.057355(29)δK 0.410 0.6928(19) 0.7024(85)   N -- 481 302

σ (kHz) -- 6.39 11.1

Donor A (MHz) B C   N σ (kHz)

1-13C 1413.0983(14) 312.52409(32) 287.16539(32) 120 8.082-13C 1412.74240(48) 312.75594(19) 287.52814(20) 136 5.773-13C 1403.8469(63) 312.00616(35) 286.73261(34) 111 8.254-13C 1406.0358(10) 310.68675(28) 285.40748(28) 129 7.98

5-13C 1413.37840(97) 310.05088(26) 285.13627(28) 133 8.75

6-13C 1411.85779(88) 311.08467(29) 286.08891(29) 113 6.98

1-18O 1387.525(25) 312.54360(80) 286.32269(76) 60 10.4

Acceptor

1-13C 1413.23440(69) 312.50304(16) 287.15363(16) 123 5.532-13C 1412.24751(35) 312.72811(22) 287.52476(22) 133 5.383-13C 1403.92180(82) 311.90897(22) 286.72685(23) 129 7.11

4-13C 1405.66540(96) 310.71737(19) 285.40780(19) 123 6.79

5-13C 1413.02009(74) 310.10090(26) 285.17718(26) 121 6.416-13C 1411.56899(64) 311.12121(17) 286.14677(18) 136 6.842-18O 1388.945(42) 312.70639(67) 286.47370(65) 69 7.02