+ All Categories
Home > Documents > NMR analysis of Ethylbenzene report

NMR analysis of Ethylbenzene report

Date post: 12-Apr-2017
Category:
Upload: daniel-gonzalez
View: 348 times
Download: 2 times
Share this document with a friend
28
Physical Measurements Goux Spring 2016 Daniel Gonzalez NMR analysis of Ethylbenzene Physical Measurements Goux Spring 2016 Daniel Gonzalez Partner: Dorothy N. 1
Transcript
Page 1: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

NMR analysis of Ethylbenzene

Physical Measurements

Goux Spring 2016

Daniel Gonzalez

Partner: Dorothy N.

1

Page 2: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

ABSTRACT:

An HNMR experiment was performed on ethylbenzene using a JEOL System with DELTA

software. A regular 90o pulse experiment was run to produce a typical spectrum with

deuterated chloroform as the solvent system. Peaks were recorded at 7.3ppm 2.7ppm and

1.3ppm with multiplicities of 6,4, and 3 respectively. A 1800 pulse experiment was performed

and the minimum intensity was used to calculate the optimal pulse width of a 90o pulse which

was determined to be 13.75 μs. Subsequently, using a double pulse experiment with a 180o

followed by a 90o pulse a variable time τ was used to calculate three T1 values, one for each

NMR peak on the ethylbenzene spectrum. T1’s were recorded as 4.216, 4.567, 5.191 seconds

with T1 max being 5.191 seconds. The relaxation delay corresponding to 95% peak intensity

was determined to be 15.57 seconds, with a total delay time D of 16.57 seconds. Optimum flip

angles were calculated for the peak at 7.3ppm and a plot of D, total delay time verses φ was

made. A linear positive relationship was evident in the said plot with y intercept laying around

40o.

INTRODUCTION:

Proton NMR is a spectroscopic technique that takes advantage the magnetic properties

of hydrogen nuclei to create spectra with peaks corresponding to particular proton

environments. The positively charged hydrogen nuclei are in continuous rotation, and as a

charged particle spins, a magnetic moment is formed. This magnetic moment denoted as μ is

related to the angular momentum by the equation:

μ=γJ (1)

where J is the angular momentum and γ is the gyromagnetic ratio.

2

Page 3: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

The gyromagnetic ratio is simply a measure of how well molecules will interact with a magnetic

field, much like εo , the molar extinction coefficient, is simply a measure of how well molecules

absorb light in Beer’s law. γ will very amongst different molecules and will change in different

chemical environments.

The energy E, of a magnetic field interaction is given by the equation

E=-μB (2)

Where B is the magnetic field strength.

When considering the proton nucleus, there are only two possible spin states as predicted by

quantum mechanics, that is ½ and – ½ . These states are simply solutions to quantum numbers

in Schrodinger’s equation. The ½ and – ½ state can be described respectively as the α and β

states.

Figure 1[1]. Splitting of α and β states

The α spin state is nearly equally occupied as the beta spin state and in the presence of an

external magnetic field, it is possible to separate the two states with the degree of separation

being proportional to the strength of the external magnetic field. If the magnetic fields of the

3

Page 4: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

two spin states are vectorily summed, the result is a net magnetic moment M in the direction of

the alpha states.

Figure 2: Net Magnetic Moment

The net magnetic moment then will point towards the positive Z axis and is spinning at a

frequency in the MHz region. The diagram in figure 2 represents all magnetic dipole

orientations rotating as a conical surface. As the nuclei spin they can be perturbed by external

fields to produce a wobble in the nuclei spin as the net magnetization vector is thrown off the z

axis. The common analogy used to describe this movement is that of a spinning top wobbling

about its axis. However, unlike the top eventually falling over, M will return to equilibrium on

the z axis; given the magnetic field stays constant.

