Observation of Trans-Ethanol and Gauche-Ethanol Complexes with Benzene Using Matrix Isolation Infrared Spectroscopy Jay C. Amicangelo, Matthew J. Silbaugh, Tracy A. Jones, Benjamin C. Mull

CH3OH-C6H6 Complex 1:1 H- complex observed in low temperature argon and nitrogen matrices 2013 ISMS TE08; 2015 ISMS RJ14 CH3OH+C6H6 CH3OH C6H6 20 K Ar Matrix 17 K N2 Matrix 1:1 complex 1:1 complex O–H Frequency Shifts expt Ar = -50.7 cm-1 expt N2 = -53.2 cm-1 MP2 = -50.1 cm-1 MP2/aug-cc-pVDZ 16.0 kJ/mol 2.42 Å

C2H5OH Rotation of OH relative to CH3 → 2 conformers Trans (180°) & gauche (60°) MP2/aug-cc-pVDZ relaxed potential energy scan gauche trans gauche trans ΔEg-t (MP2/aDZ) = 1.1 kJ/mol ΔEg-t (B3LYP/aDZ) = 0.78 kJ/mol ΔEg-t (Exp, MW) = 0.49 kJ/mol (J. Chem. Phys. 1980, 76, 4300) ΔEg-t (Exp, far-IR) = 0.50 kJ/mol (J. Mol. Struct. 1989, 238, 195) ΔEg-t (Exp, Xe soln-IR) = 0.74 kJ/mol (J. Mol. Struct. 2011, 985, 202)

Gas Phase IR Spectra of C2H5OH MP2/aug-cc-pVDZ trans gauche J. Mol. Spect. 2011, 985, 202

Matrix Isolation IR Spectra of C2H5OH Nitrogen vs argon matrices 1:1800 C2H5OH:M Nitrogen Matrix Argon Matrix B3LYP/aug-cc-pVDZ trans 30 K 36 K 28 K gauche 8 K 8 K Nitrogen matrix  both trans and gauche observed trans → gauche at 30 K Argon matrix  only trans observed J. Phys. Chem. A 1998, 102, 5789

Matrix Isolation Infrared Spectroscopy Experiments C2H5OH + C6H6 co-depositions in Ar & N2 matrices 17 - 20 K deposition; 4 – 6 mmol/hr; 4 - 8 hours; 0.5 cm-1 1:100 - 1:1600 S:M ratios Annealing: Ar → 35 K & N2 → 30 K Isotopic experiments: C2D5OD + C6H6 C6H6:Ar/N2 C2H5OH:Ar/N2

MP2/aug-cc-pVDZ Optimized Structures C2H5OH-C6H6 Complex Calculations B3LYP/aug-cc-pVDZ & MP2/aug-cc-pVDZ Optimization & BSSE corrected energies (kJ/mol) MP2/aug-cc-pVDZ Optimized Structures 2.41 Å 2.41 Å 2.38 Å 2.37 Å trans eclipsed trans staggered gauche eclipsed gauche staggered Method Trans eclipsed Trans staggered Gauche eclipsed Gauche staggered B3LYP/aDZ 5.2 5.3 5.1 MP2/aDZ 16.3 17.6 17.5

gauche barrier ≈ 0.11 kJ/mol C2H5OH-C6H6 Rigid Potential Energy Scan MP2/aug-cc-pVDZ; C6H6 rotation & C2H5OH fixed trans eclipsed gauche eclipsed trans barrier ≈ 0.06 kJ/mol gauche barrier ≈ 0.11 kJ/mol kT = 0.14 – 0.17 kJ/mol at 17 – 20 K

C2H5OH-C6H6 Complex Calculations MP2/aug-cc-pVDZ frequency analysis 48.8 cm-1 49.5 cm-1 trans staggered trans eclipsed Δt-g complex ≈ 20 cm-1 trans monomer 57.2 cm-1 58.0 cm-1 gauche staggered gauche eclipsed gauche monomer

