ABSTRACT

The report was done to identify nine different compounds (A-I) all with the same elemental compositions which Carbon (64.3%), Oxygen (28.6%) and Hydrogen (7.2%); and relative molecular mass of 112.1 g mol-1 by using the knowledge of Nuclear Magnetic Resonance (NMR). This experiment was rather different than the other experiments done previously as we did not conduct any experiment of some sort but instead, we were just given the proton, 1H NMR as well as 13C NMR data of a set of 9 compounds noted as Compound A to I. The tasks here were to draw and name viable structure for those compounds with the aid of the data given. As a result:

Compound A : buta-1,2-dien-1-yl acetateCompound B : 2-methylcyclopentane-1,3-dioneCompound C : cyclohexane-1,4-dioneCompound D : ethenly (2E)-but-2-enoate

Compound E : prop-1-en-2-yl-2-enoate

Compound F : cyclobutylideneacetic acid

Compound G: (3E)-5-hydroxyhex-3-en-2-one

Compound H: 6-methyl-2,3-dihydro-4H-pyran-4-one

Compound I : hex-2-ynoic acid

This experiment was successful since we can identify all the compounds (A-I) by using the knowledge of Nuclear Magnetic Resonance (NMR).

INTRODUCTION

Nuclear magnetic resonance (NMR) is just 50 years old but yet it has shown a remarkable growth in that short lifetime, surpassing the analytical application of infrared spectroscopy, medical diagnostics with x-ray scanner, and molecular structure determination by x-ray crystallography. Whenever NMR seemed to have reached a comfortable plateau, someone managed to discover a completely new aspect of the subject.

It all started with the independent and virtually simultaneous discoveries of NMR in bulk matter by Harvard and Stanford physicists in late 1945. At the time the main application appeared to be accurate determination of the nuclear magnetic moments of all the elements in the periodic table. Very soon, discrepancies in these “ constant “ came to light and it was realized that there was a small but significant shielding effect by the extranuclear electrons – the chemical shift was born. This may have remained a mere regrettable complication for the physicists, had not a

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chemist pointed out that if the chemical shift was indeed real, then the proton NMR spectrum of ethanol should have three separate resonance.

No one had ever thought to build magnets with field magnet with fields so homogenous that high resolution proton spectra could be properly resolved, since this required uniformity in space of the order of 1 part in 106. Undeterred by this, Arnold set out to construct such a magnet, and Anderson demonstrated the first truly high resolution proton spectra with linewidths as a low 0.5 Hz, using a spinning sample as proposed by bloch. Soon after this, may chemists realized the enormous potential of NMR and the race was on. New techniques seemed to spring up almost overnight – spin echoes, double resonance, time averaging, the Overhouser effect, and Hartmann – Hahn cross polarization.

But the technique suffered from an inherently poor sensitivity and it seemed that this could only be improved by operating at higher magnetic fields. Furthermore, higher fields mean better chemical shift dispersion, so that more complex molecules could be studied. Since the iron - cored electromagnets then in use had already reached their limit (determined by magnetic saturation) Nelson and Weaver turned to superconducting solenoids where the current continued indefinitely provided that coils were cooled to 4 K in liquid helium. First at 200 MHz, now at 750 MHz, and soon to 1000 MHz, this new generation of spectrometers gradually superseded all the iron magnets and they were hardly ever heard of again.

Soon after this, Ernst and Anderson introduced Fourier transform NMR, which delivers approximately two orders of magnitude improvement in sensitivity through the multiplex advantage, by recording all the resonance line in the spectrum at the same time. Together with the invention of noise decoupling this brought the “difficult” nuclei like 13C and 15N with reach. Inorganic chemist and biochemist began to take serious interest in NMR, the latter through studies of 31P spectra in molecule of biological interest.

At that time, NMR spectroscopy of the solid state was rather a stagnant topic because most of the information of interest was hidden by the strong dipolar broadening (which disappear in a liquid through the rapid molecular tumbling). Then Andrew showed how to circumvent this problem by rapid spinning about the “magic” angle, and Waugh devised a series of intricate pulse sequences that solved the same problem in another manner. Suddenly high resolution NMR of solid was a viable proposition.

Meanwhile a Belgian physicist ( Jeener ) was planning to revolutionize the methodology by introducing the concept of two dimensional Fourier spectroscopy. By spreading the information into a second frequency dimension, this allows all kind of interesting correlation to be made and permits studies of normally forbidden NMR transitions. Above all it emphasizes that the spins can be manipulated in a myriad different ways by designing te appropriate pulse sequence.

