[org chem] NMR spec SN1 SN2 reaction - 香港寵物小精靈村落論壇

由於打不開帖子的關係，所以我找了abc666哥哥替我(fish)post以下內容 =]

事先聲明，看不明以下內容我會盡量去答，但連我(fish)自己也未必可以正確解答到
偶的professor也說他不完全且不需完全明白NMR theory去解NMR spectra,所以我也不能確定自己的explanation是否完全正確啦 (被打

在看之前我想說我在#2 post的rules是沒有問題的,chemicalequivalence (environment) 也是可以如此去tract的，對於分析simple spectra 亦是沒有問題的
下文亦只是補充內容，對於Basic的Organic chem只為參考之用
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對於此帖#2的halobenzene 例子，我想說因為今個quarter讀了一班專讀spectroscopy的班而發現自己用了最爛(?)的例子 (雖然個人認為ring + chiral + aromatic + 19 F + 31P 等物可以令例子更爛-w-)

爛的主因是benzene ring，解釋如下：

1. When two protons are in the samechemicalenvironment (as illustrated in the halobenzene example), they will bechemicallyequivalent.

2. A proton sometimes couples with protons that are within 4 or more sigma bonds. This is known as long-range coupling, which usually occurs in ring systems and allylic systems. Although long-range couplings usually have small coupling constants (1 to 2), it can go up to 8, which is the same as alkyl proton-alkyl proton coupling.

3.Chemicalequivalence does NOT necessarily meanmagneticequivalence. Two protons are said to bemagneticallyequivalent when they couple with the same atom with the same coupling constant. If two protons are notmagneticallyequivalent, they will show different splitting patterns around the same region.
As in the halobenzene example, the top green H will couple to the grey H next to it in an ortho- manner while the bottom green H will couple to that grey H in a para- manner. Thus, the two green protons are chemically equivalent BUT not magnetically non-equivalent.

總合以上三點，我當初的用字應改為samechemicalenvironment
而long range coupling 及magnetically non-equivalence 亦解釋了為何”When there is a bunch of junk around 7ppm, I would think about aromatic compounds.”

而對於之前有關irregular height 的問題,我想補充一下:

1. An NMR spectrum is said to be first order when the coupled protons are chemically equivalent and ∆ν/ J is greater than 10.** (∆ν is the distance between 2 signals while J is the coupling constant)
∆ν is positively related to the (external magnetic) field strength.
J is a constant that depends on the molecule.
** The value 10 does not guarantee if a spectrum is first order or not; it is just an approximation.

2. First order spectra are always symmetric, and the peak heights follow the rule of Pascal triangle.

3. When coupled to chemically and/ or magnetically non-equivalent protons, the peaks would have higher chance to have ∆ν/ J less than 10 (not first order).

4. If the spectrum is NOT first order, the rule of height by Pascal triangle would be violated.

大概就是以上了…..我只是想補完(?)一下欠缺的東東而已....要咀咒吐糟的隨便
我已盡量寫得簡易了-w-

香港寵物小精靈村落 論壇

香港寵物小精靈村落論壇