The microwave spectroscopy study of 1,2-dimethoxyethane Weixing Li, Annalisa Vigorito, Camilla Calabrese, Luca Evangelisti*, Laura B. Favero, Assimo Maris, Sonia Melandri* Supervisor: Prof. Sonia Melandri Università di Bologna

Why we study… Some interesting facts about 1,2-dimethoxyethane

Rich configuration variations Trans (T) Gauche (G) Gauche’ (G’) Three bond rotations generate 33 possible rotamers giving 10 non-equivalent molecular configurations

Two rotors with internal rotation Internal rotations of the two CH3 rotors can split the spectra. 0,0 0,±1 ±1,0 ± 1, ± 1 ± 1, ∓1 σ1= -1,0,1 σ2= -1,0,1

How we measure… Two kinds of MW spectrometers were used

Free-jet Absorption Spectrometer Millimeter-Wave Stark modulated Free-Jet Absorption Spectrometer BUILT in 1993 by Prof. P. FAVERO, Prof. W. CAMINATI and Prof. S. MELANDRI on the model of the MONASH UNIVERSITY one. FREE –JET EXPANSION Tvib 100-200 K (depends on the vibrational gap) Trot 1-10 K (Boltzmann distribution) SWEEP RATE 1 GHz per hour FREQUENCY RANGE 54-118GHz SENSITIVITY RESOLUTION @ Bologna 10-7-10-8 cm-1 200-300 kHz Camilla Calabrese

Fourier Transform Microwave Spectrometer Covering range: 6.5 ─18 GHz Resolution: 5 kHz Sensitivity: 10-10 - 10-12 cm-1 Pressure: 2 bar → ~10-5 mbar Temperature: 1 K Caminati, W; Evangelisti, L; Feng, G; Giuliano, B. M.; Gou, Q; Melandri, S; Grabow, J-U, PCCP, 2016, 18, 17851 – 17855.

Energy schematic diagram of the conformers transforming cm-1 1500 θ1 θ1 θ3 θ1 θ2 θ1 GTG GTG’ 1000 θ3 θ1 θ2 GGG’ TGG θ3 TTG GGG 500 θ3 9 non-equivalent conformers TGT TGG’ TTT Calculated at MP2/6-311++G** level

Stuctures and energies with and without zero-point energy -------Calculated at MP2/6-311++G** B3LYP/6-311++G** TGT conformer is the most abundant conformer in liquid and crystalline. a TGG’ conformer is the most abundant conformer in gas phase. a TTT conformer is the lowest energy conformer. a Degeneracy: TTT : 1 TGT : 2 TGG’ : 4 aG.D Smith et. al J. Am. Chem. SOC.1995,117,530-531.

Predicted spectroscopic parameters of three most stable conformers Calculated at MP2/6-311++G** level

Example spectra of two assigned conformers Free-jet Absorption MW FTMW

Spectroscopic parameters of two assigned conformers MP2 10.0 6.9 9.7 10.0 6.9 9.7 XIAM: H.Hartwig and H.Dreizler, Z. Naturforsch 51a, 923-932 (1996). ERHAM: P. Groner, J. Chem. Phys.107, 4483-4498 (1997).

V3 comparison of different molecules 1) Esters with –OCH3 V3 below 6 kJ/mol V3=6.58 kJ/mol V3=13.2 kJ/mol 2) Ethers with –OCH3 V3 between 6-11 kJ/mol 3)–OCH3 takes part in an intra-HB V3 around 13 kJ/mol

Rotational parameters of 13C substituted species

Kraitchman’s coordinates of 13C substituted species compared with calculation results (MP2/6-311++G**) C1 C6 C1 C6 C3 C4 C3 C4

Relative population of two conformers in molecular beam ’ ’ ’ ’ ’ g = gTGG’/gTGT = 4/2 NTGG’/NTGT = 2.1/1 vs 1.8/1 Experiment ab initio

To Conclude… Two conformers were assigned in rotational spectra. All the 13C substituted species were identified. V3 barrier of each -OCH3 rotor in both conformers were measured, and intra- HB plays an important role.

Acknowledgements Prof. Sonia Melandri Prof. Walther Caminati Dr. Assimo Maris Dr. Luca Evangelisti Dr. Laura Favero Dr. Camilla Calabrese Dr. Annalisa Vigorito Dr. Lorenzo Spada Chinese Scholarship Council