Ethylene glycol has atransiently chiral, asymmetric global minimum structure,b ut it favors ah ighly symmetric, achiral dimer arrangement which has not been considered or found in previous quantum-chemical studies.C omplementary FTIR and Raman spectroscopyinsupersonic jets allows for the detection and straightforwarda ssignment of this four-fold hydrogen-bonded dimer,w hich introduces an interesting supramolecular binding motif for vicinal diols and provides astrong case for transient chirality synchronization.Carbohydrates are among the most versatile molecules employed by nature in molecular recognition processes and their biomimetic detection is challenging.[1] When reducing their molecular complexity to am inimum, one naturally arrives at vicinal diols.A mong them, the simplest representative ethylene glycol has even been detected in interstellar space.[2] Ethylene glycol or 1,2-ethanediol is ap articularly valuable model system for the study of multiple hydrogenbonded dimers and carbohydrate interactions, [3] because it allows for av ast range of quantum-chemical options to be explored. Indeed, the question of its preferred dimer structure has been addressed repeatedly by theory, [4][5][6] but not by experimental techniques.B ecause of the torsional flexibility of ethylene glycol, there are several possibilities how the four OH groups of the dimer can engage in hydrogen bonds.None of the previously proposed structures involves more than three hydrogen bonds among these OH groups. [4][5][6][7][8][9][10][11] We present unambiguous combined IR and Raman spectroscopic evidence that ethylene glycol in fact forms an S 4 -symmetric and thus non-polar and achiral dimer with four equivalent hydrogen bonds.U sing Gaussian 09, [12] an extensive search of the intermolecular potential-energy hypersurface at B3LYP-D3(BJ)/6-311 + G(2d,p) level (in short B3LYP) confirms that this structure wins over all triply hydrogen-bonded dimer structures by as ubstantial energy margin.Thet wo lowest monomer structures found at the harmonically zero-point energy-corrected B3LYP level are depicted in Figure 1. They have in common a gauche OCCO backbone,aweakly hydrogen-bonded OH group, and af ree OH group pointing in trans (M) or gauche (M') direction relative to the CÀCb ackbone.T hese monomer conformations are chiral and differ only by 1.4 kJ mol À1 at B3LYP level, making both of them potentially observable in aseeded adiabatic jet expansion. They have been investigated previously in much detail, [13][14][15][16][17][18][19][20][21][22][23][24][25] mostly by microwave spectroscopy, [13][14][15][16][17][18][19][20] and experimental estimates for their energy difference are rather close to our theoretical prediction, with 1.4(4) and 2.5(5) kJ mol À1 . [16,20] In jet experiments,t ypically only Mi so bserved in substantial abundance because of efficient conformational relaxation, [18] whereas at room temperature,M ' is also populated. Donor-acceptor OH torsional tunneling splits the levels by 0.2 and 0.05 cm À1 , respectively. ...
