Associations of alkyl carbonates:: Intermolecular C-H•••O interactions

Self- and cross-associations of cyclic as well as linear carbonates such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), and dimethyl carbonate (DMC) are investigated with ab initio (MP2) and density functional theory (DFT) methods. The results show that cyclic and linear carbonates associate mainly through the intermolecular interactions of C-H . . .O. The basis set superposition error and zero-point energy-corrected binding energy, D-0(BSSE), for the global minimum of the linear carbonate DMC dimer (1.7 kcal/mol at B3LYP/6-311++G**) is much lower than those of the cyclic carbonates, and for the involved cyclic carbonates, it decreases in the following order: 5.1 (EC) approximate to 5.1 (EC/PC) > 4.7 (PC) > 3.9 (PC/VC) approximate to 3.8 (EC/VC) > 3.0 (VC). The dimer of DMC with EC (D-0(BSSE) = 2.8 kcal/mol) is also much less stable than the EC and PC dimers. Consistent with experimental findings, the results indicate that PC may also associate with EC molecules in their mixture; therefore, the cyclic carbonate molecules apparently still behave like associates instead of free molecules. However, EC molecules would be more free in a DMC/EC mixture because of weak intermolecular interactions. On the basis of the atoms-in-molecules (AIM) calculations, the C-H . . .O interactions may be classified as hydrogen bonds, although the C-H . . .O interaction exhibits a bond contraction and a blue vibrational frequency shift as compared with the monomer when the proton donor is linked with saturated molecules such as EC/PC/DMC, whereas it shows a C-H stretch and a red shift for the case of a proton linking with an unsaturated carbon such as VC. Such a difference perhaps arises from the opposite effect of a C-H bond stretch upon the monomer dipole moment. In line with the AIM analysis, the electron localization function technique (ELF) nicely demonstrates the existence of clear C-H . . .O interactions for VC dimers and trimers, which demonstrates the ability of the ELF procedure to characterize C-H . . .O systems.