Ep 04 — How to Read an Infrared Spectrum? Basic Concepts of X-axis, Y-axis, and Peaks

Series: Infrared Spectroscopy Encyclopedia: From Principles to Practice
Chapter 1: Introductory Chapter — The Code of Light
Target Audience: High school students, undergraduate students, beginners in chemistry/materials/pharmacy
Prerequisites: Ep 01 (Infrared Light), Ep 02 (Molecular Vibrations), Ep 03 (Dipole Moment Change Rule)
Reading Time: About 18 minutes

Introduction: Facing an "Upside-Down" Graph

If you've ever picked up an infrared spectrum for the first time, you were probably confused: the peaks point downward, the numbers on the x-axis decrease from left to right, and the y-axis sometimes says "%T" and sometimes "Absorbance" — all contrary to the UV-Vis absorption spectra we are familiar with [1][2].

Don't worry; this "upside-down" plotting is not arbitrary. It has both historical origins and physical meaning. Today, we will thoroughly explain the x-axis, y-axis, and peaks of an IR spectrum, so that you can confidently "read" information from any spectrum.

📷 Figure 1: A typical infrared spectrum (Ethanol)
Source: NIST Chemistry WebBook (Ethanol gas-phase IR) [1]
https://webbook.nist.gov/cgi/…

1. X-Axis: Wavenumber (cm⁻¹) — The "Common Currency" of Spectroscopists

1.1 Why Use Wavenumber Instead of Wavelength?

The x-axis of IR spectra almost always uses wavenumber (symbol ṽ) with units of cm⁻¹ (number of waves per centimeter) [1][3]. The relationship between wavenumber and wavelength is:

$$\tilde{\nu} = \frac{1}{\lambda}$$

where λ is in centimeters. If wavelength is in micrometers (μm), the more practical conversion formula is [3]:

$$\tilde{\nu}\,(\text{cm}^{-1}) = \frac{10^4}{\lambda\,(\mu\text{m})}$$

For example:

• Wavelength 2.5 μm → wavenumber 4000 cm⁻¹
• Wavelength 25 μm → wavenumber 400 cm⁻¹

Why do spectroscopists prefer wavenumber? Because wavenumber is directly proportional to the energy and frequency of photons [1][3]:

$$E = hc\tilde{\nu}$$

This means: larger wavenumber → higher energy → higher frequency. Using wavenumber as the x-axis is equivalent to using energy as the x-axis, making the physical meaning clear. In contrast, wavelength is inversely proportional to energy, making it less intuitive for plotting [3].

💡 Memory Aid: Wavenumber = 1/wavelength. Large wavenumber = short wavelength = high frequency = high energy.

1.2 The "Main Battlefield" of Mid-IR: 4000–400 cm⁻¹

The mid-IR region covers 4000–400 cm⁻¹ (corresponding to wavelengths 2.5–25 μm), which is the "active range" for most organic molecular vibrations and the standard measurement interval for commercial FTIR instruments [1][3].

Table 1: Subdivisions of the three IR regions (Data source: LibreTexts [3], Bruker NIR tutorial [4])

1.3 Why Is the X-Axis Arranged from High to Low?

This is one of the most confusing aspects of IR spectra for beginners: the x-axis goes from 4000 to 400 cm⁻¹ from left to right, with numbers decreasing [1][5].

This convention has an interesting history. A widespread discussion on ResearchGate [5] was summarized by Lisa V. Brown of 3M Company:

• 1892: Willem Henri Julius used a bolometer to plot spectra, with the x-axis being the angular displacement of a prism, not wavelength or wavenumber [5].
• 1905–1908: IR spectroscopy pioneer William Coblentz, in his multi-volume work Investigations of Infra-Red Spectra, plotted wavelength increasing from left to right (in micrometers) [5].
• Mid-20th century: With the popularization of grating spectrometers, the scientific community began using wavenumber instead of wavelength. To keep the "red end" (low energy, long wavelength) on the right — consistent with the visual convention of visible spectra ("red to the right") — people arranged wavenumber from high to low, so that low wavenumbers (corresponding to long wavelengths, the red direction) fall on the right [5].