C2H5OH-C6H6 Complex Calculations B3LYP/aug-cc-pVDZ frequency analysis 34.2 cm-1 34.8 cm-1 trans eclipsed trans staggered Δt-g complex ≈ 18 cm-1 trans monomer 33.2 cm-1 33.5 cm-1 gauche eclipsed gauche staggered gauche monomer

C2H5OH:Ar vs C2H5OH:N2 Matrix Spectra Ar  1:1600, 20 K deposition & 35 K anneal N2  1:3200, 17 K deposition & 30 K anneal Δt-g (O−H) = 3.9 cm-1 N2 30 K trans gauche N2 17 K Ar 35 K trans Ar 20 K N2 30 K N2  trans & gauche observed trans → gauche at 30 K gauche trans trans N2 17 K Ar  only trans observed Ar 35 K trans trans Ar 20 K

C2H5OH:N2 + C6H6:N2 Concentration Study 1:200 – 1:1600 S:M ratios; O−H stretching region 17 K co-depositions gauche complex C6H6 trans complex ? 1:200 1:400 1:800 1:1600 1:3200 monomer Δt-g (O−H) = 3.9 cm-1 17 K co-depositions + 30 K anneal higher complex gauche complex trans complex C6H6 1:200 ? 1:400 1:800 1:1600 1:3200 monomer

C2H5OH:N2 + C6H6:N2 Isotopic Spectra C2H5OH/C2D5OD; O−H/O−D stretching region 1:1600 C2H5OH + 1:1600 C6H6 1:800 C2D5OD + 1:800 C6H6 17 K 17 K trans complex gauche complex E + B gauche complex E + B E trans complex E B B 30 K 30 K gauche complex gauche complex E + B trans complex E E + B trans complex E B B

C2H5OH:Ar + C6H6:Ar Concentration Study 1:100 – 1:800 S:M ratios; O−H stretching region trans complex 20 K co-depositions higher complex 1:100 1:200 1:400 1:800 1:1600 monomer trans complex 20 K co-depositions + 35 K anneal higher complexes 1:100 1:200 1:400 1:800 1:1600 monomer

C2H5OH:Ar + C6H6:Ar Isotopic Spectra C2H5OH/C2D5OD; O−H/O−D stretching region 1:800 C2H5OH + 1:800 C6H6 1:800 C2D5OD + 1:800 C6H6 trans complex 20 K trans complex 20 K E + B E + B E E B B trans complex 35 K trans complex 35 K E + B E + B E E B B

Nitrogen Matrix Spectra Complex Frequencies & Shifts (cm-1) B3LYP/aug-cc-pVDZ; freqs scaled by 0.9713 MP2/aug-cc-pVDZ; freqs scaled by 0.9615 Nitrogen Matrix Spectra Conformer Monomer Complex Shift experiment trans 3652.8 3603.4 −49.4 B3LYP/aDZ trans ecl 3717.4 3684.2 −33.2 MP2/aDZ 3682.1 3634.5 −47.6 gauche 3648.9 3599.5 gauche ecl 3699.1 3666.8 −32.3 3670.9 3615.1 −55.8 Argon Matrix Spectra Conformer Monomer Complex Shift experiment trans 3655.6 3612.3 −43.3 B3LYP/aDZ trans ecl 3717.4 3684.2 −33.2 MP2/aDZ 3682.1 3634.5 −47.6

Summary Calculations predict weakly bound complexes for trans and gauche C2H5OH with C6H6 B3LYP ≈ 5.1 – 5.3 kJ/mol & MP2 ≈ 16.3 – 17.6 kJ/mol O-H freq shifts ≈ – 30 cm-1(B3LYP) & ≈ – 50 cm-1 (MP2) Co-deposition of C2H5OH (& C2D5OD) with C6H6 in N2 & Ar matrices performed N2 Matrix O-H peaks observed for gauche & trans complexes Ar Matrix O-H peaks observed for only trans complex MP2 freq shifts compare more favorably with expt