What is spectroscopic data?

The interaction of electromagnetic radiation with the structure of molecules in the sample and their study is referred spectroscopy. Several spectroscopic methods are used to acquire sample information, and these methods are proved to be accurate, fast, and non-destructive. The information obtained about the content and composition of molecules from these spectroscopic methods is called spectroscopic data.

What are the types of spectroscopic methods?

Various techniques are used to produce spectroscopic data. The obtained data is predicted and analyzed by referring to the literature values. It helps to identify the matter by examining its composition. Spectroscopic methods operate on the principle of spectroscopy to examine the structure in the data.

Based on the interaction of electromagnetic radiation and sample content, the spectroscopic methods are classified into three types: Absorption spectroscopy, emission spectroscopy, and scattering spectroscopy.

Absorption spectroscopy

The absorbed energy of radiation is measured when the given samples undergo interaction with the electromagnetic radiation. Wavelength or frequency acts like a function, and an absorption spectrum in dark lines depicts the absorbed energy. Ultraviolet (UV) visible spectroscopy, infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and atomic absorption (AA) spectroscopy are examples of absorption spectroscopy.

Emission spectroscopy

The atoms or molecules radiate or emit photons of specific wavelengths while undergoing an electronic transition from a lower to a higher energy level. The emission spectrum depicts the measurement of emitted photons in colored lines. Atomic emission (AE) spectroscopy, fluorimetry, and flame photometry are examples of emission spectroscopy.

Scattering spectroscopy

The atoms or molecules in the sample scatter light in specific wavelengths and angles. This amount of light is measured and obtained as a scattering spectrum. Raman spectroscopy is an example of scattering spectroscopy.

Interpretation of UV spectra

When the molecule interacts with ultraviolet or visible light, its spectroscopic data are obtained through UV visible spectroscopy. The peaks in UV visible spectra are broad. The spectrum is obtained by plotting absorbance in the y-axis and wavelength in nanometers in the x-axis.

The wavelength of the greatest intensity is depicted by λmax. This spectrum rectifies the functional groups present in a compound since every functional group has its specific value of maximal intensity εmax and wavelength λmax . Several articles depict the values of each functional group in tabular form.

Woodward and later Fieser formulated the rules for calculating the total absorption position by the wavelength values of principal and additional functional groups attached to it.

For example, naphthalene has a λmax value of 275 nm, and its approximate intensity was found to be 5,600. The spectral data can evaluate the sample’s concentration. And it can plot a graph of absorption intensity on the y-axis and known concentration on the x-axis.

An illustration of the ultraviolet spectrum of naphthalene, wherein absorbance is plotted on the y-axis and wavelength in nanometers on the x-axis. The maximum wavelength of naphthalene is 275 nm.
The ultraviolet spectrum of naphthalene

Interpretation of IR spectra

The IR spectrum represents the percent transmittance (%T) of incident light and its wavelength (in microns) or its frequency (in wavenumber). The molecule experiences a uniform motion, and the motion causes the deformation of bonds, angles, and lengths. The alteration of the distance of atoms and bond angles are shown by stretching and bending vibrations, respectively. Each stretching vibration, such as C-H, N-H, O-H, C-C, C-N, C-O, C=C, C=O, C≡C, and C≡N, lies in a specific absorption range expressed in centimeters inverse. Their absorption ranges are mentioned in articles and papers upon prolonged study.

For example, the IR spectrum of benzonitrile demonstrates the functional groups present in it by observing the value of its wavenumber. A sharp line appeared at 2,200, represents a triple bond between carbon and nitrogen in the structure. It has an aromatic ring, which is proved by the absorption at 1,600  and 1,450. The C-H stretching vibration in the benzene ring is represented at 3060 cm.

