Chapter 14 NMR Spectroscopy
-
Upload
gray-maddox -
Category
Documents
-
view
209 -
download
15
description
Transcript of Chapter 14 NMR Spectroscopy
![Page 1: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/1.jpg)
© 2011 Pearson Education, Inc.1
Chapter 14
NMR Spectroscopy
Organic Chemistry 6th Edition
Paula Yurkanis Bruice
![Page 2: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/2.jpg)
© 2011 Pearson Education, Inc.2
Nuclear Magnetic Resonance (NMR) Spectroscopy
Identify the carbon–hydrogen framework of an organiccompound
Certain nuclei, such as 1H, 13C, 15N, 19F, and 31P, havenon-zero value for their spin quantum number; this property allows them to be studied by NMR
![Page 3: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/3.jpg)
© 2011 Pearson Education, Inc.3
The spin state of a nucleus is affected by an appliedmagnetic field:
![Page 4: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/4.jpg)
© 2011 Pearson Education, Inc.4
The energy difference between the spin states increases with the strength of the applied magnetic field:
![Page 5: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/5.jpg)
© 2011 Pearson Education, Inc.5
-spin states -spin states
absorb E
release E
Signals detected by NMR
![Page 6: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/6.jpg)
© 2011 Pearson Education, Inc.6
An NMR Spectrometer
In pulsed Fourier transform (FT) spectrometers, the magnetic field is held constant, and a radio frequency (rf) pulse of short duration excites all the protons simultaneously
![Page 7: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/7.jpg)
© 2011 Pearson Education, Inc.7
The electrons surrounding a nucleus decrease the effective magnetic field sensed by the nucleus:
Beffective = Bo – Blocal
![Page 8: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/8.jpg)
© 2011 Pearson Education, Inc.8
Chemically equivalent protons: protons in the same chemical environment
Each set of chemically equivalent protons in a compoundgives rise to a signal in an 1H NMR spectrum of that compound:
![Page 9: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/9.jpg)
© 2011 Pearson Education, Inc.9
![Page 10: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/10.jpg)
© 2011 Pearson Education, Inc.10
The Chemical ShiftThe reference point of an NMR spectrum is defined bythe position of TMS (zero ppm):
The chemical shift is a measure of how far the signal isfrom the reference signal
The common scale for chemical shifts =
=distance downfield from TMS (Hz)
operating frequency of the spectrometer (MHz)
![Page 11: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/11.jpg)
© 2011 Pearson Education, Inc.11
1H NMR spectrum of 1-bromo-2,2-dimethylpropane
The greater the chemical shift, the higher the frequencyThe chemical shift is independent of the operating frequency of the spectrometer
![Page 12: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/12.jpg)
© 2011 Pearson Education, Inc.12
![Page 13: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/13.jpg)
© 2011 Pearson Education, Inc.13
Electron withdrawal causes NMR signals to appear athigher frequency (at larger values):
Protons in electron-poor environments show signals at high frequencies
![Page 14: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/14.jpg)
© 2011 Pearson Education, Inc.14
Characteristic Values of Chemical Shifts
![Page 15: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/15.jpg)
© 2011 Pearson Education, Inc.15
![Page 16: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/16.jpg)
© 2011 Pearson Education, Inc.16
![Page 17: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/17.jpg)
© 2011 Pearson Education, Inc.17
Diamagnetic Anisotropy
The unusual chemical shifts associated with hydrogens bonded to carbons that form bonds:
The electrons are freer to move than the electrons in response to a magnetic field
![Page 18: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/18.jpg)
© 2011 Pearson Education, Inc.18
The protons show signals at higher frequencies because they sense a larger effective magnetic field:
benzene
![Page 19: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/19.jpg)
© 2011 Pearson Education, Inc.19
The alkene and aldehyde protons also show signals at higher frequencies:
alkene aldehyde
![Page 20: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/20.jpg)
© 2011 Pearson Education, Inc.20
The alkyne proton shows a signal at a lower frequency than it would if the electrons did not induce a magnetic field:
alkyne
![Page 21: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/21.jpg)
© 2011 Pearson Education, Inc.21
1H NMR spectrum of 1-bromo-2,2-dimethylpropane
The area under each signal is proportional to the number of protons giving rise to the signal:
![Page 22: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/22.jpg)
© 2011 Pearson Education, Inc.22
The area under each signal is proportional to the numberof protons that give rise to that signal
