Spectroscopy is a scientific technique that involves the study of the interaction between matter and electromagnetic radiation (such as light) to gain information about the properties of the matter. It is a powerful tool used in various scientific fields, including chemistry, physics, astronomy, and biology.
The basic principle of spectroscopy is based on the fact that different types of matter absorb, emit, or scatter electromagnetic radiation in unique ways. By analyzing the pattern of interaction between matter and radiation, scientists can obtain valuable information about the composition, structure, and behavior of substances.
Spectroscopy involves measuring the intensity of radiation as a function of wavelength or frequency. The resulting spectrum, which is a plot of the intensity of radiation versus wavelength or frequency, can provide insights into a wide range of properties, such as chemical composition, molecular structure, energy levels, and physical characteristics of the matter under investigation.
Spectroscopy plays a crucial role in scientific research, allowing
scientists to identify substances, monitor chemical reactions,
investigate the properties of materials, study the behavior of
molecules, and contribute to our understanding of the fundamental
principles of the universe.
Various types of spectroscopy techniques exist which are as follows:
(1) Infrared (IR) spectroscopy
(2) Ultraviolet (UV) spectroscopy
(3) Mass spectrometry
(4) X-ray spectroscopy
(5) Nuclear Magnetic Resonance (NMR) spectroscopy
Each technique utilizes different regions of the electromagnetic spectrum and specific interactions between matter and radiation to extract valuable information about the sample being analyzed. Consider that the sample under investigation is the ethyl benzoate compound whose molecular structural information we want to investigate with the different spectroscopy for comparison purpose.
(1) Infrared (IR) spectroscopy
Infrared (IR) spectroscopy is a technique that allows us to gather information about the chemical groups within a molecule, such as COR, COOR, CN, NO2, and more. By analyzing the IR spectrum of ethyl benzoate, we can identify the presence of an ester and a double bond. However, it's important to note that IR spectroscopy does not provide details about where these functional groups are located within the molecule or how they are connected to one another.
(2) Ultraviolet (UV) spectroscopy
UV spectroscopy is primarily employed to detect structures that possess
conjugated systems. If ethyl benzoate's UV spectrum indicates the
presence of a benzene ring in conjugation with another system, it
confirms the existence of such a configuration. Occasionally, the UV and
IR spectroscopic analysis alone may not provide a comprehensive
interpretation of this spectrum.
(3) Mass spectrometry
Mass spectrometry is a technique that provides valuable information
about the molecular weight or formula weight of a compound. For example,
when analyzing ethyl benzoate, the analysis reveals that its formula
weight is 150 atomic mass units (amu). High-resolution mass spectroscopy
(HRMS) can also be employed to determine the empirical formula of the
molecule. In the case of ethyl benzoate, the empirical formula is found
to be C9H10O2. However, there can be numerous constitutional isomers
with the same empirical formula, which makes it challenging to
differentiate between them. While it is possible to detect ethyl
benzoate among these isomers, it becomes exceedingly difficult for
molecules with more complex structures.
(4) X-ray spectroscopy
X-ray spectroscopy is a highly effective approach for determining the
arrangement of atoms within a compound. However, this technique
necessitates the presence of a compound in the form of a single crystal.
Many chemists find this process to be arduous, time-consuming, and
requiring great manual dexterity. When alternative spectroscopic methods
fail to uncover the identity of a compound, X-ray spectroscopy becomes
the preferred choice for determining its structure, including details
such as bond lengths and bond angles. Nonetheless, technological
advancements have now enabled the acquisition of a complete NMR spectrum
in as little as one minute.
(5) Nuclear Magnetic Resonance (NMR) spectroscopy
NMR spectroscopy is a useful technique for identifying and analyzing organic compounds. This extremely important experimental technique is based on magnetic nuclear spin of 1H, 13C, 15N, 19F, 31P, and so forth. Consider 1H and 13C NMR analysis of the ethyl
benzoate compound. 
The above shown Proton and carbon NMR spectra of ethyl benzoate contain more detailed and definitive information as follows.
(a) 1H-NMR provides information about the number of protons in the molecule.
(b) 1H-NMR reveals the existence of an aromatic moiety that is monosubstituted. This is indicative of 5 protons attached to an aromatic ring.
(c) The existence of a methyl group and its attachment to a CH2 group is indicated.
(d) Analysis of the signals provides information about a CH3 group being adjacent to a methylene group that is attached to electronegative atom (oxygen).
(e) 13C NMR provides information about the number of carbons in the molecule.
(f) The 13C NMR analysis indicates the presence of a carbonyl carbon and
four distinct carbon atoms attached to an aromatic ring, specifically a
benzene ring with a single substituent.
Based on the collective analytical data, it is evident that the molecule in question is ethyl benzoate. Therefore, when comparing information gathered from NMR and other analytical techniques, it becomes clear that NMR spectra provides the most detailed and crucial information for identifying a structure. However, it should be noted that NMR alone may not always suffice for complete structural analysis. To achieve a comprehensive understanding of structures, it is necessary to utilize all available spectroscopic methods. When a new compound is synthesized, it is imperative for the researching chemist to possess all spectroscopic data pertaining to that specific chemical compound. While an NMR spectrum offers valuable insights, its usefulness relies on effective interpretation.
