Energetics and cooperativity in three-center hydrogen bonding interactions. I. Diacetamide-X dimers (X=HCN, CH3OH)

Gong

Zeng³

2001

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High level ab initio calculations have been carried out on a number of intramolecular two-and three-center H-bonding structures containing the amide motif, NHCϭO. These structures serve as model systems for probing the stability of intramolecular three-center H bonds of the type A 1 HA 2 . Particular attention is given to cooperative effects in the three-center H-bonding structures. A systematic assessment of cooperativity for intramolecular H-bonding systems is presented.

Section: Resultsmentioning

confidence: 99%

“…3͑a͒ of Paper I. An H bond in a sixmembered ring is described in terms of r, d 1 , R 1 , and 1 . An H bond in a five-membered ring is described in terms of r, d 2 , R 2 , and 2 .…”

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Energetics and cooperativity in three-center hydrogen bonding interactions. II. Intramolecular hydrogen bonding systems

Gong

Zeng³

2001

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“…These studies support the notion that intermolecular bifurcated H-bond formation is a process that gives rise to negative cooperative effects. 36,37 For intramolecular bifurcated H bonds, examples of positive and negative cooperative effects have been reported. [38][39][40] The importance of resonance-assisted hydrogen bonding ͑RAHB͒ in bifurcated hydrogen bonds was recently pointed out by Rybarczyk-Pirek et al 41 for the crystal structures of some new phosphorochromone derivatives.…”

Section: Introductionmentioning

confidence: 99%

Cooperative effects in two-dimensional ring-like networks of three-center hydrogen bonding interactions

Bulusu

Zeng

2005

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Cooperative effects in two-dimensional cyclic networks containing intermolecular three-centered hydrogen bonding interactions of the type H 1 …A…H 2 are investigated by means of ab intio molecular orbital and density functional theory calculations. Ring-like clusters consisting of three and up to nine monomers of the cis-cis isomer of carbonic acid H 2 CO 3 are used as basic models, where each unit acts simultaneously as a double hydrogen-bond donor and double hydrogen-bond acceptor. Cooperative effects based on binding energies are evident for ͑H 2 CO 3 ͒ n , where n goes from 2 to 9. Thus, the ZPVE-corrected dissociation energy per bifurcated hydrogen bond increases from 11.52 kcal/ mol in the dimer to 20.42 kcal/ mol in the nonamer, i.e., a 77% cooperative enhancement. Cooperative effects are also manifested in such indicators as geometries, and vibrational frequencies and intensities. The natural bond orbital analysis method is used to rationalize the results in terms of the substantial charge delocalization taking place in the cyclic clusters. Cooperativity seems close to reaching an asymptotic limit in the largest ring considered, n = 9.

“…These studies support the notion that intermolecular bifurcated H-bond formation is a process that gives rise to negative cooperative effects. 24,25 For intramolecular bifurcated H bonds, examples of negative and positive cooperative effects have been reported. 26 -28 Very few studies of cooperative effects in chains of bifurcated H bonds have been reported.…”

Section: Introductionmentioning

confidence: 99%

Cooperative effects in one-dimensional chains of three-center hydrogen bonding interactions

Bulusu

Zeng

2003

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Cooperative effects in a one-dimensional network of intermolecular bifurcated hydrogen bonding interactions are investigated by means of ab initio calculations. The trans-trans conformation of the diformamide molecule is used as a basic motif to model a chain of bifurcated H bonds. In this model system, the two proton-acceptor atoms belong to the same molecule. The one-dimensional network is modeled then by periodically stacking up to 12 molecules of the unit motif. Different indicators of H-bond strength such as energetic, structural, dielectric, vibrational frequencies, and isotropic chemicals shifts consistently show significant cooperative effects in the chains. The dissociation energy in the dimer is calculated to be 9.88 kcal/mol, while that of the strongest interaction in the decamer is calculated to be 26.12 kcal/mol ͑164% increase in cooperativity͒. Thus, although three-center H bonds can be viewed as a consequence of proton deficiency, in some cases they may also be viewed as the natural result of an interaction that is itself energetically favorable and capable of competing with the more conventional two-center H bonds. Natural bond orbital analysis reveals substantial charge delocalization within each molecule, and charge transfer along the chains. Interestingly, this charge delocalization makes the system a good candidate for resonance-assisted H bonding which in turn increases the covalent character of this type of bifurcated H-bonding interaction.