Asuka Fujii, Naohiko Mikami Complete Infrared Spectroscopic Characterization of Phenol-Borane-trimetylamine Complex in the Gas Phase G. Naresh Patwari Department of Chemistry Indian Institute of Technology Bombay, Mumbai, India & Asuka Fujii, Naohiko Mikami Department of Chenistry Tohoku University, Sendai, Japan

Hydrogen Bonding X = O, N, F, Cl, C Y = O, N, Cl, (F)? + - X = O, N, F, Cl, C Y = O, N, Cl, (F)?   10 – 40 kJ mol-1

Hydrogen Bonding + - X = O, N, F, Cl, C   8 – 20 kJ mol-1

Hydrogen Bonding + - Electron Density X = O, N, F, Cl, C

Hydrogen Bonding? + - + - X = O, N, F, Cl, C E = M, B

Simulated structure of the BH3-NH3 dimer Dihydrogen Bonding (N—H•••H—B) + - Simulated structure of the BH3-NH3 dimer   50 kJ mol-1 - + + Network of dihydrogen bonds in solid BH3-NH3

Fluorescence Excitation Spectrum A+B hUV S1 S0 hfl hUV S1 S0 hfl Laser Induced Fluorescence spectrum Laser Induced Fluorescence Spectrum The formation of complexes can be inferred

Fluorescence Excitation Spectrum Phenol-BTMA D ; 384 cm-1 Phenol-Water Phenol

Hydrogen Bonding; IR Spectroscopy Because of the sensitivity of the vibrational spectrum (ns in particular) to the hydrogen bond formation, IR spectroscopy provides • a definitive criterion for the detection of hydrogen bonds • direct evidence of the role of the proton in the association • a quantitative index of the physical and chemical properties of the hydrogen bonded systems • a convenient tool in a wide variety of hydrogen bond studies G. C. Pimentel & A. L. McCleallan in “The Hydrogen Bond”

Dihydrogen Bonding of Phenol - + Y = H3B←NMe3 Should lead to lowering of OH

Fluorescence Detected Infrared Spectroscopy hfl t hfl hUV hUV n" n" hIR S0 S0 In this technique the selectivity comes form the S1←S0 electronic transition, and is sensitivity because few percent change in the population of ground state can be detected.

IR-UV Double Resonance Spectroscopy IR spectrum of phenol in the OH stretching region. OH ; 3657 cm-1 Fluorescence intensity IR spectrum of phenol-BTMA in the OH stretching region. OH ; 3514 cm-1 Energy / cm-1 OH = 143 cm-1

Dihydrogen Bonding of Phenol + - Y = H3B←NMe3 phenol-water (OH = 133 cm-1 ) phenol –borane-trimethylamne (OH = 143 cm-1 ) phenol-methanol (OH = 210 cm-1)

What happens to the BH stretches? + - Y = H3B←NMe3

IR-UV Double Resonance Spectroscopy BH IR spectrum of phenol-BTMA in the BH stretching region. Fluorescence intensity Energy / cm-1

BH Stretching Vibrations BH3 Group → C3 Symmetry → Two Bands Totally Symmetric BH Vibration Doubly Degenerate Non-totally Symmetric Vibrations Dihydrogen Bonding → Loss of Symmetry → Three Bands Dihydrogen Bonded BH Group Two Free BH Groups

IR-UV Double Resonance Spectroscopy BH IR spectrum of phenol-BTMA in the BH stretching region. Fluorescence intensity Energy / cm-1

Dihydrogen Bonding (O—H•••H—B) + - Structure of phenol–borane-trimethylamine dihydrogen-bonded gas-phase complex  = 24 kJ mol-1

Summary The 384 cm-1 red shift in the phenolic chromophore suggest BTMA hydrogen bonds with phenol The shift in the 143 cm-1 shift in the OH stretching vibration of phenol also suggest that the BTMA is hydrogen bonded to phenol The appearance of three bands in the BH stretching region indicates the BH3 group of BTMA is interacting with the phenolic OH group The above results unequivocally characterize the foramtion of the O-H…H-B dihydrogen bonded complex between phenol and borane-trimethylamine.