Alkyl groups: group frequencies

The substitution of alkyl groups for a hydrogen atom in strained ring systems serves to increase the frequency of C=C absorption. Cyclobutene absorbs at 1566 cm-1, 1-methylcyclobutene at 1641 cm-1. 

Relative initial rates of polymerization of aliphatic aldehydes catalyzed by [Me2A10CPhNPh]2 catalyst followed the order of MeCHO > EtCHO > nPrCHO > i PrCHO (Fig. 9). It is apparent from the order that the rate of polymerization is lower when the alkyl group of aldehyde monomer is bulkier. It is conceivable that the steric factor, not the ionic factor, is predominant for the relative rates of polymerization, because the frequencies of IR bands assigned to the carbonyl group are nearly constant for these aldehydes. 

The generally accepted valence bond and molecular orbital (MO) approach to the bonding of metal isocyanides has been well described in Treichel s review (6), and has been used to rationalize (i) variations in IR stretching frequencies between bonded and nonbonded isocyanides, and (ii) the better Tt-acceptor qualities of aryl versus alkyl isocyanide groups (53,54). In valence bond theory the canonical forms involved in metal isocyanide bonds are... 

If the alkyl groups in any R SnX4 series are replaced by phenyl, the U9Sn signal moves to lower frequencies (Table XI), which is inconsistent with the greater electron withdrawing capability of a phenyl substituent on tin. Similar effects are found with benzyl, vinyl, and allyl groups (Tables XXX and XXXI) and may be due to the increased polarizability of these unsaturated substituents. (23) This topic is discussed further in Section III.B.3. 

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