When viewing the spinning nuclei, if a rotating frame of reference is used to move with the

rotation, then any movement of M from the z axis will appear as a rotation downwards towards

the XY plane by an angle φ the flip angle

4

Page 5: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 3

A coil can be placed perpendicular to the magnetic field vector B, and as M wobbles an induced

magnetic field will occur in the coil which can be measured by a computer. This IMF will appear

as a sinusoidal wave that decays as the vector M returns to its equilibrium position of Mo which

can be Fourier transformed to produce peaks in an NMR spectra.

Figure 3: IMF and Signal for NMR

5

Page 6: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

As the pulse angle changes, more nuclei switch between the alpha and beta state with 90˚

being where the two states are nearly equal in population. As more nuclei flip to the beta state

the angle approaches 180˚ where there are nearly twice as many beta states than alpha states.

Similarly, approaching 0˚ there are nearly twice as many alpha states than beta states.

Figure 4: Population of α and β states at different flip angles.

Clearly the acquisition of signal is dependent upon the degree of flip angle and the amount of

time it takes for the net magnetization vector M to return to the position Mo, also known as the

relaxation delay. As a 90˚ pulse is applied, M will lay parallel and coplanar to the YX axis. As time

progresses though, the M will relax back towards the Z axis position Mo.

6

Page 7: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 5: Relaxation of the net magnetic moment M after a 90˚ pulse

The relaxation of M is different for each hydrogen nucleus due to different electronic and

magnetic environments contributed by neighboring bonded atoms. This unique delay time is

known as the variable T1 which is simply a time constant.

(K)= 1T 1 (3)

In this case, the rate of relaxation K, is inversely proportional to T1.

When performing an NMR experiment then, it is of interest to pulse the sample and perturb M,

and then depending upon T1, (which is dependent upon unique electronic environments) the

time taken to relax will produce a IMF that can be transformed to intensities in a frequency

domain. This frequency domain will have peaks that correspond directly to unique differences

in T1’s of different protons and thus allows for identification of particular groups of hydrogens

on an NMR spectra.

To measure T1 a 180o pulse followed by a delay , and then a 90o read pulse experiment can be

used. This essentially forces M towards the negative Z axis ,allows for M to start recovering

7

Page 8: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

towards the positive Z axis, and then forces M back down to the XY plane, thus allowing one to

measure how quickly M can recover between the two pulse periods. When is very short M

has very little time to recover and the 180o pulse and 90o pulse essential combine yielding a

270o pulse, which is opposite the detecting coil, and will produce an inverted spectrum. As

lengthens M begins to relax along the z-axis to its equilibrium value which is expected as τ is

simply a delay time and given long enough, M will always return to Mo.

M o−M z=2M o e−τ /T1

(4)

Using equation 4 τ can be solved for algebraically.

MM o

=(1−2e−τT 1 )

Recovery% ¿1−2e− τT 1

If 95% recovery is assumed,

(.95-1)/2 =-e− τT 1

τ = -ln(.025)T1

where T1 can be automatically obtained from the JOEL NMR Software.

It is important to consider that there is a relationship between resolution, the number of scans

one performs, the pulse angle, and the acquisition time it takes to perform each h NMR run.

The more scans one does, the longer the acquisition time. Also, if the recovery time of the M is

too long before the next subsequent pulse, resolution and signal will be lost because M can not

return to its equilibrium position. With these factors in mind, it is possible to acquire an optimal

pulse width for each T1 of the sample and used these values to acquire a better spectrum.

8

Page 9: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

The purpose of this experiment then is to first, take a regular NMR of the sample of

ethylbenzene and analyze the peaks to confirm it is indeed ethylbenzene. Second, run a single

180o pulse NMR and extrapolate the optimal pulse width for a 900 pulse that gives a maximum

signal. Third, to use a double pulse experiment to measure the T1s of each peak in ethyl

benzene. Fourth and finally, to measure the optimum flip angle at 3 different values of D, the

total duration.

