A Guide to Inorganic Spectroscopy

Inorganic compounds are all around us, from the calcium hydroxyapatite, calcium phosphate, and calcium carbonate in our bones to the silicates that form the Earth’s crust (1–3). Despite their ubiquity, there are many misconceptions regarding how these substances interact with infrared (IR) light. Drawing from Brian C. Smith’s “IR Spectral Interpretation” column, this Q&A article explores the unique characteristics, definitions, and spectral signatures of inorganic compounds.

Q: There is a common belief that inorganic compounds do not have mid-infrared spectra. Is this true?

A: No. Although some students and scientists object to the topic, inorganics decidedly have mid-infrared (MIR) spectra (1). For instance, the MIR spectrum of calcium sulfate dihydrate (gypsum) clearly shows rich spectral features (1). The field is so established that entire textbooks have been dedicated to the infrared (IR) spectroscopy of inorganic compounds (1).

Q: How do we strictly define an "inorganic compound" for the purposes of spectroscopy?

A: In short, it’s complicated. Chemistry often involves "fuzzy concepts," a term noted by Nobel Prize winner Roald Hoffmann (1). Although one could argue that organics have carbon and inorganics do not, exceptions like calcium carbonate (an inorganic substance containing carbon) make this definition tricky (1). To resolve this for spectral study, inorganic compounds are usually defined by three properties: they contain no C-H bonds; engage in ionic bonding; and consist of a positively charged metal cation and a negatively charged anion (1).

Q: Why do the peaks in inorganic spectra often appear at lower wavenumbers than organic ones?

A: This observation is primarily because of the high atomic mass of the elements involved. In the equation for peak positions, the reduced mass (M_R) is in the denominator (1). Because most metal atoms in inorganics are heavier than the non-metals (C, H, N, O) found in organics, their stretching and bending vibrations fall at lower wavenumbers or higher frequencies (1). A general rule of thumb is that while organics tend to absorb above 400 cm⁻¹, many inorganic vibrations fall in the far-IR (below 400 cm⁻¹), which is beyond the range of many standard Fourier transform IR (FT-IR) spectrophotometers (1,2).

Q: What are the most useful features to look for in the MIR spectrum of an inorganic?

A: The most diagnostically useful peaks typically come from the vibrations of polyatomic anions, which are negatively charged ions containing more than one atom. These include sulfates (SO₄), carbonates (CO₃), and nitrates (NO₃) (1,2). Additionally, because inorganic bonds are frequently ionic and have large dipole moments, their IR peaks are often intense (1,2). This high intensity also makes overtone and combination bands, which are usually weak in organics, appear with significant intensity in inorganic spectra (1).

Q: How do inorganic polyatomic anions differ from their organic analogs?

A: They differ because of the bonds they can form. In organic molecules, bonds are covalent and usually fit into a clear single or double bond picture (2). In contrast, the bonds in inorganic polyatomic anions are equivalent, with a bond order often falling between one and two (2). For example, in an inorganic carbonate, the carbon-oxygen bond order is approximately 1.5, resulting in a single stretching peak around 1450 cm⁻¹ rather than separate C=O and C−O peaks (2).

Q: Can you distinguish between different families of inorganics, like sulfates and phosphates, if their peaks overlap?

A: Although the stretching vibrations (X−O stretches) of silicates, phosphates, and sulfates often overlap in the 1100 to 1000 cm⁻¹ region because of similar atomic masses, bending vibrations help differentiate between different families of inorganics (3). For example, carbonates, sulfates, and nitrates illustrate this concept. Carbonates are unique with bending vibrations above 860 cm⁻¹; nitrates have a signature bend between 840 and 810 cm⁻¹; sulfates are found between 680 and 610 cm⁻¹; and phosphates fall below 600 cm⁻¹ (3). Silica is distinct because of its low-wavenumber Si−O−Si bend vibration near 450 cm⁻¹ (3).

Q: Why is water so frequently mentioned when discussing inorganic spectra?

A: Many inorganics are hygroscopic, which means that they readily absorb water from the atmosphere (1,3). This water can appear in three forms. The three forms are adsorbed water (a common contaminant), waters of hydration (molecules trapped within the crystalline lattice), and water chemically bonded to metal ions (1). In silicates, atmospheric water even reacts with the surface to form silanols (Si−OH), which produce distinct peaks depending on whether they are "lone" or hydrogen-bonded (3).

Q: Does the physical structure of the compound affect the results?

A: Yes. Inorganics are often crystalline, and different crystalline forms of the same molecule (polymorphs) yield different spectra (1). For example, calcium carbonate exists as calcite (trigonal) and aragonite (rhombohedral); because IR spectroscopy is sensitive to the geometry of molecular packing, these two forms produce clearly different spectra (1,2).

References

1. Smith, B. C. Inorganics I: Introduction. Spectroscopy 2023, 38 (11), 18–21. DOI: 10.56530/spectroscopy.rp3780c4
2. Smith, B. C. Inorganics II: The Spectra. Spectroscopy 2024, 39 (1), 14–17. DOI: 10.56530/spectroscopy.gh3585q6
3. Smith, B. C. Inorganics III: Even More Spectra, and the Grand Finale. Spectroscopy 2024, 39 (3), 11–15. DOI: 10.56530/spectroscopy.hp2485x8