"It is recommended that both infrared and ultraviolet spectra be plotted in cm⁻¹ with wavenumber decreasing from left to right (note the mnemonic 'red to the right', derived from the visible region)."
— IUPAC recommendation, quoted from ResearchGate discussion [5]

Thus, "from high to low" is a human convention intended to visually align IR spectra with visible spectra, keeping "red on the right" [5]. Today, all commercial software (OMNIC, OPUS, Spectrum) and journals follow this arrangement [1][5].

📷 Figure 2: Comparison of Coblentz's 1905 original spectrum (wavelength increasing from left to right) with a modern FTIR spectrum (wavenumber from high to low)
Source: ResearchGate historical discussion thread [5]
https://www.researchgate.net/…

2. Y-Axis: Transmittance (%T) vs. Absorbance (A)

2.1 Transmittance (%T)

Transmittance is defined as the ratio of light intensity detected through the sample (I) to the incident light intensity (I₀), expressed as a percentage [1][2]:

$$\%T = \frac{I}{I_0} \times 100\%$$

• %T = 100%: Light passes through completely, no absorption (baseline at the top)
• %T = 0%: Light is completely absorbed (peak bottom)

In transmittance spectra, absorption peaks point downward ("upside-down"), which is the most prominent visual feature of IR spectra [1][2].

2.2 Absorbance (A)

Absorbance is the negative logarithm of transmittance [1][2]:

$$A = -\log_{10}(T) = \log_{10}\left(\frac{I_0}{I}\right)$$

Absorbance is proportional to sample concentration c and path length b (Beer-Lambert law) [1]:

$$A = \varepsilon \cdot b \cdot c$$

where ε is the molar absorptivity. In absorbance spectra, peaks point upward, consistent with UV-Vis spectroscopy conventions [1].

2.3 Conversion Between the Two

Table 2: Conversion between transmittance and absorbance (Data source: LibreTexts [1], Small Woodworm Forum discussion [2])

A key fact: The same spectrum, whether plotted as %T or A, has identical peak positions, shapes, and intensities, except that peaks and valleys are inverted [2]. A succinct summary from the Small Woodworm Forum [2]:

"For infrared spectra with transmittance as the ordinate and with absorbance as the ordinate, the peak positions, intensities, and shapes are exactly the same, except that peaks and valleys are inverted! However, the ordinate of IR spectra is usually transmittance."
— Small Woodworm Forum reply by zhbsky [2]

2.4 Why Is Transmittance Traditionally Used in IR?

LibreTexts explains it straightforwardly [1]:

Unlike UV/Vis absorption spectra, the vertical axis of infrared spectra is displayed as percent transmittance (%T) rather than absorbance, reflecting that infrared spectroscopy is mainly used for qualitative analysis rather than quantitative analysis; in quantitative analysis, Beer’s law makes absorbance a more useful quantity.

Early infrared instruments (prism/grating type) directly measure the intensity of light transmitted through the sample, transmittance is the instrument's raw reading, no additional logarithmic conversion is needed for display, so it became the historical default [1][2].

2.5 Modern Trends: Absorbance Increasingly Favored

Nowadays, more and more journals and quantitative analysis require the use of absorbance representation, because [1][2]:

1. Absorbance is linearly related to concentration (Beer’s law), facilitating quantification
2. Absorbance is additive: the spectrum of a mixture equals the sum of spectra of its components (not true for transmittance)
3. Baseline correction, derivative processing, etc. are more reasonable in absorbance coordinates
4. Spectral subtraction (difference spectra) must be performed in absorbance coordinates

📷 Figure 3: Comparison of the same CCl₄ spectrum in transmittance (left) and absorbance (right) coordinates
Source: Socratic.org discussion post [6]
https://vespr.org/questions/w…

III. Peak Description Language: Three Elements of Position, Intensity, Shape

When you obtain a spectrum, describe each peak from three dimensions: position (where), intensity (how strong), shape (what it looks like) [7][8]. These are the "three elements" of infrared spectroscopy.

3.1 Peak Position – "Where"

The horizontal coordinate position (wavenumber cm⁻¹) tells us what bond is vibrating [7][8]. This is the most core information in infrared spectroscopy.

