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l08a - CH 203 O R G A N I C C H E M I S T R Y I...

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Conformational analysis I © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Conformational analysis I Newman projections Molecules are always executing some sort of motion. In general, molecular mo- tion increases as temperature increases, but even at absolute zero (0 K) the atoms in a molecule continue to move. Molecular motions are classified as ei- ther translational, vibrational, or rotational motions. Translational motion is realized when a molecule’s center of mass moves from one location to another, say, from some initial position denoted in the Car- tesian coordinate system by { x i , y i , z i } to a final position { x f , y f , z f }: x y z initial location { x i , y i , z i } final location { x f , y f , z f } A molecule undergoes vibrational motion when (1) bond lengths increase and decrease or (2) bond angles increase and decrease. Vibrations in which bond lengths change are called stretches, whereas vibrations in which bond angles change are called bends: stretch bend For now, we will be concerned with a molecule’s rotational motions, which are classified as either (1) rotations about the center of mass or (2) internal
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Conformational analysis I © Bruno I. Rubio 2 rotations about sigma ( ! ) bonds. A rotation about a molecule’s center of mass is a spinning motion about an axis drawn through that molecule’s center of mass such as O C Cl H O C Cl H O C H Cl or O C Cl H H C O Cl Cl C H O Internal rotational motion takes place when sigma ( ! ) bonds rotate. Consider the butane (CH 3 CH 2 CH 2 CH 3 ) molecule in the drawing below. In Structure 1 the two methyl (CH 3 ) groups are drawn on solid lines implying that both lie in the plane of the paper. As the C2–C3 bond rotates, the methyl group in bold type swings toward you and is thus drawn on a wedge (Structure 2). As the C2– C3 bond continues to rotate, the methyl group swings away from you and is thus drawn on a dash (Structure 3). Eventually, the methyl group completes the full 360° rotation and returns to the plane of the paper (back to Struc- ture 1). H 3 C H H CH 3 H H H 3 C CH 3 H H H H H 3 C H H H CH 3 H 1 2 3 Structures 1–3 have various interchangeable names: they are called conforma- tions, or conformational isomers, or rotational isomers, or rotamers.
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Conformational analysis I © Bruno I. Rubio 3 A traditional tool used to visualize rotations about ! bonds is the Newman projection (named for Professor M.S. Newman of The Ohio State University who introduced its use in 1952). To construct a Newman projection, sight down a carbon–carbon ! bond, for ex- ample, the C2–C3 bond of butane. The carbon atom nearer you in the carbon– carbon ! bond down which you are sighting resides at the center of an imagi- nary disk. You can see three groups bonded to the carbon atom at the center of the imaginary disk, but the other carbon in the carbon–carbon ! bond down which you are sighting is hidden behind the imaginary disk and is invisible.
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l08a - CH 203 O R G A N I C C H E M I S T R Y I...

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