/sup 18/O-isotope effect in /sup 13/C nuclear magnetic resonance spectroscopy. 3. Additivity effects and steric effects

Journal Article · Wed Oct 22 00:00:00 EDT 1980 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00542a007· OSTI ID:6800586

Risley, J M; Van Etten, R L

An upfield shift was previously shown to occur in /sup 13/C NMR spectroscopy upon replacement of /sup 16/O by /sup 18/O. The additivity of this effect is now demonstrated in two types of compounds. The multiple replacement of equivalent oxygen-16 atoms by oxygen-18 atoms results in a shift of the /sup 13/C NMR signal of the oxygen-bearing carbon upfield by an equal amount upon each equivalent substitution. This direct additivity effect is demonstrated by a study of the mixed methyl n-(/sup 18/O)butyl orthocarbonates: the shift of the orthocarbonate carbon is 0.015 ppM//sup 18/O. The individual contributions of the carbonyl oxygen and of the ether oxygen to the total shift of the carboxyl carbon in /sup 18/O-labeled carboxylic esters are measured. The sum of the individual effects equals the shift observed for the totally labeled ester. The additivity observed for the effect of oxygen-18 on /sup 13/C NMR signals is qualitatively similar to effects previously observed for analogous isotope shifts with other nuclei. The possible importance of steric effects in influencing the magnitude of the /sup 18/O shift was explored by synthesizing /sup 18/O-labeled, sterically hindered alcohols and a ketone. It is concluded that electronic substituent effects rather than steric effects are primarily responsible for the large /sup 18/O-isotope shifts on the hydroxyl carbon of tertiary alcohols as compared to typical primary or secondary alcohols.

• Research Organization: Purdue Univ., West Lafayette, IN
• OSTI ID: 6800586
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 102:22; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400302* -- Organic Chemistry-- Isotope Effects-- (-1987)
ALCOHOLS
CARBOXYLIC ACID ESTERS
ESTERS
EVEN-EVEN NUCLEI
HYDROXY COMPOUNDS
ISOTOPE EFFECTS
ISOTOPES
KETONES
LIGHT NUCLEI
MAGNETIC RESONANCE
NUCLEAR MAGNETIC RESONANCE
NUCLEI
ORGANIC COMPOUNDS
OXYGEN 18
OXYGEN ISOTOPES
RESONANCE
SPECTRAL SHIFT
SPECTROSCOPY
STABLE ISOTOPES