From shenkin-!at!-still3.chem.columbia.edu Thu May 29 13:45:32 1997 Received: from mailrelay1.cc.columbia.edu for shenkin #at# still3.chem.columbia.edu by www.ccl.net (8.8.3/950822.1) id NAA27025; Thu, 29 May 1997 13:17:01 -0400 (EDT) Received: from still3.chem.columbia.edu (still3.chem.columbia.edu [128.59.112.36]) by mailrelay1.cc.columbia.edu (8.8.5/8.8.5) with SMTP id NAA02956; Thu, 29 May 1997 13:16:55 -0400 (EDT) Received: by still3.chem.columbia.edu (950413.SGI.8.6.12/930416.SGI.AUTO) id NAA29274; Thu, 29 May 1997 13:16:54 -0400 From: "Peter Shenkin" Message-Id: <9705291316.ZM29272 ":at:" still3.chem.columbia.edu> Date: Thu, 29 May 1997 13:16:54 -0400 In-Reply-To: Jorge Seminario "CCL:rotational barrier: experiment and theory" (May 29, 3:16am) References: <199705290316.DAA12324-: at :-cosmos.psc.sc.edu> X-Mailer: Z-Mail (3.2.3 08feb96 MediaMail) To: Jorge Seminario , CHEMISTRY;at;www.ccl.net Subject: Re: CCL:rotational barrier: experiment and theory Cc: jorge:~at~:cosmos.psc.sc.edu Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii On May 29, 3:16am, Jorge Seminario wrote: > Subject: CCL:rotational barrier: experiment and theory > Hello dear members: > > I wonder if you could help me to clear up some problems to interpret my > colleagues' experiment by using our computational tools. The molecule > consists in two benzene rings connected by two carbon atoms. > The experiment on Ph-C-C-Ph (where the central C-C bond is triple and > the adjacents are single) indicates that a sudden rotation of one of > the benzene rings with respect to the other is initiated when the > temperature reaches 30 K. What is meant by "sudden"? All your other arguments seem to argue for an activated process, which would imply Arrhenius-type behavior; this is not usually called "sudden", though in colloquial sense I suppose it is. > ... The calculations > yield a barrier of about 0.5 kcal/mol for the rotation of the benzene > rings. The argument against this theoretical barrier is that > at 30K, the kT (or RT) available for this mode is only 0.06 kcal/mol and > therefore, the rotation of the phenyl groups is not possible because the > barrier is almost ten times bigger than the available energy. Sorry, but this argument isn't right. RT= ca. 2.5 kJ at 300K, yet most chemical reactions of everyday interest have activation energies many times greater than this. For instance, we're told in school that "typical" room-temperature reactions double in rate when the temperature is raised by 10 K. A typical exam question is to compute the activation energy of such a reaction. The answer is about 53 kJ/mol, or more than 20 RT. The action proceeds because the "pre-exponential factor" is enormously high at room temperature. We now turn to the back of the proverbial envelope. Using Eyring formalism to estimate the pre-exponential factor, kT/h_bar is equal to about 4x10^13/s at 300 K; at 30 K, we get 4x10^12/s, which is still huge. The Boltzmann factor comes out to about 2x10^-4 for your reaction at 30 K, but multiplying this by the corresponding pre-exponential factor gives a rate constant of about 8x10^8/sec, which is pretty darn fast. -P. -- **** "Deep Blue can't triumph in the game of life" (NY Times, 5/13/97) **** * Peter S. Shenkin; Chemistry, Columbia U.; 3000 Broadway, Mail Code 3153 * ** NY, NY 10027; shenkin.,at,.columbia.edu; (212)854-5143; FAX: 678-9039 *** *MacroModel WWW page: http://www.columbia.edu/cu/chemistry/mmod/mmod.html *