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THEORY

Nuclear Magnetic Resonance spectroscopy is a powerful and theoretically complex analytical tool.

Subatomic particles (electrons, protons, neutrons) can be imagined as spinning on their axes. In many atoms such as 12C these spins are paired against each other, such that the nucleus of the atom has no overall spin. However, in some atoms such as 1H and 13C the nucleus does possess an overall spin. The rules for determining the net spin of a nucleus are as follows:

1. If the number of neutrons and the number of protons are both even, then the nucleus has NO spin.

2. If the number of neutrons and the number of protons is odd, then the nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2)

3. If the number of neutrons and the number of protons are both odd, then the nucleus has an integer spin.

The overall spin, I, is important. Quantum mechanics tells us that a nucleus of spin I will have 2I + 1 possible orientations. A nucleus with spin ½ will have 2 possible orientations. In the absence of an external magnetic field, these orientations are of equal energy. If a magnetic field is applied, then the energy levels split. Each level is given a magnetic quantum number, m.

When the nucleus is in a magnetic field, the initial populations of the energy levels are determined by thermodynamics, as described by the Boltzmann distribution. This is very important, and it means that the lower energy level will contain slightly more nuclei than the higher level. It is possible to excite these nuclei into the higher level with electromagnetic radiation. The frequency of radiation needed is determined by the difference in energy between the energy levels.

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The nucleus has a positive charge and is spinning. This generates a small magnetic field. The nucleus therefore possesses a magnetic moment, m, which is proportional to its spin, I.

The constant, g, is called the magnetogyric ratio and is a fundamental nuclear constant which has a different value for every nucleus. h is Plancks constant. The energy of a particular energy level is given by:

Where B is the strength of the magnetic field at the nucleus. The difference in energy between levels can be found from:

This means that if the magnetic field, B is increased, and so is DE. It also means that if a nucleus has a relatively large magnetorytic ratio, then DE is correspondingly large.

Imagine a nucleus of spin ½ in a magnetic field. This nucleus is in the lower energy level. The nucleus is spinning on its axis. In the presence of a magnetic field, this axis of rotation will precess around the magnetic field:

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The frequency of precession is termed the Larmor frequency, which is identical to the transition frequency. The potential energy of the precessing nucleus is given by:

E = -m B cos q

Where q is the angle between the direction of the applied field and the axis of nuclear rotation.

If energy is absorbed by the nucleus, then the angle of precession, q, will change. For a nucleus of spin ½, absorption of radiation “flips” the magnetic moment so that it opposes the applied field.

It is important to realize that only a small proportion of “target” nuclei are in the lower energy state and can absorb radiation. There is the possibility that by exciting these nuclei, the populations of the higher and lower energy levels will become equal. If this occurs, then there will no further absorption of radiation. The spin system is saturated. The possibility of saturation means that we must be aware of the relaxation processes which return nuclei to the lower energy state.

The magnetic field at the nucleus is not equal to the applied magnetic field; electrons around the nucleus shield it from the applied field. The difference between the applied magnetic field and the field at the nucleus is termed the nuclear shielding.

Consider the s-electrons in a molecule. They have spherical symmetry and circulate in the applied field, producing a magnetic field which opposes the applied field. This means that the applied field strength must be increased for the nucleus to absorb at its transition frequency. This upfield shift is also termed diamagnetic shift.

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Electrons in p-orbital have no spherical symmetry. They produce comparatively large magnetic fields at the nucleus, which give a low field shift. This “deshielding” is termed paramagnetic shift.

In proton (1H) NMR, p-orbital plays no part, which is why only a small range of chemical shift (10 ppm) is observed. We can easily see the effects of s-electrons on the chemical shift by looking at substituted methane, CH3X. As X becomes increasingly electronegative, so the electron density around the protons decreases, and they resonate at lower field strengths.

Chemical shift is defined as nuclear shielding / applied magnetic field. Chemical shift is a function of the nucleus and its environment.

