METHYL METHACRYLATE 甲基丙烯酸甲酯 NMR

80-62-6

METHYL METHACRYLATE


13C NMR










1H NMR














MS



IR

OFF-RESONANCE DECOUPLING

OFF-RESONANCE DECOUPLING

	Although broad-band decoupled spectra are much simple, important information may be lost, that is the number of attached hydrogen atoms. A more advanced technique, off-resonance decoupling, can restore this information while still presenting an easily interpretation.
	In this technique, the sample is irradiated by a radio frequency generator which is either slightly upfield or downfield of normal proton resonances (i.e., off resonance). When off-resonance decoupling is used, the apparent coupling constant is greatly reduced, and peak overlap is minimized.
	Off-resonance spectra often show only singlets for each carbon atom, but the multiplicity of the peak is reported as a letter (s , d, t, or q) above the peak.

Example 1: The off-resonance proton-decoupled ¹³C NMR spectrum for ethyl phenylacetate

Example 2: ¹³C-NMR spectrum of diethyl phthalate

Advantages of Off-Resonance Spectra
       determine the number of types of carbon in a molecule
       clarify of the chemical shift
       retain multiplicities with reducing of J

Disadvantage of Off-Resonance Spectra
       if signals are closed, spectrum may be higher order and difficult to interpret

Ethyl 3-oxohexanoate エチル=3-オキソヘキサノアテ NMR

Ethyl 3-oxohexanoate

Cas Number: 3249-68-1

Formula: C₈H₁₄O₃

The Journal of Organic Chemistry, 44, p. 310, 1979 DOI: 10.1021/jo01316a039

C₈H₁₄O₃
Rule 2, omit O, gives C₈H₁₄
8 - 14/2 + 1 = 2 degrees of unsaturation.
Look for 2 pi bonds or aliphatic rings, or 1 of each.

Even though A, B, C and D are all 2H peaks, they can be distinguished by chemical shift and splitting. B is outside the normal range for protons next to carbonyls, because it's adjacent to both carbonyls and the combined deshielding is higher than normal.

The bands at 1745 and 1716 indicate that there are two carbonyls, probably an aliphatic ester and an aliphatic ketone. The bands at 3000-2850 indicate C-H alkane stretches.

H1 NMR Spectrum:

Predict NMR spectrum

ir

The bands at 1745 and 1716 indicate that there are two carbonyls, probably an aliphatic ester and an aliphatic ketone. The bands at 3000-2850 indicate C-H alkane stretches.

MASS

Ethyl 3-oxohexanoate エチル=3-オキソヘキサノアテ NMR

Ethyl 3-oxohexanoate

Cas Number: 3249-68-1

Formula: C₈H₁₄O₃

The Journal of Organic Chemistry, 44, p. 310, 1979 DOI: 10.1021/jo01316a039

C₈H₁₄O₃
Rule 2, omit O, gives C₈H₁₄
8 - 14/2 + 1 = 2 degrees of unsaturation.
Look for 2 pi bonds or aliphatic rings, or 1 of each.

Even though A, B, C and D are all 2H peaks, they can be distinguished by chemical shift and splitting. B is outside the normal range for protons next to carbonyls, because it's adjacent to both carbonyls and the combined deshielding is higher than normal.

The bands at 1745 and 1716 indicate that there are two carbonyls, probably an aliphatic ester and an aliphatic ketone. The bands at 3000-2850 indicate C-H alkane stretches.

H1 NMR Spectrum:

Predict NMR spectrum

ir

The bands at 1745 and 1716 indicate that there are two carbonyls, probably an aliphatic ester and an aliphatic ketone. The bands at 3000-2850 indicate C-H alkane stretches.

MASS

MULTIPLICITY. SIGNAL SPLITTING

When one nucleus is influenced by another nucleus with spin quantum number I0, the two nuclei are said to be coupled, both will show 2 lines instead of 1 line for each nucleus.

                   

 The reason is that the field caused by H_x can be parallel or antiparallel so the H_a can experience not only the applied field but also combination of both fields.

 One signal absorbs at higher field (if H_x is parallel) and one signal absorbs at lower field (if H_x is antiparallel). The two lines are observed for each nucleus. This is called spin-spin splitting. The signal of H_a is split into 2 peaks (doublet) because there is spin-spin coupling between nonequivalent protons (H_a and H_x).

 In order to obtain the signals, the B₀ will be adjusted to maintain the resonance conditions, either lower or higher field than without H_x.

 Each peak of the doublet has nearly equal area and is half in the absence of H_x because the population of H_a in the two spin states are nearly equal according to the Boltzmann Distribution.      - Since H_a and H_x is split by each other, the distance between 2 lines of both H_a and H_x signals is equal.      - If the two nuclei are only neighbors, then their signals are 2 lines (doublet).

 The number of line splitting can be depicted by using the following equation;

multiplicity  =  2nI + 1, where n is no. of neighboring protons.

nuclei	I	multiplicity
1H	1/2	n + 1
2H and 14N	1	2n + 1
35Cl	3/2	3n + 1

Since the spin quantum no. (I) of ¹H is 1/2, therefore,

multiplicity  =  2n + 1

Consider these examples,

Splitting patterns for a proton with 0, 1, 2, and 3 equivalent neighboring protons

COUPLING CONSTANT

The signal splitting in ¹H-NMR spectra is usually small, ranging from fractions of a Hz to as much as 18 Hz, and is designated as J (referred to the coupling constant).

Coupling Constants (J)
        J  is the distance between peaks in a splitting signal
        the magnitude of J is
                  - dependent only on fields that are caused by magnetic atoms within a molecule but
                  - independent of the external field strength (B₀)

The most common patterns that have been seen are doublet and triplet.

n = 1	n = 2

First-Order and Second-Order spectra

 Spectra that can be interpreted by using n+1 rule are said to be first-order spectra. In first-order spectra, the difference in chemical shifts (in Hz) between 2 groups of protons is large. When Dn/J  is large and we see first-order splitting, the system is said to be weakly coupled.

 In some spectra, using the n+1 rule is not enough, therefore, such advanced analysis may be required for interpreting spectra. These spectra are said to be second-order spectra. In second-order spectra, the difference in chemical shifts between 2 groups of protons is similar in magnitude to the J. When Dn/J  is large and we see second-order splitting, the system is said to be strongly coupled.

Example of first-order spetra

        1. The ethyl acetate spectrum displays the typical quartet and triplet of a substituted ethyl group.

        2. The spectrum of 1,3-dichloropropane demonstrates the triplet (B) and the quintet (A).

Example of second-order spetra

If a given nucleus is spin-coupled to two or more sets of neighboring nuclei by different J values, the n+1 rule does not predict the entire splitting pattern. The spectra may be complicated due to the strong coupling.

These are some examples of more complicated spectra that can be 2^nd order spectra.

Magnitude of Some Typical Coupling Constants