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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.
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
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.
- Spring '09