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zhangrobber至尊木虫 (正式写手)
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[交流]
【求助】 分子自组装中的氢键问题
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请较各位大虾：分子自组装中有很多都是氢键的作用，但是氢键的有效作用范围是多少啊？也就是说两个原子相距多远可以认为有氢键生成？ 积极回复，有金币奖励！ [ Last edited by wellok101 on 2008-4-16 at 11:37 ] |
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lee2002hu
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4楼2008-04-24 23:37:49
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lee2002hu
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Hydrogen bond From Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Hydrogen_bond [edit] Bonding A hydrogen atom attached to a relatively electronegative atom is a hydrogen bond donor. This electronegative atom is usually fluorine, oxygen, or nitrogen. An electronegative atom such as fluorine, oxygen, or nitrogen is a hydrogen bond acceptor, regardless of whether it is bonded to a hydrogen atom or not. An example of a hydrogen bond donor is ethanol, which has a hydrogen bonded to oxygen; an example of a hydrogen bond acceptor which does not have a hydrogen atom bonded to it is the oxygen atom on diethyl ether. Carbon can also participate in hydrogen bonding, especially when the carbon atom is bound to several electronegative atoms, as is the case in chloroform, CHCl3. The electronegative atom attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge. Because of the small size of hydrogen relative to other atoms and molecules, the resulting charge, though only partial, nevertheless represents a large charge density. A hydrogen bond results when this strong positive charge density attracts a lone pair of electrons on another heteroatom, which becomes the hydrogen-bond acceptor. The hydrogen bond is often described as an electrostatic dipole-dipole interaction. However, it also has some features of covalent bonding: it is directional, strong, produces interatomic distances shorter than sum of van der Waals radii, and usually involves a limited number of interaction partners, which can be interpreted as a kind of valence. These covalent features are more significant when acceptors bind hydrogens from more electronegative donors. The partially covalent nature of a hydrogen bond raises the questions: "To which molecule or atom does the hydrogen nucleus belong?" and "Which should be labeled 'donor' and which 'acceptor'?" Usually, this is easy to determine simply based on interatomic distances in the X-H...Y system: X-H distance is typically ~1.1 Å, whereas H...Y distance is ~ 1.6 to 2.0 Å. Liquids that display hydrogen bonding are called associated liquids. Hydrogen bonds can vary in strength from very weak (1-2 kJ mol−1) to extremely strong (>155 kJ mol−1), as in the ion HF2−.[3] Typical values include: F—H...:F (155 kJ/mol or 40 kcal/mol) O—H...:N (29 kJ/mol or 6.9 kcal/mol) O—H...:O (21 kJ/mol or 5.0 kcal/mol) N—H...:N (13 kJ/mol or 3.1 kcal/mol) N—H...:O (8 kJ/mol or 1.9 kcal/mol) HO—H...:OH3+ (18 kJ/mol[4] or 4.3 kcal/mol) {Data obtained using molecular dynamics as detailed in the reference and should be compared to 7.9 kJ/mol for bulk waters, obtained using the same molecular dynamics.} The length of hydrogen bonds depends on bond strength, temperature, and pressure. The bond strength itself is dependent on temperature, pressure, bond angle, and environment (usually characterized by local dielectric constant). The typical length of a hydrogen bond in water is 1.97 Å (197 pm). |
5楼2008-04-24 23:43:24