13 C NMR

For a molecule that is lack of C-H bonds, less information is forthcoming. For examples, polychlorinated compounds, polycarbonyl compounds, and compounds containing triple bonds.

For a molecule that has many C-H groups, interpreting from only ¹H nmr spectrum may be complicated. Depict 2 pairs of isomers (A & B) which display similar ¹H nmr spectra. These 2 cases might be difficult to identify by ¹H nmr alone.

These difficulties would be largely resolved by using ¹³C-NMR spectroscopy.

Why did ¹³C-NMR spectroscopy is developed after ¹H-NMR spectroscopy?

¹³C-NMR spectroscopy was developed after ¹H-NMR spectroscopy because

           ¹²C gives no signal (I = 0)
           natural abundance of ¹³C is particularly low (1.1%) and the resulting signal is weak
              ¹³C-NMR signals in natural samples are only about 10^-4 times the strength of ¹H-NMR signals

However, development of FTNMR techniques helps ¹³C-NMR spectroscopy become widely available because

              FTNMR spectrometers give a time-averaged spectrum and noise is random and partially cancels out
              sample signals is accumulated and becomes stronger than those from a single spectrum

An important parameter derived from ¹³C NMR spectra is the chemical shift.
    Normal range of ¹³C chemical shift is 0 - 220 ppm (d_C » 20 x d_H).

In order to see the correlation chart, you have to complete it by draging each type of carbon and droping it into the yellow goal.

The correlation chart of ¹³C chemical shift list in ppm from a referent compound, TMS

The correlation chart is devided into 4 parts;
	1. saturated C atoms appear at highest field (8 - 60 ppm) 2. saturated C atoms with EN effects appear in the range of 40 -80 ppm 3. unsaturated C atoms (alkenes, alkynes, aromatic compounds) appear in the range of 100- 175 ppm 4. carbonyl C atoms appear in the range of 155 - 220 ppm

	Like in 1H chemical shifts, 13C chemical shifts can be affected by EN and hybridization.
	EN produces more deshielding effect (down field shift) in 13C NMR than in 1H because          - the electronegative atom is directly attached to C atom and          - the effect occurs through only one bond (C-X). In the other hand, H is attached to C atom not directly to the electronegative atom so the effect occurs through only one bond (H-C-X).
	Changes in hybridization produce larger shift for directly involved 13C than they do for 1H attached to that 13C.
	Carbonyl carbons have the largest chemical shift due to both sp2 hybridization and an O (electronegative) atom.

The amount of ¹³C in a molecule is very small. Therefore, the chances of two adjacent carbon atoms being both ¹³C are very small. For this reason, homonuclear (C-C) coupling is rarely observed.
Protons attached to ¹³C atoms will couple with the carbon nucleus and then the resonance peaks are observed.

Proton-coupled ¹³C splitting patterns

Spectra which show the spin-spin splitting between ¹³C and the protons directly attached to it are called proton-coupled spectra or nondecoupled spectra.
The figure below shows the proton-coupled ¹³C NMR spectrum for ethyl phenylacetate. Although the alkyl carbon peaks are widely separated in this example, in more complicated structures the alkyl portion of the spectrum can be very difficult to interpret.

Proton-coupled ¹³C spectra for ethyl phenylacetate

Example : ¹³C-NMR spectrum of diethyl phthalate

Proton-coupled spectra are often difficult to interpret because
          1. Js for ¹³C-H couplings are between 100 to 250 Hz. ¹³C-H coupling constants are frequently larger than the d differences of the carbons in the spectrum.
          2. ¹³C normal signals with ¹H coupling has low signal/noise ratio due to the multiplet structure.
For these reasons, ¹³C-H coupling constants should be reduced by using spin decoupling techniques.
The intensities of many carbon resonances in a proton-decoupled spectrum increase significantly above those observed in a proton-coupled spectrum. Carbon atoms with Hs directly attached are more enhanced (more Hs attached, more enhancement). This effect is called Nuclear Overhauser Enhancement or NOE. Intensity of ¹³C signal increases with ¹H decoupling as shown below.

Click here to see more details about NOE

NMR spectrum can be complicated as the number of resonating protons increases due to overlap of resonating peaks and complex splitting patterns. To simplify these spectra, one can use spin-decoupling, often referred to as double resonance.

Double-Resonance Experiments

In double resonance experiments, a second RF signal is applied while the spectrum is scanned in the usual fashion. The RF can be set to irradiate a selected group of protons in the molecule being measured.

Irradiation causes the selected protons to become decoupled from all other protons in the compound. Protons that are coupled to the irradiated group of protons will have simplified signals due to the loss of coupling with the irradiated protons. Selective irradiation will allow us to assign peaks more readily to a structure.

