Nathan R. Pillsbury, Timothy S. Zwier, Department of Chemistry, Purdue University, West Lafayette, IN 47907; David F. Plusquellic, NIST, Gaithersburg, MD 20899; W. Leo Meerts, Radboud University Nijmegen, The Netherlands. CONFORMATION-SPECIFIC ELECTRONIC SPECTROSCOPY OF JET-COOLED BIS(2-HYDROXYPHENYL)METHANE

Why study Bis(2-hydroxyphenyl)methane (2HDPM)? 1. It is similar to diphenylmethane which was recently studied in our lab which was found to have very interesting spectroscopy 2. No synthesis needed!

0.00 kcal/mole0.69 kcal/mole DFT Calculations π H-bonded conformerOH…OH H-bonded conformer 2 low-energy minima

LIF Spectrum of 2HDPM

Records the UV spectrum of a single conformation free from interference from others present in the expansion Laser Timing nsec UV Hole-burn UV probe UV-UV Hole-burning Spectroscopy UV Hole-burn laser fixed: Provides selectivity UV probe laser tuned Boltzmann distribution of conformers in the pre-expansion Collisional cooling to zero-point vibrational level B* C A C C B A A A C A A B C C A A B B B B B B B UV C 2HDPM * (S 1 ) 2HDPM (S 0 ) Hole-burn Probe Conformer A Conformer B Hole-burn Probe TF

LIF and UVHB Spectra of 2HDPM

Origin Regions of Conformers A and B A B

Fluorescence-Dip Infrared Spectroscopy (FDIRS) Records IR spectrum of single species free from interference from others present in the expansion 2HDPM * (S 1 ) 2HDPM (A) (S 0 ) 2HDPM * (S 1 ) 2HDPM (A) (S 0 ) S 0 FDIRS S 1 FDIRS Total Fluorescence IR UV IR UV

S 0 FDIRS OH…OH H-bonded conformer π H-bonded conformer OH … OH(f) OH(d)/torsion combination band OH(d) … O OH(d) …  leak-thru from A

Conformer A Experimental OH-stretch vs. Calculated

Conformer B Experimental OH-stretch vs. Calculated

18 30 S 1 FDIRS S0S0 S0S0 OH  /torsion combination bands Ring 1(?) Ring 2(?) Two rings now inequivalent! Strong OH stretch/torsion coupling leak-thru from A

V OH=1  Energy OH vibration and ring torsion are very strongly coupled in the excited state Why do we see seven OH stretches instead of one in the excited state of B?   = dihedral angle between the two rings ring torsion levels S 1 zero-point level V OH=0

High Resolution UV Spectrum of 2HDPM A Origin Experimental Spectrum Simulated Fit D. Plusquellic (NIST)

What information can we get from the high resolution spectra? 2HDPM Conformer A Origin: A = B = C = ΔA = ΔB = 9.5 ΔC = 1.2 %A = 67 %B = 24 %C = 9 Ground State Rotational Constants Change in Constants Upon Excitation Transition Moment Direction Predicted Ground State Constants A = B = C = DFTMP2 A = B = C = a b c

High Resolution UV Spectrum of 2HDPM B Origin Region vibronic bands of A B origin Experimental Spectrum Simulated Fit

High Res. Spectrum of 2HDPM B +22 cm -1 Vibronic Band Experimental Spectrum Simulated Fit

2HDPM Conformer B +22 cm -1 Band: A = B = C = ΔA = 27.7 ΔB = -1.2 ΔC = -5.7 %A = 85 %B = 15 %C = 0 Ground State Rotational Constants Change in Constants Upon Excitation Transition Moment Direction Predicted Ground State Constants A = B = C = DFTMP2 A = B = C = Conformer B Vibronic Band Fit Results a b c

Dispersed Fluorescence Spectra of A

Dispersed Fluorescence Spectra of A Cont.

Dispersed Fluorescence Spectra of B * * * * * * *

What could be causing these strange intensities? 1.There could be some vibronic coupling to the nearby S 2 state 2.Duschinsky mixing 3.It is possible that we may be seeing fluorescence to another minimum in the ground state

Future Studies of 2HDPM SEP-Population Transfer Study: What are the barriers to isomerization between conformers A and B? Predicted Transition States (DFT): A B 1885 cm cm -1 TS

Conclusions 1.The two conformations of 2HDPM have been assigned using FDIRS and high resolution UV spectroscopy 2.We observed strong coupling between the OH-  stretch and inter-ring torsion in the S 1 state of conformer B 3.Dispersed fluorescence spectra show interesting features which we are currently attempting to explain

Acknowledgements Professor Timothy Zwier The Zwier Group Jasper Clarkson* Talitha Selby Esteban Baquero* V. Alvin Shubert Tracy LeGreve Josh Newby Bill James Chirantha Rodrigo Dr. Ching-Ping Liu Collaborators Dr. David F. Plusquellic (NIST) Professor W. Leo Meerts (Radboud University) Funding Frederick N. Andrews Fellowship U.S. Department of Energy

S0S0 S1S1 Zero-point level C B A IV. UV Probe, 3 II. UV Dump, 2 I. UV Pump, 1 Excited vibrational Level A*A* III. Collisional cooling, isomerization SEP-Population Transfer Spectroscopy