Orgel and Tanabe–Sugano Diagrams: Interpreting d–d Spectra

September 13, 2026

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Spectra Orgel and Tanabe–Sugano Diagrams: Interpreting d–d Spectra Two diagrams for the same purpose, differing in whether they can handle a spin state change. BSc & MSc · Inorganic Chemistry · Concept The short answer: Both plot the energies of electronic terms against ligand field strength. Orgel diagrams cover weak-field high-spin complexes only. Tanabe–Sugano diagrams cover the whole range including the high-spin to low-spin crossover, and take the ground state as the horizontal axis so transition energies are read directly. What the diagrams show A free ion’s electronic states are described by term symbols. Placing that ion…

Mass Spectrometry: Reading Fragmentation Patterns

September 11, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Spectroscopy Mass Spectrometry: Reading Fragmentation Patterns The molecular ion gives the mass. The fragments give the structure, and they fragment in predictable ways. BSc & MSc · Spectroscopy · Method The short answer: The molecular ion peak gives the molecular mass, and isotope peaks reveal certain elements immediately. Fragmentation follows rules based on which cation is most stable, so the losses observed identify the groups present. The nitrogen rule links an odd molecular mass to an odd number of nitrogens. What the spectrum shows The sample is ionised, and the resulting ions are separated by mass-to-charge ratio….

Metal Carbonyls: Synergic Bonding and What the Spectrum Reveals

September 9, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Organometallics Metal Carbonyls: Synergic Bonding and What the Spectrum Reveals The infrared stretching frequency of bound carbon monoxide is a direct readout of electron density at the metal, which makes it one of the most informative single measurements in the subject. BSc & MSc · Inorganic Chemistry · Concept The short answer: Carbon monoxide donates a lone pair to the metal and simultaneously accepts electron density from filled metal d orbitals into its own antibonding orbital. That back-donation weakens the C–O bond, lowering its stretching frequency — so the observed frequency measures how much back-donation is occurring….

Vibrational Spectroscopy: Force Constants and Anharmonicity

September 7, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Spectroscopy Vibrational Spectroscopy: Force Constants and Anharmonicity The harmonic oscillator explains where the band is. Anharmonicity explains everything the harmonic model gets wrong, including why molecules can dissociate at all. BSc & MSc · Spectroscopy · Concept The short answer: A vibrating bond behaves approximately as a harmonic oscillator with evenly spaced levels and a selection rule of one quantum. The frequency depends on the force constant and the reduced mass, so a stiffer bond or lighter atoms absorb at higher wavenumber. Real bonds are anharmonic, which produces converging levels and permits overtones. The harmonic oscillator Model…

Rotational Spectroscopy: Measuring a Bond Length From a Spectrum

September 6, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Spectroscopy Rotational Spectroscopy: Measuring a Bond Length From a Spectrum Line spacing gives the rotational constant, the rotational constant gives the moment of inertia, and that gives the bond length. Three steps, no ambiguity. BSc & MSc · Spectroscopy · Method The short answer: For a rigid diatomic rotor the energy levels are E = BJ(J+1), so successive transitions are separated by exactly 2B. Measure that spacing, extract B, convert to the moment of inertia and then to the bond length. The whole chain rests on the selection rule that J changes by one. The rigid rotor…

Term Symbols and Russell–Saunders Coupling, Step by Step

September 4, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Spectra Term Symbols and Russell–Saunders Coupling, Step by Step A mechanical procedure that looks abstract. Follow the five steps in order and any ground-state term symbol takes about a minute. BSc & MSc · Inorganic Chemistry · Method The short answer: Combine the individual orbital angular momenta into L and the spins into S, then couple them into J. The term symbol is written 2S+1LJ. Hund’s rules then pick the ground state: maximum multiplicity first, then maximum L, then J by whether the shell is less or more than half filled. What a term symbol encodes 2S+1LJ…

Structure Determination by NMR: A Method That Works Every Time

August 31, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Spectroscopy Structure Determination by NMR: A Method That Works Every Time Structure problems reward a fixed procedure far more than chemical intuition. Follow the same order every time and the answer assembles itself. BSc & MSc · Spectroscopy · Method The short answer: Work out the degrees of unsaturation from the molecular formula first, then read integration for how many hydrogens, chemical shift for their environment, and multiplicity for their neighbours. Assemble fragments last. Candidates who guess a structure early and then try to justify it lose far more marks than those who build it piece by…