Blue- and red-shifting CH•••O hydrogen bonded complexes between haloforms and ethers: correlation of donor ν_C−H spectral shifts with C−O−C angular strain of the acceptors

Abstract

In 1:1 CH···O hydrogen bonded complexes between haloforms and ethers, a correlation of the spectral shifts of ν_C−H bands (Δν_C−H) of the donors (haloforms) with C−O−C angular strain of the acceptors (ethers) is investigated by the electronic structure theory method at the MP2/6-311++G** level. The calculation predicts that the three-member cyclic ether (oxirane) that has the smallest C−O−C angle induces a much larger blue shifting effect on ν_C−H transition of fluoroform compared with that by the open chain analogue, dimethyl ether. The natural bond orbital (NBO) analysis reveals that the effect originates because of higher “s” character in the hybrid lone electron pair orbital of the oxygen atom of the former, which is responsible for a smaller contribution to n(O) → σ*(C−H) hyperconjugation interaction energy between the donor−acceptor molecules. The optimized structures of the two complexes are largely different with respect to the intermolecular orientational parameters at the hydrogen bonding sites, and similar behavior is also predicted for the two chloroform complexes. Partial optimizations on a series of structures show that the total binding energy of the complexes are insensitive with respect to those geometric parameters. However, the Δν_C−H, hyperconjugation interaction energies and hybridization of the carbon-centric bonding orbital of the C−H bond are sensitive with respect to those parameters. The predicted Δν_C−H of each complex is analyzed with respect to the IR spectral shift measured by van der Veken and coworkers in cryosolutions of inert gases.(27, 31, 32) The disagreement found between the measured and calculated IR shifts is interpreted to be the outcome of deformation of the complex geometries along shallow binding potential energy surfaces owing to solvation in the liquefied inert gases.