(Source: http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm)

Figure 1: 1H NMR Chemical Shifts

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Figure 2: 13C NMR Chemical Shifts

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RESULT AND DISCUSSION

COMPOUND A

1H NMR 13C NMR

Compound A is buta-1,2-dien-1-yl acetate

From 1H NMR data clearly showed the presence of four hydrogen peaks corresponding to the integration of 1:1:3:3. The three-proton absorption at 1.4 δ was due to methyl group-like environment, and the doublet splitting pattern implies that the CH3 is next to CH. The three-proton singlet at 2.2 δ due to methyl group attached to a carbon with no hydrogens, CH3C. A proton quartet at absorption 4.5 δ indicate that three neighboring protons and slightly due to the electronegative effect of the neighboring oxygen; while another single proton at 5.4 δ which singlet splitting having no neighboring hydrogens and due to the electronegative effect of the neighboring oxygen.

From 13C NMR data on the other hand showed the presence of six carbon peaks. A carbon having a chemical shift of 205.6 δ corresponds to the presence of a ketone. A second carbon having a chemical shift of 189.8 δ corresponds to the presence of an ester. A third carbon having a chemical shift of 103 δ corresponds to the presence of RHC=CHR. A fourth carbon having a chemical shift of 82.8 δ corresponds to the presence of C-OR. A fifth carbon having a chemical shift of 16.9 δ as well as a sixth carbon having a chemical shift of 16.3 δ, both correspond to C-H saturated alkanes.

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COMPOUND B

1H NMR 13C NMR

Compound B is 2-methylcyclopentane-1,3-dione

The 1H NMR data clearly showed the presence of three hydrogen peaks corresponding to the integration of 1:3:4. A hydrogen having three neighboring hydrogens, giving rise to a quartet splitting had a chemical shift of 2.2 δ in the shielded region and was said to be slightly deshielded due to the electronegative effect of the neighboring oxygen. Three hydrogens having single neighboring hydrogen, giving rise to a doublet splitting had a chemical shift of 1.5 δ in the shielded region. Another four hydrogens, a symmetrical of two hydrogens, both having two neighboring hydrogens, giving rise to a triplet splitting had a chemical shift of 2.4 δ in the shielded region and was said to be slightly deshielded, again due to the electronegative effect of the neighboring oxygens.

The 13C NMR data on the other hand showed the presence of four carbon peaks. A symmetrical of two carbons having a chemical shift of 194.0 δ corresponds to the presence two ketones. A second carbon having a chemical shift of 111.6 δ corresponds to the presence of an aromatic ring. A symmetrical of two carbons having a chemical shift of 30.1 δ corresponds to the presence of two C-CORs. Another symmetrical of two carbons having a chemical shift of 5.7 δ corresponds to the presence of C-H saturated alkanes.

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COMPOUND C

1H NMR 13C NMR

Compound C is Cyclohexane-1,4-dione

The 1H NMR data clearly showed the presence of only one hydrogen peak. Eight hydrogens, a symmetrical of two hydrogens, all having two neighboring hydrogens, giving rise to a triplet splitting had a chemical shift of 2.7 δ in the shielded region was said to be slightly deshielded due to the electronegative effect of the neighboring oxygens.

The 13C NMR data on the other hand showed the presence of only two carbon peaks. A symmetrical of two carbons having a chemical shift of 208.3 δ corresponds to the presence two ketones. A symmetrical of four carbons having a chemical shift of 37.6 δ corresponds to the presence of C-H saturated alkanes.

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COMPOUND D

1H NMR 13C NMR

Compound D is ethenly (2E)-but-2-enoate

From 1H NMR data clearly showed the presence of five hydrogen peaks corresponding to the integration of 1:1:1:2:3. The three-proton absorption at 1.9 δ was due to methyl group-like environment, and the doublet splitting pattern implies that the CH3 is next to CH. The two-proton doublet at 4.7 δ due to vinylic attached to a carbon with one hydrogen, CH2=CH-. A proton quintet at absorption 7.1 δ indicate that four neighboring protons around it. Single proton at 5.9 δ which doublet splitting having one neighboring hydrogen and due to the electronegative effect of the neighboring oxygen; the other singles proton at 7.4 δ which triplet splitting having two neighboring hydrogens and due to the electronegative effect of the neighboring oxygen.

From 13C NMR data on the other hand showed the presence of six carbon peaks. A carbon having a chemical shift of 163.2 δ corresponds to the presence of an ester. A second carbon having a chemical shift of 147.0 δ corresponds to the presence of R2C=CH2. A third carbon having a chemical shift of 141.3 δ corresponds to the presence of RHC=CHR. A fourth carbon having a chemical shift of 121.7 δ corresponds to the presence of R2C=CH2. A fifth carbon having a chemical shift of 97.4 δ corresponds to the presence of C-OR. A sixth carbon having a chemical shift of 18.2 δ corresponds to the presence of C-H saturated alkanes.