	Identifying structure from proton-coupled spectra may difficult because peak overlap. Broad-band decoupling technique is developed to solve this problem.
Principle of broad-band decoupling (BB) :
	The decoupling process is accomphished by irradiating the sample with a broad spectrum of high intensity RF radiation between 40 -70 MHz while scanning at normal intensities at frequencies about 15 MHz. All protons become saturated and can no longer couple with 13C nuclei.

Example 1: ¹³C NMR spectra for ethyl phenylacetate
A. The Proton-coupled ¹³C NMR spectrum

B. The BB-decoupled ¹³C NMR spectrum : each carbon atom gives a single signal (multiplicity is lost).

Example 2: 22.63 MHz ¹³C NMR spectra of crotonic acid in CDCl₃

a) spectrum with proton-coupling	b) spectrum with 1H broad-band (BB) decoupling

Example 3: The ¹³C-NMR spectrum of 1-chloro-2-propanol

	This 3-C containing compound shows well separated peaks because they are different in shielding effects caused by circulating electrons.
	The lower electron density at the carbon, the less shielded, and the more downfield signal.
	For this reason, the carbon bearing the -OH group is the most deshielded carbon and shows the furthest downfield signal at d 67.
	Chlorine is less electronegative than oxygen and the carbon bonded to it gives more upfield signal at d 51.
	The methyl carbon (-CH3) has no electronegative groups attached, so it occurs the most upfield at d 20.

Example 4: ¹³C-NMR spectrum of diethyl phthalate

Advantages of BB-decoupling:
          clarify of the chemical shift difference among carbon signals
          increasing of signal intensity of ¹³C signals (up to 200%)
          short recording time

Disadvantages of BB-decoupling:
          lost of coupling information (s, d, t, and q)

	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

 Although off-resonance proton-decoupled spectra contain a great deal of information, the technique has been supplanted by the DEPT (Distortionless Enhancement by Polarization Transfer) experiment.

Proton – coupled ¹³C-NMR spectra are often difficult to interpret due to large coupling constants and overlaping of signals. For this reason, ¹³C-NMR spectra are taken with proton–decoupled mode in which C/H ratios is lost.

To provide this information while retaining signal strength, DEPT (Distortionless Enhancement by Polarization Transfer) is developed.

In DEPT experiments, methyl, methylene, and methine protons can be distinguishable. There are several variations on the experiment.

sub-spectrum	technique
CH	DEPT 90o
CH2	DEPT 45o - DEPT 135o
CH3	DEPT 45o + DEPT 135o - 0.707DEPT 90o
C	comparing the DEPT with the BB decoupled spectrum

The types of carbon observed with various of DEPTs.

1.	DEPT 45o	signals of all protonated carbons
2.	DEPT 90o	signal of CH groups
3.	DEPT 135o	negative signal of CH2, positive signal of CH and CH3, and no signal of C with no attached H

Nomally, only two DEPT experiments are sufficient, DEPT 90^o and DEPT 135^o.

we can distinguish C, CH, CH2 and CH3 because;          - there is no signal of C with no attached H          - CH2 shows negative signal whereas CH and CH3 show positive signal          - CH carbons absorb at lower field and lower signal intensity than CH3carbons

Example 1: DEPT spectrum of isobutyl acetate

Interpretation :

d (ppm)	type of signal in DEPT	represent
22 (b)	positive	2 CH3
24 (c)	positive	CH3
24 (a)	positive	CH
37 (d)	negative	CH2
62 (e)	negative	CH2
170 (f)	not present	C of  >C=O

Example 2: DEPT spectrum of caryophyllene oxide

بقري، وتعلم معي، قطرات صغيرة من الماء جعلالمحيطات، وسوف يكون خبيرا في هذا

what is this, why are you worried, brush up with simple things, you will come out genius, learn with me, small drops of water make an ocean, you will be an expert in this

あなたが心配している理由は、これは簡単なことでブラッシュアップ、、何か、あなたは、天才が出てくる私と一緒に学ぶことが、水の小さな滴が海を作るには、この専門家になる

Name: 1,2-dimethoxymethane

C₄H₁₀O₂

From the molecular formula, the compound has "0 degrees of unsaturation" (no double bonds or rings).

The ¹³C NMR has two peaks, a quartet at  54 (a CH₃) and a triplet at  80 (a CH₂). Since the molecule has four carbons and only two ¹³C NMR peaks, there must be symmetry. Both peaks are in the regions where carbons next to electronegative atoms occur (oxygen).

