19F NMR chemical shifts. 1. Aliphatic fluorides

Area of Science:

• * Quantum chemistry
• * Computational spectroscopy
• * Nuclear magnetic resonance (NMR) spectroscopy

Background:

• * Understanding the factors influencing NMR chemical shifts is crucial for molecular structure elucidation.
• * Alkyl fluorides exhibit significant variations in fluorine-19 (19F) NMR shielding.
• * The relationship between molecular orbital (MO) contributions and NMR shielding requires detailed investigation.

Purpose of the Study:

• * To calculate and analyze the 19F NMR shielding in a series of alkyl fluorides.
• * To determine the contributions of individual molecular orbitals to the observed shielding trends.
• * To investigate the role of orbital mixing and electronic structure on fluorine NMR chemical shifts.

Main Methods:

• * Calculation of 19F NMR shielding using the Independent Gauge Including AtomiczaXRvcn (IGAIM) method.
• * Decomposition of shielding into contributions from each molecular orbital.
• * Analysis of tensor components and orbital mixing effects.

Main Results:

• * Significant changes in 19F NMR shielding were observed across the alkyl fluoride series.
• * Shielding variations are primarily attributed to tensor components perpendicular to the C-F bond axis.
• * Increased electron orbital mixing, especially involving fluorine lone-pair orbitals, leads to paramagnetic deshielding.
• * Similar trends were identified in related hydrocarbon and fluorocarbon series, including acetylene and fluoromethanes.

Conclusions:

• * The 19F NMR shielding in alkyl fluorides is strongly influenced by the electronic environment and molecular orbital interactions.
• * Paramagnetic deshielding increases with the number of adjacent p-orbitals due to enhanced MO mixing.
• * The findings provide insights into the electronic basis of NMR chemical shifts in organic fluorine compounds.