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COMPOUND E

1H NMR 13C NMR

Compound E is prop-1-en-2-yl-2-enoate

The 1H NMR data clearly showed the presence of four hydrogen peaks corresponding to the integration of 1:2:2:3. Two hydrogens having one neighboring hydrogen, giving rise to a doublet splitting had a chemical shift of 4.7 δ in the shielded region.

A single hydrogen having two neighboring hydrogens, giving rise to a triplet splitting had a chemical shift of 7.3 δ in the deshielded region due to the electronegative effect of the neighboring oxygen. Two hydrogens having no neighboring hydrogens, giving rise to a singlet splitting had a chemical shift of 6.0 δ in the deshielded region. Three hydrogens also having no neighboring hydrogens, giving rise to a singlet splitting had a chemical shift of 2.0 δ in the shielded region.

The 13C NMR data on the other hand showed the presence of six carbon peaks. A carbon having a chemical shift of 164.3 δ corresponds to the presence of an ester. A second carbon having a chemical shift of 141.5 δ corresponds to the presence of RHC=CHR. A third carbon having a chemical shift of 135.5 δ corresponds to the presence of R2C=CH2. A fourth carbon having 127.2 δ corresponds to the presence of R2C=CH2. A fifth carbon having a chemical shift of 97.8 δ corresponds to the presence of C-OR. A sixth carbon having a chemical shift of 18.2 δ corresponds to the presence of C-H saturated alkanes.

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COMPOUND F

1H NMR 13C NMR

Compound F is Cyclobutylideneacetic acid

The 1H NMR data clearly showed the presence of five hydrogen peaks corresponding to the integration of 1:1:2:2:2. A hydrogen having no neighboring hydrogen, giving rise to a singlet splitting had a chemical shift of 11.3 ppm in the deshielded region due to the electronegative effect of the neighboring oxygen. Another single hydrogen also having no neighboring hydrogen, giving rise to a singlet splitting had a chemical shift of 5.5 δ in the deshielded region, again due to the electronegative effect of the neighboring oxygen. Four hydrogens, a symmetrical of two hydrogens, both having two neighboring hydrogens, giving rise to a triplet splitting had a chemical shift of 2.4 δ as well as 2.5 ppm in the shielded region respectively. Two hydrogens having four neighboring hydrogens, giving rise to a quintet splitting had a chemical shift of 2.0 δ in the shielded region.

The 13C NMR data on the other hand showed the presence of five carbon peaks. A carbon having a chemical shift of 193.0 δ corresponds to the presence a carboxylic acid. A second carbon having a chemical shift of 120 δ corresponds to the presence of RHC = CHR. A third carbon having a chemical shift of 104.3 δ corresponds to the presence of R2C = CH2. A fourth and fifth

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carbon having a chemical shift of 32.3 δ, and 21.1 δ respectively all corresponds to the presence of C-H saturated alkanes.

COMPOUND G

1H NMR 13C NMR

Compound G is (3E)-5-hydroxyhex-3-en-2-one

The 1H NMR data clearly showed the presence of five hydrogen peaks corresponding to the integration of 1:1:1:2:3. The three-proton absorption at 2.3 δ was due to methyl group-like environment, and the singlet splitting pattern implies that having no neighboring hydrogen. The two-proton singlet at 4.5 δ due to vinylic attached to a carbon with no hydrogen, CH2=C. A proton doublet at absorption 5.9 δ indicates that one neighboring proton around it. Single proton at 6.1 δ which doublet splitting having one neighboring hydrogen and due to the electronegative effect of the neighboring oxygen. The remaining hydrogen, which appears as a one-proton singlet at 2.3 δ, is probably due to OH group.

The 13C NMR data on the other hand showed the presence of six carbon peaks. A carbon having a chemical shift of 152.6 δ corresponds to the presence R2C=CH2. A second carbon having a chemical shift of 152.1 δ corresponds to the presence of R2C=CH2. A third carbon having a chemical shift of 108.6 δ corresponds to the presence of RHC=CHR. A fourth carbon having a chemical shift of 106.3 δ corresponds to the presence of R2C=CH2. A fifth carbon having a chemical shift of 57.1 δ corresponds to the presence of C-OH. A sixth carbon having a chemical shift of 13.5 δ corresponds to the presence of C-H saturated alkanes.