Peak position is determined by two physical quantities (Hooke’s law, see Ep 02) [8]:

$$\tilde{\nu} = \frac{1}{2\pi c}\sqrt{\frac{k}{\mu}}$$

• k: bond force constant (stronger bond, larger k, higher frequency)
• μ: reduced mass (lighter atoms, smaller μ, higher frequency)

Empirical rules [8]:

• Bond strength: triple bond > double bond > single bond → frequency: C≡C (~2100) > C=C (~1650) > C-C (~1000)
• Atomic mass: H lightest → X-H stretching peaks in the highest frequency region (2800–3600 cm⁻¹)

💡 ftir.fun tool tip: Quickly find which functional groups correspond to a certain peak? Visit ftir.fun peak page, for example https://ftir.fun/ir/peak/1700 will list all known functional groups near 1700 cm⁻¹ and their literature sources. Complete peak-functional group reference table at ftir.fun frequency table.

3.2 Peak Intensity – "How Strong"

Peak intensity is described using three levels: strong (S), medium (M), weak (W) [7][8]. Note: on a transmittance plot, a deeper peak (lower %T) indicates stronger absorption [7].

What determines intensity? LibreTexts and Cabrillo College lecture notes give two key factors [7][8]:

1. Bond polarity (magnitude of dipole moment change)
   Larger dipole moment change → higher transition probability → stronger peak [7][8].
   Example: C=O (strong polarity) → strong peak; C=C (weak polarity) → weak peak [8].

2. Number of bonds participating in the vibration
   More similar bonds → stronger peak [8].
   Example: CH₂ stretching peaks in long-chain alkanes are stronger than in short-chain.

"The intensity of an absorption band depends on the polarity of the bond; more polar bonds show stronger absorption bands. Intensity also depends on the number of bonds participating in the absorption; the more bonds, the higher the intensity."
—— LibreTexts Organic Chemistry Textbook [8]

Empirical intensity grading (based on molar absorptivity ε, see Ep 20 Quantitative Analysis):

Table 3: Empirical grading of peak intensity (references: Socrates Infrared and Raman Characteristic Group Frequencies [9], Cabrillo College lecture [8])

3.3 Peak Shape – "What It Looks Like"

Peak shape is mainly described as broad (b) vs sharp (sh) [7][8].

Most common cause of broad peaks: hydrogen bonding [7][8].

Cabrillo College lecture provides an excellent comparison [8]:

Table 4: Relationship between hydrogen bonding degree and peak shape (data source: Cabrillo College IR lecture [8])

Why does hydrogen bonding broaden peaks? Because hydrogen bonding forms a continuously varying network, each molecule's O-H bond experiences slightly different hydrogen bonding environments, causing absorption frequencies to be distributed over a range, which superimposes to form a broad peak [7][8].

Other factors causing peak shape changes:

• Fermi resonance: when a fundamental frequency is close in energy and has the same symmetry as an overtone/combination frequency, coupling occurs, causing one peak to split into two [10][11]. The doublet near 1388 cm⁻¹ in CO₂ is a classic example of Fermi resonance [11].
• Rotational fine structure: gas-phase molecules show dense rotation-vibration coupling peaks, appearing like a "comb"; liquid and solid phases, where molecules cannot rotate freely, usually display smooth broad peaks [1].

📷 Figure 4: Comparison of O-H (alcohol), N-H (amine), O-H (carboxylic acid) peak shapes
Source: Cabrillo College IR lecture [8]
https://cabrillo.instructure.…

IV. Functional Group Region and Fingerprint Region: A "Map" of 4000–400 cm⁻¹

The full infrared spectrum can be divided into two major regions [12][13]:

Table 5: Comparison of functional group region and fingerprint region (data sources: LibreTexts Organic Chemistry [12], thinka.ai A-Level [13])

4.1 Functional Group Region (4000–1500 cm⁻¹)

The functional group region can be further divided into three sub-regions, each corresponding to a class of vibrations [12][13][14]:

4000 ─────── 2500 ─────── 2000 ─────── 1500 ───────── 400 cm⁻¹
│   X-H stretching │ Triple bond │ Double bond │   Fingerprint      │
│   O-H,N-H,C-H    │ C≡C,C≡N     │ C=O,C=C     │   complex skeleton │
└──────────────────┴─────────────┴─────────────┴────────────────────┘

Functional group region (4000–1500) Fingerprint region (1500–400)