τι είναι αυτό, γιατί είσαι ανήσυχος, βούρτσα με απλά πράγματα, θα βγει ιδιοφυΐα, να μάθουν μαζί μου, μικρές σταγόνες του νερού κάνει έναν ωκεανό, θα είστε ένας εμπειρογνώμονας σε αυτό

আপনি চিন্তিত কেন এই সহজ জিনিস নিয়ে ব্রাশ,, কি, আপনি, প্রতিভা বাইরে আসতে আমার সাথে শিখতে হবে, জলের ছোট ঝরিয়া একটি মহাসাগর না, আপনি এই একজন বিশেষজ্ঞ হতে হবে

C₅H₇O₂N

From the molecular formula, the compound has "3 degrees of unsaturation" (3 double bonds or rings).

: ethyl cyanoacetate

The ¹³C NMR has 5 peaks, a quartet at  14 (a CH₃), a triplet at  59 (a CH₂), another triplet at  22 (another CH₂), and two singlets, one at  118 and one at  172. Since the molecule has five carbons and five ¹³C NMR peaks, there must be no symmetry. The singlet at  172 is in the carbonyl region, most likely an acid or an ester. The CH₂ at  59 is in the region where carbons next to electronegative atoms occur (i.e., oxygen) and the CH₃ at  14 is a simple terminal methyl, suggesting an -O-CH₂CH₃ residue. The singlet at  118 would be consistent with a nitrile carbon and the shielded CH₂ at  22 suggests that it may be adjacent to the sp-carbon of the nitrile

מה זה, למה אתה מודאג, לרענן עם דברים פשוטים, אתה תצא גאון, ללמוד איתי, טיפות קטנות של מיםיגרמו לים, אתה תהיה מומחה בזה

C₆H₁₀OFrom the molecular formula, the compound has "2 degrees of unsaturation" (2 double bonds or rings).

2-butanon-4-ene

The ¹³C NMR has 6 peaks, a quartet at  25 (a CH₃), a triplet at  49 (a CH₂), another quartet at  17 (another CH₃), two doublets (a CH) , one at  124 and one at  131, and one singlet at  207. Since the molecule has six carbons and six ¹³C NMR peaks, there must be no symmetry. The singlet at  207 is in the carbonyl region, most likely an aldehyde or ketone. The CH₃ groups at  17 and 25 are consistent with simple terminal methyl groups, with one slightly shifted by an mildly electronegative group (a carbonyl?). The doublets at  124 and 131 are in the alkene region, suggesting a -CHCH- group. The remaining CH₂ group at  49 is probably deshielded by two electronegative groups.

o que é isso, por que você está preocupado, retocar com coisas simples, você vai sair gênio, aprender comigo, pequenas gotas de água fazem um oceano, você vai ser um especialista neste

C₈H₈O

From the molecular formula, the compound has "5 degrees of unsaturation" (5 double bonds or rings).

acetophenone

The ¹³C NMR has 6 peaks, a quartet at  27 (a CH₃), three doublets (CH groups), at  129, 128 and 133, and two singlets, one at  137 and one at  197. Since the molecule has eight carbons and six ¹³C NMR peaks, there must some degree of symmetry. The singlet at  197 is in the carbonyl region, most likely an aldehyde or ketone. The CH₃ groups at  27 is consistent with a simple terminal methyl group, slightly shifted by an mildly electronegative group (a carbonyl?). The doublets at  129, 128 and 133 and the singlet at 137 are in the aromatic region, suggesting a monosubstituted aromatic group, with symmetry in four of the six carbons.

நீங்கள் கவலை ஏன் இந்த எளிய பொருட்களை கொண்டு துலக்க, என்ன, நீங்கள், மேதை வெளியே வர எனக்கு கற்று, நீர் சிறு துளிகள் ஒரு கடல் செய்ய, நீங்கள் இந்த ஒரு நிபுணர் இருக்கும்

के तपाईं चिंतित हो किन यो सरल कुरा संग ब्रश,, के हो, तपाईं, प्रतिभा बाहिर आउन मलाई संग सिक्न हुनेछ, पानी सानो थोपा एक महासागर बनाउन, तपाईं यस मा एक विशेषज्ञ हुनेछ\

તમને ચિંતા થતી હોય કે શા માટે આ સરળ બાબતો સાથે બ્રશ, શું છે, તમે પ્રતિભા બહાર આવે મારી સાથે શીખશે, પાણી નાના ટીપાં સમુદ્ર કરો, તો તમે આ એક નિષ્ણાત હશે

kas tas ir, kāpēc jūs uztraucaties, suka ar vienkāršām lietām, jūs iznākt ģēnijs, mācīties kopā ar mani, nelieli ūdens pilieni veikt okeānu, jums būs eksperts šajā

