Gauche correction

If one side of the double bond is substituted as in 20 or 21, in which R stands for an alkyl group, an alkene gauche correction of 0.50 kcal mole-1 must be added. Thus the calculated AH° for 2,3-dimethylbut-l-ene (22) is as follows ... 

Cd represents a double-bonded C atom, Ct a triple-bonded C atom, and Cb a C atom in a benzene ring. Values in parentheses are to be used when corrections for gauche configurations are made. The correction applies to each such configuration. Thus 2-methyl butane has one gauche correction, 2,3-dimcthyl butane has two, and 2,2,3-trimethyl pentane has five. 

Many of the torsional terms in the AMBER force field contain just one term from the cosine series expansion, but for some bonds it was found necessary to include more than one term. For example, to correctly model the tendency of O-C—C-O bonds to adopt a gauche conformation, a torsional potential with two terms was used for the O—C—C—O contribution ... 

Examine space-filling models for the two conformers and identify any likely unfavorable nonbonded interactions. Based on steric effects, which conformer would you anticipate would be the more stable Compare energies of anti-1,2-ethanediol and gauche-1,2-ethanediol to see if you are correct. Is this the same ordering of conformer energies as seen for n-butane (see Chapter 5, Problem 3)7... 

The sensitivity of 13C solid-state chemical shifts to small conformational changes is well illustrated by the case of i-PB. Table 1 summarizes some chemical shifts differences for the forms, which have been interpreted in terms of variations of the y-shielding parameter corrected for the deviations, with respect to the exactly G conformations, in the slightly different nearly gauche — nearly trans sequences, characterizing the three crystalline forms of i-PB [116]. 

Corrections for gauche configurations and cis isomers are given in the footnotes to Table 6. Corrections for ring structures are given in Table 8. The sources of the ring correction terms are discussed in Section IV. 

This work, except as noted. Corrections for geometric isomers from Ref. 19 cis = 4.1 alkane gauche = 3.4 alkene gauche = 2.. This reference also gives other correction terms and discusses their application. 

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