**① X-H Stretching Region (2500–4000 cm⁻¹)** [12][14]
- O-H stretch: 3200–3600 cm⁻¹ (alcohol/phenol, broad); 2500–3300 cm⁻¹ (carboxylic acid, very broad)
- N-H stretch: 3300–3500 cm⁻¹ (amine/amide, medium sharp)
- C-H stretch: 2800–3100 cm⁻¹
  - sp³ C-H (alkane): 2850–2960 cm⁻¹
  - sp² C-H (alkene/arene): 3000–3100 cm⁻¹
  - sp C-H (alkyne): ~3300 cm⁻¹
- A **key criterion**: 3000 cm⁻¹ is the boundary between saturated and unsaturated C-H [14]——slightly above 3000 is mostly unsaturated, slightly below 3000 is mostly saturated.

> 🔗 **In-depth reference**: Detailed peak positions, literature sources, and assignment reasoning for O-H functional groups can be found at [ftir.fun hydroxyl functional group page](https://ftir.fun/ir/group/hydroxyl); N-H at [ftir.fun amine functional group page](https://ftir.fun/ir/group/amine).

**② Triple Bond Region (2000–2500 cm⁻¹)** [12][14]
- C≡C alkyne: 2100–2260 cm⁻¹ (weak)
- C≡N cyano: 2220–2260 cm⁻¹ (medium–strong)

**③ Double Bond Region (1500–2000 cm⁻¹)** [12][14]
- C=O carbonyl: 1650–1750 cm⁻¹ (**strong**, one of the most important peaks in infrared spectroscopy)
  - Aldehyde/ketone/acid/ester: 1700–1750 cm⁻¹
  - Amide (C=O lowered by conjugation): 1650–1700 cm⁻¹
- C=C alkene: 1620–1680 cm⁻¹ (weak–medium)
- C=N: 1610–1680 cm⁻¹
- Aromatic ring skeletal vibrations: ~1600, ~1500 cm⁻¹

> 🔗 **In-depth reference**: Detailed peak position differences for carbonyl subclasses (aldehyde/ketone/acid/ester/amide) and literature basis can be found at [ftir.fun carbonyl functional group page](https://ftir.fun/ir/group/carbonyl).

### 4.2 Fingerprint Region (1500–400 cm⁻¹)

The fingerprint region is the **"ID card" of molecules** [12][13]:

> "Just as every person has a unique fingerprint, every molecule (even isomers) has a unique combination of peak shapes in the fingerprint region."  
> —— thinka.ai Cambridge A-Level Chemistry [13]

The fingerprint region includes [12]:
- C-O, C-C, C-N single bond stretching
- C-H bending vibrations
- Benzene ring out-of-plane bending (700–900 cm⁻¹, for determining substitution patterns)
- Long-chain CH₂ in-plane rocking (~720 cm⁻¹)

**Strategy for analyzing the fingerprint region**: Typically **do not assign peaks individually**, but compare the overall peak pattern with a standard spectral library [12][13]. This content will be covered in depth in Ep 07 (Fingerprint Region Special Topic).

> ⚠️ **A-Level/College Entrance Exam Tip**: The Cambridge A-Level Chemistry syllabus explicitly states that the fingerprint region is "too complex to be used for simple functional group identification," and exam focus is above 1500 cm⁻¹ [13].

---

## 5. Systematic Reading Steps: 5-Zone Analysis

When facing a complete infrared spectrum, beginners often "don't know where to start." The Cabrillo College lecture notes propose a very practical **5-Zone Analysis** [8], dividing 4000–400 cm⁻¹ into five consecutive zones, **scanning from high wavenumber to low wavenumber**:

### Step 1: Look at 3700–3000 cm⁻¹ (Zone 1: X-H Region)
- Broad peak → O-H (alcohol/water/carboxylic acid)
- Sharp peak → N-H (amine/amide) or ≡C-H (alkyne)
- Peak around ~3050 → aromatic or unsaturated C-H
- No peak → no O-H, N-H

### Step 2: Look at 3000–2800 cm⁻¹ (Zone 2: Saturated C-H Region)
- Peak present → alkyl groups present (almost all organic compounds have them)
- **Check the 3000 boundary**: above 3000 = unsaturated C-H; below 3000 = saturated C-H [14]

