Molecular orbital theory of the hydrogen bond. XIV. Disubstituted carbonyl compounds as proton acceptors

• Published in: The Journal of Chemical Physics
• Main Author: Del Bene, Janet E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: AIP Publishing 1975
• Subjects: Physical and Theoretical Chemistry; General Physics and Astronomy; Carbonic acid
• Online Access: http://dx.doi.org/10.1063/1.431251; https://pubs.aip.org/aip/jcp/article-pdf/63/11/4666/8130768/4666_1_online.pdf

A b initio SCF calculations with a minimal STO-3G basis set have been performed to determine the ground state equilibrium structures and energies of dimers in the series HOH...OCR2, where R is CH3, NH2, OH, or F. The equilibrium structures of the water–acetone, water–urea, and water–carbonyl fluoride dimers are similar to those of the water–acetaldehyde, water–formamide, and water–formyl fluoride dimers, respectively, in which the substituents are ’’cis’’ to water with respect to the carbonyl CO bond. No equilibrium water–formic acid dimer of this type was found, and no equilibrium water–carbonic acid dimer exists in which water is the proton donor molecule. There is a greater differentiation of dimer stabilities in the series having the disubstituted carbonyl compounds as proton acceptors, which may be related to enhanced pi-donating and sigma-withdrawing effects of the substituents. Thus, the order of increasing dimer stability in the combined series HOH...OCRR′ is F2<H,F<H2<H,CH3< (CH3)2<H,NH2< (NH2) 2. CI calculations show that the dimer singlet n→π* transition energies are well approximated as the sum of the n→π* transition energy in the monomer R2CO and the hydrogen bond energy in the corresponding dimer HOH...OCR2, suggesting that the hydrogen bond is broken in the dimer n→π* state.