METHODS:

All methods were carried out at the University of Texas at Dallas’ undergraduate teaching

laboratory in Berkner Hall under the guidance of Dr. Warren Goux. A JEOL FT-NMR was used to

acquire all data and spectra were printed and analyzed. Procedural details can be found in

appendix C and were provided by Dr Goux and his team of graduate TA’s.

1) A normal NMR spectrum was run by placing a sample of ethylbenzene in deuterated

chloroform in n NMR tube and subsequently after turning on the nitrogen gas, placing

the sample in the magnet. The sample was lowered, the auto lock was adjusted

accordingly the pulse angle was set to 90o the relaxation delay was set to 25[s] and the

receiver gain was 11. The NMR was run, the peaks were labeled, and the spectrum was

printed.

2) A single pulse experiment was run however the auto gain was turned off and the scans

were adjusted to 4. The pule was set to 45o and the listed values next to the pulse were

set to start at 5[us] and end at 35[us] with 5[us] increments. The relaxation value was

then changed to 25[s] and the NMR was run. Choosing the spectra once finished the

9

Page 10: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

spectrum was analyzed using the DELT curve fit analysis software. The spectra was

transferred over, the peaks were phased up, and the graph of intensity vs time was

printed and used to calculate the value of the 90o pulse width.

3) T1s were measured by the inversion recovery method where a 180o pulse was followed

by a 90o pulse . A double pulse experiment was performed in which the receiver gain

was set to 11 and a 90o pulse was optimized using a pulse width of 13.75 as determined

form part 2. The τ values were arrayed and set to start at .05s and to end at 25s. 8 scans

were performed, and the relaxation delay was set to 25[s]. The experiment was then

submitted and the spectra were collected for each of the 3 T1 peaks. The JEOL software

was used to calculate and fit the T1 data by using the curve analysis function and

applying an unweighted T1 for the function . the Data was fit to equation 4 and the

spectra were printed.

4) To measure the flip angle a single pulse experiment was run. The receiver gain was set

to 11 and the acquisition time was set to 1 second automatically after the number of

data points was set to 4096. The pulse with was set to 13.75 and the relaxation delay

was set to .1 sec. the Pulse angle was arrayed by setting the parameters to linear and

setting the start angle to 30 and end angle to 90 with 15 increments. The experiment

was submitted and the spectra were analyzed using the Delta software with the angle

measured at with d1 set to 2sec and 4 sec.

10

Page 11: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

RESULTS:

Figure 6 HNMR of Ethylbenzene

Table 1 NMR Analysis of Ethylbenzene

Peak Multiplicity Intensity (millions) Character 1.3ppm Triplet 14.8 -CH3

2.7ppm quartet 4.5 -CH2-7.32ppm Sextet 6 Shielded Aromatic

hydrogen7.39ppm Aromatic hydrogens

11

Page 12: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 7: Measurement of a 90o pulse width from minimal 180o signal

12

Page 13: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 8: T1 at 7.3ppm

13

Page 14: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 9: T1 at 2.7ppm

14

Page 15: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 10: T1 at 1.3ppm

15

Page 16: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 11: Flip Angle φ at D= 1.1 sec.

16

Page 17: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 12: Flip Angle φ at D= 3 sec.

17

60 degrees

Page 18: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Figure 13: Flip Angle φ at D= 5 sec.

18

Page 19: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

Table 2. T1 and optimal Flip angle at Values of D

Peak ppm

intensity (mil) T1 (S)

d1 for95%

Intensity3(T1)

(s)

Total DelayAT+d1

for 95% intensity

(S)

K1/T1

D(AT+d1)

ΦFlip angle

1.4 14.8 4.21612 12.64836 13.65 0.237185 5 752.8 4.5 4.56756 13.70268 14.70 0.218935 3 607.3 6 5.19108 15.57324 16.57 0.192638 1.1 45

Figure: 14 Optimal Pulse Angle vs total Duration

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.50

10

20

30

40

50

60

70

80

Pulse Angle vs D

D = total delay time [AT+ d1] (s)

optim

al P

ulse

Ang

le D

egre

es

DISCUSSION:

Overall this experiment was indeed successful. The HNMR of Ethylbenzene was taken and when

compared to spectra obtained by others [2], peak values of 7.3ppm, 2.7ppm and 1.3ppm with

multiplicities of 6, 4, and 3 aligned neatly with their results.

