Proton NMR, Chemistry tutorial

Introduction:

Proton Magnetic Resonance (1H-NMR or PMR) spectra are obtained by employing a solution of the sample having a little TMS in a solvent which doesn't absorb in the radio frequency region being studied. Tetrachloromethane (CCl4) and deuterated solvents, like deuterated trichloromethane (CDCl3), methanol (CD3OD) and propanone (CD3COCD3) are generally employed. These deuterated solvents all have a small amount of the corresponding protonated compound due to incomplete deuteration throughout their manufacture.

Deuterotrichloromethane (CDCl3), for illustration, will have a little trichloromethane (CHCl3). As 1H-NMR spectra are additives, the signals of these impurities will take place in the spectrum and it is significant that they are recognized and taken into account whenever interpreting a spectrum. The nature of solvent will as well influence the value of the chemical shift of a signal. Changing from tetrachloromethane to deutrotrichloromethane consists of little effect on the chemical shift; however changing to more polar solvents can cause an important change in its value. These changes are at times used to assist in identifying the signals.

1H-NMR Spectra:

Most of the proton chemical shifts take place in the 0 to 12 ppm region. Identification is made in the primary instance by employing broad correlation charts as illustrated in the figure given below. This is followed through an estimate the chemical shift of a proton in a model compound and compares it by the value obtained in practice. Whenever the comparison provides a good match it is taken as proof that the signal on the spectrum is due to an identical kind of structure.  

817_Proton magnetic resonance spectroscopy.jpg

Fig: Proton magnetic resonance spectroscopy

Protons which are in the similar chemical compound and in the same magnetic environment will have similar chemical shifts. They are termed as equivalent protons example: the three protons of the methyl group of bromoethane will be equivalent and encompass the similar chemical shift (figure shown below).

151_Equivalent Protons in Bromoethane.jpg

Fig: Equivalent Protons in Bromoethane

Likewise, the two protons of the methylene group will as well be equivalent as there is unrestricted free rotation regarding the C-C bond. Though, it is not safe to suppose that protons linked to the same atoms are for all time equivalents.

Spin-Spin Coupling:

The signals observe on a 1H-NMR spectrum differs from a single peak to a group of peaks. The division of a signal to the group of peaks takes place as the magnetic fields of adjacent protons affect the magnetic field strength at which a proton comes into resonance. Consider, for illustration, the 1H-NMR spectrum of a sample of dichloroethanal. For ease, the protons of this compound will be termed to as HA and HB correspondingly (figure shown below).

1819_H-NMR Spectrum of Dichloroethanal.jpg

Fig: H-NMR Spectrum of Dichloroethanal 

The only other nuclei in dichloroethanal that encompass local magnetic fields that will influence the HA protons is the proton HB. As an outcome the spectrum exhibits a pair of peaks (a doublet) for the absorption signal of the proton HA. Likewise, the signal of HB will as well be a doublet as it is influenced in the similar manner by HA. This behavior is termed as spin-spin coupling or splitting.  The value of chemical shift separating the peaks in each and every doublet is termed as the coupling constant J. This is a constant feature of protons which are spin-spin coupled. In the 1H-NMR spectrum of dichloroethanal, for illustration, HA will encompass the similar coupling constant as HB.  This lets one to pick out the signals of hydrogen atoms which are spin-coupled and therefore adjacent to one other in a structure that is of considerable help in the interpretation of the spectrum.

It must be noted that: 

a) Chemically equivalent protons don't couple with one other even if they are bonded to dissimilar carbon atoms.

b) Protons that is further than two single 'bond lengths' apart do not generally couple.

c) Protons which are spin-coupled with one other encompass the similar J values.