### Step 3: Look at 2800–2000 cm⁻¹ (Zone 3: Triple Bond Region)
- Peak at ~2250 → C≡N (cyano)
- Peak at ~2100 → C≡C (alkyne, usually weak)
- Doublet at ~2300 → **CO₂ interference peak** (see Section 6)

### Step 4: Look at 2000–1500 cm⁻¹ (Zone 4: Double Bond Region)
- Strong peak at 1650–1750 → **C=O carbonyl** (most important diagnostic peak)
- Weak peak at 1620–1680 → C=C alkene
- ~1600, ~1500 → aromatic ring skeleton

### Step 5: Look at 1500–400 cm⁻¹ (Zone 5: Fingerprint Region)
- Do not assign peaks individually
- Compare with standard library to confirm molecular identity
- Note a few features: ~720 (long-chain CH₂), 700–900 (aromatic substitution patterns)

> 📷 **Figure 5**: Visual map of the 5-Zone Analysis  
> Source: Cabrillo College IR Lecture Notes [8]  
> https://cabrillo.instructure.com/courses/20923/pages/2e1-theory-ir-spectrscopy

**HCFTIR's practical guide** adds an important recommendation [15]:

> "First identify main functional groups in the functional group region, then use the fingerprint region to confirm molecular identity—this is a 'hypothesis-verification' process."

---

## 6. Beware of "False Peaks": Atmospheric Interference and Overtone Bands

When reading spectra, one must be aware of several types of peaks **that do not come from the sample** [12][15]:

### 6.1 Atmospheric CO₂ Peak (~2350 cm⁻¹)

CO₂ in air has a strong absorption doublet (asymmetric stretch) near **2349 cm⁻¹** [12][15]. If your instrument is not adequately purged or a background has not been subtracted, this peak may appear in the sample spectrum.

**Identification key**: **Paired sharp peaks** near 2349 cm⁻¹, and if the sample itself does not contain carbonate or carbonyl, it is almost certainly CO₂ interference [15].

### 6.2 Water Vapor Peaks (Multiple Positions)

Water vapor in air exhibits **fine structure (rotational lines)** near **3400 cm⁻¹** (O-H stretch) and **1640 cm⁻¹** (H-O-H bend), appearing like "comb teeth" [1][15].

**Elimination methods**:
- Thoroughly purge the instrument optical path (dry air or nitrogen) before measurement
- Collect background spectrum and immediately measure sample
- Use atmospheric compensation software (e.g., VaporFit [16])

### 6.3 Overtone Bands and Combination Bands

Apart from fundamental vibrations (v=0→1), molecules can also produce [10][17]:

- **Overtone bands**: transitions v=0→2, v=0→3, etc., with frequencies approximately 2×, 3× the fundamental, but very weak in intensity
- **Combination bands**: sum (ν₁+ν₂) or difference (ν₁-ν₂) of two or more fundamentals

These peaks usually appear in the **functional group region**, are weak, and can be easily mistaken for fundamentals [10][17]. For example:
- Aldehyde C-H stretches appear as doublets at ~2720 and ~2820 cm⁻¹, one due to Fermi resonance [17]
- Aromatic compounds exhibit a series of weak overtone bands at 1700–2000 cm⁻¹, which can be used to determine substitution patterns

### 6.4 Fermi Resonance

When a fundamental vibration is **close in energy and has the same symmetry** as an overtone or combination band, they can couple, leading to [10][11]:

1. An original peak **splits into two**
2. The normally weak overtone **gains intensity** ("borrowing" intensity from the fundamental)

Classic example: **CO₂'s symmetric stretch ν₁ (~1330 cm⁻¹) and its bending overtone 2ν₂ (~1334 cm⁻¹) undergo Fermi resonance**, producing two peaks at 1388 and 1286 cm⁻¹ [11]. Interestingly, this seemingly "accidental" quantum resonance is a key reason why CO₂ is a strong greenhouse gas [11].

> "The accidental resonance ν₁ ≈ 2ν₂ in CO₂ makes the otherwise infrared-inactive symmetric stretch observable—this 'accidental' quantum effect profoundly influences Earth's climate."  
> —— Wordsworth et al., arXiv:2401.15177 [11]

---

## 7. Standard Spectrum Example: Polystyrene Film

Almost every infrared laboratory has a "standard spectrum" – **a polystyrene (PS) film**. It is used by NIST as a standard reference material (SRM 1921a) for calibrating the wavenumber axis of instruments [18].