The height of each integration step is proportional to thearea under a specific signal
The integration tells us the relative number of protonsthat give rise to each signal, not the absolute number
Integration Line
![Page 23: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/23.jpg)
© 2011 Pearson Education, Inc.23
![Page 24: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/24.jpg)
© 2011 Pearson Education, Inc.24
Splitting of the Signals
• An 1H NMR signal is split into N + 1 peaks, where N is the number of equivalent protons bonded to adjacent carbons
• Coupled protons split each other’s signal
• The number of peaks in a signal is called the multiplicity of the signal
• The splitting of signals, caused by spin–spin coupling, occurs when different kinds of protons are close to one another
![Page 25: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/25.jpg)
© 2011 Pearson Education, Inc.25
It is not the number of protons giving rise to a signal that determines the multiplicity of the signal
It is the number of protons bonded to the immediately adjacent carbons that determines the multiplicity
a: a tripletb: a quartetc: a singlet
![Page 26: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/26.jpg)
© 2011 Pearson Education, Inc.26
Equivalent protons do not split each other’s signal:
![Page 27: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/27.jpg)
© 2011 Pearson Education, Inc.27
![Page 28: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/28.jpg)
© 2011 Pearson Education, Inc.28
![Page 29: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/29.jpg)
© 2011 Pearson Education, Inc.29
The ways in which the magnetic fields of three protonscan be aligned:
![Page 30: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/30.jpg)
© 2011 Pearson Education, Inc.30
![Page 31: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/31.jpg)
© 2011 Pearson Education, Inc.31
Splitting is observed if the protons are separated by no more than three bonds:
Long-range coupling occurs over systems, such as benzene
![Page 32: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/32.jpg)
© 2011 Pearson Education, Inc.32
More Examples of 1H NMR SpectraTriplet: two neighboring protons
Quintet: four neighboring protons
![Page 33: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/33.jpg)
© 2011 Pearson Education, Inc.33
Doublet: one neighboring proton
Septet: six neighboring protonsTriplets: two
neighboring protons
Sextet: five neighboring protons
![Page 34: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/34.jpg)
© 2011 Pearson Education, Inc.34
The three vinylic protons are at relatively high frequencybecause of diamagnetic anisotropy
![Page 35: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/35.jpg)
© 2011 Pearson Education, Inc.35
The signals for the Hc, Hd, and He protons overlap because the electronic effect of an ethyl substituent is similar to that of a hydrogen:
![Page 36: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/36.jpg)
© 2011 Pearson Education, Inc.36
The signals for the Ha, Hb, and Hc protons do not overlapbecause of the strong electron-withdrawing property of the nitro group:
![Page 37: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/37.jpg)
© 2011 Pearson Education, Inc.37
Coupling ConstantsThe coupling constant (J) is the distance between twoadjacent peaks of a split NMR signal in hertz:
Coupled protons have the same coupling constant
![Page 38: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/38.jpg)
© 2011 Pearson Education, Inc.38
![Page 39: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/39.jpg)
© 2011 Pearson Education, Inc.39
Summary
1. The number of chemical shifts specify the number of proton environments in the compound
2. The chemical shift values specify the nature of the chemical environment: alkyl, alkene, etc.
3. The integration values specify the relative number of protons
4. The splitting specifies the number of neighboring protons
5. The coupling constants specify the orientation of the coupled protons
![Page 40: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/40.jpg)
© 2011 Pearson Education, Inc.40
A Splitting Diagram for a Doublet of Doublets
![Page 41: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/41.jpg)
© 2011 Pearson Education, Inc.41
Complex Splitting
Ha
JAC
JAB
JAC = JAB
Triplet
Ha
JAC
JAB
JAC > JAB
Doublet of doublets
![Page 42: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/42.jpg)
© 2011 Pearson Education, Inc.42
The trans coupling constant is greater than the cis coupling constant:
![Page 43: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/43.jpg)
© 2011 Pearson Education, Inc.43
A Splitting Diagram for a Quartet of Triplets
![Page 44: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/44.jpg)
© 2011 Pearson Education, Inc.44
Why is the signal for Ha a quintet rather than a triplet of triplet?