More complex splitting patterns are noticed whenever more than two protons are comprised in the coupling. In theory, the number of peaks occurring in a signal will be n + 1; here 'n' is the number of equivalent protons whilst their relative intensities are predicted via Pascal's triangle. Consider, for illustration, the 1H-NMR spectrum of bromoethane (CH3CH2Br).  Unrestricted free rotation regarding the C-C bond signifies that the three protons of the methyl group are equivalent and the two protons of the methylene group are equivalent. Thus, in theory, the methyl protons with their two equivalent neighboring protons will encompass a signal that is a triplet (2+1) having the intensities of the peaks in the ratio 1:2:1. On the other hand, the methylene group consists of three equivalent neighbors and as a result its signal will be predicted to be a quartet (3+1) having the peaks having relative intensities of 1:3:3:1. This agrees reasonably well by the spectrum of bromoethane that as well exhibits that the signal for the methyl's protons is upfield from that of the methylene (figure shown). This kind of prediction is reasonably precise for simple molecules however less accurate for more complex molecules as the three-dimensional nature of these molecules at times makes it difficult to recognize all the nuclei which can influence a signal. 

2345_Pascals Triangle.jpg

Fig: Pascal's Triangle

Signal Intensity:

The area under a signal in a 1H-NMR spectrum is proportional to the number of equivalent protons responsible for that signal. As a result, electronic configuration of the area under each signal in a 1H-NMR spectrum lets one to find out the ratio of the numbers of equivalent protons responsible for each and every signal. This can be of considerable assistance in interpreting 1H-NMR.

Deuterium Exchange:

Deuterium doesn't absorb radio frequency radiation in the similar region as protons. Though, it will undergo fast exchange reactions by some acidic protons like those in the hydroxyl and amino groups. The mixing of D2O having a sample (termed as D2O shake) causes either a reduction in the intensity or the complete elimination of the signals in a spectrum because of the exchangeable protons. This loss will be accompanied via the appearance of a weak signal at 4.8 ppm generated by the formation of HOD given the HOD is soluble in the solvent employed. The changes in a spectrum caused through a D2O shake let one to recognize the signals of groups which have exchangeable protons.

Interpretation of Proton NMR Spectra:

The given is a general guideline to 1H-NMR spectral interpretation:

a) Note the absence or presence of saturated structures, most of which provide resonances between 0 and 5 ppm.

b) Note that the presence or absence of unsaturated structures in the region between around 5 and 9 ppm (that is, alkene protons between 5 and 7 ppm and aromatic protons between 7 and 9 ppm, alkyne protons are an exception appearing at around 1.5 ppm).

Note that any low field resonances (9 to 16 ppm) that are related with aldehydic and acidic protons, particularly those comprised in strong hydrogen bonding.

c) Measure the integrals, if recorded and compute the numbers of protons in each and every resonance signal.

d) Check for spin-spin splitting patterns provided by adjacent alkyl groups according to the (n + 1) rule and Pascal's triangle. (The position of the lower field multiplet of the two is extremely sensitive to the proximity of electronegative elements and groups like O, CO, COO, OH, Cl, Br, NH2 and so on).

e) Observe the splitting pattern provided by the aromatic protons, that couple around the ring and are frequently complex due to the second order effects.

f) 1,4- and 1,2-disubstituted rings provide complex however symmetrical looking patterns of peaks, while mono-, 1,3- and tri-substituted rings provide more complex asymmetrical patterns.

g) Note that any broad single resonances, that are evidences of labile protons form alcohols, phenols, acids and amines that can go through slow exchange by other labile protons. Comparison of the spectrum with the other after shaking the sample by a few drops of D2O will verify the presence of an exchangeable proton via the disappearance of its resonance signal and appearance of the other at 4.7 ppm due to HOD.

Examples of 1H-NMR Spectral Interpretation:

The two aromatic protons, A and X, in cytosine are coupled to provide an AX pattern of two doublets. The A proton is deshielded as compare to the X proton because of its closer proximity to nitrogen atoms and the oxygen atom. The intensities of the doublets are slightly distorted via second order effects. The OH and NH2 protons have been exchanged by D2O, and their resonance substituted by a HOD peak at 4.7ppm.

The other illustration is in figure shown below.

288_H-NMR Spectral Interpretation.jpg

Fig: H-NMR Spectral Interpretation

The CH (methane) resonance in 1,1,2-trichloroethane is at a much lower field as compare to CH2  (methylene) resonance due to the very strong deshielding via two chlorines. The protons provide an AX2 coupling pattern of a triplet and a doublet and an integral ratio of 1:2.

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