Organic Chem End of MS

Carbon NMR Spectroscopy

  • Introduction to Carbon NMR

    • Importance of understanding different types of carbons in NMR spectra.

    • Location of different hybridized carbons on the NMR scale, from right to left.

  • Categories of hybridization in Carbon NMR

    • sp³ Hybridized Carbon: Located on the right-hand side of the carbon NMR spectra.

    • sp² Hybridized Carbon: Found at an intermediate position in the spectra.

    • sp Hybridized Carbon: Slightly beside the sp² carbons.

    • Carbonyl Carbon: A specific example of sp², which is often found toward the far left, near a shift of up to 220 ppm.

  • Carbon NMR vs. Proton NMR

    • Carbon NMR does not provide detailed information about hydrogen atoms.

    • Typically, both Carbon NMR and Proton NMR are run on the same sample for comprehensive data analysis.

    • Rationale for combined analysis: Achieves a clearer picture of the molecular structure by correlating carbon and hydrogen environments.

Depth Experiments in NMR

  • Purpose of Depth Experiments: To deduce the number of hydrogen atoms on certain carbons without running additional proton NMR.

  • Types of Depth Experiments:

    • DEPT 90:

    • Shows only CH signals.

    • Does not provide peaks for CH₂ or C.

    • DEPT 135:

    • Displays positive peaks for CH₃ and CH groups, and a negative peak for CH₂.

  • Process of Elimination: Using DEPT experiments reduces ambiguity about carbon types based on peak presence and absence.

  • Example Analysis:

    • Sample Analysis with DEPT NMR indicating types of carbons (CH₃, CH₂, CH, C) based on resulting peaks.

    • Positive peak in DEPT 90 for CH₃ confirms its presence.

    • Negative peak in DEPT 135 signifies absence of CH₂.

    • Each peak pattern confirms different carbon types based on depth experiment outcomes.

Mass Spectrometry (Mass Spec) Fundamentals

  • Introduction to Mass Spectrometry: Allows determination of mass of compounds through ionization and fragmentation.

  • Operational Steps in Mass Spec:

    1. Injection of sample: The sample is injected into the instrument.

    2. Vaporization: The sample is vaporized.

    3. Ionization: The vaporized sample gets ionized.

    4. Fragmentation: Some ions fragment, forming charged particles.

    5. Detection: Ions are accelerated towards a detector using a magnetic field.

    6. Output: A graph representing relative intensity versus mass-to-charge (m/z) ratio is produced; the charge is typically one.

  • Understanding Fragmentation:

    • Cation Radical: Result of electron ejection leads to a positively charged ion (molecular ion).

    • Fragment Ions: Molecular ion can lead to various fragment ions due to further splitting.

Analyzing Mass Spectra

  • Graph Interpretation:

    • Relative Intensity: Highest peak (base peak) represents 100% intensity, serving as reference for other peaks.

    • Molecular Ion Identification: Example with methane where molecular ion peak appears at m/z of 16 (C₁H₄).

    • Fragmentation Patterns:

    • Decrease of mass observed from 16 to 15, 14 indicates hydrogen atom loss.

  • Fragment Stability:

    • Base peak not always identifying the molecular ion, depends on stability of produced ions during fragmentation.

    • Common Fragment Losses:

    • Each fragment reflects loss based on structure: one hydrogen results in m/z decrease by 1, and similar patterns follow for larger groups.

Isotopic Patterns and Their Significance

  • Isotopic Variance in Mass Spec:

    • Natural isotopes, e.g., Carbon-12 and Carbon-13, result in peaks that help determine number of carbons in a sample.

    • n+1 Peak: Represents carbon-13, where its integration can provide carbon count based on relative intensity to molecular ion peak.

  • Application to Other Elements:

    • Chlorine Isotopes: Presence of chloride can be indicated by m/z patterns with m:2 peaks.

    • Proportions of two chloride isotopes lead to a distinctive peak pattern aiding structure identification.

  • Bromine Isotopes:

    • Similar to chlorine; Bromine isotopic patterns display equivalently sized m and m+2 peaks indicating high probability of bromine presence.

Fragmentation of Functional Groups

  • Alcohol Fragmentation:

    • Alpha Cleavage: Cleavage occurs along the carbon bond adjacent to hydroxyl.

    • Dehydration Reaction: Loss of water (mΔ18) characterized by identification of peaks in mass spectra.

  • Amine Fragmentation:

    • Similar to alcohol, can experience alpha cleavage without fermentation to produce fragments.

  • Carbonyls Fragmentation:

    • Undergo rearrangements known as McLafferty rearrangements yielding unique fragments.

Advanced High-Resolution Mass Spectrometry

  • Types of Mass Spectrometry:

    • High-resolution mass spectrometry for precise mass measurements (up to four decimal places).

    • Allows differentiation of compounds that otherwise may show similar mass values using standard rounding methods.

Analysis Practice and Review

  • Practice Problems: Encouragement for students to solve exercises based on NMR and Mass Spec principles discussed.

  • Understanding Fragmentation:

    • Encourage familiarity with expected losses of carbon, hydrogen, or other groups based on common fragment patterns.

  • Final Notes:

    • Reminder to approach questions and areas of confusion with instructor for further clarity and understanding.