что это такое, почему ты беспокоишься, освежить с простых вещей, вы будете выходить гений, узнать со мной, маленькие капли воды сделать океан, вы будете экспертом в этом

 hvað er þetta, af hverju ert þú áhyggjur, bursta upp með einföldum hlutum, verður þú að koma út snillingur, læra með mér, litla dropa af vatni gera haf, verður þú að vera sérfræðingur í þessu

C₆H₈OFrom the molecular formula, the compound has "3 degrees of unsaturation" (3 double bonds or rings).

cyclohexanon-2-ene

The ¹³C NMR has 6 peaks, three triplets (CH₂ groups) at  46, 30 and 41, two doublets (CH groups), at  129 and 145, and one singlet at  198. Since the molecule has six carbons and six ¹³C NMR peaks, there must be no symmetry. The singlet at  198 is in the carbonyl region, most likely an aldehyde or ketone. Two of the three CH₂ groups are shifted by electronegative groups, suggesting a X-CH₂-CH₂-CH₂-Y unit. The doublets at  129 and 145 are in the alkene region, suggesting a -CHCH- group. The three degrees of unsaturation suggests that the molecule also has a ring.

这是什么，你为什么担心，刷了简单的事情，你会出来的天才，学我，小水珠做出的海洋，你将在这方面的专家

 당신이 걱정하는 이유는 간단한 것들로 브러시, 무엇인가, 당신은 천재 나올 나와 함께 배울 것, 물 작은 방울은 바다를 만들어,이 분야의 전문가가 될 것입니다

• Electronegative groups are "deshielding" and tend to move NMR signals from attached carbons further "downfield" (to higher ppm values).
• The -system of alkenes, aromatic compounds and carbonyls strongly deshield C nuclei and move them "downfield" to higher ppm values.
• Carbonyl carbons are strongly deshielded and occur at very high ppm values. Within this group, carboxylic acids and esters tend to have the smaller  values, while ketones and aldehydes have values  200.

The ¹³C chemical shift is dependent both on the presence of electronegative groups and on the steric environment. This is best demonstrated by examining a variety of hexane isomers:

Simple interior (primary and secondary) carbons tend to be in the range  25 - 45. Methyl groups which terminate unbranched alkyl chains, however, are significantly shielded (moved to lower  values), as shown by the examples above ( 14, 14.3 and 8.7). The origin of this effect is thought to be steric compression in the gamma () position due to gauche interactions. This is shown schematically below and the gamma position is marked above in the example for hexane.

The presence of an electronegative atom such as oxygen tends to move the chemical shift of the Œ-carbon down into the region  65 - 90, as shown in the examples below:

Halogens, however, have effects which are difficult to predict and carbons adjacent to halogens tend to have chemical shifts in the  30 - 50 region, as shown below. The effects are not simply additive, however, and multiple substitution can often be shielding (move the signal to lower  values). The nitrile carbon is significantly shielding and adjacent carbons tend to occur in the  20 - 25 region.

Alkene carbons tend to have chemical shifts in the range  110 - 140, as shown in the examples below. Conjugation between alkene centers has little effect, as demonstrated by the two middle structures shown below. Conjugation with an oxygen, however, has a dramatic shielding effect, which is attributed to contributions from the resonance forms shown below.

Alkyne carbons occur in the region  65 -85, and are significantly shielding to the carbons which are immediately adjacent ( 1.5 for the terminal methyl of 2-pentyne).

Carbonyls are the most highly deshielded carbons which are typically encountered. Their intensity is usually weak, since there are no attached hydrogens to contribute to the Nuclear Overhauser Effect enhancement (with the exception of aldehydes). Typical chemical shifts occur in the region  170 - 210 with esters, carboxylic acids and amides at the low end, and simple ketones and aldehydes at the high end of the range.

Aromatic carbons have chemical shifts in the range  120 - 140 and are shifted within this range by the nature of the attached substituent. The multiplicity of aromatic peaks in the non-decoupled spectrum is useful for identifying aromatic substitution patterns.

آپ پریشان کیوں ہیں اس سادہ چیزوں کے ساتھ برش،، کیا ہے، آپ، ہوشیار باہر آ میرے ساتھ سیکھ جائے گی، پانی کے چھوٹے چھوٹے قطرے ایک سمندر بنانے کے لئے، آپ کو اس میں ایک ماہر ہو جائے گا

Shift (ppm)
145.2	128.0
132.2	56..5
130.2	21.4

THANKS AND REGARD'S
DR ANTHONY MELVIN CRASTO Ph.D
amcrasto@gmail.com

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