Ab initio calculations are used to compare the hydrogen bonding ability of acetylene and ethylene, and various derivatives, with the analogous properties of alkanes. Water is used as the universal proton acceptor and paired with HCCH, FCCH, ClCCH, and NCH as well as with ethylene, and its mono-, di-, and trifluorosubstituted derivatives. With regard to the hydrocarbons, sp-hybridized acetylene forms the strongest bond, followed by sp2 and then sp3. Halogenation of the proton donor strengthens each type of hydrogen bond, particularly when the substitution takes place on the C involved in the hydrogen bond. sp3-hybridized systems are most sensitive to this substitution-induced bond strengthening, followed in order by sp2 and sp. For each hybridization type, the length of the hydrogen bond shortens in proportion to the strengthening, with alkanes exhibiting the greatest sensitivity and alkynes the least. Whereas formation of the hydrogen bond causes the sp3 CH bond to contract and undergo a blue shift, the opposite trend is seen in the alkynes; the CH bond length is essentially unaffected in the alkenes. All types of CH··O hydrogen bonds are weakened more gradually as the two subunits are drawn apart than are OH···O bonds. Whereas alkyne CH···O bonds behave very similarly to OH···O interactions with regard to angular distortions, the hydrogen-bond energy of alkenes is less sensitive to such nonlinearity.
- hydrogen bond