Chemical Shift and Coupling Constant (J value) , 1H-NMR

 

Chemical Shift

Chemical shift is the difference between the signal from the reference molecule and the resonance frequency of spinning protons. The chemical shift refers to the location on the plot where the nuclei absorb.

The term "chemical shift" refers to the shifting in the locations of NMR absorptions (reference and sample) that results from the shielding or deshielding of protons by electrons. The two most popular standards are CDCl3 (deuterochloroform), which has a chemical shift of 77 for 13C NMR and 7.26 for 1H NMR, and TMS (tetramethylsilane, chemical formula (Si(CH3)4)), which has a chemical shift of zero. So, it is necessary to utilize a common reference point because this can have any value.

Chemical shift

Parts per million (ppm), which are independent of spectrometer frequency, are a standard way to express the scale. One of the most important characteristics that can be used to determine molecular structure is nuclear magnetic resonance chemical change.

Different resonance frequencies and energy levels are produced in a magnetic field by atomic nuclei that have magnetic moments (also called nuclear spin). Local magnetic fields produced by electron flow in the molecular orbitals are included in the overall magnetic field that a nucleus experiences. The local geometry (bond lengths, binding partners, and angles between bonds, etc.) and, consequently, the local magnetic field at each nucleus, typically affect the electron allocation of the same type of nucleus (e.g. 13C, 15N & 1H).

Chemical shift Scale

The values of chemical shift usually come out in delta scale (⸹), its value ranges from 0 to 10 ppm. Zero value given to TMS as a reference standard.

The other scale that is used is Tau scale (ᵼ), its value for TMS is 10 and ranges between 0 to 10.

The relationship between these two scales is given as

ᵼ = 10 - ⸹

Chemical shift Values

Chemical Shift Formula

⸹ = ν _sample – ν _reference / ν _reference

⸹ = shift in Hz / frequency of spectrometer in MHz

The chemical shift value is calculated by using the above formula, sigma symbol (⸹) shows the value of chemical shift and ν is the frequency. First, we find the difference between the frequency of sample and reference and then divide this obtained value by the frequency of reference. Usually, we take TMS as a reference substance for finding chemical shift value. As we also know the signal for TMS in NMR Spectrum always occurs at zero on the scale so, we can simply say that the sigma value is the value of our sample. Also, the frequency of reference is actually refers to the frequency of the spectrometer because TMS is used as a reference it has highly shielded protons. By using the second formula we easily find the value of chemical shift. A shift in hertz means how much a proto in our sample is shifted from the reference substance that is TMS.

The values in the numerator are taken in hertz and in denominator values are shown in megahertz. Thus the obtained value will be in ppm, which is normally shown in the graph. As an example, if the NMR signal appears at 400 Hz then the reference signal will be 400 MHz.

⸹ = 400 Hz / 400 MHz

= 1 Hz / 1 * 10 ⁶ Hz

= 1 * 10 ^-6

= 1 ppm

Coupling Constant or J Value

The coupling constant, represented by the letter J, is a measure of the strength of the splitting effect. It is the distance between the peaks in a certain multiplet. Simply speaking, if we have doublet signals it means that signals are shown by two protons having the same chemical environment, it also described this signal consists of two closely spaced peaks when these peaks are split then the distance between the center of these two peaks gives a coupling constant value. The coupling means the joining of two protons and their peaks difference gives a constant value, which is known as the coupling constant.  J numerical value is given in cycles per second or Hertz. See the figure below,

 

Coupling Constant or J Value

The value of J, in contrast to chemical shifts, is completely dependent on the molecule structure and is independent of externally applied magnetic field strength.

Method for calculating Coupling Constant

The coupling constant is caused by the splitting of one proton that has the same value as the coupling constant caused by the splitting of the second proton for a pair of protons that are mutually connected. In other words, protons that are mutually connected exhibit the same degree of signal splitting.

The size of the coupling constant is generally influenced by the quantity and type of chemical connections that are present between the protons as well as their spatial relationships. For instance, protons with gauche conformation have a J value near 2-4 Hz whereas protons with anti-conformation have a J value close to 5-12 Hz in the case of freely rotating groups.

(1) Based on the spatial placements and overall structure of the molecules, J varies from 2-18 Hz for protons connected to nearby carbon atoms (vicinal protons).

(2) Based on the bond angle and general structure of the molecule, the values of J for protons bound to the same carbon atom (also known as germinal protons) range from 0 to 20 Hz.

(3) For ordinary molecules coupling constant ranges from 0-18 Hz.

The coupling constant for a doublet is the difference between its two peaks in the simplest situation. J is measured in Hz, not ppm, which is where the problem lies. The peaks from ppm must first be converted to hertz.

ppm = Hz / MHz

Hz = ppm * MHz

 Let's say there are two peaks, one at 5.250 ppm and the other at 5.237 ppm. Simply multiply these values by the field strength in MHz (400 MHz) to obtain Hz. Our peaks would be at 2100 Hz and 2094.8 Hz if we were using a 400 MHz NMR machine, respectively. So, the difference is represented by the J value. 2100-2094.8 in this instance equals 5.2 Hz. If a proton is split by more than one other proton, especially if the protons are not the same, this may become more challenging.