![Page 45: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/45.jpg)
© 2011 Pearson Education, Inc.45
![Page 46: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/46.jpg)
© 2011 Pearson Education, Inc.46
The Difference between a Quartet and a Doublet of Doublets
Methylene has three neighbors, appears as a quartet
Doublet Doublet
![Page 47: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/47.jpg)
© 2011 Pearson Education, Inc.47
When two different sets of protons split a signal, themultiplicity of the signal is determined by using the N + 1rule separately for each set of the hydrogens, as long as the coupling constants for the two sets are different
When the coupling constants are similar, the multiplicity of a signal can be determined by treating both sets of adjacent hydrogens as though they were equivalent
![Page 48: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/48.jpg)
© 2011 Pearson Education, Inc.48
Replacing one of the enantiotopic hydrogens by a deuterium or any other atom or group other than CH3 or OH forms a chiral molecule:
prochiral carbon
Ha is the pro-R-hydrogen, whereas Hb is the pro-S-hydrogen; and they are chemically equivalent
![Page 49: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/49.jpg)
© 2011 Pearson Education, Inc.49
Diastereotopic hydrogens have different chemical shifts:
![Page 50: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/50.jpg)
© 2011 Pearson Education, Inc.50
Diastereotopic hydrogens are not chemically equivalent:
![Page 51: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/51.jpg)
© 2011 Pearson Education, Inc.51
The three methyl protons are chemically equivalent because of rotation about the C—C bond:
We see one signal for the methyl group in the 1H NMRspectrum
![Page 52: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/52.jpg)
© 2011 Pearson Education, Inc.52
1H NMR spectra of cyclohexane-d11 at various temperatures:
H
HH
H
axial
equatorial
axial
equatorial
The rate of chair–chair
conversion istemperaturedependent
![Page 53: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/53.jpg)
© 2011 Pearson Education, Inc.53
Protons Bonded to Oxygen and Nitrogen
These protons can undergo proton exchange
They always appear as broad signals
The greater the extent of the hydrogen bond, the greater the chemical shift
![Page 54: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/54.jpg)
© 2011 Pearson Education, Inc.54
pure ethanol
ethanol with acid
![Page 55: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/55.jpg)
© 2011 Pearson Education, Inc.55
A 60-MHz 1H NMR spectrum
A 300-MHz 1H NMR spectrum
![Page 56: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/56.jpg)
© 2011 Pearson Education, Inc.56
To observe well-defined splitting patterns, the differencein the chemical shifts (in Hz) must be 10 times thecoupling constant values
![Page 57: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/57.jpg)
© 2011 Pearson Education, Inc.57
13C NMR Spectroscopy
• The number of signals reflects the number of different kinds of carbons in a compound.
• The overall intensity of a 13C signal is about 6400 times less than the intensity of an 1H signal.
• The chemical shift ranges over 220 ppm.
• The reference compound is TMS.
![Page 58: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/58.jpg)
© 2011 Pearson Education, Inc.58
![Page 59: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/59.jpg)
© 2011 Pearson Education, Inc.59
![Page 60: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/60.jpg)
© 2011 Pearson Education, Inc.60
Proton-Decoupled 13C NMR of 2-Butanol
![Page 61: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/61.jpg)
© 2011 Pearson Education, Inc.61
Proton-Coupled 13C NMR of 2-Butanol
![Page 62: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/62.jpg)
© 2011 Pearson Education, Inc.62
The intensity of a signal is somewhat related to the number of carbons giving rise to it
Carbons that are not attached to hydrogens give very small signals
![Page 63: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/63.jpg)
© 2011 Pearson Education, Inc.63
DEPT 13C NMR distinguishes CH3, CH2, and CH groups:
![Page 64: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/64.jpg)
© 2011 Pearson Education, Inc.64
The COSY spectrum identifies protons that are coupled:
Cross peaks indicate pairs of protons that are coupled
![Page 65: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/65.jpg)
© 2011 Pearson Education, Inc.65
COSY Spectrum of 1-Nitropropane
![Page 66: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/66.jpg)
© 2011 Pearson Education, Inc.66
The HETCOR spectrum of 2-methyl-3-pentanoneindicates coupling between protons and the carbon to which they are attached:
![Page 67: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/67.jpg)
© 2011 Pearson Education, Inc.67
Unknown Identification Using Spectroscopy
Example 1: 13C-NMR of C5H9Br
![Page 68: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/68.jpg)
© 2011 Pearson Education, Inc.68
Example 1: 1H-NMR of C5H9Br
![Page 69: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/69.jpg)
© 2011 Pearson Education, Inc.69
Example 1: IR of C5H9Br
Answer:
![Page 70: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/70.jpg)
© 2011 Pearson Education, Inc.70
Example 2: 13C-NMR of C6H10O4
24.133.4
174.4
Solvent:
![Page 71: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/71.jpg)
© 2011 Pearson Education, Inc.71
Example 2: 1H-NMR of C6H10O4
11.97
1.502.21
![Page 72: Chapter 14 NMR Spectroscopy](https://reader033.fdocuments.net/reader033/viewer/2022061404/56812b5e550346895d8f81f0/html5/thumbnails/72.jpg)
© 2011 Pearson Education, Inc.72
Example 2: IR of C6H10O4
Answer: