NMR Spectroscopy- Diamagnetic Shielding, Electronegativity, Hybridization Effects

Introduction

Welcome to chemarticle. Today we will discuss chemical environment and chemical shift of NMR Spectroscopy- Anisotropic effect in different molecules like Benzene, Alkyne, Alkene, Aromatics compounds, antiaromatic compounds. Also, we talk about approximate chemical Shift Ranges for selective types of protons. The effect of Shielding effect and De-shielding effect for various molecular structures. Mainly Protons NMR Spectroscopy is used for the determination of molecular structure. In molecules that have protons, they are different chemical environments correspond to their chemical shift are different.

NMR Spectroscopy-Created by: Chemarticle

       

Chemical environment and chemical shift 

In case the resonance frequencies of the proton are the same in a molecule, then NMR spectroscopy does not affect the determination of molecular structure being a chemist. Different types of protons have different types of chemical shifts value, that is not a guess value. They have a proper reason or characteristic chemical shift value. 

Particular protons have a limited range of δ value (ppm), which have proper resonance.  And the chemical shift value for a proton signified the NMR signal, that mains the chemical environment make how many NMR signals are formed for a particular molecule. Similarly use of infrared frequency – the determination of types of bonds or functional group.

Chemical shift list: (0 to 12 ppm)

Chemical equivalence in NMR spectroscopy

All protons are chemically identical environments or the same chemical environment in a molecule, then we have to call chemically equivalent. Correspond to they are chemical shift value is the same. 

For example- tetramethylsilane (TMS), Which has one type of chemical environment protons. Similarly, the Benzene molecule, Cyclopentane, acetone molecule all protons are in the same environment. They are corresponding to one NMR signal type. Or we can say one type of chemical shift value is formed (one type Nuclear Magnetic Resonance) for a particular same chemical environment molecular structure. 

Chemically non-equivalent proton has different types of NMR spectroscopy are observed. Mainly, we have to consider the chemically equivalent proton in component- using symmetry plane. And on the other hand, the non-equivalent proton in the same component- has no symmetry plane/ or we can say that asymmetric compound.

Chemical equivalence in NMR spectroscopy-signal counts -chemarticle: (a) two NMR signal are observe- where two different types of  chemically equivalent protons are observe (b) one NMR signal is observe -all protons are chemically equivalent. (c) Three NMR signal are observe - three different types of chemically equivalent protons are observe. 

We can see that NMR spectroscopy peaks are different in the same molecule. Few protons are chemically equivalent that mains which are also magnetically equivalent. In some instances, protons are chemically equivalent which are not may not be magnetically equivalent.

Read More: NMR spectroscopy- Basic principle, Nuclear spin, Absorption process for a proton

The effect of Local Diamagnetic Shielding

The local Diamagnetic shielding effect has affected some factors which are responsible for increasing or decreasing the chemical shift value to the expected normal value. The chemical shift value depends on some factors, which are decreases or increases the shielding effect respectively. This type of effect is known as the Local Diamagnetic shielding Effect.

The effect of Diamagnetic Shielding is divided into three types: are below

• Electronegativity Effects
• Hybridization Effects
• Acidic protons effects
• Exchangeable proton effects
• Hydrogen Bond effects

 Now we are briefly discussing -how to change the chemical shift value for changing their local diamagnetic shielding? During the effective NMR spectrum signal determination.

Electronegativity effects in the proton NMR spectrum

How to work Electronegativity Effect in NMR spectroscopy?

The chemical shift value is determent to easily include electronegative elements or groups which are attached to a carbon atom. Correspond to attach to protons are affected -The NMR spectroscopy peak observes different expectations due to change their chemical shift value. 

The chemical shift value increases as the electronegativity increases of attaching to the main element. We are considering the CH₃X where X=Cl, Br, I, etc. x is electronegative. In case electronegative substituents/ groups are attached to a carbon atom, and central carbon atom corresponds to attached with the Hydrogen atom. In case the electronegative substituents have electron-withdrawing effected. Therefore, reduced the valence electron density of the carbon and correspond to attached to protons. Which electrons clouds around protons -attached with carbon-The electrons are recalled agents applied magnetic field (external magnetic field) in NMR Spectroscopy, this is called shielding effect. And which protons are called shield protons. This is called the local diamagnetic shielding effect.

In case using the electronegativity effect reduced the electron density of around the protons attached to the carbon atom. Correspond to reduced their local diamagnetic shielding effect on the proton. These types of protons are called Deshields protons.

The greater electronegativity of the attach group, the protons are more Deshields and the greater the chemical shift value of those protons.

Multiple electronegative substituents

Similarly, the multiple substituents group attached to carbon atom then work stronger electronegativity compared to single substituents.  Therefore, the electron density around the protons attached to the carbon atom becomes decreases. Hence corresponds to their Deshields protons, which means the local diamagnetic shielding effect is reduced. 

The Hybridization Effects

The second most important effect – Hybridization of the atom has different types of bond are attach to central carbon atom-like – single bond, double bond, triple bond, etc. the different hybridization occurs in a central atom which is attached with hydrogen. 

The hybridization is sp³, sp², and sp. Which are affected the chemical shift value (in ppm) in the proton NMR spectrum. 

Sp³ Hydrogens

All hydrogens are attached to sp³ carbon atom (C-CH₃, C-CH₂-C, CH-CH(C)-C, Cycloalkanes). In case sp³ hydrogens chemical shift ranges from 0 to 2 ppm. We consider the cyclopropyl hydrogens (0-1 ppm), methyl-group hydrogens (0.5-1 ppm), secondary hydrogens (1.2-1.4 ppm), and tertiary methine hydrogens (1.4-2 ppm) from the chemical shift range table. The chemical shift compared to TMS (0 ppm) and hydrogens attached to carbon in highly strained rings like – cyclopropyl hydrogens. 

Therefore, the chemical shift of cyclopropyl hydrogen is lower value comparatively. The methyl group hydrogen occurs 1 ppm where all three hydrogens are attached to sp³ carbon. On the other hand, secondary carbon hydrogens/ Methylene- group hydrogens have a greater chemical shift value (1.2-1.4 ppm) than Methyl -group hydrogens. And tertiary methine hydrogen is occurring high chemical shift value (1.4-2 ppm) than secondary and primary hydrogens/methyl hydrogens.

Sp3 Hydrogens-aliphatic regain (1-2 ppm)-chemical shift order: 

• The chemical shift order: 3° > 2° >1° > Strained Ring.
• Aliphatic region: 0-2 ppm. (In presents of sp³ hydrogens and no electronegative elements, no π-bond group).
• The hydrogens on a carbon atom which is attached with hetero-atom (-O-CH₂-, O2¬N-CH₂-, etc.) or any unsaturated carbon (attached with double bond groups: -C=C-CH₂-). These types of hydrogens do not occur or do not match the Aliphatic region (0-2 ppm). In case chemical shift value is higher competitively. 

Sp² hydrogens

We have to consider vinyl hydrogens (-C=C-H) -the chemical shift value range 4.5- 7 ppm. In case sp²-1s or C-H bond, the vinyl carbon atom has greater s character (33.3% s) which is affected in chemical shift due to greater electronegative than sp³ carbon (25% s). 

We know that s-orbitals are closer to the nucleus than carbon p-orbitals. In case clear that the sp² carbon atom has more electrons, which are more tightly bound. That causes a less shielding effect for the vinyl hydrogens nucleus compared to sp³-1s (C-C-H, where H is attached with sp³ carbon atom) bond. That is why the chemical shift of the vinyl hydrogens (5-6 ppm) is greater than aliphatic hydrogen (sp³ carbon hydrogens, 1-4 ppm). 

• Aliphatic hydrogens: -C-C-H, 1-4 ppm. → (sp³ carbon hydrogens)
• Vinyl hydrogens: -C=C-H, 5-6 ppm. → (sp² carbon hydrogens)
• Aromatic hydrogens: Ex- Benzene, 7-8 ppm. → (sp² carbon hydrogens)
• Aldehyde protons: R-(CO)-H, 9-10 ppm. → (sp² carbon hydrogens)

The aromatic hydrogens belong to the 7 to 8 ppm chemical shift range- due to downfield region. The vinyl and aromatic hydrogens shift are downfield that have two reasons-

• Differences the hybridization
• second most affective in shift -Anisotropy effect, which is based on inductive effect ( magnetic force of electron could around protons).

we have to briefly discussed where-The anisotropy effect is the largest part of the variation of the chemical shift in NMR Spectrum (widely use of these effect in chemical shift value δ in ppm prediction in molecular structure determination). 

Aldehyde protons are attached to sp² carbon, the chemical shift occurs in the range of 9 to 10 ppm. In case aldehyde proton is more downfield (Chemical shift value range δ = 9-10 ppm) compared to aromatic protons. Because the inductive effect of an electronegative oxygen atom, which is decreases the electrons' density on the protons- (attached to sp² carbon). 

Finally, we have to see that the alkene protons, aromatic protons, and aldehyde protons appear in larger chemical shifts due to the anisotropy effect.

Sp hydrogens

The simplest example of sp hydrogens – Acetylenic hydrogens (C-H, sp-1s) shift rage 2 to 3 ppm due to anisotropy effect. Using the concept of hybridization – we are expected the chemical shift value greater than vinyl protons. We expected an sp carbon should behave as a more electronegative component than sp² carbon. This concept does not work. In this case, the more preferred diamagnetic anisotropy effect. The observed molecule acetylene looks cylinder ( in case of work high shielding effect) and two hydrogens are one is top, another is below of the cylinder.

• Chemical shift order: Aldehyde Proton [-(CO)-H] > Aromatics protons [ e.g- Benzene) > Vinyl protons [-C=C-H] > Acetylenic protons.