前面和背面進攻以N為中心的類SN2反應的理論研究

丁艷麗，母繼榮

(沈陽化工大學 數(shù)理系,沈陽 110142)

前面和背面進攻以N為中心的類SN2反應的理論研究

丁艷麗，母繼榮

(沈陽化工大學 數(shù)理系,沈陽 110142)

利用從頭算方法研究了HF前面和背面進攻以N為中心的類SN2反應機理，并與F-進攻以C為中心的SN2反應進行了對比分析。討論了反應路徑上固定點的幾何構型，勢能面輪廓，全局活化勢壘及中心活化勢壘等信息。研究表明，以N為中心的類SN2反應前面進攻路徑比背面進攻路徑容易進行，這與傳統(tǒng)的以C為反應中心的SN2反應完全不同。

前面進攻的類SN2反應；背面進攻的類SN2反應；構型；能量

0 引 言

1 計算方法

表1 MP2/6-311++G(3df,3pd)理論水平計算的全局活化勢壘中心活化勢壘，絡合能Ecomp

圖1 MP2/6-311++G(3df,3pd)方法計算的F-進攻SN2(C) 反應能量與先前理論計算值之間的線性關聯(lián)Fig.1 Linear correlations between our results for the SN2(C) reaction with F- at the MP2/6-311++G(3df,3pd) level and theoretical values

2 結果和討論

2.1 IRC路徑固定點的幾何構型

2.1.1 前面進攻保留路徑

前面進攻以N為中心的類SN2反應從HF進攻NH3…HF的N原子開始，經(jīng)由C1對稱性的過渡絡合物，到Cs對稱性的過渡態(tài)，再經(jīng)由C1對稱性的反應后過渡絡合物，其完全等同于反應前絡合物，達到分立的產(chǎn)物，反應具有保留構型。NH3…HF+HF與CH3F+F-反應的主要結構參數(shù)見圖2。圖2(a)和圖2(b)為反應前絡合物，圖2(c)和圖2(d)為過渡態(tài)，圖2(e)和圖2(f)為反應后絡合物。這兩個反應過渡態(tài)的結構類似，均為Cs對稱性，主要結構參數(shù)也類似，親核試劑和離去基團與反應中心原子之間的距離接近，虛頻振動模式類似，均為三中心過渡態(tài)，但是前者的反應絡合物為C1對稱性，后者的反應絡合物為C3v對稱性。

圖2 B3lyp/6-311++G(3df,3pd)方法優(yōu)化的前面進攻類SN2(N)和SN2(C)反應的固定點的幾何構型，單位nmFig.2 Geometries of the stationary points for the front-side attack identity SN2(N)-like and SN2(C) reactions optimized at the B3lyp/6-311++G(3df, 3pd) level of theory, distances(nm)

2.1.2 背面進攻翻轉路徑

背面進攻以N為中心的類SN2反應從HF進攻NH3…HF的NHHH面開始，經(jīng)由C3v對稱性的過渡絡合物，到D3h對稱性的過渡態(tài)，再經(jīng)由C3v對稱性的反應后過渡絡合物，達到分立的產(chǎn)物階段，反應經(jīng)過構型翻轉。NH3…HF+HF與CH3F+F-反應的主要結構參數(shù)見圖3，各圖說明與前面進攻的反應類似。背面進攻的這兩個反應固定點的結構類似，反應絡合物均為C3v對稱性，只不過前者為分子-分子型過渡絡合物，而后者為離子-分子型絡合物，過渡態(tài)均為D3h對稱性，且親核試劑和離去基團與反應中心原子之間的距離接近，虛頻振動模式類似，且反應均經(jīng)歷三中心過渡態(tài)。

圖3 B3lyp/6-311++G(3df,3pd)方法優(yōu)化的背面進攻類SN2(N)和SN2(C)反應的固定點的幾何構型，單位nmFig.3 Geometries of the stationary points for the back-side attack identity SN2(N)-like and SN2(C) reactions optimized at the B3lyp/6-311++G(3df, 3pd) level of theory, distances (nm)

2.2 勢能面的輪廓

圖4 前面和背面恒等交換NH3…HF+HF反應沿IRC路徑的能量輪廓Fig.4 Energy profiles for the front-side and back-side attack identity exchange reactions NH3…HF+HF along the IRC routes

NH3…HF+HF和CH3F+F-反應的勢能面輪廓見圖4和圖5，其中IR表示反應物，IRC表示反應前絡合物，ITS表示過渡態(tài)，IPC表示反應后絡合物，IP表示產(chǎn)物。MP2/6-311++G(3df,3pd)計算的各種能量值見表1。由圖4及表1可見， NH3…HF+HF反應無論是全局活化勢壘還是中心活化勢壘，前面進攻均比背面進攻的反應低，表明前者比后者容易進行。而CH3F+F-反應背面進攻與前面進攻的反應經(jīng)歷相同的離子-分子絡合物，但背面進攻的活化能遠低于前面進攻的反應，與NH3…HF+HF反應比背面進攻的反應路徑更容易進行。這表明HF進攻的類SN2(N)反應與傳統(tǒng)的F-進攻SN2(C)反應完全不同。

圖5 前面和背面恒等交換CH3F+F-反應沿IRC路徑的能量輪廓Fig.5 Energy profiles for the front-side and back-side attack identity exchange reactions CH3F+ F- along the IRC routes

3 結 論

1)給出了以N為中心HF前面和背面進攻的類SN2反應的能量，沿IRC路徑固定點的幾何構型，并對比了以C為反應中心F-進攻的SN2反應，給出了反應機理。

2)以N為中心的類SN2反應前面進攻路徑比背面進攻路徑容易進行，這與傳統(tǒng)的以C為反應中心的SN2反應完全不同。

致謝：非常感謝楊忠志教授對本研究工作的幫助和支持!

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Theoretical study on front-side and back-side attack SN2(N)-like reactions

DING Yan-Li, MU Ji-Rong

(Shenyang University of Chemical Technology, Department of Mathematics and Physics, Shenyang 110142, China)

Front-side and back-side attack SN2(N)-like reactions with HF have been investigated by abinitio method. And the SN2(N)-like reactions have been analyzed compared to SN2(C) reactions. The geometries of the stationary points, energy profiles, overall activation energies and central activation energies have been detailed research. Studies indicated that the front-side attack pathway was relatively easy to compare to back-side attack ones for the SN2(N)-like reaction. This was completely different from traditional SN2(C) reaction.

front-side attack SN2(N)-like reaction; back-side attack SN2(N)-like reaction; geometries; energies

10.13524/j.2095-008x.2015.01.010

2014-06-05;

2014-08-30

http://www.cnki.net/kcms/detail/23.1566.T.20141111.0930.001.html

國家自然科學基金資助項目(201403142)；遼寧省自然科學基金資助項目(2013020139)；遼寧省教育廳一般項目(L2013165)

丁艷麗(1979-),女,遼寧遼陽人，副教授，博士,研究方向：理論與計算化學,E-mail:yanliding@yeah.net。

O641.12

A

2095-008X(2015)01-0048-04