An illustration of the IR spectrum of benzonitrile, where percent transmittance is plotted on the y-axis and wavenumber is plotted on the x-axis.
The infrared spectrum of Benzonitrile

Interpretation of NMR spectra

The measurement of nuclear spin transition by a nucleus is done by NMR spectroscopy. The nuclei that have an odd value of atomic number or mass number display the NMR spectrum. The chemical shift displays NMR spectra expressed in parts per million (ppm), and at the extreme right, a peak of tetramethylsilane (TMS) is placed, which acts as a reference.

Studies and articles accept TMS as an internal standard because of its inert nature and a distinct signal. The TMS peak and the protons present in the sample create a difference in frequency value. This difference is used for the prediction of protons’ position and analyzing their chemical shifts.

For example, in 2,2-dimethylpropane and TMS, there are a total of 12 hydrogen atoms in each structure, and all the hydrogen atoms are chemically equivalent, so only one peak is obtained. Inequivalent protons will display separate peaks lying closer (upfield) or far away (downfield) from TMS.

An illustration of the NMR spectrum of 2,2-dimethylpropane in which chemical shift expressed in parts per million (ppm) is on the x-axis. Tetramethylsilane is the reference used, and its peak appears at the extreme right side of the spectra.
Nuclear magnetic resonance spectrum of 2,2-dimethylpropane

Mass spectroscopy and mass spectra

Mass spectroscopy is another spectroscopic method by which the molecular weight or mass of sample content is evaluated without the involvement of any electromagnetic radiation. It also creates the structural aspect of the compound and detects any halogen atom in it.

The mass spectra are represented by a compound’s unit mass m/z value on the x-axis and the relative abundance of ions in percentage on the y-axis. The values of relative abundance (%) of carbon or halogens are found in various articles. The sample molecule (M) gets hit by an electron of high energy, which liberates a molecular ion, i.e., positively charged.

For example, 2,2-dimethylpropane has a molecular weight of 72, and its molecular ion is not attained because its fragmentation occurs very readily.

An illustration of a mass spectrum of 2,2-dimethylpropane where the molecular mass is plotted on the x-axis and relative abundance expressed in percentage is on the y axis.
Mass spectrum of 2,2-dimethylpropane

Common mistakes

Common mistakes encountered while analyzing or predicting spectroscopic data are neglection of calibration of instruments, data handling, and solvent selection.

  • Calibration of instruments is often neglected during the experiment, which leads to inaccurate results and wrong interpretations.
  • Data handling should be efficient considering temperature, signal intensities of equipment, voltage difference, impurities, environmental effect on the sample, etc.
  • The proper solvent selection has to be done so that the component in the solvent does not create any interference with the sample reading.

Context and applications

This topic is significant in the professional exams for both undergraduate and postgraduate courses, especially

Bachelors in Chemistry

Masters in Pure and Applied Chemistry

Ph.D. in Chemistry

Bachelors in Biochemistry

Masters in Biochemistry

Ph.D. in Biochemistry

Spectroscopic methods

Data analysis

Spectral analysis

Detection of compounds

Practice problems

Q1: Who formulated the rules for calculating the maximum absorption spectra of the compound?

(a) Woodward and later by Fieser

(b) Fieser

(c) Fieser and later by Woodward

(d) Woodward

Correct option: (a)

Q2: What is the unit of chemical shift?

(a) Parts per million

(b) Parts per billion

(c) Parts per hundred

(d) Parts per thousand

Correct option: (a)

Q3: Flame photometry is an example of which type of spectroscopy?

(a) Absorption spectroscopy

(b) Emission spectroscopy

(c) Scattering spectroscopy

(d) None of the above

Correct option: (b)

Q4: What is placed on the y-axis in an IR spectrum?

(a) Wavelength

(b) Absorbance

(c) Percent transmittance

(d) Wavenumber

Correct option: (c)

Q5: What is placed on the y-axis in a mass spectrum?

(a) Unit mass

(b) Chemical shift

(c) Absorbance

(d) Relative abundance

Correct option: (d)

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