### 7.1 NIST SRM 1921a Certified Peak Positions

NIST has certified 13 absorption band positions for the polystyrene film, with uncertainties better than 1 cm⁻¹ [18][19]:

| Peak No. | Wavenumber (cm⁻¹, vacuum) | Intensity | Assignment |
|--------|-------------------|------|------|
| 1 | 3027.1 | m | Aromatic =C-H stretch |
| 2 | 2924 | m | Saturated CH₂ asymmetric stretch |
| 3 | 2850.7 | m | Saturated CH₂ symmetric stretch |
| 4 | 1944.0 | w | Aromatic combination/overtones |
| 5 | 1871.0 | w | Aromatic combination/overtones |
| 6 | 1801.6 | w | Aromatic combination/overtones |
| 7 | **1601.4** | **s** | **Aromatic C=C skeletal stretch** |
| 8 | 1583.1 | m | Aromatic C=C skeletal stretch |
| 9 | **1495** | **s** | **Aromatic C=C skeletal stretch** |
| 10 | 1454 | m | CH₂ bending |
| 11 | 1154.3 | m | C-H in-plane bending |
| 12 | **1028.0** | m | **Aromatic C-H in-plane bending** |
| 13 | **906.7** | m | **Aromatic C-H out-of-plane bending** |
| 14 | 752 | s | Monosubstituted benzene ring C-H out-of-plane bending |
| 15 | **698.9** | **s** | **Monosubstituted benzene ring C-H out-of-plane bending** |

> Table 6: NIST SRM 1921a polystyrene certified peak positions (Data sources: NIST certificate [18], IUPAC calibration table [19])

> 📷 **Figure 6**: NIST SRM 1921a polystyrene film standard spectrum, arrows indicate the 13 certified bands  
> Source: NIST SRM 1921a certificate [18]  
> https://tsapps.nist.gov/srmext/certificates/archives/1921a.pdf

### 7.2 Educational Value of the Polystyrene Spectrum

Polystyrene is considered the "gold standard" for calibration because [18][19]:

1. **Stable peak positions**: For a 38 μm thick film, the peak positions are highly reproducible.
2. **Wide coverage**: From 540 to 3125 cm⁻¹, covering most of the mid-infrared region.
3. **Easy to store**: Solid film, no special storage conditions required.
4. **Distinctive features**: The monosubstituted benzene ring has two strong peaks near 700 and 760 cm⁻¹, a classic example for determining substitution type.

> 💡 **Practical tip**: After each startup or accessory change, it is recommended to run a spectrum of the polystyrene film and check whether the positions of the peaks at 1601, 1583, 1493, 906.7, 752, and 698.9 cm⁻¹ are within the allowable error (±0.5–1 cm⁻¹) [18].

---

## 8. Quick Assessment of Spectrum Quality

Before starting the analysis, spend 30 seconds assessing the spectrum quality to avoid a lot of subsequent misinterpretation [15]:

| Check Item | Normal | Abnormal Cause |
|--------|------|---------|
| Baseline level | Transmittance spectrum baseline at 90–100% | Baseline too low → Sample too thick or severe scattering |
| Strongest peak depth | Not lower than 5% T | Hits 0% → Total absorption (flat-top), unable to quantify |
| Baseline tilt | Nearly horizontal | Severe tilt → Scattering or KBr moisture absorption |
| Signal-to-noise ratio | Smooth without spikes | Obvious jaggedness → Insufficient scans or too little sample |
| Water peak interference | No obvious water peaks at 3400/1640 | Comb-shaped peaks → Insufficient purge |

> Table 7: Quick checklist for spectrum quality assessment (Reference: HCFTIR guide [15])

---

## 9. Section Summary: "Five-Step Method" to Read an Infrared Spectrum

Condense today's content into an actionable workflow:

