丁艷麗，母繼榮

(沈陽化工大學(xué) 數(shù)理系,沈陽 110142)

前面和背面進(jìn)攻以N為中心的類SN2反應(yīng)的理論研究

丁艷麗，母繼榮

(沈陽化工大學(xué) 數(shù)理系,沈陽 110142)

利用從頭算方法研究了HF前面和背面進(jìn)攻以N為中心的類SN2反應(yīng)機理，并與F-進(jìn)攻以C為中心的SN2反應(yīng)進(jìn)行了對比分析。討論了反應(yīng)路徑上固定點的幾何構(gòu)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(yīng)前面進(jìn)攻路徑比背面進(jìn)攻路徑容易進(jìn)行，這與傳統(tǒng)的以C為反應(yīng)中心的SN2反應(yīng)完全不同。

前面進(jìn)攻的類SN2反應(yīng)；背面進(jìn)攻的類SN2反應(yīng)；構(gòu)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(luò)合能Ecomp

圖1 MP2/6-311++G(3df,3pd)方法計算的F-進(jìn)攻SN2(C) 反應(yīng)能量與先前理論計算值之間的線性關(guān)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(jié)果和討論

2.1 IRC路徑固定點的幾何構(gòu)型

2.1.1 前面進(jìn)攻保留路徑

前面進(jìn)攻以N為中心的類SN2反應(yīng)從HF進(jìn)攻NH3…HF的N原子開始，經(jīng)由C1對稱性的過渡絡(luò)合物，到Cs對稱性的過渡態(tài)，再經(jīng)由C1對稱性的反應(yīng)后過渡絡(luò)合物，其完全等同于反應(yīng)前絡(luò)合物，達(dá)到分立的產(chǎn)物，反應(yīng)具有保留構(gòu)型。NH3…HF+HF與CH3F+F-反應(yīng)的主要結(jié)構(gòu)參數(shù)見圖2。圖2(a)和圖2(b)為反應(yīng)前絡(luò)合物，圖2(c)和圖2(d)為過渡態(tài)，圖2(e)和圖2(f)為反應(yīng)后絡(luò)合物。這兩個反應(yīng)過渡態(tài)的結(jié)構(gòu)類似，均為Cs對稱性，主要結(jié)構(gòu)參數(shù)也類似，親核試劑和離去基團與反應(yīng)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(yīng)絡(luò)合物為C1對稱性，后者的反應(yīng)絡(luò)合物為C3v對稱性。

圖2 B3lyp/6-311++G(3df,3pd)方法優(yōu)化的前面進(jìn)攻類SN2(N)和SN2(C)反應(yīng)的固定點的幾何構(gòu)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(jìn)攻翻轉(zhuǎn)路徑

背面進(jìn)攻以N為中心的類SN2反應(yīng)從HF進(jìn)攻NH3…HF的NHHH面開始，經(jīng)由C3v對稱性的過渡絡(luò)合物，到D3h對稱性的過渡態(tài)，再經(jīng)由C3v對稱性的反應(yīng)后過渡絡(luò)合物，達(dá)到分立的產(chǎn)物階段，反應(yīng)經(jīng)過構(gòu)型翻轉(zhuǎn)。NH3…HF+HF與CH3F+F-反應(yīng)的主要結(jié)構(gòu)參數(shù)見圖3，各圖說明與前面進(jìn)攻的反應(yīng)類似。背面進(jìn)攻的這兩個反應(yīng)固定點的結(jié)構(gòu)類似，反應(yīng)絡(luò)合物均為C3v對稱性，只不過前者為分子-分子型過渡絡(luò)合物，而后者為離子-分子型絡(luò)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(yīng)中心原子之間的距離接近，虛頻振動模式類似，且反應(yīng)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(jìn)攻類SN2(N)和SN2(C)反應(yīng)的固定點的幾何構(gòu)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(yīng)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(yīng)的勢能面輪廓見圖4和圖5，其中IR表示反應(yīng)物，IRC表示反應(yīng)前絡(luò)合物，ITS表示過渡態(tài)，IPC表示反應(yīng)后絡(luò)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(yīng)無論是全局活化勢壘還是中心活化勢壘，前面進(jìn)攻均比背面進(jìn)攻的反應(yīng)低，表明前者比后者容易進(jìn)行。而CH3F+F-反應(yīng)背面進(jìn)攻與前面進(jìn)攻的反應(yīng)經(jīng)歷相同的離子-分子絡(luò)合物，但背面進(jìn)攻的活化能遠(yuǎn)低于前面進(jìn)攻的反應(yīng)，與NH3…HF+HF反應(yīng)比背面進(jìn)攻的反應(yīng)路徑更容易進(jìn)行。這表明HF進(jìn)攻的類SN2(N)反應(yīng)與傳統(tǒng)的F-進(jìn)攻SN2(C)反應(yīng)完全不同。

圖5 前面和背面恒等交換CH3F+F-反應(yīng)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(jié) 論

1)給出了以N為中心HF前面和背面進(jìn)攻的類SN2反應(yīng)的能量，沿IRC路徑固定點的幾何構(gòu)型，并對比了以C為反應(yīng)中心F-進(jìn)攻的SN2反應(yīng)，給出了反應(yīng)機理。

2)以N為中心的類SN2反應(yīng)前面進(jìn)攻路徑比背面進(jìn)攻路徑容易進(jìn)行，這與傳統(tǒng)的以C為反應(yīng)中心的SN2反應(yīng)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(xué)基金資助項目(201403142)；遼寧省自然科學(xué)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(xué),E-mail:yanliding@yeah.net。

O641.12

A

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