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COMPOUND H

1H NMR 13C NMR

Compound H is 6-methyl-2,3-dihydro-4H-pyran-4-one

The 1H NMR data clearly showed the presence of four hydrogen peaks corresponding to the integration of 1:2:2:3. A lone hydrogen having no neighboring hydrogen, giving rise to a singlet splitting had a chemical shift of 6.7 δ in the deshielded region due to the electronegative effect of the neighboring oxygen. Three hydrogens having no neighboring hydrogen, giving rise to a singlet splitting had a chemical shift of 2.0 δ in the shielded region. Four hydrogens, a symmetrical of two hydrogens, both having two neighboring hydrogens, giving rise to a triplet splitting had a chemical shift of 2.3 δ as well as 2.4 δ in the shielded region respectively were said to be slightly deshielded due to the electronegative effect of the neighboring oxygen.

The 13C NMR data on the other hand showed the presence of six carbon peaks. A carbon having a chemical shift of 203.8 δ corresponds to the presence a ketone. A second carbon having a chemical shift of 149.5 δ corresponds to the presence of R2C=CH2. A third carbon having a chemical shift of 145.9 δ corresponds to the presence of an aromatic ring. A fourth carbon having a chemical shift of 31.2 δ corresponds to the presence of C-COR. A fifth carbon having a chemical shift of 27.3 δ corresponds to the presence of C-C=C. A sixth carbon having a chemical shift of 14.4 δ corresponds to the presence of C-H saturated alkanes.

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COMPOUND I

1H NMR 13C NMR

Compound I is hex-2-ynoic acid

The 1H NMR data clearly showed the presence of four hydrogen peaks corresponding to the integration of 1:2:2:3. Three hydrogens having two neighboring hydrogens, giving rise to a triplet splitting had a chemical shift of 1.0 δ in the shielded region. Two hydrogens having five neighboring hydrogens, giving rise to a sextet splitting had a chemical shift of 1.6 δ in the shielded region. Another two hydrogens having two neighboring hydrogens, giving rise to triplet splitting had a chemical shift of 2.3 δ in the shielded region. A lone hydrogen having no neighboring hydrogen, giving rise to a singlet splitting had a chemical shift of 9.7 δ due to the electronegative effect of the neighboring oxygen.

The 13C NMR data on the other hand showed the presence of six carbon peaks. A carbon having a chemical shift of 158.5 δ corresponds to the presence a carboxylic acid. A second carbon having a chemical shift of 92.6 δ corresponds to the presence of C-OH. A third carbon having a chemical shift of 72.9 δ corresponds to the presence of an alkyne. A fourth, fifth and sixth carbon having a chemical shift of 21.0 δ, 20.7 δ and 13.4 δ respectively all corresponds to the presence of C-H saturated alkanes.

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CONCLUSION

The experiment was successful since we can identify all nine different compounds (A-I) by using the data given of NMR spectroscopy consisting those of proton 1H NMR as well as carbon 13C NMR. We can derive plausible structures of a number of compounds. As mentioned earlier, this report was not based on any actual experiment as we had been unfortunate as do not be able to use any NMR spectrometer. However, we have still managed to determine the set of compounds given to us. Compound A was buta-1,2-dien-1-yl acetate, compound B was 2-methylcyclopentane-1,3-dione, compound C was cyclohexane-1,4-dione, compound D was ethenly (2E)-but-2-enoate, compound E was prop-1-en-2-yl-2-enoate, compound F was cyclobutylideneacetic acid, compound G was (3E)-5-hydroxyhex-3-en-2-one, compound H was 6-methyl-2,3-dihydro-4H-pyran-4-one and compound I was hex-2-ynoic acid.

After this experiment was carried out, we had learnt some experiences and new knowledge from it. We had understood the concept of Nuclear Magnetic Resonance (NMR) which is powerful non-selective analytical tool that enables us to ascertain molecular structure. We also learnt how to draw the structures by using computer and confirm it.

REFERENCES

John Mc Murry, 2008. Organic Chemistry. 7th ed. Belmont CA: Thomson Higher Education

NMR Spectroscopy [Online] Available at: http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm[Accessed 30 December 2011]

Theory NMR Analysis [Online] Available at: resources.metapress.com [Accessed 30 December 2011]

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