From owner-chemistry@ccl.net Sun Oct 31 04:47:01 2010 From: "=?ISO-8859-1?Q?Ulf_Ekstr=F6m?= ulfek^_^few.vu.nl" To: CCL Subject: CCL: question on Born-Oppenheimer approx. Message-Id: <-43043-101031044355-1431-GIjxcwZcuzXcnh929bvV5g[-]server.ccl.net> X-Original-From: =?ISO-8859-1?Q?Ulf_Ekstr=F6m?= Content-Type: text/plain; charset=ISO-8859-1 Date: Sun, 31 Oct 2010 09:43:46 +0100 MIME-Version: 1.0 Sent to CCL by: =?ISO-8859-1?Q?Ulf_Ekstr=F6m?= [ulfek..few.vu.nl] Eric, please limit your signature to a few lines! > Ulf, please say a little more about super-selection rules etc. The last > time I spoke about this topic at a conference in London on "Emergence", a > philosopher of physics also brought up super-selection but did not go into > any detail. I am not an expert on this topic, most of my knowledge come from http://en.wikipedia.org/wiki/Einselection .. However, the idea is that a quantum subsystem will get entangled with its environment. You could say that the environment is measuring the subsystem. This causes the subsystem to decoher - when viewed "alone" it will appear in classical superpositions which are the eigenstates of the subsystem density matrix. States that are robust with respect to environment interactions are called pointer states, since they can be observed in the laboratory. Now, for your molecules, which are the pointer states? If the pointer states are the "chemists" structures then I think we are perfectly right to think of nuclei as really beeing almost localized. The most important point is how the molecule interacts with the environment. For example, if going between the two isomers requires a large rearrangement of the solvent then the solvent is strongly measuring the geometry of the solute. This is called the 'interaction operator' between the subsystem and the environment, and usually determines the pointer states. Surprisingly the interaction operator is not the only important thing in this analysis. It turns out[1] that if the environment is very 'slow', then the environment will actually stabilize the eigenstates of the subsystem Hamiltonian, no matter what form the interaction operator takes. This is possibly important for energy transfer during photosynthesis! Coming back to your question, I wonder how big the effect is for ammonia. In the gas phase ammonia is in a superposition of the two "umbrella" positions, but in a solvent I am not really sure. This could be simulated by path integral molecular dynamics, probably someone already did. If ammonia becomes classicalized in a solvent then I would guess that the bigger molecules you mention will suffer the same fate. [1] Juan Pablo Paz and Wojciech H. Zurek, Quantum limit of decoherence: Environment induced superselection of energy eigenstates, Phys.Rev.Lett. 82 1999, 5181-5185 > And what is the difference in the energies of these two isomers? Can anyone > help? Perhaps this is not so important, I would look for quantum MD simulations that look at the issue in detail. Sincerely, Ulf Ekstrom From owner-chemistry@ccl.net Sun Oct 31 06:58:00 2010 From: "Karol M. Langner karol.langner**gmail.com" To: CCL Subject: CCL: question on Born-Oppenheimer approx. Message-Id: <-43044-101031065708-29930-nDXR3C1IR8e46YpYpRaNqg^server.ccl.net> X-Original-From: "Karol M. Langner" Date: Sun, 31 Oct 2010 06:57:07 -0400 Sent to CCL by: "Karol M. Langner" [karol.langner-$-gmail.com] > > And what is the difference in the energies of these two isomers? Can anyone > > help? > > Perhaps this is not so important, I would look for quantum MD > simulations that look at the issue in detail. I don't have access to quantum MD, but the difference in RHF energies (using the cc-pp-VDZ basis set) between C2H5OH and CH3OCH3 in their optimized conformations is almost 13 kcal/mol. Of course, C2H5OH has the lower energy. You can find the input and output files here: http://www.mmqc.org/kml/tmp/isomers/ Best, Karol From owner-chemistry@ccl.net Sun Oct 31 10:47:00 2010 From: "uekstrom++gmail.com uekstrom++gmail.com" To: CCL Subject: CCL: question on Born-Oppenheimer approx. Message-Id: <-43045-101031044219-32111-vLTdYrR+gURSBNfHUFnERg|-|server.ccl.net> X-Original-From: "uekstrom _ gmail.com" Content-Type: text/plain; charset=ISO-8859-1 Date: Sun, 31 Oct 2010 09:42:11 +0100 MIME-Version: 1.0 Sent to CCL by: "uekstrom ~~ gmail.com" [uekstrom ~~ gmail.com] Eric, please limit your signature to a few lines! > Ulf, please say a little more about super-selection rules etc. The last > time I spoke about this topic at a conference in London on "Emergence", a > philosopher of physics also brought up super-selection but did not go into > any detail. I am not an expert on this topic, most of my knowledge come from http://en.wikipedia.org/wiki/Einselection .. However, the idea is that a quantum subsystem will get entangled with its environment. You could say that the environment is measuring the subsystem. This causes the subsystem to decoher - when viewed "alone" it will appear in classical superpositions which are the eigenstates of the subsystem density matrix. States that are robust with respect to environment interactions are called pointer states, since they can be observed in the laboratory. Now, for your molecules, which are the pointer states? If the pointer states are the "chemists" structures then I think we are perfectly right to think of nuclei as really beeing almost localized. The most important point is how the molecule interacts with the environment. For example, if going between the two isomers requires a large rearrangement of the solvent then the solvent is strongly measuring the geometry of the solute. This is called the 'interaction operator' between the subsystem and the environment, and usually determines the pointer states. Surprisingly the interaction operator is not the only important thing in this analysis. It turns out[1] that if the environment is very 'slow', then the environment will actually stabilize the eigenstates of the subsystem Hamiltonian, no matter what form the interaction operator takes. This is possibly important for energy transfer during photosynthesis! Coming back to your question, I wonder how big the effect is for ammonia. In the gas phase ammonia is in a superposition of the two "umbrella" positions, but in a solvent I am not really sure. This could be simulated by path integral molecular dynamics, probably someone already did. If ammonia becomes classicalized in a solvent then I would guess that the bigger molecules you mention will suffer the same fate. [1] Juan Pablo Paz and Wojciech H. Zurek, Quantum limit of decoherence: Environment induced superselection of energy eigenstates, Phys.Rev.Lett. 82 1999, 5181-5185 > And what is the difference in the energies of these two isomers? Can anyone > help? Perhaps this is not so important, I would look for quantum MD simulations that look at the issue in detail. Sincerely, Ulf Ekstrom