① Look at the horizontal axis: wavenumber cm⁻¹, from high (4000) to low (400)
② Look at the vertical axis: %T (peaks down) or A (peaks up)
③ Scan by regions: X-H → Triple bond → Double bond → Fingerprint
④ Describe each peak: position + intensity (S/M/W) + shape (sharp/broad)
⑤ Eliminate interferences: CO₂ (2350), water vapor (3400/1640), overtones/combination bands
```

Mastering this method, you can "read" 90% of routine infrared spectra. In the next episode (Ep 05), we will enter the topic of functional groups and characteristic absorption frequencies, starting from the most important peak in IR spectra – the C=O carbonyl group – and learn the characteristic frequencies of each functional group one by one.

Summary of This Episode

Questions for Thought

1. Why is the horizontal axis of IR spectra often plotted from high to low wavenumber?
2. When the same spectrum is displayed in %T and absorbance, how do the intensity relationships correspond?
3. A broad peak appears near 3400 cm⁻¹; besides alcohol O–H, what common interference should be suspected?
4. Visit ftir.fun peak page 1700 and list at least two possible assignments near that wavenumber.

References

[1] NIST Chemistry WebBook. Ethanol IR Spectrum. https://webbook.nist.gov/cgi/…

[2] Xiaomachong Forum. Why are the vertical coordinates of IR spectra different. 2009. https://muchong.com/t-1550007…

[3] LibreTexts. 6.11.1: Theory of Infrared Absorption Spectrometry. https://chem.libretexts.org/C…

[4] Bruker. What is FT-NIR Spectroscopy? https://www.bruker.com/en/pro…

[5] Brown LV. Does anyone know why FTIR spectra are typically plotted from high to low wavenumbers? ResearchGate, 2017. https://www.researchgate.net/…

[6] Truong-Son N. Why is the IR spectrum inverted? Socratic.org, 2016. https://vespr.org/questions/w…

[7] LibreTexts. 6.3: IR Spectrum and Characteristic Absorption Bands. https://chem.libretexts.org/@…

[8] Cabrillo College. 2E1 Theory: IR Spectroscopy. https://cabrillo.instructure.…

[9] Socrates G. Infrared and Raman Characteristic Group Frequencies. 3rd ed. Wiley, 2004.

[10] LibreTexts. 3.7: Non-Fundamental Transitions - Hot Bands, Combination Bands, and Fermi Resonances. UC Davis CHE 205. https://chem.libretexts.org/C…

[11] Wordsworth R, Seeley JT, Shine KP. Fermi Resonance and the Quantum Mechanical Basis of Global Warming. arXiv:2401.15177, 2024. https://arxiv.org/html/2401.1…

[12] LibreTexts. 15.3: Interpreting IR Spectra. Organic Chemistry. https://chem.libretexts.org/@…

[13] thinka.ai. Cambridge A-Level Chemistry (9701): Infrared spectroscopy. https://www.thinka.ai/en/camb…

[14] Liu X. 6.3 IR Spectrum and Characteristic Absorption Bands. LibreTexts. https://chem.libretexts.org/@…

[15] HCFTIR. A Practical 5-Step Guide: How to Read FTIR Spectra for Accurate Results in 2025. 2025. https://www.hcftir.com/a-prac…

[16] Bruzdza P. VaporFit: Automatic atmospheric subtraction. Phys. Chem. Chem. Phys. 2025, 27. DOI:10.1039/D5CP01007A. GitHub: https://github.com/piobruzd/V…

[17] LibreTexts. Combination Bands, Overtones and Fermi Resonances. https://chem.libretexts.org/@…

[18] NIST. Certificate of Analysis, SRM 1921a: Infrared Transmission Wavelength Standard (Polystyrene Film). 2014. https://tsapps.nist.gov/srmex…

[19] IUPAC. Tables of Wavenumbers for the Calibration of IR Spectrometers. Butterworths, London, 1961. (Reprinted from Minnesota State University Experiment Handout [20])

[20] Minnesota State University. Infra-Red Spectroscopy of Solids and Solutions, Experiment. CHEM 380 Lab. https://web.mnstate.edu/maras…

Next Episode Preview: Ep 05 — Functional Groups and Characteristic Absorption Frequencies (Part 1): C=O, O-H, N-H. We will start with the most "eye-catching" carbonyl peak in infrared spectra and analyze the characteristic frequencies, peak shape rules, and influencing factors of common functional groups one by one.