We're told that the photon energy of microwave frequency radiation ($\sim 10^{-5}~\text{eV}$) is not high enough to break hydrogen bonds. But if that's true, how does dielectric heating of water work? Liquid water is a network of polar molecules held together by H bonds so that they CAN'T rotate, in concert with the microwave beam or anything else ....

Seems like this is a problem of the two ways of looking at radiation -- classical wave vs photon stream -- being incompatible.

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    $\begingroup$ Molecules of liquid water are certainly not held in place, unable to move. Otherwise it wouldn't be a liquid! $\endgroup$ – Al Nejati Nov 12 '19 at 2:55
  • $\begingroup$ But people talk about "the well-known network of liquid water". $\endgroup$ – user247059 Nov 12 '19 at 2:59
  • $\begingroup$ Specifically, Marcus (1995): "the well known three-dimensional network of liquid water". in Water binding by organic molecules. Cell Biochemistry and Function 13: 157-163. $\endgroup$ – user247059 Nov 12 '19 at 3:03
  • $\begingroup$ Google doesn’t find that phrase. Reference please? $\endgroup$ – Bob Jacobsen Nov 12 '19 at 3:03
  • $\begingroup$ Marcus (1995) Water binding by organic molecules. Cell Biochemistry and Function 13: 157-163 $\endgroup$ – user247059 Nov 12 '19 at 3:06

In a solid or liquid we have collective vibrations of the whole system. It can be useful to think of these as quasiparticles called phonons, that is when we add vibrational energy to the system as a whole it generates a phonon, or conversely a phonon can decay and emit energy.

Black body radiation is (mostly) the emission of photons from the collective vibrations i.e. the decay of phonons to emit photons, and dielectric heating is the reverse process i.e. the absorption of photons to create phonons. This is what happens in your microwave oven. The heating is due to the excitation of the collective vibrations, not to the interaction of photons with hydrogen bonds. The quanta of these collective excitations (i.e. their phonons) are generally very small so they can absorb photons of even very low energies.

In real materials the collective vibrational modes are anharmonic oscillators so they all interact with each other and the vibrational energy is distributed between them in accordance with the Boltzmann distribution. That means the vibration energy from the absorbed microwave photons is quickly equilibrated with higher energy modes such as the vibrational excitations of hydrogen bonds, and it can break those bonds. This means energy can be absorbed in small units from microwave photons and still build up sufficiently to break the much higher energy hydrogen bonds.

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  • $\begingroup$ John Rennie says: "This means energy can be absorbed in small units from microwave photons and still build up sufficiently to break the much higher energy hydrogen bonds." So does that mean it's wrong to say (as the cellphone industry does) that because single microwave photons do not have enough energy to break covalent bonds, microwaves cannot break molecules like DNA? $\endgroup$ – user247059 Nov 13 '19 at 1:53
  • $\begingroup$ @user247059 in the example you discuss the microwave photons heat the water and the thermal energy in the water can break the hydrogen bonds. However even if the water is heated to 100°C there is still not enough thermal energy to break the covalent bonds in DNA. In any case you would cease to care after the microwaves had heated you by more than a few degrees :-) $\endgroup$ – John Rennie Nov 13 '19 at 5:28
  • $\begingroup$ Sorry, this doesn't make sense to me. The generally accepted model for how microwaves heat water is that they make the water molecules (dipoles) rotate, and this rotation causes friction, which manifests as heat. My original question was how this can be correct, when the presence of H-bonds between water molecules would prevent the water molecules from rotating. You now appear to be saying that the heat PRODUCED BY the rotation of water molecules causes breakage of the H-bonds that prevent the rotation of water molecules. As I said, this does not seem to me to make sense. $\endgroup$ – user247059 Nov 13 '19 at 8:04
  • $\begingroup$ The generally accepted model for how microwaves heat water is that they make the water molecules (dipoles) rotate - that is incorrect. $\endgroup$ – John Rennie Nov 13 '19 at 8:49
  • $\begingroup$ Well that's what it says in Chapter 2 of Kappe CO Stadler A and Dallinger D (2012 ) Microwaves in Organic and Medicinal Chemistry, Second Edition Wiley-VCH Verlag GmbH & Co. KGaA. What do you think is the generally accepted model of how microwaves heat water? $\endgroup$ – user247059 Nov 13 '19 at 9:08

I would think that microwaves are only sufficient to factor into the rotational modes of the water molecule and that they couldn't "break" per se, the hydrogen bond. Even if they did, I think the Van Der Walls forces would make a quick repair!

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You don’t need an individual quantum of energy to break any waves; you just need the energy to be absorbed.

It’s like pushing someone on a swing (with rigid arms on the swing). You don’t need to be strong enough to push them so they make a complete loop after a single push: it would be enough to keep pushing at the right time, and if there is sufficiently low friction in the system you will eventually push them “over the top”.

The vibration / rotation modes of water molecules are coupled - excite one, and over time some of the energy transfers to other modes. So the entire system absorbs energy, until bonds start to break. In fact, even without adding energy bonds are breaking (and forming) all the time - again, because the energy of an individual water molecule will change as they “jiggle”.

We can say something about the average energy of a molecule - but usually not about the energy of the individual molecule.

Does that make sense?

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  • $\begingroup$ Floris -- I presume you mean you don't need an individual quantum of energy to break a bond (not a wave). But that actually is the exact reason why people say low intensity microwaves (as in cell phone emissions) are not harmful. They say microwave photons don't have enough energy to ionize molecules and non-ionizing is non-harmful. The accepted model is that it's all down to single-photon interactions with matter. $\endgroup$ – user247059 Nov 12 '19 at 3:27
  • $\begingroup$ @user247059 In microwave ovens energy is given to the water in food and raises its temperature. vibrational and rotational levels. It is only when the temperature reaches the boiling point that the liquid phase is broken and H2O becomes steam $\endgroup$ – anna v Nov 12 '19 at 5:48
  • $\begingroup$ @user247059 “harm” can happen in two ways. Heating is a real effect, and sufficient heating can denature proteins. This is the reason that an MRI scanner (with 10’s of kW of RF transmit power) computes the SAR (specific absorption ratio) to make sure the patient doesn’t overheat (body temperature DOES go up... just not enough to harm you. Limits are set at 2 W/kg (“normal mode”) but can be higher for specific applications and with sufficient care and supervision). Ionization usually requires higher energy quanta (although a flame is ionized just because of the energy of endothermic reaction) $\endgroup$ – Floris Nov 12 '19 at 15:39
  • $\begingroup$ Note also that “breaking a hydrogen bond” is not at all the same thing as “ionizing”. The hydrogen bond is between the hydrogen atom of one molecule and the oxygen (electrons) of the other molecule; when the bond is broken the hydrogen “returns to” its parent molecule, usually. And anyway, water molecules are always ionizing (OH-, H3O+) and recombining; their ionization is not what is damaging. It’s complex molecules that change their configuration / create new reaction products, that are “damaged” by radiation. $\endgroup$ – Floris Nov 12 '19 at 15:41

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