CHEM 344-345 - Spectroscopy and Spectrometry in Organic Chemistry - Handouts

Oct 13, 2026

CHEM 344/345 handouts summarize core spectroscopic methods used in organic chemistry: UV/Vis, IR, mass spectrometry, and 1H NMR. The UV/Vis section links electronic structure to absorption and explains excitation through HOMO–LUMO energy, including conjugation effects on λmax. The IR section connects molecular vibrations to functional groups via absorption at characteristic frequencies, guided by Hooke’s law and factors like force constants and nuclear masses. Mass spectrometry outlines ionization, fragmentation, and mass/connectivity determination, while NMR describes structural information from magnetic-field interactions and radiofrequency excitation.

LanguageEnglish

CategoryPresentations

FormatPDF

Pages98

Reading Time34 min

AccessFree to download

Spectroscopy and Spectrometry in Organic Chemistry

Ultra-violet and Visible (UV/Vis) Spectroscopy

Infrared (IR) Spectroscopy

Factors Affecting IR Absorption

Mass Spectrometry (EI-MS)

Nuclear Magnetic Resonance (NMR) Spectroscopy

CHEM 344/345 – Spectroscopy and Spectrometry in Organic Chemistry infrared spectroscopy (IR) 300 MHz ¹H NMR In C DC l3 | nuclear magnetic resonance spectroscop~~y~~ | | | | | --- | --- | --- | --- | | ( 1 H-NMR)
| | | | | | | | | | | | | | 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 mass spectrometry (EI-MS) ultra-violet and visible light absorption spectroscopy (UV-vis) Spectroscopy and Spectrometry in Organic Chemistry 1. Ultra-violet and visible (UV/Vis) spectroscopy provides information about the electronic structure of a molecule through the excitation of electrons. (15 . 2, 16 .3D, 19 .3D) 2. Infrared (IR) spectroscopy can be used to determine functional groups and bond strengths based upon molecular vibrations. (12 . 1 – 12 .5, 14 .3B, 16 .3A, 19 .3A, 20 .3A, 21 .4A, 23 .4A, & A-3 – A-4) 3. Mass Spectrometry (MS) ionizes and energizes a molecule to determine its mass and connectivity through fragmentation. (12 .6, 19 .3E, 26 .8A) 4. Nuclear Magnetic Resonance (NMR) spectroscopy provides detailed information about the structure of a molecule by its interaction with a magnetic field and a radio frequency pulse. (13, 14 .3C, 16 .3B – 16 .3C , 19 .3B – 19 .3C, 20 .3B, 21 .4B, 23 .4A, & A-5 – A-8) 12.1 Electromagnetic Radiation Absorption spectroscopy in 3 easy steps. 1) Make light. 2) Pass light through sample. 3) Check for absorption. 15.2 UV/Vis Absorption Spectroscopy Ultra-violet and visible (UV/Vis) spectroscopy provides information about the electronic structure of a molecule – particularly useful for conjugated p systems. UV/Vis photons excite electrons to an excited state if E photon = E LUMO-HOMO . 15.2 UV/Vis Absorption Spectroscopy This explains why many organic compounds tend to be boring (white or off-white powders or crystals) in the visible spectrum. Molecules need to absorb in the visible range in order to be colored. 16 Conjugation in 4-ethylanisole The more conjugated the p-system, the lower the HOMO-LUMO gap, the larger the lmax will be. The p-orbital lone pair of the oxygen atom conjugates to the aromatic ring p-system. 6 15.2 UV/Vis Spectrum of Crystal Violet Molecules need to absorb in the visible range in order to be colored. l max = 590 nm 12.2 Physical Basis for IR Spectroscopy Chemical bonds are not rigid, but in continuous states of vibration. 3585 cm-1 3506 cm-1 1885 cm-1 Normal modes of vibration of water If a molecule absorbs a photon of a frequency of light that matches the frequency of a vibration, an absorption occurs and that vibrational state is excited. 12.3 Factors that Affect IR Absorption Frequency 􀟥 = 21􀟨 Chemical bonds can be approximated to be springs. Hooke’s Law governs the energy of vibration. The frequency (n) is dependent on the force constant (k ~ bond strength) and the masses of the nuclei. | Bond | Approximate vibrational frequency | | --- | --- | | C–C | 1000 cm-1 | | C=C | 1600 cm-1 | | C≡C | 2200 cm-1 | | Bond | Approximate vibrational frequency | | --- | --- | | C(sp)–H | 3300 cm-1 | | C(sp2 )–H | 3100 cm-1 | | C(sp3 )–H | 2900 cm-1 | | Bond | Approximate vibrational frequency | | --- | --- | | C(sp3 )–D | 2200 cm-1 | | C(sp3 )–H | 2900 cm-1 |

FAQs

What information does UV/Vis spectroscopy provide in organic molecules?

UV/Vis spectroscopy gives information about a molecule’s electronic structure by exciting electrons. It is especially useful for conjugated systems and relates absorption to the HOMO–LUMO energy difference.

How does IR spectroscopy identify functional groups?

IR spectroscopy determines functional groups and bond strengths based on molecular vibrations. When light frequency matches a vibrational mode, absorption occurs and a vibrational state is excited.

What does mass spectrometry (EI-MS) measure and how?

Mass spectrometry ionizes and energizes molecules, then uses fragmentation to determine mass and connectivity. The resulting pattern supports interpretation of molecular structure.

Oct 13, 2026

Upload by นรินทร์