Upon determining the optimal pulse width of a 90o pulse, it was determined that 13.75

microseconds seemed to appear as the correct value as determined graphically.

19

Page 20: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

The T1 values collected of 4.216, 4.56756, 5.19108 seconds all appear to be reasonable.

According to Dr. Hu form the University of Massachusetts, typical T1 values of proton NMRs will

lay between 1-5 seconds, which agree well with results obtained.

The value of d1 to obtain 95% intensity should be such that d1 > 3(max T1)[5]. In this case the

largest T1 was 5.191 (s) which yields a d1 of 15.5732 (s) for 95% integration. D, or total delay

would be 16.5732 seconds given that acquisition time was set to 1 second.

It was noted that upon adjusting the τ values in the acquisition of T1 values, that if τ was too

small, the spectra would appear inverted. This could be explained by the fact that by decreasing

τ there is less time for relaxation between the 180o and 90o pulses, essentially resulting in the

two pulses nearly adding to create a 270o pulse. A 270o pulse orients M opposite relative the

detecting coils, which results in an EMF verses time graph in the opposite direction than a 900

pulse would create. The spectrum will thus appear inverted, and as τ increases, the pulse will

act more as the subsequent 90o one, which should produce a normal upright spectrum.

Upon research of T1 values for ethylbenzene it was noted that T1 vales are dependent on

temperature and concentration in addition to the character of the species being analyzed. In

this particular instance however, the temperature of the sample, nor the connection of the

ethyl benzene were noted as the sample was prepared by a graduate teaching assistant and

this information was not provided.

It was determined that when plotting the flip angle yielding max intensity verses different

values of total delay time D, a linear plot (figure 14) was obtained. According to this plot,

extrapolating backwards to the y axis where delay time is 0 seconds, the flip angle would

20

Page 21: NMR analysis of Ethylbenzene report

Physical Measurements Goux Spring 2016 Daniel Gonzalez

appear to lay around 40 degrees. This is expected though as typical NMR experiments are run

using multiple scans, and if a pulse of 90 is used with a sample having a particularly long T1,

signal could be lost if M does not have enough time to recover to Mo before the next pulse.

With this in mind, adjusting the time intervals between each pulse to accommodate a long T1

will result in an experiment that will take a long time to complete. Instead, if the pulse is

somewhere less than 90, perhaps 75[5], the time to recover will be less and more scans can be

completed in a shorter amount of time without losing much signal and therefore could be more

practical than running an NMR for multiple hours.

REFFERENCES:

1) Prof.N.L.Bauld, Final Exam, University of Texas at Austin, 2002 http://research.cm.utexas.edu/nbauld/ex4_key.htm (accessed Apr, 23,2016)

2) Gravitywaves.com. H NMR of Ethylbenzene http://www.gravitywaves.com/chemistry/CHE303L/OxidationArene_10.htm (Accessed Apr, 23, 2016)

3) University of Illinois. NMR Basic Concepts, http://scs.illinois.edu/nmr/handouts/getting_started/NMR_basic_theory.pdf (Accessed Apr, 23, 2016)

4) Weiguo, Hu, Introduction to 1D and 2D NMR Spectroscopy, university of Massachusetts http://nmrwiki.org/wiki/images/8/8e/NMRcourse2009.4.pdf (accessed Apr,24,2009)

5) Zhou, H. ;Optimal Pulse Width and Recycle Delay in a Single Pulse Experiment, Oct 20106) Goux,Warren; NMR Experiment/Report CHEM 4473 Spring 2015 (accessed Apr, 21,

2016)

APPENDIX:

-A- Rough Spectra

-B- Notebook Pages

-C- Procedural Instructions for Data Acquisition

21


Recommended