Conformational effects on13C NMR parameters in alkyl formates

Kowalewski, Dora G. de; Kowalewski, Valdemar J.; Contreras, Rubén H.; Esteban, Ángel L.

doi:10.1002/(sici)1097-458x(199805)36:5<336::aid-omr273>3.0.co;2-c

Cited by 14 publications

(4 citation statements)

References 5 publications

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“…Populations, 13 C chemical shifts, and 1 J C - H couplings were reported for the E and Z conformations of methyl, ethyl, n -propyl, tert -butyl, n -butyl, isobutyl, and benzyl formate at room temperature and 125 MHz. However, it is actually not possible to see separate signals for E and Z conformations of alkyl formates under these conditions, and the study is invalid.…”

Section: Resultsmentioning

confidence: 99%

Barriers to Rotation in Methyl Formate by Dynamic NMR Spectroscopy and Barriers to 1,3 Oxygen-to-Oxygen Migration in Methyl Formate and Trifluoromethyl Formate by ab Initio Calculations

et al. 2001

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Free-energy barriers of 9.85 and 11.91 +/- 0.15 kcal/mol at -70.8 degrees C were found by dynamic NMR spectroscopy for the E-to-Z and Z-to-E conversions, respectively, of methyl formate (1) enriched in 13C to 99% for the carbonyl carbon [methyl formate 13C (2)]. These barriers are higher than the literature values reported for -53 degrees C. The free-energy barrier to 1,3 oxygen-to-oxygen migration of the methyl group in methyl formate was determined by ab initio calculations at several levels. The value of 58.7 kcal/mol obtained at the MP2/6-311+G (df,pd) level was compared to a literature barrier for this process (MINDO/3) and to barriers for related compounds. A free-energy barrier of 63.0 kcal/mol for the oxygen - to - oxygen migration of the CF3 group in trifluoromethyl formate (3) was calculated at the MP2/6-31+G level.

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Section: Resultsmentioning

confidence: 99%

Barriers to Rotation in Methyl Formate by Dynamic NMR Spectroscopy and Barriers to 1,3 Oxygen-to-Oxygen Migration in Methyl Formate and Trifluoromethyl Formate by ab Initio Calculations

et al. 2001

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“…For seven alkyl formates, the more abundant s‐ cis rotamer had 1 J C–H ~7 Hz greater relative to that in the minor s‐ trans conformer. In the s‐ trans geometry, the n O → σ* CH interaction weakens the C–H bond and decreases the magnitude of C–H spin coupling (Figure ) …”

Section: Spectroscopic Signatures Of Hyperconjugationmentioning

confidence: 99%

Hyperconjugation

Alabugin

Gomes

Abdo

2018

WIREs Comput Mol Sci

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This review outlines the role of hyperconjugative interactions in the structure and reactivity of organic molecules. After defining the common hyperconjugative patterns, we discuss the main factors controlling the magnitude of hyperconjugative effects, including orbital symmetry, energy gap, electronegativity, and polarizability. The danger of underestimating the contribution of hyperconjugative interactions are illustrated by a number of spectroscopic, conformational, and structural effects. The stereoelectronic nature of hyperconjugation offers useful ways for control of molecular stability and reactivity. New manifestations of hyperconjugative effects continue to be uncovered by theory and experiments. This article is categorized under: Structure and Mechanism > Molecular Structures Software > Molecular Modeling Structure and Mechanism > Reaction Mechanisms and Catalysis

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“…The s-cis conformation ( Fig. 3) is preferred [22] and the in-plane O Me lone-pair, LP 1 (O Me ), is syn to the formyl σ Cc−H f bond, i.e. this is a configuration similar to that of 2 J NHsyn = −13.9 Hz in 2a.…”

Section: One Of the Coupling Nuclei Corresponds To A Lone-pair Bearinmentioning

confidence: 94%

Stereochemical dependence of NMR geminal spin–spin coupling constants

Contreras

Provasi

Santos

et al. 2008

Magnetic Reson in Chemistry

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In this work it was sought to explore the versatility of geminal spin-spin coupling constants, (2)J(XY) SSCCs, as probes for stereochemical studies. A set of compounds, where their experimental (2)J(XY) SSCCs through the X-C-Y molecular fragment are predicted to be sensitive to hyperconjugative interactions involving either bonding or antibonding orbitals containing the C carbon atom ('coupling pathway'), were analyzed. SSCC calculations were performed for some selected examples using the second order polarization propagator approximation (SOPPA) method or within the DFT-B3LYP framework. Hyperconjugative interactions were calculated within the Natural Bond Orbital (NBO) approach. Results are condensed in two qualitative rules: Rule I(M)-hyperconjugative interactions transferring charge into the coupling pathway yield a positive increase to the Fermi contact (FC), contribution to (2)K(XY) reduced spin-spin coupling constants (RSSCC), and Rule II(M)-hyperconjugative interactions transferring charge from the coupling pathway yield a negative increase to the FC contribution to (2)K(XY) RSSCC.

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Conformational effects on13C NMR parameters in alkyl formates

Cited by 14 publications

References 5 publications

Barriers to Rotation in Methyl Formate by Dynamic NMR Spectroscopy and Barriers to 1,3 Oxygen-to-Oxygen Migration in Methyl Formate and Trifluoromethyl Formate by ab Initio Calculations

Barriers to Rotation in Methyl Formate by Dynamic NMR Spectroscopy and Barriers to 1,3 Oxygen-to-Oxygen Migration in Methyl Formate and Trifluoromethyl Formate by ab Initio Calculations

Hyperconjugation

Stereochemical dependence of NMR geminal spin–spin coupling constants

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