氢键

你们可能在课本上读过氢键的这种定义部分带正电的氢和部分带负电的氧或氮之间的弱吸引力。你可能不太明白，因为你知道离子键和共价键是什么但是它是怎么跟这个联系起来的呢?氢键本身是一种奇怪的键，因为与离子键或共价键不同，离子键或共价键将单个原子连接在一起形成分子，氢键通常是两个不同分子之间的弱吸引力，使这两个不同的分子彼此靠近。这是怎么做到的呢?这都是基于原子所称的负电荷性。简而言之，电负性本质上是原子对电子的拉力有多大。

现在碳在这里，我看到一个甲烷分子CH4。它的电负性和连着它的氢的电负性大致相同，用这些黑线表示因为它们的电负性大致相同。形成这个键的两个电子被碳和氢平分了，这边也一样。另一方面，氧是电负性强的。所以它倾向于把电子拉得更近而远离氢因为氢太弱了，不能很好地抓住这些电子。所以这些电子大部分时间都绕着氧转，绕着氢转的时间很少。他们基本上只有周末的探视权，而不是平等的共同权利。这就意味着这些负电子在氧周围的时间要比在氢周围的时间长。这使得氧带一点负电荷。如果它带一点负电，就意味着这个氢带一点正电。 Now scientists don't like writing stuff that means it's easy to understand. So they use the Greek letter delta [IB] delta to represent this slightly. I've always thought of it as kind of this weirdly twisted S, so I just think sort of. So here's my partially positive hydrogen or sort of or slightly positive hydrogen. So slightly positive, slightly positive, wait a second this is slightly negative but I know that water is a neutral atom, or a neutral molecule, so it's got to be, yes you guessed it, two slightly negatives, alright?

这是怎么起作用的呢这是另一个水分子，这个带正电，这个带正电这是两个带负电，这个带正电，这个带正电，这是两个稍微带负电。所以，当这个水分子漂浮的时候，这个水分子，带一点正电荷，带一点负电荷，还有一点引力。这两个分子之间形成了弱引力。同样地，这个水分子，它的氢在说，嘿，你有点吸引人所以这里有一个弱氢键。

这在很多方面都非常重要，因为你可能会认为这是一个巨大的惊喜但它实际上是生物学中一个非常大的惊喜。因为这会影响很多东西，尤其是水。现在你可能在化学课上学过水，了解了水所有奇妙的性质，其中大部分是由于氢键。例如，水很难煮沸。这需要大量的能量。为什么呢?为什么把它变成气体需要这么多能量?这是因为所有其他的水分子开始试图凝聚试图粘在水分子上，水分子会离开，就像分子魔术贴一样把它拉回来。真的很难让它消失。这需要大量的能量。 The same quality of water, this cohesiveness is how tree is able to evaporate water from its leaves and by cohesion it's able to pull water molecules all the way down from the bottom of the tree in the roots, pulling the water in from the soil.

水的另一种能力是附着力，它会粘在东西上。这是一张标准的纸巾。我把它放在手上，它就开始掉下来，没什么大不了的。为什么?重力。另一方面，如果我把它弄湿了，就加点水。现在，我要给你表演一些魔术。同样的纸巾，我所做的就是加水这样它就会更重所以它下落得更快，对吧?这[IB]是对的，等一下，它没有下降。最终会的，为什么? Did I generate antigravity here? Why did it stay? Because of hydrogen bonds. The water molecules were binding and sticking to the molecules that make up the paper towel and also binding and sticking to my hand, so acting as a molecular glue. Now, you may be thinking, okay so we can induce some evaporation stuff. So it's just water, no it's more, proteins, hydrogen bond all over the place and this gives them many of their abilities to form different shapes. It even causes the double helix of the DNA to spiral up and hold together. It is involving so many things that this is a trick that I've taught my AP Biology students. Whenever I call them and they don't know the answer and the answer that they can always pull out of their pocket is, "it's due to hydrogen bonds" and chances are it's right. If your teacher's one of those really annoying teachers, if he knows lots of stuff and he's really smart knows a lot and he calls on you, just say well, it's probably due to hydrogen bonds and if he's smart enough, he'll be able to invent a way that yes it is due to hydrogen bonds.