On the other hand the timescale of the process

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Unformatted text preview: . On the other hand, the timescale of the process underlying infrared spectroscopy (vibrational excitation) is short relative to that for conformational equilibration. This means that an infrared spectrum is actually made up of spectra of the individual conformers. Only a few conformers are likely to significantly contribute to the average (or likely to be seen individually). 3 Infrared Spectra of Acrolein Conformers: Acrolein (propenal, H2C=CC(H)=O) exists as a mixture of syn and anti conformers, with the latter dominating. O H H C C H H C H C C H H O C H Is it likely that these could be distinguished by way of their infrared spectra, in particular the frequency corresponding to the CO stretching motion? Obtain equilibrium geometries and infrared spectra for both conformers. Use the B3LYP/6-31G* model. While the anti conformer is planar, the syn conformer may not be, so start with a non-planar geometry. Which conformer is lower in energy? What percentage of a room-temperature equilibrium mixture corresponds to the higher-energy conformer? If it lower than 5%, what temperature would be needed to bring it to 5%? Compare the infrared spectra for the two conformers. Are the CO stretching frequencies sufficiently different (by 5 cm-1 or more) to allow them to be distinguished? Is there anything else that would help to distinguish the spectra? Elaborate. The properties of some molecules will be correctly described in terms of a single unique conformer. This is either because these molecules are rigid, or because all conformers are equivalent, or because one particular conformer is strongly preferred over all others. For example, ethylene is rigid, the two conformers of hydrogen peroxide are identical and the cis conformer of formic acid is strongly preferred over the trans conformer. H H O C H O O C O H Dipole Moment of Formic Acid: Use the HF/6-31G* model to obtain equilibrium geometries for both cis and trans conformers of formic acid. Use the Boltzmann equation to calculate an average dipole moment at room temperature. Is it dominated by one conformer or do both conformers contribute significantly? However, the majority of molecules exist in terms of a collection of several distinct conformers or shapes. These result from rotation about single bonds (as illustrated up to this point in the chapter), from constrained rotation about single bonds incorporated into rings, from inversion of pyramidal nitrogen and phosphorus centers and from pseudorotation about fivecoordinate centers. Except for very simple molecules with one or two degrees of conformational freedom, the number of unique shapes that can arise may not be obvious. Suffice it to say that this number increases rapidly with the number of single bonds and pyramidal and pentacoordinate centers and with the number and size of the rings, and may grow into the hundreds or even thousands of conformers. 4 The majority of this chapter concerns molecules with only a single degree of conformational freedom. This immediately highlights one advantage of calculations over experiment. Experiments may only directly probe equilibrium structures (and only then if they are present in sufficient amounts to actually be observed) and, for very simple molecules, the barriers to conformational change. Calculations may examine all conformers, irrespective of their abundance and all interconversion barriers. Pathways may also be described, although as pointed out in Chapter P4 they contain a degree of ambiguity. We start by introducing a Fourier series as a simple tool to “dissect” the energy profile for rotation about a bond, and to “interpret” conformational energy preferences in terms that are familiar to chemists, most notably, steric crowding, dipole-dipole interactions and conjugation. Next, we examine conformational energy profiles for a small but diverse selection of simple molecules. Here, the focus is in seeing how the fundamental preferences (identified in the Fourier series) combine to give rise to an actual energy profile. We will also seek to identify commonalities among energy profiles for different molecules. The third goal of this chapter will be to judge the ability of both “limiting” (large-basis-set) and “practical” Hartree-Fock, B3LYP and MP2 models to properly assign the lowest-energy conformer and to reproduce the known Boltzmann distribution of conformers. As indicated earlier, rotation about single bonds is not the only type of conformational change, and we will provide examples constrained rotation in rings, inversion and pseudorotation. The chapter concludes by “answering” a question that at first glance appears to be obvious, but on more careful consideration reveals is not: “What is the important conformer? The process of changing from one conformer to another is in fact a chemical reaction, and maxima on a conformational energy profile are transition states. There are, however, two significant differences between a conformational change and...
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This note was uploaded on 02/22/2010 for the course CHEM N/A taught by Professor Head-gordon during the Spring '09 